Pyrite iron isotope compositions track local sedimentation conditions through the Smithian-Spathian transition (Early Triassic, Utah, USA)

International audience ; The late Smithian and the Smithian-Spathian boundary (SSB) are associated with harsh environmental conditions, including abrupt temperature changes, oceanic acidification and oxygen deficiency causing an additional marked loss of biotic diversity in the aftermath of the end-Permian mass extinction. Such environmental disturbances are documented worldwide through large fluctuations of the C, O, S and N biogeochemical cycles. This study presents secondary ion mass spectrometry pyrite Fe isotope analyses from the Lower Weber Canyon (LWC) section (Utah, USA) combined with bulk rock δ34Spy and δ34SCAS analyses in order to better understand the redox changes in different environmental settings along a ramp depositional system through the SSB. δ56Fe analyses show a large variability along the studied ramp system of ∼7‰ (from −1.99 to +5.39‰), over a set of 350 microscale analyses. Bulk sulfide sulfur isotope analyses, performed on 30 samples, show δ34Spy varying from −20.5 to +16.3‰. The inner ramp domain is characterized by a mean negative δ34Spy values of −11.4‰. A progressive 34S-enrichment (up to +16.3‰) is recorded in pyrite from mid and outer ramp settings. Carbonate associated sulfate (CAS) sulfur isotope analyses, performed on 5 samples, show relatively steady δ34Scas of +30.2 ± 2.2‰. Variations in δ34Spy are interpreted as reflecting the degree of connection between sediment porewaters and the overlying water column. Multiple lines of evidence point to a fully oxygenated water column and thus restricts pyrite formation to the sediments. Both the sedimentary environment and the nature of deposits seem to control δ56Fepy. In the inner ramp, high δ56Fepy values averaging +2.05‰ are only observed in microbially induced sedimentary structures (MISS), which record partial Fe-oxide reduction and oxidation reactions occurring at biofilms scale. In the absence of MISS, δ56Fepy inner ramp values are lighter (δ56Femean = +0.90‰) and reflect total reduction of Fe-oxides. In more distal and deeper ...