Sand ridges from the coastal zone of southern Thailand reflect late quaternary sea-level history and environmental conditions in Sundaland

Abstract

At many times during the Quaternary, when the global sea level was much lower than today (at least 30 m), the Thai-Malay Peninsula was connected by land to the Indonesian archipelago. This now submerged region is known as Sundaland and played a key role in the expansion of flora, fauna, and humans during the Quaternary. However, information regarding past environmental conditions is sparse as post-glacial sea-level rise submerged and eroded most sedimentary archives. In this study, a combination of geomorphological mapping, investigating sedimentary features, and optically stimulated luminescence dating is used to decipher the evolution of sand ridges found in the coastal zone of the Nakhon Si Thammarat Province, southern Thailand. The two main ridges (eastern and western) are composed of moderately to poorly sorted medium to coarse, structureless massive sands. The eastern ridge deposited between ca. 8.3 ka and 0.16 ka and formed by coastal processes related to sea-level change and storm surge during the Holocene. Surprisingly, the western ridge yields a minimum depositional age of ca. 82 ka, and further deposition dates to different periods of the Late Pleistocene. We suggest that the initial formation of the western ridge possibly occurred during the Last Interglacial (Marine Isotope Stage 5e). During periods of low global sea level, when the region was far away from the coast, the ridge accumulated further sand, most likely due to aeolian activity. This would imply a rather limited vegetation cover in the interior of Sundaland and support previous assumptions of savannah-like conditions during global-scale glacial periods.