Stratigraphic ranges of foraminiferal assemblages of the Miocene-Pliocene boundary sections in the Niigata oil-fields region of northeastern Japan are described, and their paleoceanographic implications are discussed on the basis of modern foraminiferal ecology and additional geochemical data from the Rock-Eval analysis.

Q-mode cluster analysis grouped five benthic foraminiferal associations named by their dominant taxa: the Guppyella miocenica-Cribrostomoides subglobosus, Karrerulina coniformis-C. subglobosus, Recurvoidella spadix-Cyclammina pseudopusilla, C. pseudopusilla-Uvigerina yabei, and Martinottiella communis associations. Paleoenvironmental implications of each association and their stratigraphic distributions show that the bottom water in the Japan Sea generally changed as follows. 1) In the Late Miocene, bottom waters were depleted in oxygen, cold, and nearly undersaturated with calcium carbonate, conditions consistent with weak communication of seawater between the Japan Sea and the Pacific Ocean. 2) In contrast, Pliocene bottom waters were oxic, occasionally warm and saturated with CaCO3, conditions consistent with enhanced communication of seawater between the Japan Sea and Pacific Ocean perhaps due to raised sea level. Late Miocene hypoxia, which is suggested by the occurrence of Spirosigmoilinella compressa and other abundant infaunal agglutinated foraminifera, may be due to water stratification induced by freshwater input from surrounding land-masses as well as slower influx of Pacific water.

Several synonym relationships are discussed to resolve taxonomic confusions that have prevented studies of biostratigraphy and paleoenvironment. A new species, Cyclammina pseudopusilla, which has been erroneously identified in previous reports, is described herein.

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