Mg/Ca heterogeneity in the benthic foraminifera Cibicidoides wuellerstorfi and Uvigerina peregrina

Abstract

Laser ablation derived Mg/Ca ratios in the epibenthic foraminifera Cibicidoides wuellerstorfi and infaunal Uvigerina peregrina were analyzed from diverse Pacific Ocean sites, covering depths from 600 to 1800 m and bottom water temperatures (BWT) of 2–9 °C. Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), multiple measurements were performed across specimen chambers to assess Mg/Ca variability. For C. wuellerstorfi, intra-chamber Mg/Ca ratios showed an average standard deviation of 0.319 mmol/mol with a mean of 1.41 mmol/mol (relative standard deviation, RSD, of 23.2 %), while inter-chamber variability reached 39.2 %. U. peregrina exhibited slightly lower variability, with a standard deviation of 0.184 mmol/mol and a mean ratio of 1.03 mmol/mol (RSD of 17.0 %). No consistent relationship was observed between Mg/Ca ratios and BWT for either species when using published temperature calibration eqs. A calibration considering carbonate ion concentration (Δ[CO₃²⁻]) improved the fit for C. wuellerstorfi samples at temperatures below 3 °C, suggesting Δ[CO₃²⁻] influences Mg incorporation in epifaunal foraminifera in colder waters. Further analysis of intertest Mg/Ca ratios across 23 specimens of Cibicidoides wuellerstorfi and 40 of Uvigerina peregrina revealed a lack of correlation with temperature, with R² values of 0.004 and 0.003, respectively. The lack of a temperature correlation with Mg/Ca values supports the hypothesis that factors beyond ambient temperature, such as biomineralization processes and carbonate ion concentration, affect Mg/Ca incorporation in the studied species. These results highlight the complexities in interpreting Mg/Ca ratios for these species as temperature proxies and suggest the need for further research to account for geochemical variability in these proxies, especially in LA-ICP-MS based datasets.