CARBON-ISOTOPE RELATIONSHIPS BETWEEN SULFIDE-BOUND STEROIDS AND PROPOSED FUNCTIONALIZED LIPID PRECURSORS IN SEDIMENTS FROM THE SANTA-BARBARA BASIN, CALIFORNIA

In this study, molecular and isotopic analyses were used to evaluate r elationships between sulfide-bound compounds and their proposed lipid precursors in surficial sediments from the Santa Barbara Basin (SBB), California. The hydrocarbons released from the polar total lipid extra ct fraction from sediments by reaction with Raney nickel were dominate d by phytane with a lesser amount of 5 alpha(H) and 5 beta(H) C-27, C- 28 and C-29 steranes. The steroidal hydrocarbons released were compare d with extractable steroidal alcohols from the same depth. Wide and sy stematic variations were discovered in the range of isotopic compositi ons of extracted steroidal alcohols and their sulfide-bound analogs wi th the same carbon skeleton. The isotopic data offer no clear delineat ion of the product/precursor relationship but do demonstrate that conc entration alone does not determine the extent to which a steroidal alc ohol is sulfurized. Compound-specific isotope analyses of extractable 24-ethylcholesterols and S-bound 24-ethylcholestane suggest that C-29 steroidal alcohols of terrestrial origin are more susceptible to sulfu rization than those that are derived from marine organisms. This inves tigation of the early diagenetic sulfurization process shows that the amount of steranes released by Raney nickel from the SBB sediments acc ounts for between 0.2 and 3.0% of the available 4-desmethyl steroidal alcohol precursor pool with a similar carbon skeleton over the first 1 2 cm; also that the C-27 steroidal alcohols were consistently more rea ctive to sulfurization than the C-28 steroidal alcohol pool. The 4-met hyl steroidal alcohols were unreactive toward sulfurization. The prese rvation of steroidal biomarkers by the sulfurization process, as revea led by Raney nickel treatment of the polar total lipid extract, indica tes that sulfur incoporation does occur to a minor extent at this very early stage of diagenesis and is quite variable among different stero idal alcohol groups. (C) 1997 Elsevier Science Ltd.