Geological controls on the distribution of chemosynthetic communities in the Gulf of Mexico.

摘要

Communities of chemoautotrophic organisms have been observed at multiple sites on the continental slope of the Gulf of Mexico where natural seepage of hydrocarbons has been recognized. Effects of this seepage are readily seen as modification of the seafloor in both remotely-sensed data and direct observation and sampling. Though faunal distribution within these sites displays a distinct clustering, indicating a likely external control on community siting, the mechanism affecting the uneven distribution of organisms within a seep site is poorly understood. This work examines geological controls likely to influence development of environments conducive to colonization within four regions on the continental slope where either chemoautotrophic fauna have been previously documented or where indications of active seepage and seafloor modification are known.;Distribution maps of organisms from submersible observations were merged with the surface and near-surface structure mapped from 3-D CDP-processed seismic data. Where organisms are present a first-order correlation of community occurrence with surface expression of faulting was noted. Complex communities containing vestimentiferan tubeworms were generally found to exist in areas where deeply-rooted faulting initiated in response to simple shear, is present. In general, biomass is concentrated along the surface traces of antithetic faults where extensive outcrops of authigenic calcium carbonate at the seafloor and evidence of active, continuous seepage is present. Other sites contain simple communities generally consisting of a single species and lacking a vestimentiferan component. These communities, containing either methanotrophic mytilids or calyptogenid or lucinid clams, exist in areas dominated by Coulomb shear either as the result of halokinesis or mass failure. The geochemical substrate at the mytilid sites is dominated by light, biogenic hydrocarbons and, at the clam sites, by highly degraded thermogenic components consisting almost exclusively of unresolved complex mixture (UCM) and lacking indicators of continued migration. Additional sites containing insignificant biomass while exhibiting indicators of extensive hydrocarbon seepage were also visited. Structure at these sites was dominated by shallow-piercement halokinesis whereas the geochemical substrates reflected spatial and temporal discontinuities in the seepage history, or recent initiation of seepage.;Based on these observations a model protocol for evaluating a probable seep site for the occurrence of significant chemosynthetic communities is proposed.