D.M. Schultz H. Lerch D. Ligon D. Owings C. Gary Raymond E. Miller 1979 <p>Concentration levels of hydrocarbon in 122 composite mid-Atlantic shelf surface sediment samples have been examined during the second year (1976-1977) of the Bureau of Land Management environmental assessment program. The objective of this study was to establish baseline concentrations and provide data for the "natural variability" curve for the mid-Atlantic region.</p><p>Concentration levels (<span>μ</span>g/g) have been determined for the total lipids recovered by liquid column chromatography, and hexane-soluble fraction (aliphatics) benzene fraction (aromatics) as resolved by gas chromatography for each composite sediment sample. Several geochemical parameter values were also determined among which were: 1) n-alkane odd-even ratio; 2) isoprenoid alkane/ n-alkane ratio; 3) pristine/phytane ratio; 4) pristane/n-C₁₇ ratio; and 5) phytane/n-C₁₈ ratio. Summary analyses of the concentrations and geochemical 18 parameters are included in this report.</p><p>Surface sediments examined from the mid-Atlantic shelf contained very low concentrations of resolved aliphatic hydrocarbons, less than 5.0 <span>μ</span>g/gm sediment, and very low concentrations of resolved aromatic hydrocarbons, usually less than 1.0 <span>μ</span>g/gm sediment. Mass spectral analyses of selected hexane eluates confirmed the presence of a series of normal aliphatic alkanes ranging from to n-C₁₆ to n-C₃₁ as well as the isoprenoids pristane and phytane. A series of three hydrocarbon peaks in the n-C₂₀ to n-C₂₁ range were tentatively identified as n-C₂₅unsaturated, branched homologs, The aromatic fractions contained a number of polynuclear aromatics including phenanthrenes, methylphenanthrenes, fluoranthene and pyrcne, benzofluoranthene, and alkyl benzenes.</p><p>Data accumulated in the second year of the program, coupled with data from the first year's investigations (Miller et al., 1977), have provided the basis for establishing a preliminary natural. "variability curve" for the hydrocarbon concentration levels and distribution in these selected mid-Atlantic shelf samples. This curve shows the seasonal variability noted in both the aliphatic and aromatic hydrocarbons in these samples.</p> application/pdf 10.3133/ofr79363 en U.S. Geological Survey Hydrocarbon geochemical analyses of Mid-Atlantic Outer Continental Shelf sediments: an environmental assessment reports