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.

Homes for hominin evolution

African savannah exhibit at the National Zoolo...
Typical African savannah. Image via Wikipedia

Friedrich Engels’s notion in The Part Played by Labour in the Transition from Ape to Man (1876), encouraged by Darwin’s The Descent of Man (1871), that the road to modern humans began with walking on two legs, thereby freeing the hands for work and tool making has been central to discussion of human evolution for more than a century. The ‘descent from the trees’ that bipedalism signifies has long been supposed to stem from the replacement of tropical forests in East Africa by open woodland or savannah, but evidence to support that environmental change has been difficult to glean from the fossil record  since the Late Miocene. Even in terrestrial sediments plant remains are rare, so that much has rested on animal fossils in relation to the habitats of their living descendants: opinion is divided.

There is a round-about means of resolving this central issue: using the carbon-isotope record in fossil soils that depends on the fractionation effects of broadly different kinds of plants that once grew in the soils and the signature of that fractionation in carbonate nodules that formed in the soils. The d13C value (crudely the difference between the 13C/12C ratio of a sample and that of a carbon-rich standard) found in C4 plants (many grasses) is -16 to -10 ‰ whereas that in C3 plants (including almost all trees) it is much more depleted in the heavier 13C isotope (-33 to -24‰). Exchange of carbon between living and dead organic matter, and carbonates that are precipitated from soil waters through the intermediary of gases in the soil should leave a d13C signature in the carbonates that reflects the overall proportions of different photosynthetic plant groups living at the time the soil formed. The approach was developed in the early 1990s by Thure Cerling and Jay Quade of the US universities of Utah and Arizona respectively.

After a long gestation period, involving calibration using soils from different modern ecosystems, the soil C-isotope method has been applied painstakingly to palaeosols in which African hominin remains have turned-up (Cerling, T.E. and 9 others 2011. Woody cover and hominin environments in the past 6 million years. Nature, v. 476, p. 51-56). All the famous hominin sites from the Awash and Omo Valleys of Ethiopia and around Lake Turkana in Kenya, figure in this important study, in which the authors devise a proxy for ‘palaeo-shade’ based on their carbonate d13C data from 76 modern tropical soils: a good ‘straight-line’ plot of d13C against the fraction of woody cover at the different calibration sites. Applying the proxy to their 1300 samples of palaeosols they show convincingly that since about 6 Ma tree cover rarely rose above 40% in the homelands of all the East African hominins. From the times of Ardepithecus ramidus (~4 Ma) at Aramis in Ethiopia, through those of ‘Selam’ and ‘Lucy’, the 2.5 Ma first stone tools at Gona, the times when Africa was dominated by Homo erectus(1.8 to 1 Ma) to perhaps the first signs of modern human cranial remains (those with chins!) around 1 Ma, all hominins strode through open, grassy environments. One can imagine pleasured nods from the shades of Darwin and Engels now their prescience has finally been confirmed.