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

Abstract

Chemoautotrophic clam-dominated assemblages are commonly associated with petroleum seepage on the continental slope of the Gulf of Mexico. We examine the persistence and resilence of these communities by evaluating downcore trends in abundance, biomass, and trophodynamics in communities from four separate petroleum seep sites on the Louisiana continental slope. Some petroleum seep sites retained optimal habitat for some species continuously over geologically-relevant periods of time. More commonly, however, habitat optimality varied substantially over time scales of hundreds of years. Thus, one important characteristic of these sites was the degree of persistence of the chemoautotrophic biota. A fauna typically was persistent over a time span of a few hundred years, but was typically not persistent over a longer time span. The mechanisms producing local extinction remain unclear, however temporal variations in juvenile survivorship seem to be substantially larger than temporal variations in larval settlement, to the extent that the heavily 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 slope biota and the juveniles of chemoautotrophic species that failed to survive to adulthood. Thus, the only faunal transitions were between specific chemoautotrophic faunas and the non-chemoautotrophic fauna. Not one distinctive faunal transition between two chemoautotrophic faunas was observed. Accordingly, each discrete chemoautotrophic fauna was resilient over long time scales; time scales of geological importance.