Holocene to anthropocene burial of organic carbon in the Yangtze delta

Holocene organic carbon (OC) burial in mega-deltas is considered to have played a crucial role in modulating long-term atmospheric CO₂ levels, but this role has likely been significantly altered by human activities during the Anthropocene. The absence of precise estimates for Holocene deltaic OC burial rates hinders a comprehensive understanding of carbon cycle evolution. This study, using data from 50 Holocene boreholes and 216 modern surface sediment samples, examines changes in OC sources and their controlling factors, and quantifies OC burial rates in the Yangtze Delta (YD) from the mid-Holocene to the Anthropocene. The results reveal three distinct stages of OC burial evolution. From 8 ka to 2 ka, the weakening East Asian Summer Monsoon reduced terrestrial OC contributions, but the YD maintained slow progradation and stable OC burial rates (∼0.79 Mt/yr). After 2 ka, human activities emerged as the dominant driver, triggering a 78 % increase in OC burial rates (1.40 – 1.44 Mt/yr). Following the impoundment of the Three Gorges Dam, the YD entered an erosion-driven destruction phase, with OC burial rates declining by 59 % compared to pre-dam levels. Accounting for subaqueous delta erosion, the YD has shifted from a net OC burial system to a net OC source, contributing ∼0.81 Mt/yr of OC to the Zhejiang-Fujian mud belt. These findings underscore the pivotal role of sediment burial rates in regulating OC sequestration in mega-deltas and highlight the global implications of human-altered sediment dynamics, suggesting that deltas worldwide may similarly transition from positive and negative OC sequestration.