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

We test a hypothesis relating large pore water sulfate gradients to upward methane flux and the presence of underlying methane gas hydrate on continen tal rises by examining: (1) pore water geochemical data available from the global data set of Deep Sea Drilling Project-Ocean Drilling Program (DSDP-O DP) sites; (2) sulfate data from 51 coring sites located at the Carolina Ri se and Blake Ridge (offshore southeastern United States); and (3) the relat ionship 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 recover y and the presence of BSRs). This correlation is in part due to the rapid c onsumption of sedimentary organic matter by sulfate reduction and early mic robial production of methane during burial and early diagenesis. However, d etailed interstitial geochemical evidence from sediments of the CR-BR area strongly suggests that sulfate and methane co-consumption (anaerobic methan e oxidation) at the sulfate-methane interface (SMI) is an important additio nal process in depleting interstitial sulfate, producing steep (and perhaps linear) sulfate gradients and shallow depths to the SMI. The presence of B SRs is currently the only routine technique used to identify gas hydrate lo calities. However, BSRs seem to represent an abrupt interface at the base o f 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 representin g accumulations of free methane gas that pool within structural and stratig raphic traps near the crest and on the flanks of the Blake Ridge. In contra st, steep sulfate gradients (and proxy indicators of gas hydrate) are perva sive components of the CR-BR area, suggesting that steep sulfate gradients may be a better general indicator of gas hydrate potential. Steep sulfate g radients apparently identify large upward fluxes of methane, indicating con ditions conducive to the formation of gas hydrates, given favorable pressur e and temperature conditions. Global DSDP-ODP geochemical data identify man y additional deep-water marine sites with large sulfate gradients that lack BSRs, perhaps suggesting the occurrence of previously unrecognized gas hyd rate localities. (C) 1999 Elsevier Science B.V. All rights reserved.