Authigenic carbonates derived from oxidized methane vented from the Makran accretionary prism off Pakistan

Abstract

We report the first discovery and sampling of a methane-hydrogen sulfide rich “cold seep” from the Makran accretionary prism off Pakistan (Arabian Sea). A variety of cm- to m-scale pockmarks and gas seepage structures were identified from underwater TV-photo sled profiles crossing the oxygen minimum zone (OMZ), related to highresolution seismic (PARASOUND) records. From the seeps isotopically light, bacterially formed methane is emanating that is partly oxidized to HCO 3 − in the bacterial sulfate reduction zone. This results in the precipitation of irregular dark-gray to black crusts of indurated authigenic carbonates, mainly cryptocrystalline magnesian calcite and Ca-rich dolomite, near the sediment/seawater interface within the OMZ. Downcore, the crusts grade into hemipelagic carbonate-poor silty clays with transitional lithologies in between. Similar to the pockmark carbonates from the Oregon margin and in the North Sea, the authigenic carbonates are extremely depleted in 13C ( δ 13 C carbonate < − 40%.). This suggests that they were derived from bacterial methane ( δ 13 C methane: − 77.8%.) that was oxidized under anaerobic conditions. The authigenic carbonates are associated with white fluffy mats of chemoautotrophic H 2S-oxidizing bacteria. Small pockmarks appear to be concentrated at small-scale escarpments, suggesting focussed fault-controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism, whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes.