BIO 320 Carbon Sink

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Courtney Peloso

Biology 320

Dr. StabileJune 18, 2008

The Global Carbon Cycle: Oceanic Uptake

Earths climate relies heavily upon the levels of greenhouse gases, including

carbon dioxide, in the atmosphere. These greenhouse gases capture radiation, which

would otherwise be released, and cause it to remain contained in the atmosphere. With

the continuous growth of industrialization, humans bear much responsibility for the

increase in the concentration of carbon dioxide in the atmosphere, which is calculated at

about 380 parts per million, up from 280 ppm in the mid-19th century1 The trapping of

radiation by greenhouse gases causes temperature to increase, leading to the melting of

ice in the polar regions, resulting in a subsequent rise in sea level, and leading to a

multitude of issues related to climate change. Because of carbons role in greenhouse gas

emissions, the global carbon cycle has become a key subject of interest for those

investigating the implications of climate change and possible ways to alleviate and deal

with these issues. The worlds oceans are of particular importance, for they function as a

key component of the carbon cycle. Much research is currently underway involving the

vital role the oceans play in determining carbon dioxide levels in the atmosphere.

Researchers hope that by obtaining greater insight into the oceans role in the carbon

cycle, they will be able to determine how it will respond to even greater increases in

carbon dioxide atmospheric concentration, as well as formulate possible ways to use the

oceans to keep this concentration in check.

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The global carbon cycle functions as a system of carbon exchanges between the

atmosphere, hydrosphere, biosphere, and geosphere. Some elements of these spheres are

sources of carbon, while others operate as sinks, or storage reservoirs for the carbon

produced by the sources. The major sources for carbon include the burning of fossil fuels,

the production of cement, and the burning and clearing of forests for agricultural and

architectural purposes. On the other side of the equation lie the carbon sinks, which

include atmospheric storage, oceanic uptake, and forest regrowth, as well as an

undetermined factor known as the missing sink. Natural processes of photosynthesis

and respiration are also involved, but without the excess carbon emissions due to

manmade causes, they are close to equilibrium.2 Oceanic uptake is an extremely

important part of this equation, currently removing about 30% of the carbon released into

the atmosphere by the burning of fossil fuels.3 The deep ocean is the greatest reservoir of

carbon, containing about 40,000 gigatons, but the exchange occurs over several hundred

years. The upper layers of the ocean, on the other hand, store much less carbon, at about

1,000 gigatons, but the exchange with the atmosphere, biosphere, and geosphere occurs

much more quickly.4

Carbon dioxide in the atmosphere is taken up by the ocean, going into solution

and becoming dissolved carbon dioxide. Some of this dissolved carbon dioxide combines

with water to form carbonic acid, which rapidly dissociates into a bicarbonate ion and a

hydrogen ion. Bicarbonate ions account for 90% of the carbon dissolved in seawater, but

some undergo a second ionization, becoming carbonate and a second hydrogen ion.

However, not all the dissolved carbon dioxide taken up from the atmosphere proceeds

2 www.metoffice.gov.uk/research/hadleycentre3 Ocean Carbon and Climate Change: An Implementation Strategy for US Ocean Carbon Research4 www.metoffice.gov.uk/research/hadleycentre


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through this series of chemical reactions, but is, instead used in the process of

photosynthesis by the microscopic phytoplankton that exist in the surface layers of the

ocean. Many of these phytoplankton also take up carbonate ions, which they use in

combination with calcium to form calcium carbonate shells. Upon their death, the

calcium carbonate shells are decomposed, returning the carbon to the water, most of

which sinks to the deeper waters of the ocean. By using carbon in their life processes,

phytoplankton are vital to the carbon cycle; by removing carbon dioxide and carbonate

ions from the water, they thus create more space in which atmospheric carbon dioxide

can be taken up by the oceans and dissolved.

5

The movement of carbon within the oceans themselves is controlled by a system

of downwelling and upwelling. These processes, which are intertwined with the use of

carbon by phytoplankton, function in the transfer of carbon between deep and surface

layers. With the cooling and increased salinity of surface waters near the polar regions,

density of the water increases, and consequently, it sinks below to deeper levels of the

water column. This downwelling causes the movement of carbon from the surface to the

depths of the ocean. However, an opposite process occurs, called upwelling, which

occurs at areas of current divergence, such as the equatorial region. This allows for the

upward transfer of carbon as deep water rises, as well as a mix of nutrients, such as

nitrates and phosphates, which are now available for use in the growth of phytoplankton

and various other forms of marine biota.

Because phytoplankton are so vital to the exchange of carbon between organic

and inorganic forms, accounting for close to half of the entire biospheres net primary

5 www.carleton.edu/departments/geol


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productivity6 and thus establishing the basis of a global food chain, investigating the

impact of climate change on their growth and prosperity is highly important. Research

determining the productivity levels of these phytoplankton can aid in predicting how they

will behave under changes in climate and amount of carbon dioxide. Currently, research

on this topic is being conducted in order to find out whether there are any promising ways

to use these phytoplankton in reducing the concentration of carbon dioxide in the

atmosphere by increasing the levels they take up.

The GlobColour Project, organized by the European Space Agency, was

established in 2005, and as its name implies, compiles satellite data concerning the colors

of the ocean.7 Color sensors on the three satellites, which are MERIS on ESAs Envisat,

MODIS on NASAs Aqua, and SeaWiFS on GeoEyes Orbview-2, detect concentrations

of phytoplankton in the oceans. The chlorophyll contained in the microscopic plankton

accounts for the colors picked up by the sensors. The combination of data collected from

these different sources ensures its accuracy, and provides a 10 year sample that can be

used by researchers to determine the productivity and growth of phytoplankton, and

predict future levels of the marine organisms. This data is especially helpful and

important for carbon cycle researchers, such as those of the Integrated Global Carbon

Observation (IGCO) and the Global Carbon Project, which involve international efforts.

Continued data collection by the GlobColour Project will be extremely helpful in

monitoring changes in phytoplankton concentrations, which could in turn have a

profound impact on the ability of the ocean to serve as a carbon sink. Modeling based on

6Climate-Driven Trends in Contemporary Ocean Productivity7ESA Contributes to Ocean Carbon Cycle Research


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this data can help researchers determine how phytoplankton will behave under even

greater carbon dioxide concentrations due to continued industrial emissions.8

Looking at the data collection of the SeaWiFS satellite, which was the first able to

take measurements at night, evidence of changing chlorophyll levels with climate change

can be seen. These chlorophyll levels function as the gage used to measure primary

productivity in the photic zone.9 The results and observations of the data collected by the

SeaWiFS show a correlation between increased atmospheric carbon dioxide levels and

increased sea surface temperatures. With the warming of the upper level of the water

column, density disparity between that and subsurface layers increases leading to greater

stratification. In turn, less nutrient mixing occurs, and less primary productivity can be

seen.10With the decrease in productivity of the phytoplankton, they take up less carbon

dioxide from the air, and thus perpetuate global warming. This cycle of changes implied

by the primary productivity data collection shows that with increased carbon emissions,

atmospheric and ocean temperatures will rise, which is likely to decrease productivity in

the oceans. This decrease in the ability of phytoplankton to operate as a carbon sink is

likely to then increase the concentration of atmospheric carbon dioxide even further.

Because of this startling, and seemingly unending downward spiral of oceanic carbon

uptake, which is likely to disrupt oceanic and terrestrial ecosystems, further research into

developing mechanisms to deal with increased carbon emissions is currently being

undergone and will certainly continue into the future.

Increased levels of emission of carbon dioxide and other greenhouse gases also

cause ozone depletion, which, on the basis of a four-year international study of the

8 www.globcolour.info/data_access.html9Climate-driven Trends in Contemporary Ocean Productivity10Climate-driven Trends in Contemporary Ocean Productivity


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Southern Ocean, can be damaging to the oceans role as a carbon sink. Climate change in

the form of increased winds over the Southern Ocean due to ozone depletion, along with

the buildup of greenhouse gases, has caused this ocean, which functions as the primary

oceanic carbon sink, to reach its saturation point. This study, undertaken in part by the

National Institute of Water and Atmospheric Research, concerned measuring atmospheric

carbon dioxide over the Southern Ocean, which has shown that the carbon sink has

weakened. The study found that with rising carbon dioxide concentration accelerating to

an increase of about 2 ppm per year, since the start of the 20 th century, the Southern

Ocean can no longer accept any excess carbon dioxide. The processes of carbon

exchange still continue, with the saturation point meaning that, in its current conditions,

the Southern Ocean will not be able to handle an increase in carbon concentration, which

will cause more carbon dioxide to build up in the atmosphere. This continued increase in

concentration will likely cause even greater climate change. Again, a cycle of disruption

can be seen here. Not only will the climate change resulting from increased carbon

emissions cause a decrease in the primary productivity, but it will also cause the carbon

saturation of the oceans, weakening their ability to accept excess atmospheric carbon.11

Some of the research and experimental programs to develop techniques to

alleviate the concentration of carbon dioxide in the atmosphere by increasing oceanic

uptake surround what is known as the iron hypothesis. This hypothesis focuses on the

key role iron plays in the carbon cycle and its function as the limiting nutrient on primary

productivity. Because iron is a necessary component for chlorophyll synthesis and the

synthesis of organic compounds by its role in the reduction of carbon dioxide, a link can

be seen between it and the uptake of carbon by oceanic phytoplankton. This important

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element thus bears influence on primary production rates by these phytoplankton in areas

such as the Southern Ocean previously discussed. The iron hypothesis proposes that

iron, in fact, is responsible for controlling the levels of carbon dioxide in the atmosphere,

and therefore controls climate change. Based on this, some researchers have proposed the

fertilization of oceans with iron, making greater quantities available to phytoplankton, in

order to enhance carbon dioxide uptake.12 Small scale experiments with artificial iron

fertilization have confirmed that iron does have a vital role in productivity by ocean

plankton, but have not shown to aid in the transfer of large quantities of carbon to the

deep ocean reservoirs, which is the main goal of increasing oceanic uptake. The short

time scale of these experiments also presents a limitation, in that it is difficult to project

the overall viability of iron fertilization as a way to alleviate carbon excess and climate

change, which occurs on a longer time scale.

One recent study involving iron as the limiting nutrient in the primary

productivity of the oceans involved the study of the Kerguelen plateau phytoplankton

bloom in the Southern Ocean. This bloom is the result of natural iron fertilization, and its

study by the Kerguelen Ocean and Plateau Compared Study (KEOPS) found that this

made a difference in making the sequestration of carbon more efficient. The ratio of

carbon export to the amount of iron supplied was calculated at about ten times higher

than that of smaller-scale artificial experiments. This difference is likely due to the slow

and continuous addition of dissolved iron in the natural bloom, versus the rapid addition

of large amounts added in the experiment. In this rapid addition, much iron is lost to the

deeper waters and not taken into use by the phytoplankton. Therefore, while the

Kerguelen bloom does show the importance of iron fertilization to the carbon cycle, it

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does not provide support for enacting the process artificially to alleviate atmospheric

carbon concentration. Further investigation and experimentation on a longer time scale is

necessary to determine the merit of such proposals.13

Overall, with the continued increase in carbon dioxide emissions into the

atmosphere, widespread climate change has already begun and will continue to occur

without innovation in the development of techniques to alleviate and decrease its

concentration. The oceans, in their role as an important carbon sink, may hold the key to

finding the solution. Climate change due to unchecked carbon emission will also lead to

the possibly irreversible damage of the valuable ecosystems within the ocean. Further

research and study of the carbon cycle and the oceans role within it is imperative for the

maintenance of the oceans themselves, as well as the rest of the Earths systems and

structures.

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Works Cited

Behrenfeld, Michael J., et al. Climate-driven trends in contemporary ocean

productivity. Nature 444 (Dec. 2006). Science Reference Center. EBSCO. Iona

Coll. Lib., New Rochelle, NY. 19 June 2008 .

Blain, Stephanie, Bernard Queguiner, and Bruno Bombled. Effect of natural iron

fertilization on carbon sequestration in the Southern Ocean. Nature 446 (Apr.

2007). Science Reference Center. EBSCO. Iona Coll. Lib., New Rochelle, NY. 19

June 2008 .

Climate change weakens carbon sink. ScienceAlert 17 May 2007. 19 June 2008

.

Doney, Scott C. Ocean Carbon and Climate Change: An Implementation Strategy for

U.S. Ocean Carbon Research. US Carbon Cycle Science Scientific Steering

Group and Inter-Agency Working Group. Carbon Cycle Science Ocean Interim

Implementation Group. 19 June 2008 .

ESA Contributes to Ocean Carbon Cycle Research. European Space Agency 5 May

2008. 19 June 2008 .

Geology. Dept. home page. Carleton College. 19 June 2008 .


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GlobColour Project. European Space Agency. 19 June 2008

.

Houghton, Richard. Understanding the Global Carbon Cycle. The Woods Hole

Research Center. 19 June 2008 .

ODonnell, Erin. Not Easy Being Green: Climate Change Solutions? Harvard

Magazine May-June 2008. 19 June 2008 .

Street, JH, and A Paytan. Iron, phytoplankton growth, and the carbon cycle. Metal Ions

in Biological Systems 43 (2005): 153-93. Abstract. Science Reference Center.

EBSCO. 19 June 2008 .

United Kingdom. Met Office. Met Office Hadley Centre: The Carbon Cycle. 19 June

2008 .

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BIO 320 Carbon Sink