The Acid Test

So, what is this Ocean’s Acid Test all about? Obviously, it refers to the ocean acidification phenomenon. In order to explore this global phenomenon further, we need to first understand the mechanism and chemistry of the ocean.

Source: http://na.oceana.org/en/our-work/climate-energy/ocean-acidification/learn-act/what-is-ocean-acidification

The above diagram gives a nice summary of a chain of chemical reactions happening in the ocean with the dissolved CO2 coming from the atmosphere. Like carbonated fizzy drinks, when more CO2 dissolves in the water, it becomes more acidic. When more atmospheric carbon dioxide is in contact with surface water of the ocean, more is dissolved to form carbonic acid. This carbonic acid dissociates to form bicarbonate ion and hydrogen ion (proton) which therefore decreases the seawater pH. These protons are likely to aggregate with carbonate ion present in seawater to form bicarbonate ion, which decreases the amount of carbonate ion available in seawater.  As most marine organisms, especially coccolithophorids, pteropods and foraminiferans require carbonate ions to build their calcium carbonate shells and skeletons, the decrease in carbonate ion makes them harder to calcify or even unable to survive. Therefore, less carbon can be deposited at the bottom of the ocean in the form of calcium carbonate via this carbonate pump. 

In fact, our ocean has been able to buffer upon this constant increase in acidity for the last 420,000 years at least (Hoegh-Guldberg et al., 2007). This natural mechanism is mainly driven by the biological and carbonate pump to transport carbon from the surface waters to the bottom of the ocean. Hence, the ocean’s deep water is rich in carbonate ion which is able to neutralise its natural acidity through the mixing of surface and deep waters via turbulences and ocean currents (Rahmstorf and Richardson, 2008).

However, this neutralisation process takes over 100,000 years! Since Industrial Revolution (1750), atmospheric CO2 concentration has increased from the pre-industrial level of 285ppm to the present level of 389ppm today in less than 300 years. Ocean pH has also decreased by 0.1 unit from a level of about 8.2 (Fenchell, 2011). According to the past geological records, our world’s ocean has never absorbed such vast amount of CO2 in the atmosphere and experienced this sharp decrease in pH in such a short timescale, which I am going to discuss in the following weeks.

Many scientists have projected a further decrease of 0.1 pH unit of our ocean by the end of this century based on the current anthropogenic CO2 emissions. Undoubtedly, this acid test is indeed our oceans’ biggest global challenge ever in history. Calcifying marine organisms and the coral communities are the first biggest victims. Next week, I will be looking at the effects of acidified oceans on these marine organisms and how corals can be used as an important proxy to understand the past ocean chemistry. Stay tuned!

Reference:

Rahmstorf, S. and K. Richardson (2008) Our Threatened Ocean, Haus Publishing: London