Two contributors to the glacial CO₂ decline

It is generally accepted that the glacial drawdown of atmospheric CO₂ content is the sole result of uptake by the ocean. Here we make a case that the reduction of planetary CO₂ outgassing made a significant contribution. We propose that the ocean contribution to CO₂ reduction closely followed Northern Hemisphere summer insolation and was superimposed on a ramp-like decline resulting from a reduction in the input of planetary CO₂. We base this scenario on new records of δ¹³C and B to Ca ratio in cores from the upper and lower portions of the deep Atlantic. They demonstrate that the waxing and waning of the stratification of Atlantic deep water follows summer insolation. Our thoughts were driven by the observation that over the last 30 kyr the extent of mountain glaciation in both hemispheres appears to have tracked the atmosphere's CO₂ content, suggesting that the connection between orbital cycles and land ice cover is via the ocean. Instead of a direct connection between ice extent and summer insolation, the tie is a modulation of the heat and fresh water budgets of the northern Atlantic. Changes in the boundary conditions lead to reorganizations of ocean circulation and, as a consequence, changes in CO₂ storage in the ocean.