[M-IS20] Evolution of the Pelagic Realm

[MIS20-P06] Sr isotope variations in the Upper Triassic succession at Pizzo Mondello, western Sicily, Italy

*Tetsuji Onoue¹, Katsuyuki Yamashita², Chise Fukuda⁶, Katsuhito Soda⁵, Yuki Tomimatsu⁵, Benedetto Abate³, Manuel Rigo⁴ (1.Faculty of Advanced Science and Technology, Kumamoto University, 2.Graduate School of Natural Science and Technology, Okayama University, 3.Department of Earth and Sea Sciences, University of Palermo, 4.Department of Geosciences, University of Padova, 5.Graduate School of Science and Technology, Kumamoto University, 6.Faculty of Science, Okayama University)

Keywords:Norman, Tethys, Cimmerian Mountains

The Norian stage in the Late Triassic is exceptionally long (23 Myr) and was subdivided into three substages: the Lacian, Alaunian, and Sevatian. In order to infer the Norian environmental changes in the western Tethys Ocean, the stratigraphic variations of ⁸⁷Sr/⁸⁶Sr in the Upper Triassic limestone succession in Sicily were examined. The Pizzo Mondello section studied here mainly consists of a pelagic carbonate sequence of the Scillato Formation, and ranges in age from Tuvalian (late Carnian) to Rhaetian. The Scillato Formation represents a deep-water pelagic facies deposited along the Sicanian Basin in the western Tethys Ocean.

We selected fine-grained limestone samples from both the microfacies of lime-mudstone and wackestone to approximate the primary ⁸⁷Sr/⁸⁶Sr signature of the limestone beds. The ⁸⁷Sr/⁸⁶Sr values are relatively constant in the Tuvalian and Lacian (early Norian). However, the remarkable rise in ⁸⁷Sr/⁸⁶Sr occurred across the Lacian–Alaunian (early–middle Norian) transition. Variations in ⁸⁷Sr/⁸⁶Sr values show an increasing trend in ⁸⁷Sr/⁸⁶Sr from 0.7077 at the base of Lacian to 0.7080 in the Sevatian (late Norian). In the Sevatian, the ⁸⁷Sr/⁸⁶Sr ratios display a sudden negative excursion toward lower values and show a relatively quick recovery to pre-excursion ⁸⁷Sr/⁸⁶Sr ratios.

Korte et al. (2003) suggested that the rise in the ⁸⁷Sr/⁸⁶Sr values from the middle Carnian to the late Norian coincide with the Cimmerian orogeny. Our new ⁸⁷Sr/⁸⁶Sr data from the Pizzo Mondello section reveal a comparable trend, with a sharp increase in ⁸⁷Sr/⁸⁶Sr within the Alaunian, suggesting the rapid uplift and erosion in the Cimmerian Mountains at this time. The cause of the ⁸⁷Sr/⁸⁶Sr excursion in the Sevatian remains uncertain. However, the biostratigraphic record of conodonts suggests that a morphological evolution towards platform-less elements occurred with the beginning of the Sr-isotope excursion.