Millennial-scale carbon accumulation and dynamics in mangroves: A case study from the Upper Gulf of Thailand

Mangroves are among the most carbon-rich tropical ecosystems; however, the long-term dynamics of their carbon storage in Thailand remain poorly understood. This study integrates palaeoecological records, sedimentological variables, and soil carbon to evaluate the drivers that influence millennial-scale carbon accumulation in two estuarine mangrove ecosystems from two case study sites (Bang Khun Thian and Klong Kone) in the upper Gulf of Thailand. Across the study sites, total soil carbon (Soil C) storage, determined from accumulating the organic carbon density throughout each sediment profile, ranged from approximately 3300 to 3970 Mg C ha^-1. The carbon accumulation rates (CAR) varied substantially, ranging from 185 to 1256 g C m^-2 yr^-1 over the last millennium. The results showed that long-term soil carbon storage was not controlled by a single factor but by interactions among dry bulk density, CAR, sediment texture, mangrove occurrence, and past environmental conditions. Soil C was more closely linked to mangrove presence and fine-grained sediment retention, whereas CAR was more sensitive to hydrological connectivity. Palaeoecological records indicate that estuarine geomorphology was strongly influenced by sea-level change, sediment inputs from both allochthonous and autochthonous sources, and mangrove vegetation cover. During periods of rising sea levels and wet climatic conditions, mangrove expansion promoted high sediment accretion and carbon burial, whereas periods of marine regression led to mangrove retreat. Reduced tidal influence and land-use changes have lowered carbon input and storage. These patterns reflect the combined influences of sediment supply, hydrological connectivity, and vegetation change. Gathering data from deeper soil layers and conducting site-specific factor analyses are crucial for a more precise and comprehensive evaluation of coastal carbon storage. Therefore, long-term data are critical for designing climate adaptation policies that safeguard mangrove persistence and maximise the role of blue carbon in climate change mitigation.