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 by Harvard team members addresses the initial oxygenation of the atmosphere and surface ocean 2400-2200 million years ago (Ma), renewed oxygen influx near the end of the Proterozoic Eon, and life and environments between those two events.
In 1998, Donald Canfield proposed that the cessation of iron formation deposition ca. 1850 Ma reflected the expansion of sulfidic deep oceans and not, as traditionally understood, the spread of oxygen throughout ocean basins. Research by Harvard team members lends support to this hypothesis, demonstrating the presence of a strong redoxcline within the a ca.1500 Ma Roper group, Australia, and documenting an environment-specific pattern of sulfur isotopic fractionation that documents low sulfate levels in Roper seawater. Roper and older sedimentary rocks in northern Australia suggest sulfate (and, hence, probably oxygen) limitation through 250 million years of mid-Proterozoic history. One wishes, however, for geochemical markers that might provide globally integrated records of Proterozoic redox conditions. As shown by team member A. Anbar, Mo isotopes may serve this purpose. Anbar has observed systematic differences between the Mo isotopic compositions of sediments accumulated under oxic and sulfidic conditions. 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 the Black Sea. Finally, as Year 4 drew to a close, Anbar made preliminary measurements of Mo isotopes in mid-Proterozoic black shales that are consistent with the hypothesis of extensive ocean anoxia at this time. In a paper slated for publication in Science, Anbar and A. Knoll explored the implications of sulfidic deep waters for the distribution of biologically important trace metals in Proterozoicc oceans.
What factors led to the formation of sulfidic Proterozoic oceans remains uncertain, as do the processes that facilitated renewed oxygen increase toward the end of the eon. In modeling the initial rise of atmospheric oxygen levels 2400-2200 Ma, H.D. Holland has proposed that a minor increase in the oxygen fugacity of volcanic gases may have triggered the Paleoproterozoic rise of atmospheric oxygen.