中国黄土记录的季风快速变化

RAPID MONSOON CHANGES RECORDED BY CHINESE LOESS DEPOSITS

中国黄土高分辨率代用指标研究表明东亚季风气候存有亚轨道尺度的快速波动，与北半球高纬气候和大西洋经向环流变化关系密切。本文回顾前人在黄土高原地区多个典型剖面的研究结果，分末次冰期、最后两个冰期旋回及极端冰期（L9和L15）3个时段，阐述了粒度、元素比值、风化指数等代用指标揭示的冬、夏季风快速变化特征，并与冰芯、石笋和深海记录进行对比，探讨了季风突变事件的区域一致性和遥相关动力过程。结果表明，黄土高原西北部高分辨率黄土剖面代用指标对季风快速变化敏感，而东南缘黄土记录的突变事件不太清晰，这种差异受控于沉积速率和风化成壤的空间变化；在冰期-间冰期不同气候背景下，黄土粒度和元素比值的变化幅度和周期表现出较大差异，冰期幅度大、频率快，间冰期幅度小、频率慢，表明冰量大小对季风快速变化有调制作用；在两个极端寒冷期（氧同位素阶段22～24和38），黄土粒度同样表现出快速变化特征，但与北大西洋浮冰碎屑记录似乎难以对应，可能与海陆记录的分辨率差异和年代标尺不确定性有关。综上所述，季风快速变化特征在沉积速率相对较高、成壤相对较弱的黄土剖面比较清晰，在冰期一问冰期不同下垫面条件下季风快速变化的特征和机理可能不同。未来的季风快速变化研究，一方面在时间尺度上拓展高分辨率敏感代用指标研究，揭示季风快速变化的起源和演化特征；另一方面加强地质记录和数值模拟结果的对比研究，获得季风快速变化动力机理的全面认识。

Suborbital-scale monsoon variations, revealed by multiple proxies of high-resolution loess sequences, are dynamically linked with changes in high-latitude climate and the Atlantic Meridional Overturning Circulation. In this paper, we synthesized previous invesitgation on representative loess profiles （i.e., Luochuan, Weinan, Xifeng, Lingtai, Jiyuan, Mangshan, Jingyuan, Linxia and Gulang profiles） from the Chinese Loess Plateau （CLP） to elaborate the characteristics of rapid monsoon changes over three time intervals ： the Last Glacial stage, the last two glacial-interglacial cycles and the two extreme glacial intervals （corresponding to loess units L9 and L15）. Rapid variations of winter and summer monsoons are assessed using mean grain size, elemental ratios and chemical weathering index. By comparing loess proxies with indicators from ice core, marine sediments, and speleothem, we addressed the temporal-spatial characteristics of suborbital monsoon variability and their dynamic links to global climate change. It is suggested that abrupt monsoon events are well recorded by proxy indicators of several representative high-resolution loess profiles, matching well with the Heinrich events and Dansgaard-Oeschger cycles revealed by speleothem and ice-core records for the Last Glaciation. Spatially, loess proxies from high-resolution loess profiles in the northwest Chinese Loess Plateau （CLP） are sensitive to rapid monsoon changes, whereas the loess profiles in the southeast CLP are less sensitive to abrupt monsoon oscillations. Such a spatial difference is likely due to two main reasons： varying sedimentation rates and chemical weathering intensities across the CLP. During the last two climatic cycles, the amplitude and periodicities of loess proxies varied at glacial-interglacial timescales, characterized by large-amplitude and high-frequency fluctuations during the glacial stages. During the interglacials, however, relatively small-amplitude and low-frequency oscillations imply an interglacial modulation on the sub-orbital monsoon variability. Moreover, the winter monsoon also fluctuated on suborbital timescales during two cold climatic extremes （corresponding to marine isotope stages 22 ~ 24 and 38）. However, it is difficult to match sub-orbital winter monsoon variations with the ice rafting events （IRD） in the North Atlantic during these two glaciations, probably due to the resolution discrepancy and chronological uncertainties between terrestrial and marine records. In summary, previous loess studies reveal that high-resolution and weakly weathered loess sequences can well documented rapid monsoon changes. The features and mechanisms of rapid monsoon changes seem dissimilar under different glacial-interglacial boundary conditions and their amplitude and frequencies are likely modulated by varying glacial-interglacial conditions. So far, the characteristics and dynamics of abrupt monsoon changes during the early to Middle Pleistocene remains poorly constrained. In the future, more high- resolution and sensitive loess proxies should be investigated to better understand the onset and evolution of suborbital-scale monsoon variability during the Quaternary or Neogene. Furthermore, data-model integration should be strengthened to provide a thorough assessment of the origin and dynamics of abrupt monsoon changes.