Introduction

The chemical composition of river sediments does not only contain information about that of the bedrock in the area of erosion, but also shows the degree and trend of weathering in the catchment area. Taking this into consideration, research of chemical composition of river sediments plays an important part in study of processes of continental crust formation, and contributes to the mechanistic understanding of geochemical cycles in river basins. The latter is proved by significant interest of researchers in different aspects of the problem. So, Savenko (2006) and Gordeev (1983) have synthesized in their works data from their own research and data from publications concerning major and trace elements. Also, there is a large stock of analytical findings about different continents which are summarized in series of articles (Gaillardet et al. 1999; Viers et al. 2009; Li and Yang 2010).

Collectively, it has been shown that weathering and transfer of materials by river is controlled not only by topography and climate (Liu et al. 2007, 2008), but also by rock composition and tectonic conditions in the drainage area (Dalai et al. 2002; Afshar et al. 2010; Hren et al. 2007; Liu et al. 2007; Singh 2010; Besalatpour et al. 2013). Therefore, an important aspect in studying weathering is the analysis of relationship between chemical weathering and physical denudation (erosion) (Edmond and Huh 1997; Gaillardet et al. 1999; Millot et al. 2002; Yang et al. 2004; Li and Yang 2010).

So far, there has been detailed research on the chemical composition of river sediments in major rivers of tropical and subtropical regions of Asia including the Huang He and the Yangtze (Yang et al. 2004), the Xi Jiang, the Hong Ha, the Mekong (Liu et al. 2007), as well as the rivers of Taiwan (Selvaraj and Chen 2006; Liu et al. 2008), Malaysian Peninsular, Northern Borneo (Wang et al. 2011) and Tibetan highlands (Borges et al. 2008). The authors of the publications have scrutinized the dominant factors of weathering process. For example, observations (Yang et al. 2004) (in the Changjiang and Huanghe drainage basin) showed that climate dominated in regulating silicate weathering in both river basins, whereas parent rock and landscape were secondary factors. Similar conclusions were made by Wang et al. (2011), they designated that climatic conditions with warm tropical temperature and monsoon rainfall were primary factors controlling rate of chemical weathering in Eastern Asia, whereas tectonic activity and parent rock lithology were secondary. Other authors (Liu et al. 2007; Selvaraj and Chen 2006) indicated that in the south of China, Hindustan peninsular and Taiwan Isle, it was not the warm monsoon climate that played the most significant part in control of weathering processes, but the tectonics caused by physical erosion. Finally, group of authors (Borges et al. 2008) studying sediments of large rivers in the east of Tibetan Plateau concluded that sediment composition had maturity stage which did not directly relate to climatic or physiographic regime; the maturity was a paleomorphic feature inherited from previous sedimentary cycles. However, the processes of formation of river sediments in the rivers of temperate and high-temperate latitude in Asia have not been studied systematically (Sorokina and Zarubina 2011, 2013; Makhinova et al. 2014; Sirotskii et al. 2014).

As it is mentioned above, the chemical composition of river sediments does not only provide information about the composition of bedrock in the place of erosion, but shows the degree and trend of weathering. It is known that peculiar features of weathering are identified with upper continental crust (UCC)-normalized concentration method (Chen et al. 2008; Ahmad and Chandra 2013; Yang et al. 2013) as well as indices (WIP, CIA, CIW, PIA, V, etc.) and modules, which have been developed for the exploration of sedimentary rock (Chen et al. 2008; Wu et al. 2013; Yang et al. 2013; Babechuk et al. 2014). Since weathering of bedrock normally results in the elimination of basic cations and Si as well as relative accumulation of aluminum and iron, the processes of weathering are often illustrated with A–CN–K or SiO2–Al2O3–Fe2O3 diagrams (Babechuk et al. 2014; Chen et al. 2008; Qiu et al. 2014; Yang et al. 2013).

Weathering indices are generally used for measuring the degree of weathering (Table 1). These indices are expressed as molar ratios of immobile to mobile rock-forming oxides (Yousefifard et al. 2012, 2015), for example, indices based on ratio of mobile and immobile elements (Al is considered immobile in the variables) and considered to measure the extent of feldspar-to-clay conversion.

Table 1 Representative chemical weathering indices and lithochemical modules
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Nesbitt and Young (1982) developed the Chemical Index of Alteration (CIA) in reconstructing paleoclimate from Early Proterozoic sediments of the Huronian Supergroup, north of Lake Huron. The CIA is interpreted as a measure of the extent of conversion of feldspars (which dominate the upper crust) to clays such as kaolinite.

Harnois (1988) developed the Chemical Index of Weathering (CIW). This index is similar to the CIA, except that it eliminates K2O from the equation. But the CIW may yield very high values for K-feldspar-rich rocks, whether they are chemically weathered or not (Fedo et al. 1995).

Fedo et al. (1995) proposed the Plagioclase Index of Alteration (PIA) as an alternative to the CIW. As plagioclase is abundant in silicate rocks and dissolves relatively rapidly, the PIA may be used when plagioclase weathering needs to be monitored (Fedo et al. 1995).

Other weathering indices evaluate the degree of chemical weathering by tracing the mass transfer of labile elements, as in the Weathering Index of Parker (WIP; Parker 1970) or using simple molecular ratios such as SiO2/Al2O3 (Ruxton 1968) (Table 1). Most of these indices exploit Al2O3, TiO2 and Zr as normalizing elements on the assumption that their mass remained invariant during the weathering process.

Weathering Index of Parker (WIP) (Parker 1970) is used to estimate degree of weathering of silicate rock, and is based on ratio of alkaline and alkaline-earth elements in weathering product. Parker applied the index to acid and moderate extrusive rock.

Index R represents loss of Si oxide in relation to Al oxide presumably due to stability of the latter during weathering. Ruxton (1968) tested R on weathering profiles developed on igneous and metamorphic rocks in humid regions around the world, and found that R correlated well with total-element loss.

In 1927, the Vogt’s Residual Index V was introduced (Vogt 1927). It was later used to compare maturity of moraine and marine clays in Norway (Roaldset 1972). Nowadays, it is applied to characterize less mature rock.

In our work, we introduce the results on chemical composition of river sediments in the Zeya River (Middle reach) (Fig. 1), one of the largest watercourses in the basin of the Amur River. The objective of the research is examination of the main trend in the alteration of geochemical features of river sediments as a reflection of the weathering processes in bedrock within the catchment area.

Fig. 1
figure 1

Map showing the location of study area

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Materials and methods

Description of the study area

The Zeya River is one of the largest inflow watercourses in the Russian Far East. The drained area is more than 230 × 103 km2. A total of 30 river sediment samples were collected between 53.39° N and 127.06° E, and 51.39° N and 128.52° E in the drainage basin of the Zeya. A large part of the Zeya watershed is represented by low and medium–high mountains. Average absolute heights of interfluve area and hill groups amount approximately 400 m, maximum height is about 900 m. In the northwest, north and east, the mountains surround the central flat part, represented by a combination of plateaus, plains and ridges. The landscape of the river basin area is composed of tundra and taiga of mountain ranges, taiga forests of its central part, and mixed forests of plains, which are replaced by forest-steppe vegetation in the south.

In the upper part, the Zeya River drains the geological formations, represented by metamorphic and igneous complexes, attributable to a wide age interval from the Precambrian to the Mesozoic. Paleozoic and Mesozoic sedimentary and igneous rocks exposed in the base of the terraces (Krasny and Peng 1999) are dominant among original rocks outcropping in the Amur–Zeya plain. In the lower reaches of the Zeya River, the main source of material is Cenozoic unconsolidated deposits of the Amur–Zeya depression.

Due to sharp continental climate with summer monsoon features, the majority of rivers in the studied region belong to the group of rivers fed by rainwater. Annual precipitation in the basin area changes from 500–550 mm in plains to 1000–1100 mm at ridges: Stanovoy, Gianga and Turana. The rate of precipitation in river volume is no less than 50–80%, whereas snowmelt and groundwater contribute a modest 10–20% and 10–30% (Voskresensky 1973). All these climatic features result in irregularity of the river flow. The largest volume (about 70% of year norm) is reached in the warm season, mostly in July–September. Mean January temperature amounts to − 30 °C, mean July temperature is + 18 °C.

The Zeya River is divided into three parts according to the features of valley landscape and water regime: Upper, Middle and Lower Zeya (Muranov 1966). In present study, we consider the Middle Zeya part which is located from Zeya Gate to the mouth of the Selemdzha River.

Sampling and analytical method

Sampling of river sediments was conducted along the bank of the Middle Zeya in intervals of 10–15 km. In total, 30 samples were taken. Within sampled area, the sediment texture varied from sandy–clayey to clayey. Total weight of the samples was approximately 500 g. Samples were dried to gain air-dry condition and then fractions less than 1 mm were isolated from the material with the mechanical attritor.

The chemical composition of the samples was determined by X-ray fluorescence spectrometry on ICAP 6500 Duo (Thermo Electron Corporation, USA) and Pioneer S4 (Bruker, USA) spectrometers at the Institute of Geology and Nature Management, the Far East Branch of the Russian Academy of Sciences. For XRF analysis, powdered samples were homogenized by melting with a mixture of lithium metaborate and tetraborate in an electric furnace at 1050–1100 °C. The measurements were conducted on a Pioneer 4S (Bruker, Germany) spectrometer. The intensity of the analytical lines was corrected for the background, the absorption effects, and the secondary fluorescence.

Results

Main elements

River sediments are wide ranging in composition: SiO2 (50.30–67.39, medium 62.61), TiO2 (0.49–1.21, medium 0.84), Al2O3 (9.66–15.74, medium 14.41), Fe2O3 (4.16–7.93, medium 6.34), MnO (0.10–0.71, medium 0.21), MgO (0.88–1.99, medium 1.60), CaO (1.41–3.92, medium 2.66), Na2O (1.42–3.41, medium 2.29), K2O (1.41–2.29, medium 1.97), and P2O5 (0.12–0.27, medium 0.20). Analysis of absolute (Table 1) and UCC-normalized concentrations of elements (Fig. 2) shows that the composition of river sediments in the sampling area is depleted in K, Na, Ca and Mg oxides, whereas concentration of Si, Ti, Al and Fe oxides is close to UCC. All in all, concentration of elements and composition of river sediments in sampling area are close to geochemical standard PAAS—Post-Archean Australian Shale (Taylor and McLennan 1985).

Fig. 2
figure 2

UCC-normalized major elemental ratios of river sediments in Middle Zeya River. Dotted line represents same ratios of PASS (Taylor and McLennan 1985)

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Chemical Index of Alteration, Chemical Index of Weathering and Plagioclase Index of Alteration

The indices CIA (Nessbit and Young 1982), CIW (Harnois 1988) and PIA (Fedo et al. 1995) have a lot in common. They are based on the ratio of mobile and immobile elements (the indices treat Al as an immobile element during weathering process) and used to measure the degree of feldspar-to-clay conversion. Alkaline and alkaline-earth elements are leached during weathering. Simultaneously, CIA, CIW and PIA values increase up to 50 for unweathered rock and to about 100 for kaolin (Babechuk et al. 2014). Perfect trend of CIA is positive.

River sediments in the study area have narrow range of values, like CIA ranging from 49 to 65, CIW ranging from 53 to 72, and PIA ranging from 48 to 68 (Table 2).

Table 2 Chemical composition of representative samples of the river sediments of the Zeya Rivers, %
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It is obvious from the diagrams (Fig. 3) that the values of CIA, CIW and PIA gradually increase down the river flow which indicates positive trend.

Fig. 3
figure 3

The spatial variation of weathering indices of river sediments in Middle Zeya River. The dotted line indicates the trend. Samples are located downstream the river valley

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Weathering Index of Parker

The WIP values usually lie between ≥ 100 and 0, the least weathered rocks have the highest values, while strong chemical weathering is indicated by lower WIP values; perfect trend of index values is negative. WIP calculation is applicable to mobile alkaline and alkaline-earth elements, so use of the index with highly weathered rock is not always reliable. In the study area, the index values ranged from 39 to 65 (Table 2). WIP values decrease and show obvious descending trend (Table 1, Fig. 3) in the part of the river.

Ruxton coefficient

Index R represents loss of Si oxide in correlation to Al oxide presumably due to stability of the latter during weathering. Perfect trend of index R is negative. In Middle Zeya, R values of river sediments range widely from 3.6 to 5.5 (Table 2). However, as it is seen from the diagram, the weathering trend is descending (or negative) (Fig. 3).

Vogt’s Residual Index

In Middle Zeya, V index of river sediments increases from 1.0 to 2.0 (Table 2). V values gradually increase demonstrating a rising trend in the diagram (Fig. 3).

Lithochemical modules

Another measurement we used to study the weathering process was the method of lithochemical modules (Sklyarov 2001).

The value of potassium module (PM) (Yudovich and Ketris 2000) gives important information about K and Al distribution in rock-forming minerals. In the studied sediments, the values of the module are low and amounted 0.12–0.17 (average 0.14). The range of PM values alteration is not wide. Although the alteration trend is not very clear, the diagram still reflects descending trend of PM (Table 2, Fig. 4).

Fig. 4
figure 4

The spatial variation of potassium module (PM) and hydrolyzate module (HM) of river sediments in Middle Zeya River. The dotted line indicates the trend. Samples are located downstream the river valley

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Hydrolysate module (HM) ((Al2O3 + TiO2 + Fe2O3 + FeO + MnO)/SiO2) (Taylor and McLennan 1985) is used to characterize sedimentary rocks. Mean TM value of the studied river sediments is 0.35, range of values is quite wide from 0.25 to 0.45 (Table 2). The diagram reflects rising trend (Fig. 4).

A–CN–K and SAF diagrams

A–CN–K diagram is often used to illustrate the degree and trend of weathering (Fig. 5). The points reflecting composition of river sediments form a distinct line from CN (CaO + Na2O) apex towards A (Al2O3) apex. The line goes parallel to A–CN (CaO + Na2O–Al2O3) line.

Fig. 5
figure 5

Weathering trend from A-CN-K (a) and SAF diagrams of river sediments in Middle Zeya River, river sediments of south-east, east, north-east China. A = Al2O3; CN = CaO + Na2O; K = K2O (a); S = SiO2, A = Al2O3, F = Fe2O3 (b); major elements as molecular ratios; dotted area represent data of the soils of south-east, east, north-east China (Qiu et al. 2014), data of UCC, PAAS from ref. (Taylor and McLennan 1985)

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In SiO2–Al2O3–Fe2O3 diagram (Fig. 5), the composition points plot at SiO2 apex along the line which is perpendicular to Fe2O3–Al2O3. The points form a distinct trend towards A (Al2O3) apex.

Discussion

Data about chemical composition, content and ratio of elements of bedrock are widely used to analyze genesis of sediments, including river sediments, which inherit the composition of rock. Rock weathering in the drainage area influences the rock composition dramatically. Soluble elements are leached. Degree of transformation of parent rock in the zone of erosion can be estimated using data about chemical composition of sediments. According to the analysis of absolute and UCC-normalized concentrations of elements, compositions of channel deposits are substantially depleted in K, Na, Ca and Mg oxides. Concentrations of Si, Ti and Al oxides are close to UCC. Overall, the level of element concentration in river sediments of Middle Zeya is close to the composition of PAAS (Taylor and McLennan 1985), which is subjected to significant weathering.

The values of weathering indices and lithochemical modules present distinct trends down the river flow.

CIA, CIW, PIA and V indices, which are calculated from the ratio of mobile and immobile elements, reflect the range which is characteristic of medium weathering degree. Increase of weathering means that the rock is depleted in alkaline and alkaline-earth elements which is reflected in the increase of the index values.

CIA, CIW, PIA and V index values increase dynamically from Zeya Gate to the mouth of the Selemdzha River (Figs. 1, 3). In contrast, WIP and R index values decrease. These alterations have common trend which demonstrates significant contribution of well-weathered rocks into river sediments.

Changes of lithochemical modules prove the identified trend. PM values slightly decrease down the river, which could be caused by significant contribution of plagioclases in sediment composition. Hydrolysate module is used to differentiate the rocks containing hydrolysates from the rock with dominant earth silicon. The higher the HM in clayey rocks or sandstone, the more weathered the parent rock is in the area of erosion, consequently, the more mature the sediment is. HM of river sediments in the studied area increases (down the river), which reflects high maturity of the sedimentary rock in the area of erosion.

Diagrams

A–CN–C and SAF diagrams allow examining the rate and trend of rock weathering. UCC and PAAS gages are shown in A–CN–K diagram, and CIA table is placed next to the diagram. Weathering trend is marked with an arrow and correlates with the fundamental law of rock transformation from UCC to PAAS. Points representing composition of river sediments from Middle Zeya plot on the line between UCC and PAAS gages and illustrate increase of weathering degree. Besides, composition of river sediments from Middle Zeya reflects significant loss of K2O which designates high degree of bedrock weathering. Placement of the composition points of river sediments from Middle Zeya in SAF diagram proves increase of weathering in drainage area. This placement of the points indicates strong Fe/Al correlation in river sediments, but it also indicates significant difference in the content of Si. As weathering of parent rock is usually accompanied by loss of Si and accumulation of Al and Fe (Chesworth 1973; Chetsworth et al. 1981; Taylor and McLennan 1985; Singh 2010; Babechuk et al. 2014), we may presume that sediments of Middle Zeya, which are characterized by low content of Si, have mostly been formed from rock transformed by chemical weathering.

All in all, the results of the study show that the ratio of the main components of river sediments in Middle Zeya has changed which proves increase of chemical weathering.

Comparison with the rivers of southeastern Asia

Collected data enable us to compare geochemical features of river sediments in the Zeya and the Selemdzha Rivers to the sediments of the rivers of southeastern Asia. According to the data we have obtained and the publications we have reviewed (Gaillardet et al. 1999; Viers et al. 2009; Li and Yang 2010; Shao et al. 2012), river sediments of the region are characterized by a wide range of element concentrations which define the rate of bedrock weathering in the area of erosion. This feature is illustrated in A–CN–K diagram. The composition points in the diagram reveal common trend towards A (Al2O3) apex and plot parallel A–CN (CaO + Na2O–Al2O3) line. Composition of sediments in the Yantze and the Xi Jiang Rivers in the southeast of China is mostly depleted in CaO and Na2O. The comparison suggests that the source of sediments is the material which has strongly been transformed by chemical weathering. In particular, CIA values in the studied sediments often reach 80–85, which is higher than PAAS values (Shao et al. 2012). Index values of the sediments of the rivers Huanghe, Yalujiang, Nenjiang, Laohe and Seonghujiang (Songhua) (northeastern China) are remarkably lower and close to the values in Middle Zeya.

In conclusion, river sediments of the compared Asian region are characterized by the increase of values of chemical indices which describe the degree of weathering of parent rock in the area of erosion in the south direction. We presume that the process can be explained by climatic conditions, namely rise of temperature and humidity.

Conclusion

The collected data helped us to describe chemical composition of sediments in the Zeya River, which is one of the largest tributaries of the Amur River. Overall, river sediments in Middle Zeya are characterized by relative depletion in alkaline and alkaline-earth elements.

The obtained values of weathering indices and lithochemical modules reflect wide range of changes from insignificantly weathered to medium-weathered rock. So, the data indicate that channel deposits of the river are formed by rocks which have been transformed by chemical weathering. The changes of weathering values make trends which point to increase of the weathering processes down the river.

It is proved that the features of chemical composition of the river sediments and values of weathering indices in the studied river are close to the features of northeastern China and can be explained by common geographical and climatic conditions.