Major Fields of Paleoceanography
A prerequisite to reconstruct ocean circulation in the past is
that marine sediments have not been buried in the mantle at
subduction zones. Due to the renewal of the ocean floor by
plate tectonics, the oldest sediments date back to the Triassic, about 200 million years ago. These very old sediments
are rare, and found only in the Pacific Ocean. In practice, we
can hope to obtain global reconstructions for the whole
Tertiary era, but they become increasingly scarce as we go
back to the Secondary era. Here, we will restrict ourselves to
the analysis of some of the major aspects of the evolution of
climate and paleoceanography over the last 25 million years,
which correspond to the progressive establishment of glacial
conditions in the high latitudes of both hemispheres.
From ‘The Greenhouse Effect Era’ to
the ‘Ice Ages’
The Quaternary period, which covers the last 2.6 million
years of the history of our planet, is characterized by persistent major ice sheets in the high latitudes of both hemispheres. The volume of these caps and their geographical
expansion fluctuate over timescales of 10
4 to 10
5 years, in
response to changes in insolation controlled by the orbital
parameters of the Earth (see Chap. 28).
The origin of these major glacial phases is discussed in
several chapters that present the point of view of geophysicists, geochemists and modelers (see Chaps. 22, 26 and
27). At time scales exceeding a million years, the major
causes of the development of glaciations are feedbacks
related to plate tectonics. Among these feedbacks, the following may be highlighted:
– the varying shapes and positions of landmasses and ocean
basins,
– the existence of passages between basins,
– the location and altitude of mountain ranges, both in the
ocean and on land, affecting oceanic and atmospheric
circulation and heat transfer,
– volcanism and erosion, and their impact on atmospheric
chemistry and on the pCO 2 of the atmosphere.
The record describing the evolution of the d
18 O of benthic foraminifera over time shown in Fig. 21.2 is the result
of a compilation of analyses made in more than forty
ocean-drilling sites. This global compilation registers both
the changes in d w and temperature, in accordance with the
paleotemperature formula (Eq. 21.1). It shows that the current climate is the result of a long decline that started at the
end of the climate optimum of the early Eocene (52–50 Ma
ago). This decline is characterized by an increase in d
18
O
over time that reflects the drop in ocean temperatures at
higher latitudes (where deep waters are formed) in a world
without major glaciers, followed by the growth of ice caps.
The d
18 O of benthic foraminifera therefore dropped from an
average of *0.2‰ during the climate optimum of the early
Eocene, to an average of *4‰ at the end of the Quaternary.
This slow climate decline is generally attributed to two
main factors: a gradual reduction in atmospheric CO 2 concentration, and the growing thermal isolation of the Antarctic
continent due to the widening of the ocean passages that
surround it.
Relatively abrupt incidents (that is, events happening over a
much shorter time scale than the general trend), of large
amplitude, are superimposed onto this slow drift, indicating
that other climate drivers such as thresholds or rapid feedbacks
are also involved. The first of these changes occurred at the
very end of the Paleogene, at the boundary of the
Eocene/Oligocene, about 33.5 Ma ago. It resulted in a rapid
increase in benthic foraminiferal d
18 O from +1.6 to +2.8‰
over only 100–200 ka (Fig. 21.2). This change in d
18 O is
attributed to the development of the Antarctic ice cap. The
northward drift of the Australian continent allowed the opening of the Strait of Tasmania and the establishment of the
Antarctic Circumpolar Current (Zachos et al. 2001). The isolation of this large land mass resulted in a drop in temperatures
and the establishment of a permanent ice cap on East
Antarctica, with an ice volume that may have reached about
50% of its current size. After a period of ten million years when
the climate changed little, two warming phases, one at the end
of the Oligocene and the other during the Middle Miocene,
caused a significant reduction of the Antarctic ice sheet. The
increase in temperature and the decrease in ice volume were
reflected in a 1.2‰ decrease in the benthic d
18 O signal. The
causes of these climatic changes are not clear yet.
The final phase of the climatic decline leading to the
major glaciations of the Quaternary began from the Middle
Miocene, between 14.2 and 12.2 Ma, and was marked by an
increase in benthic foraminiferal d
18 O of 1.0‰ over two
million years. Paleotemperature reconstructions based on
Mg/Ca ratios in benthic foraminifera suggest that deep water
temperature varied little, and that the growth of the Antarctic
ice cap was the main contributor to the benthic d
18 O change
(around *0.8‰). If these estimates were correct, the ice cap
located on the eastern part of Antarctica would have reached
85% of its current volume. As for the West Antarctic ice cap,
it seems to have only developed from 6 Ma on, as evidenced
by the first coarse sediment deposits in the Weddell Sea,
coming from melting icebergs emitted at the edge of this
cap. These sediments transported by drifting ice result from
land erosion caused by the friction of glaciers, and are often
referred to by their acronym IRD (ice rafted debris).
242
T. Caley et al.
A prerequisite to reconstruct ocean circulation in the past is
that marine sediments have not been buried in the mantle at
subduction zones. Due to the renewal of the ocean floor by
plate tectonics, the oldest sediments date back to the Triassic, about 200 million years ago. These very old sediments
are rare, and found only in the Pacific Ocean. In practice, we
can hope to obtain global reconstructions for the whole
Tertiary era, but they become increasingly scarce as we go
back to the Secondary era. Here, we will restrict ourselves to
the analysis of some of the major aspects of the evolution of
climate and paleoceanography over the last 25 million years,
which correspond to the progressive establishment of glacial
conditions in the high latitudes of both hemispheres.
From ‘The Greenhouse Effect Era’ to
the ‘Ice Ages’
The Quaternary period, which covers the last 2.6 million
years of the history of our planet, is characterized by persistent major ice sheets in the high latitudes of both hemispheres. The volume of these caps and their geographical
expansion fluctuate over timescales of 10
4 to 10
5 years, in
response to changes in insolation controlled by the orbital
parameters of the Earth (see Chap. 28).
The origin of these major glacial phases is discussed in
several chapters that present the point of view of geophysicists, geochemists and modelers (see Chaps. 22, 26 and
27). At time scales exceeding a million years, the major
causes of the development of glaciations are feedbacks
related to plate tectonics. Among these feedbacks, the following may be highlighted:
– the varying shapes and positions of landmasses and ocean
basins,
– the existence of passages between basins,
– the location and altitude of mountain ranges, both in the
ocean and on land, affecting oceanic and atmospheric
circulation and heat transfer,
– volcanism and erosion, and their impact on atmospheric
chemistry and on the pCO 2 of the atmosphere.
The record describing the evolution of the d
18 O of benthic foraminifera over time shown in Fig. 21.2 is the result
of a compilation of analyses made in more than forty
ocean-drilling sites. This global compilation registers both
the changes in d w and temperature, in accordance with the
paleotemperature formula (Eq. 21.1). It shows that the current climate is the result of a long decline that started at the
end of the climate optimum of the early Eocene (52–50 Ma
ago). This decline is characterized by an increase in d
18
O
over time that reflects the drop in ocean temperatures at
higher latitudes (where deep waters are formed) in a world
without major glaciers, followed by the growth of ice caps.
The d
18 O of benthic foraminifera therefore dropped from an
average of *0.2‰ during the climate optimum of the early
Eocene, to an average of *4‰ at the end of the Quaternary.
This slow climate decline is generally attributed to two
main factors: a gradual reduction in atmospheric CO 2 concentration, and the growing thermal isolation of the Antarctic
continent due to the widening of the ocean passages that
surround it.
Relatively abrupt incidents (that is, events happening over a
much shorter time scale than the general trend), of large
amplitude, are superimposed onto this slow drift, indicating
that other climate drivers such as thresholds or rapid feedbacks
are also involved. The first of these changes occurred at the
very end of the Paleogene, at the boundary of the
Eocene/Oligocene, about 33.5 Ma ago. It resulted in a rapid
increase in benthic foraminiferal d
18 O from +1.6 to +2.8‰
over only 100–200 ka (Fig. 21.2). This change in d
18 O is
attributed to the development of the Antarctic ice cap. The
northward drift of the Australian continent allowed the opening of the Strait of Tasmania and the establishment of the
Antarctic Circumpolar Current (Zachos et al. 2001). The isolation of this large land mass resulted in a drop in temperatures
and the establishment of a permanent ice cap on East
Antarctica, with an ice volume that may have reached about
50% of its current size. After a period of ten million years when
the climate changed little, two warming phases, one at the end
of the Oligocene and the other during the Middle Miocene,
caused a significant reduction of the Antarctic ice sheet. The
increase in temperature and the decrease in ice volume were
reflected in a 1.2‰ decrease in the benthic d
18 O signal. The
causes of these climatic changes are not clear yet.
The final phase of the climatic decline leading to the
major glaciations of the Quaternary began from the Middle
Miocene, between 14.2 and 12.2 Ma, and was marked by an
increase in benthic foraminiferal d
18 O of 1.0‰ over two
million years. Paleotemperature reconstructions based on
Mg/Ca ratios in benthic foraminifera suggest that deep water
temperature varied little, and that the growth of the Antarctic
ice cap was the main contributor to the benthic d
18 O change
(around *0.8‰). If these estimates were correct, the ice cap
located on the eastern part of Antarctica would have reached
85% of its current volume. As for the West Antarctic ice cap,
it seems to have only developed from 6 Ma on, as evidenced
by the first coarse sediment deposits in the Weddell Sea,
coming from melting icebergs emitted at the edge of this
cap. These sediments transported by drifting ice result from
land erosion caused by the friction of glaciers, and are often
referred to by their acronym IRD (ice rafted debris).
242
T. Caley et al.