To elucidate the geochemical behavior of labile organic matter (OM) in sediments under the pressures of intensified coastal eutrophication, we examined the fatty acid abundance and their carbon stable isotope values of a sediment core from the shelf of the northern South China Sea (NSCS), Across this sediment core, relatively low total organic carbon content of around 0.60% was observed. Fatty acids with less carbon numbers exhibited exponential decreases downward with degradation rate constants ranging from 0.10 to 0.17 y^-1, no matter they were from single sources (14:0 presumably from marine OM and ai-15:0, i-15:0, 15:0 from bacterial OM) or mixed sources (16:0). Meanwhile fatty acids with more carbon numbers, either 18:0 and 24:0 from mixed sources, or 26:0 and 28:0 presumably from terrestrial input, were less varied in concentrations with depth. This demonstrates a preferential decomposition of labile fatty acids during the early diagenesis of coastal sediment organic matter. A decrease of labile fatty acids δ¹³C values was observed with the decomposition, from which kinetic isotope fractionations were predicted ranging from 0.7 to 1.5‰. Using the closed-system KIE model, the isotope fractionation by decomposition has been quantitatively assessed, and applied to constrain the mixing model with two end-members, marine and terrestrial organic matter respectively. The model results indicate that a maximum difference of up to 22% can be observed in the relative abundance of fatty acids from marine phytoplankton. This finding together with the low organic carbon content and rapid removal of fatty acids in the top sediment, demonstrates the poor preservation of labile organic matter on the shelf of NSCS.

fatty acid, compound specific isotope analysis, decomposition, coastal carbon sequestration, South China Sea