  Ultraviolet (UV) light plays a key role in surficial theories of the origin
of life, and numerous studies have focused on constraining the atmospheric
transmission of UV radiation on early Earth. However, the UV transmission of
the natural waters in which origins-of-life chemistry (prebiotic chemistry) is
postulated to have occurred is poorly constrained. In this work, we combine
laboratory and literature-derived absorption spectra of potential aqueous-phase
prebiotic UV absorbers with literature estimates of their concentrations on
early Earth to constrain the prebiotic UV environment in marine and terrestrial
natural waters, and consider the implications for prebiotic chemistry. We find
that prebiotic freshwaters were largely transparent in the UV, contrary to
assumptions by some models of prebiotic chemistry. Some waters, e.g.,
high-salinity waters like carbonate lakes, may be deficient in shortwave
($\leq220$ nm) UV flux. More dramatically, ferrous waters can be strongly
UV-shielded, particularly if the Fe$^{2+}$ forms highly UV-absorbent species
like Fe(CN)$_6^{4-}$. Such waters may be compelling venues for UV-averse
origin-of-life scenarios, but are disfavorable for some UV-dependent prebiotic
chemistries. UV light can trigger photochemistry even if attenuated through
photochemical transformations of the absorber (e.g., $e^{-}_{aq}$ production
from halide irradiation), which may have both constructive and destructive
effects for prebiotic syntheses. Prebiotic chemistries invoking waters
containing such absorbers must self-consistently account for the chemical
effects of these transformations. The speciation and abundance of Fe$^{2+}$ in
natural waters on early Earth is a major uncertainty, and should be prioritized
for further investigation as it plays a major role in UV transmission in
prebiotic natural waters.
