CHEMICAL AND ISOTOPIC EVIDENCE OF GAS-INFLUENCED FLOW AT A TRANSFORM PLATE BOUNDARY - MONTEREY BAY, CALIFORNIA

Chemical and isotopic compositions of pore fluids document upward flow through communities of vesicomyid clams in Monterey Bay, California. Within the clam communities, the sulfate reduction zone is only 10 cm thick, and Ca and Mg concentrations decrease to values as low as 2.2 m M and 34.5 mM, respectively, at depths less than 30 cm below the sedim ent-water interface. Less than 5 m outside the communities, the base o f the sulfate reduction zone is deeper than the greatest penetration o f the cores (similar to 30 cm), and Ca and Mg exhibit only minor chang es from seawater values. The sediment exhibits no significant variatio n in grain size, mineralogy, organic carbon, nitrogen, or carbonate co ntent throughout the region. The composition of pore fluid within clam communities results from upward flow of altered fluid rather than dif ferent diagenetic reactions within and outside the communities. Isotop ically light dissolved inorganic carbon (DIG), with delta(13)C values ranging from -3.2 to -54.1 parts per thousand, could reflect carbon so urces from either oxidized thermogenic methane and/or a mixture of oxi dized microbial methane and solid organic carbon. The C-1/(C-2+C-3) ra tios (ranging from 34 to 1142) and the hydrogen and carbon isotopic co mpositions of methane (delta D values of -109 to -156 parts per thousa nd; delta(13)C values of -30.6 to -86.6 parts per thousand) suggest th at methane is primarily microbial but that a minor component could be thermally generated. Any thermogenic methane would have migrated from great depths, possibly >2 km. The presence of methane is likely to con tribute to fluid flow by reducing the density of the fluids. Past flui d migration and venting are reflected by widespread carbonate minerali zation at the sediment-water interface. This mineralization and the ge ographic distribution and proportions of microbial and thermogenic met hane suggest that vent sites migrate when permeability is reduced duri ng carbonate cementation. These results demonstrate that along with co nvergent and divergent plate boundaries, transform plate boundaries ar e characterized by fluid flow and that the flow may be widespread, occ urring at sites away from fault zones.