1100万年以来中国北方风尘堆积与古气候变化的周期及驱动因素分析

Cycles and Forcing Mechanism of Wet-Dry Variations in North China during the Past 11.0 Million Years Revealed by Wind-Blown Silt Deposits

中国北方连续堆积的晚新生代风成红粘土-黄土,是气候干湿变化的良好地质记录。对第四纪黄土的研究,揭示了中国北方干湿的周期变化,但是中新世以来的古气候变化规律及其驱动机制还不清楚。通过对具有代表性的红粘土-黄土序列的粒度测试,并根据独立的古地磁地层年代控制,建立了我国北方1100万年以来气候变化的时间序列。在此基础上,应用传统谱分析和小波变换的方法,对1100万年以来的干湿变化进行了分析,讨论了11-7.3Ma,8.0-5.0Ma,5.3-2.5Ma,2.5-1.2Ma和1.2-0Ma期间我国北方和亚洲中部干湿变化的周期,发现在不同时间段分别存在着-20ka的岁差周期、-40ka的黄赤交角周期和-100ka的地球轨道偏心率周期,获得了地球轨道运动变化引起太阳辐射变化驱动晚新生代亚洲内陆干湿变化的新证据。同时,还发现风尘沉积的粒度时间序列存在非轨道周期,可能是轨道周期谐振和时间标尺不确定性造成的。地球轨道运动变化引起的北半球太阳辐射量的变化,从而引起北半球冰期-间冰期旋回、西风带位置移动和亚洲季风强度的共同变化,可能控制着晚新生代我国北方粉尘沉积过程和古气候变化。加强粉尘堆积记录和古气候模拟研究,可深入认识我国北方晚新生代干湿变化的规律和机制。

There occur thick wind-blown silt deposits which are regarded as a good geological archive of paleoclimatic changes in Late Cenozoic in North China and the central Asia. In these aeolian sequences, the Quaternary loess deposits were widely investigated and the Milankovitch cycles were found in the paleoclimatic changes. However, the frequencies and forcing mechanism of wet-dry changes in Pliocene and Miocene are not clear to date. In this study, the representative aeolian silt sequences in Chinese Loess Plateau and northeastern Qinghai-Tibetan Plateau are systemically sampled and measured for grain size distribution. On the basis of the previous optically stimulated luminescence dating and magnetostratigraphic investigations, the time series of the grain size variations of the past 11.0 Ma is established, which is a good proxy indicator of the wet-dry changes in North China and the interior Asia. We slice the new time series into 11-7.3 Ma, 8.0-5.0 Ma, 5.3-2.5 Ma, 2.5-1.2 Ma and 1.2-0 Ma, and utilize Blackman-Turkey spectrum analysis and wavelet transfer to detect cycles of the long-time climatic changes. Our results show that the cycles of -20 ka of the precession, -40 ka of the obliquity and-100 ka of the eccentricity have been dominated in the time series, demonstrating that insolation has forced the paleoclimatic changes in the past 11.0 Ma. At the same time, the non-orbital cycles found in this new time series might be caused by harmonics of the orbital cycles and/or the spurious time scale, induced by a vulnerable depositional process of the aeolian sediments. We suggest perturbation of the Earth orbital trajectory, which caused the insolation changes in the north Hemisphere, might drive the replacement of the Westerlies and the Asian monsoon changes, and controlled the wet-dry variations in North China and the central Asia. Further investigation on seeking for complete aeolian records and strengthening modeling work would improve our understanding of environment changes in North China and the central Asia.