Diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems

Mineral magnetic results and electron microscope observations from gas hydrate-bearing marine sediments cored at southern Hydrate Ridge during Ocean Drilling Program Leg 204 (Sites 1244 to 1252, Cascadia Margin, offshore Oregon) demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially-mediated diagenetic reactions in the sulphate, the anaerobic oxidation of methane (AOM), and the methanic/gas hydrate zones. Geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide, whether it derives from microbially-driven sulphate reduction in the sulphate zone, in the AOM zone or in deep sediments undergoing AOM, so that pyritization reactions are not driven to completion. Our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record, because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred. Formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times, which is likely to compromise paleomagnetic results from greigite- and pyrrhotite-bearing marine sediments.