Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments
Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments
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DOI：
10.1016/j.epsl.2021.117240
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发表时间：
2021-11-03
影响因子：
5.3
通讯作者：
Tissot, Francois L. H.
中科院分区：
文献类型：
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作者：
Kipp, Michael A.;Tissot, Francois L. H.
铀同位素(U-238)已迅速成为古海洋学研究中应用最广泛的氧化还原替代物之一。Delta U-238代理的定量能力来自于铀在海洋中的长停留时间和海水中铀同位素在去除过程中的还原优势。因此，海水三角洲U-238值对缺氧池的大小很敏感，进而对缺氧水域覆盖的海底面积很敏感。利用碳酸盐记录和保留海水三角洲U-238值的能力，以及碳酸盐沉积物在地质记录中的普遍存在，许多研究量化了海洋缺氧事件、大规模灭绝和全球气候变化中的海底缺氧。在大多数情况下，已经使用了海洋铀同位素质量平衡正演模型，说明了这些事件期间海底缺氧的潜在历史。在这里，我们表明，这种正演模拟可以通过多种方式导致错误的缺氧推断，包括(I)当缺氧分数较高时，Delta U-238替代模型的灵敏度较低，以及(Ii)在具有预期缺氧间隔的地层剖面中生成说明性正演模型输出的固有偏差。因此，我们探索了反向建模方法来约束给定增量U-238数据集最可能的海底缺氧历史，并最终通过马尔可夫链蒙特卡罗模拟开发了一个使用贝叶斯推理来实现这一点的框架。我们表明，该方法可以恢复模拟的趋势，并进一步重建了8个已公布的Delta U-238数据集的海洋缺氧。我们发现，以前对缺氧海底范围的一些解释是不准确的，要么是因为对稳态的假设不正确，要么是因为所用的说明性正演模型与数据拟合不佳。为了在未来与Delta U-238氧化还原代理的工作中解决这些问题，我们公开了该模型，并为合理使用远期模型提供了建议。通过建立这一框架，可以一致地对未来在瞬时环境扰动期间的海洋缺氧进行量化，从而促进事件之间缺氧程度的可靠比较。(C)2021年爱思唯尔B.V.保留所有权利。
Uranium isotopes (delta U-238) have quickly become one of the most widely-used redox proxies in paleoceanographic studies. The quantitative power of the delta U-238 proxy derives from the long marine residence time of uranium and the dominance of reduction in fractionating uranium isotopes during removal from seawater. The seawater delta U-238 value is therefore sensitive to the size of the anoxic sink, and by extension, the area of the seafloor overlain by anoxic waters. Leveraging the ability of carbonates to record and retain the seawater delta U-238 value, and the ubiquity of carbonate sediments in the geologic record, numerous studies have quantified seafloor anoxia across ocean anoxic events, mass extinctions, and global climatic changes. In most cases, forward models of marine uranium isotope mass balance have been used, illustrating potential histories of seafloor anoxia during these events.Here we show that there are multiple ways in which such forward modeling can lead to spurious inferences of anoxia, including (i) the poor sensitivity of the delta U-238 proxy when fractional anoxia is high, and (ii) the inherent bias in generating illustrative forward model outputs in stratigraphic sections with expected anoxic intervals. We thus explore inverse modeling approaches to constrain the most likely history of seafloor anoxia for a given delta U-238 dataset, and ultimately develop a framework for doing so using Bayesian inference via Markov Chain Monte Carlo simulation. We show that this approach can recover simulated trends, and further reconstruct marine anoxia for eight published delta U-238 datasets. We find that some previous interpretations of anoxic seafloor extent were inaccurate, either because steady state was improperly assumed, or because the illustrative forward models used were poor fits to the data. In order to overcome these issues in future work with the delta U-238 redox proxy, we have made this model publicly available, and also offer suggestions for the judicious use of forward models. By building on this framework, the future quantification of marine anoxia during transient environmental perturbations can be performed consistently, thereby facilitating robust comparison of anoxic extent between events. (C) 2021 Elsevier B.V. All rights reserved.