Estimates of late Early Cretaceous atmospheric CO2 from Mongolia based on stomatal and isotopic analysis of Pseudotorellia

Premise

The Aptian–Albian (121.4–100.5 Ma) was a greenhouse period with global temperatures estimated as 10–15°C warmer than pre-industrial conditions, so it is surprising that the most reliable CO₂ estimates from this time are <1400 ppm. This low CO₂ during a warm period implies a very high Earth-system sensitivity in the range of 6 to 9°C per CO₂ doubling between the Aptian-Albian and today.

Methods

We applied a well-vetted paleo-CO₂ proxy based on leaf gas-exchange principles (Franks model) to two Pseudotorellia species from three stratigraphically similar samples at the Tevshiin Govi lignite mine in central Mongolia (~119.7–100.5 Ma).

Results

Our median estimated CO₂ concentration from the three respective samples was 2132, 2405, and 2770 ppm. The primary reason for the high estimated CO₂ but with relatively large uncertainties is the very low stomatal density in both species, where small variations propagate to large changes in estimated CO₂. Indeed, we found that at least 15 leaves are required before the aggregate estimated CO₂ approaches that of the full data set.

Conclusions

Our three CO₂ estimates all exceeded 2000 ppm, translating to an Earth-system sensitivity (~3–5°C/CO₂ doubling) that is more in keeping with the current understanding of the long-term climate system. Because of our large sample size, the directly measured inputs did not contribute much to the overall uncertainty in estimated CO₂; instead, the inferred inputs were responsible for most of the overall uncertainty and thus should be scrutinized for their value choices.