X-ray computed tomography of deep-sea clay as tools to detect rare earth elements and yttrium enrichment

Pelagic clay that contains rare-earth elements and yttrium (REY) at over 2000 ppm (termed “highly REY-rich mud”) has been reported from a few areas in the western North Pacific Ocean through focused geochemical analyses. Those REY are carried by biogenic calcium phosphate, suggesting that the formation of highly REY-rich mud involves enhanced biological productivity. However, detecting REY enrichment in sediment is time-consuming, and the global significance of the formation of REY-rich mud is still under debate. Here we perform x-ray computed tomography (CT) analyses on cores recovered around Minamitorishima Island and demonstrate the positive correlation with highly REY-rich mud and high CT numbers of the sediment. The variation of the CT number matches the wet bulk density. Further quantitative analyses using direct measurements of density and geochemistry suggest that a grain density increase and a porosity decrease due to the REY-carrying biogenic apatite are essential to explain the high CT numbers in the REY-enriched layer. While the chemical composition itself is of minor importance to elevate the CT numbers, our results suggest that x-ray CT can be a proxy for highly REY-rich mud.