Tracking carbon fluxes across ocean interfaces using dissolved gas observations

Abstract

The cycling and exchange of carbon between Earth’s systems play a pivotal role in regulating climate, yet two major carbon fluxes remain poorly constrained: the biological carbon pump (BCP) and carbon release from Arctic permafrost. This thesis focuses on dissolved gases as tracers and drivers of these processes through both autonomous and field-based observations. It encompasses (i) improvements to sensor-based measurements of O₂, (ii) the use of these measurements to assess the strength of the BCP in two distinct export regimes, and (iii) isotopic approaches to carbon dioxide (CO₂) and methane (CH₄) dynamics at a coastal permafrost site. The first part of the thesis is centered around the NASA EXPORTS campaign and studies the BCP at two contrasting field sites. Using autonomous platforms, carbon export was evaluated at both sites and demonstrated that at the lower productivity site, a greater proportion of fixed carbon was routed to sinking particulate organic carbon (POC), while the higher productivity site resulted in near equal proportions of dissolved organic carbon production and sinking POC. These findings underscore the value of autonomous sensors in capturing spatial and temporal variability in oceanic carbon cycling. The second part of this thesis shifts focus to the Arctic, where rapid warming threatens to mobilize vast (~1,500 Pg) amounts of carbon currently stored in permafrost. This study presents observations from the spring thaw at a coastal Arctic site and demonstrated that even sites with high CH₄ and CO₂ concentrations drew less than 10% of their carbon source from ancient permafrost sources. The variability in CH₄ and CO₂ emissions reflects the complex interplay between hydrological changes, primary productivity, and microbial processes. The research highlights the need for regular monitoring of Arctic rivers, which integrate changes in the terrestrial system, as a potential early warning system for abrupt permafrost thaw. This thesis leverages the fundamentals of dissolved gas geochemistry to examine key climate-relevant biogeochemical cycles across diverse environments that are sensitive to global change. These insights contribute to refining Earth system models and emphasize the need for expanded monitoring to predict future shifts in global carbon cycling and climate dynamics.