临夏黄土记录的26万年来季风快速变化

黄土高原西部高分辨率黄土序列开展的多代用指标研究,有助于揭示轨道-亚轨道尺度上东亚冬、夏季风的变化特征及动力机制。但针对同一地质记录提取冬、夏季风多代用指标进行对比和敏感性分析的工作尚不多见,因此制约了对季风快速变化特征和机理的理解。本文利用甘肃临夏85 m黄土岩芯开展高分辨率的粒度、磁化率和元素扫描分析,发现在轨道尺度上粒度和Zr/Rb比值显示的大幅冰期-间冰期波动与深海氧同位素记录有很好的对应关系,表明东亚冬季风的波动响应于北半球高纬冰量的变化。但在千年尺度上,粒度能更敏感地响应冬季风的快速变化;Rb/Sr比值和磁化率均能很好记录轨道尺度上的东亚夏季风波动,但在亚轨道尺度上Rb/Sr的敏感性明显高于磁化率。粒度、Zr/Rb和Rb/Sr所指示的东亚季风快速变化表现出较强的岁差信号,与石笋和北纬65°夏季太阳辐射有较好的对应关系,表明东亚季风变化也受到岁差直接驱动的夏季太阳辐射变的控制。在最后两个冰期-间冰期旋回中(即末期冰期旋回和倒数第二次冰期旋回)Rb/Sr及Zr/Rb比值和粒度记录分别可以辨别出与石笋相同数量级的变暖事件,但粒度记录的暖事件明显要多于Rb/Sr及Zr/Rb比值,且这些暖事件并不能一一对应。粒度及元素比值记录显著变冷的千年尺度的Heinrich(H)事件与石笋记录的夏季风明显减弱的事件有很好的对应关系,但其发生频率明显高于海洋记录。这些指标记录的千年尺度上波动信号的不一致性主要归因指标对不同气候要素敏感性的差异。未来需要进一步开展高分辨率多代用指标敏感性分析,选取对季风快速变化的敏感指标,有助于我们理解季风快速变化的特征(如周期和幅度等)和分析季风快速变化的时空差异性。

Rapid Asian monsoon changes recorded by loess depositions in Linxia since 260 ka B.P.

Multi-proxies studies of high-resolution loess-paleosoil sequences in the arid northwest Chinese Loess Plateau(CLP) are the key access to revealing the variation characteristics and driving mechanism of Asian monsoon in orbital-suborbital timescale. However, few comparison and sensitivity analysis researches of the multi-proxies obtained from the same geological archive are available for us to refer to, which restricts us to better understand the characteristics and driving mechanism of rapid monsoon changes. In this study, we chose the upper 85 m of two parallel loess cores(35.63°N, 103.12°E) retrieved from Linxia City in Gansu Province to conduct high-resolution mean grain size, magnetic susceptibility(2-cm resolution) and geochemical elements scanning(1-cm resolution) analysis. Ten age control points are selected to match the loess/paleosol boundaries to the glacial/interglacial transitions to reconstruct chronosequence of Linxia loess core since the past 260 ka. Based on the analysis above, the results show that mean grain size and Zr/Rb exhibit significant glacial-interglacial fluctuations, matching well with marine isotope record over the last 260 ka and confirming the strong coupling between the winter monsoon and Northern Hemisphere ice volume. But mean grain size is relatively more sensitive than Zr/Rb in millennial timescales. Rb/Sr ratio and magnetic susceptibility can well document glacial-interglacial variations of summer monsoon. Nevertheless, summer monsoon can be more elaborately recorded by Rb/Sr ratio. The Aisa monsoon implied by mean grain size, Zr/Rb and Rb/Sr ratios exhibits obvious precessional signal, corresponding well with summer insolation at 65°N and speleothem δ^18O of Sanbao-Hulu caves, manifesting that summer solar insolation is one of the forcing factors of East Asian monsoon fluctuations. Compared with speleothem curve, mean grain size, Zr/Rb and Rb/Sr ratios can detect the same order of magnitude warm events during the last two glacial cycles. But the number of warm events recorded by mean grain sizes is more than that by Zr/Rb and Rb/Sr ratios, which can not be well correlated with each other. Significantly cooling Heinrich events implied by mean grain size and Zr/Rb ratio are well associated with obviously weakening events of summer monsoon recorded by speleothem. But the frequencies of cooling events are higher than that of marine records. Mismatches of abrupt climate changes in the various proxies are mainly caused by the sensitivity discrepancy among different indicators. In the future, we should conduct more high-resolution and sensitive loess proxies research to help us to better understand the characteristics(such as timing and amplitude) and to analyze the temporal and spatial discrepancy of rapid monsoon changes.