Rare earth element geochemistry characteristics of seawater and porewater from deep sea in western Pacific

Yinan Deng,Jiangbo Ren,Chenhui Liu,Haifeng Wang,Qingjun Guo,Jun Cao

doi:10.1038/s41598-017-16379-1

Abstract

Deep-sea sediments contain high concentrations of rare earth element (REE) which have been regarded as a huge potential resource. Understanding the marine REE cycle is important to reveal the mechanism of REE enrichment. In order to determine the geochemistry characteristics and migration processes of REE, seawater, porewater and sediment samples were systematically collected from the western Pacific for REE analysis. The results show a relatively flat REE pattern and the HREE (Heavy REE) enrichment in surface and deep seawater respectively. The HREE enrichment distribution patterns, low concentrations of Mn and Fe and negative Ce anomaly occur in the porewater, and high Mn/Al ratios and low U concentrations were observed in sediment, indicating oxic condition. LREE (Light REE) and MREE (Middle REE) enrichment in upper layer and depletion of MREE in deeper layer were shown in porewater profile. This study suggests that porewater flux in the western Pacific basin is a minor source of REEs to seawater, and abundant REEs are enriched in sediments, which is mainly caused by the extensive oxic condition, low sedimentation rate and strong adsorption capacity of sediments. Hence, the removal of REEs of porewater may result in widespread REE-rich sediments in the western Pacific basin.

Highlights

The rare earth element (REE) is important in world industrial demand[1], and in tracing various geochemical processes[2]
The seawater oxygen minimum zone (OMZ) ranges from 600 m to 1000 m with oxic bottom water, and the nutrients (SiO32−, NO3− and PO43−) show the low values at surface water followed by a steep rise at OMZ and deep seawater[42]
REEs in upper seawater column are removed by adsorption on downward settling particles followed by dissolution of REEs from these particles upon settling into deep seawater[44]

Summary

Introduction

The rare earth element (REE) is important in world industrial demand[1], and in tracing various geochemical processes[2]. The Pigafetta basin is typical of pelagic sediment having no abundant riverine input and low values of terrigenous, organic contents and sedimentation rate[43] compared to the margin and slope sediments[2,28]. These sediments are dominated by zeolitic brown clay in the depth of 0–40 m (equivalent to the Late Cretaceous to the Pliocene)[43], and show REE enrichment characteristics[1] with the highest concentration exceeding 6500 ppm near the area[38]. Whereas the lower part (deep sediments, ~8–32 cm) are dominated by pelagic clay with low moisture content and relatively high viscosity

Methods

Results

Conclusion