nitrogen, oxygen, sulphur, as well as aerosols. We review
existing records from marine sediment cores and ice cores
and highlight the current knowns and unknowns.
Carbon Cycle
Two atmospheric gases containing carbon play a major role
in the interactions between climate and biogeochemistry:
carbon dioxide (CO 2 ) and methane (CH 4 ). While CO 2 is the
most famous and most studied element, CH 4 is far from
negligible and has a rather different cycle. We will first focus
on CO 2 , then on CH 4 , before reviewing past changes.
Carbon Dioxide (CO 2 )
On the timescale of a few hundred to a few tens of thousands
of years, the main carbon reservoirs are the ocean (including
sediments), atmosphere and land (including terrestrial biosphere and permafrost) (Fig. 23.2). On the longer timescale
of a few million years the lithosphere (rocks) starts to play a
major role (see Volume 1, Chap. 2, Volume 2, Chaps. 22
and 26). Exchanges between these reservoirs involve biotic
processes, due to biological activity such as photosynthesis
by plants, and abiotic processes.
In the atmosphere, CO 2 is the main form of carbon and is
relatively well mixed over a year. Its concentration is currently increasing due to anthropogenic activity and, at the
beginning of 2018, the atmospheric CO 2 concentration was
over 400 ppm (408 in March 2018, NOAA, https://www.
esrl.noaa.gov/gmd/ccgg/trends/monthly.html). At the beginning of the pre-industrial era, it was around 280 ppm, corresponding to a carbon stock of around 600 GtC.
On land, atmospheric CO 2 is taken up by plants during
photosynthesis, turning CO 2 into organic carbon. The living
biomass is then converted into dead organic carbon matter in
litter and soils. Organic carbon is progressively returned to
the atmosphere by autotrophic and heterotrophic respiration.
Currently, carbon storage in the terrestrial biosphere is
around 2500 GtC. If the conditions become cold enough, the
soil freezes, locking the carbon into permafrost, i.e. frozen
soil. During warming, the permafrost thaws, and carbon is
returned to the atmosphere. The current estimate of carbon
stored in permafrost is *1700 GtC (Tarnocai et al. 2009),
making it the single largest component of the terrestrial
carbon pool. The fluxes of carbon between the atmosphere
and land are around 120 GtC/year.
The ocean is the largest carbon reservoir (excluding the
lithosphere) with around 38,000 GtC both in organic and
inorganic forms. Carbon fluxes to the ocean come from the
atmosphere through surface exchanges and from the continent from riverine inputs. In the ocean, dissolved carbon
(CO 2(aq) ) gets hydrated into H 2 CO 3 (carbonic acid), which
then gives bicarbonate ion (or hydrogen carbonate ion,
HCO 3
− ) and carbonate ion CO 3
2−
. All these dissolved species are summed up in the term “dissolved inorganic carbon”
(DIC). Because the concentration of H 2 CO 3 is very small, it
is included in CO 2(aq) . CO 2(aq) , HCO 3
− and CO 3
2− are in
equilibrium following the chemical equations:
CO 2 aq
ð Þ þ H 2 O HCO
À
3 þ H
þ
Sediment core
with marine organism shells
Ice core
with air bubbles
Fig. 23.1 Diagram of the Earth system and schemes showing ice cores and sediment cores extraction
23 Biogeochemical Cycles and Aerosols Over the Last Million Years
273
existing records from marine sediment cores and ice cores
and highlight the current knowns and unknowns.
Carbon Cycle
Two atmospheric gases containing carbon play a major role
in the interactions between climate and biogeochemistry:
carbon dioxide (CO 2 ) and methane (CH 4 ). While CO 2 is the
most famous and most studied element, CH 4 is far from
negligible and has a rather different cycle. We will first focus
on CO 2 , then on CH 4 , before reviewing past changes.
Carbon Dioxide (CO 2 )
On the timescale of a few hundred to a few tens of thousands
of years, the main carbon reservoirs are the ocean (including
sediments), atmosphere and land (including terrestrial biosphere and permafrost) (Fig. 23.2). On the longer timescale
of a few million years the lithosphere (rocks) starts to play a
major role (see Volume 1, Chap. 2, Volume 2, Chaps. 22
and 26). Exchanges between these reservoirs involve biotic
processes, due to biological activity such as photosynthesis
by plants, and abiotic processes.
In the atmosphere, CO 2 is the main form of carbon and is
relatively well mixed over a year. Its concentration is currently increasing due to anthropogenic activity and, at the
beginning of 2018, the atmospheric CO 2 concentration was
over 400 ppm (408 in March 2018, NOAA, https://www.
esrl.noaa.gov/gmd/ccgg/trends/monthly.html). At the beginning of the pre-industrial era, it was around 280 ppm, corresponding to a carbon stock of around 600 GtC.
On land, atmospheric CO 2 is taken up by plants during
photosynthesis, turning CO 2 into organic carbon. The living
biomass is then converted into dead organic carbon matter in
litter and soils. Organic carbon is progressively returned to
the atmosphere by autotrophic and heterotrophic respiration.
Currently, carbon storage in the terrestrial biosphere is
around 2500 GtC. If the conditions become cold enough, the
soil freezes, locking the carbon into permafrost, i.e. frozen
soil. During warming, the permafrost thaws, and carbon is
returned to the atmosphere. The current estimate of carbon
stored in permafrost is *1700 GtC (Tarnocai et al. 2009),
making it the single largest component of the terrestrial
carbon pool. The fluxes of carbon between the atmosphere
and land are around 120 GtC/year.
The ocean is the largest carbon reservoir (excluding the
lithosphere) with around 38,000 GtC both in organic and
inorganic forms. Carbon fluxes to the ocean come from the
atmosphere through surface exchanges and from the continent from riverine inputs. In the ocean, dissolved carbon
(CO 2(aq) ) gets hydrated into H 2 CO 3 (carbonic acid), which
then gives bicarbonate ion (or hydrogen carbonate ion,
HCO 3
− ) and carbonate ion CO 3
2−
. All these dissolved species are summed up in the term “dissolved inorganic carbon”
(DIC). Because the concentration of H 2 CO 3 is very small, it
is included in CO 2(aq) . CO 2(aq) , HCO 3
− and CO 3
2− are in
equilibrium following the chemical equations:
CO 2 aq
ð Þ þ H 2 O HCO
À
3 þ H
þ
Sediment core
with marine organism shells
Ice core
with air bubbles
Fig. 23.1 Diagram of the Earth system and schemes showing ice cores and sediment cores extraction
23 Biogeochemical Cycles and Aerosols Over the Last Million Years
273