The dating of REY- rich deep sea sediments in the Pacific by in situ Sr and U-Pb isotope stratigraphy of bioapatite

The formation model of REY-rich deep-sea sediments in the Pacific is attempted to be established. It is considered that since late Oligocene, the deep Antarctic circumpolar current formed with the opening of the Drake Passage, and Lower Circumpolar Water (LCDW) began to affect the Pacific. The LCDW led the upwelling the nutrients to the surface ocean around the seamount simultaneously delaying the rate of sediment deposition. During the early to middle Miocene, when the LCDW intensity was moderate, the REY in the seawater enriched in the fish teeth bioapatite of the pelagic clay and zeolite clay under the carbonate compensation depth (CCD)and with low sedimentation rate. • Deep-sea sediments rich in REEs and yttrium (REY) are important economic resources. • The age of REY-rich deep-sea sedimentary layers in the western and middle Pacific were estimated to be 26–15 Ma. • Lower Circumpolar Water (LCDW) may had an important influence on the development of the REY-rich sedimentary layers. REY-rich (rare earth elements and yttrium) deep-sea sediments are potential rare earth resources and REY enrichment is believed to be associated with the fish teeth bioapatite. However, the mechanism of REY enrichment is still poorly understood, partly due to the ambiguous geochronology of REY-rich sediment layers. In this study, we conducted in situ Sr isotope stratigraphy of fish teeth bioapatites from three sediment piston cores collected from the western and middle Pacific. Additionally, in situ U-Pb dating of bioapatites from the typical layers were performed to further constrain Sr isotope stratigraphy. Our results indicate that REY-rich deep-sea sedimentary layers (REY > 1000 μg/g) were deposited between 26 and 15 Ma, consistent with the U-Pb geochronology. Coincidentally, this period coincides with the development of the Lower Circumpolar Deep Water (LCDW), suggesting a potential linkage between LCDW circulation and REY enrichment processes. Furthermore, the late Oligocene to middle Miocene (∼26–15 Ma) was characterized by a sustained warm climatic regime. We propose that these elevated temperatures may have enhanced pelagic fish productivity. The LCDW could transport these fish debris to favorable depositional environments and then fish debris were transformed into REY-enriched bioapatites after prolonged diagenesis, which became concentrated in deep-sea sediments.