SCRIPPS INSTITUTION OF OCEANOGRAPHY FACULTY CANDIDATE SEMINAR: Low Temperature Geochemistry
DATE: April 18th, Wednesday, 1p.m.
LOCATION: Eckart Lecture Hall
TITLE: Investigating the Role of Anoxia on the Global Carbon Cycle: New Uranium Isotopic Insights
Oxygenation and deoxygenation of the oceans are major controls on the evolutionary trajectory of life and biogeochemical cycles through Earth history. The connections between ancient life and oxygen have been intensively scrutinized because oxygenation determines the pathways through which organisms can obtain energy and is critical for defining habitability and environmental conditions. 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 d238U)—are the most promising for constraining global changes in redox conditions. In this talk, I will show how the d238U 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 by a factor of 100 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 d238U fluctuations in the rock record. Productivity, basin connectivity, and sedimentation rate can all influence the isotopic fractionation of uranium into organic-rich shales—the largest lever on the d238U 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 of sediment biogeochemistry. In sum, this work provides new constraints on the major controls on d238U in the past oceans, as a critical step in understanding the patterns and drivers of ancient biogeochemical cycles.