HYDROTHERMAL DISCHARGE AND ALTERATION IN NEAR-SURFACE SEDIMENTS FROM THE GUAYMAS BASIN, GULF OF CALIFORNIA

In the Guaymas Basin, Gulf of California, hydrothermal fluids discharge onto the seafloor via both direct emanation through hydrothermal spires and diffusely through the sediments. The latter mode is the focus of this study. Short (less-than-or-equal-to 20 cm) sediment cores were taken in zones of diffuse hydrothermal flow for chemical investigation of interstitial fluids and bulk solids. The hydrothermal component of the pore fluids was quite variable, indicating the heterogenous nature of the near-surface flow regimes. The bulk solids chemistry indicates that the suite of cores has encountered different amounts of cumulative hydrothermal exposure. Dissolution of biogenic silica and deposition of large amorphous silica clasts in the sediment are related to the extent of hydrothermal exposure. Surface enrichments in Ba are formed as barite precipitates when Ba-rich hydrothermal fluids mix with sulfate-rich seawater at the seafloor. Formation of pyrite and removal of Mg from the solid phases are also observed effects of hydrothermal flow through these sediments.