Change in the thickness of the warm Tsushima Current at the initiation of its flow into the Sea of Japan

The initiation of flow of the warm Tsushima Current created significant changes in Quaternary paleoceanography and ecosystems in the continental shelf of the Japanese islands in the Sea of Japan. In order to understand the behavior of the current at the initiation of its flow, we reconstruct the change in the relative thickness of the current during the initiation of flow based on combining planktonic and shallow-water molluscan fossil analyses of the Early Pleistocene Omma Formation in central Japan. The result indicates that warm-water planktonic foraminifera and cold-water molluscs coexisted during about 2300–2400 years just after the initiation of the intrusion of the Tsushima Current. Migration of a warm-water molluscan fauna to the shelf occurred shortly after the local extinction of cold-water molluscs. This means that the rate of increase in thickness of the Tsushima Current during the initiation of flow was faster than that of the sea-level rise. Using deep-sea core records and modern biogeographic data, the rates of the former and latter are calculated to be 17.2±8.9 m per 1000 years and 1.9±1.1 m per 1000 years, respectively. Because bottom water change might lag behind surface water change during a rapid climatic change in all shallow water, combining planktonic and benthic fossil analyses is necessary to reconstruct shallow paleoceanography.