吐哈盆地北缘中侏罗世植物化石稳定碳同位素的古环境意义

新疆吐哈盆地北缘三道岭煤田中侏罗统西山窑组的植物化石属于Coniopteris-Phoenicopsis植物群的典型组合。从该植物群中挑选出优势属种，分别为似银杏、拟刺葵、枝脉蕨以及斯堪尼狭轴穗，对其进行稳定碳同位素分析。前3种为叶片化石，它们的δ13C值分布在-21.66‰~-23.83‰，属于典型的C3植物。最后一种为繁殖器官化石，其δ13C值为-20.91‰，比叶片的值高。将δ13C值根据公式转换为大气CO₂的δ13C_a值，发现基于银杏类叶片获得的现代和中侏罗世的δ13C_a值与其他方法获得的值相似。这说明银杏类叶片是重建地质历史时期δ13C_a的一类可靠植物，为今后研究中生代-新生代δ13C_a变化提供了一个简便而可行的方法。C_i/C_a比值是一重要的植物生理生态特征值，因此进一步将银杏类化石叶片的δ13C值换算成C_i/C_a。化石似银杏的C_i/C_a值为0.6，而现生银杏的C_i/C_a为0.63，二者相似。虽然三道岭地区中侏罗世的CO₂浓度约为现在的4.5倍，但是化石和现生银杏类植物的C_i/C_a值仍然保持恒定，表明银杏植物的气孔参数对CO₂变化比较敏感，是重建古CO₂浓度的良好指示植物。这也为判断一种植物能否用来重建古CO₂浓度提供了一种简便而有效的检测方法。

Stable Carbon Isotope of Middle Jurassic Plant Fossils in the North Edge of Turpan-Hami Basin,Xinjiang and Their Palaeoenvironmental Implications

The plant fossil assemblage from the Jurassic Xishanyao Formation in the north edge of Turpan-Hami Basin, China, is ascribed into Coniopteris-Phoenicopsis flora.The four dominant genera of fossil plants which are Ginkgoites, Phoenicopsis, Cladophlebis, Stenorachis, are chosen to perform stable carbon isotopic analysis.The δ13C values of three fossil leaves of Ginkgoites, Phoenicopsis, Cladophlebis range from-21.66‰ to-23.83‰, which indicates they blong to C3 type.The Stenorachis is one fossil of reproductive organs.Its δ13C value is-20.91‰, more enriched than those of fossil leaves, showing the reliability of carbon isotopic data.According to formula, the δ13C of plants is converted into stable carbon isotopic values of atmospheric CO2 (δ13Ca).It is found that modern and Middle Jurassic δ13Ca values obtained from Ginkgoales leaves are similar to those obtained from other methods, suggesting Ginkgoales leaves are reliable specimens for reconstructing δ13Ca during geological period, which provides a simple and feasible method to obtain δ13Ca from the Mesozoic to Cenozoic.Ci/Ca is an important plant ecophysiological parameter.Thus, δ13C of Ginkgoales leaves is also converted into Ci/Ca.The Ci/Ca value of fossil Ginkgoites is 0.6, similar to 0.63 that is Ci/Ca value of living Ginkgo.It was known that palaeo-CO2 level in the Middle Jurassic is about 4.5 times of nowaday CO2.However, Ci/Ca keeps approximate constant between living and fossil Ginkgoales leaves.It indicates that Ginkgoales plants possess high photosynthetic rate in the Middle Jurassic, as well as favorable sensitivity of stomata to CO2 change.Thereby, stomata of Ginkgoales plants are good indicator of reconstructing palaeo-CO2 level.As a result, variation in Ci/Ca provides effective means to estimate whether one plant can be used as indicator to reconstruct palaeo-CO2 level.