Sources and enrichment processes of rare earth elements and yttrium in fish bones from deep-sea sediments: A study from the Wake Seamounts area in the western North Pacific Ocean

Fish bones (bio-phosphate) are the primary carrier of rare earth elements (REE) and yttrium (Y), collectively termed REY, in deep-sea sediments. However, the specific processes that contribute to the enrichment of REY in bio-phosphate remain unclear. In this study, we perform integrated analyses of sediments and individual minerals of bio-phosphate along the core profile of a sediment column, numbered GC12, which was retrieved from the western North Pacific Ocean. We find that strontium (Sr) and neodymium (Nd) isotopes of bio-phosphate offer a comprehensive and detailed understanding of REY sources and enrichment processes. Living fish absorbs Sr from seawater into their bones. After death, when bones settle down through the water column, they begin to absorb REY from seawater, which are released by dissolved volcanic ash. When bones settle down on the seafloor, at the initial stage of diagenesis (depths < 100 cmbsf), they preferentially absorb Sr relative to REY due to the locally high Sr concentrations created by the rapid dissolution of volcanic ash. At the advanced stage of diagenesis (depths > 100 cmbsf), where volcanic ash is exhausted, the enrichment of REY in bones occurs through replacing Sr derived from volcanic ash with REY from the dissolution of terrigenous debris. Furthermore, we find the sorting effect caused by abrupt changes in sedimentary conditions may enhance the accumulation of bio-phosphate in sediments, promoting the enrichment of REY in sediments. The findings of this study may broaden the application of bio-phosphate in marine geoscience.