In living organisms21,23,39 (Fig. 4m). Therefore, the REY-enrichment process by biogenic Stattic site Ca-phosphate in pelagic sediments has long been studied by a number of investigators17,18,21?4,40?2. Some have suggested that biogenic Ca-phosphate incorporates REY via pore water in the sediment column23,40. Others have argued that Ca-phosphate takes up REY from a variety of carrier phases (e.g. Fe n-oxyhydroxides, pellets, and organic debris) that absorb REY from seawater and are readily decomposed at the sediment ater interface17,22,41,42. Although the complete process remains an open question, the characteristic REY patterns indicate that seawater is the ultimate source of the REY presently captured in biogenic Ca-phosphate22,25,41 (Fig. 4m). Simple quantitative estimation in this study suggested the possibility that seawater can contain the flux of REY precipitation required to explain the observed very high REY concentrations ( REY + Ce > 1,000 ppm) in bulk REY-rich mud with a sedimentation rate less than 0.5 m/Myr (Supplementary Fig. S19). Such circumstances may facilitate the concentration of REY in biogenic Ca-phosphate via diffusion of REY from seawater or the transfer of REY from original carriers to Ca-phosphate SIS3 custom synthesis during the early diagenetic processes because of the prolonged exposure to seawater at the sediment surface and in the bioturbated and well-ventilated uppermost sediment layer17,18,22,41. In addition, biogenic Ca-phosphate enriched in REY might be stabilised through recrystallisation as insoluble apatite17. Moreover, the amount of Ca-phosphate in a unit volume of sediment also increases with a depression of the sedimentation rate17,18. Hence, the low sedimentation rate is considered to be crucial for the formation of REY-rich mud. Actually, the spatiotemporal distribution of high-IC1, -IC4, and -IC7 muds overlapped with the oligotrophic North Pacific and South Pacific gyres and with water depths greater than the CCD, both of which prevented the fast accumulation of dilutive components with low REY content such as biogenic carbonate and silica. If we assume that deep-sea sediments can continuously acquire REY from the overlying deep-sea water prior to burial, the REY-enrichment in sediments can occur in a time scale of 105 years considering the sedimentation rate of <0.5 m/Myr with a typical thickness of 0.1 m for the well-ventilated uppermost sediment layer43. This timescale of REY enrichment is significantly longer than that of global ocean circulation, which is 103 years44. Both IC1 and IC4 indicated statistical independent geochemical features of pelagic red clay mainly composed of detrital aluminosilicates involving abundant Si, Al, Fe, Mg, and K without significant contributions of biogenic carbonate and silica. Fe and Mn of hydrothermal or hydrogenous origins could have also been incorporated without dilution in high-IC1 and high-IC4 sediments, resulting in an increase in the contents of these elements. In addition, in deposition slow enough to allow biogenic Ca-phosphate grains to concentrate REY and to accumulate significantly in the sediment, the P and REY contents of these sediments also increase concurrently. Although sediments enriched in biogenic Ca-phosphate contain several percent of Ca, the significant dilution effect of biogenic carbonate, which contains several tens of percent of Ca, generally creates negative trends in these ICs as a whole in the spaces of Ca and other elements, including REY. The differenc.In living organisms21,23,39 (Fig. 4m). Therefore, the REY-enrichment process by biogenic Ca-phosphate in pelagic sediments has long been studied by a number of investigators17,18,21?4,40?2. Some have suggested that biogenic Ca-phosphate incorporates REY via pore water in the sediment column23,40. Others have argued that Ca-phosphate takes up REY from a variety of carrier phases (e.g. Fe n-oxyhydroxides, pellets, and organic debris) that absorb REY from seawater and are readily decomposed at the sediment ater interface17,22,41,42. Although the complete process remains an open question, the characteristic REY patterns indicate that seawater is the ultimate source of the REY presently captured in biogenic Ca-phosphate22,25,41 (Fig. 4m). Simple quantitative estimation in this study suggested the possibility that seawater can contain the flux of REY precipitation required to explain the observed very high REY concentrations ( REY + Ce > 1,000 ppm) in bulk REY-rich mud with a sedimentation rate less than 0.5 m/Myr (Supplementary Fig. S19). Such circumstances may facilitate the concentration of REY in biogenic Ca-phosphate via diffusion of REY from seawater or the transfer of REY from original carriers to Ca-phosphate during the early diagenetic processes because of the prolonged exposure to seawater at the sediment surface and in the bioturbated and well-ventilated uppermost sediment layer17,18,22,41. In addition, biogenic Ca-phosphate enriched in REY might be stabilised through recrystallisation as insoluble apatite17. Moreover, the amount of Ca-phosphate in a unit volume of sediment also increases with a depression of the sedimentation rate17,18. Hence, the low sedimentation rate is considered to be crucial for the formation of REY-rich mud. Actually, the spatiotemporal distribution of high-IC1, -IC4, and -IC7 muds overlapped with the oligotrophic North Pacific and South Pacific gyres and with water depths greater than the CCD, both of which prevented the fast accumulation of dilutive components with low REY content such as biogenic carbonate and silica. If we assume that deep-sea sediments can continuously acquire REY from the overlying deep-sea water prior to burial, the REY-enrichment in sediments can occur in a time scale of 105 years considering the sedimentation rate of <0.5 m/Myr with a typical thickness of 0.1 m for the well-ventilated uppermost sediment layer43. This timescale of REY enrichment is significantly longer than that of global ocean circulation, which is 103 years44. Both IC1 and IC4 indicated statistical independent geochemical features of pelagic red clay mainly composed of detrital aluminosilicates involving abundant Si, Al, Fe, Mg, and K without significant contributions of biogenic carbonate and silica. Fe and Mn of hydrothermal or hydrogenous origins could have also been incorporated without dilution in high-IC1 and high-IC4 sediments, resulting in an increase in the contents of these elements. In addition, in deposition slow enough to allow biogenic Ca-phosphate grains to concentrate REY and to accumulate significantly in the sediment, the P and REY contents of these sediments also increase concurrently. Although sediments enriched in biogenic Ca-phosphate contain several percent of Ca, the significant dilution effect of biogenic carbonate, which contains several tens of percent of Ca, generally creates negative trends in these ICs as a whole in the spaces of Ca and other elements, including REY. The differenc.
