青海湖鸟岛北原生砂楔的发现及其古气候意义

Fossil sand wedges of north Niaodao in Qinghai Lake：Discovery and paleoclimatic implications

青海湖鸟岛北中更新世冲洪积砂砾石层中发育7个原生砂楔。原生砂楔中的细粒砂ESR测年结果表明，该地区至少经历两次冰期：砂楔剖面南侧3个楔体发育时代分别为228±2ka、228±22ka和143±14ka，形成于倒数第二次冰期；剖面北侧3个楔体发育时代分别为17±1ka、59±10ka和60±6ka，形成于末次冰期。估算青海湖盆地在倒数第二次冰期时的年平均气温不高于-9.5^-11.0℃，降温幅度较大，比现今年均气温低11.2~12.7℃；末次冰期的年平均气温不高于-7.5^-9.0℃，较现在低9.2~10.7℃。

Qinghai Lake is located in the northeast of the Tibetan Plateau, which is the most developed area of periglacial in mid-low latitudes.Qinghai Lake is the biggest inland lake and largest saltwater lake of China and is at an elevation of more than 3000 meters with the annual mean temperature of 1.7℃.A fossil sand wedges profile was newly discovered in an alluvial-diluvial sand gravel layer, which is located in the north of Niaodao Island（37°05＇26.34＂N, 99°44＇43.93＂E） of Gangcha County in Qinghai Lake.7 fossil sand wedges were found in the profile, named NS1, NS2, NS3, NS4, NS5, NS6 and NS7.These fossil sand wedges are characterized by shape of inverted triangle or funnel-form, and are filled by medium-fine grained sandstone, wall of wedges are smooth.Surrounding rock of these fossil sand wedges is conglomerate.Based on the ESR dating, the fossil sand wedges NS2, NS3 and NS4, which located in the south of profile, were formed during Penultimate Glaciation （228~143ka）; NS5, NS6 and NS7, which located in the north of profile, were formed during Last Glaciation （60~17ka）.As an evidence for reconstructing paleotemperature of northeast Tibetan Plateau, it is inferred that the annual mean temperature in Qinghai Lake basin was about no higher than -9.5^-11.0℃ in Penultimate Glaciation, which was 11.2~12.7℃ lower than present; and about -7.5^-9.0℃ in Last Glaciation, which was 9.2~10.7℃ lower than present, temperature reduction in glaciation was conspicuous.That is the significant for the study of Quaternary paleoclimate and permafrost distribution in the northeast of Tibetan Plateau.