Late Early Cretaceous climate and pCO2 estimates in the Liupanshan Basin, Northwest China

The extinct conifer family Cheirolepidiaceae, especially Pseudofrenelopsis, is commonly used to reconstruct the atmospheric palaeo-CO2 concentration (pCO2) during the Cretaceous period. In recent years, many Pseudofrenelopsis specimens have been collected from the uppermost stratum of the Naijiahe Formation in the Liupanshan Basin, Ningxia Hui Autonomous Region, Northwest China. The stomatal parameters, such as stomatal density, stomatal index, stomatal pore length, and stomatal pore depth, and the stable carbon isotope composition of the present Pseudofrenelopsis were analyzed. The palaeo-CO2 levels of the Liupanshan Basin during the late Early Cretaceous (late Albian) were reconstructed using stomatal ratio (SR) and mechanistic model approaches. The SR method yielded a semi-quantitative pCO2 estimate of 620 ppmv on the basis of the Recent standardization and 1030 ppmv on the basis of the Carboniferous standardization, which show similarities with other SR-based and geochemical-based reconstructions for the late Early Cretaceous. The pCO2 was about 670 ppmv on the basis of mechanistic model method, similar to the estimated pCO2 values recovered by the SR method corrected with the Recent standardization. Based on comparative studies of previously published pCO2 results of the Early Cretaceous, pCO2 levels varied during the Early Cretaceous, and pCO2 was relatively stable during the late Early Cretaceous. Furthermore, environmental conditions of the Liupanshan Basin during the late Early Cretaceous were studied. The stable carbon isotope composition of Pseudofrenelopsis leaves ranged from −21.5‰ to −24.3‰, with an average of −22.8‰, indicating an arid/semi-arid and high-evaporation climate during the late Early Cretaceous in the Liupanshan Basin. These environmental conditions were also supported by palynological, lithological, and geochemical evidence.