Abstract

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 not due to the previously noted lower oxygen solubility with increasing ocean temperature, but rather to the increased efficiency with which plankton will extract nutrients from convecting waters at low latitudes. The present ocean nutrient content is sufficient to induce deep-water anoxia if circulation were to change, yet anoxia is rare in the paleoceanographic record. A simple ocean chemical model makes explicit the tradeoffs 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.

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