渭河咸阳段全新世古洪水事件光释光测年研究

古洪水水文学研究是全球气候变化研究的前沿课题，确定古洪水事件的年代是古洪水研究的重要内容之一。对渭河流域进行深入的野外考察，在中游咸阳段阶地发现全新世黄土-古土壤层里夹有古洪水滞流沉积层，表明该地层记录了古洪水事件发生的信息。在该剖面采集光释光样品，分离提取40~63 μm石英颗粒成分，应用单片再生剂量法（SAR）进行红外后蓝光(Post-IR OSL)释光测量，获得了9个OSL年龄值。结果表明由古洪水滞流沉积层记录的特大古洪水事件发生在3 200~2 800 a B.P.之间。结合沉积样品系列的磁化率和粒度成分等气候替代性指标分析，揭示了在全新世中期向晚期转折过渡时期，渭河流域处于气候向干旱化发展的转型期，气候变化剧烈，大气系统不稳定，降水变率增大等，是导致特大洪水多发的主要原因。这也是渭河流域气候水文系统对于全球性气候恶化响应的结果。

OSL Dating of the Palaeoflood Events in the Middle Reaches of the Weihe River

Through deeply field survey,a Holocene loess-soil profile with interbedded flood slackwater deposits(SWD) was found in Xianyang of the middle reaches of the Weihe River,which recorded the information of palaeoflood events.In order to achieve OSL ages,9 luminescence samples were collected and post-IR OSL measurements were used with single-aliquot regenerative-dose(SAR) protocol.Grain-size distribution and magnetic susceptibility were also analysed so as to research the occurrence and cause of palaeoflood.The result shows that the fast component is dominant in OSL signals of quartz grains(40~63 μm),which is suitale for post-IR OSL measurements.For all dating measurements,preheat temperature of each natural or regenerated dose of young samples and older samples were 260℃ and 240℃,respectively;and the cutheat was set to 220℃.OSL ages increases with depth and conform to layer where were collected except individual ages underestimated. At the NSC site,the layer of flood slackwater deposits(SWD) with horizontal bedding、silty clay-like and sallow colour,was in the depth of 95~110 cm,which has typical characteristic of SWD.This flood deposit involve 3 flood depositional layers,indicate of at least 3 palaeoflood evevts occured.The average of low-frequency magnetic susceptibility of paleosoil was 133.62 10-8 m3.kg-1,was the highest in this profile,while the value of SWD layer was 113.68 10-8m3.kg-1 which was much higher than Malan loess,but lower in paleosoil layer.Grain-size distribution of SWD layer was dominated by silt,the content of silt was 59.43% and clay was 38.107%,the content of sand(>0.05 mm)was the least that was 3.32%.The curve of sand(>0.05 mm) in the SWD layer present a small peak.The dating results show that the extreme palaeofloods occurred 3.0~3.2 ka.According to the analysis of all the result,extraordinary floods occurred 3 200~3 000 a B.P.in the Weihe river,the period of flood occurred frequently.Three flood depositional layers indicates that each of the slackwater deposit beds has recorded one individual flood event.So the Weihe river chronological framework of palaeoflood were established.This study shows that the Weihe river in transform period from rainy to dry;the highly variable,unstable and catastrophic climate was the cause of extreme palaeofloods occurrence.