Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Timmermans, Mary-Louise

Abstract

This dissertation examines the evolution of dissolved oxygen in the waters of the Arctic Ocean's Canada Basin. Dissolved oxygen concentrations in the ocean are initially set at the surface through air-sea exchange and subsequently modified by physical and biogeochemical processes as waters circulate and mix. The variability of dissolved oxygen in the Arctic Ocean across seasonal and interannual timescales provides valuable insight into the broader evolution of water masses in the Arctic Ocean. As dissolved oxygen concentrations are gradually depleted in deeper waters via biological consumption, and only replenished when waters return to the surface, their distribution provides a valuable indicator of water mass ventilation. In the Arctic Ocean, sea ice plays a crucial role in the dissolved oxygen budget. Enhancing our understanding of the influence of sea ice on dissolved oxygen concentrations is essential for interpreting its observed evolution and predicting future shifts under ongoing sea ice loss. In this thesis, the physical and biogeochemical mechanisms that drive the observed evolution of dissolved oxygen concentrations in the Arctic Ocean water column are quantified. The processes that govern the spatial, seasonal and interannual distributions of dissolved oxygen are explored using observational data, alongside analytical and one-dimensional modeling approaches. These findings provide insight on ocean ventilation, how dissolved gases are transferred between the atmosphere, sea ice, and the ocean, and how these processes are evolving in response to declines in Arctic sea ice. The seasonal and interannual variability of dissolved oxygen in the Canada Basin surface ocean are explored using year-round data from Ice-Tethered Profilers. The seasonal and spatial distributions of mixed-layer dissolved oxygen concentrations are found to be tightly linked to the seasonally varying sea ice cover. One-dimensional modeling results, supported by comparisons with Ice-Tethered Profiler observations, demonstrate that the seasonal cycle of dissolved oxygen in the mixed layer is primarily driven by sea ice formation and melt, with smaller contributions from other physical and biological processes. The influence of declining Arctic sea ice is reflected in decreasing dissolved oxygen concentrations in the upper water column. Chapter 3 explores the dissolved oxygen evolution of the Pacific Water layer in the context of previously documented warming using hydrographic data. The observed decline in dissolved oxygen concentrations is consistent with solubility-driven effects of solar warming in the source waters. The influence of enhanced biological consumption is evident in the lower portion of the Pacific Summer Water layer, which is supported by measurements of inorganic nutrients. This study illustrates how declining sea ice can lead to reduced dissolved oxygen concentrations not only at the surface, but also in the subsurface Arctic Ocean due to warming of source waters in regions with reduced sea ice cover. Finally, the evolution of dissolved oxygen concentrations in the deep waters of the Arctic Ocean are explored in Chapter 4. In the vertically homogeneous bottom waters, declining dissolved oxygen concentrations in recent decades are primarily attributed to biological consumption, which is consistent with the presumed horizontal and vertical isolation of these waters. Decreasing oxygen trends are in good agreement with independent estimates of oxygen utilization rates due to organic matter breakdown. This suggests that the influence of vertical mixing with overlying waters plays only a small role over most of the basin. The analysis of bottom water dissolved oxygen concentrations in context with temperature and salinity evolution provides important insight on water mass ventilation and presumed isolation. This dissertation presents the most comprehensive analysis to date of dissolved oxygen concentrations in the Arctic Ocean's Canada Basin water column, advancing our understanding of the physical and biological drivers of dissolved oxygen variability, particularly in the context of Arctic sea ice decline, air-sea gas exchange in polar regions, and ocean dynamics. Future research will expand on these findings by comparing Arctic dissolved oxygen trends, seasonal variability, and their governing processes with those in other ocean basins.

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