黄土粒度分布中的超细粒组分及其成因*

多个剖面的粒度测量表明,黄土高原的黄土和古土壤中普遍存在超细粒组分,其平均粒径大致0.4μm,在全样中的含量大致5 ％ ~6 ％。 超细粒组分的含量在黄土中低,在古土壤中高;并且同一地层自西北至东南增加,这些特征都显示超细粒组分的形成与成壤作用强度相关。一般情况下超细粒组分的粒度在黄土中粗,在古土壤中细。黄土高原各季节的现代天然粉尘中同样存在超细粒组分,然而干旱区成壤作用较少的天然降尘中基本不存在超细粒组分。以上特征说明,黄土中的超细粒组分的形成与成壤作用有关,成壤作用中次生矿物的粒度和含量特征也支持这一解释。

SUPPER-FINE GRAIN SIZE COMPONENTS IN CHINESE LOESS AND THEIR PALAEOCLIMATIC IMPLICATION

Grain-size data from sections over the Loess Plateau indicate that loess and paleosol in that area contain supper-fine components with average size of about 0.4μm, which generally compose about 5 %~6% of the bulk sample. It is interesting that the grain size of supper-fine components in loess is coarser than those in paleosol while their proportion in loess is lower than that in paleosol. Plus that the proportion of supper-fine components in a specific layer increases from northwest to southeast of the Loess Plateau, we hypothesize that the formation of supper-fine components was associated with pedogensis. Supper-fine components are also found in present dust from the Loess Plateau, but are absent in the dust from arid area of Northwest China. This provides further confirmation for our hypothesis.