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)(3)(4-) remain statistically invariant within uncertainty when the pH and [CO32-] increase, whereas the minor U(VI) species UO2(CO3)(2)(2-) decreases by 10-fold. The decrease of [UO2(CO3)(2)(2-)] with increasing pH and [Co-3(2-)] best predicts the decrease of U/Ca in foraminifera, suggesting that UO2(CO3)(2)(2-) is most likely the U(VI) species incorporated into calcite. The incorporation of the minor U(VI) species UO2(CO3)(2)(2-) into calcite provides an alternative interpretation of significantly lower U concentration in calcite relative to aragonite, if the more abundant UO2(CO3)(3)(4-) is incorporated into aragonite. Theoretical aqueous U(VI) speciation further demonstrates that [UO2(CO3)(2)(2-)] is very sensitive to changes in pH, [CO32-], and [Ca2+], suggesting that U/Ca is a good proxy for seawater pH and [CO32-] within , similar to 400 kyr when both U and Ca concentrations remain constant.