[B-PT02] Biomineralization and Geochemistry of Proxies

[BPT02-06] Long-term and short-term variation of shells of planktonic foraminifera Orbulina universa

*Kaoru Kubota¹, Tsuyoshi Ishikawa¹, Toshihiro Yoshimura¹, Katsunori Kimoto¹, Qing Chang¹, Minoru Ikehara², Yusuke Yokoyama^1,3, Francisco Jimenes-Espejo^4,1, Sidney Hemming⁵, Stephen Barker⁶, Ian Hall⁶ (1.Japan Agency for Marine-Earth Science and Technology, 2.Kochi University, 3.The University of Tokyo, 4.University of Granada, 5.Columbia University, 6.Cardiff University)

Keywords:Planktonic foraminifera, Mg/Ca ratio, Geochemistry, Marine sediment, IODP, Laser ablation

The shells of planktonic foraminifera preserved in marine sediments are an excellent recording medium for preserving past surface ocean environments over long timescales. For example, the Mg/Ca ratio of the shells can record surface ocean temperatures, and is widely used in paleoclimate research. In this study, we focus on the Mg/Ca ratio from geochemical analyses of planktonic foraminifera Orbulina universa (shell size 500-850 μm) collected from marine sediment cores (Site U1476) in the Western Indian Ocean during the International Ocean Discovery Program (IODP) Expedition 361. The Mg/Ca ratio in O. universa shells strongly reflects variations in the Mg/Ca ratio of seawater, showing a long-term increasing trend over the past 7 million years. However, even considering this, the reconstructed water temperatures were estimated to be about 15 degrees lower than those obtained by other methods. Considering the possibility of partial dissolution of the shells, we investigated the physical structure of the shell surface and interior, as well as the distribution of elemental concentrations within the shell, using SEM, Micro-X-ray CT, and Laser Ablation ICPMS. Our findings suggest that the thickness of the shells has halved over the past 7 million years, and the method of elemental incorporation (specifically Mg) into the shells may have changed over time.