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 Fe 2+ forms highly UV-absorbent species such as . 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. , 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 Fe 2+ 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.

No abstract

Sulfur is important to planetary habitability, but the early sulfur cycle is poorly understood. In particular, S[IV] species (HSO, SO), derived from volcanogenic SO, are critically invoked in recent proposals for origins-of-life chemistry and also influence atmospheric sulfur haze formation, but their abundance in early natural waters is unclear. Here, we combine new laboratory constraints on the kinetics of S[IV] disproportionation with a novel aqueous photochemistry model to estimate the concentrations of S[IV] in natural waters on prebiotic Earth. We show that S[IV] disproportionation is slow in pH≥7 waters, with timescale T≥1 year at room temperature, meaning that S[IV] was present in prebiotic natural waters. However, we also show that photolysis of S[IV] limits [S[IV]]< 100 μM in global-mean steady state. Marine S[IV] was sub-saturation with respect to atmospheric SO, meaning that climate-altering, UV-attenuating sulfur hazes did not persist on prebiotic Earth. [S[IV]] was much lower in natural waters compared to the concentrations generally invoked in laboratory simulations of origins-of-life chemistry (≥10 mM), meaning further work is needed to confirm whether S[IV]-dependent prebiotic chemistries discovered in the lab could have realistically functioned in nature. [S[IV]]≥1 μM in terrestrial waters for: (1) SO outgassing ≥20× modern, (2) pond depths <10 cm, or (3) UV-attenuating agents present in early waters or the prebiotic atmosphere. Our work illustrates the synergy between planetary science, geochemistry and synthetic organic chemistry experiments in understanding the emergence and maintenance of life on early Earth.