克什克腾世界地质公园晚第四纪黄土沉积特征及其古气候意义

Sedimentary Characteristics of Late Quaternary Loess in Hexigten Global Geopark and Its Paleoclimatic Implications

黄土中蕴涵着多种古气候指标,对不同地区黄土地层中的粒度和磁化率进行测定和分析,有利于进行古气候旋回的划分和全球气候变化对比。本研究选取克什克腾世界地质公园三义乡全新世黄土剖面和土城子晚更新世黄土剖面为研究对象,对代表性黄土样品进行14C和光释光(OSL)测年以及粒度及磁化率分析,以重建研究区晚第四纪以来的古气候环境变化过程。三义乡剖面的14C测年结果为2.32 ka、9.05 ka、9.94 ka、11.27 ka,土城子剖面的光释光测年结果为21.5 ka、23.5 ka、43.9 ka、53.4 ka。分析结果显示,三义乡和土城子剖面均属于晚第四纪,后者老于前者,形成一个较为连续和完整序列的黄土剖面;黄土-古土壤沉积以粉砂(16~63μm)为主,含量占61.45%~62.4%,其次为砂粒(>63μm)和黏粒(<4μm);冰期表现为较粗的粒度和较低的磁化率值,而间冰期或间冰阶磁化率值明显增高,粒度则明显变细。三义乡和土城子剖面的粒度及磁化率记录了冷-暖、干-湿的气候旋回和至少4次夏季风强化事件,其发生时限分别为60~55 ka B.P.、34~25.6 ka B.P.、11.5~10 ka B.P.、8.6~2.2 ka B.P.。该特征与黄土高原洛川剖面的结果基本相似,反映两地的古环境演化在冰期-间冰期时间尺度上均受控于东亚季风的强弱变化。本研究为季风边缘区晚更新世以来古气候重建提供了新材料和参考数据。

Loess has various paleoclimatic indicators,among which the particle size and magnetic susceptibility are important objects for paleoclimate research,and detailed studies on loess layers in different regions can faci-litate global climate change comparison and climate cycle division.Taking the Holocene Sanyixiang loess profile and the Upper Pleistocene Tuchengzi loess profile of Hexigten Global Geopark as an example,the loess samples were subjected to 14C dating,photoluminescence dating,grain size and magnetic susceptibility analyses.The regional paleoenvironmental change since the Late Quaternary was reconstructed,which provided a new and strong evidence for studying the climate types in northern China.The 14C dating on the Sanyixiang profile yielded 2.32 ka,9.05 ka,9.94 ka and 11.27 ka,whilst the OSL dating on the Tuchengzi profile yielded 21.5 ka,23.5 ka,43.9 ka,53.4 ka,which show that the Sanyixiang and Tuchengzi profiles are Late Quaternary.The Sanyixiang and Tuchengzi profiles are young and old,respectively,forming a relatively complete loess sequence.The study shows that coarse silt(16-63μm)was a major course-size component in the loess-paleosol deposition,accounting for 61.45%-62.4%,followed by sand(>63μm)and clay(<4μm).Based on stratigraphic features and grain-size curve,the Hexigten area underwent cold-hot,dry-humid cycles and at least four summer monsoon intensifying events.The grain size and magnetic susceptibility records of the Sanyi-xiang/Tuchengzi loess profiles are basically similar to those of the Luochuan profile of the loess plateau,with coarser grain size and lower magnetic susceptibility during the glacial period.Meanwhile,the magnetic susceptibility of the interglacial or interglacial step increases significantly,and the grain size becomes significantly smaller,indicating that the paleoenvironmental evolution of the two places was controlled by the East Asian monsoon intensity in a glacial-interglacial timescale.