HR: 0830h
AN: B21D-0735    [PDF]
TI: Anaerobic Methane Oxidation and the Formation of Dolomite
AU: * Moore, T S
EM: tsmoore@udel.edu
AF: Boston University, Department of Earth Sciences 685 Commonwealth Ave, Boston, MA 02215 United States
AU: Murray, R W
AF: Boston University, Department of Earth Sciences 685 Commonwealth Ave, Boston, MA 02215 United States
AU: Kurtz, A C
AF: Boston University, Department of Earth Sciences 685 Commonwealth Ave, Boston, MA 02215 United States
AU: Schrag, D P
AF: Harvard University, Department of Earth and Planetary Sciences 20 Oxford St, Cambridge, MA 02138 United States
AB: The environment and conditions necessary for the formation of dolomite has been a long standing problem in many scientific fields. In particular, the formation of organogenic dolomites and how they relate to the degradation of organic matter have been the focus of numerous studies. We examine the link between organic matter degradation, anaerobic methane oxidation (AMO), and sulfate depletion, and explore how these processes potentially influence dolomitization. We determined rates and depths of AMO and dolomite formation for a variety of organic-rich sites along the west African Margin at sites occupied during Ocean Drilling Program Leg 175. Our data set is porewater and gas data gathered during the cruise (alkalinity, sulfate, methane, Ca, and Mg), as well as postcruise analyses of $\delta$$^{13}$C of DIC. We find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming, indicating that the diffusive fluxes of Mg and Ca are not limiting. Based upon the calculated log IAP values, log K$_{sp}$ values for dolomite was found to narrowly range between -16.1 and -16.4. Dolomite formation is controlled in part by competition between AMO and methanogenesis, which controls the speciation of dissolved CO$_{2}$. AMO increases the concentration of CO$_{3}$$^{2-}$ through sulfate reduction, favoring dolomite formation, while methanogenesis increases the pCO$_{2}$ of the pore waters, inhibiting dolomite formation. By regulating the pCO$_{2}$ and alkalinity, methanogenesis and AMO can regulate the formation of dolomite in organic-rich marine sediments. In addition to providing a mechanistic link between AMO and dolomite formation, our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments, and allows prediction of regions and depth domains in which dolomite may be forming.
DE: 0330 Geochemical cycles
DE: 4805 Biogeochemical cycles (1615)
DE: 4806 Carbon cycling
SC: Biogeosciences [B]
MN: 2003 Fall Meeting