Weathering assessment in the Achala Batholith of the Sierra de Comechingones, Cordoba, Central Argentina. II: major hydrochemical characteristics and carbon dynamics

摘要

La Trucha is a granitic, second-order catchment (similar to 1.9 km(2), average slope 5 %) placed at similar to 1400-m elevation in the Sierra de Comechingones (Argentina, 31 degrees 54' S, 64 degrees 45'W; 31 degrees 53', 64 degrees 44'W). The main geochemical characteristics were studied during a hydrological year (March 2005-February 2006), attending mainly seasonal and spatial variability and weathering-induced carbon consumption. Subjected to a weathering-limited denudation regime, the catchment is representative of hundreds of the second-order streams that dissect the Achala Batholith. Water chemistry is typical of streams draining F- rich granites. High discharges (austral summer) determine the dilution of Na+, Ca2+, Mg2+, HCO3-, SO42-, and F-. Conversely, high summer rainfall increases Cl(-)concentrations. The rainfall-induced mobilization of K+ is likely masked by its affinity for adsorption onto fine-grained particles and that of SiO2 by its biological consumption. La Trucha's upper catchment shows that steep slopes and a shallow regolith layer result in a more diluted water flow, chemically closer to rainfall. Subsurface flow increases the downriver positive rate of La Trucha's electrical conductivity twice as much in winter (6.7 mu S cm(-1) per 1000 m) than during wet summer (3.9 mu S cm(-1) per 1000 m). PHREEQC allowed the calculation of free CO2 and the likelihood of its evasion in different seasons and areas of the catchment. A mean carbon efflux of similar to 180 mg C m(-2) year(-1) has been estimated for La Trucha catchment. The first-and second-order streams, which occupy similar to 62.5 km(2) of the Achala Batholith's surface area, are significant sources in the Sierra de Comechingones' carbon cycle.