PANG Yue, FAN Dejiang, SUN Xiaoxia, SUN Xueshi, LIU Ming, and YANG Zuosheng

Characteristics and Origins of Suspended Pyrite in the Mixing Zone of the Yangtze Estuary

PANG Yue1), 3), FAN Dejiang1), 2), *, SUN Xiaoxia1), SUN Xueshi1), LIU Ming1), and YANG Zuosheng1)

1),,,266100,2),,266061,3),,266061,

For a long time,most studies about pyrite have focused on sediments while only a few have focused on pyrite in water. In this study, a method that combines the scanning electron microscopy (SEM) and the energy dispersive X-ray spectrometry (EDS) was used to compare pyrite particles suspended in water to those in associated bottom sediments, both obtained from the mixing zone of the Yangtze Estuary. It was found that the pyrite particles in the two media have similar morphologies and size distributions. The particle morphology mainly includes two types, single crystal and aggregate, and the particle size mainly ranges from 0.5 to 2μm. The pyrite particles in water exhibit an increase in relative content towards the sea, and their transport and deposit processes are mainly affected by hydrodynamic conditions. It is concluded that the pyrite particles in the suspended matter mainly derived from the resuspension of sediments, which are products of the early diagenesis. Precursor minerals may appear during the formation of pyrite, but are generally restricted by the diagenetic environment and local microenvironment.

Yangtze Estuary; suspended matter; pyrite; resuspension; authigenic mineral

1 Introduction

Pyrite is a common authigenic mineral that forms in special conditions and has multiple crystal states. It mainly develops in anoxic sedimentary environments both in modern and geological time and is a key subject for the study of early diagenesis (Halbach, 1986; Jung and Lee, 1999; Leng and Yang, 2003; Ohfuji and Richard, 2005).

Pyrite widely develops in fine-grained sediments from the continental shelves in various forms of framboidal aggregates. The iron sulfide is thought to be generated during diagenesis, and therefore, it can be treated as an indicator mineral of the redox state during early diagenesis (Wilkin., 1997; Richards and Pallud, 2016). In ad- dition, pyrite exists in biologic shells in sediments as the forms of casting films, cement, and spherical aggregates, in which the idiomorphic degree, size, and morphology of pyrite crystal reflect the physical and chemical conditions of the microenvironment inside the shell (Wangand Yang, 1992). Moreover, the presence of a large amount of framboidal pyrite aggregates indicates the existence of a local reducing microenvironment in oxygen-rich water, and it also reflects the strong upward flow and the mixing of waters with different properties (Chu., 1995; Morseand Wang, 1997). The leakage of methane on the sea floor also accompanies the formation of tubular aggregate pyrite crystals, and they are the result of anaerobic oxidation of methane driven by the sulfate in oceanic sediments and accompanied by the reduction of organic sulfate. This pyrite is present in cold spring seep areas in the Black Sea and the South China Sea (Peckmann., 2001; Lin., 2017).

Given the presence of pyrite in a strong reducing environment, which is rarely provided by surface water, studies regarding pyrite particles in water are lacking. Previous studies of pyrite in water have mainly focused on a few extreme environments, such as extremely anoxic water (Wilkin., 1996) and water proximal to hydrother- mal vents on sea floor (Gartman., 2014), where fram- boidal pyrite aggregates and nanosize pyrite particles can be formed. In addition, the size of framboidal pyrite aggregates is associated with the redox state of the environment. Recently, iron sulfide was found in the oxidizing aquatic environment of the Yangtze Estuary and offshore areas and was thought to be formed in a microenvironment with organic matters (Fan., 2014). Due to the wide presence of reducing microenvironments in water, it is speculated that pyrite can be widely formed in oxidizing water (Fan., 2014). The mixing zone in the estuary is an area characterized by extremely intensive land-ocean interactions, involving physical, chemical, and biological effects on geological processes. This area also restricts some important processes such as the exchange and flux of continental and oceanic materials, as well as the formation and evolution of the estuarine maximum turbidity zone. As a unique authigenic mineral that can indicate the physical and chemical conditions in an estuary, the distribution and occurrence of pyrite in the mixing zone is of great value for understanding the processes that occur in the estuary. Therefore, the morphology, com- position, and spatial distribution of pyrite particles in the water column and the surface sediments in the Yangtze Estuary are analyzed in this study, combined with the aqueous physical and chemical properties. The goals of this paper are to elucidate the types, formation mechanisms, and transport mechanisms of pyrite minerals in this area. And the differences in spatial development of pyrite particles are demonstrated. Then the basis for the understanding of this sedimentary process in the Yangtze Estuary is provided in this study.

2 Study Area

The Yangtze River is the longest river in China and the third longest in the world, with a total length of 6.3×106m and a watershed of 1.8×106km2. Its average annual seawater inflow is 9.2×1011m3, and at natural state, the sediment discharge is 4.8×108t. It eventually pours into the East China Sea (Liu., 2007; Chai., 2009), where a large delta is formed (Yang., 2013).

The Yangtze Estuary is the mixing zone of saline and fresh water, which is characterized by a complicated hydrodynamic condition jointly controlled by currents, runoff, waves, and tides (Chen, 2000; Yang., 2000). The frequent exchange of water and sand and turbulent hydrodynamic conditions make the suspended matter in this area keep in the processes of transport, deposition, and resuspension, which affect and shape the submarine topography (Liu., 2006; Li., 2017). The maximum turbidity zone is about 25–46km, with the total su- spended matter content of 0.1–0.7kgm?3at surface layer, and 1–8kgm?3at bottom layer, which is located at estuarine bar and its upper and lower reach areas (Li., 2012; Wu., 2012). The location, area, and contention of particles about the maximum turbidity zone are mainly determined by the variation in runoff and tidal currents, total suspended matter content carried by the river, and the strength of the saline wedge density current at the estuary (Shen., 2008; Uncles., 2010; Kitheka., 2016). The water mixing in estuary makes the physical and chemical properties of water bodies change significantly, which is accompanied by the process of particle flocculation and the formation of authigenic minerals. As a result, the size and composition of suspended particles are changed, the process of sedimentation is influenced subsequently, and the pollution, such as heavy metals, are redistributed between the suspended matter and the soluble phase (Walling and Moorehead, 1987; Dyer and Manning, 1999; Xia., 2004; Fettweis., 2006; Wang., 2013).

3 Sampling and Analysis Methods

3.1 Survey and Sample Collection

The study area is located in the Yangtze Estuary and has nine stations (Fig.1), including three stations surveyed in spring and six stations surveyed in winter. The spring sta- tions were investigated by a shared voyage associated with the ‘Yangtze Estuary Scientific Investigation and Expe- rimental Research’ in March 2015, which was organized by the National Natural Science Foundation of China and used the survey vessel ‘. Water samples from the surface, middle (0.6H, three-fifths of the water depth), and bottom (H, 2m from seafloor) layers were collected by a Rosettesampler mounted on a Sea- Bird SBE25 CTD (conductivity-temperature-depth). About 5L of water samples were collected, 2mL of which was extracted and filtered through a filter membrane with distilled water washing the samples thrice. Then the membrane was air-dried to obtain suspended particle samples. The winter stations were investigated during a Yangtze Estuary field survey in December 2016 organized by the East China Normal University and the Ocean University of China. The survey vessel was ‘. Water samples from the surface, middle (0.6H, three-fifths of the water depth), and bottom (H, 2m from seafloor) layers were collected using a sampler mounted on ALEC CTD (ASTD120). The samples were each approximately 5L. According to the water turbidity, about 3–6mL subsample was extracted, diluted and filtered through a membrane. After leaching three times using distilled water, the membrane with suspended matters was air-dried. During the winter survey, a box sampler was used to obtain the surface sediment samples, which were stored in plastic sample bags.

Fig.1 Marine environmental characteristics (left) and sampling stations in the study area (right). The depicted circulation system was modified from Liu et al. (2007) and Wang et al. (2014). KC, Kuroshio Current; YSWC, Yellow Sea Warm Current; TWWC, Taiwan Warm Current; YSCC, Yellow Sea Coastal Current; SBCC, Subei Coastal Current; ECSCC, East China Sea Coastal Current; CDW, Changjiang Diluted Water.

3.2 Methods of Measurement

Approximately 10g surface sediment sample obtained in the winter survey was weighed and salt-leached three times. Then it was dried and weighed again. Lastly, it was sieved to obtain the subsample below 32 microns, which was homogenized by 400mL distilled water and 10mL 0.1molL?1sodium hexametaphosphate. About 0.3–0.5mL so- lution was diluted and filtered through a membrane and was rinsed with distilled water. The membrane was dried at room temperature to obtain the sediment particle sample.

The morphological features and chemical composition of the pyrite particles in the suspended matter and the surface sediment were analyzed by using scanning electron microscopy (SEM) and energy dispersive X-ray spe- ctrometry (EDS). Firstly, approximately 3×3mm2small square piece was cut from the membrane that held suspended particles (or sediments after dispersion processing) for the following SEM/EDS observation. The piece was placed on copper sample plate and coated with gold. Then, the sample was placed in the SEM sample chamber with working conditions including a high vacuum, a working distance of 10mm, and an accelerating voltage of 25kV. The pyrite compositionwas analyzed and imaged by using EDS under the combined spot analysis and map scanning patterns. The FEI Quanta 200 scanning electron microscope interfaced with an EDAX Inc. X-ray spectrome- ter was used.

The relative content of pyrite was calculated based on the number of suspended particles in the SEM images.Specifically, it was calculated as the ratio between the total number of pyrite particles and the total number of suspended particles at the same station.

4 Results

4.1 Pyrite Morphology

1) The morphological characteristics of pyrite in the sus- pended matter

The suspended pyrite particles observed in the study area were mainly in the form of single crystals and aggre- gates. Some pyrite particles display the corrosion structure on their faces. Based on the morphology, pyrite particles can be divided into four types: euhedral single cry- stals, irregular single crystals, framboidal aggregates and irregular aggregates.

a) Euhedral single crystal pyrite particles

The suspended euhedral single crystal pyrite (Fig.2) is either cubic or pentagonal dodecahedral with a particle size of 0.5 – 2 μm. The crystal has a clear outline and rim as well as a smooth face. Particles exist independently and they are completely lacking in corrosion.

b) Irregular single crystal pyrite particles

The suspended irregular single pyrite particles (Fig.3) have various morphologies, including long strips (Fig.3A) and ovals (Fig.3B). The particle size is larger than the eu- hedral particles, ranging from 1 to 3μm. The crystal has clear rims with smooth faces. Some particles showed weak corrosion on their rims (Fig.3C).

c) Framboidal aggregates

The framboidal pyrite aggregates display the shape of a regular sphere or an oval with a diameter of 5–10μm. The interior of the sphere is composed of tens or hundreds of pyrite microcrystals with cubic, dodecahedral, or octahedral morphologies. The microcrystals have small and ho- mogeneous sizes in the range of 0.5–0.6μm. They are ali- gned in a disordered or semi-ordered manner, with spaces among microcrystals. Some aggregates are covered by the thin membrane, which is composed of organic matters or clay minerals (Fig.4).

Fig.2 Euhedral pyrite particles in the suspended matter. A and B show cubic pyrite crystals with complete edges and rims and smooth faces. C and D show pentagonal dodecahedral pyrites with clear edges and rims, smooth faces, and complete morphology.

Fig.3 Irregular pyrite particles in the suspended matter. A shows a long strip crystal with a large particle size and smooth crystal face. B shows oval pyrite, which may result from the erosion in water for a long time. C shows a pyrite with a weakly corroded rim and a smooth crystal face. D shows the pyrite particle which looks like a triangular with vague rims and obvious edges among crystals.

Fig.4 Framboidal pyrite aggregates in the suspended matter. A1–A4 show well-preserved framboidal pyrite aggregates. In A1 and A2, the rims are adhered to other mineral particles, and A3 and A4 display complete framboidal pyrite aggregates with clear internal structures and homogeneous, ordered distributions of microcrystals. B1 shows the aggregate composed of two semi-euhedral pyrite crystals adhered to other mineral particles with clear edges and smooth faces. B2 shows the aggregate composed of two cubic pyrite crystals connected point-to-point with good microcrystal morphologies, clear edges and rims, and an apparent 90? angle. B3 shows the aggregate composed of pyrite particles connected point-to-point, point- to-line, or line-to-line with clear rims and obvious edges. B4 shows the aggregate composed of multiple pyrite particles with overlapping particle rims.

In addition, a small number of pyrite microcrystal aggregates appear in the suspended matter with intact microcrystal morphologies, high euhedral degrees and incomplete aggregate morphologies (Fig.4). The characteristics of microcrystals and the aggregate suggest that the aggregates are fragments of framboidal pyrite aggregates and preserve their some characteristics. Specifically, the microcrystal morphology and combination relationship among the microcrystals are consistent with those of fram- boidal pyrite.

d) Irregular aggregate

The irregular aggregate in suspended matter is charac-terized by an irregular shape, angular edge, uneven crystal face, and large size ranging from 5 to 8μm. Some particles show obvious corrosion grooves on their faces. It is considered that the compact pyrite aggregates are composited by multiple irregular micro-particles (Fig.5).

2) The morphological characteristics of pyrite particles in the surface sediment

The pyrite particles in the surface sediment also include single crystals and aggregates. The single crystals include euhedral and irregular morphologies and the aggregates include framboidal and irregular morphologies, similar to those in the suspended matter.

Fig.5 Irregular pyrite aggregates in the suspended matter. A and B show apparent corrosion grooves on the faces of pyrite particles. C and D show that pyrite particles were not corroded, although their faces are uneven.

a) Euhedral single crystal pyrite particles

The euhedral single pyrite particles in the surface sedi- ment (Fig.6) display cubic, octahedral, and pentagonal do- decahedral morphologies. They have clear rims, smooth faces, and obvious edges, with sizes ranging between 1 and 3μm.

b) Irregular single pyrite particle

The irregular single pyrite particles in the surface sedi- ment (Fig.7) have smooth faces and are rounded particles in the shape of a sphere. Some particle rims are absorbed onto other minerals. The particle size is finer than those observed in the suspended matter.

c) Framboidal pyrite particles

Compared with the framboidal pyrite aggregates in sus- pension, the sedimentary framboidal particles have a finer particle size (Fig.8) with poorer morphological development. The microcrystals have a strip-shaped structure with a homogeneous morphology and smaller spaces among microcrystals. Insome framboidal pyrite particles, the mi- crocrystals are tightly combined together.

d) Irregular aggregate pyrite particles

The sedimentary irregular pyrite aggregates have multiple morphologies (Fig.9). They are either wrapped in a membrane or exposed to the environment but with incom- plete morphologies. These aggregates are all composed of multiple microcrystals with good morphologies, intergra- nular spacing, and a clear internal structure. In addition, some aggregates are adhered to other mineral particles.

Fig.6 Sedimentaryeuhedral pyrite particles. A shows a pyrite crystal that has a prismatic contour, smooth faces and clear edges. B shows the pyrite has rectangle contour, clear rims, and partially well-preserved edges, but some of edges have been rounded. C and D show pentagonal dodecahedral crystals with clear faces and edges and complete morphologies.

Fig.7 Sedimentary irregular pyrite particles. A and B, the pyrite particles are rounded with smooth faces and clear rims. C, the pyrite has a heart-shaped outline with uneven edges and corrosion pits. D, the pyrite has the shape of sphere with smooth face and is absorbed to other mineral particles on the rim.

Fig.8 Sedimentary framboidal pyrite aggregates. A and B shows normal framboidal aggregates with internal microcrystals similar to those observed in the suspended matter. A, the aggregate is covered by a silicate membrane based on the EDS analysis. B, the aggregate is exposed to the environment with the rim adhered to other minerals. C, the pyrite aggregate has strip-shaped microcrystals with homogeneous morphologies, unique structures, and narrow intergranular spacing. D, the pyrite has compact internal microcrystals without intergranular spaces or clear structures, displaying a rough sphere, which is thought to result from advanced development from the C aggregate.

4.2 Size Fraction Characteristics of Pyrite Particles

The size distribution of suspended pyrite particles is si- milar to that of sedimentary pyrite ones (Fig.10). The sizes of suspended pyrite particles are all smaller than 10 μm, including 75.97% single particles and 24.03% aggre- gates. However, more rough particles are found in sediments, including 62.50% single particles and 37.50% agg-regates. The pyrite particles larger than 10μm are extracted from the surface sediments.

The dominant size of suspended single pyrite crystals is between 1 and 2μm, with a proportion of 37.21%. The secondary dominant size range is 0.5–1μm, with a proportion of 12.40%. The dominant size of pyrite in the surface sediment is similar to that in the suspended matter, with a proportion of 27.5% ranging between 1 and 2μm, and the secondary dominant size fraction is 3– 4μm with a proportion of 12.5%.

The suspended aggregate particles mainly have a size of 2–5μm with a proportion of 13.95%. The sedimentary aggregate particles are mainly distributed between 1 and 2μm and >10μm, with each fraction accounting for 10%. The contents of other fractions are relatively low.

4.3 Compositional Characteristics of Pyrite Minerals

Theoretically, pyrite (FeS2) is composed of 46.67% Fe and 53.33% S, often with trace Co and Ni isomorphism replacing Fe, and trace As and Se and Te isomorphism replacing S. In addition, there is also fine impurity composed of Sb, Cu, Au,and Ag. The suspended pyrite particles have a similar composition to sedimentary ones. EDS analyses (Fig.11) show that the weight percentages (wt%) of Fe and S are similar to the theoretical values (Fe 46.67%, S 53.33%) and that the volume percentage ratio (at%) is similar to the atomic ratio of 1:2. The EDS spectra of some pyrites show the presence of C and O, which is thought to be stimulated from the cellulose acetate membranes under the attack of the electron beam due to the fine particle size of the pyrite.

Fig.9 Sedimentary irregular pyrite aggregates. The aggregate in A is wrapped with a clay membrane with diverse microcrystalline morphology. B, the aggregate has loose microcrystals with smooth crystal faces and inhomogeneous morphologies. C, the microcrystals have clearly different sizes and are mixed with silicate particles. D, the aggregate has a homogeneous microcrystal morphology and is wrapped with the clay membrane similar to the crystal in A.

Fig.10 Size fraction distribution of pyrite particles in suspended matter (A) and sediment (B).

Fig.11 Chemical composition of pyrite. (A), EDS spectrum of framboidal pyrite only shows S and Fe with a volume ratio (%) close to 1:2. (B), EDS spectrum of framboidal pyrite aggregate fragment shows the presence of C, O, Fe and S, in which C and O are resulted from the cellulose acetate membranes. (C) and (D), EDS spectra of pyrite indicate trace Si, Al and Mg, although pyrite is still mainly composed of Fe and S.

In addition, some of the EDS images show the presence of Si, Al, Mg, and Fe, which are thought to be conta- mination from the membrane wrapped on framboidal pyrite particles or other minerals that were stuck to the rims. The EDS analysis suggests that the membranes are composed of clay minerals and that the minerals adhering to pyrite rims are silicate.

4.4 Spatial Distribution of Pyrite Particles in the Suspended Matter

Pyrite particles are found at all stations in the study area (Fig.12, left). The relative content increased toward the ocean. The content in the maximum turbidity zone is relatively high, up to 111.68×10?4.

Pyrite particles are found in the surface, middle (0.6H, three-fifths of the water depth), and bottom (H, 2m from seafloor) layers of the water column with the contents of 35.1%, 32.4%, and 32.4%, respectively, implying insignificant vertical differences. In contrast, the particle size is distributed differently in the vertical profile (Fig.12, right). The pyrite particles in the surface and middle layers are finer, mainly ranging from 0.5 to 3μm, whereas the particles at the bottom are coarser, and the particle content in the range of 4–8μm is higher than the sum of the contents in the surface and middle layers.

Fig.12 Spatial distribution (×10?4) (left) and particle size histogram (right) of pyrite particles in the suspended matter.

5 Discussion

5.1 Homology Analysis of Pyrite Particles in the Sus- pended Matter and in the Surface Sediment

According to Sections 3.1 and 3.2, no significant difference was observed in the morphology of pyrite particles found in the suspended matter and in the surface sediment. In both media, there are four morphologies of pyrite particles: regular and irregular single crystals, and framboidal and irregular aggregates. The particle fraction distributions in both media are also similar, mainly in the range of 0.5–2μm. The content of fine particles is higher, whereas the content of particles above 10μm is lower.

The similarity in the morphology and particle fraction of pyrite particles in the suspended matter and in the surface sediment suggests the exchange between the two media. The study area is located at the convergence zone of tidal currents and runoff. Such strong hydrodynamic conditions promote the resuspension of surface sediments. Thus, it is speculated that the pyrite particles in water are mainly derived from re-suspended sediments, with only a small amount of fine particles derived from authigenic processes in the water microenvironment.

In addition, a high content of suspended pyrite particles occurs inside the maximum turbidity zone (stations 3, 4, 5 and 6). The mud-type sediment aids the formation and pre-servation of pyrite particles (see Section 4.2). The most in- tensive hydrodynamics and frequent sediment resuspension in this zone make pyrite particles re-suspend and thus increase the content of pyrite in water.

5.2 Formation Mechanism of Pyrite

As shown in Section 4.1, the characteristics of pyrite in the suspended matter are similar to those of pyrite in the surface sediment, suggesting that pyrite in suspension mainly originates from the resuspension of sediments.

There are two views about the complex formation of pyrite. One view is that precursor minerals, such as grei- gite, are first formed in oceanic sediments, and then pyrite is formed through vulcanization. For example, Wilkin. (1997) experimentally proved that the formation of fram- boidal pyrite includes four stages. 1) Iron monosulfide microcrystals nucleate and start to grow. 2) Iron monosulfide microcrystals are converted to greigite (Fe3S4). 3) Gr- eigite with a homogeneous particle size starts to aggregate, and framboidal particles start to grow. Numerical si- mulations suggest that when greigite has a particle size of larger than 0.1μm, it can rapidly aggregate in seawater andfresh water. 4) Pyrite particles replace framboidal greigite.

The reactions in each stage can be summarized as follows:

Or, the above three reactions can be summarized as:

The other view is that pyrite is formed directly in se- diments without the formation of precursor minerals such as mackinawite, pyrrhotite, and greigite. For example, So- liman and Goresy (2012) analyzed the formation of py- rite at Gabal Oweina, Nile Valley, Egypt. They proposed that the oversaturation of sulfur can directly lead to the formation of framboidal pyrite aggregates through dehydration, nucleation, crystallization, and aggregation of individual framboidal pyrite. The process includes four stages: euhedralmicrocrystalline pyrite initially nucleates and grows; an aggregate of pyrite microcrystals is formed; the large, cemented, framboidal aggregate absorbs smaller, solidified, framboidal aggregates during growth; and the framboidal aggregates are grouped.

Pyrite particles mainly grow in reducing or strongly reducing environments. Toward the ocean, the size of par- ticles in the study area becomes finer in the surface sedi-ment (Fig.13, left) as the transition of sediment type from sandy to muddy. The particles of sandy sediments are coarse with large particle spaces and good air permeability, making it difficult to preserve organic matter. In comparison, the muddy sediment particles are finer with small- er particle spaces and poor air permeability, their free oxygen content is low, and Eh values can be as low as 224.1mV (Fig.13, middle). Previous studies have suggested that the Yangtze Estuary is an anoxic area that facilitates severe eutrophication (Bao., 2006; Zhou., 2008). In winter, the low oxygen solubility (Dai., 2006; Chen., 2007), rapid deposition rate and flocculation precipitation hinders the instant decomposition of organic matter in the water (Tang, 2007). After precipitating on the sea floor, the organic matters are aerobically decomposed by bacteria to form a local reducing environment. Under anoxic conditions, sulfate-reducing bacteria proliferate and reduce SO42?to S2?in the pore water of sediment. S2?reacts with active iron in the sediments to ultimately produce pyrite. The pH values of the sediments in this area range from 7 to 8 (Fig.13, right), indicating weak alkalinity. Compared to that in acidic environment, the solubility of sulfate in the alkaline environment is greater, which is beneficial to the formation of pyrite because the best pH value for sulfate-reducing bacteria to survive is between 7 and 8.

Fig.13 Average particle size distribution (Mz) and redox potential (Eh) distribution in the surface sediments and pH-Eh map for stability of various iron species (revised from Hem, 1972; Sun et al., 2017).

The pH and Eh data for the water and sediment samples from the study area are plotted in a pH-Eh map, which has been under constant improvement (Fig.13, right). The pH and Eh data are all located in the area associated with iron oxide, which is inconsistent with the presence of pyrite particles found in a broader environment. Thus, it is believed that pyrite particles found in the study area are formed in local microenvironments, where strong reducing environments can be achieved to make the Fe2+concentration reach the value that is required for the formation of pyrite (FeS2), thus leading to the appearance of low- valence iron sulfide authigenic minerals. Moreover, the EDS image of iron sulfide shows that although a majority of particles have a S:Fe volume ratio close to 2, there are also ratios of 1.3, 1.2, 1.7, and below 1, implying the formation of intermediate products during the formation process of pyrite.

5.3 Transport and Deposition of Pyrite in the Estuary

The pyrite particles in the water of the mixing zone are transported through suspension and begin to settle under suitable hydrodynamic conditions. Consider the sedimentation of pyrite particles (aggregate) in water. We assumed that the pyrite particle is spherical, the density of pyrite is 5.0gcm?3, and the density of seawater is 1.0gcm?3. Theis pyrite diameter (the diameter of suspended pyrite particle is in a range of 0.5–10μm). Theis gravitational acceleration, 9.81ms?2. Theis seawater viscosity, which is 0.012cm2s?1for a water temperature of 13℃. The de- position of pyrite particles (aggregates) with different dia- meters can be estimated by using the Stocks equation, as shown in Fig.14.

Fig.14 Relationship between theoretical settling rate and particle size of pyrite. A, all pyrite particles; B, pyrite aggregates.

At a water depth of 30m in the study area, the suspended pyrite particles have a settling rate ranging from 4.54×10?5to 1.70×10?2cm2s?1. The size of most pyrite particles ranges from 0.5 to 2μm, and the settling time is between 48 and 760days. By contrast, the settling time is no longer than one week for particles larger than 5μm. In comparison, the pyrite aggregates have a particle size ranging from 1.68 to 9.67μm and the settling rate is from 5.12×10?4to 1.69×10?2cms?1, and the settling time is approximately 2–68days (Fig.14).

The surface sediment of the study area is under an oxidizing, weakly alkaline environment, whereas reducing environments still exist inside the muddy sediments, which is favorable for the formation and preservation of pyrite. However, the unstable hydrodynamic conditions in this area cause the pyrite particles in the sediments to be resuspended in water. Then most fine pyrite particles keep suspending and transporting in water instead of settling down to sediments on the sea floor.

6 Conclusions

Pyrite particles appear in both water and surface sediments in the mixing zone of the Yangtze Estuary. They have multiple morphologies, mainly including euhedral single crystals, irregular single crystals, framboidal aggre- gates and irregular aggregates. The particle size of pyrite is relatively small, mainly ranging from 0.5 to 2μm. The dominant particle size of single crystal particles is in the range of 1–2μm, whereas the particle size of aggregates varies widely, ranging from a few μm to larger than 10μm. The morphology and size distribution of pyrite particles suggest that the main source of pyrite particles in the suspended matter is the resuspension of sediments.

In the mixing zone of the Yangtze Estuary, the relative content of pyrite particles in water increases toward the ocean. And such a spatial distribution is mainly affected by the type of bottom sediments. The pyrite particles require a relatively long time for sedimentation in water and are thus transported and deposited mainly under the effect of hydrodynamic conditions.

The pyrite in sediments and water in the Yangtze Estuary is the product of early diagenesis, during which precursor minerals may appear. In general, the formation of pyrite is restricted by diagenetic conditions and local microenvironments.

Acknowledgements

We thank the National Natural Science Foundation of China (NSFC), who organized the public survey cruise in the Yangtze River Estuary in March 2015, with special thanks to the crew of thefor kindly assisting in sediment sampling on the cruise. This study was funded by the NSFC (No. 41676036) and the National Key R&D Program of China (No. 2016YFA0600904).

Bao, X., Watanabe, M., Wang, Q., Hayashi, S., and Liu, J., 2006. Nitrogen budgets of agricultural fields of the Changjiang Ri- ver Basin from 1980 to 1990., 363 (1-3): 136-148.

Chai, C., Yu, Z., Shen, Z., Song, X., Cao, X., and Yao, Y., 2009. Nutrient characteristics in the Yangtze River Estuary and the adjacent East China Sea before and after impoundment of the Three Gorges Dam., 407 (16): 4687-4695.

Chen, C. T. A., 2000. The Three Gorges Dam: Reducing the up- welling and thus productivity in the East China Sea.,27 (3): 381-383.

Chen, C. C., Gong, G. C., and Shiah, F. K., 2007. Hypoxia in the East China Sea: One of the largest coastal low-oxygen areas in the world.,64 (4): 399-408.

Chu, F. Y., Chen, L. R., Shen, S. X., Li, A. C., and Shi, X. F., 1995. Origin and environmental significance of authigenic pyrite from the south Yellow (Huanghai) Sea sediments., 26 (3): 227-233 (in Chinese with English abstract).

Dai, M., Guo, X., Zhai, W., Yuan, L., Wang, B., Wang, L., Cai, P., Tang, T., and Cai, W., 2006. Oxygen depletion in the upper reach of the Pearl River Estuary during a winter drought.,102 (1-2): 159-169.

Dyer, K. R., and Manning, A. J., 1999. Observation of the size, settling velocity and effective density of flocs, and their fractal dimensions., 41: 87-95.

Fan, D., Chen, B., Wang, L., Sun, X., and Yang, Z., 2014. Authigenic lepidocrocite and greigite particles in aquatic environments off the Yangtze River Estuary.,39 (10): 1364-1370 (in Chinese with English abstract).

Fettweis, M., Francken, F., Pison, V., and Eynde, D. V. D., 2006. Suspended particulate matter dynamics and aggregate sizes in a high turbidity area., 235 (1-4): 63-74.

Gartman, A., Findlay, A. J., and Luther III, G. W., 2014. Nano- particulate pyrite and other nanoparticles are a widespread component of hydrothermal vent black smoker emissions., 366 (3): 32-41.

Hem, J. D., 1972. Chemical factors that influence the availability of iron and manganese in aqueous systems., 83 (2): 443-450.

Halbach, P., 1986. Processes controlling the heavy metal distribution in Pacific ferromanganese nodules and crusts., 75 (1): 235-247.

Jung, H. S., and Lee, C. B., 1999. Growth of diagenetic ferroman- ganese nodules in an oxic deep-sea sedimentary environment, northeast equatorial Pacific., 157 (3-4): 127- 144.

Kitheka, J. U., Mavuti, K. M., Nthenge, P., and Obiero, M., 2016. The turbidity maximum zone in a shallow, well-flushed Sabaki Estuary in Kenya., 110: 17-28.

Leng, Q., and Yang, H., 2003. Pyrite framboids associated with the Mesozoic Jehol Biota in northeastern China: Implications for microenvironment during early fossilization., 13 (3): 206-212.

Li, D., Li, Y., and Xu, Y., 2017. Observations of distribution and flocculation of suspended particulate matter in the Minjiang River Estuary, China., 387: 31-44.

Li, P., Yang, S. L., Milliman, J. D., Xu, K. H., Qin, W. H., Wu, C. S., Chen, Y. P., and Shi, B. W., 2012. Spatial, temporal, and human-induced variations in suspended sediment concentration in the surface waters of the Yangtze Estuary and adjacent coastal areas., 35 (5): 1316-1327.

Lin, Z., Sun, X., Lu, Y., Strauss, H., Xu, L., Gong, J., Teichert, B., Lu, R., Sun, W., and Peckmann, J., 2017. The enrichment of heavy iron isotopes in authigenic pyrite as a possible indicator of sulfate-driven anaerobic oxidation of methane: Insights from the South China Sea.,449: 15-29.

Liu, J. P., Li, A. C., Xu, K. H., Velozzi, D. M., Yang, Z. S., Milliman, J. D., and DeMaster, D. J., 2006. Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea., 26 (17): 2141- 2156.

Liu, J. P., Xu, K. H., Li, A. C., Milliman, J. D., Velozzi, D. M., Xiao, S. B., and Yang, Z. S., 2007. Flux and fate of Yangtze River sediment delivered to the East China Sea., 85 (3-4): 208-224.

Morse, J. W., and Wang, Q., 1997. Pyrite formation under conditions approximating those in anoxic sediments: II. Influence of precursor iron minerals and organic matter., 57 (3-4): 187-193.

Ohfuji, H., and Rickard, D., 2005. Experimental syntheses of fra- mboids–A review., 71 (3-4): 147-170.

Peckmann, J., Reimer, A., Luth, U., Luth, C., and Reitner, J., 2001. Methane-derived carbonates and authigenic pyrite from the northwestern Black Sea.,177 (1-2): 129- 150.

Richards, C. M., and Pallud, C., 2016. Kinetics of sulfate reduction and sulfide precipitation rates in sediments of a bar-built estuary (Pescadero, California)., 94: 86-102.

Shen, Z., Zhou, S., and Pei, S., 2008. Transfer and transport of phosphorus and silica in the turbidity maximum zone of the Changjiang Estuary.,78 (3): 481-492.

Soliman, M. F., and Goresy, A. E., 2012. Framboidal and idiomorphic pyrite in the upper Maastrichtian sedimentary rocks at Gabal Oweina, Nile Valley, Egypt: Formation processes, oxidation products and genetic implications to the origin of framboidal pyrite., 90: 195- 220.

Sun, X., Fan, D., Liu, P., Pang, Y., and Tian, Y., 2017. The states of Eh, pH of water from Yangtze River Estuary and its adjacent areas and their implications., 35 (1): 96-106 (in Chinese with English abstract).

Tang, J. H., 2007. Characteristics of fine cohesive sediment’s flocculation in the Changjiang Estuary and its adjacent sea area. Master thesis. East China Normal University.

Uncles, R. J., Bale, A. J., Stephens, J. A., Frickers, P. E., and Harris, C., 2010. Observations of floc sizes in a muddy estuary., 87 (2): 186-196.

Walling, D. E., and Moorehead, P. W., 1987. Spatial and temporal variation of the particle-size characteristics of fluvial suspended sediment., 69 (1): 47-59.

Wang, L., Fan, D., Li, W., Liao, Y., Zhang, X., Liu, M., and Yang, Z., 2014. Grain-size effect of biogenic silica in the surface sediments of the East China Sea.,81 (4): 29-37.

Wang, Q., and Yang, Z., 1992. Authigenic pyrite in the surface sediments of the southern Huanghai Sea., 12 (1): 25-32 (in Chinese with English abstract).

Wang, Y. P., Voulgaris, G., Li, Y., Yang, Y., Gao, J., Chen, J., and Gao, S., 2013. Sediment resuspension, flocculation, and settling in a macrotidal estuary., 118 (10): 5591-5608.

Wilkin, R. T., Arthur, M. A., and Dean, W. E., 1997. History of water-column anoxia in the Black Sea indicated by pyrite framboid size distributions.,148 (3-4): 517-525.

Wilkin, R. T., Barnes, H. L., and Brantley, S. L., 1996. The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions., 60 (20): 3897-3912.

Wu, J., Liu, J. T., and Wang, X., 2012. Sediment trapping of turbidity maxima in the Changjiang Estuary., 301-306: 14-25.

Xia, X. M., Li, Y., Yang, H., Wu, C. Y., Sing, T. H., and Pong, H. K., 2004. Observations on the size and settling velocity distributions of suspended sediment in the Pearl River Estuary, China.,24 (16): 1809-1826.

Yang, S. L., Eisma, D., and Ding, P. X., 2000. Sedimentary pro- cesses on an estuarine marsh island within the turbidity ma- ximum zone of the Yangtze River mouth., 20 (2): 87-92.

Yang, Y. P., Li, Y. T., Sun, Z. H., and Fan, Y. Y., 2013. Trends and causes of suspended sediment concentration variation in the turbidity maximum zone at the Yangtze River Estuary., 68 (9): 1240-1250 (in Chinese with English abstract).

Zhou, M. J., Shen, Z. L., and Yu, R. C., 2008. Responses of a coastal phytoplankton community to increased nutrient input from the Changjiang (Yangtze) River., 28 (12): 1483-1489.

. E-mail: djfan@ouc.edu.cn

December 26, 2018;

May 8, 2019;

October 11, 2019

(Edited by Chen Wenwen)