Distribution of rare earth elements and methane concentrations the east china sea water column: A comparison with the east Vietnam Sea (Bien Dong)

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” DOI: 10.15625/vap.2019.000102 128 DISTRIBUTION OF RARE EARTH ELEMENTS AND METHANE CONCENTRATIONS THE EAST CHINA SEA WATER COLUMN: A COMPARISON WITH THE EAST VIETNAM SEA (BIEN DONG) Le Duc Luong 1,2 , Nguyen Hoang 1 1 Institute of Geological Sciences, Vietnam Academy of Science and Technology, 84 Chua Lang, Hanoi, Vietnam 2 Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam Email: leducluong@igsvn.vast.vn ABSTRACT This research presents the distribution of dissolved REE and methane concentrations in the East China Sea water column and a comparison with the East Vietnam Sea (Bien Dong). In general, the REE concentrations of the shelf water are markedly higher than those of Okinawa Trough water in the East China Sea. The dissolved REE contents of the East Vietnam Sea water are relatively similar to those of the slope water and the Okinawa Trough water and significantly lower than those of the continental shelf water in the East China Sea, except for the case of Ce. Both the East China Sea and the East Vietnam Sea contain methane gas emission sites, indicating deep fluid input into the seawater that influences the chemical composition of water masses, especially near the bottom water layer. Key words: East China Sea, Okinawa Trough, East Vietnam Sea, methane anomalies. 1. INTRODUCTION Scientists have been increasingly interested in geochemistry of Rare Earth Elements (REE) and methane in seawater in recent years, because of their important roles in tracing water masses (Elderfield and Greaves, 1982) and determining the possible input sources (Anh et al., 2014; Luong et al., 2018). Previous studies (e.g., Luong et al., 2018) have investigated spatial REE concentrations in the East China Sea and some important data were reported. According to Shakirov et al. (2017), the East China Sea is belong to the Eastern Asia gas hydrate belt, including the Bering Sea, the Sea of Okhotsk, the Sea of Japan, the East China Sea (ECS), the East Vietnam Sea (EVS – Bien Dong) and direct southward to off-shore New Zealand. However, the data on methane concentration in the East China Sea water has been few to date and the relationship between REE and methane in seawater was not linked for clearer evaluation of input sources to water masses. In addition, the comparison of REE and methane concentration within the ECS and the EVS water has been made to show the differences of input sources between the two marginal sea regions. 2. METHOD Dissolved REE concentrations in the ECS was mainly studied using the published data and materials in Luong et al. (2018). An inter-basin comparison of REE has been made between the ECS water and the EVS (Bien Dong) water (Alibo and Nozaki, 2000). The analytical procedure was performed in a clean room at Laboratory of Rock and Geochemistry, Department of Physics and Earth Sciences, University of the Ryukyus, Japan following the method described in Luong and Shinjo (2011). Dissolved methane concentrations in the ECS and EVS water were obtained following method introduced by Obzhirov (1993) using an LHM 8MD, Shimadzu 14 and CrystalLux4000M gas chromatographs, equipped with a flame ionization detector (FID) and two detectors with thermal conductivity (TCD). Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 129 3. RESULTS AND DISCUSSION 3.1. Distribution of dissolved REE and water masses in the ECS water The distribution of dissolved REE in the ECS water is shown in Fig. 1 for some typical elements (La, Ce, Eu, Lu) at the depth ~ 50 m. The most noticeable feature can be seen that markedly higher values of REE concentrations in the shelf water compared to those of Okinawa Trough water, where the KC has dominated. Luong et al. (2018) emphasized the role of fluvial input from Chinese mainland and scavenging processes on the shelf water. The KC plays important role as natural boundary of dissolved REE concentrations between the shelf water and the Okinawa Trough water. Figure 1. The distribution of dissolved REE in the ECS at water depth of ∼ 50 m. 3.2. Comparison of dissolved REE in the ECS and the EVS (Bien Dong) water column Figure 2 demonstrates the inter-basin comparison of REEs vertical profiles within the ECS water with the EVS waters (Station PA-11, after Alibo and Nozaki, 2000). The first comparison (line A) shows that, the dissolved REE contents of the EVS water are relatively similar to those of the slope water and the Okinawa Trough water and significantly lower than those of the continental shelf water in the ECS. The Okinawa Trough water and the EVS water (line B) have fairly similar REE profiles within their basin, except for the case of Ce. The Ce contents in the EVS water is totally higher than those in the Okinawa Trough water, from the surface to bottom. Figure 3 presents a chain of NPDW-normalized REE patterns of the ECS shelf water (Luong et al., 2018) and the EVS water (Alibo and Nozaki, 2000). The most striking feature of these patterns is large MREE enrichment compared with LREE and HREE. However, the strong negative Gd anomalies, which are the typical property of EVS water patterns, are not observed in the ECS shelf water. Hồ Chí Minh, tháng 11 năm 2019 130 Figure 2. Inter-basin comparison of REE profiles within the ECS and the EVS (A and B). Figure 3. NPDW-normalized REE patterns of the ECS shelf water and the EVS water. 3.3. Methane concentrations in the ECS and the EVS (Bien Dong) near the bottom water 3.3.1. The ECS Obzhirov (1993) showed that the dissolved methane concentrations have a very high variability in the near-bottom water layer of ECS (Fig. 4). In the Okinawa Trough at depth about 1440m methane concentrations vary from 8 nl/l to 23730 nl/l. High anomalous methane concentrations were found in some tectonic fault areas of Okinawa Trough. The concentrations of methane exceeds 500 nl/l at 120-1440m water depths in the northwest off Okinawa Trough (Fig. 4). The maximal content of 23730 nl/l was found at site 25°04'18N, 122°35'20''E, which appears to be active Yonaguni hydrothermal vent (Fig. 4). High anomaly of methane indicates active emission of endogenic volatiles into the water column. 3.3.2. The EVS (Bien Dong) Vietnam continental shelf can be divided into 3 parts according to the methane distribution: northern, central and southern (Shakirov, 2018). The contrast variability of dissolved methane concentrations in the near-bottom water of the western part of the EVS were observed (Obzhirov, 1993). The low methane concentrations (10-20 nl/l) were found in the deep-water (500-3800m) of the sea. On the Vietnam continental shelf (depths of 30-200 m) and slope (depths of 200-500 m), the minimum (background) methane concentration increases to 30-40 nl/l. The abnormal methane concentrations within the EVS water indicate long-term diffusion of methane from the sediments and donot indicate active fluid venting into the water column, in comparison to high methane anomalies, caused by active fluid venting and gas seepage of the Okinawa Trough in the ECS. Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 131 Figure 4. Distribution of methane concentrations in the ECS. 4. CONCLUSION In the ECS, the REE concentrations of the shelf water are markedly higher than those of Okinawa Trough water. The comparison result shows that the dissolved REE contents of the EVS water are relatively similar to those of the slope water and the Okinawa Trough water and significantly lower than those of the continental shelf water in the ECS, except for the case of Ce. Further research will be needed to reveal the relationship between the shelf and slope water with the deep basin water of EVS as well as the role of fluvial input and coastal water masses. Methane concentrations in the near bottom water layer indicates that the endogenic volatiles in the ECS has significant anomalies within the hydrothermal vents of Okinawa Trough. In contrast, the EVS has a low methane concentrations in the near bottom water layer and expose anomalous fields in the northern and southern of Vietnam shelf. 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