Incorporation of uranium in benthic foraminiferal calcite reflects seawater carbonate ion concentration

Abstract

The chemical and isotopic composition of foraminiferal shells (so-called proxies) reflects the physicochemical properties of the seawater. In current day paleoclimate research, the reconstruction of past seawater carbonate system to infer atmospheric CO 2 concentrations is one of the most pressing challenges, and a variety of proxies have been investigated, such as foraminiferal U/Ca. Since in natural seawater and traditional CO 2 perturbation experiments the carbonate system parameters covary, it is not possible to determine the parameter of the carbonate system causing, e.g., changes in U/Ca, complicating the use of the latter as a carbonate system proxy. We overcome this problem by culturing the benthic foraminifer Ammonia sp. at a range of carbonate chemistry manipulation treatments. Shell U/Ca values were determined to test sensitivity of U incorporation to various parameters of the carbonate system. We argue that CO 3 2- is the parameter affecting the U/Ca ratio and consequently, the partitioning coefficient for U in Ammonia sp., D U. We can confirm the strong potential of foraminiferal U/Ca as a CO 3 2- proxy. ©2013. American Geophysical Union. All Rights Reserved.