Ocean nutrient distribution and oxygenation: limits on the formation of warm saline bottom water over the past 91 m.y.

It has been proposed that deep-water formation in the oceans would be quite different during geologic intervals with reduced equator to pole temperature gradients. Salinity, rather than temperature, differences might drive the deep-ocean circulation. Saline water would tend to form at subtropical latitudes where evaporation exceeds precipitation. We point out a likely consequence of warm saline bottom-water formation on ocean chemistry - the tendency to drive the ocean toward anoxia. This effect is due to the increased efficiency with which plankton will extract nutrients from convecting waters at low latitudes. A simple ocean chemical model makes explicit the tradoffs between mean ocean nutrient content and circulation parameters that will satisfy the geologic observations of an oxygenated ocean since the mid-Cretaceous. Barring decreases of ocean phosphate on the order of 30%-50%, deep-water formation at high latitudes was a major source of ocean ventilation in the warmer past. -from Authors