[B-CG07] 地球惑星科学　生命圏フロンティアセッション

[BCG07-09] Calibration between temperature and Mg/Ca and oxygen isotope ratios in high-magnesium calcite precipitated by asexually reproduced juveniles of large benthic foraminifera

*前田 歩¹、藤田 和彦²、堀川 恵司³、鈴木 淳⁴、大野 良和⁵、川幡 穂高⁶ (1.東京大学　理学系研究科地球惑星科学専攻、2.琉球大学　理学部物質地球科学科、3.富山大学　大学院理工学研究部（理学）、4.独立行政法人産業技術総合研究所　地質情報研究部門、5.沖縄科学技術大学院大学、6.東京大学　大気海洋研究所)

キーワード：水温プロキシ、大型底生有孔虫

The potential usefulness of test composition of reef-dwelling large benthic foraminifera as paleotemperature proxies was demonstrated by our previous study which reports that Mg/Ca and oxygen isotope ratios (δ¹⁸O) in the high-magnesium calcite tests represent a good correlation with seawater temperature. However, the distinction of temperature correlation with chemical composition in tests between perforate and imperforate species still remains unclear. We cultured asexual reproduced juveniles of four species among large benthic foraminifera at controlled temperatures (21°C–31°C). Neorotalia calcar, Baculogypsina sphaerulata, and Calcarina gaudichaudii were selected as the perforate species, whereas Amphisorus kudakajimensis was chosen as the imperforate species. The results of analyses and measurements suggest linear relations between Mg/Ca and temperature except at 31 °C, regarding A. kudakajimensis also except at 29°C, which is likely attributed to metabolic anomaly of foraminifera under chronic thermal stress. Additionally, A. kudakajimensis (imperforate species) exhibited different Mg/Ca–temperature equation from those of perforate species. The Mg/Ca–temperature and δ¹⁸O–temperature equations are yielded using the least square method as follows: Mg/Ca = 2.73 T + 74.71 for three perforate species, which is applicable to temperature range of 21°C–30°C. The temperature relations of δ¹⁸O differ across perforate species. We observed that the δ¹⁸O values were between the equilibrium of pure calcite and those of high-magnesium calcite. A species-specific calibration is necessary for the temperature relation of δ¹⁸O. Between perforate and imperforate species, respective calibrations are needed for Mg/Ca–temperature relationship, although the similar Mg/Ca range is observed in 21°C–30°C.