2.1. Optical characterization

To measure the number and diameters of micro-inclusions, the ice was sliced with a microtome to a thickness of several millimeters and observed with an optical microscope (Olympus BH2-UMA, with objective lens Olympus ULWD Neo SPlan 50, numerical aperture (NA) 0.55, 50×) in a cold clean room at −15°C. According to the Becke test, the density contrast between micro-inclusions and the surrounding ice is strong enough to distinguish by optical microscopy. We cannot distinguish between dust (insoluble) and salt (water-soluble) inclusions, however, so both types are counted. Micro-inclusions with a diameter greater than 1.0 μm can be measured reliably by this method, but diameters less than 2.0 μm have a large margin of error. All micro-inclusions in the field of view (0.14 × 0.18 × thickness mm³) of the optical microscope were counted and measured, and we randomly picked 100 to calculate the average number per mm³ and standard deviation. The density unit was calculated as count mL⁻¹ from count mm⁻³. In addition to measurements to compare the different spatial distribution of micro-inclusions in 6.9 ka BP (Holocene) and 30.0 ka BP (last glacial period) ice, micro-inclusions were measured in the field of the microscope (0.14 × 0.18 × thickness mm³), and 100 views were measured to construct a view of 1.4 × 1.8 × thickness mm³. To measure the spatial distribution, we selected one view (1.4 × 1.8 × thickness mm³) which contains a grain boundary and air hydrate, but does not include the plate-like inclusions, because we cannot see the micro-inclusions in plate-like inclusions. In these observations, we have not accounted for a possible influence of seasonal variability on the differences between samples taken from different time periods (GRIP Members, 1993).

2.2. Micro-Raman spectroscopy

To measure the chemical composition of the inclusions, we used micro-Raman spectroscopy within a cold chamber controlled to −30°C by N₂ gas flow. The micro-Raman spectroscope uses a triple monochromator (Jobin-Yvon, T64000) equipped with a charge-coupled device (CCD) detector. The CCD detector has good quantum efficiency from 500 to 900 nm. We employed a laser beam of wavelength 514.5 nm and power 20 mW. We chose an objective lens with a long working distance, 6 mm focal length and NA 0.75 (Mitutoyo, M Plan Apo 100×). The absolute frequency of the monochromator was calibrated using a neon emission line. The resolution of the Raman spectrum was 0.6 cm⁻¹ .

We measured reference spectra for various salts in advance. The characteristic features of Raman spectra have been recognized in previous works (e.g. Reference SocratesSocrates, 2001). For instance, SO₄ ²⁻ produces a strong band at 955–1065 cm⁻¹ from the S-O symmetric stretching mode. Its other modes give rise to medium–strong bands at 405–530, 580–680, 1080–1130 and 1140–1200 cm⁻¹ . NO₃ ⁻ has a strong band at 1015–1070 cm⁻¹ and a medium–weak band at 700–770 cm⁻¹. CO₃ ²⁻ has a medium–strong band at 1020–1100 cm⁻¹ and a weak band at 670–745 cm⁻¹ . CaCO₃, a key compound of many inclusions, has a strong band at 1085 cm⁻¹ . Details on the reference specimens prepared for these sulfate and nitrate salts and acid solutions have been given in a previous study (Reference Ohno, Igarashi and HondohOhno and others, 2005).

2.3. SEM-EDS

SEM-EDS analysis determines which chemical elements are present in the micro-inclusions, information that can be used to deduce the chemical compounds. This analysis was performed using a JSM-6360LV (Japan Electron Optics Ltd (JEOL)) SEM system and a JED2201 (JEOL) EDS system. The accelerating voltage of the SEM was 15 keV. The apparatus included an optical probe (Renishaw) to observe the surface of the ice samples. Cores of 5.2 mm diameter were extracted from the 6.9 ka BP (1200.40 m) and 30.0 ka BP (2099.96 m) sections using a trephine drill (Nobel Biocare). The bases of these specimens were cut down to a height of 3–4 mm using a microtome. Each specimen was put into a stainless-steel holder of diameter 5.3 mm and depth 2.5–3.0 mm, held in place with a screw. The sample was kept in an Alto2100 (Gatan) cryo-system during analysis. Inside the SEM chamber, the specimen was sublimated at −90°C until micro-inclusions were exposed on the ice surface. The sublimation rate was about 2 μm min⁻¹ under these conditions. The specimen was maintained at −135°C for the remainder of the analysis.

2.4. Ion chromatography

Ion chromatography is one of the most common methods of measuring the ionic species present in ice. We decontaminated the ice samples used for ion chromatography by cutting away the surface with a clean ceramic knife, on a clean bench in a clean cold room at −15°C. The decontaminated ice samples were immediately sealed in a clean polyethylene bag. After melting in its bag outside the cold room, the ice sample was passed through a filter with 0.45 μm pores and analyzed in a Dionex 500 chromatograph. We measured the concentrations of sodium, magnesium, calcium, chloride, nitrate and sulfate ions. To test for possible contamination, we measured ultrapure water using the same methods and found no peaks in the chromatograph spectrum. The process of sample preparation has been described in detail by Reference Iizuka, Hondoh and FujiiIizuka and others (2006).

To determine the calcium ion concentration due to micro-inclusions, for each sample we also calculated the total volume of inclusions containing CaSO₄.2H₂O and CaCO₃, based on the size distribution curve (Fig. 1) and the observed proportion of micro-inclusions of this type (Fig. 5). The calculated calcium ion is shown in Table 1 as _calc.Ca²⁺. The difference between the calcium ion concentration found by ion chromatography and that estimated from micro-inclusions of calcium salts is considered in section 4.

Fig. 1.

Percentage of micro-inclusions in six diameter ranges. Inclusions of less than 2.0 μm diameter are merged into a single bin because their sizes have a large uncertainty. Inclusions of diameter greater than 4.0 μm are merged because they are rare.

Table 1.

Micro-inclusion counts and ion concentrations in the five GRIP ice-core sections. N is the total number of micro-inclusions found by optical microscope in all 100 views (i.e. the whole 1.8 × 1.4 × thickness mm³ sample, where one view is 0.80 × 0.14 × thickness mm³); the number of inclusions measured using Raman spectroscopy is given in parentheses. n is the number density of micro-inclusions in the sample volume. d is the mean diameter of the micro-inclusions. The thicknesses of the ice sections are also shown; the number in parentheses is the thickness of the section which was re-sliced by microtome at the time the spatial distribution was measured, following measurement of the number density and mean diameter of the micro-inclusions. The ion concentrations measured at each depth are averaged over five samples, _calc.Ca²⁺ is calculated from the diameter and number density of CaSO₄·2H₂O and CaCO₃. The sum of the two major anion concentrations (NO₃ ⁻+SO₄ ²⁻) is also given

3.1. Number density and diameter of micro-inclusions

The number densities, n (mL⁻¹), and mean diameters, d(μm), of the micro-inclusions found in each sample are shown in Table 1. The number density of micro-inclusions is more than an order of magnitude lower in the Holocene than in the glacial period. The mean diameter of the micro-inclusions is about the same at each depth. The diameter distributions for each climate period are shown in Figure 1. The overall shape of the distribution is the same in each period, in that the percentage of micro-inclusions decreases with increasing diameter. The shapes of these distributions correspond well to the size distribution curves of larger dust particles (Reference SteffensenSteffensen, 1997), except for the outstanding abundance of 2.0–2.5 μm inclusions in Holocene ice. This similarity suggests that water-soluble micro-inclusions smaller than 2.0 μm may have a distribution similar to that of dust. The excess of 2.0–2.5 μm inclusions remains to be explained, but it seems likely that water-soluble compounds are responsible.

To illustrate the spatial distribution of micro-inclusions, including both dust and water-soluble compounds, Figure 2 shows the micro-inclusions, clathrate hydrates, air bubbles and grain boundaries observed in two samples of 6.9 and 30.0 ka BP ice. Note that the micro-inclusions are not distributed uniformly in either sample; they tend to be found in tight groups. Previous research has postulated that the water-soluble impurities used as climate proxies are influenced by diffusion through a vein network (e.g. Reference Rempel, Waddington, Wettlaufer and WorsterRempel and others, 2001). If the water-soluble impurities exist as liquid phase, these may diffuse through a vein network. However, we find that most of the micro-inclusions (water-soluble and dust) are distributed in the ice grain at these depths, and chemical compositions of water-soluble micro-inclusions are measured by micro-Raman spectroscopy.

Fig. 2.

Spatial distribution of micro-inclusions (black dots), clathrate hydrates (white regions), an air bubble (gray region) and grain boundaries (solid lines) in ice: (a) a 1.4 mm × 1.8 mm × 1.4 mm thick section of 6.9 ka BP ice; (b) a similar section (1.65 mm thick) of 30.0 ka BP ice.

3.2. Micro-Raman spectroscopy

In the Raman spectra of the micro-inclusions we found several sharp peaks. We compare these data to the reference spectra of pure specimens. Below, we focus on CaSO₄·2H₂O and CaCO₃, because calcium is the most abundant ion in GRIP ice.

Calcium sulfate (CaSO₄·2H₂O)

In some inclusions, a sharp primary peak is detected at 1009.05 cm⁻¹ with sub-peaks at 215.35, 414.23, 493.04, 620.75 and 670.22 cm⁻¹ (Fig. 3c and g). Inclusions of this type were found at all five depths. The reference spectrum for CaSO₄·2H₂O has its main peak at 1007.84 cm⁻¹ (from the S-O symmetric stretching mode) and secondary peaks at 216.75, 413.60, 493.07, 620.78 and 670.25 cm⁻¹ from other modes (Fig. 3b and f). The two Raman spectra correspond very well. Note that the 215.34 and 216.75 cm⁻¹ peaks (Fig. 3f and g) come from the ice itself (Fig. 3a), as shown in Figure 3a and c. This analysis thus confirms the existence of CaSO₄·2H₂O in the GRIP ice core.

Fig. 3.

Raman spectra (signal intensity in arbitrary units (AU)) of reference specimens and the measured micro-inclusions. a. GRIP ice without inclusions; b. pure specimen of CaSO₄·2H₂O in ice prepared with ultra-pure water; c. CaSO₄·2H₂O inclusion found in the GRIP ice core (the spectrum is from 9.8 ka BP); d. pure specimen of CaCO₃; e. CaCO₃ inclusion found in 30.0 ka BP ice from the GRIP core. Curves f–i are magnifications (10×) of spectra b–e respectively, from 180 to 850 cm cm⁻¹.

3.3. SEM-EDS

Micro-inclusions exposed by sublimation inside the SEM chamber (as described in section 2) were measured by EDS. SEM images of micro-inclusions found in the 6.9 and 30.0 ka BP ice are shown in Figure 4d and e respectively. X-ray spectra of three micro-inclusions observed in the 6.9 and 30.0 ka BP ice samples are shown in Figure 4a, b and c respectively. In the 6.9 ka BP samples (Fig. 4a, b and d) both S and Ca were identified. An abundance of O was also measured, which mainly comes from ice. Micro-inclusions having both S and Ca turned out to be CaSO₄·2H₂O. In the 30.0 ka BP sample, the micro-inclusion contains C, O and Ca (Fig. 4c and e). Such inclusions most likely consist of CaCO₃. The results of SEM-EDS analysis therefore support the Raman spectroscopy identification of CaSO₄·2H₂O and CaCO₃.

Fig. 4.

(a–c) X-ray spectra of micro-inclusions found in (a) 6.9 ka BP ice and (b, c) 30.0 ka BP ice. Inclusions (a) and (b) are deduced to be CaSO₄·2H₂O (containing oxygen, sulfur and calcium). Inclusion (c) is deduced to be CaCO₃ (containing oxygen, carbon and calcium). (d, e) Pictures of the inclusions yielding spectra (a) and (c) respectively.

Taking the micro-Raman spectroscopy and SEM-EDS results together, we claim the first direct evidence that CaSO₄·2H₂O and CaCO₃ exist as solid-phase micro-inclusions in the GRIP ice core.

3.4. Ion chromatography

To further support the above results, we measured the concentrations of sodium, magnesium, calcium, chloride, nitrate and sulfate ions in the same ice sections by ion chromatography (Table 1). In Holocene ice, we find fewer calcium ions than sulfate and nitrate ions: 0.58 vs 1.58 μEq L⁻¹ at 6.9 ka BP and 0.70 vs 2.67 μEq L⁻¹ at 9.8 ka BP, where the second value is the total concentration of (NO₃ ⁻ +SO₄ ²⁻). On the other hand, in the 30.0 ka BP section, we find a significant excess of calcium ions (7.17 μEq L⁻¹) compared to the sulfate and nitrate ions (totaling 2.98 μEq L⁻¹). The 43.1 ka BP ice has a more modest excess of calcium ions (1.97 μEq L⁻¹) with respect to sulfate and nitrate ions (2.16 μEq L⁻¹).

These results can support a discussion of the chemical compounds present in ice (Reference IizukaIizuka and others, 2008). The ion balance observed in Holocene ice (6.9 and 9.8 ka BP) suggests that the calcium ion could exist as both CaSO₄·2H₂O and calcium nitrate. In the 30.0 ka BP ice, the excess of calcium ions strongly suggests the presence of CaCO₃. In the 43.1 ka BP ice, a more modest excess of calcium ions suggests that a small amount of CaCO₃ should have remained. Although the existence of these compounds is inferred, the results support the findings of micro-Raman spectroscopy and SEM-EDS analysis. Note, however, that we did not find direct evidence of CaCO₃ in the 43.1 ka BP ice over the course of this analysis. This section must therefore have significantly less CaCO₃ than the 30.0 ka BP section. Analysis of more inclusions would probably reveal some, but the process is very time-consuming.

3.5. Micro-inclusions identified by Raman spectroscopy

The number of micro-inclusions measured by Raman spectroscopy is given in Table 1. The number density includes both water-soluble and water-insoluble inclusions, as shown in Figure 5. Among water-soluble inclusions, we expect to find sulfate, nitrate and chloride salts given the results of ion chromatography (see section 3.4 and Table 1). As our earlier measurements of reference chemical compounds demonstrate, both sulfate and carbonate salts have strong symmetric stretching modes (S-O and C-O respectively). The nitrate salt also has a symmetric (N-O) stretching mode, but its response is relatively weak. We therefore could not find direct evidence for nitrate salts in the ice via Raman spectroscopy in this study. Chloride salts (e.g. NaCl·2H₂O) can be identified by their hydrate (O-H) vibration mode in the range 3100–3600 cm⁻¹ (Reference Dubessy, Audeoud, Wilkins and KosztolanyiDubessy and others, 1982), but this signal may be hidden by the O-H vibration of ice in this measurement.

Fig. 5.

Number of micro-inclusions identified as containing various compounds in the GRIP ice core. CaSO₄·2H₂O is a major compound at all depths. Na₂SO₄·10H₂O and MgSO₄·11H₂O are difficult to distinguish (Reference Ohno, Igarashi and HondohOhno and others, 2005, Reference Ohno, Igarashi and Hondoh2006). CaCO3 is found only in the 30.0 ka BP ice. Nitrate and quartz are also present, and classified in the ‘other’ category along with some spectra which have not yet been identified. Inclusions for which no spectral features other than those of ice could be detected are classified as ‘no Raman signal’.

A few water-insoluble micro-inclusions (e.g. dust) were partly identified by their chemical compounds, while others could not be matched to reference spectra. The latter are classified as ‘other’ in Figure 5. Micro-inclusions with no Raman peaks except for the ice background are classified as ‘undetectable’, although they may well possess a weak signal hidden by the ice. For all the reasons given above, only water-soluble inclusions containing sulfate and carbonate salts could be clearly identified by this method.

Figure 5 shows that CaSO₄·2H₂O exists not just in glacial period ice but also in Holocene ice. On the other hand, Na₂SO₄·10H₂O and MgSO₄·11H₂O were found only in Holocene ice sections (6.9 and 9.8 ka BP). The Raman spectra of Na₂SO₄·10H₂O and MgSO₄·11H₂O are difficult to distinguish from one another, because the molecules have very similar stretching modes. These sulfate salts have been discussed in two of our previous papers (Reference Ohno, Igarashi and HondohOhno and others, 2005, Reference Ohno, Igarashi and Hondoh2006). We found CaCO₃ in the 30.0 ka BP ice section, but could not find it in the 43.1 ka BP ice section. As the ion-chromatography results suggest, the amount of CaCO₃ in the latter section is probably too small to measure easily (Table 1).

4.1. Ion-concentration estimates based on micro-inclusion volume

To determine the proportion of calcium ions originating from CaSO₄·2H₂O and CaCO₃ in the form of micro-inclusions, we estimated the total mass of such inclusions in the ice section (see section 2.4). Because it is difficult to measure the particle volume, we treat all inclusions as spheres. In ice from the glacial period and Termination I, it is clear that CaSO₄·2H₂O inclusions (and CaCO₃, especially in the 30.0 ka BP section) make the most important contribution. The calcium ion abundance calculated from micro-inclusions is comparable to that measured by ion chromatography (Table 1): at least 74%, 68% and 98% of calcium ions can be attributed to CaSO₄·2H₂O in the 13.5, 30.0 and 43.1 ka BP samples respectively. At least 8% of calcium ions come from CaCO₃ in the 30.0 ka BP sample. In Holocene ice, on the other hand, CaSO₄·2H₂O contributes only 22% and 21 % of calcium ions in the 6.9 and 9.8 ka BP sections respectively. The remaining ions probably come from other salts.

4.2. Significance of carbon dioxide

The amount of CaCO₃ deduced to exist in the ice is sensitive to the measuring method. Reference Anklin, Barnola, Schwander, Stauffer and RaynaudAnklin and others (1995) compared two CO₂ measurements in GRIP ice, obtained by melting–refreezing and dry extraction. In the first method, CaCO₃ dissolves in the melted ice and probably reacts with acid in solution. This produces more CO₂ than would be observed in a dry extraction process. The excess CO₂ can be calculated from the chemical reaction, written as

(1)

This formula implies that 1 mol of CaCO₃ produces 1 mol of excess CO₂ and 1 mol of Ca²⁺. We measured 0.28 μmol L cm⁻¹ (0.55 μEq L⁻¹) of Ca²⁺ in the form of CaCO₃, so only 0.28 μmol L⁻¹ (12.19 ppb) of excess CO₂ could be produced from our observed number density of CaCO₃. This cannot explain the observed excess of CO₂ that Reference Anklin, Barnola, Schwander, Stauffer and RaynaudAnklin and others (1995, Reference Anklin1997) suggested. This difference appears to suggest that we should have found more CaCO₃ in the 30.0 ka BP ice by Raman spectroscopy. Heterogeneous chemical reactions that take place during measurement might also have been active in the ice sheet and/or atmosphere when the layer was formed.

4.3. Significance of sulfate and carbonate salts

Equation (1) is strongly dependent on the acidity of the ice. For the GRIP ice core, this quantity has already been determined by electrical conductivity measurement (ECM) (Reference Wolff, Moore, Clausen, Hammer, Kipfstuhl and FuhrerWolff and others, 1995, Reference Wolff, Moore, Clausen and Hammer1997). In Holocene ice the existence of acid leads to the chemical reaction, whereby HCl may sublimate from reactant,

(2)

which supports our finding of the existence of Na₂SO₄·10H₂O and MgSO₄·11H₂O in Holocene ice. In contrast, less acidity is expected in the glacial period ice due to the presence of CaCO₃.

The existence of sulfates and carbonates in the atmosphere is related to the radiative forcing process. Reference Jarzembski, Norman, Fuller, Srivastava and CuttenJarzembski and others (2003) showed that sulfate and carbonate salts have significantly different refractive indices of absorption from ultraviolet to infrared. The origins of calcium ions in various climate conditions have been previously discussed by several authors (Reference MayewskiMayewski and others, 1994; Reference De Angelis, Steffensen, Legrand, Clausen and HammerDe Angelis and others, 1997; Reference LajLaj and others, 1997; Reference LegrandLegrand and others, 1997; Reference Svensson, Biscaye and GroussetSvensson and others, 2000). A study of Sr and Nd isotopic compounds and rare-earth concentrations suggests that eastern Asia was the main source of dust (including calcium ions) during the last glacial period (Reference Svensson, Biscaye and GroussetSvensson and others, 2000). With respect to the Holocene, Reference Dibb, Whitlow and ArsenaultDibb and others (2007) found that most aerosols came from Asia and the Sahara Desert. These studies suggest that sulfate and carbonate were transported by the atmosphere from Asia and/or the Sahara Desert to the Greenland ice sheet.

It is still necessary to clarify which chemical reactions occurred during formation of the ice sheet, as well as quantifying their rates. Achieving this understanding will allow the paleoclimatology community to discuss how the radiative forcing of aerosols occurred in detail.