Climate change and Palaeoclimatology

Testing hypotheses for the onset of Northern Hemisphere glaciation

September 2008

Whereas Antarctica began to develop significant ice caps in the early Oligocene (maybe in late Eocene times) those of the Northern Hemisphere, principally on Greenland, did not arise until about 3 Ma ago. There are several hypotheses for that onset of the Great Ice Age: closure of the Panama seaway and increased poleward heat transport in the North Atlantic; perhaps related development of the El Niño cycle in the East Pacific; uplift of the Himalaya and Rocky Mountains changing atmospheric circulation; lowered atmospheric CO2, and a combination of all four that allowed the Milankovich astronomical forcing to get a grip on Earth’s climate ‘machine’. Testing the hypotheses is somewhat more difficult than find empirical support for them; i.e. coincidences in timing. Climate scientists from Bristol, Cambridge and Leeds universities in the UK have attempted such a test, using a complex climate model involving coupled atmosphere-ocean circulation and ice-sheet models (Lunt, D.J. et al. 2008. Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels. Nature, v. 454, p. 1102-1105). Only a decrease in the greenhouse effect could have transformed climate over Greenland sufficiently to equip it with a large ice sheet, the other three main hypotheses falling a long way short, although each could have led to small ice volumes. Significantly, the study failed to find support for any of the terrestrial processes having been capable of ‘priming’ orbital and rotational forcing to such an extent that they triggered glaciation. Despite the claims by the authors, as computing power goes up and the resolution of feasible climate modelling comes down it is quite likely that within a few years there will be another view ‘supported’ by models.

Climate shock of the Younger Dryas

September 2008

Between 12,900-11,500 years before the present, high northern latitudes returned to almost full glacial conditions, after about 6000 years of warming since the last glacial maximum. Just prior to the Younger Dryas cooling event, conditions had warmed sufficiently that European people had migrated northwards, some to occupy what are now the British Isles. Temperate grasslands teeming with game were the probable attraction, and still-low sea levels permitted crossing of what became the North Sea. Although it is possible that some people remained in Britain through the thousand-year mini glaciation, conditions would have been at the extremes of winter cold and year-long windiness, judging from the Greenland ice-core records of air temperature and dust. Those records have shown for some time that the transition from warmth to frigidity was rapid, but not how rapid. The cold spell had much in common with sudden, millennial-scale coolings repeated several times during the run-up to the last glacial maximum. Each such event has been linked with interruptions in the shallow and deep circulation of North Atlantic ocean waters, a likely trigger having been reduction in the salinity of surface waters as a result of floods of fresh water, either through collapses of ice caps and melting of icebergs or, in the case of the Younger Dryas, release of massive amounts of fresh water from glacially-blocked lakes in North America. One result would have been failure of cold surface water to sink at high latitudes, thereby shutting down the suction effect that drags warm water northwards to raise temperatures, especially in NW Europe.

There are concerns that unsuspected climate shifts that stem from the Earth System rather than astronomical influences – the Milankovich effect – may characterise the period of global warming caused by human activities. Increased precipitation at high northern latitudes or melting of ice on Greenland could result in falling ocean salinity and slowing or shutdown of the North Atlantic heat conveyor. Two sets of data published in August 2008 highlight potential climate shifts that may arise with virtually no warning. Both rely on the potentially high resolution of cores through ice caps and stagnant lakes that are annually layered, which has hitherto not been fully exploited by climate scientists. European and North American researchers have focussed on the upper part of the latest core through the Greenland ice cap, using two or three samples from each annual layer (Steffensen, J.P. and 19 others 2008. High-resolution Greenland ice core data show abrupt climate change happens in few years. Science, v. 321, p. 680-684). Deuterium and oxygen isotopes during the onset of the Younger Dryas show a marked cooling at the source of moisture precipitated as snow within 1 to 3 years, which the authors ascribe to the Intertropical Convergence Zone migrating northwards through a major change in atmospheric circulation. Temperature over the Greenland ice cap also changed, but over about 50 years [note however, that the sharp warming of the Bolling episode took less than a decade].

The second study uses annually varved lake sediments that accumulated in an isolated lake in central Germany that filled a circular depression formed by explosive volcanism (Brauer, A. et al. 2008. An abrupt wind shift in western Europe at the onset of the Younger Dryas cold period. Nature Geoscience, v. 1, 520-523). The seasonal sediment layers change in thickness, colour and mineralogy as warmth gave way to the frigidity of the Younger Dryas. One of the proxies, the iron content of the sediments deposited under anoxic conditions during winters fell significantly within a year at 12679 BP, along with a 4-5 fold increase in the rate of sediment deposition. Together with shifts in the lake biota, these features suggest to the authors that within a year wind strength increased greatly, probably due to a greater incidence of storm-force westerlies brought on by a change in the position of the jet stream. Today, westerly winds add to warming in northern Europe, around 12.7 ka they added to cooling, which can only be explained by global cooling or a southward excursion of sea ice in the North Atlantic.

Neither abrupt climate shift can be produced by validation of today’s climate models using actual data from the time just before they took place. It follows therefore that similar shifts in the near future could make themselves felt with no warning.

Opinion has drifted back and forth regarding the global effects of the Younger Dryas, evidence for its effects in the Southern Hemisphere being scanty. The best place to look for direct evidence would be in mid-latitude glaciers, especially where they are abundant in South America and New Zealand. A study of the largest of these, the Southern Patagonian Icefield (Ackert, R.P et al. 2008. Patagonian glacier response during the late glacial-Holocene transition. Science, v. 321, p. 392-395) indicates that the ice there advanced around the time of the YD. However, its dating indicates that the advance lay outside the 1300 year span of the cold period in the Northern Hemisphere. It was more likely due to a local response to increased precipitation from air moving from the east.
See also: Flückiger, J. 2008. Did you say “fast”? Science, v. 321, p. 650-651.

A 0.8 Ma history of changing greenhouse gases

July 2008

Polar ice cores have presented us with the most exquisite records of how high-latitude climate has changed in the recent past from indirect clues presented by variations in stable isotopes of oxygen and deuterium (temperature change), dust and sulfate content (aridity and volcanicity respectively) in layers of ice. That proxy record extends back to 800 ka in the Dome C core from Antarctica, showing in great detail the course of the last nine glacial-interglacial cycles, both the astronomical effect of a changeover from a 40 ka pacing to one of around 100 ka and many intricacies on a millennial time scale. The most tangible archive of information resides in the air bubbles trapped by the original snow that eventually turned into ice. That reveals how the intricate pacing of climate change has been almost perfectly tracked by the global carbon cycle as shown by changes in the concentrations of carbon dioxide and methane. This was first demonstrated by cores through the Greenland ice cap, which penetrate just the last glacial episode and the warmth before and after. After several years of painstaking bubble analyses at many collaborating labs, the full 800 ka greenhouse-gas records from Antarctica have now appeared (Luthi, D. and 10 others 2008. High resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, v. 453, p. 379-382. Lulergue, L. and 9 others 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature, v. 453, p. 383-386). These long records demonstrate the close connection between climate and greenhouse gases that must be maintained by complex (and not fully understood) feedback mechanisms. Different Earth processes affect the two principal gases, methane probably being controlled by effects of varying temperature and rainfall on peat-rich swamps in the tropics, whereas carbon dioxide’s main driver is capture and release of carbon by the oceans. The central feature remains that of astronomical forces, with perhaps some sign of a signal from the 413 ka component of orbital eccentricity from a shift in the range of temperatures and greenhouse gases in 100 ka cycles around 450 ka ago, and a broad change in methane concentrations. Yet, despite being a pole away from high northern latitudes where comparison of the Greenland ice record with North Atlantic sea-floor sediment data revealed a northern cause for dramatic short term shifts, much the same millennial cycles characterise the whole Antarctic record. It could be that these rapid changes are proxies for the course of northern climate vagaries – there are about 75 of them in the methane Antarctic record. So stunning are the new data that they are sure to spur attempts to go back even further by more drilling in Antarctica, probably in the eastern ice cap where current air temperature and snow fall are extremely low and a greater length of time may be preserved in a smaller thickness of ice. That is because the faster snow and ice accumulate the more rapidly flow removes the record: the reason why the thick Greenland ice, although capable of yielding time resolution of as little as individual years, cannot retain records much beyond 200 ka.

See also: Brook, E. 2008. Windows on the greenhouse. Nature, v. 453, p. 291-292.

The yellowing of the Sahara

July 2008

As Earth emerged steadily from the last glacial maximum, around 14.8 ka when temperatures were close to those of the Holocene yet sea level still had a way to rise before reaching its current level, the Sahara became a land of wetlands, lakes and grassland. Many caves within its modern arid confines contain superb artwork depicting its fauna and the forager-hunters that preyed on it. Around the time of the earliest Pharaonic civilisation on the Nile floodplain (~3000 BCE) the humid episode ended, forcing inhabitants of the Sahara either to the Nile valley of the Mediterranean coast. Having spanned the millennium-long climatic upheaval of the Younger Dryas and the relative stability and warmth of the early Holocene, why it ended is something of a mystery. A small, amazingly beautiful lake in northern Chad seems to hold the key, as it has existed and gathered sediment for at least 6 thousand years (Kröpelin, S and 14 others 2008. Climate-driven ecosystem succession in the Sahara: the past 6000 years. Science, v. 320, p. 765-768), Lake Yoa is one of several permanent lakes fed by ancient groundwater from the vast Nubian Sandstone aquifer, yet receives negligible rainfall. The uppermost lake sediments are laminated in an annual fashion so that each layer and its contents of aquatic organisms, pollen and dust can be precisely dated. Between 4200 and 3900 years ago the lake changed from a freshwater habitat to a salt lake when evaporation overcame recharge by rain. However, the environment as a whole did not change suddenly, but progressively. The sudden change in salinity resulted from Lake Yoa losing any outflow, which previously had removed salts accumulated by evaporation of the inflowing groundwater. The lake would then no longer have had any use for humans and their livestock, but conditions did not drive people out of the Sahara suddenly.

Impact cause for Younger Dryas draws flak

May 2008

Almost a year ago two dozen scientists presented evidence to suggest that onset of the Younger Dryas at 12.9 ka followed upper atmosphere explosions of cometary material (Firestone, R.B. and 25 others 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences of the United States of America, v. 104, 16016-16021; see Whizz-bang view of Younger Dryas in EPN July 2007). Evidence cited included: excess iridium; tiny spherules; fullerenes containing extraterrestrial helium; nanodiamonds and evidence for huge wildfires. Not quite the Full Monty, as neither crater nor shocked mineral grains were claimed, hence the teams’ opting for a cometary airburst. In North America such signs were said to overly the last known occurrences of Clovis tools at 7 archaeological sites (see Clovis First hypothesis dumped above). It was pretty clear that the suggestion for a hitherto unnoticed event with a widespread signature – 26 sites either side of the Atlantic were cited –  was going to be challenged, and so it has (see Kerr, R.A. Experts find no evidence for a mammoth-killer impact. Science, v. 319, p. 1331-1332), perhaps not unconnected with the blaze of publicity surrounding the paper’s appearance, including several TV documentaries.

Well, say experts, sooty layers do suggest large-scale fires, but forest fires occur every year, especially when humans are around. Fullerenes or ‘buckyballs’ equally can form terrestrially, except those containing ET helium. The last is regarded by many critics as ‘inventive’; they have never been isolated since such combinations were first reported in 2001 (see Extinctions by impacts: smoking artillery in EPN March 2002). The accepted methodology for detection of tiny diamonds seems to have been ignored, and that claimed to have found them misused. The iridium ‘spike’ – crucial in identifying the global nature of the K-T event – by itself is not enough for claims of impacts. Astonishingly, the authors cited such a Younger Dryas iridium spike in a Greenland ice core, yet the originator of those data says his paper does not report abnormal iridium at 12.9 ka or anywhere during the YD. Microspherules rain down all the time with interplanetary dust, and do not constitute sound evidence either.
So, what on Earth is going on? A collaboration between 26 authors, who willingly supply other workers with materials for checking surely cannot be conspiring at a hoax. Impact experts are hinting at ‘over-enthusiasm’ by a team outside the ‘impact community’. It all sounds oddly similar to the furore that in 1980 greeted  first suggestions by the Alvarezes for the K-T impact…

Neoproterozoic climate modelling supports a ‘slushball’ Earth

January 2008

Following its first discovery, evidence for low-latitude glacial action at several times during the Neoproterozoic has fuelled one of the most publicised controversies in the geosciences. Was the Earth totally frozen over during these episodes, or was ice confined only to parts of the surface? Whatever, the last part of the Precambrian witnessed huge fluctuations of many kinds, and after the cold epochs the first large animals made a sudden appearance. The most dramatic geochemical ups and downs in Earth history took place, in the form of sudden extreme shifts in the relative proportions of the stable isotopes of carbon in seawater, as recorded by marine carbonate rocks. These fluctuations correlate closely with the evidence for low-latitude glaciations: large negative excursions of d13C with glacial epochs, and positive values developing between them. The first can be interpreted as the result of massive declines in photosynthetic fixation of organic carbon. The second suggests repeated recoveries of such biological productivity, which favours the extraction of 12C from seawater and an increase in the relative proportion of the heavier isotope as organic carbon becomes buried in seafloor sediments.

Since organic carbon is ultimately extracted photosynthetically from carbon dioxide in the atmosphere, a link between climate and living processes (and those that bury dead organisms) can be the basis for models attempting to explain the extraordinary events of Neoproterozoic times. If large amounts of organic carbon are buried or remain suspended in the oceans, the drawdown of atmospheric CO2 reduces the greenhouse effect and leads to cooling. Conceivably, the effect could be to so reduce global mean surface temperature that freezing conditions grip even the lowest latitudes. Once glacial and sea ice becomes established, its high reflectivity reduces the amount of incoming solar radiation that is absorbed to warm the Earth. The two processes combined would tend to lock frigid conditions in place until such time as gradual release of volcanic CO2 increased the atmospheric greenhouse effect. That is the theoretical essence of the Snowball Earth hypothesis in which complete ice cover sterilised surface biology for long periods. However, it leaves out two important factors: as water cools it is able to dissolve more gases from the atmosphere; organic carbon in ocean water can be transformed to dissolved CO2 if it is oxidised, thereby reducing the amount of carbon being buried. Modelling the carbon-climate link in the Neoproterozoic requires that both factors are accounted for (Peltier et al. 2007. Snowball Earth prevention by dissolved organic carbon remineralization. Nature, v. 450, p. 813-818).

The model devised by Richard Peltier and colleagues from the University of Toronto also incorporates the distribution of land at the time. Results from it show a looping behaviour, with recovery from frigidity as increases in dissolved oxygen convert organic carbon to dissolved carbon dioxide, whose increasing concentration in turn leads to more escape of the gas to the atmosphere. The model also suggests how glacial and sea ice might have developed during such a cycle, and with the late Precambrian configuration of drifting continents it allows for low-latitude continental glaciation, but not for all-enveloping sea ice. The implication is indeed glacial events vastly greater than those of the late Palaeozoic and during the present Ice Age, but less effect on marine photosynthesis than from Snowball conditions – a ‘Slushball’ Peltier et al. explain why the cyclical processes suggested by the model stopped before the start of the Phanerozoic, from carbon-isotope evidence for a massive oxidation of suspended marine organic carbon around 550 Ma. Thereafter, abundant oxygen and large animals ensured most dead organic carbon was oxidised in the oceans.

Unsurprisingly, one of the authors of the Snowball hypothesis finds flaws in the geochemical argument for its impossibility (Kaufman, A.J. 2007. Slush find. Nature, v. 450, p. 807-808). Not only was oxygen likely to have been at far lower atmospheric concentrations than it became in the Phanerozoic, the glacial epochs provide evidence that its concentration in seawater was very low. The marine diamictites associated with each contain both ironstones and iron-oxide cements. For them to have formed demands high concentrations of dissolved iron in sea water, in the form of reduced Fe2+ ions; incompatible with widespread oxidizing conditions that would favour Fe3+ whose compounds are insoluble.

Some good news about carbon burial

January 2008

The second largest ‘sink’ for atmospheric CO2, after silicate weathering and formation of carbonate sediments, is the burial of organic carbon. Derived from photosynthesis of carbon dioxide in the air or dissolved in water, organic carbon descends from the photic zone of the oceans or is carried from the land by rivers. In the second case it is often believed that more than 70% of the carbon load of rivers is oxidised back to CO2 before having a chance of being buried in marine sediments. To estimate the proportion that does contribute to carbon sequestration is a complicated matter, involving measurement of the carbon budgeting for an entire river basin and its offshore sediments. This has been done by a team of French geochemists for the huge Ganges-Brahmaputra system that drains the northern Indian subcontinent and much of the Himalaya (Galy, V. et al. 2007. Efficient carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature, v. 450, p. 407-410). This system carries a stupendous load of sediment, especially during the monsoon season. At 1 to 2 billion t of sediment deposited from suspension in the Bay of Bengal each year, this is the largest single flux of sediment from land to the ocean floor. Even more is delivered as bed load (rolling and bouncing sand particles) to build up the Ganges-Brahmaputra delta of Bangladesh and West Bengal, India. The authors found that recently produced organic carbon is about 4 to 5 times more abundant in the suspended sediment load than is reworked fossil carbon derived by erosion of ancient sedimentary rocks, which itself is predominant in the bed load. Fossil carbon makes no difference to the modern carbon cycle, provided it does not get oxidised, which is less likely than for recent organic carbon in a form that can be metabolised.

By comparing the recent organic carbon load suspended in the rivers’ flow with that in the fine sediments of the Bengal Fan, Galy et al. have been able to show that most of that carbon is conserved without oxidation. As a result, the Bengal Fan accounts annually for about 15% of global carbon burial. There are two reasons for this remarkable efficiency: the low oxygen availability in deep waters of the Bay of Bengal; the very high sediment load from erosion of the Himalaya that buries carbon before oxidation is possible. Orogenic belts in humid areas are therefore key factors in exerting negative feedback on climate, whereas drainages of flat areas, such as the Amazon and especially its main tributary the Rio Negro, encourage oxidation in their lower reaches and offshore and are less important.

Cyclicity of Neoproterozoic glacial epochs

September 2007

The concept of ‘snowball’ conditions on Earth in the late Precambrian is that climate reached a state that drove it into semi-permanent frigidity, so encasing the globe in ice. The idea has been challenged by several lines of evidence since its first proposal in the late 1990s. Few authorities doubt that the Neoproterozoic was punctuated by several periods of extreme cold whose effects extended to tropical latitudes, but many reckon that climate never reached runaway ‘snowball’ conditions. It now seems that the glacial episodes had something in common with those of the Pleistocene, and exhibited cyclicity (Rieu, R. et al. 2007. Climatic cycles during a Neoproterozoic ‘snowball’ glacial epoch. Geology, v. 35, p. 299-302). The evidence comes from one of the youngest sequences of glacial and non-glacial sediments in the Neoproterozoic: the Fiq Formation in Oman. It is in the form of mineral and geochemical indices of alteration, based on the climate dependence of chemical weathering in the source area for sediments.

Both the chemical and mineralogical indices show considerable variation in the Fiq Formation, peaks and troughs in both data sets correlating well. There appear to have been three glacial-interglacial cycles. Troughs in the indices coincide with diamictites that contain dropstones and are usually interpreted as having been deposited partly from floating ice shelves. Peaks are found in mudstone and sandstone sequences that contain no evidence for glacial conditions. Unfortunately, the lack of precise dating rules out useful knowledge of cycle duration. An extremely crude estimate based on assumed deposition rates and the 200-500 m thickness of each cycle suggests far longer periods (1 to 5 Ma) than in the present glacial epoch.

The Fiq Formation is by no means the only Neoproterozoic sequence containing repeated diamictites. For instance, the 900 m thick Port Askaig Formation from the Dalradian of Scotland and Ireland contains around 30 diamictite horizons separated by non-glaciogenic siltstones and sandstones. Were those alternations to be shown to involve similar variations in indices of alteration, a similar crude estimate of cycle duration would be of the order of hundreds of ka; i.e. comparable with Pleistocene cycles.

See also: Lorentz, N.J. & Corsetti, F.A. 2007. Another test for snowball Earth. Geology, v. 35, p. 383-384

An early Greenland ice cap

May 2007

Around 34 Ma the oxygen isotopes in deep-water forams from ocean-floor cores show a jump towards more ‘heavy’ ¹⁸O. Together with the sudden appearance of coarse debris in oceanic sediments around Antarctica, the early Oligocene isotopic shift has been taken to represent the first sizeable build-up of land ice since the Carboniferous-Permian glacial epoch over 200 Ma earlier. It is also a marker for the later decline in overall surface temperature, culminating at 2.5 Ma with the onset of cyclical glaciation of high northern latitudes. This has to be revised after discovery of older ice-rafted debris in ocean-floor sediments in the North Atlantic between Greenland and Norway (Eldrett, J.S. et al. 2007. Continental ice in Greenland during the Eocene and Oligocene. Nature, v. 446, p. 176-179). The debris occurs in layers dated between 30 to 38 Ma. Direct evidence for cooling that supported ice-cap formation in both polar areas from the end of the Eocene is not matched by indirect evidence for the contemporary state of the global ‘greenhouse’. Proxy data suggest that the CO₂ content of the atmosphere was around 7 times higher than in recent, pre-industrial times, falling dramatically in the middle Oligocene (~30 Ma) to about twice those in the Holocene, then slowly declining through the Neogene.

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Climate change and the penguin

March 2007

The Adélie penguin nests on rocky outcrops in coastal Antarctica. Because the climate is extremely dry, their nesting sites are rich in mummified remains. It would not occur to everyone to date penguin mummies (Emslie, S.D. et al . 2007. A 45,000 yr record of Adélie penguins and climate change in the Ross Sea, Antarctica. Geology, v. 35, p. 61-64), but since the Adélie, like all penguins, is oddly choosy it is a useful thing to try. Not only do Adélies demand dry sites, but they have to be within easy reach of open water during the nesting season. Abandoned sites are a proxy for those demands as they existed in the past. Emslie and colleagues were able to show that from 45 to 27 ka conditions for Adélies prevailed in the Ross Sea area, but penguins abandoned it until 8 ka, when they returned. Conditions did not suit them throughout the Holocene, however, and most modern colonies were established only in the last two millennia.

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Clear signs of a north-south climatic linkage

January 2007

The climate records obtained from cores through the ice cap of Greenland reveal so much because they enable very fine time-resolution. That is because it snows a lot in Greenland and the ice accumulation rate is high. The down-side is that Greenland 's glaciers move quickly and little more than the last glacial period (since 140 ka) is preserved. Antarctica is a great deal bigger than Greenland , and less water vapour reaches the accumulation zone of its ice cap. As a result, the record from Antarctic ice goes back much further, to 800 ka (see Yet further back in the Antarctic ice and Calibrating the deepest ice core, December 2005 and November 2006 issues of EPN). For most of the Antarctic cores the slow ice accumulation gives coarser time-resolution than available from Greenland . For the astronomically modulated long-term fluctuations in climate, that doesn't matter a great deal, but it poses a problem for understanding more rapid climate change that is bound up with the Earth system itself. Variations with periods of the order of a thousand years and less are products of atmospheric and oceanic circulation, in which climatologists expect different behaviour in the two hemispheres. An example is the thermohaline circulation of the North Atlantic, linked with the Gulf Stream . A widely held view is that the millennial variations in climate, which are such strong features in the record of the last glacial period, relate to periodic shutting down and restarting of the circulation, connected with changes in the salinity of the North Atlantic at high latitudes as ice sheets wax and wane. By delivering heat northwards, the North Atlantic thermohaline circulation may reduce available oceanic heat in the Southern Hemisphere: Greenland and Antarctica records should reveal whether or not there is a short-term ‘see-saw' effect on climate of the two hemispheres. The different time-resolutions have prevented that hypothesis from being confirmed or refuted. November 2006 saw publication of a much more detailed Antarctic record from closer to the Southern Ocean, in which 2500 m of ice represents 150 ka (EPICA Community Members 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica. Nature, v. 444, p. 195-198).

To splice the Greenland and Antarctic records together the EPICA team used the atmospheric methane record preserved in air bubble from both, calibrated to time using annual layering back to 41 ka and flow modelling for earlier periods. Indeed, there is evidence for the predicted ‘see-saw' between the hemispheres. As Greenland entered a cold stadial so Antarctica experienced a warming, and vice versa. That does not necessarily confirm a mechanism controlled by the North Atlantic thermohaline circulation: on a global scale its effect on heat transport is a lot less than processes involving atmospheric circulation. So there is a clear coupling, but also a need for a viable explanation. Interestingly, the enormous plunge back to almost full-glacial conditions in the Northern Hemisphere of the Younger Dryas (around 12 to 13 ka) still does not show up in the Antarctic record, even at a resolution of around 15 years for the new core.

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Calibrating the deepest ice core

November 2006

Although the ice that forms the upper parts of the Greenland and Antarctic ice sheets is annually layered, before about 70 ka the layering disappears because of plastic deformation. Earlier ages have to be estimated from models of the deformation, and a second check is to match the data records from ice cores against those from sea floor sediments. Different processes contribute to those records: for instance, the marine record of oxygen isotopes in benthonic forams tracks the changing volume of ice locked on land, while the same record from ice cores depends on the air temperature above the ice cap. The correlation does seem to work, however. But not, it seems, for the very deepest ice recovered from beneath Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) which extends to around 800 ka.

French scientists involved in the EPICA Dome C ice-core project have cunningly discovered a means of checking on the otherwise problematic deep Antarctic ice (Raisbeck, G.M. et al. 2006. ¹⁰Be evidence for the Matuyama-Brunhes geomagnetic reversal in the EPICA Dome C ice core. Nature, v. 444, p. 82-84). The core penetrated to an estimated time that should include the most recent magnetic reversal, dated very precisely to 778±2 ka. Although the exact details of how the magnetic field behaved during this reversal are unclear, it is known that when polarity flips the intensity of the field becomes very small. While the field is stable it is sufficiently strong to deflect charged particles, both in the solar wind and in cosmic rays, so that less pass through the atmosphere. Cosmic rays are so energetic that they can induce isotopic transformations, one product being ¹⁰Be. So if the magnetic field decreased so the proportion of ¹⁰Be in the atmosphere would go up. Raisbeck and colleagues have examined the EPICA core's ¹⁰Be record in great detail. In a 10 m thick section from a depth of almost 3.2 km the isotope rises to a peak, which they interpret as the signature of the reversal. If correct, this gives a `golden spike' against which the depth-to-age conversion can be refined.

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Balmy shores of the Precambrian

November 2006

Before the appearance of fossil organisms that could give clues to past climates the only sources of information are in the form of proxies. One of the best examples might seem to be the oxygen isotope composition of carbonate rocks that relate to sea-surface temperature. In fact it isn't useful for the Precambrian because estimates of SST depend on being able to identify the shells of planktonic animals and use their δ¹⁸O as a proxy. That is a pity, because limestones are common throughout the geological record and various aspects of their geochemistry have been used extensively as proxies for other crucial information, such as the relationship between their strontium isotope composition and the pace of continental weathering. Nonetheless, information about the mean temperature of all ocean water would be very useful. But a further problem is that many Precambrian limestones have been subject to alteration and perhaps changes in their oxygen isotope composition.

Another palaeothermometer also relies on the temperature-dependent fractionation of oxygen isotopes between seawater and silica that has precipitated from it to form cherts. In that case, the δ¹⁸O of chert decreases with temperature. The trouble is that silica is notoriously prone to being remobilised and reprecipitated as pH changes in the fluids within sedimentary rocks. Some Precambrian cherts gave such low δ¹⁸O that seawater temperature would have been tens of degrees higher than they were during the Phanerozoic. Such extraordinary results raise the suspicion of oxygen isotopes having been altered by warmer fluids passing through cherty sequences. Now the chert method has been given a boost by geochemists at the French National History Museum (Robert, F. & Chaussidon, M. 2006. A paleaotemperature curve for the Precambrian ocean based on silicon isotopes in cherts. Nature, v. 443, p. 969-972).

François Robert and Marc Chaussidon analysed the silicon isotopes in cherts for which oxygen isotope data are available. Since the two isotopic systems would both change, yet would behave differently during hydrothermal or metamorphic alteration, if the results correlate well both should be undisturbed. Except in samples that show the lowest δ¹⁸O values (i.e. highest temperatures) there is a good correlation. The finding validates many of the O-isotope seawater temperatures, but Si isotopes fractionate during precipitation too, again in relation to temperature. So Robert and Chaussidon take Precambrian ocean temperature data to a new level with estimates based on two methods. Their results are fascinating: as well as confirming a decline from around 70°C 3400 Ma ago to between 10 to 40°C in the Phanerozoic, the δ³⁰Si data show sharp downward `spikes' at about 2500 Ma and 1800 Ma. From 1500 to 600 Ma ocean temperature seems steady at around 20°C, so there is no sign of continually cold oceans through the period of `Snowball Earth' events—the number of samples cannot yet resolve the individual events, but the `Cryogenian' is an obvious target for more work. The data are also important as they hint at all kinds of possible biological outcomes for such global warmth, and explanations are definitely needed. Does the record suggest greater geothermal heating, or was it an outcome of the greenhouse effect? Will more details correlate with periods of changing burial of organic carbon? Whatever, the Precambrian has become a stranger world to contemplate.

See also: de la Rocha, C.L. 2006. In hot water. Nature, v. 443, p. 920-921.

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Another blow for `Snowball Earth'

October 2006

The so-called Cryogenian Period of the Neoproterozoic rests on evidence for coincident glaciation at all latitudes. It has been supposed to include at least two, maybe three and perhaps more frigid `snowball' events, each with a pattern of lower diamictites and an upper carbonate cap rock. The most widely supposed glacial epochs are the Sturtian at 712 Ma, the Marinoan at 635 Ma and the Gaskiers at 580 Ma, but Precambrian sedimentary sequences are notoriously difficult to tie down in time. Only if dateable igneous events bracket evidence for glaciation is an age truly valid. Yet the global 3-fold division depends largely on correlation of stratigraphic and carbon-isotope sequences with the odd few that are dated in an absolute time-frame. The developing field of rhenium-osmium (Re-Os) radiometric dating offers a more universal check, since it provides a means of dating highly reduced black shales, that are abundant in the Neoproterozoic. The first reported results come as a blow to the `Snowball Earth' community (Kendall, B. et al. 2006. Re-Os geochronology of postglacial black shales in Australia: constraints on the timing of the `Sturtian' glaciation. Geology, v. 34, p. 729-732).

Bruce Kendall and colleagues from the Universities of Alberta, Canada and Durham, UK have constrained some of the principal occurrences of the Sturtian event in Australia to between 643 and 657 Ma, by dating the shales which envelop the diamictites and cap carbonates. They are younger than even the widest range previously suggested for the Sturtian: either the glaciation was grossly diachronous, or this is yet another glaciation of `Sturtian' type. The best that can be concluded is that the `Cryogenian' was cold but glaciation shifted from place to place – a `slushball' model?

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Pliocene climate and a lesson for the near future?

August 2006

While most geoscientists use the products of processes that operate today to judge environments of the past, climatologists do the reverse: the past is the key to the present. While the climate record of the last 2.5 Ma is a key to understanding and perhaps even predicting rapid climate shifts during glacial-interglacial periods uncontaminated by human influences, such is the extent to which greenhouse emissions have affected the current climate that we have little idea what the outcomes may be. The possibility of greenhouse warming has become higher than in any previous interglacial epoch. To get even an inkling of what that might set in motion requires looking back to warmer times than the Late Pliocene and Pleistocene, at around 3 to 5 Ma. In the Early Pliocene it is very likely that CO 2 in the atmosphere was no more than nowadays. Because the Earth's geography was little different from the way it is now and the Milankovich forcing was the same too, modelling Early Pliocene climate might seem to result in similar patterns, but it doesn't (Fedorov, A.V. et al. 2006. The Pliocene paradox (mechanisms for a permanent El Niño). Science , v. 312 , p. 1485-1489). Sea level was some 25 metres higher than it is at present and mean global temperature was an extra 3 ° C, and sea-surface temperatures (from the oxygen isotopes in planktonic foraminifera) were high as well. Despite much the same forcing factors as today, the Pliocene lacked large high-latitude ice caps in Arctic regions. Milankovich-related fluctuations were damped down compared with those of the Pleistocene. Both modelling and geological evidence from the Early Pliocene suggests that Earth's climate was dominated by a perpetual El Niño in the tropical oceans, because of an inability of cold water to upwell periodically along the western tropical margins of Africa and South America . Quite probably such conditions had persisted for the previous 50 Ma, despite gradual overall cooling.

Fedorov and colleagues point to very different Early Pliocene climates in several regions: Mild winters in central and north-eastern North America; droughts in Indonesia and torrential rains in western North and South America . Overall, it was a much more humid world, and since water vapour is a powerful greenhouse gas warmth and humidity were sustained despite no higher CO 2 levels than now. At about 3 Ma, ocean surface waters began to cool, with signs that the alternations associated with El Niño and La Niña in the eastern Pacific began. An explanation for this is the gradual build up of very cold water deep in the ocean as a result of winds from continents cooling ocean surface water at high latitudes and causing it to sink. Without periodic upwellings, warm surface waters and cold deep waters could not mix, so inevitably the interface became shallower. At some critical depth, this thermocline could break surface, transforming both climate patterns and those of ocean currents, eventually to end up as the present tropical climate cyclicity which is connected with other climate features of the Great Ice Age.

Fedorov et al speculate that only a small descent of the ocean thermocline – a matter of a few tens of metres – could re-establish Pliocene conditions. That might occur because of continued anthropogenic warming, and the `flip' might be as quick as a few decades to centuries.

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The Younger Dryas and the Flood

June 2006

Between about 12.9 and 11.5 ka the progress of warming from the frigidity of the Last Glacial Maximum was rudely interrupted. For over a thousand years conditions returned to those of a mini ice age, with continental glaciers re-advancing on a large scale, an increase in aridity and a reversal of colonisation of high northern latitudes by both plants and humans. Pollen records become dominated by those of a diminutive alpine plant, the mountain avens (Dryas octopetala) from which the cold snap gets its name – the Younger Dryas. The pace at which cooling took place was dramatic, and glacial conditions swept in within a decade at most. The most likely scenario is failure of North Atlantic Deep Water to form, thereby shutting down the thermohaline circulation that draws the warming Gulf Stream into the Arctic Ocean off the northern cape of Norway. The reason for that was a massive and sudden freshening of surface water at high latitudes in the North Atlantic, but where the influx of fresh water came from is a puzzle. Wallace Broeker of the Lamont-Doherty Earth Observatory in New York State resurrected an earlier idea that a vast lake of meltwater in the region of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. Broeker has recently reviewed this hypothesis (Broeker, W.S. 2006. Was the Younger Dryas triggered by a flood? Science, v. 312, p. 1146-1148).

Oxygen isotope records from sediments in the Gulf of Mexico had been recording massive influx there of water depleted in ¹⁸O; a sure sign that the Mississippi was carrying much of the water produced by melting of the Laurentian ice sheet. That signature stops abruptly at the outset of the Younger Dryas. The meltwater must have found another outlet, but so far its oxygen isotope signature has not been conclusively discovered. As well as the St Lawrence escape route there are three other possibilities: north-westwards along the MacKenzie River valley; beneath the great ice sheet and through Hudson Bay; and by massive break-up of the ice sheet to launch an `armada' of icebergs that quickly melted to freshen northern Atlantic waters. One of the clearest signs that vast proglacial lakes suddenly emptied is that they carve immense channels resembling canyons, in which there is abundant evidence for extreme scouring. Examples are the `channelled scablands' of the state of Washington, and the Minnesota River valley. The volume escaping at the start of the Younger Dryas would have been so immense that such overflow channels would be dominant features of northern North America's terrain; but there are few that fit the bill, and those that do exist are poorly constrained by radiocarbon dating. The lack of accurate dates for sediments and channels associated with the demise of the Laurentian ice sheet is the main obstacle, and surely evidence for exactly how the sudden plunge into glacial conditions was triggered will emerge sooner rather than later. One thing seems certain, the Younger Dryas was a freak event. The new ice core from Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) penetrates the previous six glacial maxima and shows no sign of a similar event at their terminations.

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Sedimentary evolution of the Arctic Ocean: a start is made

For the Northern Hemisphere, especially around the North Atlantic, what happens in the Arctic exerts a strong influence over climate. On the one hand, ice-cover increases the proportion of solar energy that is reflected back to space, giving a cooling effect. On the other, cooling and increasing salinity of high-latitude water at the ocean surface results in its sinking to draw in warmer waters from further south, to extend warming further north. The two are linked intricately, for sea-ice formation adds to surface waters' salinity. How and when the delicate balances arose remained poorly known while thick sea ice prevented ships penetrating to the highest possible latitudes in the Arctic Ocean, because the key to climate evolution depends on access to long core through ocean-floor sediments. Ironically, the decrease in Arctic ice cover with global warming has created greater access by icebreakers and drilling vessels. A consortium of countries around the Arctic funded a major effort to resolve the gap in knowledge through such a marine drilling programme in 2004. Results from the polar expedition have just begun to emerge (Moran, K and 36 other 2006. The Cenozoic palaeoenvironment of the Arctic Ocean. Nature, v. 441, p. 601-605). The cores were taken almost at the North geographic pole on the Lomonosov Ridge, a sliver of continental crust separated from its connection with the northern Russian continental shelf when North Atlantic sea-floor spreading nosed into the Arctic about 57 Ma ago.

The core is from sediments deposited on the Lomonosov ridge since it became detached from Russia, and is over 400 m long. Analyses are not yet complete, and the report by the IODP Arctic Coring Expedition covers the simplest parameters to determine: sediment bulk density and lithology, and micro-organisms. Nonetheless, these preliminary results provide a major surprise. Previously it was believed that frigid conditions in northern polar regions became established long after the Antarctic developed an ice cap 43 Ma ago, which matches the Cenozoic fall in atmospheric CO₂ and other evidence for lower mean global temperatures. The first glaciation in the Arctic was thought to be at 2-3 Ma, when pebbles dropped by icebergs first appear in the cores from the North Atlantic floor. In the Arctic Ocean core, such pebbles appear at much the same time as those around the Antarctic. They become widespread by 14 Ma. At the time of the Palaeocene-Eocene global warming, in response to massive methane emissions at 55 Ma, the Arctic waters were as warm as 18°C. The record is one of transition from a greenhouse world to an ice house. Surprisingly, considering the later influence of thermohaline processes that draw in warm water from lower latitudes, the earliest period is marked by fresh or at most slightly brackish waters. That was probably a result of isolation from the Atlantic and an excess of precipitation over evaporation. The early sediments record abundant carbon, then at around 14 Ma, the percentage of buried organic carbon drops dramatically to mark the start of increasing frigidity, when icebergs dropped significantly more debris in the Arctic Ocean.

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Yet further back in the Antarctic ice

December 2005

The groundbreaking Vostok ice core from Antarctica is the deepest ever to have been drilled. It recorded 440 ka of climate and atmospheric history, but unfortunately the very depth of the ice beneath the drilling station made that the limit in time terms. Thick ice begins to deform and flow, and the lowest parts of the Vostok core were clearly scrambled by that. The European Project for Ice Coring in Antarctica (EPICA) focussed its effort on a region of the East Antarctic ice sheet (Dome Concordia) whose location may always have ensured low accumulation of snow. Hopefully that would ensure that ice thickness was not so much as to result in complex flow at depth and that a fuller record would be preserved. The idea paid off, and the Dome C core penetrates back as far as 740 ka, giving an additional 3 glacial-interglacial cycles during the early part of the 100 ka periodicity; but falling just short of the first of those major cycles that are reflected in the marine oxygen-isotope record.

Results are now starting to emerge from Dome C (Siegenthaler, U and 10 others 2005. Stable carbon cycle-climate relationship during the Late Pleistocene. Science, v. 310, p. 1313-1317. Spahni, R. and 10 others 2005. Atmospheric methane and nitrous oxide of the Late Pleistocene from Antarctic ice cores. Science, v. 310, p. 1317-1321). The results are high-quality, and reveal some new features. The first three cycles conform to the 100 ka signal of the very weak variation in orbital eccentricity, as expected, but show lower amplitude shifts in CO₂ and methane in air trapped in bubbles than do the later four cycles. The two `greenhouse' gases vary in concert, and their earlier low levels match with less extreme shifts in temperature as shown by the changes in deuterium content of the ice itself. This is probably due to the transition from the previous dominance by the 40 ka pace of changing axial tilt. Nitrous oxide values, although patchy down the core, seem to have fluctuated but at much the same amplitude throughout the last 720 00 ka. Dome C has yet to be `bottomed out' so there is a chance that the record may yet reach the 40-100 ka boundary around 900 ka ago. What is striking – and should ring alarm bells – from the results so far is that in each of the previous 7 interglacials atmospheric neither CO₂ nor methane levels came close to those of the last century. Whatever its eventual effects, anthropogenic addition to the `greenhouse effect' is an incontrovertible fact.

See also: Brook, E.J. 2005. Tiny bubbles tell all. Science, v. 210, p. 1285-7

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Climate and the end-Permian extinction

October 2005

A time in Earth history (~251 Ma) when life was all but snuffed out and from which the creatures most familiar to us eventually emerged is understandably revisited quite often. Causes ranging from impacts (no convincing evidence as yet), through flood-basalt emissions, catastrophic methane release, low atmospheric oxygen to ocean anoxia have all been proposed. Hesitantly, opinion is converging on a climatic crisis of some kind, and indeed the coincidence of both terrestrial and marine faunal and flora extinctions points to climate being the global transmitter of some cause or a coincidence of causes. After the waning of Southern Hemisphere glaciations, the late Permian was warm, even at high latitudes. Until recently, attempts at modelling the end-Permian climate have not been entirely convincing because of limitations in the models themselves. Jeffrey Kiehl and Christine Shields of the US National Center for Atmospheric Research in Colorado have assembled a model that couples land, atmosphere, oceans, sea-ice and palaeogeography for the period (Kiehl, J.T. & Shields, C.A. 2005. Climate simulation of the latest Permian: Implications for mass extinction. Geology, v. 33, p. 757-760).

The critical test for the model is running it with parameters for the near-present, and it performs well. Several lines of evidence point to a much higher CO₂ level in the Permian atmosphere, so this is the main input parameter. The outcome is a world with a mean surface temperature that is 8° C higher than now. Unlike today, there was no geographic hindrance to poleward heat transport, so the high mean temperature is reflected in the summer warmth and humidity of Permian high-latitude land. The sub–tropics on the other hand were scorching (around an average summer minimum of 51° C, 15° C higher than now); a clear contributor to minimising life there. Sea-surface temperatures at high latitudes are higher in the model outcomes, this warmth extending to depths of 3 km. Surprisingly, low-latitude sea temperature emerges as much the same as now. The model also suggests that seawater was saltier than now, and that results in greater uniformity of density with depth and location: a hindrance to bottomward circulation and mixing. There would probably have been no thermohaline circulation worth speaking of. The model helps confirm the likelihood of an oxygen-free lower ocean and little transfer of nutrients. The oceans too would have been inhospitable. A shutdown of biological productivity and therefore carbon burial would have accelerated warming. So, pushing the biosphere into a mass extinction would have been inevitable. The last straw may have been the additional stress of increasing acidity from sulphur dioxide emissions from the Siberian flood basalts.

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Milankovich forcing and Early Jurassic methane

October 2005

Periods of environmental crisis less severe than those leading to mass extinction appear throughout the fossil record. As well as minor extinction peaks they are often signified by departures of carbon-isotope records from long-lasting norms. Such a crisis appears in the d 13C record of the Early Jurassic, and is beautifully preserved in about 15 m of black shales on the North Yorkshire coast of England. Geoscientists from the Open University, UK and the University of Cologne, Germany have produced an extremely high-resolution time series of carbon-isotope data from the section (Kemp, D.B. et al. 2005. Astronomical pacing of methane release in the Early Jurassic period. Nature, v. 437, p. 396-399). The quality is sufficiently good to analyse the time series using Fourier analysis that yields the frequencies that contribute to the observed wave-like patterns in the data. Of course, the time in a stratigraphic time series is measured in metres, unless it is possible to calibrate the section by precise radiometric dating. The Yorkshire Jurassic contains only fossils and no dateable horizons, but the fine stratigraphic division based on ammonites is also widespread and calibration is possible from dates obtained elsewhere. The overwhelmingly dominant frequency in the carbon-isotope curve is 1.23 cycles m-1, which represents 21 ka after the calibration of depth to time. That is the signal of precession of the equinoxes, part of the astronomical forcing bound up in Miliutin Milankovich's theory of astronomical forcing of climate.

Astronomical pacing turns up throughout the stratigraphic column, wherever sediments are suitable for time-series analysis (steady, unbroken sedimentation), so a precessional signal is no great surprise. The important feature is the profundity of the d 13C excursions; a total of –7‰, largely accomplished by three abrupt shifts of –2 to –3‰. The first two coincide with bursts in extinctions. The most likely phenomenon to have produced these shifts is massive release of methane by destabilization of submarine gas hydrates. Emissions seem to have been blurting out on a regular basis as the Earth's rotational axis precessed like a gyroscope. So, the complete time period was one in which gas hydrate was unstable, probably due to overall warming. Yet something else must have triggered vast releases three times. The Lower Jurassic extinctions link in time with massive magmatism in Southern Africa and Antarctic (the Karoo-Ferrar large igneous province). Perhaps especially large volcanic events there set the stage for large precessional methane releases. An alternative view is that volcanic emissions of CO₂ gradually produced enough widespread warming for the astronomical trigger to cause breakdown of gas hydrate simultaneously over very wide areas of the ocean floor. Other explanations have been suggested for the Lower Jurassic warming and carbon-isotope excursions, such as wildfires, impacts and connections with petroleum maturation and migration. The clear cyclicity rules them out.

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Documenting the Palaeogene transition from `hothouse' to `icehouse'

August 2005

It is well-established that the first large ice sheets that presaged descent into the oscillating climate of the Neogene formed about 34 Ma ago (the Eocene-Oligocene boundary) on Antarctica. Some 21 Ma before, at the Palaeocene-Eocene boundary, global temperatures had leaped following what many believe was a massive blurt of methane previously held in cold storage in ocean-floor sediments as gas hydrate. A monstrous `greenhouse' climatic system must sometime in the interim have reverted to the cooling trend begun at the outset of the Cenozoic. Defining that transformation relies on assembling and interpreting newly available, high-resolution records of climatic proxies through the Eocene and Early Oligocene (Tripati, A. et al. 2005. Eocene bipolar glaciation associated with global carbon cycle changes. Nature, v. 436, p. 341-346). Hitherto, the Eocene part of the ocean-floor sedimentary column had been poorly sampled, so that only broad trends showed.

As you might expect, the change was not a simple transition. At about 42 Ma the record of the Pacific Ocean calcite compensation depth (CCD – the depth at which carbonate remains are dissolved in the deep oceans) shows a remarkable perturbation long before the CCD dipped decisively from about 3.5 km to around 5 km at the start of the Oligocene. A close look at the oxygen isotope record of that age in a highly detailed marine sediment core shows an increase in d 18O that corresponds to either some 6° of cooling or a 120 m fall in sea-level due to build-up somewhere of ice on land. Coinciding with this perturbation are shifts in the carbon-isotope record in carbonates. The authors suggest that the mid-Eocene cooling and continental glaciation that produced falling sea level triggered the weathering of shallow-water carbonates, which together with river transport increased the oceans' alkalinity. That would have increased deep-water carbonate formation enormously and accelerated the effective `burial' of carbon from the atmosphere.

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Tracking ocean circulation during the last glacial period

May 2005

The use of various ocean-floor sediment proxies for climate change, such as the ups and downs of heavy ¹⁸O that chart waxing and waning continental ice cover, has progressively revealed the complexity of shifts during glacial and interglacial periods. Yet more emerged from finer-resolution time-series contained with Greenland and Antarctic ice cores. The diversity of information that proxy for many different, climate-related processes has in the last decade enabled palaeoclimatologists to begin piecing together possible causative mechanisms, beyond the initial discovery of an astronomical signal in early oxygen-isotope records. One of enormous significance is the possibility that sudden millennial-scale cooling and warming link to changes in ocean circulation, especially that performed by the Gulf Stream driven by thermohaline processes at high northern latitudes. Shutting down that poleward transfer of heat, probably because freshwater made high-latitude surface water less dense, has been implicated in sudden cooling or "stadials", and its restart linked to warming or "or interstadials". The last such sudden climate event, the Younger Dryas between about 12 and 11 thousand years ago, is widely believed to have resulted from a collapse of the Gulf Stream. That has raised fears that current anthropogenic warming might achieve the same thing, thereby plunging Western Europe into a counterintuitive frigid period through loss of its maritime warming. Ocean circulation has lacked a proxy that might help resolve such worrying scenarios, but it seems that one has arrived, because of improvements in mass spectrometry (Piotrowski, A.M. et al. 2005. Temporal relationships of carbon cycling and ocean circulation at glacial boundaries. Science, v. 307, p. 1933-1938). Different bodies of ocean-surface water have subtly different chemical compositions, due to the varied geochemistry of surrounding landmasses. Weathering of exposed rocks results in some elements entering solution in river water, and that mixes with surface water in the nearby ocean. Among the most useful elements are those with an isotope to which radioactive decay of unstable isotopes of another element contributes. A good example is ⁸⁷Sr that is formed when ⁸⁷Rb decays. Where continents expose large expanses of very ancient rocks they contribute more 87Sr to seawater than do continents veneered with younger rocks. Strontium isotopes have been used successfully for charting very-long term changes in the overall erosion of continental crust, in relation to climate shifts, but being related to calcium are taken up quickly by carbonate secreting organisms, such as foraminifera, at many different levels in the ocean as it circulates. So they are not very useful for short-term studies. A more useful isotopic system involving an daughter of slow radioactive decay is that of neodymium, because it does not get taken up in this way. It does however enter the manganese minerals that slowly precipitate on the deep ocean floor. Moreover, its isotopic composition varies greatly in different ocean-water masses. Piotrowski et al. used neodymium isotopes from deep ocean cores to see if changes in this circulation proxy coincided with known climate proxies. For interstadial, warming events there is a match, so a Gulf-stream control over millennial-scale climate shifts is indeed supported. But for the start and end of the full glacial period control by ocean circulation did not happen. Instead, changes in the neodymium record lag behind the climate proxies, suggesting climatic control of circulation, which then "kicked in" to boost changes that were well underway.

See also: Kerr, R.A. 2005. Ocean flow amplified, not triggered, climate change. Science, v. 307, p. 1854.

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Snowball Earth gets a boost

May 2005

Since Paul Hoffman and others launched their hypothesis of successive Earth-enveloping glaciations during the Neoproterozoic Eon, that notion of "Snowball" conditions has received many severe knocks, charted by numerous items in EPN. Geochemists and geologists from the Universities of Vienna and Witwatersrand realised that a good test of the hypothesis would be to concentrate on a rather obvious property of an ice-bound planet (Bodiselitsch, B. et al. 2005. Estimating duration and intensity of Neoproterozoic Snowball glaciations from Ir anomalies. Science, v. 308. P. 239-242). Whatever falls on an ice sheet, whether it is cosmic dust from outside the Earth or ash from volcanoes, becomes trapped in the annual layers of ice. When the ice melts, that accumulated content is transferred to the oceans very quickly. With weathering in suspended animation during the glacial epoch, transport of many elements would have slowed to very low levels. So, marine sediments deposited immediately after the diamictites that are allegedly glaciogenic ought to contain anomalously high levels of several elements. The most important of these would be those which show very different abundance patterns in meteorites form those in terrestrial rocks.

Bodiselitsch et al. hit what seems to be "paydirt" in carbonates above a prominent diamictite in central Africa. Their samples are impeccable, being from diamond-drill cores produced during evaluation of sediment-hosted mineralization in the famous Neoproterozoic Copper Belt of Zambia and Congo. The core contains a prominent iridium anomaly at the very base of the carbonates, with a "signature" relative to other anomalous elements that points to a cosmic origin. Normally such an anomaly would be ascribed to a meteorite impact, but in this case the coincidence would be too good to be true. Instead, the authors use the magnitude of the anomaly to estimate how long cosmic dust had to accumulate to build up such a high level if it was released by rapid deglaciation. Deep-ocean sediments from the last 80 Ma are a guide to the long-term accumulation rate of cosmic material. If that rate is applied to the cap-carbonate anomaly, it gives a total time for accumulation in the hypothesised global ice cover of around 12 Ma. Presumably this would have been from ice immediately overlying the area being studied. An ice age that long defies any idea of more "normal", astronomically forced glaciation, which would be expected to have cyclically formed and receded many times, thereby releasing the dust particles much more gradually. Any anomalies would be expected in the diamictites themselves, yet there are none. Although sample spacing is rather patchy through the entire succession, they are most dense around the anomaly itself. Moreover, another suspected glaciogenic "package" higher in the sequence shows exactly the same iridium "spike".

Arguing against such support for the "Snowball Earth" hypothesis will be difficult, but other sequences require similar tests, most importantly those of Namibia, where Hoffman and colleagues developed their ideas, and the much more extensive deposits of Australia. This diamictite sequence is reckoned to represent both postulated deep-freeze events of the Neoproterozoic, around 710 Ma (Sturtian) and 635 Ma (Marinoan). There is one nagging problem. Data from one area are likely to record ice-retained cosmic dust only from ice in its immediate vicinity, and therefore do not represent the entire planet. Much of the controversy is between supporters of a whole-Earth ice cover, and those who favour patchy glaciation (the "Slushball" model). Unfortunately, Neoproterozoic stratigraphic correlation and radiometric age calibration is not sufficiently good to detect the same intervals elsewhere and look for anomalies there. In fact, the stratigraphy is generally correlated from place to place by matching the diamictites themselves. There is plenty of evidence that they may all coincide in time.

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Making sense of glacial-interglacial cycles?

April 2005

The competing periodicities of the three astronomical "drivers" of climate – orbital eccentricity (~100 ka), axial obliquity (~40 ka) and axial precession (~20 ka) – lie behind several models for the climate changes of the last 0.7 Ma. Taking in the theories that sway towards the influence of variables in the Earth system itself, around 30 models have some currency at present. Since climate forecasters have to take account of which factors drive climate in the absence of human emissions, as well as piece together their own particular models, it is easy to see how critics of global warming get a wide hearing: compared with creationists, they have it easy! Is there any way of resolving what is quite bluntly a theoretical mess? It is a mess simply because the available data are so complex, and in the case of both main sources, ocean-floor sediments and ice cores, not only are their devils in the detail, but there are whopping contradictions, such as the mismatches in timing between the Greenland and Antarctic ice cores. Add all the other sources, such as stalactites, tree rings etcetera, together with caveats like the difficulty in time calibration using 14C dating, and the volume of diverse records become bewildering.

It is tempting that a reversion to a statistical approach, that includes more bells and whistles than hitherto (see Evolutionary rhythms below), can resolve matters. Peter Huybers and Carl Wunch, of Woods Hole Oceanographic Institution and MIT, have tried that for pacing of the last 0.7 Ma of climate cycles (Huybers, P. * Wunsche, C. 2005. Obliquity pacing of the late Pleistocene glacial terminations. Nature, v. 434, p. 491-494). Generally accepted "wisdom" holds that the last 7 glacial-interglacial cycles are paced by ~100 ka eccentricity forcing, even though it has the weakest effect on solar heating, by a very long way. But there are smidgens of evidence for some interaction between that and the much stronger influence of changes in the Earth's axial tilt or obliquity. Huybers and Wunsch go for the Popperian rigor of first defining a null hypothesis, that obliquity has no effect, and then designing a test. It isn't easy to decide how the contrary hypothesis that it does can be evaluated though. The clearest features in all climate records are the ends of glacial epochs or termination: they are sudden, sharp and generally look the same. Most other features have some kind of pattern, but little consistent comparability. Using the most advanced statistical techniques, which employ many iterations to test for stability in statistical models, they can show that the null hypothesis fails. The positive result is that the time between terminations that are repeatedly modelled falls into two envelopes, around 120 and 80 ka, which simple arithmetic shows are divisible by 40 ka.

But how can axial obliquity only have an effect every two of three of its cycles, while a single cycle does not appear in the time-series; is it nature skipping beats somehow. One means that the authors suggest is that the underlying pace of eccentricity can effect the temperature at the base of ice sheets, depending on their thickness. If they are thin, then the heating is insufficient to trigger ice-sheet collapse because the base is very cold, whereas if ice is thick the effects of thermal conductivity and heat flow makes the ice base warmer and more subject to perturbation beyond its failure limit. It was at this point that I gave up, but wish the authors good luck in promoting their possibly unifying hypothesis for what finishes off glacial epochs…..

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Warming may have triggered Northern Hemisphere glaciation

February 2005

While I write this issue of EPN it is supposed to be early spring outside, and that is clearly what the ducks reckon as well – they are beginning to, er um, frolic. But there has been two weeks of snow and frost. Britain and the rest of Europe owe the frigid snap to cold air spilling westwards from northern Asia; the influence of the Siberian winter high-pressure area. Although somewhat lost in the recent kerfuffles about whether or not global warming is a fact or a misreading of data, the inevitable build up of mid-continental cold dense air in winter might have interesting consequences, should climate warm. Normally, areas far from the oceans remain dry as well as getting very cold through radiative heat loss in winter. When spring comes, such snow as there is soon disappears and the extremes of cold are replaced by surprisingly high summer temperatures, as anyone who has visited Siberia or Northern Canada will know. Should moist air find its way into such areas during winter, vastly more snow would fall. Its melting would take longer, and more solar radiation would be reflected back to space in spring. Such an albedo feedback could induce generalised cooling. Now evidence has emerged that the earliest known growth of land ice in North America was linked to warming of the ocean from which winds blew over it (Haug G.H. et al. 2005. North Pacific seasonality and the glaciation of North America 2.7 million years ago. Nature, v. 433, p. 821-825). In fact it is axiomatic that growth of continental ice sheets requires a supply of moisture and snow that exceeds the rate of summer melting and ablation, as well as cold winters.

Most theorising about the onset of Northern Hemisphere glaciation has centred on changes in North Atlantic circulation due to closures of the straits where the Isthmus of Panama now links North and South America, and the start of southward deep-water circulation from the latitude of Iceland. In fact both are known to have preceded the last Ice Age by a good 2 Ma. The actual start around 2.7 Ma coincided with an increase in obliquity of the Earth's orbit that would have led to periods with cold northern summers. Without abundant mid-continent snowfall, that in itself would not have set ice sheets forming in earnest. The multinational team of oceanographers studied sea-floor sediment cores from the sub-Arctic Pacific. To their surprise, sea-surface temperatures provided by evidence from planktonic organisms show evidence at 2.7 Ma for on the one hand cooling of the sea surface (from foraminifer oxygen isotopes) yet considerable warming on the other (from organic chemicals secreted by coccolithophores). Resolving this paradox requires a careful assessment of the ecological behaviour of the two groups of organisms. The authors' explanation involves the onset of density stratification in the North Pacific, so that the surface warmed quickly in summer, retaining warmth during autumn, and warmed slowly in spring from its minimum temperature. Both result from the high thermal inertia of water. The productivity of silica-secreting diatoms plummeted to a fifth of its earlier levels at 2.7 Ma as well, explained by ocean stratification reducing the supply of nutrients from deep water upwellings. Intuitively, a warm sea upwind of the North American continental interior should have generated high snowfall in late autumn and winter. Haug and colleagues modelled the contrasting effects of an ocean with water overturn and mixing with one that tends to become stratified, to simulate snowfall over the North American Arctic. From a situation in the Pliocene with snowfall over Greenland and the Arctic islands, the scenario shifts to heavy snow over the whole Arctic in the earliest Pleistocene. It seems that the trigger for the Great Ice Age was a hemisphere away from the "usual culprit", the North Atlantic, although its vagaries, once glacial cycles were underway, probably controlled the details thereafter.

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And was there a mighty wind?

January 2005

Readers will be familiar with the to-ing and fro-ing that surrounds the idea of Neoproterozoic Snowball Earth episodes from earlier issues of EPN. The leading proponent and sturdy defender of the hypothesis, Paul Hoffman of Harvard University, re-enters the fray as co-author of a paper that builds on the idea that following global glaciation the climate became not only very warm but also violent (Allen, P.A. & Hoffman, P.F. 2005. Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature, v. 433, p. 123-127). They document evidence from "cap carbonates" in northern Canada and Spitzbergen that succeed diamictites of "Marinoan" (~635 Ma) age, in the form of large-scale sedimentary structures. Many of these are submarine ripples with amplitudes up to 40 cm, and forms that suggest they were produced by sea-bed motion due to surface waves, down to 200-400 m, far deeper than modern storm-wave base. Central to their argument is hydrodynamic modelling of wind speeds that might have produced such large ripples, and their specific shapes – steep sided. Being based on experiment and observation of modern sea-bed processes, the theory seems quite rigorous. It retrodicts wave periods that are somewhat longer than those commonly seen in modern ocean storms. From that they derive sustained wind speeds that exceed 70 km per hour across open oceans, extraordinary by modern ocean wind standards.

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Torrid times in the Cretaceous Arctic

December 2004

Despite its latitude (above the Arctic Circle) the sedimentary depocentre of northern Alaska is becoming famous for its Cretaceous terrestrial flora and fauna. Plant remains indicate luxuriant vegetation cover, and high excitement greeted the discovery of 8 species of dinosaurs (4 herbivores and 4 theropod predators (Fiorillo, A.R. 2004. The dinosaurs of Arctic Alaska. Scientific American, v. 291(6), p. 60-67). How dinosaurs were able to survive the darkness of the Arctic winter is a bit of a mystery, unless the migrated as do modern caribou – Fiorillo cites evidence for small juveniles that would have been unlikely to have migrated far, because compared with adults they were much smaller than young caribou. There would have been sufficient winter biomass for survival during the Cretaceous, but seeing and being active as cold-blooded reptiles pose problems. At least one of the species had unusually large eyes, so one of the conditions for dinosaur's remaining year-round seems established. New data regarding climatic conditions in the far north have turned up after an most unusual and intrepid programme of drilling through a drifting island of pack ice over the Arctic Ocean's Alpha Ridge, not far short of the geographic North Pole. An extraordinary feature of the programme is that it took place between 1963-74, the core having only been examined in detail in the last year (Jenkyns, H.C. et al. 2004. High temperatures in the Late Cretaceous Arctic Ocean. Nature, v. 432, p. 888-892). The Late Cretaceous part of the cores is black mud rich in terrestrial vegetation remains and marine diatoms, and totally lacking in evidence for dropstones and other debris from floating ice shelves. Unfortunately, the Arctic sediments lack carbonate-shelled plankton remains, so the now standard method of sea-surface temperature measurement is not possible. However, Jenkyns et al. were able to use a method based on the fatty acids that survive in plankton membranes, results from which match oxygen-isotope palaeo-temperature measurements in Cretaceous cores from lower latitudes. Astonishingly, even at polar latitudes, the Cretaceous Arctic Ocean seems to have been as warm as 15 degrees C. Climate modelling based on lower latitude data and estimates of CO₂ concentration in the Late Cretaceous atmosphere falls around 10 degrees short of these levels. The conventional modelling requires 3 to 6 times more "greenhouse" warming than generally accepted, to account for Arctic sea temperatures in which we could swim in moderate comfort. Possibly the modelling is awry. One of the most important features of Late Cretaceous palaeogeography was a major seaway across North America that connected the Arctic with tropical latitudes. It existed because global sea level was far higher than now, probably due to the oceans' volume having been substantially reduced by huge magmatic outpourings on the floor of the West Pacific basin (the Ontong-Java Plateau), earlier in Cretaceous times, together with higher rates of sea-floor spreading. The seaway would have been shallow, and thereby easily warmed. Had poleward currents been possible in it, their flow would have acted very like the modern Gulf Stream to warm high latitudes. Despite palaeoclimatologists reliance on models of heat circulation, it needs to be remembered that they are based on grossly simplified geographic features. If they get it very wrong indeed for the well-studied Cretaceous, that casts doubts on climate modelling's predictive powers for the course of current climate evolution.

See also: Poulsen, C.J. 2004. A balmy Arctic. Nature, v. 432, p. 814-815

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Update on the "Snowball"

November 2004

Two recent papers add weight to the "against" view expressed in For and against "Snowball Earth in EPN of October 2004" One gives age of 709 ± 5 Ma for tuff immediately beneath a supposed Sturtian diamictite from the western USA (Fanning, C.M & Link, P.K. 2004. U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho. Geology, v. 32, p.881-884), which does not tally with the radiometric age (685 Ma) of similar rocks not far away. The other (Calver, C.R. et al. 2004. U-Pb zircon age constraints on late Neoproterozoic glaciation in Tasmania. Geology, v. 32, p.893-896), gives a 575 ± 3 Ma age for sills intruding a "Marinoan" diamictite in Tasmana, and 582 ± 4 Ma for a rhyodacite immediately beneath it. This suggests that these antipodean glaciogenic rocks are correlative with those in Newfoundland and Norway , that are supposedly representatives of the Varangerian glacial epoch. Yet the authors are pains to state that the Marinoan and the Varangerian are one and the same. Read these papers if you are still confused!

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How often did it rain?

September 2004

Geoscientists have become used to masses of climate data, often with better than 50 years resolution, from cores through ice sheets and sea-floor sediments. But all of it is from some kind of proxy; oxygen isotopes for air temperature and land-ice volume, methane for humidity, dust for windiness, and so forth. One aspect of both climate and the British obsession with weather is raininess, for which there is scant evidence. How many rainy days occur in a British summer is interesting, but for studies of past climate evidence for the onset or disappearance of seasonality, and the annual intensity and duration of rainfall would be invaluable, if it could be had. A piece of ingenious research shows that it is possible (Kano, A. et al. 2004. High-resolution records of rainfall events from clay bands in tufa. Geology, v. 32, p. 793-796). Akihiro Kano and Japanese colleagues studied the well-known layering of tufa – carbonate veneers laid down in freshwater that has high dissolved bicarbonate and calcium ions. In "hard-water" areas tufa can be deposited very quickly, at rates above a few millimetres per year, and it tends to be preserved, being quite tough. So tufas have the potential for preserving annual records of various fluctuations. Kano and colleagues saw that colour laminations represented clays deposited in the tufa when the water was turbid after prolonged rainfall. To record the variations they simply measured fluorescent X-rays emitted by silicon when slices of tufa were examined in an electron microprobe – silicon is present in clays and silt, but not in carbonate minerals. Because they used tufa deposited in recent times (1988-2002) they were able to correlate variations in clay content with detailed weather records from the site, thereby calibrating their method. The match was very good and followed rainfall closely at the level of a few days. Of 112 high rainfall days in the abnormally wet year of 1993, 100 showed up in the clay record. So, tufas are potentially more revealing than even the annual growth rings in wood, and some tufa deposits preserve long records.

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Details of the last interglacial climate

September 2004

Worries about how anthropogenic warming will affect the course of the Holocene interglacial in which we live might be tempered or exacerbated by knowing what went on during the previous, Eemian interglacial that ended about 120 ka ago. Data from cores through the Greenland and Antarctic ice sheets have been both ambiguous and plagued by resolution that does not show enough detail, but a core from a new position in Greenland seems to resolve both problems (North Greenland Ice Core Project members 2004. High-resolution record of Northern Hemisphere climate extending into the last interglacial period. Nature, v. 431, p. 147-151). Uniquely, the NGRIP ice still preserves the annual snow layering as far back as 123 ka. This is because the site shows little sign of the deformation at deep levels that characterised previous Greenland cores. That is probably because the site lies above a zone of high heat flow through the underlying crust, so that the base of the ice has melted. Melting helps prevent internal deformation, but that in itself is a surprise because the site was chosen because it is colder and drier at the surface than other sites. The drilling objective was to penetrate older ice than the Eemian to give a fuller record than from earlier cores, yet anticipated poor time resolution. The presence of resolvable annual records from depth was both a surprise and a bonus, although the melting had removed ice from the earliest part of the last interglacial. Despite that, preliminary oxygen-isotope results from the NGRIP core suggest that the Eemian had a remarkably stable climate and one that was warmer than that of the Holocene by about 5ºC; maybe it is an analogue for climate evolution during a future, artificially warmed world. That possibility stems from the observation that around 115 ka, North Atlantic climate suddenly warmed Thereafter, interglacial conditions did not suddenly change to glacial, as happened several times during the course of the last glacial epoch, but took around five millennia after the sudden warming. The authors make no claims that their preliminary data help resolve current fears of warming collapsing to glacial conditions in a matter of years to decades. That grim scenario has been widely trumpeted both by the media and some climate scientists. There is more to the Eemian than the period after 123 ka, and who knows what the eventual annual resolution will show up? The data presented in the paper are from a coarse sampling of 55 cm that represents about 40 year intervals.

See also: Kuffey, K.M. 2004. Into an ice age. Nature, v. 431, p. 133-134

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For and against "Snowball Earth"

September 2004

Reputedly glaciogenic sediments in the Neoproterozoic are reckoned to represent at least three separate cold episodes, the Sturtian (~720 Ma), Marinoan (~600 Ma) and Varangerian (~580 Ma). Sadly, the diamictites that characterise these episodes are not easily dated. Only two have well-defined radiometric ages, the Gubrah Member in the Oman (713 Ma), said to be Sturtian, and the Gaskiers Formation of Newfoundland (580 Ma), a possible example of the Varangerian that is better exposed in northern Norway. The truly whopping Sturtian and Marinoan diamictites of Australia are fitted to a global stratigraphy on the basis of carbon isotope variations, as are those of Namibia on which Paul Hoffman and colleagues stake their claims to "Snowball Earth" events. Another Hoffman, native to Namibia, and geochemists at MIT, have finally given a believable age to one of the Namibian diamictites (Hoffman, K.-H. et al 2004. U-Pb zircon dates from the Neoproterozoic Ghaub Formation, Namibia: constraints on Marinoan glaciation. Geology, v. 32, p. 817-820). Their zircons come from a thin volcanic ash within isolated Neoproterozoic diamictites in central Namibia, and yield an age of 636±1 Ma. Correlating the studied diamictites with the Namibian sequences elsewhere in the country relies on the presence of a supposed cap carbonate rather than lateral continuity. The authors link them with the younger of the two Namibian diamictites, the Ghaub Formation, rather than the Chuos Formation that lies at depth, despite the fact that both well-studied units are sometimes overlain by carbonate sediments. The conclusion is that the Ghaub is Marinoan, previously thought to be somewhere between 600 and 660 Ma. Interestingly, the new occurrence of diamictites is divided vertically by two thick sequences of volcanic lavas, neither of which have been dated by the authors.

One of the leading experts on what actually constitutes incontrovertible evidence for glacial sedimentation is Nicholas Eyles of the University of Toronto. He has become increasingly disenchanted with notions of Snowball conditions, on the basis of ambiguity in the very evidence said to signify them; diamictites with drop stones. He and Nicole Januszczac have assembled a monumental paper that counsels caution, and perhaps more (Eyles, N. & Januszczac, N. 2004. "Zipper-rift": a tectonic model for Neoproterozoic glaciations during breakup of Rodinia after 750 Ma. Earth-Science Reviews, v. 65, p. 1-73). Part of their argument rests on the very lack of robust ages for Neoproterozoic diamictites that prevents believable correlations from continent to continent. It is the globally synchronous nature assumed for these glaciations that gave rise to the "Snowball Earth" notion. The palaeomagnetic latitudes are often used to support this, but they are error prone both palaeogeographically and geochronologically. Accepting evidence for glaciation at low latitudes is no guarantee of support for even cold extremes, let alone an icebound world. Solar heating in the Neoproterozoic was lower than now, and so, therefore, would be the elevations at which glaciers might form at different latitudes. But the main problem is reconciling the features of many supposed glaciogenic diamictites with modern ideas of what truly constitutes evidence for glacial transport and deposition. Few of the units on which the "Snowball Earth" hypothesis is based stand up to modern scrutiny. Most of the diamictite packages occur in tectonically controlled basins, that were subject to episodic rifting. Each can be considered to form the base of a "tectonostratigraphic" cycle, and many show abundant evidence of having formed as mass flows from a shelf into the basin. They include olistostromes with huge rafts of carbonates likely to represent failure of carbonate platforms and huge submarine landslides, similar to those being discovered off many large islands today. The 750 to 580 Ma period was one of the most dramatic episodes of continental break-up in Earth's history as the Rodinia supercontinent was disassembled. Continental uplift, resulting either from mantle plume activity or rebound of rift shoulders, could have resulted in large areas rising above the ice limit, even at low latitudes in those cooler times. Those diamictites that are undoubtedly glaciogenic could easily have formed haphazardly in time. The carbon isotope record of immense shifts in δ¹³C during the Neoproterozoic, linked by some to repeated collapses and resurrections of life, might just as easily have occurred through efficient organic burial in active extensional basins and repeated major volcanism from plumes. Only evidence of timing will tell, and three good dates for "Snowball Earth" events are simply not enough.

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Influence of continental weathering on climate boosted

February 2004

Since the resurrection of Chamberlin's idea that the rate of chemical weathering of continental crust helps regulate atmospheric CO₂ by Maureen Raymo, the hypothesis has not yet been supported by convincing geochemical evidence. There is such a lag between changes in ocean chemistry and evidence for global climate change, that correlations are flimsy. The need is for a proxy for weathering of the land surface that resides in seawater for a geologically very short period. Such an element is osmium (Os), which passes from river water through the oceans to sea-floor sediments in about 25 thousand years, so changes in its abundance in sediments ought to match the pace of any climatic shifts. In principle, there are two main sources for elements in seawater, from sea-floor hydrothermal alteration of oceanic crust, and from continental weathering. The first can be considered to be more or less constant, except on time scales of tens of million years. Continental weathering is a response to climate change, and keeps pace with it. Researchers at the UK Open University and the University of Köln in Germany analysed samples for osmium and carbon isotopes through a sequence of Jurassic mudstones on the NE coast of England (Cohen, A.S. et al. 2004. Osmium isotope evidence for the regulation of atmospheric CO₂ by continental weathering. Geology, v. 32, p. 157-160). The carbon isotopes show a sudden drop in δ¹³C within a very hydrocarbon-rich unit famous for it contribution of jet (oil-rich lignite) to Victorian funereal jewellery. This negative excursion is recognisable world-wide at around 180 Ma. The most likely explanation is a monstrous blurt of methane from destabilised gas hydrate on the Jurassic sea floor (see Methane hydrate – more evidence for the ‘greenhouse' time bomb, August 2000 issue of EPN). The Jet Rock of the Whitby coast therefore preserves a nice example of sudden climatic change, and by the end of its deposition carbon isotopes returned to Jurassic background values. Methane, a powerful "greenhouse" gas, is rapidly oxidised to CO₂ in the atmosphere, so reducing its initial warming effect, but climate would have been hotter for some time afterwards until the excess CO₂ was drawn down somehow. Interestingly, the Jet Rock also shows a sudden leap in the abundance of ¹⁸⁷Os, reflected in the ¹⁸⁷Os/¹⁸⁶Os ratio of the samples, and an upward step in the value of the ⁸⁷Sr/⁸⁶Sr ratio – one of the fastest rises known. The latter is generally assigned to an increase in continental weathering, since continental crust contains more radiogenic ⁸⁷Sr than does oceanic crust. The implication of the osmium-isotopic shift is odd; it requires an increase in the rate of continental weathering by 4 to 8 times that in the preceding period. That is a vast change, even if it only lasted for a short period, but it tallies with what is known about the temperature dependence of the dissolved loads of rivers in more recent times. If the osmium isotope excursion truly reflects massive continental weathering, then it is possible to calculate the drawdown of the excess CO₂ in the atmosphere from a commensurate flux of calcium and magnesium ions from the continents, that would eventually form marine carbonates. The authors estimate a mere 37-123 ka to get rid of it. Yet continent-derived radiogenic ⁸⁷Sr remained high for much longer, and the authors' arguments become tricky. One interesting aside is that, unlike today, more groundwater found its way to the oceans than surface run-off during the Jurassic, perhaps 6 times more. It is easy to look on weathering as what happens at the interface between rocks and the weather; the land surface. Not so. A great deal of chemistry that releases soluble ions goes on in the subsurface, above and below the water table. It is by no means as simple as reactions between carbonic acid in rainwater and silicate minerals. Weathering is the product of hydrogen ions' (whatever their source) effects on silicates. Bacteria are extremely important actors in modifying pH below the surface, for example the sulphate-sulphide reducers, and the oxidative dissolution of sulphides produces sulphuric acid. Even more interesting for the chemistry of groundwater is the curious role of iron hydroxide. Under oxidising conditions it adsorbs many elements from solution, including platinum-group elements, such as osmium. Should conditions become reducing, dissolution of goethite skins on sedimentary grains releases the accumulated elements. A warming trend almost inevitably results in increased precipitation, and rising water tables. It also should boost biological productivity on land and an increase in the amount of buried organic matter, which create reducing conditions in groundwater.

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Collapse of the continental margin and methane release

January 2004

The vast reserves of peculiar methane-water ice deposits (gas hydrate or clathrate) in sea-floor sediments are the most likely source of methane releases that could generate sudden warming events, such as that at the end of the Palaeocene, and left traces in polar ice cores during the last few glacial-interglacial episodes. Methane probably leaks from the sea floor all the time, but is soon oxidised to the lesser "greenhouse" gas CO₂ in the atmosphere, so muting its potential effects to a low background level. For methane to have a sizeable effect on global warming, lots of it has to blurt out suddenly. Possibly the only mechanism that can trigger such explosive releases are failures of sea-floor sediments, either by those beneath a steep surface slope collapsing under gravity, or as a result of seismicity. Geoscientists from University College London and the British Geological Survey have tried to correlate known peaks in atmospheric methane from the recent past (shown by ice cores) with episodes of mass flow on the seabed (Maslin, M. et al. 2004. Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis. Geology, v. 32, p. 53-56). They found that the periods of greatest disturbance of continental-slope sediments over the last 45 ka took place at the tail-end of the last glaciation, between 13 and 15 ka and 8 to 11 ka. Each correlates with methane highs in the Greenlandic ice cores and with bouts of rapidly rising sea level (the Bølling-Ållerød and Preboreal warming periods). So they conclude that there is support for a "clathrate gun" model for sudden warming associated with glacial to interglacial transitions. However, seafloor collapses also correlate with Heinrich events (ice-sheet surges that launched iceberg "armadas" to low latitudes) that punctuated glacial times. These marked brief periods, repeating every 1000 years or so, which mark cooling when sea-levels were low. None are associated with upsurges in atmospheric methane., although the following interstadial warmings are. This lack of correlation rules out a "clathrate gun" influence on millennial-scale climate fluctuations during glaciations.

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Super-eruptions and climate

January 2004

The biggest known, young volcanic crater is that of Toba on Sumatra, which is a caldera complex measuring 30 x 100 km. Around 74 ka Toba emitted an eruption that dwarfed any in more recent times, and spread a dust cloud around the world – it is present in ice cores from Greenland, and has been linked with a cooling step during the onset of the last glaciation. It happened around the time that fully modern humans had begun to spread across Asia after migrating from NE Africa – an Acheulean hand-axe has been found in the Toba Tuff – and may have deeply affected those pioneering bands. There are older ash levels that can also be attributed to Toba eruptions, one found 2500 km away in the sediments of the South China Sea (Lee, M-Y. et al. 2004. First Toba supereruption revival. Geology, v. 32, p. 61-64) and at other sites up to 3000 km from Toba. This gives an age around 800 ka. Lee and colleagues from Academica Sinica (Taiwan), the National Taiwan University and the University of Rhode Island estimate that almost 1000 km³ of ash was expelled by the eruption. Unlike the 74 ka ash, this layer falls in the transition from a glaciation to an interglacial period; instead of a possible cooling influence through dust blocking solar heating, there is a warming trend. Although not quite as big as the 74 ka eruption of Toba, that of 800 ka is still vastly bigger than any other explosive volcanism during the Pleistocene. So, it suggests that super-eruptions are not significant climate triggers after all.

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High- and low-latitude climate changes almost match

October 2003

Ten years ago the records of climate proxies from the Greenland ice sheet set new benchmarks for understanding how climate has varied over the last 100 thousand years – annual ice layers allowed division of that data to as fine as decades. Variations in the ice cores helped explain many of the variations found in more blurred data from sea-floor sediment cores in the Northern Hemisphere. Variations could be correlated with changes in the formation of North Atlantic deep water at high latitudes and the destabilisation of North American and Scandinavian glaciers. The whole hemisphere behaved in concert, through long-distance connections in climatic processes, but high-latitude processes seemed to dominate. Development of ²³⁴U/²³⁰Th dating extended high precision to carbonates that have been precipitated from groundwater to form stalagmites or speleothem. The latest results from speleothem, collected on the Indian Ocean island of Socotra, cover 14 thousand years between 56 and 42 ka, and resolve down to only 8 year intervals (Burns, S.J. et al. 2003. Indian Ocean climate and an absolute chronology over Dansgaard/Oeschger events 9 to 13. Science, v. 301, p. 1365-1367). They show variations in rainfall on the island, though the δ¹⁸O proxy, and thus changes in the strength of the Indian Ocean monsoon. In terms of shape, the stalagmite record closely resembles δ¹⁸O changes in the Greenland ice cores, although the two have opposite senses, because the Greenland proxy is for air temperature above the ice cap. During the frigid Heinrich events that saw massive southward waves of icebergs, rainfall over Socotra was low. It became higher as high-latitude conditions warmed in Dansgaard-Oeschger events. The fine speleothem resolution shows a dramatic change-over that took only 25 years or so. The explanation is that warmer conditions increased equatorial evaporation from the oceans. But water vapour is the dominant "greenhouse" gas, and a wetter atmosphere would become warmer. So the question of whether low- or high latitudes drove the changes is still an open one. If North Atlantic events were the driver, then the tropical processes would greatly amplify their effects. One big problem emerges from the joint research by US, Swiss and Yemeni scientists. The highly reliable U/Th dating gives ages for each event that are about 3000 years older than those interpreted from the ice cores. The authors are convinced that the ice-core ages need revision, yet there are discrepancies with the event-ages from other similarly dated speleothems. Commenting on the paper, Frank Sirocko of Johannes Gutenberg University of Mainz in Germany (Sirocko, F. 2003. What drove past teleconnections. Science, v. 301, p. 1336-1337) makes the point that maybe the quality and age of ice core records lie behind the widely accepted view that high-latitude process drive climate. He presents an excellent global image of modern sea-surface temperatures that show the main oceanic shifts of energy – the leakage of cold circum-Antarctic waters northwards, the westward movement of equatorial warm waters to which the El Niño – Southern Oscillation (ENSO) is due, and the unique movement of warm water to Arctic regions in the North Atlantic that is connected to deep water formation. To that he adds the major effect of continental winter snow cover in central Eurasia, that affects albedo and the size of the winter high-pressure zone there. Is there a teleconnection between that and events in the North Atlantic? Nobody knows, because there are no data to compare, yet. Another uncharted but likely linkage is between the ENSO and processes in the circum-Antarctic current. Using currently accepted dating of ice cores, records from those in the Antarctic show air temperature changes that precede those from Greenland by several thousand years. In that respect, the Socotra record possibly has a link with the South Polar climate. Until the issue of dating is sorted out, it will always be difficult to make concrete statements about global climate change.

Interestingly, in the same issue of Science, sea-floor data (between 9 and 16 ka) from the Cariaco Basin off Venezuela, at about the same latitude as Socotra, mimic the Greenland records to within 30 to 90 years (Lea, D.W. et al, 2003. Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination. Science, v. 301, p. 1361-1364).

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"Greenhouse" controls challenged

October 2003

There's data gathering and there's theorising. In palaeoclimate studies the two come into conflict. Theory suggests that CO₂ is likely to be the principal driver for climatic ups and downs, probably on all time scales. Atmospheric CO­₂ estimates from the past are based on proxies of different kind, and the various models that they support do not tally vary well. Worst of all they do not fit climate records through the Phanerozoic at all well, except in the crudest possible way. Only the long-lived Carboniferous to Permian "icehouse" and Tertiary cooling tally, and then only in Berner's GeocarbIII model. One of the best records of major climate shifts, aside from continental tillites, are marine sediments that contain ice-rafted debris, in particular the palaeolatitudes to which they extend. They record four major cooling episodes: Late Ordovician; Devonian to Late Permian; Late Jurassic to Mid Cretaceous; and those since about 35 Ma ago. The oxygen isotope record from Phanerozoic fossils, partly correlated with ocean temperatures also suggest 4 global coolings in the last 545 Ma. Either the CO₂ modelling needs more detail, or the whole issue of the "greenhouse" effect is under question. That is the conclusion of a study by Nir Shaviv of the Hebrew University of Jerusalem, and Ján Veiser of the Ruhr University and The University of Ottawa (Shaviv, N.J. & Veiser, J. 2003. Celestial driver of Phanerozoic climate? GSA Today, Huly 2003, p. 4-10). Veiser has been analysing the chemistry of carbonates, especially their oxygen isotopes, for his 30 year career, and has amassed more data than any other geochemist on carbonate-related issues. The two have worked together because their interests fit together extremely well. Shaviv has reconstructed the variation of cosmic ray flux from studies of the exposure of iron meteorites to them, blended with analysis of how the Solar System moves through the spiral arms of our galaxy. Cosmic rays are known to affect the Earth's cloudiness and therefore albedo. Greater cosmic ray flux should increase the amount of solar energy reflected away by the Earth, thereby causing global cooling. The degree of fit between the cosmic ray flux and palaeoclimatic records is so good that up to 2/3 of climate variation may be connected with the Earth's celestial position. That is, as it passes through the star-rich spiral arms cosmic rays intensities go up. This happens every 140 Ma or so, which fits very well with the 4 icehouse periods during the Phanerozoic. They even suggest that the climate-CO₂ relationship may be the opposite of that generally agreed; climate might drive carbon dioxide levels. A secondary role for "greenhouse" gases wreaks havoc on attempts at modelling climate change feared to result from increasing anthropogenic releases. Shaviv and Veiser's work comes at a particularly awkward time for climate modellers, who have just initiated a programme for running huge simulations by corralling the combined computing power of millions of home PC users, similar to the approach pioneered by the SETI Institute (Allen, M.R. Possible or probable. Nature, v. 425, p. 242). Perhaps the view of Phillip Stott, that climate modelling is a complete waste of time (Stott, P. 2003. You can't control the climate. New Scientist, 20 September 2003, p. 25) might sink in as a result of the possible link between cosmic ray flux and climates of the past. Stott believes that acting on the output of such models might perhaps even be dangerous, since we clearly do not understand short-term climate change well enough.

Precambrian CO₂ levels

October 2003

Whether or not fluctuations in the "greenhouse" effect drive climate change, the fact remains that CO₂, methane and water vapour all act to retain solar heat in the Earth system. Were it nor for their presence in the atmosphere, the Earth would be about 33 degrees colder than it is. It would be covered by ice. Theoretical modelling of how stars evolve suggests that the Sun had progressive less energy output going back in Earth's history. Only gaseous heat retention could have prevented a sterile, frigid planet. Yet periods of cooling sufficient to hold large amounts of water in surface ice have occurred only a few times, 4 in the Phanerozoic, a flurry of so-called "Snowball" epochs in the Neoproterozoic and the earliest known glaciation around 2200 Ma ago. The earliest coincided with the first evidence for free oxygen in the atmosphere, and may have been caused by that. Methane, a more powerful "greenhouse" gas than water or carbon dioxide and abundantly produced by anaerobic decay, is easily oxidised. In later time, it has been ephemeral in the atmosphere, unless continuously released, for instance by destabilisation of gas hydrate in sea-floor sediments. Warming by CO₂ has undoubtedly kept total frigidity at bay since then. The problem is charting just how much was in the air, because most estimates have been based on studies of palaeosols that give odd and very imprecise results for the early Palaeozoic (see Shaviv and Veiser, 2003; previous item).

Photosynthetic organisms derived their carbon from CO₂, either in the air or dissolved in water through equilibration with the atmosphere. The extraction favours lighter ¹²C, so biological activity results in their products being depleted in the heavier ¹³C by about 25 parts per thousand (‰) relative to carbon in air and water. If organic carbon becomes buried, the remaining carbon in the surface environment gets richer in ¹³C, and that signature becomes fixed in contemporaneous carbonates, both organic and inorganic. It is therefore possible to use the two carbon-isotope signatures to estimate the reservoir of CO₂; its proportion in contemporary air. However, the degree of fractionation depends on the specific carbon metabolism of different organisms, yet most organic carbon in sediments is a mixed product of widely differing life styles. That severely blurs estimates of atmospheric carbon dioxide content. What is needed are data from a single source with known metabolism. Acritarchs are fossil remains of single-celled marine eukaryotes that were, and still are, marine photosynthesisers. They are made of degraded hydrocarbons. Advanced ion-microprobe resolution is now sufficient to produce carbon-isotop