有孔虫の殻の酸素・炭素同位体比に基づく過去１５万年間の日本海と鹿島沖の環境変化

Abstract

Five distinct paleoenvironmental changes in the Japan Sea over the last 95 ka are revealedfrom the oxygen and carbon isotopes of foraminiferal tests in a piston core recovered from the Oki Ridge. Between 95 and 27 ka, the warm Tsushima Current did not flow into the Japan Sea.The environment at the seafloor fluctuated between anoxic to weakly oxic conditions. Between 27and 17 ka, freshwater input to the Japan Sea, probably from the surrounding land, stratified the water column, and the resulting severe anoxic conditions eliminated most benthic fauna. Between 17 and 10 ka, the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Between 10 and 8 ka, bottom conditions changed from anoxic to oxic. At 10 ka, the warm Tsushima Current started to flow into the Japan Seathrough the Tsushima Strait, establishing the modern oceanographic regime at 8 ka.From high-resolution oxygen and carbon isotope analysis of both benthic and planktonic foraminifera from an IMAGES core collected from off Kashima, central Japan, very large (20°C) sea surface temperature (SST) fluctuations can be reconstructed for the last 150 kyr, with minimum SSTs of 3-4°C during Marine Isotope Stage (MIS) 2 and MIS 6/5e transition, and with peak SSTs of 22-23°C during early MIS 1 and MIS 5a/4, 5c/5b, and 5e/5d transitions. The SSTs varied in parallel with changing carbon isotope differences between Globorotalia inflata and Globigerina bulloides, which suggests that the SST changes were primarily caused by latitudinal displacements of the Kuroshio-Oyashio Currents. A strong correlation between the SST shifts and orbital forcing indicates that latitudinal displacements of the Kuroshio-Oyashio Currents were influenced by summer insolation at 65°N.