ABSTRACT

Geohazard and geochemical survey data consisting of high-resolution profiles, side-scan sonographs, drop cores, dredge samples, and borings have substantiated the consistent association between carbonate buildups and hydrocarbon seeps on the Louisiana continental slope.

Analyses of lithified bottom samples indicate a range of carbonate mineralogies, including aragonite, Mg-calcite, and dolomite, that are extremely depleted in the ¹³C isotope (¹³C values to -53.9 PDB). Microbial oxidation of methane (biogenic and thermogenic) and crude oil creates a source of pore water CO₂ containing isotopically light carbon that helps trigger carbonate precipitation. Geophysical and geochemical evidence suggests that both surface and subsurface lithification is taking place. Recent observations and samples collected using a Pisces II research submersible confirm the abundance of ¹³C-depleted sedimentary carbonates and massive authigenic buildups associated with the tops and flanks of shallow salt diapirs and gas hydrate hills. Although chemosynthetic communities (including tube worms and bivalves) with isotopically light carbon in their tissues have been described from gas seeps, bacterial mats sampled from several seep areas using a submersible have ¹³C values of -28 to -31 PDB, suggesting a crude oil contribution to microbial biomass. Lithoherms 50 ft (15 m) high are common to dome crests. These features dominate mesoscale seafloor topography on the slope and have important short-term impacts on platform locations as well as pipeline routing. They are of long-term importance as sites for low-sea-level reefs. Moreover, these observations provide new insight into the earliest stages of salt-dome cap rock evolution.