Preferential Loss of High Trace Element Bearing Inner Calcite in Foraminifera During Physical and Chemical Cleaning

Trace elements (TEs) in the calcite shells of foraminifera are widely used in paleoceanographic reconstructions. Nearly all TE analytical protocols include cleaning to remove contaminant phases. Understanding, how the TE composition is altered during physical and chemical cleaning is of utmost importance in ensuring the accuracy of paleoclimate reconstructions. Aggressive cleaning steps that include corrosive reagents are commonly used to clean shells with elevated Ba/Ca and Mn/Ca ratios, under the assumption that these elements are elevated due to diagenetic overprinting. We provide evidence that elevated intrashell Ba/Ca, Mn/Ca, and Zn/Ca ratios are lattice bound and a primary signal incorporated into inner calcite during calcification in the nonspinose species Neogloboquadrina dutertrei and Pulleniatina obliquiloculata . We first demonstrate, using laser ablation depth profiling analyses, that aggressive cleaning following the full cleaning protocol traditionally used to eliminate Mn and Ba contamination, preferentially removes inner (ontogenic) calcite and favors the low Ba/Ca, Mn/Ca, Zn/Ca, and Mg/Ca crust/cortex calcite. In a separate experiment, we demonstrate that mechanical fragmentation of bulk shells prepared for solution analysis also preferentially removes inner calcite and favors the low TE crust/cortex calcite. Preferential loss of the ontogenic calcite decreases Ba/Ca, Mn/Ca, and Zn/Ca ratios by 50%–90% in N. dutertrei and P. obliquiloculata shells. This biases foraminiferal TE results toward lower values and may yield data that is problematic for the interpretation of downcore records.