INTRODUCTION

Cold seeps and vents have been found along almost all convergent plate boundaries (e.g., Kulm et al., 1986; Le Pichon et al., 1992; Wallmann et al., 1997) and on many passive continental margins (e.g., Paull et al., 1984; Hovland, 1992; Aharon et al., 1997). Distinctive colonies of clams, tube worms, and precipitates of authigenic minerals mark areas of fluid discharge and are the result of both biogeochemical turnover and the interaction between fluids and ambient bottom water at cold vent sites (Suess et al., 1985; Paull et al., 1992; Roberts and Aharon, 1994; Paull et al., 1995b, Barry et al., 1996). The co-occurrence of gas hydrates and fluid venting at several locations (e.g., the Sea of Okhotsk [Soloviev and Ginsburg, 1997], the Eel River Basin [Brooks et al., 1991], the Cascadia Margin [Carson, Westbrook, Musgrave, Suess, et al., 1995; Sample and Reid, 1998; Bohrmann et al., 1998] and the Blake Ridge [Paull et al., 1995b]) indicates a relationship between gas hydrate decomposition, venting of methane-rich fluids, and the formation of authigenic carbonates at the seafloor.

The chemical environment at most vent sites appears to be controlled by the flux of gases to the seafloor and rates of biologically induced reactions, which affect both the bicarbonate and sulfate activities. Furthermore, chemosynthetic benthic fauna is supported by CH₄- and H₂S-oxidizing and SO₄^2--reducing bacteria. Whereas aerobic oxidation of methane favors calcite dissolution instead of precipitation (Wallmann et al., 1997), the anaerobic oxidation of methane via sulfate reduction generates HCO₃^- and can, therefore, produce oversaturation of pore water with respect to calcite and other carbonate minerals (Reeburgh, 1980):

CH₄ + SO₄^2- HCO₃^- + HS^- + H₂O. (1)

Drilling at the Blake Ridge Diapir vent site (Site 996) during ODP Leg 164 focused on processes related to methane migration and gas hydrate formation/decomposition in a fault zone where methane, presumably derived from gas hydrates below, is leaking out onto the seafloor. Drilling at this site provided the unique opportunity to study the connection between the seafloor phenomena of cold seeps and the reservoir from which these fluids might originate, and how the mineralogical, geochemical, and isotopic composition of mineral precipitates might be affected by the temporal or spacial evolution of fluids within such fault conduits. New mineralogical, chemical, and isotope data from authigenic carbonate precipitates presented in this paper provide a record of active fluid venting above the Blake Ridge Diapir. These results, when compared to data from the associated pore fluids and gases, provide the means to establish the connection between the fluid reservoir, fluid conduits, and the formation of mineral precipitates.

The carbon isotopic composition of authigenic carbonates serves as an indicator for the origin of carbon incorporated during carbonate precipitation (e.g., Reeburgh, 1980; Anderson and Arthur, 1983; Ritger et al., 1987). Sources of carbon to the pore fluids include (1) biogenic methane (¹³C <-65 PDB) or thermogenic methane (¹³C -30 to -50); (2) sedimentary organic carbon (¹³C ~-25); and (3) marine biogenic carbonate or seawater CO₃^2- with a ¹³C value near 0. Ultimately, the amount of mixing between these different components will determine the ¹³C value of any authigenic carbonate (Paull et al., 1992).

Oxygen isotope ratios, on the other hand, may provide information pertaining to the temperature and origin of fluids from which authigenic carbonates are precipitated. Additionally, zonation patterns of carbon and oxygen isotopes within the carbonate nodules provide insights into changes of the isotopic composition of the pore fluids during the process of cementation.

The objectives of this study are (1) to document and summarize the occurrence and distribution of authigenic carbonates recovered from several depth intervals in sediments above the Blake Ridge Diapir (Site 996); (2) to investigate the petrographic, geochemical and isotopic characteristics of these carbonates; (3) to infer the nature and source of fluids associated with carbonate formation; and (4) to address the question of whether authigenic mineral formation at vent sites is mainly restricted to the seafloor, or if there is ongoing diagenesis with depth. Authigenic carbonates recovered from as deep as 60 mbsf into this active vent site provided us with an unparalleled opportunity to investigate the spatial variability of vent-derived authigenic mineral precipitates.