TORRES, MARTA E., COAS, Oregon State University, Corvallis; JAMES P. BARRY, MBARI, 7700 Sandholdt Rd. Moss Landing, CA; and EDWIN SUESS, GEOMAR, Kiel, Germany

Abstract: Extreme Heterogeneity of Cold Seep Sites Documented in Shells of Vesicomyid Clams

We obtained high-resolution Ba/Ca profiles on shells of Calyptogena clams that were collected live from sites of cold fluid discharge in the Monterey Canyon and Cascadia margin. Significant differences in clam populations, fluid chemistry and Ba/Ca ratios of shell samples are used to document the spatial heterogeneity of fluid discharge localities. A 2-year long episode of high fluid flow centered around 1992 can be inferred from coherent changes in the Ba/Ca profiles of three Calyptogena kilmeri shells from Monterey Bay. Comparison with precipitation and d¹⁸O data indicates that this high-flow period may have been driven by an increase in rainfall after the 1986-1990 California drought.

We have estimated that during the periods of high fluid discharge rates, the Ba in surface sediments might have reached values as high as 2µmol/l, which are comparable with measured concentrations of dissolved barium at other cold-seep areas. In the Cascadia margin, the measured barium fluxes associated with venting on First Ridge are in the order of 50 µmol cm^-2yr^-1. These data, although not enough to construct a model of the role of fluid venting on barium budgets, hints at the idea that these sites may be contributing significant amounts of dissolved barium to the bottom water. The extent of the contribution of cold seepage to the geochemical budgets must be evaluated in the context of the spatial and temporal variations presented here.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California