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OA not OK part 6: Always take the weathering

Posted on 16 July 2011 by Doug Mackie

This post is number 6 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.

Weathering refers to the wearing away of rocks by, err well, weather (or birds ).

Carbonate rocks can be CaCO3 (calcium carbonate or limestone) or MgCO3 (magnesium carbonate) or mixed calcium-magnesium carbonate CaMg(CO3)2 (dolomite) and several other less common forms. We will discuss only calcium carbonate, but as calcium and magnesium are members of the same group (i.e. vertical column) on the periodic table, you would just replace Ca with Mg in the equations.

Limestone weathering occurs when rainwater reacts with the carbonate rocks. Rainwater does not have a neutral pH, but this has nothing to do with industrial pollution. Rainwater is in equilibrium with atmospheric CO2, so carbonic acid is formed via equation 7 from post 5. This leads to mildly acidic rainwater with a pH of about 5.7.

Equation 7

(Note: acid rain refers to rain with a pH less than 5.7 due to equilibration of rain with sulphate and nitrate - forming sulphuric and nitric acids - from the burning of fossil fuels and biomass.)

What happens when rain washes over limestone on its way to rivers? In post 1 we introduced Equation 4, the reverse of calcification, which describes the weathering of carbonate rocks:

Equation 4 no words

In post 1 we also said that Equation 1, calcification, was a source of CO2. So it should come as no surprise to say that the weathering of carbonate rocks consumes atmospheric CO2.

text box 4 weathering

The result of weathering is that river water contains a lot of carbon species, in the form of bicarbonate and carbonate, compared to seawater.

River water also contains salts. Stephen Jay Gould wrote an engaging essay (republished in the book Eight Little Piggies) about Edmund Halley (he of the comet). Halley, like Gould, was a polymath and one of his ideas was to work out the age of the Earth from the amount of salts in the sea. It isn't clear if Halley knew about the other salts in the sea or if he just meant NaCl, but it doesn't matter as Halley's argument works just as well for carbonate species. Halley began with the observations that:

1) The oceans are not saturated with salts (NaCl and/or carbonate species etc). That is, you can add additional salt to seawater and it will dissolve.

2) River water contains salts.

Halley knew the water was recycled and he knew the salts came from the rivers because of weathering. Halley assumed the salt content must be increasing with time as the rivers washed more salts in, but none ever left. So Halley concluded that if you measured the saltiness (salinity) at the time (1700-ish for Halley) and again in a few hundred years then Halley expected that the salinity would have measurably increased and allow a maximum the age of the Earth to be calculated.

For example, current salinity is about 35 grams per kg of seawater. If it had been 34 g per kg of seawater 300 years ago, the salinity would have increased 1 g/kg over 300 years.  Therefore, it would have taken 10,500 years to build up the current 35 g/kg - if we started from salt-free water. The maximum age part is because if the seas had started salty then the Earth would be younger than it seemed. Halley, a young-Earth creationist, expected an answer in the low thousands of years, but was wrong for several reasons. 

Halley’s biggest mistake was his assumption that it was a one way process, that salts never leave the ocean. But salts do leave the ocean and are part of a long slow cycle. Halley should have thought this one through as salt deposits (from ancient seas) have been mined by humans for thousands of years. Similarly, calcium carbonate (another salt delivered to the oceans by rivers) also leaves the ocean – as part of the carbon cycle. The most striking example of calcium carbonate leaving the ocean are the White Cliffs of Dover, which are calcium carbonate skeletons of coccolithophores.

We are so familiar with the idea of a carbon cycle now that we don't recall how astonishing the idea would have been only a few hundred years ago that the weathering of carbon from rocks could possibly be part of such a cycle. But, as we shall see in later posts, when atmospheric CO2 is high then more acidic rain causes more weathering and that consumes CO2 to lower the atmospheric CO2. Only problem is that this happens over a geological time scale.

If we do the same sort of calculation that Halley did for salt with bicarbonate HCO3 in rivers then we can get a good estimate of the amount of weathering that happens and thus how much bicarbonate added to the ocean by rivers. Now, we know from Eq. 4 that weathering consumes CO2. So the amount of bicarbonate added to the ocean is equal to the amount of CO2 consumed.

Equation 4 no words

But remember that this CO2 comes from the atmosphere. So it follows that given enough time then that the consumption of CO2 by weathering will remove all the CO2 from the atmosphere. It turns out that amount of weathering is sufficient to remove all CO2 from the atmosphere in 3500 years. Plainly this hasn't happened in the past. Something is returning CO2 to the atmosphere. That something is our Eq. 1 for calcification:

Equation 1 no words

Equation 1 explains the formation of CaCO3 in the ocean from Ca2+ and 2HCO3 derived from weathering. When this CaCO3 dissolves, by Eq. 4, it exactly reverses the original Ca2+ and 2HCO3 derived from weathering that was lost from the ocean by CaCO3 formation, and also consumes the CO2 formed at the same time. However, if CaCO3 is buried in sediments, that Ca2+ and 2HCO3 must now be replaced by further weathering of terrestrial limestone. Fortunately, the CO2 needed to do that has been returned to the atmosphere after CaCO3 formation. Thus closure of the cycle!

In the next post we consider the first stage of what happens when the equilibrium of the oceans is disturbed.

Written by Doug Mackie, Christina McGraw , and Keith Hunter . This post is number 6 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 29:

  1. It turns out that amount of weathering is sufficient to remove all CO2 from the atmosphere in 3500 years. Faster than I expected given the amount of carbon in the atmosphere. Do you have a reference or a link regarding this calculation, so I could read more about it ? Is the Urey reaction treated in another post ? Thanks in advance.
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  2. So calcification releases CO2 into the ocean (and eventually the atmosphere), while weathering removes CO2 from the atmosphere and eventually deposits it into the oceans. I assume these two processes occur at dissimilar rates, since volcanism also releases CO2 into the atmosphere, and the volcano-weathering interaction is Richard Alley's famous CO2 thermostat.
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  3. Re Composer99 - weathering also refers to the weathering of silicate rocks, such as CaSiO3. CO2 + CaSiO3 = CaCO3 + SiO2 (weathering + carbonate mineral formation, example of a net reaction) In intermediate steps additional CO2 can be taken up by the water which is then released upon formation of CaCO3, but the net reaction has a net uptake of CO2. This can balance geologic emission. Actually geologic emission includes some oxidation of organic C in rocks, and geologic sequestration includes some organic C burial. Meanwhile, carbonate minerals, so far as I know, become thermodynamically unstable at sufficiently high temperatures, and so their geological processing under some conditions should be able to feed the geologic emission of CO2. I think geologic emission may be somewhere around 0.2 Gt C per year, so this would replace an atmosphere's worth of CO2 in 3500 years if the atmosphere had 700 Gt of CO2. (I'm unsure about how much geologic emission is directly into the oceans, though, but that would still require some additional geochemical process in order for that CO2 to not be released into the air.)
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  4. ... of course, if there were no weathering, CO2 would build up in the atmosphere and ocean, as higher CO2 partial pressure would increase the equilibrium C content of the ocean. So it would take longer than 3500 years for geologic emission at 0.2 Gt C /year to actually ~double atmospheric CO2.
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  5. @JosHag: I am not trying to be flippant (when I am trying it is obvious). However, the very purpose of the Skeptical Science site and the purpose of this series of posts is to encourage people to think for themselves. A lot of what we have written so far does involve a little chemistry. We appreciate that not everyone has had this training so we have tried to keep it simple. However, this calculation involves no chemistry at all so we think it would be worthwhile having a go yourself. The calculation has been done carefully but in this case a first approximation or speherical cow calculation will give you an answer with the right order of magnitude. To do this calculation as a first approximation you need to know four things: 1) Equations 1 & 4 2) World annual river flow 3) World annual river bicarbonate concentration 4) Amount of carbon in the atmosphere. Have a go yourself and come back if you can't do it.
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  6. Thanks for clearing that up, Patrick.
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  7. Doug:
    The oceans are not full of salts (NaCl and/or carbonate species etc).
    Is the word "not" supposed to be there?
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    Moderator Response: The 'not' is correct. You can take seawater and add more salt to it. Therefore seawater is not full or saturated with salts. But good point about language use and that bullet point changed. Thanks. Doug.
  8. @Doug Mackie 5 I got the message. I just presumed there were dissolution rates involved. Sorry about that, next time I will give it some more thoughts before I pose a quick question. The chemistry is no problem though and I'm very interested in the topic. I had never heard of a spherical cow before, but this cow calculation gave me 4035 years to get rid of 3050 Gt CO2.
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  9. JosHag @8: Fantastic! Well done. Enrio Fermi was an early pioneer of the spherical cow approach with his famous piano tuners problem.
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  10. The CO2 drawdown from the weathering of the Himalayas is thought to have been the main contributing factor to the cooling trend throughout the Cenozoic and the shift to the Great Ice Age thats been dominating the Earth's climate for the past 2.5 million years.
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  11. ...Sorry, I should have mentioned it's the weathering of the Tibetan Plateau and not just the Himalayas!
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  12. Patrick 027 is quite correct to point out the importance of silicate weathering. In this post we were trying to focus on the CaCO3 cycle as a simplification. But Patrick is quite correct in pointing out anomalies in which weathering loss off CO2 is balanced by volcanic emissions. However these do not substantially change the picture we have painted. Also, we will deall with this in a future post.
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  13. I hope that in the next post will discuss the formation of ooliths (= ooids) ...
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  14. PS the thermodynamic equilibria for various carbonate-silicate reations shown in fig 1 of "Initiation of clement surface conditions on the earliest Earth" N. H. Sleep, K. Zahnle, P. S. Neuhoff (not entirely on topic but it is related to chemical weathering) (PS note the temperature plotted as a function of atmospheric CO2 - I'm pretty sure this includes the effects of the faint young Sun.) C&P w/ formatting change, the reactions shown: 1. "leonhardite + albite + CO2 = calcite + paragonite + 4 quartz + 2.5 H2O" 2. "leonhardite + CO2 = calcite + kaolinite + 2 quartz + 1.5 H2O" 3. "clinochlore-14A + 5 calcite + 5 CO2 = 5 dolomite + kaolinite + quartz + 2 H2O" 4. "clinochlore-14A + 5 CO2 = 5 magnesite + kaolinite + quartz + 2 H2O" 5. "daphnite-14A + 5 CO2 = 5 siderite + kaolinite + quartz + 2H2O" 6. "2 albite + CO2 = 3 thermonatrite + kaolinite + 4 quartz " 7. "paragonite + CO2 + 5 H2O = thermonatrite + 3 kaolinite" mineral formulas given in table 1 , and references are identified for thermodynamic data.
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  15. To extend the discussion about silicates by Patrick 027 to geo-engineering. I'm assuming that calcium and the carbon cycle are the topic here. One of the more natural methods of geo-engineering is to grind calcium silicate containing igneous rocks to a fine powder and disperse them over the oceans to help counter ocean acidification and aid in the draw down of CO2 from the air. The calcium silicate dissolves and the added calcium concentration helps maintain aragonite super-saturation and increase CO2 deposition as CaCO3 and also raises ocean pH. This is one of the two ethical methods of geo-engineering (the other being bio-char addition to farms soil) that have a lower chance of really messing up the planet. I've noticed in trying to discuss these points with environmentalist (I'm a chemist so I like discussing something I'm trained in) I find that all have so far have been convinced that geo-engineering is about sulphur burning and the sky no longer being blue. With so much emotion that geo-engineering is no longer wanted to be talked about. I've noticed that the fuel levy on petrol and diesel is higher than the cost of making bio-char from garden waist. I don't know what the cost of the grinding of igneous rocks is but I thought it worth discussing the option of pushing for liquid fuel to be offset by one of these two methods before sale. Starting to use CO2 sequestration now instead of latter makes some sense to me. Especially if it's not cost prohibitive and we have a tax that could pay for it. Oh by the way I don't think doing this for coal fired power stations is worth doing. I'd rather replace them.
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  16. Paul W, We are chemists also and it pains us to point out that your idea has not been thought through and is distressingly commonplace amongst chemists. Firstly, as a chemist, you are surely aware that as we noted in post 3, surface seawater is supersaturated in CaCO3. Secondly, please consider the response (repeated below) we made to a similar comment to post 3 here. The idea was explored with respect to limestone and what is true of adding calcium carbonate also applies to adding calcium silicate. Harvey L.D.D. (2008) Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C04028, 2008. abstract. The conclusion:
    Geographically optimal application of 4 billion t of CaCO3 a−1 (0.48 Gt C a−1) could induce absorption of atmospheric CO2 at a rate of 600 Mt CO2 a−1 after 50 years, 900 Mt CO2 a−1 after 100 years, and 1050 Mt CO2 a−1 after 200 years.
    That is, a fleet of tankers dumping 4 billion tons of powdered limestone per year could be sucking up 600 million tons of CO2 per year after 50 years. A slight problem is that current emissions are about 30,000 million tons of CO2. Yes, current emissions are 50x the amount that would be being sequestered by 2100 – and your guess is as good as mine for what emissions will be in 2100. Every little bit helps? Perhaps. But mining, crushing and transporting the limestone might be a little carbon intensive. Harvey is not seriously proposing this as a solution. He goes through those calculations too, as he says to show that:
    The calculations presented here serve to illustrate the enormity of the task of even partially reversing the acidification of the oceans that is yet to occur under even the most optimistic scenarios concerning reductions in CO2 emissions. The task is not only enormous but would need to continue for several 100 years. These calculations also underline the fact that in the absence of stringent reductions in CO2 emissions, efforts to reduce adverse impacts on ocean chemistry will be ineffective.
    Harvey goes on to say that if emissions have dropped to zero by 2100:
    then application of limestone at a rate of 4 Gt a_1 (0.48 Gt C a_1) beginning in 2020 serves to restore about 20% of the difference between the minimum pH and preindustrial pH by 2200 and restores about 40% of the difference by 2500, with the same benefits for the degree of supersaturation with respect to calcite.
    Yes quite. If emissions have dropped to zero by 2100.
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  17. Doug Mackie@16 thank for your thoughts. I agree with you. I would not think of it as a solution but of a nightmare that becomes real at some point. My Ph D topic thirty years ago was hydro treatment of heavy petroleum oils (the clean up step that allows heavy oils to be processed in current refineries). After 6 years I resigned and left that industry not wanting to be part of "baking the planet". I am heartened by the increased numbers opposing the use of fossil fuels particularly the most CO2 intensive alternatives. If I had it my way I'd want coal use to stop ASAP and other fossil fuels to only be used if carbon was put into the ground to match their extraction. Discussion of the needed geo-engineering required to return the planet to below 350 ppm CO2 in air is sobering as it is an extreme task as you rightly point out. I do expect that we will be forced to it (for a very long time) if politics don't change promptly. The discussion of what recovery processes will be needed for the ocean is also sobering. Since aragonite life forms are possibly the most vulnerable, the chemistry of ocean remediation is interesting. There is a slight reduction in the volume of calcium silicate over carbonate needed but it's still profoundly large. James Hansen made a quick reference to grinding igneous rocks in a talk. I've also been considering the work ability of it. You have been focusing on carbonate chemistry but the other part of the calcium carbonate solubility product calcium ions is worth a mention. In my readings so far I have not seen any discussion of Ca2+ concentrations. (I've not yet been looking for it) I wonder what the historical ocean Ca2+ concentrations have been over deep time. I've been curious about the [Ca2+] levels at paleo thermal maximums. I appreciate your work on this topic by the way. OA has been under discussed and is very concerning.
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  18. Supply of calcium is not an issue. [Ca2+] is well known and changes negligibly with depth and is roughly 0.010 mol kg-1. K's varies with both temperature and salinity but for example at 35S and 0oC K's for calcite is about 4.3 x10-7 and for aragonite is about 6.8 x10-7. Clearly carbonate dominates the solubility of CaCO3 in the ocean.
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  19. Thanks for your extra information. That resolves my question about calcium levels in todays oceans. There is still some un answered questions in my mind. Is the possible change in the pattern of weathering of rocks that occurs with higher CO2 levels in air an allie, enemy or not important? A change in the ratio of calcium silicate and calcium carbonate weathering is a possible positive feedback mechanism. You have focused on carbonate weathering but not silicate weathering. As rain becomes more acid for what ever reason I would expect a change in the ratio of weathering of these two minerals. i would expect more calcium carbonate to dissolve in rain that was more acid. This may not be that simple as land forms also play a role as when the Himalaya's weathered to allow the glacial/interglacial cycle to begin. As silicate precipitation will lead to pH increase which lead to bicarbonate/carbonate level changes it is also a factor. I can't tell if this is significant or not. Is the silicate cycle to some extent connected with the carbonate chemistry in the ocean? The reference (Harvey L.D.D. (2008) Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions.) does point to a limit in the use of ground limestone dissolved in ocean upwelling. It does not deal with dissolution in fresh water rivers or other outfalls which do not face that limit or a preference for silicate over carbonate. As the article points out carbonate is a very inefficient base for changing OA. In aquariums the addition of ground calcium carbonate to raise pH in the short term back fires as the pKa ratios of the dominant buffers have been moved away from 8.3 to 7.6 by the addition and so in the longer term acidification is not remediated. The correct balance of ionic species to give correct pKa (~ 8.3) is needed as well as base to correct aquarium pH not just simple addition of a base to raise pH. (Aquariums become more acid due to nitration and food breakdown products which is different from OA caused by increased CO2 in the air. But the effect of ignoring the existing buffers and there average pKa is interesting) Once a non carbon power source is used a possible method of remediation is less problematic. But as with the aquarium example the correct material will work, other less well designed interventions will just make it worse eventually. Again I'm not wanting to leave room open for fossil fuel use to be justified but to get clearer about possible workable remediation. Our fossil fuel use over the last 150 years has over whelmed the weathering effect that is now much slower. So my interest in speeding up or mimicking weathering comes from that.
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  20. Paul W, we are glad to see you are paying attention. Many of the things you ask are dealt with in the rest of the series. However one point may help you right now: The thing about future weathering is that the pH of rain just won't change much under realistic future atmospheric pCO2. As a chemist you will be able to approximate the answer quite easily. You will need to the Henry's law coefficient for CO2 in freshwater (see next post) and K1 for H2CO3 in freshwater. (Or include K2 for an exact solution). This change in rainwater pH will make only a slight difference to weathering that will take a long time (geologically long) to make a difference to the ocean carbon speciation. It is certainly possible to rapidly manipulate the chemistry of an aquarium. Then again, if it all goes badly it is possible to empty the aquarium and refill it. We suggest that the time and energy constraints on mining, grinding and dispersing any mineral precludes such an option as a realistic remediation for the ocean.
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  21. I agree with the lack of pH change from CO2 the other concern was with desperate measures like SO2 injected high into the sky to lower solar insolation. There may be a back fire with the weathering of rocks from the sulphur cycle increasing. Still the amount of sulphur will be small compared to the cooling effect. This comes back to my preferred OA remediation, (after ASAP end to fossil fuel use) charing of the organic waste stream (gardens, farms) and the build up of soil microbes as a result of the extra charcoal in the soil. Since about 60 giga tonnes is turning each year in the carbon cycle that is part of land based life, the task is not small but with out the above hazards. I wonder if we will be using waste heat from bio-char ovens to get warm in the coming generations? There is other buffers not just the ocean that will tend to unload CO2 back into the air after a reduction of the air born CO2 level begins. More reason to try and end fossil fuel use ASAP. Apart from that it's still a Gordian knot. I like to have an end point in a long journey like investigating ocean chemistry. :-)
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  22. Hi Paul, I see that Doug (comment 20) suggested you calculate the pH of rainwater in order to answer your question about the change in rainwater pH (comment 19). How did you go with this? You are correct - biochar soil remediation may help reduce atmospheric CO2. However, it is unlikely that the carbon sequestration would be any faster than the 150 years it has taken to put the CO2 into the atmosphere. This is unlikely to reverse ocean acidification (the topic of these posts) in any remotely useful time frame.
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  23. Would anyone who not a NZer spot the referencing in the title of this part? (I assume its a take on "always take the weather"?)
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  24. Are you referring to this "Australian" band? What kind of dirty creature would stoop to that sort of cultural appropriation? It makes me see red 8-)
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  25. scaddenp @23, Crowded House was an Australian band, formed in Melbourne, no less. Granted the genius behind it was a Kiwi.
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  26. Many kiwis consider Crowded House to be a reincarnation of the brilliant Split Enz, a band of obvious Kiwi origins. Anyway, what about Phar lap? And the pavlova?
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  27. @Paul W and @Doug Mackie Don't forget that weathering rates respond to changes in vegetation type & cover("bioogical enhanced weathering"), and the hydrological cycle - which will change due to increasing global temperatures at higher CO2. These changes will increase weathering rates, CO2 consumption and the supply of alkalinity to the ocean.
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  28. @rjowens: Please clarify your point. From the post above:
    But, as we shall see in later posts, when atmospheric CO2 is high then more acidic rain causes more weathering and that consumes CO2 to lower the atmospheric CO2. Only problem is that this happens over a geological time scale.
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  29. second summary post

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