AUTHIGENIC CARBONATES DERIVED FROM OXIDIZED METHANE VENTED FROM THE MAKRAN ACCRETIONARY PRISM OFF PAKISTAN

We report the first discovery and sampling of a methane-hydrogen sulfi de rich ''cold seep'' from the Makran accretionary prism off Pakistan (Arabian Sea). A variety of cm- to m-scale pockmarks and gas seepage s tructures were identified from underwater TV-photo sled profiles cross ing the oxygen minimum zone (OMZ), related to high-resolution seismic (PARASOUND) records. From the seeps isotopically light, bacterially fo rmed methane is emanating that is partly oxidized to HCO; in the bacte rial sulfate reduction zone. This results in the precipitation of irre gular dark-gray to black crusts of indurated authigenic carbonates, ma inly cryptocrystalline magnesian calcite and Ca-rich dolomite, near th e sediment/seawater interface within the OMZ. Downcore, the crusts gra de into hemipelagic carbonate-poor silty clays with transitional litho logies in between. Similar to the pockmark carbonates from the Oregon margin and in the North Sea, the authigenic carbonates are extremely d epleted in C-13 (delta(13)C(carbonate) < -40 parts per thousand). This suggests that they were derived from bacterial methane (delta(13)C(me thane): -77.8 parts per thousand) that was oxidized under anaerobic co nditions. The authigenic carbonates are associated with white fluffy m ats of chemoautotrophic H2S-oxidizing bacteria. Small pockmarks appear to be concentrated at small-scale escarpments, suggesting focussed fa ult-controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism, whereas diffuse discharge of por e fluids is inferred from the widespread occurrence of tiny gas bubble tubes.