Local sedimentary effects shaped key sulfur records after the Great Oxidation Event

Abstract

Our understanding of Earth's surface oxidant budget following the Great Oxidation Event (GOE) relies heavily on interpretations of carbon (δ¹³C) and sulfur (δ³⁴S) isotope ratios. Isotopic data have been used to argue that a rise in marine sulfate concentrations to >10 mM during the GOE was followed by a sulfate reservoir ‘collapse’. However, carbonate δ³⁴S and δ¹³C values often reflect conditions specific to depositional setting and diagenetic alteration. To assess the relative importance of global vs. local/diagenetic controls, we present δ³⁴S, δ²⁶Mg, δ^44/40Ca, and trace-metal data coupled with existing δ¹³C data from the ca. 2.1–2.0 Ga Tulomozero Formation and the overlying Zaonega Formation in the Onega Basin (Karelia, Russia). Over this transitional interval, δ³⁴S and Sr/(Ca+Mg) values increase whereas δ¹³C and δ^44/40Ca values decrease. X-ray absorption spectroscopy shows that the dominant form of sulfur within the rocks concomitantly shifted from mineralized sulfate and carbonate-associated sulfate (CAS) to variable mixtures of sulfide, sulfonate, and CAS. Those changes are associated with a depositional shift from shallow-marine to deeper-marine slope settings. δ^44/40Ca and Sr/(Ca+Mg) data indicate that depositional environmental changes drove a shift from seawater- to sediment-buffered carbonate recrystallization. Consequently, we interpret δ³⁴S trends that closely match those reported by previous workers as reflecting changing local environmental and diagenetic conditions, a finding that renders equivocal the use of such trends to assess sulfur cycle changes following the GOE. Our work establishes a blueprint for how to obtain deeper insight into the evolution of Earth's surface oxidation from stable isotope records.