黄土高原黄土．和红粘土10Be地球化学特征

充分认识元素和同位素在不同环境条件下的地球化学行为，是运用元素和同位素示 踪环境变化的前提。对来自于黄土高原的黄土、古土壤和红粘土样品的宇宙成因核素¹⁰Be测量和化学成分分析，以及各种化学淋溶实验表明：¹⁰Be主要以吸附状态赋存于粉尘沉积物粘粒矿物的表面，部分已结合进自生的粘土矿物中；在粉尘沉积物风化过程中¹⁰Be与⁹Be和～的活动性相似，基本没有发生迁移，其原因是连续沉积的粉尘含有大量的碱性物质，阻止了…Be的解吸附和淋滤；沉积和风化作用导致了¹⁰Be浓度与化学指标在黄土-古土壤和红粘土剖面中的协同变化。

GEOCHEMISTRY OF COSMOGENIC 10Be IN LOESS-PALEOSOL SEQUENCES AND RED CLAY IN THE LOESS PLATEAU

The cosmogenic radionuclide 10Be （half-life = 1.5Ma） has generated much interest because of its potential as a tracer in the environment and applications to the Earth sciences. Several studies of 10Be in Chinese loess have been made to reconstruct the histories of climate and weathering on the Loess Plateau. Nevertheless, 10Be geochemical behavior, especially the mobility in wind-blown dust sequences during weathering, needs to be more fully understood for its applications in sedimentology,pedology, paleoclimatology, and chronology. In this paper, measurements of 10Be,9 Be, and major elements in different grain size fractions, in selective chemically leached frachons, and in bulk samples from some sections of loess-paleosols and red clay in the Loess Plateau were conducted using the AMS （accelerator mass spectrometer）, XRF and ICP-MS. The obtained data provide a clear picture on geochemistry of 10Be in the dust deposits, which extend the earlier understandings.Similar to the 9Be distribuhon pattern over different grain size frachons, 10Be concentration increases with the decrease in particale size of the loess and paleosol samples. The amount of 10Be in the < 4μm grain size fraction is more than 60% of total, indicating that most of 10Be is carried by clay minerals. Since dust sediments have an appreciable endowment of 10Be from the source regions, concentrations of 10Be in loess, paleosols and red clay would depend on the dust grain size.We observed that although 10Be concentrahons in the > 4μm gtain size fractions inboth the loess and paleosol samples are similar, the 10Be frachons in the < 4μm size frachons are much higher in the paleosol samples, suggesting that most of 10Be in loess was absorbed on the surface of mineral panicles, while an appreciable amount of 10Be in paleosol and red clay was incorporated in authigenic clay minerals formed in-sita during weathering of the dust. This conclusion is also supported by several leaching experiments showing that 10Be is leached from the loess samples easier than from the paleosol samples using 1M HCl solution and the saturated EDTA solution.It is also presented in our data that the concentrations of major elements are coerelated to the grain size and weathering degree which in tum depends on the dust deposition rate. Consequenhy, 10Be concentration increases from loess to paleosol, and to red clay. Taking into account of the effect of paleoclimate on dust deposition and weathering on the Loess Plateau, we recognized the relahonships of 10Be concentration respectively with mobile and immobile elements observed eariy in the dust sediments.Mobility of Be during weathering depends on the pH value of the soil solution.In our eary observations the 10Be losses from the individual soil layers are minimal （3 ± 3%）, in contrast to a great post-depositional 10Be losses due to solution transport and soil erosion which was observed from the soil profiles in California. In this study,leaching experiments and chemical analyses reveal that there are enough amounts of alkaline materials in dust sediments to resist desorption of 10Be bound by clay minerals.Moreover, since dust falls almost continuously, a paleosol layer is qinckly overlain by a loess layer （within^~4 × 104 years）, whereby further weathering and recharge of rainwater are tterminated Comparing the 10Be concentration fluctuations with the changes in 9Be concentration, grain size proxy of Si / Al ratio, and weathering index of Na / A ratio through the loess-paleosol profiles and red clay section, we conclude that 10Be is chemically immobile in the dust deposition-weathering sequences.