[M-IS03] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS03-P03] Oxygen isotope analysis of individual planktic foraminifers to constrain depth habitat and interannual variability in the middle Okinawa Trough, East China Sea

*久保田 好美¹、入野 智久²、木元 克典³、多田 隆治⁴、サイディ マッソウ¹ (1.国立科学博物館、2.北海道大学、3.海洋研究開発機構、4.千葉工業大学)

キーワード：有孔虫、酸素同位体比、個体分析、東シナ海、沖縄トラフ、黒潮

Oxygen isotope (δ¹⁸O) of foraminiferal calcite are well-established tools to understand climatic and oceanographic changes in the past. Information on living foraminifera, such as dominant season and depth habitat in the field, is essential to interpret the paleoceanographic data better. As the planktic foraminifers dwell in the water column as plankton, it is more difficult to constrain their living season and depth habitat than benthic foraminifers. Therefore, interpreting the geochemical information of their calcite tests is more complicated. The δ¹⁸O analysis of individual planktic foraminiferal tests is a relatively new tool and has been used to reconstruct the paleoenvironment, especially in tropical regions. These studies deal with past changes in the El Niño Southern Oscillation (ENSO) by conducting the statistics (e.g., standard deviation, range, skewness) of the population of individual foraminiferal analyses (IFA). The advantage of this method is that it can extract the interannual or seasonal variability, such as ENSO, using a deepsea sediment sample that generally has a more extended time resolution than a few decades. The life span of the planktic foraminifera is typically one month. Thus, the population of foraminiferal δ¹⁸O allows for resolving the variability of the water density (e.g., temperature and salinity) on a seasonal- to interannual scale. In this study, we focus on the middle Okinawa Trough, where the Kuroshio main current flows, to investigate how the interannual variability of the water properties (temperature and salinity) is recorded in the foraminiferal δ¹⁸O. Among the planktic foraminiferal species, subsurface and deep dwellers have great potential to reconstruct the water column structure. Especially in the East China Sea, the density structure at the subsurface and deeper depths is closely related to the Kuroshio variations. Therefore, we use subsurface to deep-dwelling planktic foraminifers, Globorotalia menardii (N=127), Globorotalia truncatulinoides (N=20), Globoconella inflata (N=43), and Pulleniatina obliquiloculata (N=124), in this study and conduct individual foraminiferal analyses to constrain the variance of δ¹⁸O.
The IFA analysis indicates that the calcification depth is 130±150 m for G. menardii, 290±100 m for G. inflata, and 80±70 m for P. obliquiloculata, respectively. The δ¹⁸O of G. truncatulinoides is similar to those of G. inflata, indicating subsurface habitat depth. The δ¹⁸O of IFA for G. menardii, G. inflata, and P. obliquiloculata shows similar skewness and variance to the calculated δ¹⁸O using instrumental temperature and salinity data (1987–2016), suggesting the IFA can detect the interannual variability using fossil foraminifers.