Intratest oxygen isotope variability in the planktonic foraminifer N. pachyderma: Real vs. apparent vital effects by ion microprobe

Abstract

Intratest oxygen isotope variations in the planktonic foraminifer Neogloboquadrina pachyderma sinistral (left coiling) from North Atlantic core top and multi-net samples were assessed by ion microprobe analysis from 2 to 6 μm spots with precision and accuracy better than 0.7‰ in δ 18O (2 SD). Within a single foraminiferal test from a core top sample comprising both ontogenetic calcite and outer crust, δ 18O values vary from 0.5‰ to 3.7‰ [PDB], exceeding the range of equilibrium δ 18O in the specimens' habitat by a factor of three. The isotopic difference between the ontogenetic calcite and the crust averages 1.8‰. Neither of the two types of foraminiferal calcite precipitates in equilibrium with ambient seawater. The ontogenetic calcite exhibits a negative vital effect Δ 18O (M-E) ( δ 18O (measured) − δ 18O (equilibrium)) ranging from − 0.5 to − 1‰. The largest negative fractionation is associated to the inner walls of juvenile chambers. In contrast, a positive vital effect of about 0.8‰ was observed in the crust with respect to the highest equilibrium δ 18O values at water depths below 200 m. Hence two vital effects that are opposite in sign are effective within a single foraminiferal test, indicating that ‘whole test’ values of this species are highly sensitive to the degree of encrustation and amplify or attenuate environmental signals. The negative vital effect of the ontogenetic calcite was verified by ion microprobe analysis of four nonencrusted net-sampled specimens reflecting three different depth intervals. Intra-ontogenetic oxygen isotope ratios in these juvenile tests range from 0.6 to 3.0‰ and exhibit a negative vital effect even larger than that observed in core top samples. Based on these data, the large range of ‘apparent’ vital effects reported for this foraminiferal species can be assessed by a mass balance calculation, assuming that the degree of encrustation is variable.