Spathian to Aegean (upper Lower Triassic to lower Middle Triassic) carbon isotope stratigraphy constrained by the conodont biostratigraphy of carbonates on top of a mid‐oceanic seamount formed in the Panthalassic Ocean. (25th February 2021)

• Record Type: Journal Article
• Authors: Ha, Thuy Thi Nhu; Maekawa, Takumi; Takayanagi, Hideko; Iryu, Yasufumi
• Is Part Of: Island arc. Volume 30:Number 1(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-25
• Subjects: Anisian -- carbon isotopes -- chemostratigraphy -- conodont biostratigraphy -- limestone -- Olenekian -- Panthalassic Ocean -- Taho -- Triassic; Plate tectonics -- Periodicals; Island arcs -- Periodicals; Geodynamics -- Periodicals; 551.136
• DOI: 10.1111/iar.12391
• Languages: English
• ISSNs: 1038-4871

Abstract

Abstract: The Taho Formation in western Shikoku Island, Japan, consists of Triassic carbonates that formed on a seamount in the Panthalassic Ocean. In order to investigate the stratigraphy and paleoceanography of this carbonate succession, we analyzed the biostratigraphy and chemostratigraphy of a 17.6 m‐thick section of the upper Taho Formation at the stratotype area in Tahokamigumi, Seiyo City. This section comprises bioclastic limestone containing Triassic bivalves, ammonoids, and conodonts. We recognized six conodont zones (in ascending order): the Novispathodus pingdingshanensis, Novispathodus brevissimus, Triassospathodus symmetricus, Triassospathodus homeri, Chiosella timorensis, and Magnigondolella cf. alexanderi zones. Thus, the studied carbonate succession is latest Smithian to Aegean in age. A δ 13 C profile of this section shows elevated values during the lowest Spathian followed by a gradual negative excursion, a subsequent positive excursion near the Spathian–Aegean boundary, and relatively constant values during the Aegean. The characteristic series of negative and positive excursions correlates with other δ 13 C records for this period, including the peak of the upper Smithian–lowest Spathian positive excursion (P3), lower to middle Spathian negative excursion (N4), and middle Spathian–lowest Aegean positive excursion (P4). This represents a new high‐resolution Spathian–Aegean δ 13 C record of the Panthalassic Ocean, for which ages are constrained by conodont biostratigraphy. The Taho δ 13 C profile exhibits a consistent positive offset of ~2 ‰ as compared with those from other regions (i.e., mostly in the Tethyan Ocean). This can be explained by preferential removal of 12 C from seawater during photosynthesis and calcification by marine organisms over the platform, and/or the relatively high δ 13 C values of dissolved inorganic carbon in the Panthalassic Ocean due to less influence of 12 C‐enriched terrestrial waters and high marine organic production/burial as compared with the more restricted Tethyan Ocean.