where R represents the34S/32S ratio.
Surface water was sampled from rivers draining the Khibiny massif in
late August in 2017 using an equivalent sampling strategy as the one
performed in the Imetjoki catchment (following Fischer et al., 2020;
Fischer et al., 2022). More specifically, the Belaya catchment and
Vuonnemiok catchments were sampled within three zones: the upstream
areas unaffected by the mining sites (IDs 16‒19, 23 in Fig. 1), the
mining areas comprising the mine effluents (IDs 20‒21, 24), and the
downstream areas (IDs 22, 25). An additional sample was collected from
the Umba River (ID 26; draining the Umbozero Lake catchment) which
corresponds to about 30 km downstream of the Vuonnemiok catchment.
Isotopic fractionation and mixing scheme
Both the Imetjoki and Khibiny catchments were assumed to have two major
sulfur isotopic end-members; sulfur from atmospheric deposition
(δdep ; composed of sulfur from both sea spray and
fossil fuel emissions) and geogenic sulfur from weathered bedrock
(δrock ). This allowed us to apply the sulfur
isotopic fractionation and mixing scheme developed by Fischer et al.
(2022) to quantify MSR within each site. In summary, the method compares
stream water field measurements (representing potential post-MSR
conditions) with theoretical predictions derived from sulfur end-member
mixing (representing initial or pre-MSR conditions). Deviations between
these two are assumed to be due to MSR. Although other microbial
processes apart from MSR could contribute to isotopic deviations, they
either represent only smaller/negligible fractionation (e.g.,
assimilatory sulfate reduction; Sharp, 2017) or they can be registered
only when sulfur concentrations are much higher (e.g. sulfur
disproportionation in marine/laboratory environments; Böttcher et al.,
2005) than in our study environments. A theoretical stream water sample
(δsample ) is then in the considered conservative
case composed of proportional fractions of each end-member
(fdep and frock ,
respectively), according to the principle of mass balance: