ABSTRACT

Seepage of oil and gas occurring from faults and fractures that intersect the modern sea floor is pervasive on the north western Gulf of Mexico upper continental slope. Previous studies have focused on the near-surface expressions of hydrocarbon seepage which manifest themselves as oil-stained sediments, carbonate buildups, gas hydrates, and chemo-synthetic-based ecosystems. Here we present results of a recent study which demonstrate the usefulness of dissolved inorganic carbon (DIC) and ¹³C variables for tracing the effects of hydrocarbon seepage on the carbon chemistry of the water column. Our strategy is to contrast water column profiles overlying hydrocarbon seeps on the upper slope (Green Canyon blocks 53 and 143) with water column profiles from the lower slope (western Gulf) where hydrocarbon seeps were absent. Sea water samples (n = 12) from the lower slope spanning a water column of 6424 ft (1958 m) yield mean DIC and ¹³C values of 2.57 mmole/l and 0.63 PDB, and their profiles show little variability around the mean values with increasing water depth. By contrast, the water column above seeps at water depths ranging from 712 to 1093 ft (217 to 333 m) shows anomalously negative ¹³C values of up to -4.5 PDB, and increased levels of DIC of up to 3.03 mmole/l. The observed DIC enrichments coupled with the ¹³C-depletions are attributed to the release of CO₂ from microbial oxidation of crude oil in the seep environment, and its migration into the water column. This contention is supported by submersible observations of bacterial mats (Beggiatoa sp.) blanketing extensively biodegraded oil pools at the sampling sites. The ¹³C composition of the migrating CO₂ is estimated to be -27.1 on the basis of dissolved carbon inventory. Manifestation of DIC and ¹³C anomalies in the water column caused by hydrocarbon seeping into the near-surface sediments holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory.