Magnesium methanesulfonate salt found in the Dome Fuji (Antarctica) ice core

Abstract

Using micro-Raman spectroscopy, we identified the chemical forms of methanesulfonate salt particles in reference samples of the Dome Fuji (Antarctica) ice core. We found only (CH₃SO₃)2Mg nH₂O among methanesulfonate salts, and this salt particle is most prevalent in the Last Glacial Maximum (LGM) ice. We suggest that during the LGM, (CH₃SO₃)₂Mg nH₂O may have formed in the atmosphere through the chemical reaction of CH₃SO₃H with sea salts, but probably not in the firn and ice due to the neutralization of acid in LGM ice of inland Antarctica.

1. Introduction

Methanesulfonic acid (CH₃SO₃H) is an important compound formed by the oxidation of dimethyl sulfide (DMS) in the atmosphere (Hynes and others, 1986; Saigne and Legrand, 1987; Saltelli and Hjorth, 1995). CH₃SO₃H is specific to marine biogenic activity, so its concentration in the remote troposphere has been a subject of great interest over the past decade. In ice cores, CH3SO3H can be detected through ion chromatography (CH₃SO₃ ^-) by melting an ice sample. Its anion is of primary importance as a tool for reconstructing several activities related to climate change: marine productivity (Legrand and others, 1991, 1992), sea-ice extent (Curran and others, 2003) and major El Nino events (Legrand and Feniet-Saigne, 1991).

In the present time period, ice-core CH₃SO₃ ^- appears to derive mainly from post-depositional relocation, a process that occurs while the acid is in the gas and/or liquid state (Mulvaney and others, 1992;Jaffrezo and others, 1994; Pasteur and Mulvaney, 1999, 2000; McMorrow and others, 2004; Smith and others, 2004; Ruth and others, 2005). On the other hand, a recent study using Raman spectroscopy has provided new information on the chemical forms of certain water-soluble salts contained in the microparticles of ice cores. This evidence suggests that methanesulfonate may exist as a salt in glacial period ice (Ohno and others, 2005, 2006; Barletta and others, 2009). However, very little is known about the eutectic temperature and Raman spectra of methanesulfonate salts. This study identifies methanesulfonate salt particles in the Dome Fuji (Antarctica) ice core using micro-Raman spectroscopy, and proposes a formation process based on phase diagrams measured by differential scanning calorimetry (DSC) and Raman spectroscopy.

2. Methods

2.1. Chemical form of methanesulfonate salts

Our micro-Raman spectrometer is a triple monochromator (Jobin-Yvon, T64000) equipped with a charge-coupled device (CCD) detector (Jobin-Yvon, Spectraview-2D). The device has good quantum efficiency from 500 to 900 nm. The spectral resolution of the Raman spectrum is 0.6 cm^-1. The absolute frequency of the monochromator was calibrated using a neon emission line and silicon wafers. The cross slit of this measurement is 3 mm. The cryostat is posed on an x-y-z translation stage, and fed by a N ₂ gas flow from liquid nitrogen heated inside a dewar. More detgails of the method can be found in Genceli and others (2009) and Sakurai and others (in press).

The microparticles in the Dome Fuji ice core were analyzed using micro-Raman spectroscopy. The Dome Fuji ice-core section at Termination I (the Holocene/Last Glacial Maximum (LGM) transition) was cut with a bandsaw at depths of 340.38 (Holocene), 382.86, 414.36, 463.85, 502.35 (Termination I) and 576.50 m. The final depth corresponds to the LGM. All sections have dimensions of 10 × 10 × 3 mm³. The Raman spectra of microparticles in the ice core were compared with Raman spectra taken from reference specimens of simple salts such as reagent-grade chemicals 98wt% CH₃SO₃Na (Alfa Aesar), 99wt% (CH₃SO₃)₂Mg (Daniels Fine Chemicals), 98wt% CH₃SO₃K (Tokyo Kasei Kogyo) and 98 wt% (CH3SO3)2Ca (Tokyo Kasei Kogyo) at room temperature. CH3SO3Na-H2O, (CH₃SO₃)₂Mg-H₂O and CH₃SO₃K-H₂O were measured by Raman spectroscopy at -30°C, and (CH₃SO₃)₂Ca-H₂O was measured at -50°C, temperatures below the eutectic temperature, at which solid hydrated crystal salts formed. All four solutions were prepared with ultrapure water (18.3 mfi) by weighing out the required amounts of the aforementioned reagent-grade chemicals. Due to their high percentage purities, we assume that these chemicals are anhydrous. Note that we distinguish the spectra on the basis of peak shapes as well as peak positions.

2.2. The eutectic temperatures of methanesulfonate salts

The phase diagrams of the methanesulfonate salts, including their eutectic temperatures, are determined using a differential scanning calorimeter (DSC;Rigaku Co., DSC8230) with liquid nitrogen cooling. Further details on the analytical technique of DSC measurement are provided by, for example, Wunderlich (1973). The temperature reproducibility of this instrument is better than ±0.2°C, based on a three-point temperature calibration using indium (156.6°C), water (0.0°C) and cyclohexane (-83.0°C) (Charsley, 1993).

The DSC measurements are carried out on specimens of CH₃SO₃Na-H₂O, (CH₃SO₃ Mg-H₂O, CH₃SO₃K-H₂O and (CH₃SO₃)₂Ca-H₂O. Solutions are prepared by combining ultrapure water (18.3 mO) with reagents. The DSC cells are made of aluminum and have a capacity of 30pL. In cases where the samples sublimated, the cells were sealed before use. During the measurements for CH₃SO₃Na-H₂O, (CH₃SO₃)₂Mg-H₂O and CH₃SO₃K-H₂O, the cells were cooled to -70°C at a rate of 20°Cmin^-1 and then heated to room temperature at 10°Cmin^-1. These three types of samples typically had a mass of about 2 mg. The (CH3SO3)2Ca-H2O samples typically had a mass of about 15 mg. The (CH₃SO₃)₂Ca-H₂O cells were cooled to-80°C at a rate of 20°C min^-1 and then heated to room temperature at a rate of 1°Cmin^-1. These conditions were chosen because the latent heat of (CH₃SO₃)₂Ca-H₂O is very low.

3. Results

3.1. Raman spectra of methanesulfonate salts

We measured the Raman spectra of several reference specimens: the simple salts CH₃SO₃Na, (CH₃SO₃)₂Mg, CH₃SO₃K and (CH₃SO₃)₂Ca (Fig. 1), and their solid hydrated crystal salts with ice under eutectic conditions (Fig. 2). The methanesulfonate salts show three major peaks around 1050-1100, 2930-2950 and 3010-3030 cm^-1. The peak around 1050-1100 cm^-1 can be assigned to the SO₃ ^2- stretching frequencies (Socrates, 2001; Barletta and others, 2009). The peaks around 2930-2950 and 3010-3030 cm^-1 can be assigned to CH₃ symmetric stretching and CH₃ asymmetric stretching respectively (Durig and others, 2000; Socrates, 2001;Barletta and others, 2009).

Fig. 1. Raman spectra from (a-d) 300-1500 cm^-1 and (a’-d’) 2800-3100 cm^-1. (a-d) are reagent-grade crystals of CH₃SO₃Na, (CH₃SO₃)₂ Mg, CH₃SO₃K and (CH₃SO₃)₂ Ca respectively, which were measured at room temperature.

Fig. 2. Raman spectra from (a-d) 300-1500 cm^-1 and (a’-d’) 2800-3100 cm^-1. (a-d) are hydrated crystals of CH₃SO₃Na nH₂O, (CH₃SO₃)₂Mg nH₂O, CH₃SO₃K nH₂O and (CH₃SO₃)₂ Ca nH₂O respectively, which formed solid hydrated crystal salt under eutectic conditions. Measurement temperatures were -30_C for CH₃SO₃ Na nH₂O, (CH₃SO₃)₂ Mg nH₂O and CH₃SO₃K nH₂O, and -50_C for (CH₃SO₃)₂ Ca nH₂O.

The Raman spectra of the simple salts are significantly different from those of the corresponding hydrated crystals. The differences are especially evident in the SO₃ ^2-, OH and CH₃ stretching frequencies. The SO₃ ^2- stretching mode generates a peak at 1084.0 cm^-1 for the (CH₃SO₃)₂Mg salt. The (CH₃SO₃)₂Mg-H₂O solution, on the other hand, has a peak at 1054.2 cm^-1. The OH stretching mode does not appear for any simple salt. In contrast, all the solutions have a large peak around 3000-3500cm^-1 from OH stretching modes. A more interesting result from these measurements relates to the CH₃ symmetric and asymmetric stretching modes from 3010 to 3030cm^-1. In Raman spectra from simple salts, we detect mode splitting. For instance, the spectrum of (CH₃SO₃)₂Mg has peaks at 3021.8 and 3028.6cm^-1 from CH₃ asymmetric stretching (Fig. 3a). The (CH3SO3)2Mg-H2O sample, on the other hand, produces a single sharp peak at 3020.9 cm^-1. These disagreements must be caused by the crystal structure, suggesting that hydrate is forming in the methanesulfonate salt, i.e. the structure in solution is (CH₃SO₃)₂Mg-nH₂O.

Fig. 3. Optical microcopy of a methanesulfonate salt particle (arrow) found in the ice sample at 576.5 m depth (24.5 ka BP). The scale bar is 10 μm. The Raman spectra from (a-d) 200-1500 cm^-1 and (a’-d’) 2500-3100 cm^-1 shown below the photograph come from (a) (CH₃SO₃)₂ Mg, (b) (CH₃SO₃)₂ Mg nH₂O (after standing at room temperature for several days and measures at -30_C), (c) (CH₃SO₃)₂ Mg nH₂O and (d) the methanesulfonate salt inclusion from the Dome Fuji ice core (the particle in the photograph).

3.2. Chemical form of methanesulfonate salts in the ice core

We found microparticles with Raman spectra very similar to those of the methanesulfonate salt (CH₃SO₃)₂Mg. Among the six depths considered, particles of this kind are only present in LGM (576.50 m depth) ice. Furthermore, no other types of methanesulfonate salt were found among the 758 microparticles examined (Fig. 4). The microparticles of (CH₃SO₃)₂Mg were typically several microns in diameter (Fig. 3 photograph).

Fig. 4. The distribution of microparticle types found in ice samples taken from Termination I of the Dome Fuji ice core. The climate ranges are 340 m (Holocene), 382-502 m (Termination I) and 576 m (LGM). The numbers of microparticles measured at each depth are 87, 106, 101, 161, 237 and 66 (a total of 758 particles.) The numbers of microparticles in the ‘gypsum’ category are 6, 11, 11, 73, 69 and 27 respectively. The numbers in the ‘mirabilite and meridianiite’ category are 58, 90, 86, 29, 77 and 1 respectively. Only four (CH₃SO₃)₂Mg nH₂O microparticles were found, and these only in the LGM.

To identify the chemical form of the methanesulfonate salts present in the ice core, we compared the Raman spectra of microparticles (Fig. 3) with the reference spectra. The Raman spectra of the methanesulfonate salt particles found in the Dome Fuji ice correspond with the (CH₃SO₃)₂Mg-nH₂O reference specimen, not with the anhydrous (CH₃SO₃)₂Mg specimen or any other chemical form.

The Raman spectra of (CH₃SO₃)₂Ca-nH₂O and (CH₃SO₃)₂Mg-nH₂O are very similar in many respects. However, they can be distinguished by their symmetric and asymmetric CH₃ stretching frequencies: 2938.9 and 3020.9 cm^-1 for (CH₃SO₃)₂Mg-nH₂O, or 2941.6 and 3022.6cm^-1 for (CH₃SO₃)₂Ca-nH₂O. We used Gaussian fitting to estimate the positions and half-bandwidths of the peaks. The corresponding peaks in Raman spectra obtained from methanesulfonate salt particles in the ice core are located at 2938.9 and 3020.9 cm^-1. The half- bandwidths of the peaks are 3.9 ± 0.1 and 2.9 ± 0.9cm^-1 respectively. The peak locations and half-bandwidths of the (CH₃SO₃)₂-Ca-nH₂O reference hydrated crystal are 2941.6cm^-1, 3023.1 cm^-1 and 15.8 ± 0.2cm^-1, 23.7 ± 3.4 cm^-1 respectively. This evidence strongly suggests that the methanesulfonate salt found in the ice core is (CH₃SO₃)₂Mg-nH₂O and not (CH₃SO₃)₂Ca-nH₂O.

To evaluate the structural phase transition during preservation, we left the (CH₃SO₃)₂Mg-H₂O solution in a Petri dish at room temperature for several days to sublimate the water, then measured the Raman spectrum of the sample. The resulting spectrum remains clearly (CH₃SO₃)₂Mg-nH₂O, not (CH₃SO₃)₂Mg (Fig. 3b and c). This result suggests that once (CH₃SO₃)₂Mg-nH₂O has formed it can be preserved as a structure of hydrate. It further suggests that a structural phase transition to the anhydrous state or some other phase could not have occurred.

3.3. Phase diagrams of methanesulfonate salts

The phase transitions of CH₃ SO₃ Na-H₂ O, (CH₃SO₃)₂Mg-H₂O, CH₃SO₃K-H₂O and (CH₃SO₃)₂Ca-H₂O generate typical DSC profiles during the heating process. The phase diagrams of the solutions are shown in Figure 5.

Fig. 5. Phase diagrams (eutectic temperatures are given in parentheses) constructed from DSC measurements. (a-d) correspond to CH₃SO₃Na-H₂O (-29.3 ± 0.2_C), (CH₃SO₃)₂ Mg-H₂O (-5.0 ± 0.5_C), CH₃SO₃K-H₂O (-17.8 ± 0.1_C) and (CH₃SO₃)₂ Ca-H₂O (-32.6 ± 0.2_C) respectively. The circles, triangles and squares trace the solubility of ice, the eutectic temperature and the solubility of salt respectively.

CH₃SO₃Na-H₂O

We performed measurements in the composition range 0-52 wt% (Fig. 5a). The eutectic temperature of this salt is -29.3 ± 0.2°C. Concentrations higher than 50wt% CH3SO3Na-H2O were deposited from the solution at room temperature. The enthalpy of fusion at the eutectic composition of 50wt% is ΔH _Na = 12.6 ± 0.9 kJ mol^-1.

CH₃SO₃)Mg-H₂O

We performed measurements in the composition range 0-36 wt% (Fig. 5b). The average eutectic transition temperature over all samples is -5.0±0.5°C. Above a solute concentration of 14wt%, salt deposition occurred at room temperature. The enthalpy of fusion at the eutectic composition of 14 wt% is ΔH _Mg = 13.3 ± 0.04 kJ mol^-1.

CH₃SO₃K-H₂O

We performed measurements in the composition range 0-60 wt% (Fig. 5c). The average eutectic transition temperature is -17.8 ± 0.1 °C. The enthalpy of fusion at the eutectic composition of 44wt% is ΔH _K = 17.9± 0.08 kJ mol^-1.

(CH₃CO₃)₂ Ca-H₂O

We performed measurements in the composition range 0-44 wt% (Fig. 5d). The average eutectic transition temperature is -32.6 ± 0.2°C. At compositions of 44wt% or higher, our measurements of this compound were not easily reproducible (although the other three species were detectable at their eutectic compositions). We therefore do not have a good estimate for the enthalpy of fusion, but one profile gave ΔH _Ca = 0.46± 0.04 kJ mol^-1.

4. Discussion

The eutectic temperature of (CH₃SO₃)₂Mg-nH₂O (-5.0± 0.5°C) is much higher than any temperature within the Dome Fuji ice core, from the surface to the lowest depth. The average temperature of the core at 10 m depth is -58°C, and the deepest recorded temperature (from the first Dome Fuji ice-core project, at 2503 m) is ˜-10°C (Hondoh and others, 2002). This result indicates that (CH₃SO₃)₂Mg-nH₂O is thermodynamically stable in the Dome Fuji ice core between the surface and 2503 m. We shall now propose two formation mechanisms for (CH₃SO₃)Mg-nH₂O in Antarctica, one in the ice-sheet firn and the other in the polar atmosphere.

CH₃SO₃H is formed by the oxidation of DMS, which is emitted by biological activity (e.g. Saltelli and Hjorth, 1995) on the open sea. Sea salts are emitted by sea spray and probably from surface sea ice (e.g. Rankin and others, 2002). The chemical reaction of CH₃SO₃H with sea salt can form methanesulfonate salt. In previous studies using ion chromatography on coastal ice cores obtained at Dolleman Island and Siple Dome, Antarctica, (Mulvaney and others, 1992; Kreutz and others, 1998) the concentration of CH₃SO₃ ^- in the winter layer (which was relocated from the summer layer) determined whether sodium methanesulfonate or magnesium methanesulfonate formed at a depth of several meters. The temperatures of the cores at 10m depth were -16.8°C and -25°C at Dolleman Island and Siple Dome respectively, suggesting that deeper ice might be at even higher temperatures. Taking into consideration our data on the eutectic temperatures of methanesulfonate salts, CH₃SO₃Na-nH₂O (-29.3°C) could not be present in the Dolleman Island ice core but (CH₃SO₃)₂Mg-nH₂O (-5.0°C) should be.

The annual surface temperature (e.g. 1996/97) on the Dome Fuji ice sheet ranges from -79.7°C to -18.6°C, with a mean of -54.4°C (Yamanouchi and others, 2003). The annual average surface temperature during the LGM has been determined from stable-isotope studies as roughly -60°C (Watanabe and others, 2003). There should also have been a seasonal variation around this mean. From previous studies, Na⁺ is the most abundant ion, while Mg²⁺ and CH₃SO₃ ^- are minor ions in both periods, i.e. during the LGM and in current times (e.g. Watanabe and others, 2003; Iizuka and others, 2004, 2006). Thus, these previous studies suggest that CH₃SO₃Na-nH₂O can form at the surface of the Dome Fuji ice during both periods, and that (CH₃SO₃)₂Mg-nH₂O is probably a minor compound in the Dome Fuji ice core. However, among the 758 microparticles analyzed, we found no methanesulfonate salts except for (CH₃SO₃)₂Mg-nH₂O in LGM ice. The discrepancy can be explained by the surface temperature and acidity of the ice at the time it formed.

Taking into consideration the eutectic temperatures of the Mg and Na salts, LGM summer temperatures at the surface could have been high enough to melt CH₃SO₃Na-nH₂O but not (CH₃SO₃)₂Mg-nH₂O. Furthermore, CH₃SO₃H might sublimate from the firn after the CH₃SO₃Na-nH₂O melts, lowering the concentration of CH₃SO₃ ^- in the ice. This situation, dominant in the Holocene and in Termination I, suggests that (CH₃SO₃)₂Mg-nH₂O could have formed in the Holocene and Termination I sections of the Dome Fuji ice core. According to previous studies of the firn, however, sulfuric acid and sea salt can combine to produce sulfate salts (lizuka and others, 2006). The high acidity of Holocene and Termination I ice (Fujita and others, 2002) may have allowed the reaction

(1)

to occur during the post-depositional process, eliminating (CH₃SO₃)₂Mg-nH₂O particles from the ice.

In the LGM ice, methanesulfonate salts form by fixation on alkaline particles of marine or continental origin during long- range aerosol transport to polar areas (Delmas and others, 2003). During this period, high levels of dust neutralized the acids, greatly reducing their abundance (Röthlisberger and others, 2003;Iizuka and others, 2008). This neutralization of the LGM ice permits the existence of (CH₃SO₃)₂Mg-nH₂O.

To summarize, we propose that (CH₃SO₃)₂Mg-nH₂O may have formed in the air and been transported as an aerosol. The salt was then deposited on the ice sheet, where it had a chance to survive and be preserved in LGM ice because the acids were neutralized.

5. Conclusion

We found (CH₃SO₃)₂Mg-nH₂O in the LGM ice from Dome Fuji, and this salt is restricted to the coldest period in the glacial cycle. Our study suggests that (CH₃SO₃)₂Mg-nH₂O formed in the atmosphere of Antarctica. In acidic Holocene ice, however, (CH₃SO₃)₂Mg-nH₂O was dissolved by acid after being deposited. Only during the LGM did (CH₃SO₃)₂Mg-nH₂O that formed in the atmosphere have a chance to survive on the ice sheet and be preserved in the Dome Fuji ice core.