The dissolution of calcite in seawater from 40° to 90°C at atmospheric pressure and 35‰ salinity

Abstract

Calcite solubility experiments in 35‰ salinity seawater were carried out from 40° to 90°C at 1 atm total pressure. An open-system technique with constant CO 2 partial pressure was used. Since calcite solubility in seawater has been reported to be reversible, equilibrium was approached by undersaturation only. A fit to our own experimental data and to low-temperature data from the literature was computed, resulting in the following equation for the apparent solubility product of calcite, K′ c: log K ′ c=1.6968−1096.69/T−1.4699·10 −×T where K′ c is expressed in (mol/kg sw) 2; and T is the absolute temperature from 273.15 to 363.15 K. The standard deviation of this line is ±0.037 log unit (±90% K′ c). The mean apparent enthalpy variation between 40° and 90°C was calculated by means of the log K′ c( T) equation. After a close examination of the data from the literature, a new experimental value of the temperature coefficient, ∂K′ c/ ∂T, was assessed at 25°C. Using the thermodynamic and apparent solubility products, experimental mean activity coefficients, γ ± c, were computed for CaCO 3 and a temperature-dependent equation was derived. A comparison between solubility products in seawater and NaCl solutions confirms that, as far as calcite dissolution is concerned, seawater and NaCl solutions do not exhibit the same chemical behavior. Calcite is far more soluble in standard seawater than in NaCl solutions up to 6 m.