[M-IS12] Paleoclimatology and paleoceanography

[MIS12-P16] Analysis of the frequency of the Kuroshio large meander over the past 14,000 years from Mg/Ca-derived temperature of individual foraminiferal analysis

*Yui Fujimi¹, Keiji Horikawa⁶, Minoru Ikehara², Yusuke Okazaki³, Yoshimi Kubota⁴, Hidetaka Kobayashi⁵ (1.Graduate school of Science and Engineering, University of Toyama, 2.Marine Core Research Institute, Kochi University, 3.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 4.National Museum of Nature and Science, 5.Faculty of Science, The University of Toyama, 6.Faculty of Science, Academic Assembly, University of Toyama)

Keywords:Kuroshio large meander, analyzed individual foraminiferal

The Kuroshio large meander (KLM) is a phenomenon in which the Kuroshio Current meanders off the Kii Peninsula. Currently, the historically longest KLM (about 6 years and 6 months) has occurred since 1965, when observational data are available. The KLM causes changes in skipjack tuna fishing grounds around the southern Honshu coast, a decrease in the catch of Japanese anchovy (NRIFS 2005), and higher temperatures and humidity in the Kanto region (Sugimoto et al., 2021). Therefore, the KLM is considered as an extremely important oceanographic changes that influence the marine resources and climate conditions around Japan. However, the frequency and duration of the KLM are not well understood prior to the past 60 years, when observational oceanographic data are not available. Given this situation, Sawada and Handa (1998) used marine sediment cores, and attempted to analyze changes in the Kuroshio Current paths on a millennial scale. They focused on two areas where sea surface temperature (SST) increases and decreases during the KLM event, and assumed that the Kuroshio Current was the KLM state when the SST differences are large, and discuss the Kuroshio Current path based on the SST differences between these two sites. However, the ORAS5 reanalysis SST data for the past 65 years shows no clear SST difference between these two sites during the KLM and non-KLM periods, and the difference in 10-year mean SSTs is even more ambiguous. Furthermore, given the dating error of each sediment core, extracting SST at same time horizons at the two sediment cores is challenging. In addition, considering the sedimentation rates of less than 10 cm/1000 yr, SSTs at two sites are averaged over several 100 years, and from such averaged SST differences, currently observed in the KLM on less than decadal time scale cannot be reconstructed.
Therefore, in this study, to analyze the frequency of the past KLM, we applied the individual foraminiferal analysis (IFA, 70–100 tests/interval) for a single sediment core. We focused on the off Tokai region, which is marked by higher SST during the KLM and lower SSTs during non-KLM, and used a sediment core (KH-16-6 St2 PC) collected from the region. We analyzed individual foraminiferal Mg/Ca ratios of Globigerinoides ruber albus, which dwells in the surface layer during summer. For IF-Mg/Ca measurements, we modified the flow-through cleaning method used in a previous study (Klinkhammer et al., 2004). As the tentative IF-Mg/Ca measurement for St2 PC, we measured IF-Mg/Ca ratios of G. ruber albus deposited in the Holocene section. The average IF-Mg/Ca ratio was 3.8 mmol/mol (23.3°C), which was comparable to the present-day SST (24.5°C) off the Tokai Coast and is in general agreement with the average SST (23.8°C) for the past 10,000 years obtained from a bulk Mg/Ca analysis (20–30 tests). In our presentation, we will present the IFA-based frequency of the KLM for 5 stratigraphic intervals (1 cm thick/sample, sedimentation rate 8 cm/kyr) during the past 6000–14000 years.