The continuous decline in global riverine sediment supply has become a defining driver of human-induced coastal morphodynamic. Intriguingly, despite severe reductions in fluvial sediment inputs, many deltas exhibit remarkable resilience and self-maintenance. In the Changjiang Delta. This ongoing accretion is largely sustained by sediment reworked from the Changjiang Subaqueous Delta (CSD). This study focuses on the evolutionary mechanisms of the CSD, unravelling how it maintains regional deltaic continuity through self-cannibalization.
Using 137Cs and 210Pb radionuclide geochronology, we identified a cascading erosion process across the CSD initiated around 2003. This severe erosion induced a systematic discrepancy between 137Cs- and 210Pb-derived accumulation rates, shifting 210Pb profiles from steady-state accumulation to non-steady-state patterns. By tracking the relative depth of the 1963 137Cs time-marker horizon, we quantitatively reconstructed historical erosion rates. The results reveal that as the spatial footprint of erosion expanded, local erosion rates gradually decelerated—pointing to an intrinsic self-organizing mechanism within the CSD system.
Long-term surface grain-size time series show a continuous shrinkage of the CSD mud patch, highly consistent with the spatiotemporal pattern of cascading erosion and signifying selective winnowing of fine-grained particles. Concurrently, relict sands from the mid-to-outer shelf are actively encroaching landward onto the subaqueous delta. Multiple sediment cores reveal a distinct "sand-over-mud" stratigraphic architecture, a process driven by a compound set of stressors, including relative sea-level rise, intensified storm-wave energy, and severe sediment starvation. These morphodynamic and sedimentological shifts carry profound implications for coastal engineering safety, benthic habitats, and regional carbon sequestration.
Reconstructions of sediment accumulation rates and organic carbon (OC) burial fluxes over the past 8 ka demonstrate that both parameters intensified in tandem over the last two millennia. However, following the impoundment of the Three Gorges Dam, the system transitioned into net erosion, causing a noticeable contraction of the regional OC reservoir. Furthermore, widespread seabed coarsening impairs mineral-organic protection, potentially exacerbating OC loss via enhanced remineralization. Conversely, the newly formed superficial sand cap may also physically shield underlying muds from further resuspension. Fully resolving these competing net ecological and biogeochemical impacts demands further interdisciplinary investigation.

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