Terrestrial Fossil Plants Of The Mesozoic And The Cenozoic From West Hebei And West Yunnan And Palaeoatmospheric CO₂ Reconstruction

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.