Calcium isotopes support spatial redox gradients on the Tethys European margin across the Triassic-Jurassic boundary.

The end-Triassic mass extinction was among the most severe biotic crises of the Phanerozoic. It has been linked with the global expansion of marine anoxia, and the prolongation of these conditions within epeiric seas has been proposed as a cause for the suppression of biodiversity during the early Jurassic Hettangian Stage. Testing this interpretation is complicated by spatially heterogeneous patterns of local marine redox conditions within the western Tethys European Epicontinental Shelf. In this study, we assess the redox state within this region by focusing on two carbonate successions in Italy, a peritidal platform at Mount Sparagio, Sicily, and an offshore ramp deposit at Val Adrara in the Southern Alps. Based on previously published I/Ca ratios, these locations record distinct local background redox conditions, with Val Adrara showing a notably lower pre-extinction oxygen saturation state than Mount Sparagio. Here, we measure δ13C and δ18O at Mount Sparagio and δ44Ca and trace element ratios at both sites to identify the roles of mineralogical and diagenetic effects on the preservation of primary redox signals. A numerical framework of multiple elemental (Sr, Mg, Mn, I) and isotopic (δ13C, δ18O, δ44Ca, δ238U, and δ34S CAS) ratios was constructed to recognize modes of carbonate diagenesis and source-mixing in the data. While diagenesis is impossible to completely rule out, our state-of-the-art approach provides robust evidence against common forms of diagenetic alteration as the main drivers of the overall paleoredox proxy trends. Where the redox signals are largely preserved, we interpret differences in pre-extinction I/Ca between the two sites to reflect distinct local oxygenation states. Drawing from published Community Earth System Model simulations, we propose that ocean circulation and hydrological regime could have been important drivers of spatial heterogeneity in paleo-redox conditions across the European Epicontinental Shelf. Conceptual flow chart for the data reduction process employed in this study. Categories of geochemical tracers (major element isotope ratios, redox proxies, and minor/trace elemental ratios; rounded boxes) can be paired with calcium isotope ratios (δ44Ca) by an analytical method (blue diamonds) to identify pre- (source mixing) and post-depositional (diagenesis) geochemical transformations (grey ellipsis) which together (connected by solid curved lines) inform interpretations of original mineralogy and signal preservation (red ellipsoids). Arrow line styles connect the inputs with the outputs. [Display omitted] • Pairing δ44Ca with redox proxies (δ238U, δ34S CAS , I/Ca) offers a reliable approach to assess post-depositional geochemical alteration. • Two Italian sites, Mount Sparagio and Val Adrara, spanning the Triassic-Jurassic boundary, show diagenesis yet retain redox signals. • Ocean circulation strength and depth of vertical mixing may explain the difference in local redox conditions. [ABSTRACT FROM AUTHOR]