# Why does decreasing surface alkalinity increase surface pCO2?

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?

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.

• 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 Jan 2, 2023 at 22:18

• @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-]. Nov 28, 2022 at 8:48
• 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. Nov 28, 2022 at 9:16