Impact of lateral groundwater flow on hydrothermal conditions of the active layer in a high-Arctic hillslope setting

Modeling the physical state of permafrost landscapes is a crucial addition to field observations in order to understand the feedback mechanisms between permafrost and the atmosphere within a warming climate. A common hypothesis in permafrost modeling is that vertical heat conduction is most relevant to derive subsurface temperatures. While this approach is mostly applicable to flat landscapes with little topography, landscapes with more topography are subject to lateral flow processes as well. With our study, we contribute to the growing body of evidence that lateral surface and subsurface processes can have a significant impact on permafrost temperatures and active layer properties. We use a numerical model to simulate two idealized hillslopes (a steep and a medium case) with inclinations that can be found in Adventdalen, Svalbard, and compare them to a flat control case. We find that ground temperatures within the active layer uphill are generally warmer than downhill in both slopes (with a difference of up to ∼0.8 ∘C in the steep and ∼0.6 ∘C in the medium slope). Further, the slopes are found to be warmer in the uphill section and colder in the base of the slopes compared to the flat control case. As a result, maximum thaw depth increases by about 5 cm from the flat (0.98 m) to the medium (1.03 m) and the steep slope (1.03 m). Uphill warming on the slopes is explained by overall lower heat capacity, additional energy gain through infiltration, and lower evaporation rates due to drier conditions caused by subsurface runoff. The major governing process causing the cooling on the downslope side is heat loss to the atmosphere through evaporation in summer and enhanced heat loss in winter due to wetter conditions and resulting increased thermal conductivity. On a catchment scale, these results suggest that temperature distributions in sloped terrain can vary considerably compared to flat terrain, which might impact the response of subsurface hydrothermal conditions to ongoing climate change.

Large Herbivores Affecting Permafrost – Impacts of Grazing on Permafrost Soil Carbon Storage in Northeastern Siberia

The risk of carbon emissions from permafrost ground is linked to ground temperature and thus in particular to thermal insulation by vegetation and organic soil layers in summer and snow cover in winter. This ground insulation is strongly influenced by the presence of large herbivorous animals browsing for food. In this study, we examine the potential impact of large herbivore presence on the ground carbon storage in thermokarst landscapes of northeastern Siberia. Our aim is to understand how intensive animal grazing may affect permafrost thaw and hence organic matter decomposition, leading to different ground carbon storage, which is significant in the active layer. Therefore, we analysed sites with differing large herbivore grazing intensity in the Pleistocene Park near Chersky and measured maximum thaw depth, total organic carbon content and decomposition state by δ13C isotope analysis. In addition, we determined sediment grain size composition as well as ice and water content. 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. The intensive grazing presumably 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. We connect our findings to more animal trampling in winter, which causes snow disturbance and cooler winter ground temperatures during the average annual 225 days below freezing. This winter cooling overcompensates ground warming due to the lower insulation associated with shorter heavily grazed vegetation during the average annual 140 thaw days. We conclude that intensive grazing influences the carbon storage capacities of permafrost areas and hence might be an actively manageable instrument to reduce net carbon emission from these sites.

New insights into the drainage of inundated ice-wedge polygons using fundamental hydrologic principles

The pathways and timing of drainage from the inundated centers of ice-wedge polygons in a warming climate have important implications for carbon flushing, advective heat transport, and transitions from methane to carbon dioxide dominated emissions. Here, we expand on previous research using a recently developed analytical model of drainage from a low-centered polygon. Specifically, we perform (1) a calibration to field data identifying necessary model refinements and (2) a rigorous model sensitivity analysis that expands on previously published indications of polygon drainage characteristics. This research provides intuition on inundated polygon drainage by presenting the first in-depth analysis of drainage within a polygon based on hydrogeological first principles. We verify a recently developed analytical solution of polygon drainage through a calibration to a season of field measurements. Due to the parsimony of the model, providing the potential that it could fail, we identify the minimum necessary refinements that allow the model to match water levels measured in a low-centered polygon. We find that (1) the measured precipitation must be increased by a factor of around 2.2, and (2) the vertical soil hydraulic conductivity must decrease with increasing thaw depth. Model refinement (1) accounts for runoff from rims into the ice-wedge polygon pond during precipitation events and possible rain gauge undercatch, while refinement (2) accounts for the decreasing permeability of deeper soil layers. The calibration to field measurements supports the validity of the model, indicating that it is able to represent ice-wedge polygon drainage dynamics. We then use the analytical solution in non-dimensional form to provide a baseline for the effects of polygon aspect ratios (radius to thaw depth) and coefficient of hydraulic conductivity anisotropy (horizontal to vertical hydraulic conductivity) on drainage pathways and temporal depletion of ponded water from inundated ice-wedge polygon centers. By varying the polygon aspect ratio, we evaluate the relative effect of polygon size (width), inter-annual increases in active-layer thickness, and seasonal increases in thaw depth on drainage. The results of our sensitivity analysis rigorously confirm a previous analysis indicating that most drainage through the active layer occurs along an annular region of the polygon center near the rims. This has important implications for transport of nutrients (such as dissolved organic carbon) and advection of heat towards ice-wedge tops. We also provide a comprehensive investigation of the effect of polygon aspect ratio and anisotropy on drainage timing and patterns, expanding on previously published research. Our results indicate that polygons with large aspect ratios and high anisotropy will have the most distributed drainage, while polygons with large aspect ratios and low anisotropy will have their drainage most focused near their periphery and will drain most slowly. Polygons with small aspect ratios and high anisotropy will drain most quickly. These results, based on parametric investigation of idealized scenarios, provide a baseline for further research considering the geometric and hydraulic complexities of ice-wedge polygons.

Standardized monitoring of permafrost thaw: a user-friendly, multi-parameter protocol

Climate change is destabilizing permafrost landscapes, affecting infrastructure, ecosystems and human livelihoods. The rate of permafrost thaw is controlled by surface and subsurface properties and processes, all of which are potentially linked with each other. Yet, no standardized protocol exists for measuring permafrost thaw and related processes and properties in a linked manner. The permafrost thaw action group of the Terrestrial Multidisciplinary distributed Observatories for the Study of the Arctic Connections (T-MOSAiC) project has developed a protocol, for use by non-specialist scientists and technicians, citizen scientists and indigenous groups, to collect standardized metadata and data on permafrost thaw. The protocol introduced here addresses the need to jointly measure permafrost thaw and the associated surface and subsurface environmental conditions. The parameters measured along transects are: snow depth, thaw depth, vegetation height, soil texture, and water level. The metadata collection includes data on timing of data collection, geographical coordinates, land surface characteristics (vegetation, ground surface, water conditions), as well as photographs. Our hope is that this openly available dataset will also be highly valuable for validation and parameterization of numerical and conceptual models, thus to the broad community represented by the T-MOSAIC project.

Theory and Application of the Numerical Simulation in the Frozen Soil Problems

The freezing-thawing processes in soils are important components of terrestrial hydrology, which significantly influence energy and water exchanges between land surface and sub-surface. Long-term changes in frost and thaw depths are also an important indicator of climate change. A water-heat coupled movements model is established with frozen soil in this paper, which treats the freezing/thawing front as a moving interface governed by some Stefan problems with two free boundaries. The numerical simulation is conducted by using the modified finite difference method. The model is validated to compare its predictions with GEWEX Asian Monsoon Experiment(GAME)-Tibet observations at D66 site in Tibetan Plateau. The results show that the simulated soil temperature, soil water content and frost/thaw depth are in excellent agreement with the measured values. Finally, optimal error estimation for L^∞ norm is derived on the model problem by using coordinate transformation method. The numerical simulation system is established on the basis of rigorous mathematics and mechanics, which successfully solved the important and difficult problems of environmental science.

Extremely wet summer events enhance permafrost thaw for multiple years in Siberian tundra

Permafrost thaw can accelerate climate warming by releasing carbon from previously frozen soil in the form of greenhouse gases. Summer precipitation extremes have been proposed to increase permafrost thaw, but the magnitude and duration of this effect are poorly understood. Here we present empirical evidence showing that one extremely wet summer (+100mm; 120% increase relative to average summer precipitation) enhances thaw depth by up to 35% and prolonged the thaw period in a controlled irrigation experiment in an ice-rich Siberian tundra site. The effect persisted over two subsequent summers, demonstrating a carry-over effect of extremely wet summers. Using soil thermal hydrological modelling, we show that precipitation-induced increases in thaw are most pronounced during warm summers with mid-summer precipitation peaks. Our results suggest that, with summer precipitation and temperature both increasing in the Arctic, permafrost will likely degrade and disappear faster than is currently anticipated based on rising air temperatures alone.

PERICLIMv1.0: a model deriving palaeo-air temperatures from thaw depth in past permafrost regions

Periglacial features, such as various kinds of patterned ground, cryoturbations, frost wedges, solifluction structures, and blockfields, are among the most common relics of cold climate periods, which repetitively occurred throughout the Quaternary. As such, they are widespread archives of past environmental conditions. Climate controls on the development of most periglacial features, however, remain poorly known, and thus empirical palaeo-climate reconstructions based on them have limited validity. This study presents and evaluates a simple new inverse modelling scheme called PERICLIMv1.0 (PERIglacial CLIMate) that derives palaeo-air temperature characteristics related to the palaeo-active-layer thickness, which can be recognized using many relict periglacial features found in past permafrost regions. The evaluation against modern temperature records showed that the model reproduces air temperature characteristics with average errors ≤1.3 ∘C. The past mean annual air temperature modelled experimentally for two sites in the Czech Republic hosting relict cryoturbation structures was between -7.0±1.9 and -3.2±1.5 ∘C, which is well in line with earlier reconstructions utilizing various palaeo-archives. These initial results are promising and suggest that the model could become a useful tool for reconstructing Quaternary palaeo-environments across vast areas of mid-latitudes and low latitudes where relict periglacial assemblages frequently occur, but their full potential remains to be exploited.

Impact of lateral groundwater flow on hydrothermal conditions of the active layer in a high arctic hillslope setting

Modeling the physical state of permafrost landscapes is a crucial addition to field observations in order to understand its feedback mechanisms within a warming climate. A common hypothesis in permafrost modeling is that vertical heat conduction is most relevant to derive subsurface temperatures. While this approach is mostly applicable to flat landscapes with little topography, landscapes with more topography are subject to lateral flow process as well. With our study, we want to contribute to the growing body of evidence that lateral surface- and subsurface processes can have a significant impact on permafrost temperatures and active layer properties. We use a numerical model to simulated two idealized hillslopes with inclinations that can be found in Adventdalen, Svalbard, and compare them to a flat control case. We find that ground temperatures within the active layer uphill are generally warmer than downhill in both slopes (up to ~1.2 °C in the steep, and ~0.7 °C in the medium slope). Further, the slopes are found to be warmer in the uphill section and colder in the very bottom of the slopes compared to the flat control case. As a result, maximum thaw depth increases by about 5 cm from the flat (75 cm) to the steep slope (80 cm), while the medium case does not exhibit a deepening in thaw depth (75 cm). Uphill warming on the slopes is explained by additional energy gain through infiltration and lower evaporation rates due to a overall drier environment. The major governing process causing the cooling on the downslope side is heat loss to the atmosphere through evaporation in summer and enhanced heat loss in winter due to wetter conditions and resulting higher thermal conductivity. On a catchment scale, these results suggest that temperature distributions in hilly terrain can vary considerably compared to flat terrain, which might change the response of subsurface hydrothermal conditions to ongoing climate change.

Passive seismic investigations of subaquatic permafrost

<p>Large quantities of organic carbon are known to be sequestered within subaquatic permafrost as gas hydrates. Therefore, knowledge of the extent and thaw rate is of critical importance to our understanding of global climate change. Investigations of sub-aquatic permafrost have focussed on its physical characteristics via drilling or probing, and through the limited application of geophysical methods. Active seismic methods have been most widely employed, especially for petroleum exploration, but recently passive methods have been used to investigate the seabed using ambient noise. The Horizontal-to-Vertical Spectral Ratio (HVSR) method has previously been shown to accurately determine permafrost thaw depth below the sea floor in marine and lacustrine environments, based on the collection of seismic data over a period of weeks. In this study, we test the use of short-term seabed HVSR seismic surveys and explore possibilities for optimizing the method in a wide variety of subaquatic environments.</p><p>The method was successfully used in a thermokarst lake, a lagoon and river channels of the Lena Delta (Russia), as well as in marine shelf environments in the Laptev Sea (Russia) and Tuktoyaktuk Island (NW Canada). Study areas where validation data was available were preferred and selected when possible. A passive seismic measuring device, consisting of a watertight metal cannister containing three-component broad-band seismometers, was lowered down to the sea floor from a small boat and left to collect data for 3-4 minutes. The data was recorded at a sample rate of 100Hz.</p><p>Post-processing and analysis were done with MATLAB. The three seismic signals were individually detrended, the offset was removed and the power spectral density was calculated. The smoothing function proposed by Konno and Ohmachi (1998)  was applied to each signal with a smoothing coefficient of 40. Lastly the H/V (Horizontal / Vertical) amplitude was calculated. The H/V amplitude was plotted against signal frequencies from 0 to 50 Hz. The peak resonance frequency is believed to indicate the ice-bonded permafrost table (IBPT) thereby enabling us to determine thaw depth from the H/V plots, assuming a simple 2-layer model: thawed layer over frozen ground, characterized by low and high wave speeds, respectively.</p><p>Results generally display a good correlation, on average within 0.6 meters, between the thaw depth determined from HVSR and from physical validation, although HVSR often generates a thaw depth deeper than indicated by validation data. This may be a result of complex permafrost systems where several “zones” of frozen and unfrozen ground, of varying thickness, is present below the water bodies.</p><p>We conclude that the method has the potential to be an effective (fast) non-invasive tool for investigating the extent and, if repeated, the thaw rate of subaquatic permafrost. Further field testing is planned in order to continue the development and optimization of the method.</p>