Spectrophotometrically derived seawater CO 2 ‐system assessments: Parameter calculations using pH do not require measurements at standard temperatures

Abstract

The temperature range of Earth's open‐ocean waters is roughly 0–30°C, yet our understanding of the seawater carbon dioxide (CO 2 ) system is largely derived from analyses conducted within a narrow temperature range (e.g., laboratory temperature of 20°C or 25°C). Herein, we address two aspects of open‐ocean CO 2 ‐system measurements and modeling: (1) a highly precise spectrophotometric technique is used to determine bicarbonate dissociation constants ( K 2 ) in seawater at temperatures as low as 3°C and (2) a cruise dataset uniquely including total scale pH measurements at two temperatures is used for CO 2 ‐system internal consistency comparisons at 12°C and 25°C. Our p K 2 parameterization (where p K = −log K ) is applicable for broad ranges of salinity (20 ≤ S P ≤ 40) and temperature (3°C ≤ t ≤ 35°C). Our CO 2 ‐system internal consistency evaluation (comparison of measured and calculated CO 2 ‐system parameters) utilized data obtained during NOAA's 2021 West Coast Ocean Acidification Cruise: total alkalinity (TA), total dissolved inorganic carbon (DIC), pH measured at 25°C, and pH measured at 12°C ( n = 265). Results demonstrate that, relative to calculations utilizing the TA, DIC pair, agreement between measured and calculated parameters is improved when either TA or DIC is paired with pH measurements at either temperature. Calculations of CO 2 fugacity ( f CO 2 ) and aragonite saturation state (Ω ar ) using pH measurements made at 25°C or 12°C (paired with either TA or DIC) are statistically indistinguishable. Results also suggest that the temperature dependence of current CO 2 ‐system dissociation constants need further refinement.

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