Aragonite Undersaturation

Aragonite undersaturation is the condition where the aragonite saturation state omega falls below 1, making seawater chemically corrosive to aragonite shells and skeletons. Omega is the ratio of in-situ carbonate-ion concentration to the value at calcium carbonate equilibrium; at omega below 1, dissolution is thermodynamically favored. Rising dissolved CO2 from acidification lowers carbonate-ion concentration and drives omega down, with cold polar waters and upwelling zones reaching undersaturation first because CO2 is more soluble in cold water. Pteropods exposed to omega below 1 show visible shell pitting, and coral calcification slows as omega approaches reef-critical values near 3.