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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Palaeoclimate Around Triassic-Jurassic Boundary of Junggar Basin 24 Yuanzheng Lu, Shenghui Deng State Key Laboratory of Enhanced Oil Recovery (Research Institute of Petroleum Exploration and Development), Beijing 100083, China (E-mail: Luyz@petrochina.com.cn) The Haojiagou section, located in the southern margin of the Junggar basin, is of great significance in studying Triassic-Jurassic Boundary events. This paper deals with the climatic change during the transition period of Triassic and Jurassic. The lower part of the Haojiagou section (bed 4 to bed 44) belongs to the Haojiagou Formation that is assigned to the Upper Triassic, and the underlying interval of bed 45 to 102 is the Badaowan Formation that is considered to be the Lower Jurassic (Deng, et al., 2000, 2003; Lu and Deng, 2005). The palynological data show that the gymnosperm pollen grains dominate all the Haojiagou Formation (Fig.1), such as nonstriate bisaccate, monocolpate. On the contrary, fern spores have higher proportion than the gymnosperm pollen grains in the lower member of the Badaowan Formation (Fig.1). Among the fern spores, the genera Asseretospora, Dictyophyllidites, Concavisporites, Densoisporites, Lycopodiacidites and Cyathidites are prominent in quantity (Lu and Deng, 2009), most of them are related to the plants adapted to warm and humid climate. Therefore, we believe that it was warmer and more humid in the Early Jurassic than in the Late Triassic. Organic carbon isotope data from the Haojiagou Section demonstrate that δ 13 C curve has an obvious negative excursion (less than -24.5‰) in the interval of bed 44 to bed 68, while the underlying and overlying strata have similar δ 13 C values (greater than -24.5‰) (Fig.1). Analyses of charcoal samples show a similar negative excursion in bed 44 to bed 59 (Fig.1). Ecological researches showed that sedimentary organic matter have lighter carbon isotope values when the climate is warm and humid (Yu, et al., 2001; Medina, et al., 1986; van der Merwen and Medina, 1989), and have heavier carbon isotope values when hot and dry (Ehleringer, 1993). Therefore, it is concluded that at the beginning of the Early Jurassic the climate is warmer and more humid than before and afterward. The sudden creasing of fern spores and the distinct negative excursion of organic carbon isotope in the lowest member of Jurassic of the Haojiagou section are similar with some other sections. It may reflect a glo- bally climatic change. Key words: Junggar basin; Triassic-Jurassic boundary; Palaeoclimatic changes References: Deng S.H., Cheng X.S., Qi X.F., et al. The succession of Late Triassic-Early Jurassic plant association in Junggar basin, Xinjiang. In: Proceedings of the Third National Stratigraphical conference of China. Beijing: Geological Publishing House, 2000:174-178. Deng S.H., Yao Y.M., Ye D.Q., et al. Jurassic System in the North of China: Stratum introduction. Beijing: Petroleum Industry Press, 2003. Ehleringer J.R. Carbon and water relations in desert plants: An isotopic perspective. In: Ehieringer J.R., Hall A.E., Farquhar G.D. (ed). Stable Isotopes and Plant Carbon. San Diego: Academy Press, 1993:155-172. Lu Y.Z., Deng S.H. Triassic-Jurassic sporopollen as- semblages of the southern margin of Junggar Basin, Xinjiang and the Tr/J boundary. Acta Geologica Sinica, 2005, 79(1):15-27. Lu Y.Z., Deng S.H. Palaeoclimate around Triassic- Jurassic boundary of the Haojiagou Section in southern margin of Junggar Basin. Journal of Palaeogeography, 2009, 11(6): 652-660. Medina E., Montes G., Cueves E., et al. Profiles of CO2 concentration and δ 13 C values in tropical rain forests of the Upper Rio Negro basin, Venezuela. J Tropipcal Ecol., 1986, 2:207-217. van der Merwen J., Medina E. Photosynthesis and 13 C/ 12 C ratios in Amazonian rain forests. Geochim Cosmochim Acta, 1989, 53:1091-1094. Yu J.Q., Wang X.Y., Li J., et al. Paleoenvironmental interpretations on organic carbon isotopic records from lake sediments: A critique. Journal of Lake Sciences, 2001, 13(1):72-78.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Age Lower Jurassic Badaowan Formation Upper Tirassic Haojiagou Formation Formation Bed 74 73 72 71 70 65 62 60 59 58 57 56 52 51 49 44 42 41 40 39 38 37 36 31 28 26 24 22 21 18 17 13 11 10 9 Lithological Section δ -27 -25 -23 -21 13 δ Cwood ( ‰ ) -26 -25 -24 -23 13 Corg( ‰ ) proportion of spores main genera 0 20 40 60% or type Cyathidites, Deltoidospora, Classopollis Densoisporites, Lycopodiacidites, nonstrate bisaccate Di ctyophylli dites, Asseretospora, Concavisporites nonstrate bisaccate, Cycadopites, Chasmatosporites, Lycopodiacidites rugulatus, Li matulasporites nonstriate bisaccate Chordasporites, Aratrisporites, Li matulasporites Fig.1 The proportion of fern spores and organic carbon isotope curve across the T/J boundary of the Haojiagou section 25
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