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The tremendous potential of deep-sea mud as a source of rare-earth elements | Scientific Reports

After the KR13-02, MR13-E02, and KR14-02 cruises, three additional research cruises (MR14-E02, MR15-E01 Leg 2, and MR15-02) were conducted to reveal the detailed distribution of highly to extremely REY-rich mud in the southern part of the Minamitorishima EEZ (Fig. 1 ) 8 , 9 . During these cruises, REY-rich mud having a maximum of almost 8,000ppm of total REY content (REY) was confirmed. We estimated the resource amount of REY in the region bounded by 2148N to 2215N and 15330E to 15407E (about 2,500km2) by using whole sediment chemical data of newly analysed 573 samples and previously reported 104 samples (KR13-02 PC05: 82 samples 8 and KR13-02 PC06: 22 samples) 9 from 25 sampling points (Supplementary Tables S1 S2 , Fig. 1 ). Geographical Information System software (ArcGIS) was used to visualise the REY-rich mud distribution and evaluate its resource potential. REY maps of the average concentration values from the seafloor to 10 meters below the seafloor (mbsf) and of the values for each 1m depth interval are shown in Fig. 2 . The REY map was further divided into 24 grid squares (rows A to D and columns 1 to 6, A1D6). The calculated REY values and resource potential of each grid are listed in Table 1 . In addition, Supplementary Table S3 shows the average REY and the total resource amount from the seafloor to each target depth. There is a vast (over 400km2) area high in REY in the northwest part of the research area (see the panels for 56 and 67mbsf in Fig. 2 ), which continues loosely to the southeast (see the average panel map in Fig. 2 ). REY is relatively low in the basin in the middle of the southern area and in the topographical high area in the northeast. The calculated REY for the entire research area is more than 16 million tons of rare-earth oxides (Mt-REO) (average REY=964ppm). In addition, the mud is especially enriched in Y and HREE, which accounted for 44% (Y: 4.4Mt-REO; HREE: 2.6Mt-REO) of the total amount of REY in this region. The research area was estimated to be able to supply Y, Eu, Tb, and Dy for 780, 620, 420, and 730 years, respectively, and has the potential to supply these metals on a semi-infinite basis to the world 11 . Of the divided areas, B1 (9.9km10.6km=105km2) shows the highest REY resource potential, with an average REY of more than 1,700ppm. This area includes MR15-02 PC01, where extremely REY-rich mud was confirmed, and the REY of the 56mbsf interval exceeded 5,600ppm. The resource amount of area B1 was estimated to be 1.2Mt-REO, which would account for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively (Supplementary Table S4 ) 11 .