in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Terrestrial Carbon-isotope Stratigraphy of the Triassic-Jurassic Boundary and Early/Middle Jurassic from Tarim Basin in Xinjiang, NW China: Initial Results 10 Linhao Fang 1* , Stephen Hesselbo 1 , Shenghui Deng 2 1. Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK (E-mail: linhao.fang@earth.ox.ac.uk) 2. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 10083, China The Triassic-Jurassic (Tr-J) boundary extinction is one of the five major mass extinctions in Phanerozoic history and occurred synchronously (~200 Ma) with Central Atlantic magmatic eruption. Associated carbon-isotope anomalies have been reported from marine and terrestrial materials at various sites across the world, such British Columbia(Ward, et al., 2001); Nevada (Guex, et al., 2003); England and Greenland(Hesselbo, et al., 2002);Austria(McRoberts, et al., 1997); and Hungary(Palfy, et al., 2001)at the Tr-J boundary. Fig.1 Carbon-isotope data from fluvial stratigraphy at Haojiagou, Junggar Basin, Xinjiang, NW China However, there are few terrestrial carbon-isotope data from China, even though Mesozoic fluvial and lacustrine sediments occur widely in the provinces of the northwest and north. In this study, new organic carbon-isotope data are presented from Haojiagou in the Junggar Basin, Xinjiang, NW China. The data demonstrate that a long-term negative carbon-isotope excursion (Fig.1) occurs around the Tr-J boundary recognized on the basis of palynostratigraphy. This is broadly the same pattern reported from other locations, although the new data are not of sufficient resolution to confirm the presence of detailed features of these previously generated isotope profiles. The isotopic composition of a thick lacustrine black shale in the Yangxia Formation from Tarim Basin, Xinjiang NW China has also been examined. These sediments are dated palynologically as late Early Jurassic or early Middle Jurassic. Our hypothesis was that the black shales are a terrestrial expression of the Toarcian Oceanic Anoxic Event (T-OAE). The T-OAE (about 183Ma) in the Early Jurassic is associated with very large negative and positive carbon-isotope excursions (Hesselbo, et al., 2000; Hesselbo, et al., 2007; Jenkyns, 2003; Jenkyns, et al., 2001; Jenkyns, et al., 2002; Kemp, et al., 2005; McElwain, et al., 2005; van de Schootbrugge, et al., 2007). However, no anomaly is present in the new data, and it is concluded that the lacustrine shales are unlikely to have been associated with the T-OAE and are probably not of Early Toarcian age. Fig.2 Carbon-isotope data from lacustrine stratigraphy in Yangxia Formation from Tarim Basin, Xinjiang, NW China The new carbon isotope data obtained from terrestrial stratigraphy around the Tr-J boundary in Tarim Basin, Xinjiang, NW China are consistent in the global context, but the expression of the T-OAE has not yet been identified. We plan further study of the Early Jurassic successions of Xinjiang and to compare newly generated terrestrial carbon-isotope profiles with profiles generated from well-dated marine sections. Key words: Carbon-isotope anomalies; Terre- strial stratigraphy; Triassic-Jurassic boundary; Toar- cian References: Guex J., Bartolini A., Atudorei V., et al. Two negative
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 delta13Corg excursions near the Triassic-Jurassic boundary in the New York Canyon area (Gabbs Valley Range, Nevada). Bulletin de la Societe Vaudoise des Sciences Naturelles, 2003, 88(Part 4): 445-448. Hesselbo S.P., et al. Massive dissociation of gas hy- drate during a Jurassic oceanic anoxic event. Nature, 2000, 406: 392-395. Hesselbo S.P., Jenkyns H.C., Duarte L.V., et al. Car- bon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal). Earth and Planetary Science Letters, 2007, 253(3-4): 455-470. Hesselbo S.P., Robinson S.A., Surlyk F., et al. Terre- strial and marine extinction at the Triassic- Jurassic boundary synchronized with major carbon-cycle perturbation: A link to initiation of massive volcanism? Geology, 2002, 30(3): 251- 254. Jenkyns H.C. Evidence for rapid climate change in the Mesozoic-Palaeogene greenhouse world. Philoso- phical Transactions of the Royal Society a-Mathe- matical Physical and Engineering Sciences, 2003, 361: 1885-1916. Jenkyns H.C., Grocke D.R., Hesselbo S.P. Nitrogen isotope evidence for water mass dentrification during the early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography, 2001, 16(6): 593- 603. Jenkyns H.C., Jones C.E., Grocke D.R., et al. Chemo- stratigraphy of the Jurassic System: applications, limitations and implications for palaeoceano- graphy. Journal of the Geological Society, 2002, 159: 351-378. Kemp D.B., Coe A.L., Cohen A.S., et al. Astronomical pacing of methane release in the Early Jurassic period. Nature, 2005, 437: 396-399. McElwain J.C., Wade-Murphy J., Hesselbo S.P. Chan- ges in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals. Nature, 2005, 435: 479-482. McRoberts C.A., Furrer H., Jones D.S. Palaeoenviron- mental interpretation of a Triassic-Jurassic boundary section from Western Austria based on palaeoecological and geochemical data. Palaeo- geography, Palaeoclimatology, Palaeoecology, 1997, 136(1-4): 79-95. Palfy J., et al. Carbon isotope anomaly and other geochemical changes at the Triassic-Jurassic boundary from a marine section in Hungary. Geology, 2001, 29(11): 1047-1050. van de Schootbrugge B., Tremolada F., Rosenthal Y., et al. End-Triassic calcification crisis and blooms of organic-walled 'disaster species'. Palaeogeogra- phy, Palaeoclimatology, Palaeoecology, 2007, 244(1-4): 126-141. Ward P.D., Haggart J.W., Carter E.S., et al. Sudden productivity collapse associated with the Triassic- Jurassic boundary mass extinction. Science, 2001, 292: 1148-1151. 11
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