Tropical and subtropical rivers deliver large quantities of terrestrial organic carbon to the ocean, acting as a crucial part of the global carbon cycle, but little is known about the timescale and efficiency of their transport to and in the adjacent coastal sea. Here, we examined source-specific biomarker (normal fatty acids (n-FAs)) contents and isotope compositions in surface sediments in an alongshore transect southwestward from the Pearl River mouth. The C~28+30~ rather than other long-chain n-FAs were found to be the most representative for OC~terr~, and a plant wax mean residence time (MRT) of 3060 ± 90 yr was estimated in the Pearl River watershed from the ¹⁴C age of C~28+30~ n-FA in the river mouth sample. A four-source mixing model demonstrated that the mean burial efficiency of fossil and soil OC is 85% and 49%, respectively, indicating the refractory nature of fossil OC but a significant loss of soil OC due to remineralization during transport in the marine environment before final burial. Thus, the remineralization of soil OC in the tropical coastal sea may be an important CO₂ source.