Coastal evolution, environmental change and carbon storage in the Thung Prong Thong Mangrove, eastern Gulf of Thailand

Abstract

Palaeoenvironmental records along the eastern coast of Thailand remain sparse, with only a few studies attempting to reconstruct past climatic and environmental conditions. However, additional palaeoenvironmental, palaeoclimatic and sea-level records are needed to improve our understanding of coastal evolution and local environmental changes. This study investigates mangrove sediment from the Prasae Estuary Mangrove (PEM) complex in Rayong Province, eastern Thailand. The PEM complex is an estuarine fringing mangrove influenced by the mixed tidal-fluvial dynamics of the Prasae River mouth. The region's low-relief geomorphology includes intertidal mudflats, wetlands and relict sand deposits associated with relatively high sea levels during the Late Holocene. To reconstruct sea-level changes, beach ridge deposits from the area were dated using optically stimulated luminescence. The results indicate a progressive seaward-younging pattern of the beach ridges, implying shoreline progradation of at least 2 km over the past 1240±40 years in response to a gradual marine regression of less than 2 m during the Late Holocene. The formation of these beach ridges created favourable conditions for the mangrove forest to colonize the Thung Prong Thong area between 690 and 210 years ago. Geochemical analyses of mangrove sediments, including elemental composition (C, N) and δ¹³C values, indicate a predominance of terrestrial organic matter input, primarily from freshwater discharge and mangrove leaf litter. The accumulation of substantial amounts of organic carbon in the soil highlights the role of mangroves as important carbon sinks. Over the past 38 years, land-use changes, particularly the conversion of mangrove areas into shrimp farms, have led to an estimated carbon release of approximately 14 800 Mg C (mega gram carbon = 1000 kg) to the atmosphere. However, conservation efforts have at the same time contributed to the expansion of mangroves and increased carbon sequestration along the PEM coast, resulting in a net increase in carbon stock accumulation of ~17 500 to ~22 800 Mg C. Given the substantial impact of land-use changes on carbon stocks, this study underscores the importance of sustainable land management and long-term monitoring of coastal mangrove ecosystems.