Background

All ice-free areas of continental Antarctica, amounting to less than 5% of the total surface area, experience permafrost conditions. The extreme cold and aridity result in permafrost of low moisture content and a limited seasonally-thawed zone at the ground surface (the active layer). The Antarctic environment thus provides an opportunity to study fundamental geomorphological processes in an environmental setting that contrasts strongly with polar regions in the northern hemisphere. The 2003/04-field campaign provided the opportunity to make first measurements of the active layer climate (temperature and moisture) and to analyze its interactions with frost action processes in sediments and weathering processes in bedrock.

To date permafrost science has largely focused on northern hemisphere environments. However the inclusion of permafrost as an integral part of the global cryosphere places a new research focus on the Antarctic environment. Basic permafrost and active layer conditions remain unknown for large parts of the continent, as does an understanding of potential responses to current climate change. The recently established Global Monitoring Network on Permafrost (GTN-P) of GCOS/WCRP gives further impetus to the need to monitor active layer and permafrost conditions. Both the thickness and temperature of the active layer are recognized to provide valuable indicator signals for climate change (Brown et al., 2000). In addition, changes in active layer conditions will impact on nunatak hydrology, cryoturbation and creep rates in sediments and weathering processes. Indirectly, these will impact on habitat conditions for plant and animal life. A core activity in this study is to establish an active layer monitoring site at Basen with possible extension to Fossilryggen and Svea, the first to be established in Dronning Maud Land. This provides the baseline da ta for process studies in the active layer.

Active layer conditions provide the opportunity for self-organizing sediment movements resulting in patterned ground and down-slope sediment movements. Frost-heave mechanisms driving the formation of sorted patterned ground and creep, their interaction with climatic parameters and process rates require detailed field investigation (Matsuoka, 2001). One of the special attributes of Antarctic terrestrial environments is the scarcity of unfrozen water. This is generally considered to place a critical constraint on cryoturbation processes in sediments. In conjunction with ongoing parallel studies in northern Sweden and the maritime Subantarctic, process studies at Basen and Svea will help identify fundamental process-climate relationships. This in turn will aid the physical modeling of such processes and help understand the direction of responses in active layer cryoturbation and sediment movement in relation to climate change.

A third focus is weathering processes in the active layer of bedrock. Research in the field of cold region weathering is at present in an important state of self-reflection. Many concepts, such as the importance of frost weathering, or indeed any form of mechanical fracturing, and the lack of significance of chemical and biological processes, are under review. Hall and Andre (2001, 2003) advocate the importance of thermal stress weathering in cold regions, while Hall et al. (in press) provide new data on biogenic weathering in cold regions. The revision in understanding of cold region weathering is closely linked to the understanding of blockfield development in such regions (van Steijn et al., 2002; Boelhouwers, 2004). Once again field observations in the contrasting climatic conditions of the Antarctic environment will be used to explore their implications for blockfield development and weathering in cold regions in general.

Objectives

This study has as its first objective to establish an active layer temperature monitoring station at Wasa in support of the GTN-P, with two additional stations to be established at Fossilryggen and Svea during the 2004/05 season. The second objective is to make a first investigation of the range of frost action landforms present and of associated ground environmental conditions. Third, a similar pilot study targets weathering forms and processes, the latter focusing especially on the role of thermal stress and biogenic weathering.

Active-layer temperature monitoring

In collaboration with FINNARP a new weather station at 13asen was extended with four temperature sensors at 2, 10, 30 and 60 cm depth, recording actual temperatures every hour. Some of the first results are presented in figure 1. Summer temperatures show that the active layer reaches a depth of just 60 cm. Conditions are mild at the surface with a high frequency of diurnal frost cycles and a high potential for unfrozen water availability both in sediment and at the rock surface. The winter record is broken d u e to lack of electrical power.

Pattemed ground forms and processes

A simple morphometrical analysis of periglacial landforms gives a first indication of the type of periglacial activity on the nunatak surfaces of Basen and Fossilryggen (figure 2). Two groups of landforms are distinguished: cracking dominated and frost-heave dominated. Thermal contraction cracks are developed in both sediment and bedrock with especially well-developed forms in sandstone at Fossilryggen (photo 1). The resulting sand wedges extend downward into the permafrost. The winter ground temperature record at 60 cm in figure 1 proves thermal conditions sufficiently severe for active thermal contraction cracking. Frost crack polygons of smaller dimensions are widespread in the till on Basen and may occur as secondary forms within contraction crack polygons. These forms are developed within the active layer and some sorting may occur. Frost-heave dominated patterned ground occurs in the form of sorted circles and polygons, with sorted steps on sloping ground (photo 2). Forms occur in the till at Basen and have dimensions characteristic for diurnal to short-seasonal frost cycles. Rather than being active in the cold season, these forms are associated with frost cycles during the warm season. The availability of unfrozen water is essential for their formation. Desiccation cracking also points towards wetting and drying cycles at the till surface and interacts with sorted patterned ground formation.

Soil temperature and moisture were monitored using data loggers with thermocouples and TDR probes to analyze the summer soil frost environment and moisture availability. Daily frost cycles occurred throughout summer at the soil surface (figure 1). Soil moisture levels were highly variable, between 1-1 7 vol%, and showed distinct diurnal fluctuations in the vicinity of the freezing front. Visual observations in patterned ground showed concentrations of refrozen meltwater in coarse borders at the permafrost table. Together with the obvious presence of icings at the base of slopes, there is ample evidence for significant moisture fluxes in the active layer at Basen. More detailed measurements are planned for the 2004/05 season in order to understand how these moisture and temperature conditions drive and constrain cryoturbation in the active layer. This will be combined with monitoring of actual movement rates.

Weathering forms and processes

Basalt surfaces at Basen and Fossilryggen have in-situ developed blockfields. Coarse, angular blocky weathering in cold regions is traditionally associated with frost weathering, but cannot account for primary fracture development as observed at these sites. Recent arguments for the role of thermal stress fatigue were tested by measuring high-frequency rock temperatures at the surface and at depth. Thermal stress potential should be regarded as high at the rock surface in the light of the arguments by Hall and Andre (2001; 2003). However critical discussion points remain concerning the role of absolute temperature, seasonal temperature fluctuations at depth and critical threshold values to overcome the tensile strength of the rock. This requires further field analysis using strain gauges and acoustic emission transducers.

A further discussion point is the observed, apparently anomalous, roundness of medium-grained basalt boulders at Fossilryggen. Traditionally associated with chemical weathering, such rounded forms have been associated elsewhere with core stone development under previous warm climates and subsequent landform preservation under cold-based ice sheets (Rea et al., 1996). In contrast, fine-grained or doleritic basalt blocks are angular, leading to the blockfields discussed above. Chemical analyses using XRD of the contrasting rocks and their weathering rinds is ongoing. 1t is clear however that the fine-grained rocks possess more stable surfaces with well-developed desert varnish coatings of biogenic iron and manganese oxides. The medium-grained rounded boulders undergo more rapid weathering by granular disaggregation and flaking. These appear the combined result of biogenic, chemical and physical stress processes under present climate conditions.

Conclusions

This field campaign has established the first active layer temperature monitoring site in Dronning Maud Land. Plans are to establish two additional sites at Fossilryggen and Svea during the 2004/05 expedition. Despite the general moisture constraints, unfrozen water is sufficiently abundant to drive frost-heave processes in the active layer. Complex moisture fluxes and patterns of refreezing of water are observed in the active layer in relation to the mechanisms driving sorted patterned ground formation. These require more detailed investigation. Third, mineralogical and textural variations in bedrock appear to dominate weathering modes and processes. Analytical results are not yet available for more specific conclusions. While thermal stress potential appears high several critical issues remain unanswered and require more detailed rock mechanical analysis.