黄旗海晚第四纪湖泊演化的长石释光年代学研究

Inland closed lakes are sensitive indicators of climate change. Using lake sediment archives to reconstruct climate change on different time scales has long been one of the most important research domains of PAGES. Reliable chronological frameworks are of crucial importance for such studies. Owing to the lack of proper dating material or methods, the climate records retrieved from lake sediment with reliable absolute chronology mainly restrict to the last deglaciation and Holocene, with few that reached late Pleistocene. The optically stimulated luminescence (OSL) dating technique using quartz has been widely applied in dating lake sediment, providing age constraints within MIS 5. The quartz OSL signal, however, has been shown to saturate at relatively low doses of ~150 to 200 Gy. Like 14C ages that beyond 30-40 ka may underestimate the true age, the upper limit of quartz OSL ages lies at about 50-70 ka, that particularly observed in loess worldwide. It might therefore be problematic to employ either 14C or quartz OSL dating methods beyond the late glacial period. As an alternative dosimeter, feldspar has much higher saturation doses compared to quartz. The newly developed feldspar post-IR IRSL dating method raises the dating limit up to ~500 ka that provides great potential in dating sediment back to several glacial-interglacial cycles..Huangqihai Lake is an inland terminal lake, located at the East Asian summer monsoon (EASM) marginal area in semi- arid northern China. Its lake level fluctuations are direct indicators of regional hydrological variability related to the variation of monsoon intensity. During four field campaigns, a series of palaeo-shorelines and lake terraces were found to the southeast of the lake basin from 1280 to 1400 m a.s.l., indicating that the lake level has dramatically dropped about 130 m in the geological history. Based on quartz OSL chronology, previous studies revealed that since MIS3 the highest lake stand occurred in early Holocene at the elevation of 1340 m (~70 m above the present lake surface). In this project, we aim to apply the post-IR IRSL technique to date the higher lake terraces and beach remnants of Huangqihai Lake. The quartz OSL signal will be further investigated to determine its saturation level and reliable upper age limit. The ultimate purpose is to recover the lake level fluctuation history beyond the late glacial period (possibly back to ~200 ka) and refine our understanding of the past climate change on glacial/interglacial time scale within the EASM climate regime.

内陆封闭湖泊是环境变迁的天然记录器，而准确可靠的年代是研究湖泊沉积记录的首要前提。光释光测年是目前第四纪湖泊沉积物测年研究中应用最普遍的手段之一。最新发展的长石两步（多步）红外技术的测年范围已达~50万年，为更老的湖泊沉积物的绝对定年提供了技术支持。内蒙古黄旗海地处东亚季风边缘区，在高出现代湖面130多米的高差内发育的多级湖岸阶地（岸堤），是湖泊水位剧烈波动的直接地貌证据，也是研究湖泊演化与东亚季风变迁的理想材料。前期研究已通过石英光释光技术测定了拔湖70米以内的湖泊沉积物的年龄，而更高的湖岸沉积物的石英光释光信号已经饱和，无法获得可靠的年代框架。因此，本项目拟利用长石红外技术测定黄旗海更高海拔的湖泊沉积物年代，并结合沉积学和地貌学证据，恢复不同时期湖面覆盖范围，有望重建黄旗海二十万年以来的湖面波动历史，探讨冰期-间冰期尺度上的湖泊演化特征及其对东亚季风变迁的响应。

内陆封闭湖泊是环境变迁的天然记录器，而准确可靠的年代是研究湖泊沉积记录的首要前提。光释光测年是目前第四纪湖泊沉积物测年研究中应用最普遍的手段之一。最新发展的长石两步（，探讨多步）红外技术的测年范围已达~50万年，为更老的湖泊沉积物的绝对定年提供了技术支持。内蒙古黄旗海地处东亚季风边缘区，在高出现代湖面130多米的高差内发育的多级湖岸阶地（岸堤），是湖泊水位剧烈波动的直接地貌证据，也是研究湖泊演化与东亚季风变迁的理想材料。本项目利用长石红外技术测定黄旗海不同海拔高度上的湖泊沉积物年代，并结合沉积学和地貌学证据，重建了黄旗海MIS7以来的湖面波动历史冰期-间冰期尺度上的湖泊演化特征及其对全球变化的响应。黄旗海湖盆的高湖面出现在240-200 ka，100-70ka以及8-4 ka，而在末次冰期时段湖盆周边发育了大量的风成沙，表明湖面相对较低。我们初步认为在冰期-间冰期尺度上，黄旗海地区的高水位主要形成于间冰期，与全球的高温期对应。另一方面，在沉积地层的变化上，也揭示了末次间冰期到冰期的湖泊退缩过程，以及末次冰期到现代间冰期（全新世）的湖泊扩张历史。因此，轨道尺度上，湖面变化可能响应于区域或者全球的温度变化。