From the inversion of relative sea level data (RSL) and
observations of the isostatic rebound which is still occurring,
it is possible to estimate the ice load that produced these
isostatic variations, and therefore the thickness of the ice
sheets. This was the approach mainly used for the Last
Glacial Maximum and for the last deglaciation (Peltier 2004;
Clark et al. 2002; Lambeck et al. 2006).
Using all this information, it is possible to envisage a
scenario representing the life of the ice sheets. This is the
story outlined in the next section, with a focus on the last
glacial-interglacial cycle for which there is more information
available. It should be borne in mind, however, that although
the main points are now well understood, there are still many
regions and periods for which information is lacking.
The Last 50 Million Years
The Antarctic continent is located in the high latitudes since
from the Late Cretaceous era (*240 Ma). However, it
remained free of ice until the Eocene-Oligocene transition,
34 Ma ago (Chap. 6, Volume 2). Marine sediment recordings indicate an abrupt increase in benthic d
18 O at this date,
suggesting that water depleted in
18 O was trapped in the
form of ice. The appearance of the Antarctic ice sheet is
attributed to the opening of an oceanic corridor between
South America and the Antarctic Peninsula: the Drake
Passage. The organization of the Antarctic circumpolar
current would have led to the thermal isolation of Antarctica.
However, more recently, based on modeling work, other
authors believe that a reduction in CO 2 concentration was
the cause of the cooling in Antarctica (De Conto and Pollard
2003).
In Chap. 6, a figure shows a new threshold around
15 Ma, to which the permanency of the East Antarctic ice
sheet is attributed. There are indicators, however, that the ice
may have retreated since 3 Ma, so that at 3 Ma the ice sheet
could have been thicker than during the Last Glacial
Maximum.
As for the northern hemisphere, scientists estimate that
the first freeze-ups occurred 7 Ma ago on southern Greenland. Amplification of these glaciations then took
place *3 Ma ago. Several hypotheses have been proposed
to explain this amplification: (i) the closure of the Panama
Strait occurred at this time and modified the oceanic circulation, causing warm water to travel up to the high latitudes
of the North Atlantic, providing a considerable source of
water vapor and therefore precipitation; (ii) the reduction in
CO 2 concentration leading to cooling; (iii) an uplift of the
Rocky Mountains modifying the planetary waves. Ice
appeared between 2.75 and 2.55 Ma in Eurasia, then in
Alaska and Canada, as evidenced by debris transported by
icebergs which then appeared in North Atlantic sediments
(Shackleton et al. 1984).
The Last Three Million Years
For the past three million years, the Earth’s climate has
oscillated between glacial and interglacial periods and most
of the variations are due to the formation and subsequent
melting of ice sheets in the northern hemisphere. In
Antarctica they only fluctuated, with significant variations in
Western Antarctica and in the peninsula (Anderson et al.,
EPILOG 2002; Ritz et al. 2001).
Until *1 Ma ago, the variations in the extent and volume of the ice sheets remained moderate, with variations of
60 m in sea level. The oscillations subsequently slowed
down and gained amplitude. About 900 ka ago, the dominant periodicity of glaciations went from 40,000 to
100,000 years and the amplitude of the oscillations doubled,
with variations of more than 100 m in the sea level. This
event is known as the Mid-Pleistocene Revolution
(MPR) (Raymo et al. 2006).
The freezing-up of the northern hemisphere has varied
greatly over time: not all glaciations are alike, nor are all
interglacials. The recent maximum expansion of the continental ice sheets is fairly well documented. However, many
questions remain, in particular on the marine sides of the ice
sheets. Below. we highlight some irregularities that are of
interest.
The ice shelves in the Arctic Ocean
The Lomonosov Ridge is an underwater mountain range
located at a depth of 1000 m. It stretches across the Arctic
Ocean from Greenland to Siberia through the North Pole.
Several striations aligned at the top of this ridge suggest the
possibility of a gigantic ice-shelf that might have covered the
center of the Arctic Ocean. These undated striations are
unlikely to have been caused by icebergs which leave more
random marks. They are also corroborated by marks in other
parts of the Arctic Ocean (Jakobsson et al. 2008).
The Stage 11 Interglacial
During the Marine Isotope Stage 11, 400,000 years ago, the
Earth experienced a long interglacial. Estimates (currently
under discussion) of sea level vary between 20 m above and
8 m below the current level. If the high value is correct, it
312
C. Ritz et al.
observations of the isostatic rebound which is still occurring,
it is possible to estimate the ice load that produced these
isostatic variations, and therefore the thickness of the ice
sheets. This was the approach mainly used for the Last
Glacial Maximum and for the last deglaciation (Peltier 2004;
Clark et al. 2002; Lambeck et al. 2006).
Using all this information, it is possible to envisage a
scenario representing the life of the ice sheets. This is the
story outlined in the next section, with a focus on the last
glacial-interglacial cycle for which there is more information
available. It should be borne in mind, however, that although
the main points are now well understood, there are still many
regions and periods for which information is lacking.
The Last 50 Million Years
The Antarctic continent is located in the high latitudes since
from the Late Cretaceous era (*240 Ma). However, it
remained free of ice until the Eocene-Oligocene transition,
34 Ma ago (Chap. 6, Volume 2). Marine sediment recordings indicate an abrupt increase in benthic d
18 O at this date,
suggesting that water depleted in
18 O was trapped in the
form of ice. The appearance of the Antarctic ice sheet is
attributed to the opening of an oceanic corridor between
South America and the Antarctic Peninsula: the Drake
Passage. The organization of the Antarctic circumpolar
current would have led to the thermal isolation of Antarctica.
However, more recently, based on modeling work, other
authors believe that a reduction in CO 2 concentration was
the cause of the cooling in Antarctica (De Conto and Pollard
2003).
In Chap. 6, a figure shows a new threshold around
15 Ma, to which the permanency of the East Antarctic ice
sheet is attributed. There are indicators, however, that the ice
may have retreated since 3 Ma, so that at 3 Ma the ice sheet
could have been thicker than during the Last Glacial
Maximum.
As for the northern hemisphere, scientists estimate that
the first freeze-ups occurred 7 Ma ago on southern Greenland. Amplification of these glaciations then took
place *3 Ma ago. Several hypotheses have been proposed
to explain this amplification: (i) the closure of the Panama
Strait occurred at this time and modified the oceanic circulation, causing warm water to travel up to the high latitudes
of the North Atlantic, providing a considerable source of
water vapor and therefore precipitation; (ii) the reduction in
CO 2 concentration leading to cooling; (iii) an uplift of the
Rocky Mountains modifying the planetary waves. Ice
appeared between 2.75 and 2.55 Ma in Eurasia, then in
Alaska and Canada, as evidenced by debris transported by
icebergs which then appeared in North Atlantic sediments
(Shackleton et al. 1984).
The Last Three Million Years
For the past three million years, the Earth’s climate has
oscillated between glacial and interglacial periods and most
of the variations are due to the formation and subsequent
melting of ice sheets in the northern hemisphere. In
Antarctica they only fluctuated, with significant variations in
Western Antarctica and in the peninsula (Anderson et al.,
EPILOG 2002; Ritz et al. 2001).
Until *1 Ma ago, the variations in the extent and volume of the ice sheets remained moderate, with variations of
60 m in sea level. The oscillations subsequently slowed
down and gained amplitude. About 900 ka ago, the dominant periodicity of glaciations went from 40,000 to
100,000 years and the amplitude of the oscillations doubled,
with variations of more than 100 m in the sea level. This
event is known as the Mid-Pleistocene Revolution
(MPR) (Raymo et al. 2006).
The freezing-up of the northern hemisphere has varied
greatly over time: not all glaciations are alike, nor are all
interglacials. The recent maximum expansion of the continental ice sheets is fairly well documented. However, many
questions remain, in particular on the marine sides of the ice
sheets. Below. we highlight some irregularities that are of
interest.
The ice shelves in the Arctic Ocean
The Lomonosov Ridge is an underwater mountain range
located at a depth of 1000 m. It stretches across the Arctic
Ocean from Greenland to Siberia through the North Pole.
Several striations aligned at the top of this ridge suggest the
possibility of a gigantic ice-shelf that might have covered the
center of the Arctic Ocean. These undated striations are
unlikely to have been caused by icebergs which leave more
random marks. They are also corroborated by marks in other
parts of the Arctic Ocean (Jakobsson et al. 2008).
The Stage 11 Interglacial
During the Marine Isotope Stage 11, 400,000 years ago, the
Earth experienced a long interglacial. Estimates (currently
under discussion) of sea level vary between 20 m above and
8 m below the current level. If the high value is correct, it
312
C. Ritz et al.