青海官亭盆地史前灾难性地表过程及其影响研究

通过详细的野外考察,在官亭盆地喇家遗址东侧二方村遗址区域发现完整的土壤沉积物剖面。该剖面记录了全新世土壤土地资源的形成和发展演变,以及灾难性地表过程和人类活动的信息。在深入观察分析的基础上,系统采样并进行多种指标测试分析及年代测定。结果表明：其基本的土壤层序由上到下为：现代表土层(MS)—现代黄土(L₀)—全新世中期黑垆土(S₀)—全新世早期过渡性黄土(L_t)。该序列最显著的特征,是全新世中期黑垆土古土壤被厚达1.2~1.5 m的山洪泥流沉积层分隔为2段,表明在OSL年龄3940~3780 a,对应喇家遗址¹⁴C年龄3850~3600 a BP期间,官亭盆地北侧第三系红层丘陵沟壑区发生大规模暴雨山洪泥流过程。它们在古洪积扇前沿二级阶地溢出沟槽,扩散覆盖土地表面,形成了山洪泥流层组。而辛店文化早期(3600~3400 a BP)的透镜状灰烬层,则出现在该沉积层组的表面。这表明以农牧混合型土地利用为特色的早期辛店/卡约文化在该区域的形成发展,是以灾难性地表过程沉积的红色砂土和粘土地为基础的。这就与山洪泥流层组之下齐家文化(4150~3850 a BP)时期粟黍类谷物种植农业赖以形成发展的肥沃疏松的黑垆土类土壤形成对照。该研究结果揭示出该区域史前人类原始农业土地利用方式由谷物种植向农牧混合的转变,是在全球性气候恶化背景下,由灾难性地表过程造成的土壤和土地资源的退化所引起的。这对于深入了解青藏高原与黄土高原过渡带环境变化与土地利用方式发展演变的驱动因素具有重要的科学意义。

Prehistoric catastrophic surface processes and their impacts in the Guanting Basin,Qinghai Province

This study conducted field investigations in the Guanting Basin of the north shore of the Yellow River. A Holocene complete soil profile was identified over the Erfangcun Ruins, which is situated to the east side of the Lajia Ruins and has preserved multiple records of the information of aeolian loess and palaeosol and evolution of land resources, catastrophic surface processes, and human activities during the period of the Xindian culture. Sediment samples were taken from the profile systematically on the basis of field pedo-sedimentological observation. We established the basic pedo-stratigraphic sequence by using optically-stimulated luminescence (OSL) dating and physicochemical indices. The stratigraphic sequence of the Erfangcun profile from the top to the bottom is listed as modern soil (MS)-late Holocene aeolian loess (L₀)-Middle Holocene palaeosol (S₀)-early Holocene transitional aeolian (L_t). The most significant feature of this sequence is that the deposit of flashfloods and mudflows with a thickness of 1.2-1.5 m have split the Chernozem soil into two sublayers, which was the result of the catastrophic land surface processes and the climatic and environment changes. Optically-stimulated luminescence dating results suggest that enormous flashfloods and mudflows burst out during OSL age 3940-3780 a, equivalent to¹⁴C age 3850-3600a BP. They ran over the valley and deposited in the front of the paleo-alluvial-pluvial fans of the second terrace of the Yellow River. The deposit of the flows was covered by the ashes of the Xindian culture (3600-3400 a BP). The barren land formed by the flashfloods and mudflows caused the decline of agriculture and gave birth to stockbreeding. This soil layer is in sharp contrast to the fertile and loose soil during the period of the Qijia culture (4150-3850 a BP). Overall, the results of the study revealed that prehistoric human agricultural land use pattern in the area of the Lajia and Erfangcun Ruins in the Guanting Basin was changed by the degradation of soil caused by the catastrophic floods and climate change. It may have implications for further insights into prehistoric land use and the changes of environment in the transitional area of the Tibet Plateau and the Loess Plateau.