Observations of Antarctic permafrost
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The permafrost thermal regime is monitored by recording the temperature at different depths within boreholes. Traditionally, active layer measurements are performed by annual probing of the maximum thickness of seasonal thaw within a 100 × 100 m grid with a span of 10 m in each of the 121 grid points marked on the field, according to the CALM protocol (Nelson et al., 1998).
To monitor the depth of the 0°C isotherm, the temperature of the active layer is recorded at different depths, at least during the summer season.
However, in Antarctica the probing method has been tested for some years (Guglielmin, 2006) with poor results because of the coarse grain size of the main part of the terrain.
For these reasons, a practical and basic method to monitor the active layer thickness has been proposed for Antarctica. It consists in measuring at each grid point the ground temperature at different depths (2, 5, 10, 20 and, where possible, 30 cm) with a needle thermistor at one time during the period of maximum thawing. The active layer thickness is calculated by interpolating the two deeper temperature measurements (Guglielmin, 2006).
Active layer thickness depends primarily on ground surface temperature (GST) and the thermal properties of the ground, especially its ice/water content. Not all sites are sensitive to climate change, because heat convection, lateral heat advection and the thermal properties of the ground produce a “thermal offset” (Romanovsky and Osterkamp, 1995). This is the difference between mean annual GST and mean annual permafrost temperature, which can change with time, making difficult the modelling of the relationships between climate and active layer thickness.
Both the permafrost thermal regime and the active layer thickness are mainly related to air temperature (Guglielmin, 2004) and snow cover (Guglielmin, 2004, Zhang and Stammes, 1998), although the incoming radiation can be important especially on bare ground surfaces.
Vegetation cover can significantly influence the ground thermal regime by changing the snow thickness and permanence, the wind flow near the surface, and, therefore, the sensible heat and latent heat fluxes and, consequently, the net energy balance of the surface (Oke, 1987; Cannone et al., 2006; Guglielmin et al., 2007).
Permafrost and active layer monitoring network development in the last 50 years
Since the 1960s on many occasions, especially in maritime Antarctica, ground temperature has been measured for different purposes, through different protocols, in general for short periods (up to 2-3 years). In many cases only the active layer was investigated (Chambers, 1967; Kejna and Laska, 1999; Matsuoka et al., 1990; McKay et al., 1998; Nichols and Ball, 1964; Robertson and Macdonald, 1962; Sawagaki, 1995; Thomson et al., 1971; Walton, 1977; Wójcik, 1989).
Permafrost and active layer monitoring sites are located both in continental Antarctica (Victoria Land and Queen Maud Land) and in maritime Antarctica (Table 2.1). Five deep boreholes (up to 282 m) drilled during the 1970s during the Dry Valley Drilling Project (DVDP) may also be available to measure permafrost temperature (Decker, 1974).
Despite the relatively small extent of ice-free areas, the distribution of the monitoring sites is insufficient to characterise these areas, which may show large spatial variability in the active layer (see Figure 2.25 left). In continental Antarctica active layer monitoring is generally more difficult than in Arctic areas, because spatial variability is very large due to the heterogeneity of the coarse sediments as well as the high surface roughness.
The monitoring grid (100 × 100 m) located at Boulder Clay, a flat area close to the Mario Zucchelli station in Northern Victoria Land (Guglielmin, 2006), provides a striking example of the large areal variations in the active layer (Figure 2.25 left), which is mainly driven by different snow accumulation (Figure 2.25 right) influenced by surface roughness. There are only two other grids (Simpson Crags and Signy Island) but in both cases monitoring could not be repeated every year.
Future sites used in the framework of the IPY project “ANTPAS” will improve the network.
Table 2.1 – locations of the sites where active layer and permafrost (P) or only active layer (AL) temperature are recorded. The temperature measurements are carried out continously all year round for more than 2 years (C) or not (R). The number indicated between parentheses is the first year of measurement. The column P.I. indicates the Principal Investigator or the data source: 1) www.wcc.nrcs.usda.gov; 2) Guglielmin M. and Balks M; 3) Guglielmin M.; 4) Sletten R. ;5) Ramos M; 6) Chen X); 7) Guglielmin M., Strelin J., Sone T., Mori J.; 8) Sone et al., 2001; 9) Guglielmin M. and Worland R.; 10) Boelhouwers J.
The thickness of permafrost in Antarctica varies with region (Bockheim and Hall, 2002). In the McMurdo Dry Valleys of interior Antarctica (77º S, 161-166º E), it ranges from 240 to 970 m (Decker and Bucher, 1977). In North Victoria Land (74º S, 164º E), permafrost varies from 400 to 900 m in thickness (Guglielmin, 2006). Along the northeastern Antarctic Peninsula at Seymour and James Ross Island, it ranges from 15 to 180 m in thickness depending on elevation above sea level (Borzotta and Trombotto, 2003). Permafrost is sporadic in the South Shetland Islands and monitoring programmes show that it is generally continuous above c. 100-150 m asl and discontinuous lower down. Since the islands are very mountainous, most of the South Shetlands show the presence of permafrost (Ramos and Vieira, 2003). Permafrost temperatures, normally measured at a depth of 50 m, range from -14 to -24º C in continental Antarctica (Decker and Bucher, 1977). The temperature of permafrost at a depth of 10 m in NVL ranges between -12 to -17º C (Guglielmin, 2006).
The moisture content of permafrost in Antarctica is reflected by permafrost form. Whereas the gravimetric moisture content of ice-bonded permafrost averages 40%, the moisture content of dry-frozen permafrost may be <3% (Campbell and Claridge, 2006). A minimum of 6-7% moisture is required for ice bonding. There is considerable small-scale variation in moisture content of permafrost in Antarctica (Campbell and Claridge, 2006).
Although the age of Antarctic permafrost is not known, it is likely that it developed after the final breakup of Gondwana and the initiation of glaciers at the Eocene-Oligocene boundary, ca. 40 million years ago (Gilichinsky et al., 2007). Buried glacial ice in upper Beacon Valley (77.83° S, 159.50° E) may be 8 million years in age (Marchant et al., 2002).
The active layer refers to the layer of seasonal thawing. In Antarctica seasonal thawing is at a maximum in early February. The active layer varies between 5 and 80 cm in the MDVs (Guglielmin et al., 2003). The Circumpolar Active Layer Monitoring – Southern Hemisphere (CALM-S) Project is monitoring active layer dynamics at 16 sites in Antarctica, including 12 sites in the MDV (Figure 2.26), 2 in North Victoria Land, and 2 in the South Shetland Islands.
Variations in active layer thickness bear a strong relationship to fluctuations in air temperature, particularly during the summer months. For example, at Marble Point (77.4º S, 163.8º E), the seasonal thaw or active layer thickness varied from 30 cm in 2001 to 60 cm in 2002 (Figure 2.27); 2002 had unusually high summer temperatures and extensive flooding in the MDV. The gravimetric moisture content of the active layer in southern Victoria Land typically ranges between 1 and 10% (Campbell et al., 1997).
Ongoing ground temperature monitoring
Current ground temperature monitoring in Antarctica is done under the auspices of the Global Terrestrial Network for Permafrost (GTN-P). Ground temperature is being monitored in 11 boreholes in Antarctica to depths ranging from 2.4 to over 30 m, including five boreholes in the MDV, four in North Victoria Land, one in the South Shetland Islands, and one in the South Orkney Islands. Data loggers at these sites are monitoring temperature within the active layer and the permafrost. Electrical Resistivity Tomography (ERT) and Refraction Seismic Tomography (RST), and other electrical techniques are being used to detect and characterize structures containing frozen materials in Antarctica (Borzotta and Trombotto, 2003; Hauck et al., 2007).
At the 16 CALM-S sites, soil temperature is measured at approximately 10 cm increments in the upper 1 m and at lower frequencies to a depth of 7.8 m (Guglielmin et al., 2003).
PERMAMODEL, a project involving the University of Alcalá and the University of Lisbon, is studying permafrost dynamics on Livingston and Deception Islands in the South Shetland Group (Ramos and Vieira, 2003). The project entails (i) long-term monitoring of permafrost and active layer temperatures, (ii) identification of factors controlling ground temperatures, (iii) inverse modeling of climate signals from ground temperature data, and (iv) spatial modeling of permafrost distribution.
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- Hauck, C., G. Vieira, S. Gruber, J.J. Blanco and M. Ramos. 2007. Geophysical identification of permafrost in Livingston island, maritime Antarctica, J. Geophys. Res., doi:10.1029/2006JF0005444, in press.