28
Perennially Frozen Peatlands in the Western Arctic
and Subarctic of Canada
S. C. ZOLTA1' AND C. TARNOCAl2
Northern Forest Research Centre, Canadian Forestry Service, Department of the Environment,
5320-122 St., Edmonton, Alberta T6H 3S5
Received May 24, 1974
Revision accepted for publication August 19, 1974
Perennially frozen peatlands were divided into five morphological types: peat plateaus, polygonal peat plateaus, palsas, fen ridges and lowland polygons. One hundred and eight different
peatlands were cored, measured and sampled. The internal structure of all but the lowland
polygons suggests that the peat was deposited in wet fens unaffected by permafrost, and that
permafrost developed only after a thin layer of Sphagnum covered them. The lowland polygons
evolved in a permafrost environment. The study area was divided into four regions on the basis of
predominance of different peatlands forms.
Les tourbieres perpetuellement gelees ont ete divisees en cinq types morphologiques: les
plateaux a tourbe, les plateaux polygonaux a tourbe, les palsas, les crétes marecageuses et les
polygones de basse terre. Cent huit tourbieres differentes ont ete forties, mesurees et
echantillonnees. La structure interne de toutes, sauf celle du polygone de basse terre, suggere que
la tourbe a ete deposee dans un marais humide non affecte par le pergelisol, et que le pergelisol
s'est developpe seulement apres qu'une mince couche de tourbe a Sphagnum les ait recourvert.
Le polygone de basse terre s'est forme dans un environnement de pergelisol. L'aire etudie a ete
divi see en quatre region en s'appuyant sur la predominance des differentes formes de tourbiere.
[Traduit par le Journal]
Introduction
Permafrost occurs in all arctic areas of
Canada (Brown 1967). South of this continuous
permafrost zone unfrozen areas may be found in
the dominantly frozen landscape; this zone was
termed by Brown (1967) as widespread permafrost in a discontinuous permafrost zone. Still
farther south, the permafrost bodies become
less prevalent. At the southern fringe of permafrost, isolated permafrost lenses may be found
in the localized permafrost zone (Zoltai 1971).
Here permafrost is found only in peatlands.
Farther north the incidence of permafrost in
peatlands increases and some mineral soils may
also be perennially frozen. In the widespread
permafrost zone most unfrozen areas occur in
wet depressions in peatlands, but in the continuous permafrost zone all lands are perennially
frozen. Thus, while the most southerly incidence
of permafrost is in peatlands, much of the unfrozen spots in the north are also in peatlands.
Permafrost profoundly alters the characteristics of the peatlands by elevating them above the
water table of the lowlands (Brown 1970). The
previously water-saturated surface layers become
'Canadian Forestry Service, Edmonton Alta. T6H 3S5
'Canadian Soil Survey, Winnipeg, Man. R3T 2N2
Can. J. Earth Sci., 12, 28-43(1975)
drained, allowing an entirely different vegetation
to grow on them. The internal drainage of water
through a lowland may be blocked by frozen
lenses, altering the drainage pattern of the
peatland. The mechanical characteristics of the
peat are completely changed from its unfrozen
state. But permafrost is a temperature condition
which may not be permanent: changes in the
environment may and do induce melting. The
frozen peatlands are therefore dynamic systems
which tend to reach an equilibrium in a given
environmental setting. A thorough understanding of the natural system permits the predicting the effect of disturbances and minimizing
unwanted consequences.
In this paper the morphology and dynamics of
perennially frozen peatlands are presented,
based on field work during three summers. The
main peatland types are described and their
occurrence is related to a climatic zonation.
Description of Study Area
The study area lies north of 60°N latitude and
extends between 118°W longitude and the
Alaskan boundary (Fig. 1). Most of it is in the
widespread, but discontinuous, permafrost zone
(Brown 1967), with the most northern portion in
the continuous permafrost zone, and the most
CAN. J. EARTH SCI. VOL. 12, 1975
30
southern portion in the southern fringe of the
permafrost area.
The southern part is flanked on the west by the
Mackenzie Mountains, where elevations may
reach 3000 m ASL. The Richardson Mountains,
with peaks up to 2000 m, form the boundary
between Yukon and Northwest Territories in the
northwest. The British Mountains have peaks
up to 1750 m in the northern Yukon. The rest
of the area is gently rolling, with local lowlands.
These lowlands, lying east of the mountain
ranges, were glaciated by the Laurentide glaciers
during the Wisconsin stage (Hughes 1972). The
mountains remained unglaciated, but glaciolacustrine basins were formed in the largest
valleys of the unglaciated areas by meltwater
from the Laurentide ice sheet.
The climate is most severe in the north with
an annual average temperature of — 10.7°C at
Tuktoyaktuk and very low total annual precipitation (130 mm). The mean annual temperature
becomes warmer southward with — 6.3°C at
Norman Wells and — 3.8°C at Fort Simpson in
the southern part. Precipitation also increases,
reaching 311 mm at Fort Good Hope and 345
mm at Fort Simpson. The average number of
days per year with frost is the greatest in the
north (272 days at Tuktoyaktuk), decreasing to
239 days at Norman Wells and to 226 days at
Fort Simpson (Burns 1973).
Vegetation reflects the climate. Treeless tundra
occurs on the coastal strip nearly as far south as
Inuvik (Rowe 1959) and in the mountainous
areas. Below the tree line open, stunted spruce
forests with abundant ground lichen are prevalent. Southward from Norman Wells local broadleaved and mixedwood forests, typical of the
boreal forest zones, appear with increasing
frequency. At Fort Simpson boreal forests
dominate, but subarctic spruce-lichen woodlands still occur on local highlands.
' Methods
A total of 108 different peatlands were examined during a helicopter supported survey in
June, July and August of 1971, 1972 and 1973.
At each location one to four holes were cored
until the mineral soil was reached, using a
modified Hoffer probe (Brown 1965), which had
an inside diameter of 22 mm. Samples of measured volume were taken at 10 to 20 cm intervals. A preliminary determination of the peat
constituents and degree of decomposition was
made, using the determination of rubbed fibre
content developed by the National Soil Survey
Committee of Canada (1970). A level transect
was run along a relevant portion of the peatland
between bore locations, using the water table in
the nearby fen as datum. The elevation and
thickness of the active layer was determined at
1 m intervals along the transect. The vegetation
on various segments of the peatlands was
recorded.
In the laboratory the moisture content of the
samples was determined by weighing the samples
before and after drying at 80°C. The moisture
content was expressed on a weight and volume
basis by converting the volume of water into ice
volume by using a factor of 0.917 for the density
of ice (Pounder 1965). The plant remains were
again examined for comparison with the field
determination.
Results
Peat Types and Environments
The dominant peatlands in the area are bogs
and fens. Bogs are saturated with water nearly
to the surface, but the water is not influenced by
the mineralized groundwater of the surrounding
area, resulting in a highly acid surface. Bogs are
frequently covered by Sphagnum moss (Zoltai
et a!. 1973). Fens develop in areas having
restricted drainage that nevertheless have a slow
internal water movement by seepage. The water
is influenced by the mineral terrain and is slightly
acid to slightly alkaline (Zoltai et al. 1973). Each
of these peatlands have several forms in response
to local drainage, nutrient and permafrost conditions, and hence support different vegetation
associations. Changing environmental conditions
in the past induced changes in the vegetation,
the peatlands therefore consist of sequences of
distinctive peat materials according to the environment at the time of deposition.
Peat materials can be grouped into broad
types, according to their floristic composition.
Each material type has a corresponding vegetation association in the present environment,
permitting an extrapolation of some environmental conditions from the peat material. The
peat material types listed below were identified
in 1300 samples. They occur in slightly- to
moderately-decomposed states (fibric to mesic
peat). The peat types are described briefly below
and summarized in Table 1.
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS
29
138
136
134°
132
130°
128
/-70°
•
i
CONTINUOUS
t
6
°
9
PERMAFROST
ZONE
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Fen ridges
P olygonal peal
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132'
LEGEND
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W1 ° Esp R , 4,„ 'ulds
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Op
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65
to
o
1
plateau
Peat plateau
L
Palsy
0
1
so
1
Low Center polygons
26° -7
•--T64
High tenter polygons
R adiocarbon date. this paper
AIL
w,
Radio carbon dote. other sources
Town
L
0
100
_
0
63
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km
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blsco„,
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7200
F IG. 1. Location of peatlands sampled in the study area. Permafrost zones from Brown (1967).
32
CAN. J. EARTH SCI. VOL. 12, 1975
TABLE 2. Summary of peatland types and average thickness of peat in the
study area
No. of
No. of bore
peatlands holes
Peat plateaus
Polygonal peat plateaus
Palsas (Regions II, III)
Palsas (Region IV)
Fen ridges
Lowland polygons
High center
Low center
Thickness of peat (cm)
Ave.
Max.
Min.
49
12
18
10
5
83
34
31
12
7
217
218
267
355
246
371
365
442
646
381
84
90
140
225
165
8
6
11
18
155
66
282
210
130
25
calyculata (L.) Moench, Kalmia polifolia Wang.)
and Betula glandulosa Michx. Tree remains
(Picea mariana (Mill.) B.S.P. or Larix laricina
(Du Roi) K. Koch) are rare.
3. Forest Peat
This material develops on slightly betterdrained bogs under black spruce with the dominant peat formers being ericaceous shrubs,
feather mosses and lichens. The forest peat is
usually moderately decomposed (mesic), has a
very dark brown to dark reddish-brown matrix,
has an amorphous to very fine-fibered structure
and may have a somewhat layered macrostructure. The subtypes of forest peat were separated
based on the dominance of the plant material.
Woody-Forest Peat
The dominant peat formers are Picea mariana,
ericaceous shrubs (Ledum spp., Vaccinium vitisidea L., Empetrum nigrum L. and Larix laricina).
Present environment occurs on upraised perennially frozen peatlands.
Feather Moss — Forest Peat
The dominant peat formers are the feather
mosses (Hypnum spp., Dicranum spp., Pleurozium spp. and Hyloconium spp.) associated with
locally dense black spruce forest cover. The
present peatland environment is perennially
frozen and raised. Associated mainly with the
southern type of peatlands.
3c. Lichen — Forest Peat
The dominant peat formers are lichens
(Cladonia spp., Cetraria nivalis (L.) Ach.),
feather mosses and ericaceous shrubs. This peat
material is deposited on the surface of the
mature peat plateaus and palsas. It is better
decomposed than the other forest peat types
because of its high lichen content.
Aquatic Peat
This material usually develops in shallow lakes
and ponds. The peat is primarily derived from
various aquatic mosses, plants and algae. The
material is slightly sticky, dark brown to black
in color and is usually well-decomposed (humic).
Aquatic peat is usually found at the bottom of
the peat deposits.
Mixed Peat
This material is found in collapsed areas
where, due to the thawing of permafrost, the
peat banks are eroding and slumping into a
water saturated area. During this process the
different peat layers are mixed extensively
resulting in a peat deposit where two or more
types are usually intermixed.
Peatland Forms
Frozen peatlands occur in distinctive landforms. These are peat plateaus, palsas, polygonal peat plateaus, ridges in string fens (Zoltai
and Pettapiece 1973, Tarnocai 1973) and low
and high center polygons (Pewe 1966). A total of
108 peatlands were examined during this study,
most in the very common peat plateaus (Table 2).
Peat Plateaus
Peat plateaus are perennially frozen peat
deposits elevated about 1 m above the lowland
water table (Brown 1970; Zoltai 1971). They are
generally flat, having only minor surface irregularities. They occur as small (few m 2 ) to large
(several km 2 ) islands in unfrozen fens, or as
thick peat deposits on very slightly sloping
mineral terrain. The vegetation in an undisturbed
state are scattered black spruce (Picea mariana)
and lichen, with ericaceous shrubs and Sphagnum
cushions in some depressions. Labrador tea
31
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS TABLE 1. Summary of peat types and corresponding present environments
Peat type and subtype
Sphagnum peat
la Sphagnum (recurvum)
lb Sphagnum (fuscum)
Fen peat
2a Brown moss
2b Sedge
2c Sedge — brown moss
2d Woody sedge — brown moss
3. Forest peat
3a Woody
3b Feather moss
3c Lichen
4. Aquatic peat
5. Mixed peat
Drainage
Reaction
Permafrost
Very poor
Imperfect to poor
Acid
Acid
Absent
Present in Regions I, II, III*
Frequently absent in Region IV
Very poor
Circumneutral
Poor
Circumneutral
Very poor
Circumneutral
Poor
Circumneutral
Imperfect
Imperfect
Imperfect
Very poor
Poor
Acid to neutral
Acid to neutral
Acid
Circumneutral
Acid
Absent in Regions II, III, IV
Present in Region I
Absent in Regions II, III, IV
Present in Region I
Absent in Regions II, III, IV
Present in Region I
Absent in Regions II, III, IV
Present in Region I
Present
Present
Present
Absent
Absent
*See Fig. 11 for peatland regions.
I. Sphagnum Peat
This type of peat material develops on very
wet to wet bogs with the dominant peat former
being the Sphagnum spp. Sphagnum peat is
usually undecomposed (fibric), light yellowishbrown to very pale brown in color and loose and
spongy in consistency with the entire Sphagnum
plant being readily identifiable. In the north
somewhat decomposed (mesic) Sphagnum peat
also occurs, characterized by darker colors. Two
subtypes of Sphagnum peat (recurvum and
fuscum) were separated based on the dominant
species and the peat environment.
la. Sphagnum (recurvum) Peat
Sphagnum growing submerged in acid water
in pure colonies, consisting of Sphagnum recurvum P.—Beauv., S. squarrosum Crome, S.
cuspidatum Ehrh., etc. No permafrost.
lb. Sphagnum (fuscum) Peat
Sphagnum growing in moist to wet acid peat,
..
associated with scattered ericaceous shrubs consisting of cushion-forming Sphagnum species,
as S..fuscum (Schimp.) Klinggr. and S. rubellum
Wils. Occurs on areas with or without permafrost.
2. Fen Peat
This material develops on very wet fens with
the dominant peat formers being Carex sp.,
Drepanocladus sp. and tamarack. Fen peat is
usually moderately well-decomposed (mesic to
humic), dark brown to very dark brown, and the
fibers are fine to medium with a horizontally
matted or layered structure. The subtypes of fen
peat were separated based on the dominance of
the plant material.
Brown Moss – Fen Peat
Vegetation grows submerged in peaty ponds
of slightly acid to slightly alkaline reaction. No
permafrost, except in far north. Composed of
dark colored mosses of the Genera Drepanocla-
dus, Calliergon, Aulacomnium.
Sedge – FCI1 Peat
Vegetation grows in water saturated peat
where there is some influx of mineralized water,
the pH being neutral. No permafrost, except in
far north. Vegetation consists of Carex and some
Eriophorum species.
Sedge-Brown Moss – Fen Peat
Vegetation grows in water saturated peat and
in shallow ponds of slightly acid to slightly
alkaline reaction, influenced by some influx of
mineralized water. No permafrost, except in the
far north. Vegetation is composed of Carex spp.
and the mosses in Type 2a above.
Woody Sedge-Brown Moss – Fen Peat
Vegetation grows in water-saturated peat
which is acid or slightly acid. No permafrost,
except in far north. Vegetation consists of brown
mosses (see above), Carex spp., ericaceous
shrubs (Andromeda polifolia L., Chamaedaphne
33
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS
Marginal
I d—Peat plateau —
MES IC SEDGE PEAT
MES IC WOODY SEDGE PEAT
FIBR IC
SPHAGNUM PEAT
ME S IC
SPHAGNUM PEAT
FIBRIC FOREST PEAT
FIBR IC
BROWN MOSS
PEAT
HU M IC PEAT
ka/
Nriy.7/
,•
MESIC
BROWN
MOSS
PEAT
AQUATIC PEAT
.6,1Wagla,
FIBR IC
SEDGE
FIBRIC
SEDGE
MOSS
PEAT
MINERAL SOIL
MES IC
SEDGE
MOSS
PEAT
ICE
ORGANIC-MINERAL MIXTURE
PEAT
FIG. 2. Edge of peat plateau, showing marginal ridge of frozen peat with aggrading permafrost.
Location: 66'46' N. Lat. 13344' W. Long.
(Ledum palustre L. ssp. groenlandicuni (Oeder)
Hult.) dominates recently-burned peat plateaus.
Morphological variations include peat
plateaus with marginal ridges (Fig. 2) and peat
plateaus with collapsing edges (Fig. 3). The
ridges always occur parallel to the shore of a
pond or very wet fen. Similarly, the edges collapse into very wet fens, and are marked by dead
tress and a luxuriant growth of Sphagnum
growing submerged in water.
The internal structure of peat plateaus is
generally consistent: a cap of somewhat decomposed Sphagnum or forest peat always overlies
the main peat deposit (Figs. 2, 3). The bulk of the
peat deposit is brown-moss or moss-sedge mixture, often with shrub remains or with aquatic
peat near the base. Of the 38 peat plateaus
examined north of Norman Wells only two varied
significantly from this sequence by being composed mainly of Sphagnum peat. In the south,
however, Sphagnum peat is the dominant
material of many peat plateaus. Minor variations
are common in all peat plateaus, and layers of
different materials or similar materials in different states of decomposition may occur. Such
variations may reflect dynamic changes and
short term invasions by vegetation induced by
peat accumulation, changes in water table levels,
etc. observable in the living wetland communities. The average thickness of peat, measured in 49 different peat plateaus, is 217 cm
(Table 2).
Ice accumulations in the peat are rare, although thin (up to 20 cm) ice lenses may be
encountered. Thicker ice layers are often present
at the peat—mineral interface.
Polygonal Peat Plateaus
Polygonal peat plateaus resemble peat
plateaus, as they are elevated about 1 m above
the neighboring fens and lack any surface relief.
They are, however, criss-crossed by trenches
which form a polygonal pattern when viewed
from above (Zoltai and Pettapiece 1973;
34
CAN. J. EARTH SCI. VOL. 12, 1975
L— Peat plateau
CLADONIA
Collapse
scar
SPHAGNUM
ice wedges; apparently changes in hydrology or
climate caused the permafrost to thaw. These
wetlands, although still displaying a polygonal
pattern, are not polygonal peat plateaus.
The internal structure of the polygonal peat
plateaus is similar to peat plateaus. The basal
deposits are usually a woody sedge or aquatic
peat, followed by brown moss or brown mosssedge peat. The top layer is composed of Sphagnum remains (Fig. 4), unlike the peat plateaus
where forest peat is common. At two of the 12
locations, however, Sphagnum was the dominant
material. The total thickness of peat, based on
observations at 12 different sites, is 218 cm
(Table 2).
Palsas
m
I n,
FIG. 3. Peat plateau with collapsing edge. Note thin,
seasonally-frozen peat in wet depression. See Fig. 2 for
legend. Location: 66"49' N Lat. 135 21' W. Long.
Tarnocai 1973). The common diameter of the
polygons is about 15 m. A wedge of pure ice
extends under each trench downward for 2-4 m.
Polygonal peat plateaus are all perennially
frozen, with the permafrost extending into the
mineral soil beneath the peat. They occupy level
or depressed areas with occasional small depressions in which wet sedge or Sphagnum grows.
The peat in these depressions is not frozen, but
permafrost may occur at greater depths in the
mineral soil. The average thickness of the peat
is 218 cm, based on the examination of 12 different areas (Table 2, Fig. 1). The vegetation is
mainly a thick carpet of lichens (mainly of the
Genus Cladonia, Cetraria and Alectoria), with
only a few scattered and stunted black spruce.
The surface morphology of the polygonal peat
plateaus is dominated by polygon trenches and
the adjacent mound. The center of the polygon
is level or slightly concave. The bedding of peat
deposits is disturbed near the ice wedge; the
bedding planes appear to curve upward as they
approach the ice.
In a narrow belt near Arctic Red River, polygonal pattern was noted in shrubby, wet fens.
Probing failed to locate either permafrost or the
Palsas are mounds of peat having a permafrost core. They rise 1 to 7 m above the surrounding wet peatland (Forsgren 1968), and
have a diameter of less than 100 m (SjOrs 1961).
In the study area their diameter varied between
10 and 30 m, and the greatest height observed
was 475 cm above the water table of the neighboring fen. They invariably occur as islands or
peninsulas in very wet fens or ponds. The undisturbed vegetation is similar to that of the peat
plateaus: scattered black spruce with abundant
fruticose lichen and some ericaceous shrubs. The
average thickness of the peat in the north, based
on the examination of 18 different palsas is 267
cm. In the south, the peat is thicker, the average
thickness in 10 palsas being 355 cm (Table 2).
Layers or lenses of pure ice are sometimes
encountered, but they are generally less than
35 cm thick. Nearly pure ice is often found at the
organic—mineral soil interface and thick (up to
1 m) pure ice layers are common in the underlying mineral soil.
The sequence of peat sediments in palsas is
,„,
Peat plateau —
Flo. 4. Cross section of a polygonal peat plateau.
See Fig. 2 for legend. Location: 6719' N. Lat. 133°40' W.
Long.
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS
35
FIG. 5. Cross section of a palsa. Note thin seasonally-frozen peat layer at the base of palsa, found
at the time of survey. See Fig. 2 for legend. Location: 67'06' N. Lat. 134'17' W. Long.
similar to that in the peat plateaus. The basal
deposits, above a thin mixed organic-mineral
layer, are brown moss fen peat, often with sedges
and with shrubby wood remains. They are
capped by a thin (usually less than 50 cm)
Sphagnum or forest peat layer (Fig. 5). The
mineral soil underlying the palsas is always fine
grained, being silt or silt and clay mixtures.
Fen Ridges
Many fens have a series of narrow, low,
sinuous ridges that stretch across the fen at right
angles to the direction of drainage (SjOrs
1963). The string ridges are slightly elevated
above the fen (up to 25 cm) and are therefore
somewhat better-drained, allowing shrub and
even tree vegetation to grow on them. Thus
while the vegetation of the fen is sedge and
brown mosses, the ridges support shrubs such as
dwarf birch (Betula glandulosa) and trees such as
black spruce (Picea inariana) and tamarack
(Larix laricina). North of the central portion of
the study area permafrost lenses are found in
some ridges, often at the nodes where several
ridges meet. Further north permafrost becomes
more frequent in the strings, and it extends into
the underlying mineral soil. This is accompanied
by a broadening of the ridges, until all that
remains is a peat plateau with some oriented,
oval hollows, the remnants of interridge depressions.
The fen ridges with permafrost are similar to
peat plateaus and support the same vegetation.
Some ridges have only a thin permafrost lens
(Fig. 6) and may be developing to a stage where
the permafrost extends into the mineral soil, as
observed on the wider, better-developed ridges.
The peat sequence is to that of peat plateaus,
including the surface cap of Sphagnum or forest
peat. The average thickness of peat in ridges with
permafrost extending into the mineral soil is
246 cm, based on data at five sites (Table 2).
Lowland Polygons
Polygonal terrain occurring in the lowlands of
the tundra regions shows two basic types: poly-
36
CAN. J. EARTH SCI. VOL. 12, 1975
cunt er shoulder
MSEDSGSE-1
O
trench
SPHAo ,4,,
center
shoulder
BEN
OGE-MOSS
1m
lm
F IG. 7. Cross section of polygon trough of a low
center polygon. See Fig. 2 for legend. Location: 68°58'
N. Lat. 133"48' W. Long.
High center-1
lm
F IG. 6. Cross section of a fen ridge. Note thin
seasonally-frozen peat layer in wet fen. See Fig. 2 for
legend. Location: 6655' N. Lat. 133'00' W. Long.
gons with either low or high centers (Pewe 1966).
The middle of the low center polygons is very
wet, with standing shallow water during most of
the thawed season. The central part of high
center polygons is domed and is therefore relatively well-drained. The polygon trenches have
similar vegetation in both types, dominantly
Sphagnum spp. with some Labrador tea (Ledum
palustre L. ssp. decumbens (Ait.) Hult.). The
central part of low centre polygons supports
sedge and submerged moss growth, while the
centers of high centre polygons are often bare,
or have some wind-trained dwarf birch, with
scattered ground lichens.
The morphology of a low center polygon
resembles a bowl where the ridges pushed up by
the ice wedge formed under the trough form a
rim, creating a small pond (Fig. 7). Peat development is thin, averaging only 66 cm at six sites.
Some Sphagnum peat overlies the sedge peat in
trenches and on the shoulders, but sedge and
sedge-moss peat is dominant in the center.
Permafrost is found under the entire unit, even
under the shallow central pool.
The high center polygons form a dome, rising
from the polygon trenches (Fig. 8). The diameter
of the polygons varies, but averages about 8 m.
The depth of peat is greater than in the low
center polygons, the average depth at eight sites
being 155 cm. The surface materials in the
central part are usually too humified to permit
identification. The main peat deposit is composed of sedge, brown moss and woody shrub
remains.
BgpE_ BIRCH
:!*lt;;Nili;Z.•
lm
High
L. Polygon
center
trench
RE-BIRCH
4.Z•••
F IG. 8. Cross section of a high center polygon. See
Fig. 2 for legend. Location: 69°56' N. Lat. 131°18' W..
Long.
In many low and high center polygons the
peat is underlain by a thick layer of organic and
mineral soil mixture. Almost half of the low
center polygons examined showed such a layer
with an average thickness of 44 cm and a range
of between 22 and 88 cm. In the high center
polygons, more than half showed the presence
of this layer, having an average thickness of
152 cm, varying between 34 cm and 256 cm.
Observations of lowland polygons exposed by
shore erosion showed a pronounced mixing of
organic and mineral materials in the lower half of
the deposits. In addition, bedding planes usually
curve up to the ice wedges, with smears and
tongues of organic or mineral material intruding
into the adjacent strata (Fig. 9).
Moisture Content
The moisture (ice) content of perennially
frozen peat was determined on both a volume
and a weight basis. Moisture content, when
expressed as percentage of weight, shows a great
deal of fluctuation (200 to 40 000%), and mean
values include a wide spread of values for each
peat type. Some fluctuations are caused by contamination by dust, volcanic ash and mineral
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS
37
Im
DOMINANTLY MINERAL SOIL
DOMINANTLY ORGANIC MATERI AL
ICE
FIG. 9. Cross section of wave-eroded polygons, showing contorted mineral soil near the ice wedges.
Location: 69°23' N. Lat. 133°20' W. Long.
soil particles. When expressed on a volume basis,
these fluctuations disappear and the data show
that most perennially frozen peat has a moisture
content between 80 and 90%, with some as high
as 98%. These values are the same as the moisture content of unfrozen peat in the study area.
The moisture content of various peatlands
shows little variation. The moisture content of a
marginal ridge of a peat plateau (Fig. 10a, No. 1)
differs little from the moisture content of the
plateau interior 10 m away (Fig. 10a, No. 2). A
layer of clear ice occurring in a palsa (Fig. 10b,
No. 3) is the only significant difference from the
associated peat plateau 20 m away (Fig. 10b,
No. 4). Likewise the moisture content of a polygonal peat plateau (Fig. 10c, No. 6) and a high
center polygon (Fig. 10c, No. 5) is similar,
except for some thin ice layers in the polygonal
peat plateau.
The moisture content of the underlying
mineral soil is difficult to determine because ice
occurs in seams and layers of various thickness
along with disseminated ice crystals. Distorted
moisture data are obtained if the samples are
small, and only averaged figures are obtained if
the samples are large. In general, coarser textured soils had lower moisture content (50-60%
by volume) than the fine-grained silt loams to
clays (50-90%).
Active Layer
The maximum thickness of the active layer,
the surface layer above the permafrost which
thaws during the summer and refreezes in
winter, can be determined only after the thermal
season has ended. The timing of the study was
not suitable for the direct measurement of the
maximal active layer. It is felt, however, that by
late summer (mid-August) the peat is sufficiently
dry to insulate the permafrost, preventing any
significant melting, and that late summer observations would approximate the maximum extent
of the active layer.
The maximum development of the active
layer in peatlands by midsummer is remarkably
uniform through the area. In the south, near
Fort Simpson, it is about 50 cm, decreasing to
45 cm near Norman Wells, and further decreasing to 40 cm in the latitude of Ft. McPherson and to 38 cm near Tuktoyaktuk. In the
north, the active layer in wet low center polygons
is about 50 cm at this time. The active layer is
not completely uniform in all areas and local
variations as much as 20% were noted. These
can be related to exposure (steep peat banks),
topography (narrow ridges), or drainage (small,
wet depressions; desiccation cracks).
Age
The "C age of eight peat deposits was determined by the radiocarbon laboratory of Brock
University by analyzing peat samples from the
base of the peat deposits (Table 3, BGS 159,
144, 140 and 149). At four of these locations the
peat deposits were underlain by lacustrine
materials containing or covering organic materials; this age was also determined (Table 3,
BGS 139, 143, 142 and 148). At two locations
peat samples were taken for age determination
from immediately beneath the surface (3040 cm) of high center polygons (Table 3, BGS
196 and 216) and from the base of the peat deposits (Table 3, BGS 197 and 217). At one
additional location a sample was taken from the
base of a fibric Sphagnum layer at 55 cm below
the surface of a polygonal peat plateau (Table
38
CAN. J. EARTH SCI. VOL. 12, 1975
MOISTURE BY VOLUME (9b)
100
80
90
80
90
100
60
70
80
90
100
Ind
LEGEND
7117771
PERMAFROST TABLE
xrcr
MINERAL SOIL
FIG. 10. Volumetric moisture content of various peatlands: (a) Peat plateau: 1—marginal ridge;
2—near center, (b) Palsa and peat plateau complex: 3—palsa; 4—peat plateau, (c) 5—high center
polygon; 6—polygonal peat plateau.
3, BGS 147). A date was obtained from the top
of a Sphagnum layer on a peat plateau, immediately below a forest peat layer (Table 3, BGS
218).
Discussion
Peatland Development
Similarities in the internal structure of peat
plateaus, polygonal peat plateaus, palsas and fen
ridges indicate that their development followed
the same pattern. Reconstructing the environment from the deposits suggests three general
avenues of peatland development. One had a
shallow pond in the initial stage, as shown by
basal deposits of organic detritus, marl or
gastropods. These deposits were followed by
aquatic moss and sedge communities, still
showing the influence of mineralized water
(Korpijaakko et al. 1972). Shrub remains are
enclosed in this peat, especially near the surface.
The peat deposits are capped by Sphagnum
peat, showing a transition to an elevated bog
condition which was no longer in contact with
mineralized water.
A second developmental sequence is similar,
but the basal aquatic peat is lacking. There may
be a thin mixed organic—mineral layer at the
base, followed by brown moss-sedge deposits
with or without woody shrub remains. In some
instances there is a shrubby layer at the base,
followed by sedge-moss deposits. These sequences suggest gradual peat build-up in a
poorly drained basin, where the water table was
always at the surface and there was some influx
of water relatively rich in cations. Tree remains
are scarce, but are occasionally present. Near
the top of the sequence there is an abrupt change
to Sphagnum or forest peat, suggesting an elevation of the surface above the water table and the
invasion of trees.
A third sequence shows thick Sphagnum
deposits resting on either detrital aquatic or on
moss-sedge peat. This sequence is rarely encountered in the north and may be local. The
39
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS TABLE 3. Radiocarbon age of basal organic and peat deposits
Location
67°16' & 135°14'
Peat plateau
65°59' & 135°03'
Peat plateau
66°10' & 134°18'
67°41' & 132°05'
66°13' & 130°52'
Peat plateau
Polygonal peat plateau
69°15' & 138°02'
Peat plateau
High center polygon
69°07' & 132°56'
High center polygon
61°50' & 122°13'
68°04' & 139°50'
65°50' & 129°05'
65°15' & 126°42'
69°30' & 135°47'
Age
(yrs.)
Peatland type section
Peat plateau
Basal peat
Basal organic
Basal peat
Basal organc
Basal peat
Basal organic
Surface peat
Basal peat
Basal organic
Basal peat
Surface peat
Basal peat
Surface peat
Basal peat
Surface peat
Basal peat
Basal peat
Basal peat
Basal peat
equivalent communities are found near collapsing edges of peat plateaus in small basins,
which are completely enclosed by the peat
plateau and have no connection to the regional
water table. Forest peat, with tree and shrub
remains, is often found at the surface. An identical sequence of thick Sphagnum peat is common in the southern third of the area. It appears
that the Sphagnum peatlands can remain free
of permafrost more readily in the south than
farther north, allowing the accumulation of
Sphagnum peat.
Whatever the initial sequence of peat formation is, all were deposited in a permafrost-free
environment. Sphagnum peat near the surface
signals a significant change in the vegetation,
which included the development of an insulating
peat layer and the growth of trees. A similar
sequence is found at the southern fringe of
permafrost (Zoltai 1972) where the importance
of tree cover in intercepting snow, thereby
enhancing permafrost development, was evident
(Zoltai and Tarnocai 1971).
The encroachment of permafrost under the
somewhat better-drained fen ridge illustrates
this dynamic process (Fig. 6). Initially, the
ridges in patterned fens do not contain permafrost (Zoltai 1971). In the study area, however,
permafrost is often established under them,
because they are slightly better drained than the
inter-ridge areas, providing a suitable habitat
8190
9960
10 470
14 410
5910
10 820
2710
7200
8610
5600
8260
10 100
3150
6020
2650
6430
6120
3960
4140
± 60
± 80
± 80
+ 110
± 60
± 80
± 60
± 60
± 100
± 70
± 110
± 130
± 90
± 100
± 80
± 140
± 120
± 50
± 140
Lab no.
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
BGS
GSC
I
159
139
144
143
140
142
147
149
148
146
196
197
216
217
218
372
3735
GSC 513
Source
This report
77
99
91
If
11
99
77
79
99
91
77
77
99
99
51
77
99
99
99
59
97
99
•
77
Dyck et al. 1966
Mackay et al. 1973
Korpijaakko et al. 1972
Lowdon et al. 1971
for cushion-forming Sphagnum mosses and
scattered trees. The decreased snow cover on the
Sphagnum cushions and the presence of insulating moss allows the seasonal frost to persist
during the summer, and hence to become permafrost. Once initiated, permafrost will form
rapidly as the ridge is further elevated by the
expansion caused by the freezing of the water in
the peat. Evidences of permafrost being established in unfrozen peatlands are abundant. In
countless cases thin layers of permafrost were
found under small Sphagnum cushions, often
with a single small tree, on wet, permafrost-free
fens. These eventually become small peat plateaus which may merge to become large peat
plateaus. In the northern half of the main study
area marginal ridges around ponds or wet fens
are common. These marginal ridges are interpreted as belts which were recently affected by
permafrost. These ridges sometimes produce a
striking concentric ring around small ponds that
were filled in by peat and became affected by
permafrost (Zoltai and Pettapiece 1973).
The edges of many peat plateaus are collapsing, as shown by submerged, dead trees
where the peat slumped into the fen and by the
luxuriant growth of Sphagnum recurvum in the
acid waters at the collapsing face. It is not clear
what initiates the thawing. Perhaps a tree uprooted by wind, a deep desiccation crack, or a
fire may cause local melting which may spread
40
CAN. J. EARTH SCI. VOL. 12, 1975
along the edge of the peat plateau. Although
collapsing edges occur throughout the forested
part of the area, they are far more common in
the southern half of the area than in the north.
Sphagnum (recurvum) peat is restricted to
collapse scars in the present environment. The
occasional occurrence of such peat in now
perrennially frozen peat plateaus indicates a
collapse and re-establishment of peat plateaus.
Additional indication of such collapse cycles
comes from the presence of thin (50 cm) beds of
Sphagnum (fuscum) peat in some peat plateaus,
overlain by fen peat and finally capped by more
Sphagnum (fuscum) peat. Such Sphagnum beds
are associated with permafrost in the present
environment. Such evidences point to cyclic
development and collapse of peat plateaus and
palsas; indeed, in some instances the emergence
of small peat plateaus from collapse scars within
large peat plateaus was observed.
The internal structure of lowland polygons
suggest that many were deposited in a permafrost environment. Excessive mixing of organic
and mineral soils is a common phenomenon
under arctic conditions (Zoltai and Pettapiece
1973). Thus mixing and the presence of tongues
of different materials at the organic—mineral soil
interface suggest soil movements that are common in the active layer above the permafrost
table. Permafrost now underlies all components
of the lowland polygons, and all observations
suggest that this condition occurred while the
polygons were developing.
High center polygons have been regarded as
frost-free conditions. The bedding planes in the
peat are often contorted near the ice wedges,
showing that the ice wedges developed after the
peat was laid down. The floristic composition
of the peat is similar to that of peat plateaus,
with brown moss, sedge and the occasional thick
Sphagnum deposits. Thus both the floristic
composition and internal structure suggest that
these peatlands first developed in a permafrostfree environment, later became elevated into
peat plateaus and then ice wedges developed in
them.
Moisture Content
Palsas, peat plateaus and polygonal peat
plateaus are all elevated above the level of the
nearby fens. The thickness of peat is nearly the
same. With the exception of palsas where segregated ice layers may occur, the moisture content
of the frozen peat is about the same as the unfrozen peat, hence the elevation is due neither to
thicker peat deposits nor to ice accumulation in
the peat. Ice accumulation, however, is significant in many palsas. The moisture content of the
underlying frozen mineral soil is far greater than
in unfrozen, water-saturated mineral soil. The
moisture content of silty clay under unfrozen
fens averages 45% by volume, but in the frozen
state it frequently reaches 80 to 9070 . Additional
moisture accumulates in the frozen mineral soil
which would account for most of the elevation of
the frozen peatlands.
This vaulting of the peatlands is best illustrated in palsas which in this area always occur
as small islands or peninsulas in very wet fens or
ponds. Unfrozen water is available along most
or all of the perimeter and water migrates into
the frozen mass due to a thermal gradient
(Hoekstra 1966), as was found in a study farther
south (Zoltai and Tarnocai 1971). The pronounced marginal ridges around wet fens and
ponds may be due to the same process. In most
peat plateaus, however, the perimeter of the
frozen—unfrozen interface is low in proportion
to the mass of the peat plateau, hence water
an eroding, melting phase where the ice wedges
are inactive (Price 1972). However, the consistently greater thickness of peat in high-center
polygons suggests that peat formation contributes to the different surface morphology. In the
field a complete range of low-center polygons to
high-center polygons was observed, with the
polygon shoulders becoming thicker and the
enclosed pools smaller as the peat accumulates,
until the surface becomes level (Fig. 9). The
development of a domed center, characteristic of
penetration is low. Collapsing margins will
the high-center polygons, may be due to partial
negate any marginal ice accumulations. Thus
melting of the ice wedge during a senescent stage
little or no height increment takes place after the
(Price 1972). In some instances, near lakeshores
initial development.
or lips of plateaus, the polygon trenches may be
overdeepened by running water (Pewe 1966). Age of Peat Deposits
The radiocarbon ages of basal organic deIn polygonal peat plateaus no basal mixing of
organic and mineral soil was observed, indi- posits (Table 3) show that the continental glacier
cating that the peat was deposited under perma- had melted from the areas east of the mountains
ZOLTAI AND TARNOCAI: FROZEN PEATLANDS
41
ARCTIC OCEAN
/400
138°
1360
REGION I — Dominant peatlands:
lowland polygons
REGION II — Dominant peatlands:
polygonal peat plateaus
REGION III— Dominant peotionds,
peat plateaus, palsas
REGION IV— Dominant peatlands:
peat plateaus, palsas, bogs,
fens, collapse scors
FIG.
11. Peatland regions of the Mackenzie River valley.
and initial organic material accumulation began
between 14 400 and 10 000 years ago. The main
peat build-up began one to several thousand
years later, mainly between 10 500 and 5600
radiocarbon years ago. Regional trends are
difficult to discern, as local circumstances such
as basin formation, erosion, base drainage, etc.
played a dominant role in permitting the peat
accumulation. Generally, the oldest basal peat
deposits occur near the mountains and the ages
tend to be younger eastward.
The ages of the surface peat 2710 and 2650
years before present (BGS 147, 218), indicate the
time of establishment of Sphagnum cap at those
sites, followed by permafrost development. The
period around 3500 years B.P. and 2400 years
B.P. were times of climatic deterioration in
northern Canada (Nichols 1969), and the dates
2710 and 2650 years B.P. are probably related to
these fluctuating climatic conditions.
The meager data permit only speculation on
the chronosequence of regional peatland development. Apparently, peatland development
began soon after the melting of the continental
glaciers and peat accumulation proceeded at a
rapid rate especially beginning about 8000 years
ago. Permafrost was present, but it was less
widespread both in the north and in the south
than at the present. A minor cooling some 3000
to 4000 years ago produced an increase in peat
plateaus, restricting peat accumulation to unfrozen fens. Polygonal peat plateaus probably
CAN. J. EARTH SCI. VOL. 12, 1975
42
developed at this time. Peatlands in the tundra
were continuously subject to permafrost after
the melting of continental glaciers.
In the more southern parts of Canada the
melting of continental glaciers was followed by a
dry and warm period, permitting only restricted
peat accumulation between 7500 and 6000 years
ago (Terasmae 1972). This was followed by a
wetter and then a cooler period when rapid peat
accumulation took place. In the north, however,
the climate was favourable for peat accumulation
during the warm period (8000 to 4000 years
ago), but peat formation was reduced during the
cooler period beginning about 4000 years ago.
Such shifting of regions of peat accumulation
in accordance to climatic changes occurs
throughout the peatland regions of Canada
(Terasmae 1972).
Peatland Regions
Field observations show that different perennially-frozen peatlands occur in different broad
regions, and these regions can be characterized
by the commonly occurring peatland forms. The
western Arctic and Subarctic of Canada were
divided into four broad regions (Fig. 11) on the
basis of the present survey. These regions are
aligned in broad east—west trending belts, suggesting a latitudinal temperature gradient. The
regions can be briefly characterized as follows:
Region dominated by lowland polygons,
whether in low center, high center or intermediate forms. Other peatlands are rare; these
include thin sedge fens.
Region dominated by polygonal peat
plateaus. Other peatland forms include peat
plateaus, associated frozen or unfrozen bog
pools and patterned fens. Outliers of polygonal
peat plateaus occur farther south in exposed or
mountainous areas.
Region dominated by peat plateaus and
palsas, with few collapse edges. Other peatlands
include associated unfrozen fens, bog pools,
polygonal peat plateaus, as well as patterned and
unpatterned fens.
Region dominated by peat plateaus and
palsas, with frequent collapse edges. Large proportion of unfrozen peatlands occur; these include associated fens, bog pools and collapse
scars, as well as patterned and nonpatterned
fens and flat bogs.
Conclusions
(1) Peat landforms, having developed under
the influence of permafrost are distinctive and
can be readily characterized within the study
area.
The distribution of various perennially
frozen peatlands is similar within peatland
regions and follow a climatic zonation.
The internal structure of perennially
frozen peatlands suggests that most peat was
deposited in nonpermafrost environment, and
permafrost development followed the establishment of a Sphagnum cap at a later date.
The peat in lowland polygons accumulated
under permafrost conditions.
The internal structure and surface morphology of some peat plateaus suggest that they
underwent at least one cycle of collapse and
reconstruction.
(6) The main period of peat accumulation
occurred between 8000 and 4000 years ago, and
many peat plateaus were affected by permafrost
for the first time about 3000-4000 years ago.
Acknowledgments
The data for this report were obtained as a
result of investigations carried out under the
Environmental—Social Program, Northern Pipelines, of the Task Force on Northern Oil Development, Government of Canada. The cooperation and logistics support by the Geological
Survey of Canada, especially Drs. 0. L. Hughes
and N. W. Rutter, are gratefully acknowledged.
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