The Senegal mud-belt depocenter (NW-Africa) - an archive for climatic changes, coastal dynamics and shelf organization over the Holocene -

Abstract

NW-Africa, in particular the Sahel region, is very sensitive to climatic changes. Hence, the reconstruction of past climate variability is a valuable way to understand mechanisms of present climatic changes. In front of the Senegal River, in a mid-shelf position, lies a fine-grained depocenter. The investigation of this mud-belt depocenter forms the central theme of this thesis. Mud belts are believed to potentially be natural archives which record such variations. Continental shelves are very dynamic areas where the climatic signal recorded in the sediments may possibly be distorted by several factors.The first phase of this study investigated how the terrigenous fraction of a sediment sequence records past climate variability, specifically, which chemical elements can be employed to reconstruct climate variability off Senegal. We approached this problem with a combination of existing sedimentological techniques to unravel fluvial and aeolian sediment input. The second phase was to reconstruct the hydrological changes occurring during the late Holocene and compare them to several other records from NW-Africa. The third phase was to determine the influence of coastal and paleoclimatic changes on the Senegal mud-belt formation, specifically, to determine to what extent the climatic signal recorded by several sedimentological proxies is disrupted by the coastal changes. Then, the fourth phase of this study involved data collection on the initiation and evolution of the mud belt with the aim of reconstructing the history of this fine-grained shelf depocenter. The new scientific outcomes of this thesis are: (1) the identification of Fe and Al as being the most suitable fluvial proxies off Senegal, implying that specific chemical elements with which to record input variations must be determined for individual regions before climate reconstruction can be performed; (2) the record of two dry periods from 3010 to 2750 cal a BP and from 1900 to 1000 cal a BP, and two main humid periods from 2750 to 1900 cal a BP and from 1000 to 700 cal a BP, provide a better constrained characterization of the climate of late Holocene in this region; (3) the finding that the paleoclimatic signal can be distorted by local hydrological conditions, sediment-remobilizing events and shifts in the river-mouth location, leads to the conclusion that various paleoclimatic records can be erroneous; (4) the history of the mud belt has been reconstructed, increasing our knowledge about the initiation and evolution of these shelfal fine-grained highstand deposits.