- Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa doi link

Auteur(s): Bastian Luc, Revel Marie, Bayon Germain, Dufour A., Vigier Nathalie

(Article) Publié: Scientific Reports, vol. 7 p.44231 (2017)

Ref HAL: 01494373_v1
DOI: 10.1038/srep44231
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Chemical weathering of silicate rocks on continents acts as a major sink for atmospheric carbon dioxide and has played an important role in the evolution of the Earth's climate. However, the magnitude and the nature of the links between weathering and climate are still under debate. In particular, the timescale over which chemical weathering may respond to climate change is yet to be constrained at the continental scale. Here we reconstruct the relationships between rainfall and chemical weathering in northeast Africa for the last 32,000 years. Using lithium isotopes and other geochemical proxies in the clay-size fraction of a marine sediment core from the Eastern Mediterranean Sea, we show that chemical weathering in the Nile Basin fluctuated in parallel with the monsoon-related climatic evolution of northeast Africa. We also evidence strongly reduced mineral alteration during centennial-scale regional drought episodes. Our findings indicate that silicate weathering may respond as quickly as physical erosion to abrupt hydroclimate reorganization on continents. Consequently, we anticipate that the forthcoming hydrological disturbances predicted for northeast Africa may have a major impact on chemical weathering patterns and soil resources in this region. Erosion processes on continents include mechanical erosion and chemical weathering, both of which shape the Earth's surface and contribute to substantial drawdown of atmospheric carbon via export of organic-rich clay fractions and alteration of silicate minerals. However, while the links between climate and physical erosion rates have been well documented from river chemistry data 1 and sedimentary records 2 , the response of continental chemical weathering to climate change is not well understood and requires further investigation. At the large scale, the relative importance of physical erosion, temperature, rainfall, vegetation and lithology on chemical weathering over both long (> 10 6 yr) and short (≪ 10 4 yr) periods of time is still under debate. Most studies that have established links between climate and silicate weathering were based on the analysis of dissolved phases in modern river basins 3 or on the reconstruction of past ocean chemistry during the Cenozoic 4 from marine carbonates or deep-sea ferromanganese deposits. To date, only a few studies have investigated past variations in silicate weathering over short timescales and these have yielded contradictory results 5–9. However, this information is important for predicting the evolution of the short-term carbon cycle and its impact on the Earth's vital resources. Sediments formed in weathering profiles, exported by large rivers and accumulated on margins, can provide invaluable information on the short-term evolution of weathering at the sub-continental scale. To provide constraints on the links between hydro-climate and weathering, we have reconstructed the Late Quaternary evolution of rock chemical weathering in the Nile basin, using a marine sediment record recovered from the Nile deep-sea fan, off the coast of Egypt (Fig. 1). Tropical Africa is known to have experienced major hydrological changes during the Quaternary period, which dramatically affected fluvial discharge and particle delivery to the surrounding ocean margins 10–12. The onset of past humid periods was related to increased summer insolation in the Northern Hemisphere associated with northward migration of the Inter Tropical Convergence