Shifts in riverine POC sources reduce terrestrial OC burial in the Changjiang subaqueous delta

Abstract. Global riverine particulate organic carbon (POC) fluxes have declined worldwide due to dam-induced reductions in sediment load. However, how the composition of riverine POC has evolved in response to declining sediment supply, and how such shifts influence OC burial in subaqueous deltaic systems remain unclear. Here, we collected suspended particulate matter (SPM) from the Changjiang Estuary in summer and winter 2025 to analyze N / C molar ratios and δ13C values. These data were integrated with a four-decade (1980–2021) dataset comprising OC proxies (N / C molar ratios, δ13C and Δ14C) for SPM in the Changjiang Estuary and surface sediments from the Changjiang subaqueous delta. Our results reveal a temporal increase in N / C molar ratios and a decrease in δ13C values in riverine SPM. Based on a Bayesian end-member mixing model, we attribute these trends to an increasing proportion of POC derived from freshwater algae and a decreasing proportion of POC originating from soil/bedrock erosion. This temporal increase in river-delivered labile algae POC drove a decrease in OC preservation efficiency in the Changjiang subaqueous delta from 15.1 % (before 2003) to 10.7 % (after 2003), resulting in a pronounced reduction in deltaic OC burial. Consequently, the amount of OC retained in sediments decreased by approximately 50 %, from 68 kt per month in 2001 to an average of 34 kt per month during the flood seasons of 2011–2020. Our findings emphasize that the shifts in riverine OC sources, not merely the decline in total OC flux, may exert great effects on OC burial in deltaic systems globally.