龙山文化末期泾河特大洪水事件光释光测年研究

OSL DATING OF THE JINGHE RIVER PALAEOFLOOD EVENTS IN THE LATE PERIOD OF THE LONGSHAN CULTURE

通过对泾河流域深入的野外考察,在其中游基岩峡谷内发现了含有龙山文化末期文化层和古洪水滞流沉积单元的全新世黄土土壤剖面。利用光释光的单片测年技术,确定古洪水滞流沉积层覆盖着的黑垆土层的OSL年龄为4078±382aB.P.和4111±450aB.P.。结合其中所含龙山文化遗址的考古年代,揭示出泾河流域在4100～4000aB.P.之间出现一个古洪水多发时期。这组古洪水滞流沉积单元包含5个单层,记录了4100～4000aB.P.之间泾河流域曾经发生的5次特大古洪水事件。这个洪水期对应着我国北方在4000aB.P.前后,由全新世大暖期向着全新世晚期干旱期转折过程中的气候剧烈变化。这些史前洪水事件可能对我国龙山文化的衰落和古代华夏文明的诞生具有重大的影响。

In tributary valleys of the Yellow River over the Loess Plateau,it has been found that slackwater deposits due to past overbank floods were alternated with loess and soil formation throughout the Holocene. These bedded palaeoflood deposits without subsequent erosion and destruction recorded past hydrological events. Their age can be indirectly obtained by optically stimulated luminescence（OSL） dating of their intercalated loess and soil. During our fieldwork,such palaeoflood slackwater deposits were identified in a loess-soil sequence of the Jinghe River middle reaches. Loess and soil samples taken from this sequence were studied with polymineral fine grain OSL dating based on a single aliquot regenerative-dose （SAR）protocol. All OSL measurements were made using an automated Rise TL/OSL DA-15 system. The dose rate was converted with the concentration of U, Th and K in samples. The dating results show that the extreme palaeofloods occurred 4100 -4000aB. P., that is, the late period of Longshan Cuhure（4300 -4000aB. P. ）. This flood episode involved five flood depositional layers,indicating at the least five extreme flood events occurred on the Jinghe River. It can be seen at some of valley terraces that ruins of the Longshan Culture were inundated with these flood layers. The flooding episode corresponded to the climatic turn from the Middle Holocene Climatic Optimum to the Late Holocene dry-cold climate about 4200 - 4000aB. P. in North China. Because the climate conditions were highly instable and variable in this episode, those flood events may be of great importance to the collapse of the Neolithic Longshan Culture and the emergence of Chinese Bronze Civilization, especially the emergence of the Xia Dynasty.