Whether seawater lets a shell-forming organism build calcium carbonate — or slowly dissolves it — is set by the saturation state:
Ω = [Ca²⁺][CO₃²⁻] / K'sp
Ω > 1 → water is supersaturated, precipitation favoured
Ω < 1 → water is undersaturated, dissolution favoured
[CO₃²⁻] comes from the carbonate equilibrium system. Rising atmospheric CO₂ dissolves into the surface ocean (Henry's law, constant K₀), forms carbonic acid, and shifts the equilibrium away from carbonate ion:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻ (K₁, K₂)
Total alkalinity: TA ≈ [HCO₃⁻] + 2[CO₃²⁻]
Given pCO₂ and TA, [H⁺] is solved from the quadratic TA·[H⁺]² − K₁[CO₂][H⁺] − 2K₁K₂[CO₂] = 0, which then gives pH, [HCO₃⁻] and [CO₃²⁻]. This simulator uses the real, temperature-dependent Weiss (1974) K₀ and Mehrbach/Dickson-Millero (1987) K₁, K₂ formulas at fixed salinity 35. K'sp for aragonite and calcite both fall as water warms (aragonite is consistently ~1.5× more soluble than calcite), which is why tropical surface water is far more supersaturated than cold polar water even at the same CO₂ level.
- pCO₂ slider — raising it (fossil-fuel CO₂ uptake) lowers pH and [CO₃²⁻], pulling Ω down toward or below 1.
- Temperature slider — warmer water holds less CO₂ and has a lower K'sp, both of which raise Ω; colder water lowers it.
- Alkalinity slider — more buffering capacity (higher TA) partially offsets a CO₂ increase, keeping Ω higher.
- Mineral toggle — pteropods and many corals build aragonite (dissolves first); oysters, foraminifera and coccolithophores mostly build the more resistant calcite. The 3D coral skeleton grows or dissolves using the Ω of whichever mineral is selected, at a simplified linear rate dR/dt ∝ (Ω−1). Real biogenic calcification and abiotic dissolution kinetics are more complex (dissolution often follows a power law in (1−Ω)) — this is a teaching simplification of the correct direction and rough magnitude of the effect.
Real-world relevance: since 1850 surface-ocean pH has fallen from ~8.2 to ~8.05 and aragonite Ω has dropped by roughly 20% globally; some polar and upwelling waters are already seasonally undersaturated (Ω<1) for aragonite.