CHEMICAL-WEATHERING OF BASALT IN SOUTHWEST ICELAND - EFFECTS OF RUNOFF, AGE OF ROCKS AND VEGETATIVE GLACIAL COVER/

Compared to the global average, the chemical weathering rates of basal t in southwest Iceland are high and rather variable. This can be attri buted to soluble rock type (buasalt) and mechanical weathering, variat ion in runoff and age of rocks, and variable vegetative/glacial cover. The average temperature of the catchments in this study is near const ant, 5 degrees C. Chemical weathering of the basalt is incongruent. So me of the primary minerals do not dissolve, and secondary minerals for m, resulting In the fact that fluxes of all elements increase with run off, and there is an enormous variation in the relative mobility of el ements in the basalt during weathering. The relative mobility, in decr easing order, is: S > F > Na > K >>> Ca > Si > Mg > P > Sr much greate r than Mn > Al > Ti > Fe. Relative to Na, close to 90 percent of Mg an d 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 satur ated minerals with respect to soil solutions has decreased because of lesser amount of soluble basaltic glass and an increased vegetative co ver on old rocks. The saturation state of basaltic minerals is the mos t important variable for the dissolution and precipitation rate of min erals 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 continuo us vegetative cover the pH of the soil solutions tends to be low (<7), and glass, olivine, pyroxene, and plagioclase are unstable, but the s olutions are decreasingly saturated or more undersaturated with respec t 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 plan ts on the continents during the mid-Paleozoic may have resulted in a d rop in CO2, not necessarily because of greatly enhanced bulk chemical weathering, as suggested by Trendall (1966) and Berner (1993), but rat her due to the enhanced relative mobility and fluxes of Ca and Mg. Gla cial cover slows down the overall chemical denudation rates in southwe st Iceland. It increases the probability of high pH weathering solutio ns by excluding direct and indirect routes for the CO2 from the atmosp here to the weathering site and by continuously exposing fresh rocks t o the incoming solutions. A high pH (8-10) makes the primary Ca silica tes 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 g lacial cover and therefore retard the permanent long-term consumption of atmospheric CO2. This process supports the theory of a negative fee dback 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 C O2 degassing from the Icelandic mantle plume. However, long-term consu mption by weathering of Ca-Mg silicates and precipitation of Ca-Mg car bonates in the ocean is smaller than the CO2 degassing. The relative m obility of the least mobile elements during weathering in southwest Ic eland is similar to that observed elsewhere in the world under remarka bly variable climatic conditions. Thus we agree with Nesbitt and Wilso n (1992) and Taylor and others (1992) that laterites and bauxites are not necessarily representative of a tropical climate, but rather the r atio of mechanical versus chemical denudation rates. Icelandic precipi tation shows a normal distribution around a mean pH of 5.4. Na/Cl, K/C l, Mg/Cl, and Sr/Cl ratios in the precipitation are close to oceanic r atios, indicating that they are solely of marine origin. The concentra tions of Ca, SO4, NO3, and NH4 are higher than predicted by an unfract ionated marine contribution. The pH of spring-fed rivers In southwest Iceland is high, and they are relatively poor in total dissolved inorg anic carbon, calcium, and magnesium. The pH of other rivers ranges fro m 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 cha nnels of the rivers has enough time for significant heat exchange with its surroundings and significant gas exchange with the atmosphere, bu t the water-rock interactions are insignificant. The airborne dissolve d or soluble solids contribution to the total dissolved solids in rive rs 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 a irborne contribution is, in descending order; Cl, NO3, and NH4 (approx imate 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 primar ily, 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 nitr ogen sink in Icelandic catchment areas, caused by primary production o f progressive vegetation and biota.