Abstract
Lake Chad, the largest freshwater lake of north-central Africa and one of the largest lakes of Africa, is the relict of a giant Quaternary lake (i.e., Megalake Chad) that developed during the early- to mid-Holocene African Humid Period. Over the drylands of the Sahara Desert and the semi-arid Sahel region, remote sensing (optical satellite imagery and digital elevation models) proved a successful approach to identify the paleo-shorelines of this giant paleo-lake. Here we present the first attempt to estimate the isostatic response of the lithosphere due to Megalake Chad and its impact on the elevation of these paleo-shorelines. For this purpose, we use the open source TABOO software (University of Urbino, Italy) and test four different Earth models, considering different parameters for the lithosphere and the upper mantle, and the spatial distribution of the water mass. We make the simplification of an instantaneous drying-up of Megalake Chad, and compute the readjustment related to this instant unload. Results (i.e., duration, amplitude, and location of the deformation) are then discussed in the light of four key areas of the basin displaying prominent paleo-shoreline morpho-sedimentary features. Whatever the Earth model and simplification involved in the simulations, this work provides a strong first-order evaluation of the impact on hydro-isostasy of Megalake Chad. It demonstrates that a water body similar to this megalake would induce a significant deformation of the lithosphere in the form of a vertical differential uplift at basin-scale reaching up to 16 m in the deepest part of the paleo-lake, and its shorelines would then be deflected from 2 m (southern shorelines) to 12 m (northern shorelines), with a maximum rate of more than 1 cm y−1. As such, any future study related to the paleo-shorelines of Megalake Chad, should integrate such temporal and spatial variation of their elevations.
Highlights
Amongst the catalogue of potential links between climate change and deep Earth processes, the isostatic response of the lithosphere to loading or unloading by ice sheets or water bodies is surely the one with the most significant impact on continental surface changes and on related sedimentary systems reorganization
In order to quantitatively estimate the importance of this deformation, we model here for the first time the vertical deflection and pattern of the hydro-isostatic readjustment caused by the flooding and the drying-up of this large paleo-lake, provided that the lithosphere deflection reached the isostatic equilibrium before it started to dry up
Megalake Chad (Figure 1) is a huge extinct lake of more than 350,000 km2 that developed following the Holocene climatic optimum called the African Humid Period, known for that part of Africa as the Green Sahara Episode [42]. This climate event was forced by orbital parameters, which led to a northward shift of the Intertropical Convergence Zone (ITCZ) and a correlative increase of monsoonal rainfall over the Sahel and Sahara ([47,48], and references )
Summary
Amongst the catalogue of potential links between climate change and deep Earth processes, the isostatic response of the lithosphere to loading or unloading by ice sheets or water bodies is surely the one with the most significant impact on continental surface changes and on related sedimentary systems reorganization. Everywhere the Last Glacial Maximum ice sheets have severely deflected the continental lithosphere, the patterns of the post-glacial isostatic rebound and of its geomorphic signatures (e.g., raised beaches, deltas, and overflow location) [1,2,3,4,5] show uplift rates in the 1–10 cm yr−1 range, resulting in total vertical displacement commonly in excess of 500 m. The understanding of hydro-isostatic processes in lake studies is of prime importance, as far as large inland water bodies experiencing significant water-level fluctuations are concerned (e.g., lakes Agassiz, Bonneville, and Lahontan). Vertical deflection in meters to tens of meters has been demonstrated to deform ancient lake shorelines [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]
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