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Article

Geochemical Behavior of Rare Earth Elements in Tidal Flat Sediments from Qidong Cape, Yangtze River Estuary: Implications for the Study of Sedimentary Environmental Change

by
Yunfeng Zhang
1,2,3,
Zhenke Zhang
2,*,
Wayne Stephenson
3,* and
Yingying Chen
4
1
Research Institute of Jiangsu Coastal Development, Yancheng Teachers University, Yancheng 224007, China
2
School of Geography and Ocean Science, Nanjing University, Nanjing 210023, China
3
School of Geography, University of Otago, Dunedin 9054, New Zealand
4
School of Geography, Geomatics and Planning, Jiangsu Normal University, Xuzhou 221116, China
*
Authors to whom correspondence should be addressed.
Land 2024, 13(9), 1425; https://doi.org/10.3390/land13091425
Submission received: 1 August 2024 / Revised: 27 August 2024 / Accepted: 2 September 2024 / Published: 4 September 2024
(This article belongs to the Section Land, Soil and Water)
Browse Figures
Versions Notes

Abstract

:
Sediment transport to the sea by rivers is crucial for the stability of estuaries and coasts. The Yangtze River, the largest river in China, like many large rivers worldwide, is experiencing a decrease in sediment load reaching the coast. However, the tidal flat around Qidong Cape, located at the entrance of the North Branch of the Yangtze Estuary, is undergoing extensive siltation. The source of this sediment is unclear. In this study, a sediment core was collected and the geochemical characteristics of rare earth elements (REE) were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The results indicate the following: (1) The average content of REE is 178.57 μg/g, and the average ratio between LREE and HREE is 8.66, which is comparable to sediments from the South Yellow Sea. The chondrite-normalized and UCC-normalized patterns resemble those of the Yangtze River and the South Yellow Sea, indicating a negative gradient, a weak Ce-negative anomaly, and a distinct Eu-negative anomaly. (2) The continental shelf deposits in eastern China are primarily derived from sediment flux delivered by rivers. The sediments in the South Yellow Sea mainly originate from the Yangtze River and the Yellow River, exhibiting characteristics of a mixed source due to long-term geological processes, namely geochemical processes. The REEs in the tidal flat around Qidong Cape inherit the source area’s characteristics and originate from the weathering of upper continental rock in mainland China. Moreover, the tidal flat around Qidong Cape is influenced by both runoff and tidal actions, leading to strong land–sea interactions and reducing the environment, explaining the Eu-negative anomaly. (3) Hydrodynamic forces in the North Branch of the Yangtze River have shifted from runoff to tidal dominance since the 1930s. However, marine hydrodynamics outside the estuary have remained unchanged. Consequently, the Subei coastal current plays a key role in sediment transport and diffusion. Sediments from the south wing of the Radiative Sand Ridge in the South Yellow Sea are transported southward by the Subei coastal current, and under tidal influence, suspended sediment is deposited in the tidal flat around Qidong Cape. Therefore, the sediment source has gradually shifted from the Yangtze River to the South Yellow Sea.

1. Introduction

Sediment transport to the sea by rivers is important for the development of estuaries and coasts. To provide a preliminary assessment of current trends in the sediment loads of the world’s rivers, longer-term records of annual sediment load and runoff have previously been assembled for 145 major rivers; the majority showed evidence of declining loads, and human activity was the most important influence on sediment fluxes [1,2]. Sediment loads are reduced by human activity, which includes soil conservation and sediment control, especially dam construction, which represents the dominant cause of reduced loads [3,4]. Reduced sediment delivered to the coastal zone results in accelerated coastal erosion and a decrease in habitat [5]. One of the most typical is the Nile River: the coastline of the Nile Delta has experienced accelerated erosion since the construction of the Aswan High Dam in 1964, especially at the Rosetta and Damietta promontories [6,7].
Similar to many rivers around the world, Chinese rivers face the problem that sediment loads show significant decreases [8]. As the largest river in China, Yangtze River showed that the decadal sediment load at Datong (a key hydrological monitoring stations located in the lower reaches of the trunk river) began to decrease in the 1970s [9]. Sediment flux then decreased dramatically after the Three Gorges Dam (TGD) was built in 2006, reaching a historical low of 85 × 106 t, or just 18% of the 1970s’ sediment load [10,11]. The resulting lowering of sedimentation rates on the tidal flats is now below the critical level required to maintain the delta, triggering erosion along parts of the coast [12,13,14].
The Yangtze River Estuary has three-level bifurcation and four outlets entering the sea (refer to Figure 1b). The first bifurcation is divided by Chongming Island into the North Branch and the South Branch [15,16]. The North Branch is approximately 80 km long, extending from Chongtou in the west to Lianxing Port in the east. Most recently, the North Branch has been narrowing and shallowing because of human activity [17,18].
Rare earth elements (REEs) are stable in earth surface depositional environments. Their compositions and distribution patterns are not influenced by weathering, transportation processes, sedimentation and diagenesis [19]. As a result, REEs are an extremely useful tool for tracing sediment provenance and have been frequently used in estuary and coastal areas [20,21,22,23]. It is widely accepted that the characteristic parameters of REEs, including the concentrations in sediment, fractionation, and normalized patterns and anomalies of Ce and of Eu, can be used to discriminate sediment provenance [24,25,26,27,28,29].
Qidong Cape is located at the entrance of the North Branch. Therefore, as one of the source tracers, the geochemical features of REEs are evaluated based on a sediment core taken from the tidal flat around Qidong Cape, at the entrance to the Yangtze River Estuary. Accordingly, the objective of this study is to discriminate sediment sources in the study area. Additionally, the sediment transport process is discussed based on changes in the hydrodynamic environment. The sediment sources and transport processes are important contents of soil–sediment–water systems. We hope that this study can increase our understanding of human living environments on the Earth’s surface.

2. Study Area

The tidal flat around Qidong Cape is located at the crossing point between the North Branch of the Yangtze River and the Jiangsu coastline (refer to Figure 1b). This area along the North Branch of the Yangtze River is a key area for coastal wetland conservation, and Spartina alterniflora, a species of salt marsh plant, is the dominant species. S. alterniflora plays a key role in protecting the coast and embankments. It was introduced from the United States to China in 1979 [30] and subsequently cultivated in the tidal flats around Qidong Cape. Due to the suitable environment and vigorous reproductive growth, S. alterniflora rapidly expanded, becoming the dominant species and forming a unique ecological landscape on the muddy tidal flat. The mudflat serves as the substrate of the intertidal zone outside of the S. alterniflora salt marsh. Erosion scarps exist extensively between the S. alterniflora salt marsh and the mudflats.
In the 1950s, coastal defenses and roads were constructed to improve regional traffic and resist storm surges. Since then, people have been reclaiming the higher tidal flats for economic development, including agriculture, salt harvesting, and aquaculture. The most recent embankment dike was completed in 2006. Human reclamation activities have become increasingly significant, resulting in the coastlines along Qidong Cape advancing seaward by about 6 km from the coastal defend road, namely first dike [31]. These activities have strongly altered the original coastal morphology and sedimentary environment. The study area has been experiencing multiple pressures and rapid changes in the tidal flat and salt marsh environment over the past decade. In view of this, the government established the Yangtze River Estuary (North Branch) Wetland Nature Reserve in 2002, and the tidal flat around Qidong Cape is the core conservation area. Since then, various measures have been implemented to protect the Yangtze Estuary wetland, including ecological restoration, watershed management, and legal frameworks [32].
From field investigations, it was found that tidal flat around Qidong Cape is undergoing extensive siltation [31]. The findings seem to contradict the observation of sediment load decreasing in the Yangtze River. The question, then, is where this large amount of sediment comes from. Previous research at tidal flats around Qidong Cape includes sediment grain size characteristics [33,34], sedimentation rates [35], morphological changes [36], and human reclamation activity [37]. However, the problem of where the sediment comes from is still not well resolved.

3. Materials and Methods

3.1. Sampling

During field investigations and observations at tidal flats around Qidong Cape, we retrieved a sediment core (coded QDZ-1) 215 cm in length in August 2011. The corer was a semicircular gravity sampler produced by Dutch Eijkelkamp Ltd., Giesbeek, The Netherlands. The sampling point is located at coordinates 31°41.926′ N and 121°53.633′ E (refer to Figure 1c) in a conservation area dominated by S. alterniflora salt marsh. This site was selected because it is not directly influenced by human activities, allowing it to reflect natural changes in the tidal flat environment. After measuring and comparing the depth of the hole and the length of the core, minimal compaction was observed. It needs to be specifically explained here. Hussain et al. revisited Hybrid event beds (HEBs) and the greywacke problem [38]. The focus of their attention is the ancient sedimentary record and deep-marine depositional systems. In this paper, the focus of our attention is contemporary sedimentation processes and tidal flat depositional systems. There are essential differences between the two in terms of spatiotemporal scale and environmental characteristics. Thereby, on-site, 43 sediment samples were obtained at 5 cm intervals along the core. Soon after, all samples were sealed in sterile plastic Ziplock bags. These samples were then transferred to the laboratory and stored frozen until being prepared for analysis.

3.2. Laboratory Analyses

Sediment samples were measured using inductively coupled plasma mass spectrometry (ICP-MS) at the State Key Laboratory of Endogenous Mineral Deposits Research, Nanjing University. ICP-MA is the most powerful analytical tool in the quantification of these impurities due to its high sensitivity, wide dynamic range, and low detection limit. The steps to determine REEs are as follows [39]: (a) Sample preparation. After each sample was ground into 200 mesh powder, 25 mg powdered samples were digested with 1 mL of HF and 0.5 mL of HNO3 in PTFE-stainless steel bombs at 190 °C to evaporate. The residues were then dissolved with 5 mL of HNO3 at 140 °C. (b) Instrumentation and operational parameters. Rhodium was used as an internal standard to correct matrix effects and instrument drift. The operating conditions and settings are shown in Table 1. After cooling, the upper solution was used to measure REEs by HR-ICP-MS (Finnigan MAT Ltd., Bremen, Germany, ELEMENT2).

3.3. Data Processing

The best way to evaluate Ce- and Eu-distinctive deviations (positive or negative) from their neighboring pairs of elements is to calculate the Ce anomaly (δCe) and Eu anomaly (δEu) [20,23,25,40]. Following the approach proposed by these authors, we use their formulas:
δCe = CeN/(0.5 × (LaN + PrN))
δEu = EuN/(0.5 × (SmN + GdN)
In these equations, CeN, LaN, PrN, EuN, SmN, and GdN use the chondrite-normalized value [41]. Ce anomaly and Eu anomaly are divided into three types: (a) a value of 1 means the element is not fractionated relative to the crustal composition and the curve is smooth and straight; (b) a value < 1 indicates negative anomalies with concave curves, that is to say, depletion relative to its REE neighbor’s yields; (c) a value > 1 indicates positive anomalies with convex curves, that is to say, enrichment relative to its REE neighbor’s yields.

4. Results

4.1. REE Concentration

A total of 14 REEs were measured in this study; six elements (La, Ce, Pr, Nd, Sm, Eu) are called light rare earth elements (LREEs), and the remaining eight elements (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) are called heavy rare earth elements (HREEs). The results are shown in Figure 2 and Figure 3. The total REE contents range from 160.99 to 216.84 μg/g and the average is 178.57 μg/g. The LREEs range from 143.98 to 195.23 μg/g and the average is 160.07 μg/g. The HREEs range from 16.34 to 21.61 μg/g and the average is 18.50 μg/g. The LREE/HREE ratios range from 8.15 to 9.03, which indicates the enrichment of LREEs and HREEs in the core sediments. The enrichment of LREEs is an indicator of terrigenous sediment [42,43]. Based on average values, the LREEs are 8.66 times that of the HREEs, and this reveals that core sediments from the study area have characteristics of terrigenous sediment.
From Figure 2 and Figure 3, the 14 rare earth elements and their characteristic parameters (REE, LREE, HREE, and LREE/HREE) exhibit nearly identical vertical distribution in the core. Furthermore, these distribution curves can be segmented into two distinct stages. In the upper portion of the core, labeled as part A, there is a low-amplitude fluctuation, which is especially present above depth 52.5 cm, accompanied by a regular variation. Conversely, in the lower section of the core, labeled as part B, there is a high-amplitude fluctuation. Notably, a maximum peak is observed at a depth of 172.5 cm. As is well known, detrital materials predominantly control the REE characteristics of sediment and the transportation of detrital materials is closely related to the dynamic sedimentary environment [19]. The study area, located at the estuary, is inherently exposed to extreme environmental events. These can include the floodwaters from the upstream regions of the river or storm surges from the sea. Therefore, the maximum peak observed in the sedimentary record is likely an indication of these extreme events.

4.2. REE-Normalized Patterns

To analyze the REE patterns, we used chondrite-normalized and Upper Continental Crust (UCC)-normalized values, which have been widely applied [26,27,44,45]. In this study, chondrite normalization was performed using values from Boynton [41]. The normalized values from core sediments exhibit a rightward deviation pattern with a negative gradient (refer to Figure 4a). The curve for La to Eu is steeper, while that for Eu to Lu is smoother, indicating an enrichment of LREEs in the sediments relative to HREEs. The chondrite-normalized values also show a “V” shape with a significant negative Eu anomaly. These values are higher than those of the Yellow River sediments and closer to those of the Yangtze River and the South Yellow Sea sediments. The Yellow River flowed into the Yellow Sea in Jiangsu Province from 1128 to 1855, resulting in the formation of the Subei alluvial plain from the large amounts of sediment it transported [46]. Therefore, the Yellow River sediments were also included for comparison. It is generally accepted that there is no fractionation between light rare earth elements and heavy rare earth elements in chondrite [47]. Consequently, the chondrite-normalized pattern indicates that the sediments in the study area have been fractionated with characteristics of terrigenous sediments, which are distinct from the distribution curve of oceanic deep-sea sediments. The latter typically shows a significant Ce-negative anomaly [48,49].
The Upper Continental Crust (UCC) was normalized using the values from Rudnick and Gao [50] for average core sediments, showing a relatively flat linear shape without significant Ce and Eu anomalies (refer to Figure 4b). These normalized values are also higher than those of the Yellow River sediments and closer to those of the Yangtze River and the South Yellow Sea sediments. It is generally accepted that UCC normalization can reflect the degree of differentiation of rare earth elements in sediments during source rock weathering, transportation, and deposition, as well as the effects of mixing and homogenization [51]. Offshore sediments are primarily derived from terrigenous debris, and thus the content and distribution pattern of rare earth elements are similar to those of continental material [48]. Therefore, the chondrite-normalized pattern indicates that the sediments in the study area share characteristics with continental crust, mainly originating from the weathering material of the Chinese mainland crust. This suggests that no significant differences occurred during transportation and deposition from the source area to the estuary region [49].

5. Discussion

Regarding the sedimentation process of tidal flats, several key factors are involved: sediment source, hydrodynamic environment, and human activities. In the previous results, we discuss in detail the effects of human activity on tidal flats [37]. Reclamation activities in the past half century made the coastline move about 6 km seawards [31]. Based on continuous field observations from 2007 to 2012 and grain size analysis [37], we have come to know that the S. alterniflora plays a significant role in protecting the coastline and promoting sediment deposition. Additionally, the concave coastline formed by the embankment dike constructed in 2006 is conducive to the deposition of tidal flats. Consequently, this discussion focuses on the discrimination of sediment sources and the change in the hydrodynamic environment.

5.1. Anomalies of Ce and Eu

Ce and Eu anomalies can reflect the degree of rock weathering of the sediment source area and the oxidation–reduction conditions at the depositional area [52]. As shown in Figure 2 and Figure 3, δCe ranged from 0.88 to 1.02 and the mean was 0.96, without significant anomalies. During marine deposition, the residence time of Ce in seawater is much shorter than that of other REEs. Usually, Ce3+ will easily oxidize to insoluble Ce4+ under the pH-Eh conditions of seawater and then precipitate in the form of CeO2, which will cause rapid loss of Ce from seawater and be concentrated in bottom sediments [21]. Organic carbon production is enhanced by nutrients from the Yangtze River organic matter input, which causes oxygen deficiency and formation of the hypoxic zone [53]. Therefore, despite the presence of saline water, seawater can be excluded as the source of Ce. Consequently, in the absence of locally weathered rock, the terrigenous sediments in the study area are the source of Ce, rather than seawater.
δEu ranged from 0.62 to 0.71, and the mean was 0.67, with significant negative anomalies. Eu3+ restored to soluble Eu2+ under a strong reducing environment and then caused Eu anomalies. The estuarine tidal flat is located in the mixing zone of fresh river water and seawater and is a strong reducing environment [20]. The Yangtze Estuary is a semi-enclosed coastal body of water where salt and fresh waters meet and mix and tide and riverine currents interact in the same location [15]. Thus, chondrite indicates that sediment from the study area has produced a significant differentiation, and is similar to continental crust, reflecting the characteristics of shelf sediments [54].

5.2. Discrimination of Sediment Source

The sediment source is a crucial material basis for tidal flats. Sediments on the China Sea coastal shelf primarily originate from the Yellow River and the Yangtze River, as well as weathered erosive material from continental rocks along the coast. The Yellow River and the Yangtze River transport approximately 1 × 109 t and 5 × 108 t of sediment annually, respectively [8]. Due to its distance from the study site, as shown in Table 2, sediments from the Yellow River have only a minor influence. In contrast, the Yangtze River is the most significant source for the study area due to its proximity. The similarity in REE concentration, characteristic parameters, and normalized patterns of sediments from the study area indicates that the materials derive from the Chinese mainland rather than from seawater.
Similar to many large rivers worldwide, the Yangtze River has experienced a decline in sediment load delivered to the sea due to intense development in the river basin, particularly with the construction of dams [12,13,55]. Historically, the weathering and transport of sediment by the Yangtze River has been the primary sediment source for the Yangtze River Estuary [56]. As shown Figure 5, there has been a rapid decline in the Yangtze River’s sediment load since the 1960s. Recent research indicates that the reduced sediment supply has decreased the progradation rate of the delta and triggered erosion at the delta front [57]. Critically, the rare earth elements (REE) in the sediments are absorbed by debris or suspended material, carried into the sea by the river, and finally deposited in a stable state. According to the results of 137Cs dating, the average sedimentation rate calculated from the core sediment is 2.61 cm/a [35]. We estimate that the depth of 172.5 cm corresponds to approximately the year 1930. The time marker at a depth of 52.5 cm is estimated to be around 1972. However, the REEs did not exhibit the same downward trend. Therefore, it can be inferred that there is an additional sediment source, besides the sediments from the Yangtze River, in the study area.
Another possible sediment source is the South Yellow Sea. The sediments of the South Yellow Sea primarily originate from the Yangtze River and the Yellow River, exhibiting characteristics of a mixed source due to long-term geological processes [58]. The rare earth element (REE) parameters from the South Yellow Sea sediments indicate an enrichment of light rare earth elements, a moderate negative Eu anomaly, and no Ce anomaly [43]. As shown in Table 2 and Figure 3, the REE and LREE/HREE ratios in the study area closely resemble those of the South Yellow Sea sediments. Additionally, the normalized patterns of REEs also exhibit similar characteristics. Therefore, it is likely that the study area has received sediment from the South Yellow Sea in addition to the Yangtze River Estuary. This change is attributed to alterations in the sedimentary dynamic environment, with the runoff from the Yangtze River now playing a lesser role than in the past.

5.3. Change in Sedimentary Dynamic

Due to the study area’s location at the entrance of the North Branch of the Yangtze River, suspended sediment transport is influenced by two important currents (refer to Figure 6). The first is the runoff from the Yangtze River to the sea, and the second is the current along the northern coast of Jiangsu, known as the Subei coastal current.
At the first bifurcation of the Yangtze River, significant changes in the sediment dynamics of the North Branch have occurred over the past century. As shown in Figure 7, the discharge ratio has decreased year by year, and this evolution can be divided into three stages [18]: (1) Before 1931, runoff was the dominant hydrodynamic force, and the evolution of the North Branch channel was primarily influenced by natural variations. (2) From 1931 to 1970, the dominant hydrodynamic forces began to shift from runoff to tidal influence. This change was driven by several major flood events, such as those in 1931, 1949, and 1954, as well as reclamation activities. (3) After 1970, tidal forces became the dominant hydrodynamic influence, coupled with the impact of human activities. Consequently, the North Branch transformed into a tide-dominated estuary with bifurcation.
As shown in Table 3, the North Branch of the Yangtze River does not transport sediment into the sea, and consequently has become narrower and shallower; the net sediment transport is upstream. Sediment moves in both directions under flood and ebb tides, but there is a net movement upstream, especially with spring and neap tides [59,60].
Based on the results of 137Cs dating [35], we estimate that the depth of 172.5 cm corresponds to approximately the year 1930. Based on instrumental and historical data, there were significant floods in the Yangtze River catchment in 1931 [61], which had a considerable impact on the riverbed of the Yangtze River. Considering the dating error, the maximum REE peak observed at a depth of 172.5 cm is likely the result of the 1931 flood events. The time marker at a depth of 52.5 cm is estimated to be around 1972, which aligns closely with the year 1970 when considering the dating error. Since then, tidal processes have become dominant over runoff, and hydrodynamic forces have become less significant. Consequently, as reflected in the core profiles, REEs show regular changes with low-amplitude fluctuations. This supports previous investigations indicating that the sediment load from the Yangtze River is no longer the primary source in the study area.
The Subei coastal current consistently flows southward, with a tendency to cross the Yangtze River Estuary in winter [62]. When the sediment dynamics of the North Branch of the Yangtze River shifted from runoff to tide-dominated, the Subei coastal current remained stable without any significant changes. Consequently, suspended sediment arriving at the entrance of the North Branch of the Yangtze River moved upstream from outside the estuary due to the flood and ebb tides. This suspended sediment primarily originates from the radial sand ridges on the floor of the South Yellow Sea. Recent studies indicate that these sand ridges have been moving southward over the last 50 years under the influence of the Subei coastal current, particularly the southern wing of the sand ridges [63]. This explains why these sediments have accumulated at the entrance of the North Branch of the Yangtze River, specifically at the tidal flat around Qidong Cape.

6. Conclusions

The REE parameters recorded environmental changes based on core sediments (named QDZ-1) from tidal flats around Qidong Cape, located at the entrance of the North Branch of the Yangtze River. REE analysis provides a valuable and insightful tool for reconstructing changing environmental conditions. The concentrations and characteristic parameters of REEs are similar to those of sediments from the Yangtze River and the South Yellow Sea, indicating that the sediments have inherited the characteristics of their source areas. These also hold great potential for similar investigations in other large river deltas worldwide.
As one of source tracers, REEs are an effective tool for discriminating sediment sources. Based on the geochemical features of REEs, the tidal flat around Qidong Cape originally had two main sediment sources: one from the Yangtze River and another from the outer sea. These primary sediment sources have shifted with changes in hydrodynamic conditions since the 1930s. In addressing our research question, REE analysis has demonstrated that the main sediment source of the tidal flat has transitioned from the Yangtze River to the radial sand ridges of the South Yellow Sea.
Sediment transport cannot be separated from the hydrodynamic environment. With the gradual decrease in sediment load from the Yangtze River to the sea and the declining load–discharge ratio of the North Branch over time, the sedimentary environment at the entrance of the North Branch has changed significantly. Over the past century, sediment dynamics have shifted from being runoff-dominated to tide-dominated. Consequently, the sediment source has also changed, transitioning from primarily Yangtze River sediments transported by runoff to sediments from the radial sand ridges of the South Yellow Sea carried by the Subei coastal current.

Author Contributions

Conceptualization, Y.Z. and Z.Z.; Methodology, Y.Z. and Y.C.; Investigation, Y.Z. and Z.Z.; Resources, Z.Z.; Writing—original draft preparation, Y.Z. and W.S.; Writing—review and editing, Y.Z. and W.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Social Science Found of China, grant number 21BGJ010.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We sincerely thank the editor and all reviewers for their valuable suggestions for improving the presentation of this paper. Thank you to everyone who contributed to this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic map of the Qidong Cape and the sediment core location. (a) the location of Qidong Cape in China; (b) the location of Qidong Cape in the Yangtze River Estuary; (c) the location of the sediment core in Qidong Cape.
Figure 1. Schematic map of the Qidong Cape and the sediment core location. (a) the location of Qidong Cape in China; (b) the location of Qidong Cape in the Yangtze River Estuary; (c) the location of the sediment core in Qidong Cape.
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Figure 2. The vertical profiles of REE concentrations in core sediments.
Figure 2. The vertical profiles of REE concentrations in core sediments.
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Figure 3. The vertical profiles of characteristic parameters of REEs in core sediments.
Figure 3. The vertical profiles of characteristic parameters of REEs in core sediments.
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Figure 4. The normalized patterns of REEs in core sediments. (a) Chondrite-normalized pattern; (b) Upper Continental Crust (UCC)-normalized pattern.
Figure 4. The normalized patterns of REEs in core sediments. (a) Chondrite-normalized pattern; (b) Upper Continental Crust (UCC)-normalized pattern.
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Figure 5. Comparison between gross amount of REEs and sediment load by Yangtze River. (a) Profile curve of gross amount of REEs; (b) sediment load and water discharge by Yangtze River. Solid lines is REE in (a); Solid lines is sediment load in (b); Grey area is water discharge in (b); Dotted lines is trend line in (a,b).
Figure 5. Comparison between gross amount of REEs and sediment load by Yangtze River. (a) Profile curve of gross amount of REEs; (b) sediment load and water discharge by Yangtze River. Solid lines is REE in (a); Solid lines is sediment load in (b); Grey area is water discharge in (b); Dotted lines is trend line in (a,b).
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Figure 6. Study area (a) and regional hydrographic circulation (b). SCC is Subei coastal current; CDW is Changjiang Diluted Water; ZMCC is Zhe-Min Coastal Current; TWC is Taiwan Warm Current; KC is Kuroshio Current; YSWC is Yellow Sea Warm Current.
Figure 6. Study area (a) and regional hydrographic circulation (b). SCC is Subei coastal current; CDW is Changjiang Diluted Water; ZMCC is Zhe-Min Coastal Current; TWC is Taiwan Warm Current; KC is Kuroshio Current; YSWC is Yellow Sea Warm Current.
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Figure 7. The discharge ratio of the North Branch of the Yangtze River. (a) Discharge ration in flood season; (b) Discharge ration in dry season. Adapted from Ref. [18].
Figure 7. The discharge ratio of the North Branch of the Yangtze River. (a) Discharge ration in flood season; (b) Discharge ration in dry season. Adapted from Ref. [18].
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Table 1. Summary of the operating conditions used for ELELEMT2 (ICP-MS) measurements.
Table 1. Summary of the operating conditions used for ELELEMT2 (ICP-MS) measurements.
ParameterValue
Plasma power1280 W
Reflected power<2 W
Cooling gas14.5 L/min
Auxiliary gas0.8 L/min
Sampling gas0.85/min
Points per peak15
Resolution300, 4000, 10,000
Table 2. Comparisons of REE parameters between the core sediments and other areas.
Table 2. Comparisons of REE parameters between the core sediments and other areas.
ParametersREE/(μg/g)LREE/(μg/g)HREE/(μg/g)LREE/HREEData Source
Min160.99143.9816.348.15This paper
Max216.84195.2321.619.03This paper
Average178.57160.0718.508.66This paper
the Yangtze River211.10193.1917.9110.79[44]
the Yellow River131.56120.0011.5610.38[44]
the South Yellow Sea170.22152.8717.358.81[43]
Table 3. The flow and sediment flux at the North Branch of the Yangtze River.
Table 3. The flow and sediment flux at the North Branch of the Yangtze River.
The Type of TideFluxLianxing Port Station
Flood TideEbb TideNet Load
Spring tideFlow/108 m319.5918.09−1.50
Sediment/104 t293.7221.20−72.50
Medium tideFlow/108 m314.8113.23−1.58
Sediment/104 t165.00107.0058.00
Neap tideFlow/108 m38.646.841.80
Sediment/104 t29.2715.45−13.82
Postscript: the negative value indicates that sediment is transported upstream, that is to say, there is diffusion and backflow of water and sediment. Data from Ref. [59].
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Zhang, Y.; Zhang, Z.; Stephenson, W.; Chen, Y. Geochemical Behavior of Rare Earth Elements in Tidal Flat Sediments from Qidong Cape, Yangtze River Estuary: Implications for the Study of Sedimentary Environmental Change. Land 2024, 13, 1425. https://doi.org/10.3390/land13091425

AMA Style

Zhang Y, Zhang Z, Stephenson W, Chen Y. Geochemical Behavior of Rare Earth Elements in Tidal Flat Sediments from Qidong Cape, Yangtze River Estuary: Implications for the Study of Sedimentary Environmental Change. Land. 2024; 13(9):1425. https://doi.org/10.3390/land13091425

Chicago/Turabian Style

Zhang, Yunfeng, Zhenke Zhang, Wayne Stephenson, and Yingying Chen. 2024. "Geochemical Behavior of Rare Earth Elements in Tidal Flat Sediments from Qidong Cape, Yangtze River Estuary: Implications for the Study of Sedimentary Environmental Change" Land 13, no. 9: 1425. https://doi.org/10.3390/land13091425

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