青藏高原东北缘黄土的地球化学特征及其对物源和风化强度的指示意义

位于青藏高原东北缘的陇西黄土高原地处我国东亚季风、西南季风、西风环流及高原季风等多个大气环流的交汇部位，同时处在沙漠-黄土的边界带上，环境对气候变化的反应非常敏感。广泛分布于该区的风尘堆积是研究我国干旱、半干旱区古气候演化及高原隆升环境效应的理想材料。本文对陇西黄土高原的甘肃会宁黄土进行常、微量元素测试分析，并与陇东黄土高原黄土的地球化学特征进行了对比。结果表明，陇西和陇东黄土的地球化学组成相似，与上部陆壳（UCC）平均化学成分也基本一致，说明两者都是复杂的源岩风化物质经过高度混合堆积而成。陇西与陇东黄土的地球化特征在总体一致的基础上也存在着明显不同，反映了两者沉积后化学风化和物源的差异。与陇东黄土相比，陇西黄土具有较高的CaO、MgO、Na₂O和较低的Fe₂O₃含量，说明陇西黄土的化学风化强度相对较低；CIA值也显示，陇西黄土处于初等化学风化阶段，而陇东黄土则处于中等化学风化阶段；在化学性质相对稳定的元素中，陇西黄土与陇东黄土相比具有较高的SiO₂/Al₂O₃、SiO₂/TiO₂、TiO₂/Al₂O₃、U/Pb、Zr、Hf值和较低的Ta、Y、Rb/Sr、Ba/Sr、Ce/Yb、Eu/Yb、LREE/HREE值，反映了两者物源上的差异。受特殊地理位置的影响，我国北方戈壁荒漠、青藏高原的冰川及冰缘沉积以及邻近的第四纪松散沉积物都有可能成为陇西黄土的潜在物源。进一步研究显示，会宁黄土的地球化学特征在大约300ka B.P.前后发生了明显变化，结合以前磁组构、石英颗粒表面形态的分析结果，进一步证明青藏高原东北缘的风尘物源在该时期可能发生过比较大的变化，300ka B.P.以后高原的冰川及冰缘沉积对该区风尘物源的贡献明显增加，而青藏高原在此时期的快速隆升导致的高原季风加强可能是形成风尘物源变化的主要原因。

Geochemical characteristics of eolian deposits in the northeastern margin of the Tibetan Plateau and implications for provenance and weathering intensity

The Longxi basin of the Chinese Loess Plateau (CLP) is located in the northeastern margin of the Tibetan Plateau (TP), which is also in the desert-loess boundary zone of Northern China (NC). The climate in this region is controlled by the East Asian Monsoon, the Westerly Circulation, the TP monsoon and the India southwest monsoon, so the environmental response to climatic change is very sensitive in this area. The eolian deposits widely distributed in this region bear information about climate change for the arid or semi-arid zone of NC, and it is also an ideal material for studies on climatic response to uplift of the TP. In this study, geochemical compositions of an eolian sequence in the northeastern margin of the TP are studied and compared with loess and paleosol samples from the Longdong basin of the CLP. The main objectives are:(1) to characterize the elemental geochemistry of the eolian deposits in the northeastern margin of the TP and compare with the loess-paleosol sequence in the Longdong basin of the CLP; (2) to interpret the geological implications for dust sources; (3) to compare the chemical weathering characteristics of the eolian deposits in the Longxi and Longdong Basins. The studied section is from Huining (HN) County of Gansu Province. The present-day climate at HN County is temperate with mean annual precipitation of 330mm and mean annual temperature of 7.9℃, respectively. The HN section (36°15'N, 105°07'E) is 224.25m in thickness. In this study, only the upper part of this sequence (since L9 which is about 125m in depth) is studied, 15 samples were taken for major and trace elemental measurements, including 8 loess and 7 paleosol samples. 2 loess samples are from L1, and the other 6 loess samples are from L2, L3, L4, L6, L8, and L9; the 7 paleosol samples are from S1, S2, S3, S4, S5, S7, and S8. All the samples were measured at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences. The geochemical data of the Longdong basin loess are from Lingtai section (34°59'N, 107°45'E) in Lingtai County, Gansu Province. The results show that geochemical characteristics of the eolian deposits from the Longxi and Longdong basins are similar to the average UCC on the whole, indicating that the dust materials have experienced numerous upper crustal recycling processes. However, they also show obvious differences, reflecting different chemical weathering intensity and different eolian dust sources. The loess in the Longxi basin have higher CaO, MgO, Na2O and lower Fe2O3 concentrations in comparison with the Longdong loess samples, indicating lower weathering intensity of the Longxi loess. According to the CIA value, the Longxi loess was in the stage of incipient chemical weathering, while the Longdong loess was in the stage of intermediate chemical weathering intensity. For the relatively invariant elements, compared with the Longdong loess, the Longxi loess samples have higher Zr, Hf and lower Ta, Y value, higher SiO2/Al2O3, SiO2/TiO2, TiO2/Al2O3, U/Pb and lower Rb/Sr, Ba/Sr, Ce/Yb, Eu/Yb, LREE/HREE ratios, reflecting different eolian dust sources. Due to the influence of geographical location, the desert in NC, the glacial and periglacial sediments in the TP and the Quaternary sediments in the adjacent area is likely to become the potential sources of the Longxi loess. Further studies show obvious changes in geochemical characteristics of the HN loess since about 300ka B.P. Our geochemical data, combined with previous results of magnetic fabric and quartz grain surface morphology, further prove that the sources of the eolian deposits distributed in the northeastern margin of TP maybe obviously changed during this period. Eolian dust contribution of glacier and periglacial sediments from the TP significantly increased after 300ka B.P. This change may be related to the increased plateau monsoon caused by the rapid uplift of TP in this period.