Clays and the rise of an oxygenated atmosphere

Almost all eukaryote organisms require oxygen to be available in their environment. Therefore the eukaryote cell probably appeared only after oxygen had become a permanent component of the atmosphere and hydrosphere, which itself depended on photosynthetic metabolism outweighing the scavenging of free oxygen by abundant dissolved iron. It also depends on efficient burial of dead organic matter. For the metazoa – multicellular eukaryote animals – the oxygen demand rises with their bulk. The first tangible fossils of metazoans appear in Ediacaran, after the last global glacial episode of the late-Precambrian, around 600 Ma ago.  Apart from the evidence for an oxygen bearing atmosphere after about 2.4 Ga, not much is known about actual levels of oxygen and their changes during the Precambrian. The sudden emergence of the soft-bodied but bulky Ediacaran faunas has been ascribed by many to an equally abrupt rise in the availability of oxygen, on which their evolution must have depended. How that might have occurred has been disputed and pretty vague.

The central requirements to boost oxygen levels are increased photosynthesis – difficult if the period preceding the Ediacaran was one where large tracts of ocean were covered with ice – or increased burial of dead organic matter. The second option is also difficult to imagine if ‘snowball’ conditions had reduced living marine biomass to a very low level. What geoscientists have not been able to grasp, is information on the efficiency with which dead organic matter was buried. Mineralogists and geochemists from the Universities of California (Riverside) and Maine have addressed that aspect from the standpoint of the Precambrian history of clay mineral deposition (Kennedy, M. et al. 2006. Late Precambrian oxygenation; inception of the clay mineral factory. Science, v. 311, p. 1446-1449). If organic matter is buried in porous and permeable  sea-floor sediments, the chances of its metabolism by bacterial action is high. Research on modern sea floor sediments shows that the bulk of organic debris at continental margins is adsorbed onto clay-mineral particles, thereby increasing its chance of preservation over simple incorporation as particles in silt-sized sediment. Kennedy et al. tested the hypothesis that sedimentation in the late-Precambrian changed from dominance by physically weathered micas and other silicates to one more dominated by products of chemical weathering on the continental surface, i.e. clays.

Around 700 Ma, the record of marine strontium isotopes in limestones began a major change towards higher 87Sr/86Sr ratios, suggesting an increase in the chemical weathering of ancient continental rocks. Australia provides a continuous sequence, from 850 to 530 Ma, of quietly deposited shelf sediments that span this transition and also contain the Ediacaran. Sure enough, the mudstones in the sequence show a distinct increase in swelling clays and kaolinite, implicated in modern preservation of dead organic matter. Rather than an abrupt step, the increase is linear from about 800 Ma, and is matched by similar data from other Precambrian cratons. What might have started this chemical weathering of the land surface? Possibly it was due to a much earlier colonisation of the land than direct evidence suggests.  The DNA-based phylogeny of mosses, fungi, lichens and liverworts – all terrestrial organisms – suggests that they arose between 700 and 600 Ma ago.  All would have contributed organic acids to the process of chemical weathering.  Kennedy et al. model the rate at which free oxygen would have increased as a result of increased deposition of clays, and conclude that between 730 and 500 Ma retention of oxygen in the environment would have increased six-fold. Thereafter, land-based organisms and further colonisation permanently increased weathering, establishing increasingly efficient marine burial of organic debris, and so creating an environment in which metazoans could evolve and radiate. If confirmed by further analyses, this work establishes yet another non-uniformitarian process in the Earth system.

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