[B-CG07] Earth and Planetary Science Frontiers for Life and Global Environment

[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

*Ayumi Maeda¹, Kazuhiko Fujita², Keiji Horikawa³, Atsushi Suzuki⁴, Yoshikazu Ohno⁵, Hodaka Kawahata⁶ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Department of Physics and Earth Sciences, University of the Ryukyus, 3. Graduate School of Science and Engineering for Research, University of Toyama, 4.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 5.Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, 6.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Temperature proxy, Large benthic foraminifera

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.