Greenhouse gas production potential and associated degradation of organic carbon in warming permafrost – an incubation experiment coupled to biomarker analyses from drained lake basin sediments on the Yukon Coastal Plain, Canada

Permafrost carbon pools are vulnerable to a warming climate and bear the potential to alter the terrestrial carbon cycle. In the extensive drained lake basin wetlands that cover Arctic lowlands, enhanced degradation of organic-rich deposits upon permafrost thaw could lead to greenhouse gas emissions to the atmosphere. This study investigates processes and intensity of organic matter decomposition and associated potential greenhouse gas production in thawed sediment from drained lake basins on the Yukon Coastal Plain in the western Canadian Arctic. We conducted three-month low temperature (4 °C) incubation experiments, assessing the greenhouse gas production potential in the active layer, transition layer, and permafrost of sediment cores from two adjacent drained lake basins under aerobic and anaerobic conditons. The study was supplemented by comprehensive geochemical and biomarker analyses before and after the incubation experiments. Our findings revealed a higher carbon turnover of up to 2.7 % of the available organic carbon to CO2 under aerobic conditions. Carbon loss from mineral permafrost layers matched that of surface peat samples, whereas nitrogen limitation constrained short term carbon mineralization in pioneer peat layers that accumulated shortly after lake drainage. The GHG production under anaerobic conditions exhibited a high depth-dependency, with permafrost layer samples deviating from the otherwise observed high methanogenesis in active and transition layer samples within the short incubation period. High contributions of the potent greenhouse gas methane of up to 94 % enhanced the climate forcing effect of anaerobic emissions. Consequently, the determined relative climate forcing is higher under anaerobic compared to aerobic conditions in active and transition layers, suggesting that waterlogged conditions within drained lake basins are more unfavorable in the short term. While established degradation proxies C:N, δ13C and CPI did not distinctly trace significant degradation of terrestrial organic matter, we observed major shifts in lipid composition, reflected in increasing concentrations of n-alkanols and n-alkanes

Timing and drivers of mid- to late Holocene ice-wedge polygon development in the Western Canadian Arctic

Ice-wedge polygon formation and development from low-centred to high-centred types are thought to be either linear processes acting on long time-scales or rapid shifts between different regimes. We analyzed six sediment cores from three ice-wedge polygons on the Yukon Coastal Plain to examine the timing and drivers of these dynamics. All sites developed from shallow lakes or submerged polygon environments to low-centred polygons before rapid degradation and drying during the last century. We found that ice-wedge polygon initiation was linked to moderate climatic cooling during the mid-Holocene combined with drainage of lakes. The further conversion to high-centred polygons appeared to have been a rapid process linked to modern climatic warming. Continued warming may thus lead to increasing ice-wedge melt on larger scales and subsequent degradation of ice-wedge polygons, especially if paired with increasing geomorphic disturbances caused by thermokarst and thermo-erosion

Carbon degradation and potential greenhouse gas production in a changing Arctic thermokarst landscape

Permafrost carbon pools are vulnerable to a warming climate and bear the potential to alter the terrestrial carbon cycle. In the extensive drained lake basins that span across Arctic lowlands, enhanced degradation of organic-rich deposits upon permafrost thaw could lead to greenhouse gas emissions to the atmosphere. Yet, little is known on the geochemical properties of the sediments in these basins and on the rate of release of greenhouse gases. This study investigates processes and intensity of organic matter decomposition and associated potential greenhouse gas production in thawed sediment from drained lake basins on the Yukon Coastal Plain in the western Canadian Arctic. We conducted a three-month low temperature (4 °C) incubation experiment, during which we measured carbon dioxide (CO2) and methane (CH4) production in thawed sediment from two permafrost cores from adjacent drained lake basins. To simulate current and near future greenhouse gas production potential we incubated material from the active layer as well as from the transition layer and permafrost to account for projected active layer deepening. Four replicates of each sample were incubated under aerobic and anaerobic conditions, respectively. CO2 and CH4 concentrations were measured by gas chromatography. The experiment was supplemented by a comprehensive lipid biomarker analysis of the same sample material before and after the incubation covering n-alkanes, n-fatty acids, triterpenoids and hopanes. Biomarker concentrations and indices (average chain length, carbon preference index, higher-plant fatty acid index) gave insights on the origin and degradation state of organic matter as well as changes to carbon accompanying the incubation experiment. In a multi-proxy approach, findings are further aligned with biogeochemical and sedimentological parameters. Results will reveal organic matter vulnerability to decomposition and potential greenhouse gas production in sediments after thawing, both of which are key elements in assessing future trajectories of carbon dynamics in drained lake basins

Large Herbivores and Their Interaction with Arctic Soil Carbon Storage

Permafrost degradation and organic matter decomposition in the terrestrial Arctic are strongly depending on soil temperatures. A factor that affects these temperatures is grazing and snow trampling by large herbivorous animals, as well as animal-induced changes in vegetation cover. We analysed samples taken from adjacent areas with different grazing intensities, both in a permafrost environment (Siberia) and seasonally frozen ground (norther Finland) for TOC, C/N ratio, d13C, bulk density and radiocarbon age. While in permafrost there was a strong increase in soil carbon storage with high grazing intensity, this effect is not visible in seasonally frozen ground. However, in both areas we observed massive changes in vegetation composition and structure, following the grazing gradient. We conclude that seasonally frozen ground allows for more intensive carbon relocation and mixing, which outweighs the effects animals have in the permafrost region but state that on permafrost, animals might efficiently be utilized to stabilise permafrost temperatures and reduce organic material decomposition

Herbivore-Induced Effects on Arctic Soil Carbon Storage

Permafrost degradation and organic matter decomposition in the terrestrial Arctic are strongly depending on soil temperature throughout the year. These temperatures are affected in numerous ways by activity of large herbivorous animals. We identified snow compaction and animal-induced vegetation changes as key elements. Therefore, we analysed soil parameters along transects following grazing intensity in both a permafrost environment (northeastern Siberia) and seasonally frozen ground (norther Finland). Parameters included TOC, C/N ratio, d13C, bulk density and radiocarbon age. While we observed a strong increase in soil carbon storage with high grazing intensity under permafrost conditions, this effect does not show in seasonally frozen ground. However, an obvious animal-induced change in both areas was a shift in vegetation composition and structure, following the grazing gradient. We conclude that material and water fluxes in seasonally frozen ground outweigh the animals’ effects, contrary to permafrost environments, but state that on permafrost, animals could help maintaining low soil temperatures and hence reduce organic material decomposition

Vegetation composition and shrub extent on the Yukon coast, Canada, are strongly linked to ice-wedge polygon degradation

Changing environmental and geomorphological conditions are resulting in vegetation change in ice-wedge polygons in Arctic tundra. However, we do not yet know how microscale vegetation patterns relate to individual environmental and geomorphological parameters. This work aims at examining these relations in polygonal terrain. We analysed composition and cover of vascular plant taxa and surface height, active layer depth, soil temperature, carbon and nitrogen content, pH and electrical conductivity in four polygon mires located on the Yukon coast. We found that vascular plant species composition and cover correlates best with relative surface height. Ridges of low-centred polygons and raised centres of high-centred polygons support the growth of mesic and wetland species (e.g., Betula glandulosa, Salix pulchra, S. reticulata, Rubus chamaemorus, various ericaceous dwarf shrubs, Eriophorum vaginatum, Poa arctica). Wetland and aquatic plant species (e.g., E. angustifolium, Carex aquatilis, C. chordorrhiza, Pedicularis sudetica) grow in low-lying centres of polygons and in troughs between polygons. We also found a relationship between vascular plant species composition and substrate characteristics such as pH, electrical conductivity and total organic carbon, although the individual influence of these parameters could not be determined because of their correlation with relative surface height. Our findings stress the regulatory role of microtopography and substrate in vegetation dynamics of polygonal terrain. Ongoing warming in this region will lead to changes to polygonal terrain through permafrost degradation and subsequent conversion of low-centred into high-centred polygons. Our results indicate that shrubs, particularly Betula glandulosa and heath species, have the potential to expand most

Large herbivores on permafrost— a pilot study of grazing impacts on permafrost soil carbon storage in northeastern Siberia

The risk of carbon emissions from permafrost is linked to an increase in ground temperature and thus in particular to thermal insulation by vegetation, soil layers and snow cover. Ground insulation can be influenced by the presence of large herbivores browsing for food in both winter and summer. In this study, we examine the potential impact of large herbivore presence on the soil carbon storage in a thermokarst landscape in northeastern Siberia. Our aim in this pilot study is to conduct a first analysis on whether intensive large herbivore grazing may slow or even reverse permafrost thaw by affecting thermal insulation through modifying ground cover properties. As permafrost soil temperatures are important for organic matter decomposition, we hypothesize that herbivory disturbances lead to differences in ground-stored carbon. Therefore, we analyzed five sites with a total of three different herbivore grazing intensities on two landscape forms (drained thermokarst basin, Yedoma upland) in Pleistocene Park near Chersky. We measured maximum thaw depth, total organic carbon content, δ13C isotopes, carbon-nitrogen ratios, and sediment grain-size composition as well as ice and water content for each site. We found the thaw depth to be shallower and carbon storage to be higher in intensively grazed areas compared to extensively and non-grazed sites in the same thermokarst basin. First data show that intensive grazing leads to a more stable thermal ground regime and thus to increased carbon storage in the thermokarst deposits and active layer. However, the high carbon content found within the upper 20 cm on intensively grazed sites could also indicate higher carbon input rather than reduced decomposition, which requires further studies including investigations of the hydrology and general ground conditions existing prior to grazing introduction. We explain our findings by intensive animal trampling in winter and vegetation changes, which overcompensate summer ground warming. We conclude that grazing intensity—along with soil substrate and hydrologic conditions—might have a measurable influence on the carbon storage in permafrost soils. Hence the grazing effect should be further investigated for its potential as an actively manageable instrument to reduce net carbon emission from permafrost

The cryostratigraphy of the Yedoma cliff of Sobo-Sise Island (Lena delta) reveals permafrost dynamics in the central Laptev Sea coastal region during the last 52 kyr

The present study examines the formation history and cryolithological properties of the late-Pleistocene Yedoma Ice Complex (IC) and its Holocene cover in the eastern Lena delta on Sobo-Sise Island. The sedimentary sequence was continuously sampled at 0.5 m resolution at a vertical Yedoma cliff starting from 24.2 m above river level (a.r.l.). The sequence differentiates into three cryostratigraphic units: Unit A, dated from ca. 52 to 28 cal kyr BP; Unit B, dated from ca. 28 to 15 cal kyr BP; Unit C, dated from ca. 7 to 0 cal kyr BP. Three chronologic gaps in the record are striking. The hiatus during the interstadial marine isotope stage (MIS) 3 (36–29 cal kyr BP) as well as during stadial MIS 2 (20–17 cal kyr BP) might be related to fluvial erosion and/or changed discharge patterns of the Lena river caused by repeated outburst floods from the glacial Lake Vitim in southern Siberia along the Lena river valley towards the Arctic Ocean. The hiatus during the MIS 2–1 transition (15–7 cal kyr BP) is a commonly observed feature in permafrost chronologies due to intense thermokarst activity of the deglacial period. The chronologic gaps of the Sobo-Sise Yedoma record are similarly found at two neighbouring Yedoma IC sites on Bykovsky Peninsula and Kurungnakh-Sise Island and are most likely of regional importance. The three cryostratigraphic units of the Sobo-Sise Yedoma exhibit distinct signatures in properties of their clastic, organic, and ice components. Higher permafrost aggradation rates of 1 m kyr−1 with higher organic-matter (OM) stocks (29 ± 15 kg C m−3, 2.2 ± 1.0 kg N m−3; Unit A) and mainly coarse silt are found for the interstadial MIS 3 if compared to the stadial MIS 2 with 0.7 m kyr−1 permafrost aggradation, lower OM stocks (14 ± 8 kg C m−3, 1.4 ± 0.4 kg N m−3; Unit B), and pronounced peaks in the coarse-silt and medium-sand fractions. Geochemical signatures of intra-sedimental ice reflect the differences in summer evaporation and moisture regime by higher ion content and less depleted ratios of stable δ18O and stable δD isotopes but lower deuterium excess (d) values during interstadial MIS 3 if compared to stadial MIS 2. The δ18O and δD composition of MIS 3 and MIS 2 ice wedges shows characteristic well-depleted values and low d values, while MIS 1 ice wedges have elevated mean d values between 11 ‰ and 15 ‰ and surprisingly low δ18O and δD values. Hence, the isotopic difference between late-Pleistocene and Holocene ice wedges is more pronounced in d than in δ values. The present study of the permafrost exposed at the Sobo-Sise Yedoma cliff provides a comprehensive cryostratigraphic inventory, insights into permafrost aggradation, and degradation over the last approximately 52 kyr as well as their climatic and morphodynamic controls on the regional scale of the central Laptev Sea coastal region in NE Siberia

Organic carbon characteristics in ice-rich permafrost in alas and Yedoma deposits, central Yakutia, Siberia

Permafrost ground is one of the largest repositories of terrestrial organic carbon and might become or already is a carbon source in response to ongoing global warming. With this study of syngenetically frozen, ice-rich and organic carbon (OC)-bearing Yedoma and associated alas deposits in central Yakutia (Republic of Sakha), we aimed to assess the local sediment deposition regime and its impact on permafrost carbon storage. For this purpose, we investigated the Yukechi alas area (61.76495°N, 130.46664°E), which is a thermokarst landscape degrading into Yedoma in central Yakutia. We retrieved two sediment cores (Yedoma upland, 22.35m deep, and alas basin, 19.80m deep) in 2015 and analyzed the biogeochemistry, sedimentology, radiocarbon dates and stable isotope geochemistry. The laboratory analyses of both cores revealed very low total OC (TOC) contents (< 0.1 wt %) for a 12m section in each core, whereas the remaining sections ranged from 0.1 wt% to 2.4 wt% TOC. The core sections holding very little to no detectable OC consisted of coarser sandy material were estimated to be between 39 000 and 18 000 BP (years before present) in age. For this period, we assume the deposition of organic-poor material. Pore water stable isotope data from the Yedoma core indicated a continuously frozen state except for the surface sample, thereby ruling out Holocene reworking. In consequence, we see evidence that no strong organic matter (OM) decomposition took place in the sediments of the Yedoma core until today. The alas core from an adjacent thermokarst basin was strongly disturbed by lake development and permafrost thaw. Similar to the Yedoma core, some sections of the alas core were also OC poor (< 0.1 wt %) in 17 out of 28 samples. The Yedoma deposition was likely influenced by fluvial regimes in nearby streams and the Lena River shifting with climate. With its coarse sediments with low OC content (OC mean of 5.27 kg m-3), the Yedoma deposits in the Yukechi area differ from other Yedoma sites in North Yakutia that were generally characterized by silty sediments with higher OC contents (OC mean of 19 kg m-3 for the non-icewedge sediment). Therefore, we conclude that sedimentary composition and deposition regimes of Yedoma may differ considerably within the Yedoma domain. The resulting heterogeneity should be taken into account for future upscaling approaches on the Yedoma carbon stock. The alas core, strongly affected by extensive thawing processes during the Holocene, indicates a possible future pathway of ground subsidence and further OC decomposition for thawing central Yakutian Yedoma deposits