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Project Progress

The history of oxygen in the oceans and atmospheres is thought to have played a key role in Earth’s long term biological evolution. Ongoing research addresses the initial oxygenation of the atmosphere and surface ocean 2.4-2.2 Ga, renewed oxygen influx near the end of the Proterozoic eon, and, increasingly, the nature of the biosphere between those two events.

As described in the Year 3 report and resulting publications, Ariel Anbar has observed systematic differences between the Mo isotopic compositions of sediments accumulated under oxic and sulfidic conditions. These variations raise the possibility that the Mo isotope system may be useful for paleoredox studies, particularly in the Precambrian, where long, sustained variations in ocean redox have been postulated. Preliminary data led him to predict that Mo isotopes are fractionated during uptake by Mn-oxides. Laboratory experiments conducted in fall, 2001, confirm this hypothesis. Anbar’s lab has also extended its measurements of sulfidic sediments beyond the Black Sea, characterizing Mo isotopes in Cariaco Basin sediments. Mo isotopes here are very similar to those of the Black Sea. Finally, as Year 4 drew to a close, Anbar made preliminary measurement s of Mo isotopes in mid-Proterozoic black shales — shales deposited during a time of putative widespread ocean anoxia. The isotopic measurements in this sediment (Wollogorang Fm) are strikingly different from those in the Black Sea and Cariaco, consistent with the notion of extensive ocean anoxia at this time.

Based on the sulfur isotopic record, Canfield has proposed that the deep sea in the middle Proterozoic (1850 – 1250 Myr) was sulfidic, rather than oxic as is commonly assumed. This period of time is also characterized by unusual stability in the carbonate carbon isotope record, by unexplained delay in the diversification of eukaryotes, and by pronounced onshore-offshore decrease in diversity of fossils of eukaryotic origin. Anbar and Knoll propose that these observations are related by the extreme redox sensitivity of Fe and Mo, both metals critical to the biological N cycle. A manuscript presenting this concept was initially completed and submitted to Science in Year 3; it was revised (and lengthened into an article) and accepted for publication in Science during the current funding year.

National Research Council (NRC) postdoctoral fellow Yanan Shen has shown that S isotopic abundances in early diagenetic pyrite show strong facies dependence in the ca. 1500 Ma Roper Group, northern Australia. Fe chemistry enabled Shen to recognize a sharp redoxcline within the basin. Only in basinal sedimentary rocks deposited below the redoxcline do we see evidence of the large isotopic fractionation characteristic of dissimilatory sulfate reduction when sulfate is not limiting. Coastal sediments display markedly positive S-isotopic values, likely associated with Raleigh distillation in sulfate-depleted pore waters. Most inner and distal shelf shales show little evidence of S-isotopic fractionation, consistent with other evidence for low sulfate concentrations in mid-Proterozoic marine basins.

H.D. Holland and colleagues continued investigations of seawater composition through time. Analyses of fluid inclusions in salt crystals suggest that the chemical composition of seawater varied considerably during the last 600 Ma in response to tectonics and biological evolution. Geochemical analyses of late Archean and Paleoproterozoic sedimentary rocks suggest that the Great Oxidation Event began between 2.45±0.05 and 2.30±0.05 Ga and was largely completed by 2.0±0.05 Ga. Holland and colleagues postulate that a minor increase in the oxygen fugacity of volcanic gases may have triggered this event.