1 Unnamed Glacier 2 Bedrock

)C Squirrel Logger

ally, or near horizontally, bedded. The retreat of the surrounding ice has resulted in extensive vert- ical dilatation joints that cut the bedding at c. 90°.

FIELD APPROACH

The study was undertaken as part of a multi- disciplinary investigation of the evolution of soils and the colonization by life of an Antarctic dry valley; Viking Valley was particularly apt as the area was recently deglaciated. As part of the overall study a Campbell 21X micrologger with a full assemblage of meteorological transducers was positioned at an elevation of c. 300 m (70 m above the Mars Glacier and c. 300 m below the top of Probe Hill). The Campbell was situated 17.5 m from the south-facing valley wall, 13.6 m from the north-facing wall, 85.6 m from the lake edge and c. 31 m from the edge of the unnamed glacier. Set to record every hour, the Campbell recorded air temperature, relative humidity, radiation, wind speed and direction as well as temperatures on a dark-coloured sandstone (surface), a light-coloured sandstone (surface), and at the surface and at depths of 5, 10, 15, 30 and 40 mm of a block of local cannonball sandstone. This latter piece of sandstone had the holes for the thermistors drilled through from the "bottom" and then, after the thermistors were emplaced, it was buried in the valley floor such that the top surface was flush and thermal exchange occurred unidirectionally from the top surface. This logger was left to run throughout the summer of 1992-93 and for the winters of 1993 and 1994.

Next to the Campbell logger there was placed another piece of cannonball sandstone to which was attached a pair of 1 MHz ultrasonic transducers linked to a PUNDIT ultrasonic test apparatus. Unfortunately this could not be con- trolled by the Campbell but had to be run by the operator. As the transducers were fixed to a block of known dimensions it is possible to convert the pulse time readings (in J.1s) to a velocity (m S-I).

Readings were taken at various times on a number of days and the comparable real time values from the Campbell were noted (i.e. for the actual times rather than the hourly readings) for temperatures, radiation, etc. The ultrasonic data are potentially extremely important as they, non-destructively, indicate changes to rock properties (e.g. changes in length due to heating and cooling, cracking and the changes in moisture content). Thus, used in conjunction with the climatic data they can provide

valuable information on the behaviour of the rock and the conditions it is experiencing.

On the north-facing valley wall, at a distance of approximately 120 m and c. 30 m higher elevation, an 11 channel Squirrel logger was set up to monitor rock temperatures on a rock outcrop pitted with taffoni. On a north7facing exposure, a block of pre- drilled local sandstone (as with the blocks on the valley floor) with thermistors at the surface and at 5, 10, 15,30 and 40 mm was positioned flush with the rock exposure. In addition, one thermistor was used to measure the rock surface temperature on the top, east- and west-facing exposures of the same rock outcrop. Two other thermistors were used to monitor temperatures on the bottom and top of the inside of a small taffoni. Temperatures were recorded every two minutes throughout the period in the field (7 December 1992 until 8 January 1993).

. As a further adjunct to the overall study, Schmidt hammer readings were taken at a number of locations throughout the valley. In addition, measurements of size and orientation of the taffoni were collected as well as general observations regarding the distribution of features and the nature of the weathering. Details of Schmidt hammer and taffoni data will be presented more extensively elsewhere but information appropriate to this discussion will be given in outline here.

Samples of precipitates (sometimes up to 23.9 mm thick) found on the rocks were taken for analysis by means of X-ray diffraction and all were found to be gypsum (calcium sulphate).

RESULTS AND DISCUSSION

The information is such that it is more convenient to discuss it under a series of headings rather than as one issue. For clarity, the material can be considered in the context of: (i) the weathering regime found in Viking Valley, (ii) the perception of Antarctic weathering, and (iii) implications for weathering studies in general.

Weathering in Viking Valley

Valley floor (one hour interval) data. The Camp- bell data (i.e. at hourly intervals from the valley bottom) for the summer of 1992-93 and the winters of both 1993 and 1994 for the air temperature and the rock surface temperature are shown in Figure 3. The first and most obvious difference between the rock and the air is the

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

Summer 1992/93

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Mean s Range Min. Max. CyclesI

Table I Analysis of temperature data for the summer of1992-93(QC).

I Number of freeze-thaw cycles is here based on crossings of

o

QC boundary.See discussion for significance or otherwise of these values.

ignored the air still indicates 12 freeze-thaw cycles of amplitudes ~- 5 cC - a value clearly more likely to be geomorphologically significant, the rock experiencing only one cycle during this period.

Thus, the air temperatures are clearly not a surrogate for what is happening to the rock (this will be discussed in more detail later). In short, the available data indicate that, as a purely thermal event, the summer isnota particularly active period for freeze-thaw weathering in this area. Lastly, even if and when freeze-thaw cycles of adequate amplitude and duration are found to occur in the rock (presuming water is also present) the one hour record intervals are not adequate to provide the

16 16 2.60 12.17 -6.14 6.03 5.85 28.79 -4.41 24.38 Air -0.24

Rock +5.86 amplitude of the rock temperatures and the differ-

ence in crossings of the 0 cC isotherm. Considering the summer first, details of the rock surface and air temperature values are given in Table I. From this it can be seen that rock temperatures are significantly higher than those of the air (5.86 versus -0.24 cC) and that the maximum rock temperature is noticeably higher than that of the air (24.38 versus 6.03 cC); the range of the rock temperatures is more than twice that of the air. Using a threshold of 0 cC (but not implying any geomorphic response), it is interesting to see that both the air and the rock experienced 16 freeze-thaw cycles - but not always at the same time. The rock temperature data clearly show diurnal variability but do not always drop below 0 cC; the air temperatures, however, do not so clearly show diurnal variation and for two periods remain below 0 cC whilst the rock temperatures fluctuate across this boundary. Further, the amplitude of the sub-zero rock temperatures is, with only one or two exceptions, small(c. 2 cC) and so their geomorphic significance is very limited. Conversely, in considering the air temperatures, if the small (and frequently very short) sub-zero depressions are

76 K. Hall

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Figure 4 Ultrasonic pulse time to show the impact of water on the rock (a decrease in pulse time equates to an increase in pulse velocity owing to the presence of the better propagating medium, water).

information necessary to determine the actual freeze-thaw mechanism (see below).

In terms of weathering processes, the frequency of freeze-thaw weathering is limited by thermal constraints alone; however, moisture availability constrains the process even further. This being a

"dry valley" snowfall is low (see below) and moisture is extremely limited. Some summer snow was seen to melt on contact with rock on the valley bottom but the rock temperatures remained above

o

°C and the moisture was subsequently lost due to the radiative heating of the rock. Theonlysource of water on the valley bottom was from glacier melt and this only occurred on the warmer days with high radiation inputs. The impact of the meltwater on the rocks is clearly shown by the ultrasonic data (Figure 4). As water enters a rock so the pulse time decreases as water is a better transmitting medium than air. On 5 January as the glacier meltwater ran over the rock block with ultrasonic transducers so the pulse time can be seen to drop from 24.3 J.1s to 22.3 J.1S over one hour as the water penetrated the rock. Both air and rock temperatures remained above O°C and so the only forms of weathering likely to occur are wetting and drying and, possibly, salt weathering. During sub-zero periods the rocks were dry and ultrasonic data indicated an absence of water. Unfortunately the one hour record intervals are not adequate for any consideration of weathering by thermal stresses (see below).

The valley floor data for the winter periods are extremely interesting and very valuable as rock temperature data, particularly from the Antarctic, are rare. In broad terms the 1993 data are very similar to those of 1994 (Table 2) - there being a period of frequent crossings of O°C for the latter part of the summer in to autumn (roughly mid January through to early March) followed by the winter freeze. However, it is very clear that both air and rock temperatures fluctuate significantly through the winter freeze period and can even become positive, albeit for only a very short period (as in 1994).

Considering the first part of the records (Le. the summer to autumn section) both years clearly show frequent crossings of O°C for the rock data but significantly fewer for the air. Detailed graphs for the early period in both 1993 and 1994 (Figure 5) indicate that the rock surface experiences nearly a Table 2 Analysis of the temperature data for the total record for the winters of 1993 and 1994

eq.

Air 1993 Rock 1993 Air 1994 Rock 1994

Mean -9.65 -8.72 -9.23 -7.89

s 7.19 7.97 7.22 8.82

Range 39.85 54.9 35.55 56.48

Min. -35.18 -33.28 -31.71 -35.14

Max. 4.67 21.62 3.84 21.34

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

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Figure 5 Detail of the late summer through to early winter period of the air and rock temperatures (at one hour intervals) for the winters of 1993 and 1994 (the first section of the winter graphs in Figure 3).

78 K. Hall

Table 3 Analysis of the temperature data for the periods of across O°C oscillations for 1993 and 1994.

Air Rock Air Rock

1993 1993 1994 1994

Mean -2.65 1.51 -4.31 0.54

s 2.64 4.61 2.93 5.19

Range 14.54 29.91 17.94 30.06

Min. -9.87 -8.29 -14.28 -8.72

Max. 4.67 21.62 3.66 21.34

Cycles O°C' 14² 46 7³ 51

Cycles -2.5°C 22 20 23 39

Cycles _5°C 8 2 20 15

I Freeze-thaw cycles derived from the record in Figure 5 using thresholds ofO°C, -2.5 °C and - 5°C.

2From 51 days of record (13 January to 4 March 1993).

3From 53 days of record (15 January to 8 March 1994).

diurnal cycle whilst the air only rises above 0 °C on a few occasions. Details of the thermal conditions for these periods are given in Table 3. From the table it can be seen that the data for 1994 suggest more severe conditions, with lower temperatures and far more (thermal) freeze-thaw cycles on the rock. It is interesting that 1994 differs from 1993 in that it indicates the danger of using data for only one year (and clearly data from more than the two used here would be far better). Using the arbitrary and non-geomorphic boundary of O°C it can be seen that the rock surface experiences many more cycles than does the air (32 more in 1993 and 44 in 1994). Thus the use of air temperature would, with this threshold, give a very false picture. Using a threshold of - 2.5 QC, which is potentially geomorphologically meaningful, 1994 still shows more (16 more) on the rock than in the air but 1993 has the rock with marginally fewer (only 2). Then, using a threshold of - 5°C, at which some water will likely freeze (if water is present), the air indicates more cycles than the rock experiences in both years (6 more and 5 more respectively). Thus, the air is not reflecting what is happening on the rock. In all instances here the discussion is purely with respect to the thermal eventand no presump- tion regarding the presence of water and thus the geomorphic effectiveness is made.

Following the early winter period of intense thermal oscillations both the air and rock temperatures drop, to remain, with the one short exception in 1994, below O°c. However, the records indicate some very interesting activity that has geomorphic implications. First, they indicate that for much of the time there cannot be any significant snow on the ground (as suggested by the limited snow .f' 1997 by John Wiley&Sons. Ltd.

Table 4 Details of temperature records for (a) a period with no apparent snow cover and (b) a period with snow coverCc).

(a) Data for 5 May to 20 May 1995 to show strong relationship between air and rock: correlation r= +0.937.

Air 1994 Rock 1994

Mean -8.78 -10.95

s 3.58 3.11

Range 19.10 16.69

Min. -21.33 -21.95

Max. -2.23 -5.56

(b) Data for 11 June to 6 July 1993 to show "buffering"

of the rock by snow: correlationr= +0.787.

Air 1993 Rock 1993

Mean -13.77 -12.95

s 6.40 3.47

Range 26.61 17.98

Min. -29.56 -21.75

Max. -2.95 -3.87

observed in the field in December). The air and the rock records both show a "peakedness" with a strong correlation (values as high as +0.937°C being obtained with only a marginal difference between actual temperature values: Table 4a).

Conversely, for short periods the rock temperature data are "smoother" and less spiky than the air and this is thought to indicate the presence of snow (Table 4b). However, although snow is considered to be present it cannot be very thick as it does not obliterate completely the fluctuations shown in the air temperatures(r

=

+0.787) and its influence can rapidly disappear which, as it cannot be due to melting (owing to the sub-zero temperatures and the limited radiation input in winter), suggests it is removed by wind. Wind speed data for the time of the transition from "buffered" to "spiky" tend to support this as wind speeds changed from values close to 4 m ^S-I to values close to 18 m^S-I at the time of transition. Thus, during the winter the rock can experience substantial temperature changes, particularly as it is not protected by a thick snow cover. This also means, as observations during 1992-93 suggested, that there is very little snow available in the valley or on the nunataks to melt during the summer and provide water for the rocks.

If the winter data as a whole are considered, it can be seen that there are significant oscillations of temperature within the rock, albeit all below O°c.

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

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Air Light rock Dark rock

Mean -0.24 +3.5 +4.61

s 2.60 4.37 5.12

Range 12.17 26.96 27.75

Min. -6.14 -4.62 -4.72

Max. +6.03 +22.34 +23.03

Cycles(O°C) 16 16 14

-10 -..:.-'- - - '

-5 --- -10 - - - '

8 December 1992to11 January 1993

calculate the freezing point depression. However, as the oscillations can, particularly in 1994, come close to - I °C it is possible that thawing of any water could occur. Further, as the stress that any frozen water can exert is a function of the temperature (with maximum stresses being exerted in the region of - 22°C), so the fluctuations between c. -5°Cand c. -25°Cindicate that there is a change in the stress regime that could cause damage to rock. All of this, it is emphasized, is dependent upon water being present in the rock and here it seems unlikely that any is actually available in this area.

Lastly, from the valley floor data there is the information regarding the difference between the light-coloured and the dark-eoloured sandstones (Figure 6). So far these data are only available for the summer of 1992-93. The graphs clearly show the diurnal fluctuations, which are less pronounced in the air, as well as the larger amplitude of the events on the dark rock compared with the light- coloured rock. Details of the temperature data (Table 5) show that the dark rock was generally warmer, had a marginally larger range and did not become quite so cold, although many of these differences are actually very small. The light- coloured rock, based on a O°C threshold, experienced marginally more freeze-thaw cycles than did the dark rock. The differences were not as large as had been expected but may have been more significant had readings been taken at more frequent intervals (see below) in so far as there may be implications for thermal stress fatigue/

shock if the dark rock warms up faster.

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If there is any water available in the rock then it is not pure and so it will freeze and thaw at a temperature belowO°c. Without data on the type and amount of salts in solution (assuming that there is any water in the rock) it is not possible to

Highfrequency datafram the valley side. For the period in the field it was possible to collect temperature data on the north-facing valley side at two minute intervals. Data of this frequency (or, preferably, at a higher frequency) are required for any real understanding of weathering processes in

80 K. Hall

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-5 - - - ' 27-12-92 to 31-12-92 (2 min intervals) 27-12-92 to 31-12-92 (2 min intervals) Figure 7 An example of a five day record of temperatures for east, west and north aspects, together with a horizontal surface, collected at two minute intervalso

the field. This is for three main reasons. First, they provide information on the short term changes that are missed by observations at even five minute intervals and are certainly lost at measurement intervals of 15 minutes or more. Second, they provide the sort of information required for evaluation of weathering due to thermal stress/shock. Lastly, data of this sort are required if any understanding of the freeze-thaw mechanism is to be attempted as all the available processes are constrained by different rates of fall of temperature, the information for which cannot be derived with any certainty from temperature records collected at intervals of 15 minutes or more.

Figure 7 shows a typical example of five days records with observations every two minutes for the east·, west- and north-facing sides of a rock outcrop together with the top (horizontal) surface.

Using the 0cC boundary it can be seen that the east-facing side experienced eight freeze-thaw cycles, the west-facing seven, the top surface five and the north-facing side only four. Thus, the

different aspects would (given the presence of water and if freezing occurred) experience different weathering regimes. If a more realistic threshold of - 3cCis used then the four locations show very similar results with between one and two events each.Itis very clear that the north-facing exposure experiences the highest temperatures as well as the largest range (Table 6). The ranges for both east- and west-facing exposures are comparable but the west experiences lower minimum and maximum temperatures. The horizontal surface, which Table 6 Analysis of temperature data for north-, east- and west-facing exposures together with a horizontal surface for the period 27 to 31 December 1992(cC).

Mean s Range Min. Max.

East-facing 2.07 2.953 12.5 -2.5 10.0 West-facing 0.65 2.759 12.5 -4.0 8.5 North-facing 1.74 3.759 23.5 -3.5 20.0 Horizontal 0.77 2.869 14.5 -4.0 10.5

Dalam dokumen Mechanical weathering in cold regions with special emphasis on the Antarctic environment and the freeze-thaw mechanism in particular. (Halaman 91-104)

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