A PALEOZOIC RECORD OF URANIUM ISOTOPES CONSTRAINING FRACTIONATION FACTORS IN FERRUGINOUS SETTINGS

Uranium isotopes in shales are now widely used as a marine redox proxy through much of Earth’s history. However, the current understanding of fractionation factors during the reduction of U and removal to siliciclastic sediments is lacking. Foremost, there are minimal constraints on the fractionation of U isotopes under ferruginous conditions, which are thought to be relatively widespread in the Paleozoic based on evidence Fe speciation studies. Here we present more than 300 new U isotope data points from primarily the Paleozoic, along with additional data from the Precambrian and modern sediments. We find that; 1) there are a large range of values in both temporally equivalent strata and within individual formations, 2) based on maximum fractionations, there is evidence that seawater was generally within 0.1 ‰ of modern, 3) average fractionations in ferruginous settings have likely been overestimated in global isotope mass balance models. In sum, we suggest that these U isotope values are consistent with a large solution space dependent on the dominant reduction pathway, mass accumulation rates, and organic matter loading and are consistent with both largely oxygenated and largely anoxic depositional.