河北坝上地区中晚全新世粒度端元分析及其环境意义

MID-LATE HOLOCENE END-MEMBER ANALYSIS AND ITS ENVIRONMENTAL IMPLICATIONS IN BASHANG REGION, HEBEI

河北坝上地区地处东亚夏季风边缘,对气候变化响应敏感,是研究古气候环境演变的理想地区之一。本研究采用粒度端元模型分析方法对坝上地区2 m厚的典型风成砂-古土壤剖面沉积物粒度数据进行分离,同时结合粒度、磁化率和色度指标,探讨剖面沉积物各粒度端元所指示的环境意义以及坝上地区中全新世以来的环境演化过程。结果表明:1)白云湖剖面粒度组分分为3个端元,EM1代表区域暖湿气候背景下的风化成壤作用,间接指示区域的湿度状况;EM2代表区域风沙活动强度;EM3可能反映区域河流对基岩风化产物的搬运作用。2)研究区中晚全新世气候变化大致可以分为5个阶段:7.1 ka B. P.以前,风沙活动和河流搬运作用较强,气候表现为冷干特征;7.1~3.8 ka B. P.风沙活动减弱,气候温暖湿润,区域成壤作用加强;3.8~2.6 ka B. P.期间,气候波动频繁,处于暖湿-冷干过渡阶段;2.6~1.5 ka B. P.风沙活动增强;1.5 ka B. P.以来,气候由冷干向相对暖湿转变,成壤作用稍有加强。

Bashang region of Hebei Province is located in the edge of the East Asian summer monsoon and is one of the ideal regions for studying the evolution of paleoclimate environment due to its sensitivity to climate change. In this study, a typical aeolian sand-paleosol profile was obtained in the Bashang Region, named BYH(41° 32′26.44″N, 116° 06′51.73″E, 1563 m a. s. l.;200 cm in thickness), and sampled at 2 cm intervals. Using AMSC and OSL dating techniques combined with linear interpolation and extrapolation, the age range of the profile is determined to be around 0~7.00 ka B. P. At the same time, environmental proxy indicators such as grain size, magnetic susceptibility and chromaticity of sediments were measured.We reveal that the grain size data can be decomposed into 3 end-members. EM1 is the thinnest component in the profile with a peak value of 24.1 μm, consisting of a silt-sized main component, which belongs to the short-term suspended component, and it has the highest content in palaeosol layer(50.9%). Combined with other environmental proxy indicators, we believe that EM1 represents the weathering and pedogenesis under the background of regional warm and humid climate, and indirectly indicates the regional humidity status. The peak value of EM2 is 185.8 μm, showing a weak negative skew state, and it is dominated by ultra-fine sand and fine sand, with high values appearing in aeolian sand layers and fluvial sand-gravel layers(60.06%). Combined with other environmental proxy indicators, we think that EM2 represents the intensity of regional sandstorm activity. The peak value of EM3 is 351.7 μm, mainly medium sand, which is the coarsest component in the section. According to the change of EM3 with lithological depth, the content maximum is located in the fluvial sand-gravel layer(43.51%)before 7.1 ka B. P., and the content is less in other layers. In addition, a large number of poorly rounded gravels with a diameter of less than 1 cm can be seen in this layer. Based on this, it is speculated that during this period, the rivers in the region had large runoff and strong hydrodynamic effects, which transported the weathered and detrital materials of the upstream bedrock to this point. Therefore, EM3 is likely to be related to the effect of rivers on the transport and sedimentation of surface sediments in the basin.Based on the existing research results, we have discussed the mid-Late Holocene climate change in the study area. The results show that: The mid-Late Holocene climate change in the study area can be roughly divided into five stages:(1)Before 7.1 ka B. P., wind-sand activity and river transport were strong, the climate is cold and dry;(2)7.1~3.8 ka B. P., the aeolian sand activity is weakened, the climate is warm and humid, and the regional pedogenesis is enhanced;(3)3.8~2.6 ka B. P., the climate fluctuates frequently and is in the transitional stage of warm-wet to cold-dry;(4)2.6~1.5 ka B. P. aeolian sand activity increased;(5)Since 1.5 ka B. P., the climate has changed from cold and dry to relatively warm and humid, and soil formation has been slightly strengthened.