Negligible temperature dependence of calcium isotope fractionation in 12 species of planktonic foraminifera

The influence of temperature on calcium (Ca) isotope fractionation during biomineralization was investigated through the paired analyses of δ44/42Ca (via MC-ICP-MS) and δ18O on the calcite tests of 12 species of planktonic foraminifera from core-top sediments. Samples were collected from a suite of box-cores from sites between 60°N and 30°S in the North Atlantic and West Indian oceans, spanning a range in sea surface temperature of 9-27 °C. The δ44/42CaSRM915a of the samples ranged from +0.21‰ to +0.81‰ over a range in the δ18OVPDB of the foraminiferal calcite of between −2.3‰ and +2.1‰. The bulk of the species investigated produced calcite with an average calcite-seawater Ca enrichment factor (ε) of −0.65±0.08‰ (n=45). The globorotaliid species and a single Globigerina bulloides sample were confined to the upper 0.4‰ of the observed range in δ44/42Ca values, with the globorotaliids displaying a lesser degree of fractionation of −0.56±0.23‰ (n=15), possibly as a result of physiological or metabolic processes such as the production of calcite crusts at depth, leading to the depletion of internal stores of Ca. No significant correlation between temperature and Ca enrichment factor was observed in any of the 12 species of foraminifera investigated. The influence of temperature on ε was close to zero for the combined non-globorotaliid species, −0.07±0.10 K (ε versus 1000/K; 0.0008±0.0012‰ °C−1). The results suggest that the theoretically expected relationship between the Ca enrichment factor and temperature can be obscured by, as yet, unquantified metabolic and physiological processes in nature. These processes are particularly relevant to the globorotaliid species and G. bulloides in this core-top study. Consequently, their effects must be better understood before laboratory-determined temperature calibrations can be applied to downcore material with confidence. Provided that the effects of metabolic and physiological processes on ε remain constant through time, Orbulina universa, Globigerinoides sacculifer, Neogloboquadrina pachyderma and Globigerinella aequilateralis are suitable for the investigation of temporal changes in the calcium isotopic composition of seawater.