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