MS DEFENSE | Cameron Richardson | Carbonic Acid Dissociation In Seawater Of Non-Standard Major Ion Composition 

Title: Carbonic Acid Dissociation In Seawater Of Non-Standard Major Ion Composition

The dissolution of anthropogenic carbon dioxide (CO 2 ) and subsequent dissociation in
marine waters has led to large-scale perturbations in the global carbon cycle, most
notably a reduction in ocean pH and the saturation state of important biogenic minerals,
known as ocean acidification. The first and second stoichiometric dissociation constants
of carbonic acid (K 1 * and K 2 * ), necessary to constrain the dissolved carbonate system,
have been well-established by several investigators in dilute solutions and in seawater
of standard major ion composition. However, such constants have not been accurately
measured in seawater of non-standard major ion compositions, specifically in terms of
[Ca 2+ ], [Mg 2+ ], and [SO 4 2- ]. These constants are imperative for constraining the carbonate
chemistry speciation in various settings, including coastal areas, marginal seas,
sediment pore waters, hydrothermal fluids, and paleo-seawaters. Here we present our
measured K 1 * and K 2 * data for seawater of non-standard major ion compositions based
on closed-cell potentiometric titration of artificial seawater that systematically differ in
[Ca 2+ ], [Mg 2+ ], [SO 4 2- ], and temperature at fixed ionic strength. We conducted sensitivity
analyses for these data and compared our measured K 1 * and K 2 * data to existing K 1 * and
K 2 * data derived from select aqueous chemical speciation models. At each experimental
temperature, we found a significant and positive linear relationship between increasing
[Ca 2+ ], [Mg 2+ ], and [SO 4 2- ] and our measured K 1 * and K 2 * for each of the ions investigated
here. The implications of utilizing our constants in paleo-seawaters and in a marine CO 2
removal setting are discussed. These case studies show that neglecting changes in
seawater [Ca 2+ ], [Mg 2+ ], and [SO 4 2- ] from their standard concentration can lead to
systematic biases in calculations of the dissolved carbonate system and the saturation
state of calcium carbonate minerals in non-standard seawater. These findings highlight
the need for consideration of major ion composition in carbonate system calculations in
both observational and modeling studies of marine environments featuring non-standard
seawater compositions.

Location: POST 733

Date & Time: Tuesday, February 10th, 2026 at 9:15am