We have shown that the seawater osmium isotope composition (187Os/188Os), a proxy for the solid earth’s response to climate change, has varied during the past 300,000 years in association with glacial–interglacial cycles. Our marine Os isotope mass-balance simulation reveals that the observed Os isotopic fluctuation can be reproduced by taking account of short-term inputs of (1) radiogenic Os derived from intense weathering of glacial till during deglacial periods and (2) unradiogenic Os derived from enhanced seafloor hydrothermalism triggered by sea-level falls associated with increases of ice sheet volume. Our results constitute the first evidence that ice sheet recession and expansion during the Quaternary systematically and repetitively caused short-term (< 105 yr) solid earth responses via chemical weathering of glacial till and seafloor magmatism. This finding implies that climatic changes on < 105 yr timescales can provoke rapid feedbacks from the solid earth, a causal relationship that is the reverse of the longer-term (> 106 yr) causality that has been conventionally considered.
We have shown that the world's highest grade rare earth elements and yttrium (REY)-rich mud off the coast of Minamitorishima Island may have been generated by deep ocean currents associated with the formation of the Antarctic ice sheet 34.5 million years ago. Deep-sea bottom currents enhanced by Antarctic cooling hit huge seamounts and generated upwelling currents, bringing large amounts of nutrients to the surface, which led to a rapid increase in fish species around the seamounts. As a result, large amounts of fish bones were deposited on the seafloor, resulting in the formation of the extremely REY-rich mud. This is a groundbreaking research result, which indicates that global environmental changes during the Cenozoic era caused the accumulation of these critically important elements on the seafloor through ocean dynamics.
We have established the world's first method to visualize and calculate the area of ferromanganese oxide deposits on the deep seafloor using sound waves emitted from a research vessel. As a result, we found that manganese nodules are densely distributed in Japan’s Exclusive Economic Zone around Minamitorishima Island over at least 61,200 km2 (about the size of Shikoku and Kyushu Islands combined).
Using geochemical indicators such as osmium isotope ratios, we found an ejecta layer derived from a meteorite impact in the 11.6-million-year-old deep-sea sediments collected on a research cruise for rare earth elements and yttrium (REY)-rich mud around Minamitorishima Island. This is likely to be a signature of a previously unknown meteorite impact that hit the ocean.
We succeeded in visualizing the distribution of rare-earth resources in the southern part of the Exclusive Economic Zone near Minamitorishima Island, a promising area suitable for rare earth elements and yttrium (REY)-rich mud development. In an area of approximately 105 km2 with a particularly high concentration of rare earth elements, the amount of REY resources reached approximately 1.2 million tonnes (as REY-oxides), and the amounts of industrially important elements dysprosium, terbium, europium, and yttrium were found to be equivalent to 57, 32, 47, and 62 years of current global consumption. Furthermore, we demonstrated that the total REY concentration in REY-rich mud was increased up to 2.6 times by the selective recovery (by size separation) of biogenic calcium phosphate particles (e.g., fish teeth and bone fragments), which contain most of the REY in the deep-sea mud.
We have reconstructed a detailed record of recurrent, rapid and short-term global warming events from deep-sea sediments in the Indian Ocean, which occurred repeatedly about 56–52 million years ago. Furthermore, statistical analysis of these chemical composition data showed that, in multiple global warming events that occurred at that time, a mechanism described as “negative feedback of Earth’s system ” functioned to remove excess carbon dioxide from the atmosphere-ocean system through increased production of marine surface organisms, thus ending the warming.
Dense field of ferromanganese nodules found in Japanese Exclusive Economic Zone around Minamitorishima Island
Combining an onboard acoustic survey and a submarine survey conducted in the manned research submersible SHINKAI 6500, we discovered a large area of ferromanganese (Fe-Mn) nodules rich in industrially critical metals such as cobalt and nickel in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island. This is the first time that a vast dense area of Fe-Mn nodules has been found in Japan’s EEZ. This study demonstrated that the multi-narrow beam echo sounder system is an effective and economical method to identify the distribution of Fe-Mn nodules on the deep seafloor.
The chemical composition data of 3968 deep-sea sediment samples from 101 sites covering a wide area in the Pacific and Indian oceans were analyzed by a multivariate analysis method called independent component analysis. The following three components were found to be closely related to the formation of rare earth elements and yttrium (REY)-rich mud: Fe-oxyhydroxides from seafloor hydrothermal activities, Mn-oxides that slowly precipitate from seawater, and calcium phosphates originating from the teeth and bones of marine vertebrates. Furthermore, it was estimated that an environmental condition of very slow sedimentation (only 0.5 m per million years) is commonly sufficient to produce mineable REY concentrations in the sediments.
Several artificial hydrothermal vents were formed by the Drilling Vessel Chikyu in the Okinawa Trough. Subsequent visits by research cruises have shown that sulfide chimneys, a mineral ore deposit enriched in valuable elements such as copper, lead and zinc, grew to a height of 15 m within two years after drilling. The results show the possibility of artificially creating deposits of useful metal resources by drilling in a hydrothermally active area.
Using Re-Os radiometric dating, we have shown that Bessi-type copper deposits, which are distributed in the Sanbagawa belt in Japanese accretionary complexes, were formed in the late Jurassic period, about 150 million years ago. This indicates that the highly active volcanic and hydrothermal processes of the mid-ocean ridge formed large-scale hydrothermal sulfide deposits. At the same time, such active hydrothermal processes could have led to a series of environmental changes: an increase in atmospheric carbon dioxide concentration, the disappearance of polar ice sheets, the cessation of general oceanic circulation, and the development of a global anoxic ocean. This anoxic ocean environment should have contributed to the preservation of submarine hydrothermal sulfide deposits and petroleum deposits. This is a groundbreaking research result that elucidates the close relationship between the generation of Earth’s resources and the past evolution of the marine environment.
Rare earth elements and yttrium (REY)-rich mud deposits with total REY concentrations exceeding 6500 ppm were found just 3 m below the seafloor in Japan’s Exclusive Economic Zone (EEZ) around Minamitorishima Island. This “extremely REY-rich mud” contains the world’s highest concentration of rare earth elements in seafloor sediments. It also contains up to 20 times more heavy rare earth elements (dysprosium, terbium, etc., which are particularly important for industrial applications) than onshore deposits in China. It is therefore considered to be an extremely promising source of REY that Japan can develop independently.
Discovery of a huge new deposit of rare-earth elements in the Pacific Ocean
We discovered that there is a vast amount of deep-sea mud containing very high concentrations of rare earth elements and yttrium (REY) in the eastern South and central North Pacific Ocean. This “REY-rich mud” has multiple advantages as a mineral resource: (1) the concentration of rare earth elements is very high (especially industrially important heavy rare earths); (2) there is a vast amount of REY resources, up to 1000 times greater than the reserves of onshore deposits; (3) they are relatively easy to explore; (4) they are virtually free of radioactive elements such as uranium and thorium, which would be an obstacle to development; and (5) most of the REY can be easily extracted with dilute acid at room temperature. Therefore, this REY-rich mud is an excellent mineral resource that may lead to a solution to the world’s shortage of rare-earth resources.