Ocean - Atmospheric Exchange

Ocean - Atmospheric Exchange

Carbon dioxide can be dissolved in seawater, just as it can be dissolved in a can of soda. It can also be released from seawater, just as the CO₂ from soda can also be released. This transfer of gas back and forth between a liquid and the atmosphere is an extremely important process in the global carbon cycle, since the oceans are such an enormous reservoir with the potential to store and release significant quantities of CO₂.

The exchange of a gas like CO₂ between the air and seawater is governed by the differences in concentrations, as shown in the figure below, where the solid red line represents the concentration (increasing to the right) in the air and in the ocean. The dashed line indicates what the concentration might look like after some exchange of CO₂ occurs — lower concentration in the atmosphere, and higher in the oceans. But note that the change in concentration is less in the ocean than in the atmosphere, which is a result of some chemistry we will explore in just a bit.

Air-Sea CO₂ Exchange
Click here to see a text description.

The air-sea carbon exchange is depicted in this diagram of the air, stagnant seawater (a few microns thick), and mixed seawater. It shows how carbon dioxide moves from the air to the water. A text description says: Here, the atmosphere has a higher concentration (pCO2) than the ocean, so it will flow into the ocean; this will decrease the concentration in the atmosphere and raise it in the ocean. If the concentration gradient goes the other way–higher in the water than the air–the flow of CO2 goes up into the air.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

As depicted in the figure above, the concentrations in the air and the sea are relatively constant (spatially, not temporally) since these two media undergo rapid and turbulent mixing that would tend to even out any systematic variations. The exception to this is a thin layer of water, just 20 to 40 microns thick (a micron is one thousandth of a millimeter), which, because of the surface tension of the water, is unable to mix well. This stagnant film is the barrier across which the diffusion has to occur. The rate of gas transfer is a function of the concentration difference and the thickness of the stagnant film of water (which thins when the winds are strong).

In general, this flow depends on the concentration of CO₂ in the atmosphere and the oceans, but it gets a little complicated because the concentration of CO₂ in seawater depends on a number of other factors. To understand this process, and to see why the atmosphere's concentration changes more than the ocean, we need to have some sense of what happens to CO₂ once it gets dissolved in seawater.

Carbonate Chemistry of Seawater

When CO₂ from the atmosphere comes into contact with seawater, it can become dissolved into the water where it undergoes chemical reactions to form a series of products, as described in the following:

1. CO₂ (dissolved gas) + H₂O <=> H₂CO₃ (carbonic acid)
2. H₂CO₃ (carbonic acid) <=> H+ + HCO₃^- (hydrogen ion + bicarbonate)
3. HCO₃- <=> H⁺ + CO₃^-2 (hydrogen ion + carbonate)

The amount of CO₂ as dissolved gas is what controls the concentration of CO₂ shown in the figure above. This concentration depends strongly on the temperature — it is low in cold water and it is high in warm water, which means that the colder parts of the ocean absorb CO₂ from the atmosphere and the warm parts of the ocean release CO₂ into the atmosphere.

All of these reactions mean that in seawater, you can find all of these different forms or species of inorganic carbon co-existing, dissolved in seawater. In reality, though, bicarbonate (HCO₃^-) is the dominant form of inorganic carbon; carbonate (CO₃^2-) and dissolved CO₂ are important, but secondary (see figure below).

Relationship between carbon species and pH
Click here to see a text description.

This graph shows the relationship between carbon species and pH. A text description in the image says: The relative proportions of the different carbon species depends on the alkalinity and the total DIC; these proportions in turn determine the pH. Salinity and temperature also exert some control on the proportions of carbon species. In the oceans today, most of the carbon is in the form of HCO3-, with relatively small amounts of CO3 (2-) and dissolved CO2.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

In the above equations (1-3), the double-headed arrows mean that the reactions can go in both directions, and generally do, until some balance of the different compounds is achieved -- a chemical equilibrium.

Notice that in the equations above, the hydrogen ion appears in a number of places — the concentration of H+ in seawater is what determines the pH of the water, or in other words, its acidity. Remember that low pH means more acidic conditions. It turns out that the ratio of bicarbonate (HCO₃) to carbonate (CO₃) is proportional to the pH; at lower pH conditions, more of the carbon is in the form of bicarbonate than carbonate, and the result is that the fraction of dissolved CO₂ gas also goes up, and this tends to cause CO₂ gas to move from the seawater into the atmosphere. As we will see in Module 7, higher acidity (lower pH) also has important implications for organisms that live in the sea.

Obviously, the ratio of these two forms of carbon is important, so we need to ask what controls this. Without getting into the chemistry too much, the ratio depends on two main things, and it depends on these things because they determine the electrical charge balance in the water — the charges from all the positive and negative ions dissolved in the water have to add up to zero, and you can see that if you have more carbonate (minus 2 charge) than bicarbonate (minus 1 charge), you have more total negative charge. So, the 2 important factors here are:

1. the total amount of positive charge in the oceans that has to be canceled by the carbonate and bicarbonate — called the alkalinity;
2. the total amount of dissolved inorganic carbon in the oceans (DIC for short).

If the alkalinity is high, then more of the DIC has to take the form of carbonate (minus 2 charge), and a result is that the pH is higher, meaning less acidic. If the total DIC is large, then to get the charges to balance, more of the DIC has to be in the form of bicarbonate (minus 1 charge), which leads to lower pH and more acidic conditions.

Let’s return now to the figure that showed the exchange of CO₂ between the atmosphere and ocean and the question about why the ocean’s CO₂ concentration changed less than the atmosphere. The reason for this is the carbon chemistry reactions that shift some of the CO₂ dissolved gas into the forms of bicarbonate and carbonate — this reduces the amount of carbon in the form of dissolved CO₂ gas, so the concentration of dissolved CO₂ does not increase as much as it would if these reactions did not take place.

As you can see, the chemistry of carbon in seawater is relatively complex, but it turns out to be extremely important in governing the way the global carbon cycle operates and explains why the oceans can swallow up so much atmospheric CO₂ without having their own CO₂ concentrations rise very much.

Summary of Carbonate Chemistry

Let's see if we can summarize this carbonate chemistry — it is important to have a good grasp of this if we are to understand how the global carbon cycle works.

• Carbon can exist in three main inorganic forms in seawater — CO₂, HCO₃^-, and CO₃^2-, and there is a rapidly-achieved equilibrium between these species.
• The ratio of HCO₃^- to CO₃^2- along with the water temperature determines the CO2 concentration of seawater and also the pH.
• The temperature of the water also controls how much CO₂ occurs in the form of dissolved gas, thus affecting the concentration of CO₂ gas in seawater.
• The alkalinity of seawater represents the positively-charged ions that need to be countered by negatively-charged carbonate and bicarbonate ions.
• The concentration of the total dissolved inorganic carbon (DIC), along with the alkalinity, determines the ratio of HCO₃^- to CO₃^2-, and thus the CO₂ concentration of seawater. If we increase DIC without changing the alkalinity, then more carbon must be in the form of HCO₃^-, which increases both pH and the CO₂ concentration of seawater.
• The CO₂ concentration of seawater, relative to the atmospheric CO₂, determines whether the oceans absorb or release CO₂. Currently, the cold parts of the oceans absorb atmospheric CO₂ and the warm regions of the oceans add CO₂ to the atmosphere.
• The ability of carbon to switch back and forth between these three forms means that only a portion of the CO₂ absorbed by the oceans will remain as CO₂.

In the real world, there are important variations in the gas transfer between the ocean and atmosphere. This can be seen in the figure below, which represents a kind of snapshot of this transfer across the globe. The units here are grams of C per m² per year, and each box is about 1e6 m². The red, orange, and yellow colors represent places where the oceans are giving up CO₂ to the atmosphere; the blue and purple areas are places where the oceans are sucking up atmospheric CO₂. Summing these up, we find that the oceans are taking up around 92-93 Gt C/yr and they are releasing about 90 Gt C/yr — for a net flow of 2-3 Gt C/yr into the oceans — this represents something like 25-30% of the carbon we are adding to the atmosphere by burning fossil fuels. This exchange is variable in space and time, but a few general features can be pointed out. In general, the colder parts of the oceans absorb CO₂ and the warmer parts release CO₂ into the atmosphere. This makes sense because CO₂ is more soluble in colder water.

Flux of CO₂ across the Air-Sea Interface in 2000

Credit: Columbia University Lamont-Doherty Earth Observatory