Geochemistries of methane-derived authigenic carbonates around the largest pockmark on Umitaka Spur in the Sea of Japan

Methane-derived authigenic carbonates (MDACs) were collected around the largest pockmark on Umitaka Spur during the UT04 cruise (Hiruta et al., 2009). The pockmark is located between two areas of active gas hydrate mounds with gas seeps. Although the pockmark is dormant at present, these MDACs suggest high methane activity at the depression in the past. Geochemistries of these MDACs are valuable information for understanding the nature of methane activity at the anticline.Subsample preparation procedures and subsequent geochemical analyses followed those described in Hiruta et al. (2014). MDACs were stored in a refrigerator until washing with fresh water. After washing away sediments, these MDACs were dried at 60 oC oven, then stored in room temperature. For powder sample preparation, these stored MDACs were cut into two pieces. Both sections were polished and then dried at room temperature. Powder samples were prepared from cut surfaces using drill bit (4 mm in diameter). Carbon and oxygen isotopic compositions (d13C and d18O, respectively) of CaCO3 in MDACs were determined using an isotope ratio mass spectrometer (Finnigan, MAT252). The powdered sample was reacted with 100% phosphoric acid at 25 oC in a vacuum two-way glass tube. Analytical errors were estimated to be ±0.04 and ±0.09 ‰VPDB for d13C and d18O, respectively. Carbonate mineralogy was determined using an X-ray diffraction instrument (PANalytical, X’ Pert PRO). The d-space of calcite was calculated using 1.54187 Å for Cu radiation wavelength. Mg content was calculated from the d-space following Hiruta and Matsumoto (2022). The d18O is original value (i.e., not calibrated for Mg content). Note that remaining parts of some MDACs were used in Watanabe et al. (2008) with simplified IDs (italic IDs in remarks).Uploaded dataCross section of MDACs in PC405Geochemistries of MDACs in PC405Section after powder making (example)Related articlesHiruta, A., Matsumoto, R., 2022. Geochemical comparison of ikaite and methane-derived authigenic carbonates recovered from Echigo Bank in the Sea of Japan. Mar. Geol. 443, 106672. https://doi.org/10.1016/j.margeo.2021.106672.Hiruta, A., Snyder, G.T., Tomaru, H., Matsumoto, R., 2009. Geochemical constraints for the formation and dissociation of gas hydrate in an area of high methane flux, eastern margin of the Japan Sea. Earth Planet. Sci. Lett. 279, 326-339. https://doi.org/10.1016/j.epsl.2009.01.015.Hiruta, A., Wang, L.-C., Ishizaki, O., Matsumoto, R., 2014. Last glacial emplacement of methane-derived authigenic carbonates in the Sea of Japan constrained by diatom assemblage, carbon-14, and carbonate content. Mar. Petrol. Geol. 56, 51-62. https://doi.org/10.1016/j.marpetgeo.2014.04.005.Watanabe, Y., Nakai, S., Hiruta, A., Matsumoto, R., Yoshida, K., 2008. U–Th dating of carbonate nodules from methane seeps off Joetsu, Eastern Margin of Japan Sea. Earth. Planet. Sci. Lett. 272, 89-96. https://doi.org/10.1016/j.epsl.2008.04.012