BERNER, ULRICH, J. POGGENBURG, U. V. RAD, A. LUCKGE, and G. DELISLE, BGR, Hannover, Germany; R. SCHMALJOHANN, IFM, Kiel, Germany; and P. LINKE, GEOMAR, Kiel, Germany
Abstract: Cold Seeps of the Makran Accretionary Prism off Pakistan (NE Arabian Sea)
In order to study the fluid and gas migration in an active continental margin with high organic carbon and gas contents, we investigated the Makran accretionary wedge off Pakistan. Cold seeps were discovered along the accretion between 250 to 800 m water depth and a single location at 2500 m water depth. Here isotopically light methane of bacterial origin emanates from the sediments with water column concentrations that vary between 290 and 500 nl/l. Carbon isotope ratios of the methane plumes point to severe methane oxidation. Also, methane is partly oxidized in the sedimentary sulfate reduction zone. This results in the precipitation of irregular crusts of authigenic carbonates near the sediment/water interface that are extremely depleted in 13C. Carbon isotope analyses showed that parts of the biomass around cold seeps originated from chemoautotrophic processes. The detection of Beggiatoa and Thioploca sp. at such locations proved this assumption. Also, colonies of marco vent fauna (Calyptogena, Solemya sp., pogonophoran tubes) were observed in a canyon at about 2500 m water depth. This site is associated with high bacterial methane concentrations of the water column. Isotope analyses of Calyptogena shells showed that the shell carbonate is not influenced through uptake of carbon dioxide produced from oxidation of bacterial methane. However, carbon isotope ratios of -34‰. of the ligament and the organic coating of the shell revealed a bacterial imprint.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California