Tectonic control on the last greenhouse to icehouse transition : the rise of Africa – RISE

Recent studies have shown that the decoupling of eNd and eHf in the clay fraction of modern sediments, represented by the deviation of eHf from the «clay line« (DeHf), was strongly correlated with the intensity of weathering on the adjacent continent. In WP1, the application of this new tracer will be extended to the Upper Cretaceous, to access the local evolution of continental weathering intensity at 13 key sites in regions showing significant relief formation in Africa. and Eurasia. The dominance of illite and chlorite in the clay mineral assemblages reflects significant physical erosion of the crystalline basement, represented by a high illite/smectite ratio. In WP1, the evolution of the illite/smectite ratio of clays will also be established at the same key sites analyzed for DeHf, to obtain the relative evolution of erosion and weathering. An independent and complementary approach, based on Li isotopes in clays, will also be applied to trace the evolution of continental chemical weathering.
The approach developed in WP1 provides access to the temporal framework of weathering and erosion changes at key sites, but does not allow the calculation of chemical weathering and denudation rates. In WP2, compacted sediment volumes will be calculated in the same key regions as in WP1, using seismic profiles with dated reflectors as well as the «Volume Estimator« code developed at Geoscience Rennes. This method has already been successfully applied to the southern part of Africa and will be extended to the rest of the African and Tethyan margins. By combining this approach with the isotopic and mineralogical data established in WP1, which gives the duration of episodes of increased weathering and erosion at each site in a more precise time frame, flows of terrigenous sediments arriving in the basins main slopes will be calculated.
In WP3, the spatial distribution of continental temperatures and precipitation generated by the IPSL-CM5A2 climate model will be used to force the GEOCLIM biogeochemical model and simulate the spatial distribution of weathering and erosion fluxes, and calculate the associated CO2 consumption. The eroded sediment fluxes calculated in WP2 and the isotopic and mineralogical records established in WP1 will be used to find the relief configuration (altitude, morphology) on the paleogeographic map used for which the best model-data match is obtained. The consumption of atmospheric CO2 associated with the formation of reliefs by the Africa-Eurasia convergence during the Upper Cretaceous will then be estimated from this configuration by the GEOCLIM model.

The first part of the project is devoted to the acquisition of records of weathering and erosion of different parts of the African margin, from geochemical and mineralogical tracers (WP1). At the start of the project, the regions initially targeted were extended to the South American margin, which also presents a phase of uplift during the period studied and could thus have contributed to the cooling of the Upper Cretaceous.
During the first 18 months of the project, samples from ODP/DSDP sites 356 (Sao Paulo Basin, southeastern Brazilian margin), 1259 (Demerara Rise, off Guyana), 959 (Ivory Coast-Ghana , West African margin) were obtained and analyzed for their isotopic (eNd, eHf, DeHf) and mineralogical composition of the clay fraction, supplemented by major and trace element concentration analyses. Early results from these sites clearly show a strong response of margin erosion and silicate weathering to tectonic uplift of both the West African margin and the South American eastern margin, which manifests differently depending on the intensity of the uplift and depending on the climatic belt, and in particular the hydrolysis conditions in place. At the southeastern Brazilian margin (site 356), the data show an increase in erosion associated with an uplift episode of the Turonian to Santonian margin, which occurs in a local climate that is insufficiently hydrolyzing to generate a weathering response. The DeHf clearly shows an increase in the Campanian following this tectonic episode, the relief created allowing the establishment of locally more hydrolyzing conditions on its eastern slope. The results of this first study have been published in an international journal (Corentin et al., 2022, Chemical Geology). At Demerara Rise (site 1259), which is located in a humid climatic belt throughout the Upper Cretaceous, an intensification of continental weathering is recorded from the Middle Campanian to the Maastrichtian, concomitant with an episode of uplift of the Guiana margin . During the period of tectonic quiescence preceding this episode, a decrease in weathering is recorded, in response to global climatic cooling. The results gave rise to the writing of an article, which has just been accepted in an international journal (Corentin et al., in press, Marine Geology). At site 959, the uplift of the West African margin in a humid climatic belt also induces an increase in erosion and an intensification of weathering from the Santonian to the Middle Campanian. An article is being written to highlight these results.

The new tracer of silicate weathering on continents, based on the coupling of the isotopic composition of neodymium and hafnium in clays, has been applied for the first time to ancient environments, and shows very interesting potential and complementary to existing tracers.
The first data show an intensification of silicate weathering on a large spatial scale of the West African and East South American margin that may have contributed to the global climatic cooling recorded during the Late Cretaceous. The simulations planned in WP3 are necessary to quantify this impact, but the first results show a potentially major role of tectonic events in the evolution of the climate. This result is major because our knowledge of the tectonic-climate links is largely based on studies of the formation of the Himalayas and remains much debated.

Corentin, P., Pucéat, E., Pellenard, P., Guiraud, M., Blondet, J., Bayon, G., & Adatte, T. (2022). Late Cretaceous evolution of chemical weathering at the northeastern South American margin inferred from mineralogy and Hf-Nd isotopes. Marine Geology, 106968.
Corentin, P., Pucéat, E., Pellenard, P., Freslon, N., Guiraud, M., Blondet, J., ... & Bayon, G. (2022). Hafnium neodymium isotope evidence for enhanced weathering and uplift-climate interactions during the Late Cretaceous. Chemical Geology, 591, 120724.

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.