EMP and SIMS studies on Mn/Ca and Fe/Ca systematics in benthic foraminifera from the Peruvian OMZ: a contribution to the identification of potential redox proxies and the impact of cleaning protocols

[摘要] In this study we present an initial dataset of Mn/Ca and Fe/Ca ratios intests of benthic foraminifera from the Peruvian oxygen minimum zone (OMZ)determined with SIMS. These results are a contribution to a betterunderstanding of the proxy potential of these elemental ratios for ambientredox conditions. Foraminiferal tests are often contaminated by diageneticcoatings, like Mn rich carbonate- or Fe and Mn rich (oxyhydr)oxide coatings.Thus, it is substantial to assure that the cleaning protocols are efficientor that spots chosen for microanalyses are free of contaminants. Prior tothe determination of the element/Ca ratios, the distributions of severalelements (Ca, Mn, Fe, Mg, Ba, Al, Si, P and S) in tests of the shallowinfaunal species Uvigerina peregrina and Bolivina spissa were mapped with an electron microprobe (EMP). Tovisualize the effects of cleaning protocols uncleaned and cleaned specimenswere compared. The cleaning protocol included an oxidative cleaning step. AnFe rich phase was found on the inner test surface of uncleaned U. peregrina specimens.This phase was also enriched in Al, Si, P and S. A similar Fe rich phase wasfound at the inner test surface of B. spissa. Specimens of both species treated withoxidative cleaning show the absence of this phase. Neither in B. spissa nor in U. peregrina wereany hints found for diagenetic (oxyhydr)oxide or carbonate coatings. Mn/Caand Fe/Ca ratios of single specimens of B. spissa from different locations have beendetermined by secondary ion mass spectrometry (SIMS). Bulk analyses usingsolution ICP-MS of several samples were compared to the SIMS data. Thedifference between SIMS analyses and ICP-MS bulk analyses from the samesampling sites was 14.0–134.8 μmol mol⁻¹ for the Fe/Ca and1.68(±0.41) μmol mol⁻¹ for the Mn/Ca ratios. This is in the same order of magnitudeas the variability inside single specimens determined with SIMS at thesesampling sites (1σ_[Mn/Ca] = 0.35–2.07 μmol mol⁻¹; 1σ_[Fe/Ca] = 93.9–188.4 μmol mol⁻¹). The Mn/Ca ratios in the calcitewere generally relatively low (2.21–9.93 μmol mol⁻¹) but in the samemagnitude and proportional to the surrounding pore waters (1.37–6.67 μmol mol⁻¹).However, the Fe/Ca ratios in B. spissa show a negative correlation to theconcentrations in the surrounding pore waters. Lowest foraminiferal Fe/Caratios (87.0–101.0 μmol mol⁻¹) were found at 465 m water depth, alocation with a strong sharp Fe peak in the pore water next to the sedimentsurface and respectively, high Fe concentrations in the surrounding porewaters. Previous studies found no living specimens of B. spissa at this location. Allthese facts hint that the analysed specimens already were dead before the Feflux started and the sampling site just recently turned anoxic due tofluctuations of the lower boundary of the OMZ near the sampling site (465 mwater depth). Summarized Mn/Ca and Fe/Ca ratios are potential proxies forredox conditions, if cleaning protocols are carefully applied. The datapresented here may be rated as base for the still pending detailedcalibration.

[学科分类] 地球化学与岩石