甘肃宝积山盆地中侏罗世阿干镇似银杏(Ginkgoites aganzhenensis Yang)的古环境意义

在甘肃宝积山盆地中侏罗统窑街组发现了保存较为完整的银杏类叶片化石。对当前化石宏观特征与镜下微观构造进行研究，并将其鉴定为阿干镇似银杏（Ginkgoites aganzhenensis）。在此基础上，统计了该种下表皮的气孔参数，并利用气孔指数法和气孔比率法恢复研究区中侏罗世早期的古大气CO₂浓度，探讨这两种方法的准确性。分析结果表明，气孔指数法获得的古大气CO₂浓度为974×10-6，落在了GEOCARB Ⅲ的可信误差范围之外；气孔比率法重建的古大气CO₂浓度为1 823×10-6，处于GEOCARB Ⅲ可信误差范围之内。这说明对于当前化石而言，利用气孔比率法恢复古大气CO₂浓度更加合适，数据十分接近GEOCARB Ⅲ的最佳拟合线，表明似银杏也是恢复古大气CO₂浓度的一种理想植物。通过计算得出当时的温度比现在高7.2℃，反映了宝积山盆地中侏罗世阿林期到巴柔期为温暖潮湿的温带气候环境。

The Paleoenvironmental Significance of Ginkgoites aganzhenensis Yang from the Middle Jurassic in the Baojishan Basin,Gansu Province

Well-preserved fossil specimens of ginkgoalean leaves were collected from the Middle Jurassic Yaojie Formation of the Baojishan Basin, Gansu Province. They were identified as Ginkgoites aganzhenensis Yang according to the macro-shape and micro-characteristics. The epidermal characteristics and stomatal parameters of these fossil specimens were obtained through a cuticular analysis. The paleo-CO₂ of the early Middle Jurassic was quantitatively reconstructed using two methods, 974×10-6 by the stomatal index method which was out of the error range of CO₂ confidence from GEOCARB Ⅲ, 1 823×10-6 by the stomatal ratio method which was plotted within the error range of CO₂ confidence from GEOCARB Ⅲ based on the SI value of Ginkgoites aganzhenensis. It implied that the stomatal ratio method is more reliable in reconstructing paleo-CO₂ concentration based on Ginkgoites aganzhenensis. Compared with the paleo-CO₂ level reconstructed from Phoenicopsis and Solenites, we found that from Ginkgoites is more near to the optimum line of GEOCARB Ⅲ, suggesting that the paleo-CO₂ reconstructed from Ginkgoites is more advisable. And such high CO₂ concentration would have caused an increase in temperature of about 7.2℃, indicating that the paleoclimate in Baojishan Basin was warm and humid temperate climate in Aalenina-Bajocian of the Middle Jurassic.