DINOFLAGELLATE CYSTS AND QUATERNARY PALEOENVIRONMENT IN THE IZU-BONIN REGION, NORTH-WEST PACIFIC (ODP Leg 125, site 782A and Leg 126, site 791B)

摘要

DINOFLAGELLATE CYSTS AND QUATERNARY PALEOENVIRONMENT IN THE IZU-BONIN REGION, NORTH-WEST PACIFIC (ODP Leg 125, site 782A and Leg 126, site 791 B) Fifty species of dinoflagellate cysts, among them two new species, have been identified in oceanic sediments of the Izu-Bonin region, south of Japan. All these species have a paleoenvironmental significance. The Quaternary climatic oscillation and environment evolution in the NW Pacific have been demonstrated by the variation of dinoflagellate cyst assemblages and the contents pollen grains in the arc sediments of the Izu-Bonin region. The variation of species ratios: temperate-warm to temperate-cold dinoflagellate cysts reveals the cyclic temperature changes of sea surface waters. The data are used to reconstruct the climatic evolution since 1,79 Ma ago. The results of the present study and a consideration of relevant sedimentological factors allow us to draw the following conclusions concerning the environment of the Izu-Bonin region. It appears that the carbonate concentration in sediments was higher in the temperate-cold than in the temperate-warm phases. In comparison with the ridge of outer arc, the axial region of the inner arc rift is characterized by greater proportion of volcaniclastic materials as well as a higher sedimentation rate and the palynological contents by high percentages of coniferous pollen. These particularity suggest a possible effect of the volcanic activity on the palynological contents in the intra-arc basin since I Ma ago. Thus the axial region of the inner arc has maintained outside of the influence of warm current Kuroshio since 0.46 Ma ago; this is reflected by the very high abundance of Impagidinium patulum to Impagidinium aculeatum.