Effect of Organic Alkalinity on Seawater Buffer Capacity: A Numerical Exploration

Abstract

Organic alkalinity is a poorly understood component of total titration alkalinity in aquatic environments. Using a numerical method, the effects of organic acid (HOA) and its conjugate base (OA⁻) on seawater carbonate chemistry and buffer behaviors, as well as those in a hypothetical estuarine mixing zone, are explored under both closed- and open-system conditions. The simulation results show that HOA addition leads to pCO₂ increase and pH decrease in a closed system when total dissolved inorganic carbon (DIC) remains the same. If opened to the atmosphere (pCO₂ = 400 µatm), CO₂ degassing and re-equilibration would cause depressed pH compared to the unperturbed seawater, but the seawater buffer to pH change  $\left( {\beta _{{{\text{DIC}}}} \, = \left( {\frac{{\partial \ln \left( {\left[ {{\text{H}}^{ + } } \right]} \right)}}{{\partial {\text{DIC}}}}} \right)^{{ - 1}} } \right)$ indicates that weaker organic acid (i.e., higher pK_a) results in higher buffer capacity (greater β_DIC) than the unperturbed seawater. In comparison, OA⁻ (with accompanying cations) in the form of net alkalinity addition leads to pCO₂ decrease in a closed system. After re-equilibrating with the atmosphere, the resulting perturbed seawater has higher pH and β_DIC than the unperturbed seawater. If river water has organic alkalinity, pH in the estuarine mixing zone is always lower than those caused by a mixing between organic alkalinity-free river (at constant total alkalinity) and ocean waters, regardless of the pK_a values. On the other hand, organic alkalinity with higher pK_a provides slightly greater β_DIC in the mixing zone, and that with lower pK_a either results in large CO₂ oversaturation (closed system) or reduced β_DIC (in mid to high salinity in the closed system or the entire mixing zone in the open system). Finally, despite the various effects on seawater buffer through either HOA or OA⁻ addition, destruction of organic molecules including organic alkalinity via biogeochemical reactions should result in a net CO₂ loss from seawater. Nevertheless, the significance of this organic alkalinity, especially that comes from organic acids that are not accounted for under the currently recognized “zero proton level” (Dickson in Deep Sea Res 28:609–623, 1981), remains unknown thus a potentially interesting and relevant research topic in studying oceanic alkalinity cycle.