U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Extended Abstract 204 Thermal regime, isotopic and morphological characteristics of ice wedges in northern Victoria Land, Antarctica R. Raffi, 1 B. Stenni, 2 L. Genoni, 2 O. Flora, 2 and S. Sega 1 1 Department of Earth Sciences, Sapienza University, Piazzale Aldo Moro 5, 00185 Roma, I (rossana.raffi@uniroma1.it ), (simone_s@yahoo.it ) 2 Department of Geological, Environmental and Marine Sciences, Trieste University, Via E. Weiss 2, 34127 Trieste, I (stenni@univ.trieste.it ), (genoni@univ.trieste.it ), (flora@univ.trieste.it ) Summary This paper reports the results of a field survey carried out on an ice-wedge polygon site in the Terra Nova Bay area, in northern Victoria Land. Ice wedges were found at depths ranging from 27 cm to 55 cm from the ground surface. The ice wedges had a width at their top of 13 to 55 cm, and a height varying from 50 cm to over 80 cm. Petrofabric analyses were also performed on ice-wedge ice to investigate changes in fabric across wedges, in relation to the growth mechanism. Crystal size increased from the centre outward; c-axis directions showed a preferred vertical to sub-vertical orientation, parallel to the axial plane at the wedge centre, and parallel to the foliation at the boundaries of the wedges. A co-isotopic study was performed by measuring both the oxygen (δ18O) and hydrogen (δD) isotope compositions and the tritium activity. The measurements obtained for δ 18 O and δD had extremely negative d excess values, showing a strong divergence from the snowfalls expected to occur at the elevation of the site (874 m a.s.l.). Sublimation processes were taken into account to define the origin of the ice-wedge ice. A data logger, using NTC sensors, was installed to record hourly temperatures of the air, of the ground surface, and of the top and bottom of an ice wedge. A complete year of monitoring of the ice-wedge thermal regime (1 February 2004 – 31 January 2005) showed thermal gradients that could trigger ice-wedge cracking. Citation: Raffi, R., B. Stenni, L. Genoni, O. Flora, and S. Sega (2007), Thermal regime, isotopic and morphological characteristics of ice wedges in northern Victoria Land, Antarctica, in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10 th ISAES X, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007–1047, Extended Abstract 204, 4 p. Introduction Ice-wedge and sand-wedge polygons are common throughout southern Victoria Land, as studied by different authors (i.e., Péwé, 1959; Black and Berg, 1963; Berg and Black, 1966). In northern Victoria Land, ice-wedge and sand-wedge polygons were widely described by French and Guglielmin (2000). More recently new researches on ice wedges in northern Victoria Land was initiated by the authors, focusing on petrofabric, isotopic and thermometric analyses (Raffi, 2003; Raffi et al., 2004). In northern Victoria Land, polygons are widespread and occur over unconsolidated materials that locally cover ice-free areas. From 1998 to 2006, during four summer field seasons, we excavated numerous polygons, along the coast and in interior areas. Ice wedges were found in 45 sites at depths ranging from 10 cm to 74 cm from the ground surface. Their size ranged from 5 cm to 155 cm in width, and from 20 cm to over 150 cm in height. Three data loggers for monitoring ice-wedge thermal regime were installed at three selected sites: Boomerang Glacier, Baker Rocks and Mount Jackman. For this study we selected the ice-wedge polygon area, facing Boomerang Glacier and located in the vicinity of Terra Nova Bay, 13 km NNW of the Mario Zucchelli Italian Station (Fig. 1). Petrofabric and co-isotope analyses (δ 18 O and δD) of ice-wedge ice were performed, as well as one year of monitoring of the ice-wedge thermal regime. The purpose of the study was to define the processes which led to the formation of ice wedges and to determine if thermal conditions trigger ice-wedge cracking. Morphological features of ice wedges and ice crystal petrofabric characteristics The selected site is placed at 874 m a.s.l., on a slightly sloping bench facing Boomerang Glacier. McMurdo- volcanic regolith, formed by coarse angular clasts, covers the bench. The polygons had a convex surface, 10 to 15 m wide, and were bordered by shallow inter-polygon troughs, 0.20-0.30 m deep and 0.20-0.40 m wide. We excavated numerous sections across the polygon troughs to a depth of about 1.5 m, including one at the intersection of three polygons. In January 1999, 2001 and 2004, the frost table was 20-25 cm deep. Permafrost was Figure 1. Location of the study site. Satellite image by Italian Antarctic National Research Program.