Carbon isotopic difference of saturated long-chain n-fatty acids between a terrestrial and a marine sediment

Stable carbon isotopic compositions of individual n-fatty acids ranging from C20 to C30 in a terrestrial (riverine) and a marine (pelagic) sediment were determined using gas chromatography/isotope ratio monitoring mass spectrometry (GC/IRMS). These fatty acids in a terrestrial sediment at the Ohtsuchi river in northern Honshu Island, Japan, showed δ¹³C values (from -35 to -32%o relative to PDB) lighter than that of the total organic carbon (TOC; -26,4‰) by about 6 to 9‰, The δ^l3C values from a marine sediment in the Pacific Ocean ca. 500 km off the coast of Honshu Island varied from -31 to -26‰, which were also lighter than that of the TOC (-20.5‰) by 6 to 9‰. The δ¹³C values of n-fatty acids ranging from C₂₀to C30 in the marine sediment were heavier by about 5 to 6‰ than those at the corresponding carbon numbers in the terrestrial one. Isotopically, the marine n-fatty acids were clearly distinguished from the terrestrial ones. These results suggest that long-chain n-fatty acids in marine sediment were not derived from land higher plant but from marine organisms, although long-chain n-fatty acids (≥C₂₀) in marine sediments have often been used to assess terrestrial material contribution to the ocean.