CONSTANT GROWTH RATE OF MEKONG DELTA SHORELINES REVEALED BY OSL DATING OF MULTIPLE DELTA PLAINS

The sustainability of deltaic shorelines requires the sufficient sediment supply that is equal to or more than the sediment removal by tides and waves. Increasing human activities have altered the sediment budget of the deltaic shorelines and often caused problems represented by coastal erosion. Long-term changes in delta geomorphology, based on geological records, improve the understanding of the delta systems and are useful for clarifying the current state of delta. The Mekong River delta, southern Vietnam, is a mixed-energy, complicated system with multiple delta plain shorelines that share the sediment supply from the river. We reconstructed the long-term shoreline changes of the Mekong delta over the last 2500 years based on the architecture and optically-stimulated luminescence (OSL) dating of beach ridges on three delta plains in Ben Tre (North, Central, and South Ben Tre), and a delta plain in Tra Vinh to consider the sediment flux and its contribution to the growth of individual delta plains. While each delta plain shows temporal changes in growth rate, the sum of these four plains is nearly constant. This implies that the net sediment supply to the shorelines is constant, and that the geomorphological changes at distributary branches cause changes in sediment supply to each shoreline compartment. The distributary between North and Central Ben Tre was choked sometime between 500 and 1000 years ago, causing the rapid and stagnant shoreline progradation in North and Central Ben Tre, respectively. The shoreline of Central Ben Tre however has prograded slowly even after the choking, suggesting the beach sand can be supplied beyond the distributary or onshore from the delta front.