Using an observationally constrained earth system model, S. Khatiwala and co-workers (2019, see reference below) compare different processes that could lead to the 90-ppm glacial atmospheric CO2 drawdown, with an important improvement on the deep carbon storage quantification (i.e. Biological Carbon Pump efficiency). They demonstrate that circulation and sea ice changes had only a modest net effect on glacial ocean carbon storage and atmospheric CO2, whereas temperature and iron input effects were more important than previously thought due to their effects on disequilibrium carbon storage.
Khatiwala, S., Schmittner, A., & Muglia, J. (2019). Air-sea disequilibrium enhances ocean carbon storage during glacial periods. Science Advances, 5(6), eaaw4981. DOI: https://doi.org/10.1126/sciadv.aaw4981