Reconstruction of Sea Levels and Ice Sheets
in the Past
Over the past several millions of years, the Earth has seen a
succession of glacial periods during which huge ice sheets
covered North America, Eurasia and many mountain ranges
around the world (including the Alps). These long glacial
periods were interspersed with interglacials, as is the case in
the present day, when a warm climate prevailed, confining
freezing to the polar regions and to the highest mountains.
These evolutions of the cryosphere have been studied
since the nineteenth century, but a global understanding of
these phenomena has only become possible in the past few
decades, thanks to direct observation of the current ice sheets
and to the discovery of the paleoclimatic information contained in numerous sedimentary deposits.
Data that Enable the Reconstruction
of the Geography of Ancient Ice Sheets
Reconstruction of sea levels using the
18 O/
16 O ratio of the
ocean provides a representation of how the amount of ice
evolved over time (Fig. 24.3), which acts as a good indicator
of the overall volume of all of the ice sheets. However, to
know where these ice sheets were located (America, Eurasia,
etc.) and how far they extended, we must rely on geomorphological information which we shall briefly review. Figure 24.1 (Last Glacial Maximum in the northern
hemisphere) and 2.2 (Antarctic) show the different regions
mentioned.
The expanse and dynamics of ancient ice sheets can be
estimated directly from the deposits and traces that they left
behind on the ground. In general, pre-glacial bedrocks are
preserved in areas where the base of the ice remained cold;
while in regions where the base of the ice reached melting
point (temperate base), the flow of ice and of basal water
reshaped the bedrock. We can then distinguish between
formations due to glacial erosion and those due to sediment
deposition. Glacial erosion abrades (polishes) the rock outcrops and creates incisions (streaks caused by the scraping of
transported debris). The drop in pressure downstream of the
obstacles causes freezing of the basal water and the fracturing and plucking of rocks. This passage creates ‘roches
moutonnées’ (or ‘sheepback rocks’). On a larger scale, glacial erosion hollows out valleys into troughs (U-shaped),
especially on coasts as fjords are formed. At sea, it is possible, using multi-frequency sonars, to observe underwater
channels that have obviously been dug out by the flow of
ice, highlighting the location of ‘paleo ice streams’
(Anderson et al. EPILOG 2002). Observations of this type
have been detected around the Antarctic and the Arctic
Ocean. All these marks indicate the direction and route of
the ice flow locally. In some cases, a change in direction
over time has been observed and this information makes it
possible to infer variations in the geometry of the ice sheet,
in particular the displacement of the domes.
The deposition of transported material leads to the formation of a wide variety of moraines. We note the frontal
moraines which mark the maximum extent of each advance
of a glacier. Ground moraines are not very thick (a few meters
on average) and may be flat or irregular. Drumlins, often
grouped in fields, are elongated ovoidal hills. All these
deposits are structured by the flow of ice, and thus indicate its
direction. During periods of retreat, a sub-glacial hydrological network is formed if the base is temperate. The deposits
associated with these phases are formed when the materials
transported are abandoned, e.g. eskers at the site of former
sub-glacial canals. An esker occurs in the form of an elongated ridge sometimes over hundreds of meters in length.
These ridges are formed by materials being deposited in the
tunnels of the subglacial rivers located at the base of the
glaciers. Their often winding shape follows that of the tunnels that created them. All these traces left behind on the
ground have been observed and compiled since the nineteenth century. For about twenty years, satellites have helped
to give a large scale view of these lineations. Traces sometimes indicate a multitude of contradictory directions that
reflect changes in the flow of the ice over time. The traces
must therefore be classified chronologically and then interpreted in large coherent sets. Based on this synthesis, it is
possible to recreate the geometry of the caps at different
times. A major difficulty arises from the fact that an
advancing glacier moves the deposits of previous glaciers
and, through erosion, can erase previous traces. Therefore,
we can only reconstruct the glaciers from previous glaciations if their traces have not been erased, so only if another
more extensive glaciation has not occurred since. This is the
case for Fennoscandia, where the moraines of the penultimate
glaciation (Saalien or MIS 6, *180–140 ka BP) are still
visible much further south than those of the Last Glacial
Maximum (see Fig. 24.6). Another difficulty with formulating an overview comes from the dating of geomorphological
traces. For recent periods, radiocarbon dating allows very
precise estimates for the past 30,000 years approximately,
although it is essential to use organic matter. Prior to this
period, which only covers from the end of the last glaciation
to the present, or when there is no organic material, other
techniques are necessary. Optically stimulated luminescence
(OSL) indicates how long a rock has been exposed to solar
radiation and has been used, for example, to date quartz
crystals in moraines throughout the last glaciation. More
accurate datings have radically changed our notions of the
history of Fennoscandia (Svendsen et al. 2004).
310
C. Ritz et al.
in the Past
Over the past several millions of years, the Earth has seen a
succession of glacial periods during which huge ice sheets
covered North America, Eurasia and many mountain ranges
around the world (including the Alps). These long glacial
periods were interspersed with interglacials, as is the case in
the present day, when a warm climate prevailed, confining
freezing to the polar regions and to the highest mountains.
These evolutions of the cryosphere have been studied
since the nineteenth century, but a global understanding of
these phenomena has only become possible in the past few
decades, thanks to direct observation of the current ice sheets
and to the discovery of the paleoclimatic information contained in numerous sedimentary deposits.
Data that Enable the Reconstruction
of the Geography of Ancient Ice Sheets
Reconstruction of sea levels using the
18 O/
16 O ratio of the
ocean provides a representation of how the amount of ice
evolved over time (Fig. 24.3), which acts as a good indicator
of the overall volume of all of the ice sheets. However, to
know where these ice sheets were located (America, Eurasia,
etc.) and how far they extended, we must rely on geomorphological information which we shall briefly review. Figure 24.1 (Last Glacial Maximum in the northern
hemisphere) and 2.2 (Antarctic) show the different regions
mentioned.
The expanse and dynamics of ancient ice sheets can be
estimated directly from the deposits and traces that they left
behind on the ground. In general, pre-glacial bedrocks are
preserved in areas where the base of the ice remained cold;
while in regions where the base of the ice reached melting
point (temperate base), the flow of ice and of basal water
reshaped the bedrock. We can then distinguish between
formations due to glacial erosion and those due to sediment
deposition. Glacial erosion abrades (polishes) the rock outcrops and creates incisions (streaks caused by the scraping of
transported debris). The drop in pressure downstream of the
obstacles causes freezing of the basal water and the fracturing and plucking of rocks. This passage creates ‘roches
moutonnées’ (or ‘sheepback rocks’). On a larger scale, glacial erosion hollows out valleys into troughs (U-shaped),
especially on coasts as fjords are formed. At sea, it is possible, using multi-frequency sonars, to observe underwater
channels that have obviously been dug out by the flow of
ice, highlighting the location of ‘paleo ice streams’
(Anderson et al. EPILOG 2002). Observations of this type
have been detected around the Antarctic and the Arctic
Ocean. All these marks indicate the direction and route of
the ice flow locally. In some cases, a change in direction
over time has been observed and this information makes it
possible to infer variations in the geometry of the ice sheet,
in particular the displacement of the domes.
The deposition of transported material leads to the formation of a wide variety of moraines. We note the frontal
moraines which mark the maximum extent of each advance
of a glacier. Ground moraines are not very thick (a few meters
on average) and may be flat or irregular. Drumlins, often
grouped in fields, are elongated ovoidal hills. All these
deposits are structured by the flow of ice, and thus indicate its
direction. During periods of retreat, a sub-glacial hydrological network is formed if the base is temperate. The deposits
associated with these phases are formed when the materials
transported are abandoned, e.g. eskers at the site of former
sub-glacial canals. An esker occurs in the form of an elongated ridge sometimes over hundreds of meters in length.
These ridges are formed by materials being deposited in the
tunnels of the subglacial rivers located at the base of the
glaciers. Their often winding shape follows that of the tunnels that created them. All these traces left behind on the
ground have been observed and compiled since the nineteenth century. For about twenty years, satellites have helped
to give a large scale view of these lineations. Traces sometimes indicate a multitude of contradictory directions that
reflect changes in the flow of the ice over time. The traces
must therefore be classified chronologically and then interpreted in large coherent sets. Based on this synthesis, it is
possible to recreate the geometry of the caps at different
times. A major difficulty arises from the fact that an
advancing glacier moves the deposits of previous glaciers
and, through erosion, can erase previous traces. Therefore,
we can only reconstruct the glaciers from previous glaciations if their traces have not been erased, so only if another
more extensive glaciation has not occurred since. This is the
case for Fennoscandia, where the moraines of the penultimate
glaciation (Saalien or MIS 6, *180–140 ka BP) are still
visible much further south than those of the Last Glacial
Maximum (see Fig. 24.6). Another difficulty with formulating an overview comes from the dating of geomorphological
traces. For recent periods, radiocarbon dating allows very
precise estimates for the past 30,000 years approximately,
although it is essential to use organic matter. Prior to this
period, which only covers from the end of the last glaciation
to the present, or when there is no organic material, other
techniques are necessary. Optically stimulated luminescence
(OSL) indicates how long a rock has been exposed to solar
radiation and has been used, for example, to date quartz
crystals in moraines throughout the last glaciation. More
accurate datings have radically changed our notions of the
history of Fennoscandia (Svendsen et al. 2004).
310
C. Ritz et al.