Raman spectroscopy data of microbial methane hydrates around Sado Island

Near-seafloor gas hydrates of microbial methane were collected around Sado Island in the Sea of Japan during 1K17 and 7K18 cruises (July 2017 and September 2018, respectively) (Hiruta and Matsumoto, revised). These gas hydrate samples in piston corer were handled in same way with our previous work (Hiruta and Matsumoto, 2022). They were stored in a Dewar container with liquid-N2 and transported to onshore laboratory. These samples were CT scanned before obtaining Raman spectra (Hiruta and Matsumoto, 2022). For the CT, samples were taken out of liquid-N2 for ~20 minutes, then returned to the liquid-N2 storage. Subsamples for Raman spectroscopic analysis were prepared just before the analysis. Portions of the gas hydrate samples were crushed with a hammer to chunks of a few cm in diameter, then they were further crushed to small pieces (roughly <1cm) in a mortar storing liquid-N2. Raman spectra was obtained by a laser Raman spectroscopy (Jasco Corporation, RMP–510). Second harmonic of YAG laser (harmonic wave, 532 nm) was used. Grating was changed to 1800 gr/mm. The gas hydrate piece which was set in a chilling chamber was analyzed through glass window of the chamber. The sample was chilled to -150 degree using liquid-N2. Wide range from 1500 to 3200 cm-1 was measured. One spectra was obtained by exposing a spot (~20 μm in diameter) for 160 seconds. Although multiple measurements were conducted in most spots, single measurement was also applied to some spots. Surface of crushed gas hydrate piece was not flat. Therefore, air (mainly N2) must be included in focal plane of laser and occasionally appeared as N2 band at ~2332 cm-1. Polystyrene standard was put on glass window due to size of the chamber and measured in same conditions except for temperature. These band positions were used to calibrate spectra of gas hydrate. Related articles Hiruta, A., Matsumoto, R., 2022. Massive gas hydrates buried on Umitaka Spur in the Sea of Japan: description, origin, and significance to methane cycling in marine sediment. Geo-Mar. Lett. 42, 13. https://doi.org/10.1007/s00367-022-00735-w. Hiruta and Matsumoto, revised. Seafloor activity and deep-subsurface geology of gas hydrate areas revealed from d13C of methane-derived authigenic carbonates along the eastern margin of the Sea of Japan.2017年和2018年航次（分别为2017年7月、2018年9月，代号1K17与7K18）期间，于日本海佐渡岛周边海域采集了产甲烷微生物成因的近海底天然气水合物样品（Hiruta与Matsumoto，待修订）。 本次通过活塞式取样管（piston corer）获取的天然气水合物样品，处理流程与本团队前期研究（Hiruta和Matsumoto, 2022）完全一致。样品经液氮（liquid nitrogen）杜瓦容器（Dewar container）储存后，运送至陆上实验室。 在获取拉曼光谱前，已对样品开展计算机断层扫描（CT, Computed Tomography）（Hiruta和Matsumoto, 2022）：扫描过程中，样品从液氮储存中取出约20分钟，随后放回液氮中保存。 用于拉曼光谱分析的子样品均在分析前临时制备：先使用锤子将部分水合物样品敲碎为直径数厘米的块状物，再在盛有液氮的研钵中进一步研磨为直径小于1厘米的细碎颗粒。 拉曼光谱数据通过激光拉曼光谱仪（日本分光株式会社，型号RMP–510）采集，激发光源为钇铝石榴石激光器（YAG laser）的二次谐波（波长532 nm），光栅参数设置为1800线/毫米。将水合物样品置于低温样品室（chilling chamber）内，通过样品室的玻璃窗进行光谱采集；样品通过液氮制冷至-150 ℃。光谱扫描范围覆盖1500~3200 cm⁻¹，每个测试斑点（直径约20 μm）的曝光时长为160秒。多数斑点开展了多次重复测量，部分斑点仅进行单次测量。 由于破碎后的水合物样品表面并非平整平面，激光焦平面内可能混入空气（主要成分为氮气），因此光谱中偶见约2332 cm⁻¹处的氮气特征峰。受样品室尺寸限制，将聚苯乙烯标准样品置于玻璃窗处，在除温度外完全一致的测试条件下进行校准，以此完成水合物样品光谱的波数校准。 相关研究文献： 1. Hiruta, A., Matsumoto, R., 2022. 日本海乌米塔卡海岭（Umitaka Spur）埋藏的巨型天然气水合物：特征描述、成因及其对海洋沉积物甲烷循环的意义. 《Geo-Marine Letters》（简称Geo-Mar. Lett.）, 42, 13. https://doi.org/10.1007/s00367-022-00735-w. 2. Hiruta, A., Matsumoto, R. 待修订. 基于日本海东缘甲烷成因自生碳酸盐的δ¹³C同位素揭示的天然气水合物区海底活动与深部地下地质特征.