Sedimentology and stratigraphy

The ‘real’ Flood

January 2010

At the end of the Miocene tectonic uplift in the region of the present Straits of Gibraltar cut the Mediterranean Sea off from the Atlantic. The only water able to flow into the isolated marine basin was that carried by the major rivers: the Rhône, Danube, Dneiper and Nile. Their volume was exceeded by evaporation, so the Mediterranean became more and more salty, eventually almost drying out completely to leave thick evaporite deposits that still underlie its deepest parts. 5.33 Ma ago, the tectonic barrier was breached so that Atlantic water flooded the whole Mediterranean basin. The Zanclean flood at the start of the Pliocene has been rated as the greatest catastrophic event in the Phanerozoic history of the oceans, but just how dramatic it was has previously only been guessed at. Seismic profiles across and along the line of flooding reveal channels several kilometres across, about 200 km long and up to 250 m deep, now filled with debris (Garcia-Castellanos, D. et al. 2009. Catastrophic flood of the Mediterranean after the Messinian salinity crisis. Nature, v. 462, p. 778-781). Using a well-established model of river incision in mountain rivers, the authors have suggested how the flooding proceeded. From an initial trickle when the original barrier subsided below Atlantic sea level, flow grew exponentially over a few thousand years to about three times that of the modern Amazon discharge (~10⁸ m ^s-1), at which rate incision reached more than 0.4 m per day. Around 90% of the Mediterranean basin’s entire volume was flooded in a matter of a few months to two years, sea level rising at up to 10 m per day.

Formation of BIFs halted by Sudbury impact

January 2010

The peculiar story of banded iron formations (BIFs) is one that ‘runs and runs’, as journalists say. Most of the steel on which North American capital was built comes from gigantic BIF deposits around Lake Superior that formed during the Palaeoproterozoic. Apart from a brief return in the Neoproterozoic, associated with conditions peculiar to ‘Snowball Earth’ conditions, the Superior Province BIFs are the last of any consequence. Most geologists look to a gradual shift in the oxygen content of ocean water as photosynthetic life grew to dominate the Earth after about 2.4 Ga, but the BIFs around Lake Superior turn out to be capped by a blanket of ejecta from a massive extraterrestrial impact that formed the Sudbury Complex (Slack, J.F. & Cannon, W.F. 2009. Extraterrestrial demise of banded iron formations 1.85 billion years ago. Geology, v. 37, p. 1011-1014). But how could even a monstrous bolide have changed ocean chemistry so decisively? John Slack and William Cannon of the US Geological Survey believe that the impact was so violent that it resulted in wholesale mixing of oxygen-bearing surface waters with those of the deep ocean. The evidence they cite is a coincident change in the nature of deep-water hydrothermal deposits from sulfide-bearing to those dominated by iron-oxides.

The Sudbury impact produced a crater around 150 to 270 km across (one of the three largest known on Earth), and it is dominated by remelted basaltic rocks so almost certainly struck the Palaeoproterozoic ocean floor. Its ejected debris probably covered almost 2 million km2 and is found up to 800 km from Sudbury, Ontario. Yet, even with impact cavitation and massive tsunamis it seems barely feasible that an impact of a size dwarfed by those of the Lunar surface could completely remix the oceans. However, it is likely that in the Palaeoproterozoic continental crust was gathered together in a supercontinent so that tsunamis could scour much of the surrounding ocean. A plume of vaporised seawater may also have scavenged oxygen from the atmosphere. The evidence seems compelling, and another possibility is that Sudbury was not the only impact site...

And another oddity...

January 2010

That a major climatic warming occurred at the end of the Palaeocene (55 Ma) is now undoubted, as is its probable cause by emission from the ocean floor of vast amounts of methane. Yet oddly the Palaeocene-Eocene Thermal Maximum (PETM) coincides with a brief geomagnetic reversal 53 ka long (Lee, Y.S. & Kodama, K. 2009. A possible link between the geomagnetic field and catastrophic climate at the Paleocene-Eocene thermal maximum. Geology, v. 37, p. 1047-1050). Both events were short, so a coincidence seems unlikely, in the authors’ opinion. They suggest a connection through the massive power imparted to climatic processes by the PETM (at least a terawatt and perhaps orders of magnitude more), including the deep thermohaline circulation of the oceans that did shift during the event. Had they exceeded a threshold power for circulation of the liquid outer core they may have triggered the brief reversal, which quickly reverted to its previous magnetic polarity. Ths association is not unique, detailed magnetic studies of the K-T boundary event at 65 Ma has revealed a similar short reversal spanning the duration of the iridium peak ascribed to the Chicxulub impact. However, Chicxulub delivered a power of the order a year’s solar radiation in about one second: vastly larger than the climate perturbation of the PETM. Are we seeing here a hidden signal of an extraterrestrial impact behind the methane release? Impacts are no longer as popular as they once were...

BIFs and bacteria

November 2009

Banded iron formations from the late Archaean, Palaeoproterozoic, and in a few short time intervals linked with Neoproterozoic tillites, have long fascinated geoscientists with their counterintuitive occurrence at times when the oceans contained little if any oxygen. Anoxic water allows iron to exist in its Fe²⁺ form, thereby able to dissolve readily. The vast thicknesses and masses of BIFs demands an abundance of mobile iron, but being made predominantly of hematite (Fe₂O₃) their formation requires a balancing superabundance of oxygen. Many geochemists believe photosynthesising blue-green bacteria to have excreted oxygen to oxidise soluble iron to Fe³⁺ and precipitate it as the oxide in shallow water. Yet plenty of BIFs show such delicate banding that deep water is implicated. All the BIF paradoxes would be resolved if another mechanism had caused the oxidation and precipitation of iron. A new clue to what that may have been is the discovery of iron-oxide stromatolites in the monster BIF deposits around Lake Superior (Planavsky, N. et al. 2009. Iron-oxidizing microbial ecosystems thrived in late Paleoproterozoic redox-stratified oceans. Earth and Planetary Science Letters, v. 286, p. 230-242). Iron isotopes and rare earth elements are good indicators of redox conditions, and those in the BIFs indicate anoxic waters, so free oxygen was not available. The stromatolites, however, strongly suggest biogenic precipitation of iron oxide, which is possible through the action of specialist Fe-oxidising bacteria. Indeed, filamentous microfossils occur in the stromatolites. That opens the possibility of BIFs having formed by direct bacterial precipitation in the oxygen-free world before the Great Oxidation Event around 2.2 Ga, in the absence of cyanobacteria.

Quaternary snatched from jaws of extinction

July 2009

At a stormy meeting in August 2004at the 32nd International Geological Congress in Florence, a rearguard action was mounted by a group of stalwart geologists to thwart an attempt to expunge the last remnant of the stratigraphic divisions inspired by Giovanni Arduino’s work in the 18th century from the minds of all future geologists (see December 2004 issue of EPN). The Quaternary was under siege. Despite the fact that the International Commission on Stratigraphy (ICS) of the IUGS had already prepared the ground for a coup de gras by stating that, “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”, its defenders seem to have won (Mascarelli, A.L. 2009. Quaternary geologists win timescale vote. Nature, v. 459, p. 624). The ICS voted on 21 May 2009 to formally define the base of the Quaternary at 2.6 Ma when the Earth began to cool, glaciation began in the Northern Hemisphere and stone tools first appeared in Africa (it was formerly set at 1.8 Ma, for no obvious reason) and to pass that to IUGS for ratification. Another minority group is enraged, with rumours of chewed carpets, as the Quaternary has annexed 800 ka of what previously was designated as Pliocene: ‘It’s kind of a land grab’, commented Philip Gibbard, a Quaternary expert from Cambridge University, possibly with a hint of glee. To me, it is a milestone decision that gives a proper place to tool making, bipedal apes – ourselves – which makes a great deal more sense that the absurd notion of the Anthropocene (see Epoch, Age, Zone or Nonsense? in March 2008 issue of EPN), whose base some deluded colleagues are trying to set at the beginning of the Industrial Revolution!

Early signs of oxygen... but in the wrong place

July 2009

The so-called ‘Great Oxidation Event’ is marked by the first occurrence of iron-oxide bearing subaerial sediments or palaeosols, widely regarded as occurring at around 2400 Ma. That is probably around the time that photosynthesis overtook the rate of oxidation reactions that previously consumed the oxygen that it produced, so that oxygen could build-up continually in the air. But that date is far earlier than the origin of subaerial photosynthesis and oxygenic photosynthesis must have arisen among oceanic bacteria before then, but only those inhabiting shallow water where the sunlight is. Banded iron formations that go back into the Archaean are often cited as evidence for when such photosynthesis got underway. Their dominant mineral hematite probably formed by oxidation of soluble iron-II and combination of iron-III with free biogenic oxygen, presumed by most workers to be in shallow water. Among the oldest hematite-rich formations is the Marble Bar Chert of Western Australia, dated to 3460 Ma (Hoashi, M. et al. 2009. Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience, v. 2, p. 301-306). The hematite crystals in the chert seem to have formed at above 60ºC in ocean-floor hydrothermal springs that were discharging abundant dissolved iron-II. The authors estimate the basin in which the cherts formed to be between 200 to 1000 m deep. Since at such depths photosynthesis would not be possible, they claim that sufficient oxygen was produced by shallow-water photosynthesis to form oxygenated intermediate and deep ocean waters, reminiscent of far later times in Earth’s history. This is a minority view, and hinges on whether or not the hematite did form directly on the sea floor. One possibility is that it could have been precipitated colloidally from iron-II-rich ocean water in the photic zone where early photosynthesisers would be, to sink to the deeper sea floor. Eventually very fine iron oxide might recrystallise.

See also: Konhauser, K. 2009. Deepening the early oxygen debate. Nature Geoscience, v. 2, p. 241-242.

When the Mediterranean evaporated

March 2009

Much to geologists’ surprise seismic surveys and drilling of the Mediterranean basin revealed that it is floored by an immense thickness of evaporite salts, laid down during the Late Miocene about 6 Ma ago (Messinian Stage). The event has been dubbed the Messinian salinity crisis, and ascribed to the cutting off from the Atlantic of the Mediterranean Sea causing lowering of sea level by evaporation. The formation of the evaporite sequence has been overshadowed by what happened to restore the Mediterranean: a humongous waterfall at the Straits of Gibraltar. New modelling of the salt-forming event has had a technically surprising outcome (Govers, R. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, v. 37, p. 167-170). It suggests that most of the salt body formed before sea level fell. Sea level lowering reduced the load on the sea floor and allowed isostatic uplift to develop a flow barrier at the Straits of Gibraltar, further cutting off resupply of Atlantic water. The other factor seems to have been the effect of sluggish eastward subduction of a lithospheric slab that eventually resulted in subsidence so that the Atlantic could re-flood the Mediterranean basin.

Cycling on Mars

January 2009

High-resolution remotely sensed data (HiRISE) from the Red Planet is free of charge to registered investigators (it did cost quite a bit to acquire), whereas the Earthly equivalent costing would set you back at least US$25 per square kilometre (for Quickbird. They are wonderfully clear, as Mars’s thin atmosphere causes no haze except during dust storms. They are also in stereo, providing both 3-D views and digital terrain elevation data with a precision of 1 m. HiRISE data have revealed detail equivalent to that from aerial photos of Earth taken from about 5 km above. Not surprisingly, they show a lot of geology, including an area around 500 to 1000 km2 with clear signs of layered sediments (Lewis, K.W. et al. 2008. Quasi-periodic bedding in the sedimentary rock record of Mars. Science, v. 322, p. 1532-1535). Where large craters have exposed sequences in their walls it is possible to measure bedding thickness and count individual strata. In Becquerel crater the layering is very regular, comprising two size ranges around 3.6 and 37 m, the second being made up of several of the first sized layers. The two sets of thickness remain consistent through about 300 m of section, so probably represent cyclical processes on Mars. The most likely driving forces are rotational and orbital, as they are for the Earth’s Milankovich climatic pacing. The 10:1 ratio between the two frequencies of bedding is twice that dominating the Milankovich time series (rotational precession and orbital eccentricity). One possibility for the Martian cycles is the estimated variation of orbital eccentricity on 120 ka, 1.2 Ma and 2.4 Ma timescales, although axial tilt changes through tens of degrees; far more than does that of the Earth’s rotational axis. Thankfully, the authors stick to variations in wind-driven sedimentation to explain the bedding cycles. Changes in insolation on Mars would affect condensation and evaporation of CO₂ ice at the poles, and consequently the density of the atmosphere and its ability to move and deposit sediment. Less fortunately, they suggest water must have been involved to lithify the layers. That hardly seems necessary on a planet with low atmospheric pressure, as unconsolidated wind-blown loess in western China maintains the integrity of its layering with little cementation.

Snowball Earth challenged again

January 2009

Nobody doubts that in the Neoproterozoic there were several massive climate changes that brought frigid conditions to low latitudes. Some demand that the Earth then entered a runaway cooling because the increased albedo cause by continental ice cover would have reflected away a large amount of solar radiation; the Snowball Earth hypothesis is that the entire planet then became icebound. Evidence for the global glacial epochs is in the form of sediments clearly influenced by deposition of debris carried by ice. Later glacial episodes of Late Ordovician and Carboniferous-Permian age left thin tillites – lithified boulder clay - on glaciated land surfaces in northern and southern Africa and other parts of the southern continents, but the main evidence for the much deeper chills of late Precambrian age are thick piles of sediment studded with dropstones from floating ice. These are glaciomarine diamictites as opposed to tillites. Philip Allen and James Etienne of Imperial College, London and Neftex Petroleum Consultants of Abingdon, UK have paid particular attention to the Neoproterozoic diamictites of Oman (Allen, P.A. & Etienne, J.L 2008. Sedimentary challenge to Snowball Earth. Nature Geoscience, v. 1, p. 817-825). These prime candidates for typical products of low-latitude frigidity are over 1 km thick, and therefore require massive supply of precipitation to drive the large ice flows that could transport such large amounts of sediment. Moreover, within the sequence are many sediments that show little sign of glacial influence yet abundant signs of water transport, such as deltaic bedforms. Other strata are marine and contain ripples formed by wave action; a process that would be impossible with total ice cover. Cyclicity is present, as it is in other Neoproterozoic diamictites. That suggests repeated climate change. Snowball Earth aficionados, and others besides, claim just two and possibly three cryogenic episodes in the Neoproterozoic, but Allen and Young point to the wide range of maximum and minimum ages for those diamictites that are amenable to absolute dating. They suggest that, apart from glaciers being able to develop on land at lower latitudes than in subsequent glacial epochs, the late Precambrian was not ‘special, being merely a period of prolonged climate instability akin to those of later times paced by astronomical factors.

Holding it together

November 2008

Even for experienced geologists it is always exciting to come across direct and tangible evidence for a concept conceived in the 18^th century Scottish Enlightenment, taken up by James Hutton and immortalised by Charles Lyell as “the present is the key to the past”. The most common are ripple marks, sun cracks and raindrop impressions, usually in sandstones. Now relatively high-energy currents move sands, so that every tide on a beach or in an estuary obliterates the previous tide’s sedimentary structures: it is easy to think of them as somehow being ‘one in a billion’ chance preservations. In fact they are a lot more common than common sense might suggest. That is because photoautotropic bacteria can coat sediment surfaces quite quickly to form biofilms or microbial mats, given the right conditions. They knit the grains together, thereby armouring the structures against erosion to some extent. The October 2008 issue of GSA Today begins with a useful summary of the influence of biofilms in preserving intricate signs of sedimentary processes (Noffke, N. 2008. Turbulent lifestyle: microbial mats on Earth’s sandy beaches – today and 3 billion years ago. GSA Today, v. 18 October issue, p. 4-9). Equally important, the author shows how close examination of Archaean littoral sedimentary structures reveals clear signs of the microbial mats themselves. These are convincing evidence for ancient life, even in the absence of tangible fossil cells (the oldest undisputed fossils date back only about 2 Ga).

The Palaeozoic record of sea-level change

November 2008

Variations in global sea level shift the positions where different sedimentary facies are deposited, and also shift some aspects of oceanic chemistry. Consequently they have long been of interest to petroleum explorationists because reservoir and source facies will be laid down in different areas as the sea inundates stable continental areas or withdraws from them. Plots of changing sea level can be derived indirectly from seismic sections that reveal on- and off-lapping stratal sequences with detail added from the stratigraphy of such sequences determined in the field or from well logs, and considerable detail is available globally for the Mesozoic and Cenozoic Eras. The Palaeozoic Era is not so well known, and information has been acquired piecemeal but not correlated to time. So, a semiquantitative compilation will be welcomed in many quarters (Haq, B.U. & Schutter, S.R. 2008. A chronology of Paleozoic sea-level changes. Science, v. 322, p. 64-68). The outcome reveals a steady rise, with short term ups and downs, from about modern levels to around 220 m higher through the Cambrian and Ordovician, dropping in late Ordovician times by about 80 m, perhaps due to glaciation at that time. Through the Silurian and Devonian global sea-level stood around 180 m higher than now, with only broad fluctuation, then to fall gradually through the Carboniferous to reach modern levels around 320 Ma. The Devonian to mid-Carboniferous decline marks the onset of the longest glaciation in Earth’s history, the lasted until the late Permian. The broad shifts have a superimposed short-term eustatic fluctuations, resolved into 172 separable events that vary in amplitude from a few tens of metres to around 125 m. Parts of the record show short-term fluctuations that may correspond to the ~400 ka cycle bound up with Earth’s orbital eccentricity. Yet there is insu

May geologists now synchronise their watches?

May 2008

Calibrating the stratigraphic column to absolute time depends, of course, on radiometrically dating geochemically suitable rocks or minerals. Yet there is a range of available methods based on decay of unstable isotopes, such as 14C, 40K, 87Rb, 147Sm, uranium and thorium. All depend on a variety of assumptions, of which that of a constant, well-established half-life is common to all. If all were perfect, several methods applied to the same materials should give the same results. The trouble is, each parent isotope favours different minerals and different compositions of igneous rocks, so that discrepancies in the dates assigned by different methods to the same stratigraphic unit may either be due to disturbance of one isotopic system relative to the other or to the half-life of one (or both) parent isotope being inaccurate. Currently, the two most widely used and best-regarded methods are U-Pb and Ar-Ar, the latter depending on 40K being converted to 40Ar by neutron bombardment. The first often uses zircons, the second various potassium minerals such as alkali feldspar. Both minerals are magmatic in origin and so the same igneous rock may sometimes be dated by either method or both. It is becoming increasingly clear that the two approaches do not give the same age, which is worrisome at the detailed level permitted by the high precision of each of the methods.

A means of checking the timing parameters for radiometric dating is to compare its results with absolute age determined by a non-radiometric method. The best-calibrated and most widely possible method that does not rely on radioactive decay is based on the astronomical pacing of climate, with its 100, 41, 23 and 19 ka cycles. Analysis of cyclicity in repetitive sedimentary sequences reveals patterns of frequencies that match the astronomical signals. So, within such a sequence it is possible to chart time differences to within a few thousand years. If there are igneous rocks interlayered with the cyclical sediments it should be possible to check their radiometric age differences against the difference determined independently. A Miocene sequence in Morocco has many intercalations of igneous tephras, and therefore provides a crucial test for radiometric approaches (Kuiper, K.F. et al. 2008. Synchronizing rock clocks of Earth history. Science, v. 320, p. 500-504). The team from the University of Utrecht, the Free University of Amsterdam in the Netherlands, and the University of California, dated sanidine (K-feldspar) from the tephras using the Ar-Ar method. This involved using a standard age determined for sanidines from a similar rock type at Fish Canyon in Colorado USA. By turning the approach on its head, i.e. by using astronomically calibrated ages for the samples, they recalculated the age of the Fish Canyon standard. It seems to be 0.65% older than previously thought (from rather dodgy U-Pb dating of  zircons in the Fish Canyon Tuff).

All Ar-Ar ages involve the Fish Canyon standard. So, an underestimate of its age would imply revision of quite a lot of geological events dated by Ar-Ar, especially those that happened abruptly, such as mass extinctions, impacts and magnetic reversals. Using the new standard age puts the K/T boundary event back to 66 Ma from 65.5 Ma. The formerly 251.0 Ma mass extinction at the end of the Permian becomes 252.5 Ma, which coincides better with the outpouring of the Siberian Traps. Similarly, the once 200 Ma end-Triassic extinction, but now possibly 201.6 Ma, links better to the Central Atlantic Magmatic Province outpourings. Quite a stir may be on the horizon, if Kuiper and colleagues’ recalibration is confirmed by similar independent measures.

That radiocarbon dates need to be used with caution is well known, as the amount of 14C produced by cosmic ray bombardment of atmospheric nitrogen varies markedly over time. Again, the ‘work-around’ involves using non-radiometric ages to calibrate the fluctuating relationship between radiocarbon ages and real time. The data of choice are those from tree-ring analysis, but ice cores also preserve ages with a 1-year precision from their annual layering. The Younger Dryas cold period that interrupted the global deglaciation began when atmospheric 14C production was high. It was also a tremendously important event in the progress of human migration and perhaps even genetics – population crashes in hard times can have a ‘bottleneck’ effect on evolution. A multinational team has addressed the interrelations between radiocarbon dating, ice-core climate proxy records and tree-ring analysis for this crucial episode (Muscheler, R. et al. 2008. Tree rings and ice cores reveal calibration uncertainties during the Younger Dryas. Nature Geoscience, v. 1, p. 263-267). They combined measures of varying 14C in tree rings and 10Be in ice cores, both of which are cosmogenic. Rather than resolving the issue, they discovered that the best marine record of the carbon-cycle during the YD, in the Cariaco basin off Venezuela, has a bias caused by anomalous concentration of 14C in shallow seawater as the YD began. Their study open the possibility of resolving such changes in the marine C-cycle.
See also: Kerr, R.A. 2008. Two geological clocks finally keeping the same time. Science, v. 320, p.434-435.

Epoch, Age, Zone or Nonsense

March 2008

The International Commission on Stratigraphy lists 37 Series/Epochs and 85 Stages/Ages in the latest version of the International Stratigraphic Chart for the 11 Systems/Periods of the Phanerozoic. A great battle against ICS’s attempt to extinguish the Quaternary, the only enduring Era originated by Giovanni Arduino (1714-1795) and Johann Gotlob Lehmann (1719-1767), now seems to have ended in a compromise (Kerr, R.A. 2008. A time war over the period we live in. Science, v. 319, p. 402-403). While that vigorous struggle has apparently petered out, the Stratigraphic Commission of the Geological Society of London has launched another by proposing a new Epoch – the Anthropocene. This follow a suggestion by Nobel laureate and chemist Paul Crutzen that the Holocene Epoch ended once humanity made a significant impact on the Earth system (Zalasiewicz, J. and 20 others 2008. Are we now living in the Anthropocene? GSA Today, v.18(ii), p. 4-8).

The device intended by the ICS to mark boundaries between Periods, Epochs and Ages in the Phanerozoic is a symbolic Global Standard Section and Point (GSSP), combining an absolute age definition and a type section. A growing number of boundaries are marked by a physical ‘golden’ spike (not necessarily made of gold) including a plaque engraved with the Period or Age names, welded into the agreed boundary itself. There is good reason for this seemingly odd behaviour; geologists need to have agreed nomenclature and locations so that their discourse can be internationally sensible. It is also a deeply exciting, even exalting moment when any geologist puts her/his finger on a boundary of global significance: and how supremely triumphant actually to wield the hammer that drives the spike home. So much so, that there have been monumental squabbles, some not far short of diplomatic ‘incidents’, about exactly where GSSPs should be placed.

But the whole bureaucratic process has its awkwardly humorous side. There is a proposal that the GSSP for the Pleistocene/Holocene boundary be located in a Greenland ice core. Is that to be in the hole left by the NGRIP core drill at the centre of Greenland, at the depth at which evidence for the warming at the end of the Younger Dryas (11.5 ka) occurs? Or should it be in the core itself – a GSSP in a fridge? Either way, it is going to be difficult to put a finger on that particular boundary Moreover, global warming and the attendant social disruption might remove both. The proposed Anthropocene might have an even stranger GSSP. For a start, when did it begin? An anthropogenic human signature appears clearly in the NGRIP core around 8 ka bp, and at a variety of levels in pollen records, but the GSL’s Stratigraphic Commission wants it to start at the beginning of the Industrial Revolution. Sadly, that is a profoundly diachronous, economic boundary. To make it Eurocentric, as Crutzen suggested, would be a bit non-PC.

Let’s face it, the Holocene is just an interglacial, similar to a great many since 2.4 Ma ago. It is noted only for the brief period in which humanity became separated into two groups: a very small one owning the means of production; the other, initially diverse, being forced to work for the first in order to survive. The Industrial Revolution marked a social simplification into two opposed classes, as clearly defined by Marx, and the increased dominance of human affairs by an inhuman entity called capital. The working through of the contradictions bound up in class society and in capital itself has been largely responsible for the huge environmental changes drawn on by Zalasiewicz et al. It seems our somewhat po-faced authors forget the great many more scholars of human affairs than there are geologists: historians and political economists. Already there are plenty of anthropocentric equivalents of GSSPs in London itself, in the form of its celebrated blue plaques. Historians and political economists might well agree that the rise to dominance of capital – and hence the emergence of rapid environmental change during the uniquely short-lived Anthropocene - began outside the Banqueting Hall on Whitehall at 2.04 pm on Tuesday 30 January 1649 with the separation of the head of the divinely righteous monarch, Charles I, from his body. Ladies and Gentlemen of the SC of the GSL, that is where you place your ‘golden’ spike. However, geology might yet have its say, any time now (and geologists cannot really foretell): a super-volcanic eruption; a comet strike or a cosmic gamma-ray burst. So you had better be quick, if your aim is posterity.

A shocking discovery

January 2008

Every introductory geology course hammers home the message that the finer the grains in a sedimentary rock, the lower the energy under which it was deposited. This ‘received wisdom’ links to the ways in which grains move in moving fluids: rolling; bouncing and in suspension. A reductionist view sees this as the influence of Stokes’ Law in the boundary conditions between turbulent and laminar flow, close to the bed of flow and higher up in the fluid respectively. Stokes’ Law is invoked as that explains how spheres falling through fluids reach a steady speed related to the fluid’s viscosity. The larger the radius of the sphere, the greater that settling speed is. For the smaller size ranges settling speed is proportional to the square of the radius (laminar flow conditions), whereas for large objects it is proportional to the square root of radius (turbulent flow).  This nicely explains the upward decreasing grain sizes in graded beds, formed when a mixture of grain sizes settles from moving fluids when their speed slow, as in turbidites and the on the lee sides of sand dunes. Since we often see silts and muds being deposited in low-energy lagoons and estuaries on the coast that too seems to verify the theory. However, muds that contain clay mineral particles are quite different from scaled-down spherical grains: they are platy; often have unbalanced electrical charges and are subject to Brownian motion that helps keep them in suspension.  When clays suspended in fresh river water meet the sea, ions in sea water encourage the plates to clump together as aggregates or floccules that are much larger than the clay particles themselves. Another oddity is that, once deposited, clays are not as easily eroded as uncemented sands, partly due to their hosting biofilms that hold the particles together.

Despite the accepted explanation of mudstones as indicators of past low-energy conditions based on reductionist notions, suspicion of awkward complexity dates back to Henry Clifton Sorby, one of the founders of geology, who suggested that the study of mudstones and shales was a great challenge for sedimentology. In reality there are probably more than 30 parameters that govern the shifting and deposition of muds, many bound up with flocculation. Confidently discussing the true environmental conditions of mudstone deposition is often thwarted by their ease of weathering and by small animals that munch their way through muds to exploit often high contents of organic debris. Even the fissility of shales is a mystery in the field. Now and again, muds do reveal surprises, such as ripples and cross lamination, that surely reflect current action. Only experimentation can throw light on Sorby’s great challenge (Schieber, J. et al. 2007. Accretion of mudstone beds from migrating floccule ripples. Science, v. 318, p. 1760-1763). Schieber and colleagues from MIT and Indiana University used experimental flumes to investigate what happens to clay floccules, seeding the materials with fine hematite grains to show up any bedforms clearly. The muds used were from 5 to 63 mm in size, which produced floccules between 0.1 to 1.0 mm.  Again and again the experiments produced migrating ripples, some like tiny barchan dunes made of clay floccules. The surprise lay in the flow speeds at which they began to form: between 10 to 30 cm s-1, much the same as those needed to produce sand ripples. Floccules were preserved in the experiments, but since they are made of clay minerals, compaction tends to destroy floccule outlines when mudstones form.

No doubt some fine-grained sedimentary rocks reflect low-energy environments, but without more careful examination of their small-scale features muds formed by energies as high as those involved in producing sandstones and many limestones will go unnoticed. Since mudstones are the most common sedimentary rocks in the geological record, some big surprises are in store.

See also: MacQuaker, J.H.S. & Bohacs, K.M. 2007. On the accumulation of mud. Science, v. 318, p. 1734-1735.

Clays and the rise of an oxygenated atmosphere

April 2006

Almost all eukaryote organisms require oxygen to be available in their environment. Therefore the eukaryote cell probably appeared only after oxygen had become a permanent component of the atmosphere and hydrosphere, which itself depended on photosynthetic metabolism outweighing the scavenging of free oxygen by abundant dissolved iron. It also depends on efficient burial of dead organic matter. For the metazoa – multi-celled eukaryote animals – the oxygen demand rises with their bulk. The first tangible fossils of metazoans appear in the Ediacaran, after the last global glacial episode of the late-Precambrian, around 600 Ma ago. Apart from the earliest evidence for an oxygen-bearing atmosphere after about 2.4 Ga, not much is known about actual levels of oxygen and their changes during the Precambrian. The sudden emergence of the soft-bodied but bulky Ediacaran faunas has been ascribed by many to an equally abrupt rise in the availability of oxygen, on which their evolution must have depended. How that might have occurred has been disputed and pretty vague.

The central requirements to boost oxygen levels are increased photosynthesis – difficult if the period preceding the Ediacaran was one where large tracts of ocean were covered with ice – or increased burial of dead organic matter. The second option is also difficult to imagine if `snowball' conditions had reduced living marine biomass to a very low level. What geoscientists have not been able to grasp, is information on the efficiency with which dead organic matter was buried. Mineralogists and geochemists from the Universities of California (Riverside) and Maine have addressed that aspect from the standpoint of the Precambrian history of clay mineral deposition (Kennedy, M. et al. 2006. Late Precambrian oxygenation; inception of the clay mineral factory. Science, v. 311, p. 1446-1449). If organic matter is buried in porous and permeable sea-floor sediments, the chances of its being metabolised by bacterial action are high. Research on modern sea floor sediments shows that the bulk of organic debris at continental margins is adsorbed onto clay-mineral particles, thereby increasing its chance of preservation over that when it is simply incorporated as particles in silt-sized sediment. Kennedy et al. tested the hypothesis that sedimentation in the late-Precambrian changed from dominance by physically weathered micas and other silicates to one dominated more by products of chemical weathering of the continental surface, i.e. by clay minerals.

Around 700 Ma, the record of marine strontium isotopes in limestones began a major change towards higher 87Sr/86Sr ratios, which implies an increase in the chemical weathering of ancient continental rocks. Australia provides a continuous sequence, from 850 to 530 Ma, of quietly deposited shelf sediments that span this transition and also contain the Ediacaran. Sure enough, the mudstones in the sequence show a distinct increase in swelling clays and kaolinite, implicated in modern preservation of dead organic matter. Rather than an abrupt step, the increase is linear from about 800 Ma, and is matched by similar data from other Precambrian cratons. What might have started this chemical weathering of the land surface? Possibly it was due to a much earlier colonisation of the land than direct evidence suggests. The DNA-based phylogeny of mosses, fungi, lichens and liverworts – all terrestrial organisms – suggests that they arose between 700 and 600 Ma ago. All would have contributed organic acids to the process of chemical weathering. Kennedy et al. model the rate at which free oxygen would have increased as a result of increased deposition of organic matter adsorbed by clays, and conclude that between 730 and 500 Ma retention of oxygen in the environment would have increased six-fold. Thereafter, land-based organisms and further colonisation permanently increased weathering, establishing increasingly efficient marine burial of organic debris, and so creating an environment in which metazoans could evolve and radiate. If confirmed by further analyses, this work establishes yet another non-uniformitarian and biologically-linked process in the evolution of the Earth system.

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Sea level bonanza

December 2005

The ups and downs of sea level through geological time constitute a `beat' to which sedimentation responds by inundation of and withdrawal from the land. The `big picture' is one forced by changes in the volume of the ocean basins as plate tectonics waxed and waned, together with long periods when land ice locked sea water away. A closer focus has stemmed from the changes of oxygen isotopes in benthonic (bottom-dwelling) plankton remains that record details about advances and retreats of polar land ice, most spectacularly from the record of the Pliocene and Pleistocene. These ongoing, higher frequency fluctuations in sea level formed the key to verifying Milankovich's theory of astronomical controls over climate. There are also fluctuations of the order of thousands to tens of thousand years that seem terrestrial in origin, such as the Bond and Dansgaard-Oeschger cycles. Shorter cycles still have had various causes ascribe to them. For inhabitants of near-sea level cities and flat ocean islands, rising sea level is a realistic concern. It is rising just now at about 3 mm per year (in the 1950s the annual rise was half that), mainly because surface sea water is expanding as a result of anthropogenic warning and polar ice is melting.

November 2005 was valuable for geoscientists interested in fluctuating sea level, and most sedimentologists are in that category because the stratigraphic record is primarily governed by this eustatic (world-wide) rhythm. The earliest information on long-term sea-level change came from studies of continental transgressions and regressions that are preserved as onlap and offlap features between strata. That approach was greatly aided by detailed seismic sections gathered by petroleum explorationists, in which such features show up a great deal more readily than they do in limited exposures on land. The results of many different methods of charting eustasy are wonderfully summarised by a large team of US geoscientists (Miller, K.G. and 9 others 2005. The Phanerozoic record of global sea-level change. Science, v. 310, p. 1293-1298). Their review covers the last 543 Ma, and reveals several novel aspects. It has been known for over 30 years that the higher frequency sea-level changes correlate well with oxygen isotope records, because of the preferential evaporation of water that contains light 17O. When evaporated ocean water ends up in long-term storage as land ice, the proportion of heavier 18O rises in seawater and in carbonates extracted from it by organisms. The broad view also shows a sea-level—d 18O correlation though, and that probably reflects expansion and contraction of the volume of ocean water as mean global temperature rose and fell on the scale of tens of million years. That the Cretaceous was the period during which sea level reached an all time high during the Phanerozoic has been well known for over a century, and manifested itself in the production of giant `carbonate factories' on shallow shelves of inundated continental lowlands. Famously, that was ascribed to vast production of new oceanic crust, both by accelerated sea-floor spreading and outpouring of huge submarine flood basalts, such as the Ontong Java Plateau of the west Pacific floor. Putting together all the pertinent data, however, suggests that Cretaceous tectonics was not nearly as vigorous as once suspected.

Unsurprisingly, sea level studies are `hot' and researchers have a better than even chance of getting publications into press in the most august of journals, and a readership to boot. There is a great deal of information on past and current sea level fluctuations, and a great deal of thought has gone into acquiring data. Dotted around the world's coast lines are tide gauges of the most exquisite precision; so precise in fact that the outermost ripples of the Boxing Day tsunamis were detected at the antipode of the earthquake that caused them. Whether or not watching these gauges continuously is a fulfilling task, the long-term records have revealed a surprise (Church, J,A, et al. 2005. Significant decadal-scale impact of volcanic eruptions on sea level and ocean heat content. Nature, v. 438, p. 74-77). Since 1960, global sea level has been up and down like a yo-yo, deviating by ± 2-3 mm from the longer-term mean at a rate measured in decades. This correlates well with five major volcanic eruptions during the last 45 years, such as El Chichon and Pinatubo. The first effect is a rapid fall (6 mm in a year, after Pinatubo erupted), probably resulting from global cooling and reduced rainfall caused by sulfate aerosols injected into the stratosphere, followed by slow recovery. It seems odd that volcanoes have a bigger effect on sea level than overall global warming, yet other records show their profound global effects. The fall in sea level must be dominated by shrinkage of cooled surface water. Interesting, and quite possibly a boost for those in denial over global warming. However, my main concern, living at 250 m above mean sea level, is that my bathroom cistern is always overflowing because of a water level rise of 1 mm in a matter of a few minutes.

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A record of the Palaeoproterozoic lunar cycle

November 2004

One of the many natural processes that produce rhythmic sediments is the ebb and flow of the tide, twice a day and with an amplitude that peaks and falls twice each lunar month (today a 28-day cycle) to produce spring (new and full moon) and neap tides (the two half moons). Tidal rythmites consist of thin laminae whose thicknesses vary regularly for many cycles. Their occurrence dates back to 3.2 Ga, and along with other sedimentary structures formed by tidal action, such as "herring-bone" cross stratification formed by reversals in tidal currents, prove the presence of the Moon in orbit around the Earth. Fine rythmites can be analysed to work out the length of the lunar month in the past, and help refine ideas on the evolution of the Earth-Moon system. Rajat Mazumder of Asutosh College , Kolkata , India has analysed the earliest known tidal rythmites from the Palaeoproterozoic of NE India (Mazumder, R. 2004. Implications of lunar orbital periodicity from the Chaibasa tidal rhythmite ( India ) of late Paleoproterozoic age. Geology, v. 32, p.841-844). His work shows that between 2.1 to 1.6 Ga the lunar month was 32-days long. Remarkably, the record in these sediments is as detailed as found in modern ones from estuarine silts. As well as rhythms, they record occasional perturbations due to storms. Using the changes in the lunar month during the last 450 Ma erroneously suggests that the system emerged from a period around 1.5 to 2.0 Ga following a major collision – that of course is ruled out by a total lack of evidence of such a catastrophe. The new datum suggests instead a steady decrease in the lunar month, that corresponds with the Moon's gradually receding from the Earth. Energy apparently lost by tidal action is conserved by an increase in the angular momentum of the Earth-Moon system, and that forces the Moon ever further from us – its orbital velocity increases.

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Tying down the Devonian-Carboniferous boundary

November 2004

Getting the stratigraphic column properly calibrated from relative to absolute time is all the rage these days ( New benchmarks for geological time in EPN June 2004). On the recent stratigraphic chart published in late 2003 by the International Commission on Stratigraphy, the Devonian-Carboniferous boundary has a "golden spike" global standard section and point (GSSP) dated at 359.2 ± 2.5 Ma. Already, that is disputed because of new radiometric dating from an "auxiliary" global stratotype section (Trapp, E. et al . 2004. Numerical calibration of the Devonian-Carboniferous boundary: Two new U-Pb isotope dilution-thermal ionization mass spectrometry single-zircon ages from Hasselbachtal ( Sauerland, Germany ). Geology, v.32 , p.857-860). As well as holding the record for length of any publication title yet covered by EPN , the paper contains some intriguing points. That a carefully determined age for the strata at Hasselbachtal has been possible is thanks to about six, centimetre-thick ash beds in richly fossiliferous sediments just above the faunally determined boundary. Twenty-three single-zircon ages from the two ashes just above the accepted faunal boundary give ages of 360.5 ± 0.8 and 360.2 ± 0.7 Ma. Now, to you and I and many less pernickety geochronologists, that spells out the well-known phrase or saying, "within error", as indeed is that of the GSSP. And, for a convoluted reason based on plotting an age from another tuff with these ages against the palaeontological data, the age presented for D-C itself is 360.7 ± 0.7 Ma. This may be a better age than that of the GSSP. But, so what? The D-C boundary is not associated with any family-crushing catastrophe like the P-T or K-T boundaries, nor even that within the Late Devonian itself. Are "they" going to move the GSSP from its present location in southern France, ratified in 1990, along with the vast pyramid of precious and intricately carved crystal, which no doubts marks its spot? An altogether more serious threat to the established order is the stealthy attempt to abolish the last remnant of the great stratigraphic divisions inspired by Giovanni Arduino's work in the 18 th century; the Quaternary is besieged! One of my spies, not unconnected with this episode of our own emergence on the planet, attended a stormy meeting at the 32 nd International Geological Congress in Florence in August 2004, which seemed likely to expunge the Quaternary from the minds of all future geologists. He gleefully reported that a mighty rearguard action had put off that evil day, at least for a while. Sadly, the writing is already on the great IUGS/ICS stratigraphic wall chart – its is no longer there! The last relic in officialdom is in the latest definitive publication (Gradstein, F.M. et al . 2004. A new geologic time scale with special reference to Precambrian and Neogene. Episodes, v.27, p.83-100). On page 86, at the very top of the table conferring status on GSSPs, it is written "This composite epoch [the "Quaternary"] is not a formal unit in the chronostratigraphic hierarchy". So there you have it; the issue is getting things into proportion.

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When the Mediterranean dried up

May 2003

At the end of the Miocene (from 6 to 5.3 Ma) the connection between the Atlantic Ocean and the Mediterranean Sea was blocked somehow. Over 700 thousand years evaporation deposited a thick layer of salt that now lies beneath much of the Mediterranean basin. This is known as the Messinian salt crisis. Equally dramatic, the straits reopened suddenly to allow seawater to flood back in the early Pliocene, in a hydrological catastrophe. How the Mediterranean basin became cut off has been ascribed to a 60 m sea-level fall, crustal shortening associated with nappe formation in the Betic Cordillera of Spain and the Atlas mountains, or by some kind of tectonic uplift. Timing of the Messinian crisis rules out the first two options, but sedimentation in the former "gateway", a shallow seaway through what is now southern Spain, shows evidence of rapid shallowing that would have resulted from regional uplift. The question is, what drove this regional upwarping? A team from the GEOMAR Research Centre for Marine Geosciences in Kiel, Germany has discovered evidence from the changing geochemistry of Miocene to Pliocene volcanic rocks in the western part of the Mediterranean (Duggen, S. et al, 2003. Deep roots of the Messinian salinity crisis. Nature, v. 422, p. 602-606). Send Duggan and his co-workers found that the lavas underwent a geochemical shift from affinities with subduction-zone processes to those typical of intra-plate magmatism around 6.3 Ma ago, volcanism largely ending about 4.8 Ma. The ending in the late Miocene of eastward subduction of Tethyan sea floor beneath the Mediterranean, which had initiated volcanism around 12 Ma, led to foundering of part of the lithosphere and uprise of asthenosphere. This is marked by a change from high-silica, early magmas to alkaline, more basaltic varieties during the period of the Messinian salinity crisis. Uplift resulting from this delamination would have pushed the formed connections between the Atlantic and the Mediterranean as much as 800 m above sea level. Duggan et al. Suggest that the axis of uplift gradually migrated westwards, so that by the end of the Messinian crisis the area now centred on the Straits of Gibraltar would have been bulged up. Massive gravitational sliding from this edge of the continental lithosphere into the Atlantic may then have opened the narrow passage through which Atlantic water once again flooded.

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Biofilms and BIFs

September 2002

Biomineralization is a growing topic that ranges from life's influence on the production of economic deposits of metal ores to even the suspicion that it might play a role in Alzheimer's syndrome. The most common, and enduring evidence of the influence of micro-organisms in making rocks are stromatolites made of carbonates that blue-green bacteria have secreted, perhaps from as early as 3500 Ma ago. Something similar, though it involves eukaryotic algae, is the formation of tufa or travertine where springs emerge from limestones. Many a child, including my young self, consigned a cuddly toy to "petrifying" springs, such as Mother Shipton's Well in Knaresborough, Yorkshire. Few retrieved them, which is why there aren't many rock-like Teddies around.. Another childhood memory, that bears on biomineralization, is a spring surrounded by orange and brown slime that we supposed was so deadly that only bathing in helicopter fuel would ward off a dreadful end brought on by the faintest splash of the loathsome gunk. It is a great surprise to learn that such ochreous springs, common where coal mines drain to the surface, might hold a key to the formation of Precambrian banded iron formations (BIFs) (Brake, S.S. et al. 2002. Eukaryotic stromatolite builders in acid mine drainage: implications for Precambrian iron formations and oxygenation of the atmosphere. Geology, v. 30, p. 599-602).

Groundwater that has passed through iron-sulphide bearing rocks, becomes both acid and charged with iron-2 after oxidation of pyrite. It is high acidity and low Eh that dissolves toxic heavy metals and arsenic, rather than their iron content, that make springs of such waters so hazardous to small boys bent on careers as hydraulic engineers (check their shins and fingers for the lingering water blisters that are a sure sign of the onset of arsenic poisoning). It seems that Euglena, a common "animalcule" in such springs that is easily seen with a cheap microscope, is an ochre (iron-3 hydroxides and sulphates) forming agent. It is an acid-tolerant, oxygenic photosynthesizer that builds slimy mats. Given time and substantial supplies of dissolved iron, Euglena actually builds hard structures reminiscent of stromatolites. Brake and colleagues from Indiana State and Kansas universities, and the Colorado School of Mines, studied Euglena from coal-mine drainages under lab conditions, and provide details of their metabolism. The modern iron-stromatolites are so like some variants of BIFs from the Archaean and Palaeoproterozoic, when they were at their acme, that the authors suspect their origins in biofilms formed by prokaryotic organisms with similar metabolism to the more complex Euglena. Until their work, most geologists regarded BIFs as products of inorganic precipitation of iron-3 compounds and silica when iron-2 rich seawater met oxygen produced by photosynthesizing cyanobacteria. Indeed they speculate that the biofilm makers could have been early eukaryotes, despite the first unambiguous evidence for nucleus-bearing organisms being no older than 2100 Ma. If they are correct, then such communities would have needed free oxygen, and would themselves have contributed to oxygen build-up in the early atmosphere.

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Hydrocarbon source rocks and ocean anoxia events

January 2002

Much of the world’s oil resources formed by maturation and migration of hydrocarbons from organic-rich, marine mudrocks, which seem to have formed episodically during Earth history. A widely accepted view is that such source rocks’ content of organic matter fell to the ocean floor as the remains of tiny organism. That they were not oxidized by bacterial action seems to suggest that the periods when source rocks accumulated were characterized by low oxygen levels in bottom waters. Each major source rock has been linked to such ocean-anoxia events, and to periods when deep-ocean circulation effectively stopped, so cutting off oxygen supplies to deep levels. However, studies of modern deposition of organic matter in marine sediments at continental margins reveals that discrete particles of organic matter are far outweighed by biological molecules that coat the surfaces of minerals, particularly those of clay minerals. The amount of organic carbon in a modern sediment depends largely on its content of clay minerals derived from intense chemical weathering of continental rocks. Such coatings are protected from normal processes of decay, so that the adsorbed organic carbon compounds can be buried, more or less intact

It should be possible to check whether ancient source rocks are similar to modern carbon-rich sediments by seeking a strong correlation between clay content and organic content—mudrocks also contain fine silt particles made of non absorbent quartz. It seems that in at least one Cretaceous source rock in the US Mid-West such a correlation is clear (Kennedy, M.J., Pevear, D.R. and Hill, R.J. 2002. Mineral surface control of organic carbon in black shale. Science, v. 295, p. 657-660). This suggests that oil-shale deposition is as much related to the intensity of continental weathering of silicates as it is to ocean-water chemistry. Since clays, especially the sponge-like smectites, adsorb organic molecules from solution in seawater, they draw on a vast pool of material, so that enhanced biological productivity need not be linked to oil-shale formation either. The fact that most organic material in such rocks is structureless kerogen, rather than identifiable particles, also supports this alternative hypothesis.

Both petroleum geologists and palaeoclimatologists have assumed a source rock—ocean anoxia connection in both exploration strategies and assessment of past climate shifts. So Martin Kennedy’s painstaking findings are sure to cause a major stir. However, what cannot be avoided is that increased chemical weathering of the continents is likely to accompany globally warm conditions, and they in turn sponsor growth in planktonic productivity. Likewise, global warmth does not favour the formation of dense, cold and therefore oxygenated sea-surface water, which sinks to aerate deep oceans when the planet is cool.

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Measuring erosion rates

January 2002

So many landscapes show evidence of changes in the rate of erosion, such as terraces, waterfalls and signs of changing rates of sediment deposition, that a means of accurately measuring rates opens up an important new phase in geomorphological research. Precise dating of modern surfaces is not possible using stratigraphic or radio-carbon methods, and this has hidden much of landform history. Once a surface is exhumed, it becomes exposed to cosmic ray bombardment. These particles travel at near-relativistic speeds, and so have sufficient energy to transmute common element nuclei to unstable isotopes. The longer the exposure of a surface material, the more radioactive it becomes, albeit very weakly so Since erosion and sedimentary processes move and quickly bury particles dislodged from a surface, material has a finite time during which it can be irradiated. The particles themselves carry the isotopic signature of their surface residence time, the slower erosion is the more radioactive are particles derived from the surface.

Cosmogenic dating uses sedimentary grains from sands deposited in a drainage basin, particularly those of quartz that are common and stable. Oxygen and silicon in silica can become ¹⁰Be and ²⁶Al when struck by cosmic rays. Although sampling is fraught with pitfalls, essentially it amounts to scooping up a handful of sand that represents the past erosion of the entire catchment above a sample point. Measuring the minute concentrations of new isotopes costs of the order of $1000 per sample, using a high-energy accelerator mass spectrometer. Since dozens of samples provide sufficient data for meaningful interpretation, this is not a method that will spread widely to places that come anywhere near fully reflecting the intricacies of erosional shifts over the large age range that cosmogenic dating can address. Nonetheless, its early results are astonishing. Work in Idaho suggests that through the period of the last glacial maximum into the early Holocene the average rate of erosion was 17 times faster than it is at present. That possibly signifies either continual high erosion, that has petered out, or, more likely, that erosion has had episodic, catastrophic pulses. As might be expected, anthropogenic disturbance of the surface enhances erosion rates, but a cosmogenic study of river sediments in Sri Lanka indicates that 200 years of intensive farming in rugged highland areas have resulted in a 20- to 100-fold acceleration. Most awkward of all, another study of long-term erosion in California’s Sierra Nevada showed no relation between weathering and erosion rates and climate change. Geochemists contributing to the debate over climate controls by weathering take note. It seems that the primary control of erosion rates in western California was purely tectonic, which could tally with the notion that newly rising mountains have a major influence over sequestering of CO₂ by silicate breakdown.

The obvious next step is blending cosmogenic sediment dating with that of crustal exhumation from Ar-Ar and U-Th/He dating of cooling due to uplift and erosion.

Source: Greensfelder, L. 2002. Subtleties of sand reveal how mountains crumble. Science, v. 295, p. 256-258

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Bacteria and dolomites

January 2001

Production of carbonate rocks is one of the means of drawing down carbon dioxide from the atmosphere. During the Phanerozoic times of "greenhouse" conditions both induced and were relieved by carbonate "factories" dominated by metazoans that secreted calcium carbonate in their hard parts. An excellent example is the Chalk of the late Cretaceous. Before the evolution of the metazoa some other means was needed. Precambrian sequences contain abundant carbonate strata, but a great many of them contain lots of calcium–magnesium carbonate or dolomite. The further back in time, the more dolomitic carbonates become.

Some of these dolomites contain mounds and cauliflower–like masses built of many thin laminae. By analogy with similar structures forming nowadays in a few rare environments that are too saline to support metazoans, sedimentologists have ascribed these stromatolites to the expulsion of toxic calcium from their cells by blue–green bacteria or cyanobacteria. Blue–greens are photosynthesisers that generate oxygen. Evidence from carbon–isotope analyses of fossil organic material in old Precambrian sediments supports them having evolved very early. Despite their antiquity and ability to break down water and release its oxygen, blue–greens were unable to build up oxygen in the Earth's atmosphere until about 2 000 million years ago. For half of life's history conditions were lacking in oxygen, and bacteria that consumed dead things had to subsist with metabolisms that employed other chemical tricks than the oxidation of organic matter to carbon dioxide plus water, for which oxygen is essential. One strategy is the reduction of sulphate (SO₄^2–) to sulphide (S₂^2–) ions (see Slime to the rescue, December 2000 issue). That involves shifting of electrons so that the counterpart of a reduction is some form of oxidation, which such bacteria employ in their metabolism.

Modern environments devoid of oxygen encourage such organisms; hence their obnoxious odour from released hydrogen sulphide gas. One such habitat is a very salty lagoon in Brazil, and that is also a place where dolomite precipitates in abundance. A Swiss–French team of organic geochemists has shown experimentally that its sulphate–reducing micro–organisms actually encourage dolomite to leave solution (Warthmann, R. et al. 2000. Bacterially induced dolomite precipitation in anoxic culture environments. Geology v. 28, p. 1091–1094). Not only does that add to the ways in which modern carbon dioxide leaves the atmosphere on a long–term basis, but suggests that such bacteria played the key role in climate balance in the earliest Precambrian. The lack of oxygen before 2 000 million years ago would have made every niche available to them, for sulphate ions are continually added to sea water. They showed in vitro how bacteria from the lagoon sediments cultured in sulphate–rich water did precipitate dolomite in curious dumbbell–shaped grains that aggregated to cauliflower–shaped masses in zones around the cultures. By carefully isolating different species of bacteria, they found that sulphate reducers were the culprits. As well as helping account for the preponderance of dolomite in ancient carbonates, it expands our vista of organic diversity represented by them, albeit of a very lowly kind.

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Unravelling Neoproterozoic environments

November 2000

The latest Precambrian or Neoproterozoic, from1000 to 544 Ma ago, and especially from 700 Ma to the start of the Cambrian, is the most important episode in the history of biological evolution. That is the episode during which remains of large, soft-bodied animals (the Ediacaran fauna) first appear and at whose end animals able to secrete hard parts burst onto the scene. It marks the preparation for the beginning of life as we know it best; the Cambrian Explosion. This period is remarkable also by its huge climatic upheavals that twice turned Earth into a planetary snowball, when ice masses extended to tropical latitudes. As if these unprecedented and never repeated big freezes were not sufficient to focus geologists' undivided attention on the late-Neoproterozoic, seawater became for a time so depleted in oxygen that soluble ferrous iron entered shelf areas to precipitate out as banded iron formations, which had vanished around 2.2 Ga when oxygen first entered the oceans in any amounts. Neoproterozoic world events opened with all continental lithosphere known to be around at the time consolidated in the mother of all continents, literally called Rodinia from the Russian for motherland. Rodinia broke up with the as yet unexplained break out of Laurentia from close to its heart. A massive round of sea-floor spreading saw tiles from the Rodinian mosaic reassembled as the core of the Gondwana supercontinent beginning around 650 Ma ago. Gondwana played a massive role in subsequent tectonics until it too broke up in the Mesozoic. These were interesting times, relative to which the Phanerozoic seems somewhat tame, except for its tangible record of life's ups and downs.

But there is a problem; with magmatic activity sparsely distributed in Neoproterozoic space and time, and a lack of rapidly changing biomarkers, division of events through time and, more important, correlating events from place to place has proved difficult, except in a barely useful and often mistaken way. Geological accounts of the late-Precambrian have been permissive and provocative, to say the least. That seems likely to change rapidly. Frustration centred on the time problem set against the undoubted drama of events had spurred the development other means of stratigraphic division and correlation.

The geologically instantaneous mixing of isotopes affected by global processes forms the basis for identifying large events that fractionate them in stratigraphic sections everywhere. That has been the biggest contribution of the oxygen-isotope data in seafloor sediment cores for the Neogene, in which fluctuating volumes of land ice shifted the proportion of ¹⁶O to ¹⁸O in ocean water, so that features in δ¹⁸O records become means of fine-tuned correlation world-wide for climate shifts. Carbon isotopes play a similar role in charting changes in global bio-productivity and burial of dead matter and carbonate hard parts. Strontium serves to detect changing balances between supply of dissolved material from oceanic magmatism and from erosion of ⁸⁷Sr-enriched continental crust. Sulphur isotopes also help chart supply and demand among organic and inorganic processes. Such chemo-stratigraphic methods were recognised as a lifeline for resolving Precambrian evolution in the late 1980s. A decade on, painstaking work has begun to bear fruit, as covered by a Special Issue of the 100th volume of Precambrian Research (v. 100(1), 2000). Andrew Knoll of the Botanical Museum, Harvard, USA summarises progress (Knoll, A.H., 2000. Learning to tell Neoproterozoic time. Precambrian Research, v. 100, p. 3-20), but details of the chemo-stratigraphic approach and what the prominent isotopic markers might mean appear in a paper of monographic proportions from a team at the Department of Earth and Planetary Sciences, Macquarie University, Australia (Walter, M.R. et al., 2000. Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon and sulphur in seawater, and some interpretative models. Precambrian Research, v. 100, p. 371-433)

Chemostratigraphy seems to resolve the question of how many late-Precambrian icehouse conditions of global significance. Though some have speculated on as many as 5 or 6 from occurrences of glacigenic rocks, only two match with isotopic signals, one (Sturtian) around 700 Ma and one around 600 Ma (Marinoan). Both have associated negative δ¹³C excursions in carbonates to the level of mantle carbon, which suggest that life was reduced to a minimum by 'Snowball Earth' conditions. Associated shifts in the proportion of isotopically heavy sulphur are different. Sturtian glaciation matches with an increase in δ³⁴S, a likely product of ocean anoxia, the involvement of light ³²S in bacterial reduction of sulphate to sulphide ions, and the burial of iron sulphide at sources of ferrous iron around sea-floor hydrothermal systems. The anoxia was sufficiently extreme for Fe²⁺ to dissolve and mix throughout the ocean water column, so that precipitation as ferric oxy-hydroxides burgeoned in shelf seas to form BIFs a little younger than the glacigenic rocks. Marinoan glaciation, though equally catastrophic for bioproductivity, did not fully deplete the oceans of oxygen. Massive peaks of δ¹³C prior to glaciations suggest that intense precipitation of carbonates in the limestones so common in the run-up to frigidity, plus burial of abundant dead organic matter in the case of the Marinoan, dramatically drew down CO₂ from the atmosphere. Life's recovery after the Sturtian, together with organic burial, boosted oxygen levels, as too following the 600 Ma Marinoan. Possibly the delivery of huge amounts of glacially ground rock flour added nutrients that helped fuel this biological pump, and an increase in ⁸⁷Sr/⁸⁶Sr after the Marinoan could reflect such fertilization. There is much more in the paper that will fuel advances in ideas of the co-linkage of glaciation and biological evolution—essentially adaptive radiation by the few eukaryotes that survived anoxia and other stresses—and the evidence for large increases in oxygen production that are prerequisites for the origin of large, oxygen-demanding animals in the Ediacaran fauna. What came as complete surprise to me, a non-specialist, was clear evidence from several well-studied sections for the largest negative δ¹³C excursion in geological history only 2 Ma before the Cambrian Explosion, which took less than a million years to develop.. Other isotopic trends seem to indicate a brief but highly intense global warming that snuffed the Ediacaran animals from the fossil record. The unique depletion in heavy carbon points strongly to the seabed belching teratonnes of methane in unstable gas hydrate, a product of double selection of ¹²C by photosynthesizing plankton and methanogen bacteria metabolizing dead planktonic matter within ocean-floor sediments.

Isotopically, the late-Neoproterozoic was chaotic. Carbon in particular records ups and downs with amplitudes and frequencies that dwarf those of the far-better recorded Phanerozoic, even in later glacial epochs and mass extinctions. It was two evolutionary developments that probably damped down excursions in carbon isotopes in later times: the stirring of deep-ocean muds by burrowing animals to promote more rapid oxidation of buried organic matter; the increased efficiency of CO₂ drawdown by organisms that secreted carbonate hard parts. Perhaps Precambrian events were not so dramatic after all, equally disturbing events being smudged in the Phanerozoic by the rapid adaptive radiation following the Cambrian Explosion.

My prediction is that this issue of Precambrian Research will become the starting post for a major shift of research into Neoproterozoic and earlier Precambrian sedimentary piles, after two decades of getting things straight in the Mesozoic and Cainozoic. I feel confident in that, because the stories of Snowball Earth and near extinction of all oxygen demanding life around 700, 600 and now 545 Ma are ones that will, as the Sun might say, run and run.

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Inglorious mudstones

Because they succumb to erosion easily, mudrocks do not outcrop well, except on the coast or in arid lands. Often they show little if any stratification that field workers can distinguish from the partings imparted by compaction and dewatering, which make shales from them. Yet they are repositories of a great deal of information (see Methane hydrate—more evidence for the 'greenhouse' time bomb). In hand specimen their two main components, silt-sized quartz grains (<62 microns) and clay minerals (>4 microns) only become distinguishable by chewing! They are irresolvable using optical microscopes, and detailed work needs scanning electron microscopy.

Silt to clay proportions in mudrocks are variable. The first is generally taken as an indicator of suspended debris from land masses and its proximity to where the mud accumulated. The more clay, the further muds were from exposed continents, or so sedimentologists used to assume. That approach has taken a hard knock from some recent detailed work on Devonian mudrocks (Schieber, J. et al., 2000. Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature, v. 406, 31 August 2000, p 981-985).

Jurgen Schrieber of the University of Texas (Arlington), Dave Krinsley of the University of Oregon and Lee Riciputi of the Oak Ridge National Laboratory in Tennessee used scanning electron microscopy, cathodoluminescence and ion-probe techniques to discriminate between detrital quartz grains and those formed by precipitation of silica from pore water in the original muds. Those grains that do not luminesce probably formed by silica solution and reprecipitation, and the Devonian mudrocks contain mainly non-luminescent quartz grains. Oxygen isotope ratios from individual grains confirm this in situ origin. The researchers had no reason to suspect that their Devonian samples would give such results, and assumptions based on silt to clay ratios from any mudrock are now in doubt.

Worse still, silt in ocean-floor muds, cores of which form the linchpin for Pleistocene climate studies, has been a rough and ready way of estimating wind speeds as climate shifted from glacial to interglacial conditions. These silts could be precipitates too, and the variations in their proportions may stem from changes in the delivery of dissolved silica from land to the oceans.

See also: Kemp, A. 2000. Probing the memory of mud. Nature, v. 406, 31 August 2000, p 951-953

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