Using Element/Ca Response to Cleaning in Foraminifera From Endmember Depositional Environments to Infer Contaminants and Inform Pretreatment

Abstract

Past ocean conditions are often reconstructed using the elemental composition of foraminiferal calcite. However, foraminiferal remains are often impacted by post-depositional contaminants; thus, they require cleaning prior to element/Ca (El/Ca) analysis. To explore the impact of sample pretreatment on foraminiferal El/Ca ratios (Li, Na, Mg, Al, Mn, Fe, Zn, Sr, I, Ba, and U) we performed six cleaning procedures on four foraminifera populations from distinct depositional environments: Two from South Pacific carbonate ooze (ELT25-11) and two from the hemi-pelagic sediments of the California Margin (ODP1017E, SPR0901-04BC). Despite differences in regional oceanography, sample type (i.e., surface- or deep-dwelling planktic, benthic), and cleaning procedure, the main driver of El/Ca variability in the data set is the sedimentary depositional environment, suggesting site-specific differences in element concentrations and contaminants. This finding challenges the notion that sample cleaning procedures should be informed by the El/Ca of paleoclimate interest, as elements may be found in different contaminants and/or elemental abundances in unique environments. Our data also show that traditional cleaning methods which use a combination of rinsing, sonication, oxidation/reduction, and complexation reactions effectively remove contaminants found on foraminifera in either depositional environment. However, even after contaminant removal, some elements (i.e., U and Fe) remain higher in California Margin foraminifera relative to South Pacific foraminifera. This suggests that the range of acceptable El/Ca values in the literature must be expanded when working with foraminifera from unusual depositional environments (i.e., hemipelagic, siliciclastic sites with high sedimentation regimes) versus values associated with more typical sites (i.e., a pelagic carbonate ooze).