Late Weichselian and Holocene shore displacement history of
the Baltic Sea in Finland
MATTI TIKKANEN AND JUHA OKSANEN
Tikkanen, Matti & Juha Oksanen (2002). Late Weichselian
and Holocene shore displacement history of the Baltic Sea in
Finland. Fennia 180: 1-2, pp.
000-000. Helsinki. ISSN 0015-0010.
About 62 percent of Finland's current surface area has been
covered by the waters of the Baltic basin at some stage. The
highest shorelines are located at a present altitude of about
220 metres above sea level in the north and 100 metres above
sea level in the south-east. The nature of the Baltic Sea has
alternated in the course of its four main postglacial stages
between a freshwater lake and a brackish water basin connected
to the outside ocean by narrow straits. This article provides
a general overview of the principal stages in the history of
the Baltic Sea and examines the regional influence of the associated
shore displacement phenomena within Finland. The maps depicting
the various stages have been generated digitally by GIS techniques.
Following deglaciation, the freshwater Baltic Ice Lake (12,600-10,300
BP) built up against the ice margin to reach a level 25 metres
above that of the ocean, with an outflow through the straits
of Öresund. At this stage the only substantial land areas
in Finland were in the east and south-east. Around 10,300 BP
this ice lake discharged through a number of channels that opened
up in central Sweden until it reached the ocean level, marking
the beginning of the mildly saline Yoldia Sea stage (10,300-9500
BP). As the connecting channels rose above sea level, however,
the Baltic Sea became confined once more, to form the Ancylus
Lake (9500-8000 BP). During its existence the outflow channel
to the ocean shifted to the Straits of Denmark and the major
lake systems of central Finland became isolated from the Baltic
basin. After the brief Mastogloia transition phase, a greater
influx of saline water began to take place through the Straits
of Denmark, marking the Litorina Sea stage (7500-4000 BP), to
be followed by a somewhat less saline stage known as the Limnea
Sea. After a transgressive period early in the Litorina Sea
stage, shoreline displacement in Finland has proceeded at a
steadily declining rate.
Matti Tikkanen, Department of Geography,
P. O. Box 64, FIN-00014 University of Helsinki, Finland. E-mail:
matti.tikkanen@helsinki.fi
Juha Oksanen, Finnish Geodetic Institute, Department of Geoinformatics
& Cartography, P. O. Box 15, FIN-02431 Masala, Finland.
E-mail: juha.oksanen@fgi.fi
Introduction
The Baltic Sea has a long and chequered history, in the course
of which the area nowadays occupied by Finland has undergone
substantial changes. The phenomenon known as land uplift, which
still operates in the Baltic region and currently amounts to
some eight millimetres per year on the Finnish coast of the
northernmost Gulf of Bothnia (Kakkuri 1990; Mäkinen &
Saaranen 1998), has had the effect of reducing the sea area
over a period of several thousand years and has correspondingly
increased the land area of Finland. The present area of the
Baltic Sea is about 377,000 square kilometres, and it has a
freshwater influx of some 660 cubic kilometres a year from a
drainage basin of 1.6 million square kilometres. In addition,
some 475 square kilometres of saline water a year flows into
the Baltic through the narrow Straits of Denmark. The total
outflow of brackish water is of the order of 950 cubic kilometres
a year (Björck 1995: 19).
The Baltic Sea is the largest brackish water basin in the world,
although it is fairly shallow - its deepest point is only 459
metres. Thresholds divide the basin into a number of separate
sections (Fig. 1.). The sea's salinity varies from 0.1 percent
in the north to 0.6-0.8 percent in the central parts, and can
reach as much as 1.5-2.0 percent in the deepest waters (Björck
1995: 20). The salinity has also varied greatly in the course
of the history of the basin.
Fig. 1. The Baltic Sea and the surrounding
region.
Finland and the whole Baltic basin have been buried beneath
the continental ice sheet on several occasions during the Quaternary
period (Taipale & Saarnisto 1991: 212). These glaciations
have been separated by more favourable climatic periods, during
which the Baltic basin has been occupied by water and land uplift
has caused progressively greater areas of dry land to emerge
on its coasts, in the same manner as during this last postglacial
period. During the Eemian interglacial, about 130,000-115,000
BP, the basin contained what is referred to as the Eemian Sea.
Its waters were evidently much more saline than those of the
Baltic Sea nowadays, as deduced from diatoms recovered from
sediments dated to that period. It is possible that there was
a connection from the Baltic basin to the Arctic Ocean across
Karelia at the beginning of this Eemian Sea stage, although
it was closed later due to the effects of land uplift (Björck
& Svensson 1994).
The ice sheet associated with the Weichselian glaciation, which
followed the Eemian interglacial, reached its maximum extent
about 18,000-20,000 BP. It was more than three kilometres thick
in the area of Finland, so that it depressed the earth's crust
to such a degree that a considerable part of the country's present
area lay beneath the waters of the Baltic basin immediately
after deglaciation. As the burden of the overlying ice was released
the crust began to rise rapidly, however, so that it is estimated
that the total rise up to the present time has been 600-700
metres on the northern coast of the Gulf of Bothnia, 400-500
metres in the middle part of Finland and in central Lapland,
and around 300 metres on the coast of the Gulf of Finland and
in northern Lapland (Mörner 1980). The majority of this
rise nevertheless took place as the ice was melting, before
the ground surface became exposed.
Four main stages can be recognized in the history of the Baltic
basin since the last glaciation, influenced by a complex interaction
between deglaciation, regional differences in land uplift, and
eustatic changes in sea level (Tikkanen et al. 1999). The connection
with the outside ocean has been located variably in the Straits
of Denmark or central Sweden. There is a long tradition of research
into the history of the Baltic basin and shore displacement
in Finland and the other countries of the Baltic region (see
Eronen 1983; Björck 1995; Heinsalu et al. 2000), and the
regional changes in shore displacement during the various stages
can nowadays be traced fairly accurately by combining the vast
amount of data available with altitude data using GIS techniques
(Tikkanen & Oksanen 1999).
This article will provide a brief overview of the main stages
in the history of the Baltic Sea and their influence on the
pattern of shore displacement in Finland. The technique employed
for constructing the palaeogeographical maps of Finland is based
on a digital elevation model (DEM) for the entire country. The
model can be intersected by different surfaces in accordance
with the water level that prevailed in the Baltic at each stage.
The contour-based DEM was constructed by the National Land Survey,
and it has horizontal resolution of 200 metres.
The Baltic Ice Lake (12,600-10,300 BP)
The ice margin began to retreat from the southernmost parts
of the Baltic basin around 13,500-13,000 BP, leading to the
creation of the first postglacial water bodies in the region
(Björck 1995: 21). Deglaciation then proceeded very rapidly,
so that the margin was situated close to the south coast of
Finland by about 12,000 BP (Niemelä 1971; Saarnisto &
Salonen 1995: 10). By that stage the large volumes of meltwater
from the ice sheet were able to discharge from the Baltic Ice
Lake into the ocean via the Strait of Öresund (Agrell 1976;
Björck 1979; Björck 1995) (Fig. 2A).
Fig. 2. Subaquatic regions of the Baltic
basin and connections from the southern Baltic to the ocean,
12,000-7200 BP (Eronen 1990; Björck 1995).
Before the ice margin had reached the point now marked by the
First Salpausselkä moraines, however, the bedrock threshold
in the Strait of Öresund had risen above the ocean level
and the waters of the Baltic basin had begun to be dammed up
to form a freshwater ice lake. Then, around 11,200 BP, a (possibly
subglacial) connection to the west opened up north of Mount
Billingen in central Sweden. This remained functional for some
400 years, and allowed the water level in the Baltic to return
to that of the outside ocean, presumably implying a decline
of 5-10 metres (Björck 1995). This has been referred to
as the g-stage, derived from
a now obsolete system of labelling shore levels in the Baltic
with letters from a to p (Eronen 1990).
The climate became cooler once more during the Younger Dryas
period, causing the ice margin to advance. With the closure
of the connection with the ocean around 10,800 BP, the next
600 years proved to be a transgressive period on the shores
of the southern parts of the Baltic Ice Lake, its water level
gradually rising to about 25 metres above that of the ocean
and the outflow channel shifting to the Strait of Öresund
once again (Björck 1995). The deltas that developed in
connection with the Salpausselkä marginal formations during
this period now lie at a level of about 160 metres in the area
west of the city of Lahti and at about 100 metres near the south-eastern
boundary of Finland. The difference in altitude is due to the
differences in the rate of land uplift between these areas.
There were only a few small areas of high ground in southern
Finland that projected above the level of the Baltic Ice Lake,
together with the Salpausselkä deltas, which were laid
down at the water level. The most extensive areas of dry land
were to be found close to the eastern and south-eastern boundaries,
in the present-day districts of Ilomantsi-Tuupovaara and Ruokolahti-Rautajärvi
(Tikkanen & Oksanen 1999: 34) (Fig. 3 & CD-Fig. 1).
Fig. 3. Shore displacement
in Finland at different stages in the history of the
Baltic. For more details, see CD-Fig. 1-4.
CD-Fig. 1. Finland and the Baltic Ice Lake,
ca. 10,300 BP.
Towards the end of the Baltic Ice Lake period (stage BIII)
the depth of water at the point of the present shoreline at
Hanko was about 130 metres, that at Helsinki, 115 metres, and
that at the south-eastern boundary of Finland, 80 metres (Svensson
1989: 159). By that time the delta surfaces associated with
the First Salpausselkä, representing the BI stage, had
risen to a level of ten metres above the Baltic Ice Lake level.
By the time the Younger Dryas cold phase came to an end, the
Second Salpausselkä marginal formation was also in existence,
and the sharp warming of the climate around 10,500 BP then caused
the ice margin to retreat rapidly (Björck 1995). As a consequence,
the Billingen 'gateway' opened up again around 10,300 BP and
the waters of the Baltic Ice Lake once more began to discharge
into the ocean through central Sweden (Fig. 2B). This dropped
the water level in the Baltic basin by 25-28 metres within a
few years, regaining the level of the outside ocean.
The Yoldia Sea (10,300-9500 BP)
The new warming of the climate finally brought the Ice Age
to an end. The reopening of the Billingen channel and the drop
in the level of the Baltic Ice Lake also marked the end of glacial
conditions in the Baltic basin. The next stage in the history
of the basin is referred to as the Yoldia Sea, after the bivalve
Portlandia (Yoldia) arctica,
typical of cold, saline water and found in sediments of this
age in the Stockholm area (De Geer 1913). As the ice margin
retreated further, the Närke Strait in the lowlands of
central Sweden north of Billingen opened up about 10,000 BP,
allowing saline water from the ocean to flow into the Baltic
basin (Eronen 1990; Björck 1995) (Fig. 2C). This weakly
saline brackish water phase remained relatively short, however,
and within 100-200 years the salinity of the Baltic began to
decline again. The saline effect reached the area of Finland
with a delay. The Yoldia stage was evidently characterised by
freshwater conditions throughout in the present-day inland areas,
on account of the large volumes of meltwater (Taipale &
Saarnisto 1991). Brackish water species have been identified
in the diatom stratigraphy for this period in some places on
the Karelian Isthmus, however (Arslanov et al. 1996; Saarnisto
et al. 2000).
Ocean levels rose at a rate of more than a metre per century
around 10,000 BP, but the water level was still 30 metres below
what it is today (Taipale & Saarnisto 1991: 237) and shore
displacement continued at a considerable rate in Finland on
account of the pronounced land uplift. Current altitudes of
the Yoldia shoreline in Finland vary in the range of 120-185
metres (Saarnisto 2000: 27), and the delta surfaces laid down
in connection with ice margin formations in the interior of
the country tend to lie at current levels of 140-160 metres
above sea level. In the southern parts of the Baltic basin,
in turn, the Yoldia shoreline is located some 50 metres below
current sea level, and Björk (1995) claims that southern
Sweden was joined to the continent of Europe by an isthmus some
ten kilometres wide at that time (Fig. 2C).
The land area in what is now Finland expanded greatly during
the Yoldia Sea stage. The drop in water level that marked the
end of the Baltic Ice Lake caused land to be exposed very suddenly.
A zone ranging between 10 and 100 kilometres in width along
the present boundary of Finland in the east and south-east had
become isolated from the Baltic by just over 10,000 BP (Fig
3. & CD-Fig. 2), although a local ice lake covered part
of this area at first (Hellaakoski 1934). There were also extensive
islands in the present-day Lahti and Hyvinkää areas,
and an archipelago emerged in the interior of Finland almost
as soon as the ice had retreated (see Tikkanen & Oksanen
1999).
CD-Fig. 2. Finland and the Yoldia Sea, ca.
10,000 BP.
The Ancylus Lake (9500-8000 BP)
As the rate of land uplift in central Sweden was faster than
the rise in ocean level, the thresholds in the connecting straits
began to approach the latter level around 9500 BP. This meant
that the Baltic basin was once more isolated to form a freshwater
lake, which De Geer (1890) named the Ancylus Lake after the
gastropod Ancylus fluviatilis,
characteristic of its sediments (Munthe 1887). It was thought
earlier that the outflow channel from the Ancylus Lake had been
located at the watershed between the Baltic Sea and the Atlantic
Ocean in the Degerfors area, the flow across which was referred
to as the "Svea River" (von Post 1927). Recent research
has nevertheless shown that there was a connection from Lake
Vänern to the Baltic through the Närke-Degerfors Strait
and that the outflow threshold was located west of Lake Vänern
(Björck 1995). The channels concerned were formed by the
rocky beds of the present-day rivers Göta and Steinselva
(Fredén 1982), but these were so narrow that hydraulic
dams developed, allowing the surface of the Ancylus Lake to
rise above the ocean level (Björck 1986, 1987, 1995) (Fig.
2D).
This marked the beginning of the Ancylus transgression, which
lasted about 300 years (9500-9200 BP). During this time, the
rising water level caused extensive areas of land to be inundated
once more, especially on the south coast of the Baltic, where
practically no land uplift took place. The water was rising
at a rate of 5-10 centimetres a year, and the transgression
as a whole is estimated to have been of the order of 15-25 metres
(Eronen 1990; Björck 1995). This was also felt on the south
coast of Finland, in the form of a rise of a few metres in water
level, causing the creation of clearly defined ancient shorelines.
In the Gulf of Bothnia, the rate of land uplift consistently
exceeded the rise, so that a certain amount of new land emerged
even during this transgressive period. The uppermost shoreline
of the Ancylus Lake in the Helsinki area is located at about
60 metres above sea level (Eronen & Haila 1982: 123). There
are many places around the northern part of the Gulf of Bothnia
where it is close to 200 metres or even slightly over this (Saarnisto
1981).
The rising waters of the Ancylus Lake eventually exceeded the
threshold known as the Darss Sill in the south-western part
of the Baltic basin and water began to flow out through the
Dana River, at the site of the present-day Great Belt (Store
Bælt), around 9200 BP (Fig. 2E). This brought the Ancylus
transgression to a close and severed the land connection between
Sweden and Denmark at the same time. For a time the channels
across central Sweden functioned alongside the Dana River, but
the rapid regression of the Ancylus Lake, combined with land
uplift, soon caused these channels to dry up (Björck 1995).
The surface of the Ancylus Lake was at least ten metres above
the ocean level at the time when the Dana River arose, but as
the till of the Darss Sill was not especially susceptible to
erosion, the deepening of the Dana River and lowering of the
Ancylus Lake level did not take place very suddenly (Kolp 1990).
The drop in water level lasted altogether some 200 years, so
that the Ancylus Lake reached the level of the outside ocean
around 9000 BP. This did not mean any influx of saline water
into the Baltic basin, however, as the Dana River, being narrow
and more than 100 kilometres long, remained the only connection
for a long time (Björck 1995).
Large expanses of dry land emerged in the area of Finland during
the Ancylus regression. The great lake basins of the interior
of the country were separated from the Baltic at this time (Saarnisto
1971, 2000; Tikkanen 1990). On the other hand, when the last
of the ice disappeared from the Tornionjoki valley around 9000
BP, the outermost islands in the Gulf of Bothnia were still
more than 100 kilometres away from the present shoreline, which
in the northern part of the Gulf was still covered by more than
200 metres of water (Eronen 1990; Tikkanen & Oksanen 1999).
In the south, the Vuoksi, Kymijoki, and Kokemäenjoki watercourses
were marked by bays of the Ancylus Lake stretching far into
the interior, and narrow sounds divided the uplands of that
area into a labyrinth of islands.
The waters of the lake extended up the great river valleys of
the north as far as the southern boundaries of Pelkosenniemi,
Kittilä, and Muonio (Fig. 3 & CD-Fig. 3), but the shoreline
of the Gulf of Finland was very much closer to its present position,
extending to the First Salpausselkä in places. There were
also innumerable islands throughout the area. The shoreline
was highly irregular on account of the location of the coastal
zone in an area of bedrock faults in which the surficial deposits
played little part in levelling out the topography (Tikkanen
& Oksanen 1999).
CD-Fig. 3. Finland and the Ancylus Lake,
ca. 9000 BP.
The Litorina Sea (7500-4000 BP)
As ocean levels were still rising by more than one centimetre
per year (Fairbanks 1989), saline water eventually rose above
the threshold in the Straits of Denmark and began to enter the
Baltic basin, perhaps around 8400-8300 BP, although the Darss
Sill prevented it from spreading to the area in any great quantity
at first (Eronen 1990). The effects of this saline addition
began to be felt more clearly around 8200 BP (Berglund 1964;
Björck 1995). The transition period known as the Mastogloia
Sea is deemed to have begun at this time (the name comes from
a similarly-named diatom that favours slightly brackish water)
(Eronen 1974, 1983; Taipale & Saarnisto 1991). More and
more saline water entered the basin as the connecting channels
became broader, and by around 7500 BP the Litorina Sea (named
after the gastropod Littorina littorea)
may be regarded as having reached the south coast of Finland
(Eronen 1974; Björck & Svensson 1994). It took somewhat
longer for the saline effect to reach the head of the Gulf of
Bothnia: the Litorina Sea is deemed to have commenced in that
area around 7000 BP (Eronen 1974) (Fig. 2F). In the early part
of the Litorina Sea stage, salinity was about 0.8 percent in
the northern end of the Gulf of Bothnia and 1.3 percent in the
central Baltic (Taipale & Saarnisto 1991: 276), i.e., the
water was a good deal more saline than nowadays (0.2% and 0.7%).
The eustatic rise in ocean levels led to a transgression at
the beginning of the Litorina Sea stage. As a result, water
levels on the south-east coast of Finland rose by a few metres
and a slight rise was recorded in the Helsinki area (Eronen
1990). Little or no land uplift from the sea was recorded in
south-western Finland (Glückert 1991), and a distinct belt
of ancient shorelines was created, in the same manner as on
the south coast. On the other hand, the Litorina Sea remained
regressive throughout on the coast of the Gulf of Bothnia, i.e.,
new areas of dry land were being created constantly as a consequence
of the high rate of land uplift.
When the rise in ocean levels came to an end between 6000 and
5000 BP, the transgressive phase of the Litorina Sea also finished
(Eronen 1990). Since that time the Litorina Sea has continued
to develop without any marked changes up to the present and
new land has been laid bare on the coasts of Finland at a steadily
declining rate. No biostratigraphic evidence has yet been found
in Finland for the later transgressions identified in the southern
parts of the Baltic (Seppä & Tikkanen 1998; Seppä
et al. 2000). At the same time salinity has been declining slightly
as the Straits of Denmark have become gradually narrower and
shallower. It has thus become common to refer to the period
since around 4000 BP, when the Baltic Sea has been more or less
at its present level of salinity, as the Limnea Sea (Hyvärinen
et al. 1988; Heinsalu et al. 2000). The current altitude of
the highest Litorina Sea shoreline is around 20 metres near
the border in the south-east of Finland and just over 100 metres
around the northern part of the Gulf of Bothnia (Hyyppä
1960: 9; Eronen 1974: 158).
The shoreline still lay about 100 kilometres inland of its present
location in the river valleys of the Gulf of Bothnia coast at
the beginning of the Litorina Sea stage (Fig. 3 & CD-Fig.
4), but it was considerably smoother than at the start of the
Ancylus Lake stage. The coastal areas in the south and south-west
were at least as fragmented as at that earlier stage and have
retained such an aspect more or less up to the present. There
were a few large islands on the Gulf of Bothnia coast at the
beginning of the Litorina Sea stage, and the ring encircling
the ancient meteorite crater that now makes up Lake Lappajärvi
stood out close to the shoreline (Tikkanen & Oksanen 1999:
37).
CD-Fig. 4. Finland and the Litorina Sea,
ca. 7200 BP.
The highest shoreline of the Baltic in Finland
The highest shoreline at a particular point is the uppermost
level to which the waters of the Baltic basin have reached.
It marks the dividing line between supra-aquatic and subaquatic
terrain. It is usually represented in the landscape by belts
of washed rocks or stones (Fig. 4), and the vegetation is frequently
much lusher on the slopes above this point than below it. The
highest shoreline is a metachronic feature, and will have arisen
as the ice margin retreated, i.e., within the period 11,000-9000
BP (Taipale & Saarnisto 1991: 268). As far north as the
Salpausselkä zone, the highest shoreline will mark the
level of the Baltic Ice Lake, as reflected by the delta surfaces
on the Salpausselkä formations. Elsewhere in the southern
and middle parts of the country, it will have arisen during
the Yoldia Sea stage, and further north, in Ostrobothnia, central
Finland, and Peräpohjola, similar shore markers will have
been laid down by the Ancylus Lake (Eronen 1990). According
to Saarnisto (2000: 26-27), the dividing line between the Yoldia
Sea and Ancylus lake shorelines run from Pori through Jyväskylä
to Kajaani (Fig. 5).
Fig. 4. An exposed boulder field created
by littoral forces on the slope of Lauhanvuori in South Ostrobothnia.
The top of this hill lies slightly above the level of the highest
shoreline of the Baltic Sea. (Photo by Matti Tikkanen, 07/95)
About 62 percent of the surface area of Finland has been beneath
the waters of the Baltic at some stage. The only parts of the
country where there are extensive supra-aquatic areas are Lapland
and eastern Finland, and even here local ice lakes have covered
many places for brief periods. One outstanding supra-aquatic
area in the interior of the country consists of the extensive
Central Finland Uplands north-west of Jyväskylä, while
other such areas further south are the Tammela Uplands in the
south-west and the Rautavesi-Ruoholahti area between the Salpausselkä
formations in the south-east (Tikkanen & Oksanen 1999: 39).
The Gulf of Bothnia, in particular, is lined by a continuous
subaquatic zone some 100 kilometres wide, punctuated only by
the hill of Lauhanvuori, the top of which extended above the
highest shore level to form a tiny island (Fig. 3 & CD-Fig.
4).
Fig. 5. The highest shoreline in Finland.
The oldest shorelines of southern Finland have been subject
to the effects of land uplift for the longest time, but the
highest current altitudes for ancient shorelines are to be found
north of the Gulf of Bothnia coast, as it is here that the rate
of land uplift has been greatest. The highest known ancient
shore marker in Finland, at a current altitude of 220 metres,
is located on the slope of Vammavaara, south of Rovaniemi. It
was created by the waters of the Ancylus Lake. By comparison,
the highest Ancylus shore markers in Sweden, in the area west
of the Gulf of Bothnia, are to be found at 285 metres. The highest
markers of the Baltic Ice Lake in the area near the border in
the south-east of Finland lie at about 100 metres.
There are also numerous stone belts indicative of shorelines
and deposits laid down in water that are to be found well above
the uppermost shore of the Baltic in eastern and northern Finland.
These are the work of local ice lakes which developed in front
of the ice margin in places where this prevented the water from
draining into the Baltic basin. These lakes were short-lived
and drained as soon as a suitable route emerged from beneath
the ice. Their areas and water levels varied according to the
location of the outflow channel at a particular point in time.
One of the largest of these water bodies was the Sotkamo Ice
Lake, which discharged its water southwards through the Hiidenportti
channel in Sotkamo and the Kattilamäki channel in Kajaani
(Saarelainen & Vanne 1997). The level reached by the Ilomantsi
Ice Lake in the extreme east of the country is marked by the
Selkäkangas ice margin formation, which is regarded as
a continuation of the Second Salpausselkä (Taipale &
Saarnisto 1991). There were also extensive ice lakes in the
headwaters of the Kemijoki River and its tributaries in Lapland
(Johansson 1995). The easternmost of the Lapland ice lakes evidently
drained to the east, over the Maanselkä watershed, until
such time as the channels leading to the Kemijoki valley were
free of ice.
Recent and future shoreline changes
The shore displacement curves compiled on the basis of the
dated isolation basins in Finland indicate that since the clear
Litorina transgression at 7500-6500 BP, no transgressions have
taken place but that the shore displacement has been a stable,
gradually slowing process (Tikkanen et al. 1999; Seppä
et al. 2000; Eronen et al. 2001). The present land uplift rate
of the order of two millimetres per year on the south-eastern
coast of Gulf of Finland and eight millimetres per year on the
Finnish coast of the Gulf of Bothnia's northern part means that
shore displacement continues. Finland's area is increasing every
one hundred years by about 1,000 square kilometres, of which
two-thirds can be attributed to land uplift and the remainder
to sedimentation and colonization by vegetation (Jones 1977:
14-15). During few decades new islands emerge from water and
then gradually merge together or with the continent in the archipelago
off Vaasa, for example. Due to the land uplift many harbours
and towns have been moved and re-established on the rapidly
uplifting coast of the Gulf of Bothnia (Palomäki 1987;
Ristaniemi et al. 1997).
The geophysical data indicate that glacio-isostatic rebound
will continue for several thousands of years, even though there
are uncertainties in the numerical calculation of the remaining
land uplift (Eronen et al. 2001). The most recent calculation
suggests an amount of circa 90 metres for residual uplift (Ekman
& Mäkinen 1996). If the land uplift continues in the
same manner as in the past, the northern part of the Gulf of
Bothnia will be cut off to form an inland lake of its own after
the next 2,000 years (Kukkamäki 1956; Ristaniemi et al.
1997). The present human impact, however, can cause perturbations
in the natural development, and there are very widely differing
estimates on the future sea level rise caused by the predicted
greenhouse warming. The recent estimates suggest an average
sea level rise of about five millimetres per year over the next
century, within a range of uncertainty of 2-9 millimetres per
year (Wigley & Raper 1992). According to Watson et al. (1996),
this will produce a total increase of about 50 centimetres by
the year 2100, and means that sea level will rise two to five
times faster than over the last one hundred years. Anyhow, the
land uplift will then cancel out most of the eustatic rise,
and changes on the Finnish coastal area will be less dramatic.
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