基于硅藻转换函数定量重建的云龙湖区近百年气候变化历史

A 100-year quantitative climatic reconstruction of Yunlong Lake area based on diatom transfer functions

湖泊硅藻是进行古气候定量重建的重要指标,但是目前关于其重建气候变化的可靠性和灵敏度,尚未得到有效验证。本研究选取云南省对气候变化极其敏感的高山湖泊——云龙天池作为研究对象,通过对该湖开展连续两年(2013.09—2015.08)的硅藻季节性变化调查,构建了硅藻—湖泊平均水温转换函数;通过调查表层沉积硅藻组合随水深的空间变化特征,构建了硅藻—水深转换函数。之后结合对湖心沉积钻孔中近百年来的硅藻组合分析,用硅藻转换函数方法分别定量恢复了近百年来的湖泊平均水温和水位变化过程。笔者将基于硅藻重建的湖泊水温和水位变化分别与流域近60年来的气温、降水记录以及水库建坝记录进行对比,结果显示,硅藻重建的湖泊水位既可以灵敏地揭示短期的降水变化过程,又耦合了湖泊因建坝蓄水和大幅扩容而导致的长期逐级抬升信号。而硅藻重建的近百年来的湖泊平均水温变化,在湖泊改建为水库之前与流域的气温变化信号基本一致,但在建坝蓄水和大幅扩容以后,区域气温持续升高,水温却呈现趋势性降低,显示短期内水库水量的大幅变化可能显著改变了湖泊的热力和温度结构,并导致喜冷的硅藻组合占据优势。本研究验证了依托同一湖泊现代过程构建的硅藻转换函数在定量重建不同时间尺度的区域气候变化方面具有较高的灵敏度和可靠性,为基于代用指标的古气候重建工作奠定了重要基础。

Although lake diatoms have been commonly used for quantitative paleoclimatic reconstruction, their sensitivity and validity have rarely been tested. At Yunlong Lake, a climate-sensitive alpine lake in Southwest China, we studied the seasonal succession of diatom assemblages(September 2013 to August 2015) to produce a lake mean water temperature(MWT) transfer function. In addition, based on the spatial pattern of surface diatom assemblages with water depth, we produced a diatom-water depth transfer function. Combined with the analysis of diatom assemblages in a sediment core(YL2013-A), changes in lake mean water temperature and water level over the last ~100 years were quantitatively reconstructed using the above diatom-based transfer functions. Comparison with records of regional meteorology and reservoir water capacity revealed that the diatom-based lake water level reconstruction is a sensitive indicator of short-term fluctuations in precipitation, and it also reflects a long-term stepwise rise in water level caused by the impounding and large-scale extension of the reservoir. The diatom-based MWT reconstruction is consistent with the changes in air temperature prior to large-scale human disturbance of the site. However, after the impounding and the extension of the reservoir, although the regional air temperature continued to increase, the water temperature decreased substantially. This suggests that the large increase in lake water volume in the short term led to a decrease in the lake mean water temperature, which in turn led to the occurrence of a diatom bloom in the cold season. The results demonstrate that diatom transfer functions based on modern monitoring of the same lake has a high sensitivity and can be validly used for the quantitative reconstruction of regional climate change on various timescales. Overall, our findings provide a foundation for the proxy-based quantitative paleoclimatic reconstruction.