5:15 PM - 7:15 PM
[BCG07-P01] Organic microfossils recovered from the pelagic deep-sea sedimentary rocks of Early Triassic Panthalassa
Keywords:Mass extinction, Smithian, Deep-sea chert, Paleoecology
This study reports organic microfossils recovered from the deep-sea sedimentary rocks of the Lower Triassic (lower Olenekian, Smithian). Our studied section is the Momotaro-Jinja section which belongs to Mino-Tamba-Ashio Belt of the accretinary complex in central Japan. The depositional area of this section is the low-latitude pelagic Panthalassa.
Collected sample rocks (siliceous claystone and black claystone) from the outcrop were treated by the NaOH method (Onoue et al., 2024) to dissolve the siliceous matrix. From the residue after this treatment, 474 brownish-colored organic microfossils were discovered. Raman spectroscopy analysis on these microfossils indicated that the main composition of these fossils is carbonaceous material, that is organic matter. The discovered organic microfossils have convex thin sheet shape with a rounded outline. Their diameters were up to 100-150 μm. Some fragmented fossils suggest that the maximum size of these organic microfossils may be larger possibly up to 300-500 μm. Remarkable concentric stripes are observable on the surface of these fossils. These stripes are aligned every few micrometres.
Their external shape and the concentric striations on the surface are similar to the shells of bivalves and brachiopods. It is plausible that the organic periostracum of bivalves and brachiopods was fossilized and preserved in the sedimentary rocks, while hard tissues such as carbonate and phosphate were degraded under deep-sea conditions below the carbonate compensation depth. In fact, bivalves and brachiopods with rounded shape and concentric ornaments have been reported from the Lower Triassic (e.g. Claraira (bivalve), Orbiculodea, Borozonia and Disinisca (brachiopod)). Algal cyst and plant spore fossils are also candidates because of their similar outline. However, plant spores are too small for the fossils studied (several hundreds micrometer). It implies that recovered microfossils cannot identify as algal cysts and plant spore.
The Smithian of the lower Olenekian is a biotic recovery phase after the end-Permian mass extinction. Fossil records of the re-established ecosystem accumulate in shallow marine deposits such as the Paleo-Tethys Ocean. Meanwhile, the deep-sea sedimentary rocks of the Panthalassa Ocean, another major oceanic region of the Permian-Triassic world, have poor fossil records due to the deep-sea setting below the carbonate compensation depth. Our findings of organic microfossils have the potential to provide information on marine organisms in this oceanic region during the disaster and recovery process of the Permian-Triassic mass extinction event.
Collected sample rocks (siliceous claystone and black claystone) from the outcrop were treated by the NaOH method (Onoue et al., 2024) to dissolve the siliceous matrix. From the residue after this treatment, 474 brownish-colored organic microfossils were discovered. Raman spectroscopy analysis on these microfossils indicated that the main composition of these fossils is carbonaceous material, that is organic matter. The discovered organic microfossils have convex thin sheet shape with a rounded outline. Their diameters were up to 100-150 μm. Some fragmented fossils suggest that the maximum size of these organic microfossils may be larger possibly up to 300-500 μm. Remarkable concentric stripes are observable on the surface of these fossils. These stripes are aligned every few micrometres.
Their external shape and the concentric striations on the surface are similar to the shells of bivalves and brachiopods. It is plausible that the organic periostracum of bivalves and brachiopods was fossilized and preserved in the sedimentary rocks, while hard tissues such as carbonate and phosphate were degraded under deep-sea conditions below the carbonate compensation depth. In fact, bivalves and brachiopods with rounded shape and concentric ornaments have been reported from the Lower Triassic (e.g. Claraira (bivalve), Orbiculodea, Borozonia and Disinisca (brachiopod)). Algal cyst and plant spore fossils are also candidates because of their similar outline. However, plant spores are too small for the fossils studied (several hundreds micrometer). It implies that recovered microfossils cannot identify as algal cysts and plant spore.
The Smithian of the lower Olenekian is a biotic recovery phase after the end-Permian mass extinction. Fossil records of the re-established ecosystem accumulate in shallow marine deposits such as the Paleo-Tethys Ocean. Meanwhile, the deep-sea sedimentary rocks of the Panthalassa Ocean, another major oceanic region of the Permian-Triassic world, have poor fossil records due to the deep-sea setting below the carbonate compensation depth. Our findings of organic microfossils have the potential to provide information on marine organisms in this oceanic region during the disaster and recovery process of the Permian-Triassic mass extinction event.