INVESTIGATING URANIUM UPTAKE INTO MODERN CARBONATES BY SEQUENTIAL EXTRACTION

The Uranium isotopic system can be used to model the extent of global-scale ocean anoxia. The use of δ²³⁸U as a paleo-redox proxy is based on the equilibrium isotopic fractionation of U during redox conditions. Recent studies have shown promise with the use of this proxy but large variability seen in down-core carbonate sediment samples suggest that more work needs to be done on understanding elemental U uptake in anoxic pore waters. U geochemistry in Bahamian bulk carbonate sediments shows authigenic enrichment of U. The lack of independent constraints regarding authigenic U incorporation is one of the issues regarding the use of U in paleo-redox proxies. The numerous mechanisms involved with U incorporation make it difficult to identify where the largest authigenic U enrichment is seen in bulk carbonate sediments. One method that can be used is a sequential extraction methodology, which can help determine the distribution of Uranium within sediment phases such as exchangeable ions, carbonates, oxides, organics, and residual matter. This project utilized a modified sequential extraction specifically designed for carbonate sediments.

Samples were powdered and U concentrations of each chemical fraction were examined by ICP-MS. Significant enrichment was seen in the exchangeable fraction (11.8% U) and oxide fraction (9.5% U) with minimal contribution into the organic fraction (2.2% U) and residual fraction (3.6% U). The enrichment seen in the exchangeable phase is unexpected and could be the result of remobilization of Uranium due to oxidation of insoluble U^IV to soluble U^VI. This implies that the remobilization of U during diagenesis or redox cycling could remove a large quantity of the primary U signal from carbonates preserved in the geologic record. Enrichment in the oxide phase is expected due to Uranium incorporation into Manganese oxides. However, more work is needed to determine what other species U can be incorporating into such as carbonate flour-apatite.