RISE

Past greenhouse-icehouse transitions expose the consequences of major processes that drove and possibly helped to maintain the Earth climate into our modern icehouse mode. In the last transition, a striking temporal correspondence exists between key compressional phases affecting the African continent, linked to its motion toward Eurasia, and the two main cooling steps leading to a sharp shift into an icehouse mode at 34Ma. These critical periods correspond to increasing convergence rates between the two plates and to major changes in their deformational regime, with relief formation over a large part of Africa and around the Tethys associated to enhanced continental denudation. Focusing on the very first step of the last greenhouse-icehouse transition, the late Cretaceous cooling, RISE aims to address a critical yet overlooked process that could have played a decisive role during this transition, CO2 consumption by silicate weathering during uplift and relief formation linked to Africa-Eurasia convergence. Organized in 3 work packages, RISE aims to determine which part of the late Cretaceous atmospheric CO2 decrease is linked to this process. Recent work has demonstrated that eNd and eHf decoupling in the clay fraction of modern sediments, represented by the departure of eHf to the “clay array” (?eHf), is strongly linked to variations in chemical weathering intensity of the nearby continent. In WP1, application of this novel proxy will be extended to the late Cretaceous period, to produce local records of the evolution of silicate weathering intensity at 13 targeted key sites in regions that have encountered prominent uplift and relief formation on Africa and Eurasia. Dominance of illite and chlorite in clay mineral assemblages indicates prominent physical erosion of crystalline basement, represented by elevated illite/smectite ratio. In WP1, we will determine illite/smectite ratio evolution on the same key sites than those targeted for ?eHf analyses, to obtain a view of the relative timing of erosion and weathering evolution. An independent and complementary approach, based on clay Li isotopes, will additionally be applied to access chemical weathering evolution. The approach developed in WP1 gives access to the timeframe of sharp changes in chemical weathering and physical erosion at key sites but cannot provide quantitative estimates of chemical weathering and denudation rates. In WP2, compacted terrigenous solid volumes will be calculated in the same key regions than in WP1, using dated seismic time-lines and the Volume Estimator code developed at Géoscience Rennes. This method, successfully applied on the Austral part of Africa, will be extended to the remaining of the African and Tethyan margins. Combining this approach with ?eHf, d7Li and illite/smectite records generated in WP1, providing a more detailed timeframe to estimate the duration of enhanced weathering and erosion episodes, terrigenous sediment fluxes delivered from the main catchments will be calculated. In WP3, the IPSL-CM5A2 climate model will be used to force the global biogeochemical model GEOCLIM, that requires spatial fields of continental temperature and precipitation from a 3-D climate model to simulate the spatial distribution of continental weathering and physical erosion fluxes, and to calculate associated CO2 consumption. The eroded terrigenous sediment fluxes calculated in WP2 and the isotopic and clay mineralogical records established in WP1, providing a view of the relative importance and timing of chemical weathering and physical erosion between the targeted sites, will be used to find the combination of slight modification in relief elevation and morphology for which the model reaches the best agreement with the data set. Estimation of CO2 consumption by relief formation during the late Cretaceous in Africa and around the Tethys will then be estimated by the GEOCLIM model.