Geology Club Seminar
Oxygenation and deoxygenation of the oceans are major controls on the evolutionary trajectory of life and biogeochemical cycles through Earth history. Although quantitative reconstructions of marine redox conditions are challenging, sedimentary geochemical proxies have proven invaluable for tracking the marine environmental conditions of past oceans. Specifically, isotopic proxies of redox-sensitive metals—particularly uranium (238U/235U, or δ238U)—are the most promising for constraining global changes in redox conditions. In this talk, I will show how the δ238U paleoredox proxy can be used to determine the extent of anoxia in the aftermath of the end-Permian extinction (252 to 235 Ma), the largest catastrophe in the history of animals. The U isotope record, as archived in carbonate strata, mirrors patterns in biodiversity and carbon isotope instability, suggesting an increase of anoxia that coincides with the extinction, and a slow return to oxygenated conditions over five million years. During the recovery, shallow anoxic waters may have impinged onto the continental shelves, increasing the sensitivity of the carbon cycle. I also present a new approach for improving interpretations of δ238U in the rock record. Productivity, basin connectivity, and other depositional conditions 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, I combine a field- and laboratory-based study of the Miocene Monterey Formation with early diagenetic modeling. We find that these controls may play a large role in uranium accumulation and isotopic fractionation, and can result in diagnostic patterns in both concentrations and δ238U. Although our findings complicate the use of δ238U records for interpreting global redox patterns, these predictive patterns can provide new information about local sedimentation patterns. More broadly, this work provides new constraints on the major controls on δ238U in the past oceans.