We explore using changes in planktonic foraminiferal Mg/Ca with water depth in a single region to infer relative changes in deep water carbonate chemistry. The Mg/Ca ratio in foraminiferal calcite is quantitatively lowered by dissolution. We exploit this dissolution effect to generate a profile of carbonate ion concentration (CO32−) for the deep waters of the tropical Atlantic during the Last Glacial Maximum. We generated Mg/Ca data for three species, G. ruber, G. sacculifer, and N. dutertrei, from three cores located along a depth transect (2.8 to 4.0 km) on the Ceara Rise in the western equatorial Atlantic. We attribute lower Mg/Ca in glacial intervals from the shallow core to changes in sea surface temperature (SST). The even lower glacial Mg/Ca values in the deeper cores are attributed to dissolution and are used to reconstruct a paleo-CO32− profile. Our down core Mg/Ca data for G. ruber and G. sacculifer confirm that these species are more resistant to Mg loss due to dissolution than N. dutertrei. The greater susceptibility of N. dutertrei's Mg content to dissolution limits the utility of this species in thermocline temperature reconstructions to cores above the lysocline but, significant to this study, makes N. dutertrei useful in quantifying changes in deep water CO32−. Our paleo-CO32− reconstruction based on N. dutertrei suggests the Last Glacial Maximum CO32− gradient in the Atlantic was much steeper than today, implying that the boundary between deepwater masses in the Atlantic was shallower during the Last Glacial Maximum. Our results suggest comparison of Mg/Ca from multiple cores from one region could similarly provide constraints on changes in carbonate chemistry in other basins and other time periods.