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Project Progress

Green Lake: Intensive field sampling campaigns in November 2012 and July 2013 sampled the water column at very high resolution and provided a broad suite of analyses that will allow us to assess the detailed stratification of this ecosystem, how robust the stratification is, and through ultimate experimentation using limnocorrals, its resilience. Our ultimate goal is to use the lake as a natural laboratory to explore the nature of microbial ecosystems living in extreme environmental gradients as analogs for life in the Proterozoic.

The redox structure of the lake is more complex than we’ve realized in the past, with an oxicline between 15-20m that seems to involve a coupling between nitrate reduction to ammonium and manganese oxidation (or perhaps ammonium oxidation to nitrate at the expense of MnO₂ reduction), and a conventional chemocline with an abrupt reduction in oxidation-reduction potential, a dense community of sulfur phototrophs, and the appearance of significant quantities of hydrogen sulfide. We also have evidence suggesting methane production in the sulfate-rich monimolimnion (lower lake), although our working hypothesis is that the additional methane is being injected laterally into the lake at depth through groundwater discharge.

Permian Extinction: We are running a suite of simulations of the Permian extinction event, using an Earth system model of intermediate complexity (Genie) that allows us to simulate the ca. 200 ky event continuously. We are using a novel approach, inverting the observed and chronologically well-resolved Meishan (China) carbon isotope record to drive the model. Presently we are investigating the conditions under which severe ocean acidification may have accompanied the event. Only those scenarios that require many 10’s of thousands of gigatons of C addition (the volcanic scenarios) lead to ocean acidification; other potential sources of ¹³C-depleted C invoked to explain the accompanying negative carbon isotope excursion do not require as much C addition, and do not lead to ocean acidification.

We have also performed an analysis of redox indicators through the extinction event from rocks from South China and Turkey. Our results are consistent with those of NAI geochemist Ariel Anbar and his students, showing that anoxia developed just prior to the extinction event, rather than millions of years prior to the event, as commonly presumed.

Great Oxidation Event: Rybacki and Kump, in collaboration with Victor Melezhik (Norwegian Geological Survey) have sampled the FAR-DEEP core at high resolution through a stack of lava flows that were erupted during the Lomagundi carbon isotope anomaly, the largest in Earth history. These lavas are unusual in that they are highly oxidized, and we have evidence that the oxidation occurred contemporaneously with eruption: 1) clasts from this unit are deposited in the overlying sedimentary unit that is otherwise not significantly oxidized; 2) preliminary investigation of the U-Th-Pb systematics of these rocks, in collaboration with the U. Wisc. Astrobiology group, indicates that the oxidation occurred ca. 2.3 Ga. Careful petrography and geochemical analysis is now being conducted on the core. Bachan is constructing a box model to evaluate the hypothesis that oxidative weathering generated a positive feedback, involving the production of sulfuric acid that further stimulated weathering and biological productivity in the ocean, that generated a rapid increase in atmospheric oxidation at the end of the Lomagundi event.