Foraminiferal reaction to changes in Pliocene Climate

Anthropogenic atmospheric carbon dioxide (pCO₂^atm) is rapidly rising, affecting ocean chemistry, the biological pump, and marine organisms. To assess the potential impacts of increased CO₂^atm on marine calcifiers, this research established the relationship between changing environmental conditions and calcification in planktic foraminifera, a group essential to the carbon cycle. The Pliocene was the last time interval when pCO₂^atm concentrations were comparable to today. Pliocene core material from Ocean Drilling Program Site 999 in the Caribbean, and International Ocean Discovery Program Site U1313 in the Mid-Atlantic, was analysed for test size and weight of the dominant species, Globigerinoides ruber (white) (999) and Globigerina bulloides (U1313), to assess the impacts of pCO₂^atm on carbonate production.

At 999, test size shows high-frequency variability not related to carbonate chemistry or temperature. During marine isotope stage (MIS) M2, foraminiferal test size increased, possibly linked to intensified stratification in response to the closure of the Central American Seaway. At Site U1313 size has a positive correlation with temperature, with a general increase in size throughout the Pliocene, which matches the warming seen. The foraminiferal size-normalised weight (SNW) of G. ruber from 999 decreased during MIS M2 and is heavier at higher temperatures, thereby inferring higher pCO₂^atm and challenging the view of oceanic partial pressure of carbon dioxide (pCO₂) as a simple driver for calcification. To better understand the drivers of changes in SNW, CT scans were used to calculate calcite to volume (CV) ratios and test thickness in the penultimate chamber. This data suggests the driver of weight change in foraminifera is not likely due to increased pCO₂, but rather to biotic factors, despite reduced CV ratios during warmer interglacial periods.