We present Mg/Ca data for Globigerina bulloides from 10 core top sites in the southwest Pacific Ocean analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Mg/Ca values in G. bulloides correlate with observed ocean temperatures (7°C–19°C), and when combined with previously published data, an integrated Mg/Ca–temperature calibration for 7°C–31°C is derived where Mg/Ca (mmol/mol) = 0.955 × e0.068 × T (r2 = 0.95). Significant variability of Mg/Ca values (20%–30%) was found for the four visible chambers of G. bulloides, with the final chamber consistently recording the lowest Mg/Ca and is interpreted, in part, to reflect changes in the depth habitat with ontogeny. Incipient and variable dissolution of the thin and fragile final chamber, and outermost layer concomitantly added to all chambers, caused by different cleaning techniques prior to solution-based ICPMS analyses, may explain the minor differences in previously published Mg/Ca–temperature calibrations for this species. If the lower Mg/Ca of the final chamber reflects changes in depth habitat, then LA-ICPMS of the penultimate (or older) chambers will most sensitively record past changes in near-surface ocean temperatures. Mean size-normalized G. bulloides test weights correlate negatively with ocean temperature (T = 31.8 × e−30.5×wtN; r2 = 0.90), suggesting that in the southwest Pacific Ocean, temperature is a prominent control on shell weight in addition to carbonate ion levels.