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Project Progress

Johnston et al. (2009) illustrates how contributions to primary production by anoxygenic photoautotrophs influenced biogeochemical cycling during the Proterozoic. We suggest that the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained midwater euxinia (H₂S). On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated H₂S availability and water column oxygenation. This limited the accumulation of O₂ in the atmosphere and ocean. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced the Proterozoic O₂ budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe²⁺ rich) subsurface waters set in motion Earth’s transition from its prokaryotedominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide). This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms.

A modern analogue of this ferruginous transition may exist today beneath Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet (Mikucki et al., 2009). In this ferruginous brine that is a remnant of an ancient marine fjord, a microbial consortium facilitates a catalytic sulfur cycle. These metabolic pathways result from a limited organic carbon supply and the absence of contemporary photosynthesis. Coupled biogeochemical processes below the glacier enable subglacial microbes to grow in extended isolation, demonstrating how analogous organicstarved systems, such as Neoproterozoic oceans, accumulated Fe(II) despite the presence of an active sulfur cycle.

Related to such transitions is the possibility that the late Proterozoic oceans were rich in dissolved organic carbon (DOC). To understand how such a DOC buildup could be possible, Rothman’s group is studying the dependence of the DOC reservoir on respiration rates. In their recent (unpublished) studies of well controlled systems, respiration rates are found to be widely distributed in a predictable way, and the size of the DOC reservoir has been related to this distribution. Theoretical predictions suggest that small changes in the slowest rates, as one might expect during transitions between anoxic and oxic environments, lead to large changes in the size of the
DOC reservoir along with concomitant large fluctuations in the isotopic record.

In related work, Rothman’s group has also provided theoretical contraints on the duration and mechanism of the Shuram-Wonoka-Doushantuo carbonisotopic excursion. This isotopic event may be a signature of the oxygenation of the atmosphere at the end of the Proterozoic.