| Summary: | Stable carbon isotope ratios were determined on a number of organisms and their nonsaponifiable lipid fraction and the total organic carbon and the chloroform-soluble lipid fraction of cores from the Gulf of Mexico, Cariaco Trench, and Ross Sea. The carbon isotope compositions of the nonsaponifiable lipid fraction of plankton from the Gulf of Mexico are temperature dependent between 17 and 23°C. Muscle tissue of nekton is isotopically heavier than the whole plankton samples, while whole livers are quite similar to the plankton. Carbon isotope values, in general, reflect, an organism's habitat and food source, although there is a suggestion that the carnivores fractionate carbon isotopes in the nonsaponifiable lipid fraction of their diet. Triggerfish, as a family, have unique carbon isotope values and this appears to be related to their physiology. In open marine sediments in the Gulf of Mexico the ¹³C content of the total organic carbon decreases with depth whereas that of the lipid fraction is relatively constant. Therefore, the difference in δ¹³C between the total and lipid organic carbon (Δδ¹³C) decreases with depth. The Δδ¹³C for anoxic sediments is small and constant, suggesting that the decrease observed in the Gulf of Mexico sediments is due to post-depositional changes occurring after anoxic conditions are encountered in sedimentary columns. Carbon isotope composition and gas chromatographic analyses of saturated components of plankton indicate contamination of the plankton with petroleum residues. The potential of carbon isotope measurements as a means of monitoring Man's activities in the marine environment was demonstrated in the sediments surrounding a platform which experienced a major blowout. Gradients were observed out and down from surface sediments in the carbon isotope composition of total organic matter, the Δδ¹³C and the amount of total organic carbon and extractable material. The results suggest that the gradients are due to redeposited cuttings and petroleum residues. Carbon isotope measurements can be used to determine the source of organic matter in the sediments.
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