Isotopic tracing and source apportionment of organic matter in mangrove sediments of a subtropical semi-enclosed bay.

The Kaozhouyang Bay is a typical semi-enclosed shallow bay in South China, where understanding the distribution trend and organic matter sources in mangrove sediments is essential for elucidating regional biogeochemical processes. In this study, the surface sediment samples were collected from 16 stations across the mangrove zone and this bay in April 2024. The analysis encompassed the assessment of total organic carbon (TOC), total nitrogen (TN), stable isotopes (δ13C, δ15N), and sediment grain size composition within the mangrove sedimentary environment. TOC content ranged from 0.05% to 0.61% (mean 0.28%), TN varied between 0.02% and 0.13% (mean 0.07%), δ13C values ranged from -25.89‰ to -20.71‰ (mean -22.41‰), and δ15N values spanned from 2.09‰ to 7.70‰ (mean 4.85‰). Spatially, TOC and TN showed similar distribution patterns, whereas δ13C and δ15N exhibited more heterogeneous distributions. Sediments were dominated by fine silt, followed by clay, coarse silt, and sand. Fine-grained particles (clay and fine silt) were identified as important carriers of sedimentary organic matter. Based on the indicators of C/N, δ13C, and δ15N, the organic matter in surface sediments was derived primarily from aquatic organisms, soil, and sewage, with aquatic organisms representing the largest source. These findings enhance our understanding of the origin and burial of sedimentary organic matter in subtropical semi-enclosed bays, with implications for regional carbon and nitrogen cycling.