Distinct patterns and drivers of total and refractory terrestrial organic carbon burial in river-dominated continental margins: From climatic control to anthropogenic perturbations

Abstract

The burial of terrestrial organic carbon (OC_terr) in marine sediments is a major geological CO₂ sink. The largest sink for OC_terr burial in present-day oceans lies in river-dominated continental margins, which are key components of the global carbon cycle. Climate change and anthropogenic forcing have disrupted the cycling of OC_terr at the land–ocean interface, impacting its storage at these margins. Despite this recognition, the fate of OC_terr in marine sediments and the potential feedback mechanisms of these changes remain unclear, primarily because of the lack of reliable sedimentary records from 1850 to the present that capture the substantial internal spatial and temporal heterogeneity. Here, we present a high-resolution OC_terr record covering the past two centuries from a well-preserved sediment core in the mud depocenter of the East China Sea, using sedimentological, mineralogical, and geochemical techniques. By integrating our new results with literature data, we quantitatively evaluate the modern patterns and drivers of changes in OC_terr burial. During the climate-controlled period (pre-1950s), despite high precipitation and sediment influx driven by the East Asian summer monsoon, the observed lower and fluctuating OC_terr loadings suggest that the OC_terr accumulation rate was not directly proportional to the lithogenic sediment flux. Analyses of X-radiographs and sediment composition indicate that energetic marine processes—such as tides, waves, and typhoon events that are intensified by the East Asian winter monsoon—promoted sediment resuspension, dispersion, and redeposition. These dynamic physical processes regulate hydrodynamic particle sorting, resulting in the high variability and suppression of OC_terr burial during this interval. In contrast, the post-1950s era, marked by intensive human impact, saw a persistent decline in the overall OC_terr burial. This trend is primarily attributed to the reduced delivery of fine mineral sediments and altered sediment sources to marine environments mainly due to large-scale dam construction. Although the preservation of sedimentary OC_terr has diminished, our study provides new evidence of increased mobilization and burial of refractory OC_terr in marine sediments. Using a chemical oxidation method, we quantified the proportions of refractory organic carbon (OC) and found that, on average, 60 ± 11 % of the refractory OC exported by the Yangtze River is delivered to the adjacent continental margin. Coinciding with periods of intensive human impact (post-1950s), anthropogenic disturbances, notably inputs of fossil-derived OC, have reintroduced greater quantities of unreactive OC_terr into coastal oceans. This shift contributed to an increase of ∼ 20 % in the accumulation rate of refractory OC_terr. The contrasting responses of the total and refractory OC_terr highlight a disruption in the balance between oxidation and preservation processes within marine sediments. Such imbalances may have a substantial, yet previously unrecognized, influence on the capacity of modern continental margin sediments globally to sequester OC_terr.