Water chemistry and its diversity in relation to local factors in the Latnjavagge drainage basin, arctic–oceanic Swedish Lapland

Geomorphology 58 (2004) 125 – 143 www.elsevier.com/locate/geomorph

Water chemistry and its diversity in relation to local factors in the Latnjavagge drainage basin, arctic–oceanic Swedish Lapland Achim A. Beylich a,*, Else Kolstrup a, Tage Thyrsted b, Dorothea Gintz c a

Department of Earth Sciences, Geocentrum, Environment and Landscape Dynamics, Uppsala University, Villava¨gen 16, SE-752 36 Uppsala, Sweden b Harbacken, Stavby, Alunda, Sweden c Institute for Geological Sciences, AB Hydrogeology, Free University of Berlin, Berlin, Germany Received 29 January 2003; received in revised form 27 April 2003; accepted 11 May 2003

Abstract The chemistry of precipitation, snow pack and surface water has been analysed on 205 samples collected during the 2001 field season at 25 selected sites within the Latnjavagge drainage basin in arctic – oceanic northern Swedish Lapland. Additionally, daily discharge and yield of dissolved solids have been calculated for several subcatchments and the entire Latnjavagge catchment during the years 2000, 2001 and 2002. Chemical water analysis included the components Ca2 +, Mg2 +, Na+, K+, Fe2 +, Mn2 +, Cl , NO3 , SO24 and PO34 , with SO24 and Ca2 + being the dominant ones in the surface water. Solute concentrations and chemical denudation were low, but showed significant differences within the basin. In areas of shade, longer snow cover, frozen ground and thin regolith, concentrations over the summer were perceptible but so low that solutes brought into the basin from precipitation could be detected in the surface water. In one locality, it was even found that lake water could reflect snowmelt to such an extent that the solute concentration was less than that of summer precipitation. The highest concentrations were found at a radiation-exposed, W-facing, vegetated, moderately steep slope with relatively thick regolith that was thawed at the time of snowmelt in early June. In such well-drained sites with continuous subsurface water flow, a maximum of contact between water and mineral particles could take place. The concentration values revealed differences in the rate of thawing of frozen ground between shaded areas and/or areas at higher altitude on the one hand and radiation-exposed areas on the other. A comparison with published results from Ka¨rkevagge a few kilometres to the northwest as well as from other periglacial locations indicates that the chemical denudation values from Latnjavagge are more representative of periglacial oceanic environments than the values from the Ka¨rkevagge catchment, which shows especially high chemical denudation rates. The investigation in Latnjavagge stresses the importance of spatial variability within even small catchments of homogeneous lithology as it demonstrates that solute concentrations from different subbasins can differ substantially dependent on exposure to radiation, duration of snow cover and frozen ground conditions, regolith thickness and possibly also to vegetation cover and slope angle as factors steering water turbulence and retention of drainage. D 2003 Elsevier B.V. All rights reserved. Keywords: Water chemistry; Chemical denudation; Spatial variability; Periglacial; Arctic – oceanic; Lapland

* Corresponding author. Tel.: +46-18-471-2511; fax: +46-18-555-920. E-mail address: [email protected] (A.A. Beylich). 0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-555X(03)00228-9

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1. Introduction Several scientists (e.g. Summerfield, 1991, p. 299; Thorn et al., 2001) have stressed the paucity of investigations on chemical weathering as compared to mechanical weathering in periglacial areas; in addition, where chemical denudation is dealt with, the focus is often on calcareous rocks (see, for example, the overview in French, 1996, p. 47ff). Rapp’s (1960) paper on the Ka¨rkevagge valley in Lapland stands out as the study which opened the awareness of chemical weathering in cold regions, and is today a classic paper on chemical denudation in periglacial regions. Following Rapp’s studies, investigations have been undertaken in other cold climate areas such as in Colorado (Caine, 1979; Caine and Thurman, 1990), the northern Cascade Mountains (Reynolds and Johnson, 1972), Alaska (Dixon et al., 1984) and Iceland (Beylich, 1999, 2000a,b). In addition, investigations have recently been resumed in Ka¨rkevagge (e.g. Thorn et al., 2001; Campbell et al., 2002). Unfortunately, it seems that Ka¨rkevagge, the most-investigated area of them all, has particularly high solution and chemical denudation rates owing to local lithological factors (Darmody et al., 2001; Thorn et al., 2001; Campbell et al., 2002). Therefore, even if investigations show that chemical weathering and chemical denudation take place in cold areas to a varying extent, there is still only little information to reliably assess the general importance of chemical weathering as a denudation agent in relation to environmental factors in subarctic, arctic and alpine environments. To better relate chemical weathering and chemical denudation to environmental factors in cold areas, investigations in a range of drainage basins with different but preferably individually homogeneous lithology, and with internal differences in regolith thickness, aspect to radiation, slope angle and frozen ground conditions is needed. In the present paper, such an investigation is attempted for the Latnjavagge drainage basin, a periglacial environment within a mica – schist arctic – oceanic mountain area in northernmost Swedish Lapland (see also Beylich, 2001, 2002, 2003; Beylich et al., 2002, submitted for publication). The study focuses on differences in chemical components of

precipitation, early season snowpack and drainage water from subareas within a confined drainage basin as well as their changes over the season for detection of spatial diversity and relation to environmental factors.

2. Study area The Latnjavagge drainage basin is located at approximately 1000 m a.s.l. between Abisko and the Norwegian border at 68j20VN, 18j30VE in the Swedish Caledonian mountain range (Fig. 1). The climate is arctic– oceanic with prevailing westerly and northerly winds from the North Atlantic ocean, and at the Latnjajaure Field Station (LFS, 981 m a.s.l.) the mean annual temperature is 2.3 jC (1993 – 2001). The warmest month, July, has a mean of + 8.0 jC and the mean of February, the coldest month, is 10.1 jC. The mean annual precipitation (1990 – 2001) is 818 mm, of which about two-thirds is temporarily stored as snow during the winter. The precipitation from June to August accounts for 24% of the mean yearly total with August as the wettest month of 82 mm, and the run-off period is between mid- to end May and late October – early November (Beylich, 2003). The Latnjavagge drainage basin is approximately 9 km2 with a length of 4600 m in the N – S direction and the elevations range between 950 and 1440 m a.s.l. (Fig. 2). The bedrock consists of laminated garnet mica schist with a gentle dip towards the north and there is, thus, a rather homogeneous lithology over the basin; only in the most northern part are there minor inclusions of granites and marble (Kulling, 1964; Stephens et al., 1994; Kling, 2003). During the Quaternary, northern Scandinavia has been ice covered repeatedly. In the upper reaches of the basin above 1300 m a.s.l. and at the water divide, there are relatively flat plateaux of bare bedrock and boulder fields. In the valley, glaciers formed a Ushaped trough with glacially eroded slopes of which the steep, E-facing one is partly covered by perennial snow and ice patches (Fig. 2). The lower reaches are dominated by a lake, Latnjajaure (0.73 km2), and a series of moraine ridges across the valley (see also Kling, 2003). The soils are mainly lithosols and regosols and soil pH values within the catchment are between 4 and 6 (Molau, 2003).

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Fig. 1. Location of the study area in the northern Swedish Lapland.

Locally, in places with relatively fine-grained sediments, there is patterned ground with sorted polygons, yet geophysical investigations show that the regolith around the lake was thawed in late August. On the other hand, perennially frozen ground is probably present in higher and more shaded parts of the basin (Beylich et al., in press; Kling, 2003).

The large-scale landscape of the valley is still dominated by the activity of former glaciers, but along the slopes, there are accumulations of debris from rock falls beneath exposed rock faces as well as deposits from other mass related slope processes and further there are sediments deriving from fluvial slope denudation. In addition to till, the valley bottom is covered by stony sediments from slush flows and

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Fig. 2. Investigation areas and sampling sites in Latnjavagge, Swedish Lapland. The snow patch/field situation is for August.

fluvial activity. The sediments are of local origin and the regolith, therefore, reflects the local bedrock, but it is usually more weathered. The vegetation consists of open dwarf-shrub heaths and alpine meadows and bogs (Molau, 2001, 2003; Molau et al., 2003). Direct human impact on the natural system is small.

3. Approach and methods The major part of the fieldwork reported in this paper was conducted between the 28 May and 19 August 2001, a period that covers the run-off season from its start almost to the end. In addition, to obtain a more representative record for the area, the mean

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annual discharge weighted TDS values of different subcatchments within Latnjavagge as well as chemical denudation rates (corrected for atmospheric inputs) have been calculated on the basis of daily measurements during the summers of 2000, 2001 and 2002 (see also Beylich et al., in press, submitted for publication). In spite of the interannual variability of the climate in the area, the mean annual discharge weighted TDS values at different sampling sites in Latnjavagge showed only minor interannual variability. The data from the 2001 field season are, therefore, regarded as representative (see Beylich et al., in press, submitted for publication). The homogeneous bedrock, the largely local origin of the regolith and the small size of the basin make it possible to regard lithology and climate as uniform over the area. Thus, differences in water chemistry can be explained by local differences in aspect, snow cover duration, ground frost conditions, topography and regolith thickness. 3.1. Selection of subareas and sampling sites Subareas and sampling sites within the Latnjavagge drainage basin (see Fig. 2) were selected after analysis of topographic (1:18,000) and geological maps, aerial photographs (mean scale 1:30,000) and after detailed field investigations. Of interest was the comparison of subareas showing differences in aspect, duration of snowcover, ground frost conditions, steepness and regolith thickness. All selected sampling sites in the Latnjavagge drainage basin were within easy reach on foot for daily measurement and sampling during the entire field campaign using the Latnjajaure Field Station (LFS) as a logistic base. 3.2. Field and laboratory methods In the field, a total of 205 water samples was collected over the entire field season. Surface water samples were taken in 1000-ml polyethylene bottles at the 25 sampling sites (Fig. 2 and Table 1). All sampling sites at creeks were characterized by a high turbulence of discharges and a good mixing of the water. After sampling, all samples were filtered in the field laboratory of LFS with a pressure filter and ashfree filter papers (Munktell) and stored in 200-ml

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polyethylene bottles. Sixty-five snow cores/samples were collected with a plastic tube (10 cm diameter) at the beginning of the snowmelt period, i.e. at the end of May and in earliest June, along selected profiles with annual snow pack (Fig. 2) and subsequently melted in sterile buckets at the Latnjajaure Field Station (LFS) before electric conductivity (reference 25 jC) of all samples was measured. Eight of these snow samples were filtered and transferred into 200ml polyethylene bottles for later analysis. Precipitation samples (wet precipitation) accumulated in a precipitation gauge (Hellmann-Totalisator with windshelter, surface area 200 cm2) at the meteorological station of LFS (see Molau, 2001, 2003) were collected during the entire field season for measuring electric conductivity of all samples and 10 of them were filtered and temporarily stored in 200-ml polyethylene bottles. All the surface water, precipitation and melted snow samples were stored in a freezing box at LFS before they were analyzed in the laboratory of the AB Hydrogeology at FU Berlin, Germany. Ions of Na+ and K+ were determined with a Flammenphotometer Eppendorf Elex 6361; Ca2 +, Mg2 +, Fe2 +, Mn2 + were determined with an AAS Perkin-Elmer 5000. The SO42 , Cl and NO3 contents were measured with an ion-chromatograph DX 100 Dionex and, finally, PO43 was measured with an autoanalyzer II Technicon. Mn2 + and PO43 were below the detection limit in all samples. All analyses were conducted in accordance with the regulations given by the producers. Surface water electrical conductivity (AS/cm, reference 25 jC) was measured three times daily at all sampling sites (Fig. 2) with a temperature-corrected portable instrument (Conductivity meter Cond 315i, WTW, Weilheim, Germany) directly after discharge measurements with an Ott-propeller C2 (Ott KG, Kempten, Germany) at eight sites (see Fig. 2) and immediately prior to sampling (Beylich et al., in press). Total dissolved solids (TDS [mg/l]) were estimated by multiplying conductivity (AS/cm) by 0.7 (see Stro¨mquist and Rehn, 1981; Darmody et al., 2000; Beylich et al., in press). Daily TDS values (mg/l) were calculated by interpolation of the measurements (see Beylich, 1999). Four representative samples of fresh and weathered mica schist were collected from exposed bedrock and

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Table 1 Sampling site numbers, locations and brief outline of local geomorphologic and environmental factors (compare with Fig. 2) (2001 field season) Sampling site number

Sampling site

Description of sampling sites

1 2 3 4 5 6 7 8 9 10 11 12 13

Pipe Overland flow Temporary creek Lake SC B Outlet SC B Outlet Latnjajaure Outlet SC A Outlet Latnjavagge Tributary Temporary creek Tributary Lake SC A Creek below snow patch Temporary creek

Pipe in talus cone, lowest part of steep E-facing slope Overland flow over talus cone, lowest part of steep E-facing slope Temporary creek with water mainly from the upper parts of the steep E-facing slope Lake on small plateau, surrounded by snow and ice patches Outlet, water from lake, slope with thin regolith cover and flat area close to outlet Outlet Latnjajaure Outlet subcatchment A Outlet Latnjavagge Tributary, draining a small area with W-facing slope and relatively thick regolith Temporary creek, draining a small area with relatively thick regolith Tributary draining the plateau above with bare bedrock, boulder fields and snow patches Lake surrounded by bedrock on small plateau with boulder fields and snow patches Creek draining the gentle W-facing slope with early thawing of snow and ground frost, higher regolith thickness on the slope Creek draining the gentle W-facing slope with early thawing of snow and ground frost, higher regolith thickness on the slope Creek draining the gentle W-facing slope with early thawing of snow cover and ground frost, higher regolith thickness on the slope Creek draining the gentle W-facing slope, early thawing of snow cover and ground frost, higher regolith thickness on the slope Temporary creek with water mainly from bare bedrock of the higher plateau area plus some water draining the slope Inlet of Latnjajaure, sampling above the delta where channel changes to become braided Pipe, overland flow with water coming from the plateau above and the slope in between (thin regolith cover) Pipe, overland flow, water coming from the plateau above and the slope in between (thin regolith cover) Outlet subcatchment C with water coming mainly from the plateau areas above (bare bedrock or very thin regolith) Outlet subcatchment D with water coming mainly from the plateau areas above of bare bedrock or very thin regolith, large snow and ice patches/fields, later melting of snow cover and ground frost Creek draining a large permanent ice field

14 15

17

Creek below snow patch Creek below snow patch Temporary creek

18 19

Inlet Latnjajaure Pipe, overland flow

20

Pipe, overland flow

21

Outlet SC C

22

Outlet SC D

23

Creek below ice patch/field Lake SC D Lake SC D LFS Profiles

16

24 25 Precipitation Snow cores

Lake surrounded by steep slopes with snow/ice field and flatter areas Lake surrounded by steeper slopes with snow and ice fields and more gentle slopes Precipitation collected in a Hellman-Totalisator with windshelter (SMHI standards) Melted snow collected in May/June from snow accumulated during the previous winter

debris at the W- and E-facing valley slopes (see Fig. 2) and chemically analysed for various common and rare components by SGAB Analytica, Lulea˚ Technical University according to standard G-5. Densities (g/ cm3) of the same samples were calculated according to the pycnometer method according to Swedish Standard SS 13 21 24 at the Swedish Geological Survey (SGU), Uppsala (Table 2). Daily air temperature and precipitation data and soil temperature data were collected automatically at

the meteorological station of the Latnjajaure Field Station (see Molau, 2001, 2003). Snow depths at grids A – F (Fig. 2) were provided by the Latnjajaure Field Station (Molau, 2001, 2003; Molau et al., 2003). The snow depth data are not discussed in detail in this paper but they provide background information with the discussion of duration of snow cover and ground frost in subareas within the Latnjavagge catchment (see Beylich et al., in press, submitted for publication).

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Fig. 3. Daily air (3a) and ground temperatures (3b), daily precipitation (3c) and periods of snow cover (3b) at the Latnjajaure Field Station (LFS), together with daily specific discharges at the outlet of the Latnjavagge drainage basin (sampling site 8) (3c) during the 2001 field season.

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3.3. Cluster analysis To facilitate the overview of the water samples and how they group, cluster analysis (Daly et al., 1995, 538ff) was applied to the water chemistry data. Cluster analysis seeks to identify natural groupings in the data itself without taking into account any preoccupied knowledge of the samples. Samples were obtained from the 27 sampling sites (i.e. 25 sampling sites for surface water; LFS for precipitation samples; snow cores/samples) and each sample was analysed for TDS and 10 ions. However, two of these (Mn2 + and PO43 ) were below the detection limit and, hence, only nine attributes (Table 3) were included in the cluster analysis. The number of samples taken per locality varied between 7 and 11 with the exception of the four mountain lakes, where only one sample was taken per lake. Owing to the limited number of samples per locality, the sample distributions of the various chemical ions, as seen in histograms, could take almost any form. However, the range of the values was usually within the limit of two standard deviations from the mean. For use in the cluster analysis, therefore, the arithmetic mean of the sample values was considered as good as any representative for the sample.

The analysed data include the TDS values, which have the highest value of the samples by far. In order to eliminate the dominance of this parameter over the values of the individual ions in the cluster analysis, all values were standardized before the statistical analysis, i.e. all the data sets were transformed to sets with a mean of zero and a standard deviation of 1. The cluster analysis applied was a hierarchical single linkage method where you step by step, agglomerate nearest neighbours, as measured by Euclidean distances, into clusters. The result of the analysis is shown in a dendrogram below.

4. Results 4.1. Subareas and sampling sites (Fig. 2, Table 1) The E-facing slope west of Latnjajaure is in the shade most of the day; it is steep and has large areas with bare rockwall or only little regolith (talus cones). Sampling sites 1 –3 are situated in this area (see also Fig. 2 and Table 1 where additional details are given). Towards the south, the western slope becomes less steep and, apart from outcropping rock ledges, covered by thin regolith. In the upper part, there is a small

Table 2 Rock chemistry of selected chemical components in two fresh and two weathered samples of mica schist (SGAB analyses) together with their uncertainties and mean values, respectively Specific densities (g/cm3) SGAB (% of total) Analysis of: SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO2 Na2O P2O5 TiO2 Sum Loss on ignition

Sample 1 (2.74)

Sample 2 (2.78)

Mean of fresh samples

Sample 3 (2.55)

Sample 4 (2.42)

Mean of weathered samples

67.7 F 0.1 14.5 F 0.1 1.24 F 0.01 5.80 F 0.01 2.53 F 0.01 2.77 F 0.01 0.0718 F 0.0003 2.78 F 0.01 0.147 F 0.002 0.889 F 0.001 98.4 1.3

60.6 F 0.1 18.7 F 0.1 1.26 F 0.01 8.17 F 0.01 4.50 F 0.01 2.43 F 0.01 0.246 F 0.001 1.54 F 0.01 0.184 F 0.003 0.786 F 0.001 98.4 2.4

64.15 16.60 1.25 6.99 3.52 2.60 0.16 2.16 0.17 0.84 98.4 1.85

54.0 F 0.1 20.5 F 0.1 0.342 F 0.001 8.88 F 0.01 5.28 F 0.01 2.83 F 0.01 0.0386 F 0.0002 1.39 F 0.01 0.388 F 0.003 1.18 F 0.01 94.8 5.6

67.3 F 0.1 14.1 F 0.1 0.126 F 0.001 5.45 F 0.01 3.61 F 0.01 1.57 F 0.01 0.0232 F 0.0001 1.43 F 0.01 0.130 F 0.002 0.781 F 0.001 94.5 5.1

60.65 17.30 0.24 7.17 4.45 2.20 0.03 1.41 0.26 0.98 94.65 5.35

Particularly notable is the major percentual loss of CaO and MnO2. Specific gravities for the four samples (SGU determination by pycnometer method) are also given.

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Fig. 4. (a – g) Daily specific runoffs (mm/day), TDS values and gross yields of dissolved solids in subcatchments A – D at the inlet and outlet of the lake (sites 18 and 6, respectively) (e – f) and at the outlet of the Latnjavagge drainage basin (site 8) (g).

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Fig. 4 (continued ).

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Fig. 4 (continued ).

lake located on a small plateau amongst snow and ice patches (sample site 4, Fig. 2). Subcatchment A is located in the southeastern part of the valley in an area of relatively modest relief and it is exposed to radiation during a long part of the day. The altitudes range from the valley bottom to the watershed, and in the lower part of the reaches, there is a relatively thick regolith, up to 30 m, that thaws during July and permits subsurface flow during the remaining part of the summer season (Beylich et al., in press). Sampling sites 7 – 12 are located in this area. Sampling site 8 is at the outlet of the entire Latnjavagge drainage basin. The W-facing slope east of the lake is more gentle than the E-facing one, and its regolith varies from 0 to more than 30 m (Beylich et al., in press). The regolith in this part of the drainage basin thaws in June at the time of melting of the last snow cover there (Beylich et al., in press). From south to north, sampling sites 13 –17 are located along the lower part of this slope. The largest subcatchment is the area upstream from the lake (subcatchments C and D, sampling sites 18 –25) (Fig. 2). This part of the basin consists of an upper plateau with thin or lacking regolith as well as somewhat lower sloping areas with large perennial snow and ice patches on slopes of all aspects but most pronounced on the E- and S-facing ones. Further, except for the sediment accumulation just north of the lake, the regolith on and along the

slopes is generally thin and there is probably perennially frozen ground in most of this subcatchment (Beylich et al., in press). 4.2. Meteorological measurements (Fig. 3) Daily air (2 m above ground) and ground surface temperatures, daily precipitation and periods of snow cover at the Latnjajaure Field Station (LFS) during the 2001 field season are shown in Fig. 3. Positive air temperatures prevail from the start of June and the ground surface temperatures rose to above 0 jC at LFS directly after the snow pack disappeared. Daily precipitation is rather evenly distributed over the thaw season. It is of special interest for 2001 that relatively little snow had accumulated during the previous winter and there was relatively much precipitation (108.2 mm) and a relatively low mean temperature (7.3 jC) during July, and also August had unusually much precipitation (107.3 mm) that year including a snowstorm at the start of the month (Fig. 3). 4.3. Rock chemistry (Table 2) Specific gravities of two fresh and two weathered mica schist samples, respectively, are shown in Table 2 together with chemical components of relevance to the results from the water analysis. Lanthanides and some other elements in the rock were also determined and

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Table 3 Solute concentrations (mg/l) in water samples from the different sampling sites in Latnjavagge and in precipitation and snow samples (2001 field season) Description

GPS position

TDS (mg/l) (mean, min, max)

Ca (mg/l) (mean, min, max)

Mg (mg/l) (mean, min, max)

Na (mg/l) (mean, min, max)

K (mg/l) (mean, min, max)

Fe (mg/l) (mean, min, max)

Cl (mg/l) (mean, min, max)

NO3 (mg/l) (mean, min, max)

SO4 (mg/l) (mean, min, max)

(1) n = 8

Pipe

29.18, 20.30, 31.50

4.90, 4.23, 5.21

0.78, 0.62, 0.91

0.89, 0.73, 0.99

0.71, 0.33, 1.25

0.00

1.0, 0.8, 1.2

0.0

11.2, 9.9, 12.8

(2) n = 8

Overland flow

9.38, 5.46, 12.60

1.20, 0.79, 1.70

0.22, 0.13, 0.29

0.52, 0.44, 0.58

0.29, 0.21, 0.35

0.00

0.8, 0.7, 0.9

0.1, 0.1, 0.1

2.6, 1.5, 3.6

(3) n = 8

Temporary creek

68j21.583N, 18j28.714E, 983 m a.s.l 68j21.529N, 18j28.810E, 982 m a.s.l. 68j21.515N, 18j28.825E, 982 m a.s.l.

15.00, 12.00, 19.04

2.36, 2.10, 2.50

0.50, 0.43, 0.53

0.68, 0.62, 0.74

0.81, 0.43, 1.56

0.00

1.0, 0.8, 1.3

0.1, 0.0, 0.2

6.1, 5.4, 6.5

(4) n = 1 (5) n = 9

Lake SC B Outlet SC B

9.50 9.40, 6.23, 13.70

2.20 1.09, 0.74, 2.40

0.31 0.30, 0.25, 0.39

0.38 0.79, 0.51, 1.52

0.35 0.80, 0.24, 3.79

0.00 0.01, 0.00, 0.10

0.6 1.5, 0.8, 2.8

0.1 0.2, 0.0, 0.3

4.1 3.6, 2.7, 5.0

(6) n = 9

Outlet Latnjajaure (Lake) Outlet SC A

10.67, 5.46, 14.07

1.58, 1.20, 1.90

0.34, 0.24, 0.46

0.61, 0.45, 0.76

0.68, 0.32, 1.74

0.02, 0.00, 0.10

1.3, 0.7, 2.6

0.1, 0.0, 0.5

4.5, 3.4, 5.4

15.46, 9.10, 26.95

2.00, 1.60, 300

0.56, 0.39, 0.91

0.52, 0.46, 0.64

0.82, 0.40, 2.66

0.00

0.9, 0.6, 1.2

0.04, 0.0, 0.1

5.2, 3.7, 6.8

12.14, 8.26, 18.20

1.70, 1.30, 2.10

0.40, 0.32, 0.55

0.66, 0.50, 0.88

0.78, 0.38, 1.82

0.00

1.1, 0.6, 1.6

0.03, 0.0, 0.2

4.5, 3.4, 5.5

20.22, 12.04, 32.30

2.91, 2.20, 4.00

0.68, 0.48, 0.84

0.49, 0.40, 0.73

1.44, 0.58, 3.39

0.00

0.9, 0.5, 1.3

0.06, 0.0, 0.2

4.2, 2.5, 6.8

37.92, 20.50, 53.00

0.39, 0.21, 0.45

0.76, 0.56, 0.91

0.44, 0.31, 0.60

0.95, 0.61, 1.25

0.00

0.6, 0.4, 0.8

0.0

5.5, 4.9, 6.5

7.66, 4.20, 14.91

1.09, 0.46, 2.60

0.30, 0.14, 0.63

0.53, 0.39, 0.76

1.04, 0.19, 4.02

0.01, 0.00, 0.10

0.9, 0.6, 1.3

0.01, 0.0, 0.1

3.4, 1.3, 6.3

4.06

0.40

0.09

0.30

0.12

0.00

0.6

0.0

0.5

(7) n = 9

(8) n = 10

Outlet Latnjavagge

(9) n = 9

Tributary

(10) n = 8

Temporary creek

(11) n = 9

Tributary

(12) n = 1

Lake SC A

68°21.376N, 18°29.197E, 981 m a.s.l. 68°21.263N, 18°29.558E, 981 m a.s.l. 68°21.094N, 18°29.735E, 972 m a.s.l. 68°20.973N, 18°29.827E, 956 m a.s.l. 68j21.272N, 18j30.078E, 1005 m a.s.l. 68j21.296N, 18j30.164E, 1020 m a.s.l. 68j21.406N, 18j30.271E, 1010 m a.s.l.

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Sample site

(13) n = 9

(14) n = 9

(15) n = 9

(16) n = 9

(17) n = 8

Creek below snow patch Creek below snow patch Temporary creek

(21) n = 9

Inlet Latnjajaure (Lake) Pipe, overland flow Pipe, overland flow Outlet SC C

(22) n = 9

Outlet SC D

(23) n = 8

Creek below ice patch/field Lake SC D Lake SC D LFS Precipitation gauge Profiles

(19) n = 7

(20) n = 8

(24) n = 1 (25) n = 1 Prec. n = 10

Snow cores n = 8

68j21.569N, 18j29.999E, 981 m a.s.l. 68j21.631N, 18j29.952E, 981 m a.s.l. 68j21.667N, 18j29.941E, 983 m a.s.l. 68j21.872N, 18j29.775E, 984 m a.s.l. 68j21.944N, 18j29.513E, 983 m a.s.l. 68°22.231N, 18°29.278E, 1000 m a.s.l. 68j22.458N, 18j29.250E, 1031 m a.s.l. 68j22.472N, 18j29.220E 1032 m a.s.l. 68°22.473N, 18°29.208E, 1031 m a.s.l. 68°22.473N, 18°29.208E, 1031 m a.s.l. 68j22.497N, 18j29.092E, 1043 m a.s.l.

31.21, 20.30, 39.27

4.65, 3.70, 5.90

0.87, 0.62, 1.03

0.63, 0.46, 1.59

0.97, 0.52, 1.81

0.01, 0.00, 0.10

0.9, 0.6, 1.6

0.04, 0.0, 0.1

8.3, 5.4, 11.9

26.33, 15.54, 37.30

3.52, 2.40, 4.60

0.75, 0.58, 0.91

0.62, 0.50, 0.73

1.32, 0.73, 2.64

0.00

0.9, 0.6, 1.3

0.04, 0.0, 0.3

7.8, 6.4, 9.1

54.35, 31.85, 65.38

7.28, 5.40, 9.50

2.26, 1.60, 2.70

0.76, 0.66, 0.82

1.18, 0.71, 2.62

0.00

0.8, 0.6, 1.2

0.05, 0.0, 0.1

23.6, 17.3, 27.6

33.46, 17.57, 42.00

4.18, 1.40, 6.40

0.63, 0.45, 0.82

0.74, 0.55, 0.95

1.62, 1.00, 2.71

0.00

1.1, 0.5, 1.2

0.04, 0.0, 0.1

8.3, 5.2, 11.8

19.29, 10.01, 23.80

2.80, 1.50, 4.20

0.35, 0.21, 0.48

0.56, 0.44, 0.81

1.54, 0.65, 3.77

0.00

0.8, 0.6, 1.1

0.06, 0.0, 0.2

4.1, 2.2, 6.3

7.44, 5.60, 10.36

0.98, 0.59, 2.10

0.17, 0.13, 0.23

0.57, 0.35, 0.86

0.78, 0.24, 2.72

0.00

1.0, 0.5, 1.7

0.07, 0.0, 0.2

2.1, 1.2, 2.7

9.67, 8.75, 10.22

0.99, 0.69, 1.15

0.22, 0.19, 0.23

0.50, 0.36, 0.61

0.44, 0.32, 0.65

0.00

0.7, 0.6, 0.9

0.05, 0.0, 0.9

2.9, 1.9, 3.8

8.19, 6.20, 10.01

0.98, 0.78, 1.10

0.22, 0.20, 0.24

0.45, 0.38, 0.50

0.43, 0.31, 0.67

0.00

0.7, 0.6, 0.9

0.05, 0.0, 0.1

2.7, 2.0, 3.6

6.31, 5.10, 8.50

0.98, 0.66, 1.60

0.14, 0.10, 0.24

0.43, 0.32, 0.55

0.73, 0.25, 1.96

0.00

0.9, 0.6, 1.5

0.03, 0.0, 0.1

1.8, 1.1, 2.8

6.65, 4.90, 9.66

0.97, 0.60, 1.90

0.15, 0.12, 0.24

0.56, 0.35, 1.00

0.86, 0.24, 2.91

0.00

1.3, 0.6, 2.0

0.06, 0.0, 0.1

2.04, 1.7, 2.6

6.81, 4.90, 9.17

0.69, 0.59, 1.00

0.15, 0.14, 0.17

0.42, 0.37, 0.45

0.24, 0.23, 0.25

0.00

0.9, 0.8, 1.1

0.05, 0.0, 0.1

2.3, 1.9, 2.6

7.56 6.86 4.67, 14.84, 2.10

1.30 1.10 0.39, 0.00, 1.09

0.13 0.01 0.05, 0.02, 0.11

0.30 0.29 0.32, 0.06, 0.81

0.30 0.29 0.79, 0.14, 4.02

0.00 0.00 0.01, 0.00, 0.10

0.5 0.6 1.1, 0.4, 2.6

0.0 0.0 0.40, 0.0, 1.4

2.5 1.8 1.0, 0.5, 1.4

3.64, 4.26, 3.21

0.10, 0.05, 0.20

0.01, 0.01, 0.01

0.17, 0.12, 0.20

0.07, 0.03, 0.08

0.03, 0.00, 0.10

0.7, 0.5, 0.9

0.27, 0.0, 0.6

0.3, 0.2, 0.4

A.A. Beylich et al. / Geomorphology 58 (2004) 125–143

(18) n = 10

Creek below snow patch Temporary creek

137

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some of these show a clear loss from fresh to weathered samples, but as these components can not be compared with results from the water samples they are not shown here. The two fresh rock samples have clearly higher specific gravity than the two weathered ones and particularly the CaO and MnO2 contents are reduced in the weathered samples; also Na2O is reduced somewhat. 4.4. Specific discharges, solute concentrations and gross yields of dissolved solids (Fig. 4, Table 3) Values for seven key sites, i.e. the outlets of the subcatchments A, B, C and D, the inlet and outlet of Latnjajaure and the outlet of the entire Latnjavagge drainage basin given in Fig. 4, reflect changes over the summer of 2001 (see also sites given in boldface in Table 3). The changes for the inlet and outlet of Latnjajaure, subcatchment A and the total from the basin have been presented previously (Beylich et al., in press). In all subcatchments, there is a clear snow melt period during June, yet, in subcatchment A (Fig. 4a), it is less pronounced than in the others and it also fades out earlier than in the other parts of the basin. In the upper part of the basin (C and D and at the inlet of the lake) (Fig. 4c, d, e), there is a relatively low gross yield during the snowmelt period, and in addition, the solute concentrations are low. After the main snow melt, the discharges become more dependent on the precipitation but occasionally, such as for example during mid July, a relatively warm period caused further melting of snow. In addition, during the later part of the summer, the gross yields were low and so were the concentrations, which were rather constant between 6 and 8 mg/l over the season, and only further downstream at the inlet of the lake (Fig. 4e) where discharge from a part of the eastern slope had become added had the values become slightly higher by early August. Subcatchment B (Fig. 4b) has higher gross yields than in the upper catchment during the snow melt peak and also somewhat higher solute concentrations; also in this area, the warm July spell is reflected as snow melt in the discharge but after that there is a gentle increase in the solute concentration over the summer, i.e. during the first half of July the subarea has solute concentrations around 8 mg/l, but after the

mid July warm spell (see Fig. 3), the concentrations gradually increase to around 12 mg/l. The outlet of the lake, Latnjajaure (Fig. 4f), represents a mixture from the influx to the lake with added discharge from the W- and E-facing slopes. The snow melt discharge peak is pronounced but not excessively much higher than at the inlet, but the total gross yield is higher; the solute concentrations are very low during mid June but increase rapidly later in that month. From then on, the gross yields and specific run-offs have fluctuating but parallel trends while the solute concentrations remain around 12 mg/ l during the summer. At the outlet of the lake, there is, thus, a clear addition of solutes (Fig. 4f) from the radiation-exposed eastern slope, which has a modestly thick regolith. Clearly, the water with rather high solute concentrations along the slope with sampling site 15 having even very high values for the area has been added to the lake water. Besides, the ground temperature at this slope is above 0 jC already shortly after the end of the comparatively early snowmelt (see Fig. 3). In addition, subcatchment A faces radiation during a relatively long time of the day. This slope is more gently sloping than the east side further to the north, and in its central and lower part there is a relatively thick regolith (Beylich et al., in press). Even during the snow melt peak, the solute concentrations from this subcatchment (Fig. 4a) are higher than they were found to be at any time in the upper part of the Latnjavagge catchment, but there are differences within the subarea. In the upper reaches, which extend to the watershed and where perennial snow patches are present, the lake (sample site 12, Fig. 2) has the ‘‘cleanest’’ water of all sites with a TDS of only 4.06 mg/l, which is less than that of summer precipitation in the area (Beylich et al., submitted for publication). The concentrations at the outlet of subcatchment A remain around 12 mg/l until mid July after which they rise to more than double in less than a month, and to judge from the TDS values at the outlet of the whole Latnjavagge basin the relatively high TDS values from basin A are clearly added to those of the lake water. The mean TDS value over the summer in subcatchment A is fairly high though not as high as the values for sites at the foot segment of the eastern slope, where ions from weathered rock are relatively abundant.

A.A. Beylich et al. / Geomorphology 58 (2004) 125–143

4.5. Chemistry of surface water samples, precipitation and snow cores/samples (Figs. 4 and 5, Table 3) Based on field data from the summer seasons of 2000, 2001 and 2002, the mean annual atmospheric salt input (wet deposition) in the Latnjavagge drainage basin is 3436 kg/km2/year and the mean annual chemical denudation rates (corrected for atmospheric inputs) of the different subcatchments and the entire drainage basin were calculated as 7894 kg/km2/year for subcatchment A, 3936 kg/km2/year for subcatchment B, 1366 kg/km2/year for subcatchment C, 1411 kg/km2/year for subcatchment D, 2333 kg/km2/year at the inlet of Latnjajaure, 4554 kg/km2/year at the outlet of Latnjajaure and 5382 kg/km2/year for the entire Latnjavagge drainage basin (Beylich et al., submitted for publication). During the 2001 field season, the mean TDS values (Table 3) at the different sampling sites varied be-

139

tween 4.06 (lake site 12) and 54.35 mg/l (sampling site 15), with 12.14 mg/l at the outlet of the Latnjavagge drainage basin. The mean solute concentrations for precipitation and snow samples were 4.67 and 3.64 mg/l, respectively. Ca2 +, Mg2 +, Na+, K+, Cl and SO42 have the highest ion concentrations in the surface water samples, generally with a clear dominance of SO42 and Ca2 +. The latter of these is included with the rock sample analysis where it shows a clear loss. In the precipitation, Ca2 + is relatively minor and instead K+ and Cl are important while Cl followed by SO42 dominate in the snow samples. Fig. 5 shows that if a cut-off value of 2.3 is chosen, most of the samples group into four main clusters. One includes sampling sites 4, 12, 24 and 25, i.e. the four small lakes, and of these, the values of the upstream lakes 24 and 25 are closest together and in turn the average of these is closest to sample 12 in the

Fig. 5. Cluster analysis dendrogram based on average solute concentrations for the sample sites indicated in Table 3. The sampling sites, represented by their numbers, are given along the horizontal axis and are arranged so that the branches do not cross each other. A value of 2.3 is chosen to separate clusters. The locations of the sampling sites are indicated in Fig. 2.

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next step (Figs. 5 and 2). The two sampling sites that are closest together of all are 19 and 20. They form part of the largest group, which further comprises sites 2, 3, 7, 8, 11, 18, 21, 22, 23. However, the group can be further subdivided into one group with sites 18, 19, 20, 21, 23 and 2 which, except for the latter, come from small catchments and surface run-off samples upstream of the delta of Latnjajaure and these samples are characterised by very low ion concentrations. Another subgroup within this larger group includes sampling sites 3, 7 and 8, which can be said to represent mean concentration values for the area with site 8 being at the main outlet of the basin. Sampling site 11 also belongs to this largest cluster, but is not very close to the other samples. Amongst the group 1, 9, 13, 14, 16 and 17, sampling sites 9 and 17 are closest with 14, 16 and 13 following and all these samples are collected along the eastern side of the lake, while number 1, which is not as closely clustered to them, is from the western slope. The clustering in combination with Table 3, thus, shows that the highest concentrations of dissolved salts were measured in subcatchment A and in pipes and small creeks of the W-facing slope east of the lake (see also Fig. 2). Sampling sites 5 and 6 belong together but are only modestly close and sites 10, 26, 27 and 15 fall outside clusters, yet it may be noted that 26 (precipitation) and 27 (snow cores) are closer to each other than to other sites and that sampling site 15 is entirely different from any other sites.

5. Discussion 5.1. Subareas within Latnjavagge The combined data together with the field observations indicate that there are differences in solution and water chemistry between subareas within the Latnjavagge drainage basin. The northern part of the catchment consists of a high lying plateau with only very thin or lacking regolith; it has slopes with thin regolith and with larger snow and ice patches or fields, and large parts of the area receive only little radiation. There are rather constant low ion concentrations throughout the field season, suggesting that the ground remained frozen over the summer. The upper subcatchments are also areas with relatively little vegetation

and, thus, relatively little possibility for CO2 reactions in the ground. The TDS values are only slightly higher than that of precipitation but the composition of the cations with some Ca2 + suggests slight weathering over the surface where water can react with fresh, exposed rock, yet, at the same time the relatively high proportion of Cl with concentration values close to those of the precipitation suggests that the outflow is strongly influenced by precipitation. Subcatchment B represents an E-facing steep slope with snow and ice patches and thin regolith cover. The lake in this area clusters with the three other small lakes as to ion contents. The solution effect during snowmelt is not as pronounced as in the upper catchment, and during the first half of July, the subarea still has rather low solute concentrations. Yet, they increase after the mid July warm spell, probably as a consequence of gradual thawing of the ground. There is no collective subcatchment site for the eastern slope. In this area, the ground is above 0 jC already in mid to late June when the slope becomes snow-free some weeks before other parts of the drainage basin (Beylich et al., in press). The regolith along the slope varies in thickness between less than a metre to more than 30 m in the lower slope segment. The TDS values from individual sites are amongst the highest of the entire catchment area, most belong to the same cluster, and the water contains a relatively high proportion of ions from weathering of the bedrock while the concentrations of Cl are not higher than in the precipitation dominated upper reaches of the basin. The eastern slope is exposed to intensive summer radiation at an almost perpendicular angle during much of that season and it is, therefore, relatively warm and well vegetated with dry Dryas meadow vegetation, typical of base-rich and welldrained soils (Molau et al., 2003). It follows that this slope can be seen as a relatively warm area for the whole basin with a continuous vegetation cover in the lower part and with a reasonably thick regolith that is thawed from mid June, thus, providing free drainage with both subsurface flow and turbulent surface runoff (both creeks and diffuse flow). Daily solution rates can, therefore, become relatively high and the long period with thawed conditions provides optimal conditions for a high total output of dissolved solids (Beylich et al., in press).

A.A. Beylich et al. / Geomorphology 58 (2004) 125–143

From the total solute concentration in the outflow from Latnjajaure (Fig. 4f), there is a sudden and pronounced increase to almost double concentration values of those at the inlet (Fig. 4e) at the time when the eastern slope becomes snow free, but following that there is no major increase in ion concentrations at the lake outlet. This means that run-off from the western slope does not seem to influence the concentrations at the outlet of the lake to a large extent while that from the eastern slope clearly does. At the lake outlet, the July snow melt discharge peak that is clearly expressed at the lake inlet is still represented but it is dampened as compared to site 8 since there was no remaining snow to melt on the east slope at that time. Subcatchment A is complex but at its outlet, the solute concentration of the discharge is the highest amongst the constricted subcatchments, yet lower than individual sites along the eastern slope. The altitudes in the subcatchment vary from the watershed where there are snow patches and fields, and the low TDS value in the small lake (sampling site 12) indicates that melt-water from snow must have contributed substantially to its content. The effect from this upper part of the area is still seen at site 11, which clusters with the ion-poor sampling sites above Latnjajaure. At its outlet from the lower part of the subcatchment where the regolith varies between less than 10 and up to 30 m in thickness, the increasing ion concentrations during July indicate that the regolith gradually thawed during that month after which combined evidence from run-off and geophysical investigations (Beylich et al., in press) indicates that there was free drainage through the whole regolith in most or maybe even all of the snow-free part of subcatchment A. The solute concentrations (Fig. 4a) are amongst the highest within the Latnjavagge drainage basin and provide a sufficiently high proportion of chemical denudation material to leave a clear imprint on the total outflow from the basin (Fig. 4a and g). When seen over the whole summer, the eastern slope contributes dissolved substances for a relatively long time, at least 3 months; subcatchment A is intermediate while the western slope starts to thaw in late July and seems to still be in the process of thawing during August. The upper part of the catchment does not seem to have thawed at the end of the field season.

141

The TDS values over the area show a marked variability, with clearly higher values for the eastern slope of Latnjajaure. In this area, the chemical weathering not only lasts longest but it also shows the highest intensities with the highest mean concentrations of dissolved substances in the creeks draining the slope system, yet this area is rather small. In addition, subcatchment A (sampling site 7 in Fig. 2) contributes relatively much (7894 kg/km2/year) to the calculated total annual chemical denudation rate (corrected for atmospheric inputs) of 5382 kg/km2/year for the entire Latnjavagge drainage basin (see Beylich et al., in press, submitted for publication). The clear contrast within the basin to this situation is the upper catchment which is colder both because it is more in the shade and because relatively more of it is located at a higher altitude. Further the regolith thickness is less than along the eastern slope and the contact between drainage water and mineral particles therefore less. 5.2. Comparison to other periglacial areas The mean TDS values of 4.67 mg/l for precipitation during the summer months and 3.64 mg/l for samples from the winter snow pack in Latnjavagge are only slightly lower than the 3 – 9 mg/l with a mean of 6.3 for rain and slightly higher than the 2 –4 mg/l for snow (snow patches, sampling in August) in Ka¨rkevagge (Darmody et al., 2000) a few kilometres NW of Latnjavagge, and it is also lower than the values given by Rapp (1960) for Riksgra¨nsen (12 – 29 AS/cm, corresponding to 8.4 – 20.3 mg/l) closer to the North Atlantic. Even if the data sets are from different years, the values, especially from the Latnjavagge and the Ka¨rkevagge areas, are within the same range. On the other hand, the mean annual chemical denudation rate of 5382 kg/km2/year for the Latnjavagge drainage basin is much lower than the chemical denudation rate of 26,000 kg/km2/year calculated by Rapp (1960) and the 19,200 kg/km2/ year calculated later by Darmody et al. (2000) for Ka¨rkevagge (see also Campbell et al., 2002). The chemical denudation rate in Latnjavagge is also lower, although much less so, than the 8000 kg/ km2/year calculated by Beylich (1999, 2000a,b) for the coast near basaltic Austdalur drainage basin in East Iceland where the mean annual run-off is 1130

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mm/year and the annual atmospheric salt input reaches 18,600 kg/km2/year. The chemical denudation rate in Latnjavagge is similar to that in a number of other subarctic, arctic and alpine areas (see Table 4 in Darmody et al., 2000; Beylich, 1999, 2000a,b; Beylich et al., in press, submitted for publication). In the precipitation and annual snow pack in Latnjavagge, the mean concentrations of Na+, Mg2 +, NO3 and SO42 are lower than values given for Southern Scandinavia (see Table 4 in Darmody et al., 2000). The averages for Na+ and Fe3 + are similar, the means for Mn+ lower and the means for K+, NO3 higher than the values given by Darmody et al. (2000) for Ka¨rkevagge. The most important ion in the surface water samples from Latnjavagge is SO42 , followed by Ca2 +. Rapp (1960), Stro¨mquist and Rehn (1981) and Darmody et al. (2000) found that SO42 was also the most important ion in surface water in Ka¨rkevagge. The values of TDS, SO42 , Mg2 + and Na+ are lower in Latnjavagge than the values of the corresponding components given for Ka¨rkevagge by Darmody et al. (2000), whereas the values of NO3 and K+ are similar. As already pointed out by Thorn et al. (2001) and Darmody et al. (2001) (see also Campbell et al., 2002), local conditions with easily weathered black schist and sulphurous rocks that create a chemically aggressive environment in Ka¨rkevagge cause particularly high chemical denudation rates in that drainage basin as compared to other arctic –alpine areas. The Latnjavagge catchment, therefore, seems more representative for the northern Scandinavian mountain area than Ka¨rkevagge.

6. Conclusion Solution and water chemistry data from the arctic – oceanic Latnjavagge drainage basin, which has a homogeneous lithology, show a clear spatial variability of concentrations and yields of dissolved solids within the catchment thus reflecting different conditions of solution and chemical denudation between subareas of different aspect, snow cover duration, frozen ground conditions and regolith thickness. Cluster analysis aids the overview of data and supports the impression of a system with some areas having very low and others relatively high TDS values: There are

very low values of total dissolved solids in areas of shade, cold ground and thin regolith and even if the discharge from such areas contains ions from weathering of the rock, the ion input from the precipitation is clearly reflected in the run-off. In contrast, areas that are exposed to more intensive radiation and have a fairly thick frost-free regolith for a relatively long time have comparatively higher contents of ions that mainly reflect solution of lithological components. A comparison to Ka¨rkevagge and other periglacial areas further demonstrate that between areas of different lithology the importance of local bedrock composition can become of major importance. Because of the importance of local lithological factors for chemical denudation in Ka¨rkevagge, the Latnjavagge drainage basin appears to be more representative for chemical denudation in the periglacial mountain area of northern Scandinavia than the Ka¨rkevagge drainage basin.

Acknowledgements Research in Latnjavagge was funded by a DAAD Post Doc grant (Stipendium des DAAD im Rahmen des Gemeinsamen Hochschulsonderprogramms III von Bund und La¨ndern, 1999– 2001; grant to Achim A. Beylich). Since 2002, research has been funded by the Deutsche Forschungsgemeinschaft (Emmy Noether-Programm; grant to Achim A. Beylich). The field work was logistically supported by the Abisko Scientific Research Station (ANS), the Latnjajaure Field Station (Ulf Molau) and by the Department of Earth Sciences, Uppsala University. The water chemistry analyses were carried out in the laboratory of AB Hydrogeology, Institute for Geological Sciences, Free University of Berlin. Ulf Molau kindly provided data on snow cover and meteorological and soil temperature data from the meteorological station of the Latnjajaure Field Station (LFS) and field assistant Karin Luthbom (Uppsala/Lulea˚) gave helpful support in the field. Sven Snell (Swedish Geological Survey, SGU, Uppsala) kindly carried out the rock density measurements. The support from the mentioned persons and institutions is gratefully acknowledged. We are grateful to Nel Caine and an anonymous reviewer for critical and helpful comments on the manuscript.

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