Coastal sediments are key reservoirs of buried marine organic carbon. The burial efficiency of sedimentary organic carbon is largely regulated by microbial degradation, transformation, and resynthesis. Owing to differences in bioavailability, labile organic carbon (LOC) undergoes different extent of microbial transformation, thereby complicate the long-term preservation of organic carbon in coastal sedimens. Therefore, elucidating the sources of LOC and the extent of microbial transformation to OC with varying lability is fundamental to understanding coastal carbon cycling and carbon sequestration. In this study, two sediment cores (F101 and M301) representing contrasting depositional environments of Pearl River estuary were investigated. Based on total organic carbon (TOC), C/N ratios, fatty acid concentrations and δ¹³C values, a fatty acid lability model integrating isotopic mass balance and a genetic algorithm was developed to quantitatively apportioned the source composition of sedimentary LOC and bacterial transformation processes. Model results showed that phytoplankton-derived organic matter was the dominant source of LOC, contributing 19.3%–78.2%, followed by bacterially resynthesized organic matter (14.1%–59.9%), whereas terrestrial C3 and C4 plants accounted for relatively smaller proportions (3.7%–41.5%). The offshore station M301, which received less terrestrial input, exhibited a higher contribution of bacterial-derived organic carbon, suggesting that bacteria preferentially utilized more labile phytoplankton-derived organic matter and subsequently reshaped the composition and isotopic signatures of sedimentary organic carbon through degradation and resynthesis. This finding indicates that the lability model is an effective approach for quantifying microbial transformation of source-specific labile organic carbon, offering new insights into the mechanisms governing organic carbon burial in coastal sediments.
Keywords: Pearl River Estuary; labile organic carbon; fatty acid lability model; bacterial transformation; compound-specific isotope analysis of fatty acids; organic carbon burial

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