To clean or not to clean: Influence of cleaning procedure on trace element-to-calcium ratios measured in foraminiferal calcite

Trace element to calcium ratios (X/Ca) measured in the calcite shells of fossil foraminifera reflect the environmental conditions at the time when the organism formed their shell. During calcification, trace element ions are incorporated either in the place of a calcium cation (Ca2+) or a carbonate anion (CO32-), and bound within the calcite crystal. X/Ca measurements must represent the primary, or lattice bound, trace element-to-calcium ratio signal to reflect paleoenvironmental conditions at the time of calcification, and not a diagenetic signal after burial (e.g. through the presence of a contaminant phase adhering to the shells exterior). Therefore, cleaning methods used on foraminiferal calcite shell fragments prior to measuring X/Ca ratios on ICP-MS must only remove post-depositional contaminants and not a primary signal. Here we present 12 different trace element-to-calcium ratios (Li/Ca, Na/Ca, Al/Ca, Mg/Ca, Mn/Ca, Fe/Ca, Sr/Ca, Zn/Ca, Cd/Ca, I/Ca, Ba/Ca, and U/Ca) measured following 6 different cleaning procedures, and make methodological recommendations based on apparent strength of ion inclusion in the calcite crystal. Our data suggest the X/Ca data across cleaning procedures can be categorized into 4 groups: elements that become more concentrated with additional cleaning procedures (i.e. Na), elements most susceptible to clay removal steps (i.e. Ba and Al), elements that are relatively invariant to cleaning procedure (i.e. Sr and Li), and metals (i.e. Fe, Zn, Mg, Mn, Cd, U) plus I, which exhibit similar trends across cleaning procedures, and are most impacted by reductive cleaning. We report R2 values > 0.7 for all X/Ca ratios considered, and suggest that the elements with the strongest correlations across cleaning procedures behave similarly in the system, either as co-occurring contaminant phases (e.g. Mn-/Fe-oxides), or as elements that are similarly bound in the calcite lattice, with potential for chemical leaching (e.g. Mg, I). Such factors are important to consider when planning cleaning procedure methods for specific depositional environments, and ensuring any X/Ca variability recorded in foraminiferal calcite principally represents variations in paleoclimate conditions, not sample preparation strategy.