Carbonate‐forming organisms play an integral role in the marine inorganic carbon cycle, yet the links between carbonate production and the environment are insufficiently understood. Carbonate production is driven by the abundance of calcifiers, and the amount of calcite produced by each individual (their size and weight). Here we investigate how foraminiferal carbonate production changes in the Atlantic, Pacific and Southern Ocean in response to a 4‐5°C warming and a 0.3 surface ocean pH reduction during the Palaeocene‐Eocene Thermal Maximum (PETM). To put these local data into a global context, we apply a trait‐based plankton model (ForamEcoGEnIE) to the geologic record for the first time. Our data illustrates negligible change in the assemblage test size and abundance of foraminifers. ForamEcoGEnIE resolves small reductions in size and biomass, but these are short‐lived. The response of foraminifers shows spatial variability linked to a warming‐induced poleward migration and suggested differences in nutrient availability between open‐ocean and shelf locations. Despite low calcite saturation at high latitudes, we reconstruct stable foraminiferal size‐normalised weight. Based on these findings, we postulate that sea surface warming had a greater impact on foraminiferal carbonate production during the PETM than ocean acidification. Changes in the composition of bulk carbonate suggest a higher sensitivity of coccolithophores to environmental change during the PETM than foraminifers.This article is protected by copyright. All rights reserved.

<p>The Paleocene encompasses a series of hyperthamls including the Paleocene–Eocene Thermal Maximum (PETM) and the ETM2 which represent severe disturbances of global carbon cycling and the Earth system. Responses of marine organisms included extinction, migration and evolutionary turnover, but the role of ocean acidification on deep-sea foraminiferal calcification has not yet been quantified. Using computed tomography (CT) we investigate morphological (surface area, test volume, calcite volume, chamber number) and hence calcification response in two benthic foraminiferal species, at central Pacific Site 1210 (PaleoDepth 2100m), and Southern Ocean Maud Rise Site 690 (PD 1900m), Walvis Ridge Site 1264,  and Kerguelen Plateau Site 1135 (PD ~800m) for the PETM and ETM2.</p> <p>The relative warming during the event was the same at all sites, suggesting that biotic differences are not likely related to differential warming. The environmental change led to reduction of test volume of both species, negatively impacting their potential ability to generate gametes. Epifaunal <em>Nuttallides truempyi </em>increased its surface area relative to volume in the Southern Ocean, potentially increasing its ability to forage and take up oxygen. In contrast, there is no clear pattern of change in shallow infaunal <em>Oridorsalis umbonatus</em> which, given sufficient food, can thrive at lower oxygen conditions. Calcite volume/test volume ratio decreased in both species during the PETM in the Southern Ocean, with the lack of response at upper abyssal depth in the Pacific possibly driven by severe oligotrophy even before the excursion. Therefore, changes in food supply during hyperthermals might have been less pronounced at upper abyssal depths in the Pacific than at the other two sites. These results contrast with published results from Walvis Ridge which showed an increase in calcification in small specimens of O<em>. umbonatus</em>. Food availability at the Southern Ocean sites may have supported growth as indicated by test volumes, but did not supply enough energy for calcification to mitigate against lower carbonate ion saturation during the PETM CIE.</p>