青海湖滩坝分布规律及其古气候意义

青海湖是我国最大的内陆湖盆,对气候变化十分敏感,而滩坝是青海湖滨浅湖带最为发育的沉积类型之一,其滩坝分布规律对晚更新世以来的古气候演化具有重要指示意义。在对青海湖一郎剑剖面进行实地考察的基础之上,对滩坝的分布规律及沉积特征进行精细解剖。通过分析总结前人相关测年数据,并与青海湖滩坝分布规律进行对比,发现青海湖湖平面升降对滩坝分布有明显的控制作用,建立了18 ka以来青海湖滩坝的演化过程,并将近18 ka以来青海湖湖平面升降史分为4个阶段:(1)更新世末温湿期,湖平面在海拔3 197～3 202 m附近波动;(2)全新世冷干期,湖平面近乎干涸;(3)全新世大暖期,湖平面处于全新世以来的最高值,约为3 212 m;(4)全新世凉湿期,湖平面回落到3 200 m附近,并在近2. 5 ka湖平面加速下降。在晚更新世和晚全新世时,湖平面在海拔3 202 m附近波动时间较长,在该海拔范围内,形成了规模较大的复合滩坝;在早全新世,青海湖平面最低,多发育风成黄土和潟湖沉积;在中全新世,湖平面最高,形成了距离现今湖平面最远的数列单体滩坝。

Beach Bar Distribution and Paleoclimate Implications in Qinghai Lake

The Qinghai Lake is the largest inland lake in China, and is sensitive to climate change. As an important type of shallow lake sediments, the distribution of beach bars reflects the paleoclimatic evolution of the Qinghai Lake. The southern part of the Qinghai Lake develops a large number of beach bars, which represents an ideal place for studying the beach bar distribution in the lake. Based on field cross-section (trenches and deep pits) investigation around Yilangjian, the beach bar distribution and sedimentary characteristics there were described in detail, and are compared with the reconstructed lake level change curve. It is found that the reconstructed lake-level change curve of the Qinghai Lake has significant control over the beach bar distribution, and the evolution of Qinghai Lake beach bar with the lake-level rise and fall since 18 ka is reconstructed, which comprises four stages: (1)During the warm and humid end Pleistocene, the lake level fluctuated between 3,197-3,202 m; (2) In the cold and dry period of the Holocene, the Qinghai Lake was almost dry; (3) In the warmest period of the Holocene, the lake-level was at its highest since the beginning of the Holocene; (4) During the cool and humid period of the Holocene, the lake level fell back to 3,200 m and accelerated down in the 2.5 ka. During the Late Pleistocene and Late Holocene, the lake level fluctuated for a long time around the altitude of 3,202 m. In this altitude range, a large-scale compound beach bar was formed. In the Early Holocene, the Qinghai Lake was at its lowest level and more loess and lagoon were developed. In the Middle Holocene, the lake level was at its highest, forming a series of monolithic beach bars furthest from the current lake level.