Climate out of control after the Permian-Triassic mass extinction

The snuffing out of up to 90 percent of all terrestrial and marine species at the end of the Permian (252 Ma) was the outcome of lethal climatic warming. It probably stemmed from a stupendous episode of flood basalt volcanism and intrusions in what is now Siberia that burned vast amounts of peat or coal in the basin that the flows filled (see: Coal and the end-Permian mass extinction; March 2011). The carbon dioxide so released created planetary hyperthermia and toxic acid rain. For at least five million years Earth was an almost sterile world, a notable absence being dense vegetation on the land surface – the Early Triassic is devoid of coal, whereas there is plenty of Late Permian age. Much the same slow recovery of life is found in meagre collections of land and marine animal fossils of that age. Yet, other mass extinctions were followed by recovery and species diversification at a much faster pace.

One conceivable explanation could be the near absence of vegetation whose photosynthesis and burial would otherwise draw down CO2 and the same goes for its marine equivalent phytoplankton. But there is a powerful inorganic means of carbon sequestration: silicate weathering. The chemistry depends on carbon dioxide dissolved in water. For simple silicates it can be expressed as:

2CO2 + H2O + CaSiO3 → Ca2+ + 2HCO3 + SiO2.

The higher the ambient temperature, the faster such reactions proceed. Most silicates are more complex and many common ones, such as feldspars, include aluminium, so that another product of weathering is insoluble, fine-grained clay minerals. So various soluble metal ions (Ca, Mg, K, Na etc), dissolved bicarbonate ions, silica in various guises and clays eventually end up in the sea. Once there, it is possible for them to recombine, as for instance calcium and bicarbonate ions:

Ca2+ + 2HCO3→ CaCO3 + CO2 + H2O

Despite some CO2 gas being released, this reaction results in a net sequestration of carbon in calcium carbonate. Incidentally, the same kind of chemical reaction occurs in the soils produced by weathering. The carbonate may cement soils to form a hard crust of caliche or ‘calcrete’. Chemical weathering enhanced by a hot climate, it might seem, should reduce the greenhouse effect quickly: a feedback mechanism that normally stabilises climate. But that did not happen after the P-Tr extinction event, thereby stressing all remaining life forms. A group of scientists at the University of Waikato in New Zealand have developed a possible explanation for this potentially fatal hazard for life on Earth (Isson, T.T. et al. 2022. Marine siliceous ecosystem decline led to sustained anomalous Early Triassic warmth. Nature Communications, v. 13, article 3509; DOI: 10.1038/s41467-022-31128-3). It focuses on the silica (SiO2) released by chemical weathering, which enters the ocean in the form of a colloid: Si(OH)4, a form of silicic acid known as ‘reactive silica’. Under ‘normal’ conditions, this is removed by organisms, such as diatoms and radiolaria, and is constantly recycled on a time scale of about 400 years, some contributing to deep-ocean oozes in the form of chert. But, like all other marine organisms, they too were victims of the P-Tr mass extinction.

Examples of marine radiolaria (top)

Reactive silica colloids in seawater also participate in inorganic chemical reactions, combining with dissolved metal ions to form complex hydrated aluminosilicates, i.e. more clay minerals. The reactions change the alkalinity of seawater. As a result dissolved HCO3ions transform to CO2 gas and water. Despite the complexity of the chemistry that interweaves the carbon and silicon cycles, there is a simple conclusion. If the abundance of silica-secreting marine organisms falls drastically while continental weathering continues to deliver silica, clay-mineral formation on the ocean floor results in release of CO2 that reverses the effect of enhanced weathering and thus maintains hyperthermal conditions. The other outcome is that less chert and flint granules form Terry Isson and colleagues examined the varying proportion of chert in cores through Lower Triassic marine sediments. A ‘chert gap’characterises the 4 to 6 Ma following the P-Tr boundary event. This can be explained in part by extinction of silica-secreting organisms and by inorganic reactions converting the reactive silica that enhanced weathering delivered to the oceans to clay minerals. This supports the idea that the inorganic part of the silica cycle maintained greenhouse conditions in the absence of organic ‘competition’ for reactive silica. Many other biogeochemical cycles link biological and chemical processes that combine to affect climate: involving phosphorus, nitrogen and iron, to name but three.

Climate and tectonics since 250 Ma

A central feature of the Earth’s climate system is the way that carbon bound in two gases – carbon dioxide (CO2) and methane (CH4) – controls the amount of incoming solar energy that is retained by the atmosphere. Indeed, without one or the other our home world would have been locked in frigidity since shortly after its formation: a sterile, ice-covered planet. The ‘greenhouse effect’ has been ever-present because the material from which the Earth accreted contained carbon as well as every other chemical element from hydrogen to uranium. Naturally reactive, it readily combines with hydrogen and oxygen to form methane and carbon dioxide, which would have escaped the inner Earth as gases to enter the earliest atmosphere as a ‘comfort blanket’, along with water vapour, another greenhouse gas.  Their combined effects have remained crudely balanced so that neither inescapable frigidity nor surface temperatures high enough to boil-off the oceans have ever occurred in the last 4.5 billion years. Earth has remained like the wee bear’s porridge in the Goldilocks story! Even so, global climate has fluctuated again and again from that akin to a steamy greenhouse, through long periods of moderation to extensive glacial conditions, including three that extended from pole-to-pole – ‘Snowball’ Earths –  during in the Precambrian. During the Phanerozoic the Earth has entered three long periods of generally low global temperatures, in the Ordovician, the Carboniferous and during the last 2.5 Ma  that allowed polar ice caps and sea-ice to extend a third of the way to the Equator. These were forced back and forth repeatedly by cyclical influences apparently triggered by astronomically controlled changes to Earth’s orbital and rotational parameters – the Milankovich Effect. Anthropogenic emissions of greenhouse gases in vast and increasing amounts now threaten to disrupt natural climate variation, effectively overthrowing the gravitational influences of distant giant planets that have controlled climate changes that shaped our own evolution since the genus Homo first emerged.

Bubbles of air trapped in cores through the ice sheets of Antarctica and Greenland record decreased volumes of land ice as CO2 content increased and the opposite during glacial episodes. Somehow in step with the astronomical forcing the Earth released greenhouse gas to warm the climate and drew it down to bring on cooling. Since all life forms are built from carbon-rich compounds and some extract it from the environment to build carbonate hard parts, climate and life on land and in the oceans are interlinked. In fact life and death are involved, because once dead organisms and their hard parts are buried before being oxidised in sediments on land, as in peat and ultimately coal, and on the ocean floors as limestones or carbonaceous mudstones, atmospheric carbon is sequestered. Exposed to acid water containing dissolved CO2 from the atmosphere or to oxygen, respectively, the two forms of carbon in solid form are released as greenhouse gas once more. Both take place when sedimentary deposits are exhumed as a result of erosion and tectonics. Another factor is the abundance of available nutrients, themselves released and distributed by erosion and agents of transportation. At present surface waters of the most distant parts of the oceans contains plenty of such nutrients, except for a vital one, dissolved iron. So they are wet ‘deserts’. It seems that during the much dustier times of glacial episodes iron in fine form reached far out into the world’s oceans so that phytoplankton at the base of the food chain ‘bloomed ‘and so did planktonic animals. Dead organisms ‘rained’ to the ocean floor so drawing down CO2 from the atmosphere and decreasing the greenhouse effect. The surface parts of the carbon and rock cycles are extremely complex and climatologists have yet to come to grips with modelling its future climates convincingly. Yet the carbon cycle and much deeper parts of the rock cycle are interwoven too.

Carbon in sedimentary rock can be heated by burial, and some can be subducted to great depths at destructive plate margins together. The same applies to in ocean-floor basalts that have been permeated by circulating sea water through hydrothermal circulation to form carbonates in the altered volcanic rock. In both cases carbon stored for hundreds of million years can be released by metamorphism in orogenic belts at zones of continental collision and deep below island arcs. Carbon from mantle depths that has never ‘seen the light of day’ is also added to the atmosphere when magmas form below oceanic constructive margins, hot spots and subduction zones, and where magmas flood the continental surface. Consequently, plate tectonics and deep mantle convection have surely played a long-term role in the evolution of our planet’s climate system. Geoscientists based in Australia and the UK have used geochemical data to reconstruct the stores of carbon in oceanic plates and thermodynamic modelling to track what may have happened to it and the climate through the last 250 Ma (Müller, R.D. et al. 2022. Evolution of Earth’s tectonic carbon conveyor belt. Nature, v. 605, p. 629-639; DOI: 10.1038/s41586-022-04420-x). Their review is an important step in understanding what underpins climate on a geological time scale, onto which much shorter-term surface influences are superimposed.

The amount of carbon being outgassed as CO2 each year along plate boundaries in the early Jurassic (185 Ma) shown in dark purple (low) to yellow (high). Also shown in shades of blue is the accumulation of carbon stored in each square metre of the ocean plates. Plate motions are shown as grey arrows (credit: Müller, R.D. et al. Clip from video in Supplementary Information)

At mid-ocean ridges basaltic magma wells up from mantle depths and loses much of its content of dissolved CO2. The annual outgassing at ridges, which depends on the global rate of plate formation, has varied from 13 to 30 million tonnes of carbon  (MtC yr-1) since the start of the Mesozoic Era 250 Ma ago. Similarly, there is greenhouse-gas escape from volcanic arcs above subduction zones, estimated to have ranged from 0 to 18 MtC yr-1. As an oceanic plate moves away from its source various processes sequester CO2 into the oceanic crust and upper mantle through accumulation of deep-sea sediments and hydrothermal alteration of basaltic crust and peridotite mantle (ranging from 30 to 311 MtC yr-1). Of this influx of carbon into oceanic plates between 36 to 103 MtC yr-1 has gone down subduction zones in descending slabs. Between 0 to 49 MtC yr-1 of that has been outgassed by arc volcanic activity or absorbed into the overriding plate. The rest continues down into the deep mantle, perhaps to form diamonds. Overall, when the rate at which oceanic plates grow is rapid and plate motion speeds up, outgassing should be high. When plate growth slows, so does the rate of CO2 release. Variations in plate growth can be estimated from the magnetic reversal stripes above the ocean floors.  The authors have released an animation of the break-up of Pangaea (well worth watching at full screen – you can skip the ad at the start), with the rate of carbon emission at ridges and volcanic arcs being colour-coded. Also shown is the storage of carbon within oceanic plats plates as time passes.

Length of mid-ocean ridges (orange) and subduction zones (blue) through the last 250 Ma (top). The areas of oceanic crust produced at ridges and consumed by subduction (bottom) (credit: Müller, R.D. et al., Figs 1a, 1c)

Before Pangaea began to break up at the end of the Triassic (200 Ma) the total length of mid-ocean ridges was at a minimum of about 40 thousand km. Through the Jurassic it never exceeded 50,000 km, but rose to a maximum of 80,000 km during the Cretaceous then declined slowly to the current length of 60,000 km. Throughout the last 250 Ma the length of subduction zones stayed roughly the same at about 65 thousand km – not always in the same places – although the overall rate of subduction changed in line with the rate of oceanic plate growth  (the volume that is added must be balanced roughly by the amount that returns to the mantle).  Between the end of the Jurassic and the mid-Cretaceous crustal production and destruction doubled, shown by the bottom plot in the figure above. The very fast  movement of plates and an increase in the global length of ridges during Jurassic to mid-Cretaceous times led to a dramatic increase in CO2 outgassing from ridges so that its content in the atmosphere rose as high as 1200 ppm – more than four times that before the Industrial Revolution. That level resulted in global ‘hothouse’ conditions during the Cretaceous. Another factor behind the Cretaceous climate was a decrease in the global complement of mountains. That led to decreases in erosion and the weathering of silicates by acid rain, thus reducing natural sequestration of carbon.

During the Cenozoic (after 65 Ma) declining ridge outgassing was actually outpaced by that associated with subduction, according to the modelling. That is strange, for by around 35 Ma glaciation had begun  on Antarctica as the Earth was cooling, which implies a major, unexpected sink for excess CO2. The most likely way this might have arisen is through increased erosion and silicate weathering on the exposed continents that consumed CO2 faster than tectonics was releasing the gas. The length of continental arcs shows no sign of a major increase during the Cenozoic, which might have accelerated that kind of sequestration, but a variety of proxies for signs of weathering definitely suggests that there was an upsurge. Also there was increased storage of carbon on the deep ocean floor, shown by the video. Increased calcium released by weathering to enter ocean water in solution would allow more planktonic organisms to secrete calcite (CaCO3) skeletons that would then fall to the ocean floor when they died.

There may be more to be discovered in this hugely complex interplay between tectonics and climate. For instance, when the bottom waters of the oceans are oxygenated by deep currents of cold dense seawater sinking from polar regions, carbon in tissues of sunken dead organism is oxidised to release CO2. If bottom waters are anoxic, this organic carbon is preserved in sediments. The authors mention this as something to be considered in their future work on  the ‘tectonic carbon conveyor belt’.

Diamonds and the deep carbon cycle

When considering the fate of the element carbon and CO2, together with all their climatic connotations, it is easy to forget that they may end up back in the Earth’s mantle from which they once escaped to the surface. In fact all geochemical cycles involve rock, so that elements may find their way into the deep Earth through subduction, and they could eventually come out again: the ‘logic’ of plate tectonics. Teasing out the various routes by which carbon might get to mantle is not so easily achieved. Yet one of the ways it escapes is through the strange magma that once produced kimberlite intrusions, in the form of pure-carbon crystals of diamond that kimberlites contain. A variety of petrological and geochemical techniques, some hinging on other minerals that occur as inclusions, has allowed mineralogists to figure out that diamonds may form at depths greater than about 150 km. Most diamonds of gem quality formed in unusually thick lithosphere beneath the stable, and relatively cool blocks of ancient continental crust known as cratons, which extends to about 250 km. But there are a few that reflect formation depths as great as 800 km that span two major discontinuities in the mantle (at 410 and 660 km depth). These transition zones are marked by sudden changes in seismic speed due to pressure-induced transformations in the structure and density of the main mantle mineral, olivine.

Diamond crystal containing a garnet and other inclusions (Credit: Stephen Richardson, University of Cape Town, South Africa)

Carbon-rich rocks that may be subducted are not restricted to limestones and carbon-rich mudstones. Far greater in mass are the basalts of oceanic crust. Not especially rich in carbon when they crystallised as igneous rocks, their progress away from oceanic spreading centres exposes them to infiltration by ocean water. Once heated, aqueous fluids cause basalts to be hydrothermally altered. Anhydrous feldspars, pyroxenes and olivines react with the fluids to break down to hydrated-silicate clays and dissolved metals. Dissolved carbon dioxide combines with released calcium and magnesium to form pervasive carbonate minerals, often occupying networks of veins. So there has been considerable dispute as to whether subducted sediments or igneous rocks of the oceanic crust are the main source of diamonds. Diamonds with gem potential form only a small proportion of recovered diamonds. Most are only saleable for industrial uses as the ultimate natural abrasive and so are cheaply available for research. This now centres on the isotopic chemistry of carbon and nitrogen in the diamonds themselves and the various depth-indicating silicate minerals that occur in them as minute inclusions, most useful being various types of garnet.

The depletion of diamonds in ‘heavy’ 13C once seemed to match that of carbonaceous shales and the carbonates in fossil shells, but recent data from carbonates in oceanic basalts reveals similar carbon, giving three possibilities. Yet, when their nitrogen-isotope characteristics are taken into account, even diamonds that formed at lithospheric depths do not support a sedimentary source (Regier, M.E. et al. 2020. The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds. Nature, v. 585, p. 234–238; DOI: 10.1038/s41586-020-2676-z). That leaves secondary carbonates in subducted oceanic basalts as the most likely option, the nitrogen isotopes more reminiscent of clays formed from igneous minerals by hydrothermal processes than those created by weathering and sedimentary deposition. However, diamonds with the deepest origins – below the 660 km mantle transition zone – suggest yet another possibility, from the oxygen isotopes of their inclusions combined with those of C and N in the diamonds. All three have tightly constrained values that most resemble those from pristine mantle that has had no interaction with crustal rocks. At such depths, unaltered mantle probably contains carbon in the form of metal alloys and carbides. Regier and colleagues suggest that subducted slabs reaching this environment – the lower mantle – may release watery fluids that mobilise carbon from such alloys to form diamonds. So, I suppose, such ultra-deep diamonds may be formed from the original stellar stuff that accreted to form the Earth and never since saw the ‘light of day’.

Soluble iron, black smokers and climate

 

Phytoplankton bloom in the Channel off SW England (Landsat image)

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant food chain. The key factor that is missing is dissolved divalent iron that acts as a minor, but vital, nutrient for phytoplankton. Much of the soluble iron that does help stimulate plankton ‘blooms’ emanates from the land surface in wind blown dust (Palaeoclimatology September 2011) or dissolved in river water. A large potential source is from hydrothermal vents on the ocean floor, which emit seawater that has circulated through the basalts of the oceanic crust. Such fluids hydrate the iron-rich mafic minerals olivine and pyroxene, which makes iron available for transport. The fluids originate from water held in the muddy, organic-rich sediments that coat the ocean floor, and have lost any oxygen present in ocean-bottom water. Their chemistry is highly reducing and thereby retains soluble iron liberated by crustal alteration to emanate from hydrothermal vents. Because cold ocean-bottom waters are oxygenated by virtue of having sunk from the surface as part of thermohaline circulation, it does seem that ferrous iron should quickly be oxidised and precipitated as trivalent ferric compounds soon after hydrothermal fluids emerge. However, if some was able to rise to the surface it could fertilise shallow ocean water and participate in phytoplankton blooms, the sinking of dead organic matter then effectively burying carbon beneath the ocean floor; a ‘biological pump’ in the carbon cycle with a direct influence on climate. Until recently this hypothesis had little observational support. Continue reading “Soluble iron, black smokers and climate”

Carbon dioxide burial: an analogy of some pitfalls

Schematic showing both terrestrial and geologi...
geological sequestration of carbon dioxide emissions from a coal-fired power plant.  (Photo credit: LeJean Hardin and Jamie Payne Wikipedia)

Of all the ‘geoengineering’ approaches that may offer some relief from global warming pumping CO2 into deep sedimentary rocks, through carbon capture and storage (CCS) is one that most directly intervenes in the natural carbon cycle. In fact it adds an almost wholly  anthropogenic route to the movement of carbon. It is difficult if not impossible for natural processes to ‘pump’ gases downwards except when they are dissolved in water and most often through the conversion of CO2 to solid carbonates or carbohydrates that are simply buried on the ocean floor. Artificially producing carbonate or organic matter on a sufficient scale to send meaningful amounts of anthropogenic carbon dioxide to long-term rock storage is pretty much beyond current technology, but gas sequestration seems feasible, if costly. The main issues concern making sure geological traps are ‘tight’ enough to prevent sufficient leakage to render the exercise of little use and to understand the geochemical effects of large amounts of buried gas that would inevitably move around to some extent.

The geochemistry is interesting as reactions of CO2 with rock and subsurface water are inevitable. The most obvious is that solution in water releases hydrogen ions to create weakly acidic fluids: on the one hand that might be a route for precipitation of carbonate and more secure carbon storage, through reaction with minerals (see http://earth-pages.co.uk/2012/04/10/possible-snags-and-boons-for-co2-disposal/), but another possibility is increasing solution of minerals that might eventually cause a trap to leak. A counterpart of pH change is the release of electrons, whose acceptance in chemical reactions creates reducing conditions. The most common minerals to be affected by reducing reactions are the iron oxides, hydroxides and sulfates that often coat sand-sized grains in sedimentary rocks, or occur as accessory minerals in igneous and metamorphic rocks. Iron in such minerals is in the Fe-3 valence state (ferric iron from which an electron has been lost through oxidation) which makes them among the least soluble common materials, provided conditions remain oxidising. Flooding sedimentary rocks with CO2 inevitably produces a commensurate flow of electrons that readily interact with Fe-3. The oxidised product Fe-2 (ferrousiron) is soluble in water, and so reduction breaks down iron-rich grain coatings. Much the same happens with less abundant manganese oxides and hydroxides. One important concern is that iron hydroxide (FeO.OH or goethite) has a molecular structure so open that it becomes a kind of geochemical sponge. Goethite may lock up a large range of otherwise soluble ions, including those of arsenic and some toxic metals. Should goethite be dissolved by reduction that toxic load moves into solution and can migrate.

Bleached zone with carbonate-oxide core in Jurassic Entrada Sandstone, Green River, Utah. (Image: Max Wigley, University of Cambridge)

Except where deep, carbonated groundwater leaks to the surface in springs – the famous Perrier brand of mineral water is an example – it is difficult to judge what is happing to gases and fluids at depth. But their long-past activity can leave signatures in sedimentary rocks exhumed to the surface. Most continental sandstones, formed either through river or wind action, are strongly coloured by iron minerals simply because of strongly oxidising conditions at the Earth’s surface for the past two billion years or more. Should reducing fluids move through the, the iron is dissolved and leached away to leave streaks and patches of bleached sandstone in otherwise red rocks. In a few cases an altogether more pervasive bleaching of hundreds of metres of rock marks the site of massive fluid-leakage zones. Terrestrial Mesozoic sedimentary sequences in the Green River area of Utah, USA exhibit spectacular examples, easily amenable to field and lab study (Wigley, M. et al. 2012. Fluid-mineral reactions and trace metal mobilization in an exhumed natural CO2 reservoir, Green River, Utah. Geology, v. 40, p. 555-558). There the bleaching rises up through the otherwise brown and yellow sandstones, cutting across the bedding. In the bleached zone, secondary calcite fills pore spaces. At the contact with unbleached sandstone there are layers of carbonate and metal oxides, enriched in cobalt, copper, zinc, nickel, lead, tin, molybdenum and chromium: not ores but clear signs confirming the general model of reductive dissolution of iron minerals and movement of metal-rich fluid. Carbon isotopes from the junction are richer in 13C than could be explained by the gas phase having been methane, and confirm naturally CO2 – rich fluids.

So, Green River provides a natural analogue for a carbon capture and storage system, albeit one that leaked so profusely it would be a latter day disaster zone. In that sense the site will help in deciding where not to construct CCS facilities.

Massive event in the Precambrian carbon cycle

English: Cyanobacteria
Cyanobacteria: earliest producers of oxygen in the Precambrian. Image via Wikipedia

The entire eukaryote domain of life, from alga to trees and fungi to animals, would not exist had it not been for the emergence of free oxygen in the oceans and atmosphere about 2.4 billion years ago; thanks in large part to the very much simpler photosynthetic blue-green bacteria. The chemistry behind this boils down to organisms being able to transfer electrons from elements and compounds in the inorganic world to build organic molecules incorporated in living things. Having lost electrons the inorganic donors become oxidised, for instance ferrous iron (Fe2+ or Fe-2) becomes ferric iron (Fe3+ or Fe-3) and  sulfide ions (S2-) become sulfate (SO42-) and the organic products that receive electrons principally involve reduction of carbon, on the OilRig principal – Oxidation involves loss of electrons, Reduction involves gain. Since the Great Oxygenation Event (GOE), ferric iron and sulfate ions now account for 75% of oxidation of the lithosphere and hydrosphere while free oxygen (O2) is a mere 2-3 % (Hayes, J.M. 2011. Earth’s redox history. Science. V. 334, p. 1654-1655; an excellent introduction to the geochemistry involved in the GOE and the carbon cycle). Free oxygen is around today only because more of it is produced than is consumed by its acting to oxidize ferrous iron and sulfide ions supplied mainly by volcanism, and carbon-rich material exposed to surface processes by erosion and sediment transport.

Eukaryote life has never been snuffed out for the last two billion years or so, but it has certainly had its ups and downs. To geochemists taking the long view oxygen might well seem to have steadily risen, but that is hardly likely in the hugely varied chemical factory that constitutes Earth’s surface environments, involving major geochemical cycles for carbon, iron, sulfur, nitrogen, phosphorus and so on, that all inveigle oxygen into reactions. Tabs can be kept on one of these cycles – that involving carbon – through the way in which the proportions of its stable isotopes vary in natural systems. If all geochemistry was in balance all the time, all materials that contain carbon would show the same proportions of 13C and 12C as the whole  Earth, but that is never the case. Living processes that fix carbon in organic compounds favour the lighter isotope, so they show a deficit of 13C relative to 12C signified by negative values of δ13C. The source of the carbon, for instance CO2 dissolved in sea water, thereby becomes enriched in 13C to achieve a positive value of δ13C, which may then be preserved in the form of carbonates in, for instance, fossil shells that ended up in limestones formed at the same time as organic processes were favouring the lighter isotope of carbon. Any organic carbon compounds that ocean-floor mud buried before they decayed (became oxidised) conversely would add their negative δ13C to the sediment. Searching for δ13C anomalies in limestones and carbonaceous mudrocks has become a major means of charting life’s ups and downs, and also what has happened to buried organic carbon through geological time.

A most interesting time to examine C-isotopes and the carbon cycle is undoubtedly the period immediately following the GOE, in the Palaeoproterozoic Era (2500 to 1600 Ma). From around 2200 to 2060 Ma the general picture is roughly constant, high positive values of δ13C (~+10‰): more organic carbon was being buried than was being oxidised to CO2. However, in drill cores through the Palaeoproterozoic of NW Russia carbonate carbon undergoes a sharp decline in its heavy isotope to give a negative δ13C  (~-14‰) while carbon in organic-rich sediments falls too (to~-40‰): definitely against the general  trend (Kump, L.R. et al. 2011. Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event. Science. V. 334, p. 1694-1696). Oxygen isotopes in the carbonates affected by the depletion in ‘heavy’ carbon show barely a flicker of change: a clear sign that the 13C δ13C deficit is not due to later alteration by hydrothermal fluids, as can sometimes cause deviant δ13C in limestones. It is more likely that a vast amount of organic carbon, buried in sediments or dissolved in seawater was oxidised to CO2 faster than biological activity was supplying dead material to be buried or dissolved. In turn, the overproduction of carbon dioxide dissolved in seawater to affect C-isotopes in limestones. Such an event would have entailed a sharp increase in oxygen production to levels capable of causing the oxidation (~ 1% of present levels). Yet this was not the time of the GOE (2400 Ma) but 300-400 Ma later. A possible explanation is a burst in oxygen production by more photosynthetic activity, perhaps by the evolution of chloroplast-bearing eukaryotes much larger than cyanobacteria.

The ultra-deep carbon cycle

A scattering of "brilliant" cut diam...
Image via Wikipedia

The presence of diamonds in the strange, potassium-rich, mafic to ultramafic igneous rocks known as kimberlites clearly demonstrates that there is carbon in the mantle, but it could have come from either biogenic carbon having moved down subduction zones or the original meteoritic matter that accreted to form the Earth. Both are distinct possibilities for which evidence can only be found within diamonds themselves as inclusions. There is a steady flow of publications focussed on diamond inclusions subsidised to some extent by companies that mine them (see Plate tectonics monitored by diamonds in EPN, 2 August 2011). The latest centres on the original source rocks of kimberlites and the depths that they reached (Walter, M.J. and 8 others 2011. Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions. Science, v. 334, p. 54-57). The British, Brazilian and US team analysed inclusions in diamonds from Brazil, finding assemblages that are consistent with original minerals having formed below the 660 km upper- to lower-mantle seismic boundary and then adjusting to decreasing pressure as the kimberlite’s precursor rose to melt at shallower levels. The minerals – various forms of perovskite stable at deep-mantle pressures – from which the intricate composites of several lower-pressure phases exsolved suggest the diamonds originated around 1000 km below the surface; far deeper than did more common diamonds. Moreover, their geochemistry suggests that the inclusions formed from deeply subducted basalts of former oceanic crust.

Previous work on the carbon isotopes in ‘super-deep’ diamonds seemed to rule out a biogenic origin for the carbon, suggesting that surface carbon does not survive subduction into the lower mantle. In this case, however, the diamonds are made of carbon strongly enriched in light 12C relative to 13C, with δ13C values of around -20 ‰ (per thousand), which is far lower than that found in mantle peridotite and may have been subducted organic carbon. If that proves to be the case it extends the global carbon cycle far deeper than had been imagined, even by the most enthusiastic supporters of the Gaia hypothesis.