19 October 2026 | 16:00 - 19:00
Permafrost underlies about 25% of the Northern Hemisphere’s land surface and stores nearly twice as much carbon as the atmosphere. As the Arctic warms, permafrost thaw alters soil hydrology and redox conditions, mobilizing iron (Fe), associated metal(loid)s, and dissolved organic carbon (DOC) toward aquatic systems. Yet, the effects of these redox shifts on Fe, metal(loid)s, and DOC mobility during winter remain poorly constrained.
This thesis investigates Fe, associated meta(loid)s, and DOC mobilization in soil and river waters at a discontinuous permafrost site in Healy (Alaska, USA) and a sporadic permafrost site in Abisko (Sweden), during the late shoulder season (i.e., the transitional period from the senescence of deciduous plants until the headwaters refreeze and cease to flow) and deep winter.
We found that progressive soil freezing in Healy during the late shoulder season drives reducing conditions that mobilize metal(loid)s and DOC in soil waters. In Abisko, episodic snowmelt events during the late shoulder season induce water table fluctuations that control Fe and DOC mobilization in soil waters. In rivers, near-freezing soils in Healy reduce soil–river connectivity, whereas milder conditions with near-stagnant river flow in Abisko sustain lateral transfer of Fe and DOC. During deep winter, DOC transfer from lateral taliks to streams increases with warming without detectable increases in stream CO2 and CH4.
Overall, the late shoulder season and deep winter emerge as biogeochemically active periods with spatially and temporally heterogeneous DOC mobility, underscoring the need to include winter fluxes in Arctic carbon budget models.
Supervisor: Pr. Sophie Opfergelt (UCLouvain, Belgium)
President: Pr. Emmanuel Hanert (UCLouvain, Belgium)
Pr. Yannick Agnan (UCLouvain, Belgium)
Pr. Marc Benedetti (IPGP, France)
Dr. Laure Gandois (CNRS and INP Toulouse, France)
Dr. Alexandre Voinot (NRCan Geological Survey and Queen’s University, Canada)