It is well documented that partial dissolution of planktic foraminiferal tests results in a reduction of Mg/Ca ratios, and hence of inferred calcification temperatures; however, traditional analysis techniques have made it difficult to identify the exact mechanism through which Mg is lost. Three hypotheses have been proposed as models for Mg loss for a given extent of dissolution: (1) a percent loss of Mg in individuals, (2) a molar loss of Mg in individuals, and (3) a loss of the highest‐Mg (warmest) individuals from a population. It is vital to better constrain these models as they have very different implications for Mg/Ca paleotemperature dissolution corrections. Here we use a novel individual foraminifera Mg/Ca method to examine the effects of dissolution on the Mg/Ca paleothermometer in three species of planktic foraminifera, Globigerinoides ruber, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata, from a depth transect of core tops on the Ontong Java Plateau in the western equatorial Pacific. With the exception of the most heavily dissolved population of P. obliquiloculata, our data best support a percent Mg loss model as indicated by the preservation of inferred temperature distribution shapes among the sampled populations and the close fit of the simulated percent Mg loss model to the observed data. Coupled with estimates for foraminiferal dissolution, identification of the percent Mg loss model will allow for more accurate dissolution corrections in Mg/Ca paleothermometry work.

El Niño Southern Oscillation (ENSO) is the largest source of interannual climate variability on Earth today; however, future ENSO remains difficult to predict. Evaluation of paleo‐ENSO may help improve our basic understanding of the phenomenon and help resolve discrepancies among models tasked with simulating future climate. Individual foraminifera analysis allows continuous down‐core records of ENSO‐related temperature variability through the construction and comparison of paleotemperature distributions; however, there has been little focus on calibrating this technique to modern conditions. Here, we present data from individual measurements of Mg/Ca in two species of planktic foraminifera, surface dwelling Globigerinoides ruber and thermocline dwelling Neogloboquadrina dutertrei, from nine core tops across the equatorial Pacific (n ≈ 70 per core for each species). Population variance, kernel probability density functions, and quantile‐quantile analyses are used to evaluate the shape of each Mg/Ca‐temperature distribution and to compare them to modern conditions using monthly temperatures from the Simple Ocean Data Assimilation. We show that populations of individual Mg/Ca measurements in both G. ruber and N. dutertrei reflect site‐specific temperature distribution shapes and variances across the equatorial Pacific when accounting for regional differences in depth habitats. Individual measurements of both taxa capture zonal increases in population variance from the western equatorial Pacific to the central equatorial Pacific and a spatially heterogeneous eastern equatorial Pacific, consistent with modern conditions. Lastly, we show that populations of individual Mg/Ca measurements are able to recover meaningful differences in temperature variability between sites within the eastern equatorial Pacific, lending support to this tool's application for paleo‐ENSO reconstructions.

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