Sample preparation for determination of comminution ages in lacustrine and marine sediments
Sample preparation for determination of comminution ages in lacustrine and marine sediments
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DOI：
10.1016/j.chemgeo.2018.01.003
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发表时间：
2018-02
期刊：
影响因子：
3.9
通讯作者：
A. Francke;S. Carney;P. Wilcox;A. Dosseto
中科院分区：
文献类型：
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作者：
A. Francke;S. Carney;P. Wilcox;A. Dosseto
碎屑物质的铀同位素组成（234 U和238 U）已成为评估地质时间尺度上侵蚀和集水动力学对气候变化响应的重要工具。（234 U/238 U）活度比的相对变化可以用来估计从物理和化学风化作用形成小于63 μm的颗粒（称为粉碎年龄）以来所经过的时间，这是234 U在碎屑物质中不断消耗的结果。然而，由于非碎屑（自生的，内生的）成分通常富集在234 U，需要顺序提取方法，小心地从沉积物中去除任何非碎屑物质。在此，我们测试了不同的方法，这些方法使用类似的化学反应物，但在施加的热量和时间方面有所不同。基于这些结果，可以推断，超声辅助的方法能够从沉积物中去除非碎屑物质，而不改变碎屑颗粒的同位素比，并提供了（234 U/238 U）活度比的高再现性。此外，它是最快的，因此，最具成本效益的方法测试。快速和具有成本效益的处理方法是对长期古环境记录进行高分辨率研究的先决条件，这些结果是进一步发展铀同位素分析的基础，作为一种工具，用于重建过去的侵蚀和流域动态，以响应气候变化。样品处理后的年龄是原来的5倍，这突出了从沉积物组成中全面去除非碎屑物质的重要性。此外，气体吸附分析允许确定是否分形校正计算的反冲分数应适用。反冲分数的精确估计对于计算粉碎年龄至关重要，因为它决定了234 U从碎屑物质中的损失。本研究中分析的样品显示II型等温线，表明无孔或大孔表面。微孔和中孔，增加了气体吸附分析过程中的表面积，但不有助于损失的234 U是缺席。因此，不需要考虑微孔和中孔的分形校正。
The uranium isotope composition (234U and238U) of detrital matter has become an essential tool for evaluating the response of erosion and catchment dynamics to climate variability on geological time scales. Relative variability of the (234U/238U) activity ratio can be used as an estimate of the time that has elapsed since physical and chemical weathering has formed grains < 63 μm, termedcomminution age, as a result of ongoing depletion of234U in detrital matter. However, as non-detrital (authigenic, endogenic) components are commonly enriched in234U, sequential extraction methods are required to carefully remove any non-detrital matter from the sediments. Herein, we tested different methods, which use similar chemical reactants but differ in terms of applied heat and time. Based on the results, it can be inferred that an ultrasound-assisted approach enables the removal of non-detrital matter from the sediments without alternating the isotope ratio of the detrital grain, and provides a high reproducibility of the (234U/238U) activity ratios. Moreover, it is the fastest, and thus, most cost effective method tested. Fast and cost-effective treatment methods are a prerequisite for high-resolution studies on long paleoenvironmental records, and thus, these results are fundamental for the further development of U isotope analyses as a tool for the reconstruction of past erosion and catchment dynamics in response to climate variability.Combined U isotopes and gas adsorption surface area analyses on two sediment samples from the Mediterranean Sea yield comminution ages up to 5 times older after sample treatment, which highlights the importance of a comprehensive removal of non-detrital matter from the bulk sediment composition. Moreover, gas sorption analysis allowed determining whether a fractal correction for calculation of the recoil fraction should be applied. Precise estimates of the recoil fraction are crucial for calculating the comminution ages, as it governs the loss of234U from detrital matter. Samples analyzed in this study display Type II isotherms suggesting a non-porous or macroporous surface. Micro- and mesopores, which increase the surface area during gas adsorption analyses but do not contribute to the loss of234U are absent. Thus, a fractal correction to account for micro- and mesopores is not required.