Output from a one-dimensional, mixed-layer ocean model and a general circulation model suggests a consistent relation between surface-water residence times, large vertical salinity gradients, and anoxic bottom water during transgressive periods in the Cretaceous North American seaway. Model results show that severe storms over the seaway were not effective in mixing oxygen to the sediment-water interface when vertical salinity gradients exceeded 1‰-2‰ and depths were >300 m. At precipitation - evaporation + river runoff (P - E + R ) rates calculated from the general circulation model (up to 300 cm/yr), conditions favorable to the maintenance of anoxic bottom water would be established within a matter of months. Large vertical salinity gradients at these P - E + R values would form in time periods as short as 1-2 yr. This is considerably shorter than seaway residence times calculated from the Sverdrup relation and from wind-stress curl calculations of atmospheric general circulation models.

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