Laser ablation inductively coupled plasma mass spectrometry was used to analyze the individual chambers from tests of foraminiferal fossil and plankton tow Globigerinoides ruber from the southwest Pacific Ocean, from latitudes 3°S to 42°S. The variability of Mg/Ca between chambers of an individual (intraindividual) and individuals of the same population (interindividual), is such that when converted to temperature, the extent of intra-individual and interindividual variability appears to exceed that attributable to either calcification or seasonal temperature variability. The pooled mean chamber Mg/Ca from each core top and plankton tow site demonstrates a significant (p < 0.05) positive correlation with temperature. We derive chamber-specific calibrations where Mg/CaCh_F-2 = 0.798 exp0.070 T, Mg/CaCh_F-1 = 0.891 exp0.067 T and Mg/CaCh_F = 0.590 exp0.072 T. We do not observe any bias between the two morphotypes Gs. ruber ruber and Gs. ruber pyramidalis. The chamber-specific calibrations potentially offset Mg/Ca-based temperature reconstructions if used on bulk (whole) test Mg/Ca or applied to misidentified chambers. Nevertheless, these calibrations can be used to reliably estimate sea surface temperature. Although there is a general overriding temperature control on Mg/Ca, we show that removal of the effect of temperature at each site reveals a lognormal Mg/Ca distribution. This suggests that Mg/Ca variability at each site is also affected by biological mechanism(s) that may control the distribution of interindividual Mg/Ca. In addition, other TE/Ca data (Al/Ca and Mn/Ca) from laser ablation trace element depth profiles can be used to identify detrital or diagenetic phases that may bias the trace element/Ca signal.