CO2 levels fell during the last interglacial-glacial transition until the Last Glacial Maximum in the Pleistocene.

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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).

My question is: 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)?

H. Ren, D. M. Sigman1, A. N. Meckler, B. Plessen, R. S. Robinson, Y. Rosenthal, G. H. Haug "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

  • $\begingroup$ Your graphs only go back 1.3 million years. The Pleistocene started 2.6 million years ago or so. What is the point of including those noninclusive graphs? What is the point of this question? $\endgroup$ – David Hammen May 12 at 5:29
  • $\begingroup$ CO2 levels don't fall drastically at the beginning of the Pleistocene. nature.com/articles/s41467-019-12357-5. They fall during the Pliocene and mainly fluctuate between 180 and 320 during the Pleistocene following the Milankovic cycles. $\endgroup$ – user22279 May 12 at 9:54
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Universal_learner May 12 at 11:46
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    $\begingroup$ I suspect you are confusing cause and effect, and I know that your two causes are far from complete. You have forgotten about mountains for one thing, and for another you have forgotten ocean currents. Did the formation of the Isthmus of Panama play a role? Maybe. This is highly debated. Did the Rocky Mountains, the Alps, and the Himalaya play a role? Maybe. But this too is highly debated. Is the division between the Pliocene and the Pleistocene rather arbitrary? There is a marker for that division, but whether it is geologically / biologically significant once again is highly debated. $\endgroup$ – David Hammen May 12 at 19:46
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    $\begingroup$ @Universal_learner: I performad a casual search for the cause of the CO2 decline during oxygen isotope stages 4 and 3 (the red frame) and found work that supports your assumption that the ocean uptake played a major role. Unfortuantely I don't have time and deep enough knowledge for a good answer. $\endgroup$ – user22279 May 16 at 13:47

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