Drake Passage
Figure I Map to show the location of Rothera. Map reproduced from Antarctic Science Handy At/as. Map 12 (1995, p. 180).
.r
1998 John Wiley&Sons. Ltd.Permafrost Perig/ac. Process. 9: 47-55 (1998)
ice-covered for the greater part of the year and the site was c. 50 m above sea level. Bedrock consisted of gabbro and granodiorite - with the temperature data presented here being recorded on a medium- grained granodiorite.
FIELD METHODS
Data were collected using a 'Squirrel' data logger and 10 bead-type thermistors. On a single rock outcrop, a thermistor was attached to the rock surface and another was located at 2 cm depth for each of the cardinal directions: thermistors were also located at the surface and 2 cm depth on the upper, horizontal surface of the rock outcrop. Data were then recorded at one-minute intervals, with downloading to a laptop computer once per day.It was not possible to use pre-drilled blocks, nor were measurements made of moisture content, ultra- sonicp-wavevelocity or Schmidt hammer rebound values as in the initial study (Hall, 1997a). The aim was to focus purely on high frequency measure- ment of the thermal events in the surface layer of the rock as anindicatorof weathering process. For comparison with the rock temperature data, daily climatological data are available from the Rothera meteorological station.
RESULTS AND DISCUSSION
The impact of aspect on rock thermal conditions was not, at least for the record period shown here, as straightforward as might be expected. For a Southern hemisphere site, it should be expected that the north-facing aspect should have the highest temperatures and the south-facing the lowest.
However, this is not the case (Figure 2). For the example shown (0000 h to 2359 h on 14 January 1993), the lowest temperature (-1.5 0c) was recorded for the northern aspect and the highest temperature (27°C) on the western aspect.
A temporal shift of influence can be seen as the sun rises and then sets, from east through north and then west, to finish with south. Although this is to be expected, the actual rock temperatures offer a surprise. The influence of direct radiation can be seen from c. 0600 h on the eastern aspect as the temperature rapidly rises from a low of O°C to achieve a maximum from c. 0730 h through to 0845 h, after which temperatures decline and return towards zero close to midnight. The temperature for the northern aspect then begins to rise around
©1998 John Wiley&Sons.Lld.
0700 h and attains its maximum near 1300 hand subsequently cools from c. 1800 h. Despite its favourable orientation, the northern aspect does not achieve the highest temperatures but experi- ences the lowest.Itwas the only aspect to have sub- zero temperatures. By contrast, temperatures on the western aspect were some 4°C higher. These began to rise around 0800 h to attain a maximum close to 1800 h. The western aspect experiences an almost asymmetric temperature profile, with a (relatively) slow rise through to the maximum and then a relatively rapid decline. The southern aspect starts to warm as air temperatures rise in late morning, and peak a short time after that of the western aspect. The horizontal surface does not have the lowest overall temperature but it does have the lowest maximum values. This is the result of never receiving radiation at other than an acute angle. Noticeably, the maximum warming of this surface is not at the time when the sun is at its highest but when the sun is in the west, the tempera- ture profile being one of radiation 'accumulation' throughout the day. As the sun drops after 1900 h so temperatures (relatively) rapidly decline.
In addition to the role of aspect, the present data exemplify the influence of record interval. The 'smoothing' effect of increasing time between records is clearly shown in Figure 3. Based on the original one-minute record interval data, it is shown what would have been recorded had the logging interval been 5, 10, 15, 30 or 60 minutes.
Figure 3 indicates a significant decrease in detail between one-minute data and that of five minutes.
There is a greater variability at the record start and end for the one-minute data. This information is lost with the five-minute record. For instance, the two drops in temperature at the record end look unidirectional and 'dramatic' on the five-minute record but the one-minute data show that there were, in fact, rises and falls within the overall decline. Information loss with increased record interval dramatically changes the perception of the thermal environment. Thus, the 30-minute data are little more than a rough guide to what actually took place. Certainly any attempt to generate values of
!J.T/t from 15- or 30-minute data would be fruitless.
Examples of the important detail loss in records other than the one-minute record interval are shown in Figure 4. This 4l-minute record shows one event with a!J.T/t value of 3 °C/min and one of 2°C/min. Both values are greater than, or equal to, the theoretical threshold (2°C/min) for thermal shock (Yatsu, 1988). Other events, while not achieving the 2°C/min boundary for thermal Permafrost Periglac. Process.9: 47-55 (1998)
North East
.-
IlIlOOhts I
South West OOOOIIn
TapSurface 00lllIIn
o..,...c
.. .- .:ZO.--.--.: : : .
'.: : :: :'.~~--.:.--:.::':".'::.
Figure 2 Temperature data from ()()()() h to 2359 h for 14 January 1993 for each of the aspects plus the top surface.
shock, nevertheless indicate values ~I °C/min that must exacerbate thermal stress fatigue. In the 41-minute record shown here there are eight such events with a further 16 of 0.5 cC/min. Several of these involve consecutive positive and negative temperature changes each of 0.5 °C/min magnitude.
At depth (Figure 5), the largest temperature differential (between the surface value and that
©1998 John Wiley&Sons. Ltd.
recorded at c. 2 cm depth) is found on the eastern aspect (±120c) and the smallest (±40c) on the southern. Although it is clear from Figure 5 that the surface fluctuations are largely damped out by . 2 cm depth there are still variations of the order of I to 1.5 °C/min at times. By obtaining high fre- quency data one can demonstrate that values of I1T/t are such that thermal shock could occur.
Permafrost Perig/ac. Process. 9: 47-55 (1998)
. . . . _ . _",,-_.~..r . . . . _. -.~.~~/-- _. . -
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Figure 3 Temperature data for the period 1656 h on 15 January to 1639 h on 16 January 1993 to show, based on the original one·
minute data, what would have been recorded with collection times of 5, 10, 15,30 and 60 minutes.
Rapid thennal fluctuations produce transient high thennal gradients in the outer layer that facili tates thennal spal1ing (Thirumalai, 1970).
The surface and 2 cm depth rock temperature data show that complex stress fields must occur in the outer shel1 of the rock. For example, the southern aspect shows the 2 cm level is wanner than the rock surface at the record start. The temperature differential of c. 4cC indicates that the rock at
depth is experiencing greater expansivity than the surface. Later, there is a crossover and the surface is wanner than the 2 cm level: at this point, the exterior is expanding while the interior is contract- ing (its temperature having declined from the higher, earlier value). The resultant differential stresses could play an important role in aspect- constrained weathering. Without this one-minute data it would not be possible to attribute any
©1998 John Wiley&Sons. Ltd. Permq[rost Periglac. Process. 9: 47-55 (1998)
29 , . . . - - - ,
: : 0 0
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Figure 4 Small section of the data presented in Figure 3 (one- minute data) to show the rapid thermal changes that can occur at the rock surface.
observations of aspect influence on weathering to these thermal fatigue effects.
The north-facing record (Figure 5) may indicate other factors. Here, in the example shown, the rock at 2 cm depth stayswarmerthan the surface for the greater part of the day. The effect did not occur all the time and surface temperatures show the typical rapid changes associated with the rock surface while those at 2 cm depth show the expected dampening.
Consideration of the rest of the records for the northern aspect shows that, while the same effect didoccur on other days, it did not occur every day.
On some days (e.g. 20 to 22 January), the surface temperatures were warmer than the interior, and on other days (e.g. 25 to 26 January) the thermal conditions were mixed, with the interior warm but with significant periods when the surface was warmer than the interior.
These apparently random thermal fluctuations are possibly related to the effect of the wind. A number of Antarctic biological studies (e.g. McKay and Friedmann, 1985; Kappen et aI., 1981;
Friedmann etal., 1987) have collected temperature data at intervals of one minute or less on rock surfaces and at depths to c. 4 cm for different aspects. Modelling of thermal changes as a function of temperature, wind speed and internal light gradients was subsequently undertaken by Nienow (1987). The models suggest that the observed temperature oscillations at the rock sur- face can be caused by wind fluctuations of short duration and can be effective to a depth of c. 4 mm.
Other factors such as heating at depth by light penetration (due to translucent minerals) may also play a role although Nienow (1987) did not
©1998 John Wiley&Sons. Ltd.
consider this significant for the sandstone that he studied. Although the effect of wind offers some degree of explanation for the observed effects at Rothera, more data are needed to elucidate why the effects were only observed on the horizontal and northern surfaces.
The Rothera data also show distinct differences between the air and the rock temperatures (Figure 6). The air temperature graph is based on five-minute record intervals. The diurnal cycle is very clear. It is also very clear that maximum temperatures for the air (~70c) are substantially lower than those for the rocks (e.g. see Figure 2,
~27 cC; or Figure 4, ~28.50C). Equally, the minima for the rock can be lower than that of the air although the differences here are much less than for the maxima: Figure 2 shows the northern aspect going to -1°C whilst the air does not go lower than +0.3°C. For the other aspects shown in Figure 2, although the air went to -0.3 QC, none of them went below O°c. The most important factor here is that rock temperatures are substantially higher than those of the air and they show many and varied fluctuations. Thus, in terms of weath- ering, the air temperature data are all but useless for any interpretation of the weathering regime despite the recent arguments by such as Arnold etal. (1996).
IMPLICATIONS FOR WEATHERING
In this paper, the western and northern aspects exhibited extremes, with the north having the lowest temperatures and the west the highest.
When the 2 cm data are considered, the eastern aspect has the largest surface to depth temperature differential and thus the greatest potential for stress-induced (as a result of the thermal gradient) weathering. The higher 2 cm depth temperature than the surface for the northern aspect and the horizontal surface may have implications for weathering. Interestingly, further analysis of the thermal data referred to in Hall (1997a) failed to show the same effect on the northern exposure although the values were very close. It is possible that there is a temporal shift for this effect and that it is negligible during the summer but increases through the autumn towards the winter. Equally, it may reflect a wind effect. Accepting that thermal maxima can occurbelowthe surface (as also argued from the biological studies) then this does offer a probable weathering effect. That maximum expan- sion could occur below the surface would help Permafrost Periglac. Process.9:47-55(1998)
25,---";;iiii~jl 20
15
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,--
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-a..L---~---'
,--
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5
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,--
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15
5
o
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Figure 5 Data for the period 1639 h on 13 January to 1631 h on 14 January 1993 for the different aspects plus top surface to show the surface (thin line) and 2 cm depth (thick line) temperature curves.
explain spalling of the rock. This form of weath- ering could be particularly effective in dry areas where water-based processes are minimized.
The data presented here justify the questions posed by Thorn (1988; 1992) and White (1976) who have both questioned the ubiquitousness of freeze-thaw. The data presented here offer
©1998 John Wiley&Sons. Ltd.
solutions/causes other than freeze-thaw. In the case outlined both here and in Hall (1997a; 1997b), thermal stress fatigue or thermal shock are identified as causes worthy of detailed examina- tion; frost weathering is unlikely owing to the aridity of the study areas. Other studies suggest other mechanisms, e.g. wetting and drying (Hall, Permafrost Periglac. Process.9: 47-55 (1998)
8 . - - - ,
Air Temperatures
12Januaryto 28January1993
Figure 6 Air temperature record (five-minute intervals) for Rothera station.
1991; 1993) or biological agencies (e.g. Hall and Otte, 1990). None of this negates the occurrence of freeze-thaw but rather questions its common acceptance and explanation for landforms.
CONCLUSIONS
The information presented in this adjunct to the original study further indicates the need for more detailed studies of the rock thermal regime. These data will provide the baseline required for deter- mination of the freeze-thaw mechanism and may also indicate the occurrence of other, possibly more effective, mechanical weathering processes. Cer- tainly, the combination of high values of
aT/!
with high thermal gradients suggests the potential for thermal stress fatigue and for thermal shock. Ulti- mately, these thermal data are only one of the two significant data sets which are required, the other being moisture and its attributes and how they vary in time and space. With these sorts of data a more meaningful understanding of the role of weathering in landform and sediment origin can be achieved.ACKNOWLEDGEMENTS
This work comprised part of a larger British Antarctic Survey undertaking and I would like to sincerely thank Or David Walton and the then Director, Or David Drewry, for allowing my participation. Or lan Meiklejohn, my colleague and friend in the field, helped with the daily read- ings and I thank him for his support. Or John King of BAS kindly provided the Rothera climate data.
I would also like to thank Or Hugh French who
©1998 John Wiley&Sons. Ltd.
suggestedmanychanges to the originalmanusc~ipt
that greatly improved its content and presentatIOn and an anonymous referee who offered some very valuable suggestions regarding some of the problems that the data presented.
REFERENCES
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Friedmann,E. I.,McKay,C.P.and Nienow,J.A. (1987).
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Hall, K. (1991). Rock moisture data from the Juneau Icefield (Alaska), and its significance for mechanical weathering studies. Permafrost and Periglacial Pro- cesses, 2, 321-330.
Hall, K. (1993). Rock moisture data from Livingston Island (Maritime Antarctic) and implications for weathering studies. Permafrost and Periglacial Pro- cesses, 4, 245-253.
Hall, K. (1995). Freeze-thaw weathering: the cold region 'Panacea'.Polar Geography& Geology, 19, 79-87.
Hall, K. (1997a). Rock temperatures and implications for cold region weathering. I: New data from Viking Valley, Alexander Island (Antarctica). Permafrost and Perig/acial Processes,8,69-90.
Hall, K. (1997b). Observations on 'cryoplanation' benches in Antarctica.Antarctic Science, 9, 181-187.
Hall, K. and Hall, J. (1991). Thermal gradients and rock weathering at low temperatures: some simulation data.Permafrost and Periglacial Processes, 2.103-112.
Hall, K. and Otte, W. (1990). Observations regarding biological weathering on nunataks of the Juneau Icefield, Alaska.Permafrost and Periglacial Processes.
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Analysis of the Temperature and Light Regimes. PhD thesis, Florida State University (165 pp.).
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Thorn, C. E. (1992). Periglacial geomorphology: what, where, when? In Dixon, J. C. and Abrahams, A. D.
(eds), Periglacial Geomorphology. Wiley, Chichester, pp. 1-30.
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