TO BE CRUSTED OR NOT TO BE CRUSTED: DIFFERENCES IN TRACE ELEMENT GEOCHEMISTRY BETWEEN SHELLS OF LIVING AND DEAD PLANKTIC FORAMINIFERA

Planktic foraminiferal-based trace element-calcium ratios (TE/Ca), such as Mg/Ca-temperature calibrations, have become a cornerstone in paleoceanographic reconstructions. TE/Ca calibrations used for paleoreconstructions are often established through controlled culturing experiments. However, shell growth, and thus potentially geochemical composition, in culture is not always consistent with growth in a natural setting. For example, many species of planktic foraminifera thicken their shell following reproduction and just prior to death by growing a distinct crust. While this crust is often found in fossil shells used for paleoreconstructions, shells grown in culture do not often develop a thick crust. It is possible that applying calibrations developed from non-crusted shells to crusted-shell fossil records may result in misleading paleoceanographic reconstructions. Here we investigate this possibility by comparing the trace element composition (Mg, Na, Ba, Sr, Mn, Zn) of alive, fully mature, uncrusted shells to recently deceased, crusted shells collected from the same discrete plankton tows. Using laser ablation ICP-MS, we find that large populations of crusted (n=66) versus uncrusted (n=55) shells of Neogloboquadrina pachyderma collected off the Oregon coast have markedly different Mn, Ba, and Sr composition. The uncrusted specimens have much higher trace elements compared to their crusted counterparts. We also examined intrashell variability in these shells to characterize geochemical differences between the crust and ontogenetic portions of the shell wall. Preliminary results corroborate observed geochemical differences between crusted and uncrusted shells, showing higher TE/Ca ratios for most elements in the inner ontogenetic calcite when compared to the outer crust calcite. Results herein highlight the necessity to consider ontogeny in geochemical evaluation of planktic foraminiferal records.