The Indirect Effects of Paleogeographic Changes
The atmospheric partial pressure of carbon dioxide (pCO 2 ) is
driven by the carbon cycle. On the scale of geological time,
pCO 2 reflects the balance between the CO 2 emission fluxes
from volcanic systems (ocean ridges and aerial volcanism),
the degree of magmatic activity from the mantle (plumes),
decarbonation in subduction zones, and the CO 2 fluxes
consumed by the weathering of the rock silicates on the
surface of the Earth and in the oceanic crust, through the
burial of the organic matter. Fluctuations in pCO 2 reflect the
evolution of one or both of these flows. CO 2 emissions are
proportional to the annual rate of production of oceanic
crust. We have seen that the variability in this rate of production does not exceed 30% over the last 170 million years
(Cogné and Humler 2006). To these rates should be added
CO 2 emissions from plumes of mantle volcanism and from
decarbonation in subduction zones. Significant magmatic
events dating from the late Early Cretaceous and associated
with the establishment of some large submarine basalt provinces, such as the Ontong-Java Plateau, increase the production rate of oceanic crust by about 25% (Cogné and
Humler 2006) and about the same increase in CO 2 is injected
into the ocean-atmosphere system. Contributions from the
subduction zones are less constrained. Currently, this process is limited to a few subduction zones in the Pacific or
around the Indonesian archipelago, while the main deposition zones are in the Atlantic and Indian Ocean. In the past,
the subduction of the Tethysian Ocean could have emitted a
significant CO 2 flux (Hoareau et al. 2015).
The intensity of the chemical weathering is a function of
climate parameters such as surface temperature and runoff.
Moreover, most geochemical models considered runoff as a
function of surface temperature: the higher the temperature,
the greater the runoff, and consequently, the chemical
weathering. The chemical weathering of silicates acts as a
climate regulator. We know now that this proportional
relationship between temperature and runoff is based on
current data and is not transferable to past periods. This is
easily understood by analyzing the upper Permian climate.
Simulation of chemical weathering is effective only in some
areas experiencing a tropical and humid climate. This weak
chemical weathering of silicates in the paleogeographic
context implies a relatively high pCO 2 equilibrium of about
2500 ppm, and a high global average temperature of about
21 °C (considering that the rate of CO 2 emissions from the
ridges is comparable to the current rate). Paleoclimate data
confirm the hot and dry climate simulated over a large part
of Pangea. The paleogeography of the Triassic maintained
the Earth in a relatively stable climate dominated by a high
simulated pCO 2 of around 3000 ppm and a high simulated
global average temperature (between 21.5 and 23 °C).
During the Jurassic, the drift of the Pangea to the north and
its break-up brought about an increase in continental surfaces exposed to the hot and humid climate of the equatorial
band. This resulted in intensified chemical weathering of
silicates thus causing an increase in the consumption of CO 2 .
The simulated pCO 2 is lower at about 700 ppm as is the
average temperature of the globe at a little over 18 °C. The
end of the Mesozoic is marked by the final break-up of
Gondwana. The arid areas reduced in size, reinforcing global
chemical weathering. This resulted in a fairly low simulated
pCO 2 of between 300 and 500 ppm for the Cretaceous. The
effect of paleogeography on climate (through the regulation
of chemical weathering of silicates) is therefore an effective
process that can be seen in long-term trends, even if it does
not explain every climate variations.
The conditions necessary for the high latitudes to
freeze-up were not present at the end of the Permian, but
they were in place for a long period from the Lower Carboniferous (340 Ma) to the Lower Permian (280 Ma),
already in a supercontinent paleogeographic context. During
this glacial period, the Earth experienced a succession of
advances and retreats of continental ice over southern
Gondwana. What mechanisms would push the Earth into a
different climatic state? Although the geographical configuration of Pangea had not changed drastically between the
Carboniferous and the Upper Permian, it had been drifting
northwards during this period. Indeed, paleomagnetic data
show that southern Gondwana was located at the pole during
the Carboniferous, favoring a cooler summer in this region,
but not yet cold enough for ice to remain outside some high
reliefs. Another more effective mechanism was needed. The
decrease in pCO 2 was therefore necessary to explain this
glacial period. One suggestion was that colonization of
emerged lands by plants during the Devonian increased the
chemical weathering of silicates leading to a decrease in
pCO 2 (Berner 2001). However, this vegetal colonization of
continents took place tens of millions of years before the
beginning of the glaciation. Goddéris et al. (2017) have
shown that the Hercynian orogenesis could have played a
major role. This orogenesis, resulting from the collision of
Laurussia and Gondwana around 350 Ma, was at the origin
of the uplift of a vast chain of mountains, the Hercynian
chain, stretching for several thousands of kilometers in the
equatorial band. With no mountains, the hot and humid
climate of the lower latitudes causes a thick saprolite to
form, considerably limiting the weathering of the underlying
bedrock. With orogenesis, the presence of relief produces
strong mechanical erosion due to the slopes which considerably limit the development of thick saprolites, thus the
weathering of silicate rocks is increased, leading to a
264
F. Fluteau and P. Sepulchre
The atmospheric partial pressure of carbon dioxide (pCO 2 ) is
driven by the carbon cycle. On the scale of geological time,
pCO 2 reflects the balance between the CO 2 emission fluxes
from volcanic systems (ocean ridges and aerial volcanism),
the degree of magmatic activity from the mantle (plumes),
decarbonation in subduction zones, and the CO 2 fluxes
consumed by the weathering of the rock silicates on the
surface of the Earth and in the oceanic crust, through the
burial of the organic matter. Fluctuations in pCO 2 reflect the
evolution of one or both of these flows. CO 2 emissions are
proportional to the annual rate of production of oceanic
crust. We have seen that the variability in this rate of production does not exceed 30% over the last 170 million years
(Cogné and Humler 2006). To these rates should be added
CO 2 emissions from plumes of mantle volcanism and from
decarbonation in subduction zones. Significant magmatic
events dating from the late Early Cretaceous and associated
with the establishment of some large submarine basalt provinces, such as the Ontong-Java Plateau, increase the production rate of oceanic crust by about 25% (Cogné and
Humler 2006) and about the same increase in CO 2 is injected
into the ocean-atmosphere system. Contributions from the
subduction zones are less constrained. Currently, this process is limited to a few subduction zones in the Pacific or
around the Indonesian archipelago, while the main deposition zones are in the Atlantic and Indian Ocean. In the past,
the subduction of the Tethysian Ocean could have emitted a
significant CO 2 flux (Hoareau et al. 2015).
The intensity of the chemical weathering is a function of
climate parameters such as surface temperature and runoff.
Moreover, most geochemical models considered runoff as a
function of surface temperature: the higher the temperature,
the greater the runoff, and consequently, the chemical
weathering. The chemical weathering of silicates acts as a
climate regulator. We know now that this proportional
relationship between temperature and runoff is based on
current data and is not transferable to past periods. This is
easily understood by analyzing the upper Permian climate.
Simulation of chemical weathering is effective only in some
areas experiencing a tropical and humid climate. This weak
chemical weathering of silicates in the paleogeographic
context implies a relatively high pCO 2 equilibrium of about
2500 ppm, and a high global average temperature of about
21 °C (considering that the rate of CO 2 emissions from the
ridges is comparable to the current rate). Paleoclimate data
confirm the hot and dry climate simulated over a large part
of Pangea. The paleogeography of the Triassic maintained
the Earth in a relatively stable climate dominated by a high
simulated pCO 2 of around 3000 ppm and a high simulated
global average temperature (between 21.5 and 23 °C).
During the Jurassic, the drift of the Pangea to the north and
its break-up brought about an increase in continental surfaces exposed to the hot and humid climate of the equatorial
band. This resulted in intensified chemical weathering of
silicates thus causing an increase in the consumption of CO 2 .
The simulated pCO 2 is lower at about 700 ppm as is the
average temperature of the globe at a little over 18 °C. The
end of the Mesozoic is marked by the final break-up of
Gondwana. The arid areas reduced in size, reinforcing global
chemical weathering. This resulted in a fairly low simulated
pCO 2 of between 300 and 500 ppm for the Cretaceous. The
effect of paleogeography on climate (through the regulation
of chemical weathering of silicates) is therefore an effective
process that can be seen in long-term trends, even if it does
not explain every climate variations.
The conditions necessary for the high latitudes to
freeze-up were not present at the end of the Permian, but
they were in place for a long period from the Lower Carboniferous (340 Ma) to the Lower Permian (280 Ma),
already in a supercontinent paleogeographic context. During
this glacial period, the Earth experienced a succession of
advances and retreats of continental ice over southern
Gondwana. What mechanisms would push the Earth into a
different climatic state? Although the geographical configuration of Pangea had not changed drastically between the
Carboniferous and the Upper Permian, it had been drifting
northwards during this period. Indeed, paleomagnetic data
show that southern Gondwana was located at the pole during
the Carboniferous, favoring a cooler summer in this region,
but not yet cold enough for ice to remain outside some high
reliefs. Another more effective mechanism was needed. The
decrease in pCO 2 was therefore necessary to explain this
glacial period. One suggestion was that colonization of
emerged lands by plants during the Devonian increased the
chemical weathering of silicates leading to a decrease in
pCO 2 (Berner 2001). However, this vegetal colonization of
continents took place tens of millions of years before the
beginning of the glaciation. Goddéris et al. (2017) have
shown that the Hercynian orogenesis could have played a
major role. This orogenesis, resulting from the collision of
Laurussia and Gondwana around 350 Ma, was at the origin
of the uplift of a vast chain of mountains, the Hercynian
chain, stretching for several thousands of kilometers in the
equatorial band. With no mountains, the hot and humid
climate of the lower latitudes causes a thick saprolite to
form, considerably limiting the weathering of the underlying
bedrock. With orogenesis, the presence of relief produces
strong mechanical erosion due to the slopes which considerably limit the development of thick saprolites, thus the
weathering of silicate rocks is increased, leading to a
264
F. Fluteau and P. Sepulchre