Questions to be answered:
1. What is the main change in chemistry brought about by ocean acidification?
2. Why does adding carbon to the oceans from weathering encourage the formation of calcium carbonate,while adding carbon from dissolved CO2 cause the reverse?
3. Why will biodiversity likely decrease in the short term, despite some organisms possibly benefiting?
First a very brief consideration of weathering as part of the long term carbon cycle can help in this understanding for comparison and contrasting purposes.
The slow carbon cycle:
Carbon dioxide is released from volcanoes and is eventually returned into the Earth’s lithosphere when carbonates are eventually subducted by plate tectonic movement of the sea floor. This long, slow process takes millions of years.
Weathering of silicate rocks:
2CO2 + 3H2O + CaSiO3 = 2HCO3 – + Ca2+ + H4SiO4 (1)
If volcanic activity increases over millennia then two things on average will occur: - CO2 will build up in the atmosphere, and with a warmer climate, warmer and more rain water will fall on rocks (left side of equation 1) (extra rain water and warmer temperatures will speed up this reaction). This in turn will cause more bicarbonate ions and calcium ions to be produced by weathering and these ions to be transported to the oceans (right side of equation 1).
Bicarbonates and carbonates:
HCO3 – ⇔ H+ + CO32- (2)
Bicarbonates ions from eq 1 can dissociate into hydrogen and carbonate ions (equation 2). However, the reversibility of this equation needs to be considered. Le Chatelier’s principle can help determine which way this equation moves, on average, when changes occur. If the ocean’s pH, or acidity, is fairly constant more carbonate ions will be produced when more bicarbonate ions are added (although the relative concentrations will be determined by the equilibrium constant for this reaction). When bicarbonate ions arrive with metallic ions such as calcium ions, (instead of with extra hydrogen atoms, which would be the case with ocean acidification discussed below) then dissociation of the bicarbonate ions, as in equation 2, is encouraged.
Precipitation of carbonates in the ocean:
In the ocean, biological activity can help calcium and carbonate ions, (produced as described in the above equations), produce calcium carbonate.
Ca2+ + CO32- ⇔ CaCO3 (3)
Source and Sink of carbon:
Volcanoes act as a carbon source, and the combined weathering, precipitation of carbonates and their subduction into the Earth is a sink of carbon.
In describing the rate of weathering above, I inferred it is possible that a long term negative stabilizing feedback keeps the system in balance whereby if volcanic activity increases (decreases) then the removal of carbonates increases (decreases).
Ocean Acidification.
A problem with ocean acidification is that it changes the ocean’s chemistry. If this happens too fast then ecosystems will suffer (the faster the rate, the more disruption). The cause of this problem today is the excessive CO2 that is being dissolved into our oceans due to the buildup of CO2 in the atmosphere due to human activities. This in turn is making shell formation (calcium carbonate) harder for many organisms.
Ocean Acidification changes the chemistry but some species will benefit (If viewed in isolation but ignoring consequential interactions within the ecosystem) and others will not. In the short term this will mean a smaller and different range of species that will survive or benefit; that is, biodiversity will likely suffer. The eventual outcome (many millennia into the future) is unpredictable but highly risky on human time scales. The short term consequences, however, are also highly unpredictable, as biodiversity loss can lead to ecosystem collapse as we cross interconnected planetary boundaries.
So what happens when we have more carbon dioxide in our atmosphere?
CO2(g) ⇔ CO2(aq), (4)
CO2(aq)+ H2O ⇔ H2CO3, (5)
H2CO3 ⇔ H+ + HCO3-, (6)
Equation 4 tells us the obvious consequence that more CO2 will dissolve in the sea water. Equation 5 tells us that this will produce carbonic acid, while equation 6 tells us that this can (and readily does, in fact) dissociate into hydrogen and bicarbonate ions, (H+ and HCO3-).
Ocean acidification is the process by which extra hydrogen ions are added to the ocean and not a measure of how many hydrogen ions exist in our oceans. In fact our existing oceans are alkaline, with very few hydrogen ions, some carbonate ions and about ten times more bicarbonate ions.
To understand why this ocean acidification reduces carbonate ions we need consider equation 2 above (repeated below) concerning the balance of carbonate and bicarbonate ions;
HCO3 – ⇔ H+ + CO32- (2)
(The double arrow tells us that the equations work both ways reaching an equilibrium value. The equation is reversible).
We have already seen that, with weathering, more bicarbonate ions added to the ocean will encourage more carbonate ions to be available, but weathering did not bring additional H+ ions as is the case with ocean acidification caused by excess CO2 dissolved in water. In the case of ocean acidification (equations 4 to 6) bicarbonate ions and hydrogen are produced. The extra bicarbonate ions will push equation 2 to the right, but the extra hydrogen ions will push equation 2 to the left. It looks like a simple naive application of le Chatelier’s principle could allow one to argue that more or less carbonate ions will be produced. If one considers the existing state of the oceans, however, with few hydrogen ions and many bicarbonate ions (as described in the previous link above) it is readily seen that we have favoured the removal of carbonate ions. Ocean acidification has increased the percentage of H+ ions (due to the fact that there are very few H+ ions in the existing oceans) more than the percentage of bicarbonate ions (of which there are many). This results in equation 2 being pushed more to the left and removing valuable carbonate ions required in shell formation.
Bicarbonates from weathering will help carbonates to precipitate in the oceans.
Bicarbonates from OA will not help carbonates to precipitate in the oceans.
The difference is because in weathering the bicarbonates come along with calcium or magnesium ions (equation 1) for example and not extra hydrogen ions as in the case with ocean acidification.
Biodiversity will suffer:
Ocean acidification overall increases the bicarbonate concentration at the expense of the carbonate concentration, making it harder for many animals to make shells. However some animals can make use of either carbonate ions or bicarbonate ions to make shells and these animals will be at an evolutionary advantage. Equation 2 gives us the information to understand how this can occur. If shell making animals pump H+ ions away from the shell formation sites, using energy, then the effects of ocean acidification can be reduced by pushing the equation more to the right. Some organisms may not be able to meet these energy requirements. Other organisms will possibly do so but at a cost to other functions that will put them under stress and yet others may be able to do so easily. In this latter case the animal could possibly make benefit of the extra carbon in the oceans regardless of the relative percentage of carbonate or bicarbonate ions.
However in the short term (over decades and hundreds or years at least) there will be a loss in biodiversity as many animals will not be able to do so. These may be key animals in the food chain that are adversely affected, and the overall effect on the ecosystem is of high risk and uncertainty. Combined with the effects of warming waters that can hold less oxygen, pollution from industry and agriculture and high fishing rates, any evolutionary recovery cannot be realistically expected to occur within our current pathway.
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