Process of Paleofloods in Guanting Basin, Qinghai Province, China And

Quaternary International 321 (2014) 88e96

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Process of paleoﬂoods in Guanting basin, Qinghai Province, China and possible relation to monsoon strength during the mid-Holocene

Minmin Ma a, Guanghui Dong a,*, Fahu Chen a, Xingmin Meng a, Zongli Wang a, Robert Elston b, Guoqiang Li a a Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Research School of Arid Environment and Climate Change, Lanzhou University, Lanzhou, Gansu Province, 730000, China b Department of Anthropology, University of Nevada, Reno, USA article info abstract

Article history: Prehistoric disasters including paleofloods and earthquakes in Guanting basin, Qinghai Province, China, Available online 30 May 2012 and their impact on Neolithic cultures have attracted academic attention over the last decade. However, the process and mechanism of paleofloods in the basin remain unsolved. This paper describes studies of the periodicity and process of paleofloods in Guanting basin during the mid-Holocene and their relations to monsoon strength, based on geological and archaeological investigations, including analysis of magnetic susceptibility and color reflectance, application of radiocarbon dating and comparison with the stalagmite oxygen isotope record in Dongge Cave. During the mid-Holocene, paleofloods in Guanting basin began between 7550 cal a BP and 6510 cal a BP, and lasted to 2220 cal a BP at least. Four cycles of frequent followed by infrequent paleofloods have been detected. Three periods of infrequent paleofloods occurred during 4380e3970 cal a BP, 2850e2720 cal a BP and 2310e2140 cal a BP respectively. The other one took place around 5000 cal a BP, coinciding well with periods of weak Asian monsoon events. The process of paleofloods in Guanting basin during the mid-Holocene was possibly dominated by the precipitation changes responding to Asian monsoon strength. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction links between flood hydrology and climate variability (e.g. (Ely, 1997; Knox, 2000; Benito, 2003; Yu et al., 2003; Howard et al., The recently increasing intensity and frequency of flood disas- 2004; Baker, 2008). It is suggested that the relation between ters in the valleys of major rivers around the world pose a great paleofloods and climate change is complicated and might vary in threat to the safety of humans and the development of economy dissimilar climatic zones (Benito, 2003; Benito et al., 2003; Macklin and society (Milly et al., 2002; Wilby et al., 2008; Botzen and Van and Lewin, 2003; Macklin et al., 2006; Zielhofer and Faust, 2008) Den Bergh, 2009; de Moel et al., 2009), and these events have and during different stages (Macklin and Lewin, 2003; attracted considerable international attention. In order to predict Thorndycraft and Benito, 2006; Macklin et al., 2010). Studies of the and mitigate the hazard of future floods, the periodicity and process of paleofloods and their relations to the Asian monsoon mechanism of current floods must be thoroughly understood in the during the mid-Holocene are valuable because the relationships context of global warming. However, historical gauge records between paleofloods and climatic oscillations in the Asian provide too brief a base for thorough study of the relation between monsoon region during the Holocene are poorly understood. climate change and floods, whereas paleoflood records can be used Mid-Holocene paleofloods on the Yellow River have been to explore the linkages between climate change and hydrological studied mainly in the river’s middle and lower reaches (e.g. Yang events in the long-term and at a large-scale. Since the 1970s, et al., 2000; Yang et al., 2003; Huang et al., 2010). However, the paleoflood hydrology has been applied successfully to extend flood relations between those paleofloods and the Asian monsoon have records in North America, Europe, Australia and Japan (Kochel and received little attention. Although recent work demonstrates the Baker, 1982; Ely et al., 1993; Baker, 2008), and used to evaluate the complex linkages between anomalous climatic circulation regimes and hydrologic responses in the middle reaches of the

* Corresponding author. Yellow River (Huang et al., 2007), these are far from being fully E-mail address: [email protected] (G. Dong). understood.

Fig. 1. Location maps of study region. (a) Location of the study region and Dongge cave in China. (b) Topographical location of the sampling archaeological sites in Guanting Basin (c) Locations of the sampling proﬁles.

The Guanting basin in Qinghai Province, China, is located in the bursting of a paleo-dammed lake at the Jishi Gorge (Fig. 1b) during upper reaches of the Yellow River and the area influenced by the the mid-Holocene resulted in the paleoflood events that destroyed Asian monsoon (Fig.1a). Paleoflood deposits were found at the Lajia the Lajia site, and that the driving forces influencing Guanting site (Yang et al., 2003), a very important settlement of the Qijia basin was not climate change but tectonic events (Wu et al., 2009). culture (2130e1635 BC) (IA et al., 2002; Xie, 2002). The evidence of This is plausible, but it does not cast light on the duration and paleoflood sediments has been intensively studied at this site. It has frequency of the several other paleoflood events in Guanting basin. been suggested that the Qijia cultural layer at the Lajia site is covered Cultural layers wedged within paleoflood sediments are present by paleoflood sediments (Yang et al., 2003) and the flood is believed in Guanting basin, and these can provide reliable dating materials to have destroyed the Lajia site and greatly affected the evolution of (such as charcoal) to limit the periods of infrequent paleofloods. In Neolithic cultures in Guanting basin (Yang et al., 2003; Yang et al., addition, the resolution of those ages or cultural periods can be at 2004; Yang et al., 2005; Wu et al., 2009; Zhang et al., 2009). multi-centennial scale, which can be correlated with climate events Otherwise, some researchers argue that Lajia site is destroyed by during the Holocene as recorded in stalagmites in the Asian gully debris flows from the surrounding hills (Tarasov and Wanger, monsoon region (Wang et al., 2005). Therefore, the relation 2006) and the brownish-red clay is deposited by gully debris flows. between paleoflood process and monsoon strength can be reliably However, more sediment evidence indicates the brownish-red clay examined in Guanting basin. For these reasons, the Lajia, Lamafen was deposited by the overbank floods in Guanting basin. and Erfang sites were investigated, where paleoflood sediments It has been proposed that paleofloods in Guanting basin clus- and cultural layers had been found, and collected reliable materials tered during 3650e2750 a BP, with 14 paleoflood events occurring for radiocarbon dating. In addition, a section that included rela- during that period (Yang et al., 2005). However, recent excavation at tively intact paleoflood sediments was sampled for determining the Lajia site has shown that the Qijia cultural layer is embedded in magnetic susceptibility and color reflectance. The results were the paleoflood sediments, indicating that onset of the paleoflood compared with the stalagmite oxygen isotope record from Dongge prior to the Qijia period (Ye, 2008). The duration and characteristics Cave (Wang et al., 2005) as a mean of determining the ages and of the paleofloods remain unresolved. The paleoflood mechanism in processes involved in paleofloods in this basin and their relations to Guanting basin is also unclear. Recently, it has been argued that the the monsoon strength during the mid-Holocene. 90 M. Ma et al. / Quaternary International 321 (2014) 88e96

2. Study region

The Guanting basin (35490e35540N, 102360e102560E) is located in Minhe county in Qinghai Province, China within the Asian monsoon-affected region and in the upper reaches of the Yellow River (Fig. 1a). The basin measures about 5 km from south to north and 12 km from west to east, and has an average elevation of 1800m asl (above sea level). The basin is surrounded by mountains that consist mainly of Cretaceous purple and red sandstone, with widely developed tablelands composed of red clay and loess on the piedmonts (Yang et al., 2003, 2005; Zhang et al., 2009). The mean annual temperature is 8e10 C with a mean annual precipitation of 400e500 mm. Precipitation in the basin occurs mainly in summer (July to September), accounting for 50%e80% of the annual precipitation. The Yellow River flows from west to east across the region, forms a series of deep and narrow gorges including the Jishi and Sigou gorges and small basins such as Guanting basin (Fig. 1b). At least five terraces and a fluvial plain have been developed by the action of the Yellow River (Yang et al., 2003). The second terrace is well-preserved, located mainly in the middle of the basin. It is about 1e2 km wide on the north bank and 1 km wide on the south bank of the river. Many Neolithic sites including the Lajia, Lamafen and Erfang and paleoflood sediments are found in the second terrace. In Guanting basin, rivers in gullies on the surrounding mountains have cut deeply into the terraces, creating steep valley walls that reveal the terrace strata. The second terrace is buried, con- sisting of an old fluvial unit and a younger overbank flooding fi sediment. The old unit is predominant of the terrace, resting on the Fig. 2. Detail photo of LJ pro le. Pliocene red clay. Younger overbank flooding sediments overlie the old fluvial unit that consists of sandy gravel and yellow silt. It is covered by paleoflood sediments, which lies at the top 4 cm of old composed of alternating brownish-red clay and grayish-black silty fluvial sediment (Fig. 3). clay deposited from the Yellow River floods (Yang et al., 2003; Yang The time of the ending of flooding can be inferred from the age et al., 2004; Yang et al., 2005; Zhang et al., 2009). of the cultural layer covered by the uppermost flood sediment, which closes to the LJ profile at the Lamafen site (Figs. 1 and 4). It is a 30 cm-thick cultural layer, containing a few shards and stone 3. Materials and methods artifacts (Fig. 4a). A charcoal sample and a bulk organic sample were obtained from the cultural layer for radiocarbon dating. 3.1. Stratigraphy and materials

At the front of the second terrace, a section that contains paleoflood sediments is found at the northwest of the Lamafen site, about 1 km from the Yellow River. The section is about 4 m thick, alternating brownish-red clay and a grayish-black silty clay stratum being wedged within it. This is the LJ profile (Fig. 1c), which can be divided into 3 units on the basis of its lithology, as shown in Fig. 2. Unit 1 (0e32 cm) is composed of sand, silt and sporadic, slightly rounded gravels, ferruginous slope wash and aeolian materials. Unit 2 (32e256 cm) comprises thicker brownish-red clay layers with several grayish-black silty clay horizontal beddings. Very few root and holes can be found in the brownish-red clay, which is compact in texture. In contrast, the grayish-black silty clay is not compact and contains relatively abundant root and holes. Unit 3 (256 cm-) consists of grayish-yellow silt, old fluvial sediments. In total, 140 samples for magnetic susceptibility and color reflectance laboratory analysis were collected at 2 cm intervals from the LJ profile. In order to determine the onset of the floods, a sample of bulk organic matter for radiocarbon dating was gathered at the top 5 cm of the old fluvial deposits at profile 1 near the LJ profile (Figs. 1c and 3). In addition, 2 samples of bulk organic matter were derived from profile 2 at the northwestern margin of the Lajia site (Fig. 1). One fl was sampled from an organic-rich pit at the base of the paleo ood Fig. 3. The sampling positions of profile 1 and 2. This figure is the map of profiles. sediments. The other was collected from an organic-rich pit that is Three samples were collected for radiocarbon dating. M. Ma et al. / Quaternary International 321 (2014) 88e96 91

Fig. 4. Detail photos of sampling cultural layers. (a) Cultural layer between paleofloods deposit, Lamafen site; (b) Cultural layer between paleofloods deposit, Erfang site; (c) Majiayao ash pit containing brownish-red clay, Lajia site (d) Paleofloods deposit above the house ruins, Lajia site.

In order to obtain the duration of the flooding interval, six Furthermore, 11 14C dates from the Qijia culture and one 14C date for dating samples were collected from cultural layers wedged in the the Majiayao culture at the Lajia site have already been published flood sediments at the Lajia and Erfang sites respectively (Figs. 1 (14CLASTERC, IA, CASS, 2003, 2005). The details of the radiocarbon and 4). Another 60 cm thick cultural layer embedded within the assays are shown in Table 1. flood sediments was found at the Erfang site (Fig. 4b), which contains abundant pottery fragments, stone artifacts and animal 3.2. Methods bones regarded as relics of the Xindian culture (1400e700 BC). Two charcoal samples and a bulk organic sample were collected from The conventional radiocarbon dates were measured with a Wallac the cultural layer for radiocarbon dating. At the Lajia site, an ash pit Quantulus 1220 low-background liquid scintillation counter in the (Fig. 4c) was found, containing several pottery shards, abundant MOE Key Laboratory of West China’s Environmental System in charcoal and brownish-red clay. Most of the shards are fragments of Lanzhou University. The AMS 14C samples were prepared with the Majiayao red or grey cord-marked pottery. A charcoal sample was standard pre-treatment (acid-alkali-acid) in the MOE Key Laboratory collected from the ash pit for conventional radiocarbon dating. and then measured at Peking University. The Libby Half-Life of 5568

Table 1 Calibrated radiocarbon dates.

Laboratory code Material Traditional Mean calibrated Cultural site Source age/14CaBP age cal a BP(14) LUG06-122 TOC 5702 114 6512 114 Lamafen Measured LUG06-123 TOC 6678 64 7550 50 Lajia in this text LUG08-104 TOC 8041 86 8899 172 Lajia LUG08-106 TOC 2188 54 2224 84 Lamafen LUG09-25 TOC 2662 58 2794 51 Erfang LUG10-58 Charcoal 4408 55 5041 171 Lajia LAMS09-06 Charcoal 2185 30 2225 80 Lamafen LAMS09-29 Charcoal 2630 39 2757 19 Erfang LAMS09-30 Charcoal 2614 42 2746 23 Erfang

ZK-3132 Charcoal 3574 73 3852 124 Lajia 14CLASTERC, IA, ZK-3133 Charcoal 3685 42 4010 77 Lajia CASS (2003) ZK-3134 Charcoal 3637 75 3970 112 Lajia ZK-3137 Charcoal 4200 107 4715 138 Lajia

ZK-3179 Charcoal 3746 48 4106 115 Lajia 14CLASTERC, IA, ZK-3180 Charcoal 3740 42 4072 80 Lajia CASS (2005) ZK-3181 Charcoal 3828 43 4247 97 Lajia ZK-3182 Charcoal 3846 43 4270 114 Lajia ZK-3220 Charcoal 3684 41 4010 76 Lajia ZK-3221 Charcoal 3764 38 4120 107 Lajia ZK-3222 Charcoal 3800 56 4187 98 Lajia ZK-3223 Charcoal 3778 66 4121 126 Lajia 92 M. Ma et al. / Quaternary International 321 (2014) 88e96 years is used for all dates, and all are calibrated using the Calib5.01 to show an opposite correlation with redness and lightness, which program (Ramsey, 2001) with the IntCal04 (Reimer et al., 2004)curve. have good correlation with each other. All samples from the profile All ages reported are relative to AD 1950 (referred to as ‘Cal a BP’). have positive a* values, varying from 8.27 to 12.72. Among the three Bulk magnetic susceptibility is determined using a mass of 7 g of units, unit 2 is characterized by higher susceptibility values, ground sediment and a Bartington MS2 magnetic susceptibility moderate L* and the largest fluctuating a* values, which range from meter (0.47/4.7 kHz) in the MOE Key Laboratory of West China’s 13.80 to 54.03, from 54.69 to 62.83 and from 8.27 to 12.49 Environmental System in Lanzhou University. Color reflectance is respectively. Unit 3 is characterized by higher L*, lower a* and determined in the Color Analytical Laboratory in the Institute of moderate susceptibility values, respectively varying from 59.06 to Geology and Geophysics, Chinese Academy of Sciences in Beijing. 61.57, from 8.38 to 10.63 and from 20.09 to 35.11. Unit 1 has the Samples (2e3 g) are first dried at 40 C for 24 h and then crushed. lowest susceptibility, higher a* and moderate L*, respectively Finally, color reflectance is measured using a handheld Minolta- changing from 9.24 to 17.11, from 10.93 to 12.72 and 52.19 to 59.25. CM2002 spectrophotometer. The color reflectance values of all In addition, the nine subunits of the paleoflood sediments unit samples are given by the spherical L*a*b* color space (Robertson, are also clearly expressed in the magnetic susceptibility, redness and 1977). Red-green chromaticity a*(þa* is the red direction, a* is the lightness curves (Fig. 5; Table 2), as suggested by five valleys and four green direction) and L* (the range from 0 to 100 represents the peaks from the susceptibility curve correlated with four valleys and variations in lightness from black to white). five peaks from the color reflectance curves respectively.

4. Results 5. Discussion

4.1. Chronology 5.1. Geomorphic character of paleoﬂood sediments

In total, 20 calibrated 14C data were obtained, as shown in The evidence of paleoflood sediments has been intensively Table 1. The calibrated 14C ages of the top 5 cm of old fluvial studied at the Lajia site and in Guanting basin. Brownish-red clay deposits in profile 1, the organic-rich pits at the base of paleoflood covers on the Lajia site surface and contains wave-shaped sand sediments and the top 4 cm of old fluvial sediments in profile 2 sediments (Yang et al., 2003). This structure shows that the clay (Fig. 3) are 8899 172 cal a BP, 6512 114 cal a BP and sediments are related to running water. Besides, grain-size analysis 7550 50 cal a BP respectively. An age for the ash pit (Fig. 4c) of the clay samples reveals that the suspended fraction consists of containing brownish-red clay at the Lajia site is 5041 171 cal a BP. more than 90% of the volume, which is characteristic of flood Two calibrated dates for the cultural layer wedged within the flood deposits (Yang et al., 2003). sediments at the Lamafen site (Fig. 4a) are 2224 84 cal a BP and As shown in Fig. 3, there is a clear interface between old fluvial 2225 80 cal a BP. In addition, three dates are obtained for the sediment unit and flood sediment deposited at the second terrace in cultural layer within the flood sediments at the Erfang site (Fig. 4b); the Guanting basin. On the surface of the old fluvial sediment unit, these are 2794 51 cal a BP, 2757 19 cal a BP and 2746 23 cal geomorphic features sculpted by running water can be found, a BP respectively. The calibrated 14C data of 12 published 14C ages including sand-ripples and dragging structures. The brownish-red for the Lajia site are clustered between 4715 138 cal a BP and clay layer is widely distributed on the surface of the second 3852 124 cal a BP. terrace, and shows a low wedge-shaped layer adjacent to the channel. The layer is highest at the channel margins and tapers 4.2. Magnetic susceptibility and color reflectance of the LJ profile gradually towards the adjacent floodbasin (Yang et al., 2003; Yang et al., 2004; Yang et al., 2005). The structure geomorphic character The susceptibility, redness (a*) and lightness (L*) records of LJ of the sediment suggests that the brownish-red clay was deposited profile are displayed in Fig. 5 and Table 2. The susceptibility appears by overbank floods of the Yellow River at times of high discharge.

Fig. 5. Magnetic susceptibility and color reflection curves and calibrated 14C dates of LJ profile and its comparison to d18O values from Dongge Cave stalagmite (Wang et al., 2005). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) M. Ma et al. / Quaternary International 321 (2014) 88e96 93

Table 2 Indicators of the end of paleofloods are the cultural layers The values of susceptibility, redness and lightness in the paleoflood unit of LJ Profile. beneath the top of paleoflood sediments. Both cultural layers at the 8 3 1 fl Depth/cm Data style Xlf/10 m kg a* L* Lamafen and Erfang sites (Fig. 4a, b) are blanketed by paleo ood 36e34 average value 15.93 12.17 59.18 sediments (brownish-red clay), indicating that flooding must end range 15.78e16.09 12.12e12.23 58.74e59.62 later than that of the accumulation of cultural layers. The youngest 48e38 average value 30.75 11.55 57.52 age of those cultural layers is 2224 84 cal a BP (Table 1), indicating range 21.07e40.23 11.14e12.12 56.32e59.72 fl e that the paleo oods in Guanting basin lasted to 2220 cal a BP at 70 50 average value 21.17 12.02 57.48 fl range 13.8e32.18 11.33e12.49 56.02e58.96 least. Thus, it can be concluded that the termination of paleo oods 86e72 average value 36.93 10.27 55.91 here must be after 2220 cal a BP. range 26.62e48.48 9.74e11.18 54.69e58.01 112e88 average value 19.83 11.92 60.63 5.3. Cycles of flooding episodes and infrequent flooding intervals range 17.03e23.4 11.24e12.4 58.79e61.98 128e114 average value 27.32 10.91 60.44 range 19.85e32.07 10.42e11.72 59.43e61.53 Although paleofloods in Guanting basin persisted from 6510 cal 178e130 average value 20.14 11.77 60.92 a BP to 2220 cal a BP at least, several stable stages can be detected range 16.83e26.65 11.39e12.32 58.83e62.14 by the presence of greyish-black clay (Fig. 2) and cultural layers e 196 180 average value 37.77 9.60 58.61 embedded in brownish-red paleoflood sediments (Fig. 4a and b). range 26.1e54.03 8.27e10.91 55.27e60.76 256e198 average value 22.58 10.69 61.52 The alternation of brownish-red clay and black silty clay layers in range 17.83e30.71 9.42e11.49 60.09e62.83 the LJ profile (Fig. 2) has been discerned, suggesting the cycles of flooding episodes (when high-frequent paleofloods occurred) and infrequent flooding intervals (when low-frequent paleofloods occurred). It is inferred that brownish-red clay was deposited from The brownish-red clay of the region was probably originated the Yellow River during flooding episodes (Yang et al., 2004). The from surrounding hills and/or bedrock (e.g. Pliocene red clay) and floodplain landscape was relatively stable during intervals of then deposited by overbank floodwater of the Yellow River. The infrequent flooding. Aeolian sediments were deposited and trans- Pliocene red clay has a wide distribution in the upper reaches of the formed to the black silty clay by pedogenic process or paludification Yellow River in the northeastern margin of the Tibetan Plateau (Wu during these periods. At the same time, prehistoric humans occu- et al., 2009). The Pliocene red clay could be weathered and trans- pied the region, as indicated by the cultural layers wedged within ported into the Yellow River channel by gully/tributary flows. flood sediments. Several sand-lenses are embedded between the old fluvial Magnetic susceptibility and color reflectance can be used to sediments and the brownish-red clay, which consist of coarse sand, distinguish between the sediments deposited during flooding medium-coarse sand, and isolated gravels, with poor sorting and episodes and infrequent flooding intervals, as magnetic minerals sphericity. This indicates gully debris flows took place in Guanting will increase during the pedogenic process (Zhan and Xie, 2001; basin. Nevertheless, these gully debris flows sediments deposit at Shi, 2007). Magnetic minerals are sparse in the brownish-red clay some limited locations while the brownish-red clay almost overlies but higher in the black silty clay. There is a color contrast between the entire second terrace surface in Guanting basin and other the black silty clay and the brownish-red clay, the values of redness basins in the upper reaches (Yang et al., 2003). Moreover, the (a*) and lightness (L*) being relatively higher in the latter. Four sediments of debris flows are characterized by coarse sand, gran- obvious low values of redness (a*) and lightness (L*) and four high ules and isolated pebbles, which are sorted badly. In contrast, flood values of magnetic susceptibility in the paleoflood sediments deposits consist of fine-grained sediments such as silt or silty clay within the LJ profile (256e32 cm) indicate four infrequent flooding with horizontal bedding. The brownish-red clay with character of intervals during the mid-Holocene (Fig. 5). horizontal bedding originated from suspended sediments and Human occupation usually took place when the natural threat of deposited during overbank flooding periods. Parts of the brownish- flooding was rare. The occurrence of cultural layers wedged within red clay were transported into the Yellow River by gully/tributary paleoflood sediments indicates infrequent flooding intervals. Four flows. Therefore, the brownish-red clay was mainly deposited by cultural layers are found wedged within paleoflood sediments on overbank floods in Guanting basin during the mid-Holocene. the second terrace, implies at least four infrequent flooding intervals during the mid-Holocene (Fig. 5). These infrequent flooding 5.2. The beginning and termination of paleofloods in Guanting intervals coincide with cultural layers wedged within paleoflood basin during the mid-Holocene sediments at other sites (Xia et al., 2004; Dogan, 2005). The ages of cultural layers within brownish-red clay at the Yang et al., (2005) speculated that the bottom of LJ profile is Lamafen and Erfang sites (2310e2140 cal a BP and 2850e2720 cal 3650 a BP according to the ages of cultural layers embedded in the a BP respectively) determine two periods of infrequent flooding. brownish-red clay layers. Profile 1 and 2 (Fig. 3) displayed here Another infrequent flooding interval occurred during Qijia period, provide more precise chronological evidence about the beginning for the Lajia site was destroyed by paleofloods and earthquakes of paleoflood sediments in Guanting basin. during Qijia period (Yang et al., 2003), and Qijia house ruins are The organic-rich pit at the bottom of the paleoflood sediments in covered by paleoflood deposits (Fig. 4d). During the excavations at profile 2 probably formed during a short stable period between two the Lajia site, the Qijia cultural layer was also found to lie on paleoflood events, when plants grew and organic matter accumu- brownish-red clay that was suggested to be paleoflood deposits (Ye, lated. Its age should be younger than that of the earliest paleoflood in 2008). Ten of the eleven calibrated 14C dates from the Qijia cultural Guanting basin. Both the top of old fluvial sediments in profile 1 and layer at the Lajia site range from 4380 to 3970 cal a BP (Table 1). The the organic-rich pit that lies at the top of old fluvial sediments in youngest of these (3852 124 cal a BP) clearly deviates from the profile 2 are covered by paleoflood sediments, the ages of which must others and thus may be questionable, which might be affected by be older than that of the earliest paleofloods in Guanting basin due to the process of sampling or measurement. Archaeological investi- the surface erosion. Based on the radiocarbon results, the earliest gations show that relics of the Majiayao culture (3980-e2050 BC) paleoflood episode in Guanting basin during the Holocene occurred are present at the Lajia site (Bureau of National Cultural Relics, between 7550 cal a BP and 6510 cal a BP. 1996; Xie, 2002; Yang et al., 2004), suggesting the other 94 M. Ma et al. / Quaternary International 321 (2014) 88e96 infrequent interval occurred during Majiayao period. A Majiayao monsoon. Taking a broader view, the other infrequent flooding cultural pit was found at the Lajia site, which contains several interval should also show synchrony with periods of weak pottery shards, abundant charcoal and brownish-red clay. It monsoonal conditions. The oldest infrequent flooding interval is suggests that paleoflood occurred at the same time as the pit clustered into 5040e4720 cal a BP. Asian monsoon strength obvi- formed. The pit was dated to 5041 171 cal a BP. Another calibrated ously weakened during the period, as indicated by the oxygen 14C age of Majiayao cultural relics in the Lajia site is 4715 138 cal isotope records of the stalagmites in Asian monsoon area (Wang aBP(Table 1), suggesting the other infrequent flooding interval et al., 2005; Hu et al., 2008; Cai et al., 2010). Other records also occurred during 5040e4720 cal a BP. suggest a cold and dry climate in northwestern China during that period (Zhu et al., 2001; Zhou et al., 2010; Dong et al., 2012). 5.4. The relations between paleofloods and monsoon strength To sum up, the cyclical paleoflood episodes probably are caused by the precipitation changes responding to Asian monsoon The ages of three infrequent flooding intervals in Guanting basin strength. Infrequent flooding intervals correspond to the weak closely meet those of decline stages in strength of the Asian Asian monsoon events. Dryer climate during the mid-Holocene monsoon as recorded by d18O of stalagmites in Dongge cave (Fig. 5). leads to relatively stable and non-flooding environment in Guant- Among the three established infrequent intervals, the longest ing basin. (4380e3970 cal a BP) occurring during Qijia period correlates with the weak Asian monsoon event between 4.5 and 4.0 ka (Wang et al., 2005). Strongly enhanced aridity around 4.0 ka is universal in 5.5. Process of paleofloods in Guanting basin and possible relations records from north China and several regions of the world (Cullen to monsoon strength during the mid-Holocene et al., 2000; Gasse, 2000; Wu and Liu, 2004; An et al., 2005; Arz et al., 2006; Staubwasser and Weiss, 2006; Zhou et al., 2010), The second terrace in Guanting basin was formed by the incision a fact commonly interpreted as hastening the collapse of Neolithic of the Yellow River at around 10 ka BP and then covered by over- cultures in central China (Wu and Liu, 2004; Wang et al., 2005) and bank sediments (Yang et al., 2004). Overbank sediments were other regions in Asia (Weiss et al., 1993; Overpeck and Webb, 2000; conserved due to the general lowering of the water level. Once the deMenocal, 2001). In Guanting basin, however, the weak Asian terrace is formed, the higher surface became inactive. However, monsoon event might have induced decline in both precipitation when the water level raises high enough to cover the terrace and flood frequency, and likely have provided favorable conditions surface during extreme flood events, overbank sediments are for Neolithic farmers during the Qijia period near the Yellow River. deposited. Slope failures would increase the height of the river bed The Lajia site is a very important site of the Qijia culture. It was here and make the water level rise. that the largest Yu Qing (a sacrificial vessel usually used by chiefs) Yin et al. (2010) found that paleolandslides on the second in the Yellow River valley and the earliest recorded preparation of terraces of the Yellow River occurred frequently around 5 ka (OSL noodles were found (Lu et al., 2005), indicating a higher social and dating) in the upper reaches of Yellow River. This study suggests productive level at the Lajia site during the Qijia period. Weak Asian that a great number of materials had been transported into Yellow monsoon events very likely promoted the blossoming of the Lajia River at around 5 ka. In this case, the Yellow River bed accumulated site during the Qijia period by providing a relatively longer stable and increased to a higher level, along with the water level. The lowland landscape, fertile soil and ample water resources. Although terrace surface became active again. In addition, the Guanting basin the frequency and magnitude of paleofloods were probably low is near two narrow gorges (Fig. 1b), this would make the water of during 4380e3970 cal a BP, floods might still have threatened the Yellow River in the basin easily reach its maximum level. settlement at the Lajia site, which in turn might have forced ancient Therefore, when heavy rainfall happened, the brownish-red clay communities to refine their means of defense against flooding and originating from surrounding mountains and/or bedrock was thus accelerated differentiation within society. transported by gully/tributary flows into the Yellow River. When The infrequent flooding interval during 2850 to 2720 cal a BP is high discharge occurred, overbank flooding takes place and prob- also synchronous with a weak Asian monsoon event around 2.7 ka ably deposits sediment (brownish-red clay) on the top of the (Wang et al., 2005). Climate was cold and dry during this period in terrace. northwestern China (Chen et al., 2001; Hong et al., 2003; Wang Recently, it was suggested that paleoflooding in Guanting basin et al., 2003; Feng et al., 2006; Zhou et al., 2010). Plenty of Xindian was not induced by climate change, but by the outburst of a dam- cultural relics, including fragments of pottery and animal bones, med lake in the Jishi Gorge (in the upper reaches of the Yellow River have been found in the cultural layer wedged within the brownish- about 25 km from Guanting basin, Fig. 1b) (Wu et al., 2009). red clay at the Erfang site (Fig. 4b). Xindian sites are much less However, the work reported here does not support this view. frequent than Qijia sites on the second terrace of the Yellow River in Previous study in this region indicates that the outburst of the Guanting basin (Bureau of National Cultural Relics, 1996), which paleo-dammed lake probably occurred between 8.0 and 9.0 ka suggests that the infrequent interval was probably shorter during based on the radiocarbon dating of its sediments (Peng et al., 1997). the Xindian period compared to the Qijia period. The paleo-lake might have burst later than 8.0e9.0 ka due to the Another infrequent flooding interval occurred from 2310 to input of old sediments by the Yellow River, and probably did 2140 cal a BP, which is not closely synchronous with a weak Asian cause the first paleofloods in Guanting basin during the mid- monsoon event. However, a sharp fluctuation of d18O is obvious Holocene, raising the bed of the Yellow River in the basin and around 2.3 ka, although the duration is very short. This weak silting-up the channel of the river at the Sigou gorge (Fig. 1b). monsoon event is also represented by the redness of Tianchi Lake in Subsequently, paleofloods in Guanting basin would have been west Loess Plateau (Zhou et al., 2010). Paleofloods in Guanting basin enhanced as the Asian monsoon strengthened and precipitation also responded sensitively to rapid fluctuation of the Asian increased. Considering that precipitation in Guanting basin is monsoon. concentrated in July to September, the high discharge and sediment The high coherence between infrequent flooding intervals in load of the Yellow River and its gullies would have blocked up the Guanting basin and weak Asian monsoon events indicates that the channel at Sigou gorge during flooding, resulting in overbank cyclical paleoflood events in Guanting basin during the mid- flooding of the Yellow River in Guanting basin. However, the lake- Holocene were probably dominated by the strength of the Asian burst hypothesis needs to be tested by further work. M. Ma et al. / Quaternary International 321 (2014) 88e96 95

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