基于青冈亚属化石的气孔频度重建始新世–上新世古大气CO2浓度

Palaeoatmospheric CO2 (palaeo-CO2) concentration reconstructions provide important information between CO2 concentration and climate change. However, the reconstruction results obtained by different proxies have been quite variable. Thus more research efforts are required to estimate a more reliable atmospheric CO2 concentration history in the long geological time. The relationship between atmospheric CO2 partial pressure (pCO2) and stomatal frequency in many species of plants is an important approach to estimate palaeo-CO2 levels. Numerous genera and species have been used to estimate palaeo-CO2 concentrations, but Quercus subgenus Cyclobalanopsis fossils, which are abundant in the Cenozoic strata of East Asia, almost have not been used. The applicant has found a significant inverse correlation between stomatal index and atmospheric pCO2 in Quercus glauca. This correlation was determined from three sources: extant field samples collected along an altitudinal gradient, historical herbarium specimens collected between a time period and seedlings growing in the climatic chambers for CO2-elevated experiment. This indicates that Q. glauca is sensitive to changes of atmospheric pCO2 and is a good potential proxy for palaeo-CO2 levels. The applicant also found that the stomatal frequency of Q. delavayi shows no significant response to changes of pCO2 in both extant samples collected along an altitudinal gradient and historical herbarium samples. In this project, the relationship between atmospheric pCO2 and stomatal frequency in Q. schottkyana will be analysed. Quercus subgenus Cyclobalanopsis fossils will also be collected from different sites dated from Eocene to Pliocene. Because the relationship between stomatal frequency and pCO2 is species specific, the stomatal frequency of these three species may have different responses to changes of pCO2. We will apply the different correlations of pCO2–stomatal frequency in these three species to closely related Quercus subgenus Cyclobalanopsis fossils to estimate palaeo-CO2 concentrations during middle to late Cenozoic. Moreover, the differences of the results derived from these different correlations will be compared. The estimated palaeo-CO2 concentrations will provide important new evidence on the evolution of palaeo-CO2 concentrations, and the relationship between CO2 concentration and climate change.

重建地质时期大气CO2浓度有利于深入理解CO2浓度与气候变化的关系，但各代理指标重建的结果仍差异较大，需更多研究建立可靠的古大气CO2变化曲线。目前已有很多植物应用气孔频度与大气pCO2的相关性重建古大气CO2，而在东亚新生界地层广泛分布的栎属青冈亚属化石未被有效利用。申请人已通过三种不同来源的材料建立了青冈（Quercus glauca）气孔指数与大气pCO2的负相关关系，是潜在的重建古大气CO2的代理指标；而黄毛青冈（Q. delavayi）的气孔频度与大气pCO2无明显相关性。本项目还将研究滇青冈（Q. schottkyana）气孔频度与pCO2的相关性，并采集始新世–上新世的青冈亚属化石。本项目将充分考虑气孔频度与pCO2关系的种间特异性，用这三种植物建立不同响应模式的模型并用于相应的化石重建新生代中后期大气CO2变化历史，比较不同模型重建结果的差异，并讨论大气CO2与气候的关系。

重建地质时期的大气CO2浓度有利于深入理解CO2浓度与气候变化的关系，目前已有很多植物类群应用气孔频度与大气pCO2的相关性重建古大气CO2，而在东亚新生界地层广泛分布的栎属青冈组（Quercus section Cyclobalanopsis）化石未被有效利用。本项目以三种青冈组植物——青冈（Q. glauca）、滇青冈（Q. schottyana）和黄毛青冈（Q. delavayi）为研究对象，分析气孔频度与大气pCO2的关系，并利用不同时期的青冈组化石进行古大气CO2重建。主要进展如下：. （1）青冈三种来源样品的气孔频度均与大气pCO2呈显著负相关，说明青冈的气孔频度能敏锐地反映大气pCO2的变化。将青冈的海拔梯度样品和历史标本样品合并，优化了其气孔频度与大气pCO2呈负相关的校准曲线。将晚中新世的类青冈化石（Q. paraglauca）的气孔频度应用于该校准曲线中，重建的古大气CO2浓度为410.23和356.22 ppm，与其它重建结果十分吻合，说明青冈是重建古大气CO2浓度可靠的代理指标（proxy）。（2）滇青冈的气孔频度与大气pCO2无明显相关性，不适合单独用来重建古大气CO2浓度。（3）合并分析了青冈、滇青冈和黄毛青冈的气孔频度与大气pCO2的关系，发现呈显著的负相关。据此推测滇青冈和黄毛青冈的气孔频度与pCO2不相关可能是两者对pCO2的反应不够敏锐且数据的pCO2跨度和时间跨度不够大所致，将三种植物合并后pCO2跨度显著扩大，校准曲线也呈显著的相关性，说明亲缘关系相近的植物具有合并建立校准曲线的潜力。将时代为晚中新世至早更新世的5种青冈组化石的气孔频度应用于该合并校准曲线中，重建的晚中新世以来的古大气CO2浓度约为200-400 ppm，与其它重建结果也吻合。说明三种青冈组植物合并的校准曲线也是重建古大气CO2浓度可靠的代理指标。本项目尝试将三种形态相似、关系紧密的青冈组植物的气孔频度合并，突破了传统的种间特异性对气孔频度方法的限制，是一个新的尝试。（4）将重建的晚中新世至早更新世的古大气CO2浓度与全球深海古温度进行比较，发现古大气CO2浓度与古温度变化趋势紧密相关，验证了古大气CO2浓度变化是古温度、古气候变化的一个重要驱动因素。