Permafrost and glaciers in the high Arctic form a near-impermeable ‘cryospheric cap’ that traps a potentially large reservoir of sub-surface methane and prevents it from reaching the atmosphere. The vulnerability of the cryosphere to climate warming is making releases of this methane possible, but uncertainty in the magnitude and timing of such releases makes predictions of Arctic greenhouse gas emissions difficult. In Svalbard, where air temperatures are rising more than twice as fast as the average for the Arctic, glaciers are retreating and leaving behind exposed forefields that enable rapid methane escape. Through an extensive spatial study of proglacial groundwater springs on Svalbard, groundwater systems within glaciated catchments are found to be bringing to the surface deep-seated methane gas that was previously trapped beneath glaciers and permafrost in the Arctic. In this thesis, I estimate the amount of methane being released by such springs and discuss its origin. Through a temporal study conducted at a single glacial catchment, Vallåkrabreen, I use biogeochemical data collected from groundwaters during two melt seasons to investigate the sources of groundwaters and the origin of the methane they transport to the surface.

Waters collected from 123 groundwater springs in the forefields of 78 land-terminating glaciers are supersaturated with methane up to 600,000-times greater than atmospheric equilibration. The spatial sampling revealed a geologic control on the extent of methane supersaturation, with strong evidence of a thermogenic source. I estimate annual methane emissions from proglacial groundwaters could be up to 2.31 kt across the Svalbard archipelago. Further investigations into marine-terminating glaciers indicate emergent methane emissions as these glaciers transition into fully land-based glaciers.

My findings within the Vallåkrabreen catchment demonstrate an interconnected hydrological system where shallow and deep groundwaters mix to moderate methane emissions. During summer, deep methane-rich groundwaters sourced from upper catchment snowmelt are diluted by shallow oxygenated groundwaters, leading to some methane oxidation prior to its emergence at the surface. Microbial activity is an important methane sink along this flow-path, removing up to 62% of methane before it is brought to the surface. During winter, deep groundwaters remain active while many shallow systems shut off, reducing subsurface methane oxidation and permitting greater emissions. Ratios of the differing groundwater sources will change markedly in years to come as aquifer capacities and recharge volumes change in a warming climate.

My findings reveal that climate-driven glacial retreat facilitates widespread release of methane, a positive feedback loop that has the potential to contribute to enhanced greenhouse gas emissions in the Arctic. The findings are highly relevant for other regions of the Arctic that, due to their geology, are likely to experience similar methane emissions, either currently or with further glacial retreat.