99-11 Redox Revelations from Utah’s Wheeler Formation: A Multi-Proxy Reconstruction during the Rise of Animals

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

Presenting Author:

Kiri Maza

Authors:

Maza, Kiri¹, Thompson, Maya², Sperling, Erik A.³, Norman, Michelle⁴, Dehler, Carol M.⁵, Melnyk, Scott⁶, Birgenheier, Lauren P.⁷, Blusztajn, Jerzy⁸, Ostrander, Chadlin M.⁹

(1) Geology and Geophysics, University of Utah, Salt Lake City, UT, USA, (2) Earth and Planetary Sciences, Stanford, Stanford, CA, USA, (3) Stanford University, Stanford, CA, USA, (4) Utah State University, Logan, USA, (5) Geosciences USU, Logan, UT, USA, (6) University of Utah, Salt Lake City, USA, (7) University of Utah, Geology and Geophysics Department, Salt Lake City, UT, USA, (8) Woods Hole Oceanographic Institute, Woods Hole, USA, (9) University of Utah, Salt Lake City, UT, USA,

Abstract:

Reconstructions of Earth’s ocean oxygenation history during the Cambrian Period (538-486 Ma) provide insight into the role of free oxygen (O₂) in early animal evolution. Many Cambrian oxygenation reconstructions are derived from sedimentary sequences with no material evidence of animals. While there are benefits to this approach, it requires indirect correlations to the fossil-bearing strata, complicating temporal animal-O₂ connections. 

 

This presentation will summarize our efforts to reconstruct local- and global-scale ocean redox conditions from mid-Cambrian siliciclastic-dominated sedimentary rocks with rare and exceptional soft-body faunal preservation. Our sample targets are fossil-bearing calcareous shales of the ~503 million-year-old Wheeler Formation from western Utah recovered from drill core DM-15-1. We have applied a long (and still growing) list of paleoredox proxies to these shales. This includes iron speciation and redox-sensitive trace metals (e.g., Mo, U, V) to constrain local redox, and thallium (Tl) isotopes to potentially track global-scale changes in deep ocean oxygenation.

 

Geochemical data generated thus far reveal novel paleoenvironmental information. Iron speciation data suggest a highly anoxic and at-times even sulfidic (euxinic) local environment, whereas trace metal data support suboxic to oxic conditions. All data considered, local scale redox conditions seemed to straddle the oxic-anoxic boundary, with perhaps short-term redox oscillations but tending toward reducing conditions. This broadly reducing characterization is consistent with the local fossil preservation, in that soft-bodied fauna can only be preserved in reducing sediments.

 

Locally reducing conditions are ideal for applying Tl isotopes as a global-scale paleoredox proxy. Sedimentary sulfides formed under such conditions should, by analogy with today, reliably capture the globally homogenous seawater Tl isotope composition (e²⁰⁵Tl). Seawater e²⁰⁵Tl values are very sensitive to Mn oxide burial on the seafloor because preferential ²⁰⁵Tl removal during this process drives a complimentary seawater ²⁰³Tl enrichment. Wheeler shale e²⁰⁵Tl values rest primarily between averaged ocean inputs and modern seawater, implying that the Cambrian global deep ocean was more reducing than today.

 

Our analyses provide important information about ancient local conditions for a soft body fossil-rich Cambrian shale unit, as well as insight into global-scale deep ocean oxygenation during one of the most explosive periods of animal diversification in Earth history.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10265

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