Ocean Chemistry & Acidification

Main Points

  • The oceans have absorbed about 30% of CO2 that have been put into the atmosphere since the Industrial Revolution.
  • When dissolved in seawater, CO2 creates bicarbonate ions, carbonate ions, carbonic acid (collectively called dissolved inorganic carbon), and hydrogen ions.
  • The oceans are able to absorb large amounts of CO2 from the atmosphere, but it takes 1,000 to 100,000 years for the entire ocean system to become buffered.
  • Rapidly increasing the amount of CO2 dissolved in the surface oceans leads to lower pH of seawater, or increased acidity, which is ocean acidification.
  • Ocean acidification is hurting marine life, especially those animals who make calcium carbonate skeletons and shells, as calcium carbonate dissolves in acidic waters.

Ocean Chemistry

The ocean covers approximately 70% of the Earth’s surface, and holds 97% of all water on our planet. There is one connected global ocean, but within it there are 5 recognized oceans: the Arctic, Atlantic, Indian, Pacific, Southern Oceans. Because the ocean is so large, it plays a crucial role in moderating climate over longer time periods.

The first important piece of information to understand about our oceans is the chemistry of seawater itself. If you’ve ever swam in the ocean, you’ve noticed the water is very salty. In addition to that dissolved salt, there are other dissolved ions (an atom or molecule with a positive or negative charge) and dissolved gases. Some of the most common ions in seawater are calcium (Ca2+), sodium (Na+), chloride (Cl), sulfate (So42-), magnesium (Mg2+), and potassium (K+). Some common dissolved gases in the ocean include nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). The ocean exchanges these gases with the atmosphere, most important of which is carbon dioxide, so that they remain almost in equilibrium. This means that the dissolved gases can leave the ocean and go into the atmosphere, and atmospheric gases can be absorbed by the ocean and dissolved into seawater. The rate of this exchange is almost constant, so the amount of gas in the atmosphere dissolving in the ocean, and the amount of gas in the ocean entering the atmosphere, is almost equal, which is called equilibrium. Since the Industrial Revolution, the oceans have absorbed approximately 30% of all CO2 released. What does this do, specifically, to our oceans?

When carbon dioxide is absorbed by the ocean, it is dissolved into the water. This dissolution of carbon dioxide can lead to the formation of three ‘species’ of carbon: bicarbonate ions (HCO3), carbonate ions (CO32-), and carbonic acid (H2CO3). Together, these three species are often referred to as dissolved inorganic carbon, or DIC.  In addition, free hydrogen (H+) ions are also produced. You have most likely come into contact with some or all of these DIC. Bicarbonate  combined with sodium makes baking soda; carbonate combined with calcium is chalk, limestone, and eggshells. Carbonic acid provides the fizz in carbonated drinks!

Now for some chemistry. Even if you haven’t taken a chemistry class before, no worries! We’ll break down the reaction to better explain what is going on when the ocean absorbs CO2 from the atmosphere:

Equation 1. Dissociation of carbon dioxide in seawater.

In the above equation, the => symbol means “yields”, or produces. The items on the left side of the => sign are called the reactants, and can be thought of as the chemicals that are being reacted together. The items on the right side of the => signs are called products, as they are what the reactants produce. It is important to notice in the above equation that when CO2 and H2O mix, they produce a free hydrogen ion in seawater.

The above reaction is taken further to produce even more free hydrogen ions when bicarbonate ions are dissociated. When bicarbonate ions are dissociated more in seawater, they produce two more free hydrogen ions and a carbonate ion:

Equation 2. Dissociation of bicarbonate ions.

In chemical reactions, if more reactants are added, more products are produced. If more products are put into the equation (for example, if more carbonate ions were dumped into seawater), then the chemical reaction would “shift” to the left and create more reactants (continuing the example, it would create more bicarbonate ions). In other words, the chemical reactions above can move both to the left and to the right depending upon how much reactants and how much products are in seawater. This is best represented by the reaction below, in which the <=> symbol indicates that the chemical equation can go in either direction:

Equation 3. Equations 1 and 2 combined.

Ocean Acidification and Buffering

The pH scale with examples of common substances and foods. The blue dashed line indicates where neutral pH is.

Now that we have laid out the chemical reactions that occur when carbon dioxide (CO2) is absorbed into seawater, we can talk about how this leads to the acidification of our oceans.

First, it’s important to understand what exactly acidification is. An acid is an ion or molecule that can donate a hydrogen ion (H+). In simpler terms, an acid is a substance that has a sour taste in aqueous solutions, and have a low pH. The pH scale, which ranges from 0-14, is a scale to measure the amount of hydrogen ion present in an aqueous solution (a solution that contains water), with lower numbers being very acidic, 7 neutral, and higher numbers very basic. Acids with a very low pH have the ability to burn or dissolve other materials. Thus, acidification is to make a solution become acid or acidic.

The acidity of any substance on the pH scale is measured by the amount of hydrogen ions in the solution, or the hydrogen ion concentration. pH is measured as the negative log of hydrogen ion concentration:

The bottom line here is: when there are more hydrogen ions (H+) in a solution, the more acidic it becomes.

Now, let’s refer back to our original equations at the top of the page. To remind you, these equations show what happens when CO2 is absorbed by the ocean from the atmosphere. Remember, gases that are put into the atmosphere are absorbed by the ocean. So the more CO2 that is put into the atmosphere, the more that is absorbed by the ocean.

Buffering Capacity of the Oceans

Notice that as CO2 is put into the atmosphere and absorbed into the oceans, this pushes Equations 1 and Equations 2 to the right, or towards the products, which creates more hydrogen ions in seawater. From the above pH equation, we know with more hydrogen ions (H+) in a solution, it becomes more acidic. But this isn’t as straightforward as it seems. As more CO2 is absorbed by the oceans, the dissolved inorganic carbon molecules buffer, or resists a decrease in pH, by the reaction in Equation 3 moving from the right to the left, instead of from the left to right. In other words, as more CO2 is added, this initially pushes Equation 3 to the left, which creates more carbonate ions (CO32-) and hydrogen ions (H+):

As more carbon dioxide is absorbed by the ocean, this pushes Equation 3 towards the right, which produces more reactants (carbonate and hydrogen ions).

As more CO2 is continued to be absorbed by the ocean, there will be too much carbonate ions (CO32-), so the acidity of the seawater (amount of hydrogen ions) is buffered by the equation by moving back to the left and creating more bicarbonate ions (HCO3).

If too much carbon dioxide is added to seawater, which creates too much carbonate ions, the reaction will shift back to the left (back towards the reactants) to buffer (uptake some of the free hydrogen ions) the solution. This buffering helps to keep the acidity of the seawater from dropping.

Buffering by Carbonate Dissolution

Recall from the previous page, ‘Ocean Layers and Mixing‘, that the bottom of the ocean contains carbonate shells of organisms, most of which come from planktic foraminifera. In the presence of acidic solutions, these calcite sediments, the carbonate shells, will dissolve. But remember, it takes a very long time for the surface ocean and deep ocean to mix (~1,000 years), so buffering by fossils on the seafloor is not instantaneous. By the below equation, calcite dissolves to produce a calcium ion and two bicarbonate ions:

Dissolution of calcite in seawater.

In the above equation, dissolution of calcite produces a calcium ion (Ca2+) and two bicarbonate ions (HCO3-). Recall from the above paragraphs that as more bicarbonate ions are added to seawater, this will drive Equation 3 further to the left to produce more carbonic acid, which will act to buffer seawater.

As calcite sediments on the seafloor dissolve with the addition of carbon dioxide, these produce more bicarbonate ions. The addition of bicarbonate ions will lead to buffering by moving to the left and producing more carbonic acid in seawater.

Thus, the buffering capacity of the ocean, or its ability to resist a change in pH, is actually quite large. In general, the ocean contains around 38,000 gigatons (or 38 billion tons) of bicarbonate, carbonate, and carbonic acid. This means that the entire ocean can absorb a lot of CO2 without becoming too acidic.

The Problem is Time

You may be asking yourself, ‘So, what’s the big deal?!‘. Let’s refer back to the previous page where we talked about the 3 main layers of the ocean and circulation. Remember that the atmosphere and the surface ocean exchange gases on a rate of 10-100 years, the surface ocean and deep ocean mix on a scale of about 1,000 years. The calcite sediments on the ocean floor are only important as buffers after the surface and deep ocean layers have mixed. Therefore, the timescale on which the bottom sediments can buffer the oceans is on a timescale between 1,000-100,000 years (Zeebe, 2012).

Thus, once anthropogenic carbon emissions cease, the entire ocean will eventually absorb the excess carbon dioxide, which will be neutralized. But, this will take at least 1,000 years. Therefore, the amount of carbon dioxide being released by humans today is in part a massive problem because the rate at which it is being released. Our oceans simply can’t keep up.

Chart showing increasing atmospheric carbon dioxide concentrations and falling pH in the surface ocean. The red line is the amount of CO2 in the atmosphere; the green data points are the partial pressure of CO2 in seawater; the blue data points are surface ocean pH measurements. The black lines in each data series represent the average trends through time. Notice that as atmospheric CO2 increases, pH in the surface oceans is rapidly declining (becoming more acidic). Data from Mauna Loa; ALOHA pCO2 and pH from Dore et al. (2009).

Scientists state that the oceans have absorbed about 30% of all CO2 that has been put into the atmosphere by humans since the Industrial Revolution. Scientists have concluded that this has led to a 26% increase in the acidity of our oceans. The rate at which the oceans are becoming more acidic is totally unprecedented, and research indicates that ocean acidification is happening faster today than at any time in the last 300 million years!

Marine Life and Acidification

A vast majority of marine organisms make their shells from calcium carbonate (CaCO3), or calcite. Many of these animals we are very familiar with, such as clams, oysters, snails, sea urchins, starfish and some plankton, like planktic and benthic foraminifera. Some really important animals that make and provide habitats for other animals, such as corals, also make their skeletons out of calcium carbonate.

Thus, as our oceans become more acidic, animals that build their skeletons and shells from calcite are becoming more stressed. It is becoming increasingly harder for these animals to build their skeletons and shells, and even animals that don’t utilize calcite for their skeletons are responding in a negative manner to ocean acidification. If you want to know more about how ocean acidification is harming marine life, check out these websites and articles below:

More resources on ocean acidification:

Proceed to ‘Ocean Circulation & Stratification’