泸沽湖地区晚全新世气候和环境演变

Climatic and Environmental Changes of the Lugu Lake Area During the Late Holocene

泸沽湖地区受人类活动影响较小、对气候变化较为敏感,是开展古气候研究的理想区域。本文以孢粉为重要研究手段,对采自泸沽湖草海地区103cm的浅钻沉积物开展研究,采样间距2cm,共取得52块样品用于孢粉分析。沉积物年龄采用AMS-^(14)C测定,经校正之后钻孔底部年龄为3455cal.a BP。经实验分析之后,所有样品均含有丰富的孢粉类群,反映的植被组成主要以松(Pinus)林为主,铁杉(Tsuga)也较为常见,被子植物中则以栎属(Quercus)占绝对优势,栗属(Castanea)和木犀科(Oleaceae)植物也频繁出现,而草本植物则以禾本科(Gramineae)为主,蓼科(Polygonaceae)、菊科(Asteraceae)和莎草科(Cyperaceae)植物也是重要的组成成分,林下生长有大量的蕨类的植物,主要以紫萁属(Osmunda)、石松属(Lycopodium)和膜蕨科(Hymenophyllaceae)等为主,另外,采样点草海为季节性湿地,水生植物主要以香蒲属(Typha)和藻类植物中的双星藻(Zygnema)和水绵(Spirogyra)最为常见。为了获得该地区晚全新世以来的气候变化曲线,本研究采用共存分析法分别对划分的四个孢粉带进行了古气候的定量重建,年均温和年均降水量分别为:3455cal.a BP至2585cal.a BP,MAT(mean annual temperature)=11.5~18.6℃,MAP(mean annual precipitation)=797.5~1484.3mm;2585cal.a BP至1699cal.a BP,MAT=11.7~18.6℃,MAP=617.9~1523.1mm;1699cal.a BP至595cal.a BP,MAT=8.5~18.6℃,MAP=797.5~1484.3mm;595cal.a BP至今,MAT=5.7~18.6℃,MAP=617.9~1484.3mm。结果显示该地区3455cal.a BP以来气候经历了略微变暖-变凉-持续变凉-回暖四个阶段。本研究还将变化曲线与其他地区同时代的研究结果进行了对比分析,显示年均温的变化趋势与格陵兰冰芯δ^(18)O以及祁连山敦德冰芯所反映的温度变化趋势基本一致,并可观察到中世纪暖期、晚全新世小冰期及现代温暖期。

There is little influence by human activities in the Lugu Lake area, and it＇s sensitive to the climatic changes. Therefore, Lugu Lake is an ideal place to carry out the paleoclimate study. In this study, we drilled a 103 cm long core in Caohai, Lugu Lake area. In total of 52 samples were obtained for pollen analysis from the core with the interval of 2 cm. The dating was measured by AMS-^14C method, and the calibrated age of the bottom is 3455 cal. a BP. Abundant of pollen and spores were obtained from all the samples. The vegetation is described as follows： Pinus trees are predominant through all this period, and Tsuga trees are also with frequent appearances. Quercus trees dominate the angiosperms, Castanea and Oleaceae both occur frequently. Gramineae is the main component of herbs, together with Polygonaceae, Asteraceae and Cyperaceae. Ferns are one important composition of the understory vegetation, which including Osmunda, Lycopodium and Hymenophyllaceae etc. In addition, Caohai is a seasonal wetland and the aquatic plants are mainly composed by Typha, Zygnema and Spirogyra. In order to obtain the climatic changing curves since late Holocene in the Lugu Lake area, we applied coexistence approach to the four pollen zones. The MAT （mean annual temperature） and MAP （mean annual precipitation） are listed as follows： 3455 cal. aBPto2585 cal. a BP, MAT=11. 5～18.6 ℃, MAP=797.5～1484.3 mm; 2585 cal. aBPto 1699 cal. aBP, MAT=11.7418.6 ℃, MAP=617.9～1523.1 mm; 1699 cal. a BP to 595 cal. aBP, MAT=8.5～18.6 ℃, MAP=797.5～1484.3 mm; 595 cal. a BP to present, MAT=5.7～ 18.6 ℃, MAP=617.9～1484.3 mm. The climate since 3455 cal. a BP went through four phases： getting warmer slightly, being colder, still tend to colder and getting warmer again. We also compared our climatic changing curve with other records （such as δ^18O curves of Greenland ice core and Dunde ice core） which shows similar changing trend. The medieval warm period, little ice age and current warm period can be all observed.