Provenance and climate changes inferred from magnetic properties of the sediments from the lower Yangtze River (China) during the last 130 years

Journal of Asian Earth Sciences 175 (2019) 128–137

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Full length article

Provenance and climate changes inferred from magnetic properties of the sediments from the lower Yangtze River (China) during the last 130 years

T

Yan Zhenga,b, , Shouye Yangc, Chenglong Dengd ⁎

a

Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China b CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, China c State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China d State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

ARTICLE INFO

ABSTRACT

Keywords: Provenance Climate change Yangtze River Flood/drought events

It is essential to have a clear scientific view on paleoclimate change and its potential environmental and socioeconomic impacts for the prediction of future climate change. Fluvial sediment is one of the best archives for the investigations of high-resolution environmental variability and anthropogenic impacts on catchment over the last hundreds or even thousands years (Anthropocene). In this study, we examine magnetic properties of a 130year fluvial sediment profile from an island in the main channel of the lower Yangtze mainstream, which aims to determine the connections between modern observational data and paleoclimate signals from sediments. Sediments from the upper Yangtze River is characterized by high concentration of magnetite, which is different from the middle reach. The difference of magnetic properties allows us to trace particles in the lower reach back to upstream of the Yangtze River. Climate change that reconstructed from physical properties of sediment (susceptibility and mean grain size) is comparable to those records in historical documents, which provides a possible way to obtain precise ages. The calibrated sedimentation rate reflects long-term variation of flood/ drought events over the catchment, which is dominantly controlled by the climate circulation of Pacific. The enhanced human impacts since 1950s especially in the mid-lower reaches are observed. Therefore, riverine sediment not only contains paleoclimate information, but also reflects human activities.

1. Introduction Paleoclimate change and its potential environmental and socioeconomic impacts are essential for the prediction of future climate variation. Among many ways to obtain paleoclimate information, the riverine sediment is the most widely and available materials to obtain. The surface runoff carries a considerable amount of materials from the catchment along the pathway downstream to the lower reaches and even to marginal seas, and thus, the river sediment could provide constraints on the source to sink transport process of terrestrial material and environmental change in the catchment (e.g. Hounslow and Morton, 2004; Tripathy et al., 2014; Guan et al, 2016; Liu et al., 2018; Zhao et al., 2018). The Yangtze River (Changjiang) is the third largest river in the world and the longest one in China. It delivers large amounts of sediment to East Asian marginal seas. Geochemical and sedimentological studies have been well applied to identify sources of terrigenous

detritus along the Yangtze River (e.g. Kuhlmann et al., 2004; He et al., 2013). Environmental magnetic parameters could distinguish sediments from Yangtze River and Yellow River (e.g. Zhang et al., 2012; Wang et al., 2009) and rivers from Taiwan (Horng and Huh, 2011). Therefore, magnetic method is suitable to trace sediment origins (e.g. Horng and Huh, 2011; Kim et al., 2013; Li et al., 2012). The significant magnetic differences between upper and middle reaches of the Yangtze River (Luo et al., 2016) allow magnetic method to trace the provenance of Yangtze River sediment, which is also applied to reconstruct paleoclimate changes (e.g. Zheng et al., 2010; Hu et al., 2014; Chen et al., 2015; Duan et al., 2016; Guan et al., 2016). However, the connection between magnetic parameters of fluvial sediment and climate indicators on annual changes is still unclear. In this study, we examined the magnetic properties of a 130-year fluvial sediment profile from the lower Yangtze River, in order to trace the provenance of sediments and determine the connections between modern observation data and paleoclimate signals from sediments.

⁎ Corresponding author at: Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China. E-mail address: [email protected] (Y. Zheng).

https://doi.org/10.1016/j.jseaes.2019.01.036 Received 27 June 2018; Received in revised form 8 January 2019; Accepted 27 January 2019 Available online 04 February 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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90°

80°

N 40°

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110°

120°

sediment flux decreased to 138 Mt/yr for the period from 2004 to 2014 (Changjiang Sediment Bulletin, 2004–2014). The Yangtze River basin is characterized by a large area of Paleozoic carbonate rock or other weak magnetic rocks (Yang et al., 2009), while massive vanadium-titanium magnetite (V-Ti-Fe3O4) ore deposits are located in the upper Yangtze valley near PanZhiHua (PZH) City (Fig. 1a) that result in high magnetic concentration of upper Yangtze River, which allows for the determination of sediment provenance along the Yangtze River.

130°E

Winter Monsoon

China

Yellow River

30°

2.2. Sample collection

Lower Reaches

Yangtze TGD River YC

Upper PZH Reaches

DT

Middle Reaches

The LeiGongZui (LGZ) shoal is a small sandy bar emerging in the lower Yangtze mainstream at Yangzhong City (32°18.4'N, 119°45.2'E, about 200 km upstream from the modern river mouth; Fig. 1), which was exposed to the river surface since the regime of Empire Tong Zhi (1862–1874 CE) of the Qing Dynasty (Zhan et al., 2010). This fluvial shoal is about 3 km in length and nearly 1 km in width in the midst of the mainstream. There is no farming or other human activities on this small bar because it is still inundated periodically during flood season (May to October). The samples were collected at 2 cm intervals (101 samples in total) on a fresh profile (LGZ, 2 m depth) on the northeastern bank of the shoal. The sampling profile locates in a small harbor of this shoal, where the depositional environment is relatively stable (Fig. 1c). The 2 m-long profile sediment recorded fluvial deposition over the last 150 years as determined by 210Pb dating (average sedimentation rate is ∼1.34 cm/yr) (Zhan et al., 2010). A total of 22 samples from sea floor surface sediment (the top 5 cm sediment below sea floor, water depth is ∼20–30 m) were also collected around the Yangtze River mouth in the summer of 2011 (Fig. 1b). Sedimentation rate of the top 4 m of core YD0901 (drilled in 2009) from the subaqueous delta is nearly 1 cm/yr (Zheng et al., 2012), so the upmost 5 cm surface samples are possibly deposited during the last 5 years (from ∼2006 to 2011).

ECS

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Ri

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Fig. 1. Sketch map of China and Yangtze drainage (shaded) area as well as boundaries of upper, middle and lower reaches by dashed lines (a); location of LGZ Island in the lower Yangtze River, as well as 22 surface sampling sites from Yangtze River mouth (b); sampling site on LeiGongZui (LGZ) Island (c). Abbreviations in figure a: PZH: PanZhiHua (an important magnetite deposit at the upper Yangtze River); TGD: Three Gorges Dam (separates upper and middle reaches); YC: YiChang hydrological station between upper and middle reaches; DT: DaTong (an important hydrological station of lower Yangtze River); ECS: East China Sea.

3. Analytical methods 3.1. Basic magnetic properties Magnetic measurements were performed in the magnetically shielded room of the Paleomagnetism and Geochronology Lab (PGL), Institute of Geology and Geophysics, Chinese Academy of Sciences (IGG-CAS), Beijing. Approximately 2 g (suitable for the measurement range of magnetometers) dried sediment samples of LGZ profile and river mouth are packed in standard 8 cm3 plastic sample boxes for measurements of magnetic susceptibility and remanences. Low-frequency (lf, 976 Hz) and high-frequency (hf, 15,616 Hz) magnetic susceptibilities (χ) were measured in a 200 A/m alternating field using a MFK-FA Kappabridge magnetic susceptibility meter (sensitivity 2 × 10−8 SI). Then, superparamagnetic (SP) particles were estimated by frequency-dependent susceptibilities (χfd (%) = (χlf-χhf)/χlf × 100%). Measurements of magnetic remanence were carried out on a 2-G Enterprises pass-through high-resolution cryogenic magnetometers with a noise level of 10−12 Am2. Anhysteretic remanent magnetization (ARM) is acquired in a peak alternating field (AF) of 100 mT with a superimposed direct current (DC) bias field of 0.05 mT. Isothermal saturation remanent magnetization (SIRM) was acquired in a field of 1 T. Backfield isothermal remanence was acquired in a reverse field of 0.3 T. The S-ratio can be calculated by -IRM-0.3T/SIRM.

2. Geological setting and sampling 2.1. Features of Yangtze drainage area The Yangtze River drains a total area of 1.81 × 106 km2, with a change in elevation of 6200–6300 m (Fig. 1). It is the largest river in China and delivers considerable amounts of water (1011 m3/year) and sediment into the East China Sea, which significantly influences the sedimentation and biogeochemical cycle in this marginal sea. Annual average precipitation of the Yangtze River drainage area is about 1100 mm, which is mainly controlled by the East Asian summer monsoon (EASM). Precipitation mainly lasts from May to October and causes flood/drought cycles in eastern China. The El Nino-Southern Oscillation (ENSO) is the dominant coupled ocean–atmosphere mode of the tropical Pacific, which also influences climate change of East Asia. The relationships between ENSO, East Asian summer monsoon (EASM), precipitation and flood/drought events in eastern China are complex and have been discussed previously (e.g. Yin et al., 2009; Wei et al., 2014). The Yangtze River basin can be divided into three major sections according to tectonic relief and geographic features: upper, middle and lower reaches (Fig. 1). With the construction and impoundment of Three Gorges Dam (TGD) in the middle Yangtze mainstream in 2003, most of the upstream sediments (∼70–90%) have been trapped behind the dam (e.g. Yang et al., 2011, 2014; Li, Q. et al., 2011; Dai and Lu, 2014), and then, only fine grains (mostly < 5 μm) can reach the lower basin (e.g. Yang et al., 2014). The average sediment load between 1950 and 2003 was approximately 413 Mt/yr (Wang et al., 2011), while the

3.2. Detailed rock magnetic measurements Representative sub-samples were taken for further analysis to identify magnetic minerals. The temperature-dependence of magnetic susceptibility (χ-T) curves were measured in an argon atmosphere, using a KLY4S Kappabridge magnetic susceptibility meter with a CS-3 high129

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temperature device. Magnetic hysteresis loops and first-order reversal curves (FORC) were performed by Princeton Measurements Corporation vibrating sample magnetometer (VSM 3900). 120 FORCs (Roberts et al., 2000), with a field spacing of 4.14 mT, averaging time of 0.3 s, and a 1 T saturating field, were measured. Zero-field cooled (ZFC) remanence (imparted in a field of 2.5 T at 10 K, following warming from 10 to 300 K) of dried 50–100 mg sub-samples was performed on a Quantum Design MPMS 5 magnetic properties measurement system to observe low-temperature magnetic phase transitions characteristic of magnetic minerals.

χlf, useful indicators for magnetic grain size (Thompson and Oldfield, 1986; Evans and Heller, 2003), show fluctuations below 0.8 m and increase between 0.8 and 0.7, and are followed by relatively constant values of the top 0.7 m (Fig. 2e and f). Frequency-dependent susceptibility (χfd), a proxy for superparamagnetic (SP) materials, shows similar variations as ARM/SIRM and χARM/χlf do (Fig. 2g). These parameters suggest that fine-grained magnetic minerals fluctuate below 0.7 m, and that high and relatively constant concentration of fine grain size magnetite are present in the upmost 0.7 m of the profile. Then six representative subsamples (two samples above 0.7 m and four samples below 0.7 m) are chosen to determine the magnetic properties of this profile (Fig. 2 arrows on d).

3.3. Scanning electron microscopy (SEM) analysis Particle morphologic examination was performed by scanning electron microscopy (SEM) on magnetic extracted samples. About 0.5 g of sample was dispersed by ultrasonic agitation in distilled water. No additional chemical treatment was performed on the samples prior to magnetic extraction to avoid any alteration effects. The extraction was made by passing the dispersed sample into the poles of a magnet for about 20 mins. Magnetic extracted particles were fixed onto a carbon sticker, previously stuck on a standard stub, and then were coated with a carbon layer of a few nanometers to prevent surface charging of the sample during SEM operation. All SEM analyses were performed using LEO-1450 VP at 15 kV acceleration voltages at IGG-CAS, Beijing. Secondary electron mode was used for imaging and energy dispersive X-ray spectroscopy (EDS) for elemental composition.

4.2. Rock magnetic properties Detailed rock magnetic measurements on six subsamples include χT curves (20–700 °C), thermal demagnetization curves of zero-field cooling (ZFC) at low temperature (10–300 K) and hysteresis loops at room temperature (Fig. 3). Thermal magnetic analyses could identify magnetic minerals by Curie temperatures and transformation temperatures of magnetic minerals during the heating process. The cooling curves have similar variation with only one curie temperature of magnetite (580 °C) and the susceptibilities of cooling curves are 5–10 times higher than the heating curves (Fig. 3a), which suggests some transformation of magnetic minerals during heating process. Detail variations of susceptibility during heating (Fig. 3b) all present slightly increase from room temperature to ∼400 °C and present a peak at ∼510 °C, which might be due to the generation of magnetite resulting from the conversion from Fe-bearing minerals, such as ferric or ferrous silicate minerals (such as chlorite). Magnetic minerals have recognizable features at low temperature (10–300 K). Thermal demagnetization of IRM during zero-field cooling (ZFC) and their first derivatives show the Verwey transition of magnetite at 120 K (Fig. 3c and d). Hysteresis ratios of the six subsamples are located in a small area (2.5–3.5 for HCR/HC, while 0.125–0.225 for MRS/MS) (Fig. 3e), so one FORC distribution was selected to be shown here (Fig. 3f). FORC distributions were calculated using the FORCinel package (Harrison and Feinberg, 2008) with optimum smoothing factors (SF) of 5. FORC

4. Results 4.1. Down-core variations of magnetic properties The down-core variations of concentration dependent magnetic parameters (χlf, SIRM and ARM), magnetic mineralogy (S-ratio) and magnetic grain size (ARM/SIRM, χARM/χlf and χfd %) are given in Fig. 2. The three concentration depended magnetic parameters (χlf, SIRM and ARM) (Fig. 2a, b and c) present several fluctuations in concentration of magnetic minerals from bottom to 0.8 m, then have a slight increase from 0.32 m to 0.8 m, and keep relative constant values through the upmost 0.32 m. S-ratios show variations similar to those of SIRM (Fig. 2b and d). ARM/SIRM and χARM/

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Fig. 2. Down-core variations of rock magnetic parameters: concentration related parameters–susceptibility (χlf) (a), SIRM (b), and χARM (c); magnetic mineralogical indicator-S-ratio (d); and magnetic granularity-ARM/SIRM (e), χARM/χlf (f) and χfd (g). Arrows on the S-ratio profile denote horizons where detailed magnetic mineral analyses were carried out in Fig. 3. 130

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Fig. 3. Detailed rock magnetic analysis on six samples at different depths along the profile: typical temperature dependence of magnetic susceptibility (χ-T) curves (heated in argon) for the sample from 0.1 m (thick black and thin grey lines represent heating and cooling curves) (a); heating curves from different depths (b); Zerofield cooled (ZFC) remanence (imparted in a field of 2.5 T at 10 K) on warming for six subsamples from 10 to 300 K (c) and the first derivative of IRM with temperature (δIRM/δT) (d); Day plot (e) (domain boundaries are from Dunlop, 2002) and FORC curves of samples at 0.2 m (smooth factor is 5) (f).

(Fig. 2d), such as hematite and goethite. Based on magnetic grain size related parameters (Fig. 3e and f, Fig. 3e-g), magnetite is composed of a mixture of MD and SD grains, as well as some SP grains. SEM images show that all the magnetic materials in this profile are all irregular particles (Fig. 4), and lack of anthropogenic ferromagnetic spherical fly ashes (diameters are about 10–130 μm (e.g. Zhang et al., 2011)). Such particles are, for example, observed at Nanjing City (Li, F et al., 2011; Yan et al., 2011) and near the river mouth (Dong et al., 2014). In this study, cyclic magnetic fluctuation (Fig. 2) of this profile reflects natural environmental (depositional) changes. Nanoparticles of superparamagnetic (SP) minerals, which are indicated by χfd (Fig. 2g), keep constant and relatively high value of the top 0.7 m. This is caused by post-depositional process, pedogenesis (e.g. Jordanova et al., 2010; Lu et al., 2008, 2012). Variations of other magnetic parameters of LGZ profile reflect the climate variation during the last 130 years.

curves indicate the presence of both multi-domain (MD) (spread along the Y-axis) and single domain (SD) (elongate along the X-axis) magnetite grains in the LGZ profile (Fig. 3f). 4.3. SEM images and elemental compositions Based on elemental compositions examined by SEM-EDS, the magnetic particles present in the LGZ profile are composed by two kinds of minerals: iron oxides and iron-bearing silicate minerals. SEM images show that most of the grains are irregular shaped particles (Fig. 4). Particles with clear edges and rather clean surfaces are iron oxide (Fe:O are close to 3:1 or 4:1). Particles have small breaks on the surface composed by silicon, oxygen and cations (Mg, Ca, Fe, Na, etc.) (Fig. 4) are iron-bearing silicate, which are likely to be hornblende, biotite, and chlorite. One spherical particles is dark under backscatter images and is composed by silicon and oxygen (Si:O ≈ 1:3). This non-magnetic silica particle is in magnetic extraction samples, which must be attached to some magnetic grains (Fig. 4c).

5.2. Magnetic features along the Yangtze River

5. Discussions

The LGZ shoal is one of newly-emerged bars in the lower Yangtze River. The high sedimentation rate in the studied area makes it a suitable place for high-resolution paleoclimate study. Provenance of sediment of this shoal is the key point while discussing the climatic variations in the catchment, so it is crucial to discriminate magnetic signatures from different drainage areas. Iron oxides are widely observed in the upper Yangtze River sediment (Yang et al., 2009), and

5.1. Magnetic component of the LGZ profile Magnetic results suggest that the dominant magnetic mineral in LGZ profile is magnetite (Fig. 4). S-ratio varies between 0.8 and 0.94, indicating the additional occurrence of high coercivity magnetic minerals 131

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Fig. 4. Scanning electron microscopy (SEM) images in secondary electron (SE) mode with representative elemental spectra for sediment at the depth of 1 cm (a) and 140 cm (b and c). Marks (Fe and Si) on the particles denote that particles are mainly composed by iron/iron oxide with some Ti or iron bearing silicate minerals (such as biotite, chlorite).

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Fig. 5. Comparisons of box-whisker plots of magnetic parameters (susceptibility, S-ratio, ARM/SIRM) between Yangtze River (upper reach (U), mid-lower reaches of main stream (M&L) and tributaries from middle reach (T)) (Luo et al., 2016), LGZ profile and Yangtze River mouth (R. M) (this study). Stars denote the average values of suspended particles from Nanjing and Shanghai.

magnetite from the ore is about 55 Am2/kg, while pure magnetite is 92 Am2/kg) (Xiao, 2001). The enormous hydrological structure Three Gorges Dam (TGD) (Fig. 1)

mainly sourced from the major vanadium-titanium magnetite (V-TiFe3O4) deposits in the upper catchment (Fig. 1a). Magnetite from this ore deposit is characterized by high saturation magnetization (Ms of the 132

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Fig. 6. Two comparisons between paleoenvironmental proxies and climate data from documents and monitor of hydrological station: water and sediment discharge (solid black line and dot gray line)) at YiChang (YC) which represents features of upper Yangtze River (Shen, 2003; Liu et al., 2008) (a) versus magnetic susceptibility (b); rain fall during the flood seasons of mid-lower Yangtze River (solid black line) (Ge et al., 2008) and water discharge of DaTong (DT) (dot gray line) (Shen, 2003) (c) versus mean grain size distribution of LGZ (Zhan et al., 2010) (d). Solid and dashed lines denote comparison standards based on grain size and magnetic proxies respectively.

retains most of the upstream sediment (∼70–90%) behind the dam (e.g. Li, Q. et al., 2011; Dai and Lu, 2014; Yang et al., 2014), and only fine grains can escape the dam trapping and reach the sea (e.g. Yang et al., 2014). Luo et al. (2016) pointed out that magnetic properties of particles from the middle reach (sampled in 2011) is different from upper reach. Concentration of magnetic minerals decreases downstream with an abrupt drop between the upper and middle reaches, which is due to the water storage of TGD since 2003, and therefore, sediment was trapped in the reservoir. In this way, magnetic properties of modern suspended particles could be used as discriminator to separate sediment sources, and high concentration of magnetite could be a finger print of minerals for the upper Yangtze River. In order to characterize changes in magnetic properties along the Yangtze River, box plots of susceptibility (χlf), S-ratio, and ARM/SIRM are selected to indicate concentration, mineralogy, and grain size of magnetic minerals respectively (Fig. 5). Three magnetic parameters are different between the upstream and tributaries from middle reaches (Fig. 5), and

magnetic component from mid-lower reaches of the mainstream is a mixture. The upstream is characterized by more pure magnetite with higher concentration and relatively coarse grains of magnetic minerals, but the tributaries from the middle reaches are composed by magnetite in low concentration and fine magnetic grains (Fig. 5). The surface samples from the river mouth are mainly deposited during the last five years (after TGD was built), and their magnetic properties suggest a sediment mixture of the tributaries and mainstream of middle reaches (detailed results are shown in Appendix Fig. 1), and also indicate that the particles from the upstream are hardly transported to the modern Yangtze River mouth. 5.3. Sediment provenances of the LGZ profile Magnetic features of different reaches along the river and river mouth are good references for tracing sediment sources of the LGZ profile in the lower Yangtze reaches. Statistical results suggest that the 133

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Fig. 7. Calculated new sedimentation rate (solid line) (a) compares to digitized flood/drought years in historical documents along the Yangtze River (data are from Jiang et al., 2006) (b), as well as flood/drought events of Tai Lake area (data are from Chen, 1987; Yin et al., 2009) (c). Dots in b and c are annual records, and thin lines are 3 year smoothed results, thick solid lines represent long-term smoothing trend curve. Susceptibility (d) and ARM/SIRM (e) in the new age model, with comparison to ENSO circulation of the last 150 years (f).

magnetic minerals of the LGZ profile vary between those of the upstream and downstream materials (Fig. 5), but are more similar to that of upstream. Drainage area of the upper valley is nearly 60% of the whole basin, and the length of upper river is over 70% of the total. The dominance of sediment from the upper catchment probably accounts for the similarity of magnetic properties between the LGZ profile and upper Yangtze sediment (Fig. 5). Magnetic properties of suspended materials from Nanjing (NJ) and Shanghai (SH) (stars in Fig. 5) and river mouth are similar to those from mid-lower reaches, which indicates that the modern sediment in the lower mainstream and estuary mainly originates from the mid-lower basins since the particles from the upper Yangtze catchment are mostly trapped in the TGD. The difference in magnetic properties between the LGZ profile, the river and estuarine sediment further suggests that sediment from this profile are mostly derived from the upper catchment and deposited before the TGD impoundment in 2003. Before the TGD construction, small magnetic grain could be delivered to the lower reach and even to the sea. The distance from the large V-Ti ore deposit in the upper reaches to LGZ shoal is about 3400 km. If concentration of magnetic minerals linearly decreases downstream without the influence of hydrological dam, magnetite from the upper reaches would result in a susceptibility value of ∼160 × 10−8 m3/kg at the site of LGZ bar, which is still higher than those of sediment from the mid-lower reaches. Maximum magnetic susceptibility of the middle tributaries is approximate 90 × 10−8 m3/kg (Fig. 5, Luo et al., 2016), so the magnetic susceptibility values > 90 × 10−8 m3/kg in LGZ profile indicate more contribution from the upstream particles, while extremely low susceptibility layers are mostly related to sediment from the middle basin.

5.4. Flood/drought events recorded in lower mainstream sediment Natural processes of the Yangtze River (such as water and sediment supply, transport, sediment mobilization and deposition) are complex on annual changes (e.g. Yang et al., 2011; Dai and Lu, 2014). Observation data from hydrological gauge stations along the Yangtze River show a strong positive relationship between water flow and rainfall (Lin and Wu, 1999; Chen et al., 2014), and the annual water discharges have an overall positive correlation with sediment loads (e.g. Liu et al., 2008; Li, Q. et al., 2011) (Fig. 6c). Based on the discussion above, the sediment with high magnetic susceptibility values in LGZ profile might be predominantly derived from the upper Yangtze basin. Here, the comparisons between sediment magnetic susceptibility and water and sediment discharges at Yichang (YC) hydrological stations (Fig. 1a) are shown in order to find connections between climate signals of upper Yangtze River and physical properties of sediment in the lower reach (Fig. 6). Zhan et al. (2010) proposed that sandy layers of LGZ profile indicate flood events in the history (Fig. 6d). The coarse fraction > 100 μm indicates a strong hydrodynamic feature and a mixed erosion and reflux depositional environment, which might probably be diagnostic of flood deposition (Zhan et al., 2010). While the coarse grains are likely to deposit fast in the river, and fine grains are likely to transport a long distance. Therefore, the coarse grains of LGZ profile indicate a local high hydrodynamic environment in relation to flood events, which is compared to rain fall of mid-lower Yangtze River and river discharge of Datong (DT) hydrological station from the lower reach (Fig. 6c). The age model for LGZ profile is calculated by an average 134

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America. ENSO is considered to be one of the most important predictors for summer rainfall of Eastern China, and the extreme phases are usually in linkage with major episodes of floods and droughts disasters of the Yangtze River that are retrieved from documents (e.g. Jiang et al., 2006; Yin et al., 2009). In this study, we prefer to find the dominant control to climate changes of the Yangtze River. In the new age model, χlf and ARM/SIRM are selected to present magnetic changing patterns during the last hundred years, which could roughly represent the changes of sediment from upper and middle reaches. From 1870 to 1890, the susceptibility has same changing pattern as ENSO circulation presents, which indicates the ENSO might related to the weathering and erosion of upstream of the Yangtze River. ARM/SIRM shows high values indicating fine grains from middle reach. Given by the flood/drought year distribution, there are more water supply during these years. Between 1924 and 1934, strong ENSO corresponds to high χlf and ARM/SIRM, but sedimentation rate is low during this period. In this way, we could not simply conclude the relationship between ENSO and water/ sediment discharge of Yangtze drainage area. Power spectrum analyses of susceptibility and mean grain size were carried out by Redfit software (Schulz & Mudelsee, 2002). Susceptibility has one main periodicity of 10 years, while mean grain size has 11 and 7 years period (Fig. 8), which implies the climate variation period of upper and middle Yangtze River, respectively. Jiang et al. (2006) pointed out that ENSO circulation periods are 3–4 years, 5.67 years, while 10–12 years is the period of Pacific Decadal Oscillation. The resolution of LGZ profile is about 1–2 years, which is hardly to obtain a 3–4 years ENSO circulation period. Here, we could conclude that Pacific climate circulation has strong impacts on the sediment feature of East Asia. The increase of concentration of magnetic minerals since 1950s (Fig. 7d) is similar to the social economic development of Yangtze Catchment of the last 50 years (Wang et al., 2013; Guo and Yang, 2016). Iron mining of PanZhiHua (PZH) has been massive exploited since 1960s’, and the heavy metal concentration along this profile increase at the same depth (Guo and Yang, 2016). We therefore suppose that anthropogenic activities along the river might enhance the increasing trend of susceptibility. Grain size of magnetic minerals increases sharply between 1950 and 1956, and keeps relatively high value till the end of 1980s’. This might be due to the faster economic development in mid-lower Yangtze River valley than in the upper basin. Based on the magnetic results, we suppose that human impact on sediment erosion and quality are stronger since 1950s than before, which is more distinct in the mid-lower reaches.

Fig. 8. Power spectrum of susceptiblity and mean grain size result were analyzed by Redfit software (Settings: OFAC = 4.0; HIFAC = 1.0; Welch window; bandwidth: 0.004) (Schulz & Mudelsee, 2002). Numbers above peaks denote respective periods, over the 90% confidence interval in dashed lines (a rough estimate for a white noise component in the time series).

sedimentation rate of 1.34 cm/year based on 210Pb data (Zhan et al., 2010). Water flow and sediment discharge change every year, which lead to variable sedimentation rates. The two comparisons could help to modify the age of LGZ profile by correlations: solid correlation lines are based on grain size curve, while dashed lines are marked by susceptibility (Fig. 6). Since susceptibility of modern sediment is lower after the TGD was built than before (Fig. 5), sediments with high susceptibility of LGZ profile should deposit before 2003, and the age of top sediment should be younger than 2003. According to these correlation lines, a new age model was reconstructed and the sedimentation rate was thereby calibrated (Fig. 7a), and a smooth trend of the sedimentation rate could be also drawn (dot lines in Fig. 7a). High sedimentation rates correspond to flood years with more water and sediment supplies, while low sedimentation rates are related to drought years by low water and sediment supplies. Based on this, we observe several flood/drought period of the Yangtze River: flood years intervals are about 1880–1890 yr A.D., 1910–1925 yr A.D., 1950–1955 yr A.D., 1970–1975 yr A.D., 1985–1995 yr A.D., while serious drought years are ∼1900 yr A.D., 1930–1940 yr A.D., ∼1960 yr A.D. Flood/drought years recognized by rainfall and water level during the flood seasons (May to October) in historical documents are usually digitized by from 2 (serious flood) to −2 (serious drought) to study climate changes of the Yangtze River (flood/drought series are from Jiang et al., 2006) (Fig. 7b). Flood/drought events of Tai Lake (in the lower reach Fig. 1b) area are also retrieved from documents (data are from Chen, 1987; Yin et al., 2009) (Fig. 7c). The 3-year smoothed flood/drought changing curves of both whole Yangtze River and the Tai Lake area share similar pattern to the new sedimentation rate of LGZ, which further prove that the new age model is more reliable than average sedimentation rate of Pb-210. The more reliable and precise age controls of sediments, the better to understand annual climate and/or environment changes.

6. Conclusion High concentration of magnetite with relatively coarse magnetic grains dominates magnetic properties of upper Yangtze River, while low concentration of magnetic minerals and fine magnetic grains are related to sediment from tributaries of middle reach. Therefore, magnetic properties could be used to trace sediment provenance along the Yangtze River. Fluctuations of magnetic parameters indicate variation of sediment contributions from upper or middle reaches, which potentially infer the climate variations of the Yangtze River drainage area. Susceptibility and mean grain size of LGZ roughly denote the changes of upper and mid-lower reaches within the Yangtze basin, which have positive relationships with records in documents (water discharge and rainfall data) from upper and middle reaches respectively. A precise age model was reconstructed by the comparisons of physical parameters and proxies in documents. The new calculated sedimentation rate reflects temporal variation of flood/drought events, which shares similar long term behavior of F/D events from digitized historical data. This further proves that comparison between physical parameters and data in documents is helpful to obtain reliable ages. Magnetic results of lower Yangtze River also shows that climate of East Asia is dominantly controlled by Pacific climate circulation and impact from human activities increases since 1950s’. Our study proves that fluvial sediment contains many aspects

5.5. Climatic changes and human activities indicated by magnetic properties The El Nino-Southern Oscillation (ENSO) dominates ocean–atmosphere mode of the tropical Pacific, which not only exists in the equatorial Pacific, but also has impacts on Asia, Australia and even 135

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climate information and reflect human activities. It is suitable for annual climate change in the last hundred years, and is possibly applied in even longer period. Interest statement Having a clear scientific view on climate change and its potential environmental and socio-economic impacts is very important for past global change and climate predictions in the future. However, there are very few research to discuss the connection between modern climate data and paleoclimate changes from sediments or other records. Sediments from the upper Yangtze River is characterized by high concentration of magnetite, so the different magnetic features between upper and middle reaches of the Yangtze River allow us to trace provenance of sediments deposited in the lower reach. Then, we retrieved paleoclimate information from riverine sediment of lower Yangtze River, and found the connection between physical parameters of the

sediment (susceptibility and grain size) and climatic records from documents (rainfall, water discharge, and so on). Furthermore, flood/ drought events of Yangtze River and human impacts could be identified in the sediment profile. We suppose that riverine sediment contains plenty of information, which is a good recorder for climate variations, and also an indicator of impacts from human activities. Acknowledgements We thank anonymous reviewers for the helpful suggestions. This work was jointly supported by National Natural Science Foundation of China (NSFC) (Grant Nos. 41472146, 41730531, 41574061, 41104043) and 973 project from Ministry of Science and Technology in China (No. 2015CB953800).

Appendix A See Appendix Fig. 1.

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