Bestimmung der regional differenzierten Küstensubsidenz im südwestlichen Bangladesch anhand von versunkenen historischen Salzsiede-Öfen und Mangrovestubben
Zusammenfassung der Projektergebnisse
The densely populated, low-lying outer Ganges-Brahmaputra Delta is severely threatened by a rising global sea level, while the land surface subsides at highly and locally variable rates at the same. We reconstructed the subsidence history for the coastal Sundarbans, the largest mangrove forest of the world, in the northern Bay of Bengal/Bangladesh, as an expression of relative sea level changes. This exceptional area did not yet become modified by human activity, which excludes effects on the subsidence rate related to anthropogenic land-use activities. Proxies for the positions of the former sea level were a) the foundations of 80 submerged salt-producing kilns, specifically using their characteristic basal charcoal layer; and b) the roots of 60 in-situ preserved stumps of certain mangrove species. The originally submerged kilns and buried mangrove stumps – recently exposed by cyclone impacts and coastal erosion – were analyzed along a 25-km wide and 60-km long coastal stretch regarding their absolute elevation and age. Based on 35 AMS-14C ages of these basal charcoal layers and in-situ mangrove stumps as well as 30 OSL dates of fritted kiln chamber walls, relative sea-level changes were reconstructed for the past 1,300 years. As a result, the subsidence rate has varied considerably along the coastal area of Sundarbans over this historic time interval, both temporally as well as geographically. The latest period of accelerated subsidence started in the middle of the 18th century at the same elevation level across the entire study area from 1.5 m below present sea level to the modern level. For this event, the common initial elevation and moment in time produces a similar subsidence rate of about 5 mm/year for all sites. We propose that the Arakan subduction earthquake in 1762 CE has triggered degassing and dewatering effects of the thick Holocene depositional sequence having led to an abrupt, areawide coeval subsidence event. This event terminated the salt production in the study area, whereas this artisanal industry continued to exist in the western Indian part of the delta, according to historic commercial reports by the British rolers. Before this particular subsidence event, relative sea level has risen by a fairly low rate for about 600 years. A preceding, well-documented subsidence event with a vertical jump of almost 1 m occurred at around 850 CE, which might coincided with an earlier earthquake along the Arakan coast (805 CE) as well. The observed locally differentiated subsidence history might be rooted in geographically limited variations in the heterogeneous composition of the underlying, up to 100-m thick Holocene depositional sequence.
Projektbezogene Publikationen (Auswahl)
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(2019). Rapid coastal subsidence in the central Ganges-Brahmaputra Delta since the 17th century deduced from submerged salt-producing kiln. BAPA-BEN Conference: Delta Plan 2100 and Sustainable Development of Bangladesh, January 11-12, 2019, Dhaka
T.J.J. Hanebuth, H.R. Kudrass, J. Linstädter, B. Islam, A.M. Zander and S.H. Akhter
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(2020). Field guide for the excursion to the Sundarban mangrove forest. In cooperation with the ICEEST, Dhaka, 2020, 10 pp
H.R. Kudrass and S.H. Akhter
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(2020). High and different rates of subsidence along the coastal Sundarban since the last 1000 years: Results from dated kiln and in-situ mangrove stems. “International Conference on Earth and Environmental Sciences & Technology for Sustainable Development”, ICEEST conference, January 25-27, 2020, Dhaka
H.R. Kudrass, T.J.J. Hanebuth, A.M. Zander and S.H. Akhter
