Gas ventilation of the Saguenay fjord by an energetic tidal front
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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 of 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 to 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 significantly to air-sea gas exchange with 76% for He, 79% for Ne, 56% for Ar, 47% for Kr, and 35% for Xe. Water samples were analyzed for helium isotopes and tritium in order to explore the possibility of constraining ventilation time scales. The relationship between tritium and salinity revealed two end-member waters: a freshwater component from the Saguenay River of 23.6 +/- 0.5 TU, likely a residual of bomb-produced tritium, and a seawater end-member of approximately 1.5 TU originating in the subpolar Atlantic. An unexpected contribution of radiogenic (4)He was detected in the deep waters of the St. Lawrence Estuary, likely a consequence of out-gassing from old, uranium and thorium rich granitic terrain.