Molecular carbon isotopic fractionation of algal lipids during decomposition in natural oxic and anoxic seawaters

To evaluate molecular carbon isotopic fractionation of algal lipids during oxic and anoxic degradation, Emiliania huxleyi (a marine haptophyte) was incubated in seawater collected from two depths (30 and 930 m) in the Cariaco Basin. Three classes of algal lipids (alkenones, fatty acids, and sterols) showed a degradation pattern characterized by complete loss in oxic seawater within 2-3 months (the only exception was 16:0 fatty acid), but 10-40% of initial algal lipids remained in the anoxic seawater after 3 months incubation. Oxic degradation rate constants of alkenones and fatty acids were generally 2-3 times higher than those derived from anoxic incubations. However, two sterols had similar degradation rate constants in oxic and anoxic seawater. There was little preferential degradation of 37:3 relative to 37:2 alkenone in oxic and anoxic seawater, leading to insignificant changes for U-37(k') (paleotemperature indicator) in spite of alkenone degradation. During oxic and anoxic degradation, the same three classes of algal lipids exhibited different patterns of molecular carbon isotopic fractionation: depletion (-4parts per thousand to -6parts per thousand relative to initial value) in C-13 for alkenones, enrichment (+2parts per thousand to +7parts per thousand relative to initial value) for fatty acids, and no change for sterols. We postulate that the contrasting molecular isotopic fractionation patterns, which depend on the structure of the lipid compounds, are likely caused by different degradation reactions at specific functional groups, where the carbon atoms may have dissimilar isotopic ratios from other carbon atoms in the molecules due to the differences in original precursors and synthesis pathways. Laboratory observation of these patterns of lipid fractionations during decomposition of a single phytoplankton material suggests that diagenetic processes can alter the molecular isotopic signals of preserved organic matter in a variety of ways. (C) 2004 Elsevier Ltd. All rights reserved.