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Freeze-Thaw Cycles in Permafrost Soils and Their Impacts on the Climate: a Literature Review

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Freeze-Thaw Cycles in Permafrost Soils and Their Impacts on the Climate: a Literature Review Freeze-thaw cycles in permafrost soils and their impacts on the climate: A literature review by Maya K. Bergmann A THESIS submitted to Oregon State University Honors College in partial fulfillment of the requirements for the degree of Honors Baccalaureate of Science in Environmental Science (Honors Scholar) Presented May 27, 2020 Commencement June 2021 AN ABSTRACT OF THE THESIS OF Maya K. Bergmann for the degree of Honors Baccalaureate of Science in Environmental Science presented on May 27, 2020. Title: Freeze-thaw cycles in permafrost soils and their impact on the climate: A literature review. Abstract approved: _____________________________________________________ Rebecca A. Lybrand Climate change is occurring at an increasingly rapid rate, with impacts heightened in the cold regions of the world including the Arctic. Warming effects are widespread, with one impacted process – freeze- thaw cycles – increasing in frequency and potentially triggering additional changes in permafrost soils that have previously acted as carbon sinks. Increasing freeze-thaw cycles due to rising average temperatures are stimulating microbial activity in these soils, decomposing carbon and releasing greenhouse gases into the atmosphere. The release of greenhouse gases creates a positive feedback loop as temperatures rise with climate change where permafrost experiences more warming and freeze-thaw cycles, releasing more greenhouse gases, and thereby contributing to climate change. This literature review provides information about microbial activity, as well as the greenhouse gas emissions, from permafrost soils that have undergone freeze-thaw cycles. The overarching goal of this review is to compile the most relevant literature on freeze-thaw cycles in permafrost soils to better consider the future changes we are likely to see as climate change continues to take place. These changes include variations in season length, ability of vegetation and plants to continue to grow in affected regions, and threats to human health and safety, all of which point to the importance of considering permafrost in climate change discussions. Key Words: Permafrost, soil, climate change, freeze-thaw cycles, microbial communities, carbon dioxide, methane, nitrous oxide, future impacts. Corresponding e-mail address: [email protected]/[email protected] ©Copyright by Maya K. Bergmann May 27, 2020 Freeze-thaw cycles in permafrost soils and their impacts on the climate: A literature review by Maya K. Bergmann A THESIS submitted to Oregon State University Honors College in partial fulfillment of the requirements for the degree of Honors Baccalaureate of Science in Environmental Science (Honors Scholar) Presented May 27, 2020 Commencement June 2021 Honors Baccalaureate of Science in Environmental Science project of Maya K. Bergmann presented on May 27, 2020. APPROVED: _____________________________________________________________________ Rebecca A. Lybrand, Mentor, representing Department of Crop and Soil Science _____________________________________________________________________ Jeffery A. Hatten, Committee Member, representing Department of Forest Engineering, Resources & Management _____________________________________________________________________ Erin C. Rooney, Committee Member, representing Department of Crop and Soil Science _____________________________________________________________________ Toni Doolen, Dean, Oregon State University Honors College I understand that my project will become part of the permanent collection of Oregon State University, Honors College. My signature below authorizes release of my project to any reader upon request. _____________________________________________________________________ Maya K. Bergmann, Author Introduction The ability of soil to store or emit carbon is proving vital in the discussion of climate change as atmospheric greenhouse gas concentrations increase (Ping et al., 2015; Turetsky et al., 2020; Chadburn et al., 2017). Microbial activity increases with temperature, resulting in a shift from carbon storage to emission as permafrost soils, known as Gelisols, warm (Mu et al., 2017). Permafrost (any ground remaining below 0°C for two or more consecutive years) is estimated to contain about 1,600 billion tons of carbon (Turetsky et al., 2019). As permafrost thaws, ancient greenhouse gases including carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) are released from the soil, resulting in greater concentrations of greenhouse gases in the atmosphere and hastening climate warming in a positive feedback loop (Mu et al., 2018; Turetsky et al., 2019). Higher levels of greenhouse gas emissions are related to the stimulation of microbial activity in the soil due to thaw (Müller et al., 2018). A cycle between freezing and thawing, or the freeze-thaw cycle, occurs regularly in locations with permafrost, though freeze-thaw only occurs in the active layer overlying the permafrost layer instead of in the permafrost itself. Freeze-thaw cycles impact the soil aggregate structure, which can disrupt the organic matter and carbon stored within the soil (Liu et al., 2017). The goal of this literature review is to gain an understanding of the key concepts and features of permafrost soils, and the changes seen when undergoing freeze-thaw, specifically features associated with morphology, microbial communities, and gas concentrations. Literature ranging from the 1980s to present will be reviewed, looking at permafrost found as close as Alaska, and as far as Norway. All permafrost will be considered, though it is important to note that Arctic permafrost contains higher levels of carbon than other permafrost regions. A Definition and Understanding of Permafrost With the growing body of work surrounding permafrost, a decisive, clear definition of permafrost is critical, especially as many publications use the term permafrost in a way that can be confusing or unclear (Dobiński, 2011). While the most commonly used definition of permafrost is: any ground remaining below 0°C for two or more consecutive years (Soil Survey Staff, 2014), the definition of permafrost is actually quite complicated and has undergone several iterations in recent years. The International Permafrost Association approved the following updated definition of the term permafrost in 1998: ground (soil or rock and including ice or organic material) that remains at or below 0°C for at least two consecutive years (van Everdigen, 1998). This definition did not cover all aspects of permafrost, excluding many of the physical aspects of permafrost soil that classify it as permafrost. A newer, more inclusive definition has been posed more recently that incorporates the different aspects of permafrost and its definition as it has continued to change and become broader over the years. This definition consists of two parts, with the first stating that a frozen soil can be characterized as permafrost, even if the permafrost does not contain ice/water. Permafrost that does not contain ice is considered a dry permafrost, as the permafrost only consists of solids, such as ice, soil and rock (Dobiński, 2011). The second part of this definition is that if the permafrost does contain water, it can be frozen, partially frozen, or in a liquid state. This seems contradictory to the simplest definition of permafrost, but there are many well-known permafrost examples that are actually unfrozen. The point is that the water is still in a liquid state at 0°C, due to the phase shift that takes place at 0°C, or salts/particulates making the freezing point of soil water lower than 0°C (Dobiński, 2011). There are two types of permafrost: old permafrost and young, “ecosystem-driven permafrost.” Old permafrost is a remnant of paleoclimate, and is often characterized as being polygenetic, with syngenetic permafrost (continuous upward growth of deposits) having developed on epigenetic permafrost (growths which developed on old existing deposits) (Ping et al., 2015). The younger “ecosystem-driven permafrost” formed during the Holocene as a result of deglaciation and vegetative succession (Ping et al., 2015). Though both old and young permafrost were formed under different climate conditions, and are of drastically different ages, both are impacted by similar physical biological processes, and therefore show many of the same physical attributes. Gelisol profiles are generally comprised of three different layers: the active layer, the transient layer, and the intermediate layer. The active layer is the topmost layer of the profile, which is above the permafrost, and is subject to seasonal thaw. The transient layer is the upper part of the permafrost, which has distinct layering of cryostructures due to fluctuations in the permafrost table. The intermediate layer is below the transient layer, which is characterized by ice-rich cryostructures associated with soil climate changes. Below these three layers is the “true” permafrost, which is the part of the profile that historically has not been impacted by climate changes or freeze-thaw cycles (Ping et al., 2015). Cryostructures are ice structures or patterns that are present within the soil, usually due to freeze- thaw processes. Cryostructures are seen as ice crystals, lenses or layers within the matrix of the soil, and are found in the active layer (Ping et al., 2015). Cryoturbation, or movement within the soil due to frost action, is where we see some of these structures occur, and is one of the dominant soil processes in regions with permafrost
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