Carbon Dynamics in Arctic Ecosystem
Mineralogical Control on Permafrost Carbon Dynamics in Arctic Ecosystems
The Arctic is undergoing rapid environmental change due to enhanced warming, causing previously frozen ground (permafrost) to thaw and destabilize. This in turn exposes the vast carbon stocks that had been cryo-trapped for millennia. The potential decomposition of this ancient organic matter (OM) to greenhouse gases constitutes a potent positive feedback to ongoing climate change, yet important mechanisms regulating the fate of thawed permafrost OM remain insufficiently understood. Close interactions with mineral matrices (i.e., within aggregates, sorption and co-precipitation) have been highlighted as a primary mechanism that stabilizes and protects OM in soils as well as marine environments, but we lack information on how organo-mineral associations behave under changing environmental conditions, such as phase transitions, redox oscillations or hydrodynamic shifts. To address this knowledge gap, the proposed project aims to investigate three crucial interfaces along the Arctic land-ocean continuum where OM-mineral associations may play a pivotal role: i) thaw and erosion of permafrost soils, ii) fluvial transport, and iii) marine sediments, tracing permafrost OM from source to sink. Elucidation of OM-mineral interactions requires novel analytical strategies. We plan to utilize established concepts of density and grainsize fractionation methods combined with innovative serial oxidation, thermal slicing and isotopic and molecular fingerprinting to examine how OM-mineral associations influence permafrost OM fate.
The first part of the project focusses on method development and optimization. Briefly, we will construct a first-of-its-kind automated, coupled serial thermal oxidation radiocarbon measurement (STORM) setup, where OM in an environmental sample is successively decomposed to CO2 according to its thermal lability, which in turn is analyzed for its isotopic composition (14C and 13C). The proposed new setup will thus allow for a simultaneous assessment of the reactivity (thermal lability) spectrum of a given sample, together with the potential OM source (via 13C) and age (via 14C) of different OM fractions grouped by their activation energy.
This novel approach will be complemented by thermally-sliced gas chromatography-mass spectrometry to evaluate the molecular composition of specific thermal lability fractions, and by hydrogen-pyrolysis (HyPy) to investigate the isotopic composition of the building blocks comprising macromolecular constituents. This unique combination of techniques will allow us to determine the chemical and isotopic composition (“what is it?”), radiocarbon age (“how old is it?”) and thermal activation energy (“how reactive is it?”) of the minerally-bound OM, and to assess potential changes in OM-mineral associations at key interfaces during lateral transport.
We will use the Mackenzie river – Beaufort Sea system in northwestern Canada as a natural laboratory for our source-to-sink approach as it not only features widespread and fast-growing permafrost thaw slumps throughout the river basin but also constitutes the largest point source of sediment to the Arctic Ocean. Besides addressing current OM fluxes, we will reconstruct changes in organo-mineral delivery to the Beaufort Sea throughout the Holocene to advance our understanding of the marine OM sink. The results of the proposed project will therefore provide insights that will help to constrain the fate of permafrost OM in the aquatic environment and thereby improve current climate projections. The very strong radiocarbon gradients within arctic ecosystems lends itself to the proposed analytical approach, yet insights gleaned from this project may bear upon our broader understanding of the role of organo-mineral interactions in carbon cycle processes in general.