Global limits on silicate weathering and implications for the silicate weathering feedback
Chemical weathering of silicate rocks is considered responsible for the stabilization of atmospheric CO2 and climate on geological time scales. The strength of this negative feedback is hard to establish on a global scale due to the spatial variability of controlling factors of weathering such as climate, physical erosion and biotic activity. In order to investigate if there is a simple way of including spatial variability of climatic and tectonic drivers of weathering a new model of the regolith, which specifically considers the physical transports of water, carbon and minerals as boundary conditions for chemical weathering rates, was developed. This is in contrast to previous global models, in which chemical kinetics and thus temperature is considered of primary importance and physical transports are considered secondary in nature. The model is able to satisfactorily simulate river concentrations of elements relevant to the silicate weathering feedback for a present day scenario. If chemical weathering is limited by mineral supply, an increase in the climatic potential for dissolution and transport has no effect on chemical weathering rate. This suggests that mineral supply limitation weakens the silicate weathering feedback. Also, biotic enhancement of weathering is limited by mineral supply and by inference tectonic uplift. The model suggests that there are some inherent problems associated with the use of factorial laws in global models of the geological carbon cycle. This indicates that an integrated approach, in which all drivers of weathering are considered simultaneously, is preferable to separate empirical parameterizations, because a change in one driver might lessen the impact of the other drivers. This can only be achieved with process-based models. The principle of co-limitation associated with the physical transports of water, carbon, and minerals seems universal in setting the boundaries for the potential feedback.