[1] 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 CO2 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 CO2 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 inline image is the parameter affecting the U/Ca ratio and consequently, the partitioning coefficient for U in Ammonia sp., DU. We can confirm the strong potential of foraminiferal U/Ca as a inline image proxy.