Potential Remobilization and Sources of 234U in the Southern Ocean over Five Glacial-interglacial Cycles

Abstract:

Due to its sensitivity to changes in redox conditions, authigenic uranium is an important proxy for tracking the history of paleoproductivity or deep ocean ventilation. However, there are potential complications interpreting these records when imprinted by diagenetic processes, such as “burn-down”, or downcore remobilization after a period of re-oxygenation. To investigate these potential complications, we present here a 470 kyr record of the ²³⁴U/²³⁸U ratio (δ²³⁴U) of bulk U in South Atlantic core ODP1094. We evaluate the record using an end-member mixing model, assuming a simple mixture of high δ²³⁴U, authigenic U (seawater sourced) and low δ²³⁴U, detrital U (terrestrial sourced). The observed record displays sections with δ²³⁴U values higher than the expected seawater endmember and adjacent sections with values lower than predicted based on the amount of detrital U. Using an integrated mass balance of ²³⁴U with depth, a preferential remobilization of alpha-recoiled ²³⁴U can explain some of the isotope anomalies. However, there remains significantly more ²³⁴U than can be predicted by our assumed end-members, even allowing for preferential ²³⁴U diffusion, in two deglacial authigenic uranium peaks during Terminations II and IV. While the amount of anomalous ²³⁴U suggests the bulk authigenic U did not suffer significant remobilization, the isotope data do suggest a possible external source of U or a diagenetic process that is yet to be defined and may be a new geochemical phenomenon in pelagic sediments.