Mid to late Holocene sea-level reconstruction of Southeast Vietnam using beachrock and beach-ridge deposits

Abstract

Beachrocks, beach ridge, washover and backshore deposits along the tectonically stable south-eastern Vietnamese coast document Holocene sea level changes. In combination with data from the final marine flooding phase of the incised Mekong River valley, the sea-level history of South Vietnam could be reconstructed for the last 8000years. Connecting saltmarsh, mangrove and beachrock deposits the record covers the last phase of deglacial sea-level rise from −5 to +1.4m between 8.1 to 6.4ka. The rates of sea-level rise decreased sharply after the rapid early Holocene rise and stabilized at a rate of 4.5mm/year between 8.0 and 6.9ka. Southeast Vietnam beachrocks reveal that the mid-Holocene sea-level highstand slightly above +1.4m was reached between 6.7 and 5.0ka, with a peak value close to +1.5m around 6.0ka. This highstand is further limited by a backshore and beachridge deposit that marks the maximum springtide sea-level just below the base of the overlying beach ridge. After 5.0ka sea level dropped below +1.4m and fell almost linearly at a rate of 0.24mm/year until 0.63ka and +0.2m as evidenced by the youngest beachrocks.The Holocene sea-level fluctuations observed in Southeast Vietnam resulted from eustatic and isostatic processes. The sea-level rise up to the mid-Holocene highstand was provoked by the last melting phase of glacial polar ice-sheets. The sea-level drop after the mid-Holocene highstand was induced by the isostatic processes of continental levering with an uplift of continents in low latitudes and depression of adjacent flooded continental shelf areas and Equatorial Ocean Siphoning transferring oceanic waters from low latitudes to the increasing volume of oceanic basins in higher latitudes. The regional expression in terms of magnitude and timing of relative sea-level change might contribute to validation of geophysical model simulations.