Global trends of pCO2 across the Cretaceous-Paleogene boundary supported by the first Southern Hemisphere stomatal proxy-based pCO2 reconstruction

- A new fossil Lauraceae leaf database from New Zealand spans the K-Pg boundary.
- Latest Cretaceous-mid Paleocene pCO2 was reconstructed using the stomatal proxy.
- On average, pCO2 decreased by ~ 45%, from ~ 570 to ~ 310 ppm, during this time.
- Results are consistent with previously published Northern Hemisphere pCO2 records.
- However, a spike of extremely high pCO2 previously reported at K-Pg was not confirmed.

Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo-pCO2 levels. Such reconstructions are needed: 1, at a high temporal resolution for constraining the pre- and post-extinction atmospheres; and 2, at a sufficient spatial resolution to constrain potential inter-hemispheric differences. Here we estimate pCO2 based on fossil Lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest Cretaceous to the mid-Paleocene, including a new Cretaceous-Paleogene boundary (K-Pg) locality, in New Zealand. We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K-Pg boundary, and further to ca. 305-320 ppm in the mid-Paleocene. A 'spike' of extremely high pCO2 at the K-Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K-Pg.

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