苏干湖沉积物粒度组成记录尘暴事件的初步研究*

文章通过分析苏干湖表层沉积物、流域表层沉积物、大气降尘以及湖泊冰面囚固碎屑颗粒等的粒度组成,比较了流域地表沉积与湖泊沉积物粒度组成的差异和苏干湖表层沉积粒度组成的空间变化。初步认为苏干湖沉积物粗颗粒组分(>63μm)主要由风力搬运入湖,>63μm组分的含量可以用来指示研究区尘暴事件的演化。结合湖泊沉积岩芯纹层计数年龄,重建的1000年来尘暴演化历史显示,在1210A.D.之前尘暴事件较弱,且变化幅度较小;1210A.D.以来则表现出高频率或者高强度,其中13世纪上半叶以及17世纪的高值段与东部降尘变化历史一致。就气候变化的特征时段(中世纪暖期、小冰期等)而言,尘暴事件主要与气候变冷情况下较强的盛行风场有关;而极端干旱区有效湿度的增加可能有利于地表粉尘的释放,助长了尘暴事件的发生。

PRELIMINARY STUDY ON DUST STORM EVENTS DOCUMENTED BY GRAIN SIZE COMPONENT OF SUGAN LAKE SEDIMENTS, NORTH QAIDAM BASIN

To further understand the role of atmospheric dust in global climate system, a long sequence of dust storm history is necessary to be established using geological archives in dust provenance area. As Northwest China has been recognized as an important source area for global dust loading, hydrologically closed lakes in arid regions of China could be ideal study objectives to trace dust storm evolution history. The Sugan Lake, a closed lake located at the northern margin of the Qinghai-Tibetan Plateau, was thus employed. The lake is mainly fed by groundwater from the eastern fine earth plain of the Sugan Lake Basin. Different samples were analyzed for grain-size component. Samples include 1) deposits collected from both erosion and deposition sites in lake catchment, 2) detrital material trapped in lake ice cover in winter, 3) surface sediments of the lake, and 4) airborne dust. In winter, the lake ice cover contains some silt and sand material, which will drop into lake sediments after ice melting. The grain size of surface samples of lake sediments decreases from west to east. The decreasing direction is similar to that of prevailing wind in the study area. The settled sand and dust collected at the depth of 3.5m mainly comprises of coarse fraction with a diameter of 100~200μm. These results demonstrate that the coarser fraction (>63μm) in the lake sediments was mainly transported by winds. It is supported by the reconstructed sequence of day numbers of dust storm and day numbers of strong wind (wind speed, ≥17m/s) observed at the Lenghu Meteorological Station from 1957 to 2000A.D. Therefore, variation of >63μm fraction can be used to indicate dust storm events. Based on chronology determined by varve counts, the 1000-year dust storm history was established. Dust storm was weak and/or less frequent during 1000~1210A.D. , a time span belonging to the Mediaeval Warm Period (MWP). After 1210A.D. , dust storm became stronger and more frequent. The peaks of dust storm took place during 1210~1250A.D. , 1600~1670A.D. , and 1930 to present. In particular, strong and/or highly frequent dust storms during the Little Ice Age (LIA) occurred in the first half of the 13th century and the 17th century, which is in agreement with dust fall reconstructed by Zhang (1984) using historical records. It is plausible that dust storm in Qaidam Basin and dust fall in East China during LIA were affected by strong wind regime at a large spatial scale due to the large pressure gradient between high cell and low cell in a cold period. On the other hand, the increase in effective moisture in the hyperarid regions would be favorable to emissions of dust, which in turn fosterages the presence of dust storm. There may be also other factors causing the strong and/or highly frequent dust storms in the study area during LIA. The preliminary result presented in this paper could be helpful to our understanding the dust storm history in dust source areas of China. However, to better understand the environmental changes in the arid regions of China under the global climatic system it is necessary to carry out a further work to establish a linkage between the dust source area and the remote dust deposition sites (e.g. Greenland ice sheet, etc.).