Permafrost: distribution and relation to environmental factors in the Hudson Bay lowland

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Permafrost: distribution and relation to environmental factors in the

Hudson Bay lowland

PERMAFROST

-

DISTRLBUTION AND RELATION T O

ENVIRONMENTAL FACTORS IN THE HUDSON BAY LOWLAND

R. J . E. Brown 1

SYNOPSIS

The Hudson Bay Lowland l i e s m o s t l y in the p e r m a f r o s t r e - gion of Canada. The distribution of p e r m a f r o s t v a r i e s f r o m d i s - continuous i n the southeast, n o r t h of the 30 "F m e a n annual a i r i s o t h e r m , t o continuous i n the northwest, between the 2 5 ° F and 2 0 ° F m e a n annual a i r i s o t h e r m s . The active l a y e r v a r i e s f r o m 1 t o 3 f e e t , and p e r m a f r o s t r a n g e s in t h i c k n e s s f r o m a f e w inches a t the southern l i m i t t o 200 feet i n the continuous zone a t Churchill. In the discontinuous zone, p e r m a f r o s t i s found in p e a t p l a t e a u s and p a l s a s which a r e p r e v a l e n t , but it d o e s not o c c u r in intervening wet d e p r e s s i o n s n o r in beach r i d g e s o r r i v e r banks. P e r m a f r o s t e x i s t s e v e r y w h e r e beneath the land s u r f a c e in the continuous zone which f o r m s a n a r r o w s t r i p along t h e Hudson Bay coast. The m o s t distinctive p e r m a f r o s t f e a t u r e s a r e p a l s a s of varying s i z e t o elevated p e a t p l a t e a u s exceeding 10 f e e t in height covering s e v e r a l a c r e s . They f o r m v e r y d i s - tinctive a i r photo p a t t e r n s and t h e i r o r i g i n a p p e a r s r e l a t e d t o thin snow cover.

DISTRLBUTION O F PERMAFROST

T h e Hudson Bay Lowland l i e s a l m o s t e n t i r e l y within t h e p e r m a f r o s t region of Canada (Brown, 1968) ( F i g u r e 1). The

southern boundary of t h e Lowland extends along t h e fiftieth p a r a l l e l and the southern l i m i t of p e r m a f r o s t i s located a t

approximately latitude 51 ON. F r o m the southern t i p of

J a m e s Bay the Lowland extends in a n o r t h w e s t e r l y d i r e c t i o n t o

latitude 58 ON. About one -half of the p e r m a f r o s t region of the

'

R e s e a r c h officer, Geotechnical Section, Division of Building

R e s e a r c h , National R e s e a r c h Council of Canada, Ottawa, Ontario.

Lowland i s in the discontinuous zone and one-half in the con- tinuous zone. The distribution of p e r m a f r o s t i s notable f o r

s e v e r a l r e a s o n s . F i r s t , the m o s t s o u t h e r l y l i m i t of p e r m a - f r o s t in Canada (excluding the w e s t e r n C o r d i l l e r a region) i s at the south end of J a m e s Bay n e a r Moosonee. Second, the d i s - continuous zone i s n a r r o w e r than anywhere e l s e in Canada, being only about 250 m i l e s wide in c o n t r a s t t o about 500 m i l e s in the Mackenzie R i v e r region. T h i r d , the continuous p e r m a - f r o s t zone r e a c h e s i t s m o s t southerly extent in Canada at the n o r t h end of J a r n e s Bay (Brown, 1967).

Within the p e r m a f r o s t region of the Lowland, perenially f r o z e n ground v a r i e s f r o m s c a t t e r e d i s l a n d s in the south t o

continuous in the north. In the southern f r i n g e of the d i s -

continuous zone, p e r m a f r o s t islands v a r y in extent f r o m l e s s than 50 feet t o s e v e r a l a c r e s . The t h i c k n e s s of t h e s e patches v a r i e s f r o m a few i n c h e s to 1 o r 2 feet at the southern l i m i t of the p e r m a f r o s t region t o t e n s of f e e t w h e r e the distribution

becomes widespread in the n o r t h e r n portion of the discontinuous zone. No information i s available on the thickness of p e r m a f r o s t a t any stations in Ontario, but t h i c k n e s s e s of 100 feet have been encountered at Gillarn in n o r t h e a s t e r n Manitoba w h e r e p e r m a - f r o s t i s widespread. Northward, the p e r m a f r o s t i s continuous

reaching depths of 200 f e e t at Churchill. T h e continuous zone i s g e n e r a l l y m u c h thinner in the Lowland than e l s e w h e r e in Canada; it i s confined t o a n a r r o w c o a s t a l s t r i p . Because of i t s proximity t o Hudson Bay, the p e r m a f r o s t , although continuous, i s probably thin ( p e r h a p s only 100 feet o r l e s s in Ontario) and wedges out a t the s h o r e .

All known o c c u r r e n c e s of p e r m a f r o s t in the discontinuous zone o c c u r in peatlands which c o m p r i s e a l l of the t e r r a i n of the Hudson Bay Lowland except r i v e r banks, beach r i d g e s and a few r o c k o u t c r o p s south of Winisk. S e v e r a l of the m a j o r r i v e r s

flowing e a s t w a r d into J a m e s Bay

-

Albany, Attawapiskat and

Ekwan

-

have n o r t h - and south-facing banks. It could be

expected that p e r m a f r o s t might e x i s t in the north-facing banks

of t h e s e r i v e r s but none h a s been r e p o r t e d . In the continuous

zone, p e r m a f r o s t o c c u r s e v e r y w h e r e beneath the land s u r f a c e .

CHARACTERISTICS OF P E W A F R O S T

T r e e growth in the Lowland c o n s i s t s of s c a t t e r e d t o d e n s e s p r u c e , varying in height f r o m 2 t o 40 f e e t , and t a m a r a c k . A l d e r and willow provide the under-growth. A r e a s of burned

t r e e s a r e p r e v a l e n t e s p e c i a l l y on well d r a i n e d peat p l a t e a u s and p a l s a s . The ground vegetation i s a m o s a i c of Sphagnum,

f e a t h e r and o t h e r m o s s e s , L a b r a d o r t e a , g r a s s and m a r s h sedge in v a r i o u s combinations. The m i c r o - r e l i e f r a n g e s f r o m f l a t t o v e r y hummocky. Individual hummocks v a r y in s i z e t o a m a x i m u m of 3 f e e t high and 4 f e e t wide. V a r i a t i o n s in e l e v a - tion f r o m one a s s o c i a t i o n t o a n o t h e r r a n g e through s e v e r a l feet. P e a t p l a t e a u s r i s i n g 3 t o 4 f e e t above t h e surrounding p o o r l y d r a i n e d a r e a s a r e p r e v a l e n t . P a l s a s up t o 12 f e e t o r m o r e in height o c c u r in l a r g e n u m b e r s i n t h e Hudson Bay Lowland. S u r f a c e and s u b s u r f a c e d r a i n a g e i s v a r i a b l e . Standing w a t e r i s usually a s s o c i a t e d with m a r s h sedge a r e a s and m a n y of the lowest lying Sphagnum a r e a s ; individual h u m m o c k s , peat p l a t e a u s , and p a l s a s a r e d r i e r . Depth t o t h e m i n e r a l s o i l ( t h i c k n e s s of m o s s / l i c h e n and p e a t ) v a r i e s f r o m a m i n i m u m of 1 foot t o d e p t h s exceeding 10 feet. Hustich (1957) h a s a s c r i b e d

an a v e r a g e value of annual p e a t accumulation in t h e Hudson Bay

Lowland of about 1/20 inch b a s e d on n u m e r o u s observations. M i n e r a l s o i l s a r e predominantly fine - g r a i n e d s i l t s and c l a y s .

T h e depth t o t h e p e r m a f r o s t t a b l e r a n g e s f r o m a n o b s e r v e d m i n i m u m of 1 foot t o a m a x i m u m of about 3 feet. T h i s s i t u a - tion p r e v a i l s throughout t h e Lowland f r o m Moosonee in t h e

southeast t o C h u r c h i l l in the northwest. L o c a l d i f f e r e n c e s in

m o i s t u r e content and r e l a t e d t h e r m a l conductivity of the peat c a u s e g r e a t e r v a r i a t i o n s in t h e depth of thaw t h a n regional c l i m a t i c d i f f e r e n c e s . T h e t h i c k n e s s of p e r m a f r o s t v a r i e s f r o m l a y e r s a few i n c h e s t h i c k a t t h e s o u t h e r n l i m i t t o about 200 feet a t Churchill. Ground i c e o c c u r s m a i n l y in h o r i z o n t a l l a y e r s ranging f r o m h a i r l i n e t o about 1 inch in t h i c k n e s s . One excep- t i o n t o t h i s w a s a l a y e r of i c e 3 f e e t t h i c k encountered in t h e p a l s a i n F i g u r e 2. I c e i s a l s o found in the f o r m of s m a l l p e l l e t s and o t h e r r a n d o m inclusions.

M i c r o - r e l i e f in t h e f o r m of hummocks, peat p l a t e a u s , and

f r e q u e n t l y p a l s a s a r e widespread. T h e Sphagnum i s hurrmocky

r e g a r d l e s s of whether o r not p e r m a f r o s t i s p r e s e n t . P e a t p l a t e a u s and p a l s a s always indicate the e x i s t e n c e of p e r m a f r o s t and both t y p e s of f e a t u r e s a p p e a r t o be r e l a t e d in origin and

development. In the Hudson Bay Lowland, n u m e r o u s a r e a s of

coalescing p a l s a s e x i s t which a r e v i r t u a l l y indistinguishable f r o m p e a t plateaus. West of Winisk, f o r e x a m p l e , m a t u r e peat p l a t e a u s with lichen c o v e r ( F i g u r e 3) w e r e o b s e r v e d t o be

s i m i l a r in o r i g i n and a p p e a r a n c e of coalescing p a l s a s in F i g u r e s

F i g u r e 2

-

Stop No. 5

-

S m a l l youthful p a l s a s containing p e r m a - f r o s t in wet peatland with no p e r m a f r o s t located a t t h e s o u t h e r n l i m i t of discontinuous zone in Hudson

Bay Lowland. The peat in the l a r g e p a l s a in t h e

f o r e g r o u n d i s 5 f t 2 in. thick overlying g r e y clayey s i l t with fine sand. T h e depth t b the p e r m a f r o s t t a b l e in t h e c e n t r e of t h i s p a l s a i s 1 f t 1 in. and the p e r m a f r o s t l a y e r i s 4 ft 2 in. thick. 1 4 September

F i g u r e 3

-

A e r i a l view f r o m altitude of 500 f t of m a t u r e p a l s a s , c o a l e s c e d p a l s a s and p e a t plateaus i n t e r s p e r s e d with ponds and s m a l l shallow l a k e s in Hudson Bay

Lowland 20 m i l e s w e s t of Winisk. 17 S e p t e m b e r

F i g u r e 4

-

Stop No. 9

-

M a t u r e c o a l e s c e d p a l s a s f o r m i n g peat p l a t e a u s in Hudson Bay Lowland. Note burned s p r u c e t r e e s and d e n s e cover of L a b r a d o r t e a . Ground s u r f a c e i s v e r y hummocky and c o v e r e d with Sphagnum and lichen below which i s p e a t t o a depth

of 4 ft 9 in. overlying g r e y s i l t y clay with sand and

stones. The depth t o the p e r m a f r o s t table i s 2 ft

8 in. No p e r m a f r o s t o c c u r s in the s e d g e - c o v e r e d

F i g u r e 5

-

A e r i a l view f r o m altitude of 500 f t of p a l s a s and

I peat plateaus at S t o p N a

9.

Note wet patterned fen

1 with spruce islands in background. 16 September

1

The peat p l a t e a u s and p a l s a s in the Hudson Bay Lowland a r e the s i t e s of the w o r s t f o r e s t f i r e s b e c a u s e they a r e t h e d r i e s t a r e a s and support the m o s t vigorous t r e e and l i c h e n growth. One peat plateau south of F o r t S e v e r n which e x p e r i - enced a v e r y r e c e n t b u r n w a s 3 f e e t high with r e c e n t l y burned dead s p r u c e up t o 15 f e e t . The ground c o v e r c o n s i s t e d of c h a r r e d Sphagnum and burned lichen, with a few p a t c h e s of unburned Sphagnum. The f i r e had burned the top 1/2 inch of Sphagnum and the top 1 inch of l i c h e n down t o the wet b a s a l l a y e r . T h e depth t o the p e r m a f r o s t t a b l e in the p e a t w a s 1 foot, t h e s a m e a s in the surrounding unburned Sphagnum-covered a r e a s . It a p p e a r e d that t h e unburned peat p r o t e c t e d the p e r m a - f r o s t f r o m t h e h e a t of the f i r e and t h e i n c r e a s e d s o l a r h e a t input due to the b u r n e d black s u r f a c e .

PALSAS

P a l s a s a r e v e r y prevalent in t h e Hudson Bay Lowland. The p a l s a s shown in F i g u r e 2 constitute the m o s t s o u t h e r l y o c c u r r - ence of p e r m a f r o s t . T h e s e p a l s a s a r e s m a l l , being in t h e e a r l y

stage of development. They v a r y in s i z e f r o m a few s q u a r e

f e e t t o t h e l a r g e s t yet s e e n m e a s u r i n g 31 f e e t long by 12 f e e t wide by 2 f e e t high and a r e s u r r o u n d e d by w a t e r . Living vegetation i s a b s e n t except f o r a few willow s h r u b s and sedge p l a n t s , consisting of b a r e sedge peat. S u b s u r f a c e exploration in the l a r g e s t p a l s a r e v e a l e d a l a y e r of peat 5 f e e t 2 i n c h e s thick overlying g r e y clayey s i l t with fine sand down t o t h e 7-feet 4 - i n c h e s depth. Below t h i s depth the soil i s stoney. The depth t o t h e p e r m a f r o s t table v a r i e d f r o m 1 foot 1 inch in the m i d d l e of t h i s p a l s a t o 2 f e e t 1 inch a t t h e w a t e r ' s edge. T h e p e r m a f r o s t c o r e v a r i e s i n t h i c k n e s s f r o m 4 f e e t 2 inches, including a l a y e r of i c e 3 f e e t t h i c k in t h e middle of the p a l s a t a p e r i n g t o 7 i n c h e s a t the w a t e r ' s edge. T h e p e r m a f r o s t wedges out a few i n c h e s beyond the edge of the p a l s a u n d e r the w a t e r . No p e r m a f r o s t o c c u r s in the surrounding a r e a s u n d e r w a t e r .

M a t u r e p a l s a s ( F i g u r e s 4 a n d 5) stand a s high a s 10 f e e t above the surrounding peatland s u r f a c e and support a t r e e growth of s c a t t e r e d s p r u c e up t o 10 feet. The p r e s e n c e of m a n y

burned t r e e s both standing and lying on the ground indicated a p r e v i o u s dense growth of m a t u r e s p r u c e . T h e ground c o v e r c o n s i s t s of a l m o s t continuous l i c h e n and dense L a b r a d o r t e a . The lichen h a s undergone c o n s i d e r a b l e biological oxidation and

the ground surface i s v e r y hummocky and cracked. The peat

v a r i e s in thickness f r o m 4 f e e t

9

inches t o 2 f e e t in local

hollows overlying g r e y silty clay with a m i x t u r e of sand and stones. The depth to the p e r m a f r o s t table was observed t o

be 2 feet 8 inches but no p e r m a f r o s t was found in the local

hollows. Ice was found in the peat and m i n e r a l soil.

A l a r g e 20-feet high m a t u r e p a l s a was examined between the Ekwan and Attawapiskat R i v e r s about 100 m i l e s west of

J a m e s Bay. The t r e e growth, consisting of spruce up t o

30 f e e t high, was the m o s t vigorous of a l l the p a l s a s examined. The l a r g e number of dead m a t u r e t r e e s lying on the ground indicated previous dense t a l l growth. Several depressions up t o 20 feet in d i a m e t e r and about 12 f e e t deep o c c u r r e d in the p a l s a supporting s c a t t e r e d spruce 2 f e e t high. The ground cover of the p a l s a consisted of hummocky Sphagnum, s c a t t e r e d

lichen and Labrador tea. In the d e p r e s s i o n s the ground cover

was Sphagnum, sedge and ground birch, and the w a t e r stood a t the ground surface. The peat in the p a l s a was about 7 f e e t

i _{thick a s was that in the depressions overlying g r e y clayey silt}

with stones. The depth t o the p e r m a f r o s t table was 2 feet

3 inches in the a r e a s of growing t r e e s and 2 feet 9 inches in

the burned a r e a s . No p e r m a f r o s t was encountered in the

I depressions.

I

In addition t o the p a l s a s described above, m a n y o t h e r s in all

stages of development exist in the Hudson Bay Lowland. Near the coast of J a m e s Bay, between the Albany and Attawapiskat R i v e r s , t h e r e a r e m a n y s m a l l s c a t t e r e d palsas. Inland f r o m J a m e s Bay and south of the Attawapiskat R i v e r , s e v e r a l old p a l s a s , partially destroyed by thawing and slumping, w e r e observed. No newly forming ones w e r e seen in t h i s a r e a south t o M i s s i s s a Lake a t the southern l i m i t of the p e r m a f r o s t s i m i l a r t o those described in F i g u r e 2 n e a r Moosonee. North of the Attawapiskat River, p a l s a s s e e m to exist m o s t l y in the c e n t r e s of shallow ponds. Many a l s o occur in the vicinity of the rock outcrops a t Sutton Lake and Hawley Lake. South of F o r t Severn in the n o r t h e r n p a r t of the discontinuous p e r m a f r o s t zone extensive p a l s a s up to 10 f e e t occur in which the p e r m a f r o s t

probably exceeds 30 feet in thickness. L a r g e coalescing ones

e x i s t n e a r F o r t Severn. Small developing p a l s a s occur in a few l a k e s in the tundra coastal s t r i p on Hudson Bay between

F o r t Severn and Winisk. Northward in Manitoba, l a r g e coalesc- ing lichen-covered peat plateaus and p a l s a s cover m o s t of the t e r r a i n . One unusual f e a t u r e i s the p r e s e n c e of p a r a l l e l s t r i n g s

of peat plateaus and p a l s a s at York F a c t o r y on the peninsula be- tween the mouths of the Nelson and Hayes Rivers. These fea- t u r e s develop in concentric rows on strand lines a s the penin- sula e m e r g e s due t o isostatic rebound. No p e r m a f r o s t f o r m s in the intervening elongated depressions.

POLYGONS

Polygons and polygonal c r a c k s , another type of feature associated with p e r m a f r o s t , occur widely in the continuous zone. They have been observed from Cape Henrietta M a r i a in the e a s t to Churchill in the northwest. They a r e found in beach ridges, and lake bottoms and, n e a r Churchill, they a r e wide- s p r e a d in peatlands. They appear to be related t o t h e r m a l con- traction of the ground and the development of ice wedges.

AIR PHOTO PATTERNS

The m o s t notable feature of the a i r photographs i s the variety of patterns in the peatlands of the Hudson Bay Lowland. The lack of relief and poor drainage contribute to the jumbled appearance of surface vegetation and other t e r r a i n features. Within t h i s broad framework, the recognition of such p e r m a f r o s t features a s peat plateaus and p a l s a s i s hindered by t h e i r small size on the available photographs, and t h e i r similarity t o other

t e r r a i n features which have no permafrost. Two photographs

show some of the p a t t e r n s including p e r m a f r o s t features of the

Hudson Bay Lowland ( F i g u r e s 6 and 7).

The f i r s t a e r i a l photograph ( F i g u r e 6) was taken about 35 m i l e s northwest at Attawapiskat. The entire a r e a i s peatland in which t h r e e m a i n patterns a r e evident:

1. Medium g r e y with smooth texture covering the central

and southwest portions of the photograph on both sides

of the s t r e a m which flows f r o m west to east. This i s a

low, wet, poorly drained flat sedge-covered a r e a . The black peppery flecks a r e s m a l l pools of water l e s s than 50 feet in diameter. No p e r m a f r o s t o c c u r s in t h i s a r e a .

2 . _{Fine network of closely spaced d a r k g r e y to black flecks}

in a light g r e y mesh-like m a t r i x covering the northwest and e a s t e r n q u a r t e r of the photograph. This type of a r e a t e r m e d "patterned fen with flarks" by ~ j b ' r s (1959b)

F i g u r e 6

-

Section of RCAF a i r photo A14961 -128 a t Stop NO. 9 in the Hudson Bay Lowland northwest of Attawapiskat.

F i g u r e 7

-

Section of RCAF a i r photo A14137-65 w e s t of Winisk in t h e Hudson B a y Lowland.

i s low, wet, poorly drained.and flat, and consists of shallow pools up t o 100 f e e t in d i a m e t e r separated by low n a r r o w sedge -covered peat ridges about 1 foot high. The d a r k g r e y c i r c u l a r a r e a s in the southeast c o r n e r a r e spruce islands up t o s e v e r a l hundred feet in size. No p e r m a f r o s t was encountered in t h i s type of t e r r a i n .

3. Light g r e y c i r c u l a r and i r r e g u l a r l y shaped a r e a s with white patches adjacent to the s t r e a m and bordering P a t t e r n 2 in the southeast portion of the photograph. These a r e a s a r e l a r g e m a t u r e p a l s a s 15 feet high and high peat plateaus and coalesced p a l s a s ( F i g u r e s 4 and 5). The light g r e y tone i s caused by the dense cover of Labrador t e a growing on the lichen cover of the palsas. P e r m a f r o s t does occur in these f e a t u r e s .

The second a e r i a l photograph ( F i g u r e 7) was taken 60 m i l e s west of Winisk n e a r the treeline. The entire a r e a i s peatland in which two main p a t t e r n s a r e evident:

1. Light g r e y t o white with fine t o medium texture. This

p a t t e r n o c c u r s in irregularly-shaped a r e a s varying g r e a t l y in size f r o m s e v e r a l hundred feet to 1/2 mile. These a r e a s a r e peat plateaus, ridges and mounds rising 5 t o 10 feet above the neighbouring ponds and s m a l l lakes. The light g r e y a r e a s support low stunted spruce growing on Sphagnum-lichen, and the white a r e a s a r e lichen. P e r m a f r o s t o c c u r s in t h i s pattern (Figure 3). 2. Network of closely spaced d a r k g r e y t o black i r r e g u l a r l y

shaped flecks and minute spots in a medium g r e y m e s h - like m a t r i x producing a p a t t e r n s i m i l a r to P a t t e r n 2 of

the f i r s t a e r i a l photograph (Figure

6).

This i s also

patterned fen with f l a r k s but no spruce islands occur here. This pattern i s found mainly along the west edge of the photograph but s m a l l a r e a s a r e scattered through- out. It i s unlikely that p e r m a f r o s t e x i s t s in t h i s pattern.

RELATION OF PERMAFROST TO ENVIRONMENTAL FACTORS Climate

The continuous p e r m a f r o s t zone throughout Canada, except in the Hudson Bay Lowland, l i e s north of the 1 7 ° F m e a n annual a i r

i s o t h e r m which corresponds roughly t o a m e a n annual ground t e m p e r a t u r e of 23°F. In the Lowland, however, the n a r r o w s t r i p along the coast of Hudson Bay a p p e a r s t o lie in the con- tinuous p e r m a f r o s t zone, according to field observations, but in fact i t l i e s south of the 20°F m e a n annual a i r isotherm. This discrepancy m a y be r e l a t e d in s o m e m a n n e r to the f a c t that the a r e a i s situated north of the treeline. On the other hand, f u r t h e r field observations m a y r e v e a l discontinuities in the p e r m a f r o s t placing i t in the discontinuous zone.

T e r r a i n

The m e c h a n i s m of p e r m a f r o s t formation in peat t e r r a i n a p p e a r s t o be related t o changes in the t h e r m a l p r o p e r t i e s of the peat through the y e a r (Brown, 1966; Tyrtikov, 1959). During the s u m m e r the surface l a y e r s of peat become d r y through evaporation. The t h e r m a l conductivity of the peat i s low and warming of the underlying s o i l i s impeded. The lower peat l a y e r s gradually thaw downward and become wet a s the ice l a y e r s in the seasonably f r o z e n l a y e r m e l t . In the autumn t h e r e t e n d s t o be m o r e m o i s t u r e in the surface l a y e r s of the peat because of a d e c r e a s e d evaporation r a t e . When i t f r e e z e s the t h e r m a l conductivity of the peat i s i n c r e a s e d considerably. Thus the peat o f f e r s l e s s r e s i s t a n c e t o the cooling of the underlying soil in winter than t o the warming of i t in s u m m e r . The m e a n ground t e m p e r a t u r e under peat will t h e r e f o r e be lower than under adjacent a r e a s without peat. When conditions under the peat a r e such that the ground t e m p e r a t u r e r e m a i n s below 32°F throughout the y e a r , p e r m a f r o s t r e s u l t s and i s maintained a s long a s the t h e r m a l conditions leading t o t h i s lower t e m p e r a t u r e p e r s i s t .

The o c c u r r e n c e of p e r m a f r o s t in the discontinuous zone a p p e a r s t o be closely related t o drainage. The importance of water conditions i s shown by the absence of p e r m a f r o s t in a r e a s where the w a t e r table i s a t the ground s u r f a c e , even if the ground cover c o n s i s t s of Sphagnum. P e r m a f r o s t o c c u r s , however, in the m i c r o - r e l i e f peat f e a t u r e s such a s mounds, ridges, plateaus and p a l s a s which r i s e above the wet a r e a s . It i s suggested that these f e a t u r e s a r e morphological variations of the s a m e p r o c e s s , i. e . , the s a m e m e c h a n i s m i s responsible f o r the formation of peat plateaus and p a l s a s . The European l i t e r a t u r e a p p e a r s t o

support t h i s contention (PIyavchenko, 1955; Svensson, 1961

-

1962;

Tyrtikov, 1966). They appear t o p a s s through a life cycle of development and degradation and all s t a g e s occur in the Hudson Bay Lowland.

Initially, t h e s e f e a t u r e s a p p e a r a s low mounds of upwarping of peat protruding above w a t e r l e v e l in the middle of shallow

ponds only a few inches deep ( F i g u r e 2). The m e c h a n i s m of

t h e i r f o r m a t i o n and control of t h e i r distribution i s u n c e r t a i n , but it i s suggested that the pond f r e e z e s t o the bottom in winter and the underlying s a t u r a t e d peat i s domed up at random l o c a - t i o n s by intensive f r o s t action and i c e l e n s growth. When elevated above the pond level, the d r y l a y e r of exposed peat

insulates the underlying f r o z e n m a s s from s u m m e r thawing,

thus m a r k i n g the initiation of a p e r e n n i a l l y f r o z e n o r p e r m a f r o s t

condition. The elevation of the peat s u r f a c e above the g e n e r a l

l e v e l of the surrounding flat l e v e l s u r f a c e devoid of relief exposes it t o winter winds which reduce o r r e m o v e the insulating snow c o v e r . Winter f r o s t penetration i s t h e r e f o r e g r e a t e r than in the surrounding low, flat a r e a s , thus contributing t o f u r t h e r p e r m a - f r o s t accumulation.

A s the peat continues t o accumulate y e a r a f t e r y e a r a c c o m - panied by the i n c r e a s e in p e r m a f r o s t thickness e a c h w i n t e r , the mounds grow and c o a l e s c e t o f o r m plateaus. In the youthful

s t a g e , t h e r e i s l i t t l e o r no living vegetation on the peat surface.

During m a t u r i t y , Sphagnum and other m o s s e s and lichens be

-

come established, along with L a b r a d o r t e a and s p r u c e

( F i g u r e s 3, 4, 5). Old age and degradation begin when the

insulating ground c o v e r r u p t u r e s due to biological oxidation and g e n e r a l d e t e r i o r a t i o n , and thawing p e n e t r a t e s into the u n d e r - lying perennially f r o z e n c o r e . The s u r f a c e of the p a l s a o r peat plateau b e c o m e s v e r y uneven because of differential thawing of the underlying ground i c e and l a r g e blocks of thawed peat b r e a k off the m a r g i n s . It i s not c e r t a i n whether rejuvenation can occur.

The c r i t i c a l f a c t o r s in p a l s a and peat plateau f o r m a t i o n a r e possibly c l i m a t e , w a t e r supply, and snow cover. L i t t l e w o r k h a s been done on the c l i m a t i c r e q u i r e m e n t s f o r the f o r m a t i o n of t h e s e f e a t u r e s but s o m e information i s available in the

Scandinavian l i t e r a t u r e . It h a s been r e c o r d e d in Sweden that p a l s a s o c c u r w h e r e the a i r t e m p e r a t u r e r e m a i n s below 3 2 ° F during m o r e than 200 d a y s p e r y e a r . They a l s o a p p e a r t o be

p r e s e n t only w h e r e the precipitation

-

virtually all in the f o r m of

snow

-

during the p e r i o d of November t o A p r i l i s l e s s than

12 i n c h e s (Lundqvist, 1962). It a p p e a r s f r o m the a i r t e m p e r a t u r e and precipitation d a t a f o r m e t e o r o l o g i c a l s t a t i o n s in the Lowland a r e a that t h e s e c r i t e r i a a r e satisfied.

Drainage conditions and water supply a r e v e r y important f a c t o r s in the development of p a l s a s and peat plateaus. The ponds

in which t h e y begin t o grow probably should be sufficiently shallow t o f r e e z e t o the bottom in w i n t e r s o t h a t a f r o z e n zone

m a y develop below. The p r o c e s s of growth i s not c l e a r , but

the g r a d u a l updoming of the peat p r o c e e d s p o s s i b l y b e c a u s e of i t s high capillarity, which d r a w s considerable quantities of w a t e r t o the f r e e z i n g f r o n t f r o m the surrounding wet a r e a s . T h e s e conditions o c c u r widely in the Hudson Bay Lowland w h e r e p a l s a s and coalescing p a l s a s f o r m i n g peat p l a t e a u s grow t o heights of 10 t o 15 feet.

Snow c o v e r i s c o n s i d e r e d one of t h e c r i t i c a l f a c t o r s i n p a l s a development once f o r m a t i o n of t h e f e a t u r e h a s begun. According t o Lundqvist (37) the amount of snowfall in t h e region through

t h e w i n t e r m a y not exceed a c e r t a i n quantity

-

i. e . , 12 inches.

In addition, a differentiation o c c u r s i n that t h e snow c o v e r on t h e p a l s a s i s significantly l e s s than on the surrounding t e r r a i n . ~ j b ' r s ' (1959a, 1959b, 1961) s t u d i e s of the origin and d i s t r i b u - t i o n of p a l s a s in t h e Lowland pointed t o the c r i t i c a l r o l e of the snow c o v e r . A detailed ecological study of a p a l s a bog n e a r Winisk showed that snow c o v e r w a s t h i n n e r on p a l s a s than the

surrounding t e r r a i n (Railton, 1968). E v a p o t r a n s p i r a t i o n w a s the m a i n s o u r c e of s u m m e r heat l o s s . P a l s a c o l l a p s e w a s attributed t o d e t e r i o r a t i o n of the vegetation c o v e r by r a i n and wind e r o s i o n causing m e l t i n g of t h e ground i c e .

CONCLUSION

C l i m a t e i s the m o s t i m p o r t a n t f a c t o r influencing the f o r m a t i o n and continued e x i s t e n c e of p e r m a f r o s t . T h i s i s b o r n e out by the location of the m e a n annual a i r i s o t h e r m s r e l a t i v e t o the d i s t r i - bution of p e r m a f r o s t , and i n d i c a t e s the e x i s t e n c e of a b r o a d relationship. South of the 3 0 ° F i s o t h e r m p e r m a f r o s t i s not g e n e r - a l l y found in the Lowland. Between 3 0 ° F and 2 5 ° F i s o t h e r m s , p e r m a f r o s t i s patchy and r e s t r i c t e d t o c e r t a i n t y p e s of t e r r a i n . North of the 25OF i s o t h e r m , p e r m a f r o s t i s w i d e s p r e a d and in f a c t i s continuous along the Hudson B a y c o a s t .

P e r m a f r o s t o c c u r s only in t h e peatlands and peat bogs of t h e Hudson B a y Lowland. Even in t h e peat t e r r a i n , p e r m a f r o s t d o e s not e x i s t w h e r e w a t e r l i e s a t o r n e a r t h e ground s u r f a c e . It i s r e s t r i c t e d t o t h e positive m i c r o r e l i e f peat f e a t u r e s

-

p l a t e a u s and p a l s a s which abound in the Lowland. Drainage i s t h e r e f o r e one of the m a i n t e r r a i n f a c t o r s influencing t h e e x i s t e n c e of p e r m a f r o s t .

The r o l e of vegetation in the d i s t r i b u t i o n of p e r m a f r o s t i s complex. The t r e e growth, predominantly s p r u c e and t a m m a - r a c k , i s not a n indicator of p e r m a f r o s t distribution because t h e s e t r e e s grow on s i t e s w h e r e p e r m a f r o s t i s both p r e s e n t and a b s e n t . The only d i r e c t c o r r e l a t i o n between t r e e growth and p e r m a f r o s t o c c u r r e n c e i s in t h e extensive t a m a r a c k s t a n d s between the m a j o r r i v e r s in the Hudson Bay Lowland. The a b s e n c e of p e r m a f r o s t h e r e i s due t o t h e l a c k of d r a i n a g e and t h e t a m a r a c k r e f l e c t t h e s e conditions r a t h e r than t h e a b s e n c e of p e r m a f r o s t . The Sphagnum and lichen cannot be u s e d a s i n d i c a t o r s of the e x i s t e n c e of p e r m a f r o s t .

One of the m o s t difficult p r o b l e m s i s explaining the d i s

-

tribution of p e r m a f r o s t p e a t f e a t u r e s in the peatlands and peat bogs. T h e r e d o e s not a p p e a r t o be a n y obvious explanation of why t h e s e f e a t u r e s o c c u r in s o m e peatland a r e a s and not in o t h e r s with a p p a r e n t l y s i m i l a r conditions. L o c a l s m a l l v a r i a - t i o n s i n w i n t e r winds and snow c o v e r m a y be sufficient to t i p the net effect of a l l the e n v i r o n m e n t a l f a c t o r s e i t h e r t o w a r d s o r away f r o m the initiation of t h e s e f e a t u r e s .

R E F E R E N C E S

Brown, R. J . E . , 1966, The influence of vegetation on p e r m a f r o s t P r o c e e d i n g s of the International Conference on P e r m a f r o s t . Brown, R . J . E . , 1967, P e r m a f r o s t m a p of Canada. Division of

Building R e s e a r c h , National Re s e a r c h Council, NRC 9769, and Geological S u r v e y of Canada, M a p 1246A.

Brown, R. J . E . , 1968, P e r m a f r o s t investigations in n o r t h e r n O n t a r i o and n o r t h e a s t e r n Manitoba. National R e s e a r c h Council, Division of Building R e s e a r c h , NRC 10465, 39p. Hustich, I . , 1957, On the phytogeography of the S u b a r c t i c

Hudson B a y Lowland. A c t a G e o g r a p h i c a F e n n . , Vol. 16,

No. 1, p. 1-48.

Lundqvist, J .

,

1962, P a t t e r n e d ground and r e l a t e d f r o s t phenomena i n Sweden. S v e r i g e s Geol. ~ n d e r s b l k n , C 583.

P'yavchenko, N. I.

,

1955, B u g r i s t y e Torfyaniki (Humocky P e a t

Bogs). Akademiya Nauk SSSR, Institut L e s a ( A c a d e m y of S c i e n c e s of t h e USSR, F o r e s t r y I n s t i t u t e , Moscow, 278 p. ( i n R u s s i a n ) .

Railton, J . B . , 1968, The ecology of p a l s a bogs. M.Sc. T h e s i s , U n i v e r s i t y of Toronto, 89 p.

sjldrs, H., 1959a, Bogs and F e n s in the Hudson B a y Lowland. -4rctic, Vol. 12, No. 1, p. 3-19.

~ j g r s , H . , 1959b, F o r e s t and peatlands a t Hawley L a k e , N o r t h e r n Ontario. Contributions t o Botany, Bull. 171, National M u s e u m of Canada, p. 1-31.

~j:rs, H . , 1961, S u r f a c e p a t t e r n s i n b o r e a l peatlands. Endeavour, Vol. XX, No. 80, p. 217-224.

Svensson, H., 1961 -1962, "Nagra l a k t t a g e l s e r f r d n palsornrdden" ( O b s e r v a t i o n s on p a l s a s . Photographic i n t e r p r e t a t i o n and field s t u d i e s in N o r t h Norwegian f r o s t ground a r e a s ) .

N o r s k G e o g r a f i s k T i d s s k r i s t Bd. XVIII, H. 5-6, p. 212-227. (in Norwegian).

Tyrtikov, A. P., 1959, P e r e n n i a l l y f r o z e n ground and vegetation. P r i n c i p l e s of Geocryology, P. F. Shvetsov, ed., Academy

of S c i e n c e s of the U.S.S.R.. Vol. I, p. 399-421.

National R e s e a r c h Council. Tech. T r a n s l a t i o n 11 63. Tyrtikov, A. P., 1966, F o r m i r o v a n i y e I Razvitiye

Krupnobugristykh Torfyanikov V Severnoy Tayge Zapadnoy S i b i r i ( F o r m a t i o n and development of l a r g e hummocky peat bogs i n the n o r t h e r n t a i g a of W e s t e r n S i b e r i a ) ,

M e r z l o t n y y e I s s l e d o v a n i y a ( P e r m a f r o s t Investigations), Vol. VI, p. 144-154. ( i n R u s s i a n ) .

T h i s p a p e r i s a contribution f r o m the Division of Building Re s e a r c h , National Re s e a r c h Council of Canada, and i s published with the approval of the D i r e c t o r of the Division.

APPENDIX 1

Investigations of m i c r o c l i m a t e and t e r r a i n f a c t o r s affecting t h e d i s t r i b u t i o n of p e r m a f r o s t in the discontinuous zone have b e e n u n d e r t a k e n a t Thompson, Manitoba, s i n c e 1968. T h i s community i s about 640

lun

_{n o r t h of Winnipeg in n o r t h c e n t r a l}

Manitoba i n the m i d d l e of t h e discontinuous zone. It i s about 160

lun

_{w e s t of the Hudson Bay Lowland a t the n o r t h e r n erid of}

g l a c i a l L a k e A g a s s i z . P e r m a f r o s t o c c u r s in s c a t t e r e d i s l a n d s varying in extent f r o m a few square. m e t r e s t o s e v e r a l h e c t a r e s and in t h i c k n e s s f r o m about 1 t o 15 m o r g r e a t e r , a v e r a g i n g between 2 . 4 and 4 . 5 m (Johnston e t a l . , 1963). The p e r m a f r o s t t a b l e i s g e n e r a l l y encountered anywhere f r o m about 0. 5 t o 2.mbe low the ground s u r f a c e . M u c h i c e , p r i m a r i l y in the f o r m of h o r i z o n t a l l e n s e s up t o 20 c m t h i c k (the a v e r a g e t h i c k n e s s being l e s s t h a n 2. 5 c m ) , i s found throughout the f r o z e n g l a c i o l a - c u s t r i n e s i l t s and c l a y s underlying the a r e a . T e m p e r a t u r e s in the p e r m a f r o s t v a r y f r o m about -0. 5 t o 0°C.

O b s e r v a t i o n s a t Thompson include a i r t e m p e r a t u r e , p r e - cipitation, snow depth, d e n s i t y and t e m p e r a t u r e s , wind s p e e d s a t t h e 1 . 8 m l e v e l , net radiation, e v a p o t r a n s p i r a t i o n , ground h e a t flow a t the 30 c m depth, ground t e m p e r a t u r e s t o 7. 5 m ,

and annual depth of ground f r e e z i n g and thawing. T h e s e

m e a s u r e m e n t s a r e being c a r r i e d out a t four s i t e s located with- in a d i s t a n c e of a few hundred m e t r e s of e a c h o t h e r , two with p e r m a f r o s t and two with none.

Soil p r o f i l e s of t h e f o u r s i t e s a r e shown in F i g u r e A-1. Site A s u p p o r t s d e n s e s p r u c e growing on a 10-cm t h i c k l a y e r of f e a t h e r m o s s and f o r e s t l i t t e r overlying brown clayey s o i l ( g r a i n s i z e a n a l y s i s a t a depth of 15 t o 23 cm: c l a y

-

59 p e r c e n t , s i l t

-

39 p e r c e n t , sand

-

2 p e r cent). T h e r e i s no peat a t t h i s s i t e and no p e r m a f r o s t .

Site B s u p p o r t s open stunted s p r u c e growing on a 10-cm t h i c k l a y e r of Sphagnum o v e r 10 c m of p e a t , 23 c m of black o r g a n i c c l a y (31 p e r cent o r g a n i c content by weight) overlying brown clayey soil ( g r a i n s i z e a n a l y s i s a t a depth of 30 t o 53 cm: clay

-

77 p e r cent, s i l t

-

22 p e r cent, sand

-

1 p e r cent). T h e r e i s no p e r m a f r o s t a t t h i s site.

T h i s m a t e r i a l i s e x t r a c t e d f r o m : Brown, R. J . E . and G. P. Williams, 1972. The F r e e z i n g of P e a t l a n d . National R e s e a r c h Council of Canada, Division of Building R e s e a r c h , NRCC 12881, 24 p.

FOREST LITTER A N D FEATHER MOSS

0

SPHAGNUM PEAT O R G A N I C CLAY

/

SILT BROWN CLAY FIGURE A - 1

SOIL PROFILES OF SITES AT THOMPSON, MANITOBA

Site C s u p p o r t s open stunted s p r u c e s i m i l a r t o that a t Site B growing on a n 1 8 - c m thick l a y e r of Sphagnum o v e r 18 c m of peat, 10 c m of black o r g a n i c c l a y (26 p e r cent o r g a n i c content by weight), overlying brown clayey s o i l ( g r a i n s i z e a n a l y s i s a t a depth of 35 t o 60 cm: c l a y

-

8 5 p e r cent, s i l t

-

14 p e r cent, sand

-

1 p e r cent). T h e r e i s p e r m a f r o s t at t h i s s i t e . The p e r m a f r o s t table in 1968 w a s a t a depth of about 48 c m a l m o s t coincident with the b a s e of the o r g a n i c soil, and the p e r m a - f r o s t i s about 2.7 m thick.

Site D s u p p o r t s d e n s e s p r u c e , s i m i l a r t o that a t Site A growing on a 13-cm t h i c k l a y e r of Sphagnum o v e r 18 c m of black o r g a n i c s i l t (77 p e r cent o r g a n i c content by weight) overlying brown clayey soil. G r a i n s i z e a n a l y s i s of the o r g a n i c s i l t is: c l a y

-

28 p e r cent, s i l t

-

62 p e r cent, sand

-

10 p e r cent; in the brown c l a y a t 33 t o 48 c m it is: c l a y

-

87 p e r cent, s i l t

-

12 p e r cent, sand

-

1 p e r cent. T h e r e i s no p e a t a t t h i s s i t e s i m i l a r t o t h a t a t S i t e s B and C , but the organic s i l t h a s a v e r y high o r g a n i c content. The p e r m a f r o s t t a b l e i s a t a depth of about 75 c m and the p e r m a f r o s t e x c e e d s 6 m in t h i c k n e s s .

GROUND FREEZING REGIME

F r o s t depth-time g r a p h s a r e shown f o r e a c h s i t e f o r t h r e e complete f r e e z i n g s e a s o n s (1968 -69, 1969-70, 1970-71) in F i g u r e A-2. At t h e two s i t e s with no p e r m a f r o s t s e a s o n a l f r o s t p e n e t r a t e s t o i t s m a x i m u m depth in l a t e w i n t e r , followed by thawing f r o m the s u r f a c e downward in spring. At the s i t e s with p e r m a f r o s t s e a s o n a l thawing p r o g r e s s e s downward e a c h s u m m e r , reaching the p e r m a f r o s t t a b l e in the e a r l y autumn. T h i s i s followed by s e a s o n a l f r e e z i n g f r o m the s u r f a c e downward t o the p e r m a f r o s t table.

S e v e r a l p a t t e r n s a r e evident on the b a s i s of o b s e r v a t i o n s o v e r t h r e e f r e e z i n g s e a s o n s , f o r t h e r e a r e significant d i f f e r e n c e s in the f r e e z i n g and thawing r e g i m e s a t the four s i t e s . Site A e x p e r i e n c e s c o n s i d e r a b l y d e e p e r f r o s t penetration and a longer p e r i o d of s e a s o n a l f r e e z i n g e a c h y e a r than Site B. Both s i t e s a r e without p e r m a f r o s t and only Site B c a n be c o n s i d e r e d a peatland

s i t e . Both S i t e s C and D a r e i n p e r m a f r o s t a r e a s and s e a s o n a l f r o s t p e n e t r a t e s t o the p e r m a f r o s t table e a c h y e a r . The active l a y e r i s significantly t h i c k e r a t Site C and changes f r o m y e a r t o y e a r . I t a p p e a r s t o r e m a i n constant a t Site D.

I I I I I 0.6

2

l l I I I ' l l I ' l r , I I " I I I I- L E G E N D 1SNOW COYER I - W - - - S I T E A z - S I T E 8 t C - _{0 . 3 ;} - - S I T C 0 x 0 -

-

W 1 9 6 9 1 9 7 0

E

o 0 I- Y 4

U FROZEN GROUND O F R O Z E H GROUND O T H A l C D GROUHO 1 2 2

l f l o l l ~ O Y P I I I I FIOII P Q V I ~ ( t h o ~ t d . Gtound I e m m r o l u r t t 32.F I 2 4 s

FIGURE A - 2

S N O W C O V E R A N D GROUND F R E E Z I N G R E G I M E S AT THOMPSON M A N I T O B A

The r a t e of f r o s t penetration i s m o r e uniform at S i t e s A and B w h e r e t h e r e i s no p e r m a f r o s t than at S i t e s C and D w h e r e p e r m a f r o s t complicates the s e a s o n a l f r e e z i n g of the ground. F r o s t penetration f o r a l l four s i t e s in Table A - I i s p r e s e n t e d f o r the t h r e e f r e e z i n g s e a s o n s . Depths and p e n e t r a -

tion r a t e s a r e g r e a t e r at Site A than at Site B; penetration r a t e s

at both s i t e s a r e g e n e r a l l y two t o t h r e e t i m e s g r e a t e r in the f i r s t p a r t of the winter than in the l a t t e r p a r t , although brief p e r i o d s at lower r a t e s o c c u r at the v e r y beginning. Thawing of the ground begins in the e a r l y spring at about the t i m e the f r o s t p e n e t r a t e s a l m o s t t o i t s m a x i m u m depth. F u r t h e r ground f r e e z i n g of a few c e n t i m e t r e s at m o s t does o c c u r during the following two- t o eight-week p e r i o d but i t s effect in the total f r e e z i n g r e g i m e i s m i n i m a l . The l a s t r e m n a n t of f r o z e n ground d i s a p p e a r s in l a t e s u m m e r about one month l a t e r a t

Site A than a t Site B. At both s i t e s it w a s located at the bottom

of the f r o z e n l a y e r , indicating little o r no thawing f r o m beneath. T h e situation is somewhat different at S i t e s C and D w h e r e p e r m a f r o s t e x i s t s , although that at Site C i s m o r e m a r g i n a l than t h a t a t Site D. A t Site C the t e m p e r a t u r e of the p e r m a f r o s t i s about -O.l0C and t h u s i s v e r y sensitive t o any d i s t u r b a n c e . T h r e e y e a r s of weekly observations have r e s u l t e d in a d r o p in the p e r m a f r o s t t a b l e f r o m the original level of 48 c m t o

1 . 3 5 t o 1 . 5 m . At Site D, w h e r e the t e m p e r a t u r e of the p e r m a - f r o s t i s slightly c o l d e r a t about -0.5"C, the p e r m a f r o s t t a b l e h a s r e m a i n e d steady at a depth of about 0.75 m . Although the f r o s t gauges a t both s i t e s w e r e i n s t a l l e d in t h e f a l l of 1968 the methylene blue liquid in t h e bottom few d e c i m e t r e s of the gauges in the p e r m a f r o s t did not f r e e z e until m i d - w i n t e r .

F r o s t penetration r a t e s a r e g e n e r a l l y m o r e uneven a t S i t e s C and D than at the s i t e s with no p e r m a f r o s t . T h i s w a s p a r t i c u l a r l y t r u e i n 1968-69 and 1969-70 when penetration r a t e s w e r e r a p i d during the f i r s t two o r t h r e e weeks, levelled off in the l a t e fall, and then i n c r e a s e d a s the s e a s o n a l f r e e z i n g plane approached the p e r m a f r o s t table. At Site D the s e a s o n a l f r o s t r e a c h e d the p e r m a f r o s t t a b l e a t the end of D e c e m b e r 1969-70, the m o s t s e v e r e of the t h r e e f r e e z i n g s e a s o n s . T h i s s e a s o n p r o - duced the d e e p e s t f r o s t p e n e t r a t i o n a t the n o n - p e r m a f r o s t s i t e s . At Site C s e a s o n a l f r o s t r e a c h e d the p e r m a f r o s t t a b l e in m i d - F e b r u a r y 1969-70. Ground thawing of the active l a y e r begins a t about t h e s a m e t i m e a s f o r the two s i t e s with no p e r m a f r o s t .

No P e r m a f r o s t S i t e A S i t e B R a t e of P e n e t r a t i o n , (cm/wesk) 1968

-

69 5 . 3 (29 Oct

-

28 F e b ) 2.0 ( 2 9 Oct - 10 D e c ) 3.0 (1 M a r - 20 M a y ) 4 . 3 (10 Dec - 25 F e b ) 3.0 ( 2 5 F e b - I A p r ) 1969

-

70 5. 5 (10 Nov

-

28 A p r ) 5.0 (17 Nov - 23 M a r ) 1 . 8 (28 A p r

-

I 2 J u n e ) 1 . 8 ( 2 3 M a r - I 2 M a y ) 1970 - 71 5.5 ( I 2 Nov

-

26 F e b ) 4 . 5 ( I 8 Nov

-

26 F e b ) 2.5 (26 F e b - 21 M a y ) 2.0 ( 2 6 F e b - 7 M a y ) D e e p e s t F r o s t P e n e t r a t i o n , c m ( D a t e F i r e t A t t a i n e d ) 1968

-

69 110 (20 M a y 1969) 68 ( 1 5 A p r 1969) 1969 - 70 1 4 3 (12 J u n e 1970) 104 ( I 2 J u n e 1970) 1970 - 71 110 ( 2 3 J u l y 1971) 8 0 ( 7 M a y 1971) Bezinning of G r o u n d T h a w i n g 1968 - 69 21 A p r 1969 1969 - 70 9 M a y 1970 1970 - 71 22 A p r 1971 Thawing of L a s t F r o z e n Ground 1968

-

69 2 3 Aug 1969 1969

-

70 I 2 Sept 1970 1970

-

71 3 Aug 1971 17 A p r 1969 2 M a y 1970 27 A p r 1971 1 8 J u l y 1969 2 Aug 1970 4 J u l y 1971 P e r m a f r o s t S i t e C Site D 8 . 8 (19 Oct - 12 Nov) 6 . 8 (15 Oct - 12 Nov) 1 . 8 ( 3 Dec - 15 J a n ) 0 . 8 ( I 2 N o v - 3 1 D e c ) 27.0 (20 Oct - 27 Oct) 3. 3 (27 Oct - I 2 F e b ) 14. 5 ( 1 4 Oct

-

27 Oct) 1 . 3 ( 2 7 O c t - 1 6 D e c )

,

2 . 5 ( 1 8 Nov - 23 Dec)

'

9 . 0 ( 2 3 Dec - 6 J a n ) S e a s o n a l f r o s t p e n e t r a t e d t o p e r m a f r o s t t a b l e a t 5 3 - c m depth. S e a s o n a l f r o s t p e n e t r a t e d t o p e r m a f r o s t t a b l e a t 1 0 5 - c m depth. S e a s o n a l f r o s t p e n e t r a t e d t o p e r m a f r o s t t a b l e a t 1 2 3 - c m depth. 1 S e a s o n a l f r o s t p e n e t r a t e d t o

$

p e r m a f r o s t t a b l e a t 7 5 - c m

2

depth. S e a s o n a l f r o s t p e n e t r a t e d t o 1 _r p e r m a f r o s t t a b l e a t 75-crn T depth.

9

? 7 . S e a s o n a l f r o s t p e n e t r a t e d t o

2

p e r m a f r o s t t a b l e a t 7 5 - c m 0 Z depth. 18 A p r 1969 I M a y 1970 22 A p r 1971 about 1 5 J a n 1969 1 4 F e b 1970 about 23 J a n 1971 18 A p r 1969 28 A p r 1970 26 A p r 1971 about 1 5 J a n 1969 31 D e c 1969 9 J a n 1971

OBSERVATIONS ON CLIMATE AND TERRAIN FACTORS Observations of c l i m a t i c and t e r r a i n f a c t o r s a r e tabulated

in Table A-11. Net radiation o b s e r v a t i o n s a r e not tabulated.

T h e y w e r e commenced on a continuing b a s i s during the s u m m e r

of 1971. Wind velocities w e r e m e a s u r e d a t the f o u r s i t e s

through the f r e e z i n g s e a s o n s of 1969-70 and 1790-71. A v e r a g e wind s p e e d s a r e v e r y low, l e s s than 1 . 6 l u n / h r c o m p a r e d with an a v e r a g e of a p p r o x i m a t e l y 16 b / h r a t the s t a n d a r d m e t e o r o - logical installation 12 m above ground l e v e l a t the Thompson a i r p o r t .

The m e a n a i r t e m p e r a t u r e f o r the eight-month winter p e r i o d of e a c h of t h e t h r e e f r e e z i n g s e a s o n s w a s slightly higher a t Site A than a t Site B , both s i t e s with no p e r m a f r o s t . The s a m e w a s t r u e f o r the f o u r s u m m e r months. At the p e r m a f r o s t s i t e s the m e a n w i n t e r a i r t e m p e r a t u r e w a s g e n e r a l l y lower a t

Site C than a t Site D. The m e a n s u m m e r t e m p e r a t u r e w a s

consistently lower a t Site D, the lowest of the f o u r . The t h r e e - y e a r a v e r a g e e n c o m p a s s i n g the t h r e e f r e e z i n g s e a s o n s ,

October 1968 t o S e p t e m b e r 1971, was highest a t Site A and -0.3 C deg l e s s a t the o t h e r t h r e e s i t e s , which w e r e virtually the s a m e . T h e s e v a l u e s a r e s i m i l a r t o the value of -3.6"C f o r the s a m e p e r i o d a t t h e m e t e o r o l o g i c a l station a t Thompson.

M e a n ground s u r f a c e t e m p e r a t u r e s followed somewhat the s a m e p a t t e r n during the w i n t e r . Site A had g e n e r a l l y the h i g h e s t v a l u e s of the f o u r s i t e s and Site D the lowest. During the

s u m m e r Site A w a s c o n s i s t e n t l y low and Site B, the other s i t e

with no p e r m a f r o s t , high. The t h r e e - y e a r a v e r a g e encompassing

the t h r e e f r e e z i n g s e a s o n s , October 1968 t o S e p t e m b e r 1971, w a s highest a t Site B and lowest a t Site D.

Ground heat flow m e a s u r e m e n t s a r e available through the two f r e e z i n g s e a s o n s 1969-70 and 1970-71. T h e r e i s c o n s i d e r - 2 ~ n a l y s i s of net r a d i a t i o n i s not yet completed. P r e l i m i n a r y

examination of the d a t a i n d i c a t e s t h a t v a l u e s a r e highest a t

S i t e s B and C ( s p a r s e t r e e growth), somewhat l e s s a t Site A

( d e n s e t r e e growth

-

no p e r m a f r o s t ) and lowest a t Site D

C L I M A T I C AND T E R R A l N DATA THOMPSON. M A N l T O B A S i t e A S i t e B S i t e C S i t e D A i r T e m p c r n t u r c . 'C W i n t e r _- - mean O c t . 1968 - M a y 1969 O c t . 1969 - M a y 1970 Oct. 1970 - M a y 1971 S u m m e r - m e a n - J u n e - Sept 1969 J u n e - S e p t 1970 J u n e - S e p t 1971 A v e r a g e A n n u a l O c t 1968 - S e p t 1971 G r o u n d S u r f a c e T e m p e r a t u r e , 'C W * - m e a n Oct 1968 - M a y 1969 - 4 . 3 - 4 . 1 - 4 . 5 Oct 1969 - M a y 1970 - 4 . 9 -6. 1 - 4 . 4 Oct 1970 - M a y 1971 - 2 . 5 - 4 . 4 - 4 . 2 S w n m e r - m e a n - J u n e - S e p t 1969 5 . 8 1 1 . 9 9 . 5 J u n e - S e p t 1970 8 . 2 11.6 8 . 9 J u n e - S e p t 1971 9.2 1 1 . 9 1 0 . 3 A v e r a p A n n u a l O c t 1968 - S e p t 1971 -0. I 0 . 7 - 1 . 4 Wind S p e e d , h / h r W i n t e r - m e a n O c t 1969 - M a y 1970 0 . 4 0 0 . 3 4 0 . 4 3 Oct 1970 - >May 1971 0 . 6 1 0 . 3 7 0. 9 0 S w n m e r - m e a n J u n e - S e p t 1970 0 . 7 7 0. 51 1. 30 J u n e - S e p t 1971 1 . 3 1 0 . 2 2 0 . 5 4 G r o u n d H e a t F l o w , c a l / c m 2 / h r W i n t e r - m e a n O c t 1969 - M a y 1970 -0.16 - 0 . 1 8 - 0 . 2 3 O c t 1970 - M a y 1971 -0. 22 - 0 . 1 4 - 0 . 1 3 S w n m e r - m e a n J u n e - S e p t 1970 0 . 5 3 0 . 4 8 0 . 6 6 J u n e - S e p t 1971 0 . 5 3 0 . 6 2 0 . 7 4 S u r f a c e O r g a n i c L a y e r . ( c m ) ~ o s s / f o r e s t l i t t e r 1 0 10 17. 5 P e a t 0 1 0 1 7 . 5 O r g a n l c c l a y / s i l t 0 2 3 131) 10 (26) ( p e r c e n t o r g a n i c c o n t e n t by w e i g h t ) T o t a l t h i c k n e s s ( c m ) 1 0 4 3 4 5 Snow on G r o u n d T h i c k n e s s - c m - m e a n Oct 1968 - M a y 1969 29 3 5 38 O c t 1969 - M a y 1970 28 26 29 O c t 1970 - M a y 1971 39 4 2 4 1 Density

-

g m / c c - m e a n O c t 1968 - M a y 1969 0 . 1 9 8 0 . 2 0 6 0 . 2 1 3 O c t 1969 - M a y 1970 0. 211 0 . 1 9 7 0. 1 7 4 O c t 1970 - M a y 1971 0. 179 0. 174 0 . 1 8 0

able difference in a v e r a g e values f o r heat flow f o r the two y e a r s of o b s e r v a t i o n s . During both winter s e a s o n s Site D had the lowest value ( l e s s heat l o s t to the a t m o s p h e r e ) . During both

s u m m e r s Site C had the highest positive values ( h e a t flow f r o m

the a t m o s p h e r e t o the ground).

The s u r f a c e organic l a y e r i s a n i m p o r t a n t f a c t o r in the f r e e z i n g r e g i m e of the ground. The organic profile a t e a c h

s i t e h a s been d e s c r i b e d and i s tabulated in Table A-11. The

o r g a n i c l a y e r i s t h i c k e r a t S i t e s B and C , 43 and 4 5 c m r e s p e c t i v e l y , and thinner a t S i t e s A and D, 10 and 30 c m r e - spectively.

Snow c o v e r i s one of the m o s t i m p o r t a n t f a c t o r s influencing the f r e e z i n g of the ground. The m e a n thickness of the snow l a y e r h a s been consistently highest a t S i t e s B and C , w h e r e t r e e growth i s s p a r s e . Site D h a s the lowest snow accurnula- tion and the d e n s e s t , t a l l e s t t r e e growth. The p a t t e r n of snow density w a s l e s s c o n s i s t e n t , although values tended to be slightly l o w e r a t Site D than a t the o t h e r s i t e s .

DISCUSSION O F RESULTS

The t h r e e m a j o r f a c t o r s d e t e r m i n i n g the f r e e z i n g and thawing p a t t e r n a t a s i t e a r e :

( 1 ) heat exchange a t the s u r f a c e by radiation, convection and evaporation,

( 2 ) depth and d e n s i t y of snow c o v e r , and ( 3 ) t h e r m a l p r o p e r t i e s of the soil.

The a s s e s s m e n t of the r e l a t i v e influence of t h e s e f a c t o r s i s c o m - plicated by t h e i r c l o s e i n t e r r e l a t i o n s h i p . Often v i r t u a l l y un- m e a s u r e a b l e changes in one of the f a c t o r s o r in the w e a t h e r and

soil p r o p e r t i e s upon which they depend can cause significant d i f f e r e n c e s in s o i l f r e e z i n g o r thawing. A d i s c u s s i o n of the o b s e r v a t i o n s can, t h e r e f o r e , only i l l u s t r a t e i n a qualitative way the i m p o r t a n c e of v a r i o u s v a r i a b l e s , p a r t i c u l a r l y that of the

s u r f a c e o r g a n i c l a y e r s , in the f r e e z i n g and thawing of ground a t t h e s e four s i t e s .

T h e r e a r e not sufficient d a t a on r a d i a t i o n , wind velocity and s u r f a c e m o i s t u r e t o allow d i s c u s s i o n of the o b s e r v e d f r e e z i n g and thawing p a t t e r n s f r o m a s u r f a c e e n e r g y exchange point of view. In w i n t e r , d i f f e r e n c e s in evaporative and convective heat

l o s s e s a t the f o u r s i t e s tend t o be undetectable; wind v e l o c i t i e s a r e e x t r e m e l y low and a l l s i t e s have g e n e r a l l y the s a m e s u r f a c e conditions. D i f f e r e n c e s in net radiation m a y not be v e r y g r e a t b e c a u s e the g r e a t e r amount of s o l a r r a d i a t i o n r e c e i v e d a t exposed s i t e s could well be offset by higher long-wave radiation l o s s e s t o the a t m o s p h e r e . The f a c t t h a t a v e r a g e a i r t e m p e r a t u r e s during the winter a r e a l m o s t identical a t a l l s i t e s t e n d s t o

support t h i s conclusion. In s u m m e r , d i f f e r e n c e s in s u r f a c e h e a t balance depend to a g r e a t extent on s u r f a c e m o i s t u r e con- ditions which c o n t r o l evaporation. The r e l a t i v e i m p o r t a n c e of evaporation and o t h e r components of s u r f a c e e n e r g y exchange during t h e thaw p e r i o d will become c l e a r e r with continuing m e a s u r e m e n t s o v e r p e r i o d s of s e v e r a l y e a r s .

During t h e w i n t e r , snow c o v e r l a r g e l y c o n t r o l s the amount of h e a t l o s t f r o m the ground and subsequent ground f r e e z i n g a t the f o u r s i t e s . F i g u r e A - 3 shows a v e r a g e calculated heat flow f o r e a c h s i t e f o r e a c h snow s e s s i o n ( 1 November t o 30 A p r i l ) . T h e s e calculations w e r e m a d e by a s s u m i n g s t e a d y - s t a t e con- ditions, and an a v e r a g e snow s u r f a c e t e m p e r a t u r e e q u a l t o t h e a v e r a g e a i r t e m p e r a t u r e . The a v e r a g e t e m p e r a t u r e g r a d i e n t through the snow cover w a s a s s u m e d t o be equal t o t h e difference between a v e r a g e a i r and ground t e m p e r a t u r e s divided by the a v e r a g e depth of t h e snow. T h e t h e r m a l conductivity of the snow w a s calculated f r o m snow density v a l u e s ( W i l l i a m s and Gold, 1958).

The r e s u l t s of the calculations shown in F i g u r e A - 3 indicate that t h e r e w a s c o n s i d e r a b l e v a r i a t i o n in h e a t l o s s through t h e

snow c o v e r f r o m y e a r t o y e a r . Heat l o s s w a s highest a t a l l

s i t e s d u r i n g 1968-69 and lowest during 1970-71. On the a v e r a g e ,

Site A showed t h e l a r g e s t heat l o s s ; S i t e s B and C showed a l m o s t identical heat l o s s e s ; and Site D showed the lowest a v e r a g e h e a t l o s s .

The calculated v a l u e s f o r heat l o s s through the snow cover w e r e checked by c o m p a r i n g t h e m with heat l o s s m e a s u r e d a t t h e

30-cm depth. The t o t a l h e a t l o s s f o r two w i n t e r s e a s o n s (1969-70, 1970-71), m e a s u r e d with t h e h e a t m e t e r s , i s about 15 p e r cent h i g h e r t h a n t h e e s t i m a t e d heat l o s s through the snow c o v e r a t S i t e s A, B and C ( F i g u r e A-3). M e a s u r e d h e a t l o s s a t Site D i s slightly l o w e r than calculated heat l o s s through the snow c o v e r . Considering the u n c e r t a i n t i e s a s s o c i a t e d with m e a s u r e m e n t of h e a t flow by m e a n s of s o i l heat m e t e r s and t h e f a c t that the h e a t m e t e r s w e r e not placed a t the s u r f a c e , the a g r e e m e n t i s quite good.

A B C D

I

A B C D W I N T E R SUMMER F I G U R E A - 3 S U M M A R Y O F A I R TEMPERATURE, G R O U N D TEMPERATURE A N D H E A T LOSSES A N D G A I N S FOR W I N T E R A N D SUMMER P E R I O D S d11+99+ -6

D i f f e r e n c e s in h e a t l o s s through t h e snow c o v e r f o r the four s i t e s a r e explained by d i f f e r e n c e s in snow depth and density.

S i t e s B and C have c o m p a r a b l e a v e r a g e snow depths and d e n -

s i t i e s . Site A h a s a g r e a t e r snow depth than Site D , but

higher snow d e n s i t y offsets t o s o m e extent the g r e a t e r snow

depth. If ground t e m p e r a t u r e had been t h e s a m e a t both s i t e s ,

heat l o s s f r o m Site D would have been only slightly h i g h e r t h a n

h e a t l o s s f r o m Site A. The difference in h e a t flow between

S i t e s A and D i s t h u s p r i m a r i l y c a u s e d by d i f f e r e n c e s in ground s u r f a c e t e m p e r a t u r e .

The t h e r m a l p r o p e r t i e s of the s u r f a c e soil l a y e r have a significant influence on h e a t t r a n s f e r t o t h e s u r f a c e . F r o z e n

peat and Sphagnum in the u p p e r 3 0 - c m l a y e r a t S i t e s B and C

should have t h e r m a l conductivities ranging f r o m 0.003 t o 0.005 c a l / c m / s e c o ~ a t high m o i s t u r e contents. T h e s e v a l u e s c o m p a r e with the r a n g e f o r f r o z e n s i l t c l a y a t a v e r a g e m o i s t u r e

content, the m o s t likely conditions a t Site A. Consequently,

a v e r a g e ground t e m p e r a t u r e s a t the t h r e e s i t e s a r e quite s i m i l a r . Deviations f r o m a s s u m e d t h e r m a l conductivities and d i f f e r e n c e s in t h e r m a l g r a d i e n t and snow depth and d e n s i t y probably account f o r d i f f e r e n c e s in a v e r a g e heat l o s s during t h e winter. Site D i s a d r i e r s i t e than S i t e s B and C and if the o r g a n i c l a y e r a t Site D h a s a m a r k e d l y l o w e r m o i s t u r e content the t h e r m a l con- ductivity of the f r o z e n s u r f a c e o r g a n i c l a y e r would p r e s u m a b l y be lower. T h i s combined with the lower s u b s u r f a c e ground t e m - p e r a t u r e s a s s o c i a t e d with t h i s p e r m a f r o s t s i t e m a y account f o r the lower ground t e m p e r a t u r e s and t o t a l heat l o s s .

The heat l o s s f r o m t h e ground s u r f a c e in winter r e s u l t s in the cooling and f r e e z i n g of the soil. A s l a r g e amounts of h e a t m u s t be e x t r a c t e d t o f r e e z e w a t e r in soil, the depth and r a t e of f r o s t p e n e t r a t i o n depend l a r g e l y on the amount of m o i s t u r e p r e -

sent. Site A, the d r i e s t s i t e , h a s a . g r e a t e r depth of f r o s t p e n e t r a -

tion than Site B , which i s poorly drained. The s e a s o n a l fluctua-

tions in the r a t e of f r e e z i n g of the f i r s t few c e n t i m e t e r s of soil a t S i t e s C and D ( T a b l e A-I) a r e probably caused by v a r i a t i o n s in the m o i s t u r e content of t h e s u r f a c e o r g a n i c l a y e r s just p r i o r t o ground f r e e z i n g .

The amount of thawing during t h e s u m m e r d e t e r m i n e s not only the r a t e a t which s e a s o n a l f r o z e n ground m e l t s but a l s o

possible degradation of p e r m a f r o s t . If t h e heat gained by t h e

ground in s u m m e r e x c e e d s the h e a t l o s t in w i n t e r , t h e s u r p l u s h e a t will w a r m t h e ground a t n o n - p e r m a f r o s t s i t e s and m e l t p e r m a - f r o s t a t p e r m a f r o s t s i t e s . The only quantitative information on t h e