Hydrogen-isotopic variability in lipids from Santa Barbara Basin sediments

We conducted an extensive survey of hydrogen-isotopic compositions (D/H ratios) of diverse sedimentary lipids from the Santa Barbara Basin (SBB), offshore southern California. The main goal of this survey was to assess the diversity of D/H ratios in lipids from marine sediments, in order to provide a more detailed understanding of relevant biological and geochemical factors impacting lipid isotopic variability. A total of 1182 individual dD values are reported from two stations in SBB, one located in the suboxic basin depocenter and the other on the fully oxic flank of the basin. Sediments collected from the basin depocenter span a depth of 2.5 m and reach the methanogenic zone. Lipids that were analyzed include n-alkanes, n-alkanols and alkenols, shortand long-chain fatty acids, linear isoprenoids, steroids, and hopanoids, and exhibit several systematic patterns. First, there are no significant differences in dD values between the two sampling locations, nor with increasing depth for most lipids, indicating that degradation does not influence sedimentary lipid dD values. Second, relatively large differences in dD values among differing molecular structures are observed in all samples. n-Alkyl lipids of probable marine origin have typical dD values between 150 and 200&, those from terrestrial leaf waxes and aquatic plants range from 80 to 170&, while petroleum n-alkanes are typically 90 to 150&. Third, lipids inferred to derive from bacteria (branched fatty acids and hopanols) living at the sediment surface or in the water column tend to be D-enriched relative to similar algal products by 30& or more. At the same time, several other lipids have dD values that decrease strongly with depth, presumably as a result of in situ production by anaerobic bacteria. This dichotomy in isotopic compositions of bacterial lipids is inconsistent with a nearly constant D/H fractionation during lipid biosynthesis, and likely reflects significant variations associated with metabolism. 2009 Elsevier Ltd. All rights reserved.