Sundarbans 2016 - Subsidence estimates for southwestern Bangladesh

Estimation of the regionally differentiated coastal subsidence for southwestern Bangladesh from submerged historical kilns and associated mangrove stump horizons

The Ganges-Brahmaputra Delta is located at the margin of the northeastern Indian Plate which subsides at ca. 1 mm/a due to the nearby located subduction zone. This long-term crustal sinking created a ca. 100-m thick accommodation space since the Eemian sea-level highstand, filled by fine-grained deposits during Holocene times. Compaction of these young deposits contributes significantly to the high subsidence rate which, in combination with a rising sea level, leads to an increasing vulnerability of the delta to storm flooding. The aim of this project is to determine the local compaction rates at different locations along the Sundarbans coast for the past ca. 500 years, a forested area not affected by human activities.

In a previous pilot project, we investigated the salt-producing kiln field in Katka at the central delta`s coast, exposed during neap tide, with geological and archeological methods to obtain their vertical position, age of operation, and functionality. These rectangular kilns were placed 10-15 cm above the winter high tide level and represent, thus, unique paleo-sea level indicators. Since being used in the 18th century, the kiln level subsided by 1.5 m at an average rate of 5 mm/a. Very recently, comparable kiln places were discovered at three new sites 1, 17, and 25 km to the west of Katka. Worth to note, some of these kilns represent a new type which has a rounded shaping and is associated with a widespread mangrove stump horizon and positioned ca. 1.5 m below the formerly dated, 300-years old, rectangular kilns. This very low level is exclusively accessible during extreme spring neap tide in winter. There is clear evidence that these foundations will be lost in the near future due to ongoing coastal erosion.

The vertical position of the newly discovered rectangular kilns and the round-shaped, lower kiln fields in particular will be measured in this project. The timing of last firing will be determined by luminescence dating of the kiln walls. In addition, the 14C age of the charcoal layers present at the kilns` base and of the stump horizons will be analyzed. These data allow for reconstructing regionally differentiated subsidence rates along a 60-km long coastal section, presumably for the past ca. 500 years. The results will probably corroborate a coastal sinking in the form of event-like downward stepping rather than at a continuous rate. The functionality of the new rounded kilns will be documented archeologically, before the next storms destroy them.

The long-term historical subsidence rates from a region, not affected by human modification, will be compared to the short-term rates measured by SAR-/GPS surface monitoring to unravel geogenic from anthropogenic components. In case this project leads to the confirmation of high subsidence rates and abrupt sinking dynamics for different locations along the central delta`s coast, substantial implications for coastal zone protection and management would be the consequence.

Collaboration:

MARUM, University Bremen, Germany

Dept of Geography, University of Cologne, Germany

Dept of Geology, Dhaka University, Bangladesh

Dept of Geology and Mining, University of Rajshahi, Bangladesh

Dept of Archaeology, Comilla University, Bangladesh

Bangladesh Water Development Board, Dhaka University, Bangladesh

Institute of Water Modeling, Dhaka, Bangladesh

Sundarban East Forest Division, Bangladesh Forest Dept, Bangladesh