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

The late Smithian and the Smithian-Spathian boundary (SSB) are associated with harsh environmental condi-tions, 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 dis-turbances 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 634Spy and 634SCAS analyses in order to better understand the redox changes in different environmental settings along a ramp depositional system through the SSB. 656Fe analyses show a large variability along the studied ramp system of-7%o (from-1.99 to +5.39%o), over a set of 350 microscale analyses. Bulk sulfide sulfur isotope analyses, performed on 30 samples, show 634Spy varying from-20.5 to +16.3%o. The inner ramp domain is characterized by a mean negative 634Spy values of-11.4%o. A progressive 34S-enrichment (up to +16.3%o) is recorded in pyrite from mid and outer ramp settings. Carbonate associated sulfate (CAS) sulfur isotope analyses, performed on 5 samples, show relatively steady 634Scas of +30.2 +/- 2.2%o. Variations in 634Spy 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 656Fepy. In the inner ramp, high 656Fepy values averaging +2.05%o 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, 656Fepy inner ramp values are lighter (delta 56Femean = +0.90%o) and reflect total reduction of Fe-oxides. In more distal and deeper mid and outer ramp settings, Fe isotope compositions are controlled by microbially-produced H2S that scavenged iron into sulfides. This study unravels local redox state changes in the upper part of some marine sediments by coupling Fe and S isotope systematics. It demonstrates that pyrite grains, and their sulfur and iron isotopic compositions, formed throughout the SSB should be used with caution to infer the redox state of the ocean after the Permian-Triassic biotic crisis.

Automated summary

The late Smithian and the Smithian-Spathian boundary are associated with harsh environmental conditions, and this study reveals the redox changes in marine sediments during this period through Fe and S isotope systematics. The findings indicate that the sedimentary environment and nature of deposits play a crucial role in controlling the Fe isotope compositions and could impact interpretations of the redox state of the ocean after the Permian-Triassic biotic crisis.