Abstract:
The secular variations of the δ18O of silica from Precambrian cherts have been interpreted as a progressive change in the temperature of the oceans. However a possible drift of the seawater δ18O may also account for these variations. We have isolated the kerogen from these cherts are measured their δ18O. As shown by analyses on modern environments, the δ18O of the organic remnants reflect the isotopic composition of the seawater. The Precambrian kerogen show a constant δ18O values (within ±4‰) indicating that the seawater δ18O has remained constant through the Precambrian, hence validation the thermometric interpretation.
In the Archean cherts, the δ15N values of kerogen encompasses a large 15N isotopic heterogeneity at the scale of individual microfossils (-21.6‰ < δ15N_µm < +30.6‰) and are related to the morphology of these fossils. To account for such a large isotopic heterogeneity, we propose that uptake of ammonium by assimilation and aerobic oxidation along a redox gradient is a likely mechanism. This implies that free molecular O2 was produced by oxygenic photosynthesizers and consumed by aerobic ammonia oxidizing microorganisms. Hence, this Mid-Archean ecosystem reveals an intermediate oxygenation step in the redox evolution of the geobiological N cycle.