Side 42
Hanne Hvidtfeldt
Christiansen: Institute of Geography, Copenhagen
University, Øster Voldgade 10, DK-1350 Copenhagen
K.,
Denmark.
Danish Journal of
Geography 95: 42-48, 1995.
The purpose of this note
is to draw attention to the special topographic setting
of some pingos observed in Mellemfjord (Greenlandic:
Akugdlit), Disko Island, Central West Greenland. The
geomorphology of the area is presented as an important
base for the development of the pingos, as they are not
typical open system pingos. Using evidence on sea level
and general climatic change, the pingos are tentatively
dated to have been developed during the Little Ice Age
cold climate period.
Keywords: Marsh
initiated pingos, Greenland, Disko, periglacial
phenomena
The investigated small hills
positioned in a braided river environment in Mellemfjord
have previously been described as "mud volcanoes"
(Nielsen, 1980) formed by rising gases of methane
seeping up from coal layers below. The name is derived
from the fact that the form of the hills is like small
volcanoes, reminding of the volcanic origin of the
Tertiary basalt mountains of Disko (Nielsen, 1980).
Previously Noe-Nygaard & Rosenkrantz (1950) have
described the occurrence of these mud volcanoes (Danish:
dyndvulkaner) in West Greenland on the peninsulas of
Svartenhuk and Nugssuaq and on the Disko Island and
explained their genesis as Nielsen (1980) did. Generally
the mud volcanoes on Disko Island are known to exist in
the bottom of large fluvially dominated flat valleys
(Noe & Rosenkrantz, 1950).
Generally two types of pingos
are accepted: 1) the closed system pingo and 2) the open
system pingo (French, 1976). One of the latest
definitions of pingos given by Pissart (1988) defines a
pingo as a multiannual icecored mound produced when ice
has grown in or below permafrost as a result of water
pressure in the ground - this water pressure being
either cryostatically (closed system) or hydraulically
(open system) induced. Müller (1959) generally
interprets pingos in West Greenland as being of the same
type as the East Greenland (open system) pingos.
The aim of this paper is to
draw attention to the special character and position of
the mud volcanoes in Mellemfjord obtained by combining
geomorphological observations.
Environmental Setting
Figure 1:
Mellemfjord on Disko in Central West Greenland.
The observed
small hills are positioned in Mellemfjord, a
25 km
long fjord on the central west coast of the Disko
Side 43
Figure 2:
Aerial photograph (27.08.1964) and geomorphological map
of the Iterdlagssüp kügssua valley area. North is
upwards. The valley is approximately 600 m wide at the
mouth where the pingos are situated. The Iterdlagssüp
kügssua valley is surrounded by I ravines, 2 basalt
valley side, 3 solifluction lobes, 4 talus cones, 5
lower valley side with solifluction, soil creep and
sheet flows and 6 raised marine terraces. An icing 7 and
marsh flats 8, where the pingo remnants occur, are
present in the valley bottom 9. A glacier-derived rock
glacier 10 leads to the upper parts of the valley.
Island (Figure 1). The fjord
is a large estuary running in an NW-SE direction. Along
the sides of the fjord Tertiary plateau basalt mountains
reach 1000 m asl. The landscape is alpine and
characterised by cirques and U-shaped valleys, some of
which are partly filled by glaciers. Several active
talus-derived and glacier-derived rock glaciers
(Humlum,l9Bß) characterise the slopes of the southern
side of Mellemfjord.
The mean annual air
temperature in the inner part of Mellemfjord is -7.8° C
(1993-1994), measured at a meteorological station
positioned close to the investigated small hills (Humlum
eta 1.,1995).
This means that the area is
located in the zone of continuous permafrost. In
Mellemfjord the precipitation is not measured, but in
Godhavn the annual precipitation is around 400 mm
(Humlum eta 1.,1995).
During the maximum Wisconsin
glaciation, called the Godhavn Stage (Ingolfsson eta
1.,1990), Mellemfjord was filled by an extensive valley
glacier (Donner, 1978). The deglaciation of the inner
part of the Mellemfjord area took place about 8,000 BP
(Donner, 1978), while the outer coast was deglaciated
prior to 9,100 BP (Ingolfsson et al., 1990). Sea level
was highest between 9,250 and 9,000 BP forming the
highest marine limit at about 55 m asl. at Enoks Havn in
the Mellemfjord area (Ingolfsson eta 1.,1990).
In the inner part of the fjord
a tributary valley, Iterdlagssüp kügssua, coming from
the north, ends in Mellemfjord. This valley is the
largest leading to Mellemfjord. It has a flat bottom
with a braided glaciofluvial river system (Figure 2)
draining several glaciers in the upper part of the
valley. A delta of approximately 1 km2 is
built into Mellemfjord south of the valley and this
place is called Iterdlagssuaq. The deltaic tidal area,
which is covered by fine grained sediments, is nearly
flat with an inclination of only a few %o and ends with
a steep delta front sloping towards the bottom of
Mellemfjord.
Close to the front of the
delta the tidewater range is maximally around 2.5 m,
which makes the estuary microtidal, probably with a
saltwedge, as there is a relatively large inflow of
river water (Pethick,l9B4).
When the
Iterdlagssüp kügssua valley was first exposed
during
the deglaciation of the inner part of Mellemfjord
Side 44
around 8,000 BP, the valley
was a fjord that ended furthe up in the valley as
sedimentation in the valley had jus started. At the same
time sea level was approximately 5( m higher than the
present according to the emergence curve presented by
Ingolfsson et al. (1990).
By glaciofluvial sedimentation
going on since degla ciation, the inner parts of
Mellemfjord have been con verted to the Iterdlagssüp
kügssua valley, moving the del tafront to its present
maximal position towards the south In the same period
sea level dropped because of isostatk rebound and
probably reached its lowest position belou the present
sea level. According to a I4C dating of a
whale bone found at the Enoks Havn area in the outer
part o1 Mellemfjord, the sea level was roughly 2 m below
the present sea level at about 2,750 ± 180 years BP
(Bojsen & Jacobsen, 1979).
The
Iterdlagssûp Kûgssua Pingos
At the interface between the
inner part of the delta and the glaciofluvially
dominated part of the valley, one circulai dome and
several remnants of domes rising approximatel) 5 to 10 m
asl. can be seen (Figure 2). The diameter of the hills
varies from about 20 to 50 m. Most of them display
fluviallv eroded slopes, leaving only Darts of earlier
larse forms. As digging in the hills demonstrated the
existenc« of solid ice cores beneath updomed fluvial
sand, thes< forms are considered as pineo remnants.
In the 600 m wide
flat-bottomed Iterdlagssüp kugssu; valley the pingo
remnants are found on the highest parts o marsh flats
(Figure 3), which are sparsely covered by salt tolerant
vegetation. On the marsh flats some fossil drj shallow
channels that drained the mudflats during their de
position can be seen (Figure 4).
Fluvial dissection of the
pingos is causing slumping ol their sides (Figure 5). At
the slump scars massive and segregated ice was found
about 0.5 m below the surfaces ol the pingos. The
covering material of the pingos was predominantly
fluvial sand deposited by the glaciofluvia: rivers of
the Iterdlagssüp kügssua vallev.
In one particular pingo a
central depression, partly fillec with a lake, likewise
had a core of massive ice 0.5 m belov\ the slumping
inner side. The salinity of the water in the lake in the
central depression was measured to be 0.12 %o. while the
value for the glaciofluvial river was 0.05 %c. Ir
Mellemfjord the salinity of the water was measured to be
29 %o, which is normal for sea water in the Arctic
during the summer (Kiilerich.l96s\
Figure 3: Pingo
remnants in the Iterdlagssüp kügssua valley bottom
located on the highest marsh flats. The valley bottom is
approximately 600 m wide. Seen towards the east.
21.08.93.
Side 45
Figure 4:
Channel on marsh flat, which was active earlier as the
marsh plain was accumulating. Today the channel is
covered by vegetation. Seen towards the east. 21.08.93.
Figure 5:
Fluvially eroded pingo remnant with a fluvial channel
below the active slumping side. The pingo remnant is
about 5 m high. 21.08.93.
Side 46
According to
these observations the massive ice bodies of
the
described hills in the Iterdlagssüp kügssua valley
mouth are clearly pingo ice as defined by Pissart
(1988).
Genesis
of the Pingos
The number of pingos in the
Iterdlagssüp kügssua valley mouth is considerable
compared to other locations in West Greenland where
normally only one or two pingos are found. Recently
Yoshikawa (1991) has described two pingos from the
Sarqaq Valley, 100 km to the east of the study area, on
the Nügssuaq Peninsula, but he did, however, not discuss
their genesis in detail.
If the investigated pingos
were "mud volcanoes", one could expect their positions
to be more casual and scattered in the valley and not
confined to a specific area across the valley on the
interface between the deltaic and fluvial parts.
The pingos are probably not of
the closed system type as their position and evolution
are not controlled by the presence of former lakes. In
the Iterdlagssüp kügssua valley bottom the permafrost is
not suspected to be very thick opposed to what is
generally described for closed system pingos (Pissart,
1988).
Most of the Greenlandic pingos
are characterised as being of the open system type
(Müller, 1959). The Iterdlagssüp kügssua valley pingos,
however, cannot be characterised as typical for the open
system type as they are not found in the lower part of
the valley plain, but actually on the highest parts of
the marsh flats. Their location some distance away from
the valley sides shows that they are not dependent on
water coming from the sides, but obviously on water
coming from the valley. Finally the observed pingos are
not seasonal frost mounds because of their size and
their long existence in the valley. They can be seen in
the valley on old aerial photographs from 1964 clearly
stressing their perennial existence.
The pingos of the Iterdlagssüp
kügssua valley occur only on the slightly higher marsh
flats and this position gives crucial clues as to their
genesis. The marsh formation probably started when the
bars were raised by fluvial sedimentation just above the
mean daily high-water level and became mudflats. On the
most elevated and well-drained parts of the marsh flats,
frost penetration must have been more pronounced and
lasted for longer periods than was the case for the
surrounding lower and more water-soaked areas. Each year
this must especially have been the case during the early
autumn, when the surrounding fluvial surface was still
unfrozen, while the higher parts of the marsh flats must
have been frozen relatively early.
The measurements of pingo
water salinity show that the ice that constitutes the
frozen core of the pingos is fresh. Therefore the water
must have been coming from the glaciofluvially
interjacent channels or from the glaciofluvial sediment
now positioned over the pingos.
In the late autumn and early
winter the valley bottom is covered by a growing icing.
During a 1993 summer visit rests of an icing were still
present in the valley and rests of an icing can also be
seen on aerial photographs showing the area in different
previous years, one shown in Figure 2. As the embryonic
pingos grew high enough to protrude the icing during the
winter and the spring, the wind must have been sweeping
the pingos free of snow, improving frost penetration,
which had a positive feedback effect on the pingo
growth.
The relatively high level of
the marsh flats is thus considered as the main control
on the initiation and maintenance of the special sort of
pingos found in the Iterdlagssüp kügssua valley mouth.
Therefore, the mud volcanoes could be termed marsh
initiated pingos, a special kind of open system pingos.
The
Influence of the Local Topography on the Formation of
the Pingos
In the Mellemfjord area
several other U-shaped valleys have been filled partly
by glaciofluvial material and therefore they likewise
have a flat fluvially dominated bottom leading to the
fjord. Only one other pingo has been recognized from
aerial photographs in the innermost valley called
Kildedalen or Akugdlit itivnerat. This pingo is located
in the valley mouth in a position equal to that of the
pingos in the Iterdlagssüp kügssua valley mouth. In the
other valleys leading to Mellemfjord no pingos or
remnants of pingos have been found on aerial
photographs. In the main valley in the innermost part of
Mellemfjord (Kildedalen), several pingos have been
described by Donner (1978), the largest being 50 m in
diameter and 10 m high. These pingos are indirectly
given a maximum age of 5,000 BP by Donner (1978) based
on his reconstruction of the sea level curve.
The number of
pingos in the Iterdlagssüp kügssua valley
demonstrates that the conditions for the development
of
marsh flats in this particular valley are more
favourable
Side 47
than in any of the other
valleys ending in Mellemfjord. There are several
controlling conditions on marsh and pingo formation in
the Iterdlagssüp kügssua valley mouth and on the
distribution of the pingos in the Mellemfjord area:
1. In the Iterdlagssüp kügssua
valley the delta plain is in a much more protected and
sheltered position further inside the valley mouth than
in any of the other valleys ending in Mellemfjord, which
have recent deltafronts exposed along the Mellemfjord
coastline. This gives a wide and more flat delta plain
with much better conditions for mudflats and marsh flats
to form, just as the marsh flats are well protected
against icebergs.
2. The southerly exposure of
the Iterdlagssüp kügssua valley mouth must be crucial
for the ability of the mudflats to be vegetated and
thereby raised above the mean highwater level to become
marsh flats. As the mountains surrounding Mellemfjord
are rather high, about 1000 m asl., the Iterdlagssüp
kügssua valley mouth is the only protected valley mouth
that is not situated in shadow for longer parts of the
day during summer. This gives good conditions for plant
immigration and growth on the mudflats, making the
transition to marsh flats possible.
3. The Iterdlagssüp kügssua
valley is the largest valley ending in Mellemfjord and
has in its upper part several northerly exposed valleys
with glaciers ending in the central valley. During the
winter large amounts of cold air are flowing from these
high tributary valleys through the main valley enhancing
frost penetration of the ground at the exposed marsh
flats, where the pingos are found.
4. As the Iterdlagssüp kügssua
valley is relatively long with a large area draining to
the valley, much sediment is transported to the delta
area enabling the establishment of the marsh flats and a
large delta plain.
Dating
the Pingo Growth
The development
of the pingos in the Iterdlagssüp kügssua
valley is
closely linked to the geomorphological development
of the valley bottom.
Today the pingos are located
just above the mean highwater level, meaning that spring
tide high-water can still reach the marsh flats
surrounding the pingos. This position shows that the
pingos must be relatively young, as they have been
formed after sea level reached its present position
sometime after 2,700 BP.
According to
Mackay (1979) the maximum growing rate
for a pingo
through 5 years is measured to be 0.34 m/yr for
a mature pingo. As the pingo
grows higher, the growth decreases by the square root of
the age (Mackay & Black, 1973). Therefore the
Iterdlagssüp kügssua valley pingos can easily have
developed during a rather short time span. The Little
Ice Age (1400 to 1900 AD) would probably have offered
good conditions for frost penetration enabling the
development of the pingos. The pingos from the
Iterdlagssüp kügssua valley are tentatively dated to
this period.
Subsequent to the supposed
evolution of the pingos during the Little Ice Age, they
have been reduced in size by fluvial and thermal
erosion. However, as the area is situated in the zone of
continuous permafrost, the pingos have been preserved as
pingo remnants since the Little Ice Age. If the recent
fluvial erosion continues in the Iterdlagssüp kügssua
valley mouth or if sea level rises, the pingos might
disappear entirely.
Conclusion
A new term "marsh initated
pingos" has been suggested for a special kind of open
system pingos located on the highest parts of former
mudflats probably developed during the Little Ice Age.
Many of the so-called "mud-volcanoes" on Disko Island
and in West Greenland might be marsh initiated open
system pingos, as their geomorphic setting in the
landscape is similar to those described in this paper.
If this is true, a new and very sensitive tool for
obtaining further knowledge on sea-level change is
found, especially if absolute dating of the pingos by
the luminescence technique or relative dating by the
14C method can be performed. Simultaneously
the position of this sort of pingos can yield further
information on the influence of the local topography on
pingo growth. Finally the appearance of the pingos in
Mellemfjord shows that permafrost is present even in the
valley bottom, close to sea level.
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