Natural vs. anthropogenic sources of hydrocarbons as revealed through biomarker analysis: A case study in the southern Gulf of Mexico

Biological markers are organic compounds in geological samples with an unambiguous link to specific precursor molecules in living organisms. They provide information on the origin and depositional environment of fossil organic matter as well as about its thermal maturation caused by geothermal heat flow during burial. Petroleum and its refinery products carry this biomarker information into the environment when they are released during anthropogenically-induced pollution. Soils or aquatic surface sediments in contaminated areas usually contain binary mixtures of fossil hydrocarbons and recent biogenic hydrocarbons like higher-plant wax esters, together with (often olefinic) hydrocarbons from earliest diagenetic transformation of functionalized biomolecules. Surface sediments collected in the shelf area of the Campeche Sound, Gulf of Mexico, sampled in the course of environmental monitoring for possible petroleum pollution due to industrial activity, however, revealed a third group of hydrocarbons. GC-MS analysis of biomarkers in the nonaromatic hydrocarbon fractions of the sediment extracts yielded overlapping hydrocarbon assemblages indicating multiple sources. Samples taken close to known asphalt seeps exhibit biomarker patterns virtually identical to those of reference crude oils. Other sediments contain mature fossil hydrocarbons and biomarkers that are not typical of either fossil fuels or immature organic matter in marine surface sediments. Instead, they come from drill cuttings recovered during penetration of Tertiary to Cretaceous deposits that were littered on the shallow sea bottom.