北京王佐裂隙堆积及其记录的一次更新世暖期环境变化

THE WANGZUO FISSURE-FILLING AND ITS RECORD OF THE MIDDLE-LATE PLEISTOCENE WARM PERIOD ENVIRONMENT IN BEIJING AREA

含有哺乳动物化石的洞穴/裂隙堆积是研究北京更新世环境的重要信息载体。王佐裂隙堆积位于京西王佐镇山前地带,系富含哺乳动物化石的砖红色土状堆积,原裂隙周边灰岩已被人为剥离,残存堆积物整体沿灰岩裂隙向南倾斜状存在,长度十余米,最厚处约3.8 m。本研究首次报道了王佐裂隙堆积的发现及地层发育情况,并结合碎屑沉积物中的粒度、色度、磁化率等古环境代用指标,将其沉积过程划分为早、中、晚3个阶段。中值粒径、亮度、红度和磁化率这些古环境代用指标数值基本处于中-晚更新世黄土-古土壤序列中的古土壤层的变化范围,这一结果显示王佐裂隙堆积形成于水热条件较好的中-晚更新世暖期,记录了该暖期由盛而发展到极盛,持续较短时间后又迅速演变为相对稍弱暖期的气候变化过程。

Caves and fissure-fillings containing mammalian fossils are critical for studying Pleistocene environments.This study presents the first discovery and analysis of the Wangzuo fissure-filling(39°50′N,116°05′E;156 m a.s.l.),located in the piedmont area of Wangzuo Town,western Beijing.The surrounding limestone of the fissure has been artificially stripped,and the remaining reddish-brown soil-like filling,rich in mammalian fossils,extends southward along the limestone fissure,with a length exceeding ten meters and a maximum thickness of approximately 3.8 meters.We collected profile samples from the fissure-filling and analyzed their grain size,color,and magnetic susceptibility for paleoenvironmental reconstruction.The results show that the median grain size(Md)ranges from 6.97μm to 9.31μm with an average of 8.06μm,the redness(a*)value ranges from 5.07 to 9.92 with an average of 7.14,and the low-frequency magnetic susceptibility(χlf)ranges from 191.02×10-8 m3/kg to 343.02×10-8 m3/kg with an average of 249.90×10-8 m3/kg.By correlating these findings with the current loess-paleosol sequences in the Beijing area,we determine that this filling primarily belongs to the paleosol layer of the Middle-Late Pleistocene loess-paleosol sequence.Based on variations in median grain size,brightness,redness,and magnetic susceptibility,as well as the stratigraphy of mammalian fossils,we divided the fissure-filling into three layers from bottom to top,corresponding to the early,middle,and late stages of the Wangzuo fissure sedimentation.The Wangzuo fissure-filling formed during a warm period with favorable thermal and precipitation conditions in the Middle-Late Pleistocene.This warm period experienced rapid climate changes,evolving from a prominent warm peak to a relatively milder warm period.The paleoenvironmental proxies and mammalian fossils from the paleosol indicate that the Wangzuo fissure-filling reveals a paleo-warming event in the Beijing area during the Middle-Late Pleistocene.