CALCIUM CARBONATE SATURATION IN SEAWATER: EFFECTS OF DISSOLVED ORGANIC MATTER1

Abstract

Surface seawater is generally supersaturated with CaCO 3 ; but carbonate will not precipitate from natural seawater in convenient experimental times. If the supersaturation is increased by the addition of Ca 2+ or CO 3 2− ions, precipitation can occur in minutes or hours. Seawater, with 0.1 m Na 2 CO 3 solution added to give a p H of 9.5, will begin rapid CaCO 3 (aragonite) precipitation in as little as 15 min. The length of time between the addition of the Na 2 CO 3 and rapid precipitation increases with an increase in the dissolved organic content of the water. If CaCl 2 , which produces no p H increase, is added to seawater, CaSO 4 ·2H 2 O (gypsum) is the only precipitate that will form within 12 hr.