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University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 1 Hydrochemical Evidence for Differential Weathering in H roglacial and Deglaciated Watersheds in Western Greenland Alia J. Lesnek, Dr. Jonathan B. Martin, and Kelly Deuerling College of Liberal Arts and Sciences University of Florida Marine sediments record variations in fluxes of radiogenic isotope from continents that result from weathering processes as continental glaciers retreat, but few previous studies have examined how weathering differs in deglaciated and proglacial watersheds. In this study, we assess differences in their weathering through comparison of major element compositions of water in proglacial and deglaciated watersheds near Kangerlussuaq, Greenland. The dominant catio n in proglacial streams is Ca2+ and the dominant anion is HCO3 Concentrations of these ions increase with distance from the glacier due to the dissolution of micas, hornblende, and albite. Deglaciated watersheds are enriched in Mg2+ and HCO3 relative to proglacial watersheds and increase in Mg2+ and Ca2+ concentrations with distance from the glacier, which could reflect the dissolution of micas, hornblende, and albite. Deglaciated watersheds have higher salinity than proglacial watersheds due to evaporat ive concentration of the solutes. These differences in watershed chemistry suggest they may produce different radiogenic isotope signatures as the fraction of the types of drainage varies as glaciers advance and retreat. INTRODUCTION Glaciers increase physical weathering by producing clay sized particles (rock flour) from underlying bedrock, which is susceptible to chemical weathering because of its elevated surface area/volume ratio. The extent of chemical weathering is an important c ontrol on the fluxes of radiogenic isotopes from continents to the oceans (von Blanckenburg and Nagler, 2001; Blum and Erel, 2003). The variations in isotope fluxes that result from chemical weathering are recorded in marine sediments, which has been inter preted in terms the collapse of North American continental glaciers (Kurzweil et al., 2010; Foster and Vance, 2006). Few previous studies have examined how weathering differs as glaciers retreat, in particular differences in weathering with deglaciated wat ersheds, defined here as those from which glaciers have completely retreated, and proglacial watersheds, defined here as those which drain the glaciers. Such a study is possible in portions of Greenland from which the Greenland Ice Sheet has retreated. I n Greenland, our understanding of glacial and deglacial weathering processes is still in its infancy (Yde et al., 2004), even though melting of the Greenland Ice Sheet may have significant consequences for variations in global sea level and atmospheric CO2 concentrations (Hodson et al., 2000; Tranter et al., 2002). Additionally, the Greenland Ice Sheet represents approximately one third of modern annual glacial runoff to the oceans (Tranter, 2003), and is thus a major source of glacially derived solutes to the modern ocean, particularly in the Labrador sea, at the site of formation of most of North Atlantic Deep Water. In this study, we compare the major element chemistries of proglacial and deglaciated watersheds near Kangerlussuaq, Greenland, near the ic e margin, to assess how their weathering differs. We hypothesize that proglacial and deglaciated watersheds will have distinct major element compositions and that these compositions will reflect the variations in chemical weathering in each environment (An derson et al, 2000). The data from this investigation will be used to assess the weathering products in glacial watersheds to estimate whether fluxes of radiogenic Sr, Nd, and Pb isotope may differ between the watersheds. STUDY AREA Western Greenland repr esents the widest extent of deglaciated landscapes in Greenland ( F igure 1). Retreat and thinning of the ice sheet has previously been studied through a variety of isotopic age dating methods (Rinterknecht et al., 2009). Changes in ice location and thicknes s relative to the rate of sea level rise during the Little Ice Age (LIA) reflect short term climatic variations (Long et al., 2011). The ice sheet advanced to about 100 km seaward of the modern coast during the Last Glacial Maximum (LGM). Maximum ice elevation was about 750 to 810 m above sea level along the coast with thinning occurring between about 21 and 9.8 ka (Roberts et al., 2009). The ice retreated to as much as 50 km inland of the current ice edge, where it persisted for approximately 6000 years du ring the Holocene Climatic Optimum (ten Brink, 1975; van Tatenhove et al., 1996). During Neoglaciation and the LIA, ice advanced to approximately 12 km seaward of its present position, but has again retreated over the past few centuries (Forman et al., 20 07). This periodic advance and retreat of the ice sheet has created multiple watersheds between the ice edge and the coast, some of
ALIA J. LESNEK, DR. JONATHAN B. MA RTIN, AND KELLY DEUERLING University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 2 which drain directly from the ice and others that drain deglaciated areas. Each watershed should be characterized by differ ences in the extent of mineral suites that are weathering (e.g., Blum and Erel, 1995; Harlavan et al., 1998; Harlavan et al., 2009). Figure 1 Map of Greenland with summer 2012 study area. Our research focused on watersheds near Kangerlussuaq ( F igure 1) This region only represents a small portion of Greenland, but it provided the best accessibility to the watersheds and extensive work on the timing of retreat of the Greenland Ice Sheet has been conducted there. The area is underlain by homogenous Archea n gneiss that has been metamorphosed to amphibolite facies (Henrikson et al., 2000), which suggests that there should be little to no variation within the field area of the material that is being weathered. Precipitation in the region ranges from 150 mm/y r near the ice sheet to over 300 mm/yr near the coast. The eastern half of the region experiences a negative water balance (Helweg, 2004; Neilsen, 2010). Most of the precipitation in deglaciated watersheds discharges as surface runoff because there is a meter of active layer of permafrost (Helweg, 2004). METHODS Water samples were collected along a 20 km transect of the Watson River (a proglacial stream) and deglaciated lakes near Kangerlussuaq from 19 June 4 July 2012. Sample locations are shown in F igur e 2. For the purposes of this paper, we have divided the proglacial samples between upstream and downstream. Upstream values are the average of the three sample sites closest to the glacier (WR7, WR8 1, and WR82) and downstream values are the average of the four sites furthest away from the glacier (WR2 1, WR22, WR9 1, and WR 92). The samples were collected using a peristaltic pump and tubing connected to a trace metal clean infilter and an overflow cup. The overflow cup was instrument ed with an YSI556 multiparameter sensor, which monitored specific conductivity, dissolved oxygen, pH, and temperature. All electrodes were calibrated each day of field analyses and check standards were measured throughout the day to ensure continued calibration. Water from each sample location was collected in several 20 ml high density polyethylene (HDPE) bottles to be measured for major element concentrations and alkalinity. Figure 2. Summer 2012 sample locations. Water samples for cation measurement s were preserved by addition of ultrapure HNO3 to a pH of about 2 and anion samples were unpreserved. One aliquot of the samples was titrated for alkalinity the day of collection; cation and anion samples were shipped to University of Florida laboratories for analyses by ion chromatography and ICP MS. The coefficient of variation ranged from 1.1% to 1.6% for 5 check standards that were measured along with the major elements. Charge balances for deglaciated samples are generally within 10%. The charge balan ces for proglacial samples vary more widely because the samples were dilute, making concentrations at the lower detection limit of the instruments. Although these values are not ideal, proglacial upstream and downstream samples can still be differentiated. RESULTS Table 1 shows the concentration of dissolved major elements, charge balance, and specific conductivity in proglacial and deglaciated watersheds. The deglaciated samples are more concentrated than proglacial samples. The glaciated and deglaciated watersheds have distinct compositions of major element concentrations ( F igure 3). Anion concentrations are dominated by HCO3 in both watersheds, but Ca2+ is the dominant cation in proglacial watersheds while Mg 2+ is the dominant cation in deglaciated w atersheds. The compositions of water in the proglacial and deglaciated watersheds evolve differently with distance from the glacier, which we consider here to be a proxy for time since the glacier retreated from the watershed. Deglaciated watersheds become more enriched with Mg2+ and Na++K+ with distance from the glacier, while proglacial streams show significant increases in all dissolved components with distance. Specific conductivity increases with distance from the glacier for th e proglacial wat ershed by about a factor of 10,
DIFFERENTIAL WEATHERING IN HROGLACIAL AND DEGLACIATED WATERSHEDS IN WESTERN GREENLAND University of Florida | Journal of Und ergradua te Research | Volume 15, Issue 3 | Summer 2014 3 Note: ND= Not determined, below detection limit but decreases with distance by nearly a factor of 4 for the deglaciated watershed ( F igure 4). Based on observations in the field, we estimate that the residence time of water in the Watson River is less than one week. Figure 5 shows theincrease in cation concentrations with distance from the glacier in proglacial samples. The concentration of all cations increases, but Ca2+ and Na+ show the most increase, becoming nearly six times more concentrated between the upstream and downstream samples. The molar ratios of cations in proglacial and deglaciated samples are shown in T able 2. All samples are ordered by distance from the glacier. Samples from deglaciated watersheds are grouped by the system of the lakes from which they were collected (see F igure 2). DISCUSSION Our results show that the chemical compositions differ for water in proglacial and deglaciated watersheds. Because the bedrock in the region is homogenous (Henriksen, 2000), differences major element chemistry result from the relative importance of evaporation and chemical weathering in each environment. We obser ved changes in specific conductivity in both watershed types, which can reflect either of these processes. Evaporation in the watershed will increase the concentration of dissolved ions, but because no chemical reactions are taking place, the molar ratios of dissolved species will remain constant. In contrast, chemical weathering of the bedrock will produce both an increase in dissolved ion concentration Table 1. Major Element Chemistry o f Proglacial and Deglaciated Watersheds Sample Na+ (mM) K+ (mM) Mg2+ (mM) Ca2+ (mM) Cl(mM) SO4 2(mM) Alkalinity (mM) SiO2 (mM) Specific Conductivity Charge Balance % offset Deglaciated DG1 -1 0.11 0.042 0.11 0.19 0.087 0.026 0.58 0.02 71 2 DG1 -2 0.10 0.042 0.11 0.16 0.086 0.026 0.57 0.01 71 -1 DG1 3 0.10 0.042 0.11 0.16 0.084 0.027 0.65 0.01 71 7 DG1 -4 0.11 0.043 0.11 0.16 0.088 0.027 0.71 0.01 72 -10 DG1 5 0.10 0.043 0.11 0.16 0.089 0.028 0.68 0.01 72 9 DG2 1 0.81 0.43 1.2 0.36 0.62 0.003 3.6 0.03 396 0.3 DG2 -2 0.37 0.24 0.54 0.23 0.31 0.030 1.6 0.11 194 5 DG2 -3 0.76 0.36 0.97 0.52 0.68 0.008 3.4 0.01 374 0.2 DG3 -1 0.20 0.087 0.23 0.23 0.16 0.042 0.82 0.04 116 7 DG3 -2 0.21 0.086 0.24 0.23 0.17 0.042 0.86 0.05 118 6 DG3 -3 0.69 0.27 0.76 0.59 0.74 0.022 2.6 0.01 333 3 DG3 4 0.47 0.20 0.63 0.63 0.46 0.071 2.4 0.07 286 3 DG3 -5 0.64 0.34 0.71 0.32 0.64 0.033 2.1 0.09 280 5 DG4 1 0.27 0.077 0.23 0.31 0.24 0.051 0.95 0.04 134 5 DG4 -2 0.41 0.095 0.29 0.28 0.42 0.010 0.75 0.16 152 9 DG4 3 0.42 0.086 0.26 0.27 0.42 0.083 0.77 0.10 148 8 DG4 4 0.41 0.087 0.25 0.24 0.42 0.085 0.68 0.11 139 8 Proglacial WR10 -1 0.01 0.01 0.01 0.02 ND 0.01 0.06 0.01 7 -9 WR10 -2 0.01 0.01 ND 0.02 ND 0.01 0.04 ND 5 -3 WR11 1 0.01 0.01 0.01 0.02 ND 0.01 0.08 0.01 7 18 WR11 -2 0.01 0.01 0.01 0.03 ND 0.01 0.07 0.02 8 1 WR2 1 0.04 0.03 0.01 0.05 ND 0.02 0.20 0.02 19 14 WR2 -2 0.04 0.03 0.01 0.05 ND 0.02 0.13 0.03 19 3 WR6 1 0.03 0.02 0.01 0.04 ND 0.02 0.11 0.02 16 1 WR6 -2 0.03 0.03 0.01 0.04 ND 0.02 0.14 ND 17 -6 WR7 ND ND ND 0.01 ND ND 0.14 0.01 3 66 WR8 -1 ND ND ND 0.01 ND 0.01 0.14 0.01 5 -57 WR8 -2 ND ND ND 0.01 ND ND 0.25 ND 2 -86 WR9 -1 0.06 0.04 0.02 0.07 0.01 0.03 0.33 0.04 28 -19 WR9 -2 0.05 0.03 0.02 0.05 ND 0.02 0.17 0.03 19 -2
ALIA J. LESNEK, DR. JONATHAN B. MA RTIN, AND KELLY DEUERLING University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 4 Figure 3. Piper plot showing the major element chemistries of proglacial and deglaciated watersheds. The black lines show evolution of the dissolved ion composition with distance from the glacier. and a change in the ratios of elements that depend on the stoichiometry and extent of dissolution of minerals phases being weathered. These two processes can be differentiated by examining the molar ratios of elements in the water. Controls on Water Composition in Proglacial Watersheds The concentrations of all ions increased and specific conductivity of the water increased by a factor of 10 in the Watson River between its headwaters at the ice sheet and Kangerlussuaq where it discharges to Sndre Strmfjord ( F igures 4 and 5). The tenfold increase in specific conductivity between the upstream and downstream samples indicates that a high degree of chemical weathering is taking place over a short period of time. The negative water balance in the region drives evaporation, which likely contributes to the increase inthe ion concentrations as well. The ratios of dissolved cations were not constant among samples (Table 2), indicating that chemical weathering is occurring in proglacial watersheds. By examining these ratios, we can estimate which minerals are weathering. The K+/Mg2+ and K+/Ca2+ ratios increase with distance from t he glacier, reflecting an enrichment in K+ over Mg2+ and Ca2+. The dissolution of mica minerals could be the source of potassium in the water, especially since preferential leaching of biotite in glacial environments has Figure 4. Distance from the glac ier versus specific conductivity in proglacial and deglaciated watersheds. been observed previously (e.g. Anderson et al., 1997). The Na+/Mg2+ and Na+/Ca2+ ratios also increase with distance from the glacier. There are several possible sources for sodium i n the water, including albite (NaAlSi3O8), hornblende, and sea spray. The Na+/Cl ratio in sea spray is 0.86, and the ratio in all of the proglacial samples is much higher, reflecting the enrichment of Na+ or depletion of Cl relative to seawater. Hornblende may be a contributor of Na+ because its Na+/Ca2+ ratio cannot be higher than 0.5, and the samples near the glacier are below that value. However, the Na+/Ca2+ ratio exceeds 0.5 further downstream, suggesting that there is another source for Na+. Albite appears to be the main source of Na+ in proglacial watersheds, with the dissolution of hornblende possibly contributing Na+ closer to the headwaters of the Watson River. Figure 5 Bar graph showing the evolution of proglacial samples with distance from the glacier Deglaciated Proglacial
DIFFERENTIAL WEATHERING IN HROGLACIAL AND DEGLACIATED WATERSHEDS IN WESTERN GREENLAND University of Florida | Journal of Und ergradua te Research | Volume 15, Issue 3 | Summer 2014 5 Controls on Water Composition in Deglaciated Watersheds Like in proglacial watersheds, chemical weathering controls the composition of deglaciated lakes. However, because the concentrations of ions in this environment are much higher than in proglacial watersheds, evaporation appears to plays a role. The specific conductivity and cation ratios within any lake system are relatively constant ( T ables 1 and 2), which suggests that the same degree of evaporation and chemical weathering occurs in each area. However, between the lake systems, there are significant differences in both of these parameters. In all of the deglaciated lakes, the K+/Mg2+ ratio is near 0.3, so the dissolution of biotite may play a role in determining water composition. Hornblende dissolution may also occur in deglaciated lakes, enriching the water in Mg2+ and Ca2+. In the more saline lakes (DG2 1, DG 2 2, and DG 2 3), the Na+/Ca2+ ratio is higher than in lower salinity lakes. The Na+/Ca2+ ratio of sea spray is about 48, so sea spray can be unlikely to be a source of Na+ in Table 2. Molar Ratios o f Cations In Proglacial and Deglaciated Watersheds Sample Distance from Glacier (km) K + /Mg 2+ K + /Ca 2+ Na + /Ca 2+ Na + /Mg 2+ Ca 2+ /Mg 2+ Na + /Cl Proglacial WR8 2 0 0.9 0.3 0.2 0.7 3.4 2.4 WR7 0 0.9 0.3 0.1 0.4 3.2 2.1 WR8 1 0.6 0.9 0.3 0.2 0.7 3.4 5.6 WR10 1 10.3 1.3 0.3 0.2 0.9 4.0 3.2 WR10 2 10.3 1.3 0.3 0.3 1.1 4.2 6.4 WR11 2 15.9 1.5 0.4 0.4 1.5 3.7 7.8 WR11 1 16 1.8 0.4 0.3 1.3 4.1 7.3 WR6 1 22 3.3 0.6 0.8 4.3 5.5 11.6 WR6 2 22.2 3.3 0.6 0.8 4.3 5.3 12.2 WR2 1 35.8 2.6 12.2 11.1 2.4 0.2 12.1 WR2 2 35.8 0.5 12.2 16.5 0.7 0.1 12.5 WR9 2 41.3 2.1 0.6 0.9 3.2 3.3 14.9 WR9 1 41.4 2.1 0.5 0.8 3.4 4.2 10.1 Deglaciated DG2 1 23.5 0.4 1.1 2.2 0.7 0.3 1.3 DG2 2 23.7 0.4 1.0 1.6 0.7 0.4 1.2 DG2 3 24.3 0.4 0.7 1.5 0.8 0.5 1.1 DG3 5 34.6 0.5 1.1 2.0 0.9 0.6 1.0 DG3 4 34.7 0.3 0.3 0.8 0.7 1.0 1.0 DG3 3 34.8 0.4 0.5 1.2 0.9 0.8 0.9 DG3 2 35.7 0.4 0.4 0.9 0.9 1.0 1.3 DG3 1 35.7 0.4 0.4 0.9 0.9 1.0 1.2 DG1 1 34.5 0.4 0.2 0.6 1.0 1.7 1.2 DG1 2 34.7 0.4 0.3 0.6 0.9 1.4 1.2 DG1 3 34.8 0.4 0.3 0.6 0.9 1.5 1.2 DG1 4 35.7 0.4 0.3 0.6 0.9 1.4 1.2 DG1 5 35.7 0.4 0.3 0.6 1.0 1.5 1.2 DG4 1 47.3 0.3 0.3 0.9 1.2 1.3 1.1 DG4 2 46.8 0.3 0.3 1.5 1.4 1.0 1.0 DG4 3 46.8 0.3 0.3 1.5 1.5 1.0 1.0 DG4 4 47.3 0.3 0.4 1.7 1.6 1.0 1.0
ALIA J. LESNEK, DR. JONATHAN B. MA RTIN, AND KELLY DEUERLING University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 6 deglaciated watersheds. Therefore, we can conclude that albite is the main contributor of sodium in the water. Figure 5 Bar graph showing the evolution of proglacial samples with distance from the glacier Implications for Radiogenic Isotope Fluxes to the Oceans The differences in major element chemistry between proglacial and deglaciated watersheds reflect variations in the extent of chemical weathering in each environment. Although evaporation increases the overall concentration of deglaciated watersheds and the dissolution of micas, hornblende, and albite may occur in both environments, the degree of weathering appears to be different between the two watershed types. Since the radiogenic isotope ratios will depend on the amount of weathering, as well as the minerals being weathered, distinct isotopic compositions and concentrations of Sr Nd, and Pb should occur in water draining the landscapes. As the relative proportions of proglacial and deglaciated watersheds vary through time with the advance and retreat of ice sheets, so should the fluxes of radiogenic isotopes to the oceans. CONCLUSIONS Examination of the major element chemistry of proglacial and deglaciated watersheds near Kangerlussuaq provides insight into chemical weathering and evaporation processes occurring in glacial environments. Our results indicate that proglacial and deg laciated watersheds have distinct major element chemistries, reflecting differences in chemical weathering. These chemistries evolve differently with distance from the glacier, and the dissolution of micas, hornblende, and albite may occur in both watershe d types, controlling the composition of the water. Therefore, proglacial and deglaciated streams should produce different radiogenic isotope fluxes to the oceans as glaciers advance and retreat. Understanding how these fluxes vary with changing fractions of deglaciated and proglacial watersheds should allow refined interpretations of the marine records of periods of deglaciation. ACKNOWLEDGEMENTS We would like to acknowledge Ellen Martin and Cecelia Scribner for assistance with field work and laboratory an alyses. Funding for this project was provided by the National Geographic Society, grant number 907612, and the University of Florida University Scholars Program. REFERENCES Anderson, S. P., J. I. Drever and N. F. Humphrey (1997) Chemical weathering in gla cial environments, Geology 25: 399 -402. Blum, J. D. and Y. Erel (1995) A silicate weathering mechanism linking increases in marine 87Sr/86Sr with global glaciation, Nature 373: 415418. Blum, J. D. and Y. Erel (2003) Radiogenic isotopes in weathering and h ydrology. Surface and Ground Water, Weathering, and Soils J. I. Drever. Amsterdam, Elsevier. 5, Treatise on Geochemistry: 365392. Forman, S. L., L. Marin, C. van der Veen, C. Tremper and B. Csatho (2007) Little Ice Age and neoglacial landforms at the Inl and Ice margin, isunguata Sermia, Kangerlussuaq, west Greenland, Boreas 36: 341-351. Foster, G. L. and D. Vance (2006) Negligible glacial -interglacial variation in continental chemical weathering rates, Nature 444: 918921. Harlavan, Y., Y. Erel and J. D. Blum (1998) Systematic changes in lead isotopic composition with soil age in glacial granitic terrains, Geochimica Et Cosmochimica Acta 62(1): 33 -46. Harlavan, Y., Y. Erel and J. D. Blum (2009) The coupled release of REE and Pb to the soil labile pool with time by weatehring of accessory phases, Wind River Mountains, WY, Geochimica Et Cosmochimica. Acta 73: 320 -336. Helweg, C. (2004) Water balance in a west Greenlandic watershed, Northern Reserach Basins Water Balance, IAHS Publ. 290: 143 -151. Henriksen, N. A. K. Higgins, F. Kalsbeek, T. Christopher and R. Pulvertaft (2000) Greenland from Archaean to Quaternary, Descrptive text to the Geological map of Greenland. Geology of Greenland Survery Bulletin 185. Copenhagen. Hodson, A., M. Tranter and G. Vatne (2000) Contemporary rates of chemical denudation and atmospheric CO2 sequestration in glacier basins: an Arctic perspective, Earth Surface Processes and Landforms 25(13): 1447-1471. Kurzweil, F., M. Gutjahr, D. Vance and L. D. Keigwin (2010) Authigenic Pb isto pes from the Laurentian Fan: changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation, Earth and Planetary Science Letters 299: 458465. Long, A. J., S. A. Woodroffe, G. A. Milne, C. L. Bryant, M. J. R. S impson and L. M. Wake (2011) Relative sea -level change in Greenland during the last 700 yrs and ice sheet response to the Little Ice Age, Earth and Planetary Science Letters 315: 76-85. Nielsen, A. B. (2010) Present conditions in Greenland and the Kangerlussuaq Area. Eurajoki, Finland, Posiva Oy.
DIFFERENTIAL WEATHERING IN HROGLACIAL AND DEGLACIATED WATERSHEDS IN WESTERN GREENLAND University of Florida | Journal of Und ergradua te Research | Volume 15, Issue 3 | Summer 2014 7 Rinterknecht, V., Y. Gorokhovich, J. Schaefer and M. Caffee (2009) Preliminary 10Be chronology for the last deglaciation of the western margin of the Greenland Ice Sheet, Journal of Quaternary Science 24(3) : 270 278. Roberts, D. H., A. J. Long, C. Schnabel, B. J. Davies, S. Xu, M. J. R. Simpson and P. Huybrechts (2009) Ice sheet extent and early deglacial history of the southwestern sector of the Greenland Ice Sheet, Quaternary Science Reviews 28(2526): 2760 -2773. ten Brink, N. W. (1975) Holocene history of the Greenland ice sheet based on radiocarbon-dated moraines in West Greenland, Meddelelser om Gronland Gronlands Geologiske Undersogelse 113: 41. Tranter, M., M. J. Sharp, H. R. Lamb, G. H. Brown, B. P. H ubbard and I. C. Willis (2002) Geochemical weathering at the bed of Haut Glacier d'Arolla, Switzerland a new model, Hydrological Processes 16(5): 959993. Tranter, M. (2003) Geochemical Weathering in Glacial and Proglacial Environments. Surface and Ground Water, Weathering, and Soils J. I. Drever. Amsterdam, Elsevier. 5, Treatise on Geochemistry: 189207. van Tatenhove, F. G. M., J. J. M. van der Meer and E. A. Koster (1996) Implications for deglaciation chronology from new AMS age determinations in central west Greenland, Quaternary Research 45(3): 245253. von Blanckenburg, F. and T. F. Nagler (2001) Weathering versus circulation -controlled changes in radiogenic isotope tracer composition of the Labrador Sea and North Atlantic Deep Water, Paleoceanography 16: 424 -434. Yde, J. C., N. Tvis Knudsen and O. B. Nielsen (2004) Glacier hydrochemistry, solute provenance, and chemical denudation at a surge -type glacier in Kuannersuit Kuussuat, Disko Island, West Greenland, Journal of Hydrology 300(14): 172187.