2.2. Local and global redox proxies
Iodine-to-calcium (I/Ca+Mg) ratios in carbonate minerals are thought to capture local changes in water column oxygen contents (Lu et al., 2010). When discussing concentration patterns, iodine abundances are normalized to carbonate contents and magnesium (Mg) concentrations are included to account for varying carbonate mineralogy (Hardisty et al., 2014, 2017; Lu et al., 2018). Although iodine has a relatively long residence time in the modern ocean (~300 kyr), iodine responds rapidly to changes in local reducing conditions due to its redox potential (Rue et al., 1997; Hardisty et al., 2020). Under well-oxygenated local conditions, iodate (IO3-) is the dominant species of iodine, while under reducing conditions iodate is converted to iodide (I-) (Rue et al., 1997). The redox potential of iodate to iodide is similar to that of O2, Mn2+, and NO3-, thus high iodate concentrations (>2.6 μmol/mol) tend to correlate with well-oxygenated portions of the modern surface oceans and predictably decrease in oxygen minimum zones (OMZs; 0.5–2.5 μmol/mol) (Rue et al., 1997; Lu et al., 2010). Within this framework, concentrations of iodate within carbonate minerals can be used to track local paleoredox conditions, as iodate readily substitutes for the carbonate ion, while iodide is excluded from the lattice structure (Lu et al., 2010). Furthermore, iodine has a higher reduction potential than more widely used U, Mo, and S proxies and thus responds more readily to low-oxygen conditions (Lu et al., 2010).
Sulfate-S isotope compositions and concentrations in the global oceans are controlled by the input and output fluxes of sulfur to and from the oceans. The two major input fluxes are riverine sulfate and volcanic outgassing, which have a combined value of 1.5 × 1018mol/Myr, and both have isotopic compositions that range between 0‰ to +9‰ (Burke et al., 2018). Important output fluxes for sulfur are the burial of sulfate-evaporites, which carries a small isotopic fractionation and a flux of 0.83 × 1018 mol/Myr, and sedimentary pyrite with a flux of 0.67 × 1018 mol/Myr (Burke et al., 2018). Pyrite formation via microbial sulfate reduction (MSR) records up to a 70‰ sulfur isotope fractionation between sulfate and the product sulfide (approximated by Δ34S; Δ34S= δ34SSO4 - δ34SH2S) (Lang et al., 2020; Pasquier et al., 2021). This process occurs in anaerobic environments and is dependent on the availability of labile organic matter, reactive iron, and sulfate (Gomes and Hurtgen, 2015; Sim, 2019). Sulfur isotopes of carbonate-associated sulfate (δ34SCAS) are commonly used to generate high-resolution spatiotemporal records of global marine sulfate-sulfur isotope compositions. Since marine sulfate throughout the Phanerozoic had a significantly longer residence (105–107 yrs) time than inter-ocean mixing timescales (103 yrs) and thus is homogenous throughout ocean basins, δ34SCASvalues are generally representative of the global seawater reservoir. Pyrite sulfur (δ34Spyr) isotopes, in contrast, are best used as a local proxy for MSR activity and the associated factors that control the magnitude of fractionation, such as rates of sulfate reduction, iron availability for pyrite formation, and interplays between open and closed system dynamics (Lang et al., 2020; Pasquier et al., 2021).
Materials and methodsWeathered surfaces, when present, were removed from samples via a water-cooled saw to ensure the fresh material was utilized for geochemical analysis. In-depth details regarding sample processing and purification for carbonate-associated sulfate (CAS), pyrite sulfur, and I/(Ca+Mg) are described in the supplemental methods section. Extracted CAS precipitated as BaSO4 and sedimentary pyrite as Ag2S were weighed into tin capsules with excess V2O5 and analyzed for their δ34S values using a ThermoFisher Delta V at the University of California Riverside or a Finnigan MAT 252 at Indiana University. All sulfur isotopic ratios are reported in standard per mil (‰), using delta notation (δ) relative to Vienna Canyon Diablo Troilite (V-CDT) with reproducibility for all sulfur analyses better than ±0.2‰ based on replicates of the samples and standards. Standards used for sulfur isotopic analysis include the international standards NBS-127 =21.1‰; IAEA S-1 = -0.30‰; IAEA S-2 = 22.7‰; IAEA S-3 = -32.3‰; and EMR-CP = 1.07‰ an internal lab standard at Indiana University. I/(Ca+Mg) ratios were analyzed using an Agilent 7500cs inductively coupled-plasma mass spectrometer (ICP-MS) at the National High Magnetic Field Laboratory at Florida State University following standard methods (Lu et al., 2010). Internal standard curves were made fresh daily from high purity standards and compared to in-house and previously published geo-standards KL 1-2 and KL 1-4 from Hardisty et al., (2017) and were found to be within ±0.5% of the reported value. The precision of duplicate samples and replicate analysis were within ±0.08 μmol/mol or better.
Results