新疆准噶尔盆地早侏罗世掌鳞杉科植物化石 及古大气CO2浓度

植物化石气孔参数分析是目前恢复古大气二氧化碳浓度较为精准的方法之一,银杏类和松柏类等是恢复古大气CO_2浓度常用的化石类群。本文利用新疆准噶尔盆地下侏罗统三工河组的松柏类掌鳞杉科Brachyphyllum(Hirmeriella?)sp.化石对早侏罗世大气CO_2浓度进行了重建,获得早侏罗世大气CO_2浓度为～1200ppm,丰富了早侏罗世大气CO_2浓度信息,进一步说明掌鳞杉科植物通过气孔比率法在重建侏罗纪大气CO_2浓度方面的可靠性。掌鳞杉科植物的旱生构造和较高的大气CO_2浓度表明早侏罗世Toarcian期大洋缺氧事件在陆地生态系统内可能产生了一定的响应。

Palaeo-CO2 estimation of the Early Jurassic based on Cheirolepidiaceous conifer in the Junggar Basin, Xinjiang, China

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).