globe by satellite, for past periods, the vast majority of
observations are taken at the coasts. Variations in relative sea
level (RSL) is tracked at particular coastal points. It is
important to emphasize that isostasy has a reaction time of
several thousand years and that Scandinavia, for example, is
still rising (at the rate of about one meter per century) following
the disappearance of the ice sheet that covered northern Europe
during the last glacial maximum. At each point, the relative sea
level is therefore a combination of eustatic (global), isostatic,
and possibly tectonic (local) variations. There are several ways
to estimate this relative sea level, using traces of ancient beaches, coral analysis, speleothems etc.
Box 1. Glacial Isostasy
Post-glacial rebound: The relative changes in sea
level are the result of tectonic processes and climate
changes. These latter involve the accretion or melting
of the ice sheets on the continents (more than 5 km
thick). During ice ages, the pressure exerted by the
accumulated weight of the ice sheets on the continents
modified the shape of the surface creating large
depressions in the lithosphere. These changes in shape
brought about disturbances of the geoid (equipotential
surface of the Earth’s gravity field corresponding to
the theoretical surface of the oceans) as well as vertical
and horizontal displacements of the lithosphere and
the mantle. When the shape of the Earth or the volume
of the oceans changes, the geoid is thus modified as is
the sea level. When the continental ice sheets melt, the
lithosphere is released from the weight of the ice and
the deflection is reabsorbed at a speed which depends
on the viscosity of the mantle. This return is called the
post-glacial rebound, post-glacial describing the period from −6000 (ka BP) to today. (6 ka BP marks the
end of the melting of the ice caps of the northern
hemisphere: the Laurentide and the Eurasian ice sheets
having completely melted). For the Last Glacial
Maximum (LGM), the relaxation time of the lithosphere is estimated at about 3000 years.
Sea level: Variations in sea level are caused by
changes in the geoid surface, in the Earth’s topography
and in ocean mass. Sea level can be broken down into
three contributing factors: the ice load (S
ICE ), the
oceanic charge (S
OCE
) and the glacio-eustatic level
(S
EUS ).
S ¼ S
ICE
þ S
EUS
þ S
OCE
ð24:2:1Þ
Glacio-eustatic changes in sea level are controlled
by the amount of ice stored on the continents. In
glacial isostasy models, the glacio-eustatic sea level is
defined for a totally rigid Earth without any disturbance of the gravity field. The glacial isostasy models
allow us to estimate that the eustatic level during the
Last Glacial Maximum was 130 ± 10 m below the
current sea level (Lambeck, et al., EPILOG 2002).
This estimate of the eustatic level is mainly based on
the measurement of coral terraces in sites not affected
by the postglacial rebound, such as the Seychelles, but
also from calculations of the glacial isostasy models
attempting to reproduce relative variations in sea level.
The sea level at a given time and place in relation to
the current sea level is defined as the relative sea level
and can be described by the following equation:
RSL x; t
BP
À
Á ¼ Sðx; t
BP
Þ À S x; t
P
À
Á
ð24:2:2Þ
with x being the geographical coordinates, t
BP , a time
Before Present, et t
P , the present time. Equation (24.2.2) describes the relative level as the difference in sea level between the t
BP level and the current
level. During the formation of ice sheets on the continents, the nearby regions and those further afield
were affected differently depending on their distance
from the ice masses and on the movements of the
lithosphere and the mantle and therefore presented a
sea level graph reflecting the trend of the regional
ocean variations. These trends have been grouped into
so-called ‘Clark zones’. Figure 24.4 presents the result
of modelled sea level changes due to postglacial
rebound following the LGM according to the ICE-5G
ice model (Peltier 2004). The graph shows changes in
sea level for 18,000 years. The graphs are associated
with their respective colors on the globe and each
corresponds to a different regional trend of sea level
variations. The pink and purple zones correspond to
the regions that were either covered with ice during the
LGM or were nearby, and so were very affected by the
rebound of the lithosphere after the LGM. The dark
blue zone represents regions showing subsidence of
the lithosphere after the LGM. The light blue zone
represents areas influenced by the late melting of ice
from Antarctica after 9 ka BP (peak observed above
the current sea level).
The level of the seas is an equipotential surface of the
Earth’s gravity field (the geoid) which is linked to the distribution of mass. This changes in the presence of an ice
sheet or as a result of displacements of the asthenosphere
(the ductile part of the Earth’s mantle). This phenomenon
must therefore also to be taken into account in the interpretation of relative sea levels. Although isostasy is a considerable complication for the interpretation of observations,
this mechanism, on the other hand, it also provides
24 The Cryosphere and Sea Level
305
observations are taken at the coasts. Variations in relative sea
level (RSL) is tracked at particular coastal points. It is
important to emphasize that isostasy has a reaction time of
several thousand years and that Scandinavia, for example, is
still rising (at the rate of about one meter per century) following
the disappearance of the ice sheet that covered northern Europe
during the last glacial maximum. At each point, the relative sea
level is therefore a combination of eustatic (global), isostatic,
and possibly tectonic (local) variations. There are several ways
to estimate this relative sea level, using traces of ancient beaches, coral analysis, speleothems etc.
Box 1. Glacial Isostasy
Post-glacial rebound: The relative changes in sea
level are the result of tectonic processes and climate
changes. These latter involve the accretion or melting
of the ice sheets on the continents (more than 5 km
thick). During ice ages, the pressure exerted by the
accumulated weight of the ice sheets on the continents
modified the shape of the surface creating large
depressions in the lithosphere. These changes in shape
brought about disturbances of the geoid (equipotential
surface of the Earth’s gravity field corresponding to
the theoretical surface of the oceans) as well as vertical
and horizontal displacements of the lithosphere and
the mantle. When the shape of the Earth or the volume
of the oceans changes, the geoid is thus modified as is
the sea level. When the continental ice sheets melt, the
lithosphere is released from the weight of the ice and
the deflection is reabsorbed at a speed which depends
on the viscosity of the mantle. This return is called the
post-glacial rebound, post-glacial describing the period from −6000 (ka BP) to today. (6 ka BP marks the
end of the melting of the ice caps of the northern
hemisphere: the Laurentide and the Eurasian ice sheets
having completely melted). For the Last Glacial
Maximum (LGM), the relaxation time of the lithosphere is estimated at about 3000 years.
Sea level: Variations in sea level are caused by
changes in the geoid surface, in the Earth’s topography
and in ocean mass. Sea level can be broken down into
three contributing factors: the ice load (S
ICE ), the
oceanic charge (S
OCE
) and the glacio-eustatic level
(S
EUS ).
S ¼ S
ICE
þ S
EUS
þ S
OCE
ð24:2:1Þ
Glacio-eustatic changes in sea level are controlled
by the amount of ice stored on the continents. In
glacial isostasy models, the glacio-eustatic sea level is
defined for a totally rigid Earth without any disturbance of the gravity field. The glacial isostasy models
allow us to estimate that the eustatic level during the
Last Glacial Maximum was 130 ± 10 m below the
current sea level (Lambeck, et al., EPILOG 2002).
This estimate of the eustatic level is mainly based on
the measurement of coral terraces in sites not affected
by the postglacial rebound, such as the Seychelles, but
also from calculations of the glacial isostasy models
attempting to reproduce relative variations in sea level.
The sea level at a given time and place in relation to
the current sea level is defined as the relative sea level
and can be described by the following equation:
RSL x; t
BP
À
Á ¼ Sðx; t
BP
Þ À S x; t
P
À
Á
ð24:2:2Þ
with x being the geographical coordinates, t
BP , a time
Before Present, et t
P , the present time. Equation (24.2.2) describes the relative level as the difference in sea level between the t
BP level and the current
level. During the formation of ice sheets on the continents, the nearby regions and those further afield
were affected differently depending on their distance
from the ice masses and on the movements of the
lithosphere and the mantle and therefore presented a
sea level graph reflecting the trend of the regional
ocean variations. These trends have been grouped into
so-called ‘Clark zones’. Figure 24.4 presents the result
of modelled sea level changes due to postglacial
rebound following the LGM according to the ICE-5G
ice model (Peltier 2004). The graph shows changes in
sea level for 18,000 years. The graphs are associated
with their respective colors on the globe and each
corresponds to a different regional trend of sea level
variations. The pink and purple zones correspond to
the regions that were either covered with ice during the
LGM or were nearby, and so were very affected by the
rebound of the lithosphere after the LGM. The dark
blue zone represents regions showing subsidence of
the lithosphere after the LGM. The light blue zone
represents areas influenced by the late melting of ice
from Antarctica after 9 ka BP (peak observed above
the current sea level).
The level of the seas is an equipotential surface of the
Earth’s gravity field (the geoid) which is linked to the distribution of mass. This changes in the presence of an ice
sheet or as a result of displacements of the asthenosphere
(the ductile part of the Earth’s mantle). This phenomenon
must therefore also to be taken into account in the interpretation of relative sea levels. Although isostasy is a considerable complication for the interpretation of observations,
this mechanism, on the other hand, it also provides
24 The Cryosphere and Sea Level
305