12. Chemical weathering, chemical denudation and the CO2 budget for Iceland

Abstract

The average relative mobility of elements in SW Iceland during chemical weathering and denudation is: S>F>Na>K≫Ca>Si>Mg>P>Sr≫Mn>Al>Ti>Fe. The sequence of mobility of the elements in the youngest rocks is different from that of the oldest rocks and it changes with increased vegetation cover. The secondary minerals that are left behind in soil are predominantly allophane, poorly crystalline ferrihydrite, and some imogolite. The total chemical denudation rate of the rock derived elements (TCDR) increases with increasing runoff, but decreases with increasing age of the rocks. Where the variation in the primary variables, runoff and age of rocks, is small the denudation rate for Ca, Mg and Si increases with increased vegetative cover. The TCDR for the major Icelandic catchments range from 19 to 146 t km −2 yr 1 . The calculated average for Iceland is 37 compared to the 26 t km −2 yr −1 for the continents. The high rate in Iceland is attributed to high runoff and high reactivity of glassy and crystalline basalt. The chemical denudation rate of relatively mobile elements like S,F,Na and Ca increased by a factor of 2 to 5 at constant runoff and vegetative cover but 0% to 100% glass content of the rocks. The chemical denudation rates of 5 m yr old crystalline basalt, including data on cation uptake by growing trees, indicate that the rate of release of Ca and HCO 3 to streams is about 3 times higher in tree-covered areas than in unvegetated areas. The variation in the chemical denudation rate in Iceland is dictated by the composition of the soil solutions and the saturation state of primary and secondary minerals as reflected in the dependence on runoff. More atmospheric CO 2 is fixed during transient chemical weathering in Iceland than is released to the atmosphere from Icelandic volcanoes and geothermal systems. However, on a geological time scale, more CO 2 is released to the atmosphere than will eventually precipitate in the ocean as Ca and Mg carbonates, as the result of Ca and Mg released during chemical weathering. The overall net transient CO 2 budget for Iceland in 1990, including CO 2 fixation by chemical weathering and vegetation, and the anthropogenic release of CO 2 and the release from volcanoes, was ‘positive’. That is there was a net CO 2 release to the atmosphere of the order 0.3–1.3 m t CO 2 yr −1 .