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dc.contributor.authorMoos, Simone B.  Concept link
dc.date.accessioned2018-01-18T15:55:03Z
dc.date.available2018-01-18T15:55:03Z
dc.date.issued2018-02
dc.identifier.urihttps://hdl.handle.net/1912/9489
dc.descriptionSubmitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2018en_US
dc.description.abstractIn the ocean, chromium (Cr) is a redox-sensitive trace metal. The reduction of Cr(VI) to Cr(III) occurs in oxygen deficient zones (ODZs), and Cr reduction in general has been identified as a significant Cr isotope fractionation mechanism. This thesis presents the first Cr isotope variations (d53Cr) in ODZs of the ocean and adds to the sparse Cr isotope data published for modern seawater. I developed a precise and accurate Cr isotope method for seawater samples. Seawater acidification converts total Cr to Cr(III) which is preconcentrated by Mg(OH)2 coprecipitation. A three-column anion exchange chromatography scheme separates Cr from isobaric and polyatomic interferences present in the seawater and reagent matrixes. Isotope analysis is performed on a MC-ICP-MS IsoProbe. The addition of a 50Cr-54Cr double spike allows for accurate correction of procedural and instrumental Cr mass fractionations. The first Cr isotope ratio data for a full water column profile in the Pacific Ocean is presented. This station serves as a fully oxic counterpart to stations located within the ODZ of the Eastern Tropical North Pacific. At one station, Cr concentrations are lower and d53Cr values are heavier within the ODZ. This is consistent with Cr reduction resulting in isotopically lighter, particlereactive Cr(III), which is scavenged and exported from the water column. A strong correlation of d53Cr and d15NNO3- at this station suggests that Cr reduction may be microbially mediated instead of simply being a product of thermodynamic equilibrium. Alternatively, Cr may be reduced by Fe(II). In the anoxic bottom waters of the Santa Barbara Basin a strong Cr reduction signal (lower [Cr], heavier d53Cr) is observed, which may result from the same aforementioned Cr reduction mechanisms. A shift to the heaviest seawater Cr isotope signatures yet observed was detected in the oxic bottom waters of the shallow Arctic Chukchi shelf, while Cr concentrations decreased. This extreme isotope signal may result from Cr reduction by a reduced species (e.g. Fe(II)), which was released from the underlying anoxic shelf sediments. Cr in the Atlantic layer and in the bottom water of a central Arctic station appears to be shaped by a novel, unidentified process.en_US
dc.description.sponsorshipThis research was supported by the US National Science Foundation (NSF Award No. OCE- 0926197, OCE-1233749, OCE-1357224), the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology (Award No. WBS 6916070), and the Center for Microbial Research and Education (NSF-OIA Award No. EF-0424599). In the last year of my studies, I was supported by a MIT-WHOI Joint Program Science Fellowship.en_US
dc.language.isoen_USen_US
dc.publisherMassachusetts Institute of Technology and Woods Hole Oceanographic Institutionen_US
dc.relation.ispartofseriesWHOI Thesesen_US
dc.subjectOcean
dc.subjectChromium
dc.subjectSeawater
dc.subjectIsotopes
dc.titleThe marine biogeochemistry of chromium isotopesen_US
dc.typeThesisen_US
dc.identifier.doi10.1575/1912/9489


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