^(238)U/^(235)U Ratio in Carbonates as a Global Paleoredox Proxy

Abstract

Uranium has two main oxidation states: 4+ and 6+. In the modern ocean, U is present in its most oxidized, highly soluble form, U^(6+) [1]. In the Archean, however, when the oxygen concentration in the atmosphere was low, U was likely present in its reduced 4+ state, which has low solubility. This contrast in solubility behavior depending on the redox conditions makes U a very useful proxy of the global oxygenation state of the Earth. Indeed, one of the most important indicators that the pO_2 was low in the Archean is the survival of detrital uraninite [2], which is unstable under the modern oxic conditions.