Seawater fCO2 in seawater profiles below ice and from the ice-water interface, collected during the on-ice work on RRS Sir David Attenborough (Cruise SD035 in the Weddell Sea in February 2024 as part of the PICCOLO project).

Seawater CO2 levels determine how much CO2 is removed or added to the atmosphere and atmospheric CO2 levels influence the Earth's climate. Seawater carbonate system measurements (inc. CO2) close to sea ice are influenced by ice formation and melt, and are also close to the ocean-atmosphere interface. The exchanges of carbon at these interfaces is poorly understood and important. Measurements were made below the ice, either by pumping interfacial water from below the ice onto the ice to be measured by a ProOceanus ProCV sensor, or by strapping the sensor to a cable and lowering it through and under the ice to collect a vertical profile (stopping every few metres to collect a stable reading). Seawater sensor CO2 mole fraction (xCO2, ppmv) is converted to CO2 partial pressure (pCO2, µatm) by accounting for the gas pressure (P, atm) inside the measurement cell (pCO2 = xCO2 * P). pCO2 is then converted to CO2 fugacity (fCO2, µatm) using the fugacity coefficient (ɣ = 0.996) using fCO2 = pCO2 * ɣ . These data were collected by scientists from Plymouth Marine Laboratory during the NERC-funded PICCOLO research cruise SD035 in the Weddell Sea in February 2024.