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Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments

Kipp, Michael A. and Tissot, François L. H. (2022) Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments. Earth and Planetary Science Letters, 577 . Art. No. 117240. ISSN 0012-821X. doi:10.1016/j.epsl.2021.117240.

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Uranium isotopes (δ²³⁸U) have quickly become one of the most widely-used redox proxies in paleoceanographic studies. The quantitative power of the δ²³⁸U 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 δ²³⁸U 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 δ²³⁸U 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 δ²³⁸U 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 δ²³⁸U 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 δ²³⁸U 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 δ²³⁸U 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.

Item Type:Article
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Kipp, Michael A.0000-0003-1844-3670
Tissot, François L. H.0000-0001-6622-2907
Additional Information:© 2021 Elsevier B.V. Received 2 January 2021, Revised 30 August 2021, Accepted 6 October 2021, Available online 3 November 2021. This manuscript benefitted from the insightful comments of Kimberly Lau, Chris Reinhard, an anonymous reviewer, and Editors Lou Derry and Laurence Coogan. We also thank Joshua Krissansen-Totton and Jess Adkins for helpful discussions. Some of the computations presented here were conducted in the Resnick High Performance Computing Center, a facility supported by the Resnick Sustainability Institute at the California Institute of Technology. This work was supported by a Postdoctoral Fellowship in Geobiology from the Agouron Institute to MAK, and NSF grants EAR-1824002 and MGG-2054892, and start-up funds (provided by Caltech) to FLHT. CRediT authorship contribution statement: Michael A. Kipp: Conceptualization, Methodology, Software, Writing – original draft. François L.H. Tissot: Conceptualization, Methodology, Writing – review & editing. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Group:Resnick Sustainability Institute
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Agouron InstituteUNSPECIFIED
Subject Keywords:paleoredox; isotope mass balance; Bayesian inference; MCMC; carbonate
Record Number:CaltechAUTHORS:20211110-225053786
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Official Citation:Michael A. Kipp, François L.H. Tissot, Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments, Earth and Planetary Science Letters, Volume 577, 2022, 117240, ISSN 0012-821X,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:111835
Deposited By: Tony Diaz
Deposited On:11 Nov 2021 17:58
Last Modified:11 Nov 2021 17:58

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