Buoyancy-driven coastal current blocks ventilation of an anoxic fjord on the Pacific coast of Canada

Shallow sills restrict the ventilation of deep coastal fjords. Dense oceanic water seaward of the sill and lower density water within the receiving basin are generally required for oxygenated water to cross the sill and descend deep into the fjord. Here we use concurrent 10 year time series from current meters in the fjord and on the continental shelf to examine ventilation of the 120 m deep, anoxic inner basin of Effingham Inlet on the west coast of Vancouver Island. Whereas density currents traverse the 40 m deep sill and flow into the inner basin at mid-depth at quasi-fortnightly tidal intervals, only five current intrusions descended to the bottom of the basin over the decade-long measurement period. The deep intrusions had a mean (SD) return interval of 1.75 (1.33) years and induced bottom-water changes that persisted for 1-2 months. We show that, in addition to conditions within the inlet, deep ventilation of the inner basin is dependent on a coordinated sequence of external processes: (1) the onset of upwelling winds along the outer coast; (2) reversal of the buoyancy-driven coastal current that normally flows poleward over the inner shelf off Vancouver Island; and (3) reversal of the estuarine circulation in the channel connecting the inlet to the ocean. As the observed ventilation intervals are short compared to the decadal intervals derived from the spacing of homogenites (sedimentation sequences disrupted by intruding bottom currents), the use of homogenites as proxies for past upwelling conditions in the northeast Pacific may need to be reexamined. Plain Language Summary Shallow sills restrict the ventilation of deep coastal fjords. In extreme cases, the failure of the system to deliver new oxygen deep into the fjord - to offset the loss of oxygen through biological decay-causes the bottom waters of the basin to become anoxic and no longer capable of supporting most forms of life. This study uses decade-long times series of oceanic and meteorological observations within the fjord and on the adjacent continental shelf to examine the ventilation of the 120-m deep, anoxic inner basin of Effingham Inlet on the west coast of Vancouver Island, British Columbia. We show that for deep oxygen renewal of the inner basin to occur there must be a coordinated sequence of external processes starting with the onset of strong upwelling winds along the outer coast, followed by a reversal of the buoyancy-driven coastal current that normally flows poleward over the inner shelf off Vancouver Island, and a reversal of the estuarine circulation in the channel connecting the inlet to the ocean. This sequence is sufficiently infrequent that ventilation of the bottom waters presently only occurs on average every 1.75 years.