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According to Takahashi et al. (1993), pCO2 (partial pressure of CO2) in the surface ocean changes due to 4 surface variables: temperature, salinity, total carbon (dissolved inorganic carbon, DIC) and total alkalinity (ALK). The latter relationship, in specific, is inversely proportional. Increasing alkalinity decreases surface pCO2 and vice versa. Why?

If anything, I would expect that decreasing alkalinity would decrease pCO2. I would believe this because removing carbonate and bicarbonate ions (which compose alkalinity), at constant pH, would force some CO2 to dissolve into these ions again to maintain the relative distribution at a constant pH, thus decreasing pCO2. This relative balance is illustrated for example in the figure below from chapter 8 in Sarmiento (2013). Why is the opposite true?

enter image description here

Taro Takahashi; Jon Olafsson; John G. Goddard; David W. Chipman; S. C. Sutherland (1993). Seasonal variation of CO2 and nutrients in the high-latitude surface oceans: A comparative study. , 7(4), 843–0. doi:10.1029/93gb02263

Sarmiento, Jorge L. "Ocean biogeochemical dynamics." Ocean Biogeochemical Dynamics. Princeton University Press, 2013.

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  • $\begingroup$ Is this about The Revelle Factor? Revelle Factor/buffer factor is a measure of the resistance of the ocean surface layer to absorption of atmospheric CO2. en.wikipedia.org/wiki/Revelle_factor $\endgroup$
    – Ken Fabian
    Jan 2, 2023 at 22:18

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The answer is right there on the figure, also known as Bjerrum plot. If you decrease alkalinity (i.e. increase pH) you shift the relative proportions of the plot to the right, that is, decreasing H2CO3 (or dissolved CO2) concentrations, and increasing bicarbonate and carbonate ions concentrations. And vice versa. Increasing alkalinity decreases pH, increasing pCO2.

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  • $\begingroup$ One can change alkalinity without changing pH. This is why I mentioned "at constant pH" in the question. For example, if one adds OH- ion to sea water, it doesn't change pH. Why does it decrease pCO2? $\endgroup$
    – earthyguy
    Nov 27, 2022 at 13:15
  • $\begingroup$ Sorry, this doesn't answer the question. "At constant pH" is important here. $\endgroup$
    – earthyguy
    Nov 27, 2022 at 13:24
  • $\begingroup$ Decreasing alkalinity is not a synonym for increasing pH. Alkalinity is a sum of non-conservative ions, including H+ but not restricted to it. $\endgroup$
    – earthyguy
    Nov 27, 2022 at 13:27
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    $\begingroup$ @ouranos - Changing concentration of OH- (e.g.due dissolving natural carbonates and their hydrolysis) directly impacts concentration of H+ via $\ce{H+ + OH- <=> H2O}$ with Kw=[H+][OH-]. $\endgroup$
    – Poutnik
    Nov 28, 2022 at 8:48
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    $\begingroup$ Yes, it does, by increasing the OH- concentration // $\ce{MgO(s) + H2O(l) -> Mg(OH)2(s) <=>[H2O]Mg^2+(aq) + 2 OH-(aq)}$ // $\ce{OH-(aq) + H+(aq) <=>>H2O(l)}$ // pH increases as OH- ions are scavenging H+ ions until the equilibrium is reached. $\endgroup$
    – Poutnik
    Nov 28, 2022 at 9:16

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