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Seasonal variations of snow chemistry at NEEM, Greenland

Published online by Cambridge University Press:  14 September 2017

Takayuki Kuramoto
Affiliation:
National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan E-mail: kuramoto.takayuki@nipr.ac.jp
Kumiko Goto-Azuma
Affiliation:
National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan E-mail: kuramoto.takayuki@nipr.ac.jp
Motohiro Hirabayashi
Affiliation:
National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan E-mail: kuramoto.takayuki@nipr.ac.jp
Takayuki Miyake
Affiliation:
National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan E-mail: kuramoto.takayuki@nipr.ac.jp
Hideaki Motoyama
Affiliation:
National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan E-mail: kuramoto.takayuki@nipr.ac.jp
Dorthe Dahl-Jensen
Affiliation:
Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
Jørgen Peder Steffensen
Affiliation:
Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
Corresponding
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Abstarct

We conducted a pit study in July 2009 at the NEEM (North Greenland Eemian Ice Drilling) deep ice-coring site in northwest Greenland. To examine the seasonal variations of snow chemistry and characteristics of the drill site, we collected snow/firn samples from the wall of a 2 m deep pit at intervals of 0.03 m and analyzed them for electric conductivity, pH, Cl, NO3 , SO4 2–, CH3SO3 (MSA), Na+, K+, Mg2+, Ca2+ and stable isotopes of water (δ18O and δD). Pronounced seasonal variations in the stable isotopes of water were observed, which indicated that the snow had accumulated regularly during the past 4 years. Concentrations of Na+, Cl and Mg2+, which largely originate from sea salt, peaked in winter to early spring, while Ca2+, which mainly originates from mineral dust, peaked in late winter to spring, slightly later than Na+, Cl and Mg2+. Concentrations of NO3 showed double peaks, one in summer and the other in winter to spring, whereas those of SO4 2– peaked in winter to spring. The winter-to-spring concentrations of NO3 and SO4 2– seem to have been strongly influenced by anthropogenic inputs. Concentrations of MSA showed double peaks, one in spring and the other in late summer to autumn. Our study confirms that the NEEM deep ice core can be absolutely dated to a certain depth by counting annual layers, using the seasonal variations of stable isotopes of water and those of ions. We calculated the annual surface mass balance for the years 2006–08. The mean annual balance was 176 mm w.e., and the balances for winter-to-summer and summer-to-winter halves of the year were 98 and 78 mm, respectively. Snow deposition during the winter-to-summer half of the year was greater than that during the summer-to-winter half by 10–20mm for all three years covered by this study.

Type
Research Article
Copyright
Copyright © the Author(s) [year] 2011

Introduction

Deep ice cores from Greenland and Antarctica have provided us with excellent records of the late Quaternary climate and environment (e.g. Reference DansgaardDansgaard and others, 1982; GRIP Members, 1993; Reference Grootes, Stuiver, White, Johnsen and JouzelGrootes and others, 1993; EPICA Community Members, 2004, 2006; NorthGRIP Members, 2004). Although ice-core records from Antarctica cover the past eight glacial cycles (EPICA Community Members, 2004), those from Greenland only date back to the middle of the Last Interglacial, which is known as the Eemian period (NorthGRIP Members, 2004). Since the Eemian (130–115 ka BP) was substantially warmer than today (NorthGRIP Members, 2004), having the full record of the Eemian would enable us to study in detail climatic and environmental changes in a warmer climate, which are crucial for predicting future global changes.

To reconstruct the climate and environment during the Eemian, the NEEM (North Greenland Eemian Ice Drilling) project has been initiated. The new drill site NEEM was selected at 77˚26'55" N, 51˚03'20"W (2445ma.s.l.) in northwest Greenland (Fig. 1). Based on radio-echo sounding and ice-flow modeling, Reference Buchardt and Dahl-JensenBuchardt and Dahl-Jensen (2008) have shown that a full record of the Eemian can be obtained at this site ~100m above bedrock.

Fig. 1. Location map of NEEM, Greenland.

The chemical components in ice cores provide valuable information on past atmospheric conditions and changes in aerosol sources (e.g. Reference LegrandLegrand and Mayewski, 1997; Reference PetitPetit and others, 1999). In Greenland, many of the chemical components and stable isotopes of water show clear seasonal variations (e.g. Reference Finkel, Langway and ClausenFinkel and others, 1986; Reference SteffensenSteffensen, 1988; Reference BeerBeer and others, 1991; Reference Whitlow, Mayewski and DibbWhitlow and others, 1992; Reference Candelone, Jaffrezo, Hong, Davidson and BoutronCandelone and others, 1996). Patterns of these seasonal variations are useful tools for investigating the sources of aerosols, atmospheric circulation and deposition processes onto the ice sheet. Seasonal variations back to 60 ka BP have been used for ice-core dating in Greenland, as the annual accumulation rate is sufficiently high to allow for sub-annual sampling (Reference SvenssonSvensson and others, 2008).

Under the ongoing NEEM project, the ice core will be absolutely dated further back in the Eemian using the annual-layer counting method. To fully interpret the chemistry data obtained from the NEEM deep ice core and for accurate absolute dating, it is necessary to assess the seasonal variations of the chemical components in the present-day snow. It is also a prerequisite for understanding the characteristics of the NEEM site, including the surface mass balance and melt effects (if any). In this paper, we present the results of a pit study conducted at NEEM in July 2009 to address these issues.

Methods

We conducted a snow/firn pit study on 5 July 2009 at a site located 2600 m east (77˚26'49" N, 50˚56'47" W) of the NEEM deep drilling site (Fig. 1). As the prevailing wind direction in this area is south, the study site was selected to be least influenced by the NEEM camp. A ~2m deep pit was dug from the surface. Snow densities and temperatures were measured every 0.03 m. A stainless-steel snow sampler with a height of 0.03m was used for the density measurements. After these measurements, samples for chemical analyses were collected at 0.03m intervals with a pre-cleaned stainless-steel sampler and were then placed into individual Whirl-Pak® sample bags (Nasco). The samples were melted in the bags and transferred to pre-cleaned polypropylene bottles in the NEEM camp. The samples were then refrozen and transported to the National Institute of Polar Research (NIPR) in Japan.

At NIPR, the samples were melted again, then pH, electric conductivity and ions (Cl, NO3 , SO4 2–, CH3SO3 (MSA hereafter), Na+, K+, Mg2+ and Ca2+) were analyzed in a class 10 000 clean room with a pH meter (TOADKK: MM-60R with a pH sensor GST-5720C), a conductivity meter (TOADKK: MM-60R with an order-made conductivity sensor CT-87101B(S)) and ion chromatographs (Dionex: DX-500), respectively. Dionex AS11-HC and CS14 columns were used for anion and cation analyses, respectively. The analytical precision was better than 2% at 1 ppb level for all the ions. Stable isotopes of water (δ18O, δD) were analyzed with an isotope mass spectrometer (Thermo: Delta plus) by an equilibrium method in a laboratory next to the clean room. The precision (1σ) of determination was 0.05% for δ18O and 0.5% for δD (Reference Uemura, Yoshida, Kurita, Nakawo and WatanabeUemura and others, 2004).

Results and Discussion

Seasonal variations of stable isotopes, density and ion concentrations

Figure 2 shows the vertical profiles of the stable isotopes of water (δ18O, δD) and deuterium-excess (d-excess; d=δD– 8δ18O) obtained from the pit. The δ18O and δD varied synchronously and showed distinct seasonal variations, with the most recent peak occurring near the snow surface of 5 July 2009. Reference SteffensenSteffensen (1985) reported that the δ18O values of snow vary in parallel with the temperatures at Dye 3, Greenland. A similar seasonality in δ18O has also been reported at other Greenland sites (e.g. Reference Finkel, Langway and ClausenFinkel and others, 1986; Reference BeerBeer and others, 1991). Therefore, we can reasonably assume that the layers with δ18O and δD maxima and minima were summer and winter layers, respectively. We dated the pit using the seasonal variation of δ18O and δD. The 2m deep pit contained snow deposited over a 4 year period. The seasonal variation of d-excess, which depends on the sea surface temperature and evaporation processes of sea water in the vapor source region (e.g. Reference UemuraUemura, 2007), differed from that of δ18O and δD, as it showed maxima in autumn and minima from spring to early summer. The seasonal variability of d-excess observed at NEEM is similar to that reported at other Greenland ice-coring sites (Reference Johnsen, Dansgaard and WhiteJohnsen and others, 1989).

Fig. 2. Vertical profiles of stable isotopes of water (δ18O and δD) and d-excess. Summer and winter were defined from maximum and minimum values of δ18O and δD, respectively.

The snow density also showed seasonal variation, which is superimposed on the general trend of increasing density with depth (Fig. 3). Low-density layers were observed at depths of approximately 0.05, 0.50, 0.90, 1.40 and 2.00 m. A comparison with the stable-isotope profiles (Fig. 2) indicates that these were summer layers. The seasonality of snow density at NEEM was similar to that observed at Summit, Greenland (Reference Albert and ShultzAlbert and Shultz, 2002), which was caused by a wind-pack effect resulting in a higher density of snow in winter. Although the wind speed at NEEM appears to be greater in winter than in other seasons, which may account for the increased snow density, this speculation needs to be confirmed using the data from the automatic weather station that was installed at NEEM during the 2009 season. The snow temperature (Fig. 3) continued to decrease from the surface to the bottom of the pit (–23.7˚C at 2.01m depth). As the visual stratigraphy of the pit showed little horizontal variability, we assume that snow and ion depositions were not seriously disturbed by wind scouring.

Fig. 3. Vertical profiles of snow temperature and density.

We found thin ice layers with thickness of ~1mm or less in the pit, at depths of approximately 0.11, 0.18 and 0.23 m, which were formed by minor surface snowmelt during the summer months at NEEM. The occurrence of slight summer surface melting did not have a noticeable impact on the profiles of stable isotopes and ions, since both showed regular variations (Fig. 2 and 4).

Fig. 4. Vertical profiles of electric conductivity, pH, and ion concentrations. Black and red curves indicate total concentrations and non-sea-salt (nss) fractions, respectively.

The vertical profiles of pH, electric conductivity and concentrations of ions (Cl, N3 , S4 2–, MSA, Na+, K+, Mg2+ and Ca2+) are shown in Figure 4. Non-sea-salt (nss) components Oof Cl, SO4 2–, K+, Mg2+ and Ca2+ were calculated using sea-water ratios of these ions with respect to Na+, assuming that Na+ is solely of sea-salt origin, and are plotted in Figure 4. For instance, concentration of nssCl, [nssCl], was calculated with the following equation:

where (Cl/Na+)sea is the equivalent concentration ratio of Cl/Na+ in the sea water, which is 1.17. Similarly, concentrations of nssSO4 2–, nssK+, nssMg2+ and nssCa2+ were calculated using sea-water ratios 0.12, 0.021, 0.23 and 0.044, respectively. On average, non-sea-salt fractions of Cl, SO4 2–, K+, Mg2+ and Ca2+ were 26%, 96%, 66%, 47% and 92%, respectively.

Concentrations of Na+ and Cl showed very similar profiles, with pronounced seasonal variations. Both ions peaked in winter to early spring. Concentrations of Mg2+ also peaked in winter to early spring, while those of Ca2+ peaked in late winter to spring, slightly later than Na+, Cl and Mg2+. The concentrations of K+ showed a peak in winter to spring and possibly a secondary peak in summer. Concentrations of NO3 showed double peaks, one in summer and the other in winter to spring, whereas those of SO4 2– peaked in winter to spring. Concentrations of MSA seem to show double peaks, one in spring and the other in late summer to autumn. Seasonal variations of pH and electric conductivity were not very clear in the present-day snow at NEEM.

The winter-to-early-spring peak observed for Na+ and Cl is in agreement with previous studies carried out on the Greenland ice sheet (Reference SteffensenSteffensen, 1988; Reference Whitlow, Mayewski and DibbWhitlow and others, 1992; Reference Candelone, Jaffrezo, Hong, Davidson and BoutronCandelone and others, 1996; Reference Dibb, Whitlow and ArsenaultDibb and others, 2007). In order to examine seasonal changes of Na+ and Cl sources, we computed Cl/Na+ ratios (Fig. 5). The ratios showed clear seasonal variations, with peaks occurring in summer when Cl concentrations were higher than Na+ concentrations with respect to the sea-water ratio. The Cl/ Na+ peaks can thus be used as an indicator of summer layers at NEEM. The high Cl/Na+ ratios together with elevated nssCl concentrations (Fig. 4) in summer suggest that Cl ions in summer Greenland snow were supplied from sources other than sea-salt aerosols, such as gaseous HCl due to sea-salt dechlorination (e.g. Reference Legrand and DelmasLegrand and Delmas, 1988; Reference Legrand, Preunkert, Wagenbach and FischerLegrand and others, 2002). On the other hand, the Cl/Na+ ratios in winter to early spring were very close to that of sea water, suggesting that in winter to early spring, Na+ and Cl mainly originated from sea-salt aerosols.

Fig. 5. Vertical profiles of Cl/Na+ and MSA/nssSO4 2–. The dashed line represents the Cl/Na+ ratio in sea water (1.17).

The seasonal variation of Ca2+ concentrations was similar to that observed by Reference Dibb, Whitlow and ArsenaultDibb and others (2007), who collected and analyzed surface snow every day for 3 years at Summit, Greenland. They reported that Ca2+, which mainly originates from dust, showed a strong peak in April. As the sea-salt fraction of Ca2+ is small (Fig. 4), the major source of Ca2+ at the NEEM site would be mineral dust. Dust layers have been observed in late-winter-to-spring snow layers in Yukon, Canada (Reference Goto-Azuma, Koerner, Demuth and WatanabeGoto-Azuma and others, 2006), and over wide areas of the Greenland ice sheet (e.g. Reference SteffensenSteffensen, 1988; Reference Whitlow, Mayewski and DibbWhitlow and others, 1992; Reference Mosher, Winkler and JaffrezoMosher and others, 1993; Reference Drab, Gaudichet, Jaffrezo and ColinDrab and others, 2002). Although these sites are distant from Asia, their major sources of dust were attributed to those in Asia. Therefore, nssCa2+/dust transported to the NEEM site in spring would have also likely originated from Asian sources. Apportionment of Mg2+ by calculation of sea-salt and nss components (Fig. 4) suggests that the rising leg of the Mg2+ peak in winter to early spring is of sea-salt origin and that the later part of the peak is due to the influx of mineral dust.

The summer NO3 peak (Fig. 4) was consistent with previous studies carried out on the Greenland ice sheet (Reference SteffensenSteffensen, 1988; Reference BeerBeer and others, 1991; Reference Whitlow, Mayewski and DibbWhitlow and others, 1992; Reference Fischer, Wagenbach and KipfstuhlFischer and others, 1998; Reference MotoyamaMotoyama and others, 2001; Reference Dibb, Whitlow and ArsenaultDibb and others, 2007). The winter-to-early-spring peak in the NO3 concentration observed at NEEM was also observed in modern snow at Summit, Greenland (Reference Whitlow, Mayewski and DibbWhitlow and others, 1992; Reference Dibb, Whitlow and ArsenaultDibb and others, 2007), and on Agassiz Ice Cap in the Canadian High Arctic (Reference Goto-Azuma, Koerner, Nakawo and KudoGoto-Azuma and others, 1997), which was attributed to anthropogenic sources. Though Reference Finkel, Langway and ClausenFinkel and others (1986) found a major single summer peak in NO3 concentrations both in pre- and post-industrial south Greenland snow, they reported an increase of NO3 concentrations in late winter/early spring since the 1950s due to anthropogenic inputs. To determine the sources of NO3 , Reference Hastings, Steig and SigmanHastings and others (2004) analyzed nitrogen and oxygen isotopes of NO3 in snow at Summit, Greenland. Based on seasonal variations of δ15N, they reported that the predominant source of NO3 in summer was natural NOx produced by biomass burning, biogenic soil emissions and lightning, whereas that in winter was anthropogenic NOx from fossil fuel combustion. Since the seasonal variation of NO3 at NEEM resembled those documented at other sites in the Arctic, we suggest that the summer and winter-to-early-spring peaks of NO3 at NEEM are also mainly of natural and anthropogenic origin, respectively. For confirmation, further studies using isotopes, etc., are necessary at NEEM.

The seasonal variations of MSA in the snow at NEEM coincided with those in the air and snow at Dye 3 and Summit, Greenland (Reference Li, Barrie, Talbot, Hariss, Davidson and JaffrezoLi and others, 1993b; Reference Jaffrezo, Davidson, Legrand and DibbJaffrezo and others, 1994), and with those in the air at Alert in the Canadian High Arctic (Reference Li, Barrie and SiroisLi and others, 1993a). MSA is an oxidization product from dimethylsulfide (DMS), which is mainly produced by marine phytoplankton (e.g. Reference Legrand and MayewskiLegrand and Mayewski, 1997). The spring peak in MSA may have been caused by enhanced photochemical activity after the polar sunrise, when solar radiation becomes available to oxidize the winter reservoir of DMS (Reference Li, Barrie, Talbot, Hariss, Davidson and JaffrezoLi and others, 1993b). Alternatively, it could have been caused by long-range transport from lower-latitude oceans (Reference Li, Barrie, Talbot, Hariss, Davidson and JaffrezoLi and others, 1993b). The MSA peak in late summer to autumn, when sea ice around Greenland has retreated, could have been produced by regional DMS emissions.

The potential sources of SO4 2– include sea salt, volcanic eruptions, SO2 produced by fossil fuel combustion, dust, and DMS emissions produced by marine phytoplankton (Reference Legrand, De Angelis, Cachier, Gaudichet and DelmasLegrand and others, 1995; Reference LegrandLegrand, 1997; Reference Legrand and MayewskiLegrand and Mayewski, 1997). As can be seen in Figure 4, sea salt is only a minor component of SO4 2– at the high-elevation site NEEM. Large-magnitude volcanic eruptions produce spikes in SO4 2– concentration (e.g. Reference Legrand and MayewskiLegrand and Mayewski, 1997). They are, however, sporadic and do not make a substantial contribution to the SO4 2– budget in years without large explosive volcanism. The SO4 2– peak in winter to spring (Fig. 4) has also been documented at other ice-sheet sites in north, central and south Greenland (Reference Finkel, Langway and ClausenFinkel and others, 1986; Reference BeerBeer and others, 1991; Reference Whitlow, Mayewski and DibbWhitlow and others, 1992; Reference Jaffrezo, Davidson, Legrand and DibbJaffrezo and others, 1994; Reference Fischer, Wagenbach and KipfstuhlFischer and others, 1998; Reference BiglerBigler and others, 2002; Reference Dibb, Whitlow and ArsenaultDibb and others, 2007) and at an ice-cap site in the Canadian High Arctic (Reference Goto-Azuma, Koerner, Nakawo and KudoGoto-Azuma and others, 1997). A similar seasonal variation was seen in nssSO4 2– in the air in Greenland and the Canadian High Arctic (Reference Barrie and BarrieBarrie and Barrie, 1990; Reference Jaffrezo, Davidson, Legrand and DibbJaffrezo and others, 1994; Reference NormanNorman and others, 1999). Studies in central and south Greenland and in the Canadian High Arctic suggest that the winter-to-early-spring peak was due to the inflow of anthropogenic air pollutants. Similarly, the winter-to-early-spring peak of SO4 2– at NEEM may also be attributed to air pollutants produced by fossil fuel combustion. On the other hand, Reference BiglerBigler and others (2002) reported that a single winter-to-early-spring peak has been observed both in preindustrial and modern snow in north Greenland, though a small shift in the seasonality between them was seen. This would imply that in north Greenland both natural and anthropogenic SO4 2– peak in winter to early spring, in contrast to central and south Greenland and to the Canadian High Arctic, where natural SO4 2– peaks in summer (Reference Finkel, Langway and ClausenFinkel and others, 1986; Whitlow and others, 1992; Reference Goto-Azuma, Koerner, Nakawo and KudoGoto-Azuma and others, 1997; Reference Dibb, Whitlow and ArsenaultDibb and others, 2007). If this is the case, the winter-to-early-spring SO4 2– peak in the modern snow at NEEM could also have both anthropogenic and natural sources.

We now discuss the contributions of mineral dust and oxidation products of DMS to the SO4 2– budget at NEEM. The spring maxima of nssSO4 2– were seen at depths around 0.11, 0.57, 1.14 and 1.71 m (Fig. 4), where nssCa2+ also displayed the spring maxima, indicating that CaSO4 is one of the major sources of both SO4 2– and Ca2+ in spring. Earlier studies showed that SO4 2– and Ca2+ peaks in spring were associated with episodes of increased atmospheric dust (e.g. Reference BarbanteBarbante and others, 2003). The reaction of CaCO3 from Asian dust sources with H2SO4 (e.g. Reference Legrand and MayewskiLegrand and Mayewski, 1997) during the transport to northwest Greenland may have largely contributed to the spring maximum of SO4 2– and Ca2+ concentrations. Nevertheless, nssSO4 2–/nssCa2+ ratios at the spring maxima were larger than the stochastic ratio of CaSO4, which suggests that other nssSO4 2– sources are also important. Anthropogenic input of nssSO4 2– would be an important source particularly during winter and early spring, as was the case for Summit and Dye 3, Greenland (Reference Finkel, Langway and ClausenFinkel and others, 1986; Reference Mann, Shuman, Kelly and KreutzMann and others, 2008), and for Alert, a Canadian High Arctic site (Reference Nriagu, Coker and BarrieNriagu and others, 1991).

The two major final products of the DMS oxidation processes are nssSO4 2– and MSA. Different timings of nssSO4 2– and MSA peaks could imply that DMS is not a predominant source of nssSO4 2– at NEEM. A closer look at the nssSO4 2– seasonality, however, might indicate that the falling leg of the nssSO4 2– peak in spring overlapped at least part of the MSA peak in spring, and that a nssSO4 2– peak in summer to autumn, which was generally very small except in 2007, was seen in the rising leg of the MSA peak in summer to autumn. Hence, we cannot yet rule out the possibility that nssSO4 2– originated from DMS might have shared a noticeable fraction of nssSO4 2– in spring and late summer to autumn. We plot the MSA/nssSO4 2– ratios in Figure 5. The ratio showed a major peak in late summer to autumn, with a minor peak or shoulder in spring. As our study covers only 4 years, and as our pit data are subject to noise, we cannot conclude that the minor peaks or shoulders in nssSO4 2–, MSA and MSA/nssSO4 2– occurred regularly over multiple years. In addition, we cannot yet draw a firm conclusion about the source apportionment of nssSO4 2–. Accordingly, interpretation of the seasonality of MSA/nssSO4 2– is rather complex. Studies using sulfur isotopes and those covering longer periods are needed.

Surface mass balance

We calculated the annual surface mass balance for the years 2006–08 at NEEM (Table 1). Each year was defined using the profile of the stable-isotope ratio, so that the border between the two adjacent years was located at the layer with a stable-isotope minimum. To examine the seasonal distribution of snow accumulation, we divided one year into two parts: one representing the first half of the year (stable-isotope minimum to maximum) and the other representing the second half of the year (stable-isotope maximum to minimum). Here we define the former as ‘winter to summer’ (winter–spring– summer) and the latter as ‘summer to winter’ (summer– autumn–winter). The annual mean surface mass balance during the 3 years was 176 mm w.e., which is substantially less than the 0.26ma–1 estimated by Reference Buchardt and Dahl-JensenBuchardt and Dahl-Jensen (2008). The surface mass balance in 2006 (2005 winter to 2006 winter) was 203 mm, which was the maximum of the three years. On the other hand, the surface mass balance in 2008 (2007 winter to 2008 winter) was less than in 2006 by ~50mm. We found that the year-to-year variability of surface mass balance at NEEM was >20%. The snow depositions during the winter-to-summer and summer-to-winter seasons were 98 and 78 mm, respectively. Snow deposition during the winter-to-summer half of the year was greater than that during the other half of the year by 10– 20 mm for all three years. As these findings are based only on the past 3 years, they need to be further investigated by examining both shallow and deep ice cores drilled at NEEM.

Table 1. Annual surface mass balances from 2005/06 winter to 2008/09 winter. Concentrations of H + were calculated from pH values.

The depositions of Na+, K+, Mg2+, Ca2+, Cl, NO3 and SO4 2– showed large seasonal differences (Table 1). The 2007 winter to 2008 summer season had substantially greater depositions of Na+, K+, Mg2+ and Cl than any other seasons. During this season, deposition of these ions was >1.5 times the seasonal mean, which is due to the high peak around 0.6 m depth. Separation of non-sea-salt and sea-salt fractions (Fig. 4) indicates that the sea salt, not dust components, showed a pronounced peak in early spring of 2008. In general, all the ions but MSA show greater deposition during winter to summer than summer to winter. For instance, average depositions of major sea-salt components Na+, Cl and Mg2+ during winter to summer were about two times greater than those during the summer to winter. Ca2+ showed an even greater seasonal difference, which was more than threefold. These seasonal differences were greater than those observed for H2O. Depositions of K+, NO3 and SO4 2– were also greater during the winter to summer. Among all the ionic species, only MSA showed greater deposition in summer to winter. This study demonstrated that the depositions of most chemical components studied were highest in the winter-to-summer half of the year at NEEM.

4. Conclusions

We conducted a pit study in July 2009 at NEEM. High-resolution sampling at every 0.03 m allowed for identification of detailed seasonal chemical characteristics in the surface snow/firn. Distinct seasonal variations in the stable isotopes of water (δ18O, δD) were observed. Concentrations of Na+, Mg2+ and Cl peaked in winter to early spring. of the latter two ions, only the fractions originated from sea-salt aerosols peaked in this season. Concentrations of nssCa2+ and nssMg2+, which were of dust origin, peaked in late winter to spring, following the peak of Na+, Mg2+ and Cl. Concentrations of K+ showed a peak in winter to spring, and possibly a secondary peak in summer. The summer peak of K+, if this does exist, may have been caused by increased biomass burning in summer (Reference Whitlow, Mayewski and DibbWhitlow and others, 1992), whereas the origin of the winter-to-spring peak is not clear. Concentrations of NO3 showed double peaks, one in summer and the other in winter to spring, whereas those of SO4 2– peaked in winter to spring. The winter-to-spring concentrations of NO3 and SO4 2– were likely influenced by anthropogenic inputs. Concentrations of MSA seems to have shown double peaks, one in spring and the other in late summer to autumn. While the seasonality of MSA seems to reflect the seasonality of emissions, oxidation and transport processes of DMS, the sulfur budget at NEEM is not yet clearly understood. Further investigations using nitrogen and sulfur isotopes as well as analyses of ice cores covering the pre-industrial period will be necessary. Nevertheless, seasonal variations of ion concentrations and also those of the Cl/Na+ ratio, which displayed a distinct summer peak, can be used to absolutely date the NEEM deep ice core by annual-layer counting.

The annual mean surface mass balance during 2006–08 was 176 mm w.e. The snow deposition during the winter-to-summer half of the year exceeded that in the summer-to-winter half of the year by 10–20mm for all three years. The deposition of Na+, K+, Mg2+, Ca2+, Cl, NO3 and SO4 2– was also greater in the first half of the year (winter to summer) than the latter half (summer to winter), and showed larger seasonal differences than those of H2O. The information about present-day surface mass balance, and depositions of ions and their variability will provide the basis to interpret data from the NEEM deep ice core.

Acknowledgements

NEEM is directed and organized by the Center for Ice and Climate at the Niels Bohr Institute and the US National Science Foundation (NSF) Office of Polar Programs (OPP). It is supported by funding agencies and institutions in Belgium (FNRS-CFB and FWO), Canada (NRCan/GSC), China (CAS), Denmark (FIST), France (IPEV, CNRS/INSU, CEA and ANR), Germany (AWI), Iceland (RannIs), Japan (NIPR), Korea (KOPRI), the Netherlands (NWO/ALW), Sweden (VR), Switzerland (SNF), the UK (NERC) and the USA (US NSF OPP). This work was also supported by the Advance Project of NIPR ‘Approaching Earth System Dynamics through the Past Polar Changes: Reconstruction of Quaternary Polar Environmental and Global Atmospheric Changes with High Accuracy and High Temporal Resolution’, and Grants-in-Aid for Scientific Research 21710017 and 22221002 of the Japan Society for the Promotion of Science. The support from NEEM field members and the laboratory technicians at NIPR is appreciated.

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