New Data from Viking Valley, Alexander Island, Antarctica

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I: New Data from Viking Valley, Alexander Island, Antarctica

Kevin Hall

Geography Programme, University of Northern British Columbia, 3333 University Way, Prince George, BC, V2N 4Z9, Canada

ABSTRACT

A principal tenet of cold region weathering studies is that of temperature. Unfortunately, despite the appearance to the contrary, actual data are still sadly lacking in many instances. Here data are provided for the best part of two Antarctic winters plus, at two minute resolution, for one summer, from a variety of positions within a dry valley. The data clearly show the dangers of using air temperature as a surrogate for thermal conditions either at the rock surface or at depth in the rock. Although detailed rock moisture data are absent in this study, indirect evidence from both observation and non-destructive ultrasonic testing shows that water is extremely limited during the period of freeze-thaw cycles. Thus, despite the occurrence of the thermal events no damage can result from frost action. Detailed data at two minute intervals show the importance of such high resolution observations. It is argued that without data acquisition at two minute or preferably one minute intervals, it is not possible to discern the weathering regime, including interpreting the freeze-thaw process. These data show that processes such as thermal stress fatigue/shock are possible and that rates of change of temperature _~2 QC min-¹,as required for thermal shock, do occur. Measurements of taffoni size and occurrence, coupled with Schmidt hammer rebound values, show that the eastern aspect experiences the least weathering whilst the northern and western exposures have the greatest amount. This observation is in accord with separate findings of aspect-controlled orientation of "cryoplanation" terraces at higher eleva- tions. Some speculative suggestions for the cause of this aspect-controlled weathering are given, the most important of which is that it is unlikely that freeze-thaw plays any significant role - a factor that underscores its qualitative presumption in cold regions and questions the origin and development of cryoplanation forms. Beyond anything else, this paper indicates the complexity of rock temperature regimes and suggests that it is the synergistic relationships between different weathering processes that are important.

©

1997 by John Wiley & Sons, Ltd.

RESUME

On croit generalement que l'alteration des roches dans les regions fro ides est contr61ee par les fluctuations de temperatures. Malheureusement malgre les apparences, des observations precises manquent cruellement dans bien des cas. Dans le present article, des donnees sont fournies pour differentes situations dans une vallee seche de I'Antarctique pendant la plus grande partie de deux hivers, ainsi que, avec une resolution de deux minutes, pendant un ete. Les donnees montrent clairement les dangers d'utiliser les temperatures de l'air et non les temperatures observees

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surface des roches ou en profondeur. Bien que des donnees detaillees d'humidite soient absentes dans la presente etude, des evidences indirectes resultant

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la fois d'observations et de tests par ultrasons indiquent que l'eau est extremement limitee pendant la periode ou se produisent les cycles de gel-degel. De ce fait malgre l'occurrence d'evenements thermiques, aucun dommage ne peut resulter de l'action du gel. Des donnees detaillees avec un intervalle de mesures de deux CCC 1045-6740/97/010069-22$17.50

Accepted8 October /996

minutes montrent I'importance d'observations

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haute resolution. Il est demontre que sans observation avec cet intervalle, ou ce qui serait encore mieux avec des intervalles d'une minute, il n'est pas possible de discerner le regime d'alteration, et me~ed'interpreter les proces.sus de gel- degel. Ces donnees montrent que des processus commeI~fatigue(d~e.aux chocs.therml~ue;)sont possibles et que ?es vi.tesses de ch~ngements ^de te~peratu~e supeneur~s o~ egales a 2 .C p~r minute valeur necessalre pour aVOlr un choc thermlque, eXistent dans I envlronnement etudle.

Desm~suresde la taille et de l'occurrence des taffonis, mises en rapport avec les valeur obtenues avec le marteau de Schmidt montrent que les expositions

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l'est presentent le moins d'alteration tandis que les expositions au nord et

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l'ouest en subissent le plus. Cette observation est en accord avec des observations independantes sur l'orientation des terrasses de cryoplanation

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altitudes plus elevees. Quelques suppositions quant au controle de l'alteration sont donnees, la plus importante etant qu'il est improbable que le gel-degel joue un role significatif. Tout cela minimise I'importance de ce facteur dans les regions fro ides et pose des questions sur l'origine et le developpement des formes de cryoplanation. Avant tout, le present article montre la complexite des regimes thermiques des roches et suggere qu'il y a des relations importantes de synergie entre differents processus d'alteration.

©

1997by John WHey& Sons, Ltd.

Permafrost Periglac. Process..Vol. 8: 69-90 (1997).

(No. of Figs: 12. No. of Tables: 9. No. of Refs: 43)

KEY WORDS: Antarctic; weathering; thermal conditions; aspect; cryoplanation; taffoni; Schmidt hammer

INTRODUCTION

In studies of cold region weathering it is frequently

"freeze-thaw" (or one of its many synonyms) that is cited as the dominant process and, in most cases, it is air temperature that is used as the foundation for this premise. Both of these assump- tions err. First, it may well be that freeze-thaw is not the dominant process, indeed it may not even be operative. Second, air temperatures are not a surrogate for rock temperatures (Debenham, 1921;

Thorn, 1988) and thus any deductions therefrom are doubly misleading. Frequently it seems that frost action is universally considered the dominant process in polar or mountain areas. Hogbom (1914) suggested that the more rigorous the climate the more effective the frost weathering but van Autenboer (1964, p. 101), after visiting Spitsbergen, states he "was struck by the fact that in spite of a much milder climate the frost activity was considerably greater" than the more "rigorous" (i.e. colder) climate of the Antarctic. Van Autenboer goes on to say that, with respect to the Antarctic, "the extreme severity or rather the consequent aridity ... is even less favourable to the rapid evolution of landscape forms by frost action. "

In any consideration of freeze-thaw weathering, moisture and rock properties are as important as the thermal conditions (McGreevy and Whalley, 1982; Hall and Walton, 1992; Hall, 1993). How-

ever, in this present discussion it is solely the thermal component that is discussed in detail. With respect to the thermal conditions, Thorn (1980, p. 85) states "The presence of thresholds in the temperature-weathering relationship would pro- duce substantial changes in the likely zones of maximum weathering." This holds true not only for freeze-thaw weathering but for other processes as well, and it is the recognition of these thermal thresholds for processes other than freeze-thaw that is significant. Another key issue is the complexity of rock temperatures and a variety of factors must be considered in any evaluation of weathering where thresholds regarding the magni- tude of warming or cooling events, the rates of change of temperature and the thermal gradients are all important (e.g. Matsuoka, 1994).

In this study it was possible to obtain temperature data from a variety of positions and depths at relatively short time intervals (two minutes) as well as longer term data at hourly intervals. Although the latter lack the detail required for a satisfactory analysis of freeze-thaw or thermal stress fatigue (or shock) they do, nevertheless, provide extremely good insight into the character of the weathering regime in an Antarctic dry valley for the greater part of the winter. The combination, presented here, of winter rock temperature data and short time interval summer data allow for a better insight into the nature of Antarctic weathering. In addi- tion, the variety of temperature data obtained Permafrost and Periglacial Processes. Vol. 8: 69-90 (1997)

Natal Ridge SyrtIsHili Vild alley

Probe--9idee=-2....

Mars Glacier

51<m Figure 1 Location of the study area.

allows for a discussion of the interpretation of such data in weathering studies.

STUDY AREA

The study was undertaken in Viking Valley which is located on a north-south aligned nunatak (Figure I) along the north-eastern side of the

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^study^Main^Area

~ Bedrock

~ Contour

Mars Glacier at the southern end of Alexander Island (71 °50'S, 68°2I'W). This east -west orien- tated "dry valley" is situated approximately 70 m above the Mars Glacier but has a very small remnant glacier (unnamed) at its northern end between Syrtis Hill and Probe Ridge and a lake (Secret Lake) at the western end (Figure 2). The valley is very small, being less than 200 m between the lake and glacier boundaries, and roughly 40 m

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

)C Squirrel Logger

Dalam dokumen Mechanical weathering in cold regions with special emphasis on the Antarctic environment and the freeze-thaw mechanism in particular. (Halaman 88-91)