[M-IS08] 古気候・古海洋変動

[MIS08-P24] 最終氷期以降の東シナ海における底生有孔虫の炭素・酸素安定同位体比変動

*村中 美樹¹、岡崎 裕典²、久保田 好美³ (1.九州大学理学部地球惑星科学科、2.九州大学大学院理学府研究院地球惑星科学部門、3.国立科学博物館)

The continental shelves shallower than 200 m water depths occupies more than 70 % of the area of East China Sea (ECS). During the last glacial maximum (LGM), sea-level dropped by 120 m. Therefore, vast area of the ECS continental shelves were aerially exposed. Here we present carbon and oxygen isotopes of benthic foraminifera in sediment core from the Danjo Basin, northern ECS (KY07-04-PC01, 31°38.35’ N, 128°56.64’E, 758 m water depth). Foraminiferal shells are composed of calcite (CaCO₃). Carbon isotope ratio (δ¹³C) of benthic foraminifera is used as a proxy for nutrient concentrations Therefore, benthic foraminiferal δ¹³C is useful for reconstructing past water mass structures. Oxygen isotope ratio (δ¹⁸O) of benthic foraminifera is used as a proxy for ice volume and water temperature. Two benthic foraminiferal taxa Uvigerina spp. and Hyalinea balthica were picked up from KY07-04-PC01 sample and were used for δ¹³C and δ¹⁸O measurements. The δ¹³C of Uvigerina spp. during LGM was ~2 ‰ heavier than those of Holocene. This offset can be explained by oscillation of two water masses in ECS: cold and nutrient-rich Chinese coastal water was prevailing in glacial semi-closed ECS; warm and nutrient-depleted Kuroshio water increased since deglaciation with sea-level rise. The δ¹⁸O of Uvigerina spp. gradually decreased from 3.76 ‰ during LGM to 2.39 ‰ during late Holocene. The 1.36 ‰ δ¹⁸O offset is caused by ice volume and water temperature. Because the ice volume effect for benthic δ¹⁸O between LGM and present is estimated to be 1.1 ‰, the residual 0.27 ‰ is equivalent to water temperature change of 1 ℃. This suggests intermediate water temperature in northern ECS has risen by 1 ℃ since LGM. There are two morphology in Uvigerina spp., with more spiny and less spiny. Because offsets of δ¹³C and δ¹⁸O between the two morphologic types, it is desirable to measure separately. Hyalinea balthica showed ~1 ‰ lighter δ¹³C and δ¹⁸O values than those of Uvigerina spp. In addition, it was time-consuming for cleaning Hyalinea balthica shells. Thus, H. balthica is not a proper benthic foraminiferal species for δ¹³C and δ¹⁸O measurements.