Fluid Flow and Carbonate Precipitation in Continental Margin Settings. Naehr, T.1, Stakes, D. S.1, Paull, C. K.1, Eichhubl, P.1, Ussler, W.1, Greene, H. G.2, Lorenson, T.3, Hovland, M.4 1 Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 2 Moss landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039 3 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 4 Statoil, N-4035, Stavanger, Norway Carbonate precipitation associated with the expulsion of hydrocarbon-rich fluids occurs along almost all continental margins. Distinctive colonies of clams, tube worms and carbonate buildups mark areas of fluid discharge and seem to be the result of biogeochemical turnover and interaction between fluids and ambient bottom water at cold vent sites. However, the details of carbonate precipitation in these environments are only poorly understood. Building on past MBARI investigations of cold seeps, the objective of this study was to investigate the relationship between the mineralogical and isotopic characteristics of authigenic carbonates and the geochemical and tectonic settings in which they form. Tectonic settings included active margins (Santa Barbara Basin, Eel River Basin, Monterey Bay), as well as passive margins (Blake Outer Ridge, North Sea). Differences in the geochemical environment were represented by sulfide-rich (Monterey Bay), methane-rich (Eel River Basin), gas hydrate bearing (Blake Outer Ridge, Sea of Okhotsk) and non gas hydrate bearing fluid expulsion sites (Monterey Bay). Authigenic carbonates from different study areas exhibit a wide range of mineral and stable isotopic compositions. The mineralogy of the precipitates varies from dolomite and high-Mg-calcite to aragonite. The carbon isotopic composition of carbonates also shows a wide variation, ranging from +9.7 permil to -57.5 permil, which indicates a complex carbon source from both 13C-depleted and residual, 13C-enriched, carbon pools. The delta18O values of the authigenic carbonates vary between +3.2 permil and +7.4 permil, pointing toward a heavy, 18O-enriched oxygen source for some of the samples, possibly related to the decomposition of gas hydrate. In Monterey Bay, delta18O values range from +6.8 permil to -8.5 permil indicating the local presence of meteoric water. Although no gas hydrates have been detected in Monterey Bay sediments, heavy oxygen isotope values may indicate their presence in the past. The wide range of isotopic and mineralogical variation in authigenic carbonates from different study areas demonstrates that these parameters alone are not indicative for certain tectonic settings or geochemical environments. Rather, these variations reflect local controls on the flux of carbon, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the local carbon reservoir.