Changes in accumulation of land-based organic matter under recent climate change and anthropogenic impact: A tropical coastal perspective

• Molecular records of sediment organic matter (OM) were reconstructed in the GOT. • Significant increase in terrestrial degraded OM was observed since the 1980s. • This OM increase was linked to coastal erosion due to sea level rise and mangrove degradation. • Shifts in regional energy consumption altered occurrences of sediment pyrogenic OM. • Changes in land-based OM records were due to joint effects of climate change and human activities. Tropical continental shelves play an important role in the global carbon cycle especially in the context of increased anthropogenic interference and climate change. However, the long-term fate and response of sedimentary organic carbon (OC) in these regions remain poorly understood. In this study, we analyzed bulk OC and molecular biomarkers in a sediment core collected from the lower Gulf of Thailand, and compiled with several published records of sedimentary organic matter (OM) from other tropical Asian margins. Our results reveal a dramatic ∼ 40 % increase in terrestrial OC inputs since the 1980s, likely driven by the effects of accelerated coastal erosion from rising sea levels and the degradation of mangrove ecosystems. In addition, shifts in regional human activities, including changes in energy consumption patterns, have altered the sources of pyrogenic OM, contributing to the observed spatial and temporal variability of anthropogenic OM across tropical coastal margins. Molecular fingerprints highlight recent changes in the accumulation of land-based OM, showing an increased presence of erosion-derived and degraded OC, along with pyrogenic OM. This shift is linked to the combined effects of coastal retreat and basin-wide emissions, influenced by both natural climate forces and anthropogenic activities.