Precision of terrestrial exposure ages and erosion rates estimated from analysis of cosmogenic isotopes produced in situ
Precision of terrestrial exposure ages and erosion rates estimated from analysis of cosmogenic isotopes produced in situ
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通过分析原位产生的宇宙同位素估算出陆地暴露年龄和侵蚀率的精度
DOI：
10.1029/95jb02911
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发表时间：
1995
影响因子：
--
通讯作者：
P. Bierman
中科院分区：
文献类型：
--
作者：
A. Gillespie;P. Bierman
矿物颗粒中产生的宇宙成因同位素的浓度随暴露年龄和岩石表面的侵蚀速度而变化。因此，原则上，在侵蚀速率恒定的情况下，可以通过分析同一样品中的两个宇宙成因同位素来确定暴露年龄和侵蚀速率。如果可以独立确定另一个参数，也可以从一个同位素中找出年龄或侵蚀速率。简单的数学模型预测暴露年龄和腐蚀率的精确度和准确度。这些结果为宇宙成因同位素的使用提供了洞察，并为优化测年实验和预测容易处理的地质问题提供了一个框架。暴露年龄和侵蚀速率的精确度和准确度取决于测量的同位素浓度、同位素的半衰期以及取样岩石表面的年龄和侵蚀速率的精确度。它们也随着分析策略的不同而不同。由一些同位素对估算的暴露年龄接近于晚更新世地表侵蚀的测量精度，在<；1厘米KYR−1处。对于相同表面，侵蚀速率的不确定度可能好于±0.2厘米KYR−1。对于较老的表面，年龄和侵蚀速率估计的上限变为无穷大。不同半衰期的同位素配对可能会产生误导性的结果，除非侵蚀速率是恒定的，因为它们记录了不同的侵蚀历史。分析单一的同位素可以消除这一误差来源，但只有在年龄或侵蚀速率可以很好地独立确定的情况下才是合适的。如果侵蚀速率约束较差，则某些实际情况下的年龄可能不准确50%或更多，即使它们的精度可能接近测量精度。虽然了解宇宙成因测年的局限性很重要，但同样重要的是不要忽视这种用于定量地貌研究的强大工具的潜力。
The concentration of cosmogenic isotopes produced within mineral grains varies with both the exposure age and erosion rate of the rock surface. In principle therefore, exposure age and erosion rate may be determined by analyzing two cosmogenic isotopes from the same sample, provided the erosion rate is constant. It is also possible to find either age or erosion rate from one isotope if the other parameter can be determined independently. Simple mathematical models predict the precision and accuracy of the exposure ages and erosion rates. The results provide insight into the use of cosmogenic isotopes and a framework to optimize dating experiments and predict tractable geologic questions. The precision and accuracy of the exposure ages and erosion rates depend on the precision of the measured isotope concentrations, the half-lives of the isotopes, and the age and erosion rate of the sampled rock surface. They also vary with the analytic strategy. Exposure age estimates from some isotope pairs approach measurement precision for late-Pleistocene surfaces eroding at <1 cm kyr−1. Uncertainties in erosion rate for the same surfaces may be better than ±0.2 cm kyr−1. For older surfaces the upper limits for both age and erosion rate estimates become infinite. Pairing of isotopes with different half-lives may give misleading results unless the erosion rate is constant because they record different erosion histories. Analyzing a single isotope removes this source of error but is appropriate only if either the age or erosion rate can be well determined independently. If the erosion rate is poorly constrained, ages for some realistic situations may be inaccurate by 50% or more even though their precision may approach the measurement precision. Although it is important to understand the limitations of cosmogenic dating, it is equally important not to lose sight of the potential of this powerful tool for quantitative geomorphologic studies.