| 摘要: |
| 地表侵蚀速率是衡量地貌演化的一个重要因子。本研究利用原地宇宙成因核素10Be和26Al对合黎山西南部地表岩石侵蚀速率进行了首次测定。结果显示：约30 ka以来，合黎山西南部的地表岩石侵蚀速率约为24 mm ∙ ka−1。这一结果与已见报道的其他基岩侵蚀速率值一致。这一结果与Small et al获得的非干旱地区的基岩侵蚀速率也基本一致，但是显著高于干旱的南极地区和半干旱的澳大利亚。10Be和26Al获得的侵蚀速率的良好一致性表明本研究中所用侵蚀模式的有效性。所得的侵蚀速率小于Palumbo et al 测定的合黎山平均流域侵蚀速率（99 mm ∙ ka−1），原因解释尚待更多地点和样品的研究。 |
| 关键词: 宇宙成因核素10Be和26Al 合黎山西南部 侵蚀速率 |
| DOI：10.7515/JEE182003 |
| CSTR：32259.14.JEE182003 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41602195）；中国科学院西部博士基金 |
| 英文基金项目:National Natural Science Foundation of China (41602195); Western Light Talent Culture Project of CAS |
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| Erosion rate in the southwest of Helishan from in-situ cosmogenic nuclides 10Be and 26Al |
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ZHANG Li, WU Zhenkun, LI Ming, DONG Guocheng, ZHAO Guoqing,FU Yunchong
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Shaanxi Key Laboratory of Accelerator Mass Spectrometry Technology and Application, Xi’an 710061, China
2. Xi’an Accelerator Mass Spectrometer Center, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope Surface erosion rate is an important subject in evaluating geomorphic evolution of landforms. Our understanding of landform evolution, however, is limited by the lack of the data about landscape change rate in appropriate geological time scale. The accelerator mass spectrometry (AMS)- based cosmogenic nuclide technique is an increasingly utilized method that can measure long-term (>103—104 years) average erosion rates. Determination of in-situ 10Be and 26Al in bedrock surfaces on summit flats constrains erosion rate for mountain ranges within various climatic environments. Studies on 10Be environmental tracer and exposure dating/erosion rate have lagged behind in China due to a lack of AMS facilities for routine 10Be measurements. Based on the 10Be and 26Al samples preparation method in our laboratory, the article presents the erosion rate determined from 10Be and 26Al in quartz samples measured by AMS from outcrop bedrock collected from southwest Helishan, at the northeast margin of Tibetan Plateau. We hope to understand the effects of climate and tectonics on erosion rate better and especially to provide hints on the cause of the abrupt increase of erosion rates in northeast Tibet. Materials and methods Four rock samples were collected from the outcrop. The rocks were brought into the lab, be cleaned, dry, crush, and sieved. The 0.25 — 0.50 mm grain size were selected. The magnetic minerals were removed with magnetic separation. Pure quartz samples were completely dissolved with 0.3 — 0.5 mg 9Be carrier in hydrofluoric acid. Fluorides were removed by perchloric acid fuming. Beryllium and aluminum were separated by cation and anion exchange chromatography, and leached with 1 mol ∙ L−1 HCl and an H2C2O4-HCl acid mixture. Ammonium hydroxide was added to the 10Be and 26Al fractions to precipitate beryllium and aluminum hydroxide gels at pH = 8 — 9 and pH ≈ 8, respectively, and the hydroxides were oxidized by ignition at 900℃ in quartz crucibles. The beryllium oxide and aluminum oxide products were mixed with copper powder and pressed into target holders prior to measurement on the AMS. Results The 10 Be/9 Be ratios of the samples were normalized to the NIST AMS standard SRM-4325 with a nominal value of 10Be/9Be = 2.68×10−11. The 26Al/27Al ratios of the samples were normalized to the ICN AMS standard with a nominal value of 26Al/27Al = 1.065×10−11. We calculate the 10Be and 26Al concentrations in quartz samples, combined the 10Be/9Be, 26Al/27Al ratio determined by AMS, with the sample weight, 9Be carrier weight and 27Al weight. The erosion rates are determined by the CRONUS-Earth online calculator version 2.2. The minimum exposure ages are also shown. The maximum steady erosion rates of HLS-1-4 are (23.4 ± 1.8) mm ∙ ka−1 (Be) and (24.4 ± 2.0) mm ∙ ka−1 (Al). The maximum steady erosion rates of HLS-1-3 are well agreement with that of HLS-1-4, (33.4 ± 12.9) mm ∙ ka−1 (Be) and (24.2 ± 1.9) mm ∙ ka−1 (Al), respectfully. While A two-isotope plot of 10Be and 26Al data shows that samples HLS-1-4 and HLS-1-3 are consistent with a model of constant exposure. HLS-4-2 may have complex exposure histories, suggesting burial and re-exposure. The determined erosion rate of 24 mm ∙ ka−1 is comparable with the erosion rates in other mountainous regions. Discussion Both tectonics and climate affect erosion of landforms. The obtained average erosion rate for southwest Helishan is about 24 mm ∙ ka−1, are comparable to those with bedrock from non-arid climatic regions, but significantly higher than those of arid Antarctica and semi-arid Australia. It appears that arid regions with high erosion rates are basically all tectonically active. It may suggest tectonic activity plays a more important role in controlling erosion of landforms. The focused area is located at the northeast margin of Tibetan Plateau. Along both its northern and southern sides, reverse faults bound the core of the range. Helishan is dominated by slip with a reverse component. So the erosion rate of the southwest Helishan could be effected by the tectonic activity to some extent. Outcrop erosion rate is slower than that inferred from drainage basin study (99 mm ∙ ka−1). It is similar to that reported by Bierman et al, Small et al and Quigley et al. The results suggest that soil cover, even if it is quite shallow, speeds the rate of rock weathering. Outcrops are situated above the landscape and exposed to a limited suite of what must be largely ineffective subaerial erosion processes that both physically and chemically wear away exposed rock. Conclusions Using in-situ cosmogenic nuclide dating technique, we have determined steady-state erosion rates in arid southwest Helishan. The consistency of erosion rates derived from 10Be and 26Al suggests validity of the erosion model used in this study. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long-term erosion rates. The obtained erosion rate is, however, significantly lower than the catchment erosion rate. Recommendations and perspectives Our result should provide the basic data for the study of the erosion rate in arid regions and provide hints on the cause of the abrupt increase of erosion rates in northeast Tibet. |
| Key words: in-situ cosmogenic nuclides southwest of Helishan erosion rate |
