Authigenic pyrite-magnetic susceptibility relationships influenced by methane seepage (Site U1445, Bay of Bengal)

The sulfate-methane transition zone (SMTZ) is a pivotal biogeochemical site within marine sediment columns where anaerobic oxidation of methane (AOM) facilitates the reductive dissolution of Fe-(oxyhydr)oxides and forms iron sulfide minerals (ultimately preserved as pyrite) in marine sediments. However, the long-term relationship between pyrite accumulation, sulfur isotope signatures, and magnetic susceptibility (MS) variations in methane-seepage environments remains poorly understood. In this study, we analyzed 132 marine sediment samples from Hole U1445A in the Bay of Bengal, Indian Ocean, generating data for the content and sulfur isotopic composition of pyrite, total organic carbon (TOC), and magnetic susceptibility (MS). Hole U1445A cores provide a nearly continuous sediment record since 6.20 Ma. Pyrite morphologies are dominantly rod-like and tubular aggregates, interpreted as recording methane upward migration channels. The non-significant correlation between pyrite content and TOC content (R2 = 0.07, p = 0.31, bulk pyrite; R2 = 0.00, p = 0.97, hand-picked pyrite grains) suggests that AOM played an important role in pyrite formation. We identified approximately 8 zones of definite methane influence based on multiple proxy indicators (i.e., FeS2 > 0.5 wt.% coupled with δ34SCRS >0‰), although the actual number of such zones in the study core might be higher. We infer that each of these zones represents a discrete methane release event associated with vertical migration of the paleo-SMTZ. Low MS and ARM/SIRM values coincident with increased pyrite contents within several intervals are consistent with the hypothesis of reductive dissolution of magnetic Fe-oxide minerals by AOM. Thus, low-MS values may reflect the location of paleo-SMTZs in marine sediment columns, although this magnetic signature should be interpreted in conjunction with pyrite-based geochemical indicators to avoid ambiguity. Our findings improve the understanding of coupled S-Fe-MS diagenesis in methane-seep systems.