地表岩石侵蚀速率对宇生核素暴露测年影响的研究

Impact of Surface Rock Erosion Rate on In-situ Cosmogenic Exposure Dating Method

利用原地生宇生核素测定暴露年代时，通常会假设地貌体侵蚀速率为0。研究表明，该假设会低估地貌体的真实暴露年代。搜集2009-2012 年全球不同区域56 个岩石样品的宇生核素10Be 测年数据，探讨侵蚀速率为0对于侵蚀速率为0.5、1 以及2 mm/ka的样品，在不同暴露尺度上对暴露年代计算的影响幅度。结果表明，对于1×104a 尺度的样品暴露年代可能低估约0.5%，1%，2%；对于10×104a 尺度的样品可能低估约5%，7%，20%；对于50×104a尺度的样品可能低估约40%，70%甚至100%以上。

The cosmogenic exposure dating method that measures trace cosmogenic nuclide concentrations present in terrestrial surface rocks developed in the late 1980s. This method has become a widely used tool to address scientific questions in the fields of geomorphology, glaciology, palaeoclimatology, palaeoseismology, soil science, volcanology and geohazard research. Until now, three types of uncertainty affect the precision and accuracy of cosmogenic isotope dates; these include： analytical uncertainties including systematic errors, production rate uncertainties and sampling uncertainties. The cosmogenic nuclide concentration on the surface of rocks is a function of exposure time and erosion rate. However, it is usually hypothesized that the sample erosion rate is zero when dating the age using cosmogenic exposure dating method due to poorly constrained rock surface erosion rates. Recent research suggests that the assumption of ＂zero erosion＂ may underestimate true exposure age and the effect will increase with increasing sample age. In order to explore the impact of different erosion rates to the different exposure ages sample, we gathered 56 rock cosmogenic 10Be datum from different areas in the literature of 2009-2012. We analyzed the effects of zero erosion between 0.5, 1 and 2 mm/ka for the 1×104 a, 10×104 a and 50×104 a scale samples respectively. The erosion rates datum stem from mircro-weathering measurements and maximum erosion rates of the oldest sample using the cosmogenic exposure dating method in different areas. The results suggest, that the age for 1×104 a is underestimated approximately by 0.5%, 1% and 2% for erosion rates of 0.5, 1 and 2 mm/ka, respectively. For the age of 10×104 a, the underestimate is approximately should be 5%, 7% and 20%, and for 50×104 a approximately 40%, 70% and even more than 100%, respectively. The results are consistent with previous research results which suggest that our results are reliable. At the same time the result means that the erosion rates of surface rock significantly affect the accuracy of cosmogenic exposure dating method, and in particular for the older exposure age samples. Thus, future studies should pay attention to this issue.