pH evolution in sea ice grown at an outdoor experimental facility

• Sea ice pH near 0 °C is measured with spectroscopic and potentiometric methods.
• Lowest pH in brine and interior ice, highest pH in exterior ice and frost flowers
• The freezing process produces “C-shaped” vertical pH profiles in abiotic conditions.
• pH evolution in sea ice is explained by a conceptual model of CO2 dynamics.

The pH of sea ice and brine was experimentally determined during initial ice growth and melt at the Sea-ice Environmental Research Facility (SERF), an outdoor experimental sea ice facility in Winnipeg, Canada. pH measurements were performed potentiometrically and spectroscopically at near-freezing temperatures. Vertical pH profiles from bulk ice cores revealed a consistent C-shaped pattern during columnar ice growth, with highest pH values (> 9) in both exterior (top and bottom) ice sections and in frost flowers, and lowest pH (~ 7) in interior ice sections. Brine pH typically remained below that of the source seawater pH (~ 8.4). The distinct differences between these ice features and the underlying seawater source demonstrate the effect of the natural freezing process and associated changes in the CO2-carbonate system on the pH of the sea ice environment. Interpreting this effect provides new insight into the conditions leading to CO2 exchange across the ocean–sea ice–atmosphere interface. A conceptual model of pH evolution in seawater, sea ice and brine, and frost flowers is proposed to explain the observed pH characteristics of seawater components during sea ice growth and melt.