I can think of two causes for the reduction of CO2:
- Ocean absorption. The temperature dropped and the ocean began to absorb more CO2, according to Henry's law.
- Biosphere absorption. For example, fixed nitrogen is a limiting nutrient for algae in the low-latitude ocean, and its oceanic inventory may have been higher during the ice ages, thus helping to lower atmospheric CO2 during those intervals (Ren et al.,2009).
What was the relative role (quantitatively) that both process played in the CO2 fall?
Was the ocean the main CO2 sink and the biosphere absorption secondary? Or conversely both played a similar role (or even biosphere was a larger sink)?
Are there other sinks than biosphere and oceans?
H. Ren, D. M. Sigman1, A. N. Meckler, B. Plessen, R. S. Robinson, Y. Rosenthal, G. H. Haug (2009): "Foraminiferal isotope evidence of reduced nitrogen fixation in the ice age Atlantic Ocean" Science 09 Jan 2009: Vol. 323, Issue 5911, pp. 244-248 DOI: 10.1126/science.1165787