Physical properties of 39 sediment cores from the Bengal Fan and northeast Indian Ocean

We obtained sediment physical properties and geochemical data from 47 piston and gravity cores located in the Bay of Bengal, to study the complex history of the Late Pleistocene run-off from the Ganges and Brahmaputra rivers and its imprint on the Bengal Fan. Grain-size parameters were predicted from core logs of density and velocity to infer sediment transport energy and to distinguish different environments along the 3000-km-long transport path from the delta platform to the lower fan. On the shelf, 27 cores indicate rapidly prograding delta foresets today that contain primarily mud, whereas outer shelf sediment has 25% higher silt contents, indicative of stronger and more stable transport regime, which prevent deposition and expose a Late Pleistocene relic surface. Deposition is currently directed towards the shelf canyon 'Swatch of No Ground', where turbidites are released to the only channel-levee system that is active on the fan during the Holocene. Active growth of the channel-levee system occurred throughout sea-level rise and highstand with a distinct growth phase at the end of the Younger Dryas. Coarse-grained material bypasses the upper fan and upper parts of the middle fan, where particle flow is enhanced as a result of flow-restriction in well-defined channels. Sandier material is deposited mainly as sheet-flow deposits on turbidite-dominated plains at the lower fan. The currently most active part of the fan with 10-40 cm thick turbidites is documented for the central channel including inner levees (e.g., site 40).
Site 47 from the lower fan far to the east of the active channel-levee system indicates the end of turbidite sedimentation at 300 ka for that location. That time corresponds to the sea-level lowering during late isotopic stage 9 when sediment supply to the fan increased and led to channel avulsion farther upstream, probably indicating a close relation of climate variability and fan activity. Pelagic deep-sea sites 22 and 28 contain a 630-kyear record of climate response to orbital forcing with dominant 21- and 41-kyear cycles for carbonate and magnetic susceptibility, respectively, pointing to teleconnections of low-latitude monsoonal forcing on the precession band to high-latitude obliquity forcing. Upper slope sites 115, 124, and 126 contain a record of the response to high-frequency climate change in the Dansgaard-Oeschger bands during the last glacial cycle with shared frequencies between 0.75 and 2.5 kyear. Correlation of highs in Bengal Fan physical properties to lows in the d18O record of the GISP2 ice-core suggests that times of greater sediment transport energy in the Bay of Bengal are associated with cooler air temperatures over Greenland. Teleconnections were probably established through moisture and other greenhouse-gas forcing that could have been initiated by instabilities in the methane hydrate reservoir in the oceans.

本研究从孟加拉湾采集了47根活塞岩心与重力岩心，获取沉积物物理性质与地球化学数据，以探究恒河-布拉马普特拉河晚更新世径流的复杂演化历史，及其对孟加拉扇（Bengal Fan）的沉积印记。研究人员通过密度与速度测井曲线反演沉积物粒度参数，以此推断沉积物搬运能量，并区分从三角洲平台延伸至扇体下部的3000 km长搬运路径上的不同沉积环境。在陆架区域，27根岩心显示现今正快速进积的三角洲前积层以泥质沉积物为主；而外陆架沉积物的粉砂含量比前者高25%，表明存在更强且更稳定的搬运机制，该机制抑制了沉积作用并暴露了晚更新世残留沉积面。现今沉积作用指向陆架峡谷“Swatch of No Ground”，浊流在此被输送至全新世时期扇体上唯一活跃的水道-堤岸系统（channel-levee system）。该水道-堤岸系统的活跃发育期贯穿海平面上升与高水位期，在新仙女木事件（Younger Dryas）末期存在显著的生长阶段。粗粒物质会绕过扇体上部与中部扇体上段，在这些区域，由于规整水道内的流束约束，颗粒流作用得以增强。砂质沉积物主要以席状流沉积的形式，沉积在扇体下部以浊积岩（turbidites）为主的平原区域。扇体现今最活跃的区域位于包括内堤在内的中央水道（例如站位40），该区域发育厚度为10~40 cm的浊积岩。位于活跃水道-堤岸系统东侧远端扇体下部的站位47显示，该区域的浊积岩沉积作用在300 ka时终止。该时间对应同位素9期晚期的海平面下降阶段，此时向扇体的沉积物供给量增加，引发水道在更上游位置发生决口，这或许表明气候波动与扇体活动之间存在密切关联。远洋深海站位22与28保存了630 ka的气候对轨道驱动响应的沉积记录：其中碳酸盐含量与磁化率分别以21 ka和41 ka的周期为主导，这表明低纬度季风岁差波段强迫与高纬度黄赤交角强迫之间存在遥相关关系。陆坡上部站位115、124与126保存了末次冰期旋回中丹斯果-奥什格尔（Dansgaard-Oeschger）波段高频气候变化的响应记录，其共享周期介于0.75~2.5 ka之间。孟加拉扇物理性质的高值与格陵兰冰芯计划2（GISP2）的δ¹⁸O（d18O）低值具有相关性，这表明孟加拉湾沉积物搬运能量较高的时期，对应格陵兰地区气温较低的时段。这种遥相关关系可能通过水汽与其他温室气体强迫得以建立，而该强迫作用或许由海洋甲烷水合物储层的不稳定性所触发。