The ratio of magnesium to calcium (Mg/Ca) in foraminiferal shells is commonly used as a proxy for past ocean temperature. Recent advances in elemental analyses now enable single-specimen measurements of planktic foraminifera and thus, can provide information on past seasonal and interannual variability, owing to the near-monthly lifespan of foraminifera. In this study, we explore the temperature variance recorded by Mg/Ca in tests of foraminifera Globigerinoides ruber, a planktic species that occurs throughout the year in tropical waters. Using LA-ICP-MS, we characterize Mg/Ca variability in single specimens of two morphotypes of G. ruber picked from a sediment core retrieved offshore New Caledonia. We provide an estimate of the range of calcification temperatures for these morphotypes during five interglacial-glacial cycles over the last 1.55 Ma. First, we find significant and systematic differences between the morphotypes and second, the temperature difference between morphotypes does not remain constant through time. Our results highlight a progressive increase in surface-water temperatures during interglacials and a progressive decrease in glacial subsurface water temperatures. These changes in surface and subsurface temperatures potentially highlight a change in the stratification of the water column over the Mid-Pleistocene Transition. We conclude that single-specimen Mg/Ca on foraminiferal morphotypes can offer unique perspectives on paleoenvironmental reconstructions.