次生碳酸盐结核的物质组成及元素富集特点

Substance composing and elements enrichment characteristics of pedogenic carbonate nodules

以黄土高原南缘武功次生碳酸盐结核为研究对象,通过分析各级结核中有机质和碳酸盐的分布特征、粒度及矿物组成,结合酸溶性元素分析,明确次生碳酸盐结核的形成过程及元素富集特点。结果表明,黄土高原南缘武功全新世古土壤发育期间形成的碳酸盐结核主要由粒径>250μm结核和粒径<50μm结核相互胶结而成。次生碳酸盐首先在粘粒及细粉粒表面淀积,土壤颗粒间的孔隙随着淀积的进行逐渐被次生碳酸盐填充,碳酸盐结核逐渐形成并不断生长。随着结核的生长,碳酸盐含量大幅增加,从而稀释降低了有机质含量;同时,其颗粒组成逐渐变粗,粘土矿物含量减少。各级结核中酸溶性元素均以钙(61.03~207.36g/kg)为主,且与碳酸盐含量呈正相关,因此,CaCO_3是结核中碳酸盐的主要形式。此外,酸溶性元素中含有一定量的次生无定形硅(8.90~10.70g/kg)和铝(2.78~5.06g/kg)以及粘土矿物中的钾(3.34~4.32g/kg)和镁(2.22~3.64g/kg);同时含有少量可溶性盐中的钠(0.70~0.85g/kg)。

The pedogenic carbonate nodules formed during the Holocene in the Wugong section on southernmost Loess Plateau were selected to study the mineralogical and acid-leachable elemental composition of carbonate nodules together with the specific properties including grain size and distribution of organic matter and carbonate in 250μm,50-250μm and50μm nodules,to understand the formation of carbonate nodule and assess its elemental enrichment characteristics.The results showed that the pedogenic carbonate nodules were cemented by mainly 250μm and 50μm nodules.Pedogenic carbonate first deposited on the surface of clay and fine silt and then soil pores between the grains were gradually filled by pedogenic carbonate along with the deposition resulted in the formation of carbonate nodules and continued growing.The carbonate content in the nodules increased sharply with the increasing size of nodules and consequently diluted and decreased the organic matter content.The grain composition became coarse and the content of clay minerals decreased with increasing size of carbonate nodules.Elemental composition of the acetic acid-leachable fraction in each size of carbonate nodules was characterized by extremely high Ca concentration（61.03-207.36 g/kg）directly related with carbonate content.It is indicated that calcium carbonate was the dominant form of carbonate in the nodules.The acetic acidleachable fraction is composed of certain amounts of Si（8.90-10.70 g/kg）and Al（2.78-5.06 g/kg）from amorphous phases of pedogenic origin,K（3.34-4.32 g/kg）and Mg（2.22-3.64 g/kg）from clay minerals,and a small amount of Na（0.70-0.85 g/kg）from highly soluble salts.