Aqueous Uranium Speciation on U/Ca in Foraminiferal Calcite: The Importance of Minor SpeciesUO2(CO3)22–

The decrease of uranium to calcium ratios (U/Ca) in biogenic CaCO3 with increasing pH and/or [CO32–] is being explored as a proxy to reconstruct past seawater carbonate chemistry. This inverse relationship was interpreted to reflect the changes in the abundances of dissolved U­(VI) species that were incorporated into CaCO3 with pH and/or [CO32–]. Aqueous U­(VI) speciation calculations in all published work on U/Ca in CaCO3 only considered UO2-CO3 and UO2-OH complexes. The recent discovery that Ca/Mg-UO2-CO3 complexes are the dominant U­(VI) species in seawater rather than UO2-CO3 complexes calls this interpretation into question. Considering the Ca/Mg-UO2-CO3 complexes, I revisited published data of foraminifera culturing experiments to examine the effects of U­(VI) speciation on U/Ca in these calcite precipitates. My results reveal that concentrations of Ca/Mg-UO2-CO3 complexes and UO2(CO3)34– remain statistically invariant within uncertainty when the pH and [CO32–] increase, whereas the minor U­(VI) species UO2(CO3)22– decreases by 10-fold. The decrease of [UO2(CO3)22–] with increasing pH and [CO32–] best predicts the decrease of U/Ca in foraminifera, suggesting that UO2(CO3)22– is most likely the U­(VI) species incorporated into calcite. The incorporation of the minor U­(VI) species UO2(CO3)22– into calcite provides an alternative interpretation of significantly lower U concentration in calcite relative to aragonite, if the more abundant UO2(CO3)34– is incorporated into aragonite. Theoretical aqueous U­(VI) speciation further demonstrates that [UO2(CO3)22–] is very sensitive to changes in pH, [CO32–], and [Ca2+], suggesting that U/Ca is a good proxy for seawater pH and [CO32–] within ∼400 kyr when both U and Ca concentrations remain constant.