黄河上游官亭盆地喇家遗址地层光释光测年研究

通过对黄河上游官亭盆地喇家遗址周围黄河第二级阶地广泛的野外考察,在官亭镇下喇家村段发现了典型的风成黄土土壤层。在地层剖面可见其齐家文化层古地面被数组地震裂隙分割。有一层团块状的、鲜艳的红色粘土质泥流层覆盖了遗址的古地面,并且填充了地震裂隙。经过详细观测和采集光释光样品,应用单片再生剂量法(SAR) 获得该剖面样品的光释光(OSL) 年龄值。在此基础上,结合大量¹⁴C年龄数据和地层对比方法,确定官亭盆地黄河第二级阶地形成在35.00 ka BP,其后风成黄土土壤剖面开始发育。同时断定全新世古土壤S₀中间所夹红色粘土泥流沉积层及其下的地震裂隙的形成年代为3.95 ka BP。结合磁化率和粒度成分等气候替代指标分析,表明在35.00~11.50 ka BP 晚更新世晚期,亦即末次冰期的晚期(MIS-2),气候寒冷干燥,沙尘暴活动旺盛,在官亭盆地第二级阶地上堆积了典型的马兰黄土层(L_1-1)。在11.50~8.50 ka BP全新世早期,沙尘暴活动堆积了具有过渡性质的黄土层(L_t),反映气候逐渐变暖。在8.50~3.10 ka BP 全新世中期大暖期,气候变得温暖湿润,降水量增多,沙尘暴活动减弱,风化成壤作用强烈,堆积形成了黑垆土类古土壤层(S_0下、S_0上)。

OSL dating of the sediment stratigraphy of the Lajia Ruins in the Guanting Basin in the Upper Yellow River Basin

The palaeo-ground of the Qijia Culture in the Lajia Ruins was found to be broken by several groups of earthquake fissures. One layer of conglomerated clayey mudflow deposit has blanketed the Lajia Runis and filled with the earthquake fissures. After detailed field observations, samples for OSL dating were taken from the sediment profile. OSL ages of the samples were obtained by using the single-aliquot regenerative-dose method. A chronological framework was established in the XLJ profile based on the OSL ages and in combination with the ¹⁴C ages and stratigraphic correlations, which indicates that the second terrace of the Yellow River was formed at 35.00 ka BP. Then, accumulation of the eolian loess-soil profile started. The red clayey mudflow deposit and the earthquake fissures were formed at 3.95 ka BP in the second terrace in the Guanting Basin. Sedimentary samples were taken from the profile and magnetic susceptibility and particle-size distribution were analyzed in the laboratory. The results show that the Malan loess (L_1-1) was deposited during 35.00~11.50 ka BP. It was accumulated by dust storms and dust falls under a dry-cold environment during the late last glacial without being modified by pedogenesis. The transitional loess (L_t) was dated to be 11.50~8.50 ka BP. It was slightly affected by weathering and pedogenesis during the early Holocene because the climate was dry. The palaeosol (S₀), i.e., Heilusol or Chernozem, was dated to be 8.50~3.10 ka BP. It was strongly affected by weathering and pedogenesis during the mid-Holocene Climatic Optimum. However, a layer of conglomerated red clayey mudflow, which blanketed the Lajia Ruins at 3.95 ka BP, made the palaeosol (S₀) split into two sub-layers (S_0-lower and S_0-upper).