Circulation, salinity, and dissolved oxygen in the cretaceous North American Seaway

Abstract

Critical paleoceanographic problems regarding the maximum transgressive phase of the Cretaceous North American seaway have been studied with a three-dimensional ocean circulation model. Four simulations employing minimum and maximum solar insolation winds from the Cretaceous and a wide range of precipitation -evaporation (P-E) rates have been conducted. Winter, minimum solar insolation winds are similar to modern zonal wind patterns and produce a subtropical gyre with a strong western boundary current and broad, easterly return flow. Winter and summer maximum solar insolation winds and summer minimum solar insolation winds do not have significant wind stress curl and produce western and eastern boundary currents of equal intensity. Residence time of shallow (<100 m) water ranges from 0.6 to 2.5 yrs while deep water mass residence time varies from 1.3 to 4.6 yrs. Maximum vertical salinity differences within the seaway are approx 3 permil (10 percent freshwater dilution) when average P-E is ∼1.5 m/yr and 6 permil (20 percent freshwater dilution) when P-E is ∼3.5 m/yr. Relatively short surface water residence times prevent significant freshening of surface water. The influence of freshwater from rivers discharging at the western boundary is not pronounced due to limited drainage area and the strong western boundary currents. Dissolved oxygen has been modeled by assuming a productivity of 100 gC m-2/yr throughout the seaway and employing empirical carbon flux-depth relationships from the modern ocean in conjunction with Redneld O2:C ratios. Apparent oxygen utilization is modest (<20 μmol/kg) for all simulations due to the short deep water residence times in the seaway. The numerical simulations and simple box model calculations suggest that the widely documented anoxic episodes of the transgressive Cenomanian-Turonian North American seaway are a result of incursions of mid-depth, suboxic, or anoxic water from the open ocean or restricted deep water circulation due to a sill at the seaway entrance. Brackish surface water caused by elevated precipitation prevents mixing of oxygenated water from the surface although other factors (deep water residence time, water depth, and magnitude of biological productivity) determine whether anoxia develops in the deep seaway waters.