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<p>During the four most recent glacial maxima, atmospheric <span class="inline-formula">CO₂</span> has been lowered by about 90–100&thinsp;ppm with respect to interglacial concentrations. It is likely that most of the atmospheric <span class="inline-formula">CO₂</span> deficit was stored in the ocean. Changes in the biological pump, which are related to the efficiency of the biological carbon uptake in the surface ocean and/or of the export of organic carbon to the deep ocean, have been proposed as a key mechanism for the increased glacial oceanic <span class="inline-formula">CO₂</span> storage. The biological pump is strongly constrained by the amount of available surface nutrients. In models, it is generally assumed that the ratio between elemental nutrients, such as phosphorus, and carbon (<span class="inline-formula">C∕P</span> ratio) in organic material is fixed according to the classical Redfield ratio. The constant Redfield ratio appears to approximately hold when averaged over basin scales, but observations document highly variable <span class="inline-formula">C∕P</span> ratios on regional scales and between species. If the <span class="inline-formula">C∕P</span> ratio increases when phosphate availability is scarce, as observations suggest, this has the potential to further increase glacial oceanic <span class="inline-formula">CO₂</span> storage in response to changes in surface nutrient distributions. In the present study, we perform a sensitivity study to test how a phosphate-concentration-dependent <span class="inline-formula">C∕P</span> ratio influences the oceanic <span class="inline-formula">CO₂</span> storage in an Earth system model of intermediate complexity (cGENIE). We carry out simulations of glacial-like changes in albedo, radiative forcing, wind-forced circulation, remineralization depth of organic matter, and mineral dust deposition. Specifically, we compare model versions with the classical constant Redfield ratio and an observationally motivated variable <span class="inline-formula">C∕P</span> ratio, in which the carbon uptake increases with decreasing phosphate concentration. While a flexible <span class="inline-formula">C∕P</span> ratio does not impact the model's ability to simulate benthic <span class="inline-formula"><i>δ</i>¹³C</span> patterns seen in observational data, our results indicate that, in production of organic matter, flexible <span class="inline-formula">C∕P</span> can further increase the oceanic storage of <span class="inline-formula">CO₂</span> in glacial model simulations. Past and future changes in the <span class="inline-formula">C∕P</span> ratio thus have implications for correctly projecting changes in oceanic carbon storage in glacial-to-interglacial transitions as well as in the present context of increasing atmospheric <span class="inline-formula">CO₂</span> concentrations.</p>