Silicic acid leakage from the Southern Ocean: A possible explanation for glacial atmospheric pCO₂

Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO₂. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO₃ production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO₂ from the interglacial 277 to 230-242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO₂ sensitivity of general circulation models to carbonate pump forcing is ∼ 3.5-fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO₂. We discuss the importance of the initial rain ratio of CaCO₃ to organic carbon on atmospheric pCO₂ and relevant sedimentary records that support and constrain this "silicic acid leakage" scenario.