Environmental Archaeology Of The Mid-Holocene Palaeofloods In The Jianghan Plain, Central China

The Jianghan Plain lies in the northern part of the middle Yangtze River area, Central China and is a region that has recently formed from the fluvial processes of the Yangtze River and the Hanshui River, as well as from the later tectonic subsidence and lacustrine processes. It is known for its well-developed Neolithic rain-fed agriculture and for its accommodation of many historical urban and administrative centers during the last4000years. Numerous Neolithic and Bronze Age ruins have been found in and around this plain of the mainstream and tributaries. In recent years, on the protective archaeological excavation of South-to-North Water Diversion Project, archaeological departments have discovered that palaeoflood deposits exist in many archaeological ruins in the Jianghan Plain. Among them, three archaeological sites of Zhongqiao in Shayang, Tanjialing and Sanfangwan in Shijiahe townsite of Tianmen constitute a complete chronological sequence of archaeological strata in the Jianghan Plain area since the late Neolithic Age. They are typical archaeological sites with catastrophic palaeofloods information and a wealth of dating material of the late Mid-Holocene, which possess ideal conditions for environmental archaeology of Holocene palaeoflood events in the Jianghan Plain.Based on the detailed analyses of three typical archaeological sites, multidisciplinary approach such as the archaeological stratum, AMS14C and OSL dating, pollen, micromorphology of zircon, grain size, magnetic susceptibility and geochemistry are used to research the late Mid-Holocene palaeofloods. Then, by the integrated study with a comparative analysis for characteristics of the modern flood sediments, quantity change and spatial-temporal distribution of archaeological sites, characteristics of stratigraphic accumulation and settlement changes in the study area, as well as the geographic location, chronological data, terrain elevation, depth of palaeoflood layers and the thickness of cultural layers in numerous archaeological sites of the Jianghan Plain during the Mid-Holocene, systematic environmental archaeology of the interactive response relationships between Mid-Holocene palaeofloods and evolution of human civilization in the Jianghan Plain are conducted to reveal the chronology, features of the flood process and environment background of Mid-Holocene palaeoflood events, and to examine their relations with monsoon rainfall changes of East Asia. The impacts of palaeoflood events on the Neolithic cultures and evolution process of human civilization are also clarified. 1) The5th,7th and10th natural layers of the Zhongqiao site are palaeoflood deposits, which represented three palaeoflood episodes. The9th early Shijiahe cultural layer of the Tanjialing site is an palaeoflood related lacustrine sediments generally after the weaker hydrodynamic transport process; the14th yellow gray soil layer in the bottom of Sanfangwan site is palaeoflood deposit, while the15th gray silt layer may also have experienced the impact of floods, especially the upper layer, but the13th cinerous mud layer shall be lacustrine sediments, which may have experienced a limnology environment with moderate flow rate and shallow water. The identifying features are as follows:①The palaeoflood sediments in the Zhongqiao and Sanfangwan site take on great similarities with modern flood sediments in the study area in frequency distribution curves and other grain size parameters;②Magnetic susceptibility values are lower than those of sediments from cultural layers;③There are some resemblance of the zircon shape characteristics between palaeoflood deposits and those of modern flood deposits, that shapes of zircon are mainly oval and nearly spheral, rounded due to long distance transport;④Rb/Sr contents are higher than those of sediments from cultural layers;⑤Cu contents are lower than those of sediments from cultural layers;⑥Compared with the site cultural layers, total amount of pollen in palaeoflood deposits is relatively less, often containing aquatic herbs and algae, and the contents of Pinus, Cupressaceae are higher due to long distance transport redeposition in new stacked body.2) According to the results of AMS14C, OSL and archaeological artifacts dating, combined with the cultural stratigraphic correlation of three typical archaeological sites in the Jianghan Plain, three palaeoflood episodes (i.e.4800～4597cal.a BP,4479～4367cal.a BP and4168～3850cal. a BP) occurred in the Zhongqiao site, while palaeofloods of4913～4600cal.a BP are recorded at the Sanfangwan site. From comparisons of typical palaeoflood deposit layers in numerous Neolithic sites, two Mid-Holocene extraordinary palaeoflood events occurred commonly over the Jianghan Plain area of the middle Yangtze River at approximately4900～4600cal.a BP (i.e. mid-late Qujialing cultural period) and4100～3800cal.a BP (i.e.from late Shijiahe cultural period to Xia Dynasty). From106radiocarbon data and12luminescence data dating from cultural layers in archaeological sites around the Jianghan Plain during7000～3000cal.a BP, above two palaeoflood episodes correlate well with two lowest frequency of site occurring numbers between5000～3000cal.a BP. This mutually reflected the conclusion of chronology of the palaeoflood events.3) Geomorphic and palaeo-hydrologic process analyses indicate that, the flood scale reflected by the palaeoflood layers in the Zhongqiao site should be at least above the moderate-large scale. These three palaeoflood layers may not be the main stream flood deposition directly; they should be diluvial sediments formed by mainstream of the Yangtze River flood water backwater into the ancient Yangshui channel and stacked to area of the site. The area of ancient Shijiahe townsite still belongs to the center position of the Luohansi Irrigated Area, Hanjiang River. Thus, there are close relationships between palaeoflood deposits at the bottom of the Sanfangwan site and the Hanjiang River floods during prehistoric era. High-resolution investigations of the cave speleothem and of the lake and subalpine peat sediments have documented prominent climatic fluctuations between5000and3000cal.a BP in Jianghan Plain and surrounding monsoonal regions of China, but, as a whole, reflecting the long drying trend. The main droughts, floods and cooling climatic events of the mid-Holocene have occurred at this time, especially between approximately5000-4500cal. a BP and4000cal. a BP, with high climate variability and obvious expansion of the Jianghan lakes. These two fluctuations were just associated with the above two identified palaeoflood events at approximately4900～4600cal.a BP and4100-3800cal.a BP, respectively. It was suggested that these two palaeoflood events have close relationships with the expansion of Jianghan lakes driven by the climatic change in this area.4) From the Qujialing cultural period to the Western Zhou Dynasty in and around the Jianghan Plain, elevation distribution of archaeological sites gradually decreased, but during this time there were two periods (i.e. late Qujialing cultural period, late Shijiahe cultural period to Xia Dynasty) that the sites elevation increased faster, which may indicate the rises of flood level. Therefore, in the Jianghan Plain and its surrounding area, changes of sites elevation, palaeofloods and lakes are consistent, that is, from early Qujialing cultural period to late Qujialing cultural period, and late Shijiahe cultural period to the Xia Dynasty, extensive flood events and lake expansion led to the increase of cultural sites’elevation. During the early Qujialing cultural period, the Jianghan Lakes begun expansion; due to suitable and stable climate, fewer floods occurred; as the stimulation of the development of rice agriculture production, human living places were mainly in lowland. The late Qujialing cultural period appeared an extensive flood event with lake expansion, and human habitation was limited to higher ground hillock and plain areas. From the late Qujialing cultural period to the middle Shijiahe cultural period, with the Jianghan Lakes shrinking, sites re-appeared in the lowland plains. After the late Shijiahe cultural period, extraordinary flood events occurred around4000a BP with lakes expanding again, human then move out from the lowland plains to the higher altitude areas. There were only ten sites in the Xia Dynasty all over the Hubei Province and all of them were located in areas with an altitude above50m, and this distribution trend fully demonstrated that this period must have witnessed significant catastrophic flood events, as a result, human beings affected by the flood could only survive in areas with an elevation above50m.5) The water level rising of lakes and intensive flood disasters in the Jianghan Plain during the late Shijiahe cultural period, and human settlements expanding into the low-lying plain during the early-middle Shijiahe cultural period, all these factors intensified the discrepancy between social development and environmental change processes (especially the hydrological process) in the late existence of Shijiahe cultural period might be the key controlling factor resulting in the collapse of Shijiahe Culture. The severe extraordinary floods related to the climatic anomaly at4000cal. a BP and political and military conflicts from internal or other cultural areas all accelerated the collapse of the Shijiahe Culture. This view provides an improved insight for the collapse of Shijiahe Culture that is different from the study results at the Chengtoushan site in Hunan Province.Prehistoric floods and their social impacts on the termination of Neolithic cultures and the rise of the state level society at approximately3500BC～1500BC (i.e. Chinese civilization with the early stages of development) have always been hot topics among the Chinese historians. However, most of the discussions were based on an ancient legend of Dayu Zhishui (Flood Mitigation by Great Yu) passed down from the last phase of the Neolithic Age. There is a lack of well-dated geologic and sedimentologic evidence for these floods in and around the Jianghan Plain area. This study not only increases our knowledge on rapid climate change at ca4000a BP, but also provides solid evidence for the prehistoric flood events in the Great Yu age, which is considered the turning point from the Neolithic Age to the Bronze Age in China.