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Large amounts of organic carbon (OC) and inorganic carbon (IC) are stored in coastal sediments. These carbon compounds play an important role in coping with global climate change. In this study, the surface sediments of four tidal flat sections in northern Jiangsu were analysed for their OC and IC contents, as well as changes in the physical and chemical properties of the sediments (particle size, pH, salinity (SAL), and N, P, and S contents). A partial least squares regression (PLSR) was conducted to explore the factors affecting OC and IC variations. According to the results, the OC and IC contents exhibited increasing trends when moving from the seaward to landward parts of the sections. The OC variations were large and IC variations were small in all four sections and the vegetation coverage contributes to the carbon content of tidal flat sediments. The PLSR analysis indicates that the main factors influencing OC and IC contents in the tidal flat system were particle size (sand, silt, and clay) and N content, while SAL had the least influence on OC and IC contents. The IC and OC contents are positively coupled, which may indicate that the nearshore vegetation has a greater contribution to OC, affects the coastal hydrodynamic conditions, and enhances the retention of IC. Additionally, the transformation of OC to IC also positively contributed to the coupling of their contents.