Clam distribution and subsurface hydrothermal processes at Chowder Hill (Middle Valley), Juan de Fuca Ridge

This study used seafloor imaging, near-surface sampling and drilling results to examine hydrological constraints on vent organism distribution and productivity in a sedimented hydrothermal setting at Middle Valley, Juan de Fuca Ridge in the northeast Pacific, where venting is focused around seafloor mounds. A vesicomyid clam bed on one of these mounds (Chowder Hill) was reconstructed from imagery acquired by the remotely operated vehicle ROPOS, using a new image enhancement/mosaicking system. The mosaic was fitted to a Universal Trans-Mercator (UTM) coordinate grid and a contour map of clam density and local bathymetry was derived from visual counts within grid squares and ROPOS navigation logs. Highest clam density occurred on a slightly raised area of seafloor in the upper-central portion of the clam bed, where sediment temperatures suggested hydrothermal flux to be greater than further downslope. Results from Ocean Drilling Program (ODP), College Station, Texas, USA, Leg 139 suggested that a local hydrothermal reservoir exists within Chowder Hill and that erosion of low-permeability surface sediments may be critical to allowing fluids to escape. A 1- dimensional advection-diffusion model was used to derive an estimate for H₂S supply to the clam bed from a subsurface reservoir, based on sediment temperature profiles and pore-water H₂S concentrations. Estimated H₂S flux (250 mol m^{- 2} yr^-1) would support a maximum chemosynthetic CO₂ fixation rate of 8.3 gC m^-2 yr^-1. More detailed in situ data are required to study links between local fluid flow variations and clam density, and to resolve near- surface processes that laterally delimit the fluid conduit beneath the clam colony.