Calcite and Aragonite Saturation States of the Red Sea and Biogeochemical Impacts of Excess Carbon Dioxide

Abstract

This chapter discusses the saturation states of the Red Sea with respect to both calcite and aragonite and their possible biogeochemical impacts as a result of ocean carbonate chemistry changes. The saturation levels of the Red Sea surface waters are several-fold supersaturated with respect to calcite and aragonite; they range from 634 to 721 % and from 446 to 488 %, respectively. The saturation levels of the deep waters range from 256 to 341 % with respect to calcite and from 177 to 230 % with respect to aragonite. They generally increase from south to the north. The lowest values of seawater supersaturation with respect to both calcite and aragonite were found at water depths >1,400 m. Changes in the seawater acid–base chemistry due to excess CO₂ emission and oceanic acidification affect the saturation states of calcium carbonate. Based on reported results of the excess CO₂ sink in the northern part of the Red Sea (Krumgalz et al. 1990), the estimated degree of saturation with respect to calcite and aragonite was higher by 1.9 ± 0.4 % at >200 m, 4.9 ± 0.7 % at 200–600 m, and 2.5 ± 0.1 % at >600 m in preindustrial times than in 1982. A projected drop in pH by a 0.1 unit decreases the saturation level by a factor of 1.2, whereas a drop by 0.4 pH unit decreases the saturation level by a factor of 2.1. These changes in saturation levels will have major impacts on the calcifying pelagic and benthic organisms as well as the distribution and depth of coral reefs. Low magnesian calcite and pure calcite are expected to be the dominant carbonate minerals at these low supersaturation levels.

Appendix

Estimation of pH and pCO₂

The pH values were estimated using the following equation:

$$ \frac{\text{CA}}{{{\text{TCO}}_{2} }} = \frac{{[{\text{H}}^{ + } ]K_{1} + 2K_{1} K_{2} }}{{[{\text{H}}^{ + } ]^{2} + [{\text{H}}^{ + } ]K_{1} + K_{1} K_{2} }} $$

(6)

The iteration method is used to estimate the concentration of hydrogen ion [H⁺] in the solution. The pH is:

$$ {\text{pH}} = - \log [{\text{H}}^{ + } ] $$

(7)

The pCO₂ in seawater samples can be calculated from equation (6) or the following relationship:

$$ {\text{pCO}}_{2} = K_{0} {\text{TCO}}_{2} \left( {\frac{{[{\text{H}}^{ + } ]^{2} }}{{[{\text{H}}^{ + } ]^{2} + [{\text{H}}^{ + } ]K_{1} + K_{1} K_{2} }}} \right) $$

(8)

The solubility constant of CO₂ in seawater can be calculated from Weiss (1974).