Abstract
Abstract. Seawater absorption of anthropogenic atmospheric carbon dioxide (CO2) has led to a range of changes in carbonate chemistry, collectively referred to as ocean acidification. Stoichiometric dissociation constants used to convert measured carbonate system variables (pH, pCO2, dissolved inorganic carbon, total alkalinity) into globally comparable parameters are crucial for accurately quantifying these changes. The temperature and salinity coefficients of these constants have generally been experimentally derived under controlled laboratory conditions. Here, we use field measurements of carbonate system variables taken from the Global Ocean Data Analysis Project version 2 and the Surface Ocean CO2 Atlas data products to evaluate the temperature dependence of the carbonic acid stoichiometric dissociation constants. By applying a novel iterative procedure to a large dataset of 948 surface-water, quality-controlled samples where four carbonate system variables were independently measured, we show that the set of equations published by Lueker et al. (2000), currently preferred by the ocean acidification community, overestimates the stoichiometric dissociation constants at temperatures below about 8 ∘C. We apply these newly derived temperature coefficients to high-latitude Argo float and cruise data to quantify the effects on surface-water pCO2 and calcite saturation states. These findings highlight the critical implications of uncertainty in stoichiometric dissociation constants for future projections of ocean acidification in polar regions and the need to improve knowledge of what causes the CO2 system inconsistencies in cold waters.
Highlights
In the last decades, oceans have absorbed over a quarter of the anthropogenic carbon dioxide (CO2) emitted to the atmosphere (Le Quéré et al, 2018; Gruber et al, 2019)
Using all the data from which T, SP, DIC, pH, CA, and pressure of CO2 in seawater (pCO2) are available as high-quality, independent measurements, we were able to derive expressions for K∗1 and K∗2 with a new temperature dependence
An iterative procedure allowed us to estimate the temperature dependence of the first and second carbonic acid stoichiometric dissociation constants (K∗1 and K∗2, respectively) from a large dataset of high-quality oceanographic measurements. Both K∗1 and K∗2 were similar to the constants of Lueker et al (2000) that are currently used by most of the oceanographic community, as recommended by Dickson (2007), but the K∗1 and K∗2 values were lower in cold seawater, below a temperature of ∼ 8–9 ◦C. At these temperatures, pCO2 computed using the constants of Lueker et al (2000) may be underestimated and [CO23−] overestimated, meaning that the cold oceans are more undersaturated with respect to CaCO3 minerals than expected
Summary
Oceans have absorbed over a quarter of the anthropogenic carbon dioxide (CO2) emitted to the atmosphere (Le Quéré et al, 2018; Gruber et al, 2019). Together, these three reactions and their species constitute the marine CO2−H2O system, which is responsible for about 95 % of the acid–base buffering capacity of seawater and maintains the pH of the ocean within a narrow range (Bates, 2019; Zeebe and Wolf-Gladrow, 2001). These three reactions and their species constitute the marine CO2−H2O system, which is responsible for about 95 % of the acid–base buffering capacity of seawater and maintains the pH of the ocean within a narrow range (Bates, 2019; Zeebe and Wolf-Gladrow, 2001) Each of these reversible reactions is associated with a thermodynamic equilibrium constant, a number that expresses the relationship between the activities of products and reactants present at equilibrium at a given temperature and pressure.
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