No evidence for a large atmospheric CO2 spike across the Cretaceous‐Paleogene boundary

By Joseph N. Milligan et al.

Abstract

Currently there is only one paleo‐CO2 record from plant macrofossils that has sufficient stratigraphic resolution to potentially capture a transient spike related to rapid carbon release at the Cretaceous‐Paleogene (K‐Pg) boundary. Unfortunately, the associated measurements of stomatal index are off‐calibration, leading to a qualitative interpretation of >2300 ppm CO2. Here we re‐evaluate this record with a paleo‐CO2 proxy based on leaf gas‐exchange principles. We also test the proxy with three living species grown at 500 and 1000 ppm CO2, including the nearest living relative of the K‐Pg fern, and find a mean error rate of ~22%, which is comparable to other leading paleo‐CO2 proxies. Our fossils record a ~250 ppm increase in CO2 across the K‐Pg boundary from ~625 to ~875 ppm. A small CO2 spike associated with the end‐Cretaceous mass extinction is consistent with many temperature records and with carbon cycle modeling of Deccan volcanism and the meteorite impact.

Plain Language Summary

Currently there is only one paleo‐CO2 record close enough to the Cretaceous‐Paleogene (K‐Pg) boundary to record a rapid release in atmospheric CO2, a greenhouse gas. This record is based on the stomatal frequencies of fern fossils at the K‐Pg boundary and Ginkgo fossils before and after the boundary. Unfortunately, due to deficiencies with the method, the CO2 inferences are only qualitative. Here we look at the same fossils with a proxy based on leaf gas‐exchange principles (i.e. photosynthesis). We first test the proxy with three living species grown at 500 and 1000 ppm CO2, including the nearest living relative of the K‐Pg fern, and find a comparable accuracy to other quantitative paleo‐CO2 proxies. The fossils record a modest ~250 ppm increase in CO2 across the K‐Pg boundary. These estimates are consistent with most temperature records and with carbon cycle modeling of Deccan volcanism and the meteorite impact.

This paywalled article appeared on the Geophysical Research Letters website at https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL081215?af=R&