Geologic Controls on the Distribution of Chemosynthetic Communities in the Gulf of Mexico

Abstract

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 sea floor in remote sensing data, direct observation, and sampling. Though faunal distribution at these sites is clustered, indicating an external control on community siting, the mechanism affecting the uneven distribution of organisms within a seep site is poorly understood. Possible geologic controls are likely to have influenced the development of environments conducive to colonization in four regions on the continental slope where chemoautotrophic fauna have been documented or where indications of active seepage and sea floor modification are known. Distribution mapping of organisms from submersible observations were merged with 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 found in areas where deeply rooted faulting occurred in response to simple shear. Biomass is concentrated along the surface traces of antithetic faults where extensive outcrops of authigenic calcium carbonate on the sea floor and evidence of active continuous seepage occur. Other sites contain simple communities of a single species lacking a vestimentiferan component. These communities, containing methanotrophic mytilids or calyptogenid or lucinid clams, exist in areas dominated by Coulomb shear resulting from halokinesis or mass failure. Other sites contain insignificant biomass but have extensive hydrocarbon seepage. Structure at these sites is dominated by shallow-piercement halokinesis reflecting spatial and temporal discontinuities in the seepage history, or recent initiation of seepage. A correlation is proposed between the occurrence of significant chemosynthetic communities and structural forcing mechanisms that developed the seep substrate.