Modelling the impact of palaeogeographical changes on weathering and CO ₂ during the Cretaceous–Eocene period

Abstract. The feedback between atmospheric CO2 concentrations and silicate weathering is one of the key controls on the long-term climate of the Earth. The potential silicate weathering flux (as a function of conditions such as temperature, runoff, and lithology), or “weatherability”, is strongly affected by continental configuration; thus the position of continental landmasses can have substantial impacts on CO2 drawdown rates. Here, we investigate the potential impact of palaeogeographical changes on steady-state CO2 concentrations during the Cretaceous–Eocene period (145–34 Ma) using a coupled global climate and biogeochemical model, GEOCLIM, with higher-resolution climate inputs from the HadCM3L general circulation model (GCM). We find that palaeogeographical changes strongly impact CO2 concentrations by determining the area of landmasses in humid zones and affecting the transport of moisture, that runoff is a strong control on weatherability, and that changes in weatherability could explain long-term trends in CO2 concentrations. As Pangaea broke up, evaporation from the ocean increased and improved moisture transport to the continental interiors, increasing runoff rates and weathering fluxes, resulting in lower steady-state CO2 concentrations. Into the Cenozoic, however, global weatherability appears to switch regimes. In the Cenozoic, weatherability appears to be determined by increases in tropical land area, allowing greater weathering in the tropics. Our modelled CO2 concentrations show some strong similarities with estimates derived from proxy sources. Crucially, we find that even relatively localised changes in weatherability can have global impacts, highlighting the importance of so-called weathering “hotspots” for global climate. Our work also highlights the importance of a relatively high-resolution and complexity-forcing GCM in order to capture these hotspots.