Indicator of paleosalinity: Sedimentary sulfur and organic carbon in the Jurassic–Cretaceous Tetori Group,central Japan |
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| Authors: | Takashi Hasegawa Tsuyoshi Hibino Shunji Hori |
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| Institution: | 1. Department of Earth Sciences, Faculty of Natural systems, Institute of Science and Engineering, Kanazawa University, Kanazawa 920‐1192 Japan (email: jh7ujr@kenroku.kanazawa‐u.ac.jp);2. Shiramine Institute of Paleontology, Kuwajima, Hakusan, Ishikawa 920‐2502 Japan;3. Department of Earth Sciences, Faculty of Science, Kanazawa University, Kanazawa 920‐1192 Japan |
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| Abstract: | A simple geochemical technique using the ratio of total sulfur (TS) to total organic carbon (TOC) was successfully used to reconstruct paleoenvironments in the Jurassic–Cretaceous Tetori Group, central Japan. The TS to TOC ratio is often employed as an effective parameter to separate modern marine or brackish sediments from freshwater deposits. To test the TS/TOC method for paleoenvironmental interpretation of the Tetori Group, we first analyzed TS/TOC for samples for which depositional conditions (i.e. marine, brackish or freshwater) had been recognized paleontologically. The results indicate that the method can effectively separate sedimentary rocks deposited under freshwater from marine and brackish settings. Once we had established the effectiveness of this method, we applied it to three sections of the Tetori Group, central Japan. Stratigraphic fluctuations in TS/TOC values revealed episodic incursions of marine or brackish conditions in the dominantly freshwater depositional sequence in the middle of the Jobu Formation of the Itoshiro Subgroup at the Izumi section, Fukui Prefecture. The same paleoenvironment is also suggested to occur at the top of the Tetori Group in the Tateyama section, Toyama Prefecture. This research provides important information to paleogeographers who currently lack evidence from facies fossils to indicate if the uppermost part of the Tetori Group represents marine or brackish settings. A chemostratigraphy of TS/TOC parameters potentially could provide a correlation among Jurassic–Cretaceous sequences along continental margins over East Asia. |
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| Keywords: | brackish Cretaceous freshwater organic carbon paleoenvironment pyrite sulfur Tetori Group |
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