黄土记录的全新世千年-百年尺度冬夏季风强度变化

It is clear that changes of East Asian winter and summer monsoons intensity show anti-phased at orbital- and millennial-scale during the Pleistocene. In contrast, their changes at various timescale during the Holocene is still in debat. It is widely accepted that Chinese loess is an ideal material to revisit both the East Asian winter and summer monsoons. However, due to resolution, disturbance and chronology limit of the Holocene loess, there is still a lack of detailed reconstruction of changes of Holocene East Asian winter and summer monsoons intensity based on Chinese loess. In this study, after detailed field investigation on the Chinese Loess Plateau, approximately 2-4 high resolution Holocene loess sections will be selected, with 1 section already collected, and closely-spaced powder and luminescence samples will be collected for proxy and chronology measurement respectively. The reliable paleoclimate proxy of grain size and magnetic susceptibility are used to represent East Asian winter and summer monsoons intensity respectively, and fine-grained quartz optically stimulated luminescence (OSL) dating technique is used to get the high-resolution chronology. Then, changes of East Asian winter and summer monsoons intensity at orbital-scale, particularly at millennial- and centennial-scale can be reconstucted, and correlated with other regional paleoclimate reconstruction based on various materials. Also, the reliability of changes of East Asian winter and summer monsoons intensity reconstructed in this study can be discussed. Finally, considering the possible controlling factors of changes of East Asian winter and summer monsoons intensity, such as solar activity, Atlantic meridinal oceanic circulation etc., the possible dynamic mechanism of above monsoon changes at millennial- and centennial-scale will be suggested.

在更新世，东亚冬夏季风强度在轨道和千年尺度上呈现清楚的反相位变化，其在全新世多种时间尺度上的变化仍存一定争议。中国黄土被认为是同步记载东亚冬夏季风强度变化的良好载体，但由于全新世黄土地层的分辨率、扰动、年代制约等因素，导致基于中国黄土重建的全新世东亚冬夏季风强度变化资料较为稀有。本研究将在大量踏勘基础上，在黄土高原选取2-4个全新世高分辨率黄土剖面（已有1个），采取高密度的粉末样和释光样。用可靠的粒度和磁化率分别作为东亚冬夏季风强度的替代性指标，并重点以高精度的石英光释光测年技术为基础建立年代标尺，进而重建全新世或者全新世特征时段东亚冬夏季风在轨道，尤其千年和晚全新世百年尺度上的强度变化，并与区域其他重建结果进行对比，探讨本研究重建结果的可靠性。通过将重建结果与可能的气候控制因素（如太阳活动、北大西洋经向翻转流等）相联系，探讨全新世东亚冬夏季风在千年和百年尺度上强度变化的可能动力学机制。

针对东亚冬夏季风在全新世亚轨道（这里指多千年）和百年尺度上强度变化及其相互关系研究的广泛争议，基于黄土高原多个剖面，在高准确度释光年代和可靠气候代用指标的支撑下，主要开展如下研究：（1）全新世亚轨道尺度上东亚冬夏季风变化及其相互关系与动力学机制；（2）晚全新世百年尺度上东亚冬夏季风变化及其相互关系与动力学机制；（3）中国黄土记录的多尺度冬季风变化对于目前和未来全球变暖背景下其变化的预测意义。通过相关研究，主要取得如下重要结果：（1）来自于三个全新世高分辨率黄土剖面的独立和集成结果均显示：早全新世（约11.7-6.5 ka BP）东亚冬季风呈现持续衰退趋势，在约6.5 ka BP达到最弱，之后的中-晚全新世则表现为逐渐增强趋势，而东亚夏季风具有相反的变化模式。我们认为，早全新世和中-晚全新世冬季风强度变化分别主要受控于北半球高纬冰量和中-高纬度温度变化；（2）来自渭南晚全新世高分辨率剖面结果指示：东亚冬季风和夏季风频繁波动，呈反相位并表现出明显的约700-800年周期。我们认为太阳活动控制了上述百年尺度上东亚冬季风和夏季风强度的变化，另外北大西洋经向翻转流的强度变化亦有影响；（3）来自黄土高原4个高分辨率的剖面反映出：在多种地质时间尺度上，东亚冬季风在相对暖期强度较弱且变化幅度较小。而器测观测记录也支持东亚冬季风在1985年中期以后的全球变暖背景下处于较弱且年际变率较小的状态。因此我认为未来21世纪全球变暖背景下，东亚冬季风将在长趋势上持续减弱且（年际）振幅减小。上述研究结果有效解决了全新世多尺度上东亚冬夏季风强度变化及其相互关系的科学问题，并对认识目前和未来全球变暖背景下东亚冬季风的变化有重要现实意义。