斜率调制周期影响黄土高原夏季风降雨机制初探

Preliminary study on the mechanism of obliquity modulation affecting the summer monsoon rainfall in the Chinese Loess Plateau

晚中新世以来,全球气候和生态系统发生了显著变化。中国黄土高原地区广泛堆积的风成红粘土沉积层序蕴含着晚中新世-上新世时期丰富的古气候变化记录,对破译地质历史时期的古气候和理解现代全球气候变化具有重要意义。本研究选取了黄土高原由西向东的6个代表性风成红粘土剖面进行分析,旨在探索其中的周期性降雨模式。借助磁化率这一对夏季风活动较为敏感的代用指标,对黄土高原不同地区的红粘土剖面进行了系统的频谱分析。结果揭示,红粘土中表现出明确的400 ka偏心率周期,以及1.2 Ma和173 ka的斜率周期。进一步的研究中,分析了位于20°N的中国东南沿海和40°N的黄土高原北界的日照量以及两者之间的日照梯度。在这一范围内,太阳辐射的振幅调制(AM)主要显示出400 ka和100 ka的偏心率周期,而其纬向日照梯度不仅呈现偏心率周期,更明显地展现了1.2 Ma和173 ka的斜率调制周期。这也解释了黄土高原的季风气候除了受常规的偏心率周期调控外,还受到1.2 Ma和173 ka斜率周期的制约,尤其是1.2 Ma周期的影响尤为显著,为晚中新世至上新世季风变化提供了新的时间参考框架。斜率周期通过调整太阳辐射的纬向差异,进而影响热带辐合带(ITCZ)的位置和海陆温差,从而对夏季风的强度产生作用。

During the Late Miocene,the Earth witnessed marked shifts in its climatic and ecosystem dynamics,evolving into patterns discernible today.Amongst the pivotal archives that contain insights into these transformative epochs is the eolian red clay sedimentation observed extensively in the Chinese Loess Plateau(CLP).This significant deposit not only captures the details of Mio-Pliocene paleoclimatic transitions but also sheds the light on the links between the past and present global climatic patterns.In our study,we carried on a comprehensive analysis of various representative eolian red clay sections,across the west(Dongwan),central(Lingtai and Jingchuan),and east CLP(Shilou,Liulin and Wujiamao).Our goal was to understand the patterns within these eolian records,particularly in relation to orbital periodicities that have historically influenced Earth's climatic systems.Utilizing magnetic susceptibility(MS)—An effective proxy for summer monsoon activity,we began a detailed analysis across various areas of the CLP.This led to the identification of pronounced eccentricity periodicity(400 ka),supplemented by obliquity cycles(1.2 Ma and 173 ka).Diving deeper,we conducted a spectral analysis targeting the amplitude modulation(AM)of insolation at latitudes 20°N(similar to the Southern China coast)and 40°N(similar to the north end of the CLP).Additionally,we examined the insolation gradient distinguishing these latitudinal bands.Pertinently,our analysis unearthed significant power contributions of AM of insolation across both 40°N and 20°N latitudes within the 400 ka and 100 ka eccentricity bands.However,the latitudinal insolation gradient was predominantly influenced by the 1.2 Ma and 173 ka bands.From these findings,we infer a broader climatic orchestration where the monsoonal patterns are regulated by not just the traditional eccentricity bands but are substantially modulated by obliquity cycles,especially the 1.2 Ma and 173 ka spectrums.Among these,the AM cycle within the 1.2 Ma band emerges as a dominant influence,steering monsoonal rhythms.Crucially,our study underscores the importance of the 1.2 Ma band as an alternative temporal framework for reconstructing the complex dance of the Mio-Pliocene monsoon epochs.Such obliquity-induced shifts directly impact insolation dynamics at higher latitudes,thereby modulating the latitudinal insolation gradient.This,in turn,alters the thermal gradient across land and sea,moving the Intertropical Convergence Zone(ITCZ)northward.This complex interplay of obliquity,insolation,and latitudinal thermal gradients plays a key role in modulating monsoon strength and influencing the summer precipitation patterns on the CLP,offering critical insights into past and present climate dynamics.