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Abstract: The chemistry of molybdenum species in artificial corrosion pits on 316L stainless steel was investigated using X-ray absorption spectroscopy (XANES). It was found that the K-edge spectra are consistent with the presence of an Mo(III) species. The presence of Mo(VI) polymolybdates as been proposed to explain the well-known beneficial effects of Mo in decreasing the susceptibility of stainless steel to localised corrosion, but no evidence of these species was found. High resolution measurements (with a spatial resolution of 3.5 μm in the vertical direction) through the dissolving metal interface did not detect the presence of any different Mo-containing species in the vicinity of the salt film. However, the spatial resolution was insufficient to detect the presence of submonolayer species on the dissolving metal surface that have been proposed to block active dissolution.

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Abstract: Tl stable isotopes recorded in marine ferromanganese crusts show great promise as a tracer of past marine and climatic conditions. Key to interpreting recent Tl stable isotope time-series data is a detailed, molecular-level understanding of Tl scavenging by ferromanganese crust minerals and Tl stable isotope fractionation occurring during uptake. To this end, we determine the mechanism of Tl sorption to the primary ferromanganese minerals in crusts, namely hexagonal birnessite, todorokite and ferrihydrite, using XAS. We compliment our data with micro-focus XAS of a Tl-enriched hydrogenetic ferromanganese crust. We show that Tl(I) is oxidised to Tl(III) during sorption to hexagonal birnessite, but not during sorption to todorokite, triclinic birnessite and ferrihydrite. Tl(III) forms an inner-sphere complex at the hexagonal birnessite surface, located at vacant octahedral sites in the phyllomanganate sheets. We show that oxidation of Tl(I) to Tl(III) during reductive dissolution of birnessite is thermodynamically unfavourable; and propose that oxidation of Tl(I) is driven by the formation of the Tl(III) surface complex. Recent theoretical calculations predict a large equilibrium stable isotope fractionation between Tl(I) and Tl(III), leading to Tl(III) species that are enriched in the heavy 205Tl isotope. In light of this work, we propose a molecular sorption–oxidation–fractionation mechanism that provides a unifying explanation for the recently observed geochemical behaviour of Tl in marine ferromanganese-rich sediments. In this mechanism, the proportion of hexagonal birnessite dictates the extent of Tl oxidation, which controls the extent of Tl enrichment and isotope fractionation. This work is among the first to provide a molecular explanation for reported trends in trace element enrichments and stable isotope compositions in geologic deposits. Our molecular sorption–oxidation–fractionation mechanism will ultimately help interpret Tl signals in marine sedimentary archives to provide new constraints on past oceanic and climatic change. In addition, our mechanism should also help explain compositional relationships of other redox-sensitive elements in ferromanganese-rich marine sediments that might also be used as paleoceanographic and paleoclimate proxies.

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Abstract: We report R2 and R2* in human hippocampus from five unfixed post-mortem Alzheimer's disease (AD) and three age-matched control cases. Formalin-fixed tissues from opposing hemispheres in a matched AD and control were included for comparison. Imaging was performed in a 600 MHz (14 T) vertical bore magnet at MR microscopy resolution to obtain R2 and R2* (62 ?m × 62 ?m in-plane, 80 ?m slice thickness), and R1 at 250 ?m isotropic resolution. R1, R2 and R2* maps were computed for individual slices in each case, and used to compare subfields between AD and controls. The magnitudes of R2 and R2* changed very little between AD and control, but their variances in the Cornu Ammonis and dentate gyrus were significantly higher in AD compared for controls (p < 0.001). To investigate the relationship between tissue iron and MRI parameters, each tissue block was cryosectioned at 30 ?m in the imaging plane, and iron distribution was mapped using synchrotron microfocus X-ray fluorescence spectroscopy. A positive correlation of R2 and R2* with iron was demonstrated. While studies with fixed tissues are more straightforward to conduct, fixation can alter iron status in tissues, making measurement of unfixed tissue relevant. To our knowledge, these data represent an advance in quantitative imaging of hippocampal subfields in unfixed tissue, and the methods facilitate direct analysis of the relationship between MRI parameters and iron. The significantly increased variance in AD compared for controls warrants investigation at lower fields and in-vivo, to determine if this parameter is clinically relevant.