Monsoon over Quaternary climatic cycles control on denudation rates of southwestern Madagascar over the past 900 ka

Denudation is the sum of chemical weathering and physical erosion. It is a key parameter controlling the evolution of the Earth’s surface, the production of soils, the stability of relief or the long-term evolution of climate through silicate alteration and sedimentary fluxes that control the burial of organic carbon. Through these controls, denudation influences large scale biogeochemical cycles. In turn, climate is supposed to have a strong control over denudation through a number of processes such as precipitation, temperature or vegetation distribution. In order to comprehend the past evolution of the Earth’s surface and to better predict future changes that will affect our habitat, it is crucial to constrain links that exist between climate and denudation, notably because the intensity of these feedback mechanisms is still debated. This requires precise quantification of past denudation rates. We propose here to pursue this goal using cosmogenic radionuclides (¹⁰Be), a method which has already proven its efficiency for this particular kind of study. We apply this method to a Quaternary sedimentary archive from the Mozambique Canal, offshore southwestern Madagascar.

The choice of the study area is motivated not only by data availability, but also by the absence of intense tectonic activity or glaciations over the Quaternary, limiting changes in denudation rates over time to climatic forcings, hence simplifying the system we wish to study. We measured in situ cosmogenic ¹⁰Be in quartz grains of turbiditic layers from a marine sedimentary core that has been dated between 50 and 900 ka, and that was drilled on a terrace of the underwater Tsiribihina valley, in the Mozambique Canal. A preliminary study demonstrated that this core covers several glacial-interglacial cycles. New ¹⁰Bedata hence allows us to document paleo-denudation rates integrated over a large drainage basin through several Quaternary glacial-interglacial cycles. In order to investigate the sources of the terrigenous sediments brought from Madagascar to the Mozambique Canal during the past 900 ka we also used εNd, which is an efficient source tracer for ancient lithologies, such as in Madagascar (Mesoarchean to Neoproterozoic), as well as heavy mineral counting. This integrated approach allows us to reconstruct paleo-denudation rates from a well delimited region during the Pleistocene.

Overall, the Mangoky river appears to be the dominant source of sediments to the Mozambique Channel. Variations in sources are limited and do not follow climate cyclicity. Our ¹⁰Be-based denudation rates range from 21 ± 7 mm/ka at 136 ka to 89 ± 36 mm/ka at 614 ka. Denudation rates appear generally higher during interglacial periods, when the monsoon periods are believed to be either longer or stronger in terms of precipitation intensity. Between 600 ka and 400 ka glacial denudation rates are modified, but not interglacial denudation rates, meaning that there must be a change in a process controlling denudation during glacial, even in a region unaffected by glaciers.