Early Cretaceous atmospheric CO2 estimates based on stomatal index of Pseudofrenelopsis papillosa (Cheirolepidiaceae) from southeast China

Estimates of the atmospheric carbon dioxide concentration levels were made for three time intervals of the Early Cretaceous using the stomatal indices of an extinct fossil conifer, Pseudofrenelopsis papillosa (Chow et Tsao) Cao ex Zhou. The fossil materials were collected from the upper Hauterivian, upper Aptian, and upper Albian of the Lower Cretaceous in the Fujian and Jiangxi Provinces of southeast China. The estimated paleo-CO2 (pCO2) was 600-1300 ppmv from the late Hauterivian to the late Albian based on the ratio between the stomatal indices of fossil species P. papillosa and those of four modern nearest living equivalent (NLE) species, using Carboniferous and recent standardizations. The results showed a low value of 595-957 ppmv in the late Hauterivian, and a high value of 805-1292 ppmv in the late Albian, which was only slightly higher than the value of 753-1210 ppmv in the late Aptian. Our calculated CO2 values were consistent with GECORB II, and similar to the previously published estimates of CO2 based on stomatal indices or carbon isotopes. Thus, it could be inferred that the stomatal index of P. papillosa is a potential indicator for estimating the pCO2 content during the geological history. Furthermore, the global mean land surface temperature (GMLST) was estimated based on the CO2 data. This showed that the change ratios of GMLST increased from 2.8 to 4.7 °C in the late Hauterivian to 3.6-5.5 °C in the late Albian and 3.8-5.7 °C in the late Aptian. It appears that the temperature gradually increased from the early to late Early Cretaceous.