What can happen when the oceans become more acidic
Climate change has many impacts on our planet. Some of the ones you might hear about in the news include sea-level rise caused by melting ice and ocean heating, increases in wildfires because of long-term droughts, changes in extreme weather events due to variations in atmospheric heating, and acidification of the oceans as they absorb carbon dioxide from the atmosphere. Here I will discuss some of the mineral science research that I’m doing with other scientists, and follow that up with a more general discussion of ocean acidification.
Mineral stability in today’s changing ocean
Water, living organisms, and minerals formed organically by living things and inorganically through other natural processes are all parts of our oceans. Many atoms and molecules are dissolved in seawater, including sodium (Na) and chlorine (Cl) that combine to form the mineral halite (table salt), and others like calcium (Ca) and magnesium (Mg). Animals such as oysters, corals, and microscopic pteropods take some of these dissolved seawater components to build shells and other skeletal structures out of minerals like calcite and aragonite (see minerals section below). The availability of the chemical ingredients to build shells is related to the pH of seawater. When the pH of seawater lowers (that is, becomes more acidic), animals struggle to build new shells and existing shells can even dissolve.
We can look at the stability of minerals in the oceans to learn more about the changing acidity of seawater through time. One way to measure ocean acidity is to take measurements of seawater with a pH meter or pH strip. Another way is to figure out how fast ocean minerals are dissolving. At one extreme, if the oceans were acidic (pH<7), then common shell-forming minerals like aragonite and calcite would not be stable and would dissolve before they got down to the seafloor. In other words, in very acidic oceans, there will be no record of the carbonate minerals in the rocks because they wouldn’t make it to the bottom the sea floor to be lithified to stone. Though the oceans are trending towards lower pH values, seawater is still more basic than acidic (but this might not always have been the case in Earth’s history).
A team of scientists, including Will Berelson and Sijia Dong from the University of Southern California, Jess Adkins from Caltech, and me at the Natural History Museum, collaborated on a project to measure the concentration of carbonate minerals in the Pacific Ocean today, and to determine how fast these minerals dissolve in seawater and in experimental conditions in a lab. This work was published in Earth and Planetary Science Letters. My main contribution to this project is the direct measurement of the mineralogy, the ratios of calcite and aragonite in the ocean water and sediment, and not just the total amount of dissolved carbon, as has been more commonly done in the past.
The C DisK (Carbon Dissolution Kinetics) project is a team of scientists investigating the rates (i.e. kinetics) that calcite and aragonite dissolve in the ocean.
This poster of the overall process of sampling from the ocean. In the summer of 2017, scientists and crew members set sail for Alaska from Hawaii.
On board the ship were instruments to take mineral and water samples from the ocean. A lot of pteropods were seen and collected during the cruise of which their shells are made out of the mineral aragonite.
The pumps are custom made to filter huge amounts of seawater at various depths in order to get a mostly representative sample of what the calcite to aragonite ratios as you go from the surface to the bottom of the ocean.
Sediment was collected to compare how much calcite and aragonite make it to the bottom of the seafloor.
One important result of this work is the development of a cross-sectional map (below) showing how quickly aragonite can be dissolved in different parts of the ocean. Aragonite is the least stable carbonate mineral, and it is also the mineral that makes up many of the solid parts of shelled organisms. This map was made from the direct observations of marine mineralogy, lab-based dissolving rates of aragonite and calcite, and the experiential studies of how quickly carbonate minerals dissolved at sea where we collected all the other data. Near Hawaii (left side of figure) there is a great deal of depth where aragonite is stable, meaning that these warmer water waters are less acidic, likely because less CO2 (carbon dioxide) can be dissolved in warm water.
As you move toward Alaska (to the right in the above figure), there is a pretty big jump toward the shallowing of aragonite stability, meaning that aragonite is not stable below 200 meters depth as the waters are more acidic. The effect could be from the Northern Pacific Gyre (where the North Pacific water is separated from the Central Pacific), and also temperature gradients. What is interesting is that this work represents the first study to measure marine mineralogy, marine dissolution experiments at sea, and how they correlate to previous models people have proposed. This work will ultimately lead to better predictions of how carbon behaves in the oceans.
So what is ocean acidification?
Carbon dioxide (CO2) exists as a gas in Earth’s atmosphere. Currently, the concentration of CO2 in our atmosphere is small, less than 0.05%, but at times in Earth’s past, it is thought to have been much higher. When CO2 mixes with water, for example at the interface between the surface of the ocean and the bottom of the atmosphere, chemical reactions lead to the production of carbonic acid. These reactions are common and have been happening on Earth since the oceansfirst formed. The exchange of CO2 between the air in our atmosphere and water in our hydrosphere allows these two reservoirs to balance each other in an attempt to stay in equilibrium. For example, extremely large volcanic eruption events will increase CO2 in the air, so the ocean will absorb some of this added CO2 to correct the imbalance in the atmospheric reservoir. When the oceans absorb CO2, they become increasingly acidic.
At the moment, the oceans have a pH of about 8.1, which is slightly on the basic side of the pH scale. A pH of 7.0 is a completely neutral water (neither acidic nor basic); a pH below 7.0 is acidic. Around 150 years ago, the average ocean pH was 8.3, so the oceans are moving toward being more acidic. We call this an acidification process because the trend of ocean pH change is moving toward lower pH values.
When the ocean pH changes, marine organisms that grow hard parts like shells are impacted. The organisms become stressed and have to put more energy into shell production rather than feeding, photosynthesis, or reproduction.
While ocean acidity changes have been happening for billions of years, for most of Earth’s history these changes were gradual and subtle. Organisms could adapt when Earth’s ocean pH shifted slowly over thousands to millions of years. As more CO2 gets into the seawater, then the amount of carbonate available for shell formation is depleted as it reacts to form carbonic acid, and this can harm marine organisms in the short term.
Lower pH values can be good in some situations, like in your stomach: your body pumps out acid to dissolve the food you eat so that it can provide your body with the nutrients it requires. Or it can be bad: like when hydrofluoric acid (used in glass etching and other processes) dissolves the mineral apatite, which is the mineral that makes up your bones. If the pH of our blood changes just a little from its tightly regulated value of 7.4 (bad things happen when it’s lower than 7.35 or higher than 7.45), then many problems can develop: brain disease, muscle disorders, vomiting, etc. — queue up the awful stuff that you see on TV pharmaceutical commercials.
But how can you measure the pH for all of Earth’s oceans over time — from today to millions of years ago — and get a sense of how healthy the oceans are? What is a healthy pH for the oceans and why is it changing now? You can put a pH strip in seawater, but that only gives you an instantaneous measure of the water right next to you. To determine the pH of all the oceans, scientists rely on computational models that take into account a whole host of variables. These calculations incorporate our current understanding of mineral behavior in water, that is, how fast they dissolve, how fast they grow, and how fast they sink to the seafloor. As new experiments better characterize these mineral reactions in the ocean, the computational models can be updated to generate better and more robust predictions. Also, you also can’t directly measure the pH of the oceans from millions of years ago because that past watery environment doesn’t exist anymore, and we don’t have a literal time machine.
In short, it’s critically important to fully understand the mineral chemistry reactions so that we can model what happened in the past and make better predictions for seawater changes in the future.
What processes can cause oceans to become more acidic?
The pH of the oceans is largely driven by how much CO2 is in the atmosphere. If there is no CO2 in the air, then this limits the carbon that is available in the oceans, thereby limiting the amount of carbonate available for shell formation. Carbon dioxide comes from volcanoes, natural gas seeps, exhaling animals, and from burning of organic material like trees, crops, garbage, oil, gas, etc. The more CO2 in the atmosphere, the more carbonic acid is produced, and the lower the pH of the oceans.
How do minerals react with changing ocean chemistry?
This is a million dollar question, and there is a lot of work being done on the subject, but we do know a few things. Carbonate minerals act like a buffer, meaning, that the minerals react with the carbonic acid to neutralize the acid and keep everything in check. This is a good thing, and it keeps the acidity of the oceans from experiencing extreme deviations. However, the CO2 balance process works in both directions; when the atmosphere has less CO2 than the oceans, CO2 is released from the ocean into the atmosphere. How long can the ocean buffer the increasing amount of CO2 in the atmosphere? What happens when the ocean can’t keep up, or can no longer absorb CO2?
There are two main carbon-bearing minerals that animals make to form their shells and solid frameworks (like corals): aragonite and calcite. These minerals are different forms of calcium carbonate (CaCO3). Aragonite, where the calcium (Ca) is bound by nine oxygen atoms, is highly susceptible to dissolving with low pH. In calcite, the Ca atoms are bound to six oxygens. The bonds in the calcite mineral structure are relatively stronger than the bonds in the aragonite mineral structure. Strong bonds = less likely to dissolve. Most calcite shells in the ocean have a little bit of magnesium (Mg) substituting for some of the atoms of Ca, because the water that they formin contains Mg. The inclusion of Mg distorts the atomic structure of the calcite shells a bit. Shells that have more Mg relative to Ca have more distortions and are therefore less stable. In acidifying oceans therefore, shells and other hard-parts built from aragonite and Mg-calcite will be more readily dissolved and therefore more vulnerable.
When the pH drops below 7, then none of these minerals are stable.
If we know precisely how each of these minerals react with ocean waters of varying pH (and temperature, Ca concentration, and other factors), then we could figure out how much of each mineral is in the different parts of the ocean, and how much of it should be found at the bottom of the seafloor. When the carbonate minerals get to the bottom of the ocean, some will eventually become the sedimentary rock limestone, and locking the carbon away. If the rocks don’t get changed over time, scientists can use them to infer past ocean conditions. This is exactly what people are trying to do because marine sediment and rock records go back millions of years. So why is this so hard?
A big problem is that we actually don’t know how much of each mineral is in the ocean water right now. Hard to believe, but it’s true. We don’t know how much of the carbonate minerals forming as shells in the upper parts of the ocean work their way down to the seafloor. And to complicate matters, the ocean is not homogenous and the amount of CO2 in the water is temperature dependent (cold water holds more CO2). There are parts of the ocean that produces more aragonite or more calcite shells, and there are parts of the ocean where these shells dissolve faster than other parts. Because we don’t know how the ocean mineralogy is precisely behaving now, we have a hard time knowing how it has behaved in the past. And that is where the big ‘SO WHAT’ part of my work comes in. Because these minerals have different chemical behaviors in the ocean, you need to know their mineral-ocean behavior to get the needed constraints for pH measurements. Measuring just the total amount of dissolved carbon doesn’t capture the whole picture.
Just like you can measure the acidity of vinegar or the basicity of baking soda, youput a pH strip in water, and voilà, you know the pH. It is a quantifiable measurement that records how active hydrogen is in the water. However, you can’t directly measure the entirety of the ocean easily, and especially can’t directly measure it through geologic time. We have to use proxies to infer what conditions are like now, and what they were in the distant past. This is why it’s so important to understand the water chemistry conditions of how minerals form in the oceans today, because these minerals are the only things that last for many millions of years. Knowing the conditions of how minerals form today allows us to understand the water chemistry conditions that formed those minerals millions of years ago. We can infer what the climate like back then, how it changed, and how it’s going to change in the future.
Aaron Celestian is the Mineralogy Curator at the Natural History Museum of Los Angeles. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.
Link to original paper:
Dong, S., Berelson, W.M., Rollins, N.E., Subhas, A.V., Naviaux, J.D., Celestian, A.J., Liu, X., Turaga, N., Kemnitz, N.J., Byrne, R.H., Adkins, J.F. (2019) Aragonite dissolution kinetics and calcite/aragonite ratios in sinking and suspended particles in the North Pacific. Earth and Planetary Science Letters, 515, 1–12. https://doi.org/10.1016/j.epsl.2019.03.016
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