青海苏干湖表层沉积物粒度分布模式 与大气粉尘记录

位于干旱区柴达木盆地具有年纹层的苏干湖是以地下水补给为主的内陆封闭湖泊,其表层沉积物粒度具有多组分峰态分布特征,Weibull函数的拟合结果发现,其粒度分布全部具有4个基本组分:超细粒组分,众数粒径在1μm上下;细粒组分,众数粒径在5～10 μm;粗粒组分,众数粒径在50～100 μm;砂组分,众数粒径在500 μm左右.粒度主峰出现在细粒组分或者粗粒组分.直接来自大气粉尘的冬季湖泊冰面样品及当地尘暴样品的Weibull函数粒度拟合也显现出了相似的分布模式,冰面样品的粒度主峰位于众数粒径在15～20 μm的细粒组分,砂组分(众数粒径452.9μm)的出现指示出砂粒在冬半年沿冰面而进入湖泊中心.尘暴降尘样品在尘暴季节(5月份)和非尘暴季节(11月份)的粒度分布普遍具有3个组分,缺少砂组分,强沙尘暴事件样品则由4个组分组成,粒度主峰均位于众数粒径在100～200 μm的粗粒组分.结果显示,苏干湖湖泊沉积能够较好记录当地大气粉尘,砂组分与湖泊周边干旱地表经历的近地面强沙尘活动有关,粗粒组分记录了区域性的尘暴事件,细粒组分主要反映流域径流状况,超细粒组分代表干旱区的大气背景粉尘和气溶胶.研究表明,苏干湖内陆湖泊沉积客观记录了区域大气粉尘和尘暴事件,具有重建干旱区大气粉尘变化历史的潜在优势.

Distribution Pattern of Grain Size in Surface Sediments from Sugan Lake and Its Potential in Recording Aeolian Dust in Arid China

Sediments from inland enclosed lakes in arid regions of northwestern China provide ideal documents in reconstructing high-resolution history of aeolian dust and aerosols. Sugan Lake in northern Qaidam Basin on the Tibetan Plateau is an enclosed shallow lake mainly fed by groundwater, in which annually laminated varves are well preserved since mid- and late Holocene. Surface sediments from Sugan Lake were collected and analyzed for grain size. Frequency curves of samples indicated multi-components in the surface sediments. Weibull function fitting employed to the grain size distribution suggested that there are four components from clay to sand in the surface sediments characterized by their respective modes. Super-fine component has mode size around 1 μm, fine-grained component has mode of 5-10 μm, coarse-grained component has mode of 50-100 μm, and the mode of sand component is about 500 μm. The main components in the surface sediments appear in fine or coarse grains. Particles trapped in the lake ice in winter and dust storms during the same period present the similar distribution pattern in grain size. The main component of aeolian dust from lake ice is composed of particles sized between 15 and 20 μm, and sand component (with mode size of 452.9 μm) appeared, indicating that sand particles were transported to Sugan Lake during the winter. Dust samples from dust storm period (May) or non-dust storm period (November) prevalently consist of three components without sand grains. During strong dust storm events, however, four components including sand are identified, the main component of which still has a size of 100-200 μm. Our results show that the sand component of the surface sediments in Sugan Lake has been transported by ambient strong dust storm activities; the coarse grains have most likely recorded the regional dust storm events; fine component seems to have indicated the intensity of water supply in the basin; super-fine component as extensively deposited in the Chinese loess, may have represented the background level of atmospheric dust and aerosols in the arid region of northwestern China. Therefore, the sediments from Sugan Lake have documented the regional aeolian dust and dust-storm events and have the potential in reconstructing the history of aeolian dust activities.