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Figure 11 Detail of the temperature differences between ·the top and base of the taffoni together with an example of a rate of change of temperature ~2°C min^-I.

clearly showed that these terraces only occurred in an arc from north through to west/south-west (010°

through 360° to 250°) and withnoneorientated to the east (045° through 90° to 200°). As the terraces occurred along lithologic junctions on horizontally bedded sediments that outcropped on all aspects of the nunatak, their occurrence must be related to process. Further, where the data outlined above were collected (in the valley bottom) there are no terraces, probably as a result of not having been exposed for sufficient time (by glacier retreat). With elevation up the side of the nunatak so there is a specific size distribution of these features, with the largest being found at the highest elevations (i.e.

furthest from the retreating glacier). The implica- tion is that the process association, or at least the weathering component of that process association, that is responsible for the terraces is operative, as a function of aspect, in the valley bottom. As snow is almost non-existent (see discussion above) and was certainly not seen on the valley wall at the start of summer, it is unlikely to be freeze-thaw weathering within nivation (a process normally cited as

operative in cryoplanation terrace formation) that is a major weathering component. As an aside, it could be suggested that the main transport component(s) of the process association responsible for the terrace formation (soliftuction and wash) results from the small body of snow that begins to accumulate (as seen in the field) along the riser of the terraces. Significantly though, the riser is only present once cutting back by weathering (of some form) and removal of the products has taken place.

This is very important for, on the eastern and southern aspects weathering has not yet developed a riser; consequently terraces are not found on those aspects despite uniformity of rock outcrop.

Whatever the process synergy and sequence, it seems clear that the indirect weathering evidence supplied by Schmidt hammer rebound values and the size and distribution of taffoni indicates a preferential aspect-controlled weathering regime that is highly unlikely to be dominated by freeze- thaw. This is contrary to nearly all discussions regarding the perceived formation of cryoplanation terraces Hall.

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Figure 12 To show the variation of temperature with depth (from the surface down to 40 mm) based on two minute recordings.

One important aspect of weathering in this area that has not been considered is that of salt weathering. Being a "dry valley", with all the associated limitations on water and the high evaporative conditions, there is every expectation of salt weathering being operative. This is all the more so as thick layers of gypsum were found on some exposures, especially at the undercut bases of rock outcrops where snow was found to reside (owing to the protection from the sun). The large diurnal temperature ranges monitored for the rock surfaces indicate great potential for the thermal expansion and contraction of the salts in cracks and pores near the surface of the rock. Further, the limited but occasional wetting of the surface zone of the rock would also facilitate hydration expansion and contraction of those salts. Thus, there is every expectation that salt weathering is operative in the valley and, potentially, on the rest of the nunatak as salt effiorescences were observed on the

treads of some terraces. However, beyond the identification of the dominant deposit (gypsum) no other data or information are available and so it is not possible to quantify or more rigorously interpret the role of salt weathering.

Antarctic Weathering

In the Antarctic, as with most cold regions (see Hall, 1995), it is frequently argued that the cause of rock breakdown is a result of freeze-thaw weathering although, as van Autenboer (1964) states in the -opening paragraph, many Antarctic workers are not so sure of its efficacy owing to the widespread aridity of the continent. It is worth noting three examples of early Antarctic studies that cite other than freeze-thaw as the operative mechanism for rock breakdown in this environment. First, Bernacchi (1901, p. 107) writes, 'The

Permafrost and Periglacial Processes, Vol. 8: 69-90 (1997)

daily change in temperature caused the porous volcanic rocks of the cliffs to alternately expand and contract, and the rapid nocturnal contraction produced such a superficial strain as to cause the surface to crack, peel off in irregular pieces, and fall." Second, Shackleton (1909, p.294) states,

"Such a diurnal range of temperatures, combined with the effects of summer thaw followed by the severe frosts of winter, exerts a powerful disrupting force upon the rocks." Finally, Gunn and Warren (1962, p. 60), in a discussion of the breakdown of fine-grained rocks, note that, "Although experi- mental evidence suggests that the forces exerted by rapid temperature change are too small to fracture rocks ... it is difficult to find an alternative explanation for some ... features." Others who reference the role of thermal fatigue/shock include Avsyuk et al. (1956), Markov and Bodina (1961), Stephenson (1966), McCraw (1967), Black (1973), Miotke (1980; 1982; 1984), Aleksandrov and Simonov (1981), Robinson (1982), Miotke and von Hodenberg (1983), Bliime! (1986), Brunk (1989) and Picciotto et al. (no date). In fairness, the list of authors who refer to the (suspected) role of freeze-thaw is substantially greater! Of this latter group, many do provide good evidence in favour of freeze-thaw where the prime constraint is not temperature but rather the presence of water.

Many of the early explorersdidnote the occurrence of water on the rock; for example, Armitage (1905, p. 172) states, "Water could be heard trickling down amongst the rocks; it had formed a pond at the base of the cliffs about 4 inches deep" and observes that the rocks in this area were "exceed- ingly weathered". The key here is not that water is totally absent but rather, as Scott (1905, p.141) observed, that thawing only lasts for a very short time (about three weeks in his observations) and is highly spatially constrained; these same points are more recently reiterated by Balke et al. (1991) in respect to chemical weathering in the Antarctic where they too note the only limitation on this process is the temporal and spatial availability of moisture.

The data here presented tend to support the notion, outlined above, that the main limitation on freeze-thaw weathering in Viking Valley is not thermal but rather moisture. The winter data indicate that snow was limited and transient, as suggested by visual observation on entering the valley at the start of summer. Thus, even though the rock showed a significant number of freeze- thaw cycles they were of no (cryoclastic) effect as there was little or no water available in the rock to

freeze. Any water which may have been present could have been, owing to its highly limited nature, only in a superficial surface layer and thus could not explairi the larger-scale breakdown of the rock observed in the area. Thus, the qualitative judgement of the role of freeze-thaw in this area (and comparable locations) is spurious and not supported by the data. Nonetheless, the rock does break down and shows a markedaspect~controlled

weathering effect. The winter data, at one hour intervals, are probably not adequate for an accurate determination of weathering.

Consideration of the two minute data record suggests processes such as thermal stress and/or thermal shock are highly possible. Certainly the available record shows the occurrence of changes of temperature(~2QC min -I)of a rate thought to cause thermal shock. Other data under analysis from another site nearby, where measurements were taken at one minute intervals, certainly supports the occurrence of such temperature changes. In addition, where salts are present these fluctuations in temperature are likely to be causing the expansion and contraction of salt crystals/

accumulations. Such changes, occurring as they do with such frequency during the latter part of the summer, might be very important for salt weathering. Even the winter fluctuations at sub-zero temperatures could possibly produce a varying stress field, owing to the expansion and contraction of the salts relative to that of the rock, that could cause weathering. This salt weathering is, though, spatially constrained to the occurrence of the salts and this was not ubiquitous but rather seemed to be concentrated at certain locations within the valley. For instance, no visual indication of the presence of salts could be found in the taffoni but gypsum in layered accumulations up to 29 cm thick was found at the undercut base of some outcrops.

The main question that these data suggest is, why is there evidence of such extensive weathering on the northern and western aspects but not on the eastern? This question is fuelled by the separate finding of the preferred orientation of the terraces higher up on the nunatak that complements this aspect-controlled weathering in the valley bottom.

From the preceding discussion it was suggested that the east side had the greatest number of (thermal) freeze-thaw cycles but that, as no water was present, they were not effective. The other aspects received fewer of these thermal events but as they too were devoid of moisture this was of no consequence. The role of freeze-thaw could thus

Permafrost and Periglacial Processes. Vol. 8: 69-90 (1997)

be discounted for other than a highly superficial effect affecting only the extreme outer shell of the rock where, as a result of surface ice extrusion during freezing, it is likely of limited geomorphic impact. Conversely, the eastern aspect was found to have "warm, stable" conditions whilst the north and west had more varied temperatures, with larger and more frequent oscillations. Thus, it is possible that processes such as thermal stress fatigue, thermal shock and salt weathering (where salts are present) could be more operative and effica- cious on the northern and western aspects.

It should be pointed out that no obvious signs of chemical weathering (e.g. weathering rinds) could be found anywhere in the area, nor were there any indications of chasmoendolithic organisms that might affect weathering. As another observation (which will be dealt with in detail elsewhere), it was extremely noticeable in the field that the light~

coloured sandstones were weathering in a very

"rounded" form whilst the dark-coloured sand- stone was weathering to a very angular form. The light-coloured sandstones exhibited this roundness not only on loose clasts but also on bedrock, along the bedding and jointing intersections, on both horizontal and vertical surfaces. Further, at a number of sites the two lithologies could be seen only centimetres apart, along a lithologic junction, and yet the two weathering manifestations were quite distinct; the conditions to which they were subjected having, at these places, to be identical.

The taffoni developed in the light-coloured sand- stone exhibited this "smooth, rounded" character.

The implications of these weathering responses are also now under investigation and this too may help in understanding the overall nature of weathering in the region. The palaeoenvironmental implications of such differences are also marked, for whilst the angular debris would have readily been identified as "frost-produced", in most studies the same cannot be said for the rounded clasts (see Hall, 1995 for discussion) and yet here they are contemporaneous and in close juxtaposition.

It is clear that there is some form of aspect- controlled weathering, and some indication of what processes mayor may not be active is available, but it is far from clear exactly what the actual causes of breakdown are and exactly when and how they operate. Based upon these findings new data acquisition will soon be initiated to gather year round temperatures and another field season is expected in which an attempt will be made to obtain other data which will help solve the many problems outlined above and to integrate the

findings into the explanation for the terraces and the other resultant weathering forms.

Weathering in General

From the above study a number of generalizations can be derived that may apply to cold region weathering studies and the interpretation of cold region (past and present) sediments. First, it is clear (as other authors have suggested) that the air temperature is not a surrogate for rock temperatures and that, in the context of freeze-thaw, the threshold value for the freeze cycle can have a large impact on the number of effective cycles with the arbitrary use of 0 °C over-emphasizing the number of events. There is also a marked aspect effect on the nature of the thermal environment experienced by the rock. Second, it is highly subjective to presume freeze-thaw weathering just because it is a

"cold region": the evidence here indicates that this is an unlikely process and yet the rock is still weathering. Third, as an adjunct to the preceding statement, other processes, particularly thermal stress fatigue, need more recognition. Fourth, for any meaningful judgement of process (of any kind) there is a need for temperature recording at very frequent intervals (one minute seeming to be optimal). Fifth, the monitoring of rock moisture content is a necessity and in its absence some indirect observation (e.g. via ultrasonics) is neces- sary. Without the adjunct of moisture with temperature the ability to determine and examine weathering processes is impossible. Sixth, clast shape is no (apparent) indicator of weathering process and hence no indicator of present or past climatic conditions. Itcertainly isnot an indicator of process (e.g. angular clasts can be produced by a whole range of processes other than freeze-thaw).

Lastly, aspect-controlled weathering can have an effect on larger landforms, such as terraces, but the terraces themselves do not indicate process. Again, this has implications for palaeoenvironments where frost action is presumed to be associated with cryoplanation terrace formation.

Taken as a whole, it would seem that no one process is responsible for the weathering. Rather, as in most situations, there is a synergistic combination of different processes that cause the breakdown and, although not discussed here, interact with processes that remove the weathering products (e.g. especially in the case of the taffoni and in terrace formation). As Thorn has intimated on a number of occasions (e.g. 1988, p. 13) and as Permafrost and Periglacial Processes. Vol. 8: 69-90 (1997)

was recently discussed by Hall (1995), we need to re-evaluate cold region weathering processes and not simply presume freeze-thaw, particularly w.ith- out any evidence to support the contentIOn.

Freezing conditions and angular cl~sts are not adequate evidence of frost weathenng and we cannot presume processes such as that modelled by Walder and Hallet (1985) without being able to clearly show that the rock, thermal and _moist~~e conditions were met. With increased data acqUisI- tion, it is likely we will find that processes other than freeze-thaw are often responsible and that even where that process does dominate it is working synergistically with other processes such that the breakdown and landforms are the product of process combinations, not a singular entity.

ACKNOWLEDGEMENTS

The fieldwork on Alexander Island was undertaken in conjunction with a larger study run by the British Antarctic Survey. Or David Walton of BAS is sincerely thanked for allowing my participation in this project and for facilitating the collection of the winter temperature data. I would also like to thank all the BAS personnel who helped in the field and who maintained and collected the climatic data. Or lan Meiklejohn is thanked for the time he spent with me in the field and for all the help and companionship on our daily trek to the study site.

Nancy Alexander is thanked for all her advice in the production of the maps and graphs: without her help I could not have managed. Or loe Ackerrnan kindly wrote a program for the extrac- tion of temperature data at varying time intervals.

Two anonymous referees greatly helped in correct- ing errors and English in the original text and I am extremely grateful for their help and advice.

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Permafrosl and Periglacial Processes, Vol. 8: 69-90 (1997)

Dalam dokumen Mechanical weathering in cold regions with special emphasis on the Antarctic environment and the freeze-thaw mechanism in particular. (Halaman 104-110)

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