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[1] Abstract: Molybdenum isotopic compositions are precisely determined by MC-ICP-MS measurements using a Mo double spike. The double spike is added prior to chemical purification, so that laboratory and instrumental mass fractionations are separated from natural mass-dependent fractionation. Fractionation is determined on four Mo mass ratios, providing an internal consistency check.

Figure 1. Measured Mo isotope compositions of various marine sediments and the presumed dominant electron transport processes for each environment. Oxic sediment Mo isotope data are from FeMn crust surfaces (Barling et al., 2001; Siebert et al., 2003). Suboxic sediment Mo isotope data are average site compositions from California margin sites of Siebert et al. (2006). Anoxic sediment Mo isotope data are average site compositions from three Mexican margin sites of this study. Euxinic sediment Mo isotope data are deep Black Sea sediments from Barling et al. (2001) and Arnold et al. (2004). All data are shown without lithogenic Mo correction. ABSTRACTWe present new Mo isotope data from the Mexican continental margin that, in conjunction with previous data, allow us to propose a mechanistic description of the Mo isotope system in marine sediments. We hypothesize that there are unique environmentally dependent Mo isotope signatures recorded in marine sediments that reflect the mechanisms responsible for authigenic Mo accumulation. Open-ocean anoxic sites, defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water, exhibit a ␦ 98/95 Mo isotope signature of ؉1.6‰. We believe this value reflects Mo sulfide formation via diagenetic processes within sediments. Quantitative formation of Mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value (؉2.3‰), as typified by samples from the highly sulfidic Black Sea. In contrast to these reducing settings, manganese oxide-rich sediments have measured Mo isotope values that are more negative (relative to seawater) than any other sediment samples analyzed to date, similar to FeMn crusts (ϳ؊0.7‰). Most measured Mo isotope compositions of marine sediments from open-ocean settings appear to reflect a mixture of lithogenic Mo (0.0‰) and the Mo signature of a specific authigenic Mo accumulation mechanism. We therefore suggest that Mo isotopes may record unique signatures that reflect the dominant chemical mechanism for Mo sequestration into sediments.

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[1] Isotopic analyses of dissolved molybdenum are presented for sediment pore waters from a reducing sedimentary basin and for fluids from a low-temperature ridge flank hydrothermal system. d 98/95Mo in these fluids range from 0.8 to 3.5% (relative to a laboratory standard), demonstrating that marine sedimentary reactions significantly fractionate Mo isotopes. Within the upper 3 cm of sediment, manganese oxide dissolution produces an isotopically light fluid relative to seawater (mean of four analyses = 2.1 ± 0.1% versus seawater = 2.3 ± 0.1%). Below 6 cm depth, authigenic Mo uptake results in an isotopically heavier fluid (up to 3.5%) indicating that reducing sediments are likely to be a net sink for isotopically light dissolved Mo. In contrast, fluid circulation within a low-temperature ridge-flank hydrothermal system is a source of isotopically light Mo to the ocean having an end-member fluid of $0.8%.

. (2013) 'Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering eects over the Cenozoic.', Earth and planetary science letters., 375 . pp. 148-155. Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractThe global distribution of nickel (Ni) isotope values in ferromanganese crusts has been investigated by analysing 24 surface (zero-age) samples of crusts from various oceans by MC-ICPMS, using double-spike for mass bias correction. The isotopic compositions range between 0.9 and 2.5  (60Ni), with averages for the Atlantic samples permil ± error being slightly lighter than the ones at calculated from the Pacific samples (xy permil ± error), although the data show large overlapsre is considerable overlap in the Ni isotope compositions of waters from between all the measured oceans basins mea-sured. To complement the crustal surface study also a depth profile through one crust from the Pacific ocean was also analysed. This isotope profile suggest that large variations in nickel isotope composition (1.62.3 ) has occurred within this crust throughout the last 70 Myrs, although the mean value is similar to the average isotopic composition for ferromanganese crust surfaces from the Pacific Ocean.