[B-CG05] Frontier in diversity and ecology of protists and microfossils

[BCG05-P04] Microfossils from Triassic chert and siliceous claystone treated by NaOH (Preliminary report)

*Satoshi Takahashi¹, Rie Hori, S.², Yoshiaki Aita³, Satoshi Yamakita⁴, Atsushi Takemura⁵, Kazuki Matsui¹, Shota Yamada¹ (1.Department of Earth and Environmental Sciences Graduate School of Environmental Studies Nagoya University, 2.Department of Earth Sciences, Graduate School of Science and Engineering, Ehime University, 3.Department of Geology, Faculty of Agriculture, Utsunomiya , 4.Department of Earth Sciences, Faculty of Education, University of Miyazaki, 5.Geosciences Institute, Hyogo University of Teacher Education)

Keywords:chert, microfossil, radiolarians, sodium hydroxide

Bedded cherts in accretionary complexes in the circum-Pacific area are important materials that provide oceanic records in the geologic past, such as the Paleozoic and Mesozoic. Among them, the Permian-Triassic radiolarian cherts in New Zealand are rarely preserved cherty succession across the Permian-Triassic. Meanwhile, radiolarian cherts lack at the Lower Triassic in the Japanese deposited in the low latitude paleo-Pacific. Instead, siliceous claystone is observable in the Japanese lithologic sections of the Lower Triassic, possibly due to limited radiolarian burial flux or preservations of silicic fossils.
Rigo et al. (2023) provide a new methodology of microfossil treatment that enables the separation of fossils from cherty bulk rocks using a sodium hydroxide reaction (NaOH treatment). Here, we tried to apply this method to Triassic chert samples from New Zealand and clayey samples from Japan with some procedure modifications.
Our study samples are from Waiheke sections 1 and 2 in Waiheke Island, the North Island of New Zealand (Waipapa Terrane). These sections are best preserved deep-sea, which records the pelagic Panthalassic super ocean (paleo-Pacific) in the South Hemisphere, ranging from Upper Permian to Lowe Triassic (lower Olenekian) and from Lower Triassic (upper Olenekian) to Middle Triassic (Anisian), respectively. Another sample is from the Hoshaku-ji and Momotaro-Jinja sections in central Japan (Mino Belt; Yamakita et al., 2006). These sections are Lower Triassic deposited in the low latitude pelagic Panthalassa.
After the NaOH treatment, silicic microfossils, including radiolarians and sponge skeletons, were recovered from each sample residue. Their preservation condition is relatively good. For instance, recovered individual radiolarian fossils show sharp spine structures and some internal structures. Fragmented dark-coloured phosphate fossils of conodont were also recognized in the residues. On the other hand, the residues from the Japanese siliceous claystone contain conodont fragments and sponge skeletons. Possible radiolarian fossils relatively rarely occurred in the samples. Further observations may provide information on the Triassic ecosystem and the nature of the aftermath after the Permian-Triassic mass extinction event.

Reference:
Hori et al. (2011) Palaeoworld 20, 168–178.
Rigo et al. (2023) Palaeontology 66, 4, e12672.
Yamakita et al. (2010) Annual Meeting of the Palaeontological Society of Japan, p. 47.