Is it possible to (naturally) change alkalinity without changing pH?

The [Bjerrum plot](https://en.wikipedia.org/wiki/Bjerrum_plot) shows how the different species of dissolved inorganic carbon (DIC) are distributed in sea water as a function of pH. The higher the pH, the more carbonate ion, less dissolved $\ce{CO2}$ (i.e. $\ce{H2CO3}$), and vice versa. And according to [Takahashi et al. (1993)](https://agupubs.onlinelibrary.wiley.com/doi/10.1029/93GB02263), this is relevant for the $\ce{pCO2}$. That is, the higher the pH, the lower the $\ce{pCO2}$.


Are there processes in nature that can change alkalinity, [as defined by Dickson et al. (2007)](https://cdiac.ess-dive.lbl.gov/ftp/oceans/Handbook_2007/Guide_all_in_one.pdf) (i.e. sum of negative charges of non-conservative ions), *without changing pH?*


Here is the definition:



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> 4.2 Total alkalinity
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> The total alkalinity of a sample of sea water is a form of mass-conservation
> relationship for hydrogen ion. It is rigorously defined (Dickson, 1981) as “. . . the number of moles of hydrogen ion equivalent to the excess of proton acceptors (bases formed from weak acids with a dissociation constant K ≤ 10 –4.5 at 25°C and zero ionic strength) over proton donors (acids with K > 10 –4.5 ) in 1 kilogram of sample.” Thus
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> $A\_T = [\ce{HCO3-}] + 2[\ce{CO3^2-}] + [\ce{B(OH)4^-}] + [\ce{OH-}] + [\ce{HPO4^2-}] + 2[\ce{PO4^3-}] + [\ce{SiO(OH)3^-}] + [\ce{NH3}] + [\ce{HS-}] + ...
> - [\ce{H+}]\_F - [\ce{HSO4-}] - [\ce{HF}] - [\ce{H3PO4}] ...$ (13)
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> where the ellipses stand for additional minor acid or base species that are either unidentified or present in such small amounts that they can be safely neglected.
> $[\ce{H+}]\_F$ is the free concentration of hydrogen ion
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This last term is defined later in the text according to


$[\ce{H+}] = [\ce{H+}]\_F\left(1+\frac{S\_T}{K\_S}\right)$ where $S\_T = [\ce{SO4^2-}] + [\ce{HSO4-}]$ and $K\_S$ is the $K\_a$ for $\ce{HSO4-}$.


[Zeebe & Wolf-Gradow (2001)](http://geosci.uchicago.edu/%7Ekite/doc/Zeebe_CO2_In_Seawater_Ch_1.pdf) show in their figure 1.1.3 that the air-sea exchange of $\ce{CO2}$ does not change alkalinity (the horizontal line). Although it does change pH (diagonal background lines). This is in fact a problem related to antrhopogenic climate change, as $\ce{CO2}$ is taken up by the ocean, [causing acidification](https://www.nature.com/scitable/knowledge/library/ocean-acidification-25822734/), and posing threats to marine ecosystems. I was wondering if there is a process that draws a line that is parallel to the pH isolines in the figure below.


[![enter image description here](https://i.stack.imgur.com/BRDNc.png)](https://i.stack.imgur.com/BRDNc.png)




If there are 2 controlled processes, one increasing alkalinity, e.g. by dissolving $\ce{MgO(s)}$ or alkaline earth carbonates, and the other dissolving $\ce{CO2(g)}$, so the concentration ratios of $\ce{CO2(aq)}$, $\ce{HCO3-(aq)}$ and $\ce{CO3^2-(aq)}$ are constant, then yes.


It is theoretically possible, as the above constant ratios would form pH buffers with constant [$\ce{H+}$], [$\ce{OH-}$], but with variable concentrations and therefore alkalinity.


But in geological context, it is highly inprobable both processes occur in such a balanced way, so generally no, it is not possible and can occur just accidentally.


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