What lowered atmospheric CO2 by about 90 ppm in glacial times?
In plain words
Air bubbles in Antarctic ice show carbon dioxide fell from about 280 to about 190 parts per million during each ice age and rose again afterwards. The ocean must have stored the missing carbon, but the mix of processes that did this is still debated.
Precise statement
Quantify the contributions of ocean temperature and solubility, Southern Ocean sea-ice capping and stratification, iron fertilization of the biological pump, carbonate compensation and terrestrial carbon to the roughly $90\,\mathrm{ppm}$ glacial-interglacial pCO2 change, with a budget that closes within proxy uncertainty. An answer is a quantitative carbon budget consistent with deep-ocean carbon-14 ages, delta-13C and carbonate-ion proxies at the Last Glacial Maximum.
What would settle it
An Earth-system model simulation of the Last Glacial Maximum that matches ice-core pCO2 together with radiocarbon, delta-13C and carbonate-ion proxy fields.