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Project Progress

In the last year, I have focussed on model development along two fronts: 1) the rise of oxygen and its relationship to mantle redox evolution (poster presented at the Astrobiology Science Conference) and 2) with my graduate student, Roberta Hotinski, the conditions favorable for the development of oceanic anoxia under an oxygenated atmosphere (Hotinski et al., in press; Hotinski et al., submitted). The mantle redox modeling will be incorporated into a manuscript that Jim Kasting and I are preparing for submission. I am proposing a new hypothesis, that the oxidation state of volcanic gases increased episodically through Earth history in response to mantle plume activity, with a major event near the Archean/Proterozoic boundary that is associated with widespread glaciation and a substantial increase in atmospheric pO2. During the summer of 1999, samples of glaciomarine rocks (diamictites) putatively associated with this event were collected in the Meteorite Bore section of Western Australia. These samples are now being prepared for carbon isotopic analysis (in collaboration with Jay Kaufman, a PSARC affiliate at the University of Maryland). We will attempt to relate this glacial event to those he has sampled in Canada and South Africa and to the anticipated disturbance of the carbon cycle associated with the plume activity. Finally, Andrew Kurtz (PSARC postdoc) and I have been collected samples of marine cherts of all ages so that we can attempt to reconstruct the temporal evolution of the Ge/Si ratio of the oceans. The Ge/Si ratio, like the Sr isotopic composition, reflects the interplay between hydrothermal weathering inputs to the ocean. The isotopic composition of the riverine endmember, however, reflects the intensity of chemical weathering on land. Part of our motivation, therefore, is to contrast Precambrian and Phanerozoic weathering processes, especially the relative effects of microbiota and vascular plants on the process. The ICP-MS instrument has now been modified by Andy to allow Ge determination, and Paul Knauth has graciously provided us well-characterized chert samples.