Vertical oceanic nutrient fractionation and glacial/interglacial CO²cycles

Abstract

Ice-core studies have established that atmospheric carbon dioxide is about 80 parts per million by volume (p.p.m.v.) lower during cold glacial climates than it is during warm interglacial times^1–8. A number of models have been offered for this phenomenon^9–12. These models are consistent with observation of an enhanced carbon isotope contrast between surface and deep waters, but they also predict dissolved oxygen depletions which are inconsistent with the widespread occurrence of glacial-age fossils from aerobic benthic organisms. Recent observations on changes in the vertical oceanic chemical structure provide a new mechanism to control glacial/interglacial CO² change. Palaeochemical evidence shows that nutrients and metabolic CO² are shifted from intermediate waters into deeper waters during glacial times^13–15. At the outset of this redistribution, increased deep dissolved CO² concentrations raise carbonate dissolution above that required for a steady-state input/output balance. Oceanic alkalinity then increases until steady-state dissolution rates are restored. Here I propose that atmospheric CO² decreases in direct response to increased oceanic alkalinity. Although the carbonate response factor in atmospheric CO² has been noted before, it has not been linked with vertical nutrient rearrangements. The withdrawal of nutrients from inter-mediate waters of the ocean also prevents oxygen depletion in these waters.