Luminescence dating of beach ridges for characterizing multi-decadal to centennial deltaic shoreline changes during Late Holocene, Mekong River delta

Abstract

Empirical understanding of decadal- to centennial-scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries. We tested the effectiveness of quartz optically-stimulated luminescence (OSL) dating of beach ridges to constrain shoreline changes of the Mekong River delta over the last 3500years. Forty-seven OSL ages have been analyzed from the Tra Vinh delta plain, central Mekong River delta. The Tra Vinh beach ridges are recurved and branching, showing the hierarchy of shoreline changes, which include discontinuous shifts c. 5km seawards caused by the emergence and elongation of a delta-front bar/island, and subsequent downdrift accretion of spits. The spit accretion is interrupted by changes of shoreline orientation. Each of the discontinuous shifts resulted in a cluster of beach ridges, which is referred to as ridgeset. The beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c. 5%. The OSL chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges. The chronology clearly illustrates the coastal progradation from 3500years (relative to AD 2010) onwards. Two main types of beach ridge are present: recurved and trunk ridges. OSL ages of recurved ridges are consistently younger both downdrift and seawards, documenting decadal- to centennial-scale shoreline migration especially over the last 1500years. Trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves. Ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset, representing the extensive erosion and resedimentation of the updrift coast. Major changes in shoreline orientation occurred around 400–500years, suggesting strengthening of the northeasterly winter monsoon at the beginning of the Little Ice Age. The rate of progradation of the Tra Vinh delta plain is described in two ways: frequency of the discontinuous shift, and growth rate of the delta plain area. Both suggest the coast has expanded regularly over the last 1000years. A decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred, possibly affecting the behavior of the modern and future shorelines, which can be compared with the less human-influenced, centennial- to decadal-scale past shoreline changes reconstructed in this study.