HYDROLOGICAL PROCESSES Hydrol. Process. 22, 2180–2195 (2008) Published online 4 October 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.6816 Hydrochemical appraisal of ice- and rock-glacier meltwater in the hyperarid Agua Negra drainage basin, Andes of Argentina Karina L. Lecomte, 1 * Juan Pablo Milana, 2 Stella M. Formica 1 and Pedro J. Depetris 1 1 Centro de Investigaciones Geoqu´ ımicas y de Procesos de la Superficie (CIGeS), FCEFyN, Universidad Nacional de C´ ordoba, Avenida V´ elez. Sarsfield 1611, X5016GCA C´ ordoba, Argentina 2 Instituto de Geolog´ ıa, Universidad Nacional de San Juan, Avenida I. de la Roza y Meglioli, 5400 San Juan, Argentina Abstract: The Agua Negra drainage system (30 ° 12 0 S, 69 ° 50 0 W), in the Argentine Andes holds several ice- and rock-glaciers, which are distributed from 4200 up to 6300 m a.s.l. The geochemical study of meltwaters reveals that ice-glaciers deliver a HCO 3  –Ca 2C solution and rock-glaciers a SO 4 2 –HCO 3  –Ca 2C solution. The site is presumably strongly influenced by sublimation and dry deposition. The main processes supplying solutes to meltwater are sulphide oxidation (i.e. abundant hydrothermal manifestations), and hydrolysis and dissolution of carbonates and silicates. Marine aerosols are the main source of NaCl. The fine-grained products of glacial comminution play a significant role in the control of dissolved minor and trace elements: transition metals (e.g. Mn, Zr, Cu, and Co) appear to be selectively removed from solution, whereas some LIL (large ion lithophile) elements, such as Sr, Cs, and major cations, are more concentrated in the lowermost reach. Daily concentration variation of dissolved rare earth elements (REE) tends to increase with discharge. Through PHREEQC inverse modelling, it is shown that gypsum dissolution (i.e. sulphide oxidation) is the most important geochemical mechanism delivering solutes to the Agua Negra drainage system, particularly in rock-glaciers. At the lowermost reach, the chemical signature appears to change depending on the relative significance of different meltwater sources: silicate weathering seems to be more important when meltwater has a longer residence time, and calcite and gypsum dissolution is more conspicuous in recently melted waters. A comparison with a non-glacierized semiarid drainage of comparable size shows that the glacierized basin has a higher specific denudation, but it is mostly accounted for by relatively soluble phases (i.e. gypsum and calcite). Meltwater chemistry in glacierized arid areas appears strongly influenced by sublimation/evaporation, in contrast with its humid counterparts. Copyright  2007 John Wiley & Sons, Ltd. KEY WORDS meltwater chemistry; weathering; Andes; trace elements; REE; PHREEQC Received 04 September 2006; Accepted 1 May 2007 INTRODUCTION Weathering and meltwater chemistry in glacierized areas has received considerable attention in the specialized lit- erature (Raiswell, 1984; Anderson et al., 2000; Brown, 2002; Dixon and Thorn, 2005) because such environ- ments are of importance when approximating the relative significance of physical and chemical erosion rates in cold, high altitude environments (Hodson et al., 2002a, 2002b; Lyons et al., 2003, 2005; Lafreni` ere and Sharp, 2005). Further, the role played globally by meltwaters in CO 2 sequestration during episodic deglaciation has also attracted scientific inquiry in view of its significance for the understanding of the impact at glacial–interglacial timescales (Sharp et al., 1995; Hodson et al., 2002a, Tranter et al., 2002). The landscape forms in the central Andes are mostly tectonic-dominated, with crustal thickening by tectonic wedge propagation. Recent studies have reinforced the * Correspondence to: Karina L. Lecomte, Centro de Investigaciones Geoqu´ ımicas y de Procesos de la Superfice (CIGeS), FCEFyN, Uni- versidad Nacional de C´ ordoba, Avenida V. Sarsfield 1611, X5016GCA C´ ordoba, Argentina. E-mail: klecomte@com.uncor.edu. notion that superimposed climate patterns have played a significant role in the control of orogen morphol- ogy (Montgomery et al., 2001). In the Bolivian Andes, for example, current in-depth analyses indicate that climate-driven erosion has exerted a first-order con- trol on its development (Barnes and Pelletier, 2006). In this scenario, the Andes in Argentina’s San Juan Province holds numerous temperate valley glaciers that have been retreating throughout the Holocene. In the area, at ¾30 ° S and >4200 m a.s.l., ice-glaciers coexist with rock-glaciers, and both supply a significant amount of water to the adjacent low lands, which, by virtue of a pronounced rain shadow, are intrinsically arid. There- fore, these glaciers are a major source of high-quality water and, hence, a valuable resource for the communi- ties thriving at the foothills. The origin of rock-glaciers is a subject of debate, in as much as there are two main hypotheses under discussion (Clark et al., 1998). Some authors consider rock-glaciers to be purely periglacial features (Wahrhaftig and Cox, 1959; Haeberli, 1985; Barsch, 1978, 1996). Other authors have proposed that some rock-glaciers have formed through burial and deformation of glacial Copyright  2007 John Wiley & Sons, Ltd.