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Earth

Ocean carbonate chemistry

Bubble air of a chosen CO2 through a beaker of seawater on a laboratory bench and watch the indicator change colour. The water’s dissolved carbon and alkalinity fix its pH and the shares of dissolved CO2, bicarbonate and carbonate ion, with the published constants of Lueker, Dickson, Millero, Weiss and Mucci; the Revelle factor says why the water’s CO2 pressure rises ten times faster than its carbon. Raise the air’s CO2 and the carbonate ion falls until, near four thousand micro-atmospheres, the water can no longer keep aragonite: the shell on the bottom begins to dissolve, and its two units of alkalinity for every carbon buffer the water back to just under saturation. Cold water of the same chemistry holds less carbonate. Carbon in water, shell and air is conserved to rounding.

By Mykhailo Makarov

Ocean carbonate chemistry — Actual application image
Actual application image

What happens

A kilogram of seawater with 2300 µmol/kg of alkalinity and 2000 of dissolved inorganic carbon at 25 °C and salinity 35. The pH on the total scale solves TA = DIC (K1 h + 2 K1 K2)/(h² + K1 h + K1 K2) + B_T K_B/(K_B + h) + K_w/h − h by bisection, with K1 and K2 of Lueker et al. (2000), K_B of Dickson (1990), Uppström’s boron, K_w of Millero (1995), K0 of Weiss (1974) and the aragonite product of Mucci (1983). Gas exchange relaxes the dissolved CO2 to K0 pCO2_air; a shell of 2 g of aragonite dissolves at k (1 − Omega)^1.5 when Omega < 1, adding one carbon and two alkalinity per unit. RK4 at 1 min.

Try this

Raise the air’s CO2 and watch the indicator turn; compare the carbonate ion at two temperatures; push the water past saturation and watch the shell dissolve and buffer it; read the Revelle factor.

The model and its limits

Equilibrium chemistry with published constants at one atmosphere, no nutrients, organic matter or calcite, a single exchange time, a lumped dissolution law. Values belong to this idealized beaker.

Equations used in the module
Equations
TA = DIC·(K1·h + 2K1·K2)/(h² + K1·h + K1·K2) + B_T·K_B/(K_B + h) + K_w/h − h
pCO2 = [CO2]/K0,  Ω = [Ca]·[CO3]/K_sp
dDIC/dt = (K0·pCO2_air − [CO2])/τ + R,  dTA/dt = 2R
R = k·(m/m0)^(2/3)·(1 − Ω)^1.5 for Ω < 1,  Revelle = (dp/p)/(dDIC/DIC)

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