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OA not OK part 16: Omega

Posted on 14 August 2011 by Doug Mackie

This post is number 16 in a series about ocean acidification. Other posts: Introduction, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.

Welcome to the 16th post in our series about the fundamental background chemistry to ocean acidification. In the last post we introduced the solubility product, Ksp. Ksp describes the state of dissolution at equilibrium. By analogy with the K and Q we saw in post 8 we can also describe a Qsp that describes the state of dissolution when system is not at equilibrium.

To formalise this relationship and predict what will happen in a given situation we can write this as a ratio of the concentration of dissolved ions currently present in a given solution to the concentration of dissolved ions in a saturated solution. We call this ratio omega, Ω.

Eq 24

If Ω < 1 then the solution is undersaturated and dissolution occurs. If Ω > 1 then the solution is supersaturated and dissolution does not occur (i.e. precipitation can occur). If Ω = 1 the solution is exactly saturated and nothing happens.

We now know that the amount of ions present is given by the solubility product so we can rewrite the expression for Ω as Equations 25:

Eq 25

And we can, in turn, replace this with an expression specific for calcium carbonate as Equation 26:

Eq 26

We can now go one step further (*) because the bottom line of Eq. 26 for Ω is the same as the expression for the Ksp of CaCO3 (post 15) and the top line is Qsp for a system that is not at equilibrium:

Eq 27

We recall from post 13 that the 2 main polymorphs of calcium carbonate have different solubilities and so there is an Ω for calcite and an Ω for aragonite. That is, the values for the saturated concentrations of Ca2+ and CO32– are different for aragonite and calcite.

In surface seawater at 25oC and 35 salinity the Ksp (calcite) ≈ 4.3 ×10–7 and Ksp (aragonite) ≈ 6.5 ×10–7. And, as we have come to expect, each of those conditions – pressure, temperature, and salinity influence the solubility product.

The solubility of calcium carbonate increases at lower temperatures; for surface water at constant salinity a temperature change from 25oC to 5oC corresponds to a change in Ksp of ~1% for calcite and ~5% for aragonite. If you take a heap of salt or sugar and dissolve it in a cup of water you will find the water gets noticeably colder. This happens because it takes energy to shake the ions out of their stacked array. The source of the energy is heat energy from the water. This is why it is easier to dissolve salt or sugar in hot water; the water has more heat energy that can be stolen by the salt or sugar for the dissolution process. A reaction that takes in energy is called endothermic.

Similarly, a reaction that gives out energy is called exothermic and, unlike most minerals, the dissolution of calcium carbonates is exothermic it means that solid calcium carbonate is favoured at higher temperatures i.e. the solubility is greater at lower temperatures.

The effect of salinity is not so noticeable because in deep water there is little variation in salinity (though in surface waters local variation in salinity can have an impact on Ksp (including, tautologically, because "salinity" includes Ca2+ and dissolved inorganic carbon species).

Pressure also has an effect; solubility decreases with pressure. The theory behind this is way beyond a simple blog post but mostly comes down to the change in volume with pressure (this is the reason why we said earlier that it is better to use moles per kg for concentration than moles per L). So we'll simply give some values for 3000 m (at 25oC and 35 S) to demonstrate the extent of the effect. The Ksp for calcite at 3000 m is the same as the Ksp for aragonite at the surface.

Table 2

The concentration of calcium in seawater doesn't change much with depth or location. This is because it has only one main source (the weathering of carbonate rocks) and, because weathering is slow, the ocean is well mixed in terms of the time scale for supply of calcium by weathering. But, as we have said, the relative concentration of carbonate species and the concentration of total carbonate species does change with depth. This is true even without a human influence.

Since the concentration of carbonate CO32– that is present decreases with depth, we can see that the ratio of present concentrations to the theoretical saturation values, Ω, decreases. Thus, we expect that Ω will decrease with depth in the deep ocean. And, so it does.

We can calculate Ω using the same deep ocean profiles we presented in post 14 for pH. If we draw a line at Ω = 1 then the depth at which this line crosses the actual Ω gives the depth below which aragonite and calcite dissolve. This is called the saturation depth or saturation horizon.

Fig 15 omega calcite omega aragonite

Figure 15. Depth profile of Ω for calcite and aragonite as a function of ocean basin. Data from the same WOCE project stations as the pH profile in Figure 11 from post 14.

Ω = 1 for calcite at about 3000 m and 4500 m in the Pacific and Atlantic oceans, respectively. Ω = 1 for aragonite at about 1000 m and 2500 m in the Pacific and Atlantic oceans, respectively. A poetic image used is to compare Ω to a mountain snow line; in the oceans any parts shallower than the saturation horizon will have white sediments containing calcium carbonate while the lower slopes are covered with brownish clay sediments. The real world is not quite so simple but the image is vivid and let it not be said that we will let reality get in the way of poetry.

What happens when CO2 is added to the atmosphere? Although the total amount of carbon species increases, we now know from Henry's law (post 9) and equations 7-9 (post 6 and post 7) that the net effect is to reduce the concentration of carbonate CO32– present in surface water and to leave the concentration of Ca2+ unchanged. We also know that if CO32–decreases, then Ω also decreases. That is, adding CO2 to the surface ocean will cause overall carbon to increase, but also causes the relative fraction of carbonate to decrease. In turn this decreases Ω so the profiles in Figure 15 to move to the left and the saturation horizon thus moves towards the surface, sketched in Figure 16. (Note that the future plot is not a prediction for a particular future time – we will go over such predictions in detail in the second series of posts but as a taster consider that in the Caribbean Ω aragonite is decreasing by about 0.12 per decade).

Fig 16 future omega

Figure 16. Conceptual plot of the saturation horizon moving towards the surface as CO2 is added to the surface ocean. 

txtbox: shoaling of omega

In the next post we look at how carbon is transferred from the surface ocean to the deep ocean.

Written by Doug Mackie, Christina McGraw, andKeith Hunter . This post is number 16 in a series about ocean acidification. Other posts: Introduction, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.




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Comments 1 to 12:

  1. Hmm... if I've understood this correctly, does this mean that deep coral reefs will start to dissolve first? I have to admit, that's a novel idea for me - I had assumed that increasing atmospheric CO2 would mean surface reefs would be affected first, but I guess that's just not so! Could this undermine some old, 'tall' reefs, leading to some rather dramatic collapses as their foundations literally dissolve away? I imagine any other islands or other pieces of land with significant carbonate content will also have this problem. Could be interesting (in the Chinese proverbial sense).
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  2. Great series. The interesting figure here is Aragonite in the Pacific. Already at 1000 m and farely steep slope to the surface. This looks like where we will see problems first as the curve shifts left. How much do the curves vary within and around each basin?
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  3. And WOW! There is a second series of posts on projections! How soon?
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  4. @Bern Sob. Has nobody listened? Just because a process is thermodynamically favoured does not mean it will happen instantaneously. Blog science happens when people stray from their expertise. Reefs (why not reeves?) are Ove's department over at climate shifts. But my non-specialist $0.02 is to remind you that corals are living organisms. That is they expend energy to maintain that thermodynamically non-equilibrium state we call life. They maintain and build reefs even though ambient conditions fluctuate significantly over daily, tidal, and seasonal cycles. There seems every likelihood that, for some little time at least, corals will be able to expend a more energy to build and maintain their reefs as ocean acidification progresses. Quite when that becomes unsustainable is the topic of current research. BUT I stress again that I am outside my comfort zone writing about the response of living coral reefs - go ask Ove. Or read his previous SkepticalScience posts). GBR 1, GBR 2, and GBR 3
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  5. @GlennT Don't take the figures as the final word. Each ocean basin encompasses large variations in every parameter. The figures are for WOCE data from stations I chose randomly years ago as being 'representative'. I don't honestly recall what I was looking at to make that selection at the time but on my list of things-I-pretend-I-will-do-but-really-will-never-get-round-to is to collate and analyse such profiles for all the stations (and JGOFS, GEOSECS etc) and rework with projected CO2. But hey, the data is freely available. Go to it. The next series depends on a few watched research kettles coming to the boil.
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  6. @GlenT Consider the plots in Figure 15 as "typical" of one place in each ocean. The geographical trends are interesting. The saturation depth doesn't change a lot in the Atlantic, but becomes shallower in the Pacific from north to south. The real significance of these plots is how they determine where CaCO3 fossils accumulate on the sea floor. Recall that coccolithophores are plants so they only grow in the surface ocean where the light is. After they die, they sink into the deep ocean. If the sea floor happens to be shallower than the saturation depth, they will not dissolve - thermodynamics prevents that. If the sea floor is deeper, then they will dissolve, given enough time. Studies of deep sea sediments show that CaCO3 sediments dominate only in the submarine mountain ranges that are shallower than the saturation depth. I liken this to snow on mountains - it's only cold enough for the snow to persist high up on the mountain. The really deep parts of the ocean have very little CaCO3 in the sediments.
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  7. OT Doug Mackie @4, "Why not reeves?" Because reeves where rent/tax collectors, a term now only familiar in the modern derivative of a common form of tax collector, the Shire Reeve (Sheriff). Apparently the noun still survives with close to its original meaning in Canada (which I didn't know previously.)
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  8. The avoidance of heteronymns is not the reason: I refuse to take out the refuse just because you are busy trying to refuse the lights. (The initial comment was a hat tip to Tolkien and his aside about dwarfs/dwarves in the introduction to the now filming Hobbit).
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  9. A few weeks ago I was asking about the CCD - Carbonate Compensation Depth. A website (thanks Google) connects this discussion (OA not OK #16) of CaCO3 solubility in the water column with the response of calcarious remains settling towards the bottom. Shells start to dissolve wherever Ω drops below 1 (the “lysocline”), and at a certain undersaturation (Ω =~.65 & .75 - Atlantic & Pacific) virtually all CaCO3 tests will actually dissolve (the CCD). Deposits of shells or coral on the ocean floor will begin to dissolve when the lysocline and CCD moves upward (due to OA) as long as the deposits are not protected from undersaturated fluids. From the link above: "the ocean's ability to take up atmospheric pCO2 is influenced by the balance of production and dissolution of calcium carbonate, and lifting or lowering the CCD has important consequences on the short and long term variations of CO2 in the atmosphere." Ominous - OA is definitely not OK!
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  10. Yes Tor, this is the 'snow line' we wrote of in the post and that Keith restated in his comment. In addition to the links I gave in my comment above, I think Ove did something else on this a while back at SkS (pauses to check) here. (Wow the comments then certainly present, shall we say, an interesting spectrum).
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  11. Myself @6: Spotted a mistake. The saturation depth in the Pacific gets shallower from south to north, not as I mistakenly stated @6.
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  12. second summary post

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