FORAMINIFERA OF HYDROCARBON SEEPS, GULF OF MEXICO

Abstract

One hundred and eighty-three species of benthic Foraminifera were identified in a study of sediment substrates and tubeworm surfaces in (a) the Green Canyon, Garden Banks and Mississippi Canyon (245–1081 m) and (b) the Alaminos, Farnella and De Soto Canyons (1848–2918 m), Gulf of Mexico; the samples were obtained from submersibles in both seep and non-seep (control) areas. None of the species is endemic to seeps, but 20 species were previously unknown in the Gulf of Mexico. The imprint of water depth on foraminiferal assemblages is clearly detectable, because the species are recruited from the surrounding non-seep habitats. The two high-level surface-sample groups (clusters) recognized by numerical data analysis are distinguishable based on the bathymetric location of the sample sites. The shallower-water group contains all samples (seep and non-seep) from depths of 245–1081 m; the deeper-water (deepest-bathyal and abyssal) group contains all samples from 1848–2918 m. Foraminiferal species of wide-ranging morphologic and taxonomic affinities are able to maintain sizeable populations at sites of hydrocarbon seepage; the high bacterial productivity at the seeps could be a major factor in the sustenance of these populations. The most conspicuous dominants at seep-influenced substrates (bacterial mats) in the shallower cluster are endobenthic species, especially species of Bolivina , which are possibly facultative anaerobes. The pattern is not as clear in the deepest-bathyal and abyssal group, because some epibenthic species (e.g., Nuttallides decorata ) are present among the dominants. In the shallower areas, the diversity (species richness) of both calcareous and agglutinated Foraminifera is higher in non-seep than in seep substrates. This distinction too is not clear in the deepest-bathyal and abyssal areas. Post-mortem mixing of species from different microhabitats could have elevated the diversity at some sites. Twelve sessile, epibenthic Foraminifera have been found to colonize surfaces of vestimentiferan tubeworms (and possibly other elevated microhabitats) at the seeps, centimeters to decimeters above the sediment-water interface. These attachment points are sufficiently above locations of gas escape in the seafloor to provide the species with an oxic microhabitat with little or no H2S.