Combined carbonate carbon isotopic and cellular ultrastructural studies of individual benthic foraminifera: Method description

Carbon isotopes of foraminiferal tests provide a widely used proxy for past oceanographic environmental conditions. This proxy can be calibrated using live specimens, which are reliably identified with observations of cell ultrastructure. Observations of ultrastructures can also be used for studies of biological characteristics such as diet and presence of symbionts. Combining biological and isotopic studies on individual foraminifera could provide novel information, but standard isotopic methods destroy ultrastructures by desiccating specimens and observations of ultrastructure require removal of carbonate tests, preventing isotope measurements. The approach described here preserves cellular ultrastructure during isotopic analyses by keeping the foraminifera in an aqueous buffer (Phosphate Buffered Saline (PBS)). The technique was developed and standardized with 36 aliquots of NBS-19 standard of similar weight to foraminiferal tests (5 to 123 $μ$g). Standard errors ranged from ± 0.06 to ± 0.85‰ and were caused by CO2 contaminants dissolved in the PBS. The technique was used to measure $δ$13C values of 96 foraminifera, 10 of which do not precipitate carbonate tests. Calcareous foraminiferal tests had corrected carbon isotope ratios of −8.5 to +3.2‰. This new technique allows comparisons of isotopic compositions of tests made by foraminifera known to be alive at the time of collection with their biological characteristics such as prey composition and presence or absence of putative symbionts. The approach may be applied to additional biomineralizing organisms such as planktonic foraminifera, pteropods, corals, and coccolithophores to elucidate certain biological controls on their paleoceanographic proxy signatures.