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Abstract:
Current understanding of the long-term carbon cycle posits that Earths climate is stabilized by a negative feedback involving CO2 consumption by chemical weathering of silicate minerals. This theory holds that silicate weathering responds to climate: when atmospheric pCO(2) and surface temperatures rise, chemical weathering accelerates, consuming more atmospheric CO2 and cooling global climate; when pCO(2) falls, weathering fluxes decrease, permitting buildup of CO2 and consequent warming. However, the functional dependence of global weathering rates on atmospheric pCO(2) (Earths "weathering curve") remains highly uncertain, with a variety of mathematical formulations proposed in the literature. We explore the factors influencing this relationship, and how they may have changed over Earth history. We then revisit classic carbon cycle model experiments to demonstrate how the choice of weathering curve has dramatic consequences for the response of the Earth system to several types of climatic and carbon-cycle perturbations. First, the slope of the weathering curve determines the timescale of recovery and the "long tail" of elevated pCO(2) following carbon release events. Second, the nature of Earth's weathering curve determines the response of pCO(2) to changing volcanic CO2 degassing, which has varied significantly over geologic timescales. Finally, we demonstrate how changes to Earths weathering curve over time driven by, for example, tectonic or evolutionary processes, can act as a forcing, in addition to a feedback, in the carbon cycle and climate. These examples highlight the importance of constraining Earths weathering curve, both for improving our understanding of past carbon cycle perturbations and predicting the future impact of anthropogenic carbon release on long timescales.
