Dramatic Decline in Organic Carbon Burial in Big Estuary Driven by Dam Construction: Insights From the Yellow River Estuary

Abstract Estuaries and associated shelf deposits, which bury 70%–90% of global marine organic carbon, making them key sites for marine carbon burial. Variations in total organic carbon (TOC) content and the organic carbon burial rate (OCBR) in these sediments strongly influence carbon sequestration dynamics. Using the Yellow River Estuary as a representative case, this study assessed the impact of watershed damming on estuarine organic carbon burial based on 2023 field data and records from 1984 to 2023. Following the construction of the Xiaolangdi Reservoir, sediment TOC and OCBR in the Yellow River Estuary decreased by 46% (from 0.55% ± 0.08%–0.30% ± 0.12%) and 58% (from 155.0 ± 22.0 to 66.0 ± 25.0 g m −2 yr −1 ), respectively. This change was primarily attributed to a nearly 90% decline in sediment flux to the sea following reservoir construction, accompanied by significant sediment coarsening. Similar reservoir‐induced declines in estuarine carbon sinks have been widely observed in big estuaries such as the Mekong, Mississippi, and Nile rivers. Extrapolating from the empirical relationship between sediment flux and OCBR in the Yellow River Estuary, global damming is estimated to have reduced estuarine OCBR by approximately 33.6%, equivalent to an annual loss of about 23.5 Tg C in organic carbon burial. Collectively, sediment depletion and grain‐size coarsening are identified as major drivers of estuarine OCBR decline. These findings underscore the need to incorporate evolving estuarine carbon burial “hotspots” into global carbon cycle models.