Uranium isotope variations in a dolomitized Jurassic carbonate platform (Tithonian; Franconian Alb, Southern Germany)

The uranium (U) isotopic composition (delta U-238) of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean. However, many limestones have undergone significant dolomitization during diagenesis. To assess the potential impact of diagenetic changes on the U isotope composition of dolomitized rocks, we examined the uranium isotopic composition of a Jurassic carbonate platform of the Franconian Alb in Southern Germany. This platform underwent dolomitization during shallow burial due to the supply of magnesium-rich fluids in the form of slightly modified seawater. This type of dolomitization is common in the geologic record. The carbonate platform of the Franconian Alb can therefore serve as an example for many dolomitized carbonate platforms. A positive correlation between the concentrations of redox sensitive elements (e.g., Re and V) and delta U-238 confirms that the authigenic uptake of U under reducing conditions preferentially incorporates U enriched in U-238. This positive correlation between redox sensitive elements and delta U-238 does not change within the dolomitized interval, suggesting that the delta U-238 is not altered during shallow burial dolomitization of limestones. Therefore, our results indicate that dolostones can retain information about secular variations in seawater delta U-238. The diagenetic uptake of U shifts delta U-238 of bulk carbonate sediments to values heavier than contemporaneous seawater. The magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set. Samples with high Re and V concentrations and high Re/Mo ratios are the most enriched in U-238. This positive correlation could be used to constrain the amount of diagenetic offset of bulk delta U-238 values from seawater. This is particularly important in carbonate systems that had a primary calcitic mineralogy. Primary calcite precipitates have low U concentrations and so small amounts of U-238 uptake during diagenesis can have a significant impact on the bulk composition. The samples with the most depleted delta U-238 values have high Mo concentrations and low Re/Mo ratios. As carbonate sediments are generally Mo poor, the high Mo concentrations suggests that the depleted delta U-238 values might be linked to a manganese cycle that operated across the sediment-water interface during the deposition of this Jurassic carbonate platform.