关键词: 青海湖, 粒度, 古气候, 风沙活动

Abstract: Qinghai Lake Basin is located in the joint zone of the Asia monsoon region of eastern China, the arid region of northwestern China and the cold region of Qinghai-Tibet Plateau. It is a natural barrier to prevent the land desertification from spreading eastwards and help maintain the ecological security in the northeastern Qinghai-Tibet Plateau. Its special geographical location and its high elevation makes itself highly sensitive to the climatic changes. The aeolian deposits are widely distributed around Qinghai Lake which can serve as a source of palaeoclimatic information and provide the important evidence for aeolian geomorphology and aeolian activity processes. In the stratum, aeolian sand represents dune-field expansion and/or dune buildup, whereas paleosols indicate stabilization of dunes, accompanied by ameliorated vegetation coverage. We used 4 conventional radiocarbon dates to construct chronological frameworks for QHH sections, and all of the ¹⁴C ages occur in stratigraphic order within the limits of the analytical error. The grain size distribution and parameters were analyzed to extract the sensitive components of Aeolian deposits. The results showed that medium and large sand fractions (138-156 μm) were used for indicating the strength of aeolian activity, and the fine sand component(2-5 μm)might be influenced by the formation of paleosol. The variations of the proxy indicators measured in the laboratory were largely consistent with the lithofacies variations of QHH sections. The aeolian sand in the field showed little or no evidence of pedogenic alteration, and it had a relatively coarse mean grain size(Mz), and low magnetic susceptibility(MS). In contrast, in the field the paleosols were more compact than the aeolian sand, and they have a finer Mz and much higher MS. In addition, we collected the previously published OSL and AMS¹⁴C ages of aeolian deposits in the Qinghai Lake Basin. Combined with the ages in this study, 59 ages in total of 20 sites were plotted to extract paleoenvironmental information using a probability density function (PDF) which indicated a transitional period from last glacial period to Holocene occurred in 10-9 ka, when the climate was slightly warmer and wetter but variable, as indicated by the distributions of ages of the paleosol, weak paleosol, loess, and aeolian sand. From 9-4.2 ka, the number of PDF was relatively small during the midHolocene, indicating a mid-Holocene Climatic Optimum with weak aeolian activity. After 4.2 ka, the climate tended to be dry with frequent fluctuations. Since a drier climate occurred and the Aeolian activity strengthened again the previously stabilized dunes reactivated again especially after 2 ka. However, there is considerable disagreement about the nature of centennial-scale climatic variations during the late Holocene in the region, even between studies of the same geological archive and therefore this topic requires further investigation.

Key words: Qinghai Lake, grainsize, paleoclimate, aeolian activity