Life in a dark environment – what was the physiological and calcification response of benthic foraminifera to the environmental changes of the Paleogene hyperthermals

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.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 Nuttallides truempyi 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 Oridorsalis umbonatus 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. umbonatus. 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.