Snow chemical composition

The snow-pit chemical profiles are plotted vs depth in Figure 3. The Eclipse snow-pit data are summarized on a micro-equivalent per liter (μEq L⁻¹) basis in the box plots shown in Figure 4, with summary statistics provided in Table 1. Calcium is the predominant cation in our samples, followed by Mg²⁺, Na⁺, NH₄ ⁺ and K⁺, the last of which is present at very low concentrations. Sulfate is the predominant anion, followed by NO₃ ⁻ and Cl⁻. On a per sample basis, Ca²⁺ concentrations are the highest of the cations in 82% of the samples, followed by Na⁺ (15%) and NH₄ ⁺ (3%). Sulfate is the most concentrated anion in 63% of our samples, followed by NO₃ ⁻ (21%) and Cl⁻ (16%).

Table 1.

Summary statistics of snow-pit chemical composition

Fig. 3.

Snow-pit profiles of δ¹⁸O, Na⁺, NH₄ ⁺ , K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃ ⁻ and SO₄ ^2–.

Fig. 4.

Box plot summary of snow-pit chemical composition.

The completeness of major-ion analysis can be checked by the charge balance (ratio of major cation equivalents to major anion equivalents). A value not significantly different from 1 indicates an excellent balance between measured cations and anions and suggests all major ions present have been accounted for in the analyses. The median ratio of all samples is 1.17, indicating a cation excess that is present in 63% of the samples. In particular, high dust inputs as evidenced by increased concentrations of Ca²⁺ and Mg²⁺ are associated with a charge ratio much greater than 1. Dust inputs probably include carbonate ions (HCO³ _– and CO₃ ^2–) in concentrations large enough to balance the observed cation excesses (Reference Toom-Sauntry and BarrieToom-Sauntry and Barrie, 2002). The remaining 37% of the samples have an excess of anions that is presumably balanced by H⁺ , which is generally the most abundant positive ion in water samples.

Correlation coefficients can be used to investigate interspecies relationships in our dataset. All correlations quoted are significant at p = 0.01 for n = 320. In our snow-pit samples, the sea-salt indicators Na⁺ and Cl⁺ are highly correlated (r = 0.93) while the dust indicators Ca²⁺ and Mg²⁺ are well correlated (r = 0.77). K⁺ correlates best with NH₄ ⁺ (r = 0.52), implying a biomass-burning source, but is also correlated with Ca²⁺ (r = 0.41) and Mg²⁺ (r = 0.30), suggesting a contribution from crustal dust. Sulfate correlates best with NH₄ ⁺ (r =\ 0.73), suggesting these species are deposited in the same particles as ammonium sulfate. The stoichiometry suggests the form NH₄HSO₄ rather than (NH₄)₂SO₄ because the molar ratio of NH₄ ⁺ to SO₄ ^2– averages 0.95, very close to the 1 : 1 stoichiometry of NH₄HSO₄. Both SO₄ ^2– (r = 0.67) and NO₃ ⁻ (r = 0.66) correlate well with Ca₂ ⁺, implying uptake of acidic species on dust particles. Alternatively, the correlation of SO₄ ^2– with Ca²⁺ could suggest deposition of dust as gypsum (CaSO₄), but this does not explain the correlation of NO₃ ⁻ with Ca₂₊, nor are gypsum deposits, common only in internally draining basins, found in the North Pacific region in appreciable quantities.

Sea-salt calculations

Calculations of sea-salt and non-sea-salt fractions on a per sample basis indicate that nearly all of the Ca²⁺ (99%) and most of the K⁺ (86%) and Mg²⁺ (83%) at Eclipse are from non-sea-salt sources, presumably crustal dust but also including biomass burning in the case of K⁺. Sulfate is also mostly (97%) from non-sea-salt sources, which include oxidation of sulfur dioxide from anthropogenic sources and volcanic emissions, oxidation of dimethylsulfide produced by phytoplankton, and crustal dust and evaporate deposits. Although we assumed all of the Na⁺ present is from sea salt, some of the Na⁺ may be crustal in origin, particularly in dust-laden samples. Hence, the sea-salt and non-sea-salt fractions we calculated represent maximum and minimum estimates, respectively.

The Cl⁻:Na⁺ micro-equivalent ratio in snow at Eclipse averages 1.26, a 9% enrichment in Cl⁻ relative to the seawater ratio of 1.16 (Reference Keene, Pszenny, Galloway and HawleyKeene and others, 1986). The Cl⁻ enrichment at Eclipse is considerably less than that observed at South Pole (40%) or Summit, Greenland (23%) (Reference Whitlow, Mayewski and DibbWhitlow and others, 1992). Excess chloride in polar snow may be derived from the reaction of sea-salt particles with acidic aerosols leading to release of gaseous HCl (Reference HerronHerron, 1982). Gas-phase HCl is highly soluble and readily scavenged by snowfall. Hence, chloride enrichment in snow may be interpreted as an indicator of long-range transport of marine aerosol, allowing greater volatilization of sea-salt chloride (Reference Legrand and DelmasLegrand and Delmas, 1988). Therefore, the proximity of Eclipse to sea-salt sources relative to Summit or South Pole may allow for less sea-salt acidification and volatilization of HCl, resulting in less chloride enrichment at Eclipse and a Cl⁻:Na⁺ ratio closer to that of sea water.

Calculations of sea-salt and non-sea-salt fractions do not change appreciably if an iterative approach is used to find the limiting sea-salt species instead of assuming that all of the Na⁺ is from sea salt. The limiting sea-salt species is that species present in the lowest concentration relative to the sea-water ratio. Therefore the limiting species provides an estimate of the minimum sea-salt contribution, assuming no fractionation during production, transport and deposition of sea-salt aerosol (Reference Keene, Pszenny, Galloway and HawleyKeene and others, 1986). Sodium is found to be the limiting species in 99% of the samples. The limiting species in the remaining 1% of the samples is K⁺, Mg²⁺ or Ca²⁺, while SO₄ ^2– is not found to be limiting in any of the samples. Again, nearly all of the Ca²⁺ (99%), K⁺ (88%), Mg²⁺ (84%) and SO₄ ^2– (95%) are found to be in excess of seawater ratios, while nearly all of the Na⁺ (92%) and Cl⁻ (85%) are present in sea-salt ratios. In contrast, up to 100% of the Mg²⁺ in Antarctica can be from sea salt (Reference Whitlow, Mayewski and DibbWhitlow and others, 1992). In such situations, determining the limiting sea-salt species on a per sample basis is necessary to determine whether Na⁺ or Mg²⁺ provides the better sea-salt indicator.

Dependence on accumulation rate

We used linear regression to investigate the relationship between the annually averaged concentration (μEq L⁻¹) of major ions and snow accumulation rate (mw.e. a⁻¹) at Eclipse using the 100 year record provided by the Eclipse 1996 ice core. Results were similar using the Eclipse 2002 ice-core data. For all species, correlation coefficients are <0.15, indicating that snow accumulation rate is not an important control on the concentration of major ions at Eclipse (Fig. 5). Reference YangYang and others (1995) reached the same conclusion in their study of the GISP2 and 20D (Greenland) and Mount Logan ice-core datasets.

Fig. 5.

Annual mean chemical concentration (left) and flux (right) vs snow accumulation for the Eclipse 1996 ice core. The solid line is the least-squares fitted regression line for each species, with correlation coefficients (r) as indicated. For chemical concentration, none of these correlations are statistically significant at the 95% confidence level (n = 100). For chemical flux, statistical significance levels (p) as indicated (n = 100).

We also used linear regression to evaluate the dependence of chemical flux (μg cm⁻² a⁻¹) on snow accumulation rate (Fig. 5). Positive relationships between chemical flux and snow accumulation are evident, with correlation coefficients ranging from 0.27 for K⁺ to as high as 0.51 for NO₃ ⁻. This indicates 7–26% of the variance in the flux data can be explained by a linear relationship with snow accumulation rate. Our results agree with those of Reference YangYang and others (1995) who report that the strongest correlation exists between nitrate flux and snow accumulation rate. Furthermore, our best-fit linear regression between nitrate flux and snow accumulation rate (F = 0.035 A – 0.35, where F is nitrate flux in μg cm⁻² a⁻¹ and A is the snow accumulation rate in m w.e. a⁻¹) compares favorably to the relationship derived by Reference YangYang and others (1995) for Mount Logan (F = 0.054A – 0.19). We also find that 12–25% of the variance in the NH₄ ⁺ , Mg²⁺, Ca²⁺, Cl⁻ and SO₄ ^2– flux data is explained by a linear relationship with snow accumulation. Although no more than 20% of the flux variance of these species at GISP2 was explained by a linear relationship with snow accumulation rate, 36% of the variance in the Mount Logan sulfate flux data was explained (Reference YangYang and others, 1995). This may indicate a greater importance of wet deposition at high-accumulation-rate sites such as Eclipse, relative to the sites in the interior of ice sheets such as Greenland Ice Sheet Project 2 (GISP2) where dry and especially fog deposition can be an important contributor to chemical species flux (Reference BerginBergin and others, 1995).

Seasonal variations

Comparison of individual species concentration profiles to the δ¹⁸O profile and to each other reveals distinct seasonal variations in all of the glaciochemical parameters measured, both in the snow-pit chemical profiles (Fig. 3) and in ice cores from Eclipse Icefield (Fig. 6). The most regular seasonal variations at Eclipse are seen in the δ¹⁸O record, which exhibits the classic signal of isotopically heavier (more negative) precipitation in winter and isotopically lighter (less negative) precipitation in summer (e.g. Reference Dansgaard, Johnsen, Clausen and GundestrupDansgaard and others, 1973). Typical δ¹⁸O values for summer snowfall at Eclipse are –24%, with winter snowfall typically in the –30% range. Summer–winter variations are also evident in the major-ion records and can be divided between sea-salt species (Na⁺, Cl⁻) that peak in late fall to winter and species associated with dust (Ca²⁺, Mg²⁺) and other species (NH₄ ⁺, NO₃ ⁻, SO₄ ^2–) that peak in late spring to summer.

Fig. 6.

The top 50 m of Eclipsecore3,coveringtheperiod1963–2002. Note the strong seasonality of the signal not only in the δ¹⁸O profile, but also in the major-ion records. Because seasonality in Cl⁻ deposition is highly correlated with Na⁺ (r = 0.83) and Mg²⁺ is highly correlated with Ca²⁺ (r = 0.87), neither Cl⁻ nor Mg²⁺ is shown here.

Seasonal input timing of major-ion species at Eclipse is summarized in Figure 7 with respect to an idealized oxygen isotope curve. However, in any given year the relative timing of concentration peaks can differ markedly from the pattern described, especially for species influenced by episodic events, such as volcanic eruptions (SO₄ ^2–) or forest fires (NH₄ ⁺). Concentrations of Na⁺ and Cl⁻, derived mostly from sea salt, peak in late fall or winter either coincident with the δ¹⁸O minima or, less commonly, on the descending leg. This is presumably due to greater storminess in the nearby Gulf of Alaska during fall and winter, with higher wind speeds generating more wave action and enhanced entrainment of sea-salt aerosols, as well as more frequent advection of maritime air masses over Eclipse associated with greater inland penetration of stronger synoptic frontal systems (Reference Taylor-Barge, Bushnell and RagleTaylor-Barge, 1969). Sea-salt concentrations are at their annual minima during summer.

Fig. 7.

Summary of chemical-species input timing at Eclipse with respect to an idealized oxygen isotope curve (after Reference Whitlow, Mayewski and DibbWhitlow and others, 1992). Solid lines indicate times when maximum concentrations frequently occur, while dashed extensions indicate periods when peaks are sometimes noted.

Other species peak at Eclipse during spring or summer, with generally lower concentrations during winter. First to peak are those species indicative of dust, such as Ca²⁺ and Mg²⁺ which most often peak on the rising leg of the δ¹⁸O curve, although their concentration maxima can extend throughout the summer. Slightly lagging the spring peak in crustal dust are the NH₄ ⁺ maxima, which also tend to peak on the rising leg of the δ¹⁸O curve. However, episodic biomass-burning events can overwhelm the seasonal NH₄ ⁺ cycle and produce concentration maxima any time between spring and fall. Peak oxalate (C₂O₄ ² _–) concentrations tend to coincide with the spring-to-early-summer NH₄ ⁺ peak. The low K⁺ concentrations make it difficult to infer any seasonality in K⁺ deposition at Eclipse. However, K⁺ concentration peaks often coincide with either Ca²⁺ or Mg²⁺ peaks, suggesting a K⁺ source from crustal dust, or with NH₄ ⁺ peaks, suggesting a biomass-burning source.

Sulfate and nitrate concentrations typically peak with the δ¹⁸O maxima, and hence slightly lag dust concentration peaks, although there is considerable overlap. Elevated SO₄ ^2– and NO₃ ⁻ concentrations associated with dust inputs could suggest uptake of sulfate and nitrate by dust particles during atmospheric transport from continental source areas (as demonstrated by Reference Jordan, Dibb, Anderson and FuelbergJordan and others, 2003). The summer NO₃ ⁻ peak could be due to NO_x released by thermal decomposition of peroxyacetyl nitrate (PAN) in the presence of higher OH⁻ concentrations in summer (Reference YangYang and others, 1995). Alternatively, the summer NO₃ ⁻ peak could indicate a summer peak in NO₃ ⁻ production by biological activity, biomass burning or lightning (Reference Legrand and KirchnerLegrand and Kirchner, 1990). Reference Holdsworth and PeakeHoldsworth and Peake (1985) reported similar seasonal variations in nitrate in their analysis of snow pits and shallow firn cores from Eclipse. However, extensive post-depositional alteration of NO₃ ⁻ in snowpacks by photochemical recycling, and incomplete understanding of the underlying physical mechanisms responsible, complicates interpretation of glaciochemical NO₃ ⁻ records (Reference Dibb, Arsenault, Peterson and HonrathDibb and others, 2002).

Although the observed seasonality at Eclipse is similar to that at Summit, Greenland (Reference Whitlow, Mayewski and DibbWhitlow and others, 1992), distinct differences exist. Sodium concentrations at Summit tend to lag the δ¹⁸O minima and hence frequently overlap the spring dust concentration peak. This is thought to reflect under-representation of winter snow accumulation in the snowpack at Summit, allowing winter and spring concentration peaks separated in time by 3 months or more to overlap in the snowpack (Reference Dibb, Wolff and BalesDibb, 1996). In contrast, the even distribution of snow accumulation throughout the year at Eclipse (with the exception of pit 3) preserves distinct sea-salt maxima in winter and dust maxima in spring. Ammonium concentration peaks at Summit tend to occur in summer, coincident with the δ¹⁸O maxima, while at Eclipse they most often occur on the rising (spring) leg of the δ¹⁸O curve. SO₄ ^2– concentrations tend to peak later at Eclipse in the vicinity of the δ¹⁸O maxima, while peak SO₄ ^2– concentrations at Summit are a spring phenomenon. Relative input timings for dust species (spring) and NO₃ ⁻ (summer) are the same at Eclipse and Summit.

Spatial variations

Box plots for each glaciochemical parameter measured in each individual snow pit provide a first-order perspective of the spatial variability at Eclipse (Fig. 4). Median values for oxygen isotopes are relatively constant for the four snow pits, although pit 2 displays a much greater range in oxygen isotope values. Oxygen isotope values at pit 3 are skewed towards less negative values, reflecting the relative lack of winter accumulation at that site. Not surprisingly, Na⁺ and Cl⁻ concentrations are lower at pit 3, since the time of maximum sea-salt concentrations (winter) is under-represented at that site. Median values for dust species (Ca²⁺, Mg²⁺) are lower for pits 3 and 5, while NH₄ ⁺ , NO₃ ⁻ and SO₄ ^2– have similar median values for all snow pits. For all species, the within-group variance, as indicated by the size of the box representing the variance of the samples about the median, is greater than the between-group variance as represented by the difference in median values between snow pits. This suggests that temporal or seasonal variability in snow chemistry is greater than the spatial variability.

We used empirical orthogonal function (EOF) analyses to further investigate the spatial variability between snow pits for each of the parameters measured (Table 2). The data from all four snow pits were pooled and a separate EOF decomposition was run for each of the species measured. Oxalate was excluded from this analysis because 56% of the snow-pit samples were below detection limits for this species. The EOF technique splits the temporal variance of a dataset into patterns termed empirical eigenvectors, which are orthogonal in nature (Reference Peixoto and OortPeixoto and Oort, 1992). The first eigenvector explains the greatest percentage of variance in the dataset, with each successive eigenvector describing the maximum remaining variance. In this case, the first eigenvector describes the variance common to all four snow pits, since each snow pit has a positive correlation with the first EOF. The variance or signal common to all four snow pits identified by EOF 1 ranges from 49% of the total variance for Na⁺ and Cl⁻ to as high as 80% of the total variance for SO₄ ^2–.

Table 2.

Snow-pit per cent variance explained (r ²) by EOF 1. All loadings positive

Our results indicate that chemical species found in the accumulation mode (NH₄ ⁺, SO₄ ^2–) or gas phase (NO₃ ⁻) show less glaciochemical variability in snow pits at Eclipse than those species associated with coarse-mode dust (Ca²⁺, Mg²⁺) or sea-salt (Na⁺, Cl⁻) particles (Table 2). These results are consistent with studies of spatial variability in Greenland snow chemistry (e.g. Reference Dibb and JaffrezoDibb and Jaffrezo, 1997). Sulfate and ammonium, presumably present primarily in the accumulation mode as NH₄HSO₄ aerosol (Reference Seinfeld and PandisSeinfeld and Pandis, 1998), show the least variability in our snow pits, as evidenced by high EOF 1 loadings, with 55–83% of the signal common between snow pits. Nitrate, found mostly in the gas phase, has more variability at Eclipse than accumulation mode species (50–76% of the signal common between snow pits), although less than species associated with coarse-mode dust or sea-salt particles. Uptake of nitrate by dust particles could result in an association of part of the snow nitrate with dust particles, increasing the spatial variability in nitrate concentrations. Species associated with sea salt (Na⁺ and Cl⁻) and dust (Ca²⁺, K⁺, Mg²⁺) particles display greater variability in our snow pits, as evidenced by lower EOF 1 loadings. The results obtained from the EOF analysis agree well with the box plots of Figure 4, which showed a greater range in median values for particle-associated species (Na⁺, Cl⁻, Ca²⁺, Mg²⁺) than for species with gas-phase precursors (NH₄ ⁺, NO₃ ⁻, SO₄ ^2–).