Geochemistry

Weathering the Alps

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Science  17 Oct 2003:
Vol. 302, Issue 5644, pp. 359
DOI: 10.1126/science.302.5644.359c

Increases in atmospheric CO2 are assumed to increase weathering, because silicate minerals react to form water-soluble carbonates: for instance, CaSiO3 + CO2 → CaCO3 + SiO2. This reaction draws down atmospheric CO2 levels, and it has been suggested that mountain building contributes to global cooling. In turn, it is commonly assumed that mountain building and mechanical erosion, by increasing the exposed surfaces of minerals, accelerate this chemical weathering. Quantifying these processes is important for inferring the interaction between climate change and topography on Earth, and for predicting past and future CO2 levels. Jacobson and Blum worked to quantify these fluxes by examining stream chemistry in the Southern Alps of New Zealand. Here, the west side of the range is steep and wet (because of rapid uplift) as compared to the east side, despite the presence of similar types of rock. Although levels of dissolved Mg and Ca ions are much higher in the streams draining the western side of the Alps, most of this difference can be traced to the breakdown of carbonate minerals and not to silicate weathering related to CO2 consumption. Overall, mountain building seems to increase atmospheric CO2 consumption via weathering by only a factor of 2, less than previously thought. — BH

Geology 31, 10 (2003).

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