Long-term history of chemoautotrophic clam-dominated faunas of petroleum seeps in the northwestern Gulf of Mexico

Chemoautotrophic clam-dominated assemblages are commonly associated with pe troleum seepage on the continental slope of the Gulf of Mexico. We examine the persistence and resilience of these communities by evaluating downcore trends in abundance, biomass, and trophodynamics in communities from four s eparate petroleum seep sites on the Louisiana continental slope. Some petro leum seep sites retained optimal habitat for some species continuously over geologically-relevant periods of time. More commonly, however, habitat opt imality varied substantially over time scales of hundreds of years. Thus, o ne important characteristic of these sites was the degree of persistence of the chemoautotrophic biota. A fauna typically was persistent over a time s pan of a few hundred years, but was typically not persistent over a longer time span. The mechanisms producing local extinction remain unclear, howeve r temporal variations in juvenile survivorship seem to be substantially lar ger than temporal variations in larval settlement, to the extent that the h eavily taphonomically-biased record of juvenile individuals permits such a conclusion, When local extinctions occurred in the chemoautotrophic biota, the biota was replaced by a normal slope biota or a mixture of a normal slo pe biota and the juveniles of chemoautotrophic species that failed to survi ve to adulthood. Thus, the only faunal transitions were between specific ch emoautotrophic faunas and the non-chemoautotrophic fauna. Not one distincti ve faunal transition between two chemoautotrophic faunas was observed. Acco rdingly, each discrete chemoautotrophic fauna was resilient over long time scales; time scales of geological importance.