MARTIN, J. B., and S. DAY, University of Florida, Gainesville, FL; C. G. WHEAT, West Coast and Polar Regions Undersea Research Center, Moss Landing, CA, J. PLANT, Monterey Bay Aquarium Research Institute, Moss Landing, CA; D. L. ORANGE, Univ. California, Santa Cruz, CA; T. L. LORENSON, U.S. Geological Survey, Menlo Park, CA

Abstract: Sedimentary Record of Cold Seeps

The transient nature of cold seeps is likely to be important for the ecology of chemosynthetic communities living at the seeps, for dispersal of the communities, and for calculating hydrologic budgets for the seeps. Cold seeps commonly form authigenic carbonate and sulfate minerals which reflect their locations. The distribution of ancient cold seeps, and thus possibly their longevity, may be recorded by these or other sedimentary deposits and alteration products. In order to test this hypothesis, we have used the ROV Ventana to collect cores in and around cold seeps in Monterey Bay, CA. The cold seeps were identified on the basis of the locations of chemosynthetic clam communities. Within the seeps, alkalinity and sulfide concentrations of pore waters increase and sulfate concentrations decrease more rapidly with burial depth than in pore fluids surrounding the seeps. Seep sediments contain significantly fewer benthic foraminifera than sediment outside of the seeps, probably reflecting limited growth of the foraminifera because of the chemical composition of the seep water. The lower abundance appears to reflect the original population density, rather than dissolution, because the tests show no signs of corrosion. We expect that the d¹³C values of the foraminifera within the seeps will record the light (down to -49 ‰) isotopic composition of bicarbonate dissolved in the seep water. In pore fluids from Monterey Bay seeps, d¹⁸O composition and ⁸⁷Sr/⁸⁶Sr ratios are close to seawater values, but where altered, these isotope ratios could also be good tracers. Thus, isotopic composition of foraminifera, combined with the presence of finely disseminated alteration products, may be useful tracers of locations of ancient cold seeps.

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