constant over the rest of the Phanerozoic. This fractionation
was reconstructed over the whole Phanerozoic by measuring
the d
13 C of organic carbon in marine sediments over many
different periods and comparing it to the d
13
C of sedimentary
carbonates (Hayes et al. 1999). Since this fractionation is
dependent on the concentration of H 2 CO 3 in the waters, it
can be inferred that its decline is linked to a drop in atmospheric CO 2 since the Miocene. Yet this simple interpretation is challenged by independent estimates of the CO 2 level
suggesting pressures below 300 ppmv during the Miocene
(see section on atmospheric CO 2 ). Nevertheless, the combination of the drop in e MO fractionation and the drop in
oceanic d
13
C suggests an increase in CO 2 being buried in the
form of organic carbon in sediments since 15 Ma, probably
linked to the establishment of the Himalayan orogeny.
The
87
Sr/
86 Sr Isotope Ratio of Carbonate
Sediments
Marine carbonates record the strontium isotopic ratio,
87 Sr/
86 Sr, of seawater without fractionation. The residence
time of Sr in seawater (2–5 million years) ensures an even
value throughout the ocean and therefore low dispersion of
the data. A high-resolution curve (1 million years) has been
published by Veizer et al. (1999). This signal has been widely
used to constrain the extent to which CO 2 is consumed by
alteration of continental silicates during the Cenozoic, particularly in response to the Himalayan orogeny (Raymo
1991). Two main types of Sr intake to the ocean are identified:
an exchange flux at the level of the ocean ridges, which
doesn’t affect the Sr concentration of the water but modifies its
isotopic ratio. Today, the water enters the ocean ridges with a
87 Sr/
86 Sr ratio of 0.709 and exits after contact with mantellic
rocks with a typical value of 0.703. This process therefore
tends to reduce the
87 Sr/
86 Sr ratio of seawater and bring it
closer to the mantle value. Conversely, the isotopic ratio of the
rivers, inherited from the weathering of continental rocks is
now equal to 0.712. There is some correlation between periods with high
87 Sr/
86 Sr seawater ratios and glaciation episodes. This has been interpreted as a sign of greater
weathering during cold climate periods, in response to
intensified physical erosion, which in turn promotes chemical
weathering. The signal is particularly clear for the last
40 million years, and the rapid increase in the
87 Sr/
86 Sr ratio
of seawater has been interpreted as the signature of increased
continental weathering during the uplift of the Himalayas. It
has been suggested that there is a correlation between this
increase in the
87 Sr/
86 Sr ratio and a decrease in the CO 2
concentration in the atmosphere. In this model, the Himalayas
are considered to have triggered the cooling in the Cenozoic.
This hypothesis has been extensively developed. It is
found again in the more recent literature, linking orogeny
with global cooling of the climate in response to intensified
weathering. Nevertheless, this hypothesis is in contradiction
with the paleothermostat theory. In fact, weathering becomes
a function of climate in a positive feedback loop. The colder
it gets, the more erosion increases, forcing an uptake in CO 2
consumption by weathering, in turn forcing increased cooling. The silicate weathering and volcanic degassing are
uncoupled, and the carbon content of the ocean and the
atmosphere is consumed in less than a few million years
(Goddéris and François 1996), which would lead to unregistered climate disasters during the Tertiary. It should be
underlined, however, that it is possible that orogens pump
CO 2 while respecting the paleothermostat theory. The consequences of the uplift of a mountain range are much more
complex than a simple uplift causing increased weathering.
We shall see later that they lead to a number of geological
phenomena which ultimately link orogenesis to cooling.
Moreover, the evolution of the
87 Sr/
86 Sr isotopic ratio of
seawater cannot be interpreted solely in terms of changes in
the relative importance of mantellic and continental flows.
The isotopic ratio of the source rocks of the weathering may
also have changed over time, and, particularly in the orogenic zones, which considerably complicates the interpretation of the isotopic sign of strontium. Nevertheless, the
quality of the Phanerozoic signal should motivate further
analysis of this indicator in the future.
Along with the
87 Sr/
86 Sr isotopic signal, the
187 Os/
188 Os
osmium isotopic ratio of seawater measured from sediments
taken during ocean drilling programs is also used to constrain the evolution of continental and hydrothermal
weathering fluxes. The methodology is very similar to that of
Sr, but the major advantage of osmium is its short residence
time in the ocean, around 10–30 kyr, although it is sufficiently longer than the mixing time of the water masses to
ensure values that are representative of the global ocean.
River contributions are the dominant factor, with a
187 Os/
188 Os ratio of 1.3 and a flux of 1800 mol per year
−1 .
Hydrothermal inputs have an isotopic signature of 0.13 and a
flux of around 100 mol per year
−1 . A flow linked to cosmic
dust, with a ratio of 0.13 and a flux of 80 mol per year
−1 ,
must also be added. The main sinks are ocean sediment
deposits. The ratio can therefore be indirectly linked to the
evolution of the Earth’s climate and of greenhouse gases,
through the characterization of the geological flows of the
carbon cycle. Nevertheless, osmium is a very scarce element,
which makes it difficult to measure. The mean concentration
of Os is 10 fg/g (fentogram gram
−1 , 1 fg = 10
−15 g) in
seawater and 9 fg g
−1 in rivers.
The Level of Atmospheric CO 2
There is clearly no direct measurement of this level for the
distant past beyond 900,000 years, the period covered by ice
cores drilled in the Antarctic ice. All methods are therefore
27 The Phanerozoic Climate
365
was reconstructed over the whole Phanerozoic by measuring
the d
13 C of organic carbon in marine sediments over many
different periods and comparing it to the d
13
C of sedimentary
carbonates (Hayes et al. 1999). Since this fractionation is
dependent on the concentration of H 2 CO 3 in the waters, it
can be inferred that its decline is linked to a drop in atmospheric CO 2 since the Miocene. Yet this simple interpretation is challenged by independent estimates of the CO 2 level
suggesting pressures below 300 ppmv during the Miocene
(see section on atmospheric CO 2 ). Nevertheless, the combination of the drop in e MO fractionation and the drop in
oceanic d
13
C suggests an increase in CO 2 being buried in the
form of organic carbon in sediments since 15 Ma, probably
linked to the establishment of the Himalayan orogeny.
The
87
Sr/
86 Sr Isotope Ratio of Carbonate
Sediments
Marine carbonates record the strontium isotopic ratio,
87 Sr/
86 Sr, of seawater without fractionation. The residence
time of Sr in seawater (2–5 million years) ensures an even
value throughout the ocean and therefore low dispersion of
the data. A high-resolution curve (1 million years) has been
published by Veizer et al. (1999). This signal has been widely
used to constrain the extent to which CO 2 is consumed by
alteration of continental silicates during the Cenozoic, particularly in response to the Himalayan orogeny (Raymo
1991). Two main types of Sr intake to the ocean are identified:
an exchange flux at the level of the ocean ridges, which
doesn’t affect the Sr concentration of the water but modifies its
isotopic ratio. Today, the water enters the ocean ridges with a
87 Sr/
86 Sr ratio of 0.709 and exits after contact with mantellic
rocks with a typical value of 0.703. This process therefore
tends to reduce the
87 Sr/
86 Sr ratio of seawater and bring it
closer to the mantle value. Conversely, the isotopic ratio of the
rivers, inherited from the weathering of continental rocks is
now equal to 0.712. There is some correlation between periods with high
87 Sr/
86 Sr seawater ratios and glaciation episodes. This has been interpreted as a sign of greater
weathering during cold climate periods, in response to
intensified physical erosion, which in turn promotes chemical
weathering. The signal is particularly clear for the last
40 million years, and the rapid increase in the
87 Sr/
86 Sr ratio
of seawater has been interpreted as the signature of increased
continental weathering during the uplift of the Himalayas. It
has been suggested that there is a correlation between this
increase in the
87 Sr/
86 Sr ratio and a decrease in the CO 2
concentration in the atmosphere. In this model, the Himalayas
are considered to have triggered the cooling in the Cenozoic.
This hypothesis has been extensively developed. It is
found again in the more recent literature, linking orogeny
with global cooling of the climate in response to intensified
weathering. Nevertheless, this hypothesis is in contradiction
with the paleothermostat theory. In fact, weathering becomes
a function of climate in a positive feedback loop. The colder
it gets, the more erosion increases, forcing an uptake in CO 2
consumption by weathering, in turn forcing increased cooling. The silicate weathering and volcanic degassing are
uncoupled, and the carbon content of the ocean and the
atmosphere is consumed in less than a few million years
(Goddéris and François 1996), which would lead to unregistered climate disasters during the Tertiary. It should be
underlined, however, that it is possible that orogens pump
CO 2 while respecting the paleothermostat theory. The consequences of the uplift of a mountain range are much more
complex than a simple uplift causing increased weathering.
We shall see later that they lead to a number of geological
phenomena which ultimately link orogenesis to cooling.
Moreover, the evolution of the
87 Sr/
86 Sr isotopic ratio of
seawater cannot be interpreted solely in terms of changes in
the relative importance of mantellic and continental flows.
The isotopic ratio of the source rocks of the weathering may
also have changed over time, and, particularly in the orogenic zones, which considerably complicates the interpretation of the isotopic sign of strontium. Nevertheless, the
quality of the Phanerozoic signal should motivate further
analysis of this indicator in the future.
Along with the
87 Sr/
86 Sr isotopic signal, the
187 Os/
188 Os
osmium isotopic ratio of seawater measured from sediments
taken during ocean drilling programs is also used to constrain the evolution of continental and hydrothermal
weathering fluxes. The methodology is very similar to that of
Sr, but the major advantage of osmium is its short residence
time in the ocean, around 10–30 kyr, although it is sufficiently longer than the mixing time of the water masses to
ensure values that are representative of the global ocean.
River contributions are the dominant factor, with a
187 Os/
188 Os ratio of 1.3 and a flux of 1800 mol per year
−1 .
Hydrothermal inputs have an isotopic signature of 0.13 and a
flux of around 100 mol per year
−1 . A flow linked to cosmic
dust, with a ratio of 0.13 and a flux of 80 mol per year
−1 ,
must also be added. The main sinks are ocean sediment
deposits. The ratio can therefore be indirectly linked to the
evolution of the Earth’s climate and of greenhouse gases,
through the characterization of the geological flows of the
carbon cycle. Nevertheless, osmium is a very scarce element,
which makes it difficult to measure. The mean concentration
of Os is 10 fg/g (fentogram gram
−1 , 1 fg = 10
−15 g) in
seawater and 9 fg g
−1 in rivers.
The Level of Atmospheric CO 2
There is clearly no direct measurement of this level for the
distant past beyond 900,000 years, the period covered by ice
cores drilled in the Antarctic ice. All methods are therefore
27 The Phanerozoic Climate
365