Temperature vs extinction events: the case of Rhaetian peritidal carbonate succession from westernmost Tethys (Sicily)

Abstract

A biodiversity crisis was observed in the benthic community from a latest Triassic carbonate platform of western Tethys. The studied succession, i.e. Mt Sparagio section in NW Sicily, consists of a continuous Upper Triassic to Lower Jurassic peritidal carbonate succession organized in shallowing upward cycles. The Upper Triassic horizons can be sub-divided into two informal units on the base of the benthic fossils assemblage: at the base the Unit A (thickness about 111 m) consists of a high diverse fossiliferous assemblage comprising very large megalodontoids specimens (up to 40 cm); Unit B (about 179 m) shows a reduction of biodiversity with the occurrence of rare and small megalodontoids (Conchodon?) up to 15 cm in size. Both units are Rhaetian in age as indicated by the presence of large tested specimens of the benthic foraminifer Triasina hantkeni. The top of Unit B is marked by an oolitic level, and records the total disappearance of the Triassic benthic fossil assemblage. Upward, the subtidal members of peritidal cycles (Unit C) show the presence of an oligotypic assemblage with T. parvovesiculifera, which represents a survival zone, followed in turn by a gradual recovery of the Lower Jurassic benthic community (e.g. Siphovalvulina sp.). A negative excursion of δ18Ocarb, corresponding to an increase in temperature, matches the biodiversity reduction between Unit A and Unit B. This episode correlates the Initial Carbon Isotopic Excursion already individuated in this section and related to the first pulse of the CAMP volcanic activity. A second change in temperature that is characterized by a positive excursion of δ18Ocarb matches the total extinction of the Upper Triassic benthic community. These data well fit with the environmental parameters of the modern tropical shallow water platform (T-factory). In particular, the sea surface temperature (SST) of a T-factory range from 18 °C to 30.5 °C representing respectively the minimum SST for the carbonate factory persistence and the maximum SST that a T-factory can tolerate.