Paleohydrography of the Kuroshio Extension in the Tohoku Area based on fossil diatoms

抄録

The most complicated hydrographic conditions are encountered in the Perturbed Area, where numerous eddies and thermohaline fronts are irregularly distributed between the Kuroshio Extension and Oyashio Fronts at Lat. 38° to 41°N. These features brought the abundance of fishery resources on the northern side of the Kuroshio Extension due to the multiplication of phytoplankton by upwelling. The paleohydrography of the Tohoku Area since 150 ka was reconstructed based on the high-resolution analyses of fossil diatom assemblages in seven cores. Diatom assemblages in the southern margin of the Perturbed Area have been controlled mainly by the Kuroshio and Kuroshio Extension, but in the northern margin they changed in responding to the expansion of the warm-water eddies detached from the Kuroshio Extension and of the southward Oyashio Intrusion. Diatom abundances are higher in the area near shore than in the area off shore, and the diatom assemblages were affected more seriously by the environmental changes in the area near shore. The abundances of extinct diatoms increase in the glacial phase, and diatom abundances and Td' values increase in the interglacial phase. The variation of oceanic diatom abundance is pronounced at 41-kyr and 23-kyr periods, and also Td' values predominating at 60-kyr, 30-kyr, and 23-kyr periods. The seawater mixing during the interval from glacial to interglacial transition induced by orbital-obliquity cycles played an important role in the primary production of oceanic diatoms. The predominant occurrences of extinct diatom species Melosira albicans are accompanied by that of Pseudopodosira elegans throughout the cores.