Graduation Year

2024

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Marine Science

Major Professor

Timothy M. Conway, Ph.D.

Committee Member

Amelia E. Shevenell, Ph.D.

Committee Member

Jay T. Cullen, Ph.D.

Keywords

isotopic fractionation, redox chemistry, oxidative precipitation, anoxic basin

Abstract

Iron (Fe) is required for many biogeochemical processes, with Fe bioavailability and chemistry being controlled by redox reactions that transform Fe between oxidation states and dissolved and particulate phases. In oxic seawater, Fe is present in the Fe(III) oxidation state, mainly as Fe oxyhydroxides. Under anoxic marine conditions, such as margin sediments under oxygen minimum zones or restricted basins with anoxic bottom water, Fe(III) is reduced to Fe(II), which is highly soluble and can be present at high concentrations ([Fe]). Fe isotope ratios (δ56Fe relative to IRMM-14) can be used to characterize Fe redox transformations, constrain Fe sources of Fe to the ocean, and understand Fe biogeochemical cycling. In addition, Fe isotopes recorded in Precambrian sediments may aid in tracking the oxygenation of the Earth. A time series of dissolved [Fe] and δ56Fe was measured in Saanich Inlet, a fjord with seasonally restricted bottom water circulation and a redox gradient between oxic and anoxic water which is disrupted during annual renewal events. Results show a consistent δ56Fe of approximately -0.3‰ throughout the water column in the renewal season (Oct- Nov 2016) with layers of remnant older water containing high [Fe] and light δ56Fe. During the rest of the time series (Dec 2017- Apr 2017), a strong stratification develops, separating an oxic, Fe-depleted surface layer ([Fe]= 5-30 nmol kg-1; δ56Fe ≈ -0.3‰) which overlays an anoxic and sulfidic layer with high [Fe] and light δ56Fe ([Fe]= 30-500 nmol kg- 1; δ56Fe ≈ -1‰). Below this, a slight increase in δ56Fe with depth may be linked to Fe-sulfide interactions. Stratified profiles show a consistent increase in δ56Fe from anoxic to oxic water, likely corresponding with oxidative precipitation, consistent with a dominance of kinetic effects during the precipitation of Fe(III) minerals. This work supports the need to consider both kinetic and equilibrium fractionations in order to accurately interpret ‘paleo’ Fe isotope records when considering the early redox evolution of the Earth’s oceans.

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