Persistent organic alkalinity in the ocean

Abstract

Ocean alkalinity (AT) critically affects the buffering capacity and the precipitation and dissolution of calcium carbonate minerals in the ocean. However, our knowledge of AT is still incomplete because of limited investigations into the contributions of dissolved organic matter (DOM) to alkalinity (ADOM) across diverse ocean environments. Here, to improve the accuracy of seawater ADOM measurements, we have implemented three key modifications to the conventional method based on a second-HCl-titration. These modifications include: (1) determining concentrations and pK spectra of ADOM using the same NaOH-titration data; (2) routinely evaluating residual alkalinity (ACredidual) remaining in seawater; and (3) validating the accuracy of ADOM against known concentrations of acetate ions in sodium acetate (CH3COONa) solution, where only acetate ions contribute to ADOM. In experiments involving modification (3), the ADOM values obtained through NaOH-titration agreed with those obtained through standard HCl titration, showing an average difference of − 0.4 ± 2.7 μmol kg−1. Modification (3) represents a critical improvement incorporated into the NaOH-titration method proposed here and can also improve the accuracy of ADOM obtained by the conventional second-HCl-titration method. Modification (2) would also enhance the ADOM accuracy because the uncertainty in ACresidual can be directly translated into ADOM values. When applied to certified reference materials for AT and CT, aged local seawater, and deep waters of the East Sea, the NaOH-titration method consistently yielded ADOM values approximately 5 μmol kg−1 higher than those obtained for artificial seawater containing zero ADOM. This small yet significant ADOM value was attributed to the recalcitrant DOM component, which is widespread in the global ocean.

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