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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Fig.1 Jurassic climatic regions of China A, Early-middle of the Early Jurassic: I. Cool temperate region; II. Warm temperate region with warm-humid climate; III. Subtropic-tropic hot-humid region; IV: Subtropic-tropic semi-humid and semi-arid region; V. Tropic arid region; B, Late Early of the Jurassic: I. Cool temperate region; II. Warm temperate with warm-humid climate; III. Subtropic-tropic semi-arid and semi-humid region; IV: Subtropic-tropic arid region; V. Tropic arid region; C, Early of the Middle Jurassic: I. Cool temperate region; II. Warm temperate region with warm-humid climate; III. Subtropic-warm temperate semi-arid and semi-humid region; IV: Subtropic-tropic arid climatic region; V. Tropic arid region; D, Late of the Middle Jurassic: I. Cool temperate region; II. Warm temperate region with warm-humid climate; III. Subtropic semi-arid and semi-humid region; IV: Tropic-subtropic arid region; V. Tropic arid region; E, Late Jurassic: I. Cool temperate region; II. Subtropic humid region; III. Tropic-subtropic arid region; IV: Tropic arid region. References: Deng S.H. Palaeoclimatic implications of main fossil plants of the Mesozoic. Palaeogeography, 2007, 9(6): 559-574 (in Chinese with English abstract). Zhong X.C., Zhao C.B., Yang S.Z., et al. Jurassic System in the North of China. Volume II. Palaeoenvironment and Oil-Gas Source. Beijing: Petroleum Industry Press, 2003: 201 (in Chinese 356 with English abstract). Vakhrameev V.A. Jurassic and Cretaceous floras and climates of the Earth. Cambridge: Cambridge University Press, 1991: 318. Zhang H., Jin X.L., Li G.H., et al. Original features and palaeogeographic evolution during the Jurassic- Cretaceous in Ordos Basin. Palaeogeography, 2008, 10(1): 1-11.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 The Early Toarcian OAE in Northeastern Tethys from the Viewpoint of the Bioturbation (Toyora area, West Japan) K. Izumi * , K. Tanabe Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-8654, Japan (E-mail: izumi@eps.s.u-tokyo.ac.jp ) During the Early Toarcian (Early Jurassic), a second ordered OAE prevailed in the world oceans which led to a prominent turnover of marine biota in both shallow and deep water settings. The Early Toarcian OAE event is characterized by deposition of bituminous black muddy sediments that lacks apparent remains of benthic organisms in association with negative stable carbon isotope excursion (e.g. Jenkyns, 1988). The Early Toarcian OAE deposits in the Northwest Europe standard and Tethys regions have been studied extensively from the viewpoints of geochemistry, sedimentology and palaeontology, but little was investigated for the contemporaneous strata deposited in the eastern Tethys margin and the Panthalassa. The Nishinakayama Formation distributed in the Toyora area, Yamaguchi Prefecture, West Japan is correlated to the Upper Pliensbachian to Lower Toarcian by ammonite biostratigraphy (Hirano, 1973). This formation represents a shallow marine sequence deposited at the Northeastern margin of the Tethys Sea that was connected with the Panthalassa. It contains fine-laminated, bituminous black shales with sedimentary pyrite, in the middle part, which are contemporaneous with and similar in lito- and biofacies to the Early Toarcian OAE strata in Northwest Europe. The biostratigraphical, palaeoecological and taphono- mical aspects of the molluscan fauna of Nishinakayama Formation have been extensively investigated and their relationship to the Early Toarcian OAE has been suggested (Tanabe, 1991). Nevertheless, little was known for the activity of benthic organisms in relation to the OAE event. In this study, we investigated the activity of the benthic organisms recorded in the sediments as trace fossils and/or bioturbation for the Upper Pliensbachian to the Lower Toarcian succession exposed in the Sakuraguchi-dani Valley. For this purpose, rock samples and ammonite fossils were successively obtained from the sequence, making a detailed geologic column. In the field, laminae and trace fossils which were visible to the naked eye were observed elaborately, and lamina preservation index was given to all rock samples. In addition, soft X-ray images of 17 rock samples that were cut in vertical to the bedding plane were taken in the laboratory in order to distinguish the degree of bioturbation and types of trace fossils which were invisible to the naked eye. As some studies (e.g. Rohl et al., 2001) discussed oxygen level of bottom water using various biofacies and sedimentary facies scheme, redox condition of the Lower Jurassic succession exposed in the Sakuraguchi- dani Valley was estimated by lamina preservation index in this study. As a result, OAE sediments were identified in the Sakuraguchi-dani Valley. However, beds accom- panying trace fossils and bioturbation were episodically intercalated at several horizons in the OAE sequence which lacks trace fossils and/or bioturbations. This fact strongly suggests that the Early Toarcian OAE event recorded in the Sakuraguchi-dani Valley section was not a single event but was interrupted by short-term oxygenation events during the interval. Furthermore, international correlation between Toyora area (Northeastern margin of the Tethys realm) and European Tethys regions by means of ammonite biostratigraphy suggests that the interval of the Early Toarcian OAE in the shallow setting differed between the western and eastern parts of the Tethys Sea. Key words: Toarcian; OAE; Nishinakayama For- mation; Lamina preservation index References: Hirano H. Biostratigraphic study of the Jurassic Toyora Group, Pt. 3. Transactions and Proceedings of the Palaeontological Society of Japan, 1973, 90: 45- 71. Jenkyns H.C. The Early Toarcian (Jurassic) anoxic event: Stratigraphic, sedimentary, and geochemi- cal evidence. American Journal of Science, 1988, 357
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