ALBERT

Description: Testing order to assess their potential as a proxy for redox conditions the element/Ca ratios of the redox sensitive elements Mn and Fe were determined in tests of benthic foraminifera from the Peruvian oxygen minimum zone (OMZ). Prior to the determination of the element/Ca ratios the distributions of 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). An Fe rich phase which is also enriched in Al, Si, P and S was found on the inner test surface of U. peregrina. The element distributions of a specimen treated with an oxidative cleaning procedure show the absence of this phase. EMP maps of B. spissa also identified a similar phase which too could be removed with oxidative cleaning. Neither in B. spissa nor in U. peregrina were any hints for diagenetic (oxyhydr)oxide or carbonate coatings found. 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 on single specimens 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 μmol mol−1 for the Mn/Ca ratios. This amounts to 3–29 % for the Fe/Ca and 21.5 % for the Mn/Ca ratios of the overall variability between the samples of the different sampling sites. The Mn/Ca ratios in the calcite were generally relatively low (2.21–9.93 μmol mol−1) but of the same magnitude as in the pore waters (1.37–6.67 μmol mol−1). Comparison with sediment pore water data showed that Mn/Ca in the foraminiferal calcite is proportional to the Mn/Ca ratio in the top cm of the pore water. The lowest Fe/Ca ratio in tests of B. spissa (87.0 μmol mol−1) has been found at a sampling site which was strongly depleted in oxygen and showed a high, sharp iron peak in the top interval of the pore water. This, and the fact that at this location many dead but no living specimens were found during sampling time, hints that the 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 where the sampling site is located (465 m water depth).