where the Drake Passage is kept artificially closed, the
decrease in atmospheric CO 2 concentration causes glaciation
to occur over the Antarctic. Glaciation is simply delayed by
about 2 to 3 million years.
Changes in the climate caused by the opening or
non-opening of the Drake Passage will also affect the global
carbon cycle, potentially increasing the climate response of
the system. One possible consequence of the opening of the
Drake Passage is an upheaval of the thermohaline circulation
facilitating the formation of deep waters in the Antarctic and
triggering the plunging of waters in the North Atlantic
(Huber and Nof 2006). The result is a warming of the
northern hemisphere of approximately 3 °C and a severe
cooling of the Antarctic. As most of the continental area is
located in the northern hemisphere, an increase in global
consumption of CO 2 by continental silicate weathering is to
be expected and thus a reduction in the amount of CO 2 in the
atmosphere, reinforcing the cold climate mode being established (Elsworth et al. 2017). In conclusion, although the
overall climate effect of the opening of the Drake Passage
remains weak, there may have been positive feedbacks in the
carbon cycle which substantially amplified the response.
These have yet to be documented with precision.
A second driver of the evolution of climate also took
place during the Cenozoic: the Himalayan orogen.
How orogeny affects the carbon cycle is complex. We
have identified two effects: one is the chemical weathering of
exposed silicates in the mountain chains, the other is the
sequestration of organic carbon at the foot of the mountains.
Take first the increase in weathering of continental surfaces
through increased erosion. The development of glaciers, the
alternating freezing and thawing patterns at high altitudes
and steep slopes all favor the break-up of rocks and increase
the area of contact with solutions. This results in increased
weathering and increased consumption of CO 2 . This
increased weathering is seen in an increase in the erosion
factor f 3 in Eq. (10), and the level of CO 2 is lowered until the
weathering of the silicates again compensates for the
degassing of the solid Earth (Goddéris and François 1996).
Currently, 4 Â 10
12 kg yr
−1 of suspended solids are transported to the ocean from the Himalayan zone, representing
17% of the world’s erosion flow, whereas the ratio of the
Himalayan surface to the total continental area is only 4%. It
is therefore to be expected that a major orogen would considerably increase the consumption of atmospheric CO 2
through chemical weathering of the exposed rocks (increase
in factor f 3 ). However, this result is not confirmed by current
data of fluxes of dissolved elements in the rivers from the
Himalayas. They suggest a modest consumption of
0.7 Â 10
12 mol yr
−1 of CO 2 by weathering of Himalayan
silicates, only 6% of the world total of 11.7 Â 10
12 mol
yr
−1 . One of the reasons for this low rate of chemical
weathering may be that the erosion motor is too efficient in
the Himalayas and that the discharge of the debris produced
by mechanical erosion is too fast to allow the progress of
effective chemical weathering. This would produce a
weathering system which would be very limited by the very
slow kinetics of the dissolution of minerals. In addition, the
lithology is such that calcium silicates are scarce in the
Himalayas and the weathering fluxes are mostly of sodium
and potassium silicates. Since these chemical reactions do
not lead to precipitation of carbonates, their effect on the
carbon cycle is minimal over the long term (France-Lanord
and Derry 1997).
However, the rate of sedimentation, which is extremely
high in the Bay of Bengal, is responsible for the preservation
of very large quantities of organic matter, of both continental
and marine origin. It is estimated that the sequestration of
carbon at the foot of the orogen is two to three times higher
than the consumption of CO 2 by weathering of the Himalayan silicates. A recent study shows that 100% of the
organic carbon of continental origin transported by the
Himalayan rivers is preserved in the sediments of the Gulf of
Bengal (Galy et al. 2007). France-Lanord and Derry (1997)
estimated that the sedimentary organic carbon reservoir grew
to 0.6 Â 10
12 mol yr
−1 . This value is of a similar order of
magnitude to estimates from numerical simulations, carried
out using a carbon cycle model reversing the records of d
13 C
in carbonates during the Cenozoic period (Goddéris and
François 1996). The Himalayas consume carbon
(Fig. 27.13), but in organic form, and therefore, they are, at
least partially, responsible for the cooling of the climate
during the Cenozoic. The quantification of the impact of this
mechanism on atmospheric CO 2 has yet to be completed.
Abrupt Climate Events During
the Phanerozoic
As well as defining the major climate modes of the
Phanerozoic, recent efforts have defined episodes of rapid
climate change that have punctuated the history of the Earth
at a 100,000
−year scale. We outline below three of these
events, discussing their causes.
The Callovian-Oxfordian Transition (Middle
Jurassic-Upper Jurassic)
This was a brief cooling episode during the Jurassic. Such
events occurred several times during the Jurassic and Cretaceous periods.
The d
18 O values measured in fish teeth and belemnites
suggest an abrupt drop in temperature of 8 °C starting in the
Upper Callovian and remaining until the middle Oxfordian.
At the same time, boreal ammonite fauna invaded the
27 The Phanerozoic Climate
377
decrease in atmospheric CO 2 concentration causes glaciation
to occur over the Antarctic. Glaciation is simply delayed by
about 2 to 3 million years.
Changes in the climate caused by the opening or
non-opening of the Drake Passage will also affect the global
carbon cycle, potentially increasing the climate response of
the system. One possible consequence of the opening of the
Drake Passage is an upheaval of the thermohaline circulation
facilitating the formation of deep waters in the Antarctic and
triggering the plunging of waters in the North Atlantic
(Huber and Nof 2006). The result is a warming of the
northern hemisphere of approximately 3 °C and a severe
cooling of the Antarctic. As most of the continental area is
located in the northern hemisphere, an increase in global
consumption of CO 2 by continental silicate weathering is to
be expected and thus a reduction in the amount of CO 2 in the
atmosphere, reinforcing the cold climate mode being established (Elsworth et al. 2017). In conclusion, although the
overall climate effect of the opening of the Drake Passage
remains weak, there may have been positive feedbacks in the
carbon cycle which substantially amplified the response.
These have yet to be documented with precision.
A second driver of the evolution of climate also took
place during the Cenozoic: the Himalayan orogen.
How orogeny affects the carbon cycle is complex. We
have identified two effects: one is the chemical weathering of
exposed silicates in the mountain chains, the other is the
sequestration of organic carbon at the foot of the mountains.
Take first the increase in weathering of continental surfaces
through increased erosion. The development of glaciers, the
alternating freezing and thawing patterns at high altitudes
and steep slopes all favor the break-up of rocks and increase
the area of contact with solutions. This results in increased
weathering and increased consumption of CO 2 . This
increased weathering is seen in an increase in the erosion
factor f 3 in Eq. (10), and the level of CO 2 is lowered until the
weathering of the silicates again compensates for the
degassing of the solid Earth (Goddéris and François 1996).
Currently, 4 Â 10
12 kg yr
−1 of suspended solids are transported to the ocean from the Himalayan zone, representing
17% of the world’s erosion flow, whereas the ratio of the
Himalayan surface to the total continental area is only 4%. It
is therefore to be expected that a major orogen would considerably increase the consumption of atmospheric CO 2
through chemical weathering of the exposed rocks (increase
in factor f 3 ). However, this result is not confirmed by current
data of fluxes of dissolved elements in the rivers from the
Himalayas. They suggest a modest consumption of
0.7 Â 10
12 mol yr
−1 of CO 2 by weathering of Himalayan
silicates, only 6% of the world total of 11.7 Â 10
12 mol
yr
−1 . One of the reasons for this low rate of chemical
weathering may be that the erosion motor is too efficient in
the Himalayas and that the discharge of the debris produced
by mechanical erosion is too fast to allow the progress of
effective chemical weathering. This would produce a
weathering system which would be very limited by the very
slow kinetics of the dissolution of minerals. In addition, the
lithology is such that calcium silicates are scarce in the
Himalayas and the weathering fluxes are mostly of sodium
and potassium silicates. Since these chemical reactions do
not lead to precipitation of carbonates, their effect on the
carbon cycle is minimal over the long term (France-Lanord
and Derry 1997).
However, the rate of sedimentation, which is extremely
high in the Bay of Bengal, is responsible for the preservation
of very large quantities of organic matter, of both continental
and marine origin. It is estimated that the sequestration of
carbon at the foot of the orogen is two to three times higher
than the consumption of CO 2 by weathering of the Himalayan silicates. A recent study shows that 100% of the
organic carbon of continental origin transported by the
Himalayan rivers is preserved in the sediments of the Gulf of
Bengal (Galy et al. 2007). France-Lanord and Derry (1997)
estimated that the sedimentary organic carbon reservoir grew
to 0.6 Â 10
12 mol yr
−1 . This value is of a similar order of
magnitude to estimates from numerical simulations, carried
out using a carbon cycle model reversing the records of d
13 C
in carbonates during the Cenozoic period (Goddéris and
François 1996). The Himalayas consume carbon
(Fig. 27.13), but in organic form, and therefore, they are, at
least partially, responsible for the cooling of the climate
during the Cenozoic. The quantification of the impact of this
mechanism on atmospheric CO 2 has yet to be completed.
Abrupt Climate Events During
the Phanerozoic
As well as defining the major climate modes of the
Phanerozoic, recent efforts have defined episodes of rapid
climate change that have punctuated the history of the Earth
at a 100,000
−year scale. We outline below three of these
events, discussing their causes.
The Callovian-Oxfordian Transition (Middle
Jurassic-Upper Jurassic)
This was a brief cooling episode during the Jurassic. Such
events occurred several times during the Jurassic and Cretaceous periods.
The d
18 O values measured in fish teeth and belemnites
suggest an abrupt drop in temperature of 8 °C starting in the
Upper Callovian and remaining until the middle Oxfordian.
At the same time, boreal ammonite fauna invaded the
27 The Phanerozoic Climate
377