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

Lake Alaotra, a large but shallow lake in NE Madagascar, has been the focus of prophecies about its future life span. 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 its surrounding landscape have been predicated on incomplete and uncoordinated information. Here we present a chronology of landscape evolution in the Alaotra Basin based on a coordinated reconstruction of environmental change. The evidence draws upon an inventory of regolith and landforms, hydrological data, a compilation of unpublished boreholes and geophysical surveys of the sedimentary fill sequences, and a record of radiocarbon ages obtained from peat and diatom layers in the stratigraphy. The record indicates that Lake Alaotra has been in existence for at least 30,000 years, with evidence of climatically-driven fluctuations but no evidence of the lake ever being much larger or having occupied the entire floor of the topographic basin. The spatial distribution and sedimentology 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. The tectonic regime in this distinctly seismic part of Madagascar appears to have regulated basin subsidence and accommodation space, steadily maintaining the basin floor below the level of the water table. If the lake today is under any threat of extinction from excess sediment influx, this is chiefly on account of hydrological alterations to the girdle of fenland by irrigated rice monoculture, with artificial drainage channels recently cut through the wetland buffer and fast-tracking the turbid water of tributary rivers directly to the lake.