化石硅藻新指标在西藏昂仁湖环境演变中的应用

APPLICATION OF NEW DIATOM INDEX IN RECONSTRUCTION OF HOLOCENE ENVIRONMENTAL CHANGES IN ANGREN LAKE, SOUTHERN XIZANG (TIBET)

应用化石硅藻属组合和属系数两个新指标对西藏南部昂仁湖 (2 9°18′N ,87°11′E)剖面硅藻记录进行解析 ,得出该地区在 110 0 0— 6 0 0 0aBP间经历了寒冷期 (110 95— 9970aBP)→温和期 (9970— 9870aBP)→升温期(9870— 9770aBP)→凉干期 (9770— 8740aBP)→超高温期 (8740— 85 90aBP)→高温期 (85 90 - 5 970aBP)的演变序列。同时发现在寒冷期存在增温 (10 710aBP)和降温 (10 430aBP)波动 ;凉干期 880 0— 876 0aBP附近存在强低温事件 ;超高温期中的最高温出现在 86 80aBP附近 ;在高温期 80 0 0— 792 0aBP和 7170— 70 90aBP两个小时段分别存在增降温波动。在 6 430aBP附近存在明显的旱化现象。同时发现在研究时期除了存在 10 0 0年的气候变化周期外还存在 15 0— 2 0 0年左右的气候转型期。昂仁湖发育则至少经历了缓慢扩张 (110 0 0— 8740aBP)→快速扩张 (8740— 85 90aBP)→缓慢退缩 (85 90— 5 970aBP) 3个大阶段。

Based on the analysis of diatom record in Angren, southern Xizang by using new indexes of diatom genus spectrum and ratio of genus/species, the climatic sequence between 11 000aBP and 6 000aBP at that area is established, that is: cold period ( 11 095－9 970 aBP) → temperate period ( 9 970－ 9 870 aBP) →anathermal (9 870－9 770 aBP) → cool-dry period (9 770－8 740 aBP)→super-megathermal (8 740－8 590 aBP) →megathermal (8 590－ 5 970 aBP). And some climatic fluctuations or oscillations in the period are found, those are: (1) low temperature at 10 710aBP and high temperature at 10 430 aBP in Younger Dryas, (2) an abrupt drop of temperature between 8 800－8 760 aBP in the cooldry period, (3) occurrence of the highest temperature at about 8 680 aBP in the super-megathermal, (4) the pulses of high and low temperature in the megathermal respectively in 8 000－7 920 aBP and 7 170－ 7 090 aBP, and (5) obvious dryness at 6 430 aBP in the mid-Holocene. Apart from climatic periodicity of 1 000 years, the transitional period of climatic changes with 150－200 years between 11 000－6 000 aBP is found too. Furthermore, the development of Angren Lake between 11 000－6 000 aBP underwent three main stages of gradual expansion (11 095－ 8 740 aBP), abrupt expansion (8 740－8 590 aBP) and gradual shrinkage (8 590－5 970 aBP). Moreover, the results also show that the new diatom indexes are effective and convenient at least in studying environmental changes in closet lacustrine basin, which is rich in fossil diatoms.