How does the increase in CO2 decrease ocean carbonate concentrations?

Question

I have read in my biology textbook (Campbell Concepts and Connections 10e, p.75) that increases in CO$_2$ cause acidification of oceans, which shifts the equilibrium to the side without CO$_3^{2-}$. So far so good. Multiple other sources, such as the NOAA page on ocean acidification, support this mechanism. However, the increase in CO$_2$ would also be expected to increase the total concentration of HCO$_3^-$ and related species, such as CO$_3^{2-}$. The Wikipedia article for carbonic acid states that the oceans have acidified by 0.1 pH units since pre-industrial times. The image used by the article to show relative concentrations of these species changing with pH shows CO$_3^{2-}$ change roughly linearly to the logarithmic y-axis around pH 8. This would suggest only a $10^{-0.9}\approx0.795$ or 79.5% of the pre-industrial ratio of carbonate ions.

However, the total concentration of CO$_2$ in the atmosphere has increased by about 50% from 280 ppm to around 420 ppm, meaning the concentration of CO$_3^{2-}$ would be expected to increase by about 20%.

Why is this not the case?

Answer 1

Because the data for those numbers and gradients is derived from a closed environment experiment that doesn't reflect the true nature of the ocean as a whole, even as it does describe the behaviour of it's primary element under experimental conditions. The ocean as a whole body interacts with the life in it, crustal rocks, river inputs of water, sediments, and dissolved solids, magmatic inputs, etc...; all of these alter it's behaviour away from the experimental based expectations.

Add to that the fact that Earths atmosphere and ocean both behave like buffered solutions, meaning visible change can lag inputs, in this case CO₂, quite substantially.

Furthermore human activity has also added a substantial volume of other compounds to the oceans, both organic and inorganic. The three largest contributors are sediment from deforestation, a distant second agricultural run-off, and a close third industrial process wastes. Some of these compounds ultimately limit the solubility of other species in the overall dissolved chemical equilibrium system that is salt water, this can then affect the solubility of carbonates.