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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 modeling purposes, individual  and C values are often required, which depend on specific subsurface properties; for example, parent material, organic matter content, air, and (un)frozen soil moisture content (Midttømme & Roaldset, 1998;Mustamo et al., 2019). Reported bulk thermal conductivity () of the subsurface ranges from 0.05 to 2.2 (W/m K) for active layers in Alaska, Siberia and the QTP (Brouchkov et al., 2005;Chen et al., 2020;Romanovsky & Osterkamp, 2000) and specific heat capacity (C) ranges between 580 and 690 (J/kg K) (Chen et al., 2020;L. Liu et al., 2018). ...
... The temperature data set includes continuous daily temperature observations. The data illustrate typical behavior for freeze-thaw cycles where different stages can be distinguished (Figure 1a), namely, (a) the zero-curtain period during which pore water freezes, controlled by the release of latent heat (Putkonen, 1998;Romanovsky & Osterkamp, 2000), (b) Sub-zero temperatures, with stable thermal properties, (c) ice phase transition back to the water, dynamic thermal properties, and (d) above 0°C ground temperatures, fast response to air temperature fluctuations. Figure 1b shows the maximum active layer depth where the temperature exceeds 0°C interpolated from the observed data. ...
... d Mustamo et al. (2019). e Romanovsky and Osterkamp (2000). f Putkonen (1998). ...
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p>Permafrost has become increasingly unstable as a result of surface warming; therefore it is crucial to improve our understanding of permafrost spatiotemporal dynamics to assess the impact of active layer thickening on future hydrogeological processes. However, direct determinations of permafrost active-layer thermal properties are few, resulting in large uncertainty in forecasts of active layer thickness. To assess how to reduce the uncertainty without expanding monitoring efforts, a total of 1,728 numerical 1D models were compared using three error measures against observed active layer temperature data from the Qinghai-Tibetan Plateau. Resulting optimized parameter values varied depending on the error measure used, but agree with reported ones: bulk volumetric heat capacity is 1.82–1.94 (Formula presented.) K, bulk thermal conductivity 1.0–1.2 W/m K and porosity 0.25–0.45 (Formula presented.). The active layer thickening rate varied significantly for the three error measures, as demonstrated by a (Formula presented.) years thawing time-lag between the error measures over a 100 years modeling period.</p
... The higher ice content limits thaw penetration owing to the greater latent heat demand of phase changes (Harris et al., 2009). In autumn, the wetter soil (related to the higher ice content thawed in summer) might weaken the freezing response due to the large latent heat content of soil moisture (Romanovsky and Osterkamp, 2000). Therefore, the high ground ice (soil water) content could typically buffer the permafrost area against thaw (freeze) due to thermal inertia. ...
Article
Arctic permafrost surface freeze–thaw (FT) changes related to warming could regulate the magnitude of global warming by altering the terrestrial carbon cycle and energy balances. This study investigated the sensitivity of surface FT changes to warming over Arctic permafrost regions by analyzing long-term changes in surface FT phenology from satellite remote sensing and meteorological variables from the climate data for the period from 1979 to 2017. Averaging over the entire Arctic permafrost regions, spring thawed date apparently advanced by −2.05 days decade⁻¹, whereas autumn frozen date showed weak delaying trend of 0.83 days decade⁻¹, implying the lengthening of the thawed season. Dividing the regions by permafrost types, advancing trends of thawed dates in continuous and high ice content permafrost areas (−2.57 and −2.70 days decade⁻¹) were stronger than those over the discontinuous and low ice content permafrost areas (−1.61 and −1.73 days decade⁻¹). The difference in changes in spring thawed dates between the regions is attributed to the difference in absolute magnitude of warming trends (e.g., 0.72 °C decade⁻¹ for continuous vs. 0.44 °C decade⁻¹ for discontinuous). However, the temperature sensitivity over discontinuous (low ice content) permafrost areas was 23% (10%) stronger than that over continuous (high ice content) permafrost areas for thawed date. In case of autumn, delaying trends of frozen dates were smaller over continuous and high ice content areas (0.69 and 0.74 days decade⁻¹) than those over discontinuous and low ice content areas (1.01 and 0.88 days decade⁻¹). This is mainly explained by the difference in temperature sensitivity (e.g., 1.57 days °C⁻¹ for continuous vs. 2.18 days °C⁻¹ for discontinuous) to warming between the regions rather than the difference in the absolute warming trends between the regions (e.g., 0.91 °C decade⁻¹ for continuous vs. 0.51 °C decade⁻¹ for discontinuous). The stronger temperature sensitivity of discontinuous and low ice content permafrost could be related to the lower demand of latent heat for the phase change of ground ice (or water). Overall, our results suggest that discontinuous and low ice content permafrost are more vulnerable to atmospheric warming. In addition to the magnitude of warming, the sensitivity to warming also needs to be considered when predicting permafrost FT changes.
... In contrast, the ICBM is simulating microbial CO 2 production only at temperatures above −10°C. Between about −2°C and −5°C the water availability in the soil sharply decreases (Romanovsky & Osterkamp, 2000), resulting in a drop in microbial activity (Tilston et al., 2010), which becomes practically negligible below −10°C (e.g., Mikan et al., 2002;Natali et al., 2019;Schaefer & Jafarov, 2016). CO 2 fluxes may continue even below −10°C without microbial CO 2 production, e.g., by CO 2 diffusion along the concentration gradient between the soil and the atmosphere. ...
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The decomposition of thawing permafrost organic matter (OM) to the greenhouse gases (GHG) carbon dioxide (CO2) and methane forms a positive feedback to global climate change. Data on in situ GHG fluxes from thawing permafrost OM are scarce and OM degradability is largely unknown, causing high uncertainties in the permafrost‐carbon climate feedback. We combined in situ CO2 and methane flux measurements at an abrupt permafrost thaw feature with laboratory incubations and dynamic modeling to quantify annual CO2 release from thawing permafrost OM, estimate its in situ degradability and evaluate the explanatory power of incubation experiments. In July 2016 and 2019, CO2 fluxes ranged between 0.24 and 2.6 g CO2‐C m⁻² d⁻¹. Methane fluxes were low, which coincided with the absence of active methanogens in the Pleistocene permafrost. CO2 fluxes were lower three years after initial thaw after normalizing these fluxes to thawed carbon, indicating the depletion of labile carbon. Higher CO2 fluxes from thawing Pleistocene permafrost than from Holocene permafrost indicate OM preservation for millennia and give evidence that microbial activity in the permafrost was not substantial. Short‐term incubations overestimated in situ CO2 fluxes but underestimated methane fluxes. Two independent models simulated median annual CO2 fluxes of 160 and 184 g CO2‐C m⁻² from the thaw slump, which include 25%–31% CO2 emissions during winter. Annual CO2 fluxes represent 0.8% of the carbon pool thawed in the surface soil. Our results demonstrate the potential of abrupt thaw processes to transform the tundra from carbon neutral into a substantial GHG source.
... Körner and Paulsen's (2004) analysis of soil temperature did find that subarctic and boreal treelines match an isotherm between 6 and 7°C (global mean ± SD of 6.7 ± 0.8°C), but their sample of northern treelines was small (three sites), and the authors used seasonal mean soil temperature to estimate mean GSAT. Although this method may be accurate at lower latitudes, mean seasonal soil temperature at boreal treelines can be much colder than GSAT due to permafrost and associated high soil water content (Romanovsky andOsterkamp 2000, Sullivan et al. 2015). More extensive sampling of treelines is needed, particularly in boreal regions, to assess their climatic attributes and to understand their causal factors. ...
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Understanding the key mechanisms that control northern treelines is important to accurately predict biome shifts and terrestrial feedbacks to climate. At a global scale, it has long been observed that elevational and latitudinal treelines occur at similar mean growing season air temperature (GSAT) isotherms, inspiring the growth limitation hypothesis (GLH) that cold GSAT limits aboveground growth of treeline trees, with mean treeline GSAT ~6–7°C. Treelines with mean GSAT warmer than 6–7°C may indicate other limiting factors. Many treelines globally are not advancing despite warming, and other climate variables are rarely considered at broad scales. Our goals were to test whether current boreal treelines in northern Alaska correspond with the GLH isotherm, determine which environmental factors are most predictive of treeline presence, and identify areas beyond the current treeline where advance is most likely. We digitized ~12 400 km of treelines (>26 K points) and computed seasonal climate variables across northern Alaska. We then built a generalized additive model predicting treeline presence to identify key factors determining treeline. Two metrics of mean GSAT at Alaska's northern treelines were consistently warmer than the 6–7°C isotherm (means of 8.5°C and 9.3°C), indicating that direct physiological limitation from low GSAT is unlikely to explain the position of treelines in northern Alaska. Our final model included cumulative growing degree‐days, near‐surface (≤1 m) permafrost probability and growing season total precipitation, which together may represent the importance of soil temperature. Our results indicate that mean GSAT may not be the primary driver of treeline in northern Alaska or that its effect is mediated by other more proximate, and possibly non‐climatic, controls. Our model predicts treeline potential in several areas beyond current treelines, pointing to possible routes of treeline advance if unconstrained by non‐climatic factors.
... Sites with colder minimum temperatures tended to have warmer maximum temperatures in tall shrub tundra. It is likely that sites with warmer minimum temperatures and colder maximum temperatures could experience prolonged periods of freeze/thaw cycles due to high water content in the soil (Outcalt et al 1990, Romanovsky andOsterkamp 2000). This indicates that soil thermal characteristics and snow that influence coupling with air temperature are likely a driving factor in seasonal soil temperature amplitude (Nicholson 1979, Smith andRiseborough 1996). ...
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Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (>40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw.
... At this depth, NGS soil moisture in the Mer Bleue Bog remains quite low (typically less than 0.15 m 3 m −3 , Supplementary Fig. S1) because of the relatively good drainage of the surface peat. Values of~0.04 m 3 m −3 occur when peat at 20 cm occasionally freezes indicating a continued availability of unfrozen water during winter, likely as thin films surrounding organic soil particles 58 . The positive and nonlinear correlation between soil moisture content and NGS-NEE at Mer Bleue ( Fig. 2) agrees with similar positive relationships reported by Hirano 24 , Liptzin et al. 25 , and Schindlbacher et al. 59 . ...
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Peatlands are important ecosystems that store approximately one third of terrestrial organic carbon. Non-growing season carbon fluxes significantly contribute to annual carbon budgets in peatlands, yet their response to climate change is poorly understood. Here, we investigate the governing environmental variables of non-growing season carbon emissions in a northern peatland. We develop a support-vector regression model using a continuous 13-year dataset of eddy covariance flux measurements from the Mer Blue Bog, Canada. We determine that only seven variables were needed to reproduce carbon fluxes, which were most sensitive to net radiation above the canopy, soil temperature, wind speed and soil moisture. We find that changes in soil temperature and photosynthesis drove changes in net carbon flux. Assessing net ecosystem carbon exchange under three representative concentration pathways, we project a 103% increase in peatland carbon loss by 2100 under a high emissions scenario. We suggest that peatland carbon losses constitute a strong positive climate feedback loop.
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Talik formation has long been acknowledged as an important mechanism of permafrost degradation. Currently, a lack of in situ observations has left a critical gap in our understanding of how ongoing climate change may influence future sub-aerial talik formation in areas unaffected by water bodies or wildfire. Here we present in situ ground temperature measurements from undisturbed sub-aerial sites across the discontinuous permafrost zone of Alaska between 1999 and 2020. We find that novel taliks formed at 24 sites across the region, with widespread initiation occurring during the winter of 2018 due to higher air temperatures and above-average snowfall insulating the soil. Future projections under a high emissions scenario show that by 2030, talik formation will initiate across up to 70% of the discontinuous permafrost zone, regardless of snow conditions. By 2090, talik in areas of black spruce forest, and warmer ecosystems, may reach a thickness of 12 m. The establishment of widespread sub-aerial taliks has major implications for permafrost thaw, thermokarst development, carbon cycling, hydrological connectivity and engineering. Temperature observations from across Alaska show widespread talik formation in the discontinuous permafrost zone due to higher air temperatures and above-average snowfall in recent years.
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A series of radial splitting tests were conducted on frozen subgrade soil to systematically study the effects of sample size, temperatures, ice contents, loading rates, and prefabricated cracks on the tensile deformation and failure behaviors. Based on extensive experimental results including testing data and photos during the whole loading process, a typical splitting load-splitting displacement model for frozen soil was established which can be employed to visually analyze the influencing mechanism for tensile deformation and failure behaviors under various loading conditions. Testing results indicate that the tensile deformation and failure behaviors of frozen samples are strongly affected by ice content, loading rate, and prefabricated cracks. The evaluations of load–displacement curve and crack propagation show diverse property features under different conditions of ice contents and loading rates. The influencing mechanism of the temperatures on the tensile strength behavior of frozen soil is analyzed in detail and the temperature effect on tensile strength is not influenced by loading rates. The crack initiation and propagation in the frozen sample are directly influenced by the prefabricated cracks and show different failure features at various inclination angles and coupling angles. Frozen subgrade soil exhibits obvious rate-dependent failure and deformation behaviors. Furthermore, an empirical formula was proposed to predict the long-term tensile strength of frozen subgrade soil and this formula is capable of giving a good description of the attenuation characteristics of tensile strength as the development of time. This research can provide useful insights into the strength mechanism for frozen subgrade soils.
Thesis
Snow is one of the most vital cryospheric components owing to its wide coverage as well as its unique physical characteristics. It not only affects the balance of numerous natural systems but also influences various socio-economic activities of human beings. Notably, the importance of snowmelt water to global water resources is outstanding, as millions of populations rely on snowmelt water for daily consumption and agricultural use. Nevertheless, due to the unprecedented temperature rise resulting from the deterioration of climate change, global snow cover extent (SCE) has been shrinking significantly, which endangers the sustainability and availability of inland water resources. Therefore, in order to understand cryo-hydrosphere interactions under a warming climate, (1) monitoring SCE dynamics and snowmelt conditions, (2) tracking the dynamics of snowmelt-influenced waterbodies, and (3) assessing the causal effect of snowmelt conditions on inland water resources are indispensable. However, for each point, there exist many research questions that need to be answered. Consequently, in this thesis, five objectives are proposed accordingly. Objective 1: Reviewing the characteristics of SAR and its interactions with snow, and exploring the trends, difficulties, and opportunities of existing SAR-based SCE mapping studies; Objective 2: Proposing a novel total and wet SCE mapping strategy based on freely accessible SAR imagery with all land cover classes applicability and global transferability; Objective 3: Enhancing total SCE mapping accuracy by fusing SAR- and multi-spectral sensor-based information, and providing total SCE mapping reliability map information; Objective 4: Proposing a cloud-free and illumination-independent inland waterbody dynamics tracking strategy using freely accessible datasets and services; Objective 5: Assessing the influence of snowmelt conditions on inland water resources.
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Rapid Arctic warming is causing permafrost to thaw and exposing large quantities of soil organic carbon (C) to potential decomposition. In dry upland tundra systems, subsidence from thawing permafrost can increase surface soil moisture resulting in higher methane (CH4) emissions from newly waterlogged soils. The proportion of C released as carbon dioxide (CO2) and CH4 remains uncertain as previously dry landscapes transition to a thawed state, resulting in both wetter and drier microsites. To address how thaw and moisture interact to affect total C emissions, we measured CH4 and CO2 emissions from paired chambers across thaw and moisture gradients created by nine years of experimental soil warming in interior Alaska. Cumulative growing season (May–September) CH4 emissions were elevated at both wetter (216.1–1,099.4 mg CH4‐C m⁻²) and drier (129.7–392.3 mg CH4‐C m⁻²) deeply thawed microsites relative to shallow thaw (55.6–215.7 mg CH4‐C m⁻²) and increased with higher deep soil temperatures and permafrost thaw depth. Interannual variability in CH4 emissions was driven by wet conditions in graminoid‐dominated plots that generated >70% of emissions in a wet year. Shoulder season emissions were equivalent to growing season CH4 emissions rates in the deeply thawed, warmed soils, highlighting the importance of non‐growing season CH4 emissions. Net C sink potential was reduced in deeply thawed wet plots by 4%–42%, and by 3.5%–8% in deeply thawed drier plots due to anaerobic respiration, suggesting that some dry upland tundra landscapes may transition into stronger CH4 sources in a warming Arctic.
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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.
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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
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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
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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).
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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.
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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.
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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.