Lateritic processes in Madagascar and the link with agricultural and socioeconomic conditions

Abstract

Madagascar hosts a youthful landscape atop old bedrock geology. Observed rapid surface uplift – possibly a consequence of sub-plate mantle plume activity – does not fit the concept of “The Red Island”, i.e. thick piles of laterite, formed by slow chemical weathering that requires prolonged periods of tectonic quiescence. Here, we document the results of a three-year field campaign in which we sought to interrogate and decouple the climatic and tectonic drivers behind laterite formation, and establish a tentative link to agricultural productivity, which may reinforce local levels of poverty. We installed 45 rain gauges and merged their data with a satellite precipitation product to develop a new high-resolution climatology. We also collected 11 sediment samples from which we established 10Be-derived erosion rates; and measured laterite/saprolite thickness at 60 localities. The thickest lateritic profiles coincide with a narrow band of annual rainfall, suggesting that laterite evolution processes co-evolved with climate changes. Laterite thickness correlates negatively with erosion rate, and strongly positively with the headcount index of poverty. In the absence of agricultural technology, we propose a direct link between the local prevalence of lateritic soil and poor agricultural productivity. We also suggest that the spatial distribution of uplift/erosion and precipitation governs the distribution and thickness of laterite. Uplift and erosion rates vary dramatically over small distances, restricting the development of thick laterite to two small patches that straddle the central Hauts Plateaux.