Extraordinary Floods of 4100-4000 a BP recorded at the Late Neolithic Ruins in the Jinghe River Gorges, Middle Reach of the Yellow River, China

摘要

Holocene palaeohydrological investigations were carried out in the middle reach of the jinghe River that drains the central part of the Loess Plateau. A set of palaeoflood slackwater deposits was found interbedded in the Holocene loess-soil sequence within the cliff riverbanks. Both the sedimentary criteria and the analytical results show that this sediment was sourced from a suspended sediment load of floodwater. The major cultural layer of a late Neolithic settlement (4300-4000 a BP) occurring at the same site is blanketed by this flood deposit directly. Slopewash with pottery shards, charcoal and burnt earth occurs in between the palaeoflood slackwater deposit beds. It indicates that each of the slackwater deposit beds has recorded one individual flood event, and that these extraordinary floods occurred when the late Neolithic people occupied the riverbank terrace land. This pre-historical settlement was abandoned after the floods eventually due presumably to repeated inundation by overbank floodwater. The flood events were OSL dated to between 4100 and 4000 a BP and checked by archaeological dating of the anthropogenic remains retrieved from the sequences. Peak discharges of the floods were estimated to between 19,500 and 22,000 m(3) s(-1), which is several times gauged maximum floods. During the 4200-4000 a BP climatic event, severe droughts were documented in China's monsoonal regions and great floods were known from an ancient legend. This study provides a well-dated sediment record of pre-historical floods which occurred at the transition from the Neolithic to the Bronze Age in the Yellow River basin. It shows that extraordinary floods were part of the abrupt climatic variability during the 4200-4000 a BP climatic event. The highly variable, unstable and catastrophic climate typified by both droughts and floods resulted in settlement abandonment, and possibly the decline of the highly developed late Neolithic civilizations in China's monsoonal regions.