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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 >2,300-ppm CO2. Here we reevaluate 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 1,000-ppm CO2, including the nearest living relative of the K-Pg fern, and find a mean error rate of similar to 22%, which is comparable to other leading paleo-CO2 proxies. Our fossils record a similar to 250-ppm increase in CO2 across the K-Pg boundary from similar to 625 to similar to 875ppm. 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 1,000-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 similar to 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.