ABSTRACT: Pore-Fluid Chemistry Reveal Processes Occurring In Hydrocarbon Seeps From Deepwater Gulf Of Mexico

U, BAOSHUN, and PAUL AHARON

Hydrocarbon seeps are common over the upper bathyal depth range of the northern Gulf of Mexico. These seeps are associated with active deposition of carbonates, and host abundant chemosynthetic biota. Although extensive investigations of the seeps were conducted over the past decade, the processes linking degradation of the hydrocarbons with the chemical carbonates and the chemosynthetic fauna are poorly understood.

Sediment cores, about 50 cm in length, were obtained from thiotrophic Beggiatoa mats (TBM) and methanotrophic mussel beds (MMB) during recent submersible dives. Distribution of pore-fluid constituents including SO₄, H₂S, DIC (Dissolved Inorganic Carbon), alkalinity, Ca, Mg, Sr, and delta¹³C of DIC were determined in order to shed light on the processes involving carbon transfer in seeps.

The inverse relation observed between dissolved SO₄ and H₂S indicates that microbial degradation of hydrocarbons during sulfate reduction plays a dominant control on the pore-fluid chemistry. The delta¹³C values of SigmaCO₂ in TMB cores, ranging from -16 to -28 o/oo (PDB), indicate that carbon is derived primarily from crude oils through sulfate reduction. In contrast, sulfate exhaustion coupled with observed enrichment of ¹³C in DIC (+2 to -16 o/oo PDB) in MMB cores suggest that DIC is derived there from both microbial sulfate reduction and fermentation. Thermodynamic and stoichiometric estimates indicate that authigenic carbonate precipitation and active consumption of carbon by chemosynthetic biota are the two major sinks for the hydrocarbon-derived carbon in the seeps.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana