# Are there sufficient quantities of limestone to dump in the ocean to reverse acidification?

This question asked: What is the evidence it is feasible to reverse ocean acidification by adding large quantities of a base (bicarb soda)?

The result was:

Danny Harvey of the University of Toronto has already looked into this. His solution is to deposit 4 Gt/a of limestone into the oceans. To put this in perspective, global coal production in 2013 was nearly double this at 7.823 Gt and it is more than global iron ore production in 2014 of 3.22 Gt.

The respondent then posed the question: Are there sufficient quantities of easily obtainable limestone to do this?

My question is: Are there sufficient quantities of limestone to dump in the ocean to reverse acidification?

• In the context of though-experiment, maybe limestone under the sea floor could be easier to cut and grind in situ and pump up into the water column. Thinking about the North Sea Chalk Group and Ontong Java e.g. – Tactopoda Aug 1 '15 at 20:39
• Dissolving carbonate in an acid will result in the emission of CO2... – Floris Aug 6 '15 at 3:56

Getting figures on the amount of limestone available is difficult.

Apparently "limestone makes up at least 10% of the total volume of all sedimentary rocks".

One way to answer your question is by inference. Cement is manufactured from limestone. Current global production of cement is in excess of 3.27 Gt/a and by 2030 it is forecast to be approximately 4.83 Gt/a. Thus if all limestone currently earmarked for cement production were diverted to neutralizing the acidity of the world's oceans it could be done at short notice. How sustainable it would be over the 200 years required, is another matter. It also raises the question of what happens to all the industries and projects that current rely on a cheap and ready supply of cement?

With much tongue in cheek, the Florida peninsular in the US is predominantly limestone, likewise a significant portion of eastern England, with the "white cliffs of Dover, the karst regions of the eastern Adriatic, southern Europe, southern and eastern Asia and the Nullabor Plain in southern Australia. Would these countries be will to sacrifice their lands to neutralize acidity in the worlds oceans?

• You would also have to factor in the dissolution rate of the limestone and the mixing time of the oceans. It wouldn't do much good to dump a lot of limestone in one spot because it would likely settle before dissolving. If it were fine enough to stay suspended it could have harmful effects on the local ecosystem. – haresfur Aug 2 '15 at 22:46

Yes, there is much more than enough limestone, by several orders of magnitude, to neutralize the acidity that we are creating - so much so that I am not even going to bother with the back of an envelope calculation. Given enough time (many millennia and possibly several million years) this will happen naturally. However,the process of re-equilibration with the introduced CO2 is sluggish because of both the reaction kinetics and, more particularly the deep ocean water mixing time. The current rate of CO2 emissions far exceeds both the rate of neutralization and the rate with which we could seed the oceans with CaCO3. Consider that, for limestone neutralization to succeed we would have to micronize entire hills and mountain ranges of limestone, and then distribute it, reasonably uniformly, over the world's oceans. Technically do-able, perhaps, but mind-bogglingly expensive and probably politically divisive. For many reasons it would be cheaper, easier and more ecologically appropriate to convert to renewable energy ASAP.

The above comment that the ocean's pH is temperature-controlled is wrong. There is a temperature effect upon pH - see for example http://www.aslo.org/lo/toc/vol_14/issue_5/0679.pdf . However, this is insufficient to counter the oceanic acidification. Already the pH has decreased by between 0.06 and 0.10, with a global average decrease of about 0.075 (to 1990). See https://en.wikipedia.org/wiki/Ocean_acidification. The last two decades have seen an acceleration of this trend. By century end most models project a mean pH decrease from 8.2 to 7.8 in the upper few hundred metres of ocean, where biological productivity is greatest. This is faster than at any time in the last 300 million years. To be effective, any neutralization with limestone would have to be achieved within a few decades, at a rate of neutralizing about 100 ppm CO2 over the entire ocean surface, and to a depth of several hundred metres.

There doesn't seem much point in trying to 'de-acidify' something that is fact an alkali.

The pH of the sea has far more to do with its temperature then anything else, it's ability to hold $\ce{CO2}$ for example is dependent on this.

Acidification of the oceans is a myth, it is if anything, becoming less Alkali, closer to neutral pH.

• To acidify merely means to lower the pH, so the term would still be correct for going from higher alkalinity to lower alkalinity. Yes, it is both temperature and CO2 concentration that is acidifying the oceans. Changing either of those is very difficult. "Acidification of the oceans is a myth", yeah tell that to all the dying coral and calcium carbonate shelled plankton. – Eubie Drew Nov 7 '15 at 20:55
• Ok....but the idea of dumping limestone in the ocean the raise it's ph won't work because limestone reacts with acids not alkalis – acidohm Nov 7 '15 at 22:44
• To correct what someone said earlier....There is no acidity to neutralise, the ocean is 8.1, it was 8.2...it is alkali not acid. ..to neutralise it would be the opposite way you want.... – acidohm Nov 7 '15 at 22:55
• The CaCO3 does not react directly with the cation of an acid. The CaCO3 dissolves to Ca(+2) and CO3(-2), Some of the CO3(-2) combines with some of the H+ in solution (H+ is the H in pH), making HCO3(-), lowering H+, and raising the pH.. This happens even if the seawater is "alkali", because there is still available H+, just less of it compared to neutral. You are semantically correct that this is not neutralizing, it is making the water more alkaline, restoring it to where it should be. A better word than "neutralize" would be "restore". – Eubie Drew Nov 7 '15 at 23:11
• There is nothing magic about crossing the neutral mark, pH 7, where supposedly one set of reactions would stop working and another set would take over. CaCO3, limestone, will still raise the pH, whether it is put in acidic seawater (which hopefully will never exist) or weakly alkali seawater. --- 1st sentence of previous comment should have "ions" not "cation". – Eubie Drew Nov 7 '15 at 23:17