Abstract:
Anaerobic oxidation of methane (AOM) coupled to sulfate reduction in the sulfate-methane transition zone (SMTZ) leaves prominent diagenetic carbon and sulfur signatures in pore-water and solid-phase chemistry that are preserved over geological time scales. We report on the evolution of these signals in the sediments of Site U1341 drilled during Integrated Ocean Drilling Program (IODP) to the Bering Sea to a depth of 600 meters below seafloor (mbsf). At this site, present-day microbial activity associated with organic carbon mineralization is comparably low as evidenced in low dissolved inorganic carbon (DIC) concentrations, and a minor decrease in sulfate concentrations in the top 50 mbsf. Strong decrease of sulfate concentrations below 140 mbsf mirrored by pronounced (super 34) S-sulfate enrichment at the depth of minimum sulfate concentrations suggest that the extent of microbial sulfate reduction was high in this sediment interval deposited during a period of elevated primary productivity in the water column between 2.48 and 2.56 Ma. Elevated sulfate reduction rates drove sulfate to depletion and facilitated the onset of methanogenesis, AOM and the installation of a SMTZ. Rates of these processes apparently declined as a consequence of decreased availability of organic carbon during later time periods. This interpretation is consistent with (super 34) S-enriched pyrite, (super 13) C-depleted dolomite phases and barium depletion to detrital background in distinct sediment intervals. Close examination of the diagenetic pore-water and solid-phase signals, however, revealed that their relative positions in the sedimentary sequence considerably diverge. Also, pore-water DIC, sulfate, Ca and Mg concentrations and isotope profiles do not covary systematically. Our results suggest variable responses of diagenetic signals produced by past SMTZs in the pore-water and sediment over prolonged time scales.