Marine sulphate captures a Paleozoic transition to a modern terrestrial weathering environment

Abstract

The triple oxygen isotope composition of sulphate minerals has been used to constrain the evolution of Earth's surface environment (e.g., pO₂, pCO₂ and gross primary productivity) throughout the Proterozoic Eon. This approach presumes the incorporation of atmospheric O₂ atoms into riverine sulphate via the oxidative weathering of pyrite. However, this is not borne out in recent geological or modern sulphate records, where an atmospheric signal is imperceptible and where terrestrial pyrite weathering occurs predominantly in bedrock fractures that are physically more removed from atmospheric O₂. To better define the transition from a Proterozoic to a modern-like weathering regime, here we present new measurements from twelve marine evaporite basins spanning the Phanerozoic. These data display a step-like transition in the triple oxygen isotope composition of evaporite sulphate during the mid-Paleozoic (420 to 387.7 million years ago). We propose that the evolution of early root systems deepened the locus of pyrite oxidation and reduced the incorporation of O₂ into sulphate. Further, the early Devonian proliferation of land plants increased terrestrial organic carbon burial, releasing free oxygen that fueled increased redox recycling of soil-bound iron and resulted in the final rise in pO₂ to modern-like levels.

Fig. 1. A billion-year record of the oxygen isotope composition of marine sulphate.

The oxygen isotope composition (Δ’¹⁷O) of volumetrically significant marine sulphate evaporite basins over approximately the last 1050 million years. Filled (this study) and open circles (published data^,, scaled, see SI) are represented as a function of samples per basin, centered on the median value, and with error bars that denote the 25^th and 75^th quartiles of each distribution. A smoothed fit, with a 95% confidence interval for the Cenozoic-Cretaceous sulphate record is noted in the filled blue-green region. At the top is a timeline of geologic events that have been linked to a rise in atmospheric pO₂. The two Neoproterozoic Snowball Earth events (Cryogenian Glaciations) are represented by vertical blue bars. Data from Marinoan barite crystal fans (i.e., not evaporite deposits) is excluded. The gray dashed bars represent the temporal constraint on pO₂ rise based on the marine sulphate evaporite record.

Fig. 2. Triple oxygen isotope composition of marine sulphate minerals.

The triple oxygen isotope composition (Δ’¹⁷O against δ¹⁸O) of marine sulphate evaporite deposits (filled circles=this work; open circles=literature^,,; marker size scales with sample set size). Each circle reflects the median basinal value, with error bars reflecting the 25^th and 75^th 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 record. The two triangles in the lower right are sulphate isomer equilibrium with seawater at 15 °C. 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.