Carbon dioxide and Antarctic glaciation
Pagani et al., writing in Science:
The decline in the partial pressure of atmospheric carbon dioxide during the [Eocene-Oligocene] climate event was substantial, but absolute CO2 concentrations depend on the value of εf applied. Collectively, CO2 estimates calculated by using U37K’ and TEX86 SST estimates and a range of εf values indicate that CO2 decreased ~40% from 35.5 to 32.5 million years ago (SOM). Application of reasonable εf values (25 to 28‰) indicates that the partial pressure of atmospheric CO2 fell from 1200 to 1000 ppm to 700 to 600 ppm. Interestingly, the change in CO2 determined from this study, as well as the boron-isotope methodology (11), is consistent with model estimates for a threshold CO2 level required for rapid Antarctic glaciation (8, 29).
We conclude that the available evidence supports a fall in CO2 as a critical condition for global cooling and cryosphere evolution ~34 million years ago. Whether CO2 acted alone to cause the E-O transition or whether a threshold CO2 level in combination with favorable orbital configurations (1) ultimately triggered glaciation cannot be determined from our results. However, during the E-O transition both CO2 decline and enhanced ice albedo account for global temperature changes. Lastly, the long-term permanence of the CO2 decline (10) and the impermanent inorganic carbon isotope shift (1) implicate the role of silicate weathering rates over the influence of short-term organic-carbon burial rates as the primary cause for long-term change in atmospheric carbon dioxide.
