B E R I C H T E U N D M I T T E I L U N G E N
C L I M A T E - P E R M A F R O S T I N T E R A C T I O N B E T W E E N 1 8 k a A N D 8 k a B P I N N O R T H W E S T E R N N O R T H A M E R I C A
With 1 figure
S T U A R T A . H A R R I S
Zusammenfassung: Wechselwirkungen von Klima und Per- mafrost im nordwestlichen Nordamerika zwischen 18 ka und 8 ka BP
Das Inlandeis der Spät-Wisconsin-Zeit erreichte seine Maximalausdehnung an verschiedenen Stellen zu verschie- denen Zeiten. Die früheste Maximallage fand sich im öst- lich-zentralen Yukon (35-16 ka BP), während sie in Water- ton Park (Südwest-Alberta) erst u m 12 ka BP erreicht wurde. Nachdem sich zwischen 15-12 ka ein Eisdom über dem zentralen British Columbia entwickelt hatte, zog sich der Ostteil des Cordillierischen Inlandeises ins Gebirge zurück. Die Schneegrenzen verliefen 300-700 m tiefer als heute und die mittlere J a h r e s t e m p e r a t u r der unverglet- scherten Gebiete lag bis zu 10 °C niedriger. An den Rän- dern des Gletschers bildeten sich Eiskeile und Sandkeile.
Das Cordillierische Inlandeis wurde durch auflandige Niederschläge vom Pazifischen Ozean her ernährt, wäh- rend die zum Aufbau der Laurentidischen Eisbedeckung erforderlichen gewaltigen Niederschlagsmengen wohl auf tropisch-maritime Karibikluft als U r s p r u n g hindeuten. Der Großteil der Laurentidischen Vergletscherung hatte Basis- temperaturen unter dem Gefrierpunkt.
Der Abbau des Cordillierischen Inlandeises erfolgte rasch zwischen 13,5-10 ka. Stagnierendes Eis war verbrei- Introduction
O n e of t h e l a r g e s t a r e a s of ice s h e e t s d e v e l o p e d in N o r t h A m e r i c a d u r i n g t h e L a t e W i s c o n s i n a n gla- c i a t i o n . T o g e t h e r , t h e C o r d i l l e r a n a n d L a u r e n t i d e ice s h e e t s c o v e r e d m o s t of C a n a d a as well a s a p p r e c i a b l e a r e a s of A l a s k a a n d t h e n o r t h e r n S t a t e s of t h e c o n - t i g u o u s U . S . A . T o d a y , t h e g l a c i e r s h a v e l a r g e l y dis- a p p e a r e d , b u t a l m o s t 5 0 % of C a n a d a is u n d e r l a i n b y p e r m a f r o s t . T h i s p a p e r will s u m m a r i z e t h e d e v e l o p - m e n t of this p e r m a f r o s t i n r e l a t i o n t o t h e L a t e W i s - c o n s i n glacial h i s t o r y .
Extent of Glaciation between 18 ka and 13.5 ka BP O n e of t h e c h a r a c t e r i s t i c s of t h e g r o w t h a n d d e c a y of t h e L a t e W i s c o n s i n a n ice s h e e t s in N o r t h A m e r i c a is t h a t t h e m a x i m u m ice a d v a n c e s o c c u r r e d at dif- ferent t i m e s in different p l a c e s ( F i g . 1). T h u s t h e ice m a x i m u m p o s i t i o n in t h e B o n n e t P l u m e B a s i n of east c e n t r a l Y u k o n for t h e L a u r e n t i d e ice s h e e t o c c u r r e d at a b o u t 3 6 k a - 1 6 k a B P ( H U G H E S et a l . , 1981), w h e r e a s
tet, insbesondere im Präriegebiet. Das Laurentidische Eis wies ein sehr geringes Gefälle auf u n d hinterließ Toteis- blöcke bis zu 100 x 200 km Größe. Solche Eisblöcke über- dauerten in Nord-Dakota den allgemeinen Eisrückzug u m bis zu 2,5 ka. Reste von stagnierendem Laurentidischen Inlandeis mögen noch heute u m den Arktischen Ozean h e r u m existieren. Im Gebiet waren große proglaziale Seen weitverbreitet. Ein erneuter Vorstoß einiger Eismassen bei Calgary ließ zwischen 10,6-8 ka Eisstauseen in der Fuß- hügelregion von Alberta entstehen. Dabei herrschte wahr- scheinlich eine aufwärts gerichtete Zirkulation im Uhr- zeigersinn u m das zurückgehende Laurentidische Eis h e r u m , was auch für die trocken-warmen Verhältnisse im südlichen British Columbia verantwortlich gewesen sein mag. Eine trocken-kalte Phase trat ebenfalls im Früh- Holozän auf.
Die proglazialen Seen führten zum Abschmelzen nahezu des ganzen Spät-Wisconsin-Permafrostes, so daß der heu- tige Permafrost in West-Kanada sich ganz überwiegend erst im Holozän gebildet hat. Die ehemaligen Eiskeile schmol- zen ab u n d hinterließen Eiskeil-Pseudomorphosen. Ledig- lich kleine Reste des Spät-Wisconsin-Permafrostes erhiel- ten sich an ehemaligen Nunatakkern (z. B. am Plateau M o u n t a i n ) .
t h e ice s h e e t o n l y r e a c h e d t h e C a l g a r y - W a t e r t o n P a r k a r e a after t h e m o u n t a i n ice w a s i n r e t r e a t ( A L L E Y ,
1973, A L L E Y a . H A R R I S , 1974), w h i c h is n o w b e l i e v e d t o h a v e o c c u r r e d a b o u t 1 2 . 0 k a B P .
T h e C o r d i l l e r a n ice s h e e t e x p a n d e d from a n a l p i n e p h a s e , w h i c h m a y h a v e s t a r t e d a s e a r l y a s 29 k a B P
( C L A G U E , 1976, 1977, 1980; A L L E Y , 1979) t o a p i e d - m o n t p h a s e w h i c h o n l y finally c o v e r e d t h e a r e a n e a r K a m l o o p s , B r i t i s h C o l u m b i a a b o u t 15 k a B P ( C L A G U E
et a l . , 1988). T h e r e a f t e r it f o r m e d t h e m a j o r d o m e t h a t c u t off t h e s u p p l y of m o i s t u r e to t h e e a s t e r n slopes of t h e C o r d i l l e r a n ice c a p , r e s u l t i n g in t h e r e t r e a t of t h e e a s t e r n m a r g i n i n t o t h e m o u n t a i n s at 12 k a B P . I n c o n t r a s t , t h e ice m a x i m u m p o s i t i o n o n t h e w e s t side of t h e Q u e e n C h a r l o t t e I s l a n d s w a s a c h i e v e d a b o u t 15 k a B P ( B L A I S E et a l . , 1990), a n d t h e m a x i m u m s o u t h e r n e x t e n t w a s r e a c h e d at b e t w e e n 14.0 a n d
1 4 . 5 k a B P ( M U L L I N E A U X e t a l . , 1 9 6 5 ; H I C O C K a . A R M S T R O N G , 1985).
Climatic Conditions between 18 ka and 13.5 ka BP T o d e v e l o p t h e ice s h e e t s , t r e m e n d o u s a m o u n t s of s n o w h a d to a c c u m u l a t e a n d b e c o m e m e t a m o r p h o s e d
Fig. 1: Extent of N o r t h American ice sheets d u r i n g the last glaciation according to the currently proposed reconstructions A u s d e h n u n g des letztglazialen nordamerikanischen Inlandeises aufgrund derzeitiger Rekonstruktion
into ice. T h e snow for the Cordilleran ice sheet clearly came from the Pacific Ocean, carried onshore by the westerly winds. T h e contemporary proglacial lakes lacked a fauna of molluscs and ostracods and this is consistent with the interpretation that equilibrium lines were up to 500 m lower than today at latitudes 4 0 ° - 4 8 ° N (RICHMOND, 1965) and 300-700 m lower in central Alaska (PEWE, 1975, p. 109). Although the snow deposited by the onshore westerly winds has a relatively high density (approximately 0.1 g/cm3, i.e., 9 m of snow would yield 1 m of glacial ice), the snowfall may have been higher in order to build the ice sheets across the central plateau. Evidence from the fauna and flora and from the contemporary permafrost landforms preserved in nonglaciated areas suggests a mean annual air temperature up to 10 °C colder than today in Alaska (PEWE, 1975). Ground temperature profiles along the Arctic Coast, together with stable isotope analyses of contemporary ground ice pre-
served along the Lower Mackenzie Valley and the Arctic coast, also provide evidence for cooler temper- atures. Along the southern ice margin, BARNOVSKY (1984) noted colder and more moist conditions that probably aided the final growth of the ice sheet between 15 and 12.5 ka BP.
South of the ice sheets, there are well-preserved ice- wedge casts along the outer margin of the ice sheets (e. g. PEWE, 1983) and southwards along the higher plateaus and basins of the Rocky Mountains (e. g.
MEARS, 1981, 1987). This evidence has recently been s u m m a r i z e d i n H A R R I S ( 1 9 9 4 , F i g . 4 ) . MARKER(1990) summarized the evidence for lower periglacial land- forms southwards along the nonglaciated parts of the Rocky Mountains, while HEINE (1994a) has de- scribed the evidence for continuation of this climatic change into Mexico.
In the case of the Laurentide ice sheet, a mere change in temperature would not account for its size.
There must have been a substantial increase in pre- cipitation in the form of snow. T h e present-day mean winter snow density is only about 0.03 g/cm3 over much of the Canadian Prairies, so it would take 25 m of snow to produce 1 m of glacial ice, assuming no ablation or sublimation. Typical present-day mean annual precipitation is only 300-500 m m , yet some- how enough snow fell on the Prairies to cause the Laurentide ice to advance 3000 km from Hudson Bay to Waterton National Park in about 13 ka. This is a mean rate of ice advance of about 230 m per year - a rate that today would be considered by many to be that of a surging glacier. T h e only suitable source for such enormous quantities of precipitation would be maritime tropical air from the Caribbean Sea, per- haps advancing further north and west than at present due to the ice cap in the north preventing the cold Arctic air from moving as far south as at present. This would cause heavy snowfalls along the front where it met the cold Arctic air on the southern slopes of the ice sheet.
Whatever the cause of the higher precipitation, the colder air temperatures permitted much of the Laurentide ice sheet to be a cold-based glacier. Evi- dence for this includes the preservation of erosion surfaces in the Arctic Islands ( B I R D , 1 9 6 7 ) and the karst features in the limestones beneath the glacial till near Winnipeg. Undoubtedly permafrost was widespread around the margins and beneath parts of both the Laurentide and Cordilleran glaciers.
Déglaciation
R Y D E R et al. (1991) conclude that the retreat of the Cordilleran ice sheet began along the west coast about 13.5 ka BP and was completed within two millennia
( C L A G U E , 1 9 8 1 ) . T h e southern margin was retreating northwards by 14 ka BP, but the rapid coastal dégla- ciation is quite different from the melting down and ice stagnation that occurred in the interior valleys
( F U L T O N , 1 9 6 7 ) . In the latter, the mountain peaks appeared first, but stagnant ice choked the interior valleys and large lakes formed around the ice mar- gins. O n the eastern side of the ice sheet, the ice had retreated into the mountain valleys by about 11 ka BP west of Calgary. In the southern Fraser valley, a minor ice readvance occurred at 11.5 ka BP ( S U M A S
advance of A R M S T R O N G , 1 9 8 1 ; S A U N D E R S et al., 1 9 8 7 ) ,
but the Cordilleran ice sheet ceased to be a continuous mass shortly after 11 ka BP and melted relatively rapidly, responding to a warm, dry climate that began about 12.5 ka BP ( B A R N O V S K Y , 1984). These warmer conditions persisted until after 8 ka BP.
T h e Laurentide ice sheet was much larger and probably had a low average slope to its surface. If it were 3000 m high over Hudson Bay, the slope to Waterton Park (elevation 1200 m) would have had a
drop of only 1 8 0 0 m in about 3 0 0 0 km. T h e thinner the ice sheet, the lower the slope.
As noted previously, the southwestern margin only reached its m a x i m u m extent about 12 ka BP, although the southern margin was in retreat by 12.5 ka ( C L A Y - T O N , 1 9 6 6 ) . In most areas of the western and southern Prairies south of Edmonton, ice stagnation was wide- spread ( P R E S T et al., 1 9 6 8 ) . T h e largest stranded blocks were at least 100 x 200 km in size and their downwasting took u p to 2.65 ka in one case in North Dakota ( T U T H I L L et al., 1 9 6 4 ) and 2 ka in Saskatche- wan ( P A R I Z E K , 1 9 6 4 ) . Ice stagnation also occurred in the Lower Mackenzie Valley and on the Arctic Islands, but there it seems that some of the stagnant ice may still remain today as buried massive icy beds
( M A C K A Y e t a l . , 1 9 7 9 ; L O R R A I N a . D E M E U R , 1 9 8 5 ) .
T h e abundance of these ice masses is still being debated, but the cold continental climate of north- central C a n a d a is suitable for the preservation of buried glacial ice, in contrast to the climate in the western Cordillera.
T h e persistence of buried glacial ice in the Arctic Basin is surprising since low-lying coastal areas were inundated by the sea as the ice retreated, only to reap- pear as isostatic rebound occurred. T h e latter has resulted in postglacial marine deposits occurring as raised beaches at elevations exceeding 300 m above present-day sea level along the western margin of Hudson Bay.
Large proglacial lakes developed around the western and southern margins of the Laurentide ice cap during both advance and retreat ( A L L E Y a. H A R R I S , 1 9 7 4 ; P R E S T et al., 1 9 6 9 ; T E L L E R , 1 9 8 7 ) . T h e largest of these was Glacial Lake Agassiz, but most lowland areas of the Prairies were inundated at least once during deglaciation.
T h e inferred positions of the active Laurentide ice front based on radiocarbon dates are provided by
P R E S T ( 1 9 6 8 ) a n d D Y K E a n d P R E S T ( 1 9 8 7 ) . B y 1 0 k a ,
the front of the active ice was north of Lake Winnipeg and crossed the western end of Lake Athabasca. T h e Laurentide ice still formed a formidable ice cap that only split into three parts when the ice lying in Hud- son Bay melted shortly before 8 ka. It provided a high pressure cell in the atmosphere that caused upslope winds bringing heavy precipitation to the high plains of southwestern Alberta. This resulted in reactivation of some stagnant ice masses, e. g., the Innisfail ad- vance of S T A L K E R ( 1 9 7 3 ) , causing blocking of river valleys and the formation of new proglacial lakes starting about 10 ka and lasting until 8 ka, e.g. the last Glacial Lake Calgary ( H A R R I S , 1 9 8 5 ) . This air may also have crossed the continental divide to provide dry, warm conditions in southern British Columbia.
There is also evidence of drier, warm conditions in the mountains and foothills of southwest Alberta at this time. When the main ice sheet diminished, so did the precipitation, and the ice masses on the Plains melted.
There were intermittent, short-lived readvances or surges of lobes of the Laurentide ice sheet during its retreat, but these tended to affect only one sector at any one time ( P R E S T , 1 9 6 8 ; D Y K E a. P R E S T , 1 9 8 7 ) . In the mountains, u p to five small moraines predating the M a z a m a Ash (6.83 ka) and postdating the main Cordilleran ice retreat can be found in individual valleys. Glacier Peak ash, dated at 11.3 ka, has not been found on their surface. These advances are quite minor and their age and correlation from valley to valley are problematic. However, there is also evi- dence from Jasper National park south to Plateau Mountain, southwest Alberta, for a post-glacial ad- vance of rock glaciers to 500 m below the present lower limit of permafrost. T h e exact age of this event is unknown, but an advance of rock glaciers without a substantial complementary change in the glaciers can only be achieved by a reduction in precipitation with- out a temperature change ( H A R R I S et al., 1994). Evi- dence for this type of climatic change in the early Holocene has not been recognized by palynological studies, but the geomorphic evidence for a climatic change of this type is overwhelming. Such a change would have required perhaps a millennium to pro- duce the rock glaciers, but whether this occurred before or after 8 ka is unclear. T h e small moraines and these rock glaciers could be ascribed to the Younger Dryas event, but H E I N E (1994 b) correlates similar well-dated moraines on Mexican volcanoes to the dis- charge of cold glacial meltwater into the surrounding oceans(e.g., K E N N E T T a . S H A C K L E T O N , 1975;TELLER, 1990 a, 1990b). Those same discharge events also produced changes in the vegetation in the adjacent Mexican Highlands ( S T R A K A a. O H N G E M A C H , 1 9 8 9 ) .
Effects of Déglaciation on Permafrost
In the north, the marine incursions would have tended to interfere with any permafrost left from the Late Wisconsinan glaciation. T h u s relict perma- frost is only found in the unglaciated zones east of Tuktoyaktuk, and is characterized by colder ground temperatures and greater thicknesses of permafrost
( 3 6 0 m compared with 9 0 m - J U D G E , 1 9 7 3 ) . Per- mafrost has reformed on all the suitable land areas after retreat of the glaciers, sea, and meltwater. T h e proglacial lakes undoubtedly caused the thawing of any permafrost in the underlying sediments ( J U D G E , 1 9 7 3 ) , so the present-day permafrost is postglacial.
Proof of the postglacial age of the permafrost is seen in its shallower thickness (usually 100-200 m, ranging up to 600 m on Ellesmere Island at - 2 0 ° C mean annual temperature) compared with permafrost in Siberia (up to 1500 m thick). Ice wedges in Siberia may be 30 m deep and up to 20 m wide, compared with 5 m deep and 1 m wide in Canada. The ground temperature profiles in most Canadian permafrost
only show the effects of Holocene climatic changes
( J U D G E , 1973), while the isotopic composition of the ice reflects the changing ground temperatures of the Holocene ( M I C H E L a. F R I T Z , 1978; M I C H E L , 1983, 1995; B U R N a. S M I T H , 1985a, 1985b; H A R R I S , 1988, 1989). This is in marked contrast to unglaciated areas
( G O L D a . L A C H E N B R U C H , 1 9 9 3 ; L A C H E N B R U C H a . M A R S H A L L , 1969), and the isotopic composition of buried glacial ice ( L O R R A I N a. D E M E U R , 1985).
Southwards, it is similar story, except that con- ditions suitable for the formation of permafrost under the present-day climate are only found along the higher mountains ( H A R R I S , 1 9 8 6 ) and beneath post- glacial peaty deposits ( Z O L T A I a. V I T T , 1 9 9 0 ) . Occa- sional relict permafrost may be found on former Late Wisconsin nunataks such as Plateau Mountain
( H A R R I S a. B R O W N , 1 9 7 8 ) . In Wyoming, the former ice-wedges have melted, leaving ice-wedge casts and sand wedges as evidence of the much colder climate during the Late Wisconsin glaciation. T h u s the former ice sheets have had very little effect on where permafrost occurs today in western North America.
Acknowledgements
This paper was prepared at the request of R E I N V A I K M A E as a contribution to the I G C P project #253 - Termination of the Pleistocene.
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D A S F I N N I S C H E , , M Ö K K I W E S E N " -
E I N L A N D E S T Y P I S C H E R L E B E N S S T I L I M U M B R U C H ?
Mit 4 Abbildungen und 3 Photos
K R I S T I I N A J Ä D E R H O L M - V O G E L u n d W I L H E L M S T E I N G R U B E
Summary: T h e Finnish " M o k k i s " - a change in a typically Finnish life-style?
T o have a free-time residence of one's own has been a long tradition in Finland. At the beginning (in the early 19th century) there were impressive summer villas belonging to a small and wealthy group of the population. After a period of slow but steady growth the demand for private leisure homes in Finland has risen exponentially since the Second World War. Nowadays it is no longer a privilege of a certain social or income class to own such a home. M e a n - while nearly every third family has its own " M o k k i " , as the Finns call their summer cabin. T h e period of use is getting longer and longer, and the simple s u m m e r cabin has often become a free-time residence fit to live in all year round.
Fundamental changes in political, economic and social conditions (economic crises, new planning regulations, ecological problems and so on) will, however, bring to a sudden halt a further increase in the n u m b e r of free-time residences, possibly within a few years. Nevertheless, initial findings show clearly that m a n y Finns are striving for their own " M o k k i " or are maintaining emotional bonds with their family leisure home, which is of central importance to their lives, i.e. the " M o k k i " has become an important com- ponent of their life-style. As this life-style component is spread across all social classes all over Finland we can speak of a national life-style.
Most Finns do not consider their leisure houses as normal economic goods - they have strong emotional bonds with them. T h e Finnish " M o k k i s " will therefore probably prove to be relatively resistant to negative external influences. For prognosticating the further development of the " M ô k k i "
network it is therefore not sufficient to analyze the economic and "objective" general conditions. In order to draw u p realistic scenarios we will have to pay special attention to the life-style conception or similar approaches which take the individual level of evaluation and decision into con- sideration.
1 Einführung
Die Sommerhütte am See ist in Finnland mehr als n u r ein landschaftsprägendes Element. In diesen Häusern verbrachten und verbringen immer noch viele Finnen nahezu den gesamten Sommer, darin investieren sie einen großen Teil des Familienein- kommens, das Mökki (fin. : mökki = Haus, Hütte) ist für viele Finnen Teil ihres Lebens.
Die Anfänge des Mökkiwesens gehen zwar auf die Sommerresidenzen Adliger und anderer Privilegier-
