Ninth International Conference on Permafrost ... - IARC Research

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