What caused the glacial/interglacial atmospheric pCO2 cycles?

Fifteen years after the discovery of major glacial/interglacial cycles in the CO 2 concentration of the atmosphere, it seems that all of the simple mechanisms for lowering p CO 2 have been eliminated. We use a model of ocean and sediment geochemistry, which includes new developments of iron limitation of biological production at the sea surface and anoxic diagenesis and its effect on CaCO 3 preservation in the sediments, to evaluate the current proposals for explaining the glacial/interglacial p CO 2 cycles within the context of the ocean carbon cycle. After equilibration with CaCO 3 the model is unable to generate glacial p CO 2 by increasing ocean NO 3 − but predicts that a doubling of ocean H 4 SiO 4 might suffice. However, the model is unable to generate a doubling of ocean H 4 SiO 4 by any reasonable changes in SiO 2 weathering or production. Our conclusions force us to challenge one or more of the assumptions at the foundations of chemical oceanography. We can abandon the stability of the “Redfield ratio” of nitrogen to phosphorus in living marine phytoplankton and the ultimate limitation of marine photosynthesis by phosphorus. We can challenge the idea that the pH of the deep ocean is held relatively invariant by equilibrium with CaCO 3 . A third possibility, which challenges physical oceanographers, is that diapycnal mixing in ocean circulation models exceeds the rate of mixing in the real ocean, diminishing the model p CO 2 sensitivity to biological carbon uptake.