  A key challenge in origin-of-life studies is understanding the environmental
conditions on early Earth under which abiogenesis occurred. While some
constraints do exist (e.g., zircon evidence for surface liquid water),
relatively few constraints exist on the abundances of trace chemical species,
which are relevant to assessing the plausibility and guiding the development of
postulated prebiotic chemical pathways which depend on these species. In this
work, we combine literature photochemistry models with simple equilibrium
chemistry calculations to place constraints on the plausible range of
concentrations of sulfidic anions (HS$^-$, HSO$_3^{-}$, SO$_3^{2-}$) available
in surficial aquatic reservoirs on early Earth due to outgassing of SO$_2$ and
H$_2$S and their dissolution into small shallow surface water reservoirs like
lakes. We find that this mechanism could have supplied prebiotically relevant
levels of SO$_2$-derived anions, but not H$_2$S-derived anions. Radiative
transfer modelling suggests UV light would have remained abundant on the planet
surface for all but the largest volcanic explosions. We apply our results to
the case study of the proposed prebiotic reaction network of Patel et al.
(2015), and discuss the implications for improving its prebiotic plausibility.
In general, epochs of moderately high volcanism could have been especially
conducive to cyanosulfidic prebiotic chemistry. Our work can be similarly
applied to assess and improve the prebiotic plausibility of other postulated
surficial prebiotic chemistries that are sensitive to sulfidic anions, and our
methods adapted to study other atmospherically-derived trace species.
