Grain-size distribution, sedimentation rate and x-ray fluorescence data of four sediment cores off the Senegal River estuary

Fine-grained sediment depocenters on continental shelves are of increased scientific interest since they record environmental changes sensitively. A north-south elongated mud depocenter extends along the Senegalese coast in mid-shelf position. Shallow-acoustic profiling was carried out to determine extent, geometry and internal structures of this sedimentary body. In addition, four sediment cores were retrieved with the main aim to identify how paleoclimatic signals and coastal changes have controlled the formation of this mud depocenter.

A general paleoclimatic pattern in terms of fluvial input appears to be recorded in this depositional archive. Intervals characterized by high terrigenous input, high sedimentation rates and fine grain sizes occur roughly contemporaneously in all cores and are interpreted as corresponding to intensified river discharge related to more humid conditions in the hinterland. From 2750 to 1900 and from 1000 to 700 cal a BP, wetter conditions are recorded off Senegal, an observation which is in accordance with other records from NW-Africa. Nevertheless, the three employed proxies (sedimentation rate, grain size and elemental distribution) do not always display consistent inter-core patterns. Major differences between the individual core records are attributed to sediment remobilization which was linked to local hydrographic variations as well as reorganizations of the coastal system.

The Senegal mud belt is a layered inhomogeneous sedimentary body deposited on an irregular erosive surface. Early Holocene deceleration in the rate of the sea-level rise could have enabled initial mud deposition on the shelf. These favorable conditions for mud deposition occur coevally with a humid period over NW-Africa, thus, high river discharge. Sedimentation started preferentially in the northern areas of the mud belt. During mid-Holocene, a marine incursion led to the formation of an embayment. Afterwards, sedimentation in the north was interrupted in association with a remarkable southward shift in the location of the active depocenter as it is reflected by the sedimentary architecture and confirmed by radiocarbon dates. These sub-recent shifts in depocenters location are caused by migrations of the Senegal River mouth. During late Holocene times, the weakening of river discharge allowed the longshore currents to build up a chain of beach barriers which have forced the river mouth to shift southwards.

大陆架上的细粒沉积沉积中心（fine-grained sediment depocenters）因其能够灵敏地记录环境变化而愈发受到学界关注。陆架中部沿塞内加尔海岸分布有一处南北向延伸的泥质沉积中心（mud depocenter）。研究人员通过浅地层剖面探测（shallow-acoustic profiling）明确了该沉积体的分布范围、几何形态与内部结构。此外，还获取了4根沉积物岩芯（sediment cores），核心目的是厘清古气候信号（paleoclimatic signals）与海岸变化如何调控该泥质沉积中心的形成过程。

该沉积记录（depositional archive）中保存了与河流输入相关的古气候变化总体模式。所有岩芯中均大致同时出现陆源输入量高、沉积速率快、粒径偏细的沉积层段，这被认为对应内陆湿润气候背景下增强的河流径流量。在2750~1900校准年前（cal a BP）与1000~700校准年前，塞内加尔近海均记录到湿润气候条件，这一观测结果与西北非（NW-Africa）其他相关记录相符。不过，本次研究采用的3种代用指标（proxies）即沉积速率、粒径与元素分布（elemental distribution），并未始终展现出一致的岩芯间变化模式。各岩芯记录间的显著差异可归因于沉积物再搬运（sediment remobilization），该过程与局地水文变化及海岸系统重组密切相关。

塞内加尔泥质带（Senegal mud belt）是一套赋存于不规则侵蚀面之上的层状非均质性沉积体。早全新世（Early Holocene）海平面上升速率减缓，为陆架上的初始泥质沉积提供了有利条件。这类有利于泥质沉积的环境与西北非的湿润气候期同步出现，即彼时河流径流量偏高。泥质沉积率先在该泥质带的北部区域开始发育。中全新世（mid-Holocene）时期，海侵作用形成了一处海湾。此后，北部区域的沉积作用被中断，这与活动沉积中心（active depocenter）显著向南迁移的现象相契合，该结论得到了沉积结构（sedimentary architecture）与放射性碳定年结果（radiocarbon dates）的共同验证。沉积中心的此次近晚期迁移由塞内加尔河口的迁移所导致。晚全新世（late Holocene）时期，河流径流量减弱，沿岸流（longshore currents）得以塑造一系列滨岸沙坝，进而迫使河口向南迁移。