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

恒河-雅鲁藏布江三角洲位于印度板块东北部的边缘，由于附近的俯冲带，板块以大约1 mm/a的速度下沉。这种长期的地壳下沉自伊米亚海平面高位以来创造了大约100米厚的可容纳空间，在全新世时期被细粒沉积物填满。这些年轻沉积物的压实极大地促进了高沉降率，再加上海平面上升，导致三角洲更容易受到风暴洪水的影响。这个项目的目的是确定过去约500年来孙德尔本斯海岸不同位置的局部压实率，这是一个不受人类活动影响的森林区域。在之前的一个试点项目中，我们调查了三角洲中部海岸卡特卡的制盐窑场，用地质学和考古学的方法获得了它们的垂直位置、运行年龄和功能。这些长方形窑炉位于冬季高潮潮位之上10-15厘米，因此是独特的古海平面指示器。自18世纪使用以来，窑面以平均每年5毫米的速度下降了1.5米。最近，在卡特卡以西1、17和25公里处的三个新遗址发现了类似的窑址。值得注意的是，其中一些窑炉代表了一种新的类型，它具有圆形的形状，与广泛分布的红树林树桩地平线相关，位于旧的300年历史的长方形窑炉下方约1.5米。这一极低的水位只在冬季极端的春潮期间才能到达。有明显的证据表明，这些基础将在不久的将来由于持续的海岸侵蚀而消失。在这个项目中，将特别测量新发现的矩形窑和圆形、下窑场的垂直位置。最后一次烧制的时间将由窑壁的发光测年确定。此外，还分析了窑底炭层和残渣层的~(14)C年龄。这些数据可以重建60公里长的沿海地区的区域差异沉降率，大概是过去大约500年的时间。结果可能会证实海岸下沉的形式是类似事件的下沉，而不是以连续的速度下沉。在下一场风暴摧毁它们之前，新的圆窑的功能将被考古记录下来。一个地区的长期历史沉降率，不受人类修改的影响，将与通过SAR-/GPS地面监测测量的短期沉降率进行比较，以从人为因素中解开地质成因。如果该项目证实了中部三角洲沿岸不同位置的高沉降率和突然下沉动力，将对海岸带的保护和管理产生重大影响。

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.