Late Miocene Xianfeng palaeoflora and palaeoclimate of Yunnan Province, southwestern China

The late Miocene Xianfeng flora of Yunnan Province, southwestern China, was chosen to reconstruct the paleoclimate and the intensity of the Asian monsoon. Three available quantitative climate reconstruction methods from fossil plants, i.e. Leaf Margin Analysis (LMA), the Climate Leaf Analysis Multivariate Program (CLAMP), and the Coexistence Approach (CA) were used in this study. MATs (mean annual temperatures) resulting from these three approaches (i.e., LMA: 17.2 ± 2.38 °C; CLAMP: 15.43 ± 1.25 °C; CA: 17.2-21.7 °C) appear to have been higher than the present (14.9 °C), suggesting a much warmer climate in the late Miocene. Both the growing season precipitation (GSP) from CLAMP (1908.7 ± 217.7 mm) and mean annual precipitation (MAP) from CA (1206.0-1613.0 mm) estimates are higher than modern values (1003.2 mm (GSP) and 1038 mm (MAP) respectively) indicating a more humid climate during the late Miocene. By comparing these with climates reconstructed from neighboring late Miocene floras, we conclude that the general late Miocene climate appears warmer and more humid than present conditions in southwestern China. Furthermore, the evident differences in estimated monthly temperatures between the summer and winter, and precipitations between the humid and dry seasons indicate the existence of seasonality, though not as strong as that of today. Difference in precipitation of dry season indicates a marked strengthening in the winter monsoon since the late Miocene. A new monsoon intensity index has also been defined based on precipitation seasonality to investigate the intensity of the Asian monsoon.

本研究选取中国西南云南省晚中新世先锋植物群，以重建古气候与亚洲季风（Asian monsoon）强度。本研究采用三种已有的化石植物定量气候重建方法，即叶缘分析法（Leaf Margin Analysis, LMA）、气候叶片分析多元程序（Climate Leaf Analysis Multivariate Program, CLAMP）以及共存分析法（Coexistence Approach, CA）。三种方法得到的年平均气温（mean annual temperatures, MAT）结果如下：LMA法为17.2±2.38 ℃，CLAMP法为15.43±1.25 ℃，CA法为17.2~21.7 ℃，均高于现代年均温（14.9 ℃），表明晚中新世时期气候较现代更为温暖。CLAMP法得到的生长季降水量（growing season precipitation, GSP）为1908.7±217.7 mm，CA法得到的年平均降水量（mean annual precipitation, MAP）为1206.0~1613.0 mm，两项数据均高于现代对应值（现代生长季降水量为1003.2 mm，年平均降水量为1038 mm），指示晚中新世时期气候更为湿润。通过与周边晚中新世植物群重建的气候数据进行对比，本研究认为中国西南地区晚中新世整体气候较现代更为温暖湿润。此外，估算得到的夏冬两季月均温差与干湿季降水量差均显示存在气候季节性，但其强度弱于现代。旱季降水量差异表明，自晚中新世以来冬季季风强度显著增强。本研究还基于降水季节性定义了一种全新的季风强度指数，用以探究亚洲季风的强度。