Lake Alaotra, Madagascar. A late Quaternary wetland regulated by the tectonic regime

Abstract : Lake Alaotra, a large but shallow lake in NE Madagascar, has been the focus of prophecies about its future lifespan. None of the formulated predictions, however, are calibrated against a precisely defined modern limnimetric benchmark or former lake shoreline. As a result, existing scenarios of lake evolution in relation to itssurrounding landscape have been predicated on incomplete and uncoordinated information. Here we present achronology of landscape evolution in the Alaotra Basin based on a coordinated reconstruction of environmentalchange. The evidence draws upon an inventory of regolith and landforms, hydrological data, a compilation ofunpublished boreholes and geophysical surveys of the sedimentary fill sequences, and a record of radiocarbonages obtained from peat and diatom layers in the stratigraphy. The record indicates that Lake Alaotra has been inexistence for at least 30,000 years, with evidence of climatically-driven fluctuations but no evidence of the lakeever being much larger or having occupied the entire floor of the topographic basin. The spatial distribution andsedimentology of the age-bracketed deposits suggest instead that the Alaotra is fundamentally a wetland, or fen,which has been buffering its externally-drained lake from irreversible siltation by heavily loaded streams. Thetectonic regime in this distinctly seismic part of Madagascar appears to have regulated basin subsidence andaccommodation space, steadily maintaining the basin floor below the level of the water table. If the lake today isunder any threat of extinction from excess sediment influx, this is chiefly on account of hydrological alterationsto the girdle of fenland by irrigated rice monoculture, with artificial drainage channels recently cut through thewetland buffer and fast-tracking the turbid water of tributary rivers directly to the lake.
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Submitted on : Thursday, February 22, 2018 - 6:15:47 PM
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Michel Mietton, Yanni Gunnell, Gérard Nicoud, Luc Ferry, Reine Razafimahefa Rasoanimanana, et al.. Lake Alaotra, Madagascar. A late Quaternary wetland regulated by the tectonic regime. CATENA, Elsevier, 2018, 165, pp.22 - 41. ⟨10.1016/j.catena.2018.01.021⟩. ⟨hal-01715563⟩



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