Foraminiferal events in the Pliensbachian-Toarcian transition at almonacid de la Cuba (Zaragoza, Spain), the complementary reference section of the proposed Toarcian GSSP

Abstract

This paper reports results from biostratigraphic and quantitative palaeoecological analysis of diversified boreal foraminiferal faunas found in epicontinental shelf carbonates of the Upper Pliensbachian and Lower Toarcian at Almonacid de la Cuba (northeastern Spain). The results reveal step-wise extinction, replacement and renewal events along the Pliensbachian-Toarcian transition, from the uppermost Spinatum Zone to the lowermost Serpentinum Zone. The extinction of typical, long-ranging Lower Jurassic foraminiferal species takes place during this interval, with the highest extinction rate within the Tenuicostatum Zone, Semicelatum Subzone. The most significant renewal event occurs in the Serpentinum Zone, Elegantulum Subzone, with the first appearance of the genus Citharina. European, North African and American basins show similar patterns; the main dissimilarities concern the interpretation of species such as those belonging to the genus Citharina. Quantitative diversity analysis using richness, dominance, parametric and information theory-based measures is considered the best way to detect palaeoecological events in Jurassic foraminiferal assemblages. This analysis allows four palaeoecological episodes (Episodes I-IV) to be distinguished within the studied stratigraphie interval. Episodes I and II indicate stress conditions related to the Late Pliensbachian to Early Toarcian sea-level rise and Episode IV represents environmental stress linked to the Early Toarcian extinction event. A fair coincidence was detected between Episode IV and the first stage of the pronounced negative excursion of δ13C in the Tenuicostatum Zone, Semicelatum Subzone. On the basis of quantitative abundance results, the autecological information of several taxa is refined and distinct ecological strategies and behaviours can be distinguished.