Anomalous 234U/238U isotopic composition in Southern Ocean sediments

Uranium is a redox-sensitive trace element that under certain sedimentary conditions will reflect changes in past biological productivity and/or deep ocean ventilation. Applications for the isotopic composition of U (234U:238U activity ratio, 8234U or the per mil deviation from secular equilibrium) as a paleoceanographic proxy, however, remain largely unexplored in pelagic sediments. We present 8234U of bulk U for a 504-kyr record of South Atlantic core ODP 1094, a record that has exhibited sensitivity to bottom water formation with minimal 238U remobilization due to changes in redox condition. Sedimentary 8234U at time of burial was modeled given authigenic and detrital sources, where it was found that 8234U has a reoccurring pattern of anomalous behavior around deglacial transitions. In many cases, the anomalous 8234U compositions in sediment exceeded seawater, which is not consistent with its expected sources. A mass balance of 234U across these anomalous sections showed that vertical rearrangement of 234U can explain anomalous sedimentary 8234U during Marine Isotope Stage (MIS) 7/8 and 11/12 transitions, as well as partially during MIS 5/6 and MIS 9/10. The distribution of these anomalies is likely consistent with the idea that deglacial spikes in aU, resulting from periods of high organic carbon de-livery and thereby strong reduction in the sediments, act as a persistent sink of 234U alpha-recoiled from vertically adjacent sediment layers after burial. While this mechanism explains the negative 8234U anomalies, there is a large subset of positive 8234U anomalies that cannot be explained by vertical diffusion of 234U in pore waters alone. This excess 234U points to an extremely high 8234U source to seawater during deglaciation or additional diagenetic effects such as diffusion from lateral heterogeneity in the redox state of sediment at depth. We find that 8234U of pelagic sediment may be useful to identify fine-scale pore water migration of alpha-recoil products.