[B-PT03] Biomineralization and the Geochemistry of Proxies -Field ecology, Laboratory culture and Paleo

[BPT03-12] 大型底生有孔虫の高マグネシウム方解石の殻における水温プロキシとしての酸素同位体比およびMg/Ca比

*前田 歩¹、藤田 和彦²、鈴木 淳³、堀川 恵司⁴、吉村 寿紘¹、川幡 穂高¹ (1.東京大学　大気海洋研究所、2.琉球大学　理学部物質地球科学科、3.独立行政法人産業技術総合研究所地質情報研究部門、4.富山大学　大学院理工学研究部)

キーワード：高マグネシウム方解石、水温プロキシ、大型底生有孔虫

Although metal/Ca or oxygen isotopes (δ¹⁸O) of organic precipitated calcium carbonate (calcite and aragonite) of marine species have been used to estimate paleotemperature, the relationships between components of large benthic foraminiferal shells (high-magnesium calcite) and seawater temperature has yet to be established. We investigated the possibility as a proxy for seawater temperature through culture experiments of three species of reef-dwelling large benthic foraminifera in a laboratory. Three species found commonly in the western Pacific were selected including perforate species Calcarina gaudichaudii, and imperforate species Amphisorus kudakajimensis. All of them are host to algal symbionts. They grew sufficiently during culture experiments and showed a maximum in terms of shell length and weight at 27℃ and 29℃ while they were significantly small at 30℃. Mg/Ca ratios of three species in similar range showed high correlation to water temperature, suggesting Mg/Ca ratios as a precise proxy for paleo-temperature in shallow-reef environment. In terms of δ¹⁸O, C. gaudichaudii showed strong correlation versus temperature while δ¹⁸O of A. kudakajimensis showed less significant correlation possively caused by poor growth of unhealthy individuals. The trends in the temperature and oxygen isotope ratios were similar for both species, suggesting the potential of oxygen isotope ratios in the tests of reef-dwelling foraminifera as a paleo-thermometer. Species-specific calibration may be necessary for the use of δ¹⁸O of reef-dwelling large benthic foraminifera as a proxy of paleo-temperature.