| Rare earth elements(REE+Y,referred to as REY in this paper)following nodules,crusts and sulfides,are newly found important deep-sea mineral resources,which also have become a hot research topic in recent years.Understanding the geochemical recycling process of REY at the sediment-seawater interface is of great significance for revealing the accumulation mechanism of REY in REY-rich sediments.Current studies have shown that the REY-rich sediments are mainly contained in authigenic phase,which acquired REY from seawater or pore water.However,current researches on the REY accumulation mechanism in the deep sea sediments were mainly conducted from the sediment's perspective,with only a few studies on seawater and pore water,which to a certain extent limits the understanding of REY enrichment mechanism in sediments.In order to study the early diagenesis process of REY at sediment-water interface(SWI),the remobilization and accumulation of REY,the analytical method for measuring the dissolved REY in pore water was established as the first step.Secondly,southeastern Pacific basin was selected as the study region,and seawater,pore water and sediment samples were collected and analyzed,as well as some ancillary parameters.The concentrations and spatial distributions of REY were reported in seawater,pore water,and sediment cores,along with the PAAS-normalized pattern,ratios between LREE,MREE and HREE.In addition,sequential leaching experiments were also conducted with sediment samples in order to better elucidate the binding phase of the REY.In the end,controlling factors were discussed for such distributions and accumulation pattern.The main conclusion are as follows:(1)The vertical distribution of REY in seawater is mainly influenced by dust input,water mass mixing and particle scavenging: the concentration in surface water is higher due to dust input;the concentrations of middle and deep water are lower than those in the North Pacific due to the influence of younger water masses(Antarctic circumpolar current);except for Ce,the concentration of dissolved REY in seawater increases with the increase of depth,which is caused by the degradation of particulate matter in deep water.The PAAS-normalized pattern showed the enrichment of HREY,negative Ce anomaly and positive Gd and Y anomaly.The NPDW-normalized pattern is relatively flat,and water masses such as AAIW,NPDW and LCDW can be clearly distinguished.(2)The extremely low dissolved concentrations of Fe and Mn concentrations(<0.2 ?m)and relatively high concentrations of Mo,U,and V in pore water indicated that the sediment were all in an oxic environment.The concentrations of REY in surface pore water and bottom seawater are close to each other,both of which are on the pico-molar scale.Except for Ce,the concentration ratio of surface pore water to bottom seawater is close to 1.The concentration of REY in pore water increases with the increase of depth.In surface sediment,during the process of early diagenesis,with the degradation of organic and reduction of Fe/Mn oxides,REY was released into the pore water.Compared with the bottom seawater,the pore water showed enrichment of MREY.The selected stations in the study area are all REY-rich sediments,and the REY contents vary greatly.The source of REY in the sediments is pore water,and some of the sediments are influenced by hydrothermal vents from the East Pacific Rise.Based on the PCA analysis and sequential leaching experiment results,bio-apatite is the most important carrier of REY-rich sediments in the southeastern Pacific,followed by Fe/Mn oxides.The slow deposition rate is the key parameter for biological apatite to fully contact with seawater and absorb REY in seawater or pore water.When the water depth is below the CCD,the content of biological components is higher,which can dilute the content of REY in the sediments.The strong bottom water mass activity provides an oxidizing environment for the surface sediments,which is conducive to the enrichment of REY. |
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