therefore increasingly considered to have been a long cold
period (about 470–425 Ma), sometimes referred to as ‘Early
Paleozoic Ice Age’ (Page et al. 2007), and within which the
Hirnantien only represents a glacial maximum. This vision is
supported by the most recent climate models (Pohl et al.
2016).
The causes of this glaciation are still poorly understood.
Nardin et al. (2011) showed that the long-term cooling of the
climate can be explained by the paleogeographic evolution
occurring throughout the Ordovician, and in particular the
migration of continents in the intertropical zone conducive to
weathering, which brings about a fall in the atmospheric
concentration of CO 2 . Regarding the Hirnantien glacial
peak, the best explanations also suggest a fall in atmospheric
CO 2 , but the mechanisms to achieve this are subject to
debate. Kump et al. (1999) proposed an interesting
hypothesis: the fall in CO 2 level could have been a consequence of the establishment of New Caledonian and Appalachian orogens during the Middle and Upper Ordovician,
which would have increased the vulnerability of the continental surfaces to weathering. Other mechanisms have also
been proposed, including the establishment of the first plants
on land (Lenton et al. 2012). The difficulty in explaining this
event lies in the magnitude of the cooling, which is around
−7 °C at tropical latitudes, whereas sea surface temperatures
appear to have varied by only 1–2 °C at the same latitudes
during the last glacial-interglacial cycle (CLIMAP Project
1981).
However,
numerical
modeling
of
the
ocean-atmosphere coupled system during the Ordovician
revealed climatic instability associated with the sudden
development of sea ice, which explains a sharp fall in temperatures in response to a moderate decrease in atmospheric
CO 2 concentration, thus loosening the constraints that would
be placed on CO 2 sinks to explain the geochemical data
(Pohl et al. 2016). To explain the emergence from the Hirnantian glacial maximum, Kump et al. (1999) proposed the
following mechanism: as the ice cover on the supercontinent
Gondwana increased, the available surface of continental
silicates exposed to weathering falls, thus causing an accumulation of CO 2 in the atmosphere. This persuasive scenario
was tested with a simple climate model.
The Devonian Climate
The Devonian (419–359 Ma) is marked by numerous biological disturbances. Although the first traces of vegetation
appeared during the Middle Ordovician (Rubinstein et al.
2010) and the existence of vegetation fires during the Silurian are suggested based on the presence of charcoal in the
sedimentary record, the development of a long-stemmed
biosphere begins in the Lower Devonian. Plants reaching
heights of up to 2 to 3 meters were identified during the
Eifelian (390 Ma; Stein et al. 2007). Trees, 8–10 m high,
began to colonize the land towards the end of the Givetien
(385 Ma), with, among others, giant ferns such as Archeopteris and Cladoxyopsides (Anderson et al. 1995). True
large forests are likely to have become established towards
the end of the Devonian (Frasnian; 380 Ma, Scott and
Glaspool, 2006). As trees appeared and flourished, weathering of the continental silicates rapidly accelerated, while
degassing from the solid Earth remained almost constant
(Berner 2004). Soils developed along with root systems,
increasing the acidification of the water in contact with the
minerals as well as increasing the contact time between
inland water and silicate rocks. This resulted in a rapid
decrease in the partial pressure of atmospheric CO 2 , from
2000 ppmv in the early Devonian to 1000 ppmv at the end
(Foster et al. 2017). The extent to which the climate cooled
as a result of this colonization is uncertain however because
the change of the albedo of continental surfaces, following
the replacement of bare soils by forests, compensates at least
partially for the fall in atmospheric CO 2 (Le Hir et al. 2011).
It is also interesting to note that the overall cooling of the
climate may have been beneficial to the development of
modern leaves, which are large in size and have many
stomata, encouraging primary production on land to the
detriment of more primitive plants. We can thus infer the
establishment of a positive feedback between cooling and
the colonization of land surfaces by ever more efficient
plants.
Another mechanism that could explain the drop in CO 2
during the Devonian is an increase in the amount of CO 2
trapped in sediment, also as a result of colonization of land
by continental plants. Indeed, the appearance of lignin in
plant tissues from 410 Ma onwards increased the amount of
organic carbon preserved in continental environments and
on the margins. Lignin is indeed much more resistant to
mineralization than marine organic matter. It was first
thought that lignin appeared before the development of
organisms capable of decomposing it, causing an increase in
the burial of carbon and the reduction of CO 2 , which could
have contributed to the establishment of the glaciation of the
Late Paleozoic (Nelsen et al. 2016). Nevertheless, recent
studies have shown that decomposers evolved in parallel to
lignin, which calls into question an ‘organic’ trigger for the
Permo-Carboniferous glaciation (Nelsen et al. 2016).
Finally, the end of the Devonian is characterized by a
mass extinction event, affecting tropical marine environments in particular. This event lasted approximately 1–
3 million years, culminating at the Frasnian-Famennian
boundary. It is accompanied by the deposition of anoxic
sediments (black shales), accompanied by two positive
excursions of the d
13 C of carbonate sediments. It has been
proposed that these events are the consequence of the
emergence of pulses as vascular plants colonized the land
27 The Phanerozoic Climate
371
period (about 470–425 Ma), sometimes referred to as ‘Early
Paleozoic Ice Age’ (Page et al. 2007), and within which the
Hirnantien only represents a glacial maximum. This vision is
supported by the most recent climate models (Pohl et al.
2016).
The causes of this glaciation are still poorly understood.
Nardin et al. (2011) showed that the long-term cooling of the
climate can be explained by the paleogeographic evolution
occurring throughout the Ordovician, and in particular the
migration of continents in the intertropical zone conducive to
weathering, which brings about a fall in the atmospheric
concentration of CO 2 . Regarding the Hirnantien glacial
peak, the best explanations also suggest a fall in atmospheric
CO 2 , but the mechanisms to achieve this are subject to
debate. Kump et al. (1999) proposed an interesting
hypothesis: the fall in CO 2 level could have been a consequence of the establishment of New Caledonian and Appalachian orogens during the Middle and Upper Ordovician,
which would have increased the vulnerability of the continental surfaces to weathering. Other mechanisms have also
been proposed, including the establishment of the first plants
on land (Lenton et al. 2012). The difficulty in explaining this
event lies in the magnitude of the cooling, which is around
−7 °C at tropical latitudes, whereas sea surface temperatures
appear to have varied by only 1–2 °C at the same latitudes
during the last glacial-interglacial cycle (CLIMAP Project
1981).
However,
numerical
modeling
of
the
ocean-atmosphere coupled system during the Ordovician
revealed climatic instability associated with the sudden
development of sea ice, which explains a sharp fall in temperatures in response to a moderate decrease in atmospheric
CO 2 concentration, thus loosening the constraints that would
be placed on CO 2 sinks to explain the geochemical data
(Pohl et al. 2016). To explain the emergence from the Hirnantian glacial maximum, Kump et al. (1999) proposed the
following mechanism: as the ice cover on the supercontinent
Gondwana increased, the available surface of continental
silicates exposed to weathering falls, thus causing an accumulation of CO 2 in the atmosphere. This persuasive scenario
was tested with a simple climate model.
The Devonian Climate
The Devonian (419–359 Ma) is marked by numerous biological disturbances. Although the first traces of vegetation
appeared during the Middle Ordovician (Rubinstein et al.
2010) and the existence of vegetation fires during the Silurian are suggested based on the presence of charcoal in the
sedimentary record, the development of a long-stemmed
biosphere begins in the Lower Devonian. Plants reaching
heights of up to 2 to 3 meters were identified during the
Eifelian (390 Ma; Stein et al. 2007). Trees, 8–10 m high,
began to colonize the land towards the end of the Givetien
(385 Ma), with, among others, giant ferns such as Archeopteris and Cladoxyopsides (Anderson et al. 1995). True
large forests are likely to have become established towards
the end of the Devonian (Frasnian; 380 Ma, Scott and
Glaspool, 2006). As trees appeared and flourished, weathering of the continental silicates rapidly accelerated, while
degassing from the solid Earth remained almost constant
(Berner 2004). Soils developed along with root systems,
increasing the acidification of the water in contact with the
minerals as well as increasing the contact time between
inland water and silicate rocks. This resulted in a rapid
decrease in the partial pressure of atmospheric CO 2 , from
2000 ppmv in the early Devonian to 1000 ppmv at the end
(Foster et al. 2017). The extent to which the climate cooled
as a result of this colonization is uncertain however because
the change of the albedo of continental surfaces, following
the replacement of bare soils by forests, compensates at least
partially for the fall in atmospheric CO 2 (Le Hir et al. 2011).
It is also interesting to note that the overall cooling of the
climate may have been beneficial to the development of
modern leaves, which are large in size and have many
stomata, encouraging primary production on land to the
detriment of more primitive plants. We can thus infer the
establishment of a positive feedback between cooling and
the colonization of land surfaces by ever more efficient
plants.
Another mechanism that could explain the drop in CO 2
during the Devonian is an increase in the amount of CO 2
trapped in sediment, also as a result of colonization of land
by continental plants. Indeed, the appearance of lignin in
plant tissues from 410 Ma onwards increased the amount of
organic carbon preserved in continental environments and
on the margins. Lignin is indeed much more resistant to
mineralization than marine organic matter. It was first
thought that lignin appeared before the development of
organisms capable of decomposing it, causing an increase in
the burial of carbon and the reduction of CO 2 , which could
have contributed to the establishment of the glaciation of the
Late Paleozoic (Nelsen et al. 2016). Nevertheless, recent
studies have shown that decomposers evolved in parallel to
lignin, which calls into question an ‘organic’ trigger for the
Permo-Carboniferous glaciation (Nelsen et al. 2016).
Finally, the end of the Devonian is characterized by a
mass extinction event, affecting tropical marine environments in particular. This event lasted approximately 1–
3 million years, culminating at the Frasnian-Famennian
boundary. It is accompanied by the deposition of anoxic
sediments (black shales), accompanied by two positive
excursions of the d
13 C of carbonate sediments. It has been
proposed that these events are the consequence of the
emergence of pulses as vascular plants colonized the land
27 The Phanerozoic Climate
371