Publication Date:
2022-05-26
Description:
Submitted 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 2018
Description:
In 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.
Description:
This 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.
Keywords:
Ocean
;
Chromium
;
Seawater
;
Isotopes
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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