The Influence of Oceanic Chemistry, Physics, and Biology on Atmospheric Carbon Dioxide

The ocean controls the partial pressure of carbon dioxide in the atmosphere on time scales of decades to tens of thousands of years. To predict the level of atmospheric CO(,2), resulting from given chemical, physical, and biological conditions in the ocean, a series of models is developed. The surface ocean is divided in these models into average warm surface water and a cold, high-latitude region where water sinks out of contact with the atmosphere. The cold surface region can have an importance in determining pCO(,2) much greater than its area might suggest, and the extent of this influence is shown to depend upon the proportion of the return flow from the surface to the deep ocean which passes through the cold surface region, and also upon the relative magnitudes of the rates of exchange of carbon by gas transfer and on water flux. The chemical composition of the water in the cold surface region is determined by physical mixing of low-nutrient water from the warm surface ocean with high-nutrient water from below, and also by the degree of biological activity in the region. Changes in either physical or biological conditions are shown to have the potential to alter pCO(,2) in the cold surface water and thus in the atmosphere over a total range of several hundred parts per million. A model which includes separate consideration of the high-latitude ocean is used to explore possible conditions during the last glacial period which are in accord with the geological record and which can also account for a low value of atmospheric pCO(,2) during glacial time. Finally, a possible mechanism is presented by which alterations in the distribution of sunlight due to variations in the orbital parameters of the Earth can cause climate change, based on the direct response of biological activity to changing levels of light in high -latitude regions of the ocean.