洞庭湖周边地区第四纪红土粒度特征及环境意义

第四纪风成红土广泛分布于中国南方，与北方的黄土同为反映第四纪古环境的重要载体。基于洞庭湖周边地区10 处剖面122 个红土样品的粒度分析，对该区红土沉积特征及其反映的环境意义进行探讨。结果显示：①样品均以粉砂（4~63 μm）为优势粒级，其次为粘粒（<4 μm），砂含量很少，并富集“风尘基本粒级”（10~50 μm），含量达33.23%~42.23%，粒度特征与典型的黄土、下蜀黄土、加积型红土样品相似，显著不同于湘潭洪冲积红土，也区别于本区砂岩风化壳红土，表明洞庭湖西、南、东缘丘陵岗地存在风成堆积成因的中亚热带风成加积型红土。② 黄棕色土、网纹黄棕色土、均质红土、网纹红土等4 类样品的粘粒含量由高到低的排序是网纹红土 >均质红土 >网纹黄棕色土>黄棕色土；粉砂含量由高到低的排序正好相反。结合粒度参数推测，黄棕色土发育时期，沉积作用增强，粒度变粗，分选性较好；网纹红土发育时期，沉积作用较弱，风化成壤作用强，细颗粒含量增多，分选性相对较差。这可能反映了洞庭湖地区中更新世至晚更新世冬季风加强，风化减弱的气候变化过程。③ 沿冬季风主导风向（偏北风），洞庭湖周边地区红土的M_φ、Md值（φ值）自北向南增大（变细），风成组分（10~63 μm）百分含量随之减少，<2 μm组分百分含量自北向南略有增加。这种趋势提供了组成该地区加积型红土的粉尘由偏北向南传输的风成证据和母质风化程度向南增强的环境信息。

Grain-size features of Quaternary red earth in Dongting Lake Area and their paleoenvironmental significance

Dongting Lake Area is a semi-closed basin opening toward the north. In this area, alluvial and lacustrine sediments mainly developed in fluvial plain, low level terrace of the rivers and modern lakebed, and Quaternary red earth is widespread in low mountains, hills and hillock. Scholars paid very little attention to the red earth in this area over a long period of time in the past. In order to reveal the genesis of the Quaternary red earth in Dongting Lake Area and its preliminary climatic implications, we did a series of work. The detailed field investigation has shown that there are several types of Quaternary red earth in Dongting Lake Area, and the major type is the red earth with homogenous structure without gravel. The intact stratigraphic profile consists of the lower reticulate red earth layer or reticulate yellow brown earth layer and the upper homogeneous red earth layer or yellow brown earth layer. Ten profiles of this type of Quaternary red earth and 122 samples were collected, and their grain-size was tested by using laser particle size analyzer Malvern 2000, a product of Malvern of the UK. The results showed that: (1) Silt (4~63 μm) is the major type of particle in all samples; clay (<4 μm) is the second major type; sand accounts for the smallest portion. Aeolian particle (10~50 μm) is the dominant particle, and the mean content ranges from 33.23% to 42.23%. These features are similar to those of the loess, Xiashu loess and aeolian red earth, but quite different from fluvial red earth from Xiangjiang Rriver terrace and the residual red earth formed with the rock weathering, indicating that aeolian red earth exists on the hilly land of the west, south and east sides of Dongting Lake Area and they all belong to aggradation red earth in Mid-subtropics. (2) All the samples can be separated into four categories: vermicular red earth, homogeneous red earth, reticulate yellow-brown earth and yellow-brown earth. In terms of grain size composition, their clay contents have a ranking order of vermicular red earth >homogeneous red earth >reticulate yellow-brown earth >yellow-brown earth, while their silt contents show an opposite ranking order. Combined with other information extracted from grain size parameters, we can speculate that sedimentation significantly strengthened during the period of yellow-brown earth, so the sediments become coarser and well sorted. However, in the period of vermicular red earth, sedimentation weakens and weathering strengthens, fine particle content increases, and the red earth are poorly sorted. This probably reflects the environmental change process in which winter monsoon strengthens and weathering weakens from the mid-Pleistocene to the late Pleistocene around Dongting Lake Area. (3) Along with the direction of the winter monsoon, the average particle size and median size (φ value) of the red earth become bigger (become smaller) southwards, but the percentages of aeolian component (10~63 μm) decrease while that of the<2 μm component increase slightly. These trend, therefore, provide aeolian evidence for the Quaternary aggradation red earth sediments and suggests that the intensity of weathering enhances southwards in the Dongting Lake Area.