【作者】 林志成；

【导师】 孙柏年；

【作者基本信息】 兰州大学 ， 古生物学与地层学， 2010， 博士

【摘要】 目前,从陆生高等植物化石中提取古大气CO2浓度信息的方法主要有两种：一种是通过维管植物叶角质层气孔参数,来重建古大气CO2浓度；另一种则是通过测定苔类植物化石的碳同位素组成(δ13C),进而计算其碳同位素判别(△13C)来恢复古大气C02浓度。两种方法对所使用的化石材料都有着较为严格的要求。利用维管植物叶角质层解剖结构来重建古大气CO2浓度一般需要保存良好的实体压型化石,这样才能够通过化学处理获得其角质层特征；而利用陆生苔类植物化石重建古大气CO2浓度,同样需要较难保存为实体的苔藓植物。云南腾冲是保存新生代压型植物化石最好的地区之一,河北蔚县侏罗系发现的苔藓植物实体化石种类最多,并伴生有大量中生代裸子植物压型化石,因而是本文的重点工作地区。同时,由于苔类化石较难发现,采集标本过程中在黑龙江七台河早白垩世地层发现了一种大型苔类化石Marchantiolites blairmorensis,在此一并进行了研究。本文全面综述了利用苔类植物化石碳同位素判别重建古大气CO2浓度的原理、方法及其对应模型BROCARB的建立,很好地利用了苔藓植物碳同位素作为重建古大气CO2浓度新指标。对采集自河北蔚县中侏罗统的三种苔类化石进行了分类鉴定后,测定了它们的碳同位素组成,进而计算其碳同位素判别。基于冀西三种苔类植物化石(Riccardiopsis hsui, Metzgerites yuxianensi, Hepaticites sp.)的碳同位素判别,结合其他相关环境参数,利用BRYOCARB模型恢复了中侏罗世的古大气CO2浓度：对应BRYOCARBP模型和BRYOCARBNP模型的古大气CO2浓度平均值分别为566μmol/mol和705μmol/mol。为了进行交叉验证,利用采自冀西中侏罗世与苔类化石同一层位的一种银杏类植物Baiera cf. concinna (裸子植物)的气孔参数作为另一个独立指标,亦重建了当时的古大气C02浓度。分别对化石Baiera cf. concinna和2010年采集自兰州的现生银杏Ginkgo biloba进行了角质层分析,获得了良好表皮特征。利用气孔比率法在对应于“最近的校正标准”和“石炭纪校正标准”下获得的古大气CO2浓度结果平均值分别为811μmol/mol和1622μmol/mol,在未对化石和现生材料进行海拔校正前,此结果与苔类指标相差较大。但是,由于维管植物气孔参数会随海拔高度而变化,需要通过化石植物的古海拔参数和所用NLE种的海拔参数对结果进行校正。因此,本文结合化石点的地质背景,认为中侏罗世蔚县地区古海拔与现今相当,约为500-1000m。再次利用气孔比率法对应于“最近的校正标准”和“石炭纪校正标准”,对化石植物和现生植物进行海拔参数校正后重建的古大气CO2浓度分别为725-766μmol/mol和1454-1535μmol/mol.在“最近的校正标准”下获得的结果与苔藓所获得古大气CO2浓度相一致。将两种独立指标的结果互相验证,再结合前人对中侏罗世古大气CO2浓度重建的结果,本文认为冀西蔚县地区植物群生活时期的CO2浓度应该在705-766μmol/mol范围内。但是由于化石缺乏绝对年龄资料,我们无法准确知道这次低CO2浓度事件究竟具体发生在中侏罗世何时。在与Bener的长期碳平衡模型对比后发现,这是在中侏罗世所谓“温室世界”(green house world)中的一次低CO2浓度波动事件,可能代表着一个短暂的低温期。这一结论支持了前人的中侏罗世可能存在多个低CO2浓度波动事件,并也可能出现寒冷期或冰期的观点。本文还研究了滇西芒棒组晚上新世至早更新世地层中保存非常精致的桦木属植物Betula yunnanensis(新种)叶片化石。由于被子植物较裸子植物叶片形态复杂得多,种类也更为繁多。为了进行准确的系统分类,并为重建古大气CO2浓度找到准确的现生最近对应种(NLE species),本文运用植物解剖学、植物分类学方法对它们进行了细致的宏观叶结构和微观角质层分析。将化石植物叶片特征与我国(包括一日本特有种)所有桦木属植物,以及国内外已发表的各桦木属植物化石进行了宏观和微观对比,最终将其限定在西桦组植物(Betulaster)内,认为B.yunnanensis为西桦组植物一相先种。进一步详细研究了西桦组总共六种植物叶结构和角质层特征后,发现B.yunnanensis无论在形态上还是生态特征上都与西桦组内的B.alnoides最为近似,因而将B.alnoides选为其现生最近对应种。在统计了大量B.yunnanensis及其NLE种的气孔参数,并利用气孔比率法对现生植物生存点海拔参数校正后,获得化石植物生存时期大气CO2浓度为357μmol/mol,略高于Berner碳平衡模型GEOCARBⅢ的最高CO2浓度值.本文认为这同样可能是一次由于GEOCARB粗略的时间精度忽略了的CO2浓度波动事件。结合前人芒棒组地层测年资料和腾冲地区火山活动期次结果,分析其绝对年龄为2.4-2.7Ma,属于晚上新世至早更新世。由于腾冲地区地处青藏高原东南缘,新生代以来构造运动较为剧烈频繁,当时腾冲地区古海拔暂时无法确定,因此,目前获得的古CO2浓度357μmol/mol尚待更多资料进一步校正。更多还原

【Abstract】 Nowadays, there are two approaches to extract palaeo-CO2 levels from terrestrial higher plant fossils. The first approach utilizes the stomatal parameters of terrestrial vascular plants as palaeo-CO2 proxy, which is based on the anatomical structure of the fossil leaves. The second approach is based on carbon isotope discrimination (Δ13C) of fossil bryophytes (mainly liverworts), which is derived from carbon isotope composition (813C). Both approaches require a high demand on the fossil materials. To reconstruct the palaeo-CO2 level, well-preserved compressions of the fossils are required so that the cuticular features can be achieved after cuticular analysis with chemical treatments, if based on the stomatal parameters of terrestrial vascular fossil plants. As to the second approach, well-preserved compressions of fossil liverworts are also required to get the carbon isotope composition.Tengchong County in Yunnan Province is one of the best fossil sites where compressions of Cenozoic fossil plants are well preserved. Yuxian County in Hebei Province is one of the best fossil sites of Jurassic bryophyte together with lots of compressions of gymnosperm fossil. Therefore, the two areas mentioned above are the key areas where materials in this study were collected to reconstruct palaeo-CO2 levels. Meanwhile, as one of the liverwort fossils, Marchantiolites blairmorensis, from the Lower Cretaceous of Qitaihe City in Heilongjiang Province, is also studied as liverwort fossils are difficult to be found and the fossil records are rare in China.In this paper, a comprehensive overview of the usage of carbon isotope discrimination of liverworts fossil to reconstruct the atmospheric palaeo-CO2 concentration and the way of the establishment of the corresponding model BROCARB is presented. This study introduces fossil liverworts as a new good proxy to rebuild palaeo-CO2 levels. After the systematic work of three fossil liverworts of the Middle Jurassic collected from the west Hebei, their carbon isotope composition is measured, and then the carbon isotope discrimination is calculated. On the basis of the carbon isotope discrimination of the three fossil liverworts, Riccardiopsis hsui, Metzgerites yuxianensi, Hepaticites sp., the palaeo-CO2 level of the Middle Jurassic is reconstructed running with the BRYOCARB model, associate with some other palaeoenvironmental parameters. The palaeo-CO2 level of the Middle Jurassic is reconstructed with an association of some other palaeoenvironmental parameters, based on the BRYOCARB model. The palaeo-CO2 level is 566μmol/mol and 705μmol/mol, respectively, based on the theoretical model version of BRYOCARBP and BRYOCARBNP.In order to take a cross-check of the palaeo-CO2 level from the fossil liverwort proxy, the other proxy, the stomatal parameter of fossil Baiera cf. concinna from the same strata with the same age of the fossil liverwort was also used to reconstruct the palaeo-CO2 level. Cuticular analyses were carried out on Baiera cf. concinna and it’s selected nearest living equivalent (NLE) species, Ginkgo biloba, collected from Lanzhou in 2010. Good cuticular features of both the fossils and extant plants were achieved to measure stomatal parameters for stomatal ratio method to reconstruct palaeo-CO2 level. The mean Middle Jurassic palaeo-CO2 level is 811μmol/mol and 1622μmol/mol respectively for the Recent Standard and the Carboniferous Standard, before adjusted by the elevation parameter of both fossil and extant plants. The preliminary result of the stomatal proxy does not coincide well with that of the liverwort proxy. As the stomatal parameters such as stomatal index and stomatal density are sensitive to the elevation, the preliminary palaeo-CO2 level must be adjusted by the (palaeo-) elevation. The palaeo-elevation of the fossil site in Middle Jurassic was nearly the same as present as 500-1000m after the analysis of the geological background in this area conducted. After control for the elevation and palaeo-elevation, the mean palaeo-CO2 level in the Middle Jurassic is 725-766μmol/mol and 1454-1535μmol/mol under the Recent Standard and the Carboniferous Standard respectively. The result the Recent Standard is in good agreement with that got from the liverworts proxy.After cross-checking with the two independent proxies, together with the palaeo-CO2 level in the Middle Jurassic reconstructed by the former scholars, we suggest that the palaeo-CO2 level in the period of the Middle Jurassic when the fossil plants were living is 705-766μmol/mol. After a comparison with Berner’s long-term carbon cycle model in which the temporal resolution of his study is from 5 to 10 million years and thus is too long, a conclusion was achieved that during this short period in the so-called ’green house world’of the Middle Jurassic, a low CO2 event happened with a low level of palaeo-CO2,705-766μmol/mol, was ignored by Berner’s GEOCARB model. However, as the fossils lack of absolute age, the accurate time of the brief event cannot be determined. Generally, this conclusion supports the former scholars’point of view:there were many Mesozoic climate shifts and may cause cool glacial or non-glacial events. In this study, the well-preserved fossil leaves of Betula yunnanensis from the late Pliocene to the early Pleistocene of Mangbang Formation in west Yunnan are also studied. As the leaf morphology and system of angiosperm are more complex than that of gymnosperm, a detailed study on leaf architecture, cuticular analysis and fine venation was carried out to get an accurate systematic result and find the best NLE species for the fossil leaves to reconstruct palaeo-CO2 level. Thirty-four extant Betula species occurring in China and Japan, and four selected extinct species from China and Poland were compared with the fossil leaves in macro-and micro-characters. All the features show that the fossil species is most similar to the species of section Betulaster. We suggest that the Betula yunnanensis is one of the ancestors of the section Betulaster. Cuticular analysis of all the six species found in China and Japan that belong to the section Betulaster (including Betula alnoides, B. rhombibracteata, B. cylindrostachya, B. luminifera, B. maximowicziana, B. fujianensis) was subsequently performed. The result shows that leaves Betula yunnanensis are most similar to B. alnoides, both ecologically and morphologically. Therefore, B. alnoides is selected as the NLE species for stomatal ratio method. After the stomatal parameters of B. yunnanensis and its NLE species were counted and calculated, the mean palaeo-CO2 level was 357μmol/mol after the elevation of the extant leaves was adjusted. This level is slightly higher than the GEOCARB’s prediction. Furthermore, it also might be an ignored high-CO2 event in the Cenozoic by Berner’s model due to its too wide time span. Associate with the age-dating of the Mangbang Formation and the volcanic activity of Tengchong area, the absolute age is 2.4-2.7 Ma, the late Pliocene to the early Pleistocene, according to the latest geologic time scale. However, since Tengchong area locates in the southeast boarder of Qinghai-Tibet Plateau where the tectonic activities were significant and frequent during the Cenozoic, the palaeo-elevation of the fossil site in 2.4-2.7 Ma is hard to confirm. Therefore, the palaeo-CO2 level in this period needs further calibration.更多还原