Geothermal anomalies and ground thermal regime at Irizar Alluvial fan, Deception Island (Antarctica)

GOYANES, G.; VIEIRA, G.; CASELLI, A.; WINOCUR, D.; CARDOSO, M.; MARMY, A.; BERNARDO, I.; HAUCK, C.

Évora

Congreso; 4th European Conference on Permafrost (EUCOP); 2014

Universidade de Lisboa-Universidade de Évora

Deception Island is an active strato-volcano located in the Bransfield Strait (South Shetland Islands, Maritime Antarctica) with recent eruptions in 1967, 1969 and 1970. Subaerial fumaroles, underwater degassing and anomalous ground temperature sites are common in several areas at the island. The Irizar alluvial fan is an area located close to the Argentinean station ?Decepción? and is dominated by debris-flow, run-off and rock fall processes. The lower part of the fan enters into a lagoon. Frozen ground occurs in parts of the fan, but also a geothermal anomaly without associated surficial phenomena is present in the lower part of the fan. This study aims at understanding the spatial distribution and characteristics of the frozen terrain, as well as those of the anomalous warm terrain, in order to detect the volcanic controls on permafrost and the associated landforms. Surface and air temperatures and thaw depths have been measured. In addition, active layer-permafrost thermal regime was investigated using shallow borehole temperatures. Data shows a temperature increase with depth near the lagoon in the base of the fan, reaching 13 ºC at 80 cm depth, without permafrost occurrence. The shallow borehole located at this site shows a stable thermal stratification all year-round and only the first 20 cm react to air temperature changes. In addition, the winter satellite imagery shows both the absence of lagoon ice and snow cover near this site. In the alluvial fan and debris cones, ca. 100 m from the lagoon, frozen ground is present at ~70 cm depth as shown by mechanical probing. There, the entire shallow borehole shows a good coupling between air and ground temperatures and the thermal regime agrees with the presence of permafrost. The temperature drop and the breakdown of the thermal stratification that occurred October 2011 and November 2012 at the borehole located at the alluvial fan base deserve special attention. This phenomenon lasts for a few weeks and then the thermal stratification returns. This anomalous behaviour marks both the beginning of snow melt and active layer thaw. Thus, the cold surface water infiltrates favoured by the high permeability of volcanic debris, causing the temperature to decrease and the rupture of the thermal stratification. When the thawing of the active layer occurs upslope, water infiltrates and does not reach the area of the heat anomaly, restoring the thermal stratification at the alluvial fan base. Due to the high availability of water and the occurrence of permafrost at depth, the active layer saturation occurs at that time and debris and mud flows are triggered upslope. Results show here evidence on how volcano activity inhibits permafrost development only locally. On the other side, the development of permafrost and its active layer would be responsible for the current geomorphological dynamics.