Palaeoenvironmental implications of the prehistorical catastrophes in relation to the Lajia Ruins within the Guanting Basin along the Upper Yellow River,China

Abstract

Archaeological excavations have exposed a shocking picture of the prehistorical catastrophes in the Lajia Ruins in the upper Yellow River. The grouped skeletons resting on the dwelling floors show a vivid scene of the mortal struggle of human being during catastrophes. Geological records of the major disasters in relation to the devastation of this prehistorical settlement were investigated by detailed field observations, sedimentological analysis and optically stimulated luminescence (OSL) and ¹⁴C dating. The results show that the prehistorical community of the Qijia Culture migrated into the Guanting Basin at c. 4200 yr BP. They chose the loess-blanketed riverbanks to build their settlement and cultivate millet crops. At c. 3950 yr BP, immediately followed by a major earthquake, the settlement was overtaken by immense mudflows coming along the tributary gullies from the hillsides behind. The enormous mudflows suddenly buried and destroyed the dwellings and killed the women and children at their homes. The source of the catastrophic mudflows was traced upslope to the gully heads behind the ruins, where the palaeoearthquake together with the rainstorms and flash floods induced enormous mass wasting on the hillsides. Referring to the causations of the catastrophic debris flows and landslides in the region, it is inferred that the soil erosion, mass wasting and accumulation of debris on the hillsides were intensified largely by human disturbance of the landscape by bush clearance from 6000 yr BP to 3950 yr BP. During the climate event of 4200–4000 yr BP, enhanced human activities and over-exploitation of natural resources increased the vulnerability of the communities to detrimental environmental change and catastrophe. This means that the prehistorical catastrophic mudflows were created partly by the early settlers themselves. These results are of important implications in understanding the prehistorical environmental change in the environmentally sensitive zones over the world.

Keywords

China earthquake human impact Lajia Ruins mudflow prehistory

Introduction

Humanity with greatly expanded power brings about both beneficial and detrimental changes in the Earth’s surface system. Many of these changes, as the result of previous human activities and over-exploitation of natural resources, may lead to the much-feared impacts on environment resulting in disastrous consequences to human societies. Both human activity and the environment system are always in flux. Humans have altered the environment, and environmental change has revised human activities. Questions are how far back into the past humans have been a major force in environmental transformation, how significant human impacts were on environment and how soon their feedbacks were felt. To answer these questions, interdisciplinary investigations are needed into the temporal–spatial change in specific communities and their environment. These allow diverse evidence to be integrated at a landscape scale and enable the changes to be traced back through time accurately (Crumley, 2000; Messerli et al., 2000; Redman, 2000).

The arid and semi-arid mountainous regions in the upper Yellow River basin around the east boundary of the Tibetan Plateau are particularly sensitive to environmental changes induced by both climatic change and human activities. For instance, enormous mudflows and debris flows in combination with loess landslides that occurred during storm rains on 7 July 1965 caused huge damages to Tianshui City, Gansu Province, and killed 179 people. These disasters were attributed to terracing the loess slopes, road-cutting and excavation engineering (Wu, 2003). Massive loess landslides that occurred on 7 March 1983 destroyed 4 villages and killed 277 people in Dongxiang County, Gansu Province. It was attributed to artificially damming the gullies and irrigating the terraced loess land over the hillsides that changed the surface and groundwater flows in the past (Han, 1986). A more recent case is the extraordinary flood-debris flows that occurred during storm rains on 8 August 2010 that claimed the lives of 1760 people and caused enormous damages to Zhouqü Town of Gansu Province. This was attributed to deforestation and land reclamation that resulted in intensified soil erosion, mass wasting and accumulation of debris in the headwaters during the past decades (Yan et al., 2012).

The loess-blanketed valleys along the tributary and the mainstream of the upper Yellow River have been occupied and cultivated by the Neolithic settled communities since 6000 yr BP. A great number of prehistorical settlements, including the well-known Majiayao Culture (6000–4200 yr BP) of the Neolithic, the Qijia Culture (4200–3600 yr BP) at the turn from the Neolithic to the Bronze Age and the Xindian Culture (3400–2600 yr BP) of the Bronze Age, were identified over the valleys between Xining and Lanzhou cities in the upper Yellow River basin (Xie, 2002). An interdisciplinary investigation of the history of environment change and human impacts in this region may shed lights on understanding the role of humans as an agent of large-scale, long-term and catastrophic environmental change that may threaten the human societies.

In the Guanting Basin, along the upper Yellow River, late Neolithic ruins (4200–3950 yr BP) of the Qijia Culture, named the Lajia Ruins after the adjacent village, was identified on the riverbanks (Figures 1 and 2). Archaeological excavations have revealed the remains of many prehistorical dwellings containing human skeletons, pottery, stone and jade artefacts and animal bones in the ruins (Gao et al., 2007b; Ye, 2002, 2004). A pottery bowl containing millet noodles unearthed from the ruins was claimed to be the earliest prepared noodles known over the world (Lu et al., 2005). The unearthed human skeletons rest on the dwelling floors with various strange postures in the Lajia Ruins (Figure 3). These manifested a startling scene reminding scientists that some prehistorical catastrophes occurred in the Guanting Basin that overtook this settlement, destroyed the dwellings and killed the women and children at their homes suddenly. The Lajia Ruins have attracted attention from various fields, including archaeology, anthropology, geology and seismology around the world. It was therefore claimed to be one of the China’s Top Ten Archaeological Discoveries of 2001 (China Internet News, 2009). However, it is still unclear what kinds of disasters occurred in the Guanting Basin, which devastated the prehistorical settlement of the Qijia Culture at Lajia.

This paper presents the results of our detailed investigations in the Lajia Ruins on the riverbanks of the Yellow River within the Guanting Basin. We focus on examining the geomorphology and stratigraphy, the loess accumulation and soil development in relation to climate change, the surface processes and landscape change in connection with the early human disturbance, and the geological evidence in relation to the catastrophes in the view of environmental archaeologists. It may provide new insights into the prehistorical environmental change and major disasters forced by human factors, and into the shifts of culture and collapse of the ancient civilizations in the environmentally sensitive zones over the world.

Geographic and geological settings

The Guanting Basin is c. 53 km² in area occurring along the upper Yellow River and situated in the arid zone at the border between Gansu and Qinghai provinces (Figure 1). In view of the topographic features, this basin occurs at the northeast foot of the Tibetan Plateau and the southwest border the Loess Plateau. The mean annual temperature is 8–9°C, mean annual rainfall is 250–300 mm and mean annual evaporation is 2000–2100 mm. The Yellow River passes through the sinuous Jishixia Gorges and emerges onto the Guanting Basin with a mean annual discharge of 690 m³/s and mean annual runoff volume 21.8 × 10⁹ m³. The largest flood discharge was 4410 m³/s as gauged in September 1981. The most recent large flood discharge was 3860 m³/s as gauged in July 2012.

The fluvial plain of the Yellow River within the Guanting Basin consists of several aeolian loess-blanketed river terraces distributing at the elevations of 1760–1860 m a.s.l. The thick loess-soil cover forms flat and fertile land on the second and third terrace (Figure 2). The basin is very deep in prehistory, although it is situated in a remote region isolated by high mountains and deep gorges. The basin has been occupied and cultivated extensively by the settled communities since the Neolithic. As many as 50 prehistorical ruins, belonging to the Majiayao Culture (6000–4200 yr BP), the Qijia Culture (4200–3600 yr BP) and the Xindian Culture (3400–2600 yr BP), have been discovered in the Guanting Basin till date (Yang et al., 2004).

The upper slopes on both sides of the Yellow River consist of the unconsolidated red clay (RC) formation of the Tertiary and Cretaceous Period with an incontinuous coverage of aeolian loess. Present vegetation consisting of short shrubs and grasses is very sparse under the dry environment. The exposed RC formation on the hillsides has resulted in a bare and colourful landscape of reddish hue under the dry environment. Thus, several places of the upper land hills surrounding the basin were named the Red Terrace, the Red Rocks, the Great Red Hills and so on.

Accelerated erosion in connection with human activities has resulted in intensified soil loss and mass wasting on the surrounding hillsides. Seasonal streams and dry gullies from the upland hills have incised deeply into the loess-blanketed terraces within the basin. During rainstorms in summer, flash floods, mass wasting, mudflows and debris flows occur frequently in the headwaters of these tributary gullies (Figure 2). Enormous alluvial fans have formed at the exits of the gullies onto the plain and also at the confluences of the gullies joining the Yellow River.

The Lajia Ruins, named after the adjacent village, is situated in the front part of the second terrace that is c. 30–35 m above the Yellow River water-table (Figure 4a). This loess-blanketed terrace land is dissected by several dry gullies or seasonal streams that originated from the hillsides. Three gullies pass by the Lajia Ruins and join the Yellow River. Intensified erosion and mass wasting have resulted in enormous scars in the gully heads on the hillsides in the north behind the Lajia Ruins (Figure 2a and b). The surface runoff and the sediment load, including the mudflows and debris, flow from the Great Red Hills drain into the Yellow River all the way through the areas of the Lajia Ruins.

The Lajia Ruins was identified as a major settlement of the Qijia Culture and ¹⁴C dated to 4200–3950 yr BP (Zhang et al., 2003, 2005, 2009). There were 12 other settlements of the Qijia Culture distributed within the Guanting Basin that were contemporaneous with the settlement at Lajia (Yang et al., 2004).

Study materials and methods

Our field investigations were carried through along the upper reaches of the Yellow River in 2011–2012. Detailed geomorphological, pedological, sedimentological and stratigraphic observations were carried out over the Lajia Ruins within the Guanting Basin. Fresh and complete sediment profiles without being disturbed by past human activities were found along the cliffy gully walls adjacent to the Lajia Ruins (Figure 4). Pedo-stratigraphic subdivisions and descriptions in the LJR profile are present in Table 1. Soil and sediment samples for sedimentological analysis and for optically stimulated luminescence (OSL) dating were taken from the LJR profile near the Lajia Ruins. Samples were also taken from the GRH site at the foot of the Great Red Hills where the Gang’gou gulley emerges onto the Guanting Basin. Samples of the flood slackwater deposit of the Yellow River were also taken at the JSX site in the Jishixia Gorges in the upper stream. Magnetic susceptibility was measured on a mass of 10 g of ground sediment with a Bartington MS2 magnetic susceptibility meter (0.47/4.7 kHz). Particle-size distribution of the samples was determined using a Mastersizer-S laser analyzer, with (NaPO₃)₆ employed as a dispersing agent after pretreatment with H₂O₂ (10%) to destroy the organic matter, and with HCl (10%) for eliminating the carbonates. Chemical analysis was carried out on a Panalytical PW2403 X-ray Fluorescence Spectrometer.

Table 1.

Pedological and stratigraphic descriptions of the soil and sediments in the loess-soil profile in the Lajia Ruins at the LJR site in the front part of the second terrace of the upper Yellow River, China.

Depth (age)	Stratigraphic subdivisions	Pedo-stratigraphic descriptions
40–0 cm (1500–0 yr BP)	Modern soil (MS)	Orange (7.5YR6/6), silt, granular structure, porous, friable, some earthworm burrows and excrement, abundant well-rounded spherical pellets, some plant roots, 30–50 cm in thickness. Intensively ploughed modern soil.
80–40 cm (3100–1500 yr BP)	Recent loess (L₀)	Dull orange (7.5YR7/4), silt, massive-blocky structure, some bio-pores, friable, few earthworm burrows and excrement, some well-rounded spherical pellets, 30–50 cm in thickness. Accumulated during the late Holocene.
120–80 cm (3950–3100 yr BP)	Palaeosol (S_0-upper)	Bright reddish brown (5YR5/6), silt, granular-blocky structure, porous, relatively firm, medium abundance of earthworm burrows and excrement. Abundant well-rounded spherical pellets, some secondary calcite crystallite (pseudomycelia) in the pores and crevices, 30–80 cm in thickness. Well-developed Ustic Isohumisol (Chernozem) formed during the late mid-Holocene.
250–120 cm (3950–3950 yr BP)	Red clay (RC)	Bright reddish brown (2.5YR5/8) clay with rolling, wavy and conglomerated structure, very firm, with some stones, earth clods and human remains, including pottery shards, burnt earth, ash and charcoal, enwrapped, 100–300 cm in thickness. Deposited by mudflows coming along the gullies from the hillsides consisting of the unconsolidated Tertiary red clay formation.
330–250 cm (8500–3950 yr BP)	Palaeosol (S_0-lower)	Dull reddish brown (5YR5/3) silt, blocky structure, porous, relatively firm, medium abundance of earthworm burrows and excrement, abundant secondary calcite crystallite (pseudomycelia) in the pores and crevices looking frosted, 70–90 cm in thickness. Well-developed Ustic Isohumisol (Chernozem) formed during the early mid-Holocene and the surface representing the ground occupied by the people of the Qijia Culture (4250–3950 yr BP), on which human skeletons, pottery vessels, charcoal, burnt earth and house floors were retrieved.
420–330 cm (11,500–8500 yr BP)	Transitional loess (L_t)	Dull orange (7.5YR7/3) silt, massive-blocky structure, some bio-pores, friable, few vertical earthworm burrows, 100–130 cm in thickness, accumulated aeolian dust with weak weathering during the early Holocene.
1000–420 cm (35,000–11,500 yr BP)	Malan loess (L_1-1)	Yellow orange (10YR7/4) silt, massive structure, some bio-pores, very friable, calcareous rich, 600–1000 cm in thickness, accumulated aeolian dust during the last glacial (MIS-2).
1500–1000 cm (>35,000 yr BP)	Fluvial deposits (T_2-al)	Bright yellow orange (10YR7/6) coarse sand, loose, with lenticular beddings, raised flood plain deposit overlying on the well-rounded gravels of the raised river bed deposit, forming the second terrace of the Yellow River.

Initial chronological framework was established in the LJR profile by pedo-stratigraphic correlation with the well-dated loess-soil profiles over the Loess Plateau (Huang et al., 2000, 2004, 2006, 2007). The anthropogenic remains from the cultural layers in the Lajia Ruins were closely studied and ¹⁴C dated on charcoal by archaeologists. Calibrated ¹⁴C dates were cited from the published laboratory reports Nos. 29, 31 and 35 of Archaeological Institute of Chinese Academy of Social Science (Zhang et al., 2003, 2005, 2009). The aeolian loess-soil stratum was dated by OSL method. OSL dating on quartz grains was carried out by using the double single-aliquot regenerative-dose (SAR) protocol (Banerjee et al., 2001; Murray and Wintle, 2000). All measurements were performed on a Risø-TL/OSL-DA20 dating system equipped with a combined blue (470 nm, 50 mW/cm²) and infrared (875 nm, 150 mW/cm²) light-emitting diode (LED) unit, and a ⁹⁰Sr/⁹⁰Y beta source for irradiation (Bøtter-Jensen and Duller, 1992). The content of U, Th and K in the samples was determined with an inductively coupled plasma mass spectrometry (ICP-MS). Effective dose rate (Dy) was calculated from the elemental concentrations by using the revised dose-rate conversion factors (Adamiec and Aitken, 1998). Water content of the fresh samples was measured after drying at 105°C for 12 h. The computer program Age.exe was used for calculation of OSL dates (Grün, 2003).

Results and interpretations

In the arid and semi-arid zones, the intensity of weathering and pedogenic modification to sediments can be assessed by magnetic susceptibility. The values of magnetic susceptibility in the studied sediments are very low in general (Table 2). The aeolian loess (L_1-1) in the LJR profile has a susceptibility value of 28.7 × 10⁻⁸ m³/kg comparable exactly to that of the aeolian loess distributing in the adjacent areas (Zhao et al., 2006). The mid-Holocene Chernozem soil (S_0-lower and S_0-upper) in the LJR profile has higher values of 58.7 × 10⁻⁸ and 63.5 × 10⁻⁸ m³/kg. Susceptibility values are 18.8 × 10⁻⁸ m³/kg in the conglomerated RC in the LJR profile, 14.3 × 10⁻⁸ m³/kg in the gully deposit at the GRH site and 12.5 × 10⁻⁸ m³/kg in the flood slackwater deposit of the Yellow River at the JSX site. This implies that they are fresh deposits without being modified by the environment in situ.

Table 2.

Magnetic susceptibility and particle-size distribution in the sediment samples from the Lajia Ruins in the Guanting Basin along the upper Yellow River.

Site	Sample	Magnetic susceptibility (× 10⁻⁸ m³/kg)	<2 µm (%)	2–16 µm (%)	16–63 µm (%)	63–125 µm (%)	>125 µm (%)	Md (µm)	Mz (µm)
LJR	Soil (S_0-upper)	63.5	21.80	43.72	27.77	6.67	0	7.94	15.83
	Conglomerated RC	18.9	28.46	56.64	14.89	0.01	0	4.70	6.97
	Soil (S_0-lower)	58.7	17.25	40.31	40.19	2.25	0	11.69	17.37
	Aeolian loess (L_1-1)	28.7	12.28	32.25	46.97	8.16	0.34	19.76	24.16
GRH	Gully head deposit	14.3	28.07	57.39	14.54	0.00	0	4.87	7.00
JSX	Yellow River flood SWD	12.5	31.18	61.30	7.52	0.00	0	3.68	5.02

RC: red clay; SWD: slackwater deposits.

Particle-size distribution enables a more precise identification of the texture and classification of the sediments. The sediment samples are well differentiated on the curves of particle-size distribution frequency (Figure 5). The aeolian loess (L_1-1) in the LJR profile presents a single peak at 35–40 µm in the fraction of coarse silt. The Chernozem soil (S_0-lower and S_0-upper) in the LJR profile presents a major peak at 30–35 µm in the fraction of coarse silt and a secondary peak at 5–10 µm in the fraction of fine silt. Both the conglomerated RC in the LJR profile and the gully deposit at the GRH site present a peak at 5–10 µm. The flood slackwater deposit of the Yellow River manifests a peak at 3–6 µm to the much finer side.

Also as showing in Table 2, the aeolian loess (L_1-1) in the LJR profile is dominated by coarse silt (46.97%) with the graphic medium (Md) at 19.76 µm and mean (Mz) at 24.16 µm. It is thus defined as coarse silt. The Chernozem soil (S_0-lower and S_0-upper) in the LJR profile is dominated by fine silt (43.72% and 40.31%) with the graphic medium (Md) at 7.94 and 11.69 µm and mean (Mz) at 15.83 and 17.37 µm. It is thus defined as silt. Both the conglomerated RC in the LJR profile and the gully deposit at the GRH site are dominated by fine silt (56.64% and 57.39%) with the graphic medium (Md) at 4.70 and 4.87 µm and mean (Mz) at 6.97 and 7.00 µm. They are classified as clayey silt. The flood slackwater deposit of the Yellow River at the JSX site is dominated by fine silt (61.30%) with much smaller graphic medium (Md) at 3.68 µm and mean (Mz) at 5.02 µm. It is classified as clayey fine silt.

The source of sediments and the intensity of weathering and pedogenic modification to sediments can also be assessed by their geo-chemical features. Therefore, soils and sediments can be clearly differentiated in a pedo-stratigraphic profile according to the geo-chemical concentrations. As shown in Table 3, contents of the major components such as SiO₂, Al₂O₃, Fe₂O₃, MgO, MnO and K₂O and the trace components such as N, P, Rb, Sr, Cu and Ni in the aeolian loess ((L_1-1) are similar to those in the Chernozem soil (S_0-lower and S_0-upper) in the LJR profile because of their aeolian origin. Concentrations of these elements (except for P and Sr) in the conglomerated RC in the LJR profile are higher and much similar to those in the gully deposit at the GRH site. The flood slackwater deposit of the Yellow River at the JSX site is therefore differentiated from the conglomerated RC in the LJR profile and the gully deposit at the GRH site by the higher contents of Fe₂O₃, MnO, K₂O, P, Rb, Sr, Cu and Ni and lower contents of SiO₂, Al₂O₃, MgO and N in it. As shown by the element ratio of the sample/upper continental crust (UCC) in Figure 6a, the conglomerated RC in the LJR profile is differentiated from the flood slackwater deposit of the Yellow River at the JSX site, but similar to the gully deposit at the GRH site at the foot of the Great Red Hills behind the Lajia Ruins.

Table 3.

Concentration of elements (g/kg) in the sediment samples from the Lajia Ruins in the Guanting Basin along the upper Yellow River.

Site	Sample	SiO₂	Al₂O₃	Fe₂O₃	MgO	MnO	K₂O	N	P	Rb	Sr	Cu	Ni
LJR	Soil (S_0-upper)	516.1	124.5	49.90	24.5	0.846	24.4	12.350	0.855	0.098	0.200	0.027	0.030
	Conglomerated RC	504.7	145.4	60.80	30.2	0.853	25.4	15.742	0.452	0.117	0.178	0.030	0.037
	Soil (S_0-lower)	507.2	115.8	47.70	23.3	0.801	23.9	11.437	0.764	0.099	0.280	0.024	0.031
	Aeolian loess (L_1-1)	519.5	112.3	41.90	22.6	0.733	22.6	11.234	0.620	0.092	0.281	0.020	0.027
GRH	Gully head deposit	511.5	147.7	60.60	30.0	0.829	25.1	15.716	0.402	0.117	0.150	0.030	0.037
JSX	Yellow River flood SWD	466.8	140.6	73.70	28.2	1.153	27.2	9.676	0.627	0.133	0.249	0.035	0.042

RC: red clay; SWD: slackwater deposits.

The analytical results have further confirmed our pedo-stratigraphic descriptions on the sediments in the LJR profile. It shows that the conglomerated RC in the LJR profile is different from the flood slackwater deposit of the Yellow River, but similar to the gully deposit at the GRH site at the foot of the Great Red Hills. These suggest that the conglomerated RC enwrapping the human bodies and the entire Lajia Ruins was not deposited by Yellow River floods, but deposited by flash floods and mudflows from the gully heads behind the Lajia Ruins, where the hills consisted of the unconsolidated RC formation of the Tertiary.

Discussion and conclusion

Pedo-stratigraphy and chronology in the Lajia Ruins

The landscape and the soil and sediment over the Lajia Ruins have been modified or disturbed intensively by human activities in the past. The primary pedo-stratigraphy was observed along the cliffy gully walls surrounding the Lajia Ruins during our field investigations. Detailed pedo-stratigraphic subdivisions in the LJR profile are present in Figure 7. Detailed descriptions of the sediments and soils in the LJR profile are present in Table 1. Both the soils and pedo-stratigraphic subdivisions were identified on the basis of contrasting characteristics, such as colour, texture, structure, bioturbation and so on.

In the study LJR profile, the dual structured fluvial beds of large well-rounded gravels and well-sorted loose sand at the bottom were identified as the deposits consisting second terrace of the Yellow River. The underlying RC formation of the Tertiary forms the basement of the second terrace of the Yellow River (Figure 4a). A blanket of aeolian loess was identified in the depth range of 10.0–4.2 m in the profile. It is OSL dated to 32,050–11,500 yr BP (Figure 5) and correlates well to the Malan Loess (L_1-1) that is distributed widely over the Loess Plateau (Huang et al., 2000, 2004). It has been deposited by dust storms and dust falls under a dry–cold environment during the late last glacial (MIS-2) without being modified by pedogenesis. This means that the second terrace of the Yellow River has formed since 35,000 yr BP, and thus, aeolian dust could accumulate permanently on its surface without being affected by the river water flows thereafter.

The boundary between the Holocene loess-soil sequence and the Malan Loess (L_1-1) was observed at a depth of 4.2 m in the LJR profile (Figure 7, Table 1). This boundary is one of the most important pedo-stratigraphic marking-lines in the Chinese loess-palaeosol sequences indicating the end of the last glacial and the start of the Holocene at c. 11,500 yr BP (Huang et al., 2000, 2004). The transitional loess (L_t) of the early Holocene was identified in the depth range of 4.2–3.3 m in the profile. It was only slightly affected by weathering and pedogenesis during the early Holocene because climate was still dry during 11,500–8500 yr BP over the Loess Plateau. A well-developed Chernozem (Ustic Isohumisol) was identified in the depth range of 3.3–1.2 cm in the profile. This soil (S₀) developed during the mid-Holocene Climatic Optimum, and it is widely distributed over the Loess Plateau. It was dated integratively to 8500−3100 yr BP by using the ¹⁴C, OSL and archaeological dating methods (Huang et al., 2000, 2004). The recent loess (L0) and the modern soil (MS) were identified in the depth range of 120–0 cm in the LJR profile. They have accumulated since 3100 yr BP during the late Holocene (Figure 7, Table 1). This chronological framework is confirmed by the OSL dates and anthropogenic remnants retrieved from the profiles over the Lajia Ruins and the surrounding areas.

A conglomerated RC with a rolling surface and wavy structure is inserted into the soil (S₀) of the mid-Holocene all over the area of the Lajia Ruins (Figure 7, Table 1). This colourful red band is very contrasting to the dull yellowish landscape on the riverbanks, but much similar to the unconsolidated RC formation of the Tertiary on the bare hillsides. This conglomerated RC bed has the Chernozem soil (S₀) split into two sublayers (i.e. S_0-lower and S_0-upper) in the LJR profile. The Lajia Ruins of the Qijia Culture was identified right beneath this RC (Figures 4b and 7). The human remains were ¹⁴C dated on charcoal to 4200–3950 yr BP (Figure 6b). The human skeletons and artefacts are tightly wrapped by this conglomerated RC on the dwelling floors in the Lajia Ruins (Figure 3). These indicate that the prehistorical people of the Qijia Culture moved into the Guanting Basin at 4200 yr BP. They built their settlement in the front part of the second terrace where three tributary gullies join the Yellow River. They cultivated millet crops on the fertile Chernozem soil over the riverbank loess lands. This community disappeared, and the settlement was buried and devastated by the conglomerated RC at 3950 yr BP suddenly.

Geological records of a major earthquake that devastated the settlement as the first hitting

The Qijia Culture occurred in the valleys in the upper Yellow River basin during 4200–3600 yr BP at the turn from the Neolithic to the Bronze Age (Xie, 2002). The Lajia Ruins in the Guanting Basin, as one of the most important settlements of the Qijia Culture, have been well studied (Ye, 2002, 2004, 2008). The settlement was dated to 4200–3950 yr BP by using ¹⁴C dating method on a large group of charcoal samples collected from the cultural layers of the Lajia Ruins during archaeological excavations (Figure 6b). The conditions of the human remains and the related geological evidences suggest that the Guanting Basin suffered major catastrophes, and the prehistorical settlement at Lajia was devastated at 3950 yr BP.

Archaeological excavations have revealed that the floors and walls of the dwellings in the Lajia Ruins were badly damaged by ground fissures passing through this prehistorical settlement. These widespread ground fissures, with a width varying in a range of 20–60 cm, had the riverbank loess ground broken. They were filled in with the conglomerated RC in which various human remains, including pottery shards, burnt earth, ash and charcoal, as well as stones and earth clods have been entrapped (Figures 4 and 8).

Our field investigations showed that ground fissures are distributed widely in the west part of the Guanting Basin. They extend at three main directions, including 45°NW, 15°NW and 45°NE, over the second terrace of the Yellow River. The loess-soil strata underneath the conglomerated RC have been broken badly by these ground fissures (Figure 8). Therefore, landslips and landslides occur everywhere along the Gang’gou and Lüjiagou gullies adjacent to the Lajia Ruins. The ground fissures and the infillings, that is, the conglomerated RC with the enwrapped human remains, could be observed clearly on the cliffy gully walls (Figure 8).

These ground fissures were identified as the major evidence of a palaeoearthquake at a magnitude of 7.0 on the Richter scale. The earthquake occurred along the major faults at the foot of the Lajishan and the Jishishan Mountains, that is, the west boundary of the Guanting Basin (Ye, 2008). These suggest that the prehistorical settlement of the Qijia Culture represented by the Lajia Ruins was overtaken by catastrophic major earthquake at c. 3950 yr BP as the first hitting.

Sediment records of mudflow burying that devastated the settlement as the second hitting

In the newly exposed sediment profiles along the cliffy gully walls surrounding the Lajia Ruins, the colourful band of the conglomerated RC is very distinctive over the dull yellowish landscape (Figures 4, 7 and 8). The palaeoearthquake fissures beneath are all filled in with this conglomerated RC (Figure 8). Archaeological excavations indicate that the human skeletons, pottery vessels and the bowled noodles on the dwelling floors in the Lajia Ruins have been enwrapped by this conglomerated RC tightly (Figure 3).

Our field investigations indicate that this unique clay layer distributes at the confluence where the gullies join the Yellow River and where the Lajia Ruins is situated (Figure 2). It is c. 1.0–4.0 m in thickness and is inserted into the Chernozem soil (S₀) of the mid-Holocene. It has a rolling surface and wavy structure, with stones, loess-soil clods, pottery shards, burnt earth, charcoal and ash from the palaeoground entrapped inside. In view of sedimentological features, this clay layer is classified as the deposit of dense mudflows. During our fieldwork, the source of this red mudflow was traced upslope towards the north to the gully heads on the hillsides behind the Lajia Ruins, where the unconsolidated Tertiary RC formation has been intensively eroded by storm rains, flash floods and mass wasting (Figure 2).

The physico-chemical analysis shows that this conglomerated RC is similar to the gully deposit at the GRH site at the foot of the Great Red Hills. It is differentiated from the slackwater deposit of the Yellow River at the JSX site and the aeolian loess-soil in the LJR profile (Figure 5, Tables 2 and 3). This suggests that this conglomerated RC was deposited by mudflows from the hillsides through the dry gullies that pass by the Lajia Ruins. It seems that storm rains accompanying the palaeoearthquake induced enormous mass wasting and immense mudflows in the gully heads behind the Lajia Ruins. The women and children in the prehistorical settlement were killed suddenly in their dwellings by the catastrophic mudflow wrapping and burying, which followed an earthquake. The postures of the grouped skeletons indicate that these people could not run out of their dwellings and women were protecting their children when the houses were being filled with mudflows immediately after the earthquake. The shocking pictures imply that these people might be buried alive by the deep and dense mudflows. Therefore, the grouped skeletons show a vivid scene of the mortal struggle of human beings in the thick mudflows. A lot of the pottery vessels and the bowled millet noodles have thus been preserved by mudflow packing at the original places in the settlement.

Implications of the early human impact and the feedbacks

The Guanting Basin formed at the northeast foot of the Tibetan Plateau is isolated by the high mountains and deep gorges. Earthquakes have occurred over the region frequently in the past. The upland slopes surrounding the basin consist of the unconsolidated RC formation of the Tertiary and Cretaceous Period. As situated in the arid region, the environment is very vulnerable to climate change and human impact. The bare hillsides are heavily eroded with enormous scars formed by mass wasting, including landslide, landslip, debris flow and mudflow in the headwaters of the dry gullies (Figure 2). Archaeological investigations have documented over 50 prehistorical ruins within this small basin of c. 53 km² in area (Yang et al., 2004). There were 12 other settlements of the Qijia Culture existing contemporaneously with the one at Lajia (Figure 2a). It shows that the sedentary communities had occupied this isolated small basin for nearly 2000 years prior to the devastation of the riverbank settlement at Lajia at c. 3950 yr BP. And, it is also noticeable that the late Neolithic communities of the Qijia Culture in the upper reaches of the Yellow River developed during a global climatic event during 4200–4000 yr BP (Huang et al., 2010, 2011; Wu and Liu, 2004). These settlers had to respond to the challenges of the deteriorated environment by more intensified disturbance on landscape in order to subsist in this marginal area. The greatly intensified human activities brought pressure on landscape and accelerated the surface processes, especially mass wasting, over the hillsides. The settlement of the Qijia Culture at Lajia was devastated finally by the burying due to the catastrophic mudflows, which followed a major earthquake and storm rains. This case indicates that during the climate event of 4200–4000 yr BP, enhanced human activities and over-exploitation of natural resources increased the vulnerability of the communities to detrimental environmental change and catastrophe.

Referring to the causation of the catastrophic debris flows and landslides in the adjacent regions, it is inferable that early settlers had disturbed the landscape by bush clearance for land reclamation within the basin, and construction wood and firewood cutting over the surrounding hillsides. Intensified erosion and mass wasting resulted in accumulation of large amount of debris in the headwaters of the dry gullies and seasonal streams. This must have played an important role in the incidence of catastrophic mudflows that followed the earthquake and storm rains. This means that the prehistorical catastrophic mudflows were created partly by the early settlers themselves. Early human impact undermined the subsistence base which the settlers in the Guanting Basin had worked hard for over 2000 years to establish.

Environmental archaeological investigations in Jordan documented early farming successes followed by local disasters in Mesopotamia as early as 6500 bc because of the early human impact (Rollefson and Kohler-Rollefson, 1992). The case documented in the Lajia Ruins indicates that the Neolithic farmers successfully introduced millet cultivation into the Guanting Basin along the upper Yellow River as early as 6000 yr BP. The formation of so many Neolithic settlements in a small basin as such suggests that land reclamation for arable farming led to an increase in food productivity and also an increase in population at the same time. However, their detrimental impact on environment undermined their sustainability and resulted in serious consequence. The riverbank settlement at Lajia and the surrounding land had to be abandoned eventually. Until 550 years later when the rigid mudflow bed was blanketed by a layer of soft loess/soil accumulated by dust storms and dust falls, the riverbanks at the confluence of the dry gullies were re-occupied by a different community of the Xindian Culture (3400–2700 yr BP).

By re-evaluating the literature on environmental change in the Ethiopian highlands, it has been recognized that there was a shift to more arid conditions and more soil erosion around 5000–4800 yr BP and that many phenomena that were previously interpreted as climate-driven might be of anthropogenic origin (Nyssen et al., 2004). At about 4000 yr BP in China’s prehistory, collapse of the well-developed late Neolithic civilizations, such as the Qijia Culture in the upper and the Longshan Culture in the lower Yellow River basin, the Hongshan Culture in the Liaohe River basin and the Shijiahe Culture in the middle and the Liangzhu Culture in the lower Yangtze River basin, was attributed to the climatic event of 4200–4000 yr BP (Gao et al., 2007a; Wu and Liu, 2004). Similarly, the collapse of the Old Kingdom in Egypt, the Akkadian Empire in Mesopotamia and Indus valley civilization in India was also attributed to the climatic event of 4200–4000 yr BP (Cullen et al., 2000; DeMenocal, 2001; Drysdale et al., 2006; Peiser, 1998; Stanley et al., 2003; Staubwasser et al., 2003). The case documented in the Lajia Ruins reminds us to re-appraise the anthropogenic factors apart from climatic change that was concluded as the driving force responsible for environmental deterioration that undermined the sustainability of these prehistorical societies.

Thus, the results documented from the Lajia Ruins in the upper Yellow River basin provide important implications in understanding the prehistorical environmental change and cultural shifts or collapse of ancient civilizations in the environmentally sensitive zones over the world.

Footnotes

Funding

Grants from the National Science Foundation of China (No. 41030637, 41271108) and the PhD Programs Foundation of Chinese Ministry of Education (No. 20110202130002) supported this research.

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