庐山北麓JL红土剖面粉砂粒级元素地球化学特征及其物源意义

加积型红土是中国南方第四纪环境变化研究的重要陆相载体,其沉积母质显示风成特性,后期经历较强的化学风化。为了明确加积型红土的物质来源,并减弱风化效应对其物源信息的影响,本文提取了庐山北麓JL红土剖面(厚18.46 m)部分样品的粉砂组分(4~63 μm),分析其元素地球化学特征,并通过元素比值及物源判别指数等方法讨论其蕴含的物源信息。结果显示:1)粉砂粒级化学组成中SiO2、Al2O3和Fe2O3三者的含量可达84.29%,K2O、Na2O、CaO和MgO四者含量仅1.30%,与全岩相比SiO2含量增高,Al2O3和Fe2O3含量明显降低,风化指数CIA值(74.02%)明显低于全岩样品(85.00%);微量元素中,粉砂粒级相对全岩Cu、Zn、Rb、Sr、Cr、La等含量降低,Y、Zr含量则略高。2)Na2O/Al2O3与K2O/Al2O3及Na2O/K2O与CIA的散点图中,粉砂粒级样点间的集聚程度明显降低,暗示风化效应在一定程度上被削弱。此外以洛川黄土和长江河漫滩沉积物为端元的物源指数计算表明,全岩样品消除粒级效应后的物源指数(PIw)与粉砂粒级物源指数(PIfs)吻合度较好,进一步验证了粉砂粒级地球化学特征在削弱风化效应方面的有效性。3)JL剖面上部黄棕色土的PIfs值为0.45,物源上更接近北方黄土,剖面中下部的网纹红土PIfs值为0.57,物源更接近长江沉积物。结合TiO2/Al2O3、Zr/Y、Hf/Ti等元素比值,JL剖面自下而上近源物质的贡献略有减少,远源物质的贡献则趋于增加,这可能意味着晚第四纪以来东亚冬季风不断增强的过程。

GEOCHEMICAL CHARACTERISTICS AND THEIR PROVENANCE IMPLICATIONS OF THE SILT FRACTION FROM JL RED ERATH SECTION IN LUSHAN REGION,JIUJIANG, SOUTH CHINA

Aggradation red earth is a typical aeolian deposit, and widespread in South China. The complete stratigraphy includes three soil units in general, which are vermicular red clay soil unit, red clay soil unit, and yellowish brown clayey-silt soil unit from down to up. The changes of soil pedogenic intensity is corresponding to climate and environment changes during the Quaternary. In addition, as an aeolian deposit, the aggradation red earth also documented the dust emission history in South China. Numerous studies had paid attention on their material source tracing. However, due to the deposit are strongly weathered, the signals of lithological characteristics are commonly covered by pedogenic characteristics, for which it will result in a great error on the source tracing studies. We proposed the behavior, to filter the clayey material, can avoid or significant limit the pedogenic signal of the deposit. To determine it, we investigated the geochemical characteristics of a typical aggradation red earth section, JL section (29°42'N, 116°02'E), which locates in northern piedmont of Lushan Mountain and has 18.46 m thickness, by analyzing the major element as well as trace element of 79 silt fraction (4~63 μm) samples and compared the geochemical characteristics of 79 bulk samples. Moreover, chronology studies by others of this section show that the vermicular red clay soil began to form in the Early Pleistocene, and the slightly vermicular yellow-brown soil and yellow-brown soil formed in the Middle and Late Pleistocene. The result exhibited that, although the silt fraction samples had similar geochemical composition with bulk samples, it could easily find some different between them, such as:Silt fraction had increased SiO2 (the average content of SiO2 in silt fraction was 78.76%), significant decreased Al2O3 (3.73%) and Fe2O3 (1.80%), decreased CIA value (74.02%) by comparing with the bulk samples, it indicated that the weathering effect was limited. In the scatter plots of Na2O/Al2O3, K2O/Al2O3, Na2O/K2O ratio, and CIA value, the agglomeration degree of the silt fraction was significantly reduced. Meanwhile we analyzed the provenance indexes (PI) of the JL section, finding that the PIfs (0.45, 0.53 and 0.57) was closer to PIw (0.44, 0.50 and 0.55) than PIb (0.60, 0.60 and 0.59), indicated that the geochemical characteristics of silt fraction samples were effective to illustrate the provenance of aggradation red earth. Furthermore, we used geochemical characteristics of silt fraction to tracing the aeolian material source in JL section. The previous studies suggested the desert land in Northwest China and/or Yangtze River flood plain are two most important potential sources for aggradation red earth. By comparison with calculated provenance indexes and stable element ratios of Chinese loess and sediment of Yangtze River flood plain, we proposed that the provenance of the JL aggradation red earth should include local provenance and distant provenance. The contribution of near-provenance material in JL profile was decreased from bottom to top, while that of far-provenance material tended to increase. This result was corresponding to strengthened East Asia winter monsoon since the Quaternary.