Gas Ventilation of the Saguenay Fjord by an Energetic Tidal Front

Baschek, B. and W. J. Jenkins

Atmosphere - Ocean, 47, 1, p. 308-318, 2009.

Dissolved noble gas samples were taken during a pilot study in the Saguenay Fjord, Quebec, Canada, in order to determine the contribution of different air-sea gas exchange mechanisms in an estuary and to assess the contribution of tidal fronts to the aeration of subsurface waters. The noble gases He, Ne, Ar, Kr, and Xe span a large range in molecular diffusivities and solubilities and hence constitute a useful probe of various gas exchange and bubble injection processes. Samples were taken at flood tide upstream and downstream of an energetic tidal front that is generated by a hydraulically controlled flow over a shallow sill at the entrance of the Fjord. The results are interpreted with the help of hydrographic measurements of density and  currents along cross-sill transects describing the physical forcing at the sill. High gas saturations downstream of the sill indicate the aeration of water within the frontal region. An inverse model is used to compare the contribution of bubble injection in the front to diffusion across the air-sea interface. The large ratio of completely “trapped” bubbles to diffusion suggests that bubbles injected by waves breaking in the front contribute with 76% for He, 79% for Ne, 56% for Ar, 47% for Kr, and 35% for Xe significantly to air-sea gas exchange.