Evaluation of Abandoned Huanghe Delta as an Important Carbon Source for the Chinese Marginal Seas in Recent Decades

Increasing organic carbon (OC) inputs from coastal erosion contribute to large uncertainties in carbon cycling in coastal seas. The abandoned Huanghe Delta (AHD) is a typical coastal erosion region, which transports large amounts of eroded sediments to the Chinese marginal seas (CMS, not including the South China Sea). Here we investigated the organic geochemical characteristics of sedimentary organic matter (OM) from the heavily eroded region of the AHD and estimated the OC and lignin fluxes caused by coastal erosion. The eroded sedimentary OM was characterized by low carbon loading and low lignin content with high degradation and was influenced by the Changjiang. Owing to the sharp decline in the sediment fluxes from large rivers (the Changjiang and Huanghe) since the 1990s, the total fluxes to the CMS decreased from 11.5 to 5.0 Mt/yr for OC and from 11.5 × 10⁻² to 4.7 × 10⁻² Mt/yr for lignin; therefore, the contribution from the coastal erosion of the AHD increased from 24% ± 8% to 56% ± 10% for OC and from 21% ± 9% to 52% ± 15% for lignin. The high proportion contributed by the AHD to the Central Yellow Sea mud area (74% ± 11%) also accounted for the high terrestrial OC burial efficiency there (69%). Considering the different nature of OM and its increasing contribution, as in this case, the significance of OC input from coastal erosion to the coastal seas is expected to overwhelm that from rivers, thereby influencing regional and even global carbon budgets and cycles, especially under the scenarios of global warming and sea level rise.