中国东部季风区湖泊表层沉积物细菌四醚化合物5-和6-甲基异构体与温度的定量关系及其古环境意义

The bacterial bGDGTs are important tools for the quantitative paleotemperature reconstruction in lakes, and thereby the modern investigation of the relationship between bGDGTs and environmental variables is the key to the accurate temperature reconstruction. However, the cross plot of existing MBT/CBT proxy versus mean annual temperature (MAT) exhibits large scatters and the calibration between them has a large error. One important reason is that bGDGT compounds of II and III series are essentially comprised of 5- and 6-methyl isomers. Due to the limitation of conventional liquid chromatography methodology, the isomers were not separated in previous studies. Instead, each of them was treated as a single compound. However, the response of different bGDGT isomers to environmental variable differs significantly. In this project, we will collect a series of surface sediments from lakes with a MAT spanning from -0.7°C to 24.1°C in the eastern monsoon region of China, and separated the 5- and 6-methyl bGDGTs using the newly developed liquid chromatography methodology. We will ascertain the relationship of 5- and 6-methyl bGDGTs with environmental variables, and establish novel proxies based on 5- and 6-methyl bGDGTs for quantitative paleotemperature reconstruction using the statistical methods. The optimized transfer function between bGDGTs and MAT will afford an important tool for the quantitative paleotemperature reconstruction in the lakes from the eastern monsoon region of China. In addition, the similarities and differences in the response of bGDGTs to environmental variables for lake and catchment soils will give hint to the understanding of the mechanism as to how bGDGTs respond to the environment.

细菌bGDGTs是湖泊古温度定量重建的重要工具，对其进行现代过程研究是其能否用于古温度准确重建的关键。已有的中国湖泊MBT/CBT指标与年均温之间的校正公式还存在较大误差，其中一个重要原因是未对bGDGTs-II和III系列所有化合物的5-和6-甲基同分异构体进行分离，而不同异构体对环境的响应差异显著。本项目将采集中国东部季风区大气年均温在-0.7°C～24.1°C范围内一系列浅水湖泊表层沉积物，利用申请者自己新开发的液相色谱方法分离5-和6-甲基bGDGTs，查明湖泊环境5-和6-甲基异构体与温度等环境因子之间的关系，通过统计学方法构建新的湖泊bGDGTs古温度定量重建指标，优化现有的bGDGTs与温度之间的转换方程，为东部季风区湖泊古温度等重建提供重要工具。同时，对比湖泊沉积物与其周边土壤5-和6-甲基异构体对环境因子响应异同，揭示MBT/CBT指标响应温度等环境因子表象下的真正原因。

湖泊微生物bGDGTs分子存在诸多异构体，查明这些异构体与环境因子之间的关系，不仅可以优化bGDGTs温度指标，还可以为了解bGDGTs指示温度机理提供重要参考。本项目通过调查包括中国东部在内的不同温度带湖泊表层沉积物、湖北武汉东湖（有机质内源性）和梁子湖（有机质内外来源混合型）不同季节表层沉积物和悬浮颗粒物质SPM中5-和6-甲基bGDGTs异构体与温度、pH等环境因子之间的关系，从空间和时间两个尺度探究了这类生物标志物对温度等的响应特征。在空间尺度上，湖泊表层沉积物中5-甲基bGDGTs的甲基化指数MBT’5ME与温度不存在显著关系，而6-甲基bGDGTs的甲基化指数MBT’6ME在高温区间（年均温MAAT>5 °C）和低温区(MAAT< 5 °C) 与年均温分别存在显著相关关系，但斜率和截距存在差别。这说明在碱性湖泊中6-甲基bGDGTs对温度的响应更为敏感，在低温时bGDGTs的合成具有明显的季节性。当低温区使用生长季温度时，高温区和低温区数据点与温度的关系能够拟合在一条回归线附近。通过多元线性回归，获得了中国区域内湖泊bGDGTs重建生长季平均温度的校正方程。在季节时间尺度上，东湖表层沉积物古菌和细菌GDGTs化石分子均存在显著的季节变化，古菌GDGT-0/cren与水体溶解氧DO负相关，而细菌bGDGTs的6-甲基bGDGTs甲基化指数MBT’6ME与采样前一个月温度MATm有显著正相关关系，与空间尺度上不同气候带湖泊结果相吻合。有陆源输入的梁子湖悬浮颗粒物5-甲基bGDGTs的甲基化指数MBT’5ME与温度关系显著。这表明，湖泊表层化石分子具有较快的周转速率，湖泊和土壤bGDGTs对温度的响应存在明显差别，可能具有不同的微生物来源。以内生有机质为主的湖泊中6-甲基bGDGTs响应温度，而有土壤影响的湖泊中5-甲基bGDGTs对温度响应更为敏感。