Depositional evidence of palaeofloods during 4.0–3.6 ka BP at the Jinsha site, Chengdu Plain, China

Quaternary International xxx (2016) 1e12

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Depositional evidence of palaeofloods during 4.0e3.6 ka BP at the Jinsha site, Chengdu Plain, China Tianjiao Jia a, Chunmei Ma a, *, Cheng Zhu a, Tianhong Guo a, Jiajia Xu a, Houchun Guan a, Mengxiu Zeng a, Ming Huang b, Qing Zhang b a b

School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China Institute of Cultural Relics and Archaeology of Chengdu, Chengdu 610072, China

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

At the Jinsha site it has been discovered, through field surveys and observations of the geomorphology and sedimentology, that the direction of flow of ancient rivers, obtained through debris fabric measurement, was consistent with the direction in which buried palaeotrees had been broken, indicating that the Jinsha site possibly suffered palaeofloods. By sampling the suspected palaeoflood sediments at the Jinsha site and analysing their grain size distribution and Zr/Rb and Ba/Nb ratios, the paper discovered that the sediments tend to be composed of coarse particles, with a relatively high percentage of traction load components and saltation load components. Various samples had relatively great differences in particle size distribution; the change in the percentage content of coarse sand particles being very obvious, and the sedimentary particles exhibiting upward coarsening within the same layer. The Zr/ Rb ratios and Ba/Nb ratios of the sediments reflect the changes in hydrodynamic forces within the sedimentary environment. When the hydrodynamic force was relatively weak, the Zr/Rb ratios are relatively low and the Ba/Nb ratios relatively high; whereas when the hydrodynamic force was strengthened, the Zr/Rb ratios increase and the Ba/Nb ratios decrease. On account of these sedimentological and geochemical characteristics, these sediments are judged to be palaeoflood sediments. The AMS 14C dating results of the palaeoflood sediments shows that the period when sudden flooding occurred frequently in the Chengdu Plain was 4.0e3.6 ka BP, contemporaneous with an ancient global flooding event. This palaeoflood event probably brought about the extinction of the Baodun Culture. © 2016 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Palaeofloods Ancient human settlements Jinsha site Chengdu Plain

1. Introduction Changes in the global climate have caused frequent occurrences of sudden flood disasters in many places, endangering the survival of human beings and the development of society (Milly et al., 2002; de Moel et al., 2009; Bai et al., 2016). Palaeoflood events are the instantaneous response of hydrological processes to extreme climatic events, and recognizing palaeofloods from sedimentary records is helpful in reconstructing the history of climate change, the sequence of flooding in drainage basins, and flood cycles (Nest, 2014; Petera-Zganiacz et al., 2015; Zha et al., 2015). In turn, this knowledge is extremely significant for preventing floods, relieving disasters, constructing water conservancy projects, developing and regulating water resources, etc., in the modern world (Baker, 2008;

* Corresponding author. E-mail address: [email protected] (CM Ma).

Zhang et al., 2012; Fan et al., 2015). At present, more and more research findings show that the relationship between floods and climatic change is relatively complicated, and the mutual response between the two factors is not consistent in different climatic zones and at different historic stages (Ely, 1997; Knox, 2000; Hassan, 2007; Macklin et al., 2010; Liu et al., 2013a; Ma et al., 2014). Palaeofloods occurred frequently in the period when the climate was unstable, and deterioration in climatic conditions was accompanied by increases in precipitation variability (Grossman, 2001; Benito et al., 2003; Saint-Laurent, 2004; Bonsall et al., 2015; Sridhar et al., 2015). Research on the sedimentary strata of the Yugu River floodplain in Korea shows that temperature abnormalities co-occurred with periods of frequent flooding in the Northern Hemisphere (Lim et al., 2013). Research on sediments in the Mississippi River floodplain shows that, in the period when there was major precipitation upstream of the Mississippi River, large floods occurred frequently in the middle of the North

http://dx.doi.org/10.1016/j.quaint.2016.07.008 1040-6182/© 2016 Elsevier Ltd and INQUA. All rights reserved.

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American continent (Munoz et al., 2015). Research on palaeofloods in the Yellow River Basin shows that palaeofloods occurred frequently in the period of transition from the middle to the late Holocene, when climatic fluctuation was drastic and precipitation changed abnormally (Yang et al., 2003; Xia et al., 2004; Huang et al., 2011a, 2012). In previous research on Holocene palaeofloods, the large flood occurring around 4.0 ka BP has been a focus of attention in academic circles, mainly because the timing of this event is consistent with the rise and decline of several ancient civilizations (Cui and Zhou, 2003; Wang, 2004; Wu and Liu, 2004). The evidence of palaeofloods at numerous sites in the Jianghan Plain and its surrounding areas reveals that, the large flood events around 4.0 ka BP were one of the key factors in the extinction of the Shijiahe Culture in the middle reaches of the Yangtze River (Wu et al., 2015). Archaeological studies of the environment at many sites in the Yangtze River Delta Region show that large-scale flooding occurred around 4.0 ka BP in this region, and the sudden extinction of the Liangzhu Culture was probably caused by the palaeofloods in this period (Shi, 1993; Zhu et al., 1996a, 1996b; Wu et al., 2014). The Lajia Site in the upper reaches of the Yellow River was destroyed by palaeofloods around 4.0 ka BP (Yang et al., 2003); in the same period, abnormal palaeoflood events commonly occurred in the middle and lower reaches of the Yellow River, and the frequent occurrences of flooding finally led to the fading of the Longshan Culture (Li et al., 2009; Zhang and Xia, 2011; Huang et al., 2011b). The archaeological excavations at the Baodun site, Sanxingdui site and Jinsha site, which are located in the Chengdu Plain, show that all these sites have likely experienced palaeofloods and that the palaeofloods produced notable impacts on the ancient Shu culture (Li et al., 2005; Lin, 2006; Huang, 2013). However, there is still a lack of environmental archaeological studies on the palaeoflood alluvial layers at these sites. An evaluation of the links between palaeofloods and human-earth relationships was performed in this study by analysing the fluvial facies of sedimentary formations at the Jinsha site in the Chengdu Plain. This analysis provides a reliable interpretation of the stratigraphic record for the evaluation of issues such as the manner in which the ancient culture developed in the Chengdu Plain during 4.0e3.6 ka BP. It also contributes to determining the response of the regional hydrological environment to global climate change and how prehistoric flood events influenced the development of the ancient Shu culture. 2. Study region The Chengdu Plain is located in the western Sichuan Basin, which lies between the western Longmen and eastern Longquan Mountains. It is an alluvial basin on the eastern margin of the Tibetan Plateau and is climatically dominated by the Asian monsoon and the Indian monsoon. The plain is approximately 100 km wide in the NWeSE direction and 200 km long in the NEeSW direction, and its elevation varies from 400 m to 750 m above sea level (asl) (Jiang et al., 2002). The mean annual temperature is approximately 17
C and the mean annual precipitation is approximately 1000 mm (Liang et al., 2014). Precipitation in the plain occurs primarily in July and August. The Chengdu Plain contains numerous river networks, including three major river systems: the Minjiang River, the Tuojiang River, and the Qingyi River. A large volume of sediments consisting of gravels and silts from the upstream areas of the Minjiang River and the Tuojiang River comprise the Quaternary sediments of the Chengdu Plain, with the depositional centre located in the Pixian and Wenjiang areas (Luo et al., 2008). The Jinsha site is 5 km from the centre of Chengdu, Sichuan (30
410 N, 104
000 E), and has an area of 5 km2 (Fig. 1). The Modi River flows from northwest to southeast near the site. The

topography of the site is generally flat, with an elevation ranging from 504 to 508 m asl, and is slightly lower in the southeast and slightly higher in the northwest (Zhu et al., 2004). There are many rivers distributed within and around the Jinsha site. The Qingshui River is located 1.5 km south of the site, an old channel of the Pijiang River is located on the north side of the site, and the Modi River cuts across the middle of the site from west to east (Yao et al., 2005). It has previously been reported that the area of the Jinsha site is greater than 5 km2 and that the Jinsha culture evolved during 3.2e2.6 ka BP (Wang, 2006). The Jinsha site was a new emerging political, economic, religious and cultural centre following the presence of the Sanxingdui culture in the Chengdu Plain (Lin, 2006; Zhang, 2006).

3. Materials and methods 3.1. Stratigraphy and materials The sampling site was located in the key column of the IT8007 excavation unit within the Jinsha Site Museum, where a complete cultural sequence was preserved, including the late Shang Dynasty, western Zhou Dynasty, Spring and Autumn Period, Han Dynasty, Tang Dynasty, Ming and Qing Dynasties and the modern period (Zhu et al., 2004). In January 2014, our research group excavated a natural sedimentary deposit with the help of the archaeological staff at the Jinsha Site Museum. The deposit had a thickness of 189 cm, with deposits at the bottom representing the late Shang Dynasty culture as the starting point. The sedimentary deposit was cut into 1 m  15 cm  15 cm samples that were kept in stainless steel boxes and then moved to the laboratory for further analysis. In total, 100 gravel samples at the bottom of the IT8007 profile were measured for the palaeocurrent analysis.

3.2. Accelerator mass spectrometry

14

C dating

The ages of two wood samples from a palaeotree buried at the top of the first layer of the IT8007 profile were measured using the accelerator mass spectrometry (AMS) 14C dating method. Four plant residue samples were also recovered from the profile for AMS 14 C dating. Each sample was collected from a 1-cm-thick sediment sample using the flotation process. All the AMS 14C samples were measured at the Xi'an Accelerator Mass Spectrometry Centre, Institute of Earth Environment, Chinese Academy of Sciences, and calibrated using the IntCal 13 curve and CALIB 7.04.

3.3. Laboratory methods A total of 54 samples with a resolution of 1 cm were collected from the bottom to the surface of the IT8007 profile at 2-cm depth intervals. The grain size fractions of the samples were determined using a Malvern Mastersizer-2000 laser particle analyser (error: ±1%) at the School of Geographic and Oceanographic Sciences at Nanjing University. Each sample was measured three times, with the average values as the reported data. The grain size data were processed using the GRADISTAT program developed by Blott and Pye (2001). The concentrations of Zr, Rb, Ba and Nb were analysed using an ARL-9800-type X-ray fluorescence spectrometer (XRF) (error: ±1%) at the Modern Analysis Centre, Nanjing University. The samples were prepared following the powder tableting method, and measurement quality control was performed using two national geochemical standard samples (GSS1 and GSD9).

Please cite this article in press as: Jia, T., et al., Depositional evidence of palaeofloods during 4.0e3.6 ka BP at the Jinsha site, Chengdu Plain, China, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.07.008

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Fig. 1. Location of the Jinsha site in the Chengdu Plain.

4. Results and analysis

4.2. Analysis of palaeocurrents and buried palaeotrees

4.1. Stratigraphy and chronology

Because of the effects of stable river flow, gravels typically feature a discernible directional arrangement, and the largest flat surface of a flat gravel clast always dips in an upstream direction (Zhang and Zhu, 2008). The palaeocurrent analysis of the gravel bed in the 8th layer of the IT8007 profile shows that the angles of the flat gravel surfaces trends roughly NW, indicating that the direction of the river flow was from NW to SE at that time (Fig. 3). Several palaeotrees buried in the deposit were found approximately 30 m from the excavation site. The positions of the palaeotrees correspond to the first layer of the IT8007 profile. The trunk diameters are more than 50 cm, and most of the trunks were broken close to the roots in a southeast direction (with the roots remaining in the soil), which corresponds to the flow direction of the ancient river (Fig. 4). The buried palaeotrees can be considered to be direct evidence that the Jinsha site experienced extraordinary palaeoflood events.

The IT8007 profile was divided into 8 layers from the surface to the base according to colour and textural differences (Fig. 2). The 5th layer shows distinct ripple bedding. The 8th layer is a gravel layer (which does not extend to the bottom) showing generally high psephicity and good directional properties. The upper 7 layers (1st through 7th) are composed of fine sediments and the 8th layer is a gravel bed, which indicates a clear dual structure in the IT8007 profile. It can be deduced that the IT8007 profile represents a fluvial deposit based on its dual structure and sedimentary characteristics that are similar to those of other excavated profiles at peripheral sites. In total, 6 calibrated AMS 14C dates were obtained, as shown in Table 1. The calibrated AMS 14C ages of the 4 plant residue samples are 3950 ± 59 cal a BP, 4070 ± 85 cal a BP, 3924 ± 71 cal a BP and 4005 ± 84 cal a BP. The 14C ages of the 4 plant residues show notably small fluctuations, all of which are ~4 ka BP; therefore, it can be deduced that the 4the7th layers of the IT8007 profile were formed through rapid deposition processes at approximately 4 ka BP. The calibrated AMS 14C ages of the two palaeotree samples collected at the top of the 1st layer are 3647 ± 77 cal a BP and 3852 ± 40 cal a BP, which provide the upper age limit of the IT8007 profile.

4.3. Characteristics of the granularity parameter 4.3.1. Distribution of granularity According to the F standard grade scale method (Xu et al., 1992), the particle grain sizes of the IT8007 profile were classified into six grades: clay (<4 mm), fine silt (4e16 mm), coarse silt (16e63 mm),

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Fig. 2. Characteristics of the IT8007 profile at the Jinsha site.

Table 1 AMS 14C dating results and corresponding calibration data for the Jinsha IT8007 profile. Lab I.D.

Depth(m)

Material

AMS

14

XA11906 XA11907 XA12727 XA12728 XA12729 XA12730

4.00 4.00 5.04 5.28 5.52 5.79

Palaeotree Palaeotree Plant residues Plant residues Plant residues Plant residues

3404 3535 3649 3734 3629 3672

± ± ± ± ± ±

C age (a BP) 34 24 26 28 30 27

2s calibrated age

Calendar age (cal. a BP)

1774BC(97.4%)1620BC 1941BC(54.2%)1862BC 2058BC(77.8%)1941BC 2205BC(100%)2035BC 2044BC(91.4%)1903BC 2138BC(100%)1971BC

3647 3852 3950 4070 3924 4005

± ± ± ± ± ±

77 40 59 85 71 84

silt is in the range of 26.08%e55.58%. Fine sand is in the range of 6.36%e40.37%. Medium sand is in the range of 0e6%. Coarse sand is in the range of 0e38.3%. The distribution of the particle grain sizes indicates that the content of coarse silt is highest in the IT8007 profile, followed by the content of fine silt and fine sand. Most of the profile is composed of coarse particles, which indicates that this profile is representative of deposition by strong hydrodynamic processes.

Fig. 3. Radar diagrams of the fabric of the IT8007 profile at the Jinsha site.

4.3.2. Granularity parameter The median diameter and mean grain size can indicate the primary grain composition of the sediments. The median diameter of the Jinsha site sediments ranges from 3.46 to 5.50 F, and the mean grain size ranges from 3.12 to 5.67 F, indicating that the median diameter and mean grain size of most of the samples are representative of coarse silt (Fig. 6). According to the Folk and Ward method, the standard deviation (s1) is in the range of 1.46e2.97 F, indicating weak sorting processes. The skewness (SK1) is in the range of 0.29e0.27, indicating negative to positive distributions. The kurtosis (KG) is in the range of 0.63e1.37, indicating data with very platykurtic (flat) to leptokurtic (peaked) probability distributions. The values of the skewness and the kurtosis vary over the length of the profile, as shown in Fig. 6, indicating a considerable difference in the distribution of the grain sizes in the IT8007 profile.

fine sand (63e250 mm), medium sand (250e500 mm) and coarse sand (500e2000 mm) (Fig. 5). The percentage of clay is in the range of 3.24%e10.74%. Fine silt is in the range of 13.75%e31.37%. Coarse

4.3.3. Grain size frequency distribution curve A grain size frequency distribution curve can provide straightforward information on the relative contents of different particle grain sizes within a sample, which represents a basis for

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Fig. 4. Palaeotrees buried at the Jinsha site (Photo by Cheng Zhu).

Fig. 5. Variations in the grain size in the IT8007 profile at the Jinsha site.

Fig. 6. Variations in the granularity parameters in the IT8007 profile at the Jinsha site.

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determining the formation of the sampled sediment (Friedman, 1979). The grain size frequency distribution curves for the IT8007 profile at the Jinsha site includes two types: unimodal and bimodal, as shown in Fig. 7. Most of the frequency distribution curves for the 1st layer samples are bimodal, with peak values of 0.7 F and 4.0 F. The frequency distribution curves for the 2nd layer samples are also bimodal, and can be further classified into two types: distributions in which the first kurtosis peak is not prominent and distributions in which the first kurtosis peak and the second kurtosis peak are similar, with notably small grain size values for sample data plotted between these two kurtosis peaks. The frequency distribution curve shapes for the 3rd, 4th, and 6th layer samples are similar: some individual samples at the top of each layer have distinct double peaks, while the rest of the samples have single peaks or obscure double peaks. The frequency distribution curves for the 5th layer samples have only one peak, except for one sample collected from the middle of the layer, which has double peaks. The frequency distribution curves for the samples in the 7th layer are notably similar, with a single peak at ~4.8 F and positive skewness, which indicates a single sediment composition. There are more fine particles in this layer than in the other layers, indicating deposition in response to weak hydrodynamic processes.

0.1 mm; the saltation load is transported close to the bottom of the water flow and has a typical grain size in the range of 0.15e1.0 mm; and the traction load is transported close to the river bed and features coarser particles transported via sliding and rolling along the bottom (Chengdu Institute of Geology, 1976). Fig. 8 shows the cumulative probability curves for grain size in the different layers of the IT8007 profile. The majority of the particle composition in the profile is suspension load, with a percentage of >50% for the profile and up to 90% in the 7th layer samples. The grain size of the suspension load is in the range of 2.5e12 F, and the breakpoint between the suspension load and the saltation load is in the range of 2.5e4 F. The grain size of the saltation load is in a range of 0e4 F. The percentage of the saltation load in the 1ste6th layers is divided into two types; one type accounting for ~10% and the other type for ~40%. The percentage of the saltation load in the 7th layer is <1%. Saltation loads are absent in some of the 1ste6th layer samples and in all the 7th layer samples, while the saltation load in the remaining samples ranges from 10% to 40%, with grain sizes in the range of 1 to 1 F. The breakpoint between the saltation load and the traction load in these samples is in the range of 0e1 F, indicating a strong diluvial-alluvial deposit. 4.4. Zr/Rb ratio and Ba/Nb ratio record at the Jinsha site

4.3.4. Cumulative probability curve for grain size The grain size composition can be classified into 3 forms according to the mode of river transport: suspension, saltation, and traction. The grain size distribution of each form is different: the grain size of the suspension load is fine and is usually less than

Zr is mainly found in zircons in soils and sediments (Chen et al., 1999). During the weathering process, zircons maintain their original morphology, along with other stable minerals, such as quartz and feldspar, all of which become concentrated in the coarse

Fig. 7. Grain size frequency distribution for the IT8007 profile at the Jinsha site.

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Fig. 8. Cumulative probability distribution of the IT8007 profile at the Jinsha site.

particles (Liu et al., 2002). Rb is a typical dispersed element in nature. Because Rb and K have similar particle radii, potential and other geochemical parameters, the Rb in various types of rocks is primarily concentrated in minerals containing K, such as muscovite, biotite, illite and feldspar. During the weathering process, Rb tends to be concentrated in clay minerals (Chen et al., 2003; Liu et al., 2006). Thus, the Zr/Rb ratios reflect the content of coarse minerals relative to clay minerals. When floods occur, river runoff increases rapidly, causing the transport and convergence of large amounts of coarse, weathered particles in the river. The Zr/Rb ratios in the flood deposits are much higher than those in other deposits; therefore, the Zr/Rb ratio can be used as a palaeoflood proxy (Wang et al., 2011). Ba is readily available to form chemical compounds when combined with water. When Ba2þ migrates in an aqueous solution, it is easily adsorbed by the clay component through hydrolysis precipitation processes (Liu and Cao, 1987). As the river runoff increases rapidly during a flood period, the content of Ba decreases in the deposit. In contrast, when the runoff decreases, the content of Ba in the deposit increases. Nb is a weak transition element and has less mobility and a low water migration coefficient. Minerals enriched with Nb are typically very resistant to weathering (Huang and Gong, 2002). A high Ba/Nb ratio reflects the influence of a weak hydrodynamic force. Conversely, a low Ba/Nb ratio reflects the influence of a strong hydrodynamic force. Fig. 9 shows the curve of the Zr/Rb ratios and Ba/Nb ratios in the IT8007 profile. The Zr/Rb ratios increase with decreasing profile

depth, while the Ba/Nb ratios decrease with decreasing profile depth. Both curves show notable fluctuations, indicating that the hydrodynamics of the sedimentary environment changed rapidly and frequently. There is a strong negative correlation between the Zr/Rb ratios and the Ba/Nb ratios (Fig. 9), which indicates that the Zr/Rb ratios increase and the Ba/Nb ratios decrease with increasing hydrodynamic force. Conversely, the Zr/Rb ratios decrease and the Ba/Nb ratios increase with decreasing hydrodynamic force. The Pearson correlation analysis was used to identify the relationship between the Zr/Rb ratios and grain size (Table 2). The results of the analysis show that the Zr/Rb ratios are significantly negatively correlated with the percentage of median diameter, clay and fine silt at the 99% significance level, significantly positively correlated with the percentage of fine sand at the 99% significance level, and significantly positively correlated with the percentage of medium sand at the 95% significance level. Thus, the Zr/Rb ratios can be used to reflect the relative content of coarse and fine particles in the IT8007 profile. Higher Zr/Rb ratios are indicative of a higher component of coarser particles in the deposit. 5. Discussion 5.1. Palaeofloods at the Jinsha site Based on the results of the field investigation and laboratory analysis, it can be deduced that the 1ste7th layers of the IT8007

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Fig. 9. Variations in the Zr/Rb ratios and Ba/Nb ratios in the IT8007 profile at the Jinsha site.

profile at the Jinsha site are indicative of fluvial deposition. The total thickness of the 4the7th layers is 1.03 m. These layers were rapidly deposited at approximately 4 ka BP, based on a synthesis of the dating data. According to the AMS 14C data, the upper age limit of the IT8007 profile is 3.6 ka BP, which indicates that the time required for the formation of the 1ste3rd layers (with a total thickness of 0.86 m) did not exceed 400 years. The deposition rate of the entire profile was therefore very rapid, with rates of up to 53 cm/yr. According to the grain size analysis of all 54 samples from the IT8007 profile, the sedimentary particles in this profile are primarily coarse components deposited by strong hydrodynamic forces. The coarse sand content of some individual samples in the 1ste6th layers are indicative of surge phenomena. These samples with high contents of coarse sand are poorly sorted, with grain size distributions that have a negative skewness, a platykurtic or very platykurtic kurtosis and a distinct bimodal frequency distribution curve. The percentage

of the traction load content in these samples is greater than 10%, and the content in some samples is up to ~40%. The cumulative probability curves for grain size data indicate that the breakpoint for the coarse fraction in these samples occurs at a range of 0e1 F, which indicates influence from strong pluvial-alluvial depositional processes. The samples enriched in coarse sand are probably diluvial deposits formed by flood processes. In conclusion, the 1ste7th layers of the IT8007 profile at the Jinsha site are palaeoflood alluvial layers formed by multi-phase palaeoflood events. The Zr/Rb ratios increase with decreasing profile depth, and the Ba/Nb ratios decrease with decreasing profile depth, which indicate that the palaeoflood scale increased over time. The samples in the 2nd, 3rd, 4th, and 6th layers exhibit a reverse grain size pattern in the vertically ascending order of the layers, and even multiple cycles within the same layer. A possible reason for the reversal in grain size order is that: during the palaeoflood process, a number of strong flood peaks occurred in a short time. Although the time of occurrence for flood peaks during the same palaeoflood event may be different, the time interval is very short and the sedimentary discontinuity is negligible. Because the deposit formed by multiple flood peaks in a short time is continuous, the sedimentary particles in the same layer exhibit upward coarsening (Zheng et al., 2014). The Zr/Rb ratio and Ba/Nb ratio curves fluctuate consistently, with many corresponding upward and downward peaks, which are attributed to rapid changes in hydrodynamic forces resulting from multiple flood peaks over a short period of time. Several buried palaeotrees were identified ~30 m northeast of the sampling site. They were located at the same depth as the top of the first layer of the IT8007 profile. The palaeotrees were snapped at their bases in a northwest to southeast direction, which corresponds to the paleo river flow direction based on the palaeocurrent measurements. These observations serve as direct evidence that extraordinary palaeofloods have occurred in this region. Archaeological studies confirm that continuous cultural layers from the Shang Dynasty to modern times were identified above the depth of the first layer of the IT8007 profile at the Jinsha site, which indicates that the river shifted course at approximately 3.6 ka BP and that the surface was gradually exposed and became suitable for human settlements thereafter. The river diversion may be associated with the extraordinary palaeoflood at that time. Yang et al. proved the existence of a palaeochannel at the Jinsha site through use of the highdensity resistivity method, and identified the course of the migration of the palaeochannel from south to north towards the location of the modern Modi River (Fig.10) (Yang et al., 2005), which provides strong evidence in support of the results of this study.

5.2. Climate background and palaeoflood causation The age of the IT8007 profile is in the range of 4.0e3.6 ka BP. The results of field investigations and laboratory analyses indicate that frequent palaeofloods occurred at the Jinsha site during 4.0e3.6 ka

Table 2 Correlation between Zr/Rb ratios and grain size percentages in the IT8007 profile.

Median diameter(F) Clay (<4 mm) Fine silt (4e16 mm) Coarse silt (16e63 mm) Fine sand (63e250 mm) Medium sand (250e500 mm) Coarse sand (500e2000 mm) Zr/Rb ratios

Median diameter

Clay

Fine silt

Coarse silt

Fine sand

Medium sand

Coarse sand

Zr/Rb ratios

1 0.925** 0.957** 0.924** 0.499** 0.344* 0.774** 0.357**

1 0.930** 0.861** 0.572** 0.462** 0.660** 0.406**

1 0.832** 0.605** 0.408** 0.642** 0.468**

1 0.405** 0.490** 0.813** 0.249

1 0.269* 0.138 0.569**

1 0.240 0.298*

1 0.025

1

Note: ** significant correlation at the 0.01 level (2-tailed); * significant correlation at the 0.05 level (2-tailed).

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monsoon and the rapid strengthening of the East Asian winter monsoon. In a cold and dry climate, the climatic variability tends to be high (Zheng and Feng, 1985), which may reduce the land surface vegetation cover. Thus, when multiple precipitation events occurred in quick succession, the rainfall on the dry earth surface infiltrated slowly, and the surface runoff rapidly developed into floods. The results of previous studies indicate that droughts and floods are compatible (Zhang et al., 2004b; Wu et al., 2006; Liu et al., 2013b). The rainfall easily triggered debris flows under cold and dry conditions during 4.0e3.6 ka BP in the western mountainous area of the Chengdu Plain, which likely caused environmental disasters in the lower reaches of the Chengdu Plain. As the Chengdu Plain features dense water networks, the intensive rainfall likely resulted in sluggish flow velocities in the water networks, resulting in the diluvial events. 5.3. Comparative analysis of the palaeofloods in the Yangtze River Basin

Fig. 10. Sketch map of the palaeochannel location at the Jinsha site.

BP and that the 1ste7th layers of the IT8007 profile are part of the alluvial deposit formed by multiple palaeofloods. Climatic conditions are the primary driving force of the hydrologic system. Floods usually occur during periods of regional or global climate change (Saint-Laurent, 2004; Wu, 2013). The analysis of clay minerals at the Jinsha site (Luo et al., 2007), and the degree of humification of the Hongyuan peat (Wang et al., 2003), indicate a cold, dry, and notably fluctuating climate was extant in the Chengdu Plain during 4.0e3.6 ka BP. These conditions caused the frequent flood events during that period. The Chengdu Plain belongs to the subtropical monsoon climate region, which is affected by both the Indian monsoon and the South Asian monsoon. The Northern Hemisphere cooling caused by reduced solar output since 11 ka BP may have resulted in changes in the interhemispheric thermal gradient, causing a southward shift in the Intertropical Convergence Zone and the weakening of the low-altitude monsoon (Yuan et al., 2004; Liu and Shi, 2009; Liu et al., 2013b). The results of previous studies on the d18O values of stalagmites in Donge cave, Xianren cave, Xiangshui cave, Heshang cave and Xinya cave (Zhang et al., 2004a; Wang et al., 2005; Li et al., 2006; Hu et al., 2008; Wang et al., 2010), and a study on n-alkanes in the Hongyuan peat (Zheng et al., 2008), indicate that the East Asian summer monsoon and Indian summer monsoon weakened and that the East Asian winter monsoon rapidly strengthened, at approximately 4 ka BP, causing a substantial decrease in precipitation and a shift in the climate towards cold and dry conditions in these regions. Similarly, the cold and dry climate of the Chengdu Plain during 4.0e3.6 ka BP can also be attributed to the weakening of the East Asian summer monsoon and the Indian summer

The frequent volatile palaeofloods that occurred during 4.0e3.6 ka BP as a result of the cold and dry climate were recorded at the Jinsha site in the Chengdu Plain, located in the upper reaches of the Yangtze River, which clearly responded to the 4 ka BP event. The 4 ka BP event is considered the strongest abrupt change of climate since 5 ka BP (Wang, 2011). Several studies show that the 4 ka BP event was an important turning point in human evolution, corresponding to the fall of ancient Egypt, Mesopotamia and the Indus Valley civilizations (Courty et al., 1989; Weiss et al., 1993; Cullen et al., 2000; Weiss and Bradley, 2001). Peng et al. collected 114 records from 84 different regions in China documenting the paleoclimate at approximately 4 ka BP and found that the climate was mainly cold and dry in most regions of China. In addition, the extent and scale of the human settlement associated with the Chinese Neolithic culture narrowed significantly (Wang, 2004; Peng et al., 2013). Previous studies have recorded other palaeoflood events in the upper, middle, and lower reaches of the Yangtze River at approximately 4 ka BP. In the upper reaches of the Yangtze River, Huang proved that palaeoflood events occurred at the Baodun site (Fig. 11) in Chengdu Plain during the late period of Baodun culture, based on analysis of the sedimentology of an exploratory trench, and a palaeocurrent survey at the Baodun site (Huang, 2013). Similarly, Zhu et al. discovered palaeoflood sediments from 4.0 to 3.6 ka BP in a stratigraphic profile at the Zhongba site in the Three Gorges Reservoir Region (Zhu et al., 2005). In the middle reaches of the Yangtze River, Wu et al. discovered palaeoflood formations from 4.1 to 3.8 ka BP in an excavated profile at the Zhongqiao site, in Jianghan Plain by analysing grain size and geochemical elements (Wu et al., 2015). Palaeoflood sediments for this period were also found at other sites in the Jianghan Plain, such as the Yuezhouhu site, Guihuashu site, Taihugang site, and Xiejiadun site (Jingzhou Museum, 1976; Yang, 1985; Yao, 1986; Zhu et al., 1997). At approximately 4 ka BP, the climate of the Jianghan Plain, in the middle reaches of the Yangtze River, was unstable, and tended to be cold and dry. This caused palaeofloods that may have resulted in a decline of the Shijiahe culture in this region during the same period (Li et al., 2014; Wang et al., 2012; Wu et al., 2015). In the lower reaches of the Yangtze River Delta, the developed Liangzhu culture vanished abruptly at approximately 4 ka BP, and natural silt layers were deposited afterwards without any cultural relics. The Maqiao culture developed on top of the natural silt layers (Yu et al., 1998; Shen et al., 2003; Cao and Wang, 2005; Zhu et al., 2014). Environmental archaeology studies have found that the natural silt layers at the Maqiao site were formed by floods during 4.0e3.8 ka BP (Zhu

Please cite this article in press as: Jia, T., et al., Depositional evidence of palaeofloods during 4.0e3.6 ka BP at the Jinsha site, Chengdu Plain, China, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.07.008

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Fig. 11. Typical palaeoflood deposit layers at some late Neolithic sites in the Yangtze River Basin.

et al., 1996a). Zhu et al. discovered several buried palaeotrees at the Songze, Maqiao, Tinglin, Shahe and Dongmen sites on the Yangtze River Delta, and the 14C dates of these trees correspond well to the dates of the natural silt layers within the sites, which offers strong evidence that the Yangtze River Delta region experienced a period of significant flooding at approximately 4 ka BP (Zhu et al., 1996b, 1998). The results of previous studies indicate that some clear cooling events occurred and that the climate shifted to a cold and dry state at approximately 4 ka BP in the Yangtze River Delta region (Shen et al., 2003; Chen et al., 2007; Ma et al., 2009). Thus, it can be concluded that the climatic change at approximately 4 ka BP throughout the Yangtze River Basin exhibited a consistent pattern, with synchronous floods occurring in the upper, middle, and lower reaches of the Yangtze River. 5.4. The transition of ancient human settlements during 4.0e3.6 ka BP in the Chengdu Plain The Baodun culture was named after the Baodun site and evolved during 4.5e3.7 ka BP. It was a highly developed late Neolithic culture that was mainly distributed in the Chengdu Plain area (Jiang et al., 2002). The Baodun culture is divided into four

periods. The site of the 1st and 2nd periods was located at an elevation of 470 m asl. The elevations of the Gucheng Village site and the Yufu Village site in the 3rd period, and the Hongqiao Village site in the 4th period, were 565 m, 556 m, and 527 m respectively, all of which were higher than the site of the 1st and 2nd periods. The frequent abrupt flood events at approximately 4 ka BP likely created a notable threat to the survival and security of people living on the Chengdu Plain, which caused them to move to higher locations to avoid the floods. Palaeoflood sediments associated with the late period of the Baodun culture were found at the Baodun site (Huang, 2013). In 2014, the remnants of an early flood prevention project similar to the Dujiang Dam Project were found during an archaeological excavation. This discovery provides strong evidence that human lives were affected by the frequent palaeofloods during the period when this early flood prevention project was conducted. The scale of the floods increased over time during 4.0e3.6 ka BP, eventually resulting in the destruction of the Baodun culture. In this study, the AMS 14C age of a buried palaeotree at the top of the 1st layer in the IT8007 profile at the Jinsha site is 3.6 ka BP. This suggests that the 1st layer formed earlier than 3.6 ka BP, when the large flood occurred. The upper age limit of the Sanxingdui culture is considered to be 3.7 ka BP, based on the results of previous studies.

Please cite this article in press as: Jia, T., et al., Depositional evidence of palaeofloods during 4.0e3.6 ka BP at the Jinsha site, Chengdu Plain, China, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.07.008

T. Jia et al. / Quaternary International xxx (2016) 1e12

The Sanxingdui culture is named after the Sanxingdui site, which was located at an elevation of ~488 m asl and is 38 km from the Jinsha site (Jiang et al., 1997). The site may have formed after the frequent palaeofloods during 4.0e3.6 ka BP. Based on the difference in elevation of the Jinsha site and the Sanxingdui site, it can be assumed that the climate of the Chengdu Plain improved after the frequent large floods, and that the ancient people moved back to lower-elevation areas, reflecting the dynamic relationship between human society and Earth surface processes. The palaeochannel shifted and the climate conditions improved at the Jinsha site after the last period of extraordinary flooding, which gradually made the Jinsha site suitable for human settlements.

6. Conclusions The analysis of grain size and geochemical elements in the IT8007 profile at the Jinsha site indicates that the stratigraphy represented by the profile formed in response to strong hydrodynamic forces during 4.0e3.6 ka BP, and that the Jinsha site experienced frequent palaeofloods during this period. The cold and dry climate of the Chengdu Plain during 4.0e3.6 ka BP can be attributed to the weakening of the East Asian summer monsoon and the Indian summer monsoon, and the rapid strengthening of the East Asian winter monsoon. The rainfall regularly triggered debris flows under cold and dry conditions during 4.0e3.6 ka BP in the western mountainous areas of the Chengdu Plain, which is likely to have caused environmental disasters in the lower reaches of the Chengdu Plain. The intensive rainfall probably resulted in sluggish flow velocities in the numerous water networks of the Chengdu Plain, thereby resulting in the diluvial events. The analysis of ancient human settlements shows that people moved from lowelevation areas to high-elevation areas at approximately 4 ka BP, and moved back to the low-elevation areas when the climate conditions improved in the Chengdu Plain at approximately 3.6 ka BP, which appropriately reflects the relationship between human society and Earth processes.

Acknowledgements The authors would like to thank Prof. Yi Wang and Prof. Zhanghua Jiang from the Institute of Cultural Relics and Archaeology of Chengdu, as well as all other relevant staff, for the field and laboratory assistance. This work was supported by the National Natural Science Foundation of China (No. 41371204), the Major Programme of National Social Science Foundation of China (No. 11&ZD183), and the National Key Technology R&D Programme of China (No. 2013BAK08B02).

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Please cite this article in press as: Jia, T., et al., Depositional evidence of palaeofloods during 4.0e3.6 ka BP at the Jinsha site, Chengdu Plain, China, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.07.008