JYZ11A钻孔记录的居延泽演化历史

我国内陆干旱区在末次冰期(含深海氧同位素第3阶段)是否存在古大湖近年来持续存在争论。居延泽是黑河的尾闾湖之一, 位于我国典型干旱区, 是开展这项研究的理想区域。依据居延泽盆地最低处的JYZ11A钻孔(全孔长61.13m, 本文侧重上部27m岩芯), 使用石英光释光测年获得3个可靠释光年龄并据此建立年代框架。全孔10cm间距测量沉积物粒度, 钻孔上部16.7m以2cm间距测量低频磁化率, 本文侧重分析JYZ11A钻孔上部10m的指标记录。综合岩性地层、沉积物粒度分布特征和粒度、磁化率指标记录, 并与前人已有研究进行对比, 本研究发现末次冰期时居延泽盆地主要堆积棕色冲洪积物和风沙沉积物, 早全新世为风沙沉积, 中全新世出现浅灰色湖相沉积与风沙交替沉积, 而稳定湖泊只在约3ka时才逐渐形成, 可能存在多次湖面波动并留下古湖岸堤。因此, 居延泽盆地在末次冰期包括深海氧同位素第3阶段晚期并不存在稳定大湖。居延泽早全新世干涸、中晚全新世湖泊较稳定发育的全新世成湖模式与季风区湖泊演化模式明显不同, 而与许多中亚干旱区的湖泊、风成沉积等古环境记录相似, 它们可能共同指示了一种有别于中国季风区的全新世区域湿度演化模式。

THE DRILL CORE JYZ11A REVEALED LAKE EVOLUTION IN JUYANZE BASIN,INNER MONGOLIA

Whether palaeolakes were widely distributed in Northwestern China during Marine Isotope Stage 3(MIS 3) is still under debate. Radiocarbon dating data suggested that the paleolakes developed during MIS 3. However, the Optically Stimulated Luminescence(OSL)dating data have indicated that the ages of paleolakes 14C dating dated to MIS 3 are mostly fall in MIS 5.Ejina Basin is a terminal lake basin of the Heihe River, consisting of three sub-basins of Gaxun Nur, Sogo Nur and Juyanze from east to west. The basin is surrounded by Gobi Altay Mountains in the north, Badain Jaran desert in the southeast, and alluvial fans of Heihe River in the south. The 14C dating of palaeoshorelines at the Ejina Basin have suggested that megalake appeared during some time of MIS 3, however, this is still lack of lacustrine records studies from central of the basin as nearly all lacustrine records are just focused on Holocene paleoenvironmental/climatic changes. In this study, we drilled and retrieved a 61.13m deep core JYZ11A(41°53'41.4"N, 101°51'05.4"E; 895m a.s.l.) at the central of Juyanze Basin. Environmental proxy index samples were collected at 2cm interval in the upper 10m and 5cm intervals from 10.00~61.13m of the drill core and only upper 27m of the drill core are presented in this study. According to lithology and sedimentary, the upper 27m of the drill core sequence can be divided into two stratigraphical units from bottom to top. Unit Ⅰ(27.00~8.88m)is reddish brown coarse sand or yellowish brown sand, and Unit Ⅱ(8.88m~0)is grey silty clay alternating with fine-to-medium sand. Three OSL samples were collected at the sand layer of the drill core at the depth of ca.6.2m, 9.3m and 22.2m in the darkroom, and the ages of these three samples are 3.0±0.2ka, 6.7±0.6ka and 80.4±8.4ka, respectively. A total of 508 samples at 10cm interval of JYZ11A core were collected for grain size analysis including different grain size fractions and frequency distribution for typical samples, and a total of 463 samples at 2cm interval of upper 16.7m of JYZ11A core were collected for low frequency magnetic susceptibility(χlf)analysis.Combining lithology, grain size frequency distribution for typical samples from upper 27m of JYZ11A core and previously published proxy indexes results of JYZ11A core, it is suggested that an aridity environment characterized by prevailing alluvium sands and eolian sands appeared in Juyanze Basin during the Last Glaciation and Early Holocene. Shallow lake environment with frequent wind-sand activities appeared only after ca.6.7ka and the stable lake environment occurred in the region after ca.3ka. We suggest that no permanent paleolake appeared in Juyanze area during the MIS 3 and even Last Glaciation. The evolutionary pattern of lake in Juyanze Basin during Holocene presents a difference comparing to the lake evolution in monsoonal China.