Using Noble Gases to Constrain Parameterizations of Arctic Air-Sea-Ice Gas Exchange Processes

The Arctic Ocean plays an important role in the global climate system as it acts, for example, as major reservoir of anthropogenic carbon. Despite its global significance, data on physical parameters and tracers in the Arctic Ocean are still sparse and thus carbon inventory estimates only weakly constrained, for which insights into Arctic air-sea-ice gas exchange and ventilation need to be enhanced. Noble gases, with their biological and chemical inertness and constant atmospheric abundance history, fill this gap, as their concentrations in water are set by the conditions of last atmospheric contact. In light of this, water samples taken during the Synoptic Arctic Survey (SAS) expedition to the Central Arctic Ocean with the Swedish icebreaker Oden in summer 2021 (SAS-Oden 2021) at six stations from the surface to the seafloor were analyzed for their noble gas content. This first application of the full set of the five stable noble gases (helium, neon, argon, krypton and xenon) to the Arctic Ocean marks a new step towards a comprehensive understanding of Arctic Ocean dynamics.

The measured profiles show a strong influence of rapid cooling, excess air injection and brine rejection from sea ice formation, which affect the light and heavy noble gases differently, depending on their size, solubility and diffusivity. Building upon work from groundwater hydrology and extensions to cave calcites, as well as previous ocean applications of noble gases, the concepts of recharge temperatures, excess air terms and ice fractions or freezing rates are transferred to the Arctic Ocean, enabling the development of new parameterizations of the air-sea-ice exchange processes. We present two “static” model approaches, differing in the sea ice parameterization, and a “dynamic” mixed reactor-type model for two limits (steady state and quasi-steady state), resulting in different parameterizations of rapid cooling. The fit results from a least-squares regression for all four models are able to reproduce the measured concentrations both accurately and precisely and thus allow for predictions for other gases. In our study, these are the anthropogenic transient tracers sulfur hexafluoride (SF₆) and dichlorodifluoromethane (CFC-12), which were also measured during the SAS-Oden 2021 expedition and are used to determine water ages, a task for which the intitial surface saturations need to be known. We suggest a relative oversaturation of around 6% of SF₆ to CFC-12 due to the deviating impact of excess air, compatible with previous estimates from noble gas measurements.