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Effects of unfrozen water on heat and mass transport in the active layer and permafrost
Abstract
Precise temperature data from four Alaskan permafrost sites (Prudhoe Bay, Barrow and two sites near Fairbanks) combined with computer modelling provide quantitative measures of the existence and dynamics of unfrozen water in the active layer and permafrost. Unfrozen water contents are negligible for living and dead moss layers, small in the peat layers and larger in the silts, and show significant site-to-site variation. The effect of unfrozen water on the ground thermal regime is largest immediately after freeze-up and during cooling of the active layer. It is less important during warming and thawing of the active layer and during freezing and thawing of seasonally frozen ground. The effects last less than a month in cold permafrost and throughout most of the freeze-up period in warm permafrost. Physically, unfrozen water introduces a spatially distributed latent heat and changes thermal properties which retards the thermal response of an active layer or permafrost. Unfrozen water in the freezing and frozen active layer and near-surface permafrost also protects the ground from rapid cooling and creates a strong thermal gradient at the ground surface that increases the heat flux out of the ground. This enlarged heat flux also enhances the insulating effect of the snow cover. There do not appear to be any inherent difficulties in using conductive heat modelling for the active layer during the period when the zero curtain exists.
... for some suitable f n ≈ f (t n ). Note that V n is well defined since (20) has the form (17) and thus we can use the resolvent V n = R(a, b; V n−1 + τ f n ). (21) In turn, the selection c n ∈ C(a, b; V n ) given from (20) as τ c n = V n−1 + τ f n − V n is the unique element of minimal norm [4] (pp. 66, 28) for which (20) holds. ...
... Note that V n is well defined since (20) has the form (17) and thus we can use the resolvent V n = R(a, b; V n−1 + τ f n ). (21) In turn, the selection c n ∈ C(a, b; V n ) given from (20) as τ c n = V n−1 + τ f n − V n is the unique element of minimal norm [4] (pp. 66, 28) for which (20) holds. ...
... Note that V n is well defined since (20) has the form (17) and thus we can use the resolvent V n = R(a, b; V n−1 + τ f n ). (21) In turn, the selection c n ∈ C(a, b; V n ) given from (20) as τ c n = V n−1 + τ f n − V n is the unique element of minimal norm [4] (pp. 66, 28) for which (20) holds. ...
In this paper we consider a nonlinear partial differential equation describing heat flow with ice-water phase transition in permafrost soils. Such models and their numerical approximations have been well explored in the applications literature. In this paper we describe a new direction in which the allow relaxation and hysteresis of the phase transition which introduce additional nonlinear terms and complications for the analysis. We present numerical algorithms as well as analysis of the well-posedness and convergence of the fully implicit iterative schemes. The analysis we propose handles the equilibrium, non-equilibrium, and hysteresis cases in a unified way. We also illustrate with numerical examples for a model ODE and PDE.
... S1-S3). Redox potential in soils near the permafrost boundary may increase slightly due to flow of more oxygenated water along the permafrost boundary (supra-permafrost flow) coupled with low microbial activity inhibited by both cold temperatures and less biomass than in surface soils 35,36 . ...
Permafrost thaw in warming Arctic landscapes alters hydrology and saturation-driven biogeochemical processes. Models assume that aerobic respiration occurs in drained soils while saturated soils support methanogenesis; however, saturated soils maintain redox gradients that host a range of anaerobic metabolisms. We evaluated how redox potential and redox-active solutes vary with soil moisture in the active layer of permafrost-affected acidic and non-acidic tundra hillslopes. Oxidizing conditions persisted in highly permeable organic horizons of both unsaturated tussock tundra and saturated wet sedge meadows. Redox potential decreased with depth in all soils as increasing soil bulk density restricted groundwater flow and oxygen diffusion. High concentrations of dissolved iron, phosphate, and organic carbon coincided with redox boundaries below the soil surface in acidic tundra, indicating active iron redox cycling and potential release of adsorbed phosphate during iron (oxyhydr)oxide dissolution. In non-acidic tundra, weatherable minerals affected nutrient dynamics more than redox-driven iron cycling, especially in low-lying, saturated areas where thaw reached mineral soils. The role of thaw depth and the ability of saturated soils to maintain oxidizing conditions in organic surface layers highlight the importance of soil physical properties and hydrology in predicting biogeochemical processes and greenhouse gas emissions.
... Soil moisture can alter the energy exchange processes in the soil by affecting soil heat capacity and thermal conductivity, which ultimately affects the ALT 47 . Previous observations and modelling results suggested that higher soil moisture tends to result in a shallower ALT 48 . ...
... Soil moisture also plays an important role in permafrost dynamics. High soil moisture leads to slower refreezing in autumn, as more energy is required for the water to freeze (Romanovsky & Osterkamp, 2000). Therefore, increased soil moisture after extreme rainfall in summer delays autumn freezing . ...
Svalbards permafrost is thawing as a direct consequence of climate change. In the Low Arctic, vegetation has been shown to slow down and reduce the active layer thaw, yet it is unknown whether this also applies to High Arctic regions like Svalbard where vegetation is smaller, sparser, and thus likely less able to insulate the soil. Therefore, it remains unknown which components of High Arctic vegetation impact active layer thaw and at which temporal scale this insulation could be effective. Such knowledge is necessary to predict and understand future changes in active layer in a changing Arctic. In this study we used frost tubes placed in study grids located in Svalbard with known vegetation composition, to monitor the progression of active layer thaw and analyze the relationship between vegetation composition, vegetation structure and snow conditions, and active layer thaw early in summer. We found that moss thickness, shrub and forb height, and vascular vegetation cover delayed soil thaw immediately after snow melt. These insulating effects attenuated as thaw progressed, until no effect on thaw depth was present after 8 weeks. High Arctic mosses are expected to decline due to climate change, which could lead to a loss in insulating capacity, potentially accelerating early summer active layer thaw. This may have important repercussions for a wide range of ecosystem functions such as plant phenology and decomposition processes.
... Consequently, such reconstructions necessarily assume that the depth of the permafrost table does not change over the full reconstruction period since the spatial domain of the model is typically treated as constant. Second, in regions with warmer permafrost, here defined as those with mean annual ground temperatures (MAGT) ≥ 5°C , and finer grained soils with moderate to high silt or clay content, latent heat effects may be present even well below the top of the permafrost (Nicolsky & Romanovsky, 2018;Romanovsky & Osterkamp, 2000). This is due to the persistence of unfrozen water in soil pores at subzero temperatures as a result of capillary action (Koopmans & Miller, 1966). ...
Reconstructing historical climate change from deep ground temperature measurements in cold regions is often complicated by the presence of permafrost. Existing methods are typically unable to account for latent heat effects due to the freezing and thawing of the active layer. In this work, we propose a novel method for reconstructing historical ground surface temperature (GST) from borehole temperature measurements that accounts for seasonal thawing and refreezing of the active layer. Our method couples a recently developed fast numerical modeling scheme for two‐phase heat transport in permafrost soils with an ensemble‐based method for approximate Bayesian inference. We evaluate our method on two synthetic test cases covering both cold and warm permafrost conditions as well as using real data from a 100 m deep borehole on Sardakh Island in northeastern Siberia. Our analysis of the Sardakh Island borehole data confirms previous findings that GST in the region have likely risen by 5–9°C between the pre‐industrial period of 1750–1855 and 2012. We also show that latent heat effects due to seasonal freeze‐thaw have a substantial impact on the resulting reconstructed surface temperatures. We find that neglecting the thermal dynamics of the active layer can result in biases of roughly −1°C in cold conditions (i.e., mean annual ground temperature below −5°C) and as much as −2.6°C in warmer conditions where substantial active layer thickening (>200 cm) has occurred. Our results highlight the importance of considering seasonal freeze‐thaw in GST reconstructions from permafrost boreholes.
... As time progresses, the temperature of the water within the crack decreases gradually, and the water begins to freeze. Owing to the heat released during the phase transition from water to ice, the crack temperature remains relatively constant near 0 °C, and this area is called the zero curtain zone (Scherler et al. 2010;Romanovsky and Osterkamp 2000). By the end of this stage, the internal temperature of the freeze-thaw testing chamber reached the preset freezing temperature. ...
In cold regions, rock fractures can expand under the force of frost heave pressure, potentially triggering severe geological catastrophes such as collapses and landslides on rock slopes. Nevertheless, a comprehensive study of the evolution patterns and causes of frost heave pressure within rock fractures is currently lacking. To address this gap, this paper focuses on single-cracked sandstone and identifies four distinct types of frost heave pressure curves by conducting monitoring tests under various conditions. Each type of curve was thoroughly analyzed to understand its evolution and the underlying causes. It was found that the rupture and drop type curve can be segmented into five stages: incubation, eruption, drop, equilibrium, and dissipation. The drop stage is predominantly attributed to the fracturing of the crack influenced by frost heave pressure. Compared with the rupture and drop type curve, the stable type curve, where the crack does not expand during the freeze–thaw cycle, lacks a drop stage. Furthermore, samples displaying a stable descent type curve possess a higher freezing temperature, causing a noticeable delay in the emergence of their frost heave pressure. This curve, when compared with the stable type, shows a faster reduction in frost heave pressure during the equilibrium stage. For the secondary frost heave type curve, secondary frost heave emerges within the rock fracture at the onset of thawing, with the rupture of ice proving critical to this phenomenon. The frost heave pressure distributed along the crack surfaces of the four types of curves displays a lack of uniformity. This unevenness is observed to correlate with the inconsistent growth of ice according to its self-purification principle. Finally, based on the experimental results, a novel criterion for frost heave pressure and rock fracture cracking is proposed.
Climate warming is expected to have pronounced effects on Arctic and Subarctic ecosystems, especially regions underlain by discontinuous and relatively warm permafrost. The main goal of this research is to evaluate the vulnerability and dynamics of permafrost under climate warming across the various ecotypes in respect of ecosystem stability, socioeconomic impact, and for better understanding possible future environmental changes. We suggested the new version of the spatially distributed permafrost dynamics model (GIPL2-MPI), which is developed in the Geophysical Institute, University of Alaska Fairbanks. This model is based on the ecosystem approach to simulate the permafrost dynamics, which we are discussing in this paper. We combined ground-based observations and numerical freeze/thaw modeling using climate-ecosystem-permafrost interactions to understand the physical processes and mechanisms controlling permafrost physical state. We predict the changes in permafrost conditions using output from two GCMs (NCAR-CCSM4 and GFDL-CM3) and Five-Model Average Ensemble for the RCP-4.5 and RCP-8.5 scaled down to 1 by 1 km spatial resolution (https://uaf-snap.org/) across entire Alaska. Our result shows that by the end of the current century widespread near-surface permafrost degradation could begin everywhere in Alaska southward of the Brooks Range as well as across some spots at the North Slope Alaska.
Climate warming is expected to have pronounced effects on Arctic and Subarctic ecosystems, especially regions underlain by discontinuous and relatively warm permafrost. The main goal of this research is to evaluate the vulnerability and dynamics of permafrost under climate warming across the various ecotypes in respect of ecosystem stability, socioeconomic impact, and for better understanding possible future environmental changes. We suggested the new version of the spatially distributed permafrost dynamics model (GIPL2-MPI), which is developed in the Geophysical Institute, University of Alaska Fairbanks. This model is based on the ecosystem approach to simulate the permafrost dynamics, which we are discussing in this paper. We combined ground-based observations and numerical freeze/thaw modeling using climate-ecosystem-permafrost interactions to understand the physical processes and mechanisms controlling permafrost physical state. We predict the changes in permafrost conditions using output from two GCMs (NCAR-CCSM4 and GFDL-CM3) and Five-Model Average Ensemble for the RCP-4.5 and RCP-8.5 scaled down to 1 by 1 km spatial resolution (https://uaf-snap.org/) across entire Alaska. Our result shows that by the end of the current century widespread near-surface permafrost degradation could begin everywhere in Alaska southward of the Brooks Range as well as across some spots at the North Slope Alaska.
Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan‐Arctic permafrost maps, an increase in terrestrial measurement sites for CO2 and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process‐based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO2 sink with lower net CO2 uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO2 sink was located in western Canada (median: −52 g C m⁻² y⁻¹) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m⁻² y⁻¹). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH4 m⁻² y⁻¹). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year‐round CO2 and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non‐growing season emissions and disturbance effects.
Experiments were conducted before and during spring snowmelt in 1993 and 1994 at Niwot Ridge in the Colorado Front Range to assess the degree of interaction between inorganic nitrogen (N) deposited in seasonal snowpacks and soil N pools in alpine environments. Soils typically froze in early winter with minimum soil temperatures inversely related to the depth of early season snowpacks. Minimum soil temperatures under late-accumulating, shallow snowpacks reached -10 to -14°C, while soils under deeper, earlier snowpacks reached minimum temperatures of -5 to -6°C. Mineralization and nitrification inputs to the soil inorganic N pool were an order of magnitude higher than snowmelt inputs and were controlled by the timing and depth of snowpack accumulation. Ion exchange resin bags located at the soil surface indicated that the actual N inputs at any location were highly variable. About 90% of isotopically labelled 15NH4+ applied to the snow surface before melt was recovered in soil pools. Nitrogen mineralization in 1994 was generally higher (1712-1960 mg N mf2) and exhibited relatively little spatial variability (CV 0.04-0.26) under deeper, earlier accumulating snowpacks. In contrast, N mineralization under shallower, late-accumulating snowpacks was lower (511-1440 mg N mf2) and much more variable (CV 0.42-0.83). The lowest nitrification rates were found under deep/early snowpacks (8-18% of mineralized N); the highest were found under shallow/late snowpacks (16-58% mineralized N). These results indicate the timing and depth of snowpack accumulation plays a key role in nitrogen cycling in alpine ecosystems and may control inorganic nitrogen export in surface waters.
If the conductivity function for the frozen soil is replaced by a single value, the penetration of the 0oC isotherm, for both freezing and thawing problems, is over-predicted. Where an experimentally determined conductivity value is known, the prediction error is generally less than 20%, and depends on soil type. However, when an estimated value is used, the error can reach 50%.-from Authors
Mayo, Yukon Territory, lies in the widespread discontinuous permafrost zone. Nearby, permafrost thicknesses of up to 40 m have been measured in valleys, and of up to 135 m at higher elevations. Geothermal modeling suggests that if ground temperatures were previously in equilibrium with a near-surface temperature of approximately -3.0°C, then it has taken about 20 years for permafrost to reach present conditions. Observed changes in mean winter temperature and snowfall have probably caused the ground warming. -from Author
Pulsed nuclear magnetic resonance (NMR) techniques have been developed and utilized to determine complete phase composition curves for three soils. This new technique offers a non-destructive method for measurements of unfrozen water contents in frozen soils from minus 0. 2 degree C through minus 25 degree C. The results show that unfrozen water contents determined by this technique depend upon ice content (i. e. total water content).
Three numerical models (designated the Goodrich, Guymon/Hromadka, and Seregina models) used for calculations of the ground thermal regime which are based on different numerical methods and employ different treatments of freezing and thawing were compared with each other, with analytical solutions, and with measured temperature data. Comparisons of the models with the Neumann solution show differences generally less than 0.2°C between calculated temperatures using a wide range of time and depth steps. The Goodrich and Guymon/Hromadka models have been shown to predict temperature field dynamics reliably in the active layer and permafrost using small time and depth steps. However, comparisons of the models with each other using large time and depth steps and field data for the surface boundary condition showed significant differences between them (RMS deviations exceeding 1°C) and, in addition, the development of a non-physical feature (thaw bulb after freeze-up). Therefore, with large time and depth steps, the models cannot reproduce the temperature field dynamics in the active layer and permafrost. Consequently, agreement with the Neumann solution is necessary but not sufficient to qualify the models for calculations of real temperature fields. The Goodrich model requires a time step not longer than 1 h and depth step in the upper 1 m not larger than 0.02 m to reproduce the temperature regime with reasonable accuracy. However, the choice of optimum time and depth steps appears to be specific to the application. Using the Guymon/Hromadka model, similar accuracy can be obtained with a 1 h time step and 0.1 m space step within the upper 1 m depth or a 1 day time step and 0.01 m space step. However, the use of larger steps does not necessarily decrease the calculational time compared to the Goodrich model. For the case with unfrozen water present in the frozen soil, the results of calculations using the numerical models were compared with an analytical solution and were found to agree within 0.02°C.
An analytical solution is presented for freezing and thawing of soils
and permafrost containing unfrozen water or brine and with temperature
dependent thermal properties. Latent heat effects are incorporated into
an apparent heat capacity. The partially frozen soil is divided into
layers, each with constant thermal properties and with fixed
temperatures at the layer boundaries which move with time in a multiple
moving boundary problem. Solutions are obtained for the positions of the
layer boundaries and for the temperature distribution within each layer.
The theory is used to predict the maximum depth of ice penetration and
the temperature profile in a large artificial island. Maximum ice
penetration in the island is greater than that determined from the
two-layer Neumann solution. Predicted temperature profiles are
relatively smooth and do not exhibit a sharp break at the phase
boundary. The solution procedure is also applicable to other heat
conduction problems in permafrost containing unfrozen water or brine.
Temperature measurements through permafrost in the oil field at Prudhoe Bay, Alaska, combined with laboratory measurements of the thermal conductivity of drill cuttings permit an evaluation of in situ thermal properties and an understanding of the general factors that control the geothermal regime.A sharp contrast in temperature gradient at ~600 m represents a contrast in thermal conductivity caused by the downward change from interstitial ice to interstitial water at the base of permafrost under near steady state conditions. Interpretation of the gradient contrast in terms of a simple mode for the conductivity of an aggregate yields the mean ice content (~39%), and thermal conductivities for the frozen and thawed section (8.1 and 4.7 mcal/cm s°C, respectively). These results yield a heat flow of ~1.3 HFU, which is similar to other values on the Alaskan Arctic Coast; the anomalously deep permafrost is a result of the anomalously high conductivity of the siliceous ice-rich sediments. Curvature in the upper 160 m of the temperature profiles represents a warming of ~1.8°C of the mean surface temperature and a net accumulation of 5-6 kcal/cm2 by the solid earth surface during the last 100 years or so. Rising sea level and thawing of ice-rich sea cliffs probably caused the shoreline to advance tens of kilometers in the last 20,000 years, inundating a portion of the continental shelf that is presently the target of intensive oil exploration. A simple conduction model suggests that this recently inundated region is underlain by near-melting ice-rich permafrost to depths of 300-500 m; its presence is important to seismic interpretations in oil exploration and to engineering considerations in oil production. With confirmation of the permafrost configuration by offshore drilling, heat conduction models can yield reliable new information on the chronology of arctic shorelines.