【摘要】 第四纪气候冰期-间冰期旋回与地球轨道周期性变化之间的关系是古气候研究的重点。文章利用中国黄土沉积的粒度和次生碳酸盐微量元素记录,在同一剖面中同时获得了冰量信息和地球轨道斜度信息。黄土的平均粒径反映了冬季风的强度从而指示了北半球高纬冰盖的变化,而次生碳酸盐(生物微钙体)的Sr/Ca比值记录了东亚夏季降水的变化,反映了斜度对东亚夏季风的控制作用。通过分析冰消期时粒度快速变化所处的夏季降水指示的斜度变化的相对位置,发现过去1.5 Ma以来的冰消期总在地球轨道处于较高斜度时发生,从而验证了斜度对第四纪冰期-间冰期旋回的驱动作用。更多还原

【Abstract】 Investigating the causal link between Earth’s orbital parameters and Quaternary glacial-interglacial cycles remains one of the most important branches of paleoclimatic research.This is largely due to the discrepancies between the intensity of summer insolation in the precession band and Pleistocene glacial cycles in the obliquity and eccentricity bands.In this study, we have reconstructed proxy records that document both the Northern Hemisphere ice-sheet volume and the obliquity of Earth from the same loess-paleosol profile on the central Chinese Loess Plateau over the last 1.5 million years, enabling us to explore the potential connections between them.Loesspaleosol samples were collected every 5 cm at Xifeng(35045’N,107°47’E) and Zhenyuan(35043’N,107°12’E)sections, which can be well correlated to each other using magnetic susceptibility stratigraphy.Secondary carbonate samples(microcodium) were then picked out from the >75 μm fraction of the loess-paleosol samples.Previous studies have suggested that microcodium Sr/Ca ratio can be used as a regional measure of summer precipitation.The reconstructed paleo-precipitation records at Xifeng and Zhenyuan show consistent variations over the last 480 thousand years, confirming the reliability of microcodium Sr/Ca as a summer precipitation proxy.The summer precipitation records show a persistent obliquity cycle of the 41-ka over the last 1.5 million years, which can be attributed to the gradient of boreal summer insolation that modulates the thermal contrast between the Asian continent and surrounding oceans and thus the strength of East Asian summer monsoon.In the loess deposits, the position of terminations in the record can be precisely constrained by rapid shift in grain size and magnetic susceptibility.We find that the mid-points of the terminations are always associated with the peak precipitation.By tuning the summer precipitation to the obliquity periodicity, i.e., matching the peaks and valleys of the precipitation to the obliquity highs and lows, we determine the phase of the terminations in the obliquity band of the precipitation record.We find that the terminations always occurred at high obliquity during the past 1.5 million years, which lends support to the hypothesis of obliquity pacing of glacial terminations.By controlling the insolation integrated over the duration of the boreal summer season and influencing the meridional transport of moist air, obliquity has played a major role in regulating the ice age terminations since the Early Pleistocene.The gradual transition at the Middle-Pleistocene Transition( MPT) is likely a result of the increasing energy threshold that is required to melt the ice-sheet rapidly, causing one or two obliquity maxima being skimmed and explaining the quasi 100-ka cycles after the MPT.更多还原