2021 Volume 72 Issue 6 Pages 459-477
The paleoceanic environment of the Japan Sea during the middle to late Miocene has been previously considered to be dominated by cold water masses. However, calcareous microfossil data indicate the presence of short-term warm periods with intermittent warm current inflows into the Japan Sea during this period. One of evidence for this short-term warm period is provided by planktonic foraminifera and calcareous nannofossils from the Funakawa Formation in Shimo-arakawa, Daisen City, Akita Prefecture. Diatom chronostratigraphic analysis of this study confirms that this warm-water calcareous microfossil assemblage can be assigned near the upper limit of the diatom zone NPD6A at ca 8.7 Ma. The occurrence of warm-water diatoms shows that this warm period corresponds to the interval from the uppermost NPD6A to the lowermost NPD6B, and roughly correlated to the O-1 assemblage, which is one of the calcareous foraminiferal assemblages including warm-water species in the Japan Sea. This suggests that weak warm currents flowed from the Pacific Ocean side into the Japan Sea, where cold surface water was dominant at that time.
