17:15 〜 18:30
[MIS17-P07] IODP Site U1426の更新世貝形虫と古環境の推定
キーワード：日本海、貝形虫、更新世、深海
In the Sea of Japan, the Quaternary glacial-interglacial climate changes have affected deep ocean floor environments as well as coastal environments and biota [e.g., Kitamura and Kimoto, 2006, Palaeogeogra., Palaeoclim., Palaeoeco., 236: 355–366; Watanabe et al., 2007, Palaeogeogra., Palaeoclim., Palaeoeco., 247, 50 – 64]. Little is known about the Pleistocene deep-sea ostracodes and their response to environmental changes. We present Pleistocene ostracodes at IODP Site U1426 (37°2.00′N, 134°48.00′E; 903 m water depth). Twenty-one taxa were obtained from 87 Pleistocene (2.0–1.2 Ma) sediments of the interval between 123.50 and 218.14 mcd. Ostracodes occur from sediment samples with L* values of more than 33. Benthic ostracode accumulation rates (BOARs) exhibit less than 14 ind/cm2/kyr. At 180 mcd, BOARs show a change: BOARs indicates maxima of 0.6–4.2 ind/cm2/kyr above 180 mcd (~1.7Ma), whereas BOARs show maxima of 1.7–14 ind/cm2/kyr below 180 mcd. Acanthocythereis dunelmensis, Krithe antiswanensis, Krithe hemideclivata, Krithe reversa, and Robertsonites tabukii are found ubiquitously. The ostracode fauna indicates oxic condition, alternation of export productivity, and possibly warm water-mass. Generally ostracodes are sensitive to oxygen contents. Their population vanishes in condition with oxygen contents of less than 0.4 ml/l O2[Dingle, 1995, Mar. Geol., 122, 207–225]. We infer that the sediments with high L* values (>33) were deposited at oxic condition (> 0.4 ml/l O2). Because BOARs are influenced by export productivity [Yasuhara et al., 2012, Paleobiol., 38, 162–179], the change in BOAR at 180 mcd suggests alternation of export productivity. According to Kitamura (2009, J. Quat. Sci. 24, 880–889), who discuss the surface and intermediate water-masses during the Pleistocene, the flow of the Tsushima current was intensified after 1.7 Ma. The change in surface water-mass probably made effects on surface productivity and benthic ostracodes. R. tabukii [= R. reticuliforma of Ozawa, 2003, Paleontol. Res., 7, 257–274], that lives in seafloor environments with water depth of 150–250 m and temperature of 2–5°C, indicates a warmer water-mass that the modern intermediate water with 0–1°C temperature.