Authigenic Mineral Precipitation at Cold Seeps in Continental Margin Sediments - A Comparative Study.

Chemosynthetic biological communities and authigenic mineral formation are common along continental margins worldwide where they indicate the availability of reduced compounds on or immediately below the seafloor. Some of these sites, historically termed "cold seeps", are associated with obvious hydrocarbon venting (e.g., Gulf of Mexico, Santa Barbara Basin, and Eel River Basin), whereas fluid flow at other sites has been inferred by the presence of chemosynthetic communities and authigenic mineral deposits on the seafloor (e.g., Monterey Bay). Authigenic mineral deposits include carbonate-group minerals, most commonly aragonite, Mg-calcite, and dolomite, which form via microbially mediated anaerobic oxidation of methane. However, depending on the chemistry of the expelled fluids, minerals other than carbonate (i.e., barite) may form deposits on the seafloor. To develop a more comprehensive understanding of the complex geochemical, physical, and biological interactions at seep sites, we present a comparative study that illustrates the diverse processes associated with the flux of fluids and gases to the seafloor and the precipitation of authigenic minerals at cold seeps. In this study, we will discuss the formation and early diagenesis of authigenic mineral deposits in a variety of geologic and geochemical environments including the southern Gulf of Mexico, Monterey Bay, Santa Barbara Basin, Eel River Basin, North Sea, and the Sea of Okhotsk. Authigenic carbonates from these study areas exhibit a wide range of mineralogical and stable isotopic compositions. The mineralogy of the precipitates ranges from dolomite and high-Mg-calcite to aragonite. The carbon isotopic composition of carbonates shows a wide variation, indicating a complex carbon source from both ¹³C-depleted and residual, ¹³C-enriched, fluids. The large variability of δ ¹⁸O values also demonstrates the geochemical complexity of these sites with some samples pointing toward a heavy, ¹⁸O-enriched oxygen source, possibly related to the decomposition of gas hydrate, whereas other samples indicate the local presence of meteoric water during carbonate precipitation. In one instance, fluid flow in response to slumping and mass wasting resulted in mixing of barium-rich pore fluids and sulfate-rich bottom water, causing barite, not carbonate, precipitation on the seafloor.