>These dissolved minerals are eventually carried by water to the ocean, where they are used by living organisms such as foraminifera, radiolarians, coccolithopores, and diatoms to create shells of CaCO3 (calcite) or SiO2 (opal) through the reactions Ca2+ (aq) + 2HCO3− (aq) → CaCO3(s) + CO2(g) + H2O(l) (for calcite precipitation) and SiO2(aq) → SiO2(s) (for opal precipitation).
Observe how half of the carbon is transformed back into CO2, and of the remaining half most will simply return carbonate after death and it's just a small portion of the latter half that falls to the sea floor to end up buried...
Adding alkalinity to the oceans via accelerated silicate weathering can protect sea life against acidification. More alkalinity in the ocean also increases its absorption capacity for CO2, which reduces warming feedback effects caused by radiative forcing from CO2 in the atmosphere. The ocean contains ~60x as much inorganic carbon as the pre-industrial atmosphere; to draw down the content of the present atmosphere to pre-industrial levels, it would require increasing the dissolved inorganic carbon content of the oceans by less than 1%. Converting some of it to solid carbonate minerals is a nice-to-have but not especially critical, because average residence time of dissolved inorganic carbon is about 200,000 years, which will take us well beyond the age of fossil fuels.
https://www.soest.hawaii.edu/oceanography/faculty/zeebe_file...
I was discussing the claimed carbon removal aspect, so I now disregard the acidification aspect...
In my analogy I stand corrected and should have stated that the bottle of water represented the surface seawater, since there is no free fast oceanic mixer. The surface water is equilibrating much faster with atmosphere than the whole water column of the ocean.
You defend the technology by mentioning that the average residence time is about 200.000 years, but that is for all inorganic carbon across the whole depth of the ocean, not just the surface layer! As you say there is not enough mixing, so dissolved inorganic carbon in surface seawater has a much shorter residence time. The long residence time is dominated by the slow movement of deep sea water...
So unless the proposal is augmented with either a huge oceanic mixer, or with dumping the carbonate over the mariana trench (if it dissolves there, it will take a long time before it reaches atmosphere again), lacking these augmentations we are dissolving carbonates in surface seawater, and the carbonate ions can equilibrate back to CO2 so it really is just emissions foisted of as capture...
If it stays in the solid say calcite CaCO3 state as opposed to being dissolved, then it does not affect the alkalinity of the surface water...
As a small scale example, consider a beaker of distilled water freely exposed to the atmosphere. It dissolves a small amount of CO2 and becomes slightly acidic from carbonic acid. Now add sodium hydroxide -- it becomes strongly alkaline. Wait again and the pH falls again (though not to its original level), due to dissolving more carbon dioxide, which is deprotonated to form carbonate anions. You can keep adding hydroxide and absorbing more atmospheric CO2 for quite some time, until solubility limits come into play. Even though it is just a solution at equilibrium with the atmosphere, and not a precipitated solid, the sodium carbonate solution will not spontaneously separate back to sodium hydroxide solution in the beaker and CO2 in the surrounding atmosphere. It takes thermodynamic work to reverse the carbonate-heavy equilibrium.
Consider this (caricaturized) argument: suppose most artificial mummies are mummified humans, that does not mean most humans end up mummified!
please check out the sibling comment by philipkglass, the referenced text has a section on calcite compensation, and the previous section detailing the saturation horizon. At high enough pressure calcite prefers being dissolved again.
Consider a calcite diatom cap, close to the beach, since it is above the saturation horizon, it won't dissolve soon, now consider a diatom dropping dead over the mariana trench, so the calcite starts sinking, until it passes the saturation horizon, now the pressure is clearly high enough to prefer being dissolved...
