Coral polyps build their aragonite (CaCO₃) skeleton from an isolated pocket of fluid between the tissue and the existing skeleton — the calcifying fluid. Precipitation only proceeds when that fluid is supersaturated with respect to aragonite:
Ω = [Ca²⁺][CO₃²⁻] / Ksp(aragonite)
Calcification rate: G ∝ (Ω − 1)^n , n ≈ 1.7 (Ω > 1 only)
Rising atmospheric CO₂ dissolves into the ocean, shifting the carbonate equilibrium: pH falls and CO₃²⁻ — and therefore Ω — falls with it. This sim approximates seawater pH from a Revelle-buffer-style logarithmic response to pCO₂, then converts pH to Ω via the steep, nearly-exponential sensitivity of [CO₃²⁻] to pH near seawater's natural range.
Corals are not passive: Ca²⁺-ATPase pumps actively remove H⁺ from the calcifying fluid, raising its pH — and therefore Ωcf — well above the surrounding seawater. Boron-isotope measurements (McCulloch et al. 2012) show healthy corals hold their calcifying fluid roughly 0.3–0.7 pH units above ambient, buying the skeleton a cushion against acidification. That pumping is metabolically expensive: it draws on energy the symbiotic algae (zooxanthellae) supply by photosynthesis, so thermal stress that triggers bleaching (symbiont loss) starves the pump and collapses the cushion at the same time seawater Ω is already falling — a double hit modeled here as reduced pump efficiency under warming/bleaching.
- CO₂ slider — drives seawater pH and Ωarag down as it rises from the pre-industrial 280 ppm baseline.
- Temperature anomaly — shifts reaction kinetics (mild warming speeds precipitation up to a point, sub-optimal temperatures act as a further energetic penalty on the proton pump).
- ΔpH pump strength — the coral's biological up-regulation capacity; set it to 0 to see what happens to a purely passive calcifier with no physiological buffering.
- Bleaching toggle — symbiont loss cuts the energy budget available to the proton pump, on top of whatever the temperature slider is already doing.
The coral skeleton's tint tracks Ωcf: blue-white polyps are actively depositing aragonite, orange/red polyps sit below Ωcf = 1 and are net-dissolving. Drifting particles represent CO₃²⁻ ion density in the surrounding water, thinning out as Ωsw falls.