Sources of sediment in tidal flats off Zhejiang coast, southeast China*

Jia JIA , Xingze ZHANG , Ruhui ZHOU , Michael MEADOWS ,2,,Dhritiraj SENGUPTA , Lidong ZHU

1 College of Geography and Environmental Science, Zhejiang Normal University, Jinhua 321004, China

2 Department of Environmental& Geographical Science, University of Cape Town, Rondebosch 7701, South Africa

3 Key Laboratory of Geographic Information Science, Ministry of Education, and School of Geographic Sciences, East China Normal University, Shanghai 200241, China

Abstract The growth of tidal flats off Zhejiang coasts in southwestern China has provided substantial areas for local agriculture and construction activities. To evaluate modern and future development of tidal flats in the region, a good understanding of sediment sources is necessary. Previous research has concluded that the Changjiang (Yangtze) River is the dominant supplier of sediments to this part of the southeast Chinese coast, despite the fact that sediment delivery from this source has been decreased markedly in the past two decades. In this study, we investigated the sources of tidal flat sediments, and the magnetic and geochemical properties of recent tidal flat sediments along the Zhejiang coast were compared with those discharged from rivers. Magnetic and geochemical properties of the tidal flat samples reveal statistically distinct sediment provenance groups. The magnetic and geochemical scatter plots show that the suspended sediment samples are naturally divided into two diff erent groups, one including the Changjiang River and Qiantang River material, and the other including the Jiaojiang, Oujiang, and Feiyun Rivers that located in the central and southern parts of the study areas. At last, a binary source of tidal flat sediments along the Zhejiang coast was determined, with the substantial majority originating from local rivers, while a much smaller proportion emanates from the Changjiang River to the north. We conclude that the sediment contribution of the Changjiang River to tidal flat development in the region has been markedly overestimated,with important implications for management.

Keyword: tidal flats; Zhejiang coastal area; magnetic and geochemical characteristics; sediment source tracing

1 INTRODUCTION

The coastal region of Zhejiang Province in southeast China (Fig.1) is characterized by diverse landscapes in which the mainly undulating topography is interrupted by coastal plains that account for about 20% of the area in total. This part of the coast is densely populated and is one of the most developed economic regions in China. As a result, there has been increasing pressure on limited land resources; and coastal land reclamation has become an important solution to increasing the availability ofland for the fast development (Sengupta et al., 2020). Of the 6 600 km of coastline in the Zhejiang coastal area as a whole, approximately 2 400 km2is represented by tidal flats, a substantial area that has been reclaimed and converted to aquaculture and other various forms of constructions (Liao et al., 2016; Li et al., 2019).

Fig.1 Locations of the study area, showing sampling sites and regional coastal and ocean currents (modified after Liu et al.(2007a))

Fig.2 The time series (1990-2004-2018) of seaward land growth in Oujiang River estuary illustrated by the changes in NDVI and MNDWI

Table 1 The information of sampling sites and samples

About 34 km2of new tidal flats were generated during the 1950-2004 period in Zhejiang coastal region (Lu and Zeng, 2006). Being evidently shown in the normalized diff erence vegetation index (NDVI)(vegetation) and modified normalized diff erence water index (MNDWI) (surface water) data that mapped (Fig.2), the land areas around the Oujiang River estuary have been increased markedly by sedimentation and reclamation (Wang et al., 2020) for aquaculture and commercial and residential projects.Although the rapid growth of reclaimed tidal flats generated profits to the local economy, coastal ecosystems were seriously damaged (Wu et al., 2008;Li et al., 2013; Xu et al., 2018). Therefore, a good understanding of sediment sources is necessary to provide a sound scientific basis to reach cost-benefit balance and to support appropriate management policy and strategy in response to the tidal flat change(Wang et al., 2014). A number of previous studies were conducted on inner shelf sediments of the East China Sea (e.g. Liu et al., 2006, 2007a; Xu et al.,2011; Dong et al., 2014; Gao and Collins, 2014; Bi et al., 2017), but the tidal flats were rarely been considered in this part of the coast (Li et al., 2017). A quantitative study by Lu and Zeng (2006) suggests that the vast majority of the sediment deposited in the study region is derived from the Changjiang (Yangtze)River, with a total of 120 Mt of sediment or even higher transported from that source before 2004,while local rivers together supplied only around 10 Mt. However, in view of the large number of rivers flowing into the East China Sea along the coast of Zhejiang Province, it is necessary to consider their potential contribution to coastal deposition in more detail. Therefore, the purpose of this study is to trace the material sources of modern sediments in tidal flats of Zhejiang Province, and to quantify the relative contribution of sediment sources of CJR and regional smaller rivers. The objectives are: a) to sample surface sediments from major bays that can be considered representative of the tidal flats in the area; b) to characterize the sediment “fingerprint” through magnetic properties and major element contents; c) to compare results statistically with modern suspended sediments from various fluvial sources in the area.

2 MATERIAL AND METHOD

2.1 Sample collection

In 2018, 17 locations were selected as typical sites of the intertidal zone in the study area, of which 14 were located in Zhejiang coastal section and 3 in the northern part of Fujian Province (Fig.1; Table 1). All sites were located away from seaports, main roads,and settlements to limit the influence of vehicular pollution and human interference. The samples were obtained at low tide between 20 and 80 m from the high tide mark. At each site, between 3 and10 samples were collected in space at minimum of 10 m apart and each was analyzed separately so that the magnitude of the variation could be calculated among samples at each site (Wang et al., 2012). A small shovel was used to remove the top 5-cm sediment and then approximately 1 kg of sediment was collected from the layer immediately below, and transported to laboratory in plastic bags. For the modern fluvial samples, 10 suspended sediment and six fluvial(riverbank) sediment samples were collected from the Changjiang River and five other major rivers in this region (Table 1). The fluvial sediment sample sites were in the same localities as the suspended sediment samples, and also situated away from main roads and settlements to avoid pollution and human disturbance.Once collected, the suspended sediment and fluvial samples were stored in plastic containers and frozen for transport to the laboratory. The time between collection and processing was in maximum of four days.

2.2 Sample preparation

In the laboratory, particles coarser than 1 000 μm(mainly consisting of shell and plant material) were separated through mechanical sieving. The remaining material was oven-dried at 40 ℃ and used for laboratory analyses described below.

2.3 Parameter measurement

The particle size of all samples was measured using a Malvern Mastersizer 2000 laser diff raction particle size analyzer. Samples were pretreated using the protocol of Lu and An (1998) to remove organic matter and carbonate by reaction with 10% hydrogen peroxide (H2O2) and 10% hydrochloric acid (HCl),respectively. After mixing in an appropriate volume of 10% sodium hexametaphosphate (NaPO3)6,ultrasound was used to ensure adequate dispersal of finer particles.

After oven drying at a temperature below 40 ℃,samples were packed into 10-mL plastic vials and subjected to measurement of their magnetic properties.Low-frequency (470 Hz) (χlf) and high-frequency(4 700 Hz) (χhf) magnetic susceptibility were measured using a Bartington MS2 magnetic susceptibility meter.The frequency-dependent magnetic susceptibility (χfd)was calculated as:χfd=χlf-χhf. Anhysteretic remanent magnetization (ARM) was imparted in a 100-mT alternation field with a superimposed 100- μ T direct bias field using a Dtech D2000 Demagnetizer, and expressed as the anhysteretic susceptibility (χARM).Saturation isothermal remanent magnetization (SIRM)was imparted at 1T using an MMPM10 Impulse Magnetizer. Subsequently, a reverse field of 300 mT was applied to calculate the hard isothermal remanent magnetization (HIRM) as the reference Hu et al.(2016). All remanence measurements were made with a Mag-instruments Portable Spinning Magnetometer(PSM) magnetometer.

Diff use reflectance spectroscopy (DRS)measurements of finely powdered samples (＜38 m m)was recorded from 400 to 700 nm at 1-nm steps using a UV-2600 spectrophotometer (Shimadzu Instruments Manufacturing Co., Ltd.). The ratio of hematite (Hm)and goethite (Gt) was estimated by the method of Torrent et al. (2007) and Hu et al. (2016).

Samples for major element analysis were prepared by transferring a known mass of sediment to a column apparatus, together with boric acid, and then pressurized to 30 t/m2for 20 s. The resultant sample of approximately 4 cm in diameter and 8 mm in thickness was analyzed using Panalytical Epsilon3 X-ray fluorescence (XRF) spectroscope.

All of the magnetic experiments and particle size experiments were performed at the magnetic analysis laboratory and sedimentology laboratory of Zhejiang Normal University. The DRS and geochemical experiments were performed at the Key Laboratory for Subtropical Mountain Ecology, Fujian Normal University.

3 RESULT

3.1 Particle size

All particle size results are presented in Supplementary Fig.S1, analyzed by the logarithmic method of moments (Blott and Pye, 2001). The frequency curve of the particle-size distribution is unimodal except in the case of sample S16 that is polymodal. The results suggest that sediment texture distribution is finely skewed or symmetrical, and mostly mesokurtic. Standard deviation (SD) values indicate that the sediments are poorly sorted, more especially in the southern part of the study area. Mean particle size (Mz) lies between 5 and 15 m m, similar,or slightly coarser than that of surface sediment in the subaqueous delta of the Changjiang River (Ge et al.,2017). Sample S9 has the coarsest Mz (almost 20 m m).

3.2 Magnetic susceptibility parameters

High-temperature magnetic property analysis supports the view that magnetite dominates the magnetic properties of tidal flat sediments in the research area, as evidenced by the temperature curve at 580 ℃ (Fig.3). Therefore, magnetic susceptibility can be used as a reliable proxy for magnetite concentrations in the tidal flat sediments (Thompson and Oldfield, 1986; Liu et al., 2007b). It is accepted that magnetite may transform to goethite and/or pyrite when in a waterlogged environment for a long time(e.g., Liu et al., 1999). However, the thermomagnetic properties of these sediments do not indicate any distinct evidence for such minerals, thereby supporting the conclusion that little or no transformation of magnetite has occurred since deposition.

Fig.3 Thermomagnetic activities of a typical tidal flat sediment sample of tidal flat sediment

Samples S1-S15 exhibit a stable concentration of magnetite withχlfin the range (45-80)×10-8m3/kg(Fig.4a). Samples S16 and S17, located around the estuary of the Jiaoxi and Fujian rivers towards the southern end of the study area, exhibit magnetite concentrations about two to three times higher than the rest. Median absolute deviation (MAD) is a robust measure of parameter variability and most samples here exhibit values below 5×10-8m3/kg, although samples S16 and S9 yield much higher values, of 61×10-8m3/kg and ～8×10-8m3/kg, respectively.

The magnetic parameterχfdis sensitive to 20-25-nm grain sizes of magnetite (Zhou et al., 1990; Liu et al., 2007b), which is so fine that it can be considered a product of biochemical weathering processes rather than physical processes (Maher and Taylor, 1988;Meng et al., 1997; Oldfield and Crowther, 2007). Theχfdparameter values of all tidal flat samples range of(0-7)×10-8m3/kg, although most lie in the range (2-5)×10-8m3/kg which suggests that fine-grained magnetite is limited in these tidal flat sediments.Spatially,χfdexhibits an obvious increasing trend from sites S1 to S4 but then decreases from S10 to S15 (Fig.4b).

Fig.4 Comparison of magnetic characteristics among the study sites

χARMis also partly sensitive to the concentration of fine-grained magnetite, especially in the range of 30-100 nm (Liu et al., 2007b). The tidal flat samples vary between (100-800)×10-8m3/kg, but mostly in the range (450-750)×10-8m3/kg. Spatially,χARMfollows the pattern inχfdand exhibits a clear increase from S1 to S4 and decreases from S10 to S15 (Fig.4c). There are some exceptions to this overall pattern; for example, sample S5 has the lowestχfdbut an averageχARMvalue, while sample S9 has an averageχfdvalue but the lowestχARMvalue. This may indicate that finegrained magnetite has diff erent sources in these cases.

Fig.5 Comparison of Hm and Gt contents among the study sites

3.3 Isothermal remanent magnetization

SIRM is related to the concentrations of both ferrimagnetic and antiferromagnetic minerals (Hm and Gt), but is generally regarded as only partly sensitive to ferrimagnetic minerals (Thompson and Oldfield, 1986). SIRM values are relatively uniform through sites S1-S15, all falling within the range(292-774)×10-5Am2/kg. Samples at sites S16 and S17(Fig.4d) have somewhat higher values, reaching 1 500×10-5Am2/kg and 1 000×10-5Am2/kg on average,respectively (Fig.4d).

HIRM is particularly sensitive to the concentration of Hm particles with coercivities in the range of 0.3-1 T (France and Oldfield, 2000; Maher et al., 2004;Jia et al., 2018). The values of the samples vary in the range (26-78)×10-5Am2/kg (Fig.4e). In a pattern similar to that of SIRM, S16 and S17 yield the highest HIRM value.

3.4 DRS

The Gt content index lies in the range (2-6)×10-4(Fig.5a) while the Hm content index varies between(0-3)×10-4(Fig.5b). Both parameters, most notably the Hm content index, vary spatially in parallel with changes inχARM, although there is more marked variability. The ratio of Hm/(Hm+Gt) averages 0.6 for sites S1-S15, but is much lower at sites S16 and S17,which is 0.4.

3.5 Major element concentrations

Fig.6 The relative abundances (vs UCC) plot for the major elements in the tidal flat samples

Fig.7 Comparison of geochemical characteristics among the study sites

Figure 6 illustrates that the tidal flat sediment samples are enriched in Ti and Fe, but depleted in Na and Ca. Ca and Fe are the most variable elements while Si is most consistent. Spatially, Ca exhibits a distinctly decreasing trend from north to south(Fig.7f). Meanwhile, Na, also a depleted element, has an opposing trend and increases from north to south(Fig.7a). Ti values are in general uniform, except for samples from sites S15-S17 (Fig.7g). Mg, Al, K, Fe all present relatively low concentrations in XS Bay,TZ Bay, and SS Bay, although have high concentrations in HZ Bay and YQ Bay (Fig.7b, c, e, h). The pattern of variation is similar to that ofχARM. In contrast, Si occurs in relatively high concentrations at samples from XS Bay, TZ Bay, and SS Bay, but low concentrations in those from HZ Bay and YQ Bay(Fig.7d).

4 DISCUSSION

4.1 Identification of tidal flat sediment groups

In this study, cluster analysis (using fuzzy c-means,after Vriend et al., 1988; Maher et al., 2009) was selected to facilitate identification of any coherent groups among samples of tidal flat sediments in Zhejiang Province. It is generally accepted that the geochemical and magnetic characteristics of sediments are strongly dependent on particle size(e.g. Whitmore et al., 2004; Oldfield et al., 2009;Bouchez et al., 2011; Ge et al., 2017; Li et al., 2017;Jia et al., 2019) and, since the tidal flat sediment in Zhejiang coastal area is texturally variable, a particlesize independent parameter must be selected to avoid the influence of hydrodynamic forces relating to ocean currents or tides. In order to choose the appropriate independent variable, correlation analysis was performed among fifteen parameters in all. The results suggest that Na, Mg, Al, Si, K, Ca, Ti, Fe,χlf,χARM, and SIRM are significantly correlated (P＜0.01)with mean particle size and, for Na, Mg, Al, Si, K, Fe,andχARM, whereby the correlation coeffi cients all exceed 0.5 (Supplementary Table S1). On the other hand, Hm/(Hm+Gt),χfd, and HIRM are only weakly correlated with sediment particle size. Ti,χlf, SIRM,HIRM, and Hm/(Hm+Gt) are all grouped together by the cluster analysis. And four groups had been identified by cluster analysis (Supplementary Fig.S2;Supplementary Table S2).

Cluster analysis of samples from each site are plotted in Fig.8, revealing that samples from S17 and part of S15 belong to cluster 1; S6, S9, and part of S5 belong to cluster 2; S1, S11, S12, S13, and S14 mostly belong to cluster 3; S2, S3, S4, S7, S8, and S10 mostly belong to cluster 4 (Fig.8). In summary, therefore,four groups of sediment samples across the sites are clearly identifiable (Fig.8, Supplementary Table S3)as follows:

Fig.8 The distribution of samples across each of the four (fuzzy) clusters

Group 1 samples are from sites S17 and part of S15, located in the northern part of the Fujian coastal area; these are all characterized by high values ofχlfand SIRM and low values of Ti content, HIRM, and Hm/(Hm+Gt);

Group 2 samples are from sites S6, S9, and part of S5 and S15, mostly located in the bays which adjacent to hilly areas of the mainland (Fig.2), and are characterized by relatively low values ofχlfand SIRM and high value of Hm/(Hm+Gt);

Group 3 samples are from sites S11, S12, S13,S14, and partly of S1, S2, S5, and S10, mostly located in the south of Zhejiang coastal area or in the semienclosed bay in the north of the Zhejiang coastal area;these are characterized by relatively high values of Hm/(Hm+Gt) and Ti content;

Group 4 samples are from sites S3, S4, S7, S8, and partly of S1, S2, and S10, mostly located in the north of the Zhejiang coastal area; and are characterized by significantly higher values of Ti content and HIRM;

Unclassified samples with extreme values are mostly from sites S16 and S17, located in the Fujian coastal area, and are characterized by significantly higher values ofχlfand SIRM.

4.2 Binary mixing of the muddy sediment along the Zhejiang coastal area

In order to identify the sources of the tidal flat deposits in the Zhejiang Province, suspended material was sampled and subject to a similar range of analysis as the mudflat sediments (Fig.2). In terms of the characteristics ofits major river catchments, Zhejiang Province can be geologically divided into two parts by the Jiang-Shao fault (Fig.2). The northern part belongs to the Yangtze block, where Ordovician and Silurian sandstone, siltstone, and mudstone are widely distributed (Ma, 2002). The southern part belongs to the Cathaysian block associated with Jurassic and Cretaceous rhyolite, sedimentary tuff , and sandstone(Ma, 2002). Due to their dependence on parent rock characteristics (Thompson and Oldfield, 1986), the magnetic and geochemical characteristics of the river and coastal sediments in these two regions should be diff erent. Scatter plots of the magnetic and chemical properties of suspended sediment samples do indeed indicate a natural division into two clearly distinguishable groups (Fig.9). One group, labelled the Zhejiang river group, includes samples from the OR,and FYR, that are all located in the southern part of the study area. The second group includes suspended sediment samples from the CJR and QTR, located in the north of the study area, and labelled CJR-QTR river group. The Qiantang River is characterized by a discharge that is only approximately 3%-4% of the Changjiang River and this, coupled with the fact that their estuaries are very close to each other, suggests that the material contribution of QTR is negligible compared to the other rivers.

Fig.9 Magnetic and geochemical scatter plots of the tidal flat, suspended, and fluvial samples

The ocean sediment from the estuary of JCR (YRE)in Fig.9 symbolizes the magnetic and geochemical characteristics of sediments from the southern part of the Changjiang River estuary, originally reported in Ge et al. (2017). This location approximates the sites of suspended sediment samples from CJR and QTR in this study and supports the use of the magnetic and geochemical tracing method to identify the source of coastal sediment.

As Fig.9 shows, the suspended sediment is characterized by high concentrations of magnetite and coercive hematite from the CJR-QTR river group, and low concentrations of magnetite and coercive hematite from the Zhejiang river group. The reason for low concentrations of magnetic minerals in the sediment is in part due to the acidic volcanic nature of parent rock in the area, the magnetic susceptibility of which is usually relatively low. The tidal flat samples from the Zhejiang coastline have characteristics that are intermediate between the suspended sediments of the CJR-QTR river group and the Zhejiang river group.This strongly supports the conclusion that tidal flat sediments in the Zhejiang coastal area have a binary sediment source. The material contribution from CJR and QTR decreases in the sequence of: Group 4 ＞Group 3 ＞ Group 2 due to their progressive locational distance from those rivers (Fig.9). Sediments of Group 4 are located in the open bays in the northern part of the Zhejiang coastal area, sediments of Group 3 are located in the southern part and partly in the semiclosed bays in the northern section of the Zhejiang coast, while, sediments of Group 2 are located in the bays adjacent to the hilly part of the mainland(indicating a low deposition flux). Accordingly, we propose that both distance to the CJR estuary and the geomorphological characteristics of the coast strongly influence the material contribution of CJR to Zhejiang coastal sediments.

Yueqing Bay is shown to represent an important boundary, beyond which the material contribution from CJR is markedly decreased, as indicated by the fact that samples from WZ Bay, located southward of YQ Bay, all belong to Cluster 3. This suggests that the material contribution from OR is very important to the surrounding sediment, while, tidal flat sediment in the area still has two dominated sources, both CJR and local Zhejiang rivers. SS Bay is another important boundary, since sediments here exhibit distinctly diff erent geochemical and magnetic characteristics compared to all other samples from the Zhejiang coast. This observation further supports the contention that suspended material from CJR is not the dominant source of the fresh muddy sediment in the Fujian coastal area.

5 CONCLUSION

The geochemistry and magnetic properties of tidal flat sediment samples from the Zhejiang coastal area are distinctive and, critically, display minimal overlap with the characteristics of the CJR suspended material.Comparison with suspended sediment from both local rivers and the CJR suggests that the tidal flats of the study area have binary sources of sediment, whereby the majority originates locally. Further south, the tidal flats of Fujian have sediments that are entirely sourced from local rivers and this part of the coastline has practically no material link with the CJR. It must be noted that the situation may have been diff erent before the construction of the Three Gorges Dam, which has significantly reduced sediment flux to the Changjiang estuary and certainly warrants further research.

Our study suggests that, although the CJR may transport a huge sediment load to the south, the material is only rarely deposited along the shoreline and, accordingly, the material contribution of Changjiang sediments to the tidal flats of the Zhejiang coastal area has been overestimated, with implications for future management.

6 DATA AVAILABILITY STATEMENT

All data generated or analyzed during this study are included in this published article and its supplementary information file (Supplementary Table S4).

7 ACKNOWLEDGMENT

We thank Dr. Zhiyuan WANG and Bo WANG for the fieldwork.