Late Cenozoic Planktonic Foraminiferal Biostratigraphy of Northwest Pacific Sedimentary Sequences.

摘要

A combined planktonic foraminiferal and paleomagnetic stratigraphy extending back to about 1.8 million years B.P. in northwest Pacific deep-sea cores is compared with foraminiferal bisostratigraphy of Late Cenozoic sediments in oil-producing region of Japan. Species belonging to two rapidly evolving lineages of planktonic foraminifera and systematic changes in the coiling direction of Globigerina pachyderma provide datum levels for stratigraphic correlation. Since the sinistral population of G. pachyderma are known to occur in high northern latitudes, the sudden appearance of the sinistrally coiled G. pachyderma at about 1.2m.y. B. P. indicates considerable temperature decline in the North Pacific, possibly coincident with the initiation of mid-latitude glaciations of Europe and North America. Eustatic lowering of sea level exposed the shallow Korea Strait as a land bridge, terminating the flow of warm Pacific waters into the Sea of Japan. The curtailment substantially lowered water temperatures of the Sea of Japan, bringing the left-coiling populations of G. pachyderma into areas formerly inhabited by the right-coiling populations.