Atmosphere and Ocean Research Institute, The University of Tokyo

Graduate School of Science, The University of Tokyo

Summary

During the late Miocene (approximately 7.9 to 5.8 million years ago), when the Northern Hemisphere ice sheet expanded, various organisms became extinct, while the oldest hominids emerged and a modern-type ecosystem was established. During the same period, the marine environment and ecosystem also changed drastically in the Sea of Japan, and the process of change has been the subject of ongoing debate (Figure).

A research team led by Assistant Professor Kenshi Matsuzaki of Atmosphere and Ocean Research Institute, The University of Tokyo, and Associate Professor Masayuki Ikeda of Graduate School of Science, The University of Tokyo, measured radiolarian fossil production rates in deep-sea sediment cores in the Japan Sea on the order of million years to reconstruct changes in ocean circulation in the Japan Sea and pointed out that this may have contributed to the extinction of fossil biota.

After 7.5 million years ago in the late Miocene, the influence of the Pacific Bottom Water weakened, while the influence of the North Pacific Middle Water strengthened. This is attributed to the strengthening of the North Pacific Middle Water associated with the expansion of the Northern Hemisphere ice sheet and the intensification of the winter monsoon, which agitated the stratified Japan Sea and enhanced upwelling (Figure).

Furthermore, around Japan, not only some radiolarians but also the giant fossil shark Megalodon and the marine mammal Desmostylus became extinct and were replaced by modern marine ecosystems such as diatoms and dolphins. This study is the first to examine the relationship between the establishment of modern marine ecosystems during the Miocene, changes in ocean circulation, and global cooling. However, since there are not many records of the production of large fossils, it is expected that a deeper understanding of the history of the establishment of modern-type ecosystems will be obtained by integrating paleoinvertebrate studies in the future.

The results of this study were published in the online edition of Scientific Report on July 20, 2022 at 10:00 a.m. (British Summer Time).

Figure: (A) Present-day ocean circulation in the Japan Sea and (B) water mass depth distribution in the Northwest Pacific Ocean and the depths of the radiolarian species used in this study. (C) Ocean circulation in the Sea of Japan from 9 to 7 million years ago and (D) 7 million years ago estimated from Japanese paleogeography and radiolarian fossils.

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