Influence of provenance and hydrodynamic sorting on the magnetic properties and geochemistry of sediments of the Oujiang River, China

Marine Geology 387 (2017) 1–11

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Influence of provenance and hydrodynamic sorting on the magnetic properties and geochemistry of sediments of the Oujiang River, China Wen Li a,1, Zhongxing Hu a,⁎, Weiguo Zhang b, Ru Ji a, Thi Thu Hien Nguyen c a b c

College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China State Key Laboratory of Estuarine and Coastal Reach, East China Normal University, Shanghai 200062, China Institute of Geography, Vietnam Academy of Science and Technology, Hanoi, Vietnam

a r t i c l e

i n f o

Article history: Received 7 August 2016 Received in revised form 8 December 2016 Accepted 10 March 2017 Available online 19 March 2017 Keywords: Magnetic properties Geochemistry Sorting Sediment provenance Oujiang River

a b s t r a c t We present the results of environmental magnetic, granulometric and geochemical analyses of sediments of the Oujiang River, China. The aim of the study was to assess the influence of hydrological sorting and sediment source on sediment composition from upstream down to the river mouth. The results reveal that the sedimentary magnetic properties are dominated by ferrimagnetic minerals. In the fluvial reach, the proportion of imperfect antiferromagnetic minerals is relatively high due to strong soil chemical weathering in a subtropical climate. From upstream down to the river mouth, the proportion of ferrimagnetic minerals increases consistently, and the magnetite becomes finer-grained as a result of hydrological sorting. In the tidal reach, the sediments consist of a mixture of fluvial and marine-sourced sediments derived from the Yangtze River as a result of the strong tidal influence. A combination of the demagnetization parameter S−100 mT and element ratios (Fe/K and Al/Mg) enables the discrimination of the sediments of the fluvial reach from those of the tidal reach. Due to the rock types and subtropical weathering conditions in the catchment, the sediments of the Oujiang River are significantly different to those of the Yangtze River. Our results potentially contribute to the study of the provenance of the muddy sedimentary deposits in the inner East China Sea. They demonstrate that sediment sorting has a significant effect on bulk magnetic properties and geochemical compositions during transport from source to sink. In addition, magnetic and geochemical characterization of sized fractions of sediment can provide more reliable information regarding sediment source tracing. © 2017 Published by Elsevier B.V.

1. Introduction In the inner shelf of the East China Sea, a muddy clinoform is distributed along the coastal area of Zhejiang and Fujian Provinces, China (Qin et al., 1987). This muddy deposit has a maximum thickness of ca. 40 m, and has formed over the last 7000 years (Liu et al., 2006). The sediment provenance of this deposit has attracted significant interest in recent years. It is generally agreed that sediments from the Yangtze River are the dominant source (Milliman et al., 1985; Liu et al., 2006; Xu et al., 2011; K.H. Xu et al., 2012; Z.K. Xu et al., 2012). However, a number of rivers are distributed along the coast of Zhejiang and Fujiang Provinces and their potential contributions to the muddy deposit have not been determined so far. Oujiang River is the second largest river in Zhejiang Province, and the annual sediment discharge has been reported as ~ 2.5 × 106 t ⁎ Corresponding author. E-mail address: [email protected] (Z. Hu). 1 Present address: Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China.

http://dx.doi.org/10.1016/j.margeo.2017.03.002 0025-3227/© 2017 Published by Elsevier B.V.

(Song et al., 2012). Several studies suggest that sediment from Oujiang River contributes to the muddy deposit in the inner shelf of the East China Sea (Liu et al., 1991; Xu et al., 2011). To assess its possible contribution, several studies have used clay mineralogy and geochemistry to characterize sediments derived from Oujiang River (He, 1983; Zhu, 1993; Yang, 1995). However, due to the strong tidal range at the mouth of Oujiang River (Zhu, 1993; Lu et al., 2002), sediments sampled at the river mouth consist of a mixture of fluvial and marine sources, with the latter being primarily derived from the Yangtze River. In this context, several studies have found that the sediments at the mouth of Oujiang River are very similar to those of the Yangtze River (Liu et al., 1991; Yang, 1995). However, in order to achieve a reliable characterization of the Oujiang River sediments, fluvial samples need to be collected beyond the tidal limit of the river. Magnetic minerals are ubiquitous components in sediments (Thompson and Oldfield, 1986; Verosub and Roberts, 1995; Dekkers, 1997; Evans and Heller, 2003). In recent years, environmental magnetic methods, which include the characterization of mineralogy, concentration, and the grain size of magnetic minerals, has been widely applied to establish sediment provenance in diverse environmental settings (e.g.,

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Fig. 1. Study area and locations of sampling sites. (a) Location of Oujiang River (rectangle) and the East China Sea. Coastal currents include the Taiwan Warm Current (TWC), the East China Sea Coastal Current (ECSCC) and Changjiang (Yangtze) Diluted Water (CDW). (b) Sampling sites along the Oujiang River. The Oujiang River can be divided into fluvial and tidal reaches, and the tidal reach can be further divided into three parts: river dominant, transitional zone and tidal dominant.

Fig. 2. Variations in particle-size composition among the sampling sites. The vertical dashed lines mark the boundaries of the three zones discussed in the text: fluvial reach, upper tidal reach, and lower tidal reach. The error bar is the standard deviation.

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Thompson and Oldfield, 1986; Walden et al., 1997; Duck et al., 2001; Hounslow and Morton, 2004; Rotman et al., 2008; Zhang et al., 2008; Maher et al., 2009; Wang et al., 2009; Liu et al., 2010). These studies indicate that the influence of particle-size sorting on magnetic parameters must be thoroughly addressed in sediment provenance tracing (Zhang et al., 2008, 2012; Wang et al., 2009; Liu et al., 2010). In this study, surface sediment samples on the source-to-sink pathway of the Oujiang River were subjected to combined magnetic, granulometric and geochemical measurements. The purpose was to characterize the Oujiang River sediments from upstream down to the mouth, in order to assess the influence of hydrological sorting on sediment provenance, as well as to assess the tidal influence on sediment mixing in the lower reach of the river. It was hoped that the information obtained in the course of the study would also provide insights into the sediment provenance of the muddy deposit in the inner shelf of the East China Sea, as well as contributing to the selection of suitable sediment source tracers in source-to-sink studies. 2. Materials and methods 2.1. Study area The Oujiang River is the second largest river in Zhejiang Province, China (Fig. 1). The catchment area is 1.8 × 104 km2 and it flows for 388 km before entering Wenzhou Bay in the East China Sea (Guo et al., 2012). The river basin is located in a sub-tropical monsoon climate zone, with mean annual temperature of 17.5–20.5 °C, and annual precipitation of 1500–2100 mm. The mean water discharge and sediment flux are 442 m3/s (1950–2008) and 61.9 kg/s (1956–1998), respectively. There are marked seasonal variations in water and sediment discharge, with the majority discharged in the wet season (April to September, 74.4% for water and 90% for sediment) (Song et al., 2012). Mesozoic tuff, welded tuff and rhyolite are widespread within the catchment (Tang, 1983). According to the FAO classification system, the soils in the catchment of the Oujiang River are dominated by

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Acrisols. Wenzhou Bay is a macro-tidal estuary, and the tidal range recorded at Huanghua station near the river mouth is 4.5 m on average, with a maximum value of 7.2 m (Zheng et al., 2008). In the dry season, the tidal limit is located at Wenxi Town, about 78 km away from the river mouth (Zheng et al., 2008). According to the relative influences of runoff and tide, the tidal reach (Wenxi to the river mouth) can be divided into three sections: river-dominant (Wenxi to Meiao), transitional zone (Meiao to Longwan) and tide-dominant (Longwan to Huanghua) (He, 1983). The Oujiang River Estuary is bifurcated into northern and the southern branches by LingKun Island (Fig. 1). 2.2. Samples and methods In 2014, 15 flood plain sites were sampled from upstream down to the river mouth (Fig. 1). Four to six surface (top 5 cm) sediment samples were obtained at each site. A total of 74 samples were collected. All of the samples were dried at 40 °C and then disaggregated prior to analysis. The particle-size distribution was measured using a laser-diffraction analyzer (Coulter LS-100Q) following pretreatment with 0.2 M HCl and 5% H2O2 to remove biogenic carbonate and organic matter, respectively. To ensure complete disaggregation, the samples were dispersed by adding 0.5 M sodium hexametaphosphate (0.5 M (NaPO3)6) followed by ultrasonication (Ru, 2000). Low- (0.47 kHz) and high- (4.7 kHz) frequency magnetic susceptibility (χlf and χhf, respectively) were measured using a Bartington Instruments magnetic susceptibility meter and MS2B dual-frequency sensor. Mass-specific and percentage frequency-dependent susceptibility were calculated as χfd = (χlf − χhf) and χfd% = ([χfd / χlf] × 100), respectively. Anhysteretic Remanent Magnetization (ARM) was imparted in a 0.04 mT DC field superimposed on a peak AF demagnetization field of 100 mT, and is expressed as susceptibility of ARM (χARM). Isothermal Remanent Magnetization (IRM) was measured after applying a field of 1 T, followed by back field remanence measurements at −100 mT and − 300 mT. IRM obtained at 1 T is referred to as the saturation IRM

Fig. 3. Representative thermomagnetic curves for the sediments from the sampling sites. Red and blue lines represent heating and cooling curves, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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(SIRM). Hard IRMxmT (HIRMxmT) was defined as HIRMxmT = 0.5 × (SIRM + IRMxmT) (Hu et al., 2016). S−100 mT and S−300 mT were calculated as S−100 mT = 0.5 × (SIRM − IRM−100 mT) / SIRM × 100 and S−300 mT = 0.5 × (SIRM − IRM−300 mT) / SIRM × 100 (Yu and Oldfield, 1989). According to this definition, the S-ratio values range from 0 to 100. Selected samples were subjected to thermomagnetic analysis using an AGICO MFK-1-FA Kappabridge equipped with a CS-3 high-temperature furnace. Each sample was heated from room temperature to 700 °C and then cooled to room temperature in an argon atmosphere. 45 samples were selected for analysis of the concentrations of Al, Fe, Mg, Ti, K, Na, K and Mn using inductively-coupled plasma atomic emission spectrometry (ICAP™ 7400 ICP-OES) after mixed HF-HNO3-HCIO4 digestion (NEB, 1998). The China national reference material GSD-9 was included for quality control, and the analytical precision and error are within 10%. Based on the results of the particle size, magnetic and geochemical analyses, 15 representative samples were separated into four particlesize fractions: b16 μm, 16–32 μm, 32–63 μm and N63 μm. The N63 μm fraction was separated first using wet sieving, and the b63 μm fractions were separated further according to Stokes' Law (Ru, 2000). The

resulting particle size fractions were then used for magnetic measurements and geochemical analysis, as described above. 3. Results 3.1. Particle-size distribution According to the spatial variation of particle-size composition (Fig. 2), the Oujiang River can be divided into three sections: a fluvial reach (S1–S7), upper tidal reach (S8–S11), and lower tidal reach (S12–S15). The sediments from the fluvial reach (S1–S7) are coarsest and exhibit large spatial variations. They consist on average of 13% clay (b4 μm), 31% silt (4–63 μm) and 56% sand (N63 μm), with a mean particle-size ranging from 28 to 397 μm. From upstream to downstream, the sediment first becomes coarser and then finer. In the upper part of the tidal reach, the sediments consist on average of 19% clay, 57% silt and 24% sand, with a mean particle-size ranging from 20 to 80 μm. The content of clay and silt gradually increases downstream, while the sand content decreases. The sediments from the lower part of the tidal

Fig. 4. Variations in bulk magnetic properties of the sampling sites. The vertical dashed lines are the boundaries of the three zones discussed in the text: fluvial reach, upper tidal reach, and lower tidal reach. The error bar is the standard deviation.

W. Li et al. / Marine Geology 387 (2017) 1–11

reach are the finest and exhibit only minor spatial variations. They are dominated by clayey silt, consisting on average of 61% silt, 32% clay, and 7% sand. In summary, the particle-size composition of the sediments exhibits large spatial variability, and suggests that hydrodynamic forces become weaker from the tidal limit down to the river mouth. 3.2. Bulk magnetic properties Representative thermomagnetic curves exhibit a Curie temperature of 580 °C, indicating the presence of magnetite (Fig. 3). For bulk sediments from the fluvial section, χ increases gradually before peaking at about 300 °C in the heating curves, reflecting the unblocking of finegrained ferrimagnetic minerals (Liu et al., 2005), or the transformation of ferrihydrite or lepidocrocite to maghemite (Hanesch et al., 2006). χ exhibits a slight decrease and an inflection around 400 °C, which is interpreted as the conversion of maghemite to hematite (Liu et al., 1999, 2003). With further heating, χ increases again before peaking at around 520–530 °C, which may be caused by the transformation of weakly magnetic minerals to strongly magnetic ferrimagnetic minerals. Several samples exhibit a continuous decrease in χ after heating above 580 °C, probably due to the presence of hematite. For bulk sediments from the tidal section, χ remains relative stable or increases slightly until 400 °C. It then exhibits a significant increase before peaking at around 520–550 °C, suggesting the transformation of magnetic minerals during the heating process. χ and SIRM generally reflect the concentration of magnetic minerals, especially ferrimagnetic minerals such as magnetite (Thompson and Oldfield, 1986). Unlike χ, SIRM is not influenced by the presence of superparamagnetic, para- and dia-magnetic minerals (Thompson and

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Oldfield, 1986). χARM is a concentration-related parameter, and is particularly sensitive to the presence of single domain (SD, ~0.04–0.06 μm for magnetite) ferrimagnetic grains (Maher, 1988). HIRM−300 mT is commonly used to estimate the concentration of high-coercivity minerals (e.g., hematite and goethite), while HIRM−100 mT represents the abundance of medium- (e.g., maghemite) and high-coercivity minerals (Thompson and Oldfield, 1986; Yamazaki, 2009). As shown in Fig. 4, the bulk sediments from the fluvial reach have higher χ and SIRM values, with the highest values in the lower part of the fluvial section (S4 to S7) and they exhibit marked spatial variations. In comparison, the χ and SIRM values of the sediments from the tidal reach exhibit lower and stable values. χARM exhibits an increasing trend from S1 to S5, remains relatively stable from S5 to S9, and then exhibits an increasing trend to the river mouth. HIRM−300 mT and HIRM−100 mT exhibit similar spatial trends to those of χ and SIRM. On average, the concentration of magnetic minerals in the bulk sediments from the fluvial reach is higher than in the tidal reach. S−300 mT is as a measure of the relative importance of high- and lowcoercivity magnetic mineral components, with higher values corresponding to higher proportions of low-coercivity minerals (Thompson and Oldfield, 1986; Bloemendal and Liu, 2005). The S−300 mT values vary from 85 to 99%, indicating that the magnetic properties are dominated by ferrimagnetic minerals. S−100 mT reflects the ratio of low-coercivity minerals to medium- and high-coercivity minerals (Yamazaki, 2009). S− 100 mT is lower in the fluvial section with a mean value of 76%. It exhibits an increasing trend downstream over the tidal reach, indicating that the proportion of low-coercivity minerals increases towards the river mouth. The positive correlation between χ and SIRM (r = 0.82, p b 0.01) also indicates that χ is mainly contributed by

Fig. 5. Bulk sediment geochemical composition for the sampling sites. The vertical dashed lines are the boundaries of the three zones discussed in the text: fluvial reach, upper tidal reach, and lower tidal reach. The error bar is the standard deviation. The error bar is the standard deviation.

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Table 1 Correlation coefficients between magnetic properties and particle size.

χ χfd% χARM SIRM HIRM−300 mT HIRM−100 mT S−100 mT S−300 mT χARM/χ χARM/SIRM

b4 μm

4–8 μm

8–16 μm

16–32 μm

32–63 μm

N63 μm

Mean size

−0.48 0.42 0.60 −0.49 −0.26 −0.61 0.48 −0.16 0.72 0.74

−0.48 0.46 0.62 −0.50 −0.24 −0.64 0.53 −0.22 0.76 0.79

−0.52 0.43 0.54 −0.55 −0.27 −0.67 0.52 −0.25 0.76 0.79

−0.54 0.20 0.28 −0.51 −0.31 −0.59 0.39 −0.12 0.48 0.49

−0.29 −0.22 −0.25 −0.21 −0.21 −0.16 −0.07 0.09 −0.10 −0.15

0.57 −0.35 −0.49 0.56 0.32 0.68 −0.48 0.17 −0.68 −0.70

0.43 −0.29 −0.41 0.45 0.27 0.53 −0.36 0.12 −0.56 −0.56

Note: Bold type is significant at p b 0.01, n = 74.

ferrimagnetic minerals (Thompson and Oldfield, 1986). However, the correlation coefficient between χ and SIRM in the fluvial reach (r = 0.79, p b 0.01) is lower than that in the tidal reach (r = 0.91, p b 0.01). This is due to the abundance of imperfect antiferromagnetic magnetic minerals in the former, which is consistent with their higher HIRM and lower S−100 mT values. χfd% provides an estimate of the relative contribution of fine, viscous superparamagnetic (SP, ~b0.03 μm for magnetite) grains to the total magnetic assemblage (Thompson and Oldfield, 1986). χARM/SIRM is commonly used as a grain-size indicator for ferrimagnetic minerals, and peaks in the SD range and decreases with increasing grain size. The χARM/χ ratio is a similar grain size indicator, but the presence of SP grains size can reduce its value (Maher, 1988). The grain-size indicators (χfd%, χARM/SIRM and χARM/χ) exhibit similar spatial trends. They increase from S1 to S2; decrease gradually from S2 to S7; and from S8 to the river mouth, they exhibit an increasing trend, despite the slightly lower χARM/SIRM and χARM/χ values of sites S14 and S15. On average, the tidal reach sediments have much higher values than their fluvial reach counterparts, indicating that the magnetic minerals become finer-grained from upstream down to the river mouth.

In summary, the bulk sediments from the fluvial reach have higher χ, SIRM and HIRM values, but lower S−100 mT, χfd%, χARM, χARM/SIRM, χARM/ χ values than those of the tidal reach, indicating a higher concentration of ferrimagnetic minerals and a coarser grain size in the former. 3.3. Geochemistry of the bulk sediments Major element composition and their ratios can be used to trace sediment provenance (Yang et al., 2002; Zhang et al., 2007; Z.K. Xu et al., 2012). The results of the analyses of element concentrations of the studied samples are shown in Fig. 5. Except for K, major element such as Al, Fe, Mg, Ti, Na, Ca and Mn exhibit similar spatial trends from upstream down to the mouth. There are no obvious fluctuations in major element content in the fluvial reach (S1–S7). In the upper part of the tidal reach (S8–S11), K exhibits a decreasing trend downstream, while the other elements exhibit the opposite trend. In the lower part of the tidal reach (S12–S15), K and Ca generally exhibit a decreasing and increasing trend, respectively, while the other elements exhibit only minor variations. The Al/Mg ratio is higher in the fluvial reach samples; it exhibits a decreasing trend downstream in the upper tidal reach, and then the

Fig. 6. Relationships between the N63 μm fraction versus χ (a) and SIRM (b), and the b16 μm fraction versus (c) χARM, (d) χfd%, (e) χARM/χ and (f) χARM/SIRM.

W. Li et al. / Marine Geology 387 (2017) 1–11 Table 2 Correlation coefficients between geochemical composition and particle size.

Al Fe Mg Ti K Na Ca Mn

b4 μm

4–8 μm

8–16 μm

16–32 μm

32–63 μm

N63 μm

Mean size

0.58 0.80 0.80 0.75 −0.44 0.37 0.68 0.55

0.65 0.87 0.87 0.82 −0.51 0.43 0.72 0.61

0.70 0.88 0.86 0.86 −0.52 0.41 0.71 0.68

0.67 0.70 0.65 0.74 −0.42 0.31 0.57 0.67

0.24 0.09 0.01 0.19 −0.05 −0.03 0.06 0.28

−0.69 −0.83 −0.80 −0.83 0.48 −0.38 −0.69 −0.67

−0.59 −0.72 −0.65 −0.75 0.30 −0.28 −0.56 −0.56

Note: Bold type is significant at p b 0.01, n = 45.

values become lower and relatively constant in the lower part of the tidal reach. The Fe/K ratio exhibits the opposite trend. 3.4. Relationship between sediment particle size, magnetic properties and geochemical composition The concentration-related magnetic parameters (χ, SIRM and HIRM) are significantly correlated with the sand fraction (N63 μm) (Table 1 and Fig. 6). In addition, there are strong correlations between the fractions b16 μm and χARM/SIRM, χARM/χ, χfd% and χARM (Table 1 and Fig. 6). S −300 mT is not correlated with particle size, but S−100 mT is positively correlated with the size fractions b32 μm (Table 1). Al, Fe, Mg, Ti, Ca and Mn are positively correlated with the size fractions b 32 μm, while K is negatively correlated with these fractions. Na exhibits a weaker positive correlation with the 4–8 μm and 8–16 μm fractions (Table 2). 3.5. Magnetic properties and geochemical composition of particle-size fractions The magnetic properties of the fractionated samples are shown in Fig. 7. Except for sites S2 and S13, χ increases with coarsening particle size and peaks in the 32–63 μm fraction. Generally, the χ of the N63 μm fraction is greater than that of the b16 μm and 16–32 μm fractions. HIRM−100 mT exhibit similar particle-size relationships to χ, suggesting that the magnetic mineral concentrations are higher in the coarser fractions (i.e., N 32 μm). It is noteworthy that this particle-size dependency is much more marked in the fluvial reach. In the tidal reach, this dependency is much weaker, indicating that the influence of particle size on the magnetic properties becomes less significant. Except for sample

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site S2, χARM/SIRM generally peaks in the b 16 μm fraction and decreases with increasing particle size; however, it increases slightly in the N 63 μm fraction. To further identify the contribution of different size fractions to the bulk concentration-related magnetic parameters, we multiplied the magnetic parameters for each size fraction by the relative abundance of that fraction in the bulk sediment in order to estimate its contribution to the bulk parameters. Taking χ as an example, we find that the N 63 μm fraction contributes 54% on average to bulk χ in the fluvial reach (S1– S7), while the b16 μm fraction contributes 47% on average to bulk χ in the tidal reach (S8–S15). This further supports the conclusion that the magnetic properties are dominated by coarse-grained ferrimagnetic minerals in the fluvial reach, while fine-grained ferrimagnetic minerals are more significant in the tidal reach. The geochemical composition of the b 16 μm fraction is shown in Fig. 8. Except for Na, all major element concentrations are higher than those of the bulk sediments, which is consistent with the fact that most of the element concentration in the bulk sediments are positively correlated with the b16 μm fraction (Table 2). Application of the method detailed above to calculating the contribution of size fractions to the geochemical properties reveals that the elements Al, Fe, Mg, Ti, Ca and Mn in the b16 μm fraction contribute nearly half or more to the bulk concentrations, which are 48%, 54%, 60%, 50%, 76% and 46% of the bulk concentrations, respectively. The spatial variation of the geochemical composition of the b 16 μm fraction is similar to that of the bulk sediments. 4. Discussion 4.1. Influence of particle size on magnetic properties and geochemistry In fluvial and coastal environments, hydrodynamic variations can result in particle size and mineralogical sorting during the course of sediment transport and deposition. Bulk sediment particle size variations can have an important effect on magnetic properties and geochemistry, even if the sediment source does not vary (Oldfield et al., 1985, 2009; Oldfield and Yu, 1994; Zhang and Yu, 2003; D'Haen et al., 2012). In the present study, both correlation analysis (Table 1) and magnetic measurements of sized fractions (Fig. 7) indicate that magnetic minerals are mainly concentrated within the coarse silt and sand fraction. The highest values of bulk χ and SIRM for the samples of site S4 and S5 are in accordance with the fact that these samples have the highest

Fig. 7. Magnetic properties of particle-size fractions (b16 μm, 16–32 μm, 32–63 μm and N63 μm). Two samples (S14 and S15) had insufficient material in the N63 μm fraction for magnetic measurements and therefore no data are available. The vertical dashed lines are the boundaries of the three zones discussed in the text: fluvial reach, upper tidal reach, and lower tidal reach.

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Fig. 8. Geochemical compositions of the b16 μm fraction sediments. The vertical dashed lines are the boundaries of the three zones discussed in the text: fluvial reach, upper tidal reach, and lower tidal reach.

sand content. From upstream down to the river mouth, as the river gradient decreases and the river channel broadens, the hydrodynamic energy becomes weaker, resulting in a fining trend of sediment particlesize. Since coarse sediments and associated magnetic minerals are deposited first, the quantity of magnetic minerals carried downstream is reduced, and this is reflected by the decreasing χ and SIRM values of the sediments. Sediments in the fluvial reach have lower S−100 mT and higher HIRM −300 mT values, suggesting higher imperfect antiferromagnetic mineral proportions and concentrations. This corresponds well with the soil types in the study area. Situated in the subtropical climate zone, the soils undergo strong chemical weathering, with Acrisols exhibiting red and yellow colors. In such soil types, hematite and goethite are formed during pedogenesis (Zhang, 1990). Correlation analysis reveals that these minerals are associated with the sand fraction (Table 1). This could be due to the formation of hematite and goethite as coatings on the surface of coarse quartz particles. From upstream down to the river mouth, S−100 mT and HIRM−300 mT exhibit increasing and decreasing trends, respectively, which are consistent with the fining trend of

the sediments and the lower content of imperfect antiferromagnetic minerals. It is generally agreed that immobile elements such as Al, Fe and Ti will become enriched in fine-grained fractions during soil chemical weathering in tropical or sub-tropical conditions, while mobile elements like Na and Ca will become depleted in fine-grained sediments (Buggle et al., 2011). During the process of soil erosion and sediment transport, immobile elements will become concentrated in the finer fractions of sediments (Singh, 2009). The positive relationships between most elements (Al, Fe, Mg, Ti and Mn) and the finer size fractions (b32 μm) are consistent with the chemical weathering regimes in the study area. The enrichment of Ca in fine-grained sediments in the present data can be explained by the sediment source variations discussed below. 4.2. Influence of provenance on magnetic properties and geochemistry Previous work has revealed that the sediments in the Oujiang River estuary contain marine-sourced sediment transported by flooding tides

Table 3 Geochemical compositions (mean ± SD) of the sediments from the fluvial and tidal reach of the Oujiang River, and its comparison with soils in its catchments and sediments in the Yangtze River estuary. MgO

TiO2

Fe2O3

Al2O3

CaO

MnO

Na2O

K2O

Data source

0.49 ± 0.08 0.67 ± 0.05 0.77 ± 0.02 0.61 ± 0.08 0.71 ± 0.12

3.13 ± 0.52 4.75 ± 0.29 6.38 ± 0.19 2.59 ± 0.37 4.46 ± 1.03

10.52 ± 1.41 12.76 ± 0.93 14.60 ± 0.92 12.09 ± 0.87 11.57 ± 1.95

0.36 ± 0.14 0.62 ± 0.06 1.10 ± 0.30 0.25 ± 0.08 4.49 ± 0.27

0.09 ± 0.03 0.13 ± 0.01 0.15 ± 0.01 0.05 ± 0.01 0.09 ± 0.03

1.06 ± 0.17 1.20 ± 0.10 1.53 ± 0.19 0.65 ± 0.20 1.92 ± 0.37

3.51 ± 0.27 3.25 ± 0.13 2.94 ± 0.12 3.16 ± 0.31 2.26 ± 0.34

This study

(%) Fluvial reach Upper part of tidal reach Lower part of tidal reach Soil in the catchment Yangtze River estuary

0.44 ± 0.13 1.13 ± 0.19 2.09 ± 0.21 0.47 ± 0.08 2.45 ± 0.35

Dong et al. (2007) Xing (2007)

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due to the macrotidal conditions. This marine-sourced sediment is believed to be derived from the Yangtze River and is carried southwards by the prevailing coastal current (Fig. 1) (He, 1983; Milliman et al., 1985; Qin et al., 1987; Xu et al., 2011). Geochemical composition can be used to trace sediment provenance (Yang et al., 2002; Zhang et al., 2007; Z.K. Xu et al., 2012). Clearly, element ratios such as Fe/K and Al/Mg exhibit a marked difference between the fluvial reach and tidal reach (Fig. 5). Such a difference is also evident in the b 16 μm fraction (Fig. 8). Sediments in the fluvial reach have a higher K concentration but lower Fe, Mg and Ca concentrations, which is consistent with the geochemical characteristics of the tuff and rhyolite which are widespread in the catchment. These rocks belong to the calc-alkaline series, which are enriched in Al, K and Na, but depleted in Fe, Mg and Ca (Dong et al., 2008). As show in Table 3, the concentrations of major elements, especially Mg and K, in the sediments from the tidal reach are markedly different to their counterparts in the fluvial reach, and to the soils in the catchment of the Oujiang River; however, they are very similar to those of the Yangtze River estuary (Table 3). This indicates that the tidal reach sediments are significantly influenced by the Yangtze River. From the perspective of their magnetic properties, the sediments from the fluvial reach are also different from that those of the tidal reach. In particular, they contain higher proportions of imperfect antiferromagnetic minerals due to the strong chemical weathering under subtropical conditions. As shown in Fig. 9, the combination of the demagnetization parameter S−100 mT and element ratios (i.e., Fe/K and Al/Mg) can be used to differentiate between the sediments from the fluvial and tidal reaches. Clearly, the sediments from the lower tidal reach are well-matched with those of the Yangtze River. This is finding is in agreement with the aforementioned clay mineral analysis, which revealed that the tidal reach sediments of the Oujiang River are strongly influenced by the Yangtze River sediments (Yang, 1995). In other words, our combined magnetic and geochemical methodology provides a new approach for characterizing the sediments from the Oujiang River, which can in turn be used to trace the provenance of the sediments in the muddy clinoform in the East China Sea. 4.3. Implications for provenance studies Sediment source tracing is a fundamental issue in coastal and marine studies. Sediment provenance is commonly identified by comparing magnetic properties and geochemical composition of sediments in the sink area with those of potential source areas. As shown in this study, sorting on the source-to-sink pathway can significantly alter the particle size, magnetic properties and geochemical composition of bulk sediments. A common situation is that ferrimagnetic minerals become finer-grained due to sorting on the source-to-sink pathway. In the

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sediments of the Oujiang River, χfd%, χARM/χ and χARM/SIRM exhibit a slightly decreasing trend from sites S1 to S8, and then an increasing trend from S8 to the river mouth. This is in accordance with the spatial variation of the clay fraction. This phenomenon of downstream-fining of magnetic minerals is similar to that observed in the Hangbu River (Xie et al., 2006), the Yangtze River (Li et al., 2012), the Yangtze River estuary (Dong et al., 2014) and the Red River (Nguyen et al., 2016). In the Oujiang River sediments, the magnetic grain-size indicators (e.g., χfd%, χARM/χ, and χARM/SIRM) are positively correlated with the clay or b16 μm fractions. This relationship has been observed in diverse sedimentary environments (Oldfield et al., 1985, 2009; Oldfield and Yu, 1994; Zhang and Yu, 2003; Dong et al., 2013; Hatfield et al., 2013, 2016; Lascu et al., 2015; Nguyen et al., 2016). It indicates that sorting leads to the enrichment of fine-grained particles downstream, which will alter the original composition of sediments at the source. Therefore, reliable sediment source tracing should properly address the effect of sorting on magnetic and geochemical indicators. This can be done by characterizing the size fractions of sediments, and comparing the sink with the potential source using matching size-fractions (e.g., Zhang et al., 2008). The approach described in the present study and in previous work (Zhang et al., 2008, 2012; Hatfield and Maher, 2009; Liu et al., 2010; Wang et al., 2010) can be extended to other source-to-sink systems. 5. Conclusions The magnetic properties of sediments from the Oujiang River are dominated by ferrimagnetic minerals and they also reflect an additional large contribution from imperfect antiferromagnetic minerals. In the fluvial reach, the proportion of imperfect antiferromagnetic minerals is relatively high due to strong chemical weathering in a subtropical climate, with the mineral probably forming as coatings on coarse quartz particles. From upstream down to the river mouth, the magnetite becomes progressively finer due to sorting. In the tidal reach, due to strong tidal influences, the sediments comprise a mixture of fluvial and marine sources, with the later derived from the Yangtze River. Due to the petrological composition of tuff and rhyolite, together with subtropical weathering, the sediments from the Oujiang River are different from those of the Yangtze River. A combination of the demagnetization parameter S− 100 mT and geochemical element ratios (i.e., Fe/K and Al/ Mg) can be used to discriminate the sediments from the two rivers. This study provides new indexes for characterizing the Oujiang River sediments, which can be used to trace sediment sources in the inner shelf of the East China Sea. Considering the significant effect of sorting on the magnetic and geochemical composition of sediments, characterizing different size fractions can provide a more robust approach for sediment provenance studies.

Fig. 9. Scatter plot of S−100 mT versus Fe/K and S−100 mT versus Al/Mg for samples from the Oujiang River and Yangtze River estuary (Xing, 2007).

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Acknowledgements This study was supported in part by the National Natural Science Foundation of China (grant nos. 41271223, 41576094), and the State Key Laboratory of Estuarine and Coastal Research Open Fund (grant SKLEC-KF201205). We thank Bai Xuexin and Liu Jinyan for the help in field sampling. We thank the anonymous reviewers, the editor and Jan Bloemendal for their constructive comments and language improvements. References Bloemendal, J., Liu, X.M., 2005. Rock magnetism and geochemistry of two plio–pleistocene Chinese loess–palaeosol sequences—implications for quantitative palaeoprecipitation reconstruction. Palaeogeogr. Palaeoclimatol. Palaeoecol. 226 (1– 2), 149–166. Buggle, B., Glaser, B., Hambach, U., Gerasimenko, N., Marković, S., 2011. An evaluation of geochemical weathering indices in loess–paleosol studies. Quat. Int. 240 (1), 12–21. Dekkers, M.J., 1997. Environmental magnetism: an introduction. Geol. Mijnb. 76 (1), 163–182. 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