SONG Hongying, LIU Jinqing, YIN Ping, ZHANG Yong, and CHEN Xiaoying



Characteristics of Heavy Minerals and Quantitative Provenance Identification of Sediments from the Muddy Area Outside the Oujiang Estuary Since 5.8 kyr

SONG Hongying1), 2), LIU Jinqing2), 3), 4), YIN Ping2), 3), *, ZHANG Yong2), 3), and CHEN Xiaoying2), 3)

1),,266237,2),,,266071,3),,266061,4),,266590,

The heavy mineral compositions of the sediments in core D04 with a length of 20.2m from the muddy area outside the Oujiang Estuary along the coast of Zhejiang-Fujian Provinces have been analyzed. Core D04 can be divided into three units: DU1 (0–7.4m), DU2 (7.4–11.4m) and DU3 (11.4–20.2m). The results showed that the heavy minerals are authigenic pyrite (65.6%)– hornblende (16.0%)–epidote (4.6%) assemblages. The core sediments are mainly composed of clayey silt, which belongs to the clinoform deposit formed since 5.8kyr, indicating a weak hydrodynamic environment. The deposition rates changed in the trend of high-low-high upward. Quantitative analysis shows that the core sediments were mainly from the suspended sediments of the Changjiang (Yangtze) River, accounting for 79.2%, with only 10.0% from Oujiang, and 10.8% from other provenance. The Oujiang-derived sediments were gradually increased from the bottom unit DU3 (3.1%) to the top unit DU1 (17.8%), but the Changjiang-derived sediments were gradually decreased. The source changes are closely related to the development of Yangtze River Delta and Wenzhou Bay, climate changes and human activities. During the period of 2.1–3.7kyr, the sediment supply was greatly reduced due to the poor reclamation capacity of the river basins, resulting in a low deposition rate. Since 2kyr, the enhancement of human activities in the Yangtze River Basin and the Oujiang River Basin led to more soils to be eroded and transported to the study area. Due to the short distance of the site of core D04 from the Oujiang River mouth, the study area received more Oujiang-derived sediments when upper unit (DU1) deposited than the lower units (DU2 and DU3).

mud deposit; Oujiang Estuary; mid-Holocene; heavy mineral; quantitative identification

1 Introduction

Detrital mineral assemblages (including both light and heavy minerals), clay mineral compositions and geochemi- cal compositions (major, trace, rare earth elements, carbon and nitrogen isotopes,.), are commonly used as source indicators of seafloor sediments (Vital., 1999; Garzanti and Andò, 2007; Liu., 2017; Liu., 2018b). The characteristics of the mineral assemblages include the information about sediment sources, transport routes and sedimentary differentiation, which can be used to indicate the depositional environment, climate evolutionand hydrodynamic conditions (Sun, 1990; Chen, 2008). In general, heavy mineral assemblage characteristics, characteristic mineral occurrence and mineral content ratios are used to study the sources and sedimentary environments (Morton and Hallsworth, 1994, 1999; Liu., 2018a). Hydrodynamic conditions affect the enrichment and sorting of heavy minerals (Morton and Hallsworth, 1999; Komar, 2007; Garzanti., 2009). Factors such as rock type, weathering intensity, transporting way and sedimentary diagenesis control the composition of heavy minerals in sediments (Morton and Hallsworth, 1994; Nath., 2000; Andò., 2012). Depending on the different sources, the detrital mineral assemblages exhibit different characteristics. Coastal and nearshore sediments generally have complicated sources, including materials from local rivers and coastal erosion as well as sediments transported from far-distance sources. Heavy minerals are considered as one of the most effective indicators of discriminating sediment sources (Liu., 2017, 2018a). A large number of studies have shown that heavy minerals are most abundant in very fine sand, but their distribution trends in fine sand and silt grains are relatively consistent and can reflect the overall source information to some extent (Chen, 2008).

According to previous studies, a large clinoform deposit zone extends from the Changjiang River mouth to the Taiwan Strait along the coast of Zhejiang and Fujian Province, including two depocentres, the Yangtze River Delta with thickness of 40–60m and outside Oujiang River Estuary with thickness of 30–40m (Liu., 2006, 2007a; Xu., 2012). Since the late postglacial period, the modern Yangtze River Delta has begun to receive a large amount of sediments from the Yangtze River, and the coastal current has been able to carry fine sediments away (Qin and Zhao, 1987). During the period 14.3–11.3kyrBP (between MWP-1A event and MWP-1B event), the Zhe-Min coastal muddy area began to form gradually (Liu., 2007b). It is generally proved that the mud deposits are mainly made of the Yangtze River suspended sediments transported by the coastal current (Milliman., 1985; Yang, 1988; Fan., 2002; Shi., 2010; Xu., 2009b, 2011; Xu., 2012; Zhang., 2016). However, recent studies have found that the contribution of small and medium rivers to sediments in muddy areas is also of great importance (Cai, 1982; He, 1991; Dadson., 2003; Kao and Milliman, 2008). In the muddy area along the coast of Zhejiang-Fujian Province, sediment contribution of coastal small and medium rivers has been revealed based on some boreholes. Clay mineral analysis of sediments in core MZ02 revealed that more than 60% of sediments were from Taiwan’s rivers during the period between 6.2 and 2.4kyr (Liu., 2014). According to the geochemical results of core MD06-3040, a sudden increase in sedimentation rate was due to the contribution of small local rivers in Zhejiang and Fujian Provinces since 1.5kyr (Yang., 2016). There are a number of small and medium rivers flowing into the sea along the coast of Zhejiang and Fujian provinces, including Qiantangjiang, Yongjiang, Jiaojiang, Oujiang, Feiyunjiang, Aojiang, Minjiang, Mulanxi, Jinjiang and Jiulongjiang. The fluxes of these small and medium rivers are greatly affected by the summer typhoons and floods. How do they affect the sediment composition in the muddy area? What are they different from the Yangtze River? What are the proportions of sediment contribution? How to identify and quantify the sources accurately? These are very interesting issues.

The Oujiang River, with relative large runoff and sediment discharge, is the second largest river in Zhejiang Pro vince. It is affected greatly by the land-sea interactions in flood season due to strong tide. The core D04, with a length of 20.2m analyzed in this paper, is located on the west side of the Zhe-Min depocentre, which is 40km away from the Oujiang Estuary. It is very suitable for studying the fluxes of medium and small rivers, the source and sink processes, sedimentation differences with the Yangtze River. Therefore, this paper intended to analyze the sedimentation environment and mineral assemblages using grain size analysis and heavy mineral analysis. Based on the quantitative analysis, we estimated the sediment contributions of the Oujiang River and Yangtze River to the mud deposit zone, and the source changes since the mid-Holocence. Finally, we discussed the possible influencing factors that lead to the source changes. This study is helpful to further understand the deposition mechanism of the terrigenous detrital materials in the shelf sea.

2 Regional Setting

The Oujiang River is the second largest river in Zhejiang Province, covering an area of 1.8×104km2and a length of 388km. The mean water discharge and sediment flux are 1.4×1010m3(1950–2008) and 2.0×106t (1956– 1998), respectively. The seasonal variations of runoff and sediment flux are significant, which mainly reached maximum in the flood season from April to September (74% for water and 90% for sediment) (Guo., 2012; Song., 2012). The tide in the Oujiang Estuary is a regular semidiurnal tide with an average tidal range over 4m. The tidal can enter the estuary to the upper reaches with a maxi- mum tidal range of 7.21m in Longwan. The velocities of ebb and flood tide in the north side of the estuary are higher than the south side, and the ebb tide is higher than the flood tide. The residual flow velocity is 3–4cms−1, which flows northward in summer and southward in winter. The tidal current and residual flow has significant effect on the sediment transport and the tidal flat deposition. Around the Oujiang Estuary mainly occurred Jurassic volcanic clastic rocks and Cretaceous clastic rocks of fluvial and lacustrine facies. The Quaternary loose sediments are distributed widely. The landform is characterized by the development of low mountains and hills, accumulation plains and estuaries. The main channels, waterways, mouth bars and southern sand dunes of the Oujiang estuary are mainly made of fine sand or other coarse sediments, which may deposited from the Oujiang River. Inside and outside of the Wenzhou Bay, the fine particles widespread such as clayey silt are mainly from the Yangtze River suspended matters (Xie., 1994).

3 Materials and Methods

The core D04 (120.87˚E, 27.54˚N), 20.2m in length, was collected from the area with water depth of 13m using an engineering drill (HGD-600) on the ‘research vessel in December 2015. In addition, 11 river floodplain samples were collected from the natural bank of the Oujiang River Basin in order to carry out comparative studies (Fig.1).

The core D04 sediments were subsampled at 2cm intervals (some layers were sampled at 1cm intervals) and a total of 960 samples were taken for grain-size analysis. 1– 2g sample was taken and leached by hydrogen peroxide and dilute hydrochloric acid in order to remove the organic matters and carbonate minerals, and then it was fully dispersed with sodium hexametaphosphate. Each sample treated was analyzed with a Mastersizer-2000 laser particle size analyzer (Malvern, UK). Replicated samples were measured with a relative deviation of 1%. Grain size parameters were calculated following the formula of Folk and Ward (1957).

A total of 47 subsamples were obtained at 40cm intervals from core D04 and 11 samples from the Oujiang Riverwere collected for heavy mineral analysis. After being pre- treated with hydrogen peroxide, dilute hydrochloric acid and sodium hexametaphosphate, the subsamples were wet- sieved through 62.5μm, 125μm nylon mesh. Then light and heavy minerals are separated by the tribro-methane solutions (specific gravity: 2.89gcm−3). Then, the minerals were handpicked by a polarizing microscope (BX51, Olympus) and a stereo microscope (SZX16, Olym- pus). For each sample, at least 300 grains were counted by using the ribbon method (Galehouse, 1971), and then the grain percentage of each mineral was calculated.

Fig.1 Locations of sediment samples in the study area. a), the mud thickness is modified after Liu et al. (2006).

9 samples for14C dating (molluscan and cone shells) were picked out manually and measured by using accelerator mass spectrometry (AMS) at the lab Beta Analytic (Table 1). The conventional radiocarbon ages were calculated using the Libby half-life (5568 years) and corrected based on the measured13C values, and then calibrated using CALIB7.1 (Stuiver., 2017). The regional biases caused by the regional carbon reservoir effect were corrected using the value ΔR of 113±37yr (Yoneda., 2007).

Table 1 14C ages of core D04 sediments

4 Results

4.1 Characteristics of Grain Size Compositions

According to the vertical changes of the grain-size parameters, core D04 can be divided into three units: DU1 (0–7.4m), DU2 (7.4–11.4m) and DU3 (11.4–20.2m). The deposition rate exhibits high-low-high respectively (Fig.2).

1) DU1 (0–7.4m)

It is mainly composed of brown clayey silt, occasionally with scattered shell fragments and clay interlayers, and sandy clastic shell interlayers with thickness of about 2–3cm at 4.3m, 6.0m and 6.1m. The particle size varies from 5.33Φand 7.12Φ, with the average value of 6.54Φ. This unit is characterized by a sharp single peak grain size distribution curve and the suspended component accounts for 90% (Fig.3), indicating the single and weak hydrodynamic condition. According to AMS14C ages, the sedimentation rate is 0.35cmyr−1. This relative high de- position rate indicates that the sediment supply was relatively abundant. This unit was mainly deposited on the underwater slope since 2.1kyr, the coarse sandy interlayers with clastic shells may be related to the effect of storm surge.

Fig.2 Particle size composition, calendar age and sedimentation rate of Core D04. Calendar ages are based on AMS14C.

2) DU2 (7.4–11.4m)

It is mainly composed of brown and dark clayey silt, occasionally with scattered clastic shells and a small amount of thin silt sandwich. The particle size composition is stable and basically unchanged. The average content of silt is 67.85%, and the average particle size is 6.64 Φ. Single sharp peak curve and abundant fine component with the content of more than 99% (Fig.3), reflect the weak and stable hydrodynamic environment. The deposition rate is low (0.25cmyr−1), indicating insufficient sediment supply. The unit is mainly formed during the period of 2.1–3.7kyrBP.

3) DU3 (11.4–20.2m)

It is mainly composed of gray-green and brown clayey silt, occasionally with scattered clastic shells, some silt sandwiches and coarse particle layers at 11.4–14.0m and 15.5–17.0m. The average particle size is 6.4Φ, fluctuating from bottom to top. This unit has single sharp peak grain size distribution curve and 90% of the fine components (Fig.3). In general, the hydrodynamic condition is relatively stronger than that of DU2 and the occurrence of coarse particles may be related to storm deposition. The layers were mainly formed during 3.7–5.8kyrBP with a high deposition rate (0.46cmyr−1), indicating the sufficient sediment supply compared with DU1 and DU2.

Fig.3 Frequency distribution curves and probability accumulation curves of grain size in three units.

4.2 Distribution and Assemblage of Heavy Minerals (HM)

The average weight percentage of heavy mineral component in the selected samples is 7.7%, ranging from 0.7% and 36.9% (Table 2). It decreased from the bottom up with sharp reduction during the deposition of DU3. Although the core sediments were deposited under the weak hydrodynamic conditions, the core keeps a high deposition rate. The decreasing HM indicates the hydrodynamic conditions were weak enough to be sorted.

Heavy minerals are dominated by authigenic pyrite (65.2%) and hornblende (16.0%), which account for 81.2%of the total heavy minerals. The contents of epidote (4.6%), actinolite (2.6%), biotite (2.3%) and limonite (2.1%) are relatively low. Less abundant but common occurring minerals include tremolite, zoisite, garnet, sphene, apatite, dio- pside, magnesite, ilmenite, magnetite and debris (Table 2).

Table 2 The contents of heavy minerals in Core D04

For the convenience of analysis, the main heavy minerals are divided into 5 categories according to their physical and chemical properties: amphibole group (AmpG, including hornblende, tremolite, and actinolite), epidote group (EpG, including epidote, zoisite and clinozoisite), metallic minerals (MEM, including ilmenite, magnetite, limonite and hematite), stable minerals (STM, including garnet, apatite, sphene, rutile, tourmaline and zircon), micas (Mca, including biotite, muscovite and weathered micas). In general, the content of HM changes strongly in DU1 and DU3, but is relatively stable in DU2. The content of authigenic pyrite decreases from bottom to top (Fig.4), but the contents of AmpG, EpG and MEM increase (Fig.5). The dowmcore variation of the heavy mineral assembles shows the difference in hydrodynamic conditions and provenance.

DU1 (0–7.4m): The sediments in this unit have the lowest content of HM (3.0wt%) in the core and the mineral assemblage is dominated by authigenic pyrite (49.5%)-hornblende (22.5%)-epidote (8.1%)-actinolite (3.6%)-limo- nite (3.5%). The amounts of AmpG (26.3%), EpG (10.1%), MEM (5.6%) and STM (2.3%) in this unit are the highest in the whole core, with comparatively high amount of Mca (4.3%), and the lowest amount of authigenic pyrite (49.5%).

DU2 (7.4–11.4m): The sediments in this unit have comparatively high content of HM (7.4wt%) in the core and the mineral assemblage is characterized by authigenic pyrite (78.7%)-hornblende (9.6%)-epidote (2.8%)-actino- lite (2.1%). This unit has the highest percentage of authigenic pyrite in the whole core, and the lowest amount of AmpG (11.8%), MEM (1.8%), Mca (3.0%) and STM (0.7%), and comparatively low amount of EpG (3.6%).

DU3 (11.4–20.2m): It has the highest content of HM (12.3wt%) and the mineral assemblage is characterized by authigenic pyrite (73.0%)-hornblende (13.3%)-biotite (2.6%)-epidote (2.2%) and actinolite (2.0%). This unit has a high content of terrigenous materials. The sediments have the highest percentage of Mca (4.7%) among three units, and comparatively low amount of AmpG (15.5%), EpG (3.1%), MEM (1.9%) and STM (1.1%), and comparatively high amount of authigenic pyrite.

Fig.4 Downcore variations of major heavy mineral percentages in the sediments from Core D04.

Fig.5 Downcore variations of heavy mineral assemblages in the sediments from Core D04.

5 Discussion

5.1 Indicative Significance of Heavy Mineral Compositions

The heavy minerals in sediments are dominated by au- thigenic pyrite, hornblende and epidote, followed by actinolite, biotite and limonite. The contents of the minerals such as garnet, zircon and ilmenite with larger specific gravity and better abrasion resistance are very low. These characteristics indicate that the hydrodynamic conditions in the study area are weak and insufficient to deposit the minerals with larger specific gravity (Chen., 1986). Most of hornblende, epidote and other unstable minerals are in angular and sub-angular shapes, with fresh mineral surfaces and poor grinding roundness, indicating low mineral maturities and the near provenance (Chen, 2008). The amount of authigenic pyrite in D04 is high, indicating that it was formed in a low energy reduction environment because the authigenic pyrite is generally present in the mud deposits rich in organic matters (Chu., 1995). Especially in DU2 and DU3, the contents of authigenic pyrite are much higher than DU1, indicating that the DU2 and DU3 were formed in the relatively stable and weak dynamic environment. In general, the contents of heavy minerals in D04 are decreasing from bottom to top, but the contents of unstable minerals and metallic minerals are increasing, reflecting the proportion of near-source components gradually increased.

5.2 Quantitative Identification of Provenance

According to Chen (2008), the average content of heavy minerals in the Changjiang River sediments is 12.6%, dominated by the muscovite and other flaky minerals (the average content of 28.0%, hereinafter the same), dolomite (26.0%) and hornblende (24.2%). The contents of epidote, ilmenite, pyroxene and garnet are 8.0%, 3.2%, 2.1% and 1.0% respectively, and the contents of other minerals are less than 1% (Table 3). In the coastal area of Zhejiang Province, the heavy mineral assemblage is characterized by the flaky minerals (29.7%), holoblende (20.6%) and dolomite (18.1%). The average contents of epidote, ilmenite and pyroxene are 7.8%, 4.7% and 1.6%, respectively. The mineral assemblage in Zhejiang coastal area is consistent with the modern Changjiang River because the Changjiang-derived sediments were transported southward along the Zhejiang coast. Zhang. (2016) further pointed out that the seafloor sediments outside the Oujiang estuary in the south of 28.4˚N inherited the fine grain characteristics from the Changjiang River transported by the coastal current, with high values of the flaky minerals (30.5%), hornblende (25.3%) dolomite (13.7%) and epidote (8.0%), and low values of pyroxene (1.5%) and magnetite (1.4%) (Table 3). The content of magnetite is higher than those of the sediments from the Changjiang River (0.6%) and other regions (below 0.2%) in the East China Sea shelf, which may be affected by the coastal rivers like the Oujiang River.

The heavy minerals of the Oujiang River sediments are mainly composed of magnetite (26.0%), micas (13.6%), hornblende (12.8%) and epidote (8.2%). Other mineral content is low (Table 3). The mineral assemblage of the Oujiang Estuarine sediments is consistent with the Oujiang River, but the heavy mineral content has changed significantly. The contents of micas and hornblende increased to 26.1% and 21.1% respectively, and the magnetite content reduced to 19.1%. The epidote content is almost constant (8.4%) (Table 3).

Table 3 Mean contents of heavy minerals in sediments from Core D04 and the surrounding area

Notes: N refers to the sample number; HM refers to heavy mineral content; Bt refers to Biotite; Ms refers to Muscovite; Chl refers to Chlorite; Hblo refers to Holnblende; Ep refers to Epidote; Aug refers Augite; Dol refers to Dolomite; Mag refers to Magnetite; Ilm refers to Ilmentite; Lm refers to Limonite; Hem refers to Hematite; Grt refers to Garnet; Spn refers to Sphene; Zrn refers to Zircon; Tur refers to Tourmaline. All data of Core D04 were recalculated to exclude the interference of authigenic pyrite.

This indicates that the sediments from the Oujiang Estuary not only inherited the features of Oujiang river inputting materials, but also included the information of sediments from the modern Changjiang River to get high percentages of micas and hornblende. The sediments in Core D04 also have the mineral assemblages with comparatively high amount of hornblende (44.2%), micas (17.1%) and epidote (11.8%), which is similar to the Changjiang estuarine sediments (Fig.6).

Therefore, hornblende, epidote, micas, magnetite and other metallic minerals can be used to distinguish the two end-members better. From the ternary diagram of micas- metallic minerals-hornblende (Liu., 2017, 2018a), we can also clearly see the difference in mineral compositions between the Oujiang River sediments and Changjiang River ones. In the diagram, the sediment samples from Core D04 changed gradually from the bottom up and developed a trend being close to the Oujiang River endmember (Fig.6), that is, the amount of magnetite is gradually increased accordingly. These features reflect that significant changes have taken place in the sediments source. Intermediate-acid intrusive rocks and volcanic clastic rocks are distributed in the basins of Oujiang River and the Minjiang River, rich in magnetite,. These bedrocks were weathered and eroded, then enter the rivers and were transported downstream. They generally deposited in the estuaries or nearshore areas due to its high specific gravity. Therefore, the sources of Core D04 sediments are the mixture of the materials from Changjiang River and the Oujiang River (Fig.6).

In order to obtain the contribution ratio of the two rivers, we construct the mathematical model of the two end- members. Before establishing the model, we must unify the calculation method of the mineral content in the two end-numbers. Previous studies have shown that the contents and types of heavy minerals varied with the used heavy liquids with different densities. Dolomite (2.84– 2.86gm−3) was considered to be a characteristic mineral of the Yangtze River sediments (Chen, 2008). It could be effectively sorted when the density of heavy liquid is 2.80 gm−3. However, it cannot be separated if the heavy liquid density is greater than 2.86gm−3(Liu., 2008; Liu., 2018b). Given that the different densities of heavy fluids were used by different researchers in the past, we must unify the data for the comparative study. In this study, the heavy liquid density used to separate heavy minerals was 2.89gm−3, so dolomite is not considered here. We excluded the interference of authgenic pyrite and dolomite. As shown in Fig.7, the average contribution rates of the Changjiang River component and the Oujiang River one in the sediments from core D04 are 79.2% and 10%, respectively, suggesting they are mainly from the Changjiang River. The analysis results for the adjacent core EC2005 also revealed that the sediments were primarily from the Changjiang River since 17.3kyr, and also some are from the coastal area off Zhejiang Province. Moreover, the decrease content of authigenic pyrite was closely associated with the weakening of upwelling current since 4.7kyr (Dong., 2015). There may be other sources, accounting for 10.7%, which is likely to be the materials carried by the Taiwan warm current. The proportion of the Changjiang River sediments gradually decreased upward the core. On the contrary, the proportion of the Oujiang-source sediments increased significantly. The sediments in unit DU3 are mainly from the Changjiang River, accounting for up to 90.6% on average, whereas the Oujiang River components accounted for only 3.1%. Sediment supply in this period is relatively stable. The average proportions of the components transported by two rivers in unit DU2 are 77.4% and 9.6% respectively. The sediments from the Oujiang River began to increase. In unit DU1, the average proportions of the two components are 61.1% and 23.6%, respectively.

Fig.6 Ternary diagram of Micas-Metallic minerals-Horn- blende.

Fig.7 Proportions of the components transported by the Oujiang River and the Changjiang River in the sediments from Core D04.

5.3 Sedimentary Evolution History Since 5.8kyr

The deposits in the muddy area contain abundant geological information and are an important object for studying land-sea interaction, sea-level change, sediment transport and climate events. Similarly, climate change, sediment transport and sea level fluctuations also affect the sediment sources. The study area is mainly affected by the East China Sea coastal current and Taiwan warm current. The formation of the coastal current of the East China Sea is closely related to the periodical rapid rise of the sea level, and may be formed after about 7.6 kyr BP as the sea level rose to the highest. Wang. (1981) analyzed the core sediments from the Yangtze River Delta, and found that the Holocene sediment grain size showed a coarse-fine-coarse change from bottom to top. Accordingly, the variation trends upward of the climate were cold-warm-cool and the sedimentation rates were high- low-high, respectively, consistent with the results for Core D04. This is also consistent with the results revealed by the adjacent boreholes, in which high (5–8kyrBP), low (5–2kyrBP) and high (2–0kyrBP) accumulation periods were distinguished, corresponding to different stages of sedimentary evolution (Xu., 2012).

1) 5.8–3.7kyrBP

The progradation of the Changjiang Delta has occurred since about 6–7kyrBP (Wang., 1981), when the rising sea level reached its present position (Zhao., 1979), and the average progradation rate of the delta front was approximately 50myr−1over the last 5000 years (Hori., 2001). The dynamic environment of the East China Sea was basically established, and the Changjiang-de- rived sediments were transported southward by the Zhejiang-Fujian Coastal Current. With the downwelling caused by the southward coastal current near the coast and upwelling caused by the northward Taiwan Warm Current offshore, fine sediments were deposited under the upwelling along the coast of Zhejiang-Fujian (Liu., 2006, 2007a). The period of 5–7kyrBP is generally considered to be ‘Holocene Optimum’ with very high sedimentation rate, caused by the strong effect of the East Asian summer monsoon (An., 2000). The Changjiang River with high discharge subsequently carried more sediments into the sea. The strengthened East Asian winter monsoon enhanced the transport capacity of the coastal current during the period 5.0–5.8kyrBP, and more coarser sediments were brought to the study area, leading to the high sedimentation rate (Xu., 2009a). About 3.6–5.0kyrBP, the climate became cold and dry, and ancient flood events occurred frequently. Especially about 4kyrBP, the cooling events made Liangzhu culture disappeared (Wu and Liu, 2004; Zhang., 2004). In the Oujiang River basin, the largest transgression boundary reached Qingtian, where the river mouth retreated 70km far from the present estuary, and the sea extended to the southern Zhejiang Plain. At that time, the Oujiang Estuary was a submerged valley and the southern Zhejiang plain was in a shallow sea environment with the water depth of 20m (Wang., 1982; Zhang, 1991). The Oujiang Delta is also subject to active typhoons and coastal currents by the relatively enhanced East Asian winter monsoon (EAWM) between 4.9 and 6.4kyrBP, producing high sedimentation rates (Shang., 2017). Compared to the Changjiang River, the sediments from the Oujiang River were few due to the relatively long distance.

2) 3.7–2.1kyrBP

During the period of 3.7–2.1kyrBP, the sea level fell to approach the present level and the river bed was readjusted. The coastline moved slowly to the sea. Soil erosion was mainly affected by climatic conditions. Compared with the previous stage, the temperature had picked up (Zhang., 2012), but the human reclamation capacity of the Changjiang River Basin was low and the surface was almost the original state with high watershed vegetation coverage and good soil and water conservation (Zhao., 2002). The sediments from the Changjiang River mainly deposited in the middle and lower reaches, filling the channel and estuary basins. Therefore, the main body of the Changjiang River Delta entered the lowest sedimentation period (4.5–2kyrBP) (Wang., 1981). As a result, the sediments transported to the sea by the Changjiang River decreased greatly. Moreover, the Wenzhou Bay began to be filled and the shoreline extended gradually, leading to the Oujiang-derived sediments increased slowly.

3) 2.1kyrBP–present

The sea level was basically the same as present, but the climate fluctuated greatly. A rapid progradation rate of 80kmkyr−1due to widespread human activities resulted in the increase of sediments in the Changjiang River drainage basin (Hori., 2001). In the Oujiang River basin, sediment supply was also increased greatly due to human reclamation activities (Zhang., 2012). A large amount of sediments were brought into the study area, resulting in high deposition rates. Due to the short distance from the Oujiang River mouth, the upper unit (DU1) received more Oujiang-derived sediments than the lower units (DU2 and DU3).

6 Conclusions

The upper 7.4m sediments from core D04 (2kyrBP– present) may deposit under a weak hydrodynamic condition, with a relatively high deposition rate and some interlayers associated with storm disturbances. For sediments at the depth of 7.4–11.4m (2.1–3.7kyrBP), the hydrodynamic condition was weakest with a very low deposition rate. The deposition rate of sediments at the depth of 11.4– 20.2m (3.7–5.8kyrBP) is high, with a relatively fluctuate hydrodynamic condition.

For the sediments in the core D04, mineral assemblage is mainly composed of authigenic pyrite (65.6%)-hornblende (16.0%)-epidote (4.6%). The sediments are mainly transported to this area by the Changjiang River and Oujiang River.

Quantitative identification shows that the contribution of the Changjiang River materials accounted for 79.2% and the Oujiang River components accounted for 10.0%. There-fore, the sediments of Core D04 are mainly from the Chang- jiang River, but the Oujiang-derived components increase from bottom to top. Climate changes and human activities have affected the development of the river deltas, and the change in sediment flux has led to the differences in deposition rates at different periods in the mud deposits.

This article discussed the sediment source based on the heavy mineral compositions. An endmember model was constructed to identify the sediment sources of Core D04 sediments. The contributions of the Changjiang River and Oujiang River to core D04 sediments were effectively and quantitatively revealed. It will help deepen the understanding of the influence of large rivers and small rivers on the deposition of muddy areas.

Acknowledgements

The present study was jointly funded by China-ASEAN maritime cooperation fund: Comparative Study of Holocene Sedimentary Evolution of the Yangtze River Delta and the Red River Delta, and the National Natural Science Foundation of China (Nos. 41706074 and 41506107).

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