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

Abstract

La Trucha is a granitic, second-order catchment (~1.9 km2, average slope 5%) placed at ~1400 m elevation in the Sierra de Comechingones (Argentina, 31°54´S, 64°45´W; 31°53´, 64°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 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 µS cm-1 per 1000 m) than during wet summer (3.9 µS 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 ~180 mg C m-2 yr-1 has been estimated for La Trucha catchment. First- and second-order streams, which occupy ~62.5 km2 of the Achala Batholith´s surface area, are significant sources in the Sierra de Comechingones´carbon cycle.