EFFECTS OF CLIMATE ON CHEMICAL-WEATHERING IN WATERSHEDS

Climatic effects on chemical weathering are evaluated by correlating v ariations in solute concentrations and fluxes with temperature, precip itation, runoff, and evapotranspiration( ET) for a worldwide distribut ion of sixty-eight watersheds underlain by granitoid rock types. Strea m solute concentrations are strongly correlated with proportional ET l oss, and evaporative concentration makes stream solute concentrations an inappropriate surrogate for chemical weathering. Chemical fluxes ar e unaffected by ET, and SiO2 and Na weathering fluxes exhibit systemat ic increases with precipitation, runoff, and temperature. However, war m and wet watersheds produce anomalously rapid weathering rates. A pro posed model that provides an improved prediction of weathering rates o ver climatic extremes is the product of linear precipitation and Arrhe nius temperature functions. The resulting apparent activation energies based on SiO2 and Na fluxes are 59.4 and 62.5 kJ . mol(-1), respectiv ely. The coupling between temperature and precipitation emphasizes the importance of tropical regions in global silicate weathering fluxes, and suggests it is not representative to use continental averages for temperature and precipitation in the weathering rate functions of glob al carbon cycling and climatic change models. Fluxes of Ii, Ca, and Mg exhibit no climatic correlation, implying that other processes, such as ion exchange, nutrient cycling, and variations in lithology, obscur e any climatic signal. The correlation between yearly variations in pr ecipitation and solute fluxes within individual watersheds is stronger than the correlation between precipitation and solute fluxes of water sheds with different climatic regimes. This underscores the significan ce of transport-induced variability in controlling stream chemistry, a nd the importance of distinguishing between short-term and long-term c limatic trends. No correlation exists between chemical fluxes acid top ographic relief or the extent of recent glaciation, implying that phys ical erosion rates do not have a critical influence on chemical weathe ring rates.