The global glaciations of the Neoproterozoic that reached low latitudes – the so-called ‘Snowball Earth’ events have dominated accounts of ancient glaciations since the start of the 21st century. Yet they are not the oldest examples of large-scale effects of continental ice sheets. Distinctive tillites or diamictites that contain large clasts of diverse, exotic rocks occur in sedimentary sequences of Archaean and Palaeoproterozoic age.
Diamictite from the Palaeoproterozoic Gowganda Formation in Ontario Canada (credit: Canadian Sedimentology Research Group)
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.
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