Latest Pleistocene-Holocene sedimentary evolution from sediment records of the Tam Giang-Cau Hai coastal lagoon system, central Vietnam

The latest Pleistocene-Holocene sedimentary evolution was reconstructed from two boreholes collected in the Tam Giang-Cau Hai coastal lagoon region of central Vietnam. This research was based on analyzing radiocarbon dating, grain size, sedimentary structure, petrological components, and diatom assemblages. Besides, some highresolution seismic data from the lagoon water body were interpreted to estimate Holocene sediment thickness. Sedimentary records of two boreholes revealed various sedimentary facies under the sea-level change since the Last Glacial Maximum (LGM): Fluvial channel facies, fluvial-lagoonal transition facies, lagoon facies, lagoonalto-fluvial facies, and flood plain facies. Radiocarbon dating indicated that the coastal lagoon system possibly initiated around 8.8-8.5 cal kyr BP, corresponding to the meltwater pulse 1 C when saltwater intruded into the lagoon system. The borehole-sediment data combined with the high-resolution seismic data shows the thickness of Holocene sediments is approximately 20-35 m in the lagoon region. The study also suggested that the sandy barrier complex along the coast between the lagoon system and the open sea has been likely built during the transgression since 10-8.8 cal kyr BP. Additionally, five ecological zones were identified based on diatom assemblages in borehole HK02 indicating variations in marine and fluvial influence in the coastal lagoon system during the past 8.8 kyr: Dia-HK02-1 indicates a low presence of diatoms, suggesting a weak marine influence in the coastal lagoon system; Dia-HK02-02 shows the strongest marine influence and stable ecological conditions in the coastal lagoon system; Dia-HK02-03 demonstrates a partial decrease in marine influence; Dia-HK02-4 shows an increasing fluvial influence; and Dia-HK02-5 developed under completely dominated fluvial conditions on land.