HR: 0800h
AN: B41A-0027    [Abstracts]
TI: Atmospheric oxygen levels, anaerobic methane oxidation, and the coupling of the global COS cycles by sulfate reduction
AU: * Wortmann, U G
EM: uli.wortmann@utoronto.ca
AF: University of Toronto, Department of Geology 22 Russellstr., Toronto, ON M5S 3B1, Canada
AU: Chernyavsky, B M
EM: bchern04@yahoo.com
AF: Institute for Integrated Energy Systems University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6, Canada
AB: Changes in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur are the dominant control on atmospheric oxygen levels, and the partitioning itself depends to a large degree on microbial redox processes remineralizing organic matter (OM). However, the controls of organic matter preservation in marine sediments are one of the most complex and controversial issues in contemporary biochemistry. Knowledge how the transition from one electron acceptor to another affects OM remineralization rates is scant even for the transition from aerobic to anaerobic respiration. Much less is known about the transition from anaerobic respiration to fermentation. Although the individual pathways of methane generation are known, our understanding of the complex interactions between different bacterial groups remains limited, resulting in considerable difficulties to resolve these questions in microcosm experiments. Here we show that a dramatic drop in seawater sulfate concentrations during the Early Cretaceous (Wortmann & Chernyavsky, Nature 2007) resulted in a global breakdown of microbial sulfate reduction in the marine subsurface biosphere. This event resulted in a positive excursion of the global δ¹³C-value, suggesting that organic matter remineralization rates dropped by more than 50%. This implies that

1. the methanogenic microbial community was unable to increase their metabolic rates, despite the increased supply of organic matter.
2. the reduced availability of sulfate for anaerobic methane oxidation did not increase the flux of isotopically light carbon into the ocean/atmosphere system.