Bias in detrital fission track grain-age populations: Implications for reconstructing changing erosion rates
Bias in detrital fission track grain-age populations: Implications for reconstructing changing erosion rates
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DOI：
10.1016/j.epsl.2015.04.020
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发表时间：
2015-07
影响因子：
5.3
通讯作者：
M. Naylor;H. Sinclair;M. Bernet;P. Beek;L. Kirstein
中科院分区：
文献类型：
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作者：
M. Naylor;H. Sinclair;M. Bernet;P. Beek;L. Kirstein
沉积记录是我们对过去地质构造和气候变化作出反应时，通过地球表面进行质量转移的主要档案。单个沉积物颗粒的热年代学(碎屑热年代学)已成为推断侵蚀速率和追踪山带演化的关键工具。这些推断依赖于碎屑颗粒年龄的统计反演，以无偏见地近似沉积物来源地区的冷却历史。然而，批评模拟年龄的可靠性和一致性是一件具有挑战性的事情。这既源于基本的测量不确定性，也源于我们在倒置数据时所采用的假设。对于年轻碎屑样品的碎屑裂变径迹模拟，这个问题尤其严重，因为径迹计数的不确定性导致了年龄估计的不确定性。我们应用蒙特卡罗模拟方法，在已知闭合年龄模型的条件下，生成合成碎屑数据，然后对颗粒数据进行反演，以评估不同反演方案的可靠性。结果清楚地表明，现有的做法可能会受到很大的不确定性、系统性偏差和解释的非唯一性的影响。然后，我们展示了如何将人口模型中的这些系统偏差区域映射为真实关闭年龄的函数，以及这种偏差如何传播到滞后时间模型中。将该方法应用于尼泊尔西部Siwalik群沉积物的真实数据，我们没有发现下伏气候或构造过程发生变化的证据，因为LAG的明显变化与人口模型分辨率的阈值变化相吻合。本文展示了如何将种群模型中的系统偏差区域映射为真实关闭年龄的函数，以及这种偏差如何传播到滞后时间模型中，并可以追溯到现有的研究中。然而，它同样适用于其他年龄反演方案，如最小年龄建模。这些方法的应用将加强目前的做法，并有助于更有力地解释谷物年龄，特别是在区分平稳和非平稳的地质和气候过程方面。
The sedimentary record is our principal archive of mass transfer across the Earth's surface in response to tectonic and climatic changes in the geologic past. The thermochronology of individual sediment grains (detrital thermochronology) has emerged as a critical tool to infer erosion rates and track mountain belt evolution. Such inferences are reliant upon the statistical inversion of detrital grain ages to unbiasedly approximate the cooling history of the source areas from which the sediment originated. However, it is challenging to critique the reliability and consistency of modelled ages. These arise both from fundamental measurement uncertainties and the assumptions we employ in inverting the data. For detrital fission track modelling of young detrital samples, this problem is particularly acute since the uncertainty on the track counts produces uncertainty in the age estimates. We apply Monte-Carlo modelling to generate synthetic detrital data conditioned on known closure age models, and then invert the grain data to assess the reliability of different inversion schemes. The results clearly demonstrate that existing practice can be subject to large uncertainty, to systematic bias and to non-uniqueness of interpretation. We then show how to map such regions of systematic bias in the population modelling as a function of the true closure ages, and how this bias propagates through into the lag-time modelling. Applying the method to real data from the Siwalik group sediments in western Nepal, we find no evidence for a change in the underlying climate or tectonic processes, since the apparent change in lag coincides with a thresholded change in the resolution of the population modelling. This paper shows how to map regions of systematic bias in the population modelling as a function of the true closure ages, and how this bias propagates through into the lag-time modelling and can be applied retrospectively to existing studies. However, it is equally applicable to other age inversion schemes such as minimum age modelling. The application of these methods will enhance current practice and facilitate more robust interpretation of grain ages, in particular in discriminating between stationary and non-stationary geological and climatic processes.