若尔盖盆地黄土—古土壤序列化学风化特征及环境意义

Chemical weathering characteristics and environmental significance of the aeolian loess-paleosol sequence of Zoige Basin

若尔盖盆地黄土—古土壤序列较好地记录了古湖泊消失后的地表过程及演变。本文选取玛曲段黄河二级阶地欧强村剖面为研究对象,对其粒度、磁化率、总有机碳(TOC)、色度、元素等进行分析,利用光释光(OSL)方法测年断代。结果表明:(1)黄河二级阶地在10 ka BP前后开始接受连续的风尘堆积,发育的黄土—古土壤序列年代由下至上依次为河流相沉积物(T_(2-al),>10.0 ka)→黄土(L1,10.0—8.5 ka)→古土壤(S_(0),8.5—3.0 ka)→现代表土(L_(0)+MS,3.0—0 ka)。(2)欧强村剖面整体上处于以斜长石风化分解为主、钾长石尚未分解的初等化学风化阶段(CIA值在48.20~63.08之间),与黄土层相比,古土壤S0风化程度有所增强(CIA值60.83),但增强程度有限。(3)10 ka BP前后,若尔盖盆地气候干冷,风沙活动盛行,平缓的阶地面上开始接受持续的风尘堆积(形成黄土L1);8.5 ka BP前后,气候温湿,风沙活动明显减弱和风化成壤作用占主导,区域上形成了以黑灰色为特征的古土壤S_(0);3.0 ka BP前后,气候转为较冷干,风沙活动强烈,古土壤S0发育中断和被黄土L0所覆盖,形成了现代土壤MS。(4)若尔盖盆地黄土—古土壤序列在宏观形态、地层年代和物性参数方面与黄土高原可进行良好对比,但这些参数的绝对值和变化幅度差异显著,暗示高寒区(若尔盖盆地)这些参数的环境意义可能与季风湿润区(黄土高原)有所不同。本文可为若尔盖盆地风积物的年代学研究和环境信息的提取提供基础数据支撑。

The loess-paleosol sequence in the Zoige Basin is a geological record of the surface processes and evolution after the paleolake disappeared in this region.The Ouqiangcun(OQC)profile of the secondary terrace of the Yellow River in the Maqu reach was selected as the focus of this study.Based on the comprehensive investigation of multiple proxies,such as the grainsize distribution,magnetic susceptibility,total organic carbon,chroma,elements,and optically stimulated luminescence age,the following conclusions were obtained:(1)Continuous aeolian dust accumulation began on the Yellow River secondary terrace around 10 ka BP,and the developed loess-paleosol sequence from bottom to top was fluvial sediment(T_(2-al),>10.0 ka)→loess(L1,10.0-8.5 ka)→paleosol(S_(0),8.5-3.0 ka)→topsoil(L_(0)+MS,3.0-0 ka).(2)The OQC profile was in the primary chemical weathering stage where plagioclase was mainly decomposed and potassium feldspar was not decomposed(CIA value is between 48.20 and 63.08).While the paleosol was slightly increased when compared with the loess layers,this enhancement was limited,and it had not progressed to the moderate weathering stage.(3)Around 10 ka BP,the climate of the Zoige Basin was dry and cold,aeolian sand activity was prevalent,and continuous wind-dust accumulation began to occur on the gentle terrace,which formed loess L1.Around 8.5 ka BP,the climate was warm and humid,the activity of the aeolian sand was obviously weakened,and the weathering of the pedogenesis was dominant.Paleosol with black and gray characteristics was formed during this period.Around 3.0 ka BP,the climate changed to cold and dry and the aeolian sand activity was intense.The development of paleosol was interrupted,it was covered by loess L0,and the modern soil was formed.(4)The loesspaleosol sequence of the Zoige Basin was significantly different in terms of the macroscopic morphology,stratigraphic age and physical property parameters when compared with those of the Loess Plateau.This suggests that the environmental significance of these parameters in the high-cold area(Zoige Basin)may be different from that in the monsoon humid area(Loess Plateau).The findings provide basic data to support the study of the chronology of aeolian sediments and the extraction of environmental information in the Zoige Basin.