Stepwise, earthquake-driven coastal subsidence in the Ganges–Brahmaputra Delta (Sundarbans) since the eighth century deduced from submerged in situ kiln and mangrove remnants

This study reconstructs the coastal subsidence over the past 1300 years in a mangrove region along the coast of the Ganges–Brahmaputra Delta, an area not affected by anthropogenic interference. The relative sea level (RSL) history is based on radiocarbon and luminescence ages measured on 108 submerged kilns and in situ mangrove stumps. While the regional, long-term average subsidence rate is calculated to be 2.7 ± 0.3 mm/yr, modern RSL (including eustacy, isostasy, ocean level, and subsidence) rises by 8.7 ± 0.4 mm/yr. This rate has been balanced by natural sediment accretion so far. A reduction in sediment supply by engineering projects along rivers and coasts may, however, accelerate coastal inundation and retreat. Subsidence has accelerated during specific episodes since the eighth century. A major land-sinking event happened in the eighteenth century, with a downward displacement of 1–2 m, depending on geographic area. We propose that the subduction-related 1762 Arakan earthquake caused this sudden lowering. Prior to this event, RSL was nearly stable for 900 years. An earlier major subsidence event occurred around 900 CE, when the land suddenly sank by about 1 m, which also coincided with a major earthquake along the Arakan coast. Event-driven, sudden, significant subsidence, thus, needs to be considered a potential major hazard for coastal Bangladesh.