Beschreibung: Constraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (Cr-T, where Cr-T = Cr(VI) + Cr(III)) and Cr isotope data (delta Cr-53) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44-90 mu mol kg(-1) in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater delta Cr-53, with values ranging from 1.08 to 1.72 parts per thousand. Shelf Cr-T concentrations were slightly lower (2.21 +/- 0.07 nmol kg(-1)) than in open ocean waters at the same water depth (between 0 and 160 m, 2.48 +/- 0.07 nmol kg(-1)). The shelf waters also had higher delta Cr-53 values (1.41 +/- 0.14 parts per thousand compared to 1.18 +/- 0.05 parts per thousand for open ocean waters shallower than 160 m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of delta Cr-53 values (1.19 +/- 0.09 parts per thousand) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr (III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater delta Cr-53, whereas archives of seawater delta Cr-53 derived from shelf sediments must be interpreted with caution.

Beschreibung: Iron (Fe) is an essential micronutrient for marine microbial organisms, and low supply controls productivity in large parts of the world’s ocean. The high latitude North Atlantic is seasonally Fe limited, but Fe distributions and source strengths are poorly constrained. Surface ocean dissolved Fe (DFe) concentrations were low in the study region (〈0.1 nM) in summer 2010, with significant perturbations during spring 2010 in the Iceland Basin as a result of an eruption of the Eyjafjallajökull volcano (up to 2.5 nM DFe near Iceland) with biogeochemical consequences. Deep water concentrations in the vicinity of the Reykjanes Ridge system were influenced by pronounced sediment resuspension, with indications for additional inputs by hydrothermal vents, with subsequent lateral transport of Fe and manganese plumes of up to 250–300 km. Particulate Fe formed the dominant pool, as evidenced by 4–17 fold higher total dissolvable Fe compared with DFe concentrations, and a dynamic exchange between the fractions appeared to buffer deep water DFe. Here we show that Fe supply associated with deep winter mixing (up to 103 nmol m−2 d−1) was at least ca. 4–10 times higher than atmospheric deposition, diffusive fluxes at the base of the summer mixed layer, and horizontal surface ocean fluxes.