Abstract:Over the last few years, various studies have documented significant impacts of recent global warming across the Arctic since the mid-twentieth century, with a preferential thermal degradation of ice-rich permafrost and the related release of previously sequestered carbon (e.g., Grosse et al., 2011; Romanovsky et al., 2010). These studies indicate a sensitivity of cold Arctic permafrost to climate-driven thermokarst (thaw) initiation (Olefeldt et al., 2016). The development of thermokarst results from the ther… Show more
“…In addition, thermokarst may alter the degradability of the ancient Yedoma OM by introducing younger material from overlying sediments and changing thermal and hydrological conditions (Grosse et al, 2011;Strauss et al, 2015;Wild et al, 2016). The mixing of sediments is particularly pronounced on retrogressive thaw slumps, a dynamic form of thermokarst in ice-rich areas, expanding inland by melting of the ground ice in the headwall (Lantuit and Pollard, 2008;Costard et al, 2021).…”
The release of greenhouse gases from the large organic carbon stock in permafrost deposits in the circumarctic regions may accelerate global warming upon thaw. The extent of this positive climate feedback is thought to be largely controlled by the microbial degradability of the organic matter preserved in these sediments. In addition, weathering and oxidation processes may release inorganic carbon preserved in permafrost sediments as CO2, which is generally not accounted for. We used 13C and 14C analysis and isotopic mass balances to differentiate and quantify organic and inorganic carbon released as CO2 in the field from an active retrogressive thaw slump of Pleistocene-age Yedoma and during a 1.5-years incubation experiment. The results reveal that the dominant source of the CO2 released from freshly thawed Yedoma exposed as thaw mound is Pleistocene-age organic matter (48–80%) and to a lesser extent modern organic substrate (3–34%). A significant portion of the CO2 originated from inorganic carbon in the Yedoma (17–26%). The mixing of young, active layer material with Yedoma at a site on the slump floor led to the preferential mineralization of this young organic carbon source. Admixtures of younger organic substrates in the Yedoma thaw mound were small and thus rapidly consumed as shown by lower contributions to the CO2 produced during few weeks of aerobic incubation at 4°C corresponding to approximately one thaw season. Future CO2 fluxes from the freshly thawed Yedoma will contain higher proportions of ancient inorganic (22%) and organic carbon (61–78%) as suggested by the results at the end, after 1.5 years of incubation. The increasing contribution of inorganic carbon during the incubation is favored by the accumulation of organic acids from microbial organic matter degradation resulting in lower pH values and, in consequence, in inorganic carbon dissolution. Because part of the inorganic carbon pool is assumed to be of pedogenic origin, these emissions would ultimately not alter carbon budgets. The results of this study highlight the preferential degradation of younger organic substrates in freshly thawed Yedoma, if available, and a substantial release of CO2 from inorganic sources.
“…In addition, thermokarst may alter the degradability of the ancient Yedoma OM by introducing younger material from overlying sediments and changing thermal and hydrological conditions (Grosse et al, 2011;Strauss et al, 2015;Wild et al, 2016). The mixing of sediments is particularly pronounced on retrogressive thaw slumps, a dynamic form of thermokarst in ice-rich areas, expanding inland by melting of the ground ice in the headwall (Lantuit and Pollard, 2008;Costard et al, 2021).…”
The release of greenhouse gases from the large organic carbon stock in permafrost deposits in the circumarctic regions may accelerate global warming upon thaw. The extent of this positive climate feedback is thought to be largely controlled by the microbial degradability of the organic matter preserved in these sediments. In addition, weathering and oxidation processes may release inorganic carbon preserved in permafrost sediments as CO2, which is generally not accounted for. We used 13C and 14C analysis and isotopic mass balances to differentiate and quantify organic and inorganic carbon released as CO2 in the field from an active retrogressive thaw slump of Pleistocene-age Yedoma and during a 1.5-years incubation experiment. The results reveal that the dominant source of the CO2 released from freshly thawed Yedoma exposed as thaw mound is Pleistocene-age organic matter (48–80%) and to a lesser extent modern organic substrate (3–34%). A significant portion of the CO2 originated from inorganic carbon in the Yedoma (17–26%). The mixing of young, active layer material with Yedoma at a site on the slump floor led to the preferential mineralization of this young organic carbon source. Admixtures of younger organic substrates in the Yedoma thaw mound were small and thus rapidly consumed as shown by lower contributions to the CO2 produced during few weeks of aerobic incubation at 4°C corresponding to approximately one thaw season. Future CO2 fluxes from the freshly thawed Yedoma will contain higher proportions of ancient inorganic (22%) and organic carbon (61–78%) as suggested by the results at the end, after 1.5 years of incubation. The increasing contribution of inorganic carbon during the incubation is favored by the accumulation of organic acids from microbial organic matter degradation resulting in lower pH values and, in consequence, in inorganic carbon dissolution. Because part of the inorganic carbon pool is assumed to be of pedogenic origin, these emissions would ultimately not alter carbon budgets. The results of this study highlight the preferential degradation of younger organic substrates in freshly thawed Yedoma, if available, and a substantial release of CO2 from inorganic sources.
“…In current models, the omission of conduction through, and radiation absorption by branches underestimates cooling in winter and underestimates warming in spring. The reality of the permafrost thermal regime under shrubs is not captured, with consequence on permafrost temperature 9 , nutrient recycling [10][11][12] , plant development 13 , GHG winter emissions [14][15][16] , geomorphological changes 17 , resulting in the inaccurate quanti cation of the Arctic greening-permafrost-climate feedback 5 .…”
Warming-induced shrub expansion on Arctic tundra (Arctic greening) is thought to warm up permafrost by several degrees, as shrubs trap blowing snow and increase snowpack thermal insulation, limiting permafrost winter cooling and facilitating its thaw. At Bylot Island, (Canadian high Arctic, 73°N) we monitored permafrost temperature at nearby unmanipulated herb tundra and shrub tundra sites and unexpectedly observed that low shrubs cool permafrost by 1.21°C over the November-February period. This is despite a snowpack twice as insulating in shrubs. Using heat transfer models and finite-element simulations, we show that this winter cooling is caused by thermal bridging through frozen shrub branches. This effect largely compensates the warming effect induced by the more insulating snow in shrubs. The cooling is partly canceled in spring when shrub branches under snow absorb solar radiation and accelerate permafrost warming. The overall effect is expected to depend on snow and shrub characteristics and terrain aspect. These significant perturbations of the permafrost thermal regime by shrub branches should be considered in projections of permafrost thawing, nutrient recycling and greenhouse gas emissions.
“…In current models, the omission of conduction through, and radiation absorption by, branches underestimates cooling in winter and underestimates warming in spring. The reality of the permafrost thermal regime under shrubs is not captured, with consequence on permafrost temperature 10 , nutrient recycling 11 – 13 , plant development 14 , GHG winter and spring emissions 15 – 17 , and geomorphological changes 18 , resulting in the inaccurate quantification of the shrub expansion–permafrost–climate feedback 6 . Fig.…”
Considerable expansion of shrubs across the Arctic tundra has been observed in recent decades. These shrubs are thought to have a warming effect on permafrost by increasing snowpack thermal insulation, thereby limiting winter cooling and accelerating thaw. Here, we use ground temperature observations and heat transfer simulations to show that low shrubs can actually cool the ground in winter by providing a thermal bridge through the snowpack. Observations from unmanipulated herb tundra and shrub tundra sites on Bylot Island in the Canadian high Arctic reveal a 1.21 °C cooling effect between November and February. This is despite a snowpack that is twice as insulating in shrubs. The thermal bridging effect is reversed in spring when shrub branches absorb solar radiation and transfer heat to the ground. The overall thermal effect is likely to depend on snow and shrub characteristics and terrain aspect. The inclusion of these thermal bridging processes into climate models may have an important impact on projected greenhouse gas emissions by permafrost.
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