5.2 Modeling the sulfur cycle
Our δ34SCAS trends recorded from the
Monitor Range documents an ~10‰ fall leading into the
Hirnantian. The data are the base of the Kardla core appear to capture
the very end of this fall and, when taken with the Monitor Range data,
suggest this represents a global perturbation in the sulfur cycle. Here
we constructed a forward sulfur isotope mass balance box model to
reproduce the magnitude of the isotope perturbation and to provide
quantitative constraints on global pyrite burial and weathering fluxes.
We prescribed initial conditions based on modern estimates of weathering
and pyrite burial rates (Kurtz et al., 2003), and sensitivity tests were
explored for all fluxes (for initial parameters and values explored see
SI Table 1). While this type of modeling cannot provide a single unique
solution from the changes in the sulfur cycle, it does provide a range
of plausible first-order quantifications for these various parameters.
It is important to note that all model runs presented here (Fig. 6) were
prescribed with a change in the sulfur isotope fractionation
(Δ34S). We start with an initial value of +35‰ and
decrease it to +20‰ during the perturbation, consistent with a global
average of previously published Δ 34S values (see
supplemental material for more information, Table 2) and those
determined in this study. The starting seawater sulfate-S isotope
composition for the late Katian of ~ +35‰ is based on
our δ34SCAS dataset from the Monitor
Range. We have used recent Sr isotope weathering models for the Late
Ordovician (Hu et al., 2017) and previously published Late
Ordovician–Silurian global redox studies (Bartlett et al., 2018;
Stockey et al., 2020) for further model constraints for changes in the
pyrite burial flux. The time interval adopted for all model runs is 5
Myr for the duration of the negative
δ34SCAS excursion based on graptolite
biostratigraphy of the Monitor Range section that can be well integrated
into the latest calibrated Ordovician Time Scale-2020 (Goldman et al.,
2020; see supplemental material for more details on age constraints).