<p>In answering this complex question, I think it's important to consider the form of carbon that is utilized and the overall reactions involved (warning: somewhat lengthy diversion ahead, and advance apologies if such is already understood by this audience). In the long-term carbon cycle, atmospheric <span class="math-container">$\ce{CO_{2(g)}}$</span> drives weathering on land, importantly of silicates. <span class="math-container">$\ce{CO_{2(g)}}$</span> dissolves in pure water forming (<span class="math-container">$\ce{CO_{2(aq)}}$</span>), which can react with water to form carbonic acid (<span class="math-container">$\ce{H_2CO_{3(aq)}}$</span>):
<span class="math-container">$$ \ce{CO_{2(g)} + {H_{2}O_{(l)}}  &lt;=&gt; {H_2CO_{3(aq)}} }$$</span>
but true carbonic acid is unstable, and the vast majority of this total dissolved carbon is thus present as dissolved molecular <span class="math-container">$\ce{CO_{2(aq)}}$</span>, with concentrations obeying the simple <a href="https://en.wikipedia.org/wiki/Henry%27s_law" rel="nofollow noreferrer">Henry's law</a> relation (not shown). We can write this net reaction as:
<span class="math-container">$$ \ce{CO_{2} + {H_{2}O}  &lt;=&gt; {H_2CO_{3(aq)}} &lt;=&gt; H^{+}_{(aq)} + HCO_^{-}_{3(aq)} }$$</span>
Bicarbonate also reflects the association/dissociation reaction involving pure carbonate ion,
<span class="math-container">$$
\ce{H^{+} + CO_^{2-}_{3} &lt;=&gt; HCO_^{-}_{3}}
$$</span>
Weathering of a continental silicate, e.g., a normative wollastonite (chosen for simplicity), can thus be written as<span class="math-container">$$
\ce{ CaSiO_3 + H_{2}O +2CO_{2} -&gt; 2 HCO_^{-}_{3} + Ca^{2+} + SiO_{2}}
$$</span>
The important point is that the dissolved carbon delivered to seawater by continental weathering is as bicarbonate, and surface seawater pH (<span class="math-container">$\sim 8)$</span> in approximate equilibrium with atmospheric <span class="math-container">$\ce{CO_{2}}$</span> indeed reflects this distribution of dissolved species. If we write the equilibrium for skeletal calcium carbonate as
<span class="math-container">$$\ce{ CaCO_{3} &lt;=&gt; CO_^{2-}_{3} + Ca^{2+}}$$</span>
we can combine the above equations to give the overall reaction for the dissolution/precipitation of <span class="math-container">$\ce{CaCO_3}$</span> in seawater (incorporating the mass balance on <span class="math-container">$\ce{CO_2}$</span> from reactions 1., 2., 3., and 5.) as,
<span class="math-container">$$
\ce{CaCO_{3} + H_{2}O + CO_{2} = Ca^{2+} + 2HCO_^{-}_{3} }
$$</span>
Thus in this context, an important result is that, in the overall mass balance associated with precipitation of <span class="math-container">$\ce{CaCO_{3}}$</span> (i.e., driving the above reaction to the left), <span class="math-container">$\ce{CaCO_{3}}$</span> serves as a source, not a sink of carbon, because it liberates one mole of <span class="math-container">$\ce{CO_2}$</span> originally delivered as bicarbonate to seawater from silicate weathering. In the larger view (long term geochemical cycling), marine carbonates are a temporary sink for carbon that is ultimately subducted and returned to the atmosphere by vulcanism and other magmatic processes. The carbonates residing on the continents today (e.g., Paleozoic) have, in contrast, a much longer residence or cycling time that their modern pelagic equivalents.</p>
<p>Now, to your question: ocean acidification (titration of seawater by atmospheric <span class="math-container">$\ce{CO_2}$</span>, driving the last reaction to the right) per se is buffered by the attack of this sink of <span class="math-container">$\ce{CaCO_3}$</span> producing more bicarbonate. Sequestration of carbon by growing shellfish will, according to the above, consume (some of) this bicarbonate, and produce <span class="math-container">$\ce{CO_2}$</span> as a result. The larger, key question to me, is the rate at which the existing store of sedimentary <span class="math-container">$\ce{CaCO_3}$</span>, particularly in surface water where it is being formed, dissolves in response to this attack.</p>

CC BY-SA 3.0
2018-04-09T10:23:07.737Z
"Sequestration of carbon by growing shellfish will... produce CO2 as a result" Wow, that's quite unexpected! Thanks for being so thorough on the processes, and for pointing out the important role of bicarbonate.
Warbo
CC BY-SA 4.0
2018-04-08T10:30:37.860Z
2021-09-18T10:08:18.680Z
2021-09-18T10:08:18.680Z
Rolf
<p>This might seem like a basic question, but I'm a little confused about the effects on ocean acidification of different carbon compounds.</p>

<p>I have heard that increased levels of carbon dioxide in the atmosphere are causing ocean acidification, lowering the ocean's pH.</p>

<p>I have also heard that shellfish like mussels and oysters can be used as a carbon sink, making shellfish farming a form of carbon sequestration. This makes sense, as their shells contain carbon in the form of calcium carbonate.</p>

<p>My question is how does such sequestration affect ocean acidification? On the one hand, sequestering the carbon as calcium carbonate would lower the carbon presence in the ocean, which would seem to reduce the effects of CO<sub>2</sub>. On the other hand, calcium carbonate is alkali: leaching minerals from the ocean to form such shells would seem to increase acidity (since the opposite process, dissolving calcium carbonate, reduces acidity).</p>

<climate-change><oceanography><geochemistry><carbon-cycle>
Carbon's role in ocean acidification
Warbo
# Carbon's role in ocean acidification

(Asked by: username_0 on 2018-04-05)

This might seem like a basic question, but I'm a little confused about the effects on ocean acidification of different carbon compounds.

I have heard that increased levels of carbon dioxide in the atmosphere are causing ocean acidification, lowering the ocean's pH.

I have also heard that shellfish like mussels and oysters can be used as a carbon sink, making shellfish farming a form of carbon sequestration. This makes sense, as their shells contain carbon in the form of calcium carbonate.

My question is how does such sequestration affect ocean acidification? On the one hand, sequestering the carbon as calcium carbonate would lower the carbon presence in the ocean, which would seem to reduce the effects of CO2. On the other hand, calcium carbonate is alkali: leaching minerals from the ocean to form such shells would seem to increase acidity (since the opposite process, dissolving calcium carbonate, reduces acidity).

**Tags:** climate-change, oceanography, geochemistry, carbon-cycle


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## 1 answers

### Answer by username_1 on 2018-04-08. Score: 5 | Accepted answer

In answering this complex question, I think it's important to consider the form of carbon that is utilized and the overall reactions involved (warning: somewhat lengthy diversion ahead, and advance apologies if such is already understood by this audience). In the long-term carbon cycle, atmospheric $\ce{CO\_{2(g)}}$ drives weathering on land, importantly of silicates. $\ce{CO\_{2(g)}}$ dissolves in pure water forming ($\ce{CO\_{2(aq)}}$), which can react with water to form carbonic acid ($\ce{H\_2CO\_{3(aq)}}$):
$$ \ce{CO\_{2(g)} + {H\_{2}O\_{(l)}} <=> {H\_2CO\_{3(aq)}} }$$
but true carbonic acid is unstable, and the vast majority of this total dissolved carbon is thus present as dissolved molecular $\ce{CO\_{2(aq)}}$, with concentrations obeying the simple [Henry's law](https://en.wikipedia.org/wiki/Henry%27s_law) relation (not shown). We can write this net reaction as:
$$ \ce{CO\_{2} + {H\_{2}O} <=> {H\_2CO\_{3(aq)}} <=> H^{+}\_{(aq)} + HCO\_^{-}\_{3(aq)} }$$
Bicarbonate also reflects the association/dissociation reaction involving pure carbonate ion,
$$
\ce{H^{+} + CO\_^{2-}\_{3} <=> HCO\_^{-}\_{3}}
$$
Weathering of a continental silicate, e.g., a normative wollastonite (chosen for simplicity), can thus be written as$$
\ce{ CaSiO\_3 + H\_{2}O +2CO\_{2} -> 2 HCO\_^{-}\_{3} + Ca^{2+} + SiO\_{2}}
$$
The important point is that the dissolved carbon delivered to seawater by continental weathering is as bicarbonate, and surface seawater pH ($\sim 8)$ in approximate equilibrium with atmospheric $\ce{CO\_{2}}$ indeed reflects this distribution of dissolved species. If we write the equilibrium for skeletal calcium carbonate as
$$\ce{ CaCO\_{3} <=> CO\_^{2-}\_{3} + Ca^{2+}}$$
we can combine the above equations to give the overall reaction for the dissolution/precipitation of $\ce{CaCO\_3}$ in seawater (incorporating the mass balance on $\ce{CO\_2}$ from reactions 1., 2., 3., and 5.) as,
$$
\ce{CaCO\_{3} + H\_{2}O + CO\_{2} = Ca^{2+} + 2HCO\_^{-}\_{3} }
$$
Thus in this context, an important result is that, in the overall mass balance associated with precipitation of $\ce{CaCO\_{3}}$ (i.e., driving the above reaction to the left), $\ce{CaCO\_{3}}$ serves as a source, not a sink of carbon, because it liberates one mole of $\ce{CO\_2}$ originally delivered as bicarbonate to seawater from silicate weathering. In the larger view (long term geochemical cycling), marine carbonates are a temporary sink for carbon that is ultimately subducted and returned to the atmosphere by vulcanism and other magmatic processes. The carbonates residing on the continents today (e.g., Paleozoic) have, in contrast, a much longer residence or cycling time that their modern pelagic equivalents.

Now, to your question: ocean acidification (titration of seawater by atmospheric $\ce{CO\_2}$, driving the last reaction to the right) per se is buffered by the attack of this sink of $\ce{CaCO\_3}$ producing more bicarbonate. Sequestration of carbon by growing shellfish will, according to the above, consume (some of) this bicarbonate, and produce $\ce{CO\_2}$ as a result. The larger, key question to me, is the rate at which the existing store of sedimentary $\ce{CaCO\_3}$, particularly in surface water where it is being formed, dissolves in response to this attack.

> **Comments:**
> **username_0**: "Sequestration of carbon by growing shellfish will... produce CO2 as a result" Wow, that's quite unexpected! Thanks for being so thorough on the processes, and for pointing out the important role of bicarbonate.  

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