High-resolution Atmospheric pCO2 Reconstruction across the Paleogene Using Marine and Terrestrial δ13C records

The early Paleogene (63 to 47 Ma) is considered to have a greenhouse climate¹ with proxies suggesting atmospheric CO₂ levels (pCO₂) approximately ~2× pre-industrial levels. However, the proxy based pCO₂ reconstructions are limited and do not allow for assessment of changes in pCO₂ at million to sub-million year time scales. It has recently been recognized that changes in C₃ land plant carbon isotope fractionation can be used as a proxy for pCO₂ with quantifiable uncertainty². Here, we present a high-resolution pCO₂ reconstruction (n = 597) across the early Paleogene using published carbon isotope data from both terrestrial organic matter and marine carbonates. The minimum and maximum pCO₂ values reconstructed using this method are broad (i.e., 170 +60/-40 ppmv to 2000 +4480/-1060 ppmv) and reflective of the wide range of environments sampled. However, the large number of measurements allows for a robust estimate of average pCO₂ during this time interval (~ 400 +260/-120 ppmv), and indicates brief (sub-million-year) excursions to very high pCO₂ during hyperthermal events (e.g., the PETM). By binning our high-resolution pCO₂ data at 1 million year intervals, we can compare our dataset to the other available pCO₂ proxies. Our result is broadly consistent with pCO₂ levels reconstructed using other proxies, with the exception of paleosol-based pCO₂ estimates spanning 53 to 50 Ma. At this timescale, no proxy suggests pCO₂ higher than 2000 ppmv, whereas the global surface ocean temperature is considered to be >10 ^oC warmer than today. Recent climate modeling suggests that low atmospheric pressure during this time period could help reconcile the apparent disconnect between pCO₂ and temperature and contribute to the greenhouse climate³.

References

1. Huber, M., Caballero, R., 2011. Climate of the Past 7, 603-633.

2. Schubert, B.A., Jahren, A.H., 2015. Geology 43, 435-438.

3. Poulsen, C.J., Tabor, C., White, J.D., 2015. Science 348, 1238-1241.