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

根据淹没的历史窑炉和相关的红树林树桩地平线估算孟加拉国西南部地区差异性的沿海沉降

• 批准号: 289426694
• 负责人: Professor Dr. Till Hanebuth, Ph.D.
• 金额: --
• 依托单位: Geographisches Institut
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2017-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/289426694?language=en
• 关键词: Estimation; regionally; differentiated; coastal; subsidence

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.

恒河-布拉马普特拉河三角洲位于印度东北部板块边缘，由于附近的俯冲带，其沉降速度约为1 mm/a。这种长期的地壳下沉创造了一个约100米厚的容纳空间，自埃米亚海平面高点以来，由全新世时期的细粒沉积物填充。这些年轻沉积物的压实作用极大地导致了高沉降率，再加上海平面的上升，导致三角洲越来越容易受到风暴洪水的侵袭。该项目的目的是确定过去500年来孙德尔本斯海岸不同地点的局部压实率，这是一个未受人类活动影响的森林地区。在之前的一个试点项目中，我们调查了中部三角洲海岸Katka的制盐窑田，在小潮期间暴露，用地质和考古方法获得了它们的垂直位置、运行年龄和功能。这些长方形的窑炉位于冬季涨潮高度以上10-15厘米处，因此代表了独特的古海平面指标。自18世纪使用以来，窑面以平均5毫米/年的速度下降了1.5米。最近，在Katka以西1、17和25公里的三个新地点发现了类似的窑址。值得注意的是，其中一些窑代表了一种新的类型，它们具有圆形的形状，与广泛的红树林树桩地平线有关，位于以前已有300年历史的矩形窑下方约1.5米处。这个非常低的水位只有在冬季极端的春季小潮时才能到达。有明确的证据表明，由于持续的海岸侵蚀，这些基础将在不久的将来消失。本项目将测量新发现的矩形窑，特别是圆形、较低的窑场的垂直位置。最后一次烧制的时间将由窑壁的发光定年来确定。此外，还将分析窑底和残桩层木炭层的14C年龄。这些数据可以用来重建60公里长的沿海地区的区域差异沉降率，大概是在过去的500年里。结果可能会证实海岸下沉的形式是类似事件的向下步进，而不是连续的速率。在下一次风暴摧毁它们之前，新的圆形窑的功能将被考古学记录下来。不受人类活动影响的区域的长期历史沉降率将与SAR /GPS地面监测测量的短期沉降率进行比较，以揭示地质因素和人为因素。如果该项目导致中央三角洲沿岸不同地点的高沉降率和突然下沉动力学的确认，将对沿海地区的保护和管理产生重大影响。