MEASUREMENTS, METHODS, AND MOTIVATIONS: INSIGHTS ON MG/CA FROM INDIVIDUAL FORAMINIFERA

The geochemical signatures of foraminifera have long been used as indicators of past ocean conditions. Conventional analysis of stable isotopes or of trace metals uses multiple specimens, aggregated together, to provide sufficient material for accurate results and providing a single mean value. The analysis of multiple individual foraminifera can provide additional dimensions for paleoclimatological reconstructions, and single-specimen approaches have been used to identify past changes in oceanic variability, including the El Niño Southern Oscillation (ENSO). While modern stable isotope analytical systems can achieve sufficient precision for such applications (e.g., δ¹⁸O < ± 0.1‰), oxygen isotopes may not be the most suitable proxy system for a given study. Trace metal ratios, such as Mg/Ca, are useful paleoclimatic proxies for ocean temperature, and measurement of trace metal ratios on single foraminifera specimens is possible via Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This technique provides precise trace metal measurements revealing a wealth of additional information. We show that this data reveals inter- and intra-test variability that has implications for the calculation of “whole-shell“ Mg/Ca. We also observe the impact of ablation direction on measured Mg/Ca. We compared LA-ICPMS Mg/Ca measurements to Mg/Ca ratios obtained via inductively coupled plasma - optical emission spectrometry (ICP-OES) on the same specimens of the planktonic foraminifera Globigerinoides sacculifer. We find that the correlation between paired LA-ICPMS and ICP-OES Mg/Ca measurements is high, and that the strength of this correlation can be altered by factors including the specific chamber weightings used to calculate “whole-shell” Mg/Ca from LA-ICPMS and the inclusion or exclusion of shell surfaces. We explore the implications of these findings on methodology and discuss the paleoclimate applications of individual foraminifera for reconstructing past ocean variability.