Probably the greatest ecological truism is that without oxygen there would be no life forms on Earth above the level of a restricted number of prokaryotes. Since around 2.4 Ga, when free atmospheric oxygen first appeared, levels have risen to the present 21% – it was probably as high as ~30% in the Carboniferous and Cretaceous Periods. Charting the rise has been difficult and the history of oxygen is written with a very broad brush. If there had been sudden increases in the availability of oxygen in the atmosphere and oceans there ought to have been a bursts of evolutionary radiation and diversity, but often oxygen-related causality for events such as the Cambrian Explosion have been speculative, as have cases for the inverse, declines due to downturns in oxygen levels (see Oxygen depletion before P-T extinction in the November 2003 issue of EPN). Recently a proxy for the redox chemistry of the global ocean, and therefore for relative changes in atmospheric oxygen, has been developed. It is based on the abundance and isotopic composition of the element molybdenum (Mo) in sedimentary rocks: higher 98Mo relative to 95Mo (the d98Mo value) signifies higher oxygen levels. Its recent use in relation to evolutionary radiations (Dahl, T.W. et al. 2010. Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish. Proceedings of the National Academy of the US, v. 107, p. 17911-17915) has produced interesting results. The US-Swedish-Danish-British team analysed the Mo in euxinic (reduced) marine black shales, which concentrate the element from seawater, in the Proterozoic and Phanerozoic Eons. Increases in δ98Mo occur at the time of the Cambrian Explosion, as expected, and also during the Devonian. The latter correlates with increasing diversification of large fishes and among early terrestrial plants, and may have been the greatest leap in the bioavailability of oxygen in Earth’s history, stemming from the ‘greening’ of the land. So far Mo-isotope data have not been obtained from Carboniferous, Permian or Cretaceous back shales, but the ratio of Mo to organic carbon content in black shales of those ages – a less constrained proxy – does confirm what has been suspected: highs (greater than present levels) in the Carboniferous and Cretaceous and lows during the Permian and Triassic. However, any hopes that the approach can be calibrated to actual oxygen levels seem likely to be optimistic as the controls over dissolved molybdenum supply to the oceans and its transfer to sediments are extremely complex.
Added 14 January 2011. Some of the team feature in a related article (Gill, B.C. et al. 2011. Geochemical evidence for widespread euxinia in the Later Cambrian ocean. Nature, v. 469, p. 80-83) that ticks all the geochemical boxes for the evolutionary effects of depleted oxygen; i.e. extinctions. They use new measurements of sulfur isotopes in conjunction with published carbon-isotope and other geochemical data from a wide range of Late Cambrian sediment types and environments in six well-known sections of that age. Spikes in the relative abundance of 34S match those in 13C along with a decrease in Mo in one section (see above), suggesting temporary increases in carbon and sulfide burial during periods of oxygen deficiency in the Late Cambrian ocean. Massive sequestration of organic carbon may have led to the extremely cold Late Cambrian climate, as described in A chilly Late Cambrian (this issue). Combined with changes in redox conditions associated with ocean anoxia this would have especially stressed animals, even on continental shelves had oxygen depleted water risen from the depths where sulfur and carbon burial were going on.
See also: Shields-Zhou, G. 2011. Toxic Cambrian oceans. Nature, v. 469, p. 42-43.
- Oxygen crash led to Cambrian mass extinction (newscientist.com)