Thursday, May 14, 2015

Ocean Acidification- What Exactly Happens?

I've started following @Scitable, an education resource from journal Nature. A few days back they tweeted a link to an article on ocean acidification.

Pay attention-
When CO2 dissolves in seawater to produce aqueous CO2 (CO2(aq)) it also forms carbonic acid (H2CO3) (Eq. 1; Figure 1). Carbonic acid rapidly dissociates (splits apart) to produce bicarbonate ions (HCO3-, Eq. 2). In turn, bicarbonate ions can also dissociate into carbonate ions (CO32-, Eq. 3). Both of these reactions (Eqs. 2, 3) also produce protons (H+) and therefore lower the pH of the solution (i.e., the water is now more acidic than it was — recall that pH is the negative logarithm of the proton concentration or activity, -log10[H+]. Note, as illustrated in Figure 2, Ocean Acidification does not imply that ocean waters will actually become acidic (i.e., pH < 7.0).

CO2(aq) + H2O ↔ H2CO3 (1)
H2CO3 ↔ HCO3- + H+ (2)
HCO3- ↔ CO32- + H+ (3)
However, when CO2 dissoves in seawater it does not fully dissociate into carbonate ions and the number of hydrogen ions produced (and the drop in pH) is therefore smaller than one might expect. This is due to the natural capacity of seawater to buffer against changes in pH, which can be represented simply by:

CO2(aq) + CO32- + H2O → 2HCO3- (4)

where CO2 is effectively neutralized by reaction with CO32- to produce HCO3-. The HCO3- produced by Eq. 4 then partly dissociates (Eq. 3), releasing protons and so decreasing the pH-which is where the ‘ocean acidification' actually comes from-but this drop is much smaller than for an un-buffered system. One can also think of the sequence of events resulting from dissolving CO2 in seawater as firstly the production of HCO3- and H+, but because the equilibrium between HCO3- and CO32- (Eq. 3) has now been unbalanced by excess acidity (H+), Eq. 3 goes to the left to consume some of the excess H+, and in doing so, also consumes CO32-.

This is a wonderfully clear explanation of the chemistry of ocean acidification in terms of changing  concentrations or activity of CO2 (atmospheric),  CO2(aqueous), CO3 (carbonate) and bicarbonate (HCO3) ions, as there is later in the article on the negative and positive feedbacks in terms of the capacity of the ocean to absorb CO2 with rising ocean  temperatures.

The proportion of DIC present as CO2 is also affected by temperature, as illustrated in Figure 2. The consequence of this is that, as the ocean warms, less DIC will be partitioned into the form of CO2 (and more as CO32-), hence enhancing the buffering and providing a ‘negative feedback' on rising atmospheric CO2. Here, a feedback describes a mechanism that dimishes or amplifies an initial change and asribed the sign ‘negative' or ‘positive', respectively. For example, melting polar ice caps through global warming will reduce the amount of solar radiation that is reflected back out to space (the Earth's surface becomes less reflective), so producing more warming, which in turn will melt more ice, and so on — a positive feedback. A well-known positive feedback in the carbon cycle arises due to the decrease in solubility of CO2 gas in seawater at higher temperatures. In fact, this greatly outweighs the negative feedback described above, meaning that as the ocean surface warms, even more of the emitted fossil fuel CO2 will remain in the atmosphere.

And how do saturation levels of CO3 in sea water affect the stability of CaCO3 mineral species Aragonite and Calcite which organisms use to build skeletons? What effect will increase in ocean acidification have on organisms? ...

read on.. don't miss out on this chemistry lesson.

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