Palaeo-CO2 estimation of the Early Jurassic based on Cheirolepidiaceous conifer in the Junggar Basin, Xinjiang, China
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Abstract:
The atmospheric carbon dioxide concentration of the Early Jurassic has been estimated based on the stomata ratio method by using Brachyphyllum (Hirmeriella?) sp., an extinct fossil conifer of Cheirolepidiaceae. The fossil materials were collected from the Early Jurassic Sangonghe Formation in the Junggar Basin, northern Xinjiang, China. The stomatal parameters (stomatal density and stomatal index) of cuticle were analyzed, and the stomatal index was used to estimate the Palaeo-pCO2. The average stomatal density was 96.4±19.4/mm2 and the average stomatal index value was 3.7 ± 0.3. To calculate the stomatal ratio and estimate the atmospheric carbon dioxide concentration, the Athrotaxis cupressoides (SI = 9.4 ± 3.05) was selected as a nearest living equivalent species for Brachyphyllum. The estimated atmospheric CO2 content were 940.6 ± 77.3 ppm and 1567.6 ± 128.8 ppm on the basis of Modern standardization and Carboniferous standardization respectively. The average estimate of the two standardizations indicates a Palaeo-pCO2 of 1254.1 ± 103.0 ppm in the Early Jurassic (Early Toarcian). The value of atmospheric pCO2 of the Early Jurassic, as previous studies, was 900?1500 ppm by stomata ratio method based on the fossil Ginkgo species. The present estimation from a conifer of Brachyphyllum is broadly consistent with the previous estimates which may indicate that the reliability of Cheirolepidiaceous conifer in Palaeo-pCO2 estimates during the Jurassic period. The strongly xeromorphic characters of the present fossil plant and the relatively high atmospheric pCO2 indicate that a warm and dry climate dominated the Junggar Basin during the Early Jurassic (Early Toarcian), and that may be linked to the Toarcian Oceanic Anoxic Event (T-OAE).
