The nature, timescale, and efficiency of riverine export of terrestrial organic carbon in the (sub)tropics: Insights at the molecular level from the Pearl River and adjacent coastal sea

Abstract Tropical and subtropical rivers deliver large quantities of terrestrial organic carbon (OCterr) to the ocean, acting as a crucial part of the global carbon cycle, but little is known about the timescale and efficiency of its transport to and in the adjacent coastal sea. Here we examined source-specific biomarker (fatty acids, FAs) contents and isotope compositions in surface sediments in an alongshore transect southwestward from the Pearl River mouth. The C28+30, rather than other long-chain saturated FAs, were found to be the most representative for OCterr, and a plant wax mean age of 3060 ± 90 yr (resulting from protracted storage) was estimated in the Pearl River watershed from the 14C age of C28+30 FA in a river mouth sample. A compilation of plant wax mean ages in global (sub)tropical river systems including this study suggests that regional differences in climate and morphology may have a limited impact on plant wax mean ages in (sub)tropical regions. A four-source mixing model based on bulk OC and biomarker isotope compositions demonstrated that surface sediments in the Pearl River-derived mudbelt consist of 0.15–0.36 wt.% marine OC, 0.03–0.13 wt.% riverine primary production-derived OC, 0.18–0.49 wt.% soil OC, and 0.07–0.16 wt.% fossil OC. 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. Over longer timescales, the OCterr loss experienced during transport may, thus, to some extent reduces the capacity of terrestrial ecosystems (particularly soils) as CO2 sinks.