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| 黄土高原黄土和红粘土~(10)Be地球化学特征 | |
| 引用本文： | 顾兆炎,D.Lal,郭正堂,J.Southon,M.W.Caffee,刘东生.黄土高原黄土和红粘土~(10)Be地球化学特征[J].第四纪研究,2000,20(5). |
| 作者姓名： | 顾兆炎 D.Lal 郭正堂 J.Southon M.W.Caffee 刘东生 |
| 作者单位： | 1. 中国科学院地质与地球物理研究所，北京100029 2. 美国斯克里普斯海洋研究所，拉荷亚CA92093 3. 美国劳伦斯利物茂国家实验室加速器质谱中心，利物茂CA94551 |
| 基金项目： | 国家自然科学基金!49773184 |
| 摘 要： | 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10BC与9Be和Al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10Be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。 |
| 关 键 词： | ~（10）Be 黄土－古土壤 红粘土 风化作用 |
GEOCHEMISTRY OF COSMOGENIC ~(10)Be IN LOESS-PALEOSOL SEQUENCES AND RED CLAY IN THE LOESS PLATEAU |
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| Gu Zhaoyan,D.Lal,GUO Zhengtang,J.Southon,M.W.Caffee,Liu Tungsheng.GEOCHEMISTRY OF COSMOGENIC ~(10)Be IN LOESS-PALEOSOL SEQUENCES AND RED CLAY IN THE LOESS PLATEAU[J].Quaternary Sciences,2000,20(5). | |
| Authors: | Gu Zhaoyan DLal GUO Zhengtang JSouthon MWCaffee Liu Tungsheng |
| Abstract: | The cosmogenic radionuclide 10Be (half-life = 1.5Ma) has generated much interestbecause of its potential as a tracer in the environment and applications to the Earthsciences. Several studies of 10Be in Chinese loess have been made to reconstruct thehistories of climate and weathering on the Loess Plateau. Nevertheless, 10 Begeochemical behavior, especially the mobility in wind-blown dust sequences duringweathering, needs to be more fully understood for its applications in sedimentology,pedology, paleoclimatology, and chronology. In this paper, measurements of 10 Be,9 Be, and major elements in different grain size fractions, in selective chemicallyleached fractions, and in bulk samples from some sections of loess-paleosols and redclay in the Loess Plateau were conducted using the AMS (accelerator massspectrometer), XRF, and ICP-MS. The obtained data provide a clear picture ongeochemistry of 10Be in the dust deposits, which extend the earlier understandings. Similar to the 9Be distribution pattern over different grain size fractions, 10Beconcentration increases with the decrease in particle size of the loess and paleosolsamples. The amount of 10Be in the < 4μm grain size fraction is more than 60% oftotal, indicating that most of 10Be is carried by clay minerals. Since dust sedimentshave an appreciable endowment of 10Be from the source regions, concentrations of10Be in loess, paleosols and red clay would depend on the dust grain size. We observed that, although 10Be concentrations in the > 4tm grain size fractions inboth the loess and paleosol samples are similar, the 10Be fractions in the < 4μm sizefractions are much higher in the paleosol samples, suggesting that most of 10Be inloess was absorbed on the surface of mineral particles, while an appreciable amount of10Be in paleosol and red clay was incorporated in authigenic clay minerals formed in-situ during weathering of the dust. This conclusion is also supported by severalleaching experiments showing that 10Be is leached from the loess samples easier thanfrom the paleosol samples using 1M HC1 solution and the saturated EDTA solution. It is also presented in our data that the concentrations of major elements arecorrelated to the groin size and weathering degree which in turn depends on the dustdeposition rate. Consequently, 10Be concentration increases from loess to paleosol, andto red clay. Taking into account of the effect of paleoclimate on dust deposition andweathering on the Loess Plateau, we recognized the relationships of 10Be concentrationrespectively with mobile and immobile elements observed early in the dust sediments. Mobility of Be during weathering depends on the pH value of the soil solution.In our early 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 transportand 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 ofalkaline materials in dust sediments to resist desorption of 10Be bound by clay minerals.Moreover, since dust falls almost continuously, a paleosol layer is quickly overlain bya loess layer (within～4 × 104 years), whereby further weathering and recharge ofrainwater are terminated. Comparing the 10 Be concentration fluctuations with thechanges in 9Be concentration, grain size proxy of Si/ Al ratio, and weathering indexof Na/Al ratio through the loess-paleosol profiles and red clay section, we concludethat 10Be is chemically immobile in the dust deposition-weathering sequences. |
| Keywords: | ~(10)Be loess-paleosol red clay weathering |
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