FORAMINIFERA OF DEEP-BATHYAL AND ABYSSAL HYDROCARBON SEEPS, GULF OF MEXICO

Seafloor sediment collected by submersible-based observers in two areas of active hydrocarbon seepage in the northwestern Gulf of Mexico—Alaminos Canyon (AC, water depth ~2200 m) and Mississippi Canyon (MC, ~1100 m)–– shows that numerous species of endobenthic and epibenthic Foraminifera have colonized the diverse habitats associated with seepage. The effect of the 1-km bathymetric separation of the two areas is most strongly expressed in the distribution of two species, Nuttallides decorata and Gavelinopsis translucens. The former is dominant at several AC sites but absent at the MC sites, whereas the latter is a common species in MC but absent at the AC sites. Other dominant calcareous species include Ioanella tumidula (AC and MC), Bolivina lowmani (MC), and Uvigerina peregrina (MC). Species richness is highly variable: 1-97 in AC (12 sites) and 0-62 in MC (4 sites). Some epibenthic species (e.g., Spirillina sp.) do not inhabit the seep sediment but are attached to tubeworms (Lamellibrachia). Such species living decimeters above the seafloor may be relatively unaffected by the severe physical/chemical constraints of the cold-seep sedimentary environment (e.g., CH₄ or H₂S profusion and oxygen depletion).

There is an extraordinary abundance of agglutinated Foraminifera (e.g., Hyperammina, Trochammina) at several bacterial mat sites; they may constitute >90% of the total benthic foraminiferal assemblage. At one such MC site, the shell walls of Hyperammina are made of authigenic barite microrosettes cemented by iron oxide. The formation of barite in association with hydrocarbon seepage has been reported from Mississippi Canyon by earlier workers, but from shallower depths (~640 m).

The work was supported by Minerals Management Service (Cooperative Agreement No. 1435-01-99-CA-30951, Task Order 18182) and NOAA National Undersea Research Program (Grant NURC/UNCW 2000-26).