A study of the geochemistry of chromium isotopes in the modern oceans

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).\n 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. \n 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 remineralization in the lower OMZ and deep waters, leading to Cr(III) preferentially scavenging back onto settling particles. The deficit in Cr within the OMZs is quantitatively significant to the global marine budget of Cr due to their extended coverage of the Pacific Ocean. This process is bound to become more important as the global oxygenation level of the oceans decrease as a result of the climate change.\n Chapter 4 presents measurements of dissolved rare earth element (REE) concentrations and neodymium isotope compositions (εNd) in the seawater of the Canada Arctic Archipelago (CAA). The increase in REEs and decrease in εNd as seawater flows through the CAA involves water mass mixing and addition of REEs from the weathering of the local bedrock. The application of a steady–state Nd budget shows that the local margin processes deliver as much dissolved Neodymium (Nd) as the seawater entering the CAA. The continental Nd is mainly transported by small coastal–draining rivers throughout the CAA or released from the detrital sediments and suspended particles, with geographic variations in their contribution expected to occur due to the geological and hydrological heterogeneity of the drainage systems in the CAA.\n Chapter 5 presents measurements of δ53Cr and [Cr]T from the Canada Basin, the Canadian Arctic Archipelago (CAA), Baffin Bay, and the Labrador Sea waters. The Cr composition of more than 70 % of these waters plot below the global Cr array formed in the ln[Cr]T–δ53Cr space. This chapter provides evidence for the addition of Cr with a very low δ53Cr in the Canada Basin and the CAA, as the result of two distinct processes. The first process involves a marine source of highly–fractionated Cr(III) over the Chukchi Shelf and slope that impacts the Canada Basin waters. In this instance, I suggest a release of the accumulated product of ferrous iron and/or sulfide reduction of Cr(VI) in benthic waters overlying the reducing marine sediments on productive shelves and/or in the reducing micro–environments of suspended marine particles. The second process involves a terrestrial source of highly–fractionated Cr(III) in the CAA that impacts Baffin Bay and the Labrador Sea water by lateral advection. The terrestrial Cr is delivered in the dissolved and suspended loads of rivers. It is produced from soil and bedrock weathering where isotopically–light Cr(III) accumulates during oxidative weathering over long periods of time. Comparison with [Cr]T and δ53Cr reported for Canada Basin waters sampled five years prior (Scheiderich et al., 2015) provides evidence that seawater Cr compositions in the Arctic have only recently shifted below the global Cr array, which is consistent with e.g. the acceleration of soil permafrost and coastal weathering in the Arctic landscapes from global warming. The dynamic Arctic marine Cr cycle opens the possibility for using Cr isotopes to track global impacts of climate change in the Arctic.

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