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 δ13C 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 conodont biodiversity increased with species developing characteristics of Isarcicella. This interval has been referred to the Changhsingian Late praeparvus Zone and correlated with Bed 27a-b of Meishan D section. The PTB has been identified in the Bulla section at 1.30 m from the base of the Werfen Fm. - with the first appearance of Hindeodus parvus defining the base of the parvus Zone - in a microbialitic layer correlated with Bed 27c of Meishan D section. Around the PTB the major part of the remaining Permian biota disappeared. Gymnocodiacean algae were the last group to undergo extinction within the Triassic layers. (4) Biodiversity dropped severely in the succeeding bioturbated microbialitic interval from which conodonts are absent. More humid climate is reflected in a more sandy marine substrate inhabited by Lingula, Unionites and Claraia. Conodonts recurred in the schizohaline upper part. The succeeding entry of the biozonal markers Isarcicella lobata, Isarcicella staeschei and Isarcicella isarcica allow discrimination of three conodont biozones. The layer with entry of I. staeschei has been aligned with Bed 28 of Meishan D section. The main extinction phase in the western Tethys seems to correspond to a gradual but swift transition from acid-bath to alkaline bath according to the Deev Jahi model of Heydari and Hassanzadeh [Heydari, E. and Hassanzadeh, J., 2003. Deev Jahi model of the Permian-Triassic boundary mass extinction: a case for gas hydrates as the main cause of biological crisis on Earth. Sed. Geol., 163, 147-163.]. The Bulla section with abundant data on biotic and depositional variation is here considered as the PTB parastratotype for the shallow marine western Tethys.