Carbon Cycle: Got it!!

Introduction

The ocean plays a vital dominant role in the Earth’s carbon cycle. The total amount of carbon in the ocean is about 50 times greater than the amount in the atmosphere, and is exchanged with the atmosphere on a time-scale of several hundred years. At least 1/2 of the oxygen we breathe comes from the photosynthesis of marine plants. Currently, 48% of the carbon emitted to the atmosphere by fossil fuel burning is sequestered into the ocean. But the future fate of this important carbon sink is quite uncertain because of potential climate change impacts on ocean circulation, biogeochemical cycling, and ecosystem dynamics.

Carbon atoms are constantly being cycled through the earth’s ocean by a number of physical and biological processes. The flux of carbon dioxide between the atmosphere and the ocean is a function of surface mixing (related to wind speed) and the difference the concentration of carbon dioxide in the air and water The concentration in the ocean depends on the atmosphere and ocean carbon dioxide partial pressure which, in turn, is a function of temperature, alkalinity (which is closely related to salinity), photosynthesis, and respiration. Carbon is also sequestered for long periods of time in carbon reservoirs (sinks) such as deep ocean and ocean sediment.^*

The oceans contain around 36,000 gigatonnes of carbon, mostly in the form of bicarbonate
ion. Inorganic carbon, that is carbon compounds with no carbon-carbon or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important in controlling pH in the ocean and can also vary as a source or sink for carbon. Carbon is readily exchanged between the atmosphere and ocean. In regions of oceanic upwelling, carbon is released to the atmosphere. Conversely, regions of down welling transfer carbon (CO₂) from the atmosphere to the ocean. When CO₂ enters the ocean, carbonic acid is formed:

CO₂ + H₂O → H₂CO₃

This reaction has equal forward and reverse rate, so it achieves a chemical equilibrium. Another reaction important in controlling oceanic pH levels is the release of hydrogen ions and bicarbonate. This reaction controls large changes in pH:

H₂CO₃ → H⁺ + HCO₃⁻

In the oceans, bicarbonate can combine with calcium to form limestone (calcium carbonate, CaCO₃, with silica), which precipitates to the ocean floor. Limestone is the largest reservoir of carbon in the carbon cycle. The calcium comes from the weathering of calcium-silicate rocks, which causes the silicon in the rocks to combine with oxygen to form sand or quartz (silicon dioxide), leaving calcium ions available to form limestone.^**

The carbon budget

The carbon budget is based on nine major carbon deposits or natural-exchange reservoirs (table 1). There are several pertinent comments about these.

The estimates given here are not necessary the best available, but they are consistent with most estimates and do not differ than a few percent.
The estimates for the carbon content of humus and the land or marine biosphere (0.19, 0.06, 0.002 g/cm², respectively) have not been included because these values are less precisely known.
It is useful to divide the sea into two reservoirs (item 2); one is an upper layer 75-100m deep which interacts and exchanges very slowly with the second lower layer. Some authors prefer a multi-reservoir model of the sea.

Table-1 Carbon content of Natural Deposits.

Carbon Reservoir	Carbon Content g/cm² of Earth’s Surface
Atmosphere	0.125
Oceanic
Dissolve organic carbon	0.533
Inorganic carbon (above thermocline)	0.20
Inorganic carbon (below themocline)	7.25
Plants	0.053
Animals	0.00071
Sediments, as elements C	533.00
Carbonates and sediments as CO₃^2- -C	2,340.00
Crystalline slate (total C)	1,960.00
Palingenic igneous rocks (total C)	567.00
Juvenile rocks	33.00
Total	5,541.00

Most of the carbon is stored as carbonate rock and as fossil fuel; comparatively little carbon is stored in oceanic or atmospheric reservoirs. Presumably, however, much of the carbon in the major carbon reservoirs must have been present in the atmosphere at some time (Martin, D.F. 1970).

The carbon cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, geosphere, hydrosphere, and atmosphere of the Earth.^*

The cycle is usually thought of as four major reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, the terrestrial biosphere (which usually includes freshwater systems and non-living organic material, such as soil carbon), the oceans (which includes dissolved inorganic carbon and living and non-living marine biota), and the sediments (which includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth, but the deep ocean part of this pool does not rapidly exchange with the atmosphere.^*

The Oceanic Part of the Carbon Cycle

To understand the fate of CO₂ in the atmosphere, we must understand earth’s carbon cycle because atmospheric CO₂ is only one part of the cycle.

Several important oceanic processes influence the cycle. The figure above indicates that:

The ocean stores 50 times more carbon dioxide than does the atmosphere;
Much more carbon flows through the ocean than the amount produced by burning fossil fuels;
An amount of carbon equal to the total amount stored in the atmosphere cycles through the ocean in about eight years [(750 GT) / (92 GT per year) = 8.3 years]; and
The flux in and out of the ocean is larger than the flux in and out of the land.

The carbon cycle in the ocean has two main parts, a physical part due to CO₂ dissolving into sea water, and a biological part due to phytoplankton converting CO₂ into carbohydrates.

Figure: Carbon Cycle

(Source: http://www.physicalgeography/biosphere.net.html)

Carbon dioxide dissolves into cold ocean water at high latitudes. CO₂ is carried to the deep ocean by sinking currents, where it stays for hundreds of years. Eventually mixing brings the water back to the surface. The ocean emits carbon dioxide into the tropical atmosphere. This system of deep ocean currents is the marine physical pump for carbon. It helps pumps carbon from the atmosphere into the sea for storage.

Global map of the average annual exchange CO2 flux (mol-C m-2 a-1) across the sea surface.

From Ocean Biogeochemistry and Global Change published by the International Geosphere Biosphere Program.

Phytoplankton in the ocean use CO₂, sunlight, water, and nutrients and produce carbohydrates and oxygen. Animals eat the phytoplankton contributing to the oceanic food web leading to fish. Organic material sinks when phytoplankton and animals die, carrying some reduced carbon to the sea floor (Reduced carbon is carbon that can be oxidized to yield energy, water, and CO₂.) A small fraction of the reduced carbon (0.4%) is eventually buried and stored in sediments for millions of years (Middelburg et al, 2007). But most of the reduced carbon in and below the sea floor is used by animals and bacteria, and returned to the ocean part of the carbon cycle. This is the marine biological pump for carbon. It too pumps carbon from the atmosphere into the sea for storage.

Oceanic Phytoplankton

Most of the primary production in the ocean is by single-celled microscopic organisms. The organisms include:

The Chromista, including Coccolithophorids, and Diatoms,
Dinoflagellates. And,
Photosynthetic bacteria and archaea.^***

Plankton influence the exchange of gases between the atmosphere and the sea.

In any given region, the relative amounts of CO2 contained in the atmosphere and dissolved in the ocean’s surface layer determine whether the ocean-water emits or absorbs gas. The amount of gas dissolved in the water is in turn influenced by the amount of phytoplankton (microscopic plants, particularly algae), which consume CO2 during photosynthesis. Phytoplankton activity occurs mostly within the first 50 metres of the surface and, although oceanographers don’t fully understand why, varies widely according to the season and location. Some areas of the ocean do not receive enough light or are too cold. Other areas appear to lack the nutrients or trace minerals required for life, or zooplankton (microscopic animals) that feed on phytoplankton so limit the population growth of the latter that not all of the available nutrients are consumed. (Mann, et al 1991).

Global map of the primary productivity by oceanic phytoplankton.

From the International Geosphere Biosphere Program.

The storage of reduced carbon in oceanic sediments in sediments maintains the oxygen content of the atmosphere. If no reduced carbon were stored in sediments, atmospheric oxygen would be used up in about 15 million years.

It’s a popular misconception that the concentration of oxygen in Earth’s atmosphere is controlled by photosynthesis. Photosynthesis is certainly the source of atmospheric oxygen, but the amount it produces is in almost perfect balance with the amount consumed through the respiration of living organisms. It is only when organic matter is buried in ocean sediments, and so ceases to be decomposed, that atmospheric oxygen can accumulate. This burial process also reduces the levels of the greenhouse gas carbon dioxide released into the atmosphere. The exact rate of organic-matter burial is therefore a significant determinant of atmospheric composition, and thus global climate, over geological timescales (Masiello (2007).
Animals in the ocean use carbohydrates and oxygen and emit CO₂. Plants respires CO₂ during the night. As a result, all the oxygen produced by phytoplankton is used to convert to reduce carbon into carbon dioxide except for the small amount of reduced carbon stored in sediments.

Conclusion

Of all the carbon dioxide (CO2) emitted into the atmosphere, one quarter is taken up by land plants, another quarter by the oceans. Understanding these natural mechanisms is important in forecasting the rise of atmospheric CO2 because even though plants and bodies of water now absorb surplus greenhouse gas, they could become new trouble spots. The ocean absorbs CO2 from the atmosphere in an attempt to reach equilibrium by direct air-to-sea exchange. This process takes place at an extremely low rate, measured in hundreds to thousands of years. Thus burning of fossil fuels is a source of CO₂ and the ocean is a sink of CO₂. Recently, people started burning fossil fuels, which released, in the form of CO₂, the carbon produced by plants and stored as reduced carbon (now in the form of coal, oil, and gas) in sediments millions of years ago.