Source characteristics and quantitative estimates of organic carbon composition in the intertidal zones of Jiaozhou Bay, China.

The Jiaozhou Bay (JZB) intertidal zone is a significant carbon reservoir that plays a crucial role in transporting and accumulating organic matter; however, quantitative studies of organic matter sources are scarce. In this study, we present bulk parameters of total organic carbon (TOC), TOC/TN, δ¹³C, and biomarker contents in 36 surface sediment samples from the JZB intertidal zones to quantify the contribution of organic carbon (OC) derived from terrestrial/marine sources, such as C₃ plants, C₄ plants, estuarine productivity, sewage outlets, and marine productivity. The results demonstrated that a two-end-member model based on the traditional indicators of TOC/TN or δ¹³C is not appropriate for quantifying the OC source. The presence of C₃ plants, C₄ plants, and sewage outlets in the JZB intertidal zone could lead to errors in determining OC contribution when solely using TOC/TN or δ¹³C. A classical mixing diagram (three-end-member model) utilizing TOC/TN and δ¹³C values revealed that OC contribution was dominated by marine productivity throughout the intertidal zone. In the west, the average OC contribution from marine productivity, estuarine productivity, and C₄ plants was 73.8 %, 14.2 %, and 12.0 %, respectively. In the east, the average OC contribution from marine productivity, estuarine productivity, and sewage outlets was 57.6 %, 24.9 %, and 17.4 %, respectively. The higher OC contribution from marine productivity in the west was attributed to the occurrence of Spartina alterniflora, while the OC contribution from estuarine productivity in the east was primarily due to the presence of more rivers flowing into the JZB compared to the west. By combining biomarkers and OC contents, a significant positive relationship verified the suitability of the end-member values selected for the three-end-member mixing model in the west and east intertidal zones of JZB. This finding was further supported by principal component analysis (PCA) analyses of these proxies. This study demonstrated that OC sources in intertidal zones varied among contrasting coastal environmental conditions and addressed the knowledge gap regarding biogeochemical cycles and ecological protection in the JZB intertidal zones.