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

Abstract

Stable carbon isotopic compositions of individual n-fatty acids and n-alkanes were determined in six sediments from the Ohtsuchi River to the Pacific Ocean at north Honshu Island, Japan. Long-chain n-fatty acids (LCFAs) ranging from C20 to C30 and long-chain n-alkanes (LCALs) ranging from C23 to C33 have large isotopic variations from -35 to -25‰ and -35 to 29‰, respectively, although the molecular distributions of LCFAs and LCALs are almost identical in all analyzed samples. LCFAs are more depleted in 13C than total organic carbon (TOC) by about 5 to 12‰. The δ13C values are relatively similar between n-C20 and n-C26, and gradually decrease with increasing molecular weight to n-C30 by 3 to 5‰. Each n-fatty acid component shows a systematic enrichment in 13C from river to open ocean by up to 6‰ (from -32.5 to -26‰ for n-C26), and a similar isotopic composition in the open ocean (~-26‰ for n-C26). LCALs are also more depleted in 13C than TOC by about 6 to 12‰. The δ13C values gradually decrease from n-C19 to n-C31 by up to 6‰, then increase for >n-C31. Abundant LCALs, such as n-C29 and n-C31, show a systematic enrichment in 13C from river to open ocean by up to 3‰ (from -34.6 to -31.9‰ for n-C31), and a similar isotopic composition in the open ocean (~-31.5‰ for n-C31). On the other hand, C35 n-alkane has a relatively uniform isotopic composition (~-29‰) for all sediments. Such isotopic variations exhibit good correlations with δ13C(TOC) (-26.4 to -20.4‰) and C/N ratio (12 to 7 by atom) variations as well as amount of sedimentary cutin- and lignin-derived organic compounds of terrestrial higher plant origin. The isotopic distributions can be explained by a two-component mixture model involving isotopically different terrestrial and marine LCFAs and LCALs as endmember components. Although LCFAs (>n-C20) and LCALs (>n-C23) in marine sediments have been previously presumed to be derived from terrestrial higher plants, the results presented here may indicate that some of the LCFAs and LCALs in marine sediments are actually originated in the marine environment. Compound-specific isotope signature is important for evaluating the sources, as well as transport and mixing processes for the LCFAs and LCALs in a terrestrial-marine system.