Lin-song Yu, Hong-bo Liu, Fang Wan, Zun-fang Hu, Huai-dong Luo, Xiu-wen Zhang

1 Applied Nuclear Technology in Geosciences Key Laboratory of Sichuan Province, Chengdu Universtity of Technology, Chengdu 610059,China.

2 College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China.

3 Shandong Institute of Geophysical and Geochemical Exploration, Jinan 250013, China.

Abstract: Dongping Lake area, located in the lower reaches of Yellow River, is an ideal place to study the changes of modern river and lake sedimentary environment. The sediment samples of Dawen River, Yellow River, and Dongping Lake were collected, and the major elements, trace elements and organic matter geochemical composition of the samples were analyzed. Cluster analysis, characteristic element ratio method and graphic method were used to explore the geochemical characteristics of sediments and their environmental implication. The results show that the contents of SiO2, Na2O, TiO2 and Zr in sediments of Dawen River and Yellow River are relatively high, and the contents of iron and manganese oxides, organic matter, CaO, P2O5 and Sr in lake sediments are relatively high. That reveals the differences of sedimentary environments between the rivers and the lake. The contents of Sr and Zr in Dawen River are affected by the rapid migration of clastic materials in the upstream carbonate source area during the flood season; the δCe,ΣREE and REE’s ratios in the sediments of the Yellow River reflect the influence of the Loess source; and the distribution of elements changes along the flow direction during the flood season. The characteristics of pH, element composition and LREE ＆ HREE fractionation of the lake sediments indicate that the sediment source is complex, and the lake environment is affected by the flood season. The study shows that the geochemical content and its variation characteristics of sediments effectively reveal the sedimentary environment, material composition and characteristics of flood season of rivers and the lake in the study area.

Keywords: Dongping Lake; Yellow River; Sediments; Organic matter; Element geochemistry

Introduction

Sediment is one of the main media in rivers and lakes, and it is the sink of watershed material(Håkanson and Jansson, 1983; Andjelkovic et al.2017). It accumulates important geochemical information about the sediment sources of the basin and regional environmental change (Fralick et al. 1997; Cai et al. 2013; Yan and Zheng, 2014;Bekteshi et al. 2014; Jin et al. 2004; Sun et al.2016; Wang et al. 2018; Babeesh et al. 2018).Therefore, the geochemical study of river and lake sediments is helpful to understand the distribution of chemical elements in sediments under different sedimentary conditions and their indicative significance.

Dongping Lake is located in the transition section from wandering channel to curved channel in the lower Yellow River reach, which is an ideal place to study the changes of river and lake sedimentary environment. Rich achievements have been made in the study of the geochemical composition of sediments in the Yellow River and Dongping Lake, and large amounts of research data have been accumulated. Typical studies are as follows: He et al. (1997), Gong et al. (2013) and Yang et al. (1999, 2002, 2003) systematically studied the major and the composition of trace elements, differentiation mode and control factors of the Yellow River sediments; Wang et al. (2010)and Yue et al. (2016) discussed the provenance environment of detrital sediment composition and geochronology of the Yellow River; Zhang et al.(2013) and Hu et al. (2019) evaluated the stable isotope composition and water quality evolution of the Yellow River Basin and Dongping Lake; Pang et al. (2018) and Chen et al. (2019) systematically studied the mineral composition of the Yellow River sediments; Dou et al. (2000), Rao et al.(2018) and Li et al. (2019) evaluated the evolution of organic matter ,heavy metals and hydrochemical environment in Dongping Lake sediments, and discussed the relationship between Dongping Lake changes and the Yellow River flood since Holocene.However, compared with other major rivers in the world, the source tracing and discussion on environmental changes in the Yellow River Basin are still in the exploratory stage (Yang et al. 2002;Yue et al. 2016; Chen et al. 2019). In particular,there are few reports on the elemental geochemistry of the sedimentary environment of rivers and the lake in Dongping Lake area of the lower Yellow River reach. Based on the sparse sampling analysis in the study area during the flood season, the major and trace element composition characteristics and their sedimentary geochemical significance were preliminarily discussed.

1 Materials and methods

1.1 Study area

The study area is located in Dongping Lake section of the lower Yellow River reach. Dongping Lake is the second largest lake in Shandong Province,which is located in the west of Jinan City. In Fig. 1,Dongping Lake is a typical shallow lake in the North China. The main water source comes from the Dawen River in the southeast of the lake,which flows into the Yellow River after being stored in Dongping Lake. Both Dongping Lake and Dawen River belong to the Yellow River Basin. The Yellow River sediment source is famous for the huge amount of sediments from the Loess Plateau. The sediment source of the Dawen River is metamorphic rocks of Taishan Group(Ar3T) of Archean and carbonate rocks of Cambrian and Ordovician of Paleozoic from the upper reaches. The sediments of Dongping Lake are mainly sediments carried by Dawen River and lacustrine sediments. The study area belongs to the continental semi-humid monsoon climate zone in the warm temperate zone. The annual average temperature is 13.4 ℃ and the average annual rainfall is 640 mm. The study area is located in the Taishan Uplift on the western margin of the Luzhong Uplift in the North China Plate. The basement consists of the Archean crystalline rock series, overlaid by Cambrian and Ordovician carbonate rocks of the Paleozoic and the Quaternary fluvial and lacustrine sediments.Dongping Lake is developed in the joint zone of the Yellow River fan-shaped plain and piedmont alluvial proluvial plain. The geomorphic landscape of the lake area is a low mountain and hilly area with tectonic denudation.

1.2 Sampling

In all, 6 surface sediment samples were collected in the study area (Fig. 1). Two surface sediment samples were collected in Dongping Lake by using a portable grab made of stainless steel (Fig. 2(A))in July 2019. The sampling depth was 0-15 cm,and the upper 0-10 cm were fine particles taken as the sediments of the lake during the flood season for testing and analysis. A sample of fine sediments from the Dawen River floodplain was collected. The sampling site was about 4 km away from Dongping Lake. Three samples of fine sediments from the floodplain of the Yellow River were collected (Fig. 2(B)). The sampling points were about 8 km to 12 km away from Dongping Lake (Fig. 1), and the sampling location was the Yellow River floodplain on the side of Dongping Lake. S13 was located in the Yellow River floodplain near Dongping Lake. Then these samples were taken back to the laboratory for corresponding chemical analysis. Major elements and trace elements analysis were carried out at the Test Center of Shandong Provincial Bureau of Geology and Mineral Resources.

1.3 Sample analysis

Fig. 1 Schematic representation of Dongping Lake with location of sampling sites

Fig. 2 Sampling of Dongping Lake surface sediments (A) and sediment sampling of the Yellow River floodplain (B)

The samples collected in the study area were put into polyethylene bags, and the whole rock samples of ＜63 μm were extracted by precipitation method in the laboratory and dried at low temperature (45 ℃). Ion selective electrode method(ISE) was used to analyze pH and potassium dichromate volumetric method (vol) was used to analyze organic matter. Major oxides and a few trace elements were determined by PANalytical AxiosMax X-ray fluorescence (XRF) spectrometry(Holland). The test process was carried out according to the THEORY OF XRF by Peter Brouwer with ISBN: 90-9016758-7. For each sample, a 0.600 0 g split of the sediment fraction obtained accurately was put into platinum crucible.The mixed reagent of lithium metaborate and lithium carbonate was added, and the reagent and the sample were stirred evenly. The sample was completely decomposed by melting at 1 100℃.Rare earth elements (REEs)were determined by PerkinElmer NexION 2000 inductively coupled plasma mass (ICP-MS) spectrometer (USA). For each sample, a 0.050 0 g split of the sediment fraction obtained accurately was placed in the polytetrafluoroethylene sample dissolving tank of high pressure closed tank 1.50 mL of high-purity HNO3and 1.50 mL of high-purity HF were added into the tetrafluoroethylene sample dissolving tank.The high-pressure closed tank was tightened and digested at 180℃ for 18 hours to completely decompose the sample.

The accuracy of the analytical method was checked by using the national first-level standard sediment standard material (GBW series). River sediment standard materials GBW07385,GBW07386, GBW07387 were used for quality control on the major elements; river and tidal flat sediment standard materials GBW07389,GBW07452 were used for quality control on the trace elements. The results showed that the relative deviation of the analytical quality of each element was from 1.0% to 1.5%, the maximum analytical error was less than 0.5%, and the relative deviation of all the analytical elements was less than 5%,indicating that the overall analytical quality was reliable.

1.4 Methodology

In order to analyze the correlation of each element in the sediment data of the study area, the element data were processed separately. In this study, SPSS 19.0 was used for R-type cluster analysis of major elements in sediment samples. Excel 2019 was used to process the characteristic ratio of trace elements in sediment samples, and various geochemical diagrams were made.

R-type cluster analysis is a statistical technique to classify variable groups (Han et al. 2001).Cluster analysis is widely used in the study of sediment geochemistry (Liu et al. 1984; Ji et al.1995). The main purpose of R-type cluster analysis is to divide the elements in the study area into element assemblages and find out the geochemical element assemblages that can represent the sedimentary characteristics of the area.

In terms of geochemical tracing of sediments,characteristic element ratio method and geochemical graphic method are the most commonly used and effective methods (Liu et al. 1984; Taylor 1985; Chen et al. 1999; Yang et al. 2002;Andjelkovic et al. 2017; Babeesh et al. 2018). The absolute contents of some major and trace elements may change during transportation,sedimentation and diagenesis, but the chemical properties of these elements are similar and have consistent enrichment and dispersion rules.Selecting two elements or their combination with good correlation to calculate their content ratio or make geochemical diagrams can indicate the characteristics of sediment source area and sedimentary environment. The element ratio method used in this paper includes REEs ratio(such as δEu, δCe, (La/Lu)N, ∑LREE/∑HREE) and trace element ratio (such as Sr/Rb), and geochemical graphic method includes bivariate plot of related elements and normalized geochemical diagram of REEs.

2 Results and discussion

2.1 Geochemical characteristics of major elements

2.1.1 pH and the contents of organic matter and major elements

Concentrations of major oxides and organic matter and pH values are given in Table 1. It can be seen that the pH of the sediments is relatively stable,which is moderately weakly alkaline. The tendency of pH generally follows: pH Loess in China ＞ pH Yellow River sediments (S11, S12, S13) ＞ pH Dongping Lake sediments (S6, S9) ＞ pH Dawen River sediments (S1). These test data (S11, S12,S13) are consistent with the conclusion that the pH of the Yellow River Basin is above 8.0 perennially according to Li et al. (2010). In general, the pH values of sediment samples in the study area change little. However, the pH of Dongping lake sediment is slightly lower than what was reported by Song et al. (2011), which can be related to the large amount of water supply in the lake during the flood season.

The content of organic matter in the sediments of the study area is higher than the average organic matter content of Loess in China. Comparing the content of organic matter in the sediments of river and lake, we found that the spatial distribution of organic matter is very uneven. The organic matter content of surface sediments of Dongping Lake is significantly higher than that in river sediments.The content of organic matter in sediment sample S13 of the Yellow River is high, which could be related to the wetland environment near the intersection of the Yellow River and Dongping Lake. These results suggest that the content oforganic matter and microorganisms in the surface sediments of Dongping Lake may selectively lead to the enrichment of organic matter. This conclusion is consistent with the study of Dou et al. (2000).

Table 1 Contents of metal oxides, organic matter and pH distribution in the study area / wt%

The major elements in the study area are mainly composed of SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O and K2O (Fig. 3). In the river sediments, these 7 oxides account for more than 90% of the total amount of sediments, while in the lake sedimentary environment, organic matter is one of the main components, resulting in 7 oxides accounting for less than 90%. In general, the content of metal oxides in river sediments is not different from that in the upper continental crust (UCC), slightly higher than that in the Yangtze River (CJav) and the Yellow River (HHav), while the content of MgO, CaO and MnO is higher than that in the Loess of China. The chemical compositions of the sediments, in accordance with the different sedimentary environment, exhibit relatively wide variations (Table 1andFig. 3), especially SiO2from 28.535wt% (S9) to 68.176wt% (S1), Na2O from 0.637wt% (S9) to 2.840wt% (S1), and CaO from 5.311wt% (S11) to 21.559wt% (S9) (Table 1).

Fig. 3 Diagram of major elements in surface sediments

2.1.2 R-type clustering and discussion

As shown in the R-type cluster analysis graph(Fig. 4), the major elements with “cluster recalibration distance” less than 10 are divided into four clusters. The four clusters are as follows: Iron (Fe)and manganese (Mn) oxides (Fe2O3-MnO-MgO),detrital components (SiO2-Na2O), characteristic mineral components (Al2O3-K2O-TiO2) and organic matter components (CaO-organic matter-P2O5).

Fig. 4 R-type cluster analysis of major elements

Fe and Mn oxides represent the muddy assemblage. It is generally believed that the high content of Fe2O3and MnO in the sediments represents the increase of argillaceous composition,and the content of MnO and MgO is related to the fine-grained sedimentary minerals in the lake. S6 and S13 are Lake and river wetland sediment samples respectively, and their Fe2O3, MnO and MgO contents are relatively high, which means that they have more argillaceous components;while S1, S11 and S12 are floodplain sediment samples, and their Fe2O3, MnO and MgO contents are relatively low, but still higher than HHav and close to UCC.

SiO2and Na2O are positively correlated, and they are the same cluster in cluster analysis (Fig. 4).SiO2is the main component element of terrigenous clastic quartz, and Na2O is the main component element of plagioclase. Both of them are easy to be enriched in coarse-grained sediments. The contents of SiO2and Na2O in the sediments of Dawen River and Yellow River floodplain are higher in flood season, which reflects that the river has strong hydrodynamic force in flood season and carries more coarse-grained sediments. However, the content of SiO2and Na2O are negatively correlated with that of CaO. The content of CaO in lake sediments is significantly higher than that in river sediments. Studies have shown that the ratio of CaO/MgO is usually used to indicate the change of endogenous CaCO3content in lakes (Wu et al.2004; Yang et al. 2004). The higher ratio indicates that the environment at that time had a trend of transition to warmer (Yang et al. 2004). This may be due to the fact that in the high temperature environment, plankton reproduction consumes a lot of CO2, which makes the balance of Ca2++ HCO3-→CaCO3+ CO2move to the right, resulting in the increase of sediment CaCO3content. In this study,the ratios of CaO/MgO in lake sediments were 5.4(S6) and 11.507 (S9), while those in river sediments were 1.67 (S1), 2.91 (S11), 2.88 (S12)and 3.17 (S13). Meanwhile, organic matter, CaO and P2O5belong to the same group (Fig. 4) in the cluster analysis diagram, which shows their correlation and also reflects this biogeochemical process in the Dongping Lake sediments.

Al2O3, K2O and TiO2belong to the same cluster(Fig. 4), indicating that they may have almost the same geochemical properties. It is generally believed that Al2O3and K2O in sediments mainly exist in clay minerals, while the main source of TiO2is detrital minerals (Liu et al. 1984). Studies have shown that in the clay minerals formed at low temperatures, aluminum (Al) is difficult to replace silicon (Si) in silicon tetrahedrons (Liu et al. 1984),and Al can form stable complexes in lakes rich in humus and migrate for a long distance. Potassium(K) is easily adsorbed by clay minerals, and its positive correlation with Al also shows the similar relationship between the two oxides in the selective adsorption of clay minerals. K is easily adsorbed by clay minerals, and its positive correlation with Al also shows the similar relationship between the two in the selective adsorption of clay minerals. The relationship between the organic matter in the S9 sample and Al2O3and K2O may be related to the eutrophication process represented by the lake-rich organic matter. The specific gravity of Ti bearing minerals is relatively high, and only when the flow energy is large can they be transported to the floodplain and the estuary (Lan et al. 2015).Previous studies showed that there were heavy minerals containing Ti in the study area, mainly ilmenite, anatase, leucosphenite, rutile and so on.(Chen et al. 2019). Seen from Table 1, the content of TiO2in these sediment samples is slightly higher (or close to) UCC, but significantly higher than that of Loess in China. The content of TiO2in Yellow River sediment samples (S11, S12, S13) is higher than that in Dongping Lake. This indicates that the transport capacity of detrital material increases with the increase of water volume in the flood season, which leads to the increase of TiO2content in the Yellow River sediment. In the diagram of the relationship between TiO2and organic matter (Fig. 5), the river sediments are concentrated in the lower part of the diagram,reflecting the relatively similar detrital components of TiO2. However, there is a negative correlation between the content of TiO2and organic matter in lake sediments (S6, S9). Some studies indicated that the content of Ti was inversely proportional to the content of organic matter in some soils or sediments, because humic acid could lead to the loss of Ti (Liu et al. 1984; Zhang, 2017).

Fig. 5 Bivariate plot of Organic matter vs. TiO2

From the bivariate plot of TiO2vs. Al2O3(Fig. 6 (a)) for the investigated sediments, the linear relationship between the points of the Yellow River sediments is significant (R2=0.899 8), and their evolution sequence (Fig. 6 (a) dotted line arrow) indicated the synchronous increase trend of Al2O3and TiO2content along the runoff direction. However, the lake sediments were affected by multiple factors such as hydrodynamic force and material source and so on, showing the inverse sequence (Fig. 5 (a) solid line arrow)relationship. Meanwhile, from the bivariate plot of TiO2vs. zirconium (Zr) (Fig. 6 (b)), the scatter points of sediment samples are in the felsic rocks’distribution area. However, the distribution of river sediments is more on the right side than lake sediments, indicating that the source area of river sediments is richer in felsic composition. This is consistent with the previous studies on the mineral components of the Yellow River sediment by Chen et al. (2019). In addition, it can be seen from Figure 6 (b) that the content of Zr in river sediments is relatively high. The content of Zr is characterized by sampleDawenRiver＞ HHav ＞ Cjav ＞sampleYellowRiver＞ sampleDongpingLake. Zirconium is relatively stable in geochemical properties and easy to accumulate in rivers transported in close distance (Liu et al. 1984). The Dawen River in the study area is short, and the debris source in the denuded residual mountain area is relatively rich,so it is easy to form Zr accumulation by short distance transportation; while the Yellow River carries more loess material with the increase of water volume in wet season, but it is easy to cause separation of Zr and other heavy minerals in floodplain sediments, thus reducing its content.

Fig. 6 Bivariate plot of TiO2 vs. Al2O3 (a) and TiO2 vs. Zr (b)(Hayashi et al. 1997)

2.2 Geochemical characteristics of trace elements

2.2.1 The contents of REEs

Rare earth elements (REEs) are weak alkaline elements, and their hydroxides precipitate at pH close to 6-8 under supergene action (Liu et al.1984). The pH of these sediments is 8.0±, which is conducive to the precipitation of REEs. From the trace elemental abundances of these sediments(Table 2), the content of REEs (∑REE, excluding Y) in Dongping Lake surface sediments and Dawen River sediments fluctuates between 141.85 ppm and 198.41 ppm, which is higher than that of HHav, and the mean content of ∑REE is relatively close to that of CJav and UCC. The ∑REE in the Yellow River sediment fluctuates between 133.85 ppm and 210.79 ppm, which increases with the runoff direction, and the mean content of ∑REE is relatively close to that of Loess in China (Wen,1989).

From UCC standardized spider diagram of REE in the study area (Fig. 7), it can be seen that REE patterns of the study area are similar with CJav,HHav, showing a gently inclined pattern to the right. This is different from the slightly flat distribution pattern of REE of Loess in China.Moreover, these samples’ curves of REE patterns around CJav and Loess in China show a wide range of elements abundance, which are quite different from HHav, HHW, HHjn, and Rav,indicating the abundance of grain size and provenance of the whole rock samples during the flood season.

2.2.2 Characteristic element ratio and discussion

The europium anomaly (δEu) calculated by UCC standardization is 0.92-0.98, and the δEu is not significant. This indicates that there is no significant difference of REE in the surface sediments of the study area compared with UCC.Cerium (Ce) is a variable valence element. Its ionic potential is quite different from that of REE3+, so it is easy to separate from other REEs (Liu et al.1984; Sheng et al. 2015). There is no obvious Ce anomaly (δCe) in these sediments, which is manifested as a positive δCe (S6, S9, S13) lower than Rav, and a negative δCe (S1, S11, S12) higher than HHav and Cjav. This may indicate that the material source carried by the river during the flood season is affected by the natural weak alkaline water, that is, the pH value of the precipitation during the flood season is medium alkaline. It can also be seen that the δCe in theYellow River sediments S11, S12, and S13 is slightly higher than the δCe in HHjn and HHw,and is close to the δCe in Loess in China. This directly indicates that a large amount of material carried by the river during the flood season may have the characteristics of loess source. In addition, the δCe of Dawen River sediment (S1) is negative, which may be affected by the increase of carbonate clastic components in the flood season.

Table 2 Contents of REE and trace elements in the study area / ppm

Fig. 7 Comparisons of UCC-normalized REE patterns

The UCC normalized ratio, such as (La/Sm)Nand (Gd/Yb)N, are significant for indicating the fractionation of light REEs (LREEs) and heavy REEs (HREEs). According to the relevant parameters of REEs in Table 2, the ratio of(La/Lu)Nof surface sediment in Dongping Lake is higher than that of the river sediment, and the average value of (La/Lu)Nis close to that of CJav.The high value of sample S9 is related to the content of organic matters, which possibly connects with the presence of REEs in colloidal form in organic-rich sediments. The ratio of(La/Lu)Nin the Yellow River sediments is lower than that of HHjn and HHW, close to the mean value of Rav, but higher than that of UCC, which showed the REEs composition characteristics in the flood season. The ratio of (La/Yb)Nin the Dongping Lake and Dawen River sediment is higher than that in the Yellow River sediment,which indicates that the fractionation between LREE and HREE tends to increase in the flood season, especially in the lake sediment. The ratio of (Gd/Yb)Nin the Yellow River sediment is between HHWand Rav, which indicates that the fractionation of HREE tends to decrease in the flood season. According to the bivariate plot of(La/Lu)Nvs. (La/Sm)N(Fig. 8(a)), the straight line fitted to the sediment sample points has a positive correlation with the direction of river flow in the direction extending along the straight line (River flow direction: From south to north, hydrodynamic direction indicated by the sample: S1→S9,S11→S13, R2=0.859 4). The projection points of Loess in China and Rav are outside the linear relationship, while the projection points of HHjn and HHw are near the fitting line.

Fig. 8 Bivariate plot of (La/Lu)N vs. (La/Sm)N (a) and∑HREE vs. ∑LREE (b)

From the ratio of ΣLREE/ΣHREE (Table 2) in surface sediments, the ratio of Dongping Lake is higher than that of Dawen River and Yellow River,and closer to Rav. It indicates that LREEs are relatively enriched in lake sediments. Furthermore,from the Fig. 8(b) that the sediment point of the Yellow River in the study area is near the fitting straight line, while the sediment point of Dongping Lake is relatively far away from the straight line.Distribution trend of sediment points in the study area indicates the synchronous increase between LREE and HREE along the river direction. HHw and HHjn are on the extension line of the linear relationship, which reflects the characteristics of the river sedimentation, while the Loess in China,Rav, and S9 deviate from the linear relationship,which indicates the difference of their sedimentary environments respectively.

Fig. 9 Bivariate plot of Rb vs. Sr in Dongping Lake(a) and the Yellow River (b)

Fig. 10 Bivariate plot of SiO2 vs. Rb/Sr (a), Sr vs.Rb/Sr (b) for the investigated sediments

Rubidium (Rb) and strontium (Sr) are widely used in Paleoenvironmental studies due to their distinct geochemical properties in supergene environments. In recent years, through the comprehensive analysis of the ratio of Rb/Sr and other environmental proxy indexes, some scholars had found that the Rb/Sr of lake sediments reflected the environmental changes of the watershed very well (Chen et al. 1999; Zeng et al.2011). Chen et al. (1999) believed that low value of Rb/Sr generally occurred in the period of high temperature and abundant rainfall. From the diagram of the relationship between Rb and Sr(Fig. 9), there is a clear linear relationship(R2＞0.95), which indicates the correlative changes of sedimentary samples along the runoff direction(arrow direction in Fig. 9). More concretely, the change of the Rb/Sr ratio reflects the difference in hydrodynamic carrying capacity between the Yellow River and Dongping Lake. The relationship of Rb and Sr of Dongping Lake Basin shows negative correlation. The water carrying capacity of the basin slows down from Dawen River to Dongping Lake. The decreasing trend of Rb content corresponds to the decreasing trend of the fine-grained weathered material of carbonate rocks in the upper reaches in the process of transferring to the lake, which reflects the partial hydrolysis of Rb. The relationship of the content of Rb and Sr in the sediments of the Yellow River shows a positive correlation along the river direction. It is related to the stronger hydrodynamic conditions in the flood season, and indicates that the content of Rb and Sr in the floodplain sediments experiences a gradual accumulation process, which could be related to the adsorption of clay minerals.

From Fig. 10, the river and lake sediments respectively correspond to the different characteristics of point distribution. The point sites of lake sediment samples (S6, S9) are far away from the point sites of river samples (S1, S11, S12, and S13). Figure 10 (a) shows that the sediments of Dawen River and Yellow River have relatively high SiO2abundance compared with that of Dongping Lake sediments, which indicates that the main components of the terrigenous debris are different between the lake and rive sediments.Fig. 10 (b) shows that the sediments of the Yellow River and the Dawen River have a stronger Sr leaching trend compared with those of Dongping Lake. The high content of Sr in lake sediments (S6,S9) may indicate that Sr precipitates from water or forms evaporating sediments; Dawen River Sediment (S1) is relatively high affected by carbonate rocks in the upstream source area; and Yellow River Sediments (S11, S12, S13) are close to Loess in China, which may indicate the result of rapid deposition in wet season.

3 Conclusions

This study reveals the difference of sedimentary environment between rivers and the lake. The Yellow River and Dawen River sedimentary environment are dominated by relatively coarse clastic components, while the lake sedimentary environment is relatively rich in organic and argillaceous components, and the chemical components of lake sediments are affected by biogeochemical processes.

The element content of sediments in the study area is affected by the source of sedimentary materials and flood season. The Yellow River sediment is a single source of loess, and the geochemical characteristics of elements in the sediment change along the flow direction in flood season. The source of the composition of lake sediments is complicated, and the increase of water supply has aggravated the fractionation of LREEs and HREEs in flood season. The element content of Dawen River sediment reflects the rapid migration of clastic materials in the source area of carbonate rocks in flood season.

The comparative study on the sedimentary records of Dongping Lake area in the lower Yellow River reach shows that the content and variation characteristics of chemical elements in the sediments can provide geochemical tracing tools for the study of sedimentary environment and material sources.

Acknowledgements

This study was supported by Opening Fund of Provincial Key Lab of Applied Nuclear Techniques in Geosciences (No. 201904), and Key Scientific and Technological Project (No.KY201957) of Shandong Bureau of Geology and Mineral Resources.