Effects of river damming and delta erosion on organic carbon burial in the Changjiang Estuary and adjacent East China Sea inner shelf

Abstract

River damming reduces sediment load in rivers, leading to instability and erosion of coastal deltas; this is a global issue in many large-river delta-front estuaries (LDEs). The total organic carbon, stable carbon isotope, total nitrogen, lignin phenols in suspended particulate matter, and three sediment cores collected from the Changjiang LDE were analyzed to study the effects of river damming and delta erosion on the redistribution and burial of organic carbon (OC) in the coastal margin. The OC budgets that include the delta erosion process were established to better constrain the burial fluxes of OC in the Changjiang River system before and after river damming. We used a three-end-member mixing model with Monte Carlo simulation and found that the ratio of deltaic OC to riverine OC significantly increased in estuarine SPM and sediment cores in the Changjiang LDE after the operation of the Three Gorges Dam (TGD), reflective of reduced sediment load and enhanced delta erosion. Our budget showed that, compared with the pre-dam period (1953–1985), the burial of terrestrial OC in the Changjiang LDE sediments had been reduced by ~97.3%, on the basis of 72.7% and 82.7% reductions in sediment load and particulate OC flux, respectively. Although trapping of terrestrial OC in reservoirs is the key driver in these reductions, re-exposure and decomposition of OC from eroding delta sediments also contribute significantly to OC losses in coastal sediments. On the contrary, the ratio of marine OC to riverine OC increased in sediment cores after TGD building, due in part to more inputs of marine OC because of serious eutrophication in recent decades. As continued dam construction and sea-level rise change the spatial and temporal dynamics of carbon sequestration in the coastal zones of LDEs, further consideration of alterations in these carbon burial “hotspots” is needed in global carbon models.