HISTORICAL RECORD OF BENTHIC FORAMINIFERAL SPECIES AT A METHANE-HYDRATE DISSOCIATION SITE, GREEN CANYON, GULF OF MEXICO

On the northern Gulf of Mexico slope, foraminiferal communities in surface sediments associated with methane-hydrate dissociation exhibit a reduced species diversity (compared to the diversity in isobathyal, non-seep sediments from the same general area) and a conspicuous presence of microaerophilic species. These methane-tolerant species of Foraminifera, unlike some macroinvertebrates, are not endemic to cold seeps, but represent a part of the background community. A push core (JSL2900-PC2), taken from a submersible through a bacterial (Beggiatoa) mat on a methane-hydrate mound (water depth 567 m), provided an opportunity to examine the historical record of such species (dead individuals) at a site of observable hydrate dissociation and gas expulsion, and to compare the signals of stratigraphic distribution of species with those of stable-isotope variations in foraminiferal shells (see Aharon et al., this volume). The 24-cm-long push core covers a time span of 1800 years, and shows a very sharp upward reduction of assemblage density (individuals/gm of sediment) and a slightly less pronounced reduction in equitability (derived from species richness and Shannon-Wiener diversity) in the 15-19 cm stratigraphic interval. This change probably reflects an intensification of hydrate dissociation and methane enrichment of habitat waters. The species richness is highly variable (10-46), but with the exception of the surface-sediment sample, values higher than 27 are confined to levels below 15 cm. Bolivina is the dominant genus throughout the core, and B. ordinaria is the dominant species in most stratigraphic intervals, with a relative abundance as high as 42%. Other dominant or common species include B. albatrossi, Cassidulina neocarinata, Gavelinopsis translucens, Osangularia rugosa, and Trifarina bradyi. All of these taxa are found living in anoxic substrates under Beggiatoa mats in this area.