A New Proxy Meets And Old Argument: Do Uranium Stable Isotopes Record Productivity Or Ocean Oxygenation?

The Southern Ocean is sensitive to changes in the strength of the biological pump over glacial-interglacial cycles. Authigenic U abundance is well established as a proxy that records carbon delivery to the seafloor. Uranium stable isotopes, on the other hand, are a newly emerging proxy that link variations in U isotope compositions to bottom water anoxia. Although carbon delivery and bottom water oxygen are clearly related, the current interpretation of U isotopes primarily as a water column redox proxy is inconsistent with its chemical behavior. We have measured U isotope ratios at two Southern Ocean sites spanning the past 140,000 years. Both sites experienced periods of enhanced biological productivity as reflected in increased U concentrations and opal burial. We have observed significant variations in the isotope ratios between glacial and interglacial periods at these two sites that coincide with periods on enhanced productivity. Our results suggest that U isotopes, like U elemental abundance, can be used to trace changes in the carbon cycle over a broad range of depositional redox.