泸沽湖记录的西南季风区15000aB.P.以来植被与气候变化

泸沽湖是云贵高原第二大深水湖。取自水深44.5m处长度880 cm的连续岩芯显示, 泸沽湖沉积岩芯物理特征变化不明显, 色调单一、物质构成均匀、植物残体(古树叶)丰富。本文对该岩芯上部474 cm进行了孢粉分析和TOC(总有机碳)、CaCO3(碳酸钙)百分含量的测定。在16个树叶残体 AMS14C 测年基础上建立了岩芯的年代序列。岩芯孢粉组合表明, 在泸沽湖流域, 15000~12300a B.P. 植被以松林为主, 伴有山地针叶阔叶混交林, 指示温凉偏湿的气候条件; 12300~10300a B.P. 的植被以松林减少而落叶阔叶林扩张为特点, 表明当时气候相对偏暖偏干; 在早全新世(10300~7500a B.P.)和中全新世(7500~3800a B.P.), 植被以松林占优势, 伴有铁杉林和云冷杉林, 植被盖度大, 反映温暖湿润的气候条件; 在中全新世常绿栎类偏多, 表明有效湿度不如早全新世; 在3800~3300a B.P., 松林明显减少而硬叶常绿栎林急剧扩张, 指示了一次持续的冷干事件; 在晚全新世(3300a B.P. 至今), 松林有所恢复但未达到早中全新世的水平, 常绿青冈林增加并出现较多的铁杉林和云杉林, 但植被盖度明显降低, 表明晚全新世的气温和湿度比早中全新世均偏低。TOC和CaCO3含量反映的湖泊环境和湖面变化与孢粉分析结果有较好的对应, 虽然彼此在年代上存在一定的差异。碳酸盐含量变化显示在13600~3400a B.P. 期间湖水整体较高并形成间断性外流湖。泸沽湖沉积记录不仅揭示了末次冰消期以来升温-高温-降温的变化趋势, 还记录了与Younger Dryas(YD)和约4000a B.P. 的冷事件等相关的气候异常事件。因此, 本研究对于认识我国西南地区现代气候环境变化过程、气候突变事件及植被演化具有重要意义。

VEGETATIONAL AND ENVIRONMENTAL CHANGES SINCE 15ka B.P.RECORDED BY LAKE LUGU IN THE SOUTHWEST MONSOON DOMAIN REGION

Lake Lugu, located at the southeastern Tibetan Plateau(27°40'~27°44'N, 100°46'~100°55'E), is the second largest deep plateau-lake in Yunnan-Guizhou Plateau. A 880 cm long core(at 27°40'49″N, 100°48'00″E) taken from 44.5m water depth of the lake shows that its sediments are rich in organic matters with leaf remains but without distinguishable physical features. The age-depth model of the Lake Lugu core is established from 16 AMS14 C dates. This paper presents pollen assemblages of its upper 474 cm, together with the measurement of the TOC and CaCO3 to reconstruct past vegetation and environment changes during the last 15000 years. Among the 70 samples analyzed, a total of 81 pollen and spore taxa were identified. Trees were the dominant pollen in the entire profile and the average content of trees was 92.1%, and the maximum value was as high as 97.3%. On average, herb accounts for 7.9% on average. One of the typical features from the pollen diagram identified from the sediments from Lake Lugu is that the content of Pinus spp. is high all through the profile, which averagely can reach about 38.5% ~74.6%. The samples from the top 3 cm contents too much water than sediments, therefore, it's difficult to sample and they were not considered here. Based on the pollen percentages of the major taxa and CONISS results, the pollen record from the Lake Lugu was divided into six main zones:(1)From 15000a B.P. to 12300a B.P., pine forests together with alpine coniferous and broadleaved mixed forests occupied the drainage of Lake Lugu, indicating a cool and humid conditions.(2)It was followed by a warm and dry period marked with a decline of pine forests and an increase of deciduous broadleaved forests from 12300a B.P. to 10300a B.P.(3)During the Early and Middle Holocene(10300~7500a B.P. and 7500~3800a B.P.), high coverage vegetation was dominated by pine forests, together with a few alpine coniferous forests consisting of hemlock, fir and spruce, indicating warmer and wetter conditions than those in 15000~12300a B.P. However, more evergreen oaks occurred in the Middle Holocene, suggesting that climatic conditions were not as humid as in the Early Holocene. (4)Pollen spectra from 3800a B.P. to 3300a B.P. showed a marked decline of pine forests and a distinct expansion of sclerophyllous evergreen oak forests, reflecting a 500-year cool and dry climate event.(5)From 3300a B.P. to the present, pine forests recovered from its shrink of 3800~3300a B.P., but did not reach the Early and Middle Holocene level, evergreen broadleaved Cyclobalanopsis forests and coniferous hemlock-fir-spruce forests frequently occurred in the lake drainage. Past vegetation indicated that temperature and humidity in the Late Holocene were lower than those in the Early and Middle Holocene. Lake environmental changes revealed by TOC and CaCO3 were consistent with climatic changes reflected by pollen spectra. The TOC contents vary between 3.78% ~13.80% (with average of 8.79%). Low lake levels were generally parallel with cool and dry climatic conditions. TIC content is one of the proxies for water levels. Based on this assumption, it is believed that high lake levels appeared between 13600a B.P. and 3800a B.P., during that period the water level fluctuated with a periodicity of 600~1230a. Overall, pollen, TOC, and CaCO3 records from Lake Lugu revealed a trend of temperature rise-high temperature-temperature drop, superimposed with a series of cool and dry events.