The complex response of continental silicate rock weathering to the colonization of the continents by vascular plants in the Devonian

Abstract

The first forests appeared on the continents during the Givetian stage of the Devonian. The fossil record shows that, by the end of the Devonian, vascular plants and forests were common and widespread in the wet lowlands. Although the impact of this major event on chemical weathering of the continents is reasonably known, the coeval change in physical erosion has never been explored. Here, we build a mathematical description of the coupled response of the physical erosion and chemical weathering on the continents, to the colonization by vascular plants over the course of the Devonian. This spatially-resolved erosion model is coupled to the GEOCLIM model to simulate the response of the global carbon and alkalinity cycles, and of climate, to the colonization phase. A set of simulations is described, assuming an increased weatherability of the continental surface, and a change in physical erosion which can be either a decrease or an increase in response to the spreading of vascular plants. We explore first the initial pre-colonization and the final post-colonization steady states of the surficial Earth system. Then, we simulate the transient states of the Earth system in response to theoretical randomized scenarios for the colonization. We find that the pathways of the colonization have a major impact on the CO2 history through the Devonian. Depending on the magnitude of the change in physical erodibility and chemical weatherability, and on the colonization scenario, atmospheric CO2 evolution may display contrasting behaviors: from a uniform CO2 decrease over the Devonian, to more complex patterns characterized first by a global warming from the end of the Givetian into the Frasnian, and then by a final cooling, in first order agreement with the proxy data for CO2 and reconstructed climate evolution.