Extraordinary hydro-climatic events during the period AD 200-300 recorded by slackwater deposits in the upper Hanjiang River valley, China

摘要

The Hanjiang River, a major tributary of the Yangtze River in China, is noted for the current national South-to-North Water Diversion project. Palaeo-hydrological investigations were carried out along the upper reach gorges of the Hanjiang River that drains the Qinling and the Dabashan Mountains. A set of palaeoflood slackwater deposit beds (SWDs) was identified in Holocene pedo-stratigraphy of the riverbanks along the valley. These SWDs are interbedded in the eolian loess-soil profiles in the cliffy riverbanks and they thin out toward the upper slopes. The palaeoflood SWDs were differentiated from eolian loess and soil by the sedimentary criteria and analytical results. The minimum flood peak discharges were estimated to be 65,400-65,830m~3s~(-1) by using palaeo-hydrological methods. They are about twice of the largest gauged flood (34,300m~3s~(-1)) that has ever been measured. They represent the largest flood events in the upper reaches of the Hanjiang River over the Holocene. These extraordinary flood events were dated to 1810-1710 a (AD 200-300) with the optically stimulated luminescence method and checked by archaeological dating of the human remains retrieved from the profiles. This indicates that the extraordinary floods occurred during the dynasties of Eastern Han to Western Jin (AD 25-316) in Chinese history, during which severe droughts and floods were recorded in documents. In the reconstructed 2000-year temperature time series based on high-resolution climatic proxies from tree-rings, stalagmites, ice-cores and lake sediments from over the world, these extraordinary palaeoflood events are correlated with an increased climatic variability characterized by cooling and drying during the period AD 150-350. This result is important for understanding the effects of global change on river system dynamics.