Following its first discovery, evidence for low-latitude glacial action at several times during the Neoproterozoic has fuelled one of the most publicised controversies in the geosciences. Was the Earth totally frozen over during these episodes, or was ice confined only to parts of the surface? Whatever, the last part of the Precambrian witnessed huge fluctuations of many kinds, and after the cold epochs the first large animals made a sudden appearance. The most dramatic geochemical ups and downs in Earth history took place, in the form of sudden extreme shifts in the relative proportions of the stable isotopes of carbon in seawater, as recorded by marine carbonate rocks. These fluctuations correlate closely with the evidence for low-latitude glaciations: large negative excursions of d13C with glacial epochs, and positive values developing between them. The first can be interpreted as the result of massive declines in photosynthetic fixation of organic carbon. The second suggests repeated recoveries of such biological productivity, which favours the extraction of 12C from seawater and an increase in the relative proportion of the heavier isotope as organic carbon becomes buried in seafloor sediments.
Since organic carbon is ultimately extracted photosynthetically from carbon dioxide in the atmosphere, a link between climate and living processes (and those that bury dead organisms) can be the basis for models attempting to explain the extraordinary events of Neoproterozoic times. If large amounts of organic carbon are buried or remain suspended in the oceans, the drawdown of atmospheric CO2 reduces the greenhouse effect and leads to cooling. Conceivably, the effect could be to so reduce global mean surface temperature that freezing conditions grip even the lowest latitudes. Once glacial and sea ice becomes established, its high reflectivity reduces the amount of incoming solar radiation that is absorbed to warm the Earth. The two processes combined would tend to lock frigid conditions in place until such time as gradual release of volcanic CO2 increased the atmospheric greenhouse effect. That is the theoretical essence of the Snowball Earth hypothesis in which complete ice cover sterilised surface biology for long periods. However, it leaves out two important factors: as water cools it is able to dissolve more gases from the atmosphere; organic carbon in ocean water can be transformed to dissolved CO2 if it is oxidised, thereby reducing the amount of carbon being buried. Modelling the carbon-climate link in the Neoproterozoic requires that both factors are accounted for (Peltier et al. 2007. Snowball Earth prevention by dissolved organic carbon remineralization. Nature, v. 450, p. 813-818).
The model devised by Richard Peltier and colleagues from the University of Toronto also incorporates the distribution of land at the time. Results from it show a looping behaviour, with recovery from frigidity as increases in dissolved oxygen convert organic carbon to dissolved carbon dioxide, whose increasing concentration in turn leads to more escape of the gas to the atmosphere. The model also suggests how glacial and sea ice might have developed during such a cycle, and with the late Precambrian configuration of drifting continents it allows for low-latitude continental glaciation, but not for all-enveloping sea ice. The implication is indeed glacial events vastly greater than those of the late Palaeozoic and during the present Ice Age, but less effect on marine photosynthesis than from Snowball conditions – a ‘Slushball’ Peltier et al. explain why the cyclical processes suggested by the model stopped before the start of the Phanerozoic, from carbon-isotope evidence for a massive oxidation of suspended marine organic carbon around 550 Ma. Thereafter, abundant oxygen and large animals ensured most dead organic carbon was oxidised in the oceans.
Unsurprisingly, one of the authors of the Snowball hypothesis finds flaws in the geochemical argument for its impossibility (Kaufman, A.J. 2007. Slush find. Nature, v. 450, p. 807-808). Not only was oxygen likely to have been at far lower atmospheric concentrations than it became in the Phanerozoic, the glacial epochs provide evidence that its concentration in seawater was very low. The marine diamictites associated with each contain both ironstones and iron-oxide cements. For them to have formed demands high concentrations of dissolved iron in sea water, in the form of reduced Fe2+ ions; incompatible with widespread oxidizing conditions that would favour Fe3+ whose compounds are insoluble.
Some good news about carbon burial
The second largest ‘sink’ for atmospheric CO2, after silicate weathering and formation of carbonate sediments, is the burial of organic carbon. Derived from photosynthesis of carbon dioxide in the air or dissolved in water, organic carbon descends from the photic zone of the oceans or is carried from the land by rivers. In the second case it is often believed that more than 70% of the carbon load of rivers is oxidised back to CO2 before having a chance of being buried in marine sediments. To estimate the proportion that does contribute to carbon sequestration is a complicated matter, involving measurement of the carbon budgeting for an entire river basin and its offshore sediments. This has been done by a team of French geochemists for the huge Ganges-Brahmaputra system that drains the northern Indian subcontinent and much of the Himalaya (Galy, V. et al. 2007. Efficient carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature, v. 450, p. 407-410). This system carries a stupendous load of sediment, especially during the monsoon season. At 1 to 2 billion t of sediment deposited from suspension in the Bay of Bengal each year, this is the largest single flux of sediment from land to the ocean floor. Even more is delivered as bed load (rolling and bouncing sand particles) to build up the Ganges-Brahmaputra delta of Bangladesh and West Bengal, India. The authors found that recently produced organic carbon is about 4 to 5 times more abundant in the suspended sediment load than is reworked fossil carbon derived by erosion of ancient sedimentary rocks, which itself is predominant in the bed load. Fossil carbon makes no difference to the modern carbon cycle, provided it does not get oxidised, which is less likely than for recent organic carbon in a form that can be metabolised.
By comparing the recent organic carbon load suspended in the rivers’ flow with that in the fine sediments of the Bengal Fan, Galy et al. have been able to show that most of that carbon is conserved without oxidation. As a result, the Bengal Fan accounts annually for about 15% of global carbon burial. There are two reasons for this remarkable efficiency: the low oxygen availability in deep waters of the Bay of Bengal; the very high sediment load from erosion of the Himalaya that buries carbon before oxidation is possible. Orogenic belts in humid areas are therefore key factors in exerting negative feedback on climate, whereas drainages of flat areas, such as the Amazon and especially its main tributary the Rio Negro, encourage oxidation in their lower reaches and offshore and are less important.