| Summary: | This thesis explores chromium (Cr) stable isotope cycle in the oceanic system in relation to oxygen–poor waters and shelf environments. Cr is a redox–sensitive trace metal with two oxidation states (Cr(III) and Cr(VI)) of contrasting mobility in seawater. While Cr(VI) species are highly soluble, the product of its reduction, Cr(III), is reactive to particle surfaces. In the oceans, Cr(VI) reduction occurs predominantly in the surface mixed layer in association with primary production. This process favors the partitioning of light isotopes of Cr into the produced Cr(III), which is scavenged by particles and exported to deep waters, where it is released along with major nutrients during recycling/particle dissolution and then slowly re–oxidized to Cr(VI). The biologically–mediated Cr isotope cycle is associated with a global fractionation factor of –0.85 ± 0.02 ‰ (1σ) and a strong correlation between the concentration of total dissolved Cr ([Cr]T) and its isotopic ratios (δ53Cr). Chapter 2 presents an improved method of determination of [Cr]T and δ53Cr in seawater samples. All Cr is converted to Cr(III) by acidification of the seawater and co–precipitated with Fe(III). The succession of one anion–exchange chromatographic column and two cation–exchange columns allows the effective separation of interferents such as sulfides and iron, which results in high–precision measurements of Cr stable isotopes on the Thermal Ionisation Mass Spectrometer. Chapter 3 presents measurements of [Cr]T and δ53Cr at four stations along the Line–P section, in the Oxygen Minimum Zone (OMZ; O2 < 60 µmol.kg–1) of the North–East Subarctic Pacific Ocean. This chapter presents the first evidence of a net deficit in Cr linked to a higher δ53Cr in dysoxic waters that may be due to: (1) the reduction of Cr(VI) with sulfides in the anoxic micro–environment of particle aggregates (i.e. marine snow) and export of Cr(III) in the upper OMZ; and (2) the reduction of manganese oxides that impedes the re–oxidation of Cr(III) released from particle ...
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