The projected states are composed of two sub-lattice components in the emitter and collector. As a result, momentum-dependent constructive (φe(c)= 0) or destructive (φe(c)=p) interference
between sublattice components is governed by jyAþ yBj
2
º1 þ cosφe(c), for the states both in
emitter (φe) and collector (φc) and manifests itself
in the tunneling characteristics I(Vb). Because the
magnetic field selects the pairs of particular plane wave states probed by tunneling at a particular gate or bias voltage (Fig. 4, A and B), the measured asymmetry provides a direct visualization of the pseudospin polarization of the Dirac fermions.
In the presence of the magnetic field, each res-onance peak represents tunneling from a par-ticular corner of the BZ. This allows one to inject electrons with a particular valley polarization, and from a selected corner of the BZ. We use the ex-perimental parameters to calculate the amount of polarization achieved in our experiment (Fig. 3, J and M), and estimate that the valley polar-ization, P¼ ðIK−IK′Þ=ðIKþ IK′Þ [where IK(IK′) is the current injected into the K(K′) valley] can be as high as 30% (40%) for the particular Gr/3hBN/ Gr (Gr/5hBN/BGr) devices. The main limit to the degree of polarization is the energy broadening of states at the Fermi levels caused by inelastic tun-neling processes. However, even for the current level of disorder, with the resonances at around Vb≈ 0 V (e.g., resonances marked by yellow dashed
lines on Fig. 2D at Vg> 50 V), which maximizes the
number of states participating in tunneling and sensitive to magnetic field, a polarization close to 75% could be achieved (19). By using devices with smaller misalignment between the graphene elec-trodes [on the order of 0.2°, now within the reach of the current technology (19)], valley polarization close to 100% is possible (19).
The same mechanism can also be used to se-lect ese-lectrons with a particular pseudospin polar-ization. In Fig. 4, C to R, we present results of a calculation of the contribution of different elec-tronic states in k-space to the tunnel current for the Gr/3hBN/Gr (Fig. 4, C to I) and Gr/5hBN/ BGr (Fig. 4, J to R) devices. We choose the posi-tion of the Fermi levels in the emitter and col-lector to be very close to a resonance at B = 0 T. Then, for certain directions of B, the resonant conditions are achieved only in one valley and for only a very narrow distribution in k-space (Fig. 4, G to I). Tunneling of the electrons from other parts of k-space is prohibited either because they are off-resonance or because of the pseudo-spin selection rule. Alternatively, for the Gr/5hBN/ BGr device and exploiting the difference in cur-vature of monolayer and bilayer electronic bands, we can choose the overlap between the bands in such a way that the magnetic field reduces the overlap in one valley and increases it for the other (Fig. 4, M to R). In this case, momentum conservation at B = 0 T is fulfilled for the states marked by white dashed lines (Fig. 4O). However, only one of those lines contributes to tunneling, owing to pseudospin interference (Fig. 4, M and N).
Our technique, which enables tunneling of valley-polarized electrons in monolayer and bilayer
gra-phene, also allows one to selectively inject carriers propagating in the same direction and to probe pseudospin-polarized quasi-particles. In principle, the technique can be extended to tunneling de-vices in which surface states of topological insu-lators are used as electrodes; then, all-electrical injection of spin-polarized current (28) with non-invasive tunneling contacts could reveal a number of exciting phenomena (29–31).
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This work was supported by the European Union FP7 Graphene Flagship Project 604391, European Research Council Synergy Grant, Hetero2D, Engineering and Physical Sciences Research Council (EPSRC) (Toward Engineering Grand Challenges and Fellowship programs), the Royal Society, U.S. Army Research Office, U.S. Navy Research Office, and U.S. Air Force Office of Scientific Research. M.T.G acknowledges support from the Leverhulme Trust. A.M. acknowledges support of EPSRC Early Career Fellowship EP/N007131/1. S.V.M. was supported by NUST“MISiS” (grant K1-2015-046) and Russian Foundation for Basic Research (RFBR15-02-01221 and RFBR14-02-00792). Measurements in high magnetic field were supported by High Field Magnet Laboratory–Radboud University/Foundation for Fundamental Research on Matter (HFML-RU/FOM) and Laboratoire National des Champs Magnétiques Intenses–Centre National de la Recherche Scientifique (LNCMI-CNRS), members of the European Magnetic Field Laboratory (EMFL), and by EPSRC (UK) via its membership to the EMFL (grant no. EP/N01085X/1).
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/353/6299/575/suppl/DC1 Materials and Methods
Supplementary Text Figs. S1 to S9 References (32–49)
11 February 2016; accepted 13 July 2016 10.1126/science.aaf4621
ARCHAEOLOGY
Outburst flood at 1920 BCE supports
historicity of China’s Great Flood and
the Xia dynasty
Qinglong Wu,1,2,3*† Zhijun Zhao,2,13
Li Liu,4‡ Darryl E. Granger,5Hui Wang,6 David J. Cohen,7‡ Xiaohong Wu,1Maolin Ye,6Ofer Bar-Yosef,8Bin Lu,9Jin Zhang,10 Peizhen Zhang,3,14§ Daoyang Yuan,11Wuyun Qi,6Linhai Cai,12Shibiao Bai2,13
China’s historiographical traditions tell of the successful control of a Great Flood leading to the establishment of the Xia dynasty and the beginning of civilization. However, the historicity of the flood and Xia remain controversial. Here, we reconstruct an earthquake-induced landslide dam outburst flood on the Yellow River about 1920 BCE that ranks as one of the largest freshwater floods of the Holocene and could account for the Great Flood. This would place the beginning of Xia at ~1900 BCE, several centuries later than traditionally thought. This date coincides with the major transition from the Neolithic to Bronze Age in the Yellow River valley and supports hypotheses that the primary state-level society of the Erlitou culture is an archaeological manifestation of the Xia dynasty.
C
hina’s earliest historiographies, includ-ing Shujinclud-ing (Book of Documents) and Shiji (Records of the Grand Historian, by Sima Qian), tell of the Great Flood, a lengthy, devastating flood of the YellowRiver. The culture hero Yu eventually tamed this flood by dredging, earning him the divine mandate to establish the Xia dynasty, the first in Chinese history, and marking the beginning of Chinese civilization. Because these accounts laid
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the ideological foundations for the Confucian rulership system, they had been taken as truth for more than 2500 years until challenged by the “Doubting Antiquity School” in the 1920s. Within a decade, archaeological excavations demon-strated the historicity of the second dynasty, Shang, and the search for similar evidence for Xia began (1, 2). Archaeological fieldwork since the 1950s on the Early Bronze Age Erlitou culture (~1900 to 1500 BCE) has led many scholars to associate it with the Xia (1–6) because it overlaps with the spatial and temporal framework of the Xia dynasty. Traditionally, historians have dated the start of Xia to ~2200 BCE, whereas the government-sponsored Xia-Shang-Zhou Chro-nology Project adopted the date as 2070 BCE (5), leaving a chronological gap in associating Erlitou with Xia (7–9). Other scholars see Xia pure-ly as a myth fabricated to justify political succession (10, 11).
Scholars also have long sought a scientific explanation of the Great Flood (12–14), with even Lyell mentioning it (15), yet no evidence for it has been discovered. Here, we present geological evidence for a catastrophic flood in the early second millennium BCE and suggest that it may be the basis of the Great Flood, there-by lending support to the historicity of the Xia dynasty. The evidence found in our investigations along the Yellow River in Qinghai Province in-cludes remains of a landslide dam, dammed lake sediments (DLS) upstream, and outburst flood sediments (OFS) downstream (Fig. 1 and figs. S1 to S5) that allow us to reconstruct the size of the lake and flood (16).
Field observations (fig. S2B) show that the ancient landslide dam deposits reach an ele-vation of 240 m above present river level (arl) and stretch for 1300 m (fig. S2A) along Jishi Gorge (Figs. 1A and 3A). We estimate that the saddle of the dam would have been 30 to 55 m lower than the highest preserved remnants, so
the lake would have filled to an elevation of 185 to 210 arl [2000 to 2025 m above sea level (asl)] (fig. S2B), impounding 12 to 17 km3 of water (16) (table S1). Based on typical river discharge values, the dam would have com-pletely blocked the Yellow River for 6 to 9 months before overtopping (16). DLS distributed wide-ly upstream of the dam are up to 30 m thick and have a highest elevation of ~1890 m asl (Fig. 1B and figs. S1 and S3A). We interpret this as indicating that the catastrophic breach dropped the water level 110 to 135 m (Fig. 1B), releasing ~11.3 to 16 km3of water (16) (table S1), tens of times that estimated by a previous study (17). After the breach, DLS infilled a resid-ual lake behind the lowest part of the dam that remained.
Outburst flood sediments are found down-stream at elevations from 7 to 50 m arl in the lower Jishi Gorge and in Guanting Basin (Fig. 1 and figs. S1 and S4). They are characterized by high-concentration suspension deposition and consist exclusively of angular clasts of green-schist and purple-brown mudrock sourced from Jishi Gorge (table S2). At the mouth of the gorge, where the Yellow River enters Guanting Basin, the sediments reach 20 m thickness and include boulders up to 2 m in diameter (Fig. 1B and figs. S1 and S4, C and D). We also identified the OFS at the earthquake-destroyed prehistoric Lajia site (fig.
S5), a settlement of the Qijia culture (18, 19) known for its early noodle remains (20), 25 km down-stream from the dam. OFS at Lajia covered the settlement’s last Qijia culture occupation and filled in collapsed cave dwellings (fig. S5, A and B), pottery vessels (fig. S5B), and earthquake fis-sures (fig. S5C), mixing with pottery sherds (fig. S5D) and other Qijia cultural materials, with heights of up to 38 m arl.
Stratigraphic relationships of the OFS, rem-nant dam, DLS, loess, and other deposits in Jishi Gorge and neighboring basins, along with destruction features at the Lajia site (fig. S1), allow us to reconstruct and date a sequence of events ending in the outburst flood. First, they show that the damming and outburst flood event occurred during the archaeological Qijia cul-ture period (~2300 to 1500 BCE) after the collapse of the Lajia cave-houses. Ground fis-sures caused by the earthquake at the Lajia site were entirely filled with OFS (fig. S5C) before silts from surface runoff during the annual rains could enter them, indicating that the outbreak flood must have occurred less than 1 year after the earthquake and collapse of the houses. It is likely that the same earthquake that destroyed Lajia also triggered the landslide that dammed the river, along with widespread contempora-neous rock avalanches whose deposits lay directly beneath the DLS (fig. S3A).
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1School of Archaeology and Museology, Peking University, Beijing 100871, China.2School of Geography Science, Nanjing Normal University, Nanjing 210023, China.3State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China.4Department of East Asian Languages and Cultures, Stanford University, Stanford, CA 94305, USA.5Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA.6Institute of Archaeology, Chinese Academy of Social Sciences, Beijing 100710, China.7Department of Anthropology, National Taiwan University, Taipei 10617, Taiwan (R.O.C).8Department of Anthropology, Harvard University, Cambridge, MA 02138, USA.9CCTEG Xi’an Research Institute, Xi’an 710077, China. 10Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China.11Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou 730000, China.12Qinghai Provincial Institute of Cultural Relics and Archaeology, Xining 810007, China.13Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing, Jiangsu 210023, China.14School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China.
*Corresponding author. Email: wuqinglong@pku.edu.cn †Present address: School of Geography Science, Nanjing Normal University, Nanjing 210023, China.‡These authors contributed equally to this work. §Present address: School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China.
Fig. 1. Evidence of the exceptional outburst flood in the upper valley of the Yellow River. (A) Dis-tributions of OFS, DLS, and landslide dam. Light purple and dark green shaded areas indicate purple-brown mudrock and greenschist, respectively. Line AB across the Lajia site shows the location of the recon-structed cross section in fig. S6C. (B) The vertical distribution of the OFS, landslide dam, DLS, Lajia site and reconstructed lake levels relative to the longitudinal profile of the present Yellow River. DLS are clas-sified into lacustrine sediments (LS) and fan delta deposits (FD).
RESEARCH | R E P O R T S
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To date the outburst flood, we collected car-bon samples for accelerator mass spectrome-try (AMS)14C dating (16). Seventeen charcoal samples from the OFS and the only charcoal sample from a layer overlying the OFS (fig. S1) indicate that the age for the flood is between 2129 and 1770 cal. BCE [95% confidence in-terval (CI)] (Fig. 2A and table S5) (16). Char-coal samples from DLS upstream of the dam (fig. S1) yield calibrated 14C results (95% CI)
spanning 2020 to 1506 BCE (Fig. 2A and table S5), demonstrating that the DLS is coeval with or younger than the outburst flood and con-firming that it is fill from the remnant lake. The best dating for the flood comes from the Lajia site (16), because it was destroyed within 1 year before the outburst flood. Radiocar-bon determinations of Radiocar-bone samples from three human victims, aged 6 to 13 years old, in col-lapsed Lajia dwellings (Fig. 2B) agree to within uncertainty (Fig. 2A and table S5), consistent with that of two victims reported previously (21) as well. Because the radiocarbon calibration curve is linear in this region and the bones are the same age, we use the inverse variance weighted mean of the three measurements. This yields a calibrated age with a median of 1922 ± 28 BCE (1 SD) and a 95% CI of 1976 to 1882 BCE (Fig. 2C). To simplify this range, we use 1920 BCE to indicate the approximate date of the flood.
We estimate the peak discharge of the flood in two ways. Empirical formulas considering the volume of the lake and the height of the dam lead to estimates ranging from 0.08 to 0.51 × 106m3s−1, with large uncertainties (16)
(table S3). We also reconstruct the flood channel cross section from detailed surveys in Guanting Basin and use Manning’s equation to estimate a peak discharge of 0.36 to 0.48 × 106m3s−1(16) (fig. S6 and table S4), consistent with the dam break estimations (16) (table S3). The calculated peak discharge of ~0.4 × 106m3s−1is more than
500 times the average discharge of the Yellow River at Jishi Gorge. This ranks globally among the largest freshwater floods of the Holocene (22). We do not explicitly model the inundation and effect of this outburst flood in the lower reaches of the river, but analogous events dem-onstrate that outburst floods from landslide dams can propagate long distances. In 1967, an outburst flood with a volume of just ~0.64 km3 propagated at least 1000 km along the Yalong-Yangtze Rivers (23), so the Jishi prehistoric outburst flood, with a volume of ~11 to 16 km3, could have easily travelled more than 2000 km downstream. The Jishi flood would have breached the natural levees of the Yellow River, result-ing in rare, extensive floodresult-ing. It is possible that this outburst flood was also the cause of a major avulsion of the lower Yellow River (Fig. 3A) inferred from archaeological data, with a previously estimated date of ~2000 BCE (24, 25). Widespread destruction of levees and depo-sition of tributary mouth bars may have de-stabilized the main river channel, leading to repeated flooding until a new river channel was established. Extensive flooding on the low-er Yellow Rivlow-er plain would have had a great effect on societies there. We argue that this event and its aftermath likely would have
sur-vived in the collective memories of these so-cieties for generations, eventually becoming formalized in the received accounts of the Great Flood in the first millennium BCE. In fact, early texts such as the Shujing and Shiji even record that a place called Jishi (the same characters as the gorge where the outburst flood began) was where Yu began his dredging of the Yellow River; whether this is a coincidence will require further historical geographical research. The ~1920 BCE flood shares the main char-acteristics of the Great Flood described in an-cient texts. Apart from its huge peak discharge, the secondary flooding on the lower plains may have been long-lasting, just as the Great Flood remained uncontrolled for 22 years until it was managed by dredging (rather than by blocking breaches in natural levees). There is also the issue of whether the Great Flood could have been caused by exceptional me-teorological flooding, but a speleothem record shows a generally weakened Asian summer mon-soon from 8000 to 500 years before the present (26), and proxies from lake and loess records also indicate that a cool, dry climate regime begins 2000 BCE along the lower Yellow River (27), so this would be unlikely. Furthermore, the early textual records make no mention of frequent, extreme storms related to the Great Flood.
The discovery and reconstruction here of the massive outburst flood originating in Jishi Gorge provide scientific support that the an-cient Chinese textual accounts of the Great Flood may well be rooted in a historic natural event. They also shed light on the potential
Fig. 2. Radiocarbon chronology of the prehistoric outburst flood on the Yellow River. (A) Calibrated age probabilistic histograms of radiocarbon data. The outliers of the ages inconsistent with stratigraphic sequences and indicating reworking are denoted with asterisks. Samples best constraining the age of the outburst flood are boxed in red. See fig. S1 for sample locations. (B) The radiocarbon dated skeletons in cave dwelling F4 at the Lajia site. The skeletons were identified by reference (30). (C) The calibration of the inverse variance weighted mean for three bone samples on calibration curve IntCal13 (31). All radiocarbon dates were calibrated individually with IntCal13 (31) and OxCal 4.2 (32).
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historicity of the Xia dynasty itself, as Yu’s founding of the dynasty is directly tied to his achievements in controlling the Great Flood. According to the Shiji, Yu’s father labored unsuccessfully for 9 years to tame the flood before Yu took over for 13 more years. Yu’s success led to his mandate to become found-ing kfound-ing of the Xia 22 years after the flood started. If the Jishi Gorge outburst flood of ~1920 BCE is the natural cataclysm that came to be known as the Great Flood, then we can propose a new beginning date for the Xia dy-nasty, ~1900 BCE. This date, some 2 to 3 cen-turies later than previous reckonings (1, 2, 5), is compatible with the 1914 BCE date proposed by Nivison based on astro-historiographical evidence (28). This 1900 BCE date for the found-ing of the Xia coincides with the beginnfound-ing of the Erlitou culture (6), so this finding also supports the arguments that the Erlitou cul-ture is the archaeological manifestation of the Xia and that the Erlitou site was a Xia dynastic capital (1–3). This outburst flood is
also coincident with the major sociopolitical transition from Neolithic to Bronze Age in the Yellow River valley (2, 6, 29) (Fig. 3, A and B), suggesting that the concurrence of these major natural and sociopolitical events known through the geological, historiographical, and archaeological records may not simply be co-incidence but rather an illustration of a pro-found and complicated cultural response to an extreme natural disaster that connected many groups living along the Yellow River.
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oxcal/OxCal.html. AC K N OW L E D G M E N TS
This research was supported by the National Science Foundation of China (nos. 40801010 and 41271017), Fundamental Research Grant of China (no. F-IGCEA-0607-1-10), Chinese Geological Survey Project (no. 121201102000150009-16), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and National Science and Technology Support Program (no. 2013BAK08B01). P. Dong, H. P. Zhang, and W. Qin partly contributed to the fieldwork; X. Su and P. M. Zhang provided important financial support; Y. Chen and J. Liu-Zeng provided partial financial support; and J. K. Lu gave suggestions on the discharge estimation. Three anonymous reviewers provided valuable comments on the manuscript. We especially thank and commemorate H. B. Wang, who passed away in an accident in November 2011, for his passionate and laborious work in reconstruction of the cross section in June 2010. All data are available in the main manuscript and supplementary materials.
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Figs. S1 to S7 Tables S1 to S5 References (33–47)
20 December 2015; accepted 14 June 2016 10.1126/science.aaf0842
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Fig. 3. Major transition of archaeological cultures in the Yellow River valley around 1900 BCE. C, culture; LS C, Longshan culture. (A) Distribution of the late Neolithic and early Bronze Age cultures in the Yellow River valley. Blue dashed lines show avulsion of the lower Yellow River channel ~2000 BCE (24). (B) Timeline showing ages of the archaeological cultures (6, 29) and the proposed Great Flood of China.
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