D/H ratios of fatty acids from marine particulate organic matter in the California Borderland Basins

We report the molecular and hydrogen-isotopic compositions of fatty acids extracted from suspended particulate organic matter (POM) and surface sediments at three stations off the southern California coast: Santa Barbara Basin, Santa Monica Basin, and the Gulf of Santa Catalina. Values of δD for individual fatty acids ranged from −237‰ to −73‰ in POM and from −216‰ to −124‰ in sediments. For most fatty acids, there are no significant differences in δD between sampling locations, with depth at each location, or between POM and sediments. Two fatty acids of likely bacterial origin (i-15:0, 15:0) are strongly D enriched in all samples, while a third (cyc-17:0) is not. The origins of that enrichment are uncertain, and could reflect either an anomalous D/H fractionation in certain marine bacteria, or a significant terrestrial source for those fatty acids, or both. In surface POM and sediments, even carbon numbered fatty acids become slightly D enriched as chain length increases. This isotopic ordering is similar to that observed in living organisms, and is presumably biosynthetic in origin. In contrast, all POM samples from below the mixed layer show a consistent pattern of D depletion with increasing chain length. The order of D enrichment in these fatty acids is well correlated with their solubility, and may be caused by fractionations accompanying dissolution or degradation by microbes.