CONSTRAINTS ON URANIUM ISOTOPE FRACTIONATION AND RELATED RECONSTRUCTIONS OF PAST MARINE REDOX CONDITIONS FROM THE MIOCENE MONTEREY FORMATION (Invited Presentation)

Oxygenation and de-oxygenation of the oceans are major controls on the evolutionary trajectory of life and biogeochemical cycles through Earth history. Quantitative reconstructions of global marine redox conditions are challenging, as direct measurements of the degree of oxygenation are not possible, and instead rely on a combination of proxies. The isotopic proxies of redox-sensitive metals—particularly uranium (²³⁸U/²³⁵U, commonly denoted as δ²³⁸U)—are the most promising for constraining global changes in redox conditions, in contrast to other proxies that record only basin-level conditions. However, our current understanding of the controls on the uranium paleoredox proxy is limited, complicating the interpretation of δ²³⁸U fluctuations in the rock record. Variations in productivity-driven upwelling, basin connectivity, and sedimentation rate can influence the isotopic fractionation of uranium into organic-rich shales—the largest lever on the δ²³⁸U composition of seawater. To investigate the interplay of these factors on uranium cycling, we focus on the phosphatic, carbonaceous, and silica-rich shales of the Miocene Monterey Formation. Deposition of these rocks coincides with the Mid-Miocene Climatic optimum (ca. 17 – 14.7 Ma), when a period of higher organic carbon burial—and hence anoxia—has long been hypothesized, followed by a transition to a colder, upwelling-driven period linked to global tectonic events. We present δ²³⁸U data from two depositional basins that capture these climatic and oceanographic changes, as well as variations in sedimentation rates and basin restriction. The δ²³⁸U records in both basins show an overall increase of -0.2‰ from the end of the Mid-Miocene Climatic optimum to the cessation of Monterey deposition. This change may reflect a shift in seawater δ²³⁸U and thus reduced organic carbon burial and increased seafloor oxygenation, or may reflect an increase in uranium isotope fractionation related to higher productivity. Coupled to reactive transport models, this work provides new constraints on the major controls on δ²³⁸U in the past oceans as a critical step in the development of an emerging paleoredox proxy.