Compared to the global average, the chemical weathering rates of basalt in southwest Iceland are high and rather variable. This can be attributed to soluble rock type (buasalt) and mechanical weathering, variation in runoff and age of rocks, and variable vegetative/glacial cover. The average temperature of the catchments in this study is near constant, 5 degrees C. Chemical weathering of the basalt is incongruent. Some of the primary minerals do not dissolve, and secondary minerals form, resulting In the fact that fluxes of all elements increase with runoff, and there is an enormous variation in the relative mobility of elements in the basalt during weathering. The relative mobility, in decreasing order, is: S > F > Na > K >>> Ca > Si > Mg > P > Sr much greater than Mn > Al > Ti > Fe. Relative to Na, close to 90 percent of Mg and Ca in the original rocks is left behind at the weathering site. The runoff dependence of fluxes and the variation in relative mobility is less in old rocks than in young ones. In old rocks the number of saturated minerals with respect to soil solutions has decreased because of lesser amount of soluble basaltic glass and an increased vegetative cover on old rocks. The saturation state of basaltic minerals is the most important variable for the dissolution and precipitation rate of minerals during weathering in southwest Iceland and is dictated by the pH of the weathering solutions. The overall rate of chemical denudation rate in southwest Iceland is independent of vegetative cover. However, fluxes of Ca, Mg, and Sr Increase with increasing vegetative cover at constant runoff, whereas fluxes of Na and K decrease. With a continuous vegetative cover the pH of the soil solutions tends to be low (<7), and glass, olivine, pyroxene, and plagioclase are unstable, but the solutions are decreasingly saturated or more undersaturated with respect to zeolites and smectite, thus increasing the relative mobility and fluxes of Ca, Mg, and Sr. Since the weathering of Ca-Mg silicate rocks is the principal process by which CO2 is removed from the atmosphere on a geological time scale (Berner, 1992), the spread of vascular plants on the continents during the mid-Paleozoic may have resulted in a drop in CO2, not necessarily because of greatly enhanced bulk chemical weathering, as suggested by Trendall (1966) and Berner (1993), but rather due to the enhanced relative mobility and fluxes of Ca and Mg. Glacial cover slows down the overall chemical denudation rates in southwest Iceland. It increases the probability of high pH weathering solutions by excluding direct and indirect routes for the CO2 from the atmosphere to the weathering site and by continuously exposing fresh rocks to the incoming solutions. A high pH (8-10) makes the primary Ca silicates stable and the Mg silicates stable or less unstable, and the high pH increases the probability of deposition of zeolites and smectites. Thus, the relative mobility and fluxes of Ca and Mg slow down during glacial cover and therefore retard the permanent long-term consumption of atmospheric CO2. This process supports the theory of a negative feedback mechanism for the long-term stabilization of the Earth's surface temperature (Walker, Hays, and Kasting, 1981). Transient consumption of atmospheric CO2 by chemical weathering in Iceland is greater than CO2 degassing from the Icelandic mantle plume. However, long-term consumption by weathering of Ca-Mg silicates and precipitation of Ca-Mg carbonates in the ocean is smaller than the CO2 degassing. The relative mobility of the least mobile elements during weathering in southwest Iceland is similar to that observed elsewhere in the world under remarkably variable climatic conditions. Thus we agree with Nesbitt and Wilson (1992) and Taylor and others (1992) that laterites and bauxites are not necessarily representative of a tropical climate, but rather the ratio of mechanical versus chemical denudation rates. Icelandic precipitation shows a normal distribution around a mean pH of 5.4. Na/Cl, K/Cl, Mg/Cl, and Sr/Cl ratios in the precipitation are close to oceanic ratios, indicating that they are solely of marine origin. The concentrations of Ca, SO4, NO3, and NH4 are higher than predicted by an unfractionated marine contribution. The pH of spring-fed rivers In southwest Iceland is high, and they are relatively poor in total dissolved inorganic carbon, calcium, and magnesium. The pH of other rivers ranges from 7.15 to 7.94, which is typical for waters with access to atmospheric CO2 during or after water-rock interaction. The water in the main channels of the rivers has enough time for significant heat exchange with its surroundings and significant gas exchange with the atmosphere, but the water-rock interactions are insignificant. The airborne dissolved or soluble solids contribution to the total dissolved solids in rivers in southwest Iceland (only carbon dissolved in rain is considered) ranges from 14 to 38 percent for those catchment areas closest to the coast. Most of the airborne contribution is of marine source and the airborne contribution is, in descending order; Cl, NO3, and NH4 (approximate to 100 percent) > Sr (44 percent) greater than or equal to SO4 (42 percent) > Na (33 percent) > Mg (23 percent) greater than or equal to K (21 percent) > Ca, PO4 (13 percent) much greater than SiO2, F, Al, Fe, Mn, Ti (0 percent). The dissolved carbon in the rivers is primarily, directly, or indirectly derived from the atmosphere. The average total dissolved inorganic N content of Icelandic precipitation is 124 mu g/l N, but the discharge weighted average of the total inorganic N concentration of Icelandic rivers is 62 mu g/l N. Thus there is a nitrogen sink in Icelandic catchment areas, caused by primary production of progressive vegetation and biota.
