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[BCG07-01] Investigation of a Ladinian (Middle Triassic) humid event in pelagic deep-sea sedimentary rocks of Panthalassa
Keywords:Triassic, conodont, Radiolarian, Ladinian, Anisian, Biostratigraphy
The Triassic climate is punctuated by several extreme events, such as the Smithian–Spathian boundary event and the Carnian Pluvial Event. The Middle Triassic climate is generally considered to have been arid and semi-arid conditions, but recent studies have addressed climatic variations within the Middle Triassic. For example, a humid event during the Longobardian (late Ladinian) in the Tethys has been reported1,2. Furthermore, an increase in radiolarian diversity has been observed in the Longobardian3. However, such studies have primarily focused on the Tethys, and the environmental conditions of the Panthalassa during this period remain little studied.
To investigate the environmental changes during the Longobardian humid event on a geographically broader scale, this study examined its record in sediments accumulated in a pelagic, open ocean setting based on radiolarian and conodont biostratigraphy and geochemical profiles. Two sections were investigated in this study: the Sakahogi section (Section O of Sugiyama, 19974; Inuyama area) in the Mino Belt of central Japan and the newly established Orikabe section (Akka area) in the North Kitakami Belt of northeast Japan. Both are pelagic deep-sea sedimentary rocks originally deposited in Panthalassa that are now preserved within Jurassic accretionary complexes.
The Sakahogi section consists mainly of rhythmic brick-red bedded cherts with a total thickness of 21 m. Radiolarian biostratigraphy shows that six of Sugiyama's radiolarian zones4 was recognized in the study section: the TR 2B (Triassocampe coronata) to TR 5A (Capnuchosphaera) zones. These have been compared to the late Anisian to early Carnian, which was confirmed by the occurrence of conodont fossils such as Nicoraella kockeli and Paragondolella tadpole. A radiolarian faunal change from Fassanian (early Ladinian) fauna to Longobardian fauna involving a transition to strongly twisted spine morphology occurred simultaneously in several taxa across a thick siliceous shale bed (4 cm thick) at the Fassanian–Longobardian boundary.
The Orikabe section is composed of 23-m-thick grey bedded chert. This section yielded conodonts such as Chiosella timorensis, Paragondolella bulgarica group, and Paragondolella trammeri, which indicative the early Anisian to Ladinian. This section did not yield any radiolarians. In the Orikabe section, the thick shale layer observed in the Sakahogi section was not present.
Whole-rock elemental composition of claystone interbeds in the bedded chert sequence was measured by XRF in the Sakahogi section. Results showed no significant changes in weathering indices, redox-sensitive elements or barium. In contrast, high levels of phosphorus and calcium relative to PAAS were detected around the Fassanian–Longobardian boundary. No significant increase in the abundance of conodonts or other phosphatic fossils was observed during this period, suggesting that the cause of the increase in phosphorus and calcium is probably authigenic apatite rather than biogenic apatite.
In sum, the humid event in the late Ladinian may have led to an increase in phosphorus and calcium concentrations in the pelagic realm of the Panthalassa Ocean, which could have triggered the radiolarian faunal turnover and the twisting of the spine.
1. Preto, N. et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 290, 1-10 (2010).
2. Stefani, M. et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 290, 43-57 (2010)
3. Ozsvart, P. Proceedings of InterRad XVI in Ljubljana 2022, 76-77 (2022)
4. Suguyama, K. Bull. Mizunami Fossil Mus., 24, 79-193 (1997)
To investigate the environmental changes during the Longobardian humid event on a geographically broader scale, this study examined its record in sediments accumulated in a pelagic, open ocean setting based on radiolarian and conodont biostratigraphy and geochemical profiles. Two sections were investigated in this study: the Sakahogi section (Section O of Sugiyama, 19974; Inuyama area) in the Mino Belt of central Japan and the newly established Orikabe section (Akka area) in the North Kitakami Belt of northeast Japan. Both are pelagic deep-sea sedimentary rocks originally deposited in Panthalassa that are now preserved within Jurassic accretionary complexes.
The Sakahogi section consists mainly of rhythmic brick-red bedded cherts with a total thickness of 21 m. Radiolarian biostratigraphy shows that six of Sugiyama's radiolarian zones4 was recognized in the study section: the TR 2B (Triassocampe coronata) to TR 5A (Capnuchosphaera) zones. These have been compared to the late Anisian to early Carnian, which was confirmed by the occurrence of conodont fossils such as Nicoraella kockeli and Paragondolella tadpole. A radiolarian faunal change from Fassanian (early Ladinian) fauna to Longobardian fauna involving a transition to strongly twisted spine morphology occurred simultaneously in several taxa across a thick siliceous shale bed (4 cm thick) at the Fassanian–Longobardian boundary.
The Orikabe section is composed of 23-m-thick grey bedded chert. This section yielded conodonts such as Chiosella timorensis, Paragondolella bulgarica group, and Paragondolella trammeri, which indicative the early Anisian to Ladinian. This section did not yield any radiolarians. In the Orikabe section, the thick shale layer observed in the Sakahogi section was not present.
Whole-rock elemental composition of claystone interbeds in the bedded chert sequence was measured by XRF in the Sakahogi section. Results showed no significant changes in weathering indices, redox-sensitive elements or barium. In contrast, high levels of phosphorus and calcium relative to PAAS were detected around the Fassanian–Longobardian boundary. No significant increase in the abundance of conodonts or other phosphatic fossils was observed during this period, suggesting that the cause of the increase in phosphorus and calcium is probably authigenic apatite rather than biogenic apatite.
In sum, the humid event in the late Ladinian may have led to an increase in phosphorus and calcium concentrations in the pelagic realm of the Panthalassa Ocean, which could have triggered the radiolarian faunal turnover and the twisting of the spine.
1. Preto, N. et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 290, 1-10 (2010).
2. Stefani, M. et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 290, 43-57 (2010)
3. Ozsvart, P. Proceedings of InterRad XVI in Ljubljana 2022, 76-77 (2022)
4. Suguyama, K. Bull. Mizunami Fossil Mus., 24, 79-193 (1997)