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<inf>2</inf> 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<inf>2</inf> 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<inf>3</inf>^2- is the parameter affecting the U/Ca ratio and consequently, the partitioning coefficient for U in Ammonia sp., D<inf>U</inf>. We can confirm the strong potential of foraminiferal U/Ca as a CO<inf>3</inf>^2- proxy. ©2013. American Geophysical Union. All Rights Reserved.