Environmental and biotic changes across the Permian–Triassic boundary in western Tethys: The Bulla parastratotype, Italy

The sedimentary and biotic evolution of a 190 m interval of shallow marine and lagoonal facies in the Bellerophon and Werfen formations in the Southern Alps has allowed comparison of western with eastern Tethys: Meishan D section (southern China), Salt Range (Pakistan) and Abadeh (Iran). Results are as follows: (1) The upper part of the Bellerophon Fm. (Changhsingian changxingensis–deflecta Zone) shows only modest biotic variation connected with tectonically driven local variation and perhaps to more general climatic variation. The δ 13C decrease starting in the uppermost 30 m of the Bellerophon Fm. is correlated with decrease in global organic productivity starting about 1 m below the PTB in Chinese sequences and 20 m below in the Abadeh section. This interval culminated in a regression truncated by an unconformity–paraconformity (Unconformity 1). (2) The uppermost Bellerophon Fm. is a ca. 1 m transgressive-regressive sedimentary cycle, the informally named Bulla Mbr (Changhsingian: Early praeparvus Zone). The maximum flooding interval of this unit possibly had a slight increase in biodiversity, mainly in foraminifers, algae and brachiopods. The high increase in biodiversity previously reported may, in part, reflect abundance of biota and organic matter reworked into transgressive and regressive intervals. We suggest partial correlation of the basal unconformity of the Bulla Mbr (Unconformity 1) with the regressive uppermost Bed 24e of the Meishan D section marking the disappearance of foraminifers and algae in the eastern Tethys. We also suggest diachronous disappearance of benthic taxa in Tethys, with the Southern Alps acting like a refugium. (3) The main extinction (first extinction phase, mainly regarding foraminifers) in the Southern Alps occurred in a thin ca. 25 cm interval including the uppermost regressive Bulla Mbr, Unconformity 2, and possibly, the basal transgressive bed of the Tesero Mbr of the Werfen Fm. This interval is correlated in part with regressive Bed 26 of Meishan D section. The main decrease in abundance and biodiversity in the Southern Alps coincides with appearance of small oolites with crystalline outer cortex near the basal transgressive tract of the lower Tesero Mbr, a ca. 220 cm sedimentary cycle, that is followed by extension of microbialitic layers alternating with veritable biostromes with brachiopods and byssate bivalves as salient components among the algae. Vacated niches favored evolution of conodonts. Hindeodid conodo