Holocene Climate Change And Flood Events Documented By Sedimentary Record In Sanyangzhuang Site, Henan Province, China

The alluvial plain in the lower reaches of the Yellow River provided a suitablegeographical environment for the formation and flourishing of Chinese nation and civilization.However, due to the frequent channel changes of the Yellow River, this area constantlysuffered from the outburst floods which brought serious disasters to the people who livedthere and hindered the development of the local civilization. Sanyangzhuang Site was close tothe channel of the Yellow River during the Han Dynasty, it was deeply buried by the YellowRiver’s alluvium. The strata in this area consist of several paleosol layers and palaeofloodsedimentary layers without obvious hiatus. The time scale of the sedimentary strata are fromthe Late Pleistocene to the present, so it provided a precious opportunity to investigate theprocess of environment change and the influences of the paleoflood events on Chinesecivilization in the lower reaches of the Yellow River.In this research, we evacuated a typical profile that was near the Sanyangzhuangarchaeological site, ten sedimentary layers included five paleosol layers and five palaeofloodsedimentary layers were identified, based on the results of pollen and charcoal analysis andAMS14C dating results, the process of climate-vegetation change and human activity arereconstructed, at the same time, the development of agricultural activity is revealed since theearly Holocene.Based on the dating results and the character of pollen assemblages, the process of theclimate-vegetation change in this area can be divided into three periods:10200~7200cal aB.P., the vegetation coverage was relatively low, the vegetation was coniferous andbroad-leaved mixed forest and grasslands landscape, the climate was warm and humid;7200~4500cal a B.P., the vegetation coverage increased, the pollen percentages andconcentrations of broad-leaved trees and aquatic plants increased significantly, the pollenassemblages indicated warmer climate and higher precipitation which was a character of theMegathermal period in the Holocene;4500~2000cal a B.P., the pollen percentages andappearing frequencies of some pollen types such as Pieca increased significantly, the pollenpercentages of Chenopodiacea and Nitraria increased obviously during the later stage, the pollen assemblage indicated that the climate had a trend of turning cold and aridity, but theclimate was still warm as a whole.The charcoal analysis results show that:10200~7200cal a B.P., the concentrations weregenerally low, which demonstrated the fire intensities and frequencies were both lower, thehuman activities were weak;7200~6700cal a B.P., the charcoal concentrations appeared apeak which may caused by a period of dry climate;6700～4500cal a B.P., the concentrationswere generally high and their changes were not obvious, this may reflected a warm and humidclimate, the abrupt increases of concentrations in later stage reflected the slash and burncultivation in this area;4500～2300cal a B.P., the concentrations were generally lower, butthe concentrations of large charcoal appeared two peaks which indicated the drought events;2300～2000cal a B.P., the concentrations were higher than the other periods, this mayindicate the surrounding area was inhabited by a large and prosperous community, the firewas widely used by ancient people in daily life at that time.The sedimentary characteristics of Sanyangzhuang profile indicate that the formedreasons of the five palaeoflood sedimentary layers were different. The I and the III of thepalaeoflood sedimentary layers were formed by large-scale floods that were caused by abruptclimate change of wet and cold; palaeoflood sedimentary layers II was formed by small-scaleflood sedimentations，as the higher vegetation coverage on the ground of the upper reaches, itwas not easy to form large-scale floods, although there were higher precipitations during theMegathermal period in the Holocene; palaeoflood sedimentary layer IV was formed byseveral seasonal floods, palaeoflood sedimentary layer V was formed by unconcentrated flowof floods that happened in Western Han Dynasty. In addition, the palaeoflood events wereclosely related to the evolution of Neolithic Culture (Peiligang culture-Yangshao culture), thetransition from Neolithic Culture to Chinese civilization, the distribution of ancient relics andthe social instability of Western Han Dynasty etc.Due to the differences of pollen source, grading function of river and human activity, it isclearly to discover that the arboreal pollen percentages in paleoflood sedimentary layers wereobviously higher than in paleosoil layers, and the disparity had been expanded under theeffects of serious deforestation and reclamation of intensive human activities since the middleand late Holocene. Therefore, the strata in this area can also be well distinguished through the pollen assemblages, but it still need more studies to identify whether the pollen assemblagescould be applied to classify the strata of other floodplain.The pollen records of Sanyangzhuang profile manifested that planted Poaceae pollenappeared in the early Holocene, which indicated that early agriculture may have existed in thisarea. The ancient farmland founded in middle and late Holocene is the direct evidence toprove ancient agricultural activity in this area. The ancient human had already used theslash-and-burn method for agricultural production; this judgment was supported by the highpollen percentages of herbs and Selaginella sinensis, and also by the high concentrations oflarge charcoal fragments. Later, according to the very distinctive marks of ridges and furrowsin the ancient farmland of the Warring States Period, it can be concluded that the ironimplements had already been widely used in this period. However, although Han Dynasty isone of the agriculturally-developed periods in ancient China, very few Planted Poaceaepollens were identified, so the further studies are needed to explain this phenomenon.