Hydrothermal hydrocarbon gases in the sediments of the King George Basin, Bransfield Strait, Antarctica

Abstract

Thermogenic hydrocarbons, formed by the thermal alteration of organic matter, are encountered in several piston core stations in the King George Basin, Anatarctica. These hemipelagic sediments are being deposited in an area of active hydrothermalism, associated with the back-arc spreading in the Bransfield Strait. The lateral extent of sediments infiltrated by the hydrothermally influenced interstitial fluids is characterized by basalt diapirix intrusions and is delineated by an acoustically turbid zone in the sediments of the eastern part of the basin. Iron-sulphide-bearing veins and fractures cut across the sediment in several cores; they appear to be conduits for flow of hydrothermally altered fluids. These zones have the highest C 2+ and ethene contents. The thermogenic hydrocarbons have molecular C 1/(C 2 + C 3) ratios typically < 50 and δ 13CH 4 values between −38% and −48%, indicating an organic source which has undergone strong thermal stress. Several sediment cores also have mixed gas signatures, which indicate the presence of substantial amounts of bacterial gas, predominantly methane. Hydrocarbon generation in the King George Basin is thought to be a local phenomenon, resulting from submarine volcanism with temperatures in the range 70–150°C. There are no apparent seepages of hydrocarbons into the water column, and it is not believed that significant accumulation of thermogenic hydrocarbons reside in the basin.