weathering. The result would be a drop in atmospheric
CO 2 and an overall cooling. This theory is based on
measurements of current weathering rates in mountain
ranges, on the strong correlation between physical erosion and chemical weathering in modern watersheds, and
on the
87 Sr/
86 Sr isotopic ratio of the water run-off from
the Himalayas. A second effect should also be taken into
account: the very high sedimentation rates in the seas at
the foot of the orogens lead to very efficient burial of
organic carbon and thus to increased consumption of
atmospheric CO 2 thanks to the action of the biological
pump. This process is responsible for trapping two to
three times more CO 2 than the silicate weathering in the
Himalayan orogeny (Galy et al. 2007).
2. The development of vascular vegetation on land. Tall
vegetation with a developed root system acts at three
levels on weathering rates. The roots stabilize the soils
and thus increase the contact time between inland waters
and the silicate minerals. In addition, root and microbial
respiration in soils increases the partial pressure of CO 2 ,
and thus acidifies the water which percolates towards the
bedrock. Finally, the plants secrete organic acids which
also contribute to the acidification of the waters. As a
result of these three effects, there is an increase in the
consumption of atmospheric CO 2 through dissolution of
the continental silicates. This hypothesis is based on
studies carried out in particular in Iceland on lava flows
on slopes covered and uncovered with stemmed vegetation. It appears that weathering rates are eight to ten times
larger under dense vegetation cover (Berner 2004).
A small-scale laboratory study suggests that non-vascular
plants (lichens and mosses) could have a similarly
accelerate chemical weathering of continental surfaces
(Lenton et al. 2012).
3. Increased burial of organic carbon during the anoxic
phase of the ocean. This hypothesis is often proposed to
explain positive excursions in the d
13 C ratio of oceanic
carbonates correlated with climate cooling. It requires
particular environmental conditions: either conditions
favorable to maintaining water stratification in large
ocean basins and preventing the ventilation of the deep
waters in these basins, or conditions of oceanic hyperproductivity leading to the absorption of oxygen in the
deep waters through the recycling of organic matter
produced in the euphotic zone. This burial may also
occur on land-based environments, as has happened
during the Carboniferous period.
4. The movement of the solar system into a galactic arm.
This recent hypothesis attempts to explain the periodicity
of 135 million years in cold modes. The galactic arm is
an area of formation of intense stars and of emission of
galactic cosmic rays. Reaching the atmosphere, these are
thought to participate in the nucleation of low-level
clouds, increasing the albedo of the atmosphere and
cooling the climate (Shaviv and Veizer 2003). To date,
there is no experimental evidence of the validity of this
mechanism, which remains purely speculative.
5. The fragmentation of a supercontinent. The resulting
increase in rainfall activates the consumption of CO 2 by
silicate weathering and thus cools the climate. This effect
is particularly important if the supercontinent breaks up
along the equator, the site of intense rainfall (Goddéris
et al. 2014).
6. The migration of continents towards the low latitudes,
characterized by climatic conditions favoring the
weathering of continental silicates and thus an increased
consumption of atmospheric CO 2 (Nardin et al. 2011).
7. Any reduction in degassing of greenhouse gases from the
mantle or sediments towards the atmosphere.
8. The establishment and subsequent weathering of basaltic
provinces on the continents. Basalts weather much more
efficiently than the average continental crust on which
they spread (in equivalent conditions, basalt weathers
eight times faster than granite). This finding was established from a study of weathering in basaltic watersheds
(Dessert et al. 2001). The weathering of new basalt thus
produces a long-term decrease in the partial pressure of
atmospheric CO 2 . The question remains as to the
weathering of submarine basaltic plateaus. Do they
contribute to the cooling of the climate system or not?
The pH buffer imposed by carbonate speciation in seawater nevertheless suggests that weathering of oceanic
basalts is a minor phenomenon, with basalt dissolution
being minimal at around pH 8, a value for seawater
which probably didn’t change much over the course of
the Phanerozoic.
The Causes of Warm Climate Modes
Curiously, cold modes have always been considered to be
accidents in a prolonged warm state. This is probably the
reason why the suggested causes of warm modes are fewer
and less discussed in the literature, with the exception of the
thermal event of the Palaeocene-Eocene transition.
The following mechanisms have been proposed:
1. Any increase in degassing of greenhouse gases from the
mantle or sediments to the atmosphere. This could be due
to increased volcanic activity releasing massive amounts
of CO 2 , basaltic effusion events over land (Dessert et al.
2001) or methane degassing from gas hydrates accumulated in sediments (McInerney and Wing 2011).
2. The creation of a supercontinent, reducing rainfall and
thus weathering of the continental silicates, allowing an
368
Y. Goddéris et al.
CO 2 and an overall cooling. This theory is based on
measurements of current weathering rates in mountain
ranges, on the strong correlation between physical erosion and chemical weathering in modern watersheds, and
on the
87 Sr/
86 Sr isotopic ratio of the water run-off from
the Himalayas. A second effect should also be taken into
account: the very high sedimentation rates in the seas at
the foot of the orogens lead to very efficient burial of
organic carbon and thus to increased consumption of
atmospheric CO 2 thanks to the action of the biological
pump. This process is responsible for trapping two to
three times more CO 2 than the silicate weathering in the
Himalayan orogeny (Galy et al. 2007).
2. The development of vascular vegetation on land. Tall
vegetation with a developed root system acts at three
levels on weathering rates. The roots stabilize the soils
and thus increase the contact time between inland waters
and the silicate minerals. In addition, root and microbial
respiration in soils increases the partial pressure of CO 2 ,
and thus acidifies the water which percolates towards the
bedrock. Finally, the plants secrete organic acids which
also contribute to the acidification of the waters. As a
result of these three effects, there is an increase in the
consumption of atmospheric CO 2 through dissolution of
the continental silicates. This hypothesis is based on
studies carried out in particular in Iceland on lava flows
on slopes covered and uncovered with stemmed vegetation. It appears that weathering rates are eight to ten times
larger under dense vegetation cover (Berner 2004).
A small-scale laboratory study suggests that non-vascular
plants (lichens and mosses) could have a similarly
accelerate chemical weathering of continental surfaces
(Lenton et al. 2012).
3. Increased burial of organic carbon during the anoxic
phase of the ocean. This hypothesis is often proposed to
explain positive excursions in the d
13 C ratio of oceanic
carbonates correlated with climate cooling. It requires
particular environmental conditions: either conditions
favorable to maintaining water stratification in large
ocean basins and preventing the ventilation of the deep
waters in these basins, or conditions of oceanic hyperproductivity leading to the absorption of oxygen in the
deep waters through the recycling of organic matter
produced in the euphotic zone. This burial may also
occur on land-based environments, as has happened
during the Carboniferous period.
4. The movement of the solar system into a galactic arm.
This recent hypothesis attempts to explain the periodicity
of 135 million years in cold modes. The galactic arm is
an area of formation of intense stars and of emission of
galactic cosmic rays. Reaching the atmosphere, these are
thought to participate in the nucleation of low-level
clouds, increasing the albedo of the atmosphere and
cooling the climate (Shaviv and Veizer 2003). To date,
there is no experimental evidence of the validity of this
mechanism, which remains purely speculative.
5. The fragmentation of a supercontinent. The resulting
increase in rainfall activates the consumption of CO 2 by
silicate weathering and thus cools the climate. This effect
is particularly important if the supercontinent breaks up
along the equator, the site of intense rainfall (Goddéris
et al. 2014).
6. The migration of continents towards the low latitudes,
characterized by climatic conditions favoring the
weathering of continental silicates and thus an increased
consumption of atmospheric CO 2 (Nardin et al. 2011).
7. Any reduction in degassing of greenhouse gases from the
mantle or sediments towards the atmosphere.
8. The establishment and subsequent weathering of basaltic
provinces on the continents. Basalts weather much more
efficiently than the average continental crust on which
they spread (in equivalent conditions, basalt weathers
eight times faster than granite). This finding was established from a study of weathering in basaltic watersheds
(Dessert et al. 2001). The weathering of new basalt thus
produces a long-term decrease in the partial pressure of
atmospheric CO 2 . The question remains as to the
weathering of submarine basaltic plateaus. Do they
contribute to the cooling of the climate system or not?
The pH buffer imposed by carbonate speciation in seawater nevertheless suggests that weathering of oceanic
basalts is a minor phenomenon, with basalt dissolution
being minimal at around pH 8, a value for seawater
which probably didn’t change much over the course of
the Phanerozoic.
The Causes of Warm Climate Modes
Curiously, cold modes have always been considered to be
accidents in a prolonged warm state. This is probably the
reason why the suggested causes of warm modes are fewer
and less discussed in the literature, with the exception of the
thermal event of the Palaeocene-Eocene transition.
The following mechanisms have been proposed:
1. Any increase in degassing of greenhouse gases from the
mantle or sediments to the atmosphere. This could be due
to increased volcanic activity releasing massive amounts
of CO 2 , basaltic effusion events over land (Dessert et al.
2001) or methane degassing from gas hydrates accumulated in sediments (McInerney and Wing 2011).
2. The creation of a supercontinent, reducing rainfall and
thus weathering of the continental silicates, allowing an
368
Y. Goddéris et al.