water 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 and the SCSaccumulation sediments areprocesses highly localizedof 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 adjacentmaterials to [17] the. ChineseThe 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 to sink. (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 ' to 7.28 ', with a mean value of 5.60 '. 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.