Impact of permafrost thaw on iron-organic carbon interactions in contrasted permafrost zones: Case study in Churchill (Hudson Bay Lowlands, Canada)

Details

Supervisors

Opfergelt, Sophie ; Agnan, Yannick

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

enPermafrost soils, which cover approximately 20% of the Northern Hemisphere’s exposed land surface, represent a major global carbon reservoir, storing nearly three times more carbon than the atmosphere. Rising temperatures associated with climate change are driving widespread permafrost thaw, enhancing microbial decomposition of previously frozen organic matter and leading to increased emissions of carbon dioxide (CO2) and methane (CH4). This process creates a positive feedback loop that further accelerates global warming and permafrost degradation. Northern peatlands, often underlain by permafrost and characterised by high organic carbon (OC) content due to incomplete plant decomposition, play a particularly important yet dynamic role in this system. While the historical accumulation of OC in these peatlands is primarily driven by low temperatures and reducing (waterlogged) conditions that limit decomposition, the role of mineral associations—particularly with iron (Fe) oxides—in stabilising OC remains poorly constrained and may become increasingly important as permafrost thaw alters soil physicochemical conditions. This study therefore investigates the effects of permafrost degradation on Fe–OC associations across contrasting permafrost zones near Churchill in the Hudson Bay Lowlands (Canada). Results indicate that (i) organic horizons contain, on average, more than twice the proportion of reactive metals as mineral horizons; (ii) between 2% and 12% of OC is associated with mineral phases in peatland soils; (iii) microtopographic variation, freeze–thaw dynamics, and element translocation significantly influence the proportion and distribution of mineral-bound OC; (iv) permafrost degradation is associated with an increased concentration of organic-complexed OC; and (v) the most waterlogged environments exhibit the highest proportion of OC associated with minerals and mineral elements. These findings provide new insights into the mechanisms governing carbon persistence in thawing permafrost systems, with implications for predicting future carbon–climate feedbacks.