, Volume 12, Issue 1, pp 33-40

Dissolved carbon and 33-133-133-1anomalies in the water column caused by hydrocarbon seeps on the northwestern Gulf of Mexico slope

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Abstract

Alkalinity, dissolved inorganic carbon (DIC), and δ13C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative δ13C values of up to −4.5‰ PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63‰, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the13C-depletions are attributed to the release of CO2 by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The δ13C composition of the migrating CO2 is estimated to be −26.0‰ on the basis of dissolved carbon inventory. Manifestation of DIC and δ13C anomalies in the water column caused by hydrocarbon seepage 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.