the climate equation. Because of this importance, there is a major effort to reduce its uncertainties.
Weathering rates of ancient rock are not well known because of basic uncertainties about the process. For example, newly exposed (fresh) surfaces of mineral grains weather orders of magnitude faster than long-exposed surfaces (Figure 3.9)—a factor that does not appear explicitly in chemical models of reaction kinetics. In other words, areas of active mountain building (e.g., continental collision zones) generate a large amount of fresh mineral surface area by erosion and hence should contribute much more to CO2 reduction than stable continental areas. Research advances are needed to better understand what controls mineral weathering rates, to quantitatively relate weathering rates to erosion rates and mountain building, and to evaluate how the age dependence of weathering rates affects models for the regulation of global climate.
A promising proxy for globally averaged rates of weathering in the geological past is the strontium isotopic composition of the oceans. The variation of 87Sr/86Sr provides a measure of the relative Sr inflows to the ocean from hydrothermal fluids and eroded continental material, with high 87Sr/86Sr indicating a high influx of continental silicate minerals. There is evidence that Sr isotope ratios respond to continental collisions (e.g., Derry and France-Lanord, 1996) and that periods of high 87Sr/86Sr are correlated with some glaciations. However, Sr isotope ratios also indicate