Origin of the authigenic gypsum and pyrite from active methane seeps of the southwest African Margin

Abstract

The southwest African Margin is a well-known area of active methane seeps that are related to high deposition rates of organic-rich sediments where intense methanogenesis occur in the subseafloor environment. The sediments are totally anoxic up to the surface and contain huge H2S concentrations, gas hydrates and authigenic carbonates derived from the anaerobic oxidation of methane coupled with the reduction of sulphate. Three cores collected at water depths from 3102 to 3435m from Worm Hole, Deep Hole and Regab Pockmark located offshore Gabon and Angola were selected to determine the origin of pyrite concretions and authigenic gypsum crystals that may be very abundant in the sediments down to about 2.8m below the seafloor. Pyrite concretions correspond often to tubular structures that are interpreted as possible molds of giant sulphur oxidizing bacteria. The sulphur isotopic composition of pyrite varies within a narrow range (−50.4 to −44.9‰ V-CDT) that indicates extreme isotopic fractionation due to bacterial reduction of the sea-water sulphate and disproportionation of sulphur during sulphide oxidation. The authigenic gypsum occurs as hyaline prisms more or less elongated and as dull prismatic or lenticular crystals. The oxygen and sulphur isotopic compositions of gypsum vary widely (+1.6<δ18O ‰ V-SMOW<+7.9; −38.8<δ34S ‰ V-CDT<+10.9) and are much lower than the values characteristic of marine gypsum. These values indicate that the dissolved sulphate of pore water from which gypsum precipitated was issued from two pools: 1) the 18O-34S rich residual sea-water sulphate resulting from bacterial sulphate reduction, and 2) the sulphate produced by oxidation of sulphide by dissolved oxygen and oxygen of water more or less modified by gas hydrate formation, and where the disproportionation of sulphur caused large and variable 34S and 18O enrichments in the produced sulphate.