Assessing the prehistoric shoreline changes of the Mekong delta, Vietnam

Abstract:

The environment and sustainability of deltaic lowlands have been threatened by combined impacts of climate changes, human activities and land subsidence. The Mekong delta has also been disturbed by the construction of upstream river dams and fluvial dredging, and we have a limited knowledge of its morphodynamics response to fluctuations in Asian monsoons that governs the coastal sediment dynamics. In order to understand the recent shoreline changes of the Mekong delta, we attempted to reconstruct the prehistoric changes based on landform records and their chronology. Thirteen optically-stimulated luminescence (OSL) ages of chenier, a record of past shoreline, were obtained from the Soc Trang coastal plain, southwestern part of the delta. The OSL ages are younger than 2770 yr BP, and become younger seawards concordantly with the coastal progradation that cheniers document. The progradation rate has changed drastically; it was 8 m/yr, 31 m/yr, and 5 m/yr during periods of 2770–1370 yr BP, 1370–590 yr BP, and 590 yr BP to present, respectively. The rapid progradation during 1370–590 yr BP did not form cheniers, resulting in an extensive inter-ridge swale. Cheniers generally form in relations to erosion of muddy coast. The chenier distribution and drastic changes in progradation rate were resulted from fluctuations in relative importance of longshore sediment removal to seaward sediment accretion. Beach ridges in the Tra Vinh coastal plain, northeast of the Soc Trang, also indicate similar and synchronous fluctuations in the longshore sediment transport. The dominance of the longshore sediment transport after 590 yr BP is thought caused by the strengthened winter monsoon that drives the southwestward longshore current, which is possibly related to the beginning of the Little Ice Age. Many cheniers occur along the present shoreline in relation to the low progradation rate, and possibly some sporadic erosion after 590 yr BP. Some evidences have suggested that the winter monsoon has weakened in the South China Sea after the Little Ice Age, and thus human-induced decrease of sediment supply in recent decades would have hindered the coastal progradation or even caused erosion.