Ninth International Conference on Permafrost ... - IARC Research

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Detection of Degraded Mountain Permafrost with the Help of GPR Profiling atMesón San Juan, Mendoza, ArgentinaDario Trombotto LiaudatGeocryology Ianigla-Cricyt-Conicet, Casilla de Correo 330, 5500 Mendoza, ArgentinaJandyr Menezes TravassosObservatório Nacional, Rua General José Cristino, 77, 20921-400, Rio de Janeiro, BrazilGiovanni Chaves StaelObservatório Nacional, Rua General José Cristino, 77, 20921-400, Rio de Janeiro, BrazilThis work shows the use of ground penetration radar(GPR) for detection of superficial structures of degradationin mountain permafrost imaging in an area of the CentralAndes. Andean permafrost can be classified on the basis oftopography, hydrology (or estimated ice content), and climate(global warming) (Trombotto 2003). Continuous or quasicontinuous Andean permafrost still appears at -2 to -4°C,found on the mountain summits or as “island permafrost” ifit appears as an isolated body. Permafrost types could be alsosubclassified on the basis of more or less than 10% of groundice content (Brown et al. 1998).The study area was a permafrost plateau at Mesón SanJuan (Fig. 1, 6012 m, 33°30′S and 69°49′W), located on thefoot of the glacier and bordering recent moraines at a heightof 4400 m a result of the glaciation retreat of the MesónSan Juan summit and the consequent cryoweathering anderosion of the sediments produced by cryogenic phenomena.The Cenozoic volcanic Mesón San Juan with 6012 mare located on the Argentinean-Chilean border, south ofTupungato volcano (6570 m) and Cerro Aconcagua (6962m), the highest mountain of the Western Hemisphere, in theprovince Mendoza.The permafrost plateau at Mesón San Juan is related tothe highest parts of the mountain and represents a type ofFigure 1. Study area.cryoplanation surface of polygenetic origin on the foot ofthe glacier and bordering recent moraines at a height of 4400m. It falls as glacier-shaped rocky slopes reaching down to3600 m in the area. The plateau is a result of the glaciationretreat of the Mesón San Juan summit and the consequentcryoweathering and erosion of the sediments produced bycryogenic phenomena.The GPR data were collected with PULSE EKKO IVequipment with a time window of 2048 ns, a sampling intervalof 800 ps, a 1000 V transmitter, and 50 MHz antennae. Theantennae were moved along fixed-offset profiles with aconstant step size of 0.20 m and kept 2 m apart from eachother. We kept the same step size for the central mid-point(CMP) profiles, where each antenna was moved awayfrom another symmetrically in fixed steps of 0.10 m. Weconcentrate here on two profiles: one fixed-offset that is theresult of merging two 51 m long profiles with directions 96ºMN and 134º MN, the former along the largest dimensionof the plateau; and two CMP profiles. The two mutuallyperpendicular CMP profiles were deployed crossing at thecenter point of the first fixed-offset profile. Each CMP profilewas 31 m long, one of them having the same direction of thefirst fixed-offset profile. We also restricted the time-windowto 1024 ns, where the signal-to-noise ratio is higher. Weadopted a basic processing flux in our data set. After someediting, the time window was chopped off to earlier times,dewowed, low-pass filtered to reduce high-frequency noise,and gained with an automatic gain control.The periglacial sedimentary cover on the plateau displaysan open permeable structure. The thickness of the activelayer and the depth to the permafrost table was obtained with3 superficial holes reaching a depth of 1.20 m. One hole wasdrilled at the edge of the glaciation in an ice-covered areawith transitional sediments from the SE wall of the glacierof the Mesón San Juan, thus correlating with the morainicarea. Two holes were drilled further away on cryoplanationsurfaces bordering the present glaciation and did not reacha permafrost table, but revealed the presence of a freezinglevel without any visible ice, which was interpreted asdry permafrost. Temperatures were obtained with Westonthermometers.The CMP profiles yielded an average velocity of 0.09m/ns, and a very interesting indication of lower velocity attwo-way travel time ≥ 200 ns was discovered. There wasa clear transition at 270 ns from a more conductive andinhomogeneous horizon to a less conductive, albeit still317
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 tFigure 3. Interpretation of the subsoil at Mesón San Juan.Figure 2. Migrated section with the I-zone.cluttered, subsurface. Another important characteristic of thesection was relatively higher amplitudes seen at later timesbetween 22 and 60 m of reflectors that do not display lateralcontinuity.A migrated section, which has conspicuous smiles at latertimes inside a so-called vertical I-shaped zone located between22 and 60 m, indicated a significantly smaller phase velocitywithin it (Fig. 2). The lower velocity I-zone is surroundedlaterally by two zones of phase velocity, v = 0.09 m/ns.Moreover the tails of the diffractions in the I-zone indicatethat the conductivity is lower compared to the remainder ofthe section. We found that the I-zone is correctly migratedonly with very low velocities like the water velocity, v = 0.03m/ns for migrating the section. We supposed by weight andlow ion contents that it is a region of liquid water probablydue to the degradation of permafrost. The I-zone is toppedby a cluttered, more conductive horizon, reaching 270 ns or12 m, that encompasses the active zone formed probablymainly by till and cryogenic sediments.One explanation for the I-zone is that the segment is adischarge channel linking a suprapermafrost—a laterallydiscontinuous near-surface system not seen in this work—to a subpermafrost aquifer (Lawson et al. 1996). Mostprobably the source of that water is the retreating glacierabove the plateau that finds its way through the morainictill, which composes the active layer. Figure 3 expressesthe interpretation of the frozen and unfrozen subsoil. Weexpected to find the basement at 45–50 m, but that is beyondthe limit of our data.As already described for other areas (Trombotto et al.1998), with gradual disappearance of snow patches, theretreat of the Andean glacier fronts generates a considerableinput of melting water in the open structure of the Andeancryolithozone, which enhances erosion of suprapermafrost butalso allows the water to penetrate through the discontinuitiesof the frozen soil, thus contributing to the degradation of itsinternal structure.AcknowledgmentsWe would like to thank Dr. Alberto Aristarain (LEGAN)for his financial and logistical support, and José Hernández,José Corvalán, and Rafael Bottero for their technical support.JMT acknowledges a grant from the CNPq.ReferencesBrown, J., Ferrians Jr., O.J., Heginbottom, J.A. & Melnikov,E.S. 1998. Cicum-Artic map of permafrost andground-ice conditions. U.S. Geological Survey.Lawson, D.E., Strasser, J.C., Strasser, J.D., Arcone, S.A.,Delaney, A.J. & Williams, C. 1996. Geological andGeophysical Investigations of the Hydrogeology ofFort Wainwright, Alaska, Part I: Canol Road Area.CRREL Report 96-4, 32.Trombotto, D. 2003. Mapping of permafrost and theperiglacial environment, Cordón del Plata, Argentina.Proceedings of the Eighth <strong>International</strong> <strong>Conference</strong>on Permafrost, Zurich, Extended Abstracts: ReportingCurrent Research and New Information, W. Haeberli& D. Brandová (eds.): 161-162.Trombotto, D., Buk, E., Corvalán, J. & Hernández, J.1998. Present state of measurements of cryogenicprocesses in the “Lagunita del Plata,” Mendoza,Argentina, Report Nr. II. Proceedings of the Seventh<strong>International</strong> <strong>Conference</strong> on Permafrost, Yellowknife,Canada. Program, Abstracts, and IPA Reports,Extended Abstracts: 200-201.318
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NICOP 2008 Ninth <
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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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