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).