Fig. 2: Triple oxygen isotope composition of marine sulphate minerals.
From: Marine sulphate captures a Paleozoic transition to a modern terrestrial weathering environment
The triple oxygen isotope composition (Δ’17O against δ18O) of marine sulphate evaporite deposits (filled circles=this work; open circles=literature16,18,24; marker size scales with sample set size). Each circle reflects the median basinal value, with error bars reflecting the 25th and 75th percentiles. As in Fig. 1, blue-green symbols reflect basins ≤387.7 Ma, whereas brown symbols are ≥420 Ma. The light blue-green regression line reflects the Cenozoic-Cretaceous marine barite record19. The two triangles in the lower right are sulphate isomer equilibrium with seawater at 15 °C23. The long-dashed line — approximated as linear — represents sulphate that derives between 0 to 25% of its O atoms from (modern) atmospheric O2 and the remainder from seawater H2O. Finally, a suite of vectors below the key indicates the directionality of change if (i) meteoric waters are involved, (ii) if paleo-O2 had a more depleted Δ’17O composition, or (iii) if sulphate is overprinted via thermodynamic equilibria with seawater.
