HYDROLOGICAL PROCESSESHydrol. 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 meltwaterin the hyperarid Agua Negra drainage basin, Andes of

Argentina

Karina L. Lecomte,1* Juan Pablo Milana,2 Stella M. Formica1 and Pedro J. Depetris1

1 Centro de Investigaciones Geoquımicas y de Procesos de la Superficie (CIGeS), FCEFyN, Universidad Nacional de Cordoba, Avenida Velez.Sarsfield 1611, X5016GCA Cordoba, Argentina

2 Instituto de Geologıa, Universidad Nacional de San Juan, Avenida I. de la Roza y Meglioli, 5400 San Juan, Argentina

Abstract:

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

� –Ca2C solution and rock-glaciers a SO42� –HCO3

� –Ca2C 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. 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 areashas 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 relativesignificance of physical and chemical erosion rates incold, high altitude environments (Hodson et al., 2002a,2002b; Lyons et al., 2003, 2005; Lafreniere and Sharp,2005). Further, the role played globally by meltwaters inCO2 sequestration during episodic deglaciation has alsoattracted scientific inquiry in view of its significance forthe understanding of the impact at glacial–interglacialtimescales (Sharp et al., 1995; Hodson et al., 2002a,Tranter et al., 2002).

The landscape forms in the central Andes are mostlytectonic-dominated, with crustal thickening by tectonicwedge propagation. Recent studies have reinforced the

* Correspondence to: Karina L. Lecomte, Centro de InvestigacionesGeoquımicas y de Procesos de la Superfice (CIGeS), FCEFyN, Uni-versidad Nacional de Cordoba, Avenida V. Sarsfield 1611, X5016GCACordoba, Argentina. E-mail: klecomte@com.uncor.edu.

notion that superimposed climate patterns have playeda significant role in the control of orogen morphol-ogy (Montgomery et al., 2001). In the Bolivian Andes,for example, current in-depth analyses indicate thatclimate-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 JuanProvince holds numerous temperate valley glaciers thathave been retreating throughout the Holocene. In the area,at ¾30 °S and >4200 m a.s.l., ice-glaciers coexist withrock-glaciers, and both supply a significant amount ofwater to the adjacent low lands, which, by virtue of apronounced rain shadow, are intrinsically arid. There-fore, these glaciers are a major source of high-qualitywater 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 underdiscussion (Clark et al., 1998). Some authors considerrock-glaciers to be purely periglacial features (Wahrhaftigand Cox, 1959; Haeberli, 1985; Barsch, 1978, 1996).Other authors have proposed that some rock-glaciershave formed through burial and deformation of glacial

Copyright 2007 John Wiley & Sons, Ltd.

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2181

ice (Outcalt and Benedict, 1965; Corte, 1980; Whalleyand Martin, 1992; Hamilton and Whalley, 1995). In anycase, an active rock-glacier is widely used as a goodindicator of buried ice. This implies that such bodies havea complicated drainage system involving supraglacial andsubglacial melting (Thenthorey, 1992; Croce and Milana,2002b).

A number of investigations have been carried out inthe study area, mostly addressing physical and hydrolog-ical processes. Schrott (1991, 1996, 1998, 2002) studiedsolar radiation, soil temperature, and geomorphological–hydrological aspects in the permafrost. Barsch et al.(1994) presented some information on discharge vari-ability and sediment supply for the Agua Negra basin.Perucca and Carrizo (1998) studied natural hazards inthe Agua Negra highway. Leiva (1999, 2002) examinedspatial fluctuations of some Argentine glaciers, includingthe Agua Negra Glacier, and Milana and Maturano (1999)studied the structure of the Agua Negra Glacier throughseismic surveys and radio echo sounding. Croce andMilana (2002a, 2002b) reported on the internal structureand permafrost thickness of the El Paso and Dos Lenguasrock-glaciers. More recently, electrical tomography sur-veys have been carried out on El Paso, Dos Lenguas,

and Agua Negra rock-glaciers (Croce, 2006; Croce andMilana, 2006).The geochemistry of these Andean glaciershas received insufficient consideration in the specializedliterature.

In this paper it is intended to establish: (a) the prove-nance of solutes and the relative significance of eachsource; (b) the main rock weathering characteristicsunder conditions of extreme aridity; and (c) the identi-fication of processes that govern the downstream soluteevolution.

FIELD SITE

For this study, the upper catchments of the Agua Negrariver basin (¾240 km2) was selected, which is one of thebest known glaciated drainage basins in the arid to hyper-arid subtropical Andes. It is located in western Argentina(i.e. the Cuyo region), 30°120S and 69° 500W. The roadcrossing (R.N. 150) the Argentina–Chile internationalborder (the oceanic water divide) at a height of 4880 ma.s.l. (Figure 1), runs through the area, next to rock- andice-glaciers. The outfall of the studied drainage is locatedat Ojos de Agua, at 3350 m a.s.l. The basin extends uphill

Figure 1. Location map of Agua Negra drainage basin. Stiff diagrams at each sampling point are included as a guide to follow chemical evolution.Sampling sites are keyed in Table I

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2182 K. L. LECOMTE ET AL.

to the summits of Cordillera de Olivares, reaching a max-imum height of 6266 m a.s.l.

The high Andes in the province of San Juan arecharacterized by arid to hyperarid conditions, withhigh solar radiation intensities throughout the year.Global solar radiation measured at heights of 4150and 4720 m reaches annual means of 20Ð6 MJ m�2 and22Ð3 MJ m�2, respectively (Schrott, 1996). Maximumtemperatures (¾22 °C) occur in February and minimum(¾ � 27 °C) in July. Relative humidity varies between16 and 80%. Above 4000 m a.s.l., annual precipitationoscillates between 100 and 350 mm, mostly as snowfall,and occurs almost exclusively during the (austral) wintermonths (Perucca and Carrizo, 1998).

The tundra biome dominates everywhere in the stud-ied area. The leading biological characteristics are thecomplete absence of trees and low biotic diversity, frost-moulded landscapes, and short growing seasons. Alsosignificant is the fact that energy and nutrients are storedmainly in the form of dead organic material. The plantsinclude tussock grasses, small-leafed shrubs (e.g. Stipasp., Prosopis sp.), and heaths. Most streams in the AguaNegra drainage basin are of the kryal type, since they aredirectly fed by glacial meltwater; other smaller catch-ments are of the rhithral type because they are fed bysnowmelt and occasional rainfall (Ward, 1994).

The geology of the basin is composed primarily ofphysically weathered andesites and other volcanic rocks(Choiyoi Group, Carboniferous–Permian), which are ontop of sandstones and mudstones of the Agua Negra Fm.(Carboniferous). There are minor proportions of asso-ciated acid volcanic (rhyolites) and pyroclastic rocks,whereas the basalts of the Olivares complex (Late Ter-tiary and Quaternary) crop out as thick dark layers inthe uppermost edge of the basin. Of some significanceare localized areas with hydrothermal alteration, whichexhibit a high proportion of sulphides that appear as red-dish patches. Ramos (1999) described the correspondingregional geology.

The outfall of the Agua Negra drainage basin is at thebase of what is probably the lowermost frontal moraineassociated with the Pleistocene Agua Negra Glacier. Thismoraine acted as a sediment-retaining dam that generateda several kilometre-long braid plain of highly permeablematerial.

Three main streams contribute to the main channel:the Pircas Negras; the San Lorenzo; and the Agua Negrastream itself (Figure 1).

(1) The Pircas Negras stream is fed by the largest ice-glacier in the area. A radio echo sounding survey ofthe Pircas Negras glacier showed that it is 200 m thickin the centre and has an ice volume 20 times largerthan the Agua Negra glacier (Milana and Maturano,1999). Moreover, the Pircas Negras Glacier is facingnorthwest and, therefore, it is more exposed to solarradiation and produces more meltwater than otherglaciers in the area. The Pircas Negras stream runsalong a 12 km channel before joining the Agua Negra

main stem (Figure 1). A representative sample of thisstream is M2 (Figure 1).

(2) The San Lorenzo stream is fed by meltwater inabout equal proportions from ice- and rock-glaciers. Arepresentative sample of this stream is M3 (Figure 1).

(3) The Agua Negra stream itself has, like the SanLorenzo, a mixed water supply. Each signal wassampled separately to characterize the mixing process.Representative samples for this stream are M6, M7,M8, M9 and M1 (Figure 1). Five main solute sourceswere identified for this stream:

Widespread snow penitentes (2 to 3 m high) thatdevelop seasonally due to intense snow-melting andsublimation. Meltwater usually enters the fractured rocksunderneath, whereas the largest snow patches develop asmall water stream at their base. A representative sampleis M5-snow (Figure 1).

The Agua Negra Glacier: this is a glacier withoutdebris cover (i.e. ice-glacier) located at the uppermostreaches of the Agua Negra drainage basin (M5-ice,Figure 1). The water usually does not reach the mainstream as it seeps through the frontal moraines; a pro-glacial pond collects glacier meltwater and dries upcompletely during winter (M6, Figure 1). This glacierproduces only supraglacial meltwater since it has beenrecognized—owing to its placement far above the 0 °Cannual isotherm—as a cold-based glacier (Milana andMaturano, 1999).

The Agua Negra rock-glacier: as indicated above, thisrock-glacier is formed directly downstream from thepro-glacial pond and therefore is fed mainly by glaciermeltwater. A representative sample is M7 (Figure 1).

The El Paso rock-glacier: this rock-glacier occupies theupper part of a small valley. It probably delivers the mostrepresentative chemical signal of rock-glacier meltwaterbecause there are few seasonal snow patches, and waterdrains directly at the foot of the rock-glacier terminaltalus. A representative sample is M4 (Figure 1).

The Dos Lenguas rock-glacier: this is the largest rock-glacier in the area. It causes partial blocking of the AguaNegra main stream and, hence, it was not possible tosample its meltwater discharge as it reaches the streamdirectly by subsurface intergranular flow. The samplingstrategy for this important rock-glacier was to obtain asample of the main stream a few metres upstream (M8)and downstream (M9) the intersection of the frontal taluswith the stream (Figure 1).

SAMPLING AND ANALYTICAL METHODOLOGY

During two field campaigns, surface water, ice, and snowsamples were collected to examine the differences in thechemistry of ice- and rock-glacier (permafrost) meltwa-ter. The first sampling was performed in March 2003, atthe beginning of the southern fall, when ice/snow-meltingwas somewhat suppressed and there was an impreciseconnection between meltwater discharge and total solute

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2183

production (i.e. electrical conductivity). At the time, theice-glacier meltwater contribution was probably decreas-ing. The second sampling took place in February 2004,during summer, and a clear relationship existed betweensolute/sediment production and meltwater discharge, thusindicating that ice-glacier melting processes were fullyoperational at the time. Discharge was estimated fromthe total suspended solids (TSS)–discharge relationshipcalculated by Barsch et al. (1994).

Sampling points were selected at the outfall of glaciers,wherever possible, and at the main Agua Negra stream(Figure 1). Samples are identified as 1Mx or 2Mx, iftaken during the first or second sampling, respectively; xis the sampling site. A series of samples was collectedover several hours at the lower drainage basin to analysethe daily evolution in total dissolved solids (TDS) andwater chemistry.

Water temperature, pH, electrical conductivity, andalkalinity were measured in situ. Alkalinity was measuredas CaCO3 with a 0Ð1600 N H2SO4 titration in unfilteredwater, until the end point. In this case, filtration was notperformed because it could modify the dissolved gasesconcentration in the sample (Eaton et al., 1995).

For subsequent determinations, samples were vacuum-filtered in the field with 0Ð22 µm pore size cellulose filters(HA-type, Millipore Corp, Bedford, MA 01730, USA)and divided into two aliquots. One aliquot was acidified(pH ³ 2) with concentrated and redistilled (½99Ð999%)HNO3 (Aldrich Chemical Co., Milwaukee, WI 53201,USA) for the analytical determination of major cations,minor, and trace elements by ICP-MS (Activation Lab-oratories Ltd., Ancaster, Ontario, Canada). The otheraliquot was stored in polyethylene bottles at 4 °C fordeterminations of anions by chemically suppressed ionchromatography with conductivity detection. The valid-ity of the results for major, minor, and trace elementswas carried out along with sample analysis, checkingwith NIST-1640 (Riverine Water Reference Materials forTrace Metals certified by the National Research Coun-cil of Canada) and SRLS-4 along with method blanks(Table II). Duplicate analyses for one sample (1M4) areshown in Tables II and III.

Chemical data were processed with AQUACHEM soft-ware (Waterloo Hydrogeologic, Inc., Waterloo, Ontario,Canada N2L 3L3). PHREEQC (Parkhurst, 1995) inputfiles were constructed using the AQUACHEM PHREEQCinterface. These programs were used to calculate chargebalance, TDS, and to simulate dissolution–precipitation of minerals by means of an inverse mod-elling approach. For this purpose, it is necessary to knowthe physical and chemical characteristics of an initialand a final solution, and also the reacting gas phasesand the minerals susceptible to weathering. The modelquantifies the processes of dissolution and/or precipita-tion that lead to the final solution chemistry. Applicationsusing PHREEQC in the analysis of geochemical data arecommon in the literature: Uliana and Sharp (2001) usedinverse modelling to study groundwater evolution; Eary

et al. (2003) assessed water quality changes in connec-tion with mining operations; and Lecomte et al. (2005)modelled geochemical dissolution-precipitation processesin semi-arid mountain rivers.

RESULTS AND DISCUSSION

Stream chemistry: major components

In glaciers, daily discharge variability is determinedby the energy received by the ice surface, which dependsmainly on the intensity of solar radiation, and by the sen-sible heat flux. Discharge maxima lag radiation maxima,depending on the season and gauge location. Summerobservations (November 1990 to April 1991) at the AguaNegra glacier showed a discharge mean ³0Ð35 m3 s�1

with peak discharges of ¾1Ð4 m3 s�1 at Ojos de Agua(M1) (Schrott, 2002). Downstream from this point, dis-charge decreases due to infiltration and evaporation. Dur-ing the same period, the suspended sediment transportrate was ¾0Ð057 T h�1 (¾4Ð4 g m�2 h�1) (Barsch et al.,1994). The summertime meltwater discharge at DosLenguas, which is the largest rock-glacier in the basin,fluctuates between 0Ð005 and 0Ð008 m3 s�1 (Schrott,2002).

Table I shows the main environmental types andphysicochemical characteristics, the concentration of dis-solved major components, and the charge balances deter-mined in the collected samples. Water samples areslightly acid to alkaline. Samples belonging to the March2003 campaign (first sampling) had a pH variation from¾6Ð4 (ice and snow) to ¾8Ð2 (at Ojos de Agua, 1M1). It isevident that pH increases downstream. During the secondsampling (February 2004), pH was more constant thanduring the first sampling, from ¾7Ð1 (El Paso rock-glaciermeltwater, 2M4) to ¾7Ð8 (Ojos de Agua, 2M1). Alkalin-ity, as HCO3

�, fluctuated in the upper reaches between3Ð6 mg L�1 (snow and glacial meltwater) and 36Ð6 mgL�1 at 1M7, in samples collected during the first fieldtrip. At Ojos de Agua (1M1), alkalinity varied between58Ð5 mg L�1 and 81Ð2 mg L�1 in the first sampling,and between 50Ð4 mg L�1, and 56Ð2 mg L�1 in watersamples collected in February 2004 (2M1). This pro-nounced variation between the upper and lower reacheswas also observed in water conductivity: 13 µS cm�1

(snow), 28Ð5 µS cm�1 (ice), 405 µS cm�1 during the firstsampling (1M1), and 479 µS cm�1 during the secondone (2M1). Recent investigations (Ginot et al., 2001 andreferences therein) have stressed the role played in thetropics and subtropics by high radiation, temperature andwindiness, and low relative humidity and snow accumu-lation rates, all factors that suggest that glaciochemicalrecords might be significantly affected by sublimation anddry deposition.

Figure 2a shows the daily summertime discharge varia-tion (modified from Barsch et al., 1994) measured duringthree days at Cuatro Mil (4000 m a.s.l.) and Kolibri(3150 m a.s.l.) gauge stations, which are upstream anddownstream, respectively, of Ojos de Agua (3350 m

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2184 K. L. LECOMTE ET AL.

Tabl

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ple

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iron

men

tTy

pepH

Con

duct

ivit

y(µ

Scm

�1)

Tem

pera

ture

(°C

)C

a2CN

aCK

CM

g2CC

l�

(mg

L�1

)SO

42�

NO

3�

HC

O3

�H

4Si

O4

Cha

rge

bala

nce

(%)

1°Sa

mpl

ing

cam

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1-12

:00

mix

ed7Ð7

640

5Ð07Ð3

51Ð00

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0Ð79

8Ð71

n.d.

n.d.

n.d.

61Ð37

17Ð84

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536

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1Ð75

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Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2185

Dis

char

ge (

l/s)

600

400

200

06 12 18 24 6 12 18 24 6 12 18 24

Kolibri

Cuatro Mil

6

Time (hs)

25- 1- 91 26- 1- 91 27- 1- 91 28- 1- 91

1817 19 20 21 22 23

Tem

perature (°C)

Conductivity (µS cm-1)

Temperature (°C)

200

250

300

350

400

450

0

2

4

6

8

10

12

14

16

18

20

500

22

24

250

300

350

400

450

468

1012141618

Conductivity (µS cm-1)

Tem

pera

ture

°C

February 2004March 2003

Q=~650 l/sTSS=~170 mg/l

Q=~780 l/sTSS=~450 mg/l

Q=~870 l/sTSS=~930 mg/l

Q =~950 l/sTSS =~1600 mg/l

Time (hs)

Con

duct

ivity

(µS

cm

-1)

(a)

(b)

Figure 2. (a) Daily discharge variability at Cuatro Mil (4000 m a.s.l.) and Kolibri (3150 m a.s.l.) gauging stations. Modified from Barsch et al.(1994); (b) time-dependent evolution of water temperature and electrical conductivity at 2M1; measured TSS and estimated discharge are included.

Insert shows the scatter plot of conductivity and temperature at M1 (both samplings)

a.s.l.). The meltwater flow started increasing at mid-afternoon, reaching a maximum discharge at about mid-night.

A similar situation was observed during the secondsampling (Figure 2b): the discharge increase was accom-panied by a significant decrease in water temperature andelectrical conductivity. The temperature decline rate issteeper after 19 : 00, whereas water conductivity showsa variation that is probably connected with changes insolute sources. In contrast, TSS concentration increasedtenfold in response to a ¾50% discharge increase. Dis-charge is controlled by the ice melting rate and, therefore,conductivity and water temperature are discharge prox-ies. Hence, both variables show a significant non linearrelationship for the second sampling (Figure 2b, insert).The erratic association between conductivity and temper-ature during the first sampling reveals the absence of aclear discharge control (Figure 2b, insert).

It is interesting to analyze the system’s contrastinghydrological dynamics during the samplings (Table I)performed at Ojos de Agua (M1). During the first one(March 2003), there was no time-dependant trend in theconcentration of the more soluble components (e.g., NaC,

Ca2C, Mg2C, HCO3�, Cl�), whereas it became linearly

decreasing during the second sampling (February 2004).Likewise, silicon (as H4SiO4 in Table I) shows a signifi-cant variation in concentration without any trend duringthe ¾8 h sampling series performed during the first sam-pling but, in contrast, there is a significant correlationwith conductivity during the second sampling. Similarly,NO3

� also shows a decreasing concentration trend duringthe second sampling series, which is probably associatedwith an increased net biological uptake occurring duringsummer.

Stiff diagrams were used in Figure 1 to show the meanchemical composition at each sampling spot. It is appar-ent that there is a separation between SO4

2�-dominatedsamples and HCO3

�-dominated samples. Samples corre-sponding to the first group (SO4

2�-dominated) are 1M1,2M1, 1M8, 2M8, 1M9, 2M9, 1M3, and 2M4. All thesamples collected at the outfall of ice-glaciers belongto the HCO3

�-dominated water-type (1M2, 2M2, 1M5-ice, 1M5-snow, and 1M6); 1M7, has mixed sources, butbelongs to the HCO3

�-dominated water-type because theAgua Negra rock-glacier supplies less meltwater than theAgua Negra ice-glacier.

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2186 K. L. LECOMTE ET AL.

80 20 20 80

20

80 80

20

20

80

20

80

Ca+2 Na+ + K+

Na + + K +

Cl-

SO42-

50

50

50

50

5050

M5-iceM5-snow

Upstream

Downstream

Ice-glacier meltwaterRock-glacier meltwaterMixed meltwater

Upstream

Downstream

HCO3- + CO3

2-

HC

O 3- +

CO 3

2-

Ca +2 + M

g +2SO

42-

+ C

l-

Mg+2

Figure 3. Piper diagram of Agua Negra basin water samples, showing the downstream chemical evolution (arrows). Environment-types are includedfor reference

Figure 3 shows a Piper diagram (Piper, 1944) that indi-cates that Agua Negra drainage basin waters were domi-nated by Ca2C and HCO3

�- in the uppermost, ice-glacier-fed streams. Downstream, SO4

2� became the dominantanion. In contrast, snow is of the NaC —KC —HCO3

�-type. Clearly, as the stream order and the influenceof rock-glaciers increases, water samples switch to theCa2C —SO4

2�-type. Agua Negra glacial runoff is a diluteCa2C —HCO3

� —SO42� solution, as is the case in most

glaciers throughout the world. The sum of cation equiva-lents ranges from ¾90 to ¾4500 µeq L�1, slightly higherthan the world range (from ¾10 to ¾3500 µeq L�1),with ionic values higher than other glacial runoff fromdifferent regions of the world. Moreover, with the excep-tion of Cl� and HCO3

�, which have lower values inAgua Negra, the concentrations of the remaining ions aresignificantly higher than the mean global glacial runoff(Tranter, 2005).

Figure 4 shows a scatterplot of the ratio Ca2C: Si ver-sus the ratio HCO3

�: SO42�. As has been observed in

other glaciers, Agua Negra meltwater has high Ca2C: Siratios and low HCO3

�: SO42� ratios in SO4

2�-dominatedwaters. In contrast, HCO3

�-dominated waters show alarger variation (higher HCO3

�: SO42� ratios) but also

are clearly separated from other non-glacierized, semi-arid mountainous streams (Lecomte, 2006). Because ofits fine-grain-size distribution and, hence, its increasedspecific surface area, glacial flour accelerates the hydroly-sis of carbonate and silicate species when in contact withice meltwater. In contrast, sulphide oxidation seems tobe a very important reaction in subglacial environments.Considering that ice-glaciers in the region are cold-based(Milana and Maturano, 1999), subglacial environmentsmostly occur in rock-glaciers, where there is a layer ofwet debris (Croce and Milana, 2002a). Thus, sulphide

oxidation becomes dominant in rock-glaciers (a debris-rich environment), where microbially-mediated reactionsinvolving sulphide oxidation take place when water isin contact with sulphide minerals in the comminutedbedrock:

4FeS2�s� C 16CaCO3�s� C 15O2�aq�

C 14H2O�aq� ) 16Ca2C�aq�

C 16HCO3��aq� C 4Fe�OH�3�s� C 8SO4

2��aq�

In this reaction, four moles of pyrite produce eightof sulphate. The protons resulting from the oxidationof pyrite drive further the dissolution of calcite, butthe net effect of the coupled reaction is to increasealkalinity. Moreover, in an oxygen-poor environment,

0

5

10

15

20

25

1 3 5 7 9 11 13 15

R2 = 0.88 (p < 0.05)

Ca+

2 : S

i

HCO3--dominated

Non-glacierized

SO42- -dominated

HCO3- : SO4

2-

Figure 4. Scatter plot of Ca2C : Si ratio versus HCO3� : SO4

2� ratiofor ice- and rock-glacier meltwater and for non-glacierized catchments

(Lecomte, 2006)

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2187

such as permafrost, the oxygen supply is limited to thatreleased from bubbles in the ice during regelation (i.e. theprocess of ice melting and refreezing as it flows aroundbedrock obstacles). It has been argued that under anoxicconditions, Fe (III), rather than O2, acts as an oxidizingagent (Tranter, 2005).

Figure 5a, b and c shows the theoretical stoichiometricrelationships between the ions of minerals such asanhydrite, calcite and halite. The significant correlationbetween Ca2C and SO4

2� (Figure 5a) and the coherenceof the resulting regression line with the theoreticalanhydrite dissolution indicates that gypsum is a majorsource for Ca2C. If we subtract the Ca2C accounted forby the dissolution of gypsum (or anhydrite) in HCO3

�-dominated samples, we still obtain some remnant Ca2C

that plots close to the theoretical dissolution line ofcalcite (Figure 5b). The relationship suggests, however,that there is still a third source for Ca2C (i.e. theincongruent dissolution of plagioclase) accounting forthe remainder. Likewise, Cl� is mainly contributedby meteoric precipitation, and NaC (Figure 5c) reachesconcentrations four times higher than the theoretical1 : 1 ratio. It follows then that there are other sourcesbesides halite from aerosols that supply dissolved NaC

to meltwater. All Cl� in meltwater is considered derivedfrom marine aerosols.

Figure 5d shows the molar relationship (corrected foratmospheric input) between Ca2C: NaC, andHCO3

� : NaC Ð NaC is used for normalization to mini-mize the effect of dilution and evaporation (Picouet et al.,2002). The graph discriminates SO4

2�-dominated sam-ples from HCO3

�-dominated ones that plot along theplagioclase theoretical dissolution lines.

The marine solute contribution—via the atmosphericpath—to meltwater and, hence, to stream chemistry wascalculated by means of Xa D Clriv �X/Cl�a, where Xa

is the atmospheric contribution of X in streamwater;Clriv is the Cl� concentration in the water, and �X/Cl�a

the element ratio normalized to Cl� in the atmosphere(seawater ratio) (Picouet et al., 2002). Therefore, theatmospheric contribution is insignificant for Ca2C, SO4

2�

and Mg2C (less than 5%, except in ice, snow and in verydiluted samples), is low for KC (variable, depending onthe sampling site) and it is significant for NaC, with anatmospheric contribution of approximately 30% in eachsample.

Minor and trace elements in meltwaters

Major ion concentrations in meltwaters have beenreported extensively in the specialized literature. The con-centrations of dissolved minor and trace elements, how-ever, have received limited attention in glacier-dominateddrainages (Mitchell et al., 2002). Table II shows the con-centration of dissolved minor and trace elements, exclud-ing the REE. The extended multielemental diagrams (orspidergrams) in Figure 6 show the concentration vari-ability in glacier meltwater and associated streams, afternormalization. Figure 6a was normalized to an ice and

0.1

1

10

100

0.1 1 10 100

Andesine

Ca2+ : Na+

Labradorite

Bytownite

HCO3--dominated samples

SO42--dominated samples

Ice and snow

HC

O3-

: Na+

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400

1/4 theoretical halite dissolution

Na+ (µeq L-1)

Lower Agua Negra stream

y = 0.21x + 15.71R2 = 0.63 (p < 0.05)

Cl- (

µeq

L-1)

R2 = 0.87 (p < 0.05)

500

1000

1500

2000

2500

3000

3500

500 1000 1500 2000 2500 3000

Ca2+ (µeq L-1)

Theoretical anhydrite dissolution

SO42- -dominated samples

y = 1.05 x - 173SO

42- (

µeq

L-1)

Ca*2+ (µeq L-1)

0

200

400

600

800

1000

1200

0 200 400 600 800 1000

Theoretical calcite dissolutionHCO3

- -dominated samples

y = 1.52x - 15.48

R2 = 0.99 (p < 0.05)

HC

O3-

(µeq

L-1

)

(a)

(b)

(c)

(d)

Figure 5. Theoretical stoichiometric relationships (filled lines) betweenthe ions of (a) anhydrite; (b) calcite, CaŁ2C means Ca2C concentrationminus SO4

2� concentration; (c) halite, broken lines correspond to theobserved equivalent relationships; (d) molar ratio relationship (correctedfor atmospheric input) between Ca2C : NaC ratio, and HCO3

� : NaCratio. The lines correspond to the theoretical dissolution of plagioclase

(andesine, bytownite, and labradorite)

snow mean (samples M5-ice, and M5-snow), whereasFigure 6b was normalized to the upper continental crust(UCC) (Taylor and McLennan, 1985) in order to facilitatethe comparison with non-glacierized environments.

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2188 K. L. LECOMTE ET AL.

1.10-2

1.10-1

1

1.101

1.102

1.103

Sam

ple /

(ice-

snow

)

Si Al Fe Ti Mn Ba Sr Zr Rb Zn V Cr Cu Y Li Ni Pb Ga Sc Th Co Hf Cs U As Mo Sb

Si Al Fe Ti Mn Ba Sr Zr Rb Zn V Cr Cu Y Li Ni Pb Ga Sc Th Co Hf Cs U As Mo Sb

Mean ice-snow below limit detection

Glaciers meltwater samples

Agua Negra stream samples

Non-glacierized stream samples

1.10-7

1.10-6

1.10-5

1.10-4

1.10-3

1.10-2

1.10-8

Sam

ple /

UC

C

(b)

(a)

Figure 6. (a) Ice-snow-normalized extended variability diagram (spidergram) for minor and trace elements in glacier meltwater; and (b) uppercontinental crust (UCC)-normalized samples of Agua Negra main stream and non-glacierized stream samples (Lecomte, 2006)

An inspection of the snow and ice-normalized spider-gram (Figure 6a) shows those elements that were moreconcentrated in meltwater samples than in ice and snow(e.g. Ba, Sr, V, Y, Li, Ga, U, Cs, As, Mo). Some ofthem are very soluble whereas others probably exhibita higher normalized concentration as a result of drydeposition. It also shows some elements that have ahigher concentration in ice or snow than in meltwater(ppmsample : ppmice�snow < 10�1), probably because theyare affected by sublimation. Furthermore, it is likely thatsuch elements are also partially removed from solution byadsorption when dilute meltwater contacted fine-grainedglacial flour. Notably, such elements are Mn, Zr, Zn, Cu,and Co, which are transition metals, with the exceptionof Zr.

Figure 6b shows the UCC-normalized extended vari-ability diagram for stream water samples collected alongthe system’s main stem, the Agua Negra stream. Fol-lowing the solubility of minerals, and also their abun-dance in the rock outcrops, the trace elements that inFigure 6b present the highest normalized concentrations(i.e. ppmsample : ppmUCC > 10�4) are Cr, Cu, Sr, Zn, Li,Cs, U, As, Mo, and Sb. A few of these elements (Sr, Cs)are soluble large cations, known as “large ion litophile” orLIL elements (ionic potential <40 nm�1), whereas oth-ers are transition metals and high-field strength (HFS)elements.

Those that exhibit ppmsample : ppmUCC < 10�6 at theAgua Negra stream are Al, Fe, Ti, Zr, Y, Ga, Th, and Hf.Most of these elements are very insoluble, and correspondto the HFS group, which are small, highly charged cations(ionic potential ½40 nm�1). All the remaining elementsfall within the 10�6 –10�4 concentration range. Figure 6bhas, added for comparison, the UCC-normalized extendeddiagram obtained for dissolved trace elements deter-mined in non-glacierized, semiarid, mountainous streams(catchment heights >1500 m a.s.l.), draining granites andgneisses in the Sierras Pampeanas of Cordoba (Argentina)(Pasquini et al., 2004; Lecomte, 2006). Besides someminor departures, the similarity among patterns sug-gests that concentrations in these mountainous environ-ments are governed by mineral solubility, with a proba-ble second-order control imposed by sorption processes(Gaillardet et al., 2005). Here again, the elements thatare more concentrated in snow and ice than in streamwater are from the transitional group, presenting a highaffinity for colloid particles. In the acid pH range, theseelements are more concentrated in solution, which is thecase for ice and snow (pH ³6Ð5), and as pH increases,these metals are adsorbed onto adsorbing surfaces (e.g.colloids, organic matter, clays) decreasing their con-centration in solution. Interestingly, these spidergramsappear to convey what would be the typical dissolved

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2189

Tabl

eII

.D

isso

lved

min

oran

dtr

ace

elem

ent

conc

entr

atio

ns(p

pb)

atth

eA

gua

Neg

radr

aina

geba

sin

Sam

ple

Al

FeT

iM

nB

aSr

Zr

Rb

Zn

VC

rC

uY

Li

Ni

PbG

aSc

Th

Co

Hf

Cs

UA

sM

oSb

Ge

1M1a

53Ð1

192Ð4

2Ð12Ð5

24Ð3

121Ð0

0Ð03

0Ð97

2Ð50

0Ð49

9Ð20

2Ð71

0Ð062

24Ð31

0Ð470

0Ð814

0Ð028

1Ð84

0Ð004

0Ð074

0Ð001

0Ð028

3Ð033

2Ð512

1Ð680

0Ð075

0Ð019

1M2b

39Ð3

58Ð7

1Ð61Ð4

20Ð2

64Ð8

0Ð02

0Ð82

2Ð71

0Ð37

<0Ð5

0Ð36

0Ð092

9Ð29

<0Ð3

0Ð398

0Ð027

1Ð28

0Ð002

<0Ð0

050Ð0

020Ð0

482Ð4

562Ð1

770Ð5

430Ð0

200Ð0

151M

325

Ð124

3Ð11Ð6

1Ð918

Ð776

Ð00Ð0

21Ð4

012

Ð690Ð9

9<

0Ð51Ð2

90Ð2

0414

Ð750Ð6

750Ð6

010Ð0

241Ð3

50Ð0

020Ð1

04<

0Ð001

0Ð702

1Ð331

2Ð924

0Ð662

0Ð052

0Ð021

1M4

31Ð7

57Ð3

1Ð11Ð0

5Ð040

Ð80Ð0

10Ð5

90Ð7

50Ð3

5<

0Ð50Ð5

60Ð0

614Ð3

8<

0Ð30Ð2

820Ð0

28<

1<

0Ð001

<0Ð0

05<

0Ð001

0Ð043

1Ð365

0Ð851

0Ð259

0Ð025

0Ð010

1M4

(2)

33Ð7

62Ð1

1Ð21Ð2

5Ð141

Ð30Ð0

10Ð6

12Ð2

10Ð3

8<

0Ð50Ð5

30Ð0

674Ð4

8<

0Ð30Ð3

540Ð0

26<

10Ð0

01<

0Ð005

<0Ð0

010Ð0

441Ð3

820Ð8

530Ð2

790Ð0

31<

0Ð01

1M5 s

now

28Ð2

16Ð6

1Ð26Ð0

1Ð31Ð3

0Ð05

0Ð57

12Ð78

<0Ð1

0<

0Ð54Ð7

00Ð0

35<

1<

0Ð30Ð4

26<

0Ð01

<1

0Ð002

0Ð076

0Ð002

0Ð009

0Ð008

0Ð099

<0Ð1

0Ð062

<0Ð0

11M

5 ice

30Ð2

16Ð3

1Ð219

Ð85Ð9

2Ð60Ð0

50Ð4

68Ð1

80Ð1

1<

0Ð53Ð7

70Ð0

26<

1<

0Ð30Ð2

090Ð0

11<

10Ð0

030Ð4

680Ð0

010Ð0

200Ð0

250Ð3

81<

0Ð10Ð0

65<

0Ð01

1M6

10Ð8

<10

0Ð83Ð0

1Ð53Ð2

<0Ð0

10Ð0

62Ð4

4<

0Ð10

<0Ð5

0Ð67

0Ð016

<1

<0Ð3

0Ð133

<0Ð0

1<

10Ð0

020Ð0

08<

0Ð001

<0Ð0

010Ð0

770Ð4

52<

0Ð10Ð0

23<

0Ð01

1M7

25Ð1

42Ð6

1Ð20Ð5

9Ð640

Ð80Ð0

10Ð3

5<

0Ð50Ð6

3<

0Ð50Ð3

20Ð0

484Ð7

2<

0Ð30Ð1

580Ð0

25<

1<

0Ð001

<0Ð0

050Ð0

010Ð0

221Ð7

891Ð9

611Ð2

160Ð0

58<

0Ð01

1M8

30Ð1

129Ð6

1Ð41Ð2

21Ð4

96Ð9

0Ð01

0Ð69

21Ð03

0Ð21

<0Ð5

0Ð88

0Ð053

13Ð36

<0Ð3

0Ð522

0Ð016

1Ð00

0Ð002

<0Ð0

05<

0Ð001

0Ð027

2Ð132

1Ð238

1Ð228

0Ð172

<0Ð0

11M

953

Ð125

9Ð51Ð4

10Ð2

22Ð1

110Ð8

0Ð01

1Ð08

4Ð55

<0Ð1

0<

0Ð50Ð5

10Ð0

7419

Ð120Ð5

221Ð1

960Ð0

121Ð1

00Ð0

010Ð8

06<

0Ð001

0Ð091

1Ð988

0Ð436

1Ð330

0Ð071

0Ð013

2M1c

28Ð0

33Ð7

1Ð11Ð2

21Ð6

114Ð9

<0Ð0

11Ð5

90Ð8

80Ð1

6<

0Ð50Ð5

40Ð0

4321

Ð630Ð6

040Ð0

630Ð0

261Ð1

00Ð0

02<

0Ð005

<0Ð0

010Ð1

212Ð3

591Ð6

111Ð6

050Ð0

780Ð0

112M

218

Ð8<

100Ð8

1Ð214

Ð048

Ð1<

0Ð01

0Ð92

0Ð80

0Ð30

<0Ð5

0Ð31

0Ð054

7Ð72

<0Ð3

0Ð071

0Ð032

<1

<0Ð0

01<

0Ð005

<0Ð0

010Ð0

622Ð2

702Ð1

670Ð4

080Ð0

22<

0Ð01

2M4

13Ð0

14Ð7

0Ð80Ð7

3Ð737

Ð9<

0Ð01

0Ð61

1Ð39

0Ð19

<0Ð5

0Ð38

0Ð031

4Ð31

<0Ð3

0Ð249

0Ð024

<1

<0Ð0

01<

0Ð005

<0Ð0

010Ð0

281Ð5

020Ð7

260Ð2

230Ð0

20<

0Ð01

2M8

52Ð8

18Ð8

0Ð527

Ð212

Ð969

Ð0<

0Ð01

0Ð68

0Ð83

<0Ð1

0<

0Ð50Ð2

60Ð0

409Ð4

10Ð4

830Ð0

850Ð0

15<

1<

0Ð001

0Ð157

<0Ð0

010Ð0

501Ð9

910Ð9

210Ð7

500Ð0

64<

0Ð01

2M9

57Ð1

32Ð8

0Ð617

Ð813

Ð377

Ð0<

0Ð01

0Ð84

1Ð24

<0Ð1

0<

0Ð50Ð2

10Ð0

4112

Ð65<

0Ð30Ð0

410Ð0

11<

1<

0Ð001

0Ð083

<0Ð0

010Ð0

672Ð3

090Ð9

270Ð8

440Ð0

68<

0Ð01

Bla

nk2

100Ð1

0Ð10Ð1

0Ð04

0Ð01

0Ð005

0Ð50Ð1

00Ð5

0Ð20Ð0

031

0Ð30Ð0

10Ð0

11

0Ð001

0Ð005

0Ð001

0Ð001

0Ð002

0Ð03

0Ð10Ð0

10Ð0

1

SLR

S-4

4592

1Ð53Ð2

11Ð4

28Ð8

0Ð08

1Ð59

0Ð90Ð3

1�0

Ð51Ð8

0Ð124

�10Ð7

0Ð1�0

Ð01�1

0Ð009

0Ð030

0Ð003

0Ð005

0Ð038

0Ð74

0Ð21

0Ð24

�0Ð01

NIS

T16

4049

471Ð0

110

160

130

0Ð17

2Ð10

48Ð3

12Ð3

39Ð4

84Ð0

0Ð248

5626

Ð237

Ð70Ð0

12

0Ð012

20Ð4

0Ð004

0Ð111

0Ð545

27Ð1

55Ð9

12Ð8

0Ð10

Exp

ecte

dVa

lues

SLR

S-4

5410

3—

3Ð412

26Ð3

——

0Ð90Ð3

0Ð31Ð8

——

0Ð67

0Ð09

——

—0Ð0

3—

—0Ð0

50Ð6

80Ð2

0Ð23

—

NIS

T16

4052

34—

122

148

124

—2

5312

Ð939

85—

50Ð7

27Ð4

27Ð9

——

—20

Ð3—

——

26Ð7

4713

Ð8—

am

ean

valu

e(n

D6)

;Łb

mea

nva

lue

(nD

2);

Łcm

ean

valu

e(n

D4)

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2190 K. L. LECOMTE ET AL.

signature of small and pristine basins in high moun-tains. The patterns of high-altitude glacial, hyperarid andsedimentary–volcanic catchments are remarkably similarto those obtained for mountainous rivers in granite- andgneiss-dominated semiarid catchments (Pasquini et al.,2004; Lecomte, 2006), included in Figure 6b for compari-son. The resemblance rests on the patterns of normalizedconcentrations and on the concentration range that, inboth instances, deviates between 10�2 and 103 from meansnow and ice concentrations.

The contrasting behavior—according to theenvironment—of each group of elements (LIL, HFS, andtransition metals) is presented by means of a set of boxand whiskers in Figure 7. Clearly, LIL elements are moreconcentrated in so-called mixed environments (i.e. ice-plus rock-glaciers); transition metals are more concen-trated in ice and snow, and HFS elements do not show aclear-cut pattern.

REE daily concentration variations in meltwater andstreams are remarkably flow-dependent. The concentra-tion range reaches almost two orders of magnitude; it issignificantly higher than the variation observed in othertrace elements (Tables II and III). In UCC-normalizedspidergrams (not shown), there is a slight pre-eminence ofmiddle REE (Sm to Dy) over heavy (Ho to Lu) and light(La to Nd) REE (Sholkovitz, 1995). Such predominanceis also evident in the spidergrams obtained for samplescollected in the non-glacierized mountainous landscape ofthe Sierras Pampeanas, in Argentina’s Cordoba Province(Pasquini et al., 2004; Lecomte, 2006). As a rule, dis-solved REE are more concentrated in granite-dominatedstreams and rivers of Cordoba than in Andean meltwater.

Samples collected at M1 show that the sum of REEconcentrations determined in the first sampling (March2003) was higher than that determined during the secondsampling (February 2004). Also, they showed a directrelationship (Figure 8a and b) with discharge (i.e. inverserelation with conductivity) that other trace elements (e.g.Cu, Ni, Pb) failed to show.

Another aspect worthy of attention is the conspicu-ous positive europium anomaly (Eu/EuŁ D EuN/�SmN ÐGdN�0Ð5, where the subscript N denotes normalization,and EuŁ is the expected Eu value for a smooth N-normalized REE pattern, e.g. McLennan, 1989), whichreached values of ¾3Ð8 and is absent in 1M5-snow.Eu/EuŁ is widely used in petrology, may be positive(enrichment) or negative (depletion), and results fromthe substitution of Sr by Eu in feldspars (notably in Ca-plagioclase). In this case, the dissolved Eu/EuŁ is a directconsequence of plagioclase weathering.

Likewise, the cerium anomaly (Ce/CeŁ D Ce/�LaN ÐPrN�0Ð5 where CeŁ is the expected Ce value for asmooth N-normalized REE pattern, McLennan, 1989)occurs in response to the oxidation of Ce3C to Ce4C

and its subsequent precipitation from solution as CeO2

(Brookins, 1989). Therefore, the negative Ce/CeŁ inmeltwater samples (between 0Ð23 and 0Ð96 except in1M5-snow and in 2M4) is determined by its removal fromsolution as CeO2 (Brookins, 1989). The two anomalies

Ice and snow Ice-glacier meltwater

Rock-glacier meltwater Mixed

0

2

4

6

8

10

12

14

0

20

40

60

0

500

1000

1500

2000

Σ LI

L el

emen

ts (

µg L

-1)

Σ tr

ansi

tiona

l ele

men

ts (

µg L

-1)

Σ H

FS

ele

men

ts (

µg L

-1)

(a)

(b)

(c)

Figure 7. Box and whiskers plot for each set of samples (ice and snow,rock-glacier and ice-glacier meltwater, and mixed environments). Theboxes correspond to the total (sum) concentration of (a) LIL elements(K, Rb, Sr, Ba, Cs); (b) transition metals (V, Mn, Co, Ni, Cu, Zn, Cr);and (c) HFS elements (Zr, Ti, U, Pb, Th, Y, REE, Sc, Hf, Nb). Blackdots correspond to mean values, box indicates 25% and 75% percentile

and bars show 95% and 5% range. Asterisks correspond to outliers

exhibit contrasting behavior: as the sum of dissolved REEconcentrations increases, the anomalies tend to disappear,Ce/CeŁ tends to become less negative and Eu/EuŁ lesspositive (Figure 8c). This behavior is interpreted as aconsequence of changing solute sources with increasingdischarge.

The �Ge:Si�diss. ratio was measured in 12 meltwaterand stream samples of the Agua Negra drainage basin andan arithmetic meanšs.d. of 1Ð48 š 0Ð272 pmol µmol�1

was found, which is higher than the ratios determinedby Chillrud et al. (1994) in Tronador Glacier meltwater(Rıo Negro, Argentina), close to the average continentalcrustal ratio (Ge : Si ³1Ð4). Clearly, the high �Ge:Si�diss.

ratio for the Agua Negra glacierized drainage faithfully

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2191

Table III. Dissolved REE concentrations (ppb)

Sample La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb

M1-12 : 00 0Ð053 0Ð093 0Ð013 0Ð042 0Ð009 0Ð003 0Ð008 0Ð001 0Ð005 0Ð001 0Ð002 0Ð002M1-13 : 10 0Ð219 0Ð446 0Ð052 0Ð194 0Ð044 0Ð011 0Ð038 0Ð005 0Ð021 0Ð004 0Ð010 0Ð007M1-15 : 00 0Ð020 0Ð031 0Ð004 0Ð015 0Ð002 0Ð002 0Ð003 <0Ð001 0Ð003 0Ð001 <0Ð001 0Ð001M1-17 : 00 0Ð012 0Ð014 0Ð003 0Ð009 0Ð003 0Ð002 0Ð001 <0Ð001 0Ð002 <0Ð001 0Ð001 0Ð001M1-19 : 00 0Ð058 0Ð107 0Ð013 0Ð050 0Ð010 0Ð005 0Ð007 0Ð001 0Ð005 0Ð001 0Ð002 0Ð003M1-20 : 30 0Ð021 0Ð035 0Ð006 0Ð022 0Ð006 0Ð002 0Ð004 <0Ð001 0Ð003 <0Ð001 0Ð002 0Ð002M2-12 : 00 0Ð022 0Ð031 0Ð005 0Ð020 0Ð003 0Ð003 0Ð003 <0Ð001 0Ð004 <0Ð001 0Ð003 0Ð003M2-17 : 00 0Ð128 0Ð199 0Ð033 0Ð119 0Ð026 0Ð006 0Ð023 0Ð003 0Ð011 0Ð002 0Ð005 0Ð0031M3 0Ð062 0Ð049 0Ð014 0Ð054 0Ð015 0Ð004 0Ð014 0Ð002 0Ð009 0Ð003 0Ð006 0Ð0051M4 0Ð031 0Ð050 0Ð008 0Ð036 0Ð005 0Ð001 0Ð007 <0Ð001 0Ð004 <0Ð001 0Ð002 0Ð0011M4 duplicate 0Ð033 0Ð055 0Ð009 0Ð028 0Ð008 0Ð002 0Ð008 <0Ð001 0Ð005 0Ð001 0Ð003 0Ð0011M5 - snow 0Ð030 0Ð127 0Ð008 0Ð031 0Ð010 0Ð002 0Ð009 0Ð001 0Ð006 0Ð001 0Ð002 0Ð0021M5 - ice 0Ð028 0Ð060 0Ð007 0Ð028 0Ð008 0Ð002 0Ð006 <0Ð001 0Ð004 <0Ð001 0Ð002 0Ð0021M6 0Ð031 0Ð056 0Ð008 0Ð033 0Ð008 0Ð002 0Ð006 <0Ð001 0Ð002 <0Ð001 <0Ð001 0Ð0011M7 0Ð024 0Ð026 0Ð006 0Ð022 0Ð006 0Ð002 0Ð004 <0Ð001 0Ð004 <0Ð001 0Ð002 0Ð0021M8 0Ð045 0Ð062 0Ð010 0Ð035 0Ð005 0Ð003 0Ð005 <0Ð001 0Ð004 0Ð001 0Ð002 0Ð0021M9 0Ð060 0Ð104 0Ð015 0Ð058 0Ð015 0Ð005 0Ð014 0Ð002 0Ð009 0Ð001 0Ð003 0Ð0022M1-17 hs 0Ð021 0Ð010 0Ð005 0Ð028 <0Ð001 0Ð004 0Ð001 <0Ð001 0Ð004 0Ð001 <0Ð001 0Ð0032M1-18 : 50 hs 0Ð019 0Ð015 0Ð004 0Ð016 <0Ð001 0Ð003 0Ð003 <0Ð001 0Ð001 <0Ð001 0Ð001 0Ð0012M1-21 hs 0Ð024 0Ð035 0Ð006 0Ð018 <0Ð001 0Ð002 0Ð005 <0Ð001 0Ð001 0Ð001 0Ð002 0Ð0022M1-23 hs 0Ð072 0Ð141 0Ð019 0Ð064 0Ð012 0Ð003 0Ð013 0Ð002 0Ð009 0Ð002 0Ð004 0Ð0022M2 0Ð026 0Ð036 0Ð007 0Ð025 <0Ð001 0Ð002 0Ð007 <0Ð001 0Ð002 0Ð001 0Ð003 0Ð0032M4 0Ð034 0Ð168 0Ð010 0Ð036 <0Ð001 <0Ð001 0Ð007 <0Ð001 0Ð002 <0Ð001 <0Ð001 <0Ð0012M8 0Ð019 0Ð023 0Ð004 0Ð014 <0Ð001 0Ð002 0Ð001 <0Ð001 0Ð002 0Ð001 0Ð002 0Ð0022M9 0Ð019 0Ð025 0Ð005 0Ð012 <0Ð001 0Ð002 0Ð003 <0Ð001 0Ð003 <0Ð001 0Ð001 0Ð003

Tm and Lu concentrations are below detection limit (i.e. blank D 0.001).

reflects the ratios in primary minerals and, as pointed outby Chillrud et al. (1994), there are no indications thatGe preferentially sorbs on fresh surfaces, such as thoseprovided by Fe(OH)3.

Weathering intensity through PHREEQC inversemodelling

In order to assess solute contributions originating indifferent parts of the drainage basin, weathering reac-tions in the Agua Negra drainage basin were simulated bymeans of the PHREEQC computer program (Parkhurst,1995). The relatively small size of the basin and uni-form lithology were factors that promoted a successfuloutcome of the modelling exercise.

Models were built using samples collected during bothfield trips. Figure 9 shows schematically the three streamreaches that were modelled with the PHREEQC code,and their respective length. Weathering processes werequantified as shown in Figure 9a, b, and c.

The contributing phases are salt dissolution (carbonate,halite, and gypsum), silicate incongruent dissolution(muscovite and andesine), the formation of clay minerals(kaolinite and illite), and the involvement of CO2. Theparticipating solid phases were selected following themost abundant rocks and by inspecting the clay mineralstability diagrams. It must be noted that gypsum is aproxy for the sulphate generated by sulphide oxidation,as shown above in the corresponding chemical equation.

The total sum of dissolved and precipitated speciesthat, according to the models, take part in the weatheringprocesses is about 10�3 mol L�1. In each modelled reach,the downstream rate fluctuates between 8 ð 10�2 and

6 ð 10�3 mmol L�1 km�1 and is clearly higher in theuppermost catchments (i.e. M6 to M8, Figure 9a), wherethe dominant process is gypsum dissolution as a directconsequence of sulphide oxidation. Also important in theuppermost glacierized catchments is the loss of CO2 tothe atmosphere:

Corg�s� C H2O�liq� C O2�aq� , CO2�aq� C H2O�liq�

According to the model (Figure 9a), CO2 degassingtriggers calcite precipitation in the upper catchments.

In the second reach (M8 to M9, Figure 9b, i.e. thecontribution of the Dos Lenguas rock-glacier) there is acontrasting difference between the two samplings: duringthe first one, with restricted meltwater production, car-bonation dominates over other geochemical processes,whereas during the second sampling, with ample melt-water provision, silicate and gypsum dissolution (i.e. sul-phide oxidation) becomes more important (Figure 9b).This suggests that the controls on glacial meltwater geo-chemistry in the rock-glacier switch over from calcitedissolution (due to high PCO2) to silicate and gypsumdissolution (sulphide oxidation) as meltwater evolves.However, the magnitude of moles transferred from sil-icate weathering, along with the production of illite ismore important during the first sampling. This difference(i.e. higher silicate dissolution) may be the result of shift-ing meltwater residence times.

The models obtained for the third reach (M9 toM1, Figure 9c) show clearly that solute production issignificantly higher when glaciers are in the activemelting process (the second sampling). Further, CO2 is

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2192 K. L. LECOMTE ET AL.

R2 = 0.69 (p < 0.05)

0 2.10-4 4.10-4 6.10-4 8.10-4 1.10-3

Eu/Eu*Ce/Ce*

Ano

mal

y va

lue

0

1

2

3

4

Σ REE (mg L-1)

R2 = 0.55 (p < 0.05)

370

380

390

400

410

Con

duct

ivity

(µS

cm

-1)

Con

duct

ivity

(µS

cm

-1)

0

2.10-4

4.10-4

6.10-4

8.10-4

1.10-3

Conductivity (µS cm-1)

ΣREE (mg L-1)

0

100

200

300

400

500

600

0

1.10-4

2.10-4

3.10-4

4.10-4

Σ R

EE

(mg L

-1)Time (hrs)

Σ R

EE

(mg L

-1)11 13 15 17 19 21 23

1.2.10-3

5

Conductivity (µS cm-1)ΣREE (mg L-1)

(a)

(b)

(c)

Figure 8. (a) and (b) Time-dependent variation of REE and conduc-tivity (as proxy for discharge) at 1M1 and 2M1, respectively; and(c) scatterplot of Eu/EuŁ and Ce/CeŁ versus REE. The broken line

corresponds to the absence of anomaly

no longer subjected to degassing—as it occurred in theuppermost catchment—and appears to play an active rolein ongoing chemical reactions.

A direct comparison (Figure 10a) of the total inversemodel for the Agua Negra drainage basin (1M6 to1M1) with similar results obtained in a non-glacierizedmountainous catchment (Lecomte et al., 2005) withexposed granite and gneiss, shows that, in terms ofan area-normalized approach, sulphide oxidation andCO2 consumption account for most of the total of4 ð 10�2 mmol L�1 km�2 transferred in the glacierizedarea, whereas silicate hydrolysis and dissolution are themost conspicuous processes in the non-glacierized basin,reaching a total of 9 ð 10�3 mmol L�1 km�2 of trans-ferred moles.

Figure 10b shows that in relative terms, and consid-ering dissolved phases only, sulphide oxidation is byfar the most significant solute provider in the glacier-ized environment, contrasting with the role played bysilicate hydrolysis and dissolution in a representative non-glacierized catchment of the Sierras Pampeanas.

M8 to M9

1° sampling (March 2003)2° sampling (February 2004)

0

1.10-3

8.10-4

6.10-4

4.10-4

2.10-4

-2.10-4

-4.10-4

mol

L-1

3.10-4

2.10-4

1.10-4

0

-1.10-4

-2.10-4

mol

L-1

0

7.10-5

6.10-5

5.10-5

3.10-6

2.10-6

1.10-6

-1.10-6

-5.10-6

-6.10-6

mol

L-1

1M6 to 1M8

Calcite

M9 to M1

M6 M8 M9 M1

19.2 km3.2 km

28.6 km

(a)

(b)

(c)

Halite Sulphide

oxidation

K-mica Andesine Kaolinite Illite CO2

Figure 9. Transferred moles (mol L�1) of each phase in: (a) uppermostcatchment; (b) Dos Lenguas rock-glacier; and (c) lowermost catchment.A schematic diagram is included with the length of the stream reaches

analysed in each model using the PHREEQC code

CONCLUDING COMMENTS

Several ice- and rock-glaciers with dissimilar ice masscoexist at the Agua Negra drainage basin, in the Argen-tine Andes. The area was sampled on two occasions: first,when meltwater discharge was subsiding (i.e. beginningof the southern fall); and, second, when meltwater waspeaking, during the southern summertime. In the firstinstance, the supraglacial meltwater (surface melt) con-tribution was decreasing and during the second one, thesubglacial contribution was fully operational. The asso-ciation between meltwater volume and total dissolved orsuspended solids was discernible in the latter but wasunclear during the former. In both sampling instances,serial sampling was restricted to the drainage basin outfalland, hence, it was difficult to separate the geochemicaleffects of all the intervening ice- and rock-glaciers.

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

HYDROCHEMISTRY OF AGUA NEGRA DRAINAGE BASIN 2193

-5.10-3

0

5.10-3

1.10-2

2.0.10-2

Calcite Halite/Biotite

Sulphideoxidation

K-mica Plagioclase Kaolinite Illite CO2

mm

ol L

-1 K

m-2

0

10

20

30

40

50

60

70

80

90

Calcite Silicates

% m

ol L

-1

Glacierized meltwaterNon glacierized water

2.5.10-2

Sulphideoxidation

(a)

(b)

Figure 10. (a) Area-normalized comparison of the total inverse model for the Agua Negra drainage basin (1M6 to 1M1) with a non-glacierizedmountainous catchment (Lecomte et al., 2005); and (b) relative significance of gypsum, calcite and silicate dissolution/weathering in glacierized

(Agua Negra drainage) and non-glacierized (Lecomte et al., 2005) catchments

Meltwaters in ice-glaciers basically have a HCO3� —

Ca2C composition whereas rock-glaciers have a SO42� —

HCO3� —Ca2C major solute composition. There is

a clear hydrochemical distinction between the twoglacier types, surely associated with the different rockdebris : meltwater ratios. As determined for many otherworld glaciers (Tranter, 2005), the main solute-producingprocesses are, in decreasing order of importance, sul-phide oxidation, and carbonate and silicate hydrolysis.The source of Cl�, and to a lesser extent of NaC, appearsto be marine aerosols from the Pacific Ocean (up to 30%of NaC in each sample). The study of post-depositionalprocesses at Co. Tapado Glacier, in the Chilean Andes(roughly at the same latitude as Agua Negra Glacier),has shown that dry deposition and sublimation (1Ð9 mmwater equivalent d�1) determine a concentration enhance-ment in snow and ice of irreversibly deposited chemicalspecies (Ginot et al., 2001). It is most likely that suchenrichment, in these mid-latitude glaciers, is not only lim-ited to major components but it also includes minor andtrace elements.

Examination of the dynamics of minor and traceelement concentrations was undertaken by clustering

the elements into LIL elements (e.g. Cs, Rb, Sr), HFSelements (e.g. Ti, Zr, Hf), and transition metals (e.g. Cu,Ni, Co). Analysis showed that LILs are more abundantin the lowermost reach (i.e. the combined effect of ice-C rock-glaciers). HFS elements are more abundant inboth mixed environments and in rock-glaciers, whereastransition metals are more abundant in ice and snow.Fine-grain-size rock flour scavenges transitional elementsin solution as soon as they make contact with each other.

At the lowermost reach, the sum of dissolved REEconcentrations correlates positively with meltwater dis-charge and, hence, decreases with increasing total dis-solved solids. When there is a clear association withmeltwater discharge, as during the summer sampling, dis-solved REE increase with suspended solids concentration.It seems that in this environment, REE are mobilizedand flushed-out by meltwater, and the prevailing physic-ochemical conditions are not favorable to implement thescavenging from solution. It is interesting to point out thatboth Eu/EuŁ and Ce/CeŁ tend to disappear as the REEincreases (and discharge also increases). This seems to bethe consequence of varying meltwater sources throughoutthe cycle.

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp

2194 K. L. LECOMTE ET AL.

The use of the PHREEQC computer code to simulatethe interaction of different phases in glacial weather-ing, produced the following results: (a) significant CO2

degassing appears to occur only in the upper catchments,where ice- and rock-glaciers coexist; (b) the Dos LenguasGlacier is one of the largest rock-glaciers in the drainagebasin and shows at least two modes of operation: dom-inant calcite dissolution and CO2 consumption duringearly fall, and sulphide oxidation and silicate dissolu-tion (along with CO2 consumption) during high melt-water flow (summer); (c) in the overall picture for theAgua Negra drainage basin, sulphide oxidation/gypsumdissolution (¾2Ð0 ð 10�2 mmol L�1 km�2), and CO2

consumption (¾0Ð85 ð 10�2 mmol L�1 km�2) are themost important intervening phases in the Agua NegraGlacier environment; and, finally, (d) comparison witha mountainous semiarid drainage of comparable sizein the granite- and gneiss-dominated Sierras Pampeanas(Cordoba, Argentina) shows that the glacierized areareaches a higher specific denudation, 80% of which isaccounted for by sulphide oxidation C calcite dissolu-tion. A similar proportion was determined for silicates inthe semiarid range of central Argentina.

The geochemical characteristics of a typical glacierizeddrainage basin in the arid to hyperarid Andes of Argentinahave been studied. Initial conclusions provide support forthe idea that there are significant geochemical differenceswith the humid counterparts, which basically stem fromthe higher sublimation/evaporation rate in the former.Additional research would improve current knowledgeof the differences between the two glacier types.

ACKNOWLEDGEMENTS

We wish to acknowledge the continued support ofArgentina’s CONICET (PIP 5947) and FONCYT (PICT25594), and CICITCA from Universidad Nacional de SanJuan (Argentina). We are grateful to E.L. Piovano, D.M.Gaiero, and R. Morilla for their helpful assistance in thefield, during the first sampling trip. We are grateful totwo anonymous reviewers for helpful comments that ledto an improvement of the original manuscript.

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Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 2180–2195 (2008)DOI: 10.1002/hyp