The concentrations of total organic carbon (TOC) and total nitrogen (TN) and their isotopic compositions (δ¹³C_TOC and δ¹⁵N_TN) were determined in surface sediments (0−3 cm) and sediment cores (Y3 and Y5) collected in the eastern Guangdong coast (EGDC) during spring, summer and winter cruises in 2021, in order to determine the distribution and source of sedimentary organic matter. Spatially, TOC and δ¹³C_TOC distributions in the EGDC have contrasting patterns; TOC concentrations decreased and δ¹³C_TOC values increased with distance offshore. Higher TOC concentrations and enriched δ¹³C_TOC were observed in the marine aquaculture area off EGDC. The distributions of TN and δ¹⁵N_TN were consistent with those of TOC and δ¹³C_TOC, respectively. The measured values of δ¹³C_TOC, δ¹⁵N_TN and the C/N ratio constrained the two dominant sources of organic matter, marine and terrestrial material, in the EGDC. Using a two end−member mixing model, we calculated the proportion of marine organic matter to be 75±19% in spring, 75±15% in summer and 76±11% in winter, respectively. This suggests in situ production of marine phytoplankton dominated the organic carbon pool (average=~75%) in the EGDC sediment, with no statistically significant seasonal variability. These consistent marine organic matter contributions are also observed in sediment cores, with proportions averaging 61±5% in core Y3 and 68±3% in core Y5. The observed variations in biogeochemical proxies in sediments are thus mainly caused by differing relative contributions of marine and terrestrial organic matter. Additionally, high TOC inventory in the marine aquaculture area indicates increased carbon storage and carbon fixation capacity compared to the other areas investigated. Finally, this study helps to establish a background carbon database in the coastal environment for use in future studies of blue carbon in order to achieve carbon neutrality in China.
 

Terrestrial organic carbon, δ13C, C/N ratio, Marine sediment, eastern Guangdong coast