Ocean alkalinity enhancement

The ocean carries tens of times the atmosphere’s inventory of dissolved inorganic carbon and exchanges CO2 with the air continuously. What limits its uptake is not contact with the atmosphere but its own chemistry: seawater is a buffer, and buffers resist exactly the change that CO2 imposes.

The buffer that holds the uptake back

Dissolved CO2 in seawater does not stay CO2: it reacts with water and with carbonate ion to form bicarbonate, and at today’s pH near 8.1 roughly nine tenths of the dissolved inorganic carbon sits as HCO3−. That rearrangement is what makes the ocean a buffer. When extra CO2 dissolves, it consumes carbonate ion (CO2 + CO3²− + H2O → 2 HCO3−), lowering the pH and depleting the very species that converts CO2 into its stored form; the increase in dissolved CO2 outruns the increase in total carbon — the Revelle factor — so the surface ocean’s appetite falls as it fills. Chemistry converts only part of the CO2 it contacts into bicarbonate, and the less carbonate remains, the slower the uptake.

What added alkalinity does

Alkalinity is seawater’s excess of proton-accepting bases over acids — in practice the charge carried by HCO3− and CO3²− plus minor species. Dissolving a silicate or a hydroxide raises it. Olivine, a magnesium–iron silicate, dissolves in carbonated water to give magnesium ions, silicic acid and bicarbonate; a mole of olivine adds on the order of four moles of alkalinity. Lime or brucite contributes hydroxide directly. In every case the added base shifts the carbonate equilibrium: pH rises, dissolved CO2 falls, and the surface water leaves equilibrium with the air, drawing down CO2 until the balance is restored — with the carbon locked as bicarbonate for as long as ocean circulation retains it, on the order of centuries to millennia. This is the same proton-consuming dissolution that enhanced rock weathering on land exploits; the ocean variant adds distribution by currents and an endpoint that is already in the right chemical form. In the long-run limit a mole of alkalinity yields somewhat less than a mole of CO2 taken up.

The measurement problem

Chemistry sets the potential; measurement decides what is creditable. Released alkalinity is diluted within hours into a moving, heterogeneous fluid whose own dissolved-carbon content swings with the seasons and from eddy to eddy, so the signal is small against natural variability. The removal also completes displaced in space and time: drawdown happens only as the plume exchanges gas across the sea surface, over weeks and drifting with currents. To this add the ways the accounting quietly loses tonnes: carbonate precipitation, which consumes two moles of alkalinity per mole of calcium carbonate formed; and ecological limits — pH swings near the release point, trace nickel and chromium from olivine — that cap how fast and how concentrated the addition can be. The limiting quantity of the route is therefore not tonnes of mineral but attributable uptake: moles of CO2 whose drawdown can be measured, assigned to the added alkalinity, and defended against background variation.