HR: 0800h
AN: OS21A-1204    [Abstracts]
TI: Authigenic Dolomite in Marine Marginal Sediments: An Indicator of Fossil Microbial Activity
AU: * Vasconcelos, C
EM: cris.vasconcelos@erdw.ethz.ch
AF: Geological Institute, Department of Earth Sciences, ETH-Zentrum, Zurich, CH-8092 Switzerland
AU: McKenzie, J A
AF: Geological Institute, Department of Earth Sciences, ETH-Zentrum, Zurich, CH-8092 Switzerland
AU: Meister, P
AF: Geological Institute, Department of Earth Sciences, ETH-Zentrum, Zurich, CH-8092 Switzerland
AU: Magalhaes, V H
AF: Marine Geology Department, IGM, Estrada da Portela, Alfragide, 2721-866 Portugal
AU: Warthmann, R
AF: Geological Institute, Department of Earth Sciences, ETH-Zentrum, Zurich, CH-8092 Switzerland
AU: Mauclaire, L
AF: Geological Institute, Department of Earth Sciences, ETH-Zentrum, Zurich, CH-8092 Switzerland
AB: Dolomite formation under Earth surface conditions remains a long-standing enigma in Earth science. Although it is abundant in the rock record, dolomite does not commonly form in modern marine sedimentary environments and attempts to precipitate dolomite in the laboratory at Earth surface conditions have been largely unsuccessful. In recent years, however, microbial mediation of dolomite formation has been tested in cultural experiments and in natural environments. A new microbial factor has been added to the list of factors, which are thought to promote dolomite precipitation. This microbial model for dolomite formation has found wide acceptance and has been applied to explain modern and ancient dolomite formation. In particular, deep-sea margin sediments provide an ideal setting to apply the microbial dolomite model. With the discovery of dolomite in Quaternary deep-sea sediments in the late 1970's, e.g. during Deep Sea Drilling Program (DSDP) Legs 63 and 64 on the California Margin and in the Gulf of California, a new environment for dolomite formation was recognized. We have applied the microbial dolomite model in two different marine marginal settings and will present the results of SEM, isotopic and mineralogical studies of the recovered dolomite. The discovery of a field of dolomite chimneys strewn on the Gulf of Cadiz seafloor provided an opportunity to study the fossil microbial activity, which had mediated dolomite formation in the flow path of upward migrating methane on this tectonically active margin. In contrast, modern microbial activity was investigated in organic carbon-rich hemipelagic sediments recovered in drill cores obtained from the Peru Margin during ODP Leg 201, which was dedicated to the study of the deep biosphere. The microbial impact on the pore-water geochemistry could be linked directly to the diagenetic processes occurring in the sediments, i.e. dolomite formation. Combining the results obtained from our two study areas, we propose that dolomite formation in marine marginal settings is probably directly linked with microbial activity associated with the geochemical transition between the zones of sulfate reduction and methanogenesis.
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting