Environmental implications of the mineralogical record in Olenekian (Early Triassic) units in Western Dolomites (Italy): acidity conditions and ecosystems recovery

Abstract The Smithian-Spathian boundary (SSB) was related to a global physical–chemical alteration of ecosystems that led to a Lower Triassic biotic extinction in marine and continental environments. A recent approach to SSB in continental environments has demonstrated the existence of increased levels of acidity in the SSB of different western continental peri-Tethys areas coinciding with the absence of life or its drastic decrease. A study of Sr-rich aluminum phosphate sulfate (APS) minerals, considered indirect indicators of higher acidity, has been carried out in the Olenekian members of the Werfen Formation (Early Triassic) of the Dolomites and Vicentinian Prealps (Southern Alps, Italy), complemented by analysis of clay mineralogy and geochemical proxies. The results have shown the presence of small (< 1 µm) idiomorphic pseudo-cubic APS crystals either disseminated or forming aggregates among quartz and feldspar grains and detrital mica plates. These minerals only appear in the Campil, Val Badia and Cencenighe members of the Werfen Formation (Olenekian) coinciding with a reduction in the ichnofabric index, trace fossil diversity and burrowing depth that were associated to strongly stressed environmental conditions and preceding to a progressive recovery already in the San Lucano Member, which lacks of APS minerals. These results reinforce the idea of the increase in acidity conditions during the SSB in the continental and transitional environments of the western Tethys and its influence in the development of life and subsequent recovery from the biotic crisis of that transition.