Moulins of a subpolar glacier seen as a thermal anomaly

Moulins of a subpolar glacier seen as a thermal anomaly
Jacques Schroeder
Département de géographie, Université du Québec à Montréal, CP888,
Centre-ville, Montréal (Québec), Canada, H3C 3P8, e-mail: [email protected]
ABSTRACT
The moulins of a subpolar polythermal glacier are seen as a thermal anomaly in the body of the moving
glacier during winter. The air column trapped between the base of the snowcover and the bottom of the
moulin is, at that time, subjected to a convection movement that favours the consolidation of the snowbridge
(appearance of hoarfrost, regelation in the snowbridge). The moulins fill with water when basal ice creep
cause closure of subglacial channels. The presence of water in the moulins reduces the constriction that
would be caused by the lateral shear stresses of ice and provides the potential energy necessary to initiate
intra- and subglacial water circulation at the end of winter.
KEY WORDS: subpolar glacier, Moulin, snowbridge, internal meltwater.
Introduction
The conceptual model presented in this
article is based on on-site observations of
moulins that collect supraglacial water in the
ablation zone of a medium sized subpolar
calving glacier. The Hansbreen, a valley
glacier that flows from North to South, throws
itself into the Hornsund Fjord at Svalbard (77˚
00’ N - 15˚ 50’ E) (Glazovsky et al. 1991).
The moulins that were studied are all located
in the body of the moving glacier, upstream of
a section that is so fractured that no meltwater
can flow in an organised manner. It has been
shown that moulins in that specific area of the
ablation zone have « a functional lifespan that
can exceed 20 years » (Schroeder, 1998).
This paper aims at understanding as clearly
as possible what happens in these moulins
from the end of one summer until the next, i.e.
during a period when on-site observations are
rare as the moulins are closed by snowbridges.
We know that at Svalbard, the thermal
difference between arctic summer and winter
is important (even though winters are clearly
colder at about the same latitude in the
Canadian archipelago, for example). The
cause, obviously, lies in the duration of the
polar night which, at the Hansbreen latitude
(77˚Lat. N), lasts from October 31st to
February 11th. That is not all, however. To
these 104 days of complete polar night, during
which hardly any solar radiation reaches the
ground (1), almost two months are added at the
beginning and end of the midnight sun period.
During these periods, the sun is always below
or very close above the horizon line and
provides almost no warmth to nearly
horizontal surfaces like the glacier’s. The sun
must “rise” up to 10˚ above the horizon for the
heat to start accumulating in the snow that
covers the glacier (2). However, since the snow
has, in some ways, accumulated much cold
during the polar night, this calorific energy can
do no more than raise the snow temperature
which becomes closer to 0˚. As a result, on the
(1)
It is known that the illuminance of the full moon 30˚ above
the horizon provides 0,2 lux/m2 on a horizontal surface.
With the sun 10˚ above the horizon, sun and sky provide
15 000 lux/m2.
(2)
66
Jacques Schroeder
nearly horizontal surfaces of the Hansbreen,
the snowcover will not begin to melt before
June. This means that the glacier surface
receives no significant heat from the sun from
the beginning of September, two months
before the polar night, until the month of June,
two months plus one after the end of the polar
night. During this additional month, the
snowcover on the glacier surface warms, and
then melts.
However, from the end of June to the end
of August (a period of just two months), heat
transfers occur in the moulins in the moving
glacier body; these are in relation to the
movements of exterior air, the movements of
ice itself and the catchment of liquid water,
which brings latent heat as well as energy
produced by its own turbulent flow. The
volume and shape of the moulins are a
function of these inflows. This is already
known, although the conditions of the actual
origin of the moulins are – in my opinion – a
problem to which a satisfactory solution
remains to be found. It is also during this short
summer period that data is collected
concerning the moulins’ morphometry and the
way they work as input of intra-and subglacial
drainage. However, come September, once
snow covers the moulins and isolates them
from the exterior, observations of what goes on
inside them are rare. Since these moulins still
exist at the end of winter (Schroeder, op. cit.),
we are trying, in this article, to understand
what happens inside them during this period,
in the hopes of targeting the types of
observations that are required to ensure the
validity of our understanding. It has to be
stressed: considerations are related to the case
of active moulins without turbulent water
intake.
A thermal anomaly inside the glacier
Svalbard glaciers have a polythermal
structure with upper layer of ice (called cold
ice) in temperatures lower than the pressure
melting point and underlaying temperate ice
layer (i.e. in the temperature of the pressure
melting point). Such structure is observed in
the ablation area of laciers and the cold ice
layer can not reach up to 100 m (in the
accumulation area entire glacier body has
temperate ice (Moore et al. 1999). By their
very presence in the glacier body, moulins can
be considered a thermal anomaly. This
anomaly presents boundary conditions that are
defined by ice temperature, water (trapped
inside glacier) temperature, and meteorological
fluctuations above the chimney entrance. In
the moulin, heat exchanges at the boundaries
comprise 4 types: air forced convection,
conduction throughout surrounding ice and
water, advection by meltwater circulation
through the snowbridge and seasonal thermal
radiation. Once these parameters are identified,
their interaction may be understood as a series
of scenarios spanning from the month of
August to the following month of July.
Summer (August) autumn (September)
In August, most of the moulins of the
Hansbreen are accessible. In the larger
subjects, which obviously allow for a more indepth exploration, the bottom is generally
drowned, unless it is too narrow, and the water
level varies constantly in immediate response
to meteorological conditions. Some moulins
have also been known to fill up and spill over
on the glacier surface, following important
precipitations. The circulation of intraglacial
water in the summer has thus opened a
network inside the glacier, with a finite
absorptive capacity; this maintained the water
level in the larger moulins at an average of 70
meters under the glacier surface according to
our observations, although on some occasions
we observed water levels as low as -150
metres. Air, ice and water have different
temperatures. Outside, the average temperature
remains just above 5˚. Because of the general
slope of the glacier and the coldness of its
surface, a cool air flow slides downstream on
its surface. This adiabatic air circulation brings
about a slow air convection inside the moulin.
All through summer, this low air exchange
inside the chimney generates the superficial
Moulins of a subpolar glacier seen as a thermal anomaly
melting of the moulin’s walls, which are most
often smooth and even. When the moulin is
active, the water falling inside the chimney and
the resulting air turbulence cause deeper
grooves in the moulin’s wall. Whatever the
shape of these walls, once the surface water
has run dry (this happens at the end of August
or beginning of September), the more or less
regular moulin chimney can be compared to
the simplified model proposed here.
67
especially in September (Fig. 1), snow falls on
the Hansbreen, always accompanied by violent
and cold winds. This blizzard pushes dry snow
grains that catch on the ground surface after
hitting it at an angle, sometimes even bounce
on it and eventually pile up according to the
surface roughness, not unlike deflation
processes in the desert. From the exterior edge
of the moulin, downstream of the wind, a ridge
of packed snow takes shape and slowly creates
a cover on the moulin. At Svalbard, 48-hour
blizzards frequently cover the surface of the
Hansbreen with an average 30 cm of hardened
snow. During the blizzard and immediately
after, this packed snow is hard enough for one
to walk on it without sinking. This shows the
cohesion of the snowcover which nonetheless
depresses a few centimeters on top of the
moulins it has covered (Photo 2).
Photo 2. A slight depression (around the shovel) allows
us to locate the moulin following the first blizzard.
Photo 1. Snowbridge seen from the bottom of a moulin,
in August.
In August, it was also possible to enter a
moulin that was still closed by a snowbridge,
via a crevasse that had become wide enough
during the summer (Photo 1). This proves that
snowbridges only block off the top of the
moulins and that, under certain conditions as
we will see later, the width of the moulin is not
reduced by the lateral push of the ice.
As early as the end of August and
This snow, carried by an ice cold wind,
thus creates a cover on the moulin, inside
which an air column remains trapped. The
temperature of the snowbridge that serves as
cover is obviously significantly lower than the
water at the bottom of the moulin, which
remains slightly above 0˚. Furthermore, the
closure of the moulin puts a stop to the
convection movements generated by the
adiabatic air circulation on the glacier surface.
Now, the trapped air column inside the moulin
is constrained at its base by a surface warmer
than that of the snowbridge. The air column in
68
Jacques Schroeder
the chimney is thus submitted to a thermal
inversion.
Autumn (September-October) Part one
As autumn moves along (September and
October), the number of hours during which
the sun rises above the horizon decreases and
the blizzards grow more frequent. The average
air temperature is now negative and still
decreasing. The snow, packed by the wind,
accumulates on the glacier surface; during
winter, it will reach an average of more than 3
meters, as high as 10 meters in some areas.
From a thermal point of view, the moulin is
increasingly isolated from the exterior. This
isolation is generated by two factors: the
thickness of the snowbridge itself and the
insulating property of snow. It is thus possible
to identify two sites where the boundary
conditions are different: the closed chimney
proper and the snowbridge.
In the closed chimney, the air becomes
mobile to balance the difference in temperature
between the “cold” roof and the “warm” water
at the bottom (1). The rising “warm” air is
stopped by the “cold” base of the
snowbridge and loses its heat by this
contact.
This
results
in
the
condensation of the water vapour it
contains, which in turn causes
“condensation ice [that] occurs in the
form of needles, rosettes or hexagonal
crystals” (Luetscher, 2005, p.14).
These ice crystals, or hoarfrost, are
well known in all caves that present a
thermal anomaly caused by seasonal
temperature variations or by the
presence of permafrost (Ford,
Williams, 2007, p. 294-298). When
we explored four (4) moulins in May,
before the spring thaw, a deep, narrow
opening allowed us to dig our way
into them; we observed hoarfrost
patterns with intertwined crystals that
reached lengths of 5 cm and made up
a cover of varying thickness, from 2-3
cm up to almost 20 cm. Of course, the
“warm” air inside the chimney rose in
its center, while the “cooled” and
“dry” air flowed down the sides of the
chimney. Considering that the ice
temperature, a few meters below the
glacier surface, is approximately 0,1˚(2), the air becomes warmer and
picks up humidity as it goes down and comes
in contact with the ice and water at the bottom.
The convection movement of the air trapped
inside the chimney will last as long as the
difference in temperature between the “cold”
roof and the “warm air” is sufficient. Such air
convection supplies the top of the moulin,
closed by the snowbridge, with hoarfrost.
(1)
Even if the bottom of the moulin is not filled with water,
thermal inversion still occurs in the moulin because ice at the
bottom remains close to 0˚C in this polythermal glacier.
(2)
Close to its surface, on a maximum thickness of 10 m, the ice
is colder as it has been exposed for a long time to cold air during
winter.
Moulins of a subpolar glacier seen as a thermal anomaly
Autumn (September-October) Part two
Because of the way it was created by the
blizzard (cf. supra), the snowbridge also has a
kind of cork that increases the thickness of the
snowcover above the moulin entrance. Due to
heat transfers at the boundaries (very cold air
temperature outside, close to 0˚ air temperature
inside the moulin) and to the insulating
property of snow, the snowbridge becomes
slowly warmer than the snowcover around.
When the temperature at the base of the
snowbridge approaches 0˚, some snow crystals
melt, and induced water flows down through
the snow crystals. As water moves towards the
bottom and comes in contact with ice crystals,
it freezes again. This regelation has two
consequences. From a mechanical point a
view, the water, as it refreezes, causes the ice
crystals in the wind-packed snow to bind
together, which increases the rigidity of the
cork at the base of the snowbridge. Its span is
69
autocatalysis of the process begins. In time, the
induced water percolates through the
snowbridge and creates ice stalactites on its
base, which will be covered with hoarfrost as
soon as they become inactive (Photo 3).
Photo 3. Close-up on the base of an open snowbridge, at
the end of winter. The hoarfrost covers the base of the
snowbridge and the ice stalactites.
As winter progresses, the heat
balance in the snowbridge body leans
more and more towards « cold »; this
stops the production of induced water
inside the snowbridge and keeps the
roof of the chimney at a temperature
that is lower than that of the air that is
trapped inside he moulin.
thus increased as the snowbridge’s gravity
center is lowered, which in turn increases the
lateral pressure of the snowbridge on the
moulin’s edge (P on Fig. 3).
The second consequence of the regelation
of induced water in the snowbridge’s cork is of
a thermal nature. It is well known that when it
freezes, water generates an efficient
exothermic reaction. The production of
calories in the snowbridge cork allows it to
remain “warmer” than the rest of the
snowcover. As soon as a few ice crystals begin
to melt in the snowbridge, a kind of
Early winter (November-December)
During November, the glacier
behaviour is modified. The glacier
movement is usually slowing down.
The glacier velocity and its calving
velocity decrease and seismic activities
on the glacier front is also reduced;
and the sea ice cover begins to freeze at the
glacier front (Jania, 1988).
As a consequence, the glacier’s internal
drainage is blocked by closure of in- and
sublacial tunnels. However, we know that
water remains in this polythermal glacier
(direct author’s observations and Lliboutry,
1996). This water inside glacier discontinuities
flows down by gravity into moulin chimneys,
the biggest empty voids in the glacier body.
This hypothetical scenario is supported by the
following observation: in each of the moulins
that were explored at the end of winter, we
70
Jacques Schroeder
observed the remains of horizontal and thin ice
floors at the top of the chimneys (Schroeder,
op. cit.). This can only mean that water trapped
inside the glacier rose to that level (cf. Fig. 5).
At the beginning of winter, moulins work like
kinds of surge tanks. When water rises in the
chimney, to levels observed from 10 to 30
meters below the glacier surface, the volume
of air imprisoned at the top of the chimney is
reduced significantly as compared to the
situation at the time when snowbridges close
the tops of the moulins (Fig. 1 and 2). The
reduced air volume becomes colder. There is
more “cold” available to the air’s convection
movements than “warmth” that can be
provided by the trapped water. Congelation ice
appears on the edge of the water surface. The
surface area and thickness of the congelation
ice depend on the stability of the water level in
the moulin.
If the water level rises so that it reaches the
glacier surface, the transfer of “cold” from the
outside generates fast freezing of water
surface. Then the moulin is closed by a
recrystallised ice cap. Unfortunately in
literature, this phenomenon is often called
“recrystallised moulin”. This is incorrect
because the “cold” from the outside can only
affect the superficial part of the water (no more
than a few decimeters), while the water, by its
very presence, prevents the constriction of the
moulin. As a result, when the water level in the
moulin slowly decreases (Fig. 5), the moulin
still exists as an empty void inside the glacier
body.
Winter (November-May)
It is now possible to consider what happens
in the moulins during the entire winter. Since
winter lasts more than six months, the
temperature of the snowcover decreases
slowly, despite the snow’s insulating property.
The base of the snowbridge becomes colder
and keeps the ice crystals in the cork from
melting (cf. Fig. 3), which stops the growth of
ice stalactites on the moulin’s roof.
In May, we observed thin congelation ice
“rings” on the moulin’s walls (Photo 4). These
rings were observed on a height that spanned
30 meters from the highest ring to the ice floor
in contact with the water that remains in the
moulin (see also Fig. 4). It is important to note
that at the end of winter, the water level in the
moulins is higher, everywhere, than it
is in summer, except during brief
periods of flood as explained
previously. The various “ice rings” are
0,5 to 3 cm thick and the distance
between rings varies from 20 to 50
cm. This shows that after the water
has almost completely filled the
moulin, it lowers in jerks. The
thickness of congelation ice shows
how stable the water has been at that
level. When the water level decreases
in a jerk, the water no longer supports
the congelation ice that formed on its
surface and adhered to the chimney
wall. The thin ice breaks and only a
ring remains on the wall (Photo 4a).
As a result, between regelation occurrences of
the stable water surface, air convection occurs,
activated by the thermal inversion between the
top and bottom of the chimney (Fig. 2).
The growth of hoarfrost can thus go on.
Also, the thick ice floor observed in May
(Photo 5), just before thaw, suggests that the
water level lowering stops during a long
enough period, at the end of winter, for the
congelation ice to thicken considerably (more
than 10 cm). In the four moulins explored in
May, the level of the water that drowned them
had increased 20 to 30 meters as compared to
Moulins of a subpolar glacier seen as a thermal anomaly
the level measured in August and September,
the previous year (Schroeder, op. cit.).
71
the constriction of the moulin, under the lateral
push of ice, is only slightly, if at all efficient in
a complete summer–winter cycle. The moulins
must be seen as surge tanks that remain a
thermal anomaly, year after year, in the body
of the moving glacier. As soon as, in winter, a
moulin no longer collects water from various
smaller discontinuities, it obviously collapses
under the lateral push of the ice. This is
thoroughly documented but does not explain
the durability of moulins in the body of
moving glaciers.
Photo 4. Thin sheet of congelation ice at the top of a
narrow moulin (observed in May).
Photo 5. Thick floor of ice. The water is visible
underneath, as well as the debris of thin sheets of
congelation ice (observed in May).
Photo 4a. A kind of ring is visible behind the figure after
the water level has lowered in the moulin (observed in
May).
Although
our
scenario
is
partly
hypothetical, it shows that water is present in
the moulins throughout winter. This
information allows us to better understand why
Spring - early summer (June-July)
What remains to be reviewed is what
happens in the moulins during the short period
(less than two months) that links winter to
subpolar summer. The exterior temperature,
aided by the midnight sun period remains
slightly above 0˚ for many hours during the
24-hour cycle. As a result, the snow
temperature increases rapidly on the entire
72
Jacques Schroeder
thickness of the snowcover due to the
exothermic reaction generated by the
regelation of early meltwater. When it has
been warmed and close to 0˚, the snowcover
works like a hanging aquifer, disappearing
quickly. Meltwater flows through the snow
and onto the glacier surface and is drained into
the moulins via rills that were driven
the previous summer. The water then
fills the pores of the partly icy
snowbridge and refreezes in the cork
of the snowbridge. The snowbridge
thus reaches its maximum solidity.
Once the snowcover has disappeared,
only the cork of the snowbridge
remains; made of ice and refrozen
snow, it closes the top of the moulin.
If the cork is too thin for its own
weight and span, it falls down (Fig.
6).
In the moulin chimney, the water
column contains a great deal of
potential energy, proportional to its
height in the chimney. One must also
remember that during some summers, several
moulins that had emptied of their water
content could be observed to be as deep as 150
meters under the glacier surface. So in winter,
the water occupies volumes “in the range of
thousands of cubic meters, mining the glacier’s
active body to depths at least 150 m”
(Schroeder, 1998). At the base of the moulins,
this water column exerts a hydrostatic pressure
superior to the bonding power that ensures the
cohesion of ice crystals(1). So the
retention water in each moulin sets
intraand
subglacial
water
circulation starting in keeping with
calving and glacier velocity of course.
This implies that sea ice breaks up at
the front of the glacier. What remains
to be understood is how icequakes at
the glacier front are involved in this
scenario. This, to us, is an open
question that goes widely beyond the
purpose of this article.
Another possibility is that enough
meltwater from the snowcover is
drained towards the moulins, via the
rills, for the snowbridge to become
saturated. In such a case, the meltwater would
percolate through the snowbridge and fall into
the water column that is already trapped inside
the moulin. The push of the water column
would also exceed the ice crystals’ resistance
threshold. The intra- and subglacial drainage
could be started in this manner.
Finally, the mass balance of the Hansbreen
having been negative for decades, the glacier
surface in the ablation zone where the moulins
were studied lost an average thickness of ca.
1 m/year (Jania, 1988). Currently, this ablation
(1)
One application of Glen’s Law shows that an 80-meter water
column in the polycristalline ice of a temperate glacier is heavy
enough to dissociate the ice crystals and sink into the glacier
body.
Moulins of a subpolar glacier seen as a thermal anomaly
is superior to 1 m /year. It causes the snow and
ice corks that close the moulins to disappear.
When they are too thin, the corks collapse,
73
which puts the moulin in contact with the
exterior, as illustrated at the beginning of the
various scenarios presented in this article.
Conclusion
The model, presented in a series of
scenarios that cover a complete winter,
concerns the moulins located in the ablation
zone of subpolar polythermal glaciers, where
the ice is still moving. Excluded from our
model are moulins developed in stagnant ice,
which have been described in literature for a
long time (Clayton, 1964).
These scenarios, although they are based on
a restricted number of on-site observations,
allow us to better understand what happens in
theses voids that measure thousands of cubic
meters and remain in the glaciers’ bodies
during a period when the glaciers is moving
slower than during the ablation season.
Collecting water that has not frozen in the
multiple glacier discontinuities generated by
its summer progression, the moulins work
through winter like surge tanks. Their water
levels are variable, raising close to the surface
at the beginning of winter, then lowering in
jerks, and finally remaining, at the end of
winter, at a height that is superior to the
preceding summer’s average level. It is
reasonable to think that the hydrostatic
pressure exerted by water trapped in these
moulins is a determinant factor to initiate the
circulation of intra- and subglacial water when
winter ends. The presence of water in the
moulins in winter also allows a better
comprehension of the reasons why the moulins
are submitted only to a low constriction, to
such a point that “the life expectancy of
moulins can reach several dozens of years”
(Goudie, 2004, p. 700).
However, to be validated, this model
should be confirmed by further on-site
observations and confronted to more systemic
glacier behaviour approaches such as
Mavlyudov’s (2006).
Acknowledgements
The research missions are realized through an
international cooperation agreement between
the University of Silesia and the University of
Québec in Montréal. We thank the Science
Academy of Poland and Svalbard’s Norwegian
authorities for their support. In the field, the
Polish polar base’s team has always provided
us with help. Thanks to Prof. M. Pulina and
Prof. J. Jania for their welcome and their
support. Finally, I was successively helped by
P. Gagnon, M. Tremblay, Dr. A. Tyc, C.
Paradis and R. Gagnon with glacier
exploration. F. Gionet translated the
manuscript. The figures were illustrated by A.
Parent.
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