Why does sound absorption in oceans depend on the pH?

Question

I was reading "The Sixth Extinction: An Unnatural History", by Elizabeth Kolbert, and there she comments that high level of $CO_{2}$ in the atmosphere lowers the pH of oceans (which makes sense) and, consequently, diminishes sound absorption:

WHY is ocean acidification so dangerous? The question is tough to answer only because the list of reasons is so long. Depending on how tightly organisms are able to regulate their internal chemistry, acidification may affect such basic processes as metabolism, enzyme activity, and protein function. Because it will change the makeup of microbial communities, it will alter the availability of key nutrients, like iron and nitrogen. For similar reasons, it will change the amount of light that passes through the water, and for somewhat different reasons, it will alter the way sound propagates. (In general, acidification is expected to make the seas noisier.)

I do understand that sound absorption depends on the material properties of the medium, but it is not trivial to me that more acidic water should have a different absorption than neutral water. Could anyone explain the physical mechanism behind this?

(I'm not exactly sure that this belongs here, but since it deals with material properties, I thought it would fit better here than in Chemistry SE)

Answer 1

Hard though it is to believe, pH does have an effect on sound absorption in water.

There are some reactions that are affected by pressure, that is pressure changes their equilibrium. One example is the equilibrium between boric acid and the borate ion:

$$ B(OH)_4\,^- + H^+ \rightarrow B(OH)_3 + H_2O $$

Increasing pressure pushes the reaction over to the right, and in doing so it absorbs energy. That energy comes from the energy in the sound wave, so unlikely though it may seem the chemical reaction absorbs sound. When the pressure is released the reaction moves back to the left, but the energy is released as heat not sound. The net result is sound energy gets converted to heat.

The problem with lowered pH is that as you lower the pH that moves the equilibrium to the right so there is less borate and more boric acid. That leaves less borate to absorb the sound. So lowered pH means less sound absorption.

If you're interested there is an article about this in Scientific American, and a more rigorous scientific publication here (sadly not available online). Lemon helpfully provided this link, Investigation of chemical sound absorption in sea water, and Googling should find you more relevant publications.

The effect is very small, not least because boric acid/borate concentrations in seawater are small. However sound propagates for huge distances in water - hundreds of miles. So even a small change in the water chemistry can have a measurable effect. The first of the papers I linked mentions that:

Sound attenuation in the low-frequency range is primarily due to boric acid relaxation and is a function of the seawater pH.

(my emphasis)

Whether that effect is actually big enough to deafen dolphins is a matter of some debate.

Answer 2

The author should have been more specific on the matter or at least provide a reference. The speed of sound in water depends on the bulk modulus and density of the water, so in the open oceans the factors that most affect the speed of sound are salinity, depth (pressure) and temperature. I was an ocean engineering major and have taken courses in physical and chemical oceanography, underwater instrument systems, etc., and I don't believe I've ever seen anything relating pH to sound speed.

Perhaps the author was thinking that changes in pH are related to salinity. There is a connection. But "Making the seas 'noisier'" seems to be an odd statement regarding the speed of sound. That's more a subject for sound absorption, but I don't see how pH could change that either.