4.3.2 Interaction between plate tectonics, climate and the carbon cycle

The chemical weathering of rocks (e.g., Eq. 2.49) is influenced by a large number of processes. In particular, high temperatures and the availability of water at the surface tend to induce higher weathering rates, at least for some rocks such as the basalts. As the weathering consumes atmospheric CO₂ (see section 2.3.4), this can potentially lead to a negative feedback that can regulate the long term variations in the atmospheric CO₂ concentration (Fig. 4.14). For example, more intense tectonic activity might cause the uplift of a large amount of calcium-silicate rocks to the surface, leading to an increase in the weathering rate. The atmospheric CO₂ concentration would then decrease, producing a general cooling of the climate system. This would lead to lower evaporation and so less precipitation and lower water availability. These climate changes would then reduce the weathering rates, so moderating the initial perturbation.

In a second hypothesis, we can postulate that the rate of weathering is mainly influenced by mechanical erosion, which increases the exposition of fresh, new rocks to the atmosphere and these can then be more efficiently altered. If the temperature decreases because of a higher rate of weathering, glaciers and ice sheets can cover a larger surface. As they are very active erosion agents, this would increase the mechanical weathering and so the chemical weathering, amplifying the initial perturbation (Fig. 4.14).

Although they both appear reasonable, these two feedback mechanisms are still being debated. Their exact role, if any, in the variations of CO₂ concentration on time scales of hundreds of thousands to millions of years is still uncertain.

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