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 relationships 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 extract fraction from sediments by reaction with Raney nickel were dominated by phytane with a lesser amount of 5α(H) and 5β(H) C27, C28 and C29 steranes. The steroidal hydrocarbons released were compared with extractable steroidal alcohols from the same depth. Wide and systematic variations were discovered in the range of isotopic compositions of extracted steroidal alcohols and their sulfide-bound analogs with the same carbon skeleton. The isotopic data offer no clear delineation of the product/precursor relationship but do demonstrate that concentration alone does not determine the extent to which a steroidal alcohol is sulfurized. Compound-specific isotope analyses of extractable 24-ethylcholesterols and S-bound 24-ethylcholestane suggest that C29 steroidal alcohols of terrestrial origin are more susceptible to sulfurization than those that are derived from marine organisms. This investigation of the early diagenetic sulfurization process shows that the amount of steranes released by Raney nickel from the SBB sediments accounts for between 0.2 and 3.0% of the available 4-desmethyl steroidal alcohol precursor pool with a similar carbon skeleton over the first 12 cm; also that the C27 steroidal alcohols were consistently more reactive to sulfurization than the C28 steroidal alcohol pool. The 4-methyl steroidal alcohols were unreactive toward sulfurization. The preservation of steroidal biomarkers by the sulfurization process, as revealed by Raney nickel treatment of the polar total lipid extract, indicates that sulfur incoporation does occur to a minor extent at this very early stage of diagenesis and is quite variable among different steroidal alcohol groups.