亀尾 浩司

The fluctuating position of the boundary between the Kuroshio (warm) and Oyashio (cold) currents in the mid-latitude western North Pacific affects both heat transport and air–ocean interactions and has significant consequences for the East Asian climate. We reconstruct the paleoceanography of Marine Isotope Stages (MIS) 20–18, MIS 19 being one of the closest astronomical analogues to the present interglacial, through multiple proxies including microfossil assemblage data, planktonic foraminiferal isotopes (δ O and δ C), and foraminiferal Mg/Ca-based temperature records, from the Chiba composite section (CbCS) exposed on the Boso Peninsula, east-central Japan. Principal component analysis (PCA) is used to capture dominant patterns of the temporal variation in these marine records, and shows that the relative abundances of calcareous nannofossil and radiolarian taxa are consistent with the water mass types inferred from geochemical proxies. The leading mode (36.3% of total variance) mirrors variation in the terrestrial East Asian winter monsoon (EAWM), reflecting seasonal trends dominated by the winter monsoon system. In the CbCS, this mode is interpreted as reflecting the interplay between the warm Kuroshio and cold Oyashio waters, which is likely related to the latitudinal shift of the subtropical–subarctic gyre boundary in the North Pacific. The second mode (15.4% of total variance) is closely related to subsurface conditions. The leading mode indicates that MIS 19b and 19a are represented by millennial-scale stadial/interstadial oscillations. Northerly positions for the gyre boundary during late MIS 19c, the interstadials of MIS 19a, and early MIS 18 are inferred from the leading mode, which is consistent with a weak EAWM and consequent mild winter climate in East Asia. Nonetheless, the northerly positions for the gyre boundary during late MIS 19c and early MIS 19a were not associated with subsurface warming presumably due to the suppressed gyre circulation itself caused by the weak Aleutian Low. Intermittent southerly positions for the gyre boundary are inferred for the stadials of MIS 19b and 19a. Regional sea surface temperature (SST) comparisons in the western North Pacific reveal that the moderate SSTs during MIS 19a through early MIS 18 were restricted to the mid- to high latitudes, influenced by the weak EAWM. Comparison between MIS 20–18 and MIS 2–1 suggests that glacial MIS 20 and 18 had significantly milder winters than MIS 2, likely related to the relatively weak EAWM. [Figure not available: see fulltext.]. 18 13

The Executive Committee of the International Union of Geological Sciences on January 17, 2020 ratified the Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage/Age and Middle Pleistocene Subseries/Subepoch at the Chiba section of the Chiba composite section, Japan. The Chiba composite section is a continuous and expanded marine sedimentary succession in the east-central Japanese archipelago facing the Pacific Ocean. It contains well-preserved pollen, marine micro-and macrofossils, a tightly-defined Matuyama-Brunhes (M-B) paleomagnetic polarity boundary, two geomagnetic field paleointensity proxies, and numerous tephra beds, allowing the establishment of a robust and precise chronostratigraphic framework. Its open-ocean continental slope setting has captured both terrestrial and marine environmental signals from upper Marine Isotope Stage (MIS) 20 to lower MIS 18. The M-B reversal serves as the primary guide for the Lower-Middle Pleistocene boundary, yield ing an astronomical age of 772.9 ka. The GSSP is positioned 1.1 m below the directional midpoint of the reversal, at the base of a regional lithostratigraphic marker, the OntakeByakubi-E (Byk-E) tephra bed, in the Chiba section. The GSSP has an astronomical age of 774.1 ka and occurs immediately below the top of Marine Isotope Substage 19c.

房総半島北東部の銚子地域に分布する鮮新–更新統犬吠層群は微化石を豊富に含む半遠洋性堆積物であり，これまでいくつかの年代層序学的研究が行われてきた（酒井，1990）．この犬吠層群を対象として1998年に陸上ボーリングコアが東京大学海洋研究所によって掘削され，微化石層序，古地磁気，酸素同位体比から年代層序が確立され, 海洋酸素同位体比ステージ（MIS）11～24であることが明らかになっている (Kameo et al., 2006)．このコアには，いわゆるEarly-Middle Pleistocene Transition (EMPT)を含み，氷期・間氷期サイクルが4万年周期から10万年周期へと変動する時代の環境変化を研究するためには非常によい試料である．特にMIS 19については離心率が現在と同じような状況に近いことから将来の気候変動予測に重要な時代であり（Head and Gibbard, 2015），気候変動と関連した海洋循環の消長を考える上で重要な地域といえる．そこで本研究では，浮遊性有孔虫化石群集を用いることによって，数千年スケールでの古海洋環境変動を推定することを試みた．本研究では犬吠層群に相当する地層のうち，横根層上部に相当する深度130～160 mを使用した．この層準に該当する年代は，MIS 17～19である．検討した試料間隔は，約1 mで，時間分解能は約2000～3000年である．乾燥させた各試料を熱湯で処理，ボロン法を用いることで泥化処理を行った. その後125 µmのふるいにかけて浮遊性有孔虫化石が200個体程度になるまで分割して顕微鏡下で全て拾い出し, 種の同定を行った. 検討を行った試料から15属42種の浮遊性有孔虫化石が認められた．全層準を通じて,Neogloboquadrina incomptaが高い産出頻度を占めし，次いで, Globigerinita glutinata, Turbolotalita quinqueloba, Globorotalia inflata などが高い産出頻度を示す．このことから, 銚子地域は当時ほとんど混合水域の環境下にあったと考えられる．しかし，黒潮水域に優勢なGlobigerinoides ruber, Pulleniatina obliquiloculataについては酸素同位体比曲線の温暖なピークと概ね一致することから，温暖時には黒潮の影響を強く受けていたことが考えられる．一方，親潮水域に優勢なNeogloboquadrina pachydermaの左巻も寒冷のピークと概ね一致するが, 産出頻度はそこまで高くないことから，銚子地域では親潮の影響は寒冷な時でもそこまで強い影響はなかったと考えられる. T. quinquelobaについては混合水域だけでなく塩分の低い沿岸水の指標とされている(Takemoto and Oda, 1997). T.quinquelobaが高い産出頻度を示した層準は，氷期への移行期に相当するように見えるので，寒冷化に伴って海水準の低下が起こり，その結果，西方からの沿岸水の影響が強くなったことが考えられる．以上より氷期・間氷期サイクルより短い時間スケールでの黒潮・親潮の南北振動が銚子地域で起こっていたと考えられる. 引用文献 Head and Gibbard,2015,Quat.Int.,389,7-46 五十嵐ほか，2002,地質学会講演要旨 Kameo et al.,2006,Island Arc,15,366-377 Takemoto and Oda,1997, Paleontolog. Res.,1,4,291-310 酒井,1990,宇都宮大教育紀要,23,1-34