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| Titel |
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 |
| VerfasserIn |
N. Glock, A. Eisenhauer, V. Liebetrau, M. Wiedenbeck, C. Hensen, G. Nehrke |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 1 ; Nr. 9, no. 1 (2012-01-18), S.341-359 |
| Datensatznummer |
250006669
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| Publikation (Nr.) |
 copernicus.org/bg-9-341-2012.pdf |
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| Zusammenfassung |
| In this study we present an initial dataset of Mn/Ca and Fe/Ca ratios in
tests of benthic foraminifera from the Peruvian oxygen minimum zone (OMZ)
determined with SIMS. These results are a contribution to a better
understanding of the proxy potential of these elemental ratios for ambient
redox conditions. Foraminiferal tests are often contaminated by diagenetic
coatings, like Mn rich carbonate- or Fe and Mn rich (oxyhydr)oxide coatings.
Thus, it is substantial to assure that the cleaning protocols are efficient
or that spots chosen for microanalyses are free of contaminants. Prior to
the determination of the element/Ca ratios, the distributions of several
elements (Ca, Mn, Fe, Mg, Ba, Al, Si, P and S) in tests of the shallow
infaunal species Uvigerina peregrina and Bolivina spissa were mapped with an electron microprobe (EMP). To
visualize the effects of cleaning protocols uncleaned and cleaned specimens
were compared. The cleaning protocol included an oxidative cleaning step. An
Fe 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 was
found at the inner test surface of B. spissa. Specimens of both species treated with
oxidative cleaning show the absence of this phase. Neither in B. spissa nor in U. peregrina were
any hints found for diagenetic (oxyhydr)oxide or carbonate coatings. Mn/Ca
and Fe/Ca ratios of single specimens of B. spissa from different locations have been
determined by secondary ion mass spectrometry (SIMS). Bulk analyses using
solution ICP-MS of several samples were compared to the SIMS data. The
difference between SIMS analyses and ICP-MS bulk analyses from the same
sampling sites was 14.0–134.8 μmol mol−1 for the Fe/Ca and
1.68(±0.41) μmol mol−1 for the Mn/Ca ratios. This is in the same order of magnitude
as the variability inside single specimens determined with SIMS at these
sampling 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 calcite
were generally relatively low (2.21–9.93 μmol mol−1) but in the same
magnitude 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 the
concentrations in the surrounding pore waters. Lowest foraminiferal Fe/Ca
ratios (87.0–101.0 μmol mol−1) were found at 465 m water depth, a
location with a strong sharp Fe peak in the pore water next to the sediment
surface and respectively, high Fe concentrations in the surrounding pore
waters. Previous studies found no living specimens of B. spissa at this location. All
these facts hint that the analysed specimens already were dead before the Fe
flux started and the sampling site just recently turned anoxic due to
fluctuations of the lower boundary of the OMZ near the sampling site (465 m
water depth). Summarized Mn/Ca and Fe/Ca ratios are potential proxies for
redox conditions, if cleaning protocols are carefully applied. The data
presented here may be rated as base for the still pending detailed
calibration. |
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