Investigating the fidelity of Mg/Ca and other elemental data from reductively cleaned planktonic foraminifera

There has been considerable discussion about whether reductive cleaning can or should be used when samples are prepared for foraminiferal Mg/Ca analyses. The reductive solution causes considerable sample loss through dissolution, and the natural dissolution effect on planktonic foraminiferal Mg/Ca raises the question whether the shell chemistry might be affected during cleaning. We conducted a series of experiments to explore the effects of the reductive cleaning step on shell chemistry (Mg/Ca, Na/Ca, Sr/Ca, Fe/Ca, Mn/Ca, REE/Ca, and U/Ca) of two planktonic foraminiferal species. We then compare these results with natural dissolution in core top specimens from a range of varying natural dissolution intensity. Our results confirm previous experiments by Yu et al. (2007) that the citrate used in the reductive solution can play a larger role in decreasing some element/Ca ratios than hydrazine, the reductive agent, at least in samples that are not problematic to clean (i.e., have low Mn/Ca, Fe/Ca, and Al/Ca). Solutions containing varying concentrations of citrate yield lower Mg/Ca than those without citrate; however, Mg/Ca reaches an asymptotic value following a dilute application of citrate. Other element/Ca ratios are also lowered by citrate and tend toward an asymptotic value at increasing strengths of citrate. Mg/Ca data from experiments in which the duration of the reductive cleaning step was varied or repeated suggest that the standard reductive cleaning step should yield reproducible results for most elemental ratios. Our experiments suggest that the primary mechanism of dissolution by citrate is unlikely to be preferential removal of Mg from the lattice and also demonstrate that there are differences between natural and laboratory dissolution effects. Given that there are situations in which Mg/Ca data are improved by reductive cleaning, we conclude that it is acceptable to reductively clean samples before Mg/Ca analysis.