Noble Gas Fluxes Reveal Links Between Air-sea Gas Exchange, Bubbles, and the Structure of the Air-sea Interface at High Wind Speeds

Air-sea gas exchange is a crucial part of the biogeochemical cycles of many climatically important gases. Despite much progress, air-sea gas exchange parameterizations differ from each other widely at high wind speeds. In addition, the gas flux due to bubbles, which becomes increasingly important as wind speed increases, is not explicitly represented in many commonly-used air-sea gas exchange parameterizations. Noble gases are ideal tools for studying air-sea gas exchange since they are biologically and chemically inert and hence changes in noble gas concentrations are due primarily to air-sea gas exchange. Thus, in order to improve understanding of air-sea gas exchange at high wind speeds, we measured the gas flux of five noble gases (He, Ne, Ar, Kr and Xe) and oxygen, as well as the steady-state saturation anomalies of these gases, at wind speeds of 20 to 50 m s^-1 in the University of Miami’s SUrge STructure Atmosphere InteractioN (SUSTAIN) salt-water wind-wave tank. Experiments were conducted with monochromatic and with spectral waves, and at water temperatures warmer, equivalent to, and colder than atmospheric temperatures in order to examine different atmospheric stability regimes. We observed an expected increase in noble gas fluxes and noble gas steady state saturation anomalies as wind speeds initially increased but then we found an intriguing and surprising flattening of gas fluxes and steady state saturation anomalies at wind speeds greater than 40 m s^-1.,We relate this flattening, as well as the gas fluxes in general, to observations of bubble size distribution, turbulent kinetic energy, and wave characteristics (significant wave height, steepness, etc.) that were measured concurrently with the noble gases.