Cette étude par Gaitan et al. explore le rôle de l’altération chimique des roches du plateau sud-africain dans le refroidissement climatique global survenu à la fin du Crétacé. L’analyse géochimique et isotopique de sédiments du bassin du Cap montre une intensification de l’altération des silicates dès le Cénomanien, en lien avec le soulèvement tectonique et l’érosion du plateau sud-africain. Cette altération accrue aurait contribué à diminuer le CO₂ atmosphérique, directement par les réactions d’altération des silicates, mais également en augmentant les apports de nutriments vers l’océan, favorisant la productivité marine et le stockage de carbone organique. Elle pourrait ainsi avoir participé non seulement au maintien, mais aussi au déclenchement du refroidissement global de la fin du Crétacé. PDF
ABSTRACT: Climate cooling during the late Cretaceous (ca. 100–66 Ma) has been linked to CO2 drawdown produced by enhanced denudation episodes recorded in the eastern margin of South America and western margin of South Africa. However, the onset of these episodes is recorded during rather than prior to the main climate cooling stage of the late Cretaceous, reported within the Santonian-Early Campanian interval (about 86–81 Ma). Here, we investigate the clay fraction (<2 μm) of sediments from the borehole O-A1 located in the continental slope of the Cape Basin, adjacent to the western South African Margin. We combine these results with previous records from the basin (DSDP 361) to investigate the controlling factors of the denudation record along the South African Plateau, and their relationship with the climate cooling during the late Cretaceous. The ΔεHf record (proxy for silicate weathering intensity) from site O-A1 shows an increase in silicate chemical weathering intensity during the Cenomanian-Maastrichtian (ca. 95–68 Ma) interval, predating the Campanian-Danian interval shown by the DSDP 361 record. While this offset between the sites is attributed to age model uncertainties from the site DSDP 361, the observed increase in silicate chemical weathering intensity is concomitant to an enhanced phase of physical erosion and tectonic uplift of the South African Plateau and mirrors the evolution of seawater 87Sr/86Sr at that time. Additionally, the duration of this increase even during global climate cooling suggests that the increase in weathering intensity and rate are mainly tectonically driven. Our ΔεHf(80) data from site O-A1 shows for the first time an increase in silicate chemical weathering intensity as early as the Cenomanian, suggesting not only that it could have played a role in maintaining the cool global climate trend during the late Cretaceous, but that this episode could have contributed to trigger the CO2 drawdown responsible for the observed cooling record that started by the end of the Turonian in some high-mid latitude locations and extended up to the Campanian. This process appears to be driven by two different mechanisms. First, via silicate reactions, and second through increased nutrient riverine input into the basin, which could have contributed to enhanced primary productivity and organic carbon storage as shown by the increase in Baexcess during the Santonian-Maastrichtian interval (ca. 85–65 Ma). Our study establishes enhanced weathering along the South African Plateau as a potential early contributor to the late Cretaceous cooling trend and highlights the importance of nutrient driven marine productivity as an understudied but potentially significant mechanism for CO2 drawdown.