Download Outline The Evidence For Permafrost Conditions Having Extended

Outline The Evidence For Permafrost Conditions Having Extended Beyond The
South Coast Of Mainland Britain
At various times during the Pleistocene epoch, it is argued that that the
permafrost conditions of the northern hemisphere extended beyond their present
limits and affected large areas of mid-latitude Europe and North America (Harry, in
Clark, 1988). In Europe, southeastern England (Williams, 1965; in Péwé, 1969), the
Channel Islands (James & Worsley, 1997), northern France, Belgium and The
Netherlands (Lautridou et al, 1986; Lowe & Walker, 1997) are believed to have been
underlain by permafrost during the last glacial stage (See Figure 1). French (1996)
and Pitty (1988) suggest that the extent of permanently frozen ground during the
Pleistocene and associated paleo-climatic conditions, can be mapped from the
distribution of relict ice wedges, involution structures and other remnant landforms.
Consequently, this paper will consider the nature and processes associated with
contemporary permafrost regions and then discuss the evidence, through case
studies, for those conditions having extended beyond the south coast of mainland
Britain during the last glacial stage.
[Figure 1: Distribution of Permafrost in England (A) and northern France,
Belgium and The Netherlands (B) during the last glacial maximum (Lowe &
Walker, 1997: p108]
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Permafrost is defined by Muller (1947, in Washburn, 1973) as “a thickness of
soil or other superficial deposit… at a variable depth beneath the surface… in which
a temperature below freezing has existed continually for a long time” (p18).
Washburn (1973), however, notes that this definition is usually used to mean
‘perennially’ rather than ‘permanently’ frozen ground as climatic and surface changes
can cause permafrost to thaw rapidly.
During the summer, for example, when
temperatures are above 0oC, seasonal thawing creates a saturated active layer,
which refreezes once the temperature drops (Washburn, 1973). Harry (in Clark,
1988) also suggests that although some researchers (e.g. Péwé, 1969) have
equated the periglacial domain with the extent of permafrost, “the relationship is
complex since not all periglacial processes are restricted to the permafrost zone”
(p115).
In other words, permafrost should not necessarily be equated with the
proximity and climatic influence of glaciers (Harry, ibid). In Alaska, for example, there
is an increase in permafrost intensity and activity away from the cordilleran glaciers
(French, 1996; Briggs et al, 1996).
[Figure 2: Modern Distribution of Permafrost in the Northern Hemisphere.
Briggs et al, 1997: p269]
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Figure 2 shows the current distribution of permafrost to include Alaska, northwest
Canada, northern Scandinavia and Siberia (Pitty, 1988).
Globally, there is
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approximately 40 million km of permafrost (Briggs et al, 1996), which forms
continuous cover on non-glacial polar landscapes and cold, dry continental interiors,
but becomes discontinuous, as the amount of unfrozen ground increases,
equatorwards and near coastal regions (Briggs et al, 1996; Pitty, 1988; Washburn,
1973). As French (1996) suggests, the conditions necessary for the formation of
permafrost differs from place to place. For example, in Norway, permafrost exists at
-6oC, whilst in Siberia it exists at -8oC (Lowe & Walker, 1997). Although it is accepted
that mean annual air temperature must drop below 0oC, the formation of permafrost
ultimately depends on the porosity and thermal conductivity of the local earth
materials (Harry, in Clark, 1988).
Washburn (1979) also notes that current
permafrost areas are not necessarily good analogues for paleo-climates as different
solar radiation cycles in the mid-latitudes would have meant more diurnal freeze-thaw
action than in the present arctic. Consequently, conclusions drawn about paleoclimatic conditions and the formation of permafrost must take into account these
locational differences (Washburn, 1973; Pitty, 1988).
French (1996) argues that permafrost related features can be divided into
those associated with either its aggradation or its degradation and that in both cases,
“the landforms are often associated with the build up or degradation of ground ice”
(p69). French (1996) also notes that although patterned ground and solifluction are
commonly associated with permafrost, these features are not restricted to permafrost
regions. Pitty (1988), however, argues that they are best developed in permafrost
zones and it is for this reason that both patterned ground and solifluction will be
considered.
The features associated with ground ice in permafrost areas include ice
wedges and veins, ice-cored mounds and Palsas (Washburn, 1973; See Figure 3).
Within the continuous zone, ice wedges, Pingos and cryostructures are the dominant
features (Harry, in Clark, 1988). Ice wedges form where water seeps into cracks in
the permafrost table and then subsequently freezes (French, 1996).
The ideal
conditions for the development of ice wedges are poorly drained tundra lowlands
within the continuous permafrost zone.
Size varies from place to place and is
dependent on the availability of water and the age of the ice wedge (French, 1996).
In Siberia, ice wedges approximately 3-4m wide at the surface and extending 5-10m
downwards have been discovered (French, 1996). When the ice melts, it is replaced
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by material falling into the cracks from above and from the sides (French, in Harry,
1988) and consequently, the original form of the ice wedge is preserved to become a
relict feature. Pitty suggests that relict ice wedges are the “major and almost the sole
reliable indicator of former permafrost conditions in lowland areas” (1988: p9). In
England, examples found in East Anglia, the South East and Midlands have been
used to argue for permafrost conditions during the last glacial stage (Williams, 1965
& in Péwé, 1969; French, 1996; See Figure 1).
[Figure 3: Permafrost Ground Disturbances. Briggs et al, 1997: p289]
The remains of Pingos and other frost mounds (see Figure 3) also provide
evidence for the previous existence of permafrost. In England, the remnants of frost
mounds have been found at Walton Common in Norfolk (French, 1996). Palsas,
which are mounds with a core of alternating ice and peat, are regarded by French
(1996) as one of the few reliable indicators of permafrost in the discontinuous zone.
Pingos are formed when unfrozen water that has collected in the substrate begins to
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freeze. As the water freezes, an ice lens forms and the resulting cryostatic pressures
push up the land to create domes between 10-80m high (Washburn, 1973; French,
1996). Contemporary pingos are found in the Mackenzie Delta and on the Yukon
coastal plain (French, 1996).
Frost mound remnants posses a rampart where
material has moved down the slope by either solifluction or creep and a central
depression in which the ice core developed (French, 1996).
The ramparts are
particularly important because they are the main difference between the depression
associated with thermokarst (i.e. a general thawing of the ice) and the remains of a
pingo (French, 1996).
However, as French (1996) points out, there are several
problems associated with relying solely on remnant frost mounds as evidence for
former permafrost conditions. There is a difficulty in “distinguishing between the
remnants of seasonal and perennial types, since only the latter unambiguously
requires permafrost for their formation” (French, 1996: p250) and there may be
alternative, non-permafrost explanations for the features (French, ibid).
Other processes associated with the permafrost zone can be divided into those
associated with the active layer and those associated with movement and slopes
(French, 1996). In the active layer, cryofracture, patterned ground, and cryoturbation
are the main features, whilst frost creep and gelifluction are the primary slope
processes (Washburn, 1973; French, in Clark, 1988). Cryofracture, or frost wedging,
is the prying apart of rock, into angular fragments of varying size, by the expansion of
freezing water (Lautridou, in Clark, 1988). The creation of patterned ground and the
up freezing of stones are associated with frost heaving and cryoturbation (See Figure
3). Cryoturbation is defined as the “lateral and vertical displacement of soil which
accompanies seasonal and/or diurnal freezing and thawing” of the active layer
(French, 1996: p143). Examples of cryoturbation can be found in the Axe Valley at
Broom in Devon (Bowen, 1999). The freeze-thaw cycle leads to “annual ground
displacements of several centimetres” (French, 1996: p131) and creates uneven,
patterned ground.
At the same time, stones in the soil are gradually pushed
upwards, in a vertical position, as the active layer seasonally thaws and refreezes.
Once the stones have reached the surface, they roll down to the edges of the
hummocks and create sorted polygons, circles or nets (French, 1996; Washburn,
1973). On slopes, these shapes become elongated to form stripes (See Figure 4).
French (1996) argues that the paleo-environmental significance of patterned ground
is questionable since “most patterned ground forms are not unique to periglacial
regions” (p146). However, French (1996) does accept that they are usually indicative
of cold climate conditions and Lowe & Walker (1997) suggest that there is “often a
close field relationship between cryoturbations and other undoubted periglacial
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phenomena” (p105).
The Breckland polygons and stripes in East Anglia, for
example, coincide with evidence of other former cold climate features (French, 1996).
[Figure 4: Stone Circles and Stripes. French, 1996: p141]
Two slope processes are particularly linked with permafrost zones.
Gelifluction, a form of solifluction, has been described by Benedict as “the net
downward displacement that occurs when the soil, during a freeze thaw cycle,
expands normal to the surface and settles in a more nearly vertical position (1970,
cited in French, 1996: p151). The associated landform is a uniform sheet of locally
derived surficial materials in a tongue shaped lobe (French, 1996).
Unstratified
deposits, which usually consist of angular material within a sediment matrix, are
known by a variety of names including ‘Head’ and ‘Coombe Rock’, whilst stratified
deposits are scree or grèzes litées (Dewolf, in Clark, 1988).
Frost creep is the
movement of particles down slope caused by the freeze-thaw process (French,
1996). As with patterned ground, the deposits are good indicators of cold climate
conditions, but are not without ambiguity. As French argues, “direct interpretation in
terms of frost action is not always easy” (French, 1996: p240) as it is difficult to
differentiate between a deposit caused by seasonal frost creep with retrograde
movement and one caused by gelifluction where permafrost is present.
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The distribution of relict ice wedges, pingo remnants, cryoturbations and other
periglacial landforms have been used to reconstruct the environment of western
Europe during the last cold stage (Lowe & Walker, 1997; French, 1996). Williams
(1965, & in Péwé, 1969), for example, used this data to establish the coverage of
permafrost in England and Wales (See Figure 1A), whilst similar data has
established the southern extent of permafrost in France, The Netherlands and
Belgium (See Figure 1B) (Lowe & Walker, 1997; Lautridou et al, 1986). The latter
reconstruction, “suggests that the continuous permafrost limit may have been much
further west than envisaged by Williams” (Van Vliet-Lanoe, 1988, in Lowe & Walker,
1997: p107) and it is within this context, and of the evidence outlined above, that the
research undertaken in Brittany and the Channel Islands is considered.
ALDERNEY (UK)
(UK)
NORMANDY
BRITTANY
[Figure 5: Location of the Channel Islands, Brittany and Normandy. Microsoft
Encarta Atlas, 2001]
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The Channel Islands are located in the Golfe de St-Malo between the west
coast of Normandy and the north coast of Brittany. Alderney is the furthest north of
the islands and is situated off the Normandy coast near the Cap de la Hague (See
Figure 5). Figures 1 suggests that the Channel Islands were in or very close to the
continuous permafrost zone during the last cold stage and Gurney et al (1998)
believe there is “good reason to expect evidence of periglacial structures, indicative
of at least discontinuous permafrost, if not continuous permafrost, in the Channel
Islands” (p1).
[Figure 6: Exposed Ice Wedge Cast at Fort Richmond, Guernsey. Gurney et al,
1998: p4]
Gurney et al (1998) note the existence of either seasonal or perennial frozen
ground structures in Alderney and Guernsey, including sorted patterned ground,
sediment displacement and ice wedge casts.
In Alderney, the evidence for
permafrost is based upon the distribution of head and loess deposits (Keen, 1978;
James & Worsley, 1997), a frost thrust boulder and “macrofabric evidence relating to
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former active layers above a permafrost table” (James & Worsley, 1997: p177). As
discussed above, head deposits alone are not enough to indicate the existence of
permafrost (French, 1996) and it is the disturbed raised beach at Fort Clonque,
where most of the pebbles lie near to the vertical, and frost thrust boulder at Fort
Tourgis that provides more concrete evidence (James & Worsley, ibid). James and
Worsley (ibid) suggest that the boulder at the Fort Tourgis site is incomplete ejection
from the active layer through frost heaving. The boulder was also surrounded by a
coarse head deposit, where the long axes were vertically aligned with the boulder
and a series of involutions caused by cryoturbation (James & Worsley, ibid). Further
work in The Channel Islands has provided more evidence, including head deposits at
Moulin Huet in Guernsey, Bonne Nuit Bay in Jersey and Hannaine Point in Alderney
and disturbed ground on Guernsey (Gurney et al, 1998). In addition, a wedge feature
was discovered at Fort Richmond in Guernsey (See Figure 6). Gurney et al, (1998)
suggest that the feature is a relict ice-wedge and therefore indicative of permafrost,
although they note that there is a danger in drawing conclusions from single
structures (Worsley, 1996 in Gurney et al, ibid). The sedimentological evidence from
Alderney and “the range of possible/probable frozen ground phenomena overlying
former discontinuous or continuous permafrost in Guernsey” (Gurney et al, 1998: p4)
suggest that “severe periglacial climatic conditions occurred on Alderney” (James &
Worsley, 1997: p180) and Guernsey.
[Figure 6: Sketch Map of Côtes D’Armor Sites. Renouf & James, 2001: p161]
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Brittany (See Figure 5) lies to the west of the continuous permafrost zone as
shown in Figure 1. However, work by Renouf and James (2001) in Côtes D’Armor
and Lautridou et al (1986) suggests that the northern coastline of Brittany should, like
Guernsey and Alderney, now be within its limits.
Evidence of cryoturbation was
found on a raised beach at Port l’Épine (See Figure 6) that suggested past
permafrost conditions. The sub-rounded clasts within the raised beach “display a
striking vertically inclined macrofabric similar to that previously recorded at Alderney
and Guernsey” (Renouf & James, 2001: p161). Although Renouf and James (2001)
suggest that vertically inclined clasts could be the result of frost heaving within
seasonally frozen ground or permafrost, they accept that the clasts may have been
reoriented by alternative processes. At Toul ar Roussin and the adjoining North East
Cove fractured and displaced pebbles and large angular boulders, which may have
been frost shattered, have also been recorded. The pebbles have also been “sorted
into plumes of vertically inclined material with regular spacing”, which may have been
indicative of patterned ground (Renouf and James, 2001: p163).
Renouf and James (2001) argue that the evidence from the Channel islands,
together with “the discovery of deep seated periglacial disturbances of raised beach
sediments” at Toul ar Roussin, suggest that the “southern permafrost limit needs to
be redrawn to include… the Channel Islands and the northern coastline of Brittany”
(p165). As discussed above, other mechanisms could have been responsible for the
evidence found (Renouf and James, 2001; Washburn, 1973) and periglacial
environments are not necessarily synonymous with permafrost conditions (Harry,
ibid). However, the existence of a relict ice wedge in Guernsey, which Pitty (1988)
has described as the “major and almost the sole reliable indicator of former
permafrost conditions” (p9) and compelling evidence from Alderney and Brittany
seems to support Renouf and James’ (2001) conclusion that these areas were
underlain with continuous permafrost.
[2,566 Words]
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References:
Bowen, D Q (Ed)
(1999)
A revised correlation of Quaternary deposits in the
British Isles, pp174
Bath: Geological
Society
Briggs, D, P Smithson,
K Addison & K Atkinson
(1997)
Fundamentals of the Physical Environment, 2nd
Edition, pp557
London:
Routledge
Clark, M J (Ed) (1988)
Advances in Periglacial Geomorphology, pp481
French, H M (1996)
The Periglacial Environment, 2nd Edition, pp341
Chichester:
Wiley
Harlow:
Longman
Gurney, S D, H C L
James & P Worsley
(1998)
The nature of last glacial periglaciation in the
Channel Islands. Read at the Annual Conference of
the Usher Society, January 1998, pp5
James, H C L & P
Worsley (1997)
Evidence for Last Glacial Activity in Alderney.
Proceedings of Usher Society, 9, 177-181
Keen, D H (1978)
The Pleistocene deposits of the Channel Islands:
Report 78/26 of the Natural Environment Research
Council, pp11
London: HMSO
Lautridou, J P, D Keen,
& J L Monnier (1986)
The loess and other Pleistocene Periglacial
deposits of Northwest Europe including their
relationships with marine formations and features,
pp214
Caen, France:
Centre de
Geomorphologie
du CNRS
Lowe, JJ & M J C
Walker (1997)
Reconstructing Quaternary Environments, 2nd
Edition, pp446
London:
Prentice Hall
Péwé, T L (Ed) (1969)
The Periglacial Environment: Past and Present,
pp487
Montreal:
McGill-Queen’s
University Press
Pitty, A F (1988)
Geomorphological Processes in Britain in a
Periglacial Age, pp44
Norwich:
University of E
Anglia
Renouf, J T & H C L
James (2001)
The Raised Beach at Toul Ar Roussin, Port L’Épine,
Trélévern (Côtes D’Armor, Brittany): Its significance
for the last cold stage events. Geoscience in SouthWest England, 10, 160-165
Van Vliet-Lanoe, B, B
Hallegouet & J L
Monnier (1997)
The Quaternary of Brittany. Guide Book of the
excursion of the QRA in Brittany 12-15 September
1997, pp131
Rennes, France:
University of
Rennes
Washburn, A L (1973)
Periglacial processes and environments, pp319
London: Edward
Arnold
Williams, R B G (1965)
Permafrost in England during the last Glacial
Period. Nature, v205, 27 March 1965, p1304-1305
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