Reconstructing Global Paleocean Redox Conditions: Exploring the Coupled Use of the Molybdenum and Uranium Isotope Systems in Euxinic Organic-rich Mudrocks
Abstract
The molybdenum and uranium isotope compositions of euxinic organic-rich mudrocks (ORM) are commonly used as tracers for the extent of global ocean oxygenation. However, inferring the global seawater Mo and U isotope compositions is hampered by significant uncertainties for local seawater-sediment isotope fractionation factors. Combining the Mo and U isotope systems has potential to yield more reliable information about global ocean redox conditions. Here, we measured δ238U data (relative to standard CRM145) from eight euxinic ORM units deposited from the late Neoproterozoic to middle Paleozoic. The total organic carbon (TOC), Fe speciation, and δ98Mo data for these samples were reported in previous studies.
A compilation of coupled authigenic δ98Mo and δ238U for post-Archean euxinic ORMs from this and previous studies shows no overall correlation between δ98Mo and δ238U, indicating that both local effects and global ocean redox state affected ORM isotope compositions. A coupled Mo-U isotope mass balance model, developed based on observations from modern environments, was used to simulate seawater Mo and U isotope values under various redox conditions. The local depositional environment of each ORM was examined via geological background (e.g., paleogeographic location) and geochemical data (e.g., Mo/TOC). Different Mo-U isotope patterns were observed for individual ORM units, including negative, positive, and no correlations. A positive correlation between δ98Mo and δ238U suggests a change in global ocean redox conditions, which shift seawater Mo and U isotope compositions (and thus ORM isotope compositions) in the same direction. By contrast, a strong negative correlation, which is also observed for modern euxinic sediments, reflects local depositional controls (e.g., redox conditions, basin restrictions) without requiring significant global redox change. No correlation could be caused by both local effects and global redox states and/or limited data. The possible range in coeval seawater Mo and U isotope compositions for each euxinic ORM unit was estimated via the proposed method. Our study suggests some revisions to ancient seawater Mo and U isotope compositions compared to previous studies, highlighting the importance of using multiple redox proxies to reveal global ocean redox dynamics.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51E0099L
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRY;
- 1025 Composition of the mantle;
- GEOCHEMISTRY;
- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY