Wikipedia says this, unsourced:

"...Generally, gas solubility decreases as water temperature increases. Accordingly carbon dioxide is released from ocean water into the atmosphere as ocean temperatures rise.

"Most of the CO₂ taken up by the ocean forms carbonic acid. Some is consumed in photosynthesis by organisms in the water, and a small proportion of that sinks and leaves the carbon cycle. There is considerable concern that as a result of increased CO₂ in the atmosphere the acidity of seawater will increase and may adversely affect organisms living in the water. In particular, with increasing acidity, the availability of carbonates for forming shells decreases."

I have seen elsewhere the even stronger statement that the increased acidity of the ocean from increased dissolved CO₂ could dissolve carbonate minerals on the seabed, and thus release further CO₂.

Now this is all unsourced, and therefore I am concerned it may not be entirely right. I believe that more CO₂ in the atmosphere results in more in the ocean, provided the temperature of the ocean does not change. I also believe that CO₂ forms an acidic solution. But I have two particular concerns.

(1) Principles of chemical equilibria are that increasing the concentration (or partail pressure, for gases) of one item promotes the movement of the reaction in the direction to reduce the change. So that would tend to suggest that if there is increased concentration of CO₂ in the ocean, whether in the form of CO₂(aq), HCO₃^-(aq), CO₃^2-(aq) or H₂CO₃(aq), or whatever, that would tend to promote deposition of carbonate minerals, not promote the further dissolution of carbonates so as further to increase the dissolved CO₂, regardless of the presence of increased concentration of hydrogen ions. Now what is involved here is a mainly a biological deposition process, but in general biology operates to use up increased useful ingredients supplied to it. An increased concentration of CO₂ necessarily increases the acidity. I can believe that in general increased acidity is unhelpful for carbonate availability. But here the increased acidity comes from increased dissolved CO₂, which on balance should increase the availability of carbonate by a normal understanding of chemistry. Of course biological entities can have too much of a good thing. But have we already reached a concentration of CO₂ in the oceans that more would be bad for the oceanic consumers of it?

(2) More CO₂ in the atmosphere would certainly tend to result in more dissolved CO₂ in the ocean, by precisely the equilibrium chemistry principle I mentioned - (though kinetic issues also matter, it needs to rain sufficiently to wash the CO₂ into the ocean). But the above quote also mentions the countervailing issue that a warmer ocean has lower solubility of CO₂. More CO₂ in the atmosphere warms the ocean, especially its upper parts which are in contact with the atmosphere, and that is what matters. How do these effects interact and what is the net effect?

I wish to add to the inquiry by Ivan that where I live Devonian limestones are very common and it is characteristic that they display lamination of organic stain between deposition of more limestone. The organic stain is interpreted as a period of anoxic conditions. Therefore, the deposition of the limestone occurs during periods of oxygenated seawater. Just to say that mixing conditions were poor is not enough. Deposition of limestone stopped during the formation of the organic stain. Why? I am not looking for a geologic answer but a chemical one, though geology will certainly play a primary role in setting up the conditions for the chemistry to occur.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

An interesting question, and I don't know the answer. I wonder if it is a short-term versus long-term difference. In the short term, organisms that currently make carbonate shells can't adjust to the changing pH, but eventually ones that can evolve, and then procede to tie up more of the carbonate and make carbonate minerals.

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http://www.geocraft.com/WVFossils/Ca...s_climate.html contains a curve entitled: "Global Temperature and Atmospheric CO2 over Geologic Time" which alleges that during the Devonian, Triassac, Jurassic, and Cretaceous periods the CO2 concentrations and global temperatures were well above todays levels. Life abounded during these periods. Only during the Ordovician, Carboniferous, and early Permian were levels near those of today. If this curve is moderately accurate (I wonder how it could be), we should avoid panic for a few more hundred years. I assume the science used to generate the curve is on par with that used in the better climate modeling and that the accuracies are equivalent.

Note that if this curve can be believed, the Earth is unusually cool at the current time.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

It is. The ice ages are blips on the geologic time scale, and there have been three major ones since approximately 750Mya. And we are not out of the last one.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

I'm not sure how low you can get the pH with just atmospheric CO₂; the lower pKa for carbonate is about 6.4. Note that culture media for cells grown in vitro is often buffered with carbonate/bicarbonate in equilibrium with a 5% CO₂ atmosphere (much much higher than earth's atmospheric level) and the pH of the media is about 7-ish.

Nick

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Nick Theodorakis

A solution of NaHCO₃ or Na₂CO₃ is equivalent to adding a stoichiometric amount of NaOH to a pure solution of CO₂. So I'm not surprised your carbonate/bicarbonate buffer isn't as acid as a pure solution of carbonic acid.

But as you say, the ability of CO₂ to release hydrogen ions in sea water ought to be modest. I've just googled, and the pH of the surface areas of the ocean is about 8.1, which, er, isn't very acid at all. The article I quote below says that the pH of the ocean has reduced by about 0.075 over 250 years, but is projected to reduce much more rapidly over the next 100 years, perhaps by as much as 0.3 if we continue pouring the stuff out.

It seems there is another wikipedia article on this which is fully sourced, which I didn't find first time. http://en.wikipedia.org/wiki/Ocean_acidification This article says there is experimental evidence for reduced calcification of some important species at increased CO₂ concentration, and attributes it to the fact that less of the CO₂ is in the form of carbonate ion at higher concentrations of CO₂, because carbonate forms a stable supersaturate at lower pH. This seems to be saying, yes, this is counterintuitive, but there is a kinetic effect at work (supersaturation of carbonate ions). Although presumably there must be a lower concentration of CO₂ at which these species eventually reduce calcification, and it would be interesting to understand what causes the changes in direction and why and where.

But this article also says (sourced)

"Recent work examining a sediment core from the North Atlantic found that while the species composition of coccolithophorids has remained unchanged for the industrial period 1780 to 2004, the calcification of coccoliths has increased by up to 40% during the same time."

So I'm not sure on what basis they can come to such a firm conclusion at the bottom of the article that the net direction of calcification resulting from increased acidity is downwards. It seems that what I am asking really is at the edge of science and not easily answered on the basis of established knowledge.

Keep in mind that carbonate rocks are made by more than just animals. They can be made just fine with no animals at all, which is why the earth is livable at all.

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Don't forget about calcium, very abundant in seawater. In fact, is not seawater saturated with respect to calcium carbonate -otherwise the sea shells would all re-dissolve?

Also, the carbonate/bicarbonate system is not the only buffer system in operation in the oceans, borate is also significant in this regard.

<<A solution of NaHCO3 or Na2CO3 is equivalent to adding a stoichiometric amount of NaOH to a pure solution of CO2>>

I'm not sure about this statement, well all right just the NaHCO3 part of it. Surely bicarbonate is fully saturated, it cannot take on additional CO2 equivalents?

The pH of rainwater runs about 5.5, due to the CO2 in the air. Limestone caves can result from the slow dissolution of calcium carbonate in buried strata due to the percolation of this slightly rainwater through the overlying earth. Anyone who grows a garden in a damp climate knows that you have to add limestone to the soil periodically to keep soil pH up.

The solubility of calcium carbonate is influenced by several factors, and they are all in play at once. A major one is acidity. The ratio of carbonate to bicarbonate in solution shifts toward bicarbonate as acidity increases (pH falls). Solid calcium carbonate in contact with supersaturated carbonate solution will see no net dissolution (although there will be microscopic interchange of carbonate at the surface). Lower the pH and the ratio of carbonate to bicarbonate falls. If it drops far enough then the calcium carbonate begins to undergo net dissolution.

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OK, I'm beginning to get this now. It seemed odd to me that carbonate would be more soluble in water made acid by CO₂, thus increasing the concentration of the dissolved CO₂ species. But actually the point is that calcium ions are more soluble in water made more acid by CO₂. My mistake was think of generic "carbonate" when in fact it is the calcium chemistry that is important (and apparently also borate according to kzb).

kzb: 2NaOH + CO₂ = Na₂CO₃ + H₂O
was what I had in mind.

True, there are few or no fossils in most limestone such as a white rock driveway.

How does the presence or absence of dissolved oxygen affect the chemistry?

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

Some basic chemistry here. Note that these are all equilibrium reactions, meaning that at the molecular level they are going back and forth. How far they go to one side or the other depends on the immediate properties of the solution at the time.

CO2 in the air dissolves in water

CO2(gas) <> CO2(aq)

CO2 in water reacts with water to form carbonic acid

CO2(aq) + H2O <<<> H2CO3(aq)

In pure water most of the CO2 is not in the form of carbonic acid. Then carbonic acid can dissociate to a hydrogen ion (the acid, if you will) and bicarbonate.

H2CO3(aq) <<<> H+ (aq) + HCO3-(aq)

Bicarbonate can dissociate to carbonate

HCO3-(aq) <> H+(aq) + CO3--(aq)

Carbonate can react with calcium ions to form calcium carbonate, a solid (ppt)

CO3--(aq) + Ca++(aq) <>>>> CaCO3(ppt)

There is a concept in chemistry (Le Chatelier’s principle) showing that if extra component is added to one side of the equation the reaction is pushed toward the other side. By the same reasoning, removing a component from one side will draw stuff from the other side.

Keep in mind that an equilibrium reaction is a dynamic thing. It implies that, at equilibrium, there is no net change of concentrations on either side of the equation. But at the molecular level there is a constant traffic back and forth. It is this microscopic shuttle that allows the reaction to shift as concentrations of the individual components are changed.

If you increase the concentration of CO2 in the atmosphere, the concentration of CO2(aq) also increases, increasing the concentration of H2CO3(aq), hence H+. Since the solubility of CaCO3 depends on the concentration of carbonate (CO3--), anything that lowers the concentration of CO3-- will increase the dissolution of calcium carbonate. The excess acid pushes the carbonate/bicarbonate equilibrium toward bicarbonate, increasing the solubility of calcium carbonate.

A classic experiment is dripping an acid (even vinegar, aqueous acetic acid) on a chunk of limestone, which will promptly foam up. The acid converts the carbonate at the surface of the stone to bicarbonate, the acid then protonates the bicarbonate to carbonic acid, the carbonic acid mainly converts to aqueous CO2, which then mostly escapes to the atmosphere as gaseous CO2, leaving soluble calcium acetate behind in solution.

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The Devil offered me power. I told him I preferred aperture.

Hi all,

Couple of other things.

In the 90's I became adept at keeping corals alive in captivity. Did it 50 hours a week for years. (Minimum wage too! I had my disabilty check, I was helping a good friend, and I loved the work!) Sadly, I developed one of those godawful, life-threatening, aquired allergies to the venoms and proteins of Cnidarians in general.

Three bad run-ins with carpet anemones and three severe reactions to envenomations by brain corals. Now I can't go to the beach after a big beaching of by-the-wind-sailors without getting tight chested and wheezing in the parking lot! (Is there a treatment for this? This really sucks for me.)

A big disappointment in my life. Sort of like one of you physicists developing a contact allergy to chalk dust. Or I guess "dry erase" dust nowadays.

BUT in biological systems solubility can be altered by "active transport" factors that don't occur otherwise. As living things we do violate the laws of thermo dynamics whenever possible. And vice-versa.

And forgive me, its been eight years since I've been near a coral tank, but either magnesium or manganese alter the solubilty of calcium. Both present in sea water. I forget in which direction though. Somewhere the phrase "magnesium poisoning", (with regard to the pertenant chemistry equations), keeps popping up and demanding attention from my forebrain but a quick wiki search didn't get me anything that jogged my memory properly.

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"The beauty of that discussion of averages is that you don't have to be an expert in Apollo or in photography in order to see where this time study "analysis" breaks down. You just have to be, well...not an idiot." -JayUtah

I'm not sure how exactly it affects it, but I know it is not required. Dissolved sulfur compounds can completely eliminate formation of carbonates however, which is one current theory why Mars had little to no carbonate rock, despite past oceans or lakes and a predominantly carbon dioxide atmosphere.

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Magnesium lowers the solubility of calcium carbonate. Dolomite (MgCa(CO3)2) also dissolves more slowly than calcite. Note that rate of dissolution is a kinetic property and solubility is a thermodynamic one.

Speaking of thermo... living things are just as subject to thermodynamic strictures as anything else, but you have to look at the system properly. Plants and animals maintain a lower state of entropy by increasing the entropy in their surroundings.

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The Devil offered me power. I told him I preferred aperture.

I had all this in mind, but was too lazy to write it all out, so thank you.

I agree that Le Ch implies that extra H+ pushes (4) to the left, hence (5) to the left also, assuming nothing else has changed.

But extra CO2 dissolved pushes (3) to the right, thus increasing the quantity of HCO3-- by exactly the same amount as you increase the H+, (unless there is something else not mentioned here that consumes either). It seems to me that if you increase both H+ and HCO3- in the same proportions, then (4) on balance doesn't move at all, nor (5). Though I can possibly conceive that there is something else happening outside this system which means that at the starting point H+ is at a lower concentration than HCO3- (other components in seawater are keeping the pH to 8 remember); then if the number of H+ is increased by the about the same number as HCO3-, then that is actually a larger proportionate increase in the concentration of H+ than the increase in concentration of HCO3-, so that would push (4) to the left.

But even then it still seems counter-intuitive. Carbonate-producing life forms can't produce any carbonate at all if there is no dissolved carbonate in the water. So the first addition of CO2 from zero promotes carbonate formation (assuming for simplicity that there is no other source of inorganic carbon, aside from the carbonate so deposited). Then at some concentration it reverses, and now more CO2 now reduces carbonate formation, that is what we are being asked to believe. Now there is nothing in those above equations that, as far as I can see, indicates a change in direction at any point as CO2 is increased. So I think there has to be more to it than just the equilibrium chemistry. That is why the argument about super-saturation of CO3-- being possible at higher pH, rather than equilibrium chemistry, appealed to me. Obviously there would have to be a source of supersaturated carbonate coming in from somewhere. There's probably plenty washing off the land.

Getting rather off topic, if energy is input, then the local reduced entropy is at the "cost" of increasing the entropy in the source that supplied the energy. So life on earth can reduce the net entropy of the earth by being powered by the sun. Across the universe, the energy supplied by gravitational collapse in effect is an almost unlimited engine to reduce entropy.