基于叶化石气孔及对照性碳同位素的古大气CO2重建

Atmospheric CO2 is one of important factors to influence environmental change as well as an important parameter to understand global change during geological time. Among some current methods, leaf stomatal parameter method is significant for reconstructing quantitatively ancient CO2. In general, well-preserved leaf fossils possess anatomical characters. Besides, n-alkanes are also preserved in leaf fossils. However, the paleoenvironmental significance of n-alkanes was neglected all along. Compared to total organic carbon from surrounding rock, compound-specific hydrocarbons from fossil leaves have higher precision to estimate paleo-CO2 due to the hydrocarbon sealed into fossil leaves that leads to smaller variation affected by diagenesis, as well as clear sources. As a result, in this research the dominant fossil taxa sensitive to CO2 change are chosen from the Late Miocene Shengxian flora of eastern Zhejiang, such as Liquidambar, Quercus, Machilus, Cunninghamia plants, to perform two researches of stomatal features and corresponding compound-specific hydrocarbon. Based on the two researches, paleo-CO2 levels are estimated. After that, cross validation for the two results will be used to determine which method is more feasible. Based on isotopic dating for overlying and underlying basalts, it was presented that the section bearing leaf fossils was deposited during the geological time from 10.8ma to 9.2ma. Consequently, a new method to reconstruct paleo-CO2 levels will be established in this study. Moreover, this research will contribute to reconstruct paleo-CO2 change with high precision during the Late Miocene (i.e., 10.8ma-9.2ma).

大气CO2是影响环境变迁的主要因素之一，也是深入理解地史时期全球变化的重要参量。在现有的诸多研究方法中，陆生植物叶气孔分析是以往古大气CO2定量恢复的主要手段之一。植物叶化石除解剖特征之外，还保存有少量单体正构烷烃，而其古环境意义长期被低估。相对于围岩的总有机碳而言，由于叶化石单体烃封存于叶片之内，受成岩作用影响较小，且来源明确，对古大气CO2恢复的精度也就相应更高。本项目选取浙东嵊县植物群中枫香Liquidambar、栎属Quercus、润楠Machilus、杉木Cunninghamia等对CO2变化敏感的优势类群，分别对各类群开展叶气孔和对照性的单体烃碳同位素分析，恢复古CO2浓度，并对二者进行交叉验证。此外，火山岩绝对测年显示该植物群的发展延限为1.6ma(10.8-9.2ma)。因此，本项目的实施在确立古CO2研究新方法的同时，也将高精度重建该地史阶段CO2浓度变化的详细过程。

植物是生命的主要形态之一，在生命演化和陆地生态领域开拓中具有重要作用。与动物不同，植物具有被动适应环境的特性，对气候和CO2的变化比较敏感，因而它是揭示古CO2和古气候、古环境变化的重要信息源。在现有定量恢复古大气CO2的诸多研究方法中，陆生植物叶片气孔分析是主要手段之一。但是，不同植物类群的气孔对CO2变化的响应机制和敏感性不同，需要选用对CO2变化敏感的植物；同时还需考虑气孔参数对CO2浓度的响应可能存在上限的问题，所以当采用植物叶片气孔参数重建CO2浓度，尤其是CO2浓度较高的地史时期，往往会使重建的古CO2值偏低。因而，本项目预期目标是筛选出对CO2变化敏感的植物，然后基于这些植物的气孔参数和稳定碳同位素，采用合适的方法准确重建出中新世CO2变化。在项目执行过程中，项目组成员已经证明水杉（Metasequoia）和红杉（Sequoia）对CO2变化比较敏感，可以作为重建古CO2的可靠材料；同时修订了以气孔和碳同位素作为参数的叶-气交换模型（Franks模型），重建出早中新世的CO2浓度约为400ppmv，与其它参数获得的CO2值相似，说明修订后的Franks模型具有可靠性，可以突破气孔对CO2响应的上限，从而准确重建出深时CO2值。为了找寻更多对CO2变化敏感的植物化石，本项目详细对早-晚中新世不同阶段的植物化石进行了分类学研究，已鉴定出10余个化石物种，其中作为活化石的香蕨木具有成为重建古CO2可靠材料的潜力。此外，为了了解新近纪以来我国植物与CO2和气候的变化，项目组从时间和空间两个跨度拓展了研究内容，分别开展了内蒙古早中新世、青海泽库早-中中新世、浙东晚中新世和陕西宝鸡早更新世的植物与CO2和气候重建方面的研究，揭示了新近纪以来植物与环境的演化特征，探讨了植物对环境变化的响应过程。