https://doi.org/10.1017/jog.2021.115
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2.3 SEM observations and EDS measurements on air-hydrate crystals
For the SEM observations, we used an environmental SEM (Quanta 450 FEG; Thermo Fisher Scientific) equipped with a cryogenic preparation system (PP3010; Quorum Technologies, Lewes, UK). The EDS measurements used an energy-dispersive X-ray spectrometer (X-Max 50; Oxford Instruments, plc, Abingdon, UK) with an acceleration voltage of 20 kV. To reduce buildup of surface charge, N2 gas was purged and observations were made in low vacuum mode (120 Pa). An EDS measurement of a single spot was integrated until the X-ray count reached 3 × 106 (about 1 min). To ignore the damage of the sample due to the EDS measurement, multiple measurements were carried out on different points for an air-hydrate crystal.
The EDS’s energy resolution is about 10 eV. We estimated the spatial resolution of the EDS measurements by comparing measurements across an air-hydrate crystal to the surrounding ice crystal, finding a value of about 15 μm, which is much smaller than the roughly 100 μm diameter size of the air-hydrate crystals.
Thin-section images showing the two air-hydrate crystals (labeled H1 and H2) from the 1548 m depth, and the three air-hydrate crystals (H3, H4, and H5) from the 2406 m depth are shown in Figure 2.
Fig. 2. Thin section samples with locations of the analyzed air-hydrate crystals in the sample. (a) From the 1548 m ice core (2815-C-IV-1). Box shows air-hydrate H1. (b) Air hydrate H2 from the 1548 m ice core (2815-D-I-1). (c) Air hydrate H3 from the 2406 m ice core (4375-A-V-1). (d) Air hydrate H4 and H5 from the 2406 m ice core (4375-B-II-2). The core numbers are the same as those described in another study (Shigeyama and others, Reference Shigeyama, Nakazawa, Goto-Azuma, Homma and Nagatsuka2021).
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3.2 EDS measurements of air-hydrate crystals
In examining the EDS results, we start by focusing on hydrate H3 (Fig. 4). Figure 6a shows the position of the air-hydrate crystal with an optical microscope. The part of the crystal viewed with a SEM secondary electron image is in Figure 6b. The higher magnification image in Figure 6c shows the positions of the EDS measurements on ice outside the air-hydrate (white ‘ + ’) and on the inside of the air-hydrate crystal (yellow ‘*’).
Fig. 6. Sample images of air-hydrate crystal H3 from the 2406 m ice core. (a) Optical microscope image of the thin section (same as Fig. 2 (c)). (b) SEM image. The object on the left side is the hollow of a dissociated air-hydrate crystal due either to the surface cutting or by the primary sublimations. (c) Expanded SEM image of the upper part of air-hydrate (H3) sample. The white ‘ + ’ is the EDS measurement position in the ice, the yellow ‘＊’ that for the air-hydrate crystal.
The EDS spectrum of ice in Figure 7a shows a large O peak at 0.53 keV, derived from H2O, plus smaller peaks for C at 0.28 keV, from the background (likely from the freeze adhesive), and N at 0.39 keV, derived from the chamber gas. There is also a weak, broad background peak at 1–8 keV. In contrast, the spectrum from the inside of the air-hydrate crystal (Fig. 7b) shows a much stronger N peak intensity and a weak, yet distinct, Ar peak at about 2.95 keV.
Fig. 7. EDS spectra (0–8 keV) obtained from the two points shown in Figure 6c. (a) Ice. (b) Air-hydrate crystal.
For the ice case, we attribute the N peak to the chamber gas because we observed a similar N peak in the spectrum of pure ice in a comparative measurement (Fig. S1). We also found that signals from various substances used in the sample holder and SEM chamber (Fig. S2) do not appear in Figure 7. These tests help confirm that the EDS spectra in Figure 7 come from ice, air-hydrate, and chamber gas, not the sample holder.
To remove the signal that is not from the sample and to extract the characteristic signals of the air-hydrate crystal, we subtracted an average ice spectrum from the spectrum from an air-hydrate crystal. This average ice spectrum comes from averaging the EDS spectra from several points surrounding the air-hydrate crystal (e.g. the white ‘ + ’ in Fig. 6c). The resulting difference spectrum for H3 is shown in Figure 8. The difference spectrum of H3 shows both N and Ar peaks as positive values, whereas the O peak is negative. As both spectra are from nearby positions, they should both have the same contribution of N from the atmosphere of the SEM chamber. Thus, the N signal in the difference should largely be from the air-hydrate crystal. By this argument, the three peaks in Figure 8 are considered to be from elements in the air-hydrate crystal: N2, O2, and Ar. This appears to be the first time that experiment has shown an air-hydrate crystal in an ice core to contain Ar.
Fig. 8. Differential spectrum of air-hydrate crystal (from data in Fig. 7).
If the air-hydrate crystal contains Ar at the current atmosphere proportion (0.93%), the intensity of Ar signal would be near the noise level of this sample or close to the sensitivity limit in the EDS measurement. We had chosen only air-hydrate crystals with particles to possibly increase the Ar signal, but it is difficult to estimate the intensity quantitatively. Thus, the difference-spectrum analysis method should be useful for Ar observation in the air-hydrate crystal. Concerning the composition ratio, we did not calculate this ratio from the peak intensity because we did not run an EDS measurement on a standard sample. So, the concentration of each component in the air-hydrate crystal cannot be discussed here.
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