Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: Sensitivity to underlying methane and gas hydrates

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We test a hypothesis relating large pore water sulfate gradients to upward methane flux and the presence of underlying methane gas hydrate on continental rises by examining: (1) pore water geochemical data available from the global data set of Deep Sea Drilling Project–Ocean Drilling Program (DSDP–ODP) sites; (2) sulfate data from 51 coring sites located at the Carolina Rise and Blake Ridge (offshore southeastern United States); and (3) the relationship between the distribution of bottom-simulating reflectors (BSRs) and sulfate depletion patterns at the Carolina Rise–Blake Ridge (CR–BR) area. Within continental rise sediments, large sulfate gradients are correlative with marine methane gas hydrate settings (recognized by gas hydrate recovery and the presence of BSRs). This correlation is in part due to the rapid consumption of sedimentary organic matter by sulfate reduction and early microbial production of methane during burial and early diagenesis. However, detailed interstitial geochemical evidence from sediments of the CR–BR area strongly suggests that sulfate and methane co-consumption (anaerobic methane oxidation) at the sulfate–methane interface (SMI) is an important additional process in depleting interstitial sulfate, producing steep (and perhaps linear) sulfate gradients and shallow depths to the SMI. The presence of BSRs is currently the only routine technique used to identify gas hydrate localities. However, BSRs seem to represent an abrupt interface at the base of gas hydrate stability (BGHS) where methane gas bubbles occur, rather than being a direct indicator of gas hydrates in overlying sediments. Detailed comparisons between BSR distribution and geochemical data at the CR–BR show that BSRs are patchy in their occurrence, consistent with BSRs representing accumulations of free methane gas that pool within structural and stratigraphic traps near the crest and on the flanks of the Blake Ridge. In contrast, steep sulfate gradients (and proxy indicators of gas hydrate) are pervasive components of the CR–BR area, suggesting that steep sulfate gradients may be a better general indicator of gas hydrate potential. Steep sulfate gradients apparently identify large upward fluxes of methane, indicating conditions conducive to the formation of gas hydrates, given favorable pressure and temperature conditions. Global DSDP–ODP geochemical data identify many additional deep-water marine sites with large sulfate gradients that lack BSRs, perhaps suggesting the occurrence of previously unrecognized gas hydrate localities.