SUB-SURFACE MELT POOLS IN THE MCMURDO ICE SHELF, ANTARCTICA
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
J Ollrtlal ol Claeiology, V o l. 7, TO. 51 , 1968
SUB-SURFACE MELT POOLS IN THE McMURDO ICE
SHELF, ANTARCTICA
By R. A. PAIGE
(U .S. Naval Civil Engin eering Labora tory, Port Huen em e, California 93041 , U .S. A. )
ABSTRACT. Sub-surface melt pools were discovered during constru ction of a n a irfield located on glacier
ice in th e western part of the M cMurdo I ce Shelf, Antarctica. The m elt pools occur beneath areas of blue
glacier ice a nd th ey are impossibl e to d etect by visua l exa mination. Th ey va ry in size and sha pe but are
usua ll y 1. 0 to 1. 5 m deep and spa n circul ar areas 10 to 15 m in diameter . Sub-surface melting starts in mid-
D ece mber a t depths of 4 0 cm or more a nd it progresses until late J anu ary wh en refreezing begin s. Th e ice
cover over melt pools may thin to as littl e as 7 cm and this creates a serious hazard to a ircraft opera ti ons.
The melt pools a re ca used by the greenhouse effect of intense solar radia tion, low a lbedo of th e blu e glacier
ice and heat absorption by rock particles and dust. The hi gh-a lbedo layer of chipped ice a nd powd ered ice
th at was produ ced during runway constructi on was compl etely successful in preventing sub-surface melting.
The thickn ess of this protective layer appea red to be of littl e importa nce, providing it exceed ed 3 cm .
R ESUME. Poches deionte sous-glaciaire du M cMurdo Ice Shelf, Antarctiqlle. D es eau x de font e etai ent decou vertes
da ns la couche inferieure d e la glace de glac ier a u cours d e la construction d ' un aerodrome d ans la partie
ou est d e M cMurdo I ce Shelf, Anta rct ique. Les eaux de fonte se tro uva icnt a u-d cssous cles etcndu es d e glace
bleue de glacier et il est impossibl e de les decouvrir par examen visucl . ElI es varient en volum es et formes, ma is
cll es avaient genera lem ent de I a 1, 5 m d e profondeur et de 10 a 15 m de di a metre. L a fu sion d ans la couche
inferi eure commence mi-d ecembre a la profond eur de 4 0 cm ou plus, et s'accroit jusqu ' a la fin de j anvier
qua nd le regel comm ence. La couverture de glace a u-dessus des ea ux d e [onte peut s'a mincir a 7 cm et
dev ient un risqu e seri eux a ux operations aeri enn es.
Les eaux de fonte sont les resulta ts de I'effet de serre de la radia tion sola ire intensive, la faibl e a lbedo de la
gl ace bleu e d e glacier et I'a bsorption de chal eur pa r les pa rticules d e roche et poussiere. La co uche d e forte
albedo de glace ecl atee et pul verisee, que I' on proclui t a u cours de la construction de la piste, prev ient
compl etem ent la fu sion d ans la couche inferi eure. L'epa isseur de cette co uche protective a peu d ' importance
tant qu ' elle cl ep asse 3 cm.
Z USAMM ENFASSUNG . Schmelzlocher unler der Oberjlache im Nlc M urdo Ice Shelf, Antarktika. Beim Ba u eines
Flugfelcl es a uf dem Gl etschereis des westli chen M cMurdo I ce Shelf, Antarktika, wurd en Schm elzliicher
unter der Oberflache entd eckt. Die Schmel zliicher treten unter Gebieten mit bl auem Gletschereis a uf und
sind durch blossen Augenschein ni cht a uszumachen . Sie va rii eren in Griisse und Gesta lt, sind aber gewiihn-
li ch 1, 0 - 1, 5 m ti ef unci nehmen Kreisflachen von 10 - 15 m Durchmesser ein. D as Schmel zen unt er der
Oberflache beginnt Mitte Dezember in mind estens 40 cm Ti efe und halt bis spat in den Janu ar a n , wo das
Gefri eren wi eder einsetzt. Di e Decken der Schmel zlocher konnen bis a uf den kleinen Betrag von 7 cm
a bn ehm en , womit eine ernste G efahr flir d en Flugbetrieb entsteht.
Di e Schmelzl ocher entstehen durch den Treibha useffekt intensiver Sonn eneinstra hlung, clie geringe
Albedo d es blauen Gl etschereises und die W arm eabsorption durch G esteinssplitter unci Staub. Di e Schicht
mit sta rk er Albedo a us abgehobeltem und pul verisiertem Eis, di e beim Ba u cler L andeba hn entsteht, ergab
ein en vollkommenen Schutz gegen das Schmelz en unter der Oberflache. Di e Di cke di eser Schutzschi cht
erwi es sich von geringer Bed eutung, vora usgesetzt, dass sie 3 cm llbersti eg.
INTRODUCTION
Sub-surface m elt pools occur b en eath blue g lacier ice in th e w estern p a rt of th e M cMurdo I ce S h elf,
Antarc ti ca. In th e austral summ er of 1965- 66, a reconnaissan ce of this a rea fo r an alterna tive airfield
disclosed several m elt pools; however, the ir full extent wa s not r ealized at that tim e . During con-
struction of th e "Outer Williams Fie ld" alternati ve airfi eld , th ese m elt pools w ere found to ex tend
throughout the proposed runway area and they presented a serious haza rd to aircra ft traffi c .
In " D eep Freeze 67", an investigation was initi a ted to determin e the extent, seasona l history and
cause of sub-surface melt pools in an e ffort to find th e b est m eans o f preventing the ir o ccurren ce within
the runwa y area . This re port d escribes th e melt p ools, their orig in, th e m a in fa ctors contributing to
th eir formation and th e effec tiven ess of a chipped ice and snow surface in preventing th eir form a tion .
LOCATION AND PHYSICAL SETTING
"Outer Williams Field" is located on the vast, rel a ti vel y smooth expanse of g lacier ice in the w es tern
part of the M c Murdo Ice Shelf a t approxim a tely lat. 77 0 57' 4 0" S. , long. 166 0 28 ' 30 " E . (Fig. I) .
Access from M cMurdo station to "Outer Williams Field " is provid ed b y 22 km of compacted-snow
road via S cott Base a nd th e present Williams Field Air Facility (Fig. 2) .
SI I
JOURNAL OF GLACIOLOGY
The airfield is west of a transitional climatic zone where th e annual budget changes from positive to
negative. To the east of this zone, accumulation exceeds a blation; to the west, ablation exceeds accumu-
lation and the glacier surface is partially free of snow throughout most of the summer a nd fall (Paige,
1966, p. 6) . Little is known concerning th e local climate in the western part of the ice shelf except tha t
the intensity of melting and ablation increases westward towards V ictoria Land .
w
o
tll
McM URDO SOUND
ROSS ICE SHELF
°L________-16i_________3J 2kmI
I I I
o
I,!"
5
t
10
I
15 20 miles
Fig. I. Index map of the McM1lrdo S01lnd region, Antarctica.
DESCRIPTION OF MELT POOLS
The sub-surface m elt pools occur throughout an elongated, poorly defined, I ro-km z area that trends
north-south in the western part of the McMurdo I ce Shelf. The glaci er surface in this area has a
mottled appearance caused by thin, patchy snowdrifts and low mounds of wh ite bubbly ice distributed
in a random pattern on clear, blue glacier ice. The melt pools are impossible to d etect by visual examina-
tion but they occur exclusively beneath the blue ice. The topography of the area is relatively smooth
with a northerly slope of only a few feet per mi le and a subdued hummocky surface having a relative
relief of 30 to 60 cm. The melt pools vary widely in size and shape but they are generally 0.5 to 1.0 m
SHO RT NOTES
d eep and span circular or elliptical areas 10 to 15 m in diam eter. Starting in mid-December, the ice over
th e m elt pools decreases in thickness from 30 or 40 cm to as little as 7 cm by mid-January and it may not
support even lightweight tracked vehi cles. Farther to th e west towards Vi ctoria Land, surface melt
features become increasingly common in th e form of open melt pools and surface melt-water run-off.
Th e sub-surface melt pools a lternate wit h low, gently rounded mounds of white bubbl y ice. These
ice mounds a re seldom more than 60 cm high and span a circular a rea varying from 2 m to more than
McMurdo Ice Shelf
McMUROO SOUND
(Annual Sea Ice)
Sea Ice Runway
c::
Air Facility
OF·63·65 William s Field
(Abandoned 2·65)
c:.:::::::.:::::::: __
Ice RUn--"::::::.1
Way
0~!______1~.9_0_0____2~.OpOm
o
!
4.0pO 8.090 ft
"" McMurdo Ice Shelf Scale
" ""-
"
"
Transition " \
Zone \
- - Road
/I§?I'I Pressure areas
'\. \
Ablation \\ \, Accumulation Zone_
~ Zone
\ \
\ \
\
\
Outer Williams Field
Fig . 2. Sketch map of the McMurdo station to " Outer Williams Field" area.
JOURNAL OF GLACIOLOGY
8 m. Cailleux (1962, p. 132) has described similar mounds forming on small lakes elsewhere in th e
McMurdo So und region . The ice mounds are similar in many r esp ec ts to pingos that occur in p erma-
frost regions of the Arctic (Muller, 1947, p. 59) and here they are termed ice p ingos.
ORIGI N OF T H E MELT POOLS
The melt pools are caused by the greenhouse effect of intense solar radiation, low a lbedo of th e blue
glacier ice, and h eat absorption by dark obj ects. Sub-surface melting b egins in mid-December and
it consists of intergranular moisture as eviden ced by moist auger cuttings. T hi s initial melting, w hich
usually occurs a t d epths of 40 cm or more, mayor may not be associated with the presen ce of dirt. As
melting progresses, a lenticular pool forms with sub-surface boundaries determin ed by the surface
configuration of snowdrifts and ice pingos. Melting progresses both upwards and d ownwards, and
eventually the ice cover over the sub-surface pool d ecreases to as little as 7 cm, and the total depth may
reach I m .
As winter approaches, the m elt pools begin to refreeze and the enclosed water is trapped under
increasing hydrostatic pressure. Eventuall y, this pressure is r elieved either when th e entire roof of the
melt pool is uplifted to form an ice p ingo or wh en cracking releases the water as a surface flow. Tension
cracks radia ting from th e center of the mound develop and release a ny remaining water.
T he surface features throughout the vast m elt- pool a r ea near "Outer Willia m s Field" constantly
c hange in response to th e seasons. Melt pools that form during th e summer may becom e ice pingos the
following win ter. The effects of sublim a tion, meltin g and wind may even tually r edu ce the topographi c
expression of an ice p ingo so th at it once again becomes a m elt pool.
The sub-surface melt pools appear to be a feature u nique to th e western part of the McMurdo I ce
S helf. Reports of similar features a re rare in th e literature and d escribe vario us melt phenomena occ ur-
ring on fro zen la kes or bay ice elsewh ere in Antarctica. Som e of the ice mound s d escribed by Cailleux
(1962, p. 13 I , 132) have b een studied by th e author and they were found to be ice pingos resulting from
the r efreezing of sub-surface m elt pools identical to thos e near "Outer Williams F ield".
Van Autenboer (1962, p . 35 I) has described ice mounds on a la rge frozen lake in the S0r-Rondane,
Antarctica, that appear to be identical to those in th e M cMurdo So u nd region and p robably form ed by
th e sam e process. He h as a lso d escribed oth er me lt phenomen a (p. 352 ) th at are common on the
M cMurdo I ce Shelf, especia ll y n ear Black I sland and west of " Outer Willi a ms F ield ".
T a kahashi ( [1960J, p. 322 ) has described melt pools that occur in snow and snow-ice deposited on
saline bay ice of th e Prins Olav Kyst of Antarctica. These m elt pools ar e simila r in size and shape to
those in th e M cMurdo I ce Shelf a nd they are often con cealed by a crust of ice and snow (p. 323, 325 ) .
Their origin is attributed predomin a ntly to the effects of solar rad iation and the a bsorptivi ty differ ences
of snow a nd ice (p . 326, 330), a lthough hig h ambien t temperatures a nd warm winds a re al so co nsidered .
Other sub-surface melt pools a nd ice mounds probab ly occ ur und er similar climatic condi tions elsewh ere
in Anta rcti ca but th ey have not yet been di scovered or reported.
F AGTORS AFFECTING SUB-SURFACE MELTING
A combination of factors , such as solar radiation, a lbedo, dark-colored rock particles in th e ice, air
temperature and the presence or absence of snow, contribute, with varying d egrees of impor tance, to
sub-surface melting. Of these fa ctors, sola r r adiat ion, a lbedo and th e presence or absence of snow
p robably have the g reates t effect.
SOLAR RADIATION
Solar-rad iation energy is probal:: ly th e single m est im portant fa ctor con tri buti ng to su b-surface
melting. G laciological studies in Alaska show that solar radiation furnis hes a t leas t 75 per cen t of th e
energy requ ired to melt ice (Mayo and Pewe, 1963, p. 640) . Sagar (1962, p. 18, 19) has stated that
radiation is of prime impo rtan ce to th e ablation season in the central A rctic. Solar-radiation data for
the McMurdo Sound region h ave been presented by Thompson and Macdonald (1962) . These data
show a hig h percentage of sunshine and high-intensity radiation that approaches 105 kJ jcm 1 m
mid-summer (p. 887, 880) .
SHORT NOTES
ALBEDO
The refl ectivity, or a lbedo, of th e surface regulates th e h ea t energy absorbed and it is as important as
solar radiation in causing various m elt featur es on glaciers. Diurnal a nd seasonal albedo variations a re
common and they are caused by solar angle, color, grain form , solid impurities (M elior, 1964, p. 93 ),
surface roughness, m elting and refreezing, and cloud cover (Diamond and Gerdel, 1956, p. 5) .
Albedo of snow
Freshly fall en snow, or fin e-grained drift snow, probably has th e highest albedo of a n y n atural sub-
stance. Published va lues vary from 70 to 95 per cent but th ey are mostly above 80 per cent (Mantis,
1951 , p. 64, 66; Diamond a nd Gerdel, 1956, p. 5, 6). M easurements over newly fa llen snow near
Willia ms Field gave an a lbedo of 86 per cent. As snow ages, the alb edo d ecreases because of changes in
grain-size, color and density (Mantis, 1951 , p. 64 ; Chernigovskiy, 1966, p. 154- 57). Melting and
refreezing forms a n icy surface that a lso increases th e absorption and scattering of solar energy. The
presence of a light, fine -grained snow cover, even if thin , affords considerable protec tion against solal-
radia tion .
Albedo of ice
The alb edo of ice is considerably less th a n that of snow a nd it is affected largely by color, impurities
and th e presence or absence of m elt water. Ch ernigovs kiy (1966, p. 162) has re ported a lbedo va lues from
72 p er cent (probably for clean wh ite ice) to 36 per cent for blue ice benea th m elt wa ter. L yons a nd
Stoiber ( 1959) have st ressed th e difference betwee n fl awless ice and bubbly ice in m easuring th e adsorp-
tion (p. I I) of various wavelengths of energy. They have further sta ted (p. 8) that th e energy absorbed
by bubbly ice is as much as twice th at absorbed by flawl ess ice. Air bubbles are common in glacier ice,
especially in the vicinity of " Outer Williams Field", where the ice above and below the m elt pools is
quite bubbly a nd has a d ensity of only 0.88 Mg/ m J. A lso, th e ice associated with th e m elt pools was d eep
blue or aquamarin e and had an a lbedo of 48 per cent.
EF FECTS OF DIRT I N I CE AND SNOW
The presence of dirt or other dark contaminant lowers the albedo, abso rbs h eat and accele rates
melting. Dirt of volcanic origin is common in the ice a nd snow at "Outer Willi a ms Field". This material
is transported by high winds from Black Island and Brown P eninsula, and it has a size distribution
ranging from fine dust up to particles 6 mm in diam eter. The bubbly glacier ice at "Outer Wi ll iams
Field" contains isolated grains or thin discontinuous layers and lenses of dirt throughout at least the
upper 2 m of ice. Also, the bottoms of man y m elt pools have a thin layer of dark mud and sand , usually
at depths of 1.0 to 1.5 m.
PREVEN TION OF SUB- SURFACE MEL TI NC
Sub-surface m elting does not begin until earl y or mid -D ecember, and refreezing beg ins in late
January; the maximum p eriod of sub-surface melting is therefore 10 to 12 weeks. A high-albedo surface
layer with some insulating properties is needed to protect the und er lyi ng ice from intense solar radiation
during this short period.
The runway at "Outer Wi ll iams Field" was prepa red for use by leveling and smoothing th e surface
with wheel-mounted pul vimixers. Pulvimixing produces a layer 7 to 15 cm thick consisting of chipped
ice and powdered ice mixed with snow. This white, d ense layer had an albedo of 76 p er cent, and it was
completely effec tive in preventing the formation of melt pools within the runway area.
Patches of bare ice caused by high winds or traffic occasionally d eveloped on the runway. Sue-surface
melting quickly occurred und er man y of th e larger bare areas but no distin ct voids ever form ed. Sub-
surface melting beneath large, semi-stable snowdrifts or pulvimixed ice was not d etected at any time
during th e summer of " Deep Freeze 67" or " D eep Freeze 68". The thickness of snow or pul vimixed ice
seem ed to make little difference in preventing sub-surface m elting provided that the snow or ice cover
was more than 3 cm thick.
CON C LUSIONS
It is apparent that the sue-surface melting in th e vicinity of "Outer Wi lliams Field" is controlled by
a d eli ca te balance between solar radia tion, albedo, snow cover, and air and ice temperatures. Less than
5 16 JOURNAL OF GLACIOLOGY
2 km to the east, melting does not occu r and snow accumu lation begins to exceed ablation. On ly 6.5 km
to th e west, surface melting and surface melt-water run-off predomina te over su b-su rface melting, and
abla tion grea tly exceeds accumu lation. The delicacy of th is balance is further d emonstrated by th e fact
that a relatively th in, high-a l bedo layer is a ll that is required to prevent sub-surface melting.
MS. received 29 April 1968
REFERENCES
Autenboer, T. van. 1962. Ice mounds a nd melt phenomena in the S0r-Rondane, Antarctica. Journal of Glaciology,
Vol. 4, No. 33, p. 349- 54·
Ca illeux, A. 1962. Ice mounds in frozen lakes in McMurdo Sound, Antarctica. Journal rif Glaciology, Vol. 4,
No. 3 1, p . [3[ - 33.
Chernigovskiy, N. T. [966. Radiational propert ies of the centra l Arctic ice coat. (In Soviet data on the Arctic heat
budget and its climatic influence. Santa Moni ca, Cali f. , Rand Corp., p. [5[ - 72. (Memora ndum RM50030PR .))
Diamond, M., and Gerdel, R. W. [956. Radia tion measurements on th e Green land ice cap. V.S. Snow, Ice and
Permafrost Research E stablishment. Research Report [9.
Lyons, J. B., and Stoiber, R . D. 1959. The absorptivity of ice : a critical review. Hanover, New Hampsh ire, Dartmout h
College. (AFCRC- TN- 59- 656, Scientific Report No. 3, contract AF [9 (604) 2[59.)
M antis, H. T., ed. [951. R eview of the properties of snow and ice, with reports by H. Bader, C. S. Benson, P. P .
Bey, R. H . Doh erty, R . ]. Goldstein, ] . A. ]oseph, S. W. Rasmussen, D. C. Schiavone. V.S. Snow, Ice and
Permafrost Research Establishment . Report 4.
M ayo, L. , and Pewe, T. L. 1963. Ablation a nd net total radiation, Gulka na G lacier, Alaska . (In Kingery, W. D .,
ed. Ice and snow; properties, processes, and applications: proceedings of a conference held at the Massachusetts i nstitute of
T echnology, February 12- 16, 1962. Cambridge, Mass., M .LT. Press, p. 633- 43. )
Melior, M . 1964. Snow and ice on the Earth's surface. U .S. Cold Regions Resea rch and Engineering Laboratory .
Cold regions science and engineering. Hanover, N .H. , Pt. II, Sect. C.
Mu ll er, S. W. , comp. [947 . Permafrost, or permanentlyfrozen ground, and related engineering problems. Ann Arbor, Mich ..
J. W. Edwards, Inc.
Paige, R. A . [966. I ce a nd snow terrain features- McMurdo stat ion , Antarct ica. U.S. Naval Civil Engineering
Laboratory, Port Hueneme, Calif. T echnical Note N-840.
Sagar, R. B. [962. Meteorological a nd glaciological studies, Ice R ise station, Ward Hunt Island , M ay to Septem-
ber [960. Arctic Institute of North America. Scientific Report No. 5.
T akahashi , Y . [[ 960.] On the pudd les of Lutzow-Holm Bay. (I n Antarctic meteorology. Proceedings of the symjJosium
held in M elbo:lrne, February 1959. London, New York , Perga mon Press, p . 32 [- 32.)
Thompson, D. C. , alld Ma cdonald, W . J. P. [962. R adiation measurements at Scott Base. New Z eaLand JournaL of
Geology and Geophysics, Vol. 5, No. 5, p. 874- 909.You can also read
