Spatial and Temporal Distribution and Compositional Characteristics of Organic Carbon in the Intertidal Zone of the Yangtze River Estuary

In this study, soil, high-tide beach and low-tide bare beach sediments were collected from land to sea at six stations in the Yangtze River estuary region in December, 2022 and June, 2023. All samples were used to determine the total organic carbon (TOC) content, stable carbon isotope ratios (δ¹³C), n-alkanes (n-alkanes) content, black carbon (BC) and polycyclic aromatic hydrocarbons (PAHs). The results show that the TOC content (0.08%~5.20%) decreased gradually in the coastal direction. While the stable carbon isotope ratios gradually increased, suggesting that TOC were derived from terrestrial sources and increasing contribution from marine sources. The n-alkane content ranged 0.49~56.44 μg·g^-1, with an average content of (9.41±11.61) μg·g^-1. The n-alkanes content was higher in the flood season than dry one, and the spatial variation trend was similar to that of TOC. n-alkane molecular ratios indicated that the contribution of terrestrial higher plants was dominant in the intertidal surface sediments. The BC content ranged 0.32~17.12 mg·g^-1, with an average content of (2.42±2.35) mg·g^-1. BC content decreased gradually from soil to low-tide bare beach sediments. char/soot ratio indicated that the BC source was contributed by a mixture of biomass burning and fossil fuel burning. Alternating tidal hydrodynamic processes were the main factors influencing the spatial distribution of BC. PAHs were higher in the dry season than those in the flood season, and this pattern is different from BC. Since PAHs are more susceptible to degradation by light and plants in the flood season. PAHs were predominantly coal/biomass combustion, mixed with some petroleum combustion sources according to diagnostic ratios. BC and PAHs have a better correlation in the low-tide bare beach area due to continuous hydrodynamic processes. The adsorption of PAHs by the BC subcomponent soot was an important factor. The distribution and burial of organic carbon in the intertidal zone of the Yangtze River Estuary are affected by various factors, including hydrodynamic disturbances, source differences, plant growth and anthropogenic activities. This study provides a reference for understanding the source, composition and distribution of organic carbon in the surface sediments of the intertidal zone of the Yangtze River Estuary.