Estimation of particulate organic carbon export to the ocean from lateral degradations of tropical peatland coasts

Abstract. Peatlands serve as long-term carbon sinks and are distributed across subarctic, Arctic, and tropical regions. However, in tropical and permafrost-dominated coastal areas, coastal erosion and peat mass movement events (PMMs) have emerged as major contributors to peatland degradation. These processes are thought to drive territorial loss and facilitate the export of particulate organic carbon (POC) to marine environments in the form of peaty debris. This study quantifies POC export and assesses the extent of peatland degradation driven by coastal erosion and PMMs in tropical coastal peatlands. Between 2017 and 2018, PMMs impacted approximately 0.68 km2 of land in the northern part of Bengkalis Island, Indonesia, with collapse volumes ranging from 491 to 85 173 m3. Notably, a PMM event on 27 December 2014 resulted in an elevation loss of approximately 2 m, primarily triggered by flooding associated with the failure of a peat weir following 192 mm of rainfall over four days. An analysis of coastline changes from 2018 to 2021 revealed that erosion rates varied by land cover type. Oil palm plantations experienced erosion rates of 3.5 m per 30 d, exceeding those observed in mangrove areas and peat swamp forests. The highest recorded rate – 24.8 m per 30 d – occurred during periods of elevated wind speeds and intense wave activity, highlighting the role of seasonal climatic drivers in accelerating peatland degradation. Estimated specific annual POC export ranged from 6.46 to 24.03 ktC yr−1 due to coastal erosion and 5.07–7.02 ktC yr−1 from PMMs – values approximately 295 to 1089 times greater than typical riverine POC export in tropical wet regions. These findings reveal a previously underrecognized carbon export pathway from tropical peatland coasts to the ocean and suggest that coastal peatland degradation may be a significant yet overlooked component of the marine carbon budget.