THE TAPHONOMIC SIGNATURE OF BIVALVES INHABITING HYDROCARBON SEEPS

Areas of the sea floor that experience seepage of hydrocarbons (methane gas, liquid petroleum, etc.) support high-density faunal assemblages compared to surrounding seafloor not affected by seeps. These chemosynthetically based ecosystems are not uncommon on modern offshore shelves and are likely well represented in the fossil record. The recognition of ancient seep assemblages is improving, but understanding the history of formation and duration of seepage requires a better understanding of seep death assemblages. This study compares the taphonomic signature of mollusks from grab samples from active Gulf of Mexico hydrocarbon seeps vs. shells deployed at the same seeps for twelve years. The samples were also compared to other non-seep, deep-water localities in both the Gulf of Mexico and the edge of the Bahamas Bank. Taphonomic criteria used in the comparison include dissolution, biont cover, pyrite staining/crusts, presence of grayed areas of the shell carbonate, and presence of borings and etchings on the shells.

Results of the comparison of the taphonomic signature of natural seep bivalves (primarily the mussel Bathymodiolus and the lucinid bivalve Lucinoma with some Thyasira bivalves) to lucinids and mussels experimentally deployed elsewhere show that the signature of the seep clams is different. Comparison of the natural seep clam taphonomic signature to the signature acquired over 12 years by clams deployed in mesh bags at the sites reveals differences in intensity of the signature, but similar taphonomic criteria are recognizable in both samples that separate them from non-seep sites. Previous work has shown that some bivalves retrieved from these same Gulf of Mexico seeps are quite old (up to 12,000 years; Aharon, et al., 1997, GSA Bulletin 109:568-579). The difference in signature between our experimental shells that have been decomposing on site for only 12 years may simply be that the signature is slow to fully develop. Therefore, our results indicate that shells from hydrocarbon seeps can be recognized based on the following criteria: moderate dissolution of shell carbonate, the presence of pyrite crusts and stains, sparse accumulation of sclerobionts (especially serpulid tubes), and gray stains; dissolution is enhanced by tube worms, when present.