全新世以来滇西北地区天才湖粒度特征及古降水

THE GRAIN-SIZE CHARACTERISTICS OF TIANCAI LAKE IN NORTHWESTERN OF YUNNAN PROVINCE AND PALEO-PRECIPITATION HISTORY DURING THE HOLOCENE

根据滇西北高山冰蚀湖泊——天才湖的地理位置与湖泊特征,探讨了天才湖粒度所代表的古气候意义,认为天才湖沉积物的粒度特征能有效地显示该地区古降水的变化历史。综合分析天才湖TCK1沉积钻孔的粒度指标,同时结合磁化率、烧失量指标,认为全新世以来天才湖湖区的古降水经历了6个阶段,即11940～11240cal.aB.P.期间,降水强度整体相对较弱,降水量最少,气候较干旱;11240～10300cal.aB.P.期间,降水强度明显增加,降水量相对较丰富,气候较湿润;10300～8760cal.aB.P.期间,降水强度整体较弱,降水量相对较少,气候相对偏干,仅在9090～8850cal.aB.P.期间存在较强的降水事件;8760～7230cal.aB.P.期间,降水强度最大,降水量最丰富,气候最湿润;7230～2710cal.aB.P.期间,降水强度除在5350～5140cal.aB.P.期间有强降水外,其他时段降水强度都较弱,降水量总体偏少,气候较干旱,是一个气候状况比较稳定的时期;自2710cal.aB.P.以来,降水强度变化较大,存在4次明显的强弱交替时段,强降水时段对应降水量大、气候湿润,弱降水时段对应降水量小、气候偏干。

Tiancai Lake in northwestern Yunnan Province is an alpine glacial erosion lake with an altitude of 3898m and the largest water depth as 7m and is located in the northern section of the mountains and canyons of the southeast edge of the Qinghai-Tibet Plateau. The Lake area is relatively small, only covering an area of ca. 0.02km2. Its drainage area is also relatively small, therefore the source of lake sediment is relatively simple and stable. Tiancai Lake is seldom disturbed by human activities. The lake climate belongs to a highland cold temperate humid monsoon one. The mean annual temperature in the lake is 2.5℃ and the average annual precipitation is about 910mm. A 926-cm long sediment eore（TCK1） was collected at a water depth of 6.8m in the central part of Tiancai Lake （26°38′03.80″N,99°43′00.00″E）. The core was sectioned at 1 cm intervals and 926 samples were obtained, Thus the proxies of grain size, magnetic susceptibility and loss on ignition were analyzed at lcm intervals. The sedimentary record of Tiancai Lake is continuous, sensitive and of high resolution. Based on the geographical location of Tiancai Lake and its lake characteristics, the paleo-climate significances of grain size in Tiancai Lake were discussed. In general, the grain-size characteristics of the sediment core in Tiancai Lake could effectively indicate paleo-precipitation history in the studied area. When the precipitation intensity is great and hydrodynamics is strong ,fine particles are easy to be discharged and the coarse particles will be deposited in the lake because of their biggish weight, implying that the climate is relatively humid when the grain size in the sediment is coarse. Whereas, the climate is relatively dry when the grain size in the sediment is fine. Combined with mass magnetic susceptibility,loss on ignition,it was considered that Tiancai Lake went through six major stages in the paleo-precipitation history since the Holocene. From 11940cal. aB. P. to 11240cal. aB. P., the rainfall intensity was relatively weak on the whole, which resulted in the least precipitation and dry climate condition. Between 11240cal. aB. P. and 10300cal. aB. P., the rainfall intensity increased significantly, so precipitation was relatively abundant and the climate was more humid. During the period 10300 ～ 8760cal. aB. P., the rainfall intensity was relatively weak on the whole again, leading to relatively less precipitation and a relatively dry climate condition except for the relatively strong rainfall intensity between 9090cal. aB. P. and 8850cal. aB. P. From 8760cal. aB. P. to 7230cal. aB. P.,the rainfall intensity was the strongest, indicating the most abundant precipitation and the most humid climate. During the period 7230 - 2710cal. aB. P., the rainfall intensity was relatively weak on the whole except for the period of 5350 -5140cal. aB. P., so precipitation was relatively less on the whole and the climate was relatively dry with tiny changes. Since 2710cal. aB. P., the fluctuation of precipitation intensity was relatively frequent with four obvious alternations between strong precipitation intensity and weak precipitation intensity in this period. When the rainfall intensity was relatively strong,precipitation was abundant and the climate was humid,and vice versa.