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(2020) Reconstructing Global Ocean Redox Conditions by Coupling the Mo and U Isotope Systems of Euxinic Organic-Rich Mudrocks

Lyu X, Dahl TW, Zheng W, Wang S & Kendall B

https://doi.org/10.46427/gold2020.1671

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12g: Room 4, Thursday 25th June 22:12 - 22:15

Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.

Submitted by Elizabeth King on Thursday 25th June 17:01
From Conveners: 1) Looking forward to summary of right panel (positive correlation, no correlation, negative correlation) -- how do you resolve varying degrees of correlation (e.g. slightly positive versus acutely positive)? 2) Building off previous question, still focused on the rightmost panel: what is going on in the Doushantuo Formation to make it look so distinct from other units (particularly with very light Mo isotopes)? 3) What trajectory would diagenetic, metamorphic, or recent oxidative alteration impart in d98Mo vs. d238U space? In other words, could these model outputs help to identify situations where the isotopic data are better interpreted as secondary alterations artifacts than primary signatures?
Thank you for the questions. 1) The correlations were determined based on observations. We will examine the correlation coefficients between d98Mo and d238U for each formation. Thank you for pointing this out. 2) We hypothesized that the low d98Mo of group 1 was mainly caused by the local depositional environment (e.g., the particulate Fe-Mn shuttle, bottom water redox conditions, basin restrictions) because of similar d238U but distinct d98Mo between group 1 and 2. Particularly, the Fe-Mn oxides shuttle could influence sedimentary d98Mo record (cause lower values) but not d238U. Ostrander et al. (2019) reported similar low d98Mo values for the Member II, III, IV of the Doushantuo Formation and local depositional environment were suggested to cause these low d98Mo values. 3) It is a good question. Post-depositional alterations could influence the primary signatures of rocks. However, it is not known how these processes could influence d98Mo and d238U values. As far as I know, only the ca. 1730Ma Wollogorang Formation, which was affected by the post-depositional hydrothermal fluids, have d98Mo (Kendall et al., 2009) and d238U (Yang et al., 2017) data reported. Most studies focused on using rocks that were less altered. Therefore, there are still a lot of unknowns regarding how these secondary alterations influence the d98Mo and d238U of rocks.

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