495 / 2024-09-18 08:26:03

Cascading Erosion in the Tide-Dominated Changjiang Delta and its Distal Mud Disclosed by Repeating Radionuclide Measurements

Delta erosion, Subaqueous delta, distal mud, Radionuclide data, time lag

Shoreline retreat and nearshore erosion have been well gauged and studied when river deltas confront sharp sediment reduction. However, delta erosion may extend from the proximal delta front (DF) far offshore to the pro-delta (PD) and downstream towards the distal mud (DM) where precise morphodynamic changes remain less known due to shortage of monitoring data, but they are crucial for understanding the destination of riverine sediment and organic carbon in river-dominate margins. In this contribution, the delta regime shift from accretion into erosion in response to sharp sediment decline after the closure of Three-Gorges Dam in 2003 is investigated using time-series analysis of ²¹⁰Pb_ex and ¹³⁷Cs data in the Changjiang subaqueous delta (CSD) and the northern DM. Before 2003, a relatively steady accumulation state was inferred by the overwhelmed predominance of ideal steady-state accumulation (SA) style of ²¹⁰Pb_ex depth-profiles and the highly concordant sediment accumulation rates (SARs) derived from ²¹⁰Pb_ex and ¹³⁷Cs data over the whole study area. After 2003, the cascading erosion initiated at the shallow DF and gradually spread to the PD and then to the northern DM as shown by increasing occurrences of non-steady accumulation (NSA) and non-recent accumulation (NRA) styles of ²¹⁰Pb_ex profiles and enhancing discrepancies between ²¹⁰Pb_ex and ¹³⁷Cs derived SARs. The ²¹⁰Pb_ex method is no longer applicable to date recent strata influenced by severe erosion. Hence, the method employing the 1963 ¹³⁷Cs peak as the chronostratigraphic marker to compare different aged core data over a small sampling area is developed to scale both accretion and erosion magnitudes. The results show erosion rates decrease gradually from the DF, through to the PD and towards the MD depocenters with certain lag time between each two neighboring subzones because of hysteretic and resilient effects in the tide-dominated delta. The newly developed method for scaling erosion magnitudes is potentially applicable to other tidal mega-delta systems with abundant historical ¹³⁷Cs data.