Plant Fossils From The Miocene Of Ninghai-Tiantai Region Of Zhejiang Province And The Climatic Change

The paleoenvironmental studies on the basis of the plant fossils are not only helpful for understanding the paleoecology and paleoclimate in the geological history, but also are the guidances for handling the climatic change at present. The angiosperm had been rising since the Cenozoic, while the vegetation in the late Neogene closely resembles that of modern-day. Hence most plant fossil species from the Cenozoic have the modern relative species or counterpart, which is convenient for the identification of fossils, and the discussion of the paleoecology and paleoenvironment. Recent years, based on the plant fossils, many paleobotanists reconstructed the paleoenvironment and paleoclimate of the geological history, specially in the Cenozoic, by using various methods, such as Stomatal Ratio method (SR), Leaf Margin analysis (LMA), the Climate-Leaf Analysis Multivariate Program (CLAMP), the Nearest Living Relative method (NLR), the Coexistence approach (CA) etc. Furthermore, these studies explained the formation and evolution of the East Asian Monsoon, and analysised the trend of the climatic change at present and in the future. This paper mainly studied the flora of the Shengxian Formation which is from the miocene of eastern Zhejiang. Moreover, combining with the previous studies, the paleoenvironment and the paleoclimate of the eastern Zhejiang were discussed. Besides, the research aera is affected by the East Asian Monsoon strongly because it is in the coastal aera of eastern China. Therefore, the paleoclimate of the Shengxian Formation and its neighbouring regions during the Cenozoic were quantitative reconstructed based on the pellon data. The climatic change during the Cenozoic, the formation and the evolution of the East Asina Monsoon, both were briefly discussed.The outcrops of the Shengxian Formation are multiple in Ninghai-Tiantai region of eastern Zhejiang. Abundant plant megafossils have been persaved in the units. These plant fossils included leaves, fruits, seeds and stems. Most of the fossils are angiosperms, and part of the fossils are gymnosperms and a few of ferns. The leaf fossils are compressions with intact cuticles. Some species of these fossils were identified and described in this paper, such as the shoots and seed cores of Fokienia (Cupressaceae), the seed cores of Pinus (Pinaceae), the seed scale of Tsuga (Pinaceae), the leaves of Cinnamomum (Lauraceae), the leaves of Magnolia (Magnoliaceae), the cupules of Quercus (Fagaceae), the fruits of Vitis (Vitaceae), the seeds of Chenopodium (Chenopodiaceae), the stems of Graminites (Gramineae) and the stems of Cyperacites (Cyperaceae).The identification was mainly based on the morphology. For leaf fossils, the identification also combined with microstructures of leaf cuticle. The cuticles of the fossils and modern leaves were obtained by experiments and observed by the Light Microscope (LM) and the Scanning Electron Microscope (SEM). Twenty one fossil leaves closely resemble the extant genus Fokienia while these fossils can likely be distinguished from other extant genera in the family Cupressaceae s.s. by morphology; and cuticular analysis and comparison with the only modern species in genus Fokienia, F. hodginsii Henry et Thomas, further suggests that this fossil species closely resembles the modern counterpart on the microstructures of leaf cuticle. Therefore, these fossil leaves were identified as Fokienia shengxianensis sp. nov. Besides, based on steady macromorphology of leaf, such as the base, the apex and the vein, this paper studied detailedly 2 angiosperm leaf fossils by analyzing of leaf architecture. Combining with cuticle characteristics, there are some resemblance on leaf shape, leaf vein, vein cell, stomata type and the features of trichome between the fossil leaves and living Cinnamomum bejolghota (Buch-Ham.) Sweet. Therefore, these fossil leaves were identified as Cinnamomum cf. bejolghota.The frequency of use the Scanning Electron Microscope (SEM) is very helpful for the obsversion of the microstructures of plant fossils. In this paper, the leaf cuticles were observed by using SEM, as well as the microstructures of the fossil fruits and seeds. Based on the external morphology,2 conifers seed core fossils were assiged to Cupressaceae. Furthermore, the inner surfaces of the scales are longitudinally striate under the SEM, which resemble to the modern Fokienia. Therefore, the seed core fossils here were plaeced in the new fossil species:Fokienia shengxianensis sp. nov., which were collected from the same units. Besides,3 flat and round fossil seeds resemble the seeds of genus Chenopodium (Chenopodiaceae) on morphology. Adding to the cell structures and the suspected endosperm in the seeds cross section, under the SEM, the fossil seeds clearly are Chenopodium affinity.Previous paleoecological and paleoenvironmental studies of the Shengxian flora suggested that it was warm and humid in eastern Zhejiang during the Miocene. These previous studies mainly applied the Nearest Living Relative species method (NLR). It had rarely considered the paleoenvironment significances of cuticle. In this paper, the occurrence of the present fossil species which are discribed here appears to support that the fossil site was covered by subtropical evergreen broad-leaved forest. Furthermore, combining the paleoenvironment significances of cuticle with the NLR method, it was under the warm and humid conditions in eastern Zhejiang during the late Miocene, in favor of the results of previous studies.The previous quantitative reconstruction of the flora of the Shexian Formation mainly were based on megafossils. It had rarely analyzed the pollen data. The reasons may be that many paleoclimate-reconstruction methods (e.g., Stomatal Ratio method, leaf margin analysis, the Climate-Leaf Analysis Multivariate Program) can not be used for pellon fossils. Recent years, the Coexistence Approach (CA) wildly was applied in multiple plant fossils which include the pollen. Besides, based on the pellon data at 2 sites of the Shengxian flora, the paleoclimate was quantitative reconstructed by using the CA, and the Miocene climatic intervals are a MAT of 14.4-18.9℃, a WMT of 25.2-28.4 ℃, a CMT of 6.0-8.2 ℃, a DT of 24.1-25.3 ℃, a MAP of 852.6-995.3 mm. Consequently, the paleoclimatic results suggested warm and humid conditions during the late Miocene in eastern Zhejiang Province.The paleoclimatic studies of the flora ought to be combined with the background of climatic change in a long time range. In this way, the evolution of the climate in the research area can be understanded deeply. In order to understand the paleoclimate of the flora of the shengxian Formation, the paper quantitative reconstructed the paleoclimate by using the CA based on the pellon data at other neighbouring 3 sites in eastern China from the Paleocene to the Miocene. The results mainly suggested that it was under the warm and humid conditions in eastern Zhejing and neighbouring regions during the Paleocene to the Miocene. In addition, the fluctuations of plaeoclimate in eastern Zhejing and neighbouring regions also were along with the global climate events during the Cenozoic. Moreover, the paleoclimatic parameters indicated the East Asian monsoon may have been enhanced after the late Eocene (ca. 40Ma).