The reconstruction of past changes in bottom water CO3= is central to evaluating competing oceanic scenarios that deal with long-term variations in atmospheric pCO2. In search of a quantitative bottom water CO3= proxy, we analyzed the variations of calcite crystallinity of planktonic foraminifera Globigerinoides ruber shells picked from core top samples along three depth transects: Ontong Java Plateau and the northeast margin of Irian Jaya, in the western equatorial Pacific, and the Sierra Leone Rise, in the eastern tropical Atlantic. The strong empirical relationship between calcite crystallinity (inferred from the full width at half maximum (FWHM) of calcite (104) X-ray diffraction peak) and bottom water saturation relative to calcite (ΔCO3) shows that foraminifera calcite crystallinity could be a promising proxy for the reconstruction of upper Pleistocene bottom water carbonate ion concentration.