The C15+ hydrocarbon geochemistry of sediments from Sites 618 and 619 (Orca and Pigmy basins) has been examined as part of a biogeochemical study of microbial activity and biological markers in Leg 96 sediments. Total hydrocarbon concentrations (13-78 µg/g dry weight) are elevated in comparison to those reported for Recent sediments from the Gulf of Mexico continental shelf, but there is little evidence that these sediments contain petroleum hydrocarbons originating from seepage. The presence of weathered fossil hydrocarbons is suggested, however, by the unresolved envelope feature observed in gas chromatograms of most sediments and also by the distribution of C27-C29 steranes and triterpanes. Normal alkane distributions are consistent with a predominant input or preservation of terrestrial organic matter in these offshore basins. At Site 619 an enhanced terrestrial contribution below 113 m sub-bottom is indicated by an increase in the ratio of odd to even n-alkanes over the range C26 to C30. Pristane/phytane ratios at both sites range between 0.97 and 1.74 which, in conjunction with the occurrence of significant concentrations of perylene (10-500 ng/g dry weight), suggest the prevalence of reducing depositional conditions over the time period covered by the cores. A hydrocarbon tentatively identified as a C25 multibranched compound similar to those found in Recent sediments was detected in most samples.

Supplement to: Requejo, A G; Whelan, Jean K; Boehm, Paul D (1986): Hydrocarbon geochemistry and biological markers in Orca and Pigmy Basin sediments (Sites 618 and 619). In: Bouma, AH; Coleman, JM; Meyer, AW; et al. (eds.), Initial Reports of the Deep Sea Drilling Project, Washington (U.S. Govt. Printing Office), 96, 785-793