Genesis and preservation of authigenic magnetite and greigite in the cold seep sediments, Bay of Bengal

Abstract

We report a new mechanism of authigenic greigite and magnetite formation and preservation in an anomalous “sulfide-rich” interval in the cold (methane) seep sediments from the Bay of Bengal. Rock magnetic proxies (SIRM/χlf, ARM/SIRM) coupled with microscopic (TEM, SEM) observations provided compelling evidence for the presence of nanoscale authigenic magnetite and greigite particles in the methanic sediments. Multiple peaks of SIRM/χlf coupled with high Hcr (>53 mT) and DJH values suggest significant contribution from authigenic greigite particles to the bulk sediment magnetism. High ARM/SIRM in the sulfide-rich intervals within methanic zone suggest that nanoscale authigenic magnetite formation is closely linked with the microbial iron reduction process fuelled by anaerobic oxidation of methane (AOM). We proposed that enrichment of reactive secondary iron (oxyhydr) oxides through selective leaching of Fe2+ during diagenetic dissolution and maghemitization of detrital magnetic particles and iron-sulfide mineral oxidation at the paleo-SMTZ fronts, hindered the pyritization processes and favoured microbial iron reduction which led to the formation and preservation of authigenic greigite and magnetite in methanic sediments.