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Rare Earth Element Distributions in Continental Shelf Sediment, Northern South China Sea [1]

['Ge', 'Xue', 'Z. George', 'Chu', 'Qian Ge', 'Z. George Xue', 'Fengyou Chu']

Date: 2020-12-12

Despite a relatively low abundance in sediments, heavy minerals could considerably contribute to the fraction of bulk REEs [ 2 ]. As shown in Table 2 , the mean content of heavy minerals in zone II (2.76%) is the highest among the three zones [ 28 ]. Nevertheless, the parameters for REEs fractionation, including δCe, δEu, LREE/HREE, (La/Sm), and (Gd/Lu), do not exhibit similar variations with those of the heavy minerals. As heavy minerals contribute less than 20% of the ∑REE in riverine sediments [ 6 ], the heavy minerals will not be the dominant factor on ∑REE. In general, diagenesis will change the Ce anomalies and make δCe have strong correlations with both ∑REE and δEu [ 37 ]. However, the ∑REE and δEu show weak relationships with δCe in this study area, yielding an Rof 0.17 and 0.01, respectively ( Figure 6 ), which implies that diagenesis is not a dominant factor for the ∑REE.

As unstable minerals have been removed by acid digestion, the REEs reported in this study are dominated by the weathering processes of siliciclastic minerals [ 29 ]. Chemical weathering affects the residual fractions of the REEs compositions, and such an effect is most salient in LREE, which has high contents of essential weathering minerals [ 33 ]. The chemical index of alteration (CIA) can be utilized to quantify the weathering states of riverine sediments [ 34 ]. The plot of ∑REE and CIA [ 18 ] indicates a strong positive correlation, with a correlative coefficient (R) of 0.67 ( Figure 5 ). Such correlation suggests that the different weathering mechanisms exert an important influence on the ∑REE values. The climate in the sediment provenance area has been warm and humid. Chemical weathering promotes the REEs mobilization and fractionation. While LREE is preferentially adsorbed onto suspended particles [ 35 ], HREE is preferentially migrated away in forms of bicarbonate and organic complex in solutions [ 36 ]. Compared with LREE, HREE is more mobile for weathering products from granite.

We found strong correlations between ∑REE and granulometric data (contents of sand, silt, and clay, Mz, Figure 5 ) [ 18 ]. In general, low ∑REE values are found in coarse-grained sediments, while high ∑REE values are found in fine-grained sediments. There is a strong correlation between the ∑REE of the fine-grained fraction and coarse-grained fraction. Such a correlation is consistent with the “element granularity control rate” mechanism, which indicates that ∑REE values are rich in the fine fractions and deficit in the coarse sediments [ 30 ]. Hydrodynamics controls the transportation, re-suspension, and deposition of sediments, and thus, they can explain the spatial characteristics of ∑REE and the dominant effect of grain size on the ∑REE [ 27 ]. The surface sediments are mainly transported from the surrounding rivers. The riverine sediments are deposited on the continental shelf around the estuaries in summer. In the following winter, the coastal current transports the re-suspended sediments southwestwardly [ 17 18 ]. The ∑REE values in this region are high. The low ∑REE values in Shuidong Bay ( Figure 1 and Figure 2 ) can be ascribed to the residual sand deposits [ 31 ]. The ∑REE values are low in the coastal area off Leizhou Peninsula and the eastern exit of Qiongzhou Strait, where the seismic sub-bottom profiles are dominated by sand waves [ 17 ]. These sandy deposits resulted from sediment eroded from the old stratum in the sea floor [ 32 ], implying a high-energy depositional environment. The low ∑REE value area off the eastern Hainan Island is associated with the coarse-grained terrigenous sediments from the Wanquan River.

4.2. Provenance and Transport Route of the Sediments

DF=|(C 1S /C 2S )/(C 1P /C 2P )−1| (3) 1S /C 2S and C 1P /C 2P represent the ratio of two elements, with similar chemical properties in the sediments from the study area and potential provenance. In this study, we choose Sm/Nd to calculate DF [ In general, terrigenous materials transported by rivers are the dominant sediment source in shallow waters [ 27 ]. Due to the REEs’ conservative behavior in hypergene environments, REEs characteristics can be used to trace the origins of fluvial sediments. Geologically, the South China continent mostly consists of magmatic rocks (mainly granite) and metamorphic rocks [ 21 38 ]. If the source rock is granite, the ∑REE value is relatively high and LREE is relatively rich, while the Eu represents an obvious negative anomaly [ 39 ]. The average ∑REE value (192.94 μg/g) reported in this study is consistent with the shelf sediments along the Western Pacific (156.00 μg/g, [ 30 ]). The REEs abundance of the sediments in this study are also roughly close to the Chinese Loess (171.00 μg/g; [ 30 ]), the Taiwanese rivers (193.12 μg/g; [ 23 ]), the Hainan Island (124.94 μg/g; [ 38 ]), and the Pearl River (255.40 μg/g; [ 21 ]), but they are quite different from deep sea clay (411 μg/g; [ 40 ]). Chondrite-normalized REEs of sediments are characterized by relatively high LREE and a negative Eu anomaly ( Figure 3 ). The REEs fractionation patterns of sediments in the study area are comparable of those of terrigenous materials, such as the sediments from the Pearl River [ 21 ], Hainan Island [ 24 ], southwestern Taiwanese rivers [ 23 ], Qiongzhou Strait [ 22 ], and SCS slope [ 25 ]. We also use the discrimination function (DF) to analyze the proximity of the northern SCS sediments to these potential provenances.where C/Cand C/Crepresent the ratio of two elements, with similar chemical properties in the sediments from the study area and potential provenance. In this study, we choose Sm/Nd to calculate DF [ 37 ]. It is generally considered that the chemical properties of the sediments from the study area are close to those of the potential provenance if the value of DF is less than 0.5. All DF values between the study area and Pearl River, Hainan Island, southwestern Taiwanese rivers, Qiongzhou Strait, and SCS slope sediments are very low ( Table 3 ). Both REEs fractionation patterns and DF values ( Figure 3 Table 3 ) show that the REEs have strong terrigenous succession and the surface sediments in the northern SCS shelf are mainly from the surrounding continents.

As a result of their weak correlations with grain size, fractionation parameters, such as LREE/HREE ratio and δEu, are ideal proxies to trace the sources of the fluvial sediments ( Figure 5 ). The LREE/HREE ratio in the study area ranges from 5.91 to 10.93 with an average value of 9.50, which is comparable with those reported for the Pearl River (7.83–11.23; average: 8.98, [ 21 ]), Taiwanese rivers (7.48–13.03; average: 8.88, [ 23 ]), and Hainan Island (5.16–12.33, average: 9.52, [ 38 ]). The diagram of LREE and HREE exhibits a similar positive correlation in all sediment groups ( Figure 7 ). In detail, the Pearl River samples show the highest ∑REE on average, and the Hainan Island sediments have the lowest. The ∑REE values of the study area and Taiwanese riverine samples fall in the middle. Most of the sediments from zone I have relatively uniform and higher LREE and HREE concentrations, indicating a source from the Pearl and Taiwanese rivers. The LREE vs. HREE values in zone II show scattered REEs fractionation, indicating a unique terrigenous source ( Figure 7 ). The distribution patterns indicate enhanced influences from the Hainan Island and Qiongzhou Strait. Sediments in zone III exhibit the highest mean value of LREE/HREE ratio in the study area ( Table 2 ), reflecting the highest fractionation degree. The plots of LREE vs. HREE in zone III shift to the ranges of sediments from the Hainan Island and SCS slope ( Figure 7 ), which indicates the increased influences from these areas.

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