Correlation between frozen ground thickness measured in Antarctica and permafrost thickness estimated on the basis of the heat flow obtained from magnetotelluric soundings

ENRIQUE BORZOTTA; DARIO TROMBOTTO

Cold Regions Science and Technology

Elsevier

Lugar: Amsterdam, The Netherlands; Año: 2004 p. 81 - 96

A comparative study of the results of vertical electrical soundings (VES) and magnetotelluric soundings (MTS) carried out at Seymour and James Ross Islands (NE of Antarctic Peninsula) is conducted. Frozen ground thickness, estimated by VES, is compared with permafrost thickness estimates, obtained using: (a) steady geothermal heat flow, inferred from the depths of conductive layers in the crust and upper mantle estimated by MTS, and (b) mean annual air temperatures (MAAT). At Seymour Island, 250 m of permafrost thickness, in equilibrium with an inferred heat flow of 72 mW/m2 (corresponding to a geothermal gradient of 0.037 8C/m) and with an MAAT of -9.4 ºC, is estimated for the upper terrace (about 200 m a.s.l.). This value is consistent with the frozen ground thickness (200 m in the upper terrace) previously estimated by VES. Magnetotelluric soundings carried out in the northwestern region of James Ross Island (volcanic island with recent activity) suggest a magma chamber with top at about 7 km depth in the crust. From the depth of this conductive body, a heat flow of 145 mW/m2 and a geothermal gradient of 0.074 ºC/m are estimated, suggesting 67 m of permafrost thickness in the upper terraces (35 m a.s.l.) of this island in equilibrium with the estimated steady heat flow and an MAAT= -5 ºC. The frozen ground thickness estimated by VES lies between 40 and 45 m for the same terrace. The differences between frozen ground thickness and permafrost thickness in both islands could be attributed to a cryopeg, according to the high salinity beneath the frozen ground suggested by MT soundings. Although studies indicate that a climatic warming process is currently taking place in the Antarctic Peninsula, results of this study suggest that, until approximately 1980, the region had stable geological and paleoclimatic conditions during a lapse of time long enough to reach an approximately steady temperature profile in the subsoil. Around 2000 years are estimated for Seymour Island and 140 years for the northwestern region of James Ross Island as the minimum periods of time during which the soil surface was free of ice. The magma chamber, possibly located in the northwestern region of James Ross Island, could have an important role in controlling permafrost thickness, conductance below the frozen ground and deglaciation.