A research team led by Professor Minoru Ikehara of the Center for Advanced Marine Core Research, Kochi University, has conducted a study to elucidate the actual conditions of fluctuations in the Kuroshio Current from the past to present and their relationships to climate change in East Asia based on seafloor sediments in the southern waters of the Japanese Archipelago and the East China Sea. This preliminary report shows the results of a drilling expedition conducted from August 22-31, 2021 onboard the deep-sea scientific drilling vessel, Chikyu (*1) under the Shallow Core Program (*2). The program is jointly overseen by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Japan Drilling Earth Science Consortium (*3).

※Key Points

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Continuous strata were succesfully recovered, allowing fluctuations in the Kuroshio Current to be restored and analyzed over an approximately 250,000-year period

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The continuous strata were also used to elucidate the mechanism(s) by which turbidites (*4) are generated on the shelf and slope offshore of Shikoku, as well as the frequency of their occurrences

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An educational vessel-boarding framework (*5) was implemented for the first time, wherein graduate students wishing to conduct shipboard research were recruited nationwide, allowing the next generation of researchers to train on a drilling vessel

1. Details of Implementation

The Kuroshio is the western boundary current that comprises subtropical circulation in the North Pacific Ocean and is one of the largest warm currents in the world, along with the Gulf Stream in the North Atlantic Ocean. It is responsible for carrying massive amounts of heat stored as a result of strong tropical solar irradiance, from pools of warm water in the Western Pacific Ocean northward into the mid-latitudes. In this way, the Kuroshio is associated with the distribution of heat, salt, and water vapor in the North Pacific Ocean, and it has a substantial impact on climate change and precipitation patterns (including intensity and fluctuations) in the Japanese Archipelago and East Asia.

Of the glacial and interglacial periods that have alternated with a frequency of approximately 100,000 years, the interglacial period estimated to have been warmer than pre-industrial climatic conditions is called a “super-interglacial”(*6) and has attracted considerable research attention as an analog for estimating a future warming global environment. Clarifying the state of the Kuroshio Current during this period using geological records is important because it can provide evidence of what happened on Earth in the past with regards to the potential effects of global warming, which is anticipated to further accelerate in the future, affecting both the Kuroshio and the global climate, including Japan. In the study summarized here, piston core drilling was conducted onboard the Chikyu offshore of Shikoku, where traces of fluctuations in the Kuroshio Current from the past to the present were expected to have been recorded in the seafloor sediments. A continuous stratigraphic interval was successfully collected, from which such fluctuations could be restored over an approximately 250,000-year period.

2. Summary of Results and Future Developments

A total of three holes were drilled at the Site C9037(Fig. 1) by Chikyu, and a total of 300 m of cores approximately 100 m each were recovered. The hydraulic piston coring system (*7) used involved the cutting shoe shooting the seafloor and repeatedly collecting columnar geological samples at intervals up to 9.5 m. In this case, sediment gaps occurred between each sample, so splicing the three cores collected at a single location enabled a virtually successive stratigraphic record with almost no gaps. The age of the lowest sections of the recovered strata was estimated to be between 250,000–290,000 years old based on shipboard microfossil (*8) analysis.

The drilling core was subjected to transmission imaging with an X-ray CT scanner and magnetic measurements were made with a multi-sensor core logger on board the Chikyu. After the end of the expedition, the core was transferred to the Kochi Core Center (*9), where it was stored in a refrigerated state. Since September 7,2021, core splitting, photography, color measurements, and visual core description of the drilling core have been conducted at the Kochi Core Center; these measurements have gradually clarified the state of the recovered sediments. The results of non-destructive measurements and visual core descriptions have shown that the strata extracted in this expedition formed a thick deposit of fine mud containing microfossils (Fig. 2), with the partial presence of a volcanic ash layer and turbidites. It is known that the strata below the seafloor are home to many living microorganisms, and lifting them out to the surface via drilling changes their surrounding environment; thus, their compositions are assumed to change constantly with repeated drilling. Sampling was also conducted in for the systematic and sequential verification of these subseafloor microorganisms in order to determine if and how the biological information in the strata has changed over time following drilling. Future goals in the summarized research program include plans for conducting international joint research to reconstruct and analyze fluctuations in the Kuroshio Current, especially super-interglacial fluctuations, over an approximately 250,000-year period by advancing specialized microfossil, sedimentological, geochemical, and microbiological analyses. Additionally, a detailed analysis of the patterns of deposition and composition of the volcanic ash layers and turbidites is expected to provide new insights into the frequency of large-scale flood events, which are thought to be the mechanism of turbidite generation, as well as into the effects of megathrust earthquakes that occur along the Nankai Trough.