20 Pet.Sci.(2008)5:20-23 DOI 10.1007/s12182-008-0003-7 The Chang 7 member of the upper Triassic in the southwest Ordos Basin: Carbon isotope studay of dispersed organic matter and its paleoclimate signifi cance Yang Minghui1*, Liu Chiyang2, Tang Xuan3 and Zheng Xiaofeng1 1 Basin & Reservoir Research Center; Key Laboratory of Hydrocarbon Accumulation, Ministry of Education, China University of Petroleum, Beijing 102249, China 2 Department of Geology, Northwest University, Xi’an 710069, Shaanxi, China 3 Department of Energy Geology, China University of Geosciences, Beijing 100083, China Abstract: The carbon isotope δ 13C value of dispersed organic matter in the Upper Triassic sediments in the Ordos basin is (-24.5 to -26.6) ×10-3 with the carbon being derived from C3-like pleustons. The paleoclimate of the Late Triassic, especially during the deposition of the Chang 7 member, was characterized by a hot and humid climate. It underwent four small climate cycles from humid and warm to humid and hot. Key words: Paleoclimate; stable carbon isotope; dispersed organic material; Yanchang Formation; Ordos basin Ratios of stable isotopes of organic carbon in lacustrine tract, in which the Zhangjiatan shale was deposited during sediments can provide information about climatic change. the maximum fl ooding period. The sampled well Zhen 33 is This indirect relation has led to a useful application of the located in the southwestern Ordos basin (Fig.1) with samples isotope approach to studying the paleoclimate both in China being taken from well depths of 2213-2220.4 m in which the and overseas. One method is to study δ13C and δ16O values Chang 7 OLM of the Yanchang Formation is composed of of carbonate minerals or microfossils in sediments (Kelts, lacustrine mudstone and silty mudstone. Samples were evenly 1992; Zhang et al, 1994); the other is to investigate δ13C taken at a sampling interval of 20 cm. values of organic matter in lacustrine sediments (Zhang et Dispersed organic matter was prepared with the al, 1998; Horiuchi and Hoshino, 2000; Wang et al, 2002; Separation Method of Kerogen in Sedimentary Rock, Mingram and Brüchmann, 2004; Ma et al, 2004; Liu et al, following the national standard GB/T19144-2003. The 2005), in order to understand the changes of lake environment testing procedure for obtaining the δ13C value is as follows: in which organic matter has been created and preserved (1) At a low temperature (<60°C), dispersed organic matter (Sanger, 1998; Shen et al, 1998). was obtained by drying and was pulverized to a size of 100 This study is a preliminary attempt to investigate mesh. 1-5 g samples were used depending on carbon content; paleoclimate change based on studies of stable carbon (2) Samples were put into clean beakers, and then left for isotopeas of dispersed organic matter in the Chang 7 Oil 24 hours in dilute hydrochloric acid to remove carbonate Layer Member (OLM) of the Yanchang Formation in the minerals. Afterwards, the sample was flushed with distilled Ordos basin. water till neutral and dried at low temperature (<75°C). The sample was tested with an analyzer CS-344 model; (3) The 1 Sampling and analytic method treated sample was put into a quartz boat. Then the boat was placed in high temperature vacuum oven fitted with copper The Upper Triassic Yanchang Formation in the Ordos oxide and silver for 15 min. The sample was sufficiently basin represents an integral evolution of a nonmarine basin, burnt at the constant temperature of 900°C with pure oxygen; from earlier basin subsidence (Chang 8) to widespread basin (4) The generated CO was transferred and pure CO gas was expansion (Chang 7), then to basin contraction (Chang 6) 2 2 obtained with liquid nitrogen and cold liquid; (5) The δ13C and expansion again (Chang 4+5), inversion and extinction value of CO gas was determined by a mass spectrometer of (Chang 1). The deep and shallow lacustrine deposits of the 2 MAT-251 model. Chang 7 OLM were laid down as a lake expanding system δ13C value can be calculated as follows: 13 13 12 13 12 *Corresponding author. email: [email protected] δ C = [( δ C/ δ C)sample – ( δ C/ δ C)standard] 13 12 Received October 12, 2006 / ( δ C/ δ C)standard Pet.Sci.(2008)5:20-23 21 The accuracy of instrumental analyses is ± 0.02‰. The photosynthesis is phosphoric acid and glyceric acid with analytical results are reported with the reference to the three carbon molecules, the C3 cycle. The characteristics of international standard of PDB (Fig.2). the process present a long cycle and a large fractionating level, and the δ 13C values fall in the range of -23‰ to 2 Results and environment interpretation -38‰. The other photosynthesis path is called as C4 cycle. The initial product is dihydroxy acid with four carbon The organic matter in lacustrine sediments was mainly molecules. Although its assimilatory rate of CO is two times derived from terrigenous detrital materials and aquatic plants, 2 that of C cycle, the degree of fractionation distillation is but the organic carbon isotope of lacustrine sediments was 3 smaller and δ13C values are in the range of -12‰ to -14‰. derived from the fractionation of organic carbon isotope The type of plant photosynthesis determines the isotopic of land and aquatic plants. Currently, there are two factors fractionation(Smith and Epstein, 1971; Bown, 1988) while affecting the fractionating process of carbon isotope of different sorts of plants are closely connected with their land plants. One is the manner and the other is the extent growing environment. Thus, the measured δ13 C values of of photosynthesis. Nearly 90% of the initial product from 106˚E 107˚ 108˚ 109˚ 110˚ 111˚ 112˚ Baotou Toketo Dengkou N Dalate 40˚ 200 YIMENG UPLIFT Zuozi Mt. 400 Etuoke 39˚ Helan Mt. 600 Shenmu Yinchuan Hengshan 38˚ 800 Dingbian Lishi 2000 SHAANBEI SLOPE 37˚ TIANHUAN DEPRESSION 1000 XIYUAN THRUST BELT XIYUAN THRUST JINXI FLEXURE AND FOLD BELT Yanan Huanxian YELLOW RIVE Well Zhen33 1200 Qingyang Fuxian 36˚ 1400 1000 Pingliang Chongxin 2000 1600 35˚ WEIBEI UPLIET Tongchuan 800 600 Sanmenxia 400 Baoji WEIHE RIVER Xi’an 0 50 100km 34˚N Fig. 1 Structural subdivision map of the Ordos basin showing the location of the Well Zhen 33 The contours illustrate the isopach of the Yanchang Formation 22 Pet.Sci.(2008)5:20-23 -27 temperature of plants. Except for temperature constraint, -26.5 C3 plants are also controlled by moisture. Based on such -26 reasoning, the depositional environment during the deposition of the sampled Chang 7 is deduced to be wet and hot, and it -25.5 underwent four microclimate cycles from warm and wet to -25 wet and hot with sample no. 27, 19 and 9 marking the cycle C /‰, PDB -24.5 13 boundaries). ˺ -24 3 Discussion and conclusions -23.5 -23 Pre-existing studies show that the plants with different 1 4 710131619 22 25 28 31 34 37 modes of photosynthesis (e.g. C3, C4 and CAM) have different Sample numbers δ13C values. The change of the δ13C values of sedimentary 13 13 organic matter are related to the δ C values of plant sources. Fig. 2 Sequence change of δ C values of dispersed organic matter in the When the plants absorb CO for photosynthesis, it will Upper Triassic in the southwestern Ordos basin 2 lead to carbon isotope fractionation. During this process, many environmental factors (such as pressure of CO2 in organic matter in sediments can be used to defi ne the pattern atmosphere, O2 partial pressure, temperature, moisture, light of vegetation from which organic carbon was derived and salinity etc.) affect carbon isotope fractionation and the and the pattern in turn can be utilized to study the type of isotopic composition of plants (Fig.3). paleoclimate environment (Nakai, 1972; Stuiver, 1975; Wu The relationship between temperature and the sedimentary and Lu, 1992; Cormie and Schwarcz, 1994). The aquatic organic matter δ13C value is more complex, and no consensus plants can be divided into pleustons and submerged plants. has been reached for this relationship. They may be negatively The pleustons include emerging plants, fl oating-leaved plants or positively correlated in different areas of plateau or plain and phytoplanktons. The δ13C values of the pleustons are and so on (Wu et al, 1996). The components of organic matter 13 13 close to those of the C3 plants. For example, the δ C values δ C in lacustrine sediments are enriched in the heavy carbon of emerging plants are about -24‰ to -30‰. isotope as air temperature increases in the plain area. This is Microscopic examination of dispersed organic matter because (1) C4 plants reduce rapidly as latitude increases; (2) in the Chang 7 OLM indicates that the types of organic With increasing latitude, the temperature becomes lower so matter in the sediments vary slightly with the depth and that the concentration of the CO2 dissolved in water increases. the sorts of plants are more or less homogeneous in terms The process for pleustons to synthesize organic matter has of stratigraphic distribution. The organic matter consists of an inverse relationship with the supplied volume of CO2. As dominant amorphous maceral with no fl uorescence or weak to a result, the more CO2 that is provided, the more negative moderate fl uorescence and subordinate inertinite and vitrinite. the δ13C values in synthesized organic matter. Moisture is The dispersed organic matter in the Chang 7 OLM has δ13C also one of the important factors affecting the change of values of -24.5‰ to -26.6‰. They were mainly derived from δ13C in plants.