[M-IS01_29PM2] Land-ocean linkages in East Asian marginal seas

[MIS01-07] 日本海におけるN.incomptaのMg/Ca古水温計と完新世気候復元への適用

*堀川 恵司¹、小平 智弘¹、池原 研²、村山 雅史³、張 勁¹ (1.富山大学、2.産業技術総合研究所、3.高知大学海洋コア総合研究センター)

キーワード：Japan Sea, Holocene climate change, Mg/Ca-paleothermometry, Tsushima Current, Neogloboquadorina incompta

We present new core-top calibration for Neogloboquadorina incompta Mg/Ca-paleothermometry in the Japan Sea using 15 core-top surface sediments taken from the southern Japan Sea. Using this new Mg/Ca-paleothermometry, we generate the first high-resolution Mg/Ca-derived SST record for the past 7000 years from the sediment core (YK10-7-PC09) taken from 738 m water depth off Niigata. The age model for core YK10-7-PC09 was based on 8 AMS ¹⁴C data of mixed planktic foraminifera, and the conventional ¹⁴C ages were converted to the calendar ages using Marin13 and delta R of 0±100 yr. Trace metal/Ca ratio of N.incompta was measured by a SF-ICP-MS (Thermo Fisher Element II) and the precision (1sigma) of Mg/Ca ratios of the international CaCO₃ standard (BAM-RS3) was 0.786±0.008 (n=100).We have performed paired analyses of δ¹⁸Oc and Mg/Ca ratios of N. incompta at 15 sites. First, to calculate the mean temperatures of waters in which the foraminiferal shells were formed (i.e., calcification temperature), we have used modern local salinity and temperature data (http://www.jodc.go.jp/) in the following paleotemperature equation; T (℃) = 21.4-4.19×(δ¹⁸Oc-δ¹⁸Osw) + 0.05×(δ¹⁸Oc-δ¹⁸Osw)² (Oba, 1980). The δ¹⁸Osw was calculated from the following salinity-δ¹⁸Osw equation in the Japan Sea (δ¹⁸Osw (‰ VSMOW) = 0.27×Salinity-8.98; this study). The comparison of the predicted δ¹⁸Oc values with the measured δ¹⁸Oc shows that N. incompta shells were formed at 0-125 m water depths from June to December in the Japan Sea. Given that previous studies show that N.incompta dwells in the shallow waters (< 100 m) in November to December (Kuroyanagi and Kawahata., 2004; Sagawa et al., 2013), we calculated the calcification temperatures at each site assuming shells were formed in November to December. The cross plot of the calcification temperatures and the Mg/Ca ratios for our core-top samples gives the following equation; Mg/Ca (mmol/mol) = 0.361×exp (0.043×Temp). Using this new Mg/Ca-paleothermometry, the 7000-years N.incompta Mg/Ca records (0.6 to 0.9 mmol/mol, n=127) from core YK10-7-PC09 were converted to the temperature record. Compared to the present winter SST of ca.15℃, the 7000-year SSTs varied from 13.5℃ to 20.8℃. We identified four periods (ca.6000 yr BP, 4000-3500 yr BP, 3000-2300 yr BP, and 800 yr BP) that were warmer than the present and distinct colder periods at ca.4500 yr BP and ca.1500 yr BP than the present. This SST variability for the past 7000 years was almost consistent with the record of relative abundance of F.doliolus, which is the dominant species in the Tsushima Current (Koizumi et al., 2006). This finding indicates that the Tsushima Current influx might have changed with time and altered the heat transport into the Japan Sea, and probably induced significant changes in terrestrial precipitation and vegetation over the northern part of Japan facing the Japan Sea.