HYDROCHEMICAL APPRAISAL OF ICE- AND ROCK-GLACIER MELTWATER IN THE HYPERARID AGUA NEGRA DRAINAGE BASIN, ANDES OF ARGENTINA

LECOMTE, K.L.; MILANA, J.P.; FORMICA, S.M.; DEPETRIS, P.J.

HYDROLOGICAL PROCESSES

Wiley

Año: 2008 vol. 22 p. 2180 - 2195

0885-6087

The Agua Negra drainage system (30°120S, 69°500 W), in the Argentine Andes holds several ice- and rock-glaciers, whichare distributed from 4200 up to 6300 m a.s.l. The geochemical study of meltwaters reveals that ice-glaciers deliver aHCO3-–Ca2+solution and rock-glaciers a SO42-–HCO3-–Ca2+ solution. The site is presumably strongly influenced bysublimation and dry deposition. The main processes supplying solutes to meltwater are sulphide oxidation (i.e. abundanthydrothermal manifestations), and hydrolysis and dissolution of carbonates and silicates. Marine aerosols are the main sourceof NaCl. The fine-grained products of glacial comminution play a significant role in the control of dissolved minor and traceelements: transition metals (e.g. Mn, Zr, Cu, and Co) appear to be selectively removed from solution, whereas some LIL (largeion lithophile) elements, such as Sr, Cs, and major cations, are more concentrated in the lowermost reach. Daily concentrationvariation of dissolved rare earth elements (REE) tends to increase with discharge. Through PHREEQC inverse modelling, it isshown that gypsum dissolution (i.e. sulphide oxidation) is the most important geochemical mechanism delivering solutes to theAgua Negra drainage system, particularly in rock-glaciers. At the lowermost reach, the chemical signature appears to changedepending on the relative significance of different meltwater sources: silicate weathering seems to be more important whenmeltwater has a longer residence time, and calcite and gypsum dissolution is more conspicuous in recently melted waters. Acomparison with a non-glacierized semiarid drainage of comparable size shows that the glacierized basin has a higher specificdenudation, but it is mostly accounted for by relatively soluble phases (i.e. gypsum and calcite). Meltwater chemistry inglacierized arid areas appears strongly influenced by sublimation/evaporation, in contrast with its humid counterparts.