Multi-Proxy Constraints on the Sources and Spatial Variations of Organic Matter in Surface Sediments from Lingdingyang, Pearl River Estuary: Evidence from Stable Isotopes and GDGTs

To elucidate the sources and spatial variations in organic matter in surface sediments from Lingdingyang of the Pearl River Estuary, 18 surface sediment samples were collected and analyzed for obtaining total organic carbon (TOC), total nitrogen (TN), atomic TOC/TN ratio (C/Natom), stable carbon and nitrogen isotopes (δ13C, δ15N), and glycerol dialkyl glycerol tetraethers (GDGTs). A three-endmember framework was constructed using the BIT and δ13C to constrain the sources of the organic matter. The results showed a significant positive correlation between TOC and TN, with relatively higher values in Jiaoyi Bay and western Lingdingyang, lower values in eastern Lingdingyang, and intermediate values in Shenzhen Bay. The C/Natom, δ13C, and δ15N results revealed that the sedimentary organic matter in the study area exhibits mixed-source characteristics, influenced by soil, C3 plants, and marine autochthonous organic matter. Among the subregions, Jiaoyi Bay is more strongly influenced by terrestrial inputs, while Shenzhen Bay receives relatively higher contributions from marine autochthonous organic matter. The GDGTs results showed that Jiaoyi Bay is characterized by elevated abundances of both brGDGTs and isoGDGTs, whereas isoGDGTs were also relatively enriched in Shenzhen Bay. brGDGTs exhibited a significant negative correlation with δ13C, whereas BIT showed no significant correlation with either brGDGTs or δ13C, indicating that BIT cannot be simply regarded as a unique proxy for soil input, but rather reflects the combined effects of in situ production, changes in archaeal lipids, and sedimentary preservation. The three-endmember model further revealed significant spatial variations in the sources of organic matter in surface sediments from Lingdingyang. Overall, the combined use of multiple proxies is more effective than any single proxy in revealing the sources and spatial differentiation of sedimentary organic matter in this subtropical, complex estuarine environment.