Hillsides collapsing into Arctic streams can trigger CO2 release to the atmosphere

Featured Image: Permafrost thaw slumps draining into a river on the Peel Plateau in western Canada. Photo courtesy Scott Zolkos, lead author of the paper.

Paper: Experimental Evidence That Permafrost Thaw History and Mineral Composition Shape Abiotic Carbon Cycling in Thermokarst-Affected Stream Networks

Authors: Zolkos, Scott & Suzanne E. Tank.

The rivers and streams of the Arctic transfer atmospheric heat into the surrounding permafrost (perennially frozen) soil. At the same time, surface soils up to 1 meter deep undergo annual freeze-thaw cycles. When warmer air arrives in the summer months, the combination of warming air and river water can thaw large chunks of ice-rich permafrost soil along the stream’s edge. Thawed permafrost breaks away from the surrounding hillsides and causes catastrophic slope failures, transporting huge amounts of sediment into the nearby waterways. As the stream water becomes murky it takes on the appearance of chocolate milk, and simultaneously, the geochemistry of the water changes.

When permafrost soil is released into a stream, there are two ways it is transformed: 1) the carbon in the soil can be consumed by the microbial community for energy, a process that will continually release carbon dioxide, a greenhouse gas, to the atmosphere; 2) the soil can be decomposed by chemical reactions. Chemical reactions are controlled by the soil makeup and water composition. Depending on those factors, chemical reactions either cause carbon dioxide (CO2) to be produced or decomposed.

Mineral-rich permafrost soils, which contain more carbonate, cause chemical reactions that turn carbon dioxide into bicarbonate; a form of dissolved inorganic carbon that is not easily released to the atmosphere. Whereas, soils with more sulfide can generate carbonic acid, as well as carbon dioxide, when deposited in streams. The balance between carbonate and sulfides in soils will control whether their release into a stream results in carbon dioxide removal or creates a source of carbon dioxide.

A recent paper by Zolkos and Tank explored the properties and chemistry of soil, both frozen and previously thawed permafrost, at three sites in the Peel Plateau region of Western Canada. Soil types examined included surface soil, geologically modern soil formed over the last 11,000 years, older, un-thawed permafrost, formed during the Pleistocene (which spans from ~2.5 million to 11,000 years ago), and soil previously released into streams as ‘tongues’ of debris-like sediment. Their results show that the oldest and deepest Pleistocene permafrost contains the highest amounts of carbonate and sulfide and enhances carbon dioxide production and release by chemical reactions.

Thaw slump showing the laminations, or horizontal lines below the plant cover, of permafrost soil. Exposed layers show modern soil formed during the Holocene (~11,000 years until present) down to aged soils formed in the Pleistocene (~2.5 million years until ~11,000 years ago).
Thaw slump showing the laminations, or horizontal lines below the plant cover, of permafrost soil. Exposed layers show modern soil formed during the Holocene (~11,000 years until present) down to aged soils formed in the Pleistocene (~2.5 million years until ~11,000 years ago). Photo courtesy of Scott Zolkos.

Zolkos and Tank determined that prior chemical weathering of surface soil reduces the likelihood of future carbon dioxide being released when that soil enters streams. The annual freeze-thaw cycles in surface soil promote the removal of carbonate from the soil prior to its release into water bodies. Hence, soil that has undergone significant prior chemical weathering is less susceptible to further degradation or production of carbon dioxide.

Together, increasing rainfall and permafrost thaw across the Arctic are expected to facilitate more catastrophic slope failures and release old, unweathered permafrost soil into streams. Zolkos and Tank expect the amplified chemical production of carbon dioxide to continue within streams for years to decades after the initial deposition. Since permafrost sediments can be gradually redistributed within streams, the chemical production of carbon dioxide will be maintained over long timescales. Taken in combination, the production of carbon dioxide by chemical reactions and microbial action will result in Arctic streams being an ever-expanding and more significant source of carbon dioxide to the atmosphere.

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Hillsides collapsing into Arctic streams can trigger CO2 release to the atmosphere by Hadley McIntosh Marcek is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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