What caused the glacial/interglacial atmospheric pCO₂ cycles?

Abstract

Abstract. Fifteen years after the discovery of major glacial/interglacial cycles in the CO2 concentration of the atmosphere, it seems that all of the simple mechanisms for lowering pCO 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 pCO 2 cycles within the context of the ocean carbon cycle. After equilibration with CaCO 3 the model is unable to generate glacial pCO 2 by increasing ocean NO•-but predicts that a doubling of ocean H4SiO 4 might suffice. However, the model is unable to generate a doubling of ocean H4SiO 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 pCO 2 sensitivity to biological carbon uptake. The ice age cycles underwent a midlife transition

Global temperature change

“…Global warming could result in release of large amounts of GHGs, e.g., from melting permafrost or destabilized methane clathrates on continental shelves (43). Some of the largest warmings in the Earth's history and mass extinctions may be associated with such GHG releases (39,43).…”

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…This phenomenon is known as the "carbonate counter pump" effect. Additionally, it has been suggested that during the last glaciation, lower PIC:POC export ratio due to increased organic carbon export may have contributed to higher dissolution of the deep-ocean carbonate sediments, leading to a decrease in pCO 2 compared to the interglacial periods (Archer and Maier-Reimer, 1994;Archer et al, 2000;Sigman and Boyle, 2000). Therefore the PIC:POC ratio of exported particles is likely to have a significant impact on the atmosphere-ocean CO 2 fluxes from seasonal to geological timescales Sarmiento et al, 2002).…”

Climate and marine carbon cycle response to changes in the strength of the Southern Hemispheric westerlies

“…Hence, to account for the glacial CO 2 drop, large amounts of carbon must have been sequestered in the ocean. Different mechanisms have been suggested such as increased CO 2 solubility, greater Southern Ocean stratification and sea ice cover [Francois et al, 1997;Stephens and Keeling, 2000;Gildor et al, 2002], deepening of the lysocline [Archer et al, 2000] and increased marine export production due to either greater marine nutrient inventory [Broecker, 1982[Broecker, , 1998McElroy, 1983], or a higher Redfield ratio [Broecker, 1982;Omta et al, 2006], or an increase in iron availability in surface waters [Martin, 1990]. So far, however, model results and theoretical considerations [Archer et al, 2003] suggest that there does not seem to be a single mechanism that can explain the full magnitude of the glacial-interglacial atmospheric CO 2 changes as well as their timing.…”