CARBON CYCLE
C. A. Carlson, University of California, Santa Barbara,
CA, USA
N. R. Bates, Bermuda Biological Station for Research,
St George’s, Bermuda, USA
D. A. Hansell, University of Miami, Miami FL, USA
D. K. Steinberg, College of William and Mary,
Gloucester Pt, VA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Why is carbon an important element? Carbon has
several unique properties that make it an important
component of life, energy flow, and climate regulation. It is present on the Earth in many different
inorganic and organic forms. Importantly, it has the
ability to form complex, stable carbon compounds,
such as proteins and carbohydrates, which are the
fundamental building blocks of life. Photosynthesis
provides marine plants (phytoplankton) with an
ability to transform energy from sunlight, and inorganic carbon and nutrients dissolved in sea water,
into complex organic carbon materials. All organisms, including autotrophs and heterotrophs, then
catabolize these organic compounds to their inorganic constituents via respiration, yielding energy
for their metabolic requirements. Production, consumption, and transformation of these organic materials provide the energy to be transferred between
all the trophic states of the ocean ecosystem.
In its inorganic gaseous phases (carbon dioxide,
CO 2 ; methane, CH 4 ; carbon monoxide, CO), carbon
has important greenhouse properties that can influence climate. Greenhouse gases in the atmosphere act
to trap long-wave radiation escaping from Earth to
space. As a result, the Earth’s surface warms, an effect necessary to maintain liquid water and life on
Earth. Human activities have led to a rapid increase
in greenhouse gas concentrations, potentially impacting the world’s climate through the effects of
global warming. Because of the importance of carbon for life and climate, much research effort has
been focused on understanding the global carbon
cycle and, in particular, the functioning of the ocean
carbon cycle. Biological and chemical processes in
the marine environment respond to and influence
climate by helping to regulate the concentration of
CO 2 in the atmosphere. We will discuss (1) the importance of the ocean to the global carbon cycle;
(2) the mechanisms of carbon exchange between the
ocean and atmosphere; (3) how carbon is redistributed throughout the ocean by ocean circulation; and
(4) the roles of the ‘solubility’, ‘biological,’ and
‘carbonate’ pumps in the ocean carbon cycle.
Global Carbon Cycle
The global carbon cycle describes the complex
transformations and fluxes of carbon between the
major components of the Earth system. Carbon is
stored in four major Earth reservoirs, including the
atmosphere, lithosphere, biosphere, and hydrosphere. Each reservoir contains a variety of organic
and inorganic carbon compounds ranging in
amounts. In addition, the exchange and storage times
for each carbon reservoir can vary from a few years
to millions of years. For example, the lithosphere
contains the largest amount of carbon (10
23 g C),
buried in sedimentary rocks in the form of carbonate
minerals (CaCO 3 , CaMgCO 3 , and FeCO 3 ) and organic compounds such as oil, natural gas, and coal
(fossil fuels). Carbon in the lithosphere is redistributed to other carbon reservoirs on timescales of
millions of years by slow geological processes such as
chemical weathering and sedimentation. Thus, the
lithosphere is considered to be a relatively inactive
component of the global carbon cycle (though the
fossil fuels are now being added to the biologically
active reservoirs at unnaturally high rates). The
Earth’s active carbon reservoirs contain approximately 43 Â 10
18 g of carbon, which is partitioned
between the atmosphere (750 Â 10
15 g C), the terrestrial biosphere (2190 Â 10
15 g C), and the ocean
(39 973 Â 10
15 g C; Figure 1). While the absolute sum
of carbon found in the active reservoirs is maintained
in near steady state by slow geological processes,
more rapid biogeochemical processes drive the redistribution of carbon among the active reservoirs.
Human activities, such as use of fossil fuels and
deforestation, have significantly altered the amount
of carbon stored in the atmosphere and perturbed the
fluxes of carbon between the atmosphere, the terrestrial biosphere, and the ocean. Since the emergence of the industrial age 200 years ago, the release
of CO 2 from fossil fuel use, cement manufacture, and
deforestation has increased the partial pressure of
atmospheric CO 2 from 280 ppm to present day
values of 360 ppm; an increase of 25% in the
last century (Figure 2). Currently, as a result of
human activities, approximately 5.5 Â 10
15 g of
495
C. A. Carlson, University of California, Santa Barbara,
CA, USA
N. R. Bates, Bermuda Biological Station for Research,
St George’s, Bermuda, USA
D. A. Hansell, University of Miami, Miami FL, USA
D. K. Steinberg, College of William and Mary,
Gloucester Pt, VA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Why is carbon an important element? Carbon has
several unique properties that make it an important
component of life, energy flow, and climate regulation. It is present on the Earth in many different
inorganic and organic forms. Importantly, it has the
ability to form complex, stable carbon compounds,
such as proteins and carbohydrates, which are the
fundamental building blocks of life. Photosynthesis
provides marine plants (phytoplankton) with an
ability to transform energy from sunlight, and inorganic carbon and nutrients dissolved in sea water,
into complex organic carbon materials. All organisms, including autotrophs and heterotrophs, then
catabolize these organic compounds to their inorganic constituents via respiration, yielding energy
for their metabolic requirements. Production, consumption, and transformation of these organic materials provide the energy to be transferred between
all the trophic states of the ocean ecosystem.
In its inorganic gaseous phases (carbon dioxide,
CO 2 ; methane, CH 4 ; carbon monoxide, CO), carbon
has important greenhouse properties that can influence climate. Greenhouse gases in the atmosphere act
to trap long-wave radiation escaping from Earth to
space. As a result, the Earth’s surface warms, an effect necessary to maintain liquid water and life on
Earth. Human activities have led to a rapid increase
in greenhouse gas concentrations, potentially impacting the world’s climate through the effects of
global warming. Because of the importance of carbon for life and climate, much research effort has
been focused on understanding the global carbon
cycle and, in particular, the functioning of the ocean
carbon cycle. Biological and chemical processes in
the marine environment respond to and influence
climate by helping to regulate the concentration of
CO 2 in the atmosphere. We will discuss (1) the importance of the ocean to the global carbon cycle;
(2) the mechanisms of carbon exchange between the
ocean and atmosphere; (3) how carbon is redistributed throughout the ocean by ocean circulation; and
(4) the roles of the ‘solubility’, ‘biological,’ and
‘carbonate’ pumps in the ocean carbon cycle.
Global Carbon Cycle
The global carbon cycle describes the complex
transformations and fluxes of carbon between the
major components of the Earth system. Carbon is
stored in four major Earth reservoirs, including the
atmosphere, lithosphere, biosphere, and hydrosphere. Each reservoir contains a variety of organic
and inorganic carbon compounds ranging in
amounts. In addition, the exchange and storage times
for each carbon reservoir can vary from a few years
to millions of years. For example, the lithosphere
contains the largest amount of carbon (10
23 g C),
buried in sedimentary rocks in the form of carbonate
minerals (CaCO 3 , CaMgCO 3 , and FeCO 3 ) and organic compounds such as oil, natural gas, and coal
(fossil fuels). Carbon in the lithosphere is redistributed to other carbon reservoirs on timescales of
millions of years by slow geological processes such as
chemical weathering and sedimentation. Thus, the
lithosphere is considered to be a relatively inactive
component of the global carbon cycle (though the
fossil fuels are now being added to the biologically
active reservoirs at unnaturally high rates). The
Earth’s active carbon reservoirs contain approximately 43 Â 10
18 g of carbon, which is partitioned
between the atmosphere (750 Â 10
15 g C), the terrestrial biosphere (2190 Â 10
15 g C), and the ocean
(39 973 Â 10
15 g C; Figure 1). While the absolute sum
of carbon found in the active reservoirs is maintained
in near steady state by slow geological processes,
more rapid biogeochemical processes drive the redistribution of carbon among the active reservoirs.
Human activities, such as use of fossil fuels and
deforestation, have significantly altered the amount
of carbon stored in the atmosphere and perturbed the
fluxes of carbon between the atmosphere, the terrestrial biosphere, and the ocean. Since the emergence of the industrial age 200 years ago, the release
of CO 2 from fossil fuel use, cement manufacture, and
deforestation has increased the partial pressure of
atmospheric CO 2 from 280 ppm to present day
values of 360 ppm; an increase of 25% in the
last century (Figure 2). Currently, as a result of
human activities, approximately 5.5 Â 10
15 g of
495