Ninth International Conference on Permafrost ... - IARC Research

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The Effect of Soil Moisture and Ice Content on the Thermal Conductivity ofOrganic Soil Horizons Underlain By Discontinuous PermafrostJonathan A. O’DonnellDepartment of Biology and Wildlife, University of Alaska FairbanksVladimir E. RomanovskyGeophysical Institute, University of Alaska FairbanksJennifer W. HardenUnited States Geological Survey, Menlo Park, CAKenji YoshikawaWater and Environmental Research Center, University of Alaska FairbanksIntroductionPermafrost temperatures in Alaska have warmed in recentdecades in response to changing climatic conditions athigh latitudes (Lachenbruch & Marshall 1986, Osterkamp& Romanovsky 1999). In Interior Alaska, where forestsare underlain by discontinuous permafrost, mean annualground surface temperature (MAGST) often exceeds 0°C,yet permafrost can exist in a stable state (< 0°C) due tothermal offset (the difference between MAGST and meanannual permafrost surface temperature, or MAPST; Burn& Smith 1988). Thermal offset persists because the thermalconductivity of ice is greater than that of water, so soilsconduct heat more effectively in winter than in summer.Soil thermal models typically use a fixed thermalconductivity value for frozen and thawed soils. This bimodalapproach, however, may be too simplistic to accuratelypredict soil temperatures and active layer depth (Overduinet al. 2006). Thermal conductivity of organic soil horizonsvaries considerably with moisture and ice content and is acritical control on active layer depth (Yoshikawa et al. 2003).Organic horizon type, which varies in bulk density, moisturefield capacity, and extent of decomposition, may alsoinfluence thermal conductivity values. The primary objectiveof this study is to evaluate the effects of soil moisture and icecontent on the thermal conductivity of three organic horizontypes from black spruce forests of Interior Alaska.A. David McGuireInstitute of Arctic Biology, University of Alaska FairbanksIn the lab, all samples (n = 5 per site per horizon, total= 90 samples) were saturated and dried at air temperature,during which time we weighed each soil block andmeasured thermal conductivity using a KD2 Pro ThermalProperties Analyzer (Decagon Devices, Inc., Pullman,WA, USA) to generate a thermal conductivity—moisturecontent relationship for each sample from each horizontype. Measurements of thermal conductivity as a functionof ice content are in progress. In 2005, we measured VWCin the field near Hess Creek, Alaska, using ECH2O probes(Decagon Devices, Inc., Pullman, WA) in replicate plots.Following field measurements, all probes and soil blockswere calibrated (O’Donnell et al. in review).ResultsVolumetric water content (VWC) of feather moss species,such as Hylocomium splendens, is generally around 10% (byvolume) during summer months (Fig. 1). VWC of Sphagnumspp. is generally higher, ranging from 15–40% (by volume;Fig. 1).We observed strong positive and linear correlationsbetween thermal conductivity and organic VWC across all60Methods50Feather mossSphagnum spp.We conducted a laboratory experiment to examinethe effects of moisture and ice content on the thermalconductivity of organic soils collected from black spruceforests underlain by permafrost. Soil samples were collectedfrom three moderately drained and three poorly drained blackspruce forests in Interior Alaska. Organic soils were dividedinto three distinct horizons (live/dead moss, fibric, mesic/humic), that differ with respect to bulk density and extentof decomposition. Feather moss (Hylocomium splendensand Pleurozium schreberi) was the dominant moss speciesfrom the moderately drained stands, whereas Sphagnum spp.(most commonly S. fuscum) dominated forest floor cover inthe poorly drained stands.Organic VWC (%)403020100May Jun Jul Aug SepFigure 1. Seasonal variation in volumetric water content (VWC)of fibric organic matter near Hess Creek, Alaska (O’Donnell el al.in review).227
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tThermal Conductivity(W m -1 K -1 )Thermal Conductivity(W m -1 K -1 )Thermal Conductivity(W m -1 K -1 )0.60.50.40.30.20.1Live/Dead MossFeather mossSphagnum spp.0.000.620 40 60 80 100VWC (%)0.5Fibric0.40.30.20.10.000.620 40 60 80 100Mesic/Humic VWC (%)0.50.40.30.20.10.00 20 40 60 80 100VWC (%)Figure 2. Linear regressions between thermal conductivity andvolumetric water content in live/dead moss (top panel), fibricorganic matter (middle panel), and mesic/humic organic matter(bottom panel). The regressions were fit separately for feather moss(solid line) and Sphagnum (dashed line).organic horizons and moss types (range of R 2 values: 0.53–0.76; all P values < 0.0001). Increasing VWC of feathermoss from 10 to 40% increased thermal conductivity nearlythree-fold, whereas increasing VWC of Sphagnum from 10to 60% increased thermal conductivity by five-fold (Fig.2, top panel). We did not observe a very strong differencebetween feather moss- and Sphagnum-derived organicmatter (fibric, mesic/humic). On average, increasing VWCof fibric horizons by 50% resulted in a four-fold increasein thermal conductivity (Fig. 2, middle panel). Similarly,increasing VWC of mesic/humic horizons by 70% resultedin a three-fold increase in thermal conductivity values (Fig.2, bottom panel).DiscussionThermal conductivity of organic soil horizons in blackspruce forests of Interior Alaska is positively correlatedwith moisture content. Thus, during the summer, variabilityin soil moisture content will greatly influence rates of heattransfer from surface to deep soil layers (Yoshikawa et al.2003). During winter, heat fluxes through the active layer areprimarily governed by ice content. However, unfrozen watercan persist in deep peat and mineral soils during freezeupand cooling of the active layer, modifying soil thermalproperties and heat transfer (Romanovsky & Osterkamp2000). Increased unfrozen water content during wintermay reduce the thermal offset and increase the potential forpermafrost thaw.To evaluate the stability of permafrost in black sprucestands of Interior Alaska, we have developed a simplemodel to predict thermal offset (modified from Kudryavtsev1981). This approach combines field measurements of soilmoisture with laboratory measures of thermal conductivityand moisture, reported here. A comparative analysis of theobserved thermal offset values with calculated values iscurrently in progress and will be discussed.ReferencesBurn, C.R., & Smith, C.A.S. 1988. Observations of the“thermal offset” in near-surface mean annual groundtemperatures at several sites near Mayo, YukonTerritory, Canada. Arctic 41: 99-104.Kudryavtsev, V.A. (ed.) 1981. Permafrost, short edition.MSU Press (in Russian).Lachenbruch, A.H., & Marshall B.V. 1986. Changingclimate: Geothermal evidence from permafrost in theAlaskan Arctic. Science 234: 689-696.O’Donnell, J.A., Turetsky, M.R., Harden, J.W., Manies,K.L., Pruett, L.E., Shetler, G. & Neff, J.C. In review.Interactive effects of fire, soil climate and moss onCO2 fluxes in black spruce ecosystems of interiorAlaska.Osterkamp, T.E., & Romanovsky V.E. 1999. Evidence forwarming and thawing of discontinuous permafrostin Alaska. Permafrost and Periglacial Processes 10:17-37.Overduin, P.P, Kane, D.L., & van Loon, W.K.P. 2006.Measuring thermal conductivity in freezing andthawing soil using the soil temperature response toheating. Cold Regions Science and Technology 45:8-22.Romanovsky, V.E. & Osterkamp, T.E. 2000. Effects ofunfrozen water on heat and mass transport processesin the active layer and permafrost. Permafrost andPeriglacial Processes 11: 219-239.Yoshikawa, K., Bolton, W.R., Romanovsky, V.E., Fukuda,M. & Hinzman, L.D. 2003. Impacts of wildfire onthe permafrost in the boreal forests of interior Alaska.Journal of Geophysical Research 108: (D1): 8148,doi:10.1029/2001JD000438..228
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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