A research group led by Associate Professor Keiji Horikawa of the University of Toyama, in collaboration with Nagoya University, the Ocean Research and Development Organization, and Kyushu University, analyzed fossils contained in the seafloor sediments of the Sea of Japan. For the first time, it was revealed that the closure of the Sea of Japan became stronger due to the rapid contraction and shallowening of the straits in the northeastern part of the sea.
Part of the Japanese archipelago split from Eurasia about 2500 million years ago, forming the basin that is now the Sea of Japan. Ten million years ago, western Japan was connected to the Korean Peninsula by land, but most of eastern Japan was below sea level, and there was a deep sea water exchange with the Pacific Ocean.However, since 1000 million years ago, the eastern Japan-Hokkaido region has gradually risen, and the strait between the Pacific Ocean and the Sea of Japan has shrunk and shallowed.Currently, the Sea of Japan exchanges only surface water with the open ocean connected by a strait shallower than 1000 m, and deep sea water is completely separated from the Pacific Ocean in the open ocean.The research group worked to elucidate the exact time and time scale when the closure of the Sea of Japan occurred.
In 2013, the research group collected seafloor sediments from the Sea of Japan (for the past 346 million years) on the 1000th Integrated Ocean Drilling Program (IODP) and conducted neodymium isotope ratio analysis of fish teeth / bone fragment fossils.As a result, about 450 million years ago, in a very short period of about 14 years, the strait in the Tohoku region that separates the Sea of Japan and the Pacific Ocean rapidly shrank and became shallower, and the closure of the Sea of Japan became stronger. Was clarified from geochemical data.Furthermore, after the shrinking sea became more closed, the Okhotsk Sea water began to flow into the Sea of Japan along with the strengthening of the counterclockwise circulation of the ocean, and the strait near Saharin connecting the Okhotsk Sea and the Sea of Japan closed 260 million. It also revealed that the inflow continued until about a year ago.
It is expected that the results of this research, which promotes the understanding of the formation of the Japanese archipelago and the Sea of Japan, will also contribute to the basic knowledge of disaster prevention, conservation, and development of the national land.
Paper information:[Geology] Late Miocene –mid-Pliocene tectonically induced formation of the semi-closed Japan Sea, inferred from seawater Nd isotopes
