Effects of sample pre-treatment on the δ13C and δ18O values of living benthic foraminifera

Stable carbon and oxygen isotope ratios (δ13C and δ18O) of the epibenthic foraminiferan Elphidium macellum var. aculeatum were measured and compared both in raw aliquots and after various pre-analytical treatments. We tested the effects of chemical cleaning procedures (oxygen plasma ashing, NaClO and H2O2) and physical (ultrasounds) aimed at removing allochthonous particles. We also assessed the effects of using preservatives (formaldehyde and ethanol) and stains (Bengal Rose and Sudan Black B) prior to δ13C and δ18O analyses. Although the effects were generally small, three of the pre-treatments had a significant effect on δ13C or δ18O values. The use of Sudan Black B for vital identification caused a significant enrichment in the 13C of the samples (0.24‰). In addition, sample oxidation with hydrogen peroxide and ultrasonic cleaning each led to a slight decrease in δ18O values (– 0.18‰, in both cases). Our results demonstrate that Sudan Black B should be avoided prior to isotopic analysis. Bengal Rose showed no significant effect and should be used instead for staining. Because palaeoclimatic studies are based in small isotopic shifts, we discourage the pre-analysis use of unnecessary harsh cleaning treatments of foraminifera shell samples that may lead to isotopic fractionation and hence confound ecological interpretation.