Nordic Hydrology, 25, 1994, 371-388 No Part May Be Reproduced by Any Process Wlthout Complete Rekrence

Nordic Hydrology, 25, 1994, 371-388 No part may be reproduced by any process wlthout complete rekrence

Assessment of Spatial Variability of Major-Ion Concentrations and DEL Oxygen-18 Values in Surface Snow, Upper Fremont Glacier, Wyoming, U.S.A.

D. L. Naftz U.S. Geological Survey, Salt Lake City, UT 84104, U.S.A. P. F. Schuster and M. M. Reddy U.S. Geological Survey, Boulder, CO 80303, U.S.A.

One hundred samples were collected from the surface of the Upper Fremont Glacier at equally spaced intervals defined by an 8,100 m 2 snow grid to assess the significance of lateral variability in major-ion concentrations and del oxygen-18 values. For the major ions, the largest concentration range within the snow grid was sodium (0.5056 mgll) and the smallest concentration range was sulfate (0.125 mgll). Del oxygen-18 values showed a range of 7.45 per mil. Comparison of the observed variability of each chemical constituent to the variability expected by measurement error indicated substantial lateral variability within the surface-snow layer. Results of the nested ANOVA indicate most of the variance for every constituent is in the values grouped at the two smaller geographic scales (between 506 m2 and within 506 m2 sections). Cal- cium and sodium concentrations and del oxygen-18 values displayed the largest amount of variance at the largest geographic scale (between 2,025 m 2 sections) within the grid and ranged from 14 to 26 per cent of the total variance. The variance data from the snow grid were used to develop equations to evaluate the significance of both positive and negative concentrationlvalue peaks of nitrate and del oxygen-18 with depth, in a 160 m ice core. Solving the equations indicates that both the nitrate and del oxygen-18 ice-core profiles have concentrationlvalue trends that exceed the limits expected from lateral variability. Values of del oxygen-18 in the section from 110-150 m below the surface consistently vary outside the expected limits and possibly represents cooler temperatures during the Little Ice Age from about 1810 to 1725 A.D.

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Introduction High-altitude, mid- and low-latitude glaciers and snowfields in Kenya, Peru, Chi- na, Canada, Russia, Switzerland, and the United States recently have been cored and sampled to provide useful and geographically diverse paleoenvironmental records (Thompson and Hastenrath 1981; Thompson 1981; Thompson et al. 1984; 1986; 1988a; 1988b; and 1989; Holdsworth and Peak 1985; Wagenbach 1989; Ferronsky et al. 1991; Naftz et al. 1991a; 1991b; Naftz 1993; and Naftz et al. 1993) and data on chemical loadings to high-altitude ecosystems (Galbraith et al. 1991). Because of the diverse geographic locations of these sites, they provide important global linkages to the existing and future ice-core and snow-pit records in Nordic areas. To date (1993), the expense and logistical constraints of sampling these remote, high-altitude glaciers and snowfields have limited the number of drilling and sampling sites. With a limited number of sampling locations, researchers have assumed that individual snow and ice layers have uniform chemical composition (Metcalf and Peck 1991). Unfortunately, the conditions typical of high-altitude glaciers and snowfields favor large lateral variability in chemical composition. High winds during the winter season can transport snow from high-altitude ridges and deposit the snow in more protected bowls and cirques. This process could result in wide variability in snow chemistry from the ridge top to lower altitudes. For example, the high-velocity winter winds at the Colle Gnifetti site in Switzerland prevented preservation of the winter del oxygen-18 signature in ice cores taken at the site (Wagenbach 1989). Large seasonal fluctuations in air temperature can cause redistribution or re- moval of solutes in the snowpack and ice during the springlsummer melt season (Davies et al. 1982). For example, because of temperature gradients with altitude and different slope aspects, the extent of solute redistribution could vary substantially over the surface of the glacier or snowfield. Without additional information on the lateral variability of the constituent of interest, the snow or ice core samples may not represent the average chemical condition of that particular ice or surface- snow layer. Previous studies (Kumai 1985; Brimblecombe et al. 1985; Schemenauer et al. 1985; Jones 1985; Laird et al. 1986; Holdsworth and Peake 1985; and Andersson et al. 1990) have addressed the problem of lateral variability in surface snow samples qualitatively and have evaluated only selected chemical constituents. An excellent summary of these studies is given by Metcalf and Peck (1990), who concluded that lateral variability is substantial enough to mask much of the temporal variation in pH and conductivity in ice-core records. Metcalf and Peck (1990) recommend, at a minimum, a cursory examination of lateral variability in the snow chemistry of surface layers before drill-site selection in future drilling programs. Following the suggestion of Metcalf and Peck (1990), the first objective of this

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! cr" Wyoming \ L.2 I 1 wind R ver Ranr> Fretno"; Glacier , study area 1 i ,,dl, C.SPBI 1

Rock SDrnQn Cheyenne

SURFACE-SNOW GRID

Bare from U S Geolog~calSuwey 1.24.000quadrangle: 0 0 5 KILOMETERS Flemont Rak Norm. Wyommg. 1966 -0.5 MILES EXPLANATION -4.085 - TOPOGRAPHIC CONTOUR--Shows -BOUNDARY OF GLACIER alt~tudeof Ice surface Contour Interval IS about 12 meters and IS varlable because of conversion from feet Nat~onalGeodetlc Vertical Datum of 1929 Fig. 1. Location of surface-snow grid and ice-coring site on Upper Fremont Glacier.

paper is to provide a qualitative and quantitative assessment of the spatial variability of major-ion concentrations and del oxygen-18 values in surface snow near the end of the ablation season adjacent to a high-altitude drilling site in the Wind River Range of Wyoming (Fig. 1). The second objective is to utilize these results to assess the significance of variability of selected chemical constituents with depth at the same site. The late-season snow surface was sampled to best correspond to the snow chemistry potentially preserved after the firn-to-ice transition at the site.

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Fig. 2. Photograph with superimposed schematic of the surface-snow grid and ice-coring site on Upper Fremont Glacier.

Site Description The study site is located on Upper Fremont Glacier, within the Wind River Range of Wyoming (Fig. 1). Meier (1951) gives a complete description of Fremont Glacier, which is summarized as follows. Total area of Fremont Glacier in 1950 was about 2.7 km2. Median elevation of Fremont Glacier is about 3,924 m a.s.l., the highest median elevation of any major glacier in the Wind River Range. Fre- mont Glacier is composed of an upper ice mass designated "Upper Fremont Glacier" connected to a lower ice mass designated "Lower Fremont Glacier" by a narrow ice neck (Fig. 1). Only limited recession from the maximum extent of Upper Fremont Glacier in the Little Ice Age was observed in 1950. Collection locations of surface-snow samples were determined by using an 8,100 m2 grid superimposed on the glacier (Fig. 2). The east corner of the sample grid is located about 15 m southwest of the site where a 160 m continuous ice core to bedrock was collected during 1991 (American Geophysical Union 1992). Eleva- tions of the sample grid range from about 4,000 to 4,012 m a.s.1. Both the sample grid and ice-coring site were located above the August 1991 firn limit on Upper Fremont Glacier. Accumulated seasonal snow depth adjacent to the sample grid

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(measured in April 1990) ranged from 70 to more than more than 370 cm (Naftz 1993). Snow depth increases toward the cirque wall, probably from wind loading on the leeward slopes. Cumulative ablation (including snow, firn, and ice) measured for the 1990-91 budget year ranged from 68 to 119 cm on Upper Fremont Glacier.

Methodology

On-Site The criteria for selection of the grid area were a location that 1) eliminated anticip- ated disturbances to the surface by ice drilling and helicopter operations, 2) had minimal topographic relief, 3) was not subjected to differential solar effects (such as shading from the cirque walls), and 4) was above the firn line. The surface snow was sampled on Upper Fremont Glacier on August 18,1991, at 100 equally spaced points within the 8,100 m2 grid area (Fig. 2). Each sample consisted of the top 2-3 cm of snow and was composited from a 1 m2 area using a large plastic spoon. A total of approxilnately 500 g of sample was collected from each sampling point. All sample containers and equipment were pre-soaked and pre-rinsed in 17.8 megaohm water before sampling. Composited samples were placed in plastic bags and stored in snow vaults until transport off site by helicopter to a freezer truck.

Sample Processing and Analysis Snow samples were extruded from the plastic bags into covered plastic containers and allowed to melt at room temperature (20 "C). The liquid was filtered (0.45 micrometer) and split into polyethylene bottles and glass vials. The sample splits analyzed for calcium, magnesium, and sodium were preserved with ultra-pure nitric acid to a pH of less than 2.0. The sample splits analyzed for chloride, nitrate, and sulfate were preserved by refrigeration. The sample splits analyzed for del oxygen-18 values were placed in 6 ml glass vials with polyseal caps and then sealed with paraffin until analysis. Concentrations of calcium, magnesium, and sodium were determined by induc- tively coupled plasma optical emission spectrometry (ICPIOES), using a modified Jarrell-Ash Model 975 Plasma AtomComp* emission spectrometer (Garbarino and Taylor 1979; Taylor 1987) at the U.S. Geological Survey laboratory in Boulder, Colorado. %Concentrationsof chloride, nitrate, and sulfate were determined by ion- exchange chromatography (Fishman and Friedman 1989), using a Dionex Model QIC ion chromatograph at the U.S. Geological Survey laboratory in Boulder,

* Use of brand names in this article is for identification purposes only and does not consti- tute endorsement by the U.S. Geological Survey.

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Colorado. The del oxygen-18 isotopic values were determined using the method developed by Epstein and Mayeda (1953) at the U.S. Geological Survey Stable Isotope laboratory in Menlo Park, California.

Quality Assurance U.S. Geological Survey Standard Water Reference Samples (SWRS) P-7 and P-12 were analyzed with ice and snow samples from Upper Fremont Glacier to assess the quality of major-ion concentration data. Quality-assurance results of major-ion data are summarized in Table 1. A complete summary of the quality-assurance data can be found in Naftz (1993). Mean concentrations of major ions were within one standard deviation of the accepted SWRS values. Precision of the SWRS during the analyses exceeded the reported analytical precision of the SWRS for all constituents of interest. The two sigma uncertainty for del oxygen-18 values analyzed during this study was plus or minus (k) 0.2 per mil.

Table 1 - Quality-assurance results of Standard Water Reference Samples (SWRS) and analytical detection limits during the analyses of snow and ice-core samples from Upper Fremont Glacier [concentration in milligrams per liter; --, missing data] Chemical Standard wat- Number Detec- consti- er reference of sam- Standard Standard tion tuent sample number ples Mean deviation Mean deviation limit - - Study results Accepted results Calcium P-12 139 0.90 0.04 0.91 0.06 0.001 Magnesium P-12 139 0.05 0.00 0.06 0.01 0.0002 Sodium P-12 139 0.73 0.04 0.71 0.05 0.006 Chloride P-7 52 0.23 0.06 0.36 0.09 0.010 itr rate' ------0.010 Sulfate P-7 52 1.84 0.05 1.60 0.36 0.010 1 Because of the instability of nitrate, no SWRS was used. Instead, measurement error was calculated as 2 times the relative standard deviation from 11 pairs of duplicate ice-core samples.

Results and Discussion

Data Distributions Distributions of the snow-grid data were described using classical statistics. Fre- quency distribution plots for major ions and del oxygen-18 in the surface-snow layer are shown in Fig. 3. Although nitrate, magnesium, and sodium concentrations were strongly influenced by the frequency of observations at or near the analytical detection limit, the positive asymmetry of the frequency plots indicates log-normal distributions. Further analysis using probability plots shows a linear

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50 Chloride Nitrate

000 02 04 06 08

20 , Sulfate :kCalcium

llK000 02 04 06 08

Del Oxygen-18 -- I n I 15

10 Fig. 3. Frequency distribution of major-ion concentrations and del oxygen-18 5 values for the surface-snow layer in an 8,100 m2 area on Upper Fremont 0 -205 -185 -165 -I45 -125 Glacier. VALUE, IN PER MIL relation (Fig. 4), which also indicates the data are log-normally distributed. Chloride, nitrate, magnesium, sulfate, and del oxygen-18 show a log-normal distribution. Calcium, sodium, and del oxygen-18 all contain two log-normal subpopula- tions.

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MIL Fig. 4. Probability plots of major-ion concentrations and del oxygen-18 values for the surface snow layer in an 8,100 m2 area on Upper Fremont Glacier. The expected value is the z-score that a data point would have if it were from a normal distribution. For example, if 100 data points are ranked from smallest to largest, the 5oth point would have an expected value of zero, the 6gth would have an expected value of one, and the 95th point would have an expected value of two.

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Table 2 - Variability of selected chemical constituents in the surface-snow layer in an 8,100 m2 area on Upper Fremont Glacier [mgll, milligrams per liter; <, less than] Chemical 20th 80th Geo- constituent percen- percen- metric and units Minimum tile Median tile Maximum Range mean Calcium (mgll) 0.0682 0.0936 0.1184 0.1721 0.4062 0.3380 0.1276 Magnesium(mgl1) 0.0066 0.0156 0.0302 0.0614 0.1904 0.1838 0.0298 Sodium (mgll) 0.0134 0.0220 0.0304 0.0597 0.5190 0.5056 0.0397 Chloride (mgll) 0.022 0.036 0.050 0.079 0.214 0.192 0.053 Sulfate (mgll) 0.040 0.060 0.086 0.106 0.165 0.125 0.083 Nitrate (mgll) <0.010 0.021 0.060 0.104 0.426 0.416 0.052' Oxygen-181 -20.26 -17.42 -15.92 -13.56 -12.81 7.45 -14.82 oxygen-16 (per mil) 1 Censored data; therefore, geometric mean calculated according to method outlined in Klusman et al. (1980).

Spatial Variability of Chemical Constituents in Surface Snow Because of the log-normal distributions of all chemical constituents, geometric means were calculated along with maximum, minimum, range, median, and 20th and 80th percentile values (Table 2). For the major ions, the largest concentration range within the snow grid was sodium (0.0134 to 0.5190 mgll) and the smallest concentration range was sulfate (0.040 to 0.165 mgll). Although this variation appears small on an absolute scale, the concentration range of sodium exceeds an order of magnitude. Del oxygen-18 values show a range of 7.45 per mil. Assuming that the surface-snow layer is uniform in chemical composition, the observed variability of each chemical constituent should be a function of only sampling and measurement error. Sampling error was assumed to be minimized because of composite sampling techniques. Measurement error (0.05 confidence level) was quantified for each of the chemical constituents using a variety of methods discussed in Table 3. With these assumptions, only 5 out of the 100 samples are expected to fall outside of the interval defined by the geometric mean value, + measurement error at the 0.05 confidence level. None of the chemical constituents fall within the limits estimated by measurement error, indicating a substantial amount of spatial variability within the surface-snow layer. For example, 84 of the 100 calcium concentrations and 94 of the 100 del oxygen-18 values fall outside the interval expected from measurement error (Table 3). The large lateral variability in major-ion concentrations and del oxygen-18 values observed on the Upper Fremont Glacier snow grid is similar to observations made by Metcalf and Peck (1990) on snow surfaces in Colorado, Utah, and Tibet.

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Table 3 - Calculated measurement error and number of samples that were outside the limits expected by long-term measurement error in an 8,100 m2 area on Upper Fremont Glacier Number of Chemical Measurement Lower limit Upper limit samples outside constituent error, in of measure- of measure- of measurement and units per cent ment error ment error error limits Calcium (mgll) 8.5 0.118 0.138 84 Magnesium (mgll) 19.3 0.024 0.036 85 Sodium (mgll) 11.0 0.036 0.043 93 Chloride (mgll) 5.4 0.050 0.056 93 Sulfate (mgll) 5.5 0.078 0.087 87 Nitrate (mgll) not calculated 0.044 0.060 89 Oxygen-181 not calculated -15.02 -14.62 94 oxygen-16 (per mil)

[Measurement error for calcium, magnesium, and sodium assumed to be plus or minus (f)2 times the relative standard deviation for standard water reference sample P-12 determined during the analyses of the Upper Fremont Glacier ice core; measurement error for chloride and sulfate assumed to be f 2 times the relative standard deviation for standard reference water sample P-7 determined during the analyses of the Upper Fremont Glacier ice core; measurement error for nitrate calculated to be 2 times the mean standard deviation from 11 pairs of duplicate ice-core samples with at least one measurement above the lower detection limit of 0.01 (f 0.008); measurement error for 0-1810-16 assumed to be f 0.2 per mil (C. Kendall, U.S. Geological Survey, written commun. 1992)l

Fig. 5. Three-dimensional plots of spatial variability of major-ion concentrations and del oxygen-18 values for the surface-snow layer in an 8,100 m2 area on Upper Fremont Glacier.

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.- Fig. 5. cont.

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Table 4 - Per cent of variance at different geographic scales in the snow grid on Upper Fremont Glacier [m2, square meter] Per cent variance Per cent variance Per cent variance Chemical between between within constituent 2,025 m2 sections 506 m2 sections 506 m2 section Calcium 26 Magnesium 2 Sodium 14 Chloride 1 Sulfate 10 Nitrate 8 Del oxygen-18 26

The geographic scale of the observed variance for each chemical constituent sampled in the snow grid was assessed using three-dimensional plots of concentrationlvalue variations (Fig. 5) and nested analysis of variance (ANOVA). Nested ANOVA has been used by numerous researchers to cost effectively analyze the components of variance during soil, plant, and groundwater geochemical sampling programs (Klusman et al. 1980; Klusman 1985; and Naftz and Barclay 1992). The nested ANOVA model applied to the major-ion concentrations and del oxygen-18 values assessed the variance of each constituent at three different geographic scales (Table 4) in the grid area. The variance for each constituent at each geographic scale is expressed as a percentage of the total variance for each chemical constituent. Large percentages of variance for groups of concentrations/values at the smaller geographic scales in the snow grid (within 506 m2 plus random error) are indicative of variability at small scales and the concentration/value at one site would probably represent small scale, rather than regional, conditions. Converse- ly, chemical constituents displaying large percentages of variance for groups of concentrations/values at the highest geographic scale (between 2,025 m2 cells) would be more indicative of regional trends on the glacier. In general, the nested ANOVA results show that most of the variance for every constituent is in the concentrationslvalues grouped at the two smaller geographical scales, indicating the importance of small-scale variability within the snow grid. Relative to all the constituents, calcium and sodium concentrations and del oxygen- 18 have the largest variance at the largest geographic scale. The nested ANOVA results are qualitatively confirmed by the three-dimensional plots (Fig. 5). For example, the del oxygen-18 value map (26 per cent of total variance at the largest geographic scale) is relatively smooth with a gradually increasing (less negative) trend in values from the east to the west grid corners. In contrast, the chloride concentration map (1 per cent of total variance at the largest geographic scale) has no observable trend over the grid surface, with smaller scale increases and de- creases in concentration.

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Implications of Snow-Grid Results to Ice-Core Interpretation Attempts to interpret the significance in observed variability of chemical constituents with depth in ice cores from Upper Fremont Glacier must consider the large amounts of lateral variability measured in the snow grid. A method is presented to utilize the surface-variability data to determine the significance of chemical variation with depth in an ice core collected adjacent to the snow grid. Nitrate and del oxygen-18 ice-core profiles from the Upper Fremont Glacier coring site were chosen for illustration in Fig. 6. Results for the other major ions and specific conductance are reported by Naftz (1993). Although the results presented are specific to the Upper Fremont Glacier study site, this method could be applied to variance assessment studies at other ice-coring sites throughout the world. As a result of meltwater-elution processes at the study site (Naftz 1993), nitrate concentrations decrease substantially with depth in the upper 20 m of the core and sections of the del oxygen-18 value profile are smoothed considerably (Fig. 6). Because of the effects of meltwater, the nitrate-concentration and the del oxygen- 18-value ranges between selected percentiles of the snow-grid data were utilized to evaluate the significance of chemical variation with depth. Significant positive peaks in the ice-core profile were calculated as follows

where SIG.POS - upper concentration/value expected from lateral variability; 1CE.MEDIAN - median concentration/value in ice-core data; GRID .80 - 80th percentile concentration/value of the snow-grid data; and GRIDS0 - 50th percentile concentration/value of the snow-grid data. Use of the 80th instead of 95th percentiles were justified because of the lesser amounts of variability observed in the ice core, presumably due to the long-term smoothing effects of percolating meltwaters. Concentrations/values larger than SIG.POS are outside the range of concentrations/values expected from lateral variability and considered significant. Because many of the major-ion concentrations were near or below the analytical detection limits, significant negative peaks were calculated for only del oxygen-18 values. Significant negative peaks in the del oxygen-18 profile were calculated as follows

where

SIG.NEG - lower concentration/value expected from lateral variability; 1CE.MEDIAN - median concentration/value in ice-core data; GRID.20 - 20th percentile concentration/value of the snow-grid; and GRIDS0 - 50th percentile concentration/value of the snow-grid data.

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AND BEDROCK NITRATE CONCEN'IRATION. DEL OXYGEN-18, IN PER MIL HORIZONS M MlLLlGRAMS PER LITER Fig. 6. Estimated age horizons, and nitrate and del oxygen-18 concentration/value profiles in the Upper Fremont Glacier ice core. The smallest and largest concentration/value expected from lateral variability of the surface-snow layer and the median concentrationlvalue of the ice core are superimposed on the profile.

The 20th percentile was utilized instead of the 5th percentile because of the de- creased variance in the ice core expected from long-term exposure to meltwater elution processes. Concentrations/values less than SIG.NEG are outside the range of concentrations/values expected from lateral variability and considered significant. To assess the significance of lateral variability, the largest concentration/value expected from lateral variability (calculated according to Eqs. (1) and (2)) and the median concentration/value are superimposed on the ice-core profiles. Major-ion concentrations (including nitrate) in the upper 10 m of the ice core are up to an order of magnitude larger than concentrations in the surface-snow layer. The concentrations of major ions in the surface-snow layer may be underestimated because sampling of the surface-snow layer occurred in August, representing one point in time. Meltwater movement prior to the August sampling may have caused major ions to elute from the surface of the snow. This elution process has been documented by Naftz et al. (1993) and Naftz (1993) in snow-pit data from glaciers

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in the Wind River Range. In contrast, the ice-core record represents a multi-year record subject to potentially more temporal variability of major-ion concentrations. The nitrate concentration profile shows some trends in the deeper sections of the core that appear to be remnants of at least one very large perturbation in the nitrate concentration of the snow (Fig. 6). The nitrate concentration peaks (89.9 to 94.4 m and 148.9 to 156.5 m) exceed both the largest blank concentration and the largest concentration expected from lateral variability. The close proximity of bedrock to the nitrate peak from 148.9 to 156.5 m could suggest a postdepositional origin; however, the process that would cause the peak is unclear with the available data. Maximum concentrations observed in the peaks are 0.07 mgll, a concentration level 10 times the background concentration level of 0.007 mgll(0.7 multiplied by the lower detection limit of 0.01 mgll) (Fig. 6). The del oxygen-18 value profile shows some trends that could be related to climate change (Fig. 6). Atmospheric transport of water vapor is subject to isotopic fractionation processes, which include equilibrium and kinetic effects. In general, both of these processes lead to a general decrease in the del oxygen-18 values with decreasing air temperature (Lorius et al. 1989). Therefore, isotopic values falling outside of the lower limit of lateral variability might represent cooler temperatures, and values falling outside of the upper limit of lateral variability can be interpreted as reflecting warmer temperatures. Most of the del oxygen-18 values fall within the range expected from lateral variability; however, the values in the section from 110-150 m below the surface consistently vary outside the expected limits, possibly representing a climatic shift. Median value for this section of core shows a negative 1.18 per mil shift from the entire core median value. Furthermore, values for numerous samples in this section exceed the lower and upper limit expected from lateral variability (Fig. 6). The numerous high amplitude oscillations of del oxygen-18 values in this interval are probably a result of cooler temperatures, resulting in better preservation of the annual variation in isotopic values. The calculated age of this section of core corresponds to the time interval of 1810 to 1725 A.D., which is also within the time interval of the "Little Ice Age" from 1400-1900 A.D. (Dansgaard and Oeschger 1989).

Summary One hundred samples were collected from the surface of the Upper Fremont Glacier at equally spaced intervals defined by a snow grid (area = 8,100 m2) to assess the significance of lateral variability in major-ion concentrations and del oxygen-18 values. For the major ions, the largest concentration range within the snow grid was sodium (0.5056 mgll) and the smallest concentration range was

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sulfate (0.125 mgll). Del oxygen-18 values showed a range of 7.45 per mil. Com- parison of the observed variability of each chemical constituent to the variability expected by measurement error indicated substantial lateral variability within the surface-snow layer. The variance data from the snow grid were used to develop equations to evaluate the significance of both positive and negative concentration1 value peaks of nitrate and del oxygen-18 with depth, in a 160 m ice core. Values of del oxygen-18 in the ice-core section from 110-150 m below the surface consistently vary outside the expected limits and possibly represents cooler temperatures during the Little Ice Age from about 1810 to 1725 A.D.

Acknowledgements Funding for this study was provided by the U.S. Geological SurveylWater Resour- ces Division, Shoshone and Arapaho Indian Tribes, Wyoming Water Development Commission, and Wyoming State Engineer. Assistance provided by John Gar- barino and Howard Taylor of the U.S. Geological Survey during the ICPIOES lab work is gratefully acknowledged. Technical reviews by N. Caine (Institute of Arc- tic and Alpine Research) and D. Campbell (U.S. Geological Survey) significantly improved the manuscript.

References

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