Carbon isotopic compositions of individual long-chain n-fatty acids and n-alkanes in sediments from river to open ocean: Multiple origins for their occurrence

Stable carbon isotopic compositions of individual n-fatty acids and n-alkan es were determined in six sediments from the Ohtsuchi River to the Pacific Ocean at north Honshu Island, Japan. Long-chain n-fatty acids (LCFAs) rangi ng from C-20 to C-30 and long-chain n-alkanes (LCALs) ranging from C-23 to C-33 have large isotopic variations from -35 to -25 parts per thousand and -35 to -29 parts per thousand, respectively, although the molecular distrib utions of LCFAs and LCALs are almost identical in all analyzed samples. LCFAs are more depleted in C-13 than total organic carbon (TOC) by about 5 to 12 parts per thousand. The delta(13)C values are relatively similar betw een n-C-20 and n-C-26, and gradually decrease with increasing molecular wei ght to n-C-30 by 3 to 5 parts per thousand. Each n-fatty acid component sho ws a systematic enrichment in C-13 from river to open ocean by up to 6 part s per thousand (from -32.5 to -26 parts per thousand for n-C-26), and a sim ilar isotopic composition in the open ocean (similar to-26 parts per thousa nd for n-C-26). LCALs are also more depleted in C-13 than TOC by about 6 to 12 parts per thousand. The delta(13)C values gradually decrease from n-C-1 9 to n-C-31 by up to 6 parts per thousand, then increase for >n-C-31. Abund ant LCALs, such as n-C-29 and n-C-31, show a systematic enrichment in C-13 from river to open ocean by up to 3 parts per thousand (from -34.6 to -31.9 parts per thousand for n-C-31), and a similar isotopic composition in the open ocean (similar to-31.5 parts per thousand for n-C-31). On the other ha nd, C-35 n-alkane has a relatively uniform isotopic composition (similar to -29 parts per thousand) for all sediments. Such isotopic variations exhibit good correlations with delta(13)C(TOC) (-26.4 to -20.4 parts per thousand) and C/N ratio (12 to 7 by atom) variations as well as amount of sedimentar y cutin- and lignin-derived organic compounds of terrestrial higher plant o rigin. The isotopic distributions can be explained by a two-component mixture mode l involving isotopically different terrestrial and marine LCFAs and LCALs a s endmember components. Although LCFAs (>n-C-20) and LCALs (>n-C-23) in mar ine sediments have been previously presumed to be derived from terrestrial higher plants, the results presented here may indicate that some of the LCF As and LCALs in marine sediments are actually originated in the marine envi ronment. Compound-specific isotope signature is important for evaluating th e sources, as well as transport and mixing processes for the LCFAs and LCAL s in a terrestrial-marine system.