中国南方红土记录的中更新世气候转型及其环境效应

Middle Pleistocene transition and its environmental effects revealed by red earth of South China

中更新世气候转型(MPT)是第四纪气候变化的重要过程,主要表现为全球气候系统的主导周期由41 ka转变为100 ka,全球冰量显著增多,海洋温度下降,陆地干旱化程度显著增强。与海洋沉积记录相比,陆相沉积记录的MPT研究易受载体的连续性和分辨率限制,总体上不多,而针对中国南方红土MPT的工作就更为稀见。本文基于ESR年代学框架,对江西九江九庐(JL)红土剖面的多个环境代用指标进行主成分(PCA)分析,尝试建立JL红土剖面第四纪环境演化序列,并据此与深海氧同位素记录进行对比,结果表明JL红土剖面记录了MPT过程,其中区域气候系统的相对稳定状态在MPT后可能被打破,区域气候向着冷干方向持续发展。综合考虑本研究结果与全球气候变化背景,推测MPT时期全球冰量增加、全球变冷、西伯利亚高压增强,共同驱动着中国南方地区的更新世气候演化。

The Mid-Pleistocene Transition(MPT,approximately 1.2 Ma to 0.5 Ma)was marked by fundamental changes in Earth's climate state,where the low-frequency,high-amplitude,quasi-periodic(100-ka)glacial variability emerged and characterized the Later Pleistocene and Holocene.Previously published MPT records reveal that evidences from the marine realm are abundant and high-resolution.However,discontinuity and low-resolution of the continental MPT records mainly limited the practice of further researches from land area.Moreover,only a few studies have tried to explore diverse manifestations of the Middle Pleistocene climate transition by terrestrial deposits.Consequently,further work is needed to fully understand and explore characteristics,mechanisms,and implications for MPT recorded in terrestrial deposits.In South China,red earth has been recognized as an important definition of this area,as it is covered by red soil at a large scale.Especially,red earth in South China deposited during the Quaternary period and documented the evolution of regional environment,which has enormous potential to decipher the environmental changes across the MPT.Thus,we selected the JL red earth section(29°42'N,116°02'E)located in Mount Lushan,Jiujiang,Jiangxi Province as study section,where the depth of the section is about 18.46 m.A total of 920 samples were taken at 2 cm interval from the JL red earth sections for particle size,color and magnetic susceptibility measurements.Our principal aim is to investigate the manifestations and the main causal mechanisms of the MPT based on works of ESR dating and multi-proxies analyses.A more careful analysis of chronology of our section and other red earth section in South China suggests that the chronostratigraphic framework of JL red earth section can be estimated to span ca.1200 ka to 233 ka.The multi-proxies analysis will be focused on Principal Component Analysis(PCA).Results of PCA analysis indicates that PCA F1 is a sensitive proxy for the degree of regional warming and cooling,which provides significant information for identifying the three MPT processes:early MPT(1200~738 ka),late MPT(738~536 ka)and post MPT(536~233 ka).Analyzing lithology of the typical Quaternary red earth sections in South China and the PCA F1 of JL red earth section shows that,during the early MPT,the average value of PCA F1 is 1.24 and vermiculated morphology of red earth sections is typical dense and thick,with clear stripes,which implying that the homochronous climate is warm and humid;during the late MPT,the average value of PCA F1 decreased to-0.31 with unconspicuous vermiculated characteristics,indicating a drier and cooler regional climate;during the post MPT,the average value of PCA F1 changed to-2.25 and maximum value of PCA F1 is about 0,when vermiculated morphology vanished and inversely yellowish-brown soil deposited,these evidences proved that regional climate deteriorated distinctly.However,the average value of global deep-sea oxygen isotope composition nearly maintained the same value from 1200 ka to 233 ka.Apparently the Pleistocene environment in South China recorded by JL red earth section failed to match changes in global ice volume,and after the MPT,preceding equilibrium state of regional climate system may be crossed,and the subsequent development of dry and cold climate marked that the regional environmental shifted irreversibly during this interval.The combination of our records and previous study suggests that increased global ice volume and global cooling and enhanced Siberian High Pressure integrally affected the evolution of environment in South China during the Pleistocene.In addition,the spatial distribution of MPT records in the East Asian region shows that wide-ranging aridification in the region may have exceeded the upper limit existed before the Middle Pleistocene.