Geological and geochemical process structuring of hydrocarbon seep community environments

Research relating the seepage of hydrocarbons on the Louisiana upper continental slope to the distribution of seep communities and massive volumes of authigenic carbonates is described. It is noted that extensive seepage of hydrocarbon to the sea floor throughout the upper continental slope province off Louisiana has set the stage for unexpected geological features and coexisting biological communities. Numerous salt diapirs have penetrated slope sediments, creating dome/basin topography and complex fault networks that function as pathways for the movement of gases and crude oil to the surface. Microbial degradation of hydrocarbons has resulted in the precipitation of isotopically light (C-13 depleted) authigenic carbonates as a by-product. Nodules in host sediment, hardgrounds, and mounds of various sizes (to over 20-m relief) have resulted from the process of authigenic carbonate formation. Biological communities that utilize reduced compounds for chemosynthesis in these carbonate-rich areas serve as indications of active seepage. Bacterial mats (largely Beggiatoa sp) appear to be the most reliable indicator of short-term seeps when large depth ranges are studied. It is concluded that careful study of community structures and distribution patterns can potentially produce valuable information about mechanisms and rates of hydrocarbon/sulfide flux to the sea floor