Carbon cycling in coastal sediments was investigated from the geochemistry of stable carbon, nitrogen and radiocarbon isotopes of various compound class fractions. Stable isotope ratios reflect a mixture of terrestrial and marine inputs in the Santa Monica Basin sediments. In contrast, the isotopic signature of lighter carbon in the Antarctic sediments is characteristic of planktonic contribution. New additional information on sources and rates of particle processing and organic matter preservation is obtained by integrating the stable isotope data with radiocarbon measurements of the same fractions.

Inputs of carbon from wastewaters and submarine seeps in coastal sediment is obvious from the relatively older age observed in the aliphatic, aromatic and fatty acid fractions from the sediments of the Santa Monica Basin. On the other hand, eroded sedimentary sequences containing coal could have imparted an old age to the aliphatic and aromatic fractions in some of the Antarctic sediments whereas the humic and fulvic acid fractions from both the regions exhibit relatively young age in the range of 760 to 1700 years. Protokerogen in the same sediment fractions is much older than the humic and fulvic acids in both the regions. This suggests that protokerogen is more likely derived from the refractory residues of biological organic components which have been washed into the coastal shelf sediments rather than being generated by in situ humification. A major biogenic origin is evident from the younger age of most of the Antarctic sediment organic fractions. This is in contrast to the sediments from the Santa Monica Basin, which receives significant inputs of older carbon, generally from petroleum, sewage and industrial effluents.