The Dissolution Kinetics of Calcium Carbonate in Seawater: Development and Application of an Automated pH-stat Method for the Measurement of Dissolution Rates

Abstract

The gradual decrease in the saturation state of calcium carbonate in the ocean due to the absorption of anthropogenic CO2 is expected to enhance the dissolution of skeletons and shells of calcifying organisms. Experimental tools are required to investigate and determine the extent of changing ocean chemistry and impacts on the dissolution of biogenic CaCO3 minerals. An automated method based on the pH-stat technique (Morse, 1974) has been developed to study the dissolution kinetics of calcium carbonate minerals. The method was verified by investigating the dissolution behaviour of Iceland spar in artificial seawater as a function of the degree of undersaturation at 25.0 ± 0.1ºC. A closed titration system was used to retain CO2 generated from the dissolution reaction. Artificial seawater containing weighed CaCO3 samples was titrated with standardised 0.02 mol kg-1 HCl. The cumulative volume of acid dispensed to maintain constant saturation was used to calculate the mass-normalised dissolution rates. The proportion of acid involved in dissolution is a function of the initial alkalinity (AT) and dissolved inorganic carbon (CT) of seawater, and IS NOT A SIMPLE 2:1 STOICHIOMETRY AS MIGHT BE EXPECTED for the reaction between HCl and CaCO3. The values of AT and CT were used to compute a carbonate molar ratio (CMR) that was applied to rate calculations. The mass-normalised rates were fitted to the empirical rate equation, R = k(1-Ω)n to determine the values of k and n, where Ω= ([Ca2+][CO3-2])/Ksp and Ksp is the apparent solubility constant for calcium carbonate in seawater, k is the rate constant and n is the reaction order. The dissolution of Iceland spar increased with an increasing degree of undersaturation with k = 10 0.51 ± 0.06 μmol g-1 hr-1 and n = 2.5 ± 0.1. The automated method was able to maintain the operating saturation states to better than ± 0.2% and pCO2 to better than ± 1.5% by the addition of HCl. The method was applied to determine the dissolution rates of skeletal material of three species of coral from Samoan waters and two bryozoan species from New Zealand. There was no significant difference (p < 0.05) in the dissolution rates of the corals. The rate constant, k, ranged from 10 0.04 ± 0.06 - 10 1.0 ± 0.3 and n = 0.9 ± 0.1 - 3.0 ± 0.5. There was a significant difference between the dissolution rates of the corals and the aragonite bryozoan with k and n of respectively 10 1.1 ± 0.1 and1.2 ± 0.3 for the latter species. The calcite bryozoan dissolved slowly with k = 10 0.5 ± 0.3 and n = 0.3 ± 0.5. The biogenic CaCO3 materials were treated with sodium hypochlorite before they were used in dissolution studies. This procedure removed organic matter within the skeletons while preserving their carbonate mineralogy. The 2.5, 5.0 and 10% NaOCl were all effective in the pre-treatment process without changing the mineralogy of the biological samples. The measurements using the method developed in this project show that the method could be applied to the study of dissolution patterns of a variety of minerals at constant saturation states or pH. The dissolution behaviour of biogenic CaCO3 in undersaturated seawater identifies different dissolution patterns due to variations in skeletal mineralogy and morphology. It also demonstrates the crucial need to address anthropogenic contributions to atmospheric CO2 to prevent further changes in the chemistry of the ocean and reduce impacts on marine calcifying species.