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

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Article Rare Earth Element Distributions in Continental Shelf Sediment, Northern South China Sea

Qian Ge 1,2,*, Z. George Xue 3,4,5 and Fengyou Chu 1,2

1 Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou 310012, China; [email protected] 2 Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China 3 Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA; [email protected] 4 Center for Computation and Technology, Louisiana State University, Baton Rouge, LA 70803, USA 5 Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA * Correspondence: [email protected]

Received: 16 October 2020; Accepted: 11 December 2020; Published: 16 December 2020

Abstract: A total of 388 surface sediment samples taken from the northern South China Sea (SCS) continental shelf were analyzed to characterize the signature of their rare earth elements (REEs). The average REEs concentration was 192.94 µg/g, with a maximum of 349.07 µg/g, and a minimum of 32.97 µg/g. The chondrite-normalized REEs pattern exhibits a remarkably light REEs accumulation, a relatively ﬂat heavy REEs pattern, and a negative Eu anomaly. We subdivided the study area into three zones using the characteristics of REEs and statistical characteristics. Zone I: continental shelf oﬀ western Guangdong Province. Here, the sediment provenance is mainly river-derived from the Pearl River, Taiwanese rivers, and those in the adjacent area. Zone II: Qiongzhou Strait and Leizhou Peninsula. Here, the sediment provenance consists of the Qiongzhou Strait and the Hainan Island. Zone III: Hainan Island and SCS slope sediments are dominated. The REEs compositions are mainly controlled by source rock properties, hydrodynamic conditions, and an intensity of chemical weathering. We reconstructed the sediment dispersal and transport route using the REEs compositions, grain size, and other geochemical characteristics throughout the study area.

Keywords: rare earth elements; Pearl River; provenance; transportation and depositional processes

1. Introduction The source of sediment is one of the key topics in modern marine sedimentary geology [1]. It is important to establish the provenance of sediment to understand the geological and paleoclimatic history presented therein. Terrigenous materials retain the information of the source rock properties, the sedimentation affects, and changes in the composition of the sediments. The unique stability of rare earth elements (REEs) makes them strong inheritance to the parent rock, and thus, they could be used to indicate the evolution of sediment provenance [2]. Once the REEs enter the marine environment, the REEs compositions and distribution patterns for the sediments basically do not change significantly [3]. Therefore, the study of the REEs in marine sediments can provide help for provenance determination, environmental evolution, and stratigraphic correlation [4–8]. Previous studies systematically analyzed the characteristics of REEs in seafloor sediments of the Northwestern Pacific (Bohai Sea, Yellow Sea, and East China Sea [6,9–11]). The South China Sea (SCS) contains significant information about the materials and energy exchange between the Pacific Ocean and the marginal sea in Chinese coastal waters. In recent years, a breakthrough has been made in understanding the evolution of the paleoclimate and the history of the East Asian monsoon

Water 2020, 12, 3540; doi:10.3390/w12123540 www.mdpi.com/journal/water WaterWater2020 2020,,12 12,, 3540x 22 ofof 1313

in this region [12–15]. As the area with strong weathering and erosion, the origination, transport, inand this deposition region [12 of– the15]. SCS As sediments the area with are stronghighly localized weathering and and complicated. erosion, the Comprehensive origination, transport, concepts andof the deposition transport of andthe SCS accumulation sediments are processes highly localized of the weathering and complicated. and erosion Comprehensive products still concepts remain of thepoorly transport understood and accumulation [16]. The northern processes SCS of, which the weathering is adjacent and to erosionthe Chinese products mainland still remain and Taiwan, poorly understoodreceives significant [16]. The amounts northern of SCS, terrigenous which is adjacent materials to [17] the. Chinese The norther mainlandn SCS and sediments Taiwan, are receives good signiﬁcantproxies foramounts analyzing of the terrigenous sedimentary materials process [17es]. from The northernsource to SCS sink. sediments This study are selects good 388 proxies surface for analyzingsediment thesamples sedimentary (Figure processes1) for REEs from sourceanalysis to tosink. (1) Thisdelineate study selectsthe REE 388s surfacedistribution sediment and samplesconcentrations (Figure 1in) forsurface REEs sediments analysis to (1)of the delineate northern the REEsSCS continental distribution shelf; and concentrations (2) evaluate controlling in surface sedimentsfactors of the of the REEs northern distribution SCS continental patterns; shelf;and (3) (2) investigate evaluate controlling modern sediment factors of provenance the REEs distribution and their patterns;transportation and (3) processes. investigate modern sediment provenance and their transportation processes.

FigureFigure 1.1. LocationLocation of surfacesurface sedimentsediment samples on the northern South ChinaChina Sea continental shelf. TheThe locationlocation ofof distaldistal depocenterdepocenter isis citedcited fromfrom GeGe etet al.al. [[17].17]. SeasonalSeasonal variationsvariations ofof surfacesurface circulationcirculation patternspatterns onon thethe northernnorthern SouthSouth ChinaChina SeaSea continentalcontinental shelfshelf areare revisedrevised fromfrom LiuLiu et et al.al. [16[16].]. 1.1. ChineseChinese CoastalCoastal Current;Current; 2.2. SouthSouth ChinaChina SeaSea WarmWarm Current.Current. 2. Materials and Methods 2. Materials and Methods During a joint cruise between the Second Institute of Oceanography, Ministry of Natural Resources, During a joint cruise between the Second Institute of Oceanography, Ministry of Natural and South China Sea Bureau of Ministry of Natural Resources in 2008, we collected 388 surface sediment Resources, and South China Sea Bureau of Ministry of Natural Resources in 2008, we collected 388 samples using a box corer at water depth around 4–135 m (for the samples locations, see Figure1). surface sediment samples using a box corer at water depth around 4–135 m (for the samples The samples were stored at 4 C immediately after collection. The grain size was measured using locations, see Figure 1). The samples◦ were stored at 4 °C immediately after collection. The grain size a Mastersizer-2000 laser particle size analyzer in Nanjing University and South China Sea Bureau was measured using a Mastersizer-2000 laser particle size analyzer in Nanjing University and South of Ministry of Natural Resources. The procedure of grain size analysis is reported in Ge et al. [18]. China Sea Bureau of Ministry of Natural Resources. The procedure of grain size analysis is reported The identification of REEs constituents, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y in Ge et al. [18]. The identification of REEs constituents, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, were carried out at the Institute of Geophysical and Geochemical Exploration, Chinese Academy Tm, Yb, Lu, and Y were carried out at the Institute of Geophysical and Geochemical Exploration, of Geological Sciences, China, using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) Chinese Academy of Geological Sciences, China, using an Inductively Coupled Plasma Mass (iCAP Qc, Thermo Fisher Scientific, Waltham, MA, USA). The sample was dried at 105 C for 12 h and Spectrometer (ICP-MS) (iCAP Qc, Thermo Fisher Scientific, Waltham, USA). The sample◦ was dried was crushed by agate mortar (200 mesh, 0.074 mm pore size). Then, the powdered samples (0.05 g) at 105 °C for 12 h and was crushed by≈ agate mortar (200 mesh, ≈0.074 mm pore size). Then, the were digested using high-purity HNO3 (1.5 mL) and HF (hydrofluoric acid, 1.5 mL) in a tightly closed powdered samples (0.05 g) were digested using high-purity HNO3 (1.5 mL) and HF (hydrofluoric Teflon bomb for 48 h at 190 C. To remove residual HF, the dry samples were retreated with HNO acid, 1.5 mL) in a tightly closed◦ Teflon bomb for 48 h at 190 °C . To remove residual HF, the dry3 (1 mL). Then, the samples were digested with 50% HNO3 (3 mL) in a tightly closed Teflon bomb for samples were retreated with HNO3 (1 mL). Then, the samples were digested with 50% HNO3 (3 mL) another 12 h (190 C). After cooling, the final solution was transferred to a 100 mL polyethylene bottle, in a tightly closed◦ Teflon bomb for another 12 h (190 °C). After cooling, the final solution was to which we added 1 mL (Rh + Re) of mixed standard solution and then diluted it by the addition transferred to a 100 mL polyethylene bottle, to which we added 1 mL (Rh + Re) of mixed standard solution and then diluted it by the addition of Milli-Q water for analysis. The GSD-9 and GSD-10 Water 2020, 12, 3540 3 of 13 of Milli-Q water for analysis. The GSD-9 and GSD-10 sediment samples were analyzed for quality assurance and control, which showed a less than 5% REEs error.

3. Results

3.1. Grain Size Parameters The results of grain size are reported in Ge et al. [18], and they show that the average values of sand, silt, and clay fractions are 25%, 62%, and 13%, respectively. The contents of clay and silt fractions exhibit a similar pattern, having high values on the shelf oﬀ western Guangdong Province, which are opposite to those of sand fractions. The mean grain size (Mz) ranges from 0.05 ϕ to 7.28 ϕ, with a mean value of 5.60 ϕ.

3.2. Concentrations and Spatial Characteristics of REEs P P Table1 shows the total concentrations of REEs ( REE). The mean REE of surface sediments is 192.94 µg/g, with a minimum and maximum value of 32.97 and 349.07 µg/g, respectively. The light REEs (LREE, from La to Eu) exhibit a mean value of 154.36 µg/g, dominating the REEs content; the mean value of heavy REEs (HREE, from Gd to Lu) is 16.11 µg/g. The LREE/HREE values range from 5.91 P to 10.93 with an averaged value of 9.50. The REE exhibit significant spatial variabilities in the P study area (Figure2). The high REE values are found in the western Guangdong coastal waters. Two accumulation centers are identified around the Xiachuan and Hailing Islands (Figures1 and2), P P where the REE values can reach more than 250 µg/g. Another high REE value area was found and correlated to a distal depocenter (Figures1 and2), which is mentioned in Ge et al. [ 17]. The values decrease as waters are moving off the Guangdong coast. The lowest value is located in the continental P shelf off the eastern Leizhou Peninsula and Qiongzhou Strait, where the REE values are mostly P lower than 100 µg/g. The LREE and HREE exhibit similar spatial characteristics with REE, yet with a slightly smaller spatial gradient (Figure2).

3.3. The REEs Fractionation Characteristics To further investigate the fractionation characteristics of REEs, the chondrite-normalized REEs of sediments were analyzed [19]. The trends reflect a remarkable accumulation of LREE. The even distribution of HREE is relatively flat (Figure3), with the (La /Yb)N ranging from 5.87 to 15.21 (Table1). The fractionation level of REEs was determined following, where cerium and europium anomalies (δCe and δEu) are derived by the comparison between the concentrations of Ce and Eu and those of their neighboring elements [20]: δCe = Ce /(La Pr ) 1/2 (1) N N × N δEu = Eu /(Sm Gd )1/2 (2) N N × N where N is the normalization to chondrite [19]. The δCe ranges from 0.79 to 1.08 with a mean value of 0.97. No notable Ce anomaly was identified. The δEu ranges from 0.28 to 1.05 with a mean value of 0.65 (Table1), exhibiting a negative anomaly. Water 2020, 12, 3540 4 of 13 Water 2020, 12, x FOR PEER REVIEW 4 of 13

Table 1. Statistical data of rare earth elements (REEs) values (µg/g) in surface sediments taken from the northern South China Sea continental shelf, together with some importantTable 1. Statistical fractionation data of parameters. rare earth elements (REEs) values (μg/g) in surface sediments taken from the northern South China Sea continental shelf, together with some important fractionation parameters. P Statistics La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y REE LREE HREE δEu δCe (La/Yb)N Statistics La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ∑REE LREE HREE δEu δCe (La/Yb)N MinMin 6.806.8010.00 10.001.40 1.40 4.70 4.70 0.97 0.970.21 0.21 0.950.95 0.160.16 0.910.91 0.18 0.18 0.51 0.51 0.08 0.08 0.51 0.51 0.07 0.075.40 5.4032.97 32.97 24.2024.20 3.41 3.410.28 0.280.79 0.795.87 5.87 Max 73.40 125.40 14.90 49.40 9.39 1.83 8.79 1.42 8.37 1.68 5.15 0.77 4.58 0.71 44.30 349.07 273.30 31.47 1.05 1.08 15.21 Max 73.40 125.40 14.90 49.40 9.39 1.83 8.79 1.42 8.37 1.68 5.15 0.77 4.58 0.71 44.30 349.07 273.30 31.47 1.05 1.08 15.21 Average 38.50 71.50 8.32 29.30 5.64 1.11 4.85 0.78 4.31 0.83 2.32 0.38 2.28 0.37 22.46 192.94 154.36 16.11 0.65 0.97 12.12 Average 38.50 71.50 8.32 29.30 5.64 1.11 4.85 0.78 4.31 0.83 2.32 0.38 2.28 0.37 22.46 192.94 154.36 16.11 0.65 0.97 12.12 Standard deviation 12.40 23.60 2.68 9.50 1.84 0.37 1.56 0.26 1.40 0.27 0.76 0.13 0.75 0.13 7.21 62.14 50.03 5.22 0.06 0.06 1.36 Standard deviation 12.40 23.60 2.68 9.50 1.84 0.37 1.56 0.26 1.40 0.27 0.76 0.13 0.75 0.13 7.21 62.14 50.03 5.22 0.06 0.06 1.36

P FigureFigure 2. Spatial 2. Spatial characteristics characteristics of of REEs REEs in in the the surface surface sediments sediments ofof the northern South South China China Sea Sea continental continental shelf shelf ((a (() a∑REE;) REE; (b) (LREE;b) LREE; (c) HREE). (c) HREE). Water 2020, 12, x FOR PEER REVIEW 5 of 13

δCe = CeN/(LaN × PrN) 1/2 (1)

δEu = EuN/(SmN × GdN)1/2 (2) where N is the normalization to chondrite [19]. The δCe ranges from 0.79 to 1.08 with a mean value of 0.97. NoWater notable2020, 12Ce, 3540anomaly was identified. The δEu ranges from 0.28 to 1.05 with a mean value of 0.655 (Table of 13 1), exhibiting a negative anomaly.

FigureFigure 3. Chondrite 3. Chondrite-normalized-normalized REEs REEs distribution distribution patterns patterns of the of samples the samples in the instudy the studyarea (three area (threedifferent zones)diﬀerent and zones) the samples and the from samples the Pearl from the River Pearl [21 River], Qiongzhou [21], Qiongzhou Strait [22 Strait], Taiwanese [22], Taiwanese rivers rivers [23], Hainan [23], IslandHainan [24], Island and South [24], andChina South Sea slope China [25 Sea]. slope [25].

3.4.3.4. Spatial Spatial Zones Zones of REEs of REEs TheThe Q- Q-clustercluster analysis analysis has has been been widely widely used used for for the the sediment sediment distribution distribution analyses analyses [18, [1826,,2627,].27 We]. performedWe performed the Q-cluster the Q-cluster analysis to analysis investigate to investigatethe spatial relationships the spatial among relationships all REEs among. These analyses all REEs. are carriedThese out analyses on the are original carried data out u onsing the St originalatistical dataProduct using and Statistical Service Solutions Product andsoftware Service. The Solutions Q-cluster analysissoftware. indicated The Q-cluster that the analysis study area indicated can be that grouped the study into area three can zone be groupeds (Figure into 4), threewhich zones is similar (Figure to4 ),our previouswhich is study similar based to our on previous major elements study based [18] on. Zone major I elementsdesignates [18 the]. Zone continental I designates shelf the off continental the western Guangdongshelf off the Province western Guangdongand distal Province depocenter and extending distal depocenter to the extending southwest to. The the southwest. mean ∑REE The value mean of P sedimentsREE value in this of sedimentszone is 233.96 in this μg/g. zone Zone is 233.96 II comprisesµg/g. Zone the coastal II comprises area off the Leizhou coastal areaPeninsula off Leizhou and the P easternPeninsula exit of and Qiongzhou the eastern Strait. exit ofThe Qiongzhou average value Strait. of The ∑REE average is 107.00 value μg/g. of ZoneREE isIII 107.00is locatedµg/g. on Zone the outer III P shelfis located (water ondepth the outer>50 m) shelf to the (water east depthof Hainan>50 m) Island. to the The east mean of Hainan ∑REE Island. value of The sediments mean REE in this value zone of is sediments in this zone is 180.81 µg/g (Table2). In addition, the chondrite-normalized REEs parameters in different zones, such as δCe, δEu, (La/Sm)N, and (Gd/Lu)N, are shown in Table2.

Table 2. REEs characteristic parameters and mean contents of heavy minerals in three zones.

P a Zones δCe δEu REE (µg/g) LREE/HREE (La/Sm)N (Gd/Lu)N Contents of Heavy Minerals (%) I 0.99 0.66 223.96 9.32 4.24 1.74 0.95 II 0.92 0.63 107.00 8.20 4.46 1.62 2.76 III 0.97 0.65 180.81 9.94 4.59 1.81 0.89 a cited from [28]. Water 2020, 12, x FOR PEER REVIEW 6 of 13

180.81Water μg/g2020 , (12Table, 3540 2). In addition, the chondrite-normalized REEs parameters in different zones,6 such of 13 as δCe, δEu, (La/Sm)N, and (Gd/Lu)N, are shown in Table 2.

FigureFigure 4. Three 4. Three REEs REEs geochemical geochemical zones zones on on the the northern SouthSouth ChinaChina Sea Sea continental continental shelf shelf (dashed (dashed lines lines representrepresent the theestimated estimated boundary boundary of REE of REEss zones). zones).

4. Discussion Table 2. REEs characteristic parameters and mean contents of heavy minerals in three zones.

4.1. Controlling Factors∑REE of REEs Compositions Contents of Heavy Minerals a Zones δCe δEu LREE/HREE (La/Sm)N (Gd/Lu)N Many factors are responsible(μg/g) for REEs compositions in sediments, including source rock(%) properties, Iintensity 0.99 of chemical0.66 weathering,223.96 mineralogy,9.32 grain4.24 size, and diagenesis1.74 [6,29]. Among0.95 these factors, IIsource 0.92 rock properties0.63 107.00 (sediment provenance)8.20 are regarded4.46 as the1.62 most important controlling2.76 factor [2,6], IIIwhich 0.97 will be0.65 discussed 180.81 in the next section.9.94 4.59 1.81 0.89 a P We found strong correlations between citedREE from and [28]. granulometric data (contents of sand, silt, P and clay, Mz, Figure5)[ 18]. In general, low REE values are found in coarse-grained sediments, P 4. Discussionwhile high REE values are found in fine-grained sediments. There is a strong correlation between the P REE of the fine-grained fraction and coarse-grained fraction. Such a correlation is consistent with 4.1. Controlling Factors of REEs Compositions P the “element granularity control rate” mechanism, which indicates that REE values are rich in the fineMany fractions factors and are deficitresponsible in the for coarse REE sedimentss compositions [30]. in Hydrodynamics sediments, including controls source the transportation, rock properties, intensityre-suspension, of chemical and weatheringdeposition, of mineralogy, sediments, andgrain thus, size they, and can diagenesis explain the [6, spatial29]. Among characteristics these factors of , P P sourceREE rock and properties the dominant (sediment effect prov of grainenance size) are on regarded the REE as [the27]. most The important surface sediments controlling are factor mainly [2,6], whichtransported will be discussed from the in surrounding the next section rivers.. The riverine sediments are deposited on the continental shelfWe found around strong the estuaries correlations in summer. between In∑REE the and following granulometric winter, thedata coastal (contents current of sand, transports silt, and the clay, P Mz,re-suspended Figure 5) [18] sediments. In general, southwestwardly low ∑REE values [17 are,18]. found The inREE coarse values-grained in this sediments, region are while high. Thehigh low∑REE P valuesREE are values found in in Shuidong fine-grained Bay (Figures sediments.1 and There2) can is be a ascribedstrong correlation to the residual between sand the deposits ∑REE [ 31of ].the P 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 Water 2020, 12, x FOR PEER REVIEW 7 of 13 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 ∑REEWater 2020 value, 12,s 3540in this region are high. The low ∑REE values in Shuidong Bay (Figures 1 and 2) can7 of 13be 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 resulted from sediment eroded from the old stratum in the sea floor [32], implying a high-energy by sand waves [17]. These sandy depositsP resulted from sediment eroded from the old stratum in the sea depositional environment. The low REE value area off the eastern Hainan Island is associated with floor [32], implying a high-energy depositional environment. The low ∑REE value area off the eastern the coarse-grained terrigenous sediments from the Wanquan River. Hainan Island is associated with the coarse-grained terrigenous sediments from the Wanquan River.

P FigureFigure 5. 5. CorrelationsCorrelations between between total total concentrations concentrations of ofREEs REEs (∑REE ( REE)) with with (a) (Mz,a) Mz, (b) (siltb) silt+ clay,+ clay, (c) sand (c) sand and (dand) ch (emicald) chemical index of index alteration of alteration (CIA), and (CIA), Mz andwith Mz(e) light with REE (e) lights (LREE REEs)/heavy (LREE) REE/heavys (HREE REEs), and (HREE), (f) δEu. Theand granulometric (f) δEu. The granulometricdata and CIA are data cited and from CIA Ge are etcited al. [18]. from Ge et al. [18].

AsAs u unstablenstable minerals minerals have have been been removed removed by by acid digestion, the REEs reported in this this study study areare dominateddominated by by the weathering processes of siliciclastic siliciclastic minerals minerals [[2929].]. ChemicalChemical weathering weathering a affectsffects the the residualresidual fractions fractions of of the the REEs REEs compositions compositions,, and and such such an effect effect is mostmost salientsalient inin LREE,LREE, whichwhich hashas highhigh contentscontents of of essential essential weathering weathering minerals minerals [33 [33]. ].Th Thee chemical chemical index index of of alteration alteration (CIA) (CIA) can can be be utilized utilized to P quantifyto quantify the theweathering weathering states states of riverine of riverine sediments sediments [34]. [The34]. plot The of plot ∑REE of andREE CIA and [18 CIA] indicates [18] indicates a strong a positive correlation, with a correlative coefficient (R2) of 0.67 (Figure 5). Such correlation suggests that the strong positive correlation, with a correlative coefficient (R2) of 0.67 (Figure5). Such correlation suggests different weathering mechanisms exert an important influence on the ∑REE Pvalues. The climate in the that the different weathering mechanisms exert an important influence on the REE values. The climate sediment provenance area has been warm and humid. Chemical weathering promotes the REEs 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 mobilization and fractionation. While LREE is preferentially adsorbed onto suspended particles [35], is preferentially migrated away in forms of bicarbonate and organic complex in solutions [36]. Compared withHREE LREE, is preferentially HREE is more migrated mobile for away weathering in forms products of bicarbonate from granite and organic. complex in solutions [36]. ComparedDespite witha relative LREE,ly low HREE abundance is more mobilein sediments, for weathering heavy minerals products could from considerably granite. contribute to the fractionDespite of bulk a REEs relatively [2]. As low shown abundance in Table in 2 sediments,, the mean content heavy mineralsof heavy minerals could considerably in zone II (2.76%) contribute is the highestto the fractionamong the of bulk three REEs zones [2 [].28 As]. Nevertheless, shown in Table the2 ,parameters the mean contentfor REEs of fractionation heavy minerals, including in zone δCe, II δEu,(2.76%) LREE/HREE, is the highest (La/Sm) amongN, and the three (Gd/Lu) zonesN, do [28 not]. Nevertheless, exhibit similar the variations parameters with for REEsthose fractionation,of the heavy mineralincludings. AsδCe, heavyδEu, minerals LREE/ HREE,contribute (La /lessSm) Nthan, and 20% (Gd of/ Lu)the N∑REE, do notin riverine exhibit sediments similar variations [6], the heavy with P mthoseinerals of will the not heavy be the minerals. dominan Ast factor heavy on minerals∑REE. In contributegeneral, diagenesis less than will 20% change of the the CeREE anomalies in riverine and P makesediments δCe have [6], strong the heavy correlations minerals with will both not ∑REE be the and dominant δEu [37] factor. However, on theREE. ∑REEIn general, and δEu diagenesisshow weak P will change the Ce anomalies and make δCe have strong correlations with both REE and δEu [37]. P However, the REE and δEu show weak relationships with δCe in this study area, yielding an R2 of 0.17 and 0.01, respectively (Figure6), which implies that diagenesis is not a dominant factor for the P REE. Water 2020, 12, x FOR PEER REVIEW 8 of 13

Waterrelationships2020, 12, 3540 with δCe in this study area, yielding an R2 of 0.17 and 0.01, respectively (Figure 6), which8 of 13 implies that diagenesis is not a dominant factor for the ∑REE.

P FigureFigure 6. 6.Correlations Correlations between between cerium cerium anomalies anomalies ((δδCeCe)) withwith ((aa)) ∑REEREE and (b) europium anomalies ( (δEuδEu).).

4.2.4.2. Provenance Provenance and Transport Route Route of of the the Sediments Sediments InIn general, general, terrigenous terrigenous materialsmaterials transportedtransported byby riversrivers areare thethe dominantdominant sedimentsediment sourcesource inin shallowshallow waterswaters [ 27[27].]. DueDue toto thethe REEs’REEs’ conservative behavior behavior in in hypergene hypergene environments, environments, REEs REEs characteristics characteristics can canbe used be used to trace to trace the origins the origins of fluvial of fluvial sediment sediments.s. Geologically, Geologically, the South the China South continent China continentmostly consist mostlys of consistsmagmatic of magmaticrocks (mainly rocks granite) (mainly and granite) metamorphic and metamorphic rocks [21,38 rocks]. If the [21 ,source38]. If therock source is granite, rock isthe granite, ∑REE valueP is relatively high and LREE is relatively rich, while the Eu represents an obvious negative anomaly the REE value is relatively high and LREE is relatively rich, while the Eu represents an obvious [39]. The average ∑REE value (192.94P μg/g) reported in this study is consistent with the shelf sediments negative anomaly [39]. The average REE value (192.94 µg/g) reported in this study is consistent with along the Western Pacific (156.00 μg/g, [30]). The REEs abundance of the sediments in this study are also the shelf sediments along the Western Pacific (156.00 µg/g, [30]). The REEs abundance of the sediments roughly close to the Chinese Loess (171.00 μg/g; [30]), the Taiwanese rivers (193.12 μg/g; [23]), the Hainan µ inIsland this study(124.94 are μg/g; also [38 roughly]), and the close Pearl to River the (2 Chinese55.40 μg/g; Loess [21] (171.00), but theyg /areg; [quite30]), different the Taiwanese from deep rivers sea (193.12clay (411µg /μg/g;g; [23 ]),[40 the]). Chondrite Hainan Island-normalized (124.94 REEsµg/g; of [38 sediments]), and the are Pearl characterized River (255.40 by relativelyµg/g; [21 ]),high but LREE they areand quite a negative diﬀerent Eu fromanomaly deep (Figure sea clay 3). The (411 REEsµg/g; fractionation [40]). Chondrite-normalized patterns of sediments REEs in the of sedimentsstudy area areare characterizedcomparable of by those relatively of terrigenous high LREE materials, and a negative such as Eu the anomaly sediments (Figure from the3). ThePearl REEs River fractionation [21], Hainan patternsIsland [24 of], sediments southwestern inthe Taiwanese study area rivers are [23 comparable], Qiongzhou of Strait those [22 of], terrigenous and SCS slope materials, [25]. We suchalso use as thethe sedimentsdiscrimination from thefunction Pearl (DF) River to [21 anal], Hainanyze the Island proximity [24], of southwestern the northern Taiwanese SCS sediments rivers to [23 these], Qiongzhou potential Straitprovenances. [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. DF=|(C1S/C2S)/(C1P/C2P)−1| (3)

where C1S/C2S and C1P/C2P represent the ratio of two elements, with similar chemical properties in the DF=|(C1S/C2S)/(C1P/C2P) 1| (3) 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 where C1S/C2S and C1P/C2P represent the ratio of two elements, with similar chemical properties in thethose sediments of the potential from the provenance study area if the and value potential of DF is provenance. less than 0.5. InAll this DF study,values between we choose the Smstudy/Nd area to calculateand Pearl DF River, [37]. ItHainan is generally Island, considered southwestern that Taiwanese the chemical rivers, properties Qiongzhou of the Strait, sediments and SCS from slope the sediments are very low (Table 3). Both REEs fractionation patterns and DF values (Figure 3, Table 3) show study area are close to those of the potential provenance if the value of DF is less than 0.5. All DF values that the REEs have strong terrigenous succession and the surface sediments in the northern SCS shelf are between the study area and Pearl River, Hainan Island, southwestern Taiwanese rivers, Qiongzhou mainly from the surrounding continents. Strait, and SCS slope sediments are very low (Table3). Both REEs fractionation patterns and DF values (Figure3, Table3) show thatTable the 3. REEs Discrimination have strong function terrigenous (DF) values succession of the sediments. and the surface sediments in the northern SCS shelf are mainly from the surrounding continents. Potential Provenance DF Reference Table 3. DiscriminationPearl River function (DF) values0.02 of the[21 sediments.] Hainan Island 0.05 [24] SouthwesternPotential Provenance Taiwanese rivers DF0.07 Reference[23] PearlQiongzhou River Strait 0.020.03 [22 [21] ] HainanSCS Island slope 0.050.03 [25 [24] ] Southwestern Taiwanese rivers 0.07 [23] Qiongzhou Strait 0.03 [22] SCS slope 0.03 [25]

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 (Figure5). The LREE /HREE Water 2020, 12, x FOR PEER REVIEW 9 of 13

As a result of their weak correlations with grain size, fractionation parameters, such as LREE/HREE Water 2020, 12, 3540 9 of 13 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 thoseratio reported in the study for the area Pearl ranges River from (7.83–11.23; 5.91 to 10.93 average: with an8.98, average [21]), valueTaiwanese of 9.50, rivers which (7.48–13.03; is comparable average: 8.88,with [23]), those and reported Hainan for Island the Pearl(5.16–12.33, River (7.83–11.23; average: 9.52, average: [38]). The 8.98, diagram [21]), Taiwanese of LREE riversand HREE (7.48–13.03; exhibits a similaraverage: positive 8.88, correlation [23]), and Hainanin all sediment Island (5.16–12.33, groups (Figure average: 7). In 9.52, detail, [38 the]). ThePearl diagram River samples of LREE show and the highestHREE ∑ exhibitsREE on average, a similar and positive the Hainan correlation Island in sediments all sediment have groups the lowest. (Figure The7). ∑REE In detail, values the of Pearlthe study P areaRiver and samples Taiwanese show riverine the highest samplesREE fall onin average,the middle. and Most the Hainan of the sediments Island sediments from zone have I have the lowest. relatively P uniformThe REEand valueshigher ofLREE the studyand HREE area andconcentrations, Taiwanese riverineindicating samples a source fall from in the the middle. Pearl Mostand Taiwanese of the rivers.sediments The LREE from zonevs. HREE I have relativelyvalues in uniform zone II andshow higher scattered LREE andREEs HREE fractionation, concentrations, indicating indicating a unique a terrigenoussource from source the Pearl (Figure and Taiwanese7). The distribution rivers. The patte LREErns vs. indicate HREE values enhanced in zone infl IIuences show scatteredfrom the REEs Hainan Islandfractionation, and Qiongzhou indicating Strait. a uniqueSediments terrigenous in zone III source exhibit (Figure the highest7). The mean distribution value of patterns LREE/HREE indicate ratio in theenhanced study area influences (Table 2), from reflecting the Hainan the highest Island fracti andonation Qiongzhou degree. Strait. The Sediments plots of LREE in zone vs. IIIHREE exhibit in zone the III shifthighest to the mean ranges value of ofsediments LREE/HREE from ratio the in Hainan the study Island area and (Table SCS2), reflectingslope (Figure the highest 7), which fractionation indicates the increaseddegree. influences The plots of from LREE these vs. HREEareas. in zone III shift to the ranges of sediments from the Hainan Island and SCS slope (Figure7), which indicates the increased influences from these areas.

Figure 7. Correlations between LREE and HREE in the study area and potential provenances (the Pearl

River data are cited from [21]; the Hainan Island data are cited from [24]; the Taiwanese rivers data are cited Figurefrom 7. [23 Correlations]; the Qiongzhou between Strait dataLREE are and cited HREE from [in22 ];the the stud Southy area China and Sea potential slope data provenances are cited from (the [25]). Pearl River data are cited from [21]; the Hainan Island data are cited from [24]; the Taiwanese rivers data are cited fromHere, [23]; we the summarize Qiongzhou Strait the modern data aretransportation cited from [22]; the and South depositional China Sea processes slope data of are sediments cited fromon [25]). the northern SCS continental shelf (Figure8) based on the characteristics of the REEs from this study and theHere, grain we size, summarize geochemical the characteristics modern transportation [18], sub-bottom and profilesdepositional [17], and processes hydrodynamic of sediments conditions on the northernfrom previous SCS continental studies. shelf Under (Figure the impact 8) based of the on Eastthe characteristics Asian summer of monsoon, the REEs thefrom weathering this study andand the grainerosion size, ratesgeochemical in the South characteristics Chinese Mainland [18], sub-bottom and Taiwan profiles are high;[17], and thus, hydrodynamic huge amounts conditions of riverine from previousfrom the studies. South Under Chinese the Mainland impact of rivers the East (Pearl As Riverian summer and surrounding monsoon, smallthe weathering rivers, such and as erosion Moyang rates in River,the South Jian RiverChinese and Mainland so on) and and southwestern Taiwan are Taiwanese high; thus, rivers huge could amounts flow into of theriverine ocean from and deposit the South Chinesenear the Mainland estuary. rivers The strong(Pearl EastRiver Asian and surrounding winter monsoon small strengthens rivers, such theas Moya Chineseng River, Coastal Jian Current, River and so whichon) and controls southwestern the southwestward Taiwanese longshore rivers could transport flow ofinto re-suspended the ocean and sediments. deposit During near the this estuary. process, The strongcoarse East fractions Asian of winter re-suspended monsoon sediments strengthens could the be transportedChinese Coastal over a relativeCurrent, short which distance controls and the southwestwardthen be deposited longshore firstly, transport while fine of particles re-suspended could besediments. dispersed During over 400 this km process, to the eastcoarse of Leizhoufractions of re-suspendedPeninsula. The sediments coastal currentcould be meets transported with the irregularover a rela diurnaltive short tide distance to form a and cyclonic then circulation be deposited in this firstly, area [41,42]. The seawater’s carrying capacity is reduced sharply and results in the rapid deposition on the distal depocenter (Figure8). The sediments distributed in the east end of Qiongzhou Strait P have the coarsest grain size and lowest REE in the study area. The coarse materials are mainly Water 2020, 12, 3540 10 of 13 Water 2020, 12, x FOR PEER REVIEW 10 of 13 re-suspended and transported from the Qiongzhou Strait. With transport southward, the influence of in the study area. The coarse materials are mainly re-suspended and transported from the Qiongzhou theStrait. Hainan With transport Island-derived southward, sediments the influence (such asof the Wanquan Hainan River,Island- Nanduderived Riversediments sediments, (such as and Wanquan so on) increases.River, Nandu Meanwhile, River sediments the sediments, and so from on) increases. SCS slope Meanwhile, could be delivered the sediments into thefrom relatively SCS slope deep-water could be regiondelivered during into the summer. relatively deep-water region during summer.

FigureFigure 8. Transport8. Transport patterns patterns of of modern modern sediments sediments on the northern northern South South China China Sea Sea continental continental shelf. shelf.

5.5. ConclusionsConclusions P REEsREEs analysis was was performe performedd on on 388 388 surface surface sediments sediments in the in northern the northern SCS. SCS.The ∑REE The valuesREE values range rangefrom 32.97 from to 32.97 349.07 to μg/g, 349.07 withµg /ang, average with an value average of 1 value92.94 μg/g. of 192.94 The REEsµg/g. are The dominated REEs are by dominated LREE, with by a P LREE,mean value with of a mean154.36 valueμg/g; the of 154.36mean valueµg/g; of the HREE mean is 16.11 value μg/g. of HREE The ∑REE is 16.11 valuesµg a/g.re high The in REEthe western values areGuangdong high in the coastal western waters Guangdong. Two accumulation coastal waters. centers Two are accumulation identified near centers the areXiachuan identified and near Hailing the XiachuanIslands, and and the Hailing lowest Islands, value is and located the lowest in the value eastern is located Leizhou in Peninsula the eastern and Leizhou Qiongzhou Peninsula Strait. The and Qiongzhoufractionation Strait. parameters The fractionation reflect a remarkable parameters LREE reflect accumulation, a remarkable relatively LREE accumulation, flat HREE patterns, relatively and flat HREEnegative patterns, Eu anomalies. and negative The Eu REEs anomalies. compositions The REEs are mainly compositions controlled are mainly by source controlled rock properties, by source rockhydrodynamic properties, conditions, hydrodynamic and intensity conditions, of chemical and intensity weathe ofring. chemical The study weathering. area was The divided study in areato three was dividedzones according into three the zones results according of Q-cluster the results analysis of Q-cluster. Zone Ianalysis. is around Zone on I the is around continental on the shelf continental off the Guangdong Province, with a mean ∑REE value ofP 233.96 μg/g. The sediments from zone I have relatively shelf off the Guangdong Province, with a mean REE value of 233.96 µg/g. The sediments from zone I uniform and higher LREE and HREE concentrations, indicating sources from the Pearl River and have relatively uniform and higher LREE and HREE concentrations, indicating sources from the Pearl Taiwanese rivers. Zone II is located in the coastal area off Leizhou Peninsula and eastern end of the River and Taiwanese rivers. Zone II is located in the coastal area off Leizhou Peninsula and eastern end Qiongzhou Strait, and it has an averaged ∑REE value of 107.00 μg/g. The distribution patterns of REEs P µ ofindicate the Qiongzhou elevated influences Strait, and from it has the an Hainan averaged Island REEand Qiongzhou value of 107.00 Strait. gZone/g. The III covers distribution the outer patterns shelf of REEs indicate elevated influences from the Hainan Island and Qiongzhou Strait. Zone III covers waters and eastern Hainan Island. The mean ∑REE value in this zoneP is 180.81 μg/g. The plots of LREE and theHREE outer show shelf that waters the Hainanand eastern Island Hainan and SCS Island. slope The sediments mean REE dominate value in in this this zone. zone isBased 180.81 onµ g the/g. Thecompositions plots of LREE of REEs, and grain HREE size, show geochemical that the Hainan characteristics Island, andsub-bottom SCS slope profiles, sediments and hydrodynamic dominate in thisconditions zone. Basedin the study on the area compositions, a modern sediment of REEs, dispersal grain size, route geochemical can be reconstructed characteristics,. sub-bottom profiles, and hydrodynamic conditions in the study area, a modern sediment dispersal route can beAuthor reconstructed. Contributions: Conceptualization, Q.G., Z.G.X. and F.C.; methodology, Q.G.; software, Q.G.; validation, Q.G., Z.G.X. and F.C.; writing—original draft preparation, Q.G.; writing—review and editing, Z.G.X. and F.C.; project Authoradministration, Contributions: Q.G. andConceptualization, F.C.; funding acquisition, Q.G., Z.G.X.Q.G. and and F.C. F.C.; All methodology, authors have read Q.G.; and software, agreed Q.G.;to the validation,published Q.G.,version Z.G.X. of the and manuscript. F.C.; writing—original draft preparation, Q.G.; writing—review and editing, Z.G.X. and F.C.; project administration, Q.G. and F.C.; funding acquisition, Q.G. and F.C. All authors have read and agreed to the publishedFunding: This version research of the was manuscript. funded by the National Programme on Global Change and Air-Sea Interaction (Grant No. GASI-GEOGE-03), and the National Natural Science Foundation of China (Grant Nos. 41476047 and 41106045). Funding: This research was funded by the National Programme on Global Change and Air-Sea Interaction (GrantConflicts No. of GASI-GEOGE-03), Interest: The authors and thedeclare National no conflict Natural of Science interest. Foundation of China (Grant Nos. 41476047 and 41106045). Conflicts of Interest: The authors declare no conflict of interest. Water 2020, 12, 3540 11 of 13

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