Moderate to Elevated Atmospheric CO₂ During the Early Paleocene Recorded by Platanites Leaves of the San Juan Basin, New Mexico

Abstract

Throughout the Phanerozoic, estimated CO2 levels from CO2 proxies generally correlate well with independent estimates of temperature. However, some proxy estimates of atmospheric CO2 during the Late Cretaceous and early Paleocene are low (<400 ppm), seemingly at odds with elevated sea surface temperature. Here we evaluate early Paleocene CO2 by applying a leaf gas‐exchange model to Platanites leaves of four early Paleocene localities from the San Juan Basin, New Mexico (65.66–64.59 Ma). We first calibrate the model on two modern Platanus species, Platanus occidentalis and P. × acerifolia, where we find the leaf gas‐exchange model accurately predicts present‐day CO2, with a mean error rate between 5% and 14%. Applying the model to the early Paleocene, we find CO2 varies between ∼660 and 1,140 ppm. These estimates are consistent with more recent CO2 estimates from boron, leaf gas‐exchange, liverwort, and paleosol proxies that all suggest moderate to elevated levels of CO2 during the Late Cretaceous and early Paleocene. These levels of atmospheric CO2 are more in keeping with the elevated temperature during this period.

The evolution of extant South American tropical biomes

“…There are, however, two major differences compared to extant TRF. First, MAT was c. 29°C, c. 2°C warmer than now (Jaramillo & Cardenas, 2013), and CO 2 was almost double (c. 500 ppm) (Milligan et al, 2022). Second, species diversity was significantly lower (Notes S3).…”

“…2°C warmer than now (Jaramillo & Cardenas, 2013), and CO 2 was almost double ( c . 500 ppm) (Milligan et al ., 2022). Second, species diversity was significantly lower (Notes S3).…”

Variation in organic matter across the Cretaceous-Paleogene boundary in New Zealand supports the “Living Ocean” model of biotic recovery

“…In contrast, terrestrial ecosystems are believed to have recovered more quickly from the K-Pg event than counterpart marine ecosystems (Beerling et al, 2001). A shutdown or reduction in the uptake of 12 C by photosynthetic phytoplankton and increased biomass burning (Ivany and Salawitch, 1993;Lyons et al, 2020;Wolbach et al, 1988) could together have led to elevated levels of CO 2 in the atmospheric reservoir (Milligan et al, 2022). Evidence for the transfer of CO 2 from the atmosphere to the terrestrial biosphere reservoir (via photosynthesis) is reflected in a negative carbon isotope excursion (CIE) recorded in δ 13 C of terrestrial biomarkers in Europe (Arinobu et al, 1999;Sepúlveda et al, 2019) and bulk terrestrial organic matter from sites in North America (Arens and Jahren, 2000;Schimmelmann and DeNiro, 1984).…”