Abstract
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 we consider the implications for prebiotic chemistry. We find that prebiotic freshwaters were largely transparent in the UV, contrary to assumptions in some models of prebiotic chemistry. Some waters, such as high-salinity waters like carbonate lakes, may be deficient in shortwave (≤220 nm) UV flux. More dramatically, ferrous waters can be strongly UV-shielded, particularly if the Fe2+ forms highly UV-absorbent species such as Fe(CN)4−6FeCN64-<math><mstyle><mi>F</mi><mi>e</mi></mstyle><msubsup><mrow><mfenced><mrow><mstyle><mi>C</mi><mi>N</mi></mstyle></mrow></mfenced></mrow><mrow><mn>6</mn></mrow><mrow><mn>4</mn><mo>-</mo></mrow></msubsup></math>. Such waters may be compelling venues for UV-averse origin-of-life scenarios but are unfavorable for some UV-dependent prebiotic chemistries. UV light can trigger photochemistry even if attenuated through photochemical transformations of the absorber (e.g., e−aqeaq-<math><msubsup><mrow><mi>e</mi></mrow><mrow><mi>a</mi><mi>q</mi></mrow><mrow><mo>-</mo></mrow></msubsup></math> production from halide irradiation), which may have both constructive and destructive effects for prebiotic syntheses. Prebiotic chemistries that invoke waters that contain such absorbers must self-consistently account for the chemical effects of these transformations. The speciation and abundance of Fe2+ in natural waters on early Earth is a major uncertainty and should be prioritized for further investigation, as it played a major role in UV transmission in prebiotic natural waters.