[M-IS14] Aqua planetology

[MIS14-14] Effect of photo-oxidation reactions of ferrous iron on geochemical conditions required for redox stratification on early Gale lake

*Haruhisa Tabata^1,2, Yasuhito Sekine², Kazumi Ozaki³ (1.Department of Earth and Planetary Science, University of Tokyo, 2.Earth-Life Science Institute, Tokyo Institute of Technology, 3.Department of Environmental Science, Toho University)

Keywords:Mars, Geochemistry, Photochemistry, Iron oxidation, Redox stratification

Recent observations on iron mineralogy and geochemical data by NASA’s Curiosity rover suggested that Gale Crater had sustained closed-basin, redox-stratified lakes at around Hesperian (~3.5–3.3 Ga) (Hurowitz et al., 2017). On Earth, redox stratification in closed-basin lakes is largely maintained by biological processes, e.g., photosynthesis. For Martian lakes without photosynthetic life to be redox stratified, photo-oxidation of ferrous iron could have been a source of oxidants at shallow lakes; whereas, upwelling of reducing ferrous iron is hypothesized to have occurred from the bottom (Hurowitz et al., 2017). However, no quantitative evaluations have been conducted to investigate conditions that can achieve redox stratification on early Mars.

In the present study, we investigate geohydrological and atmospheric conditions (e.g., Fe²⁺ fluxes from rivers and groundwater, advection rate and pH of lake water, and UV flux) to achieve redox stratification on early Gale lakes using a one-dimensional reaction-transport model. We took into account pH-dependent, photo-oxidation of Fe²⁺ into the model using our experimental data (Tabata et al., 2021). Attenuation of UV light due to atmospheric gas species and dissolved species was considered in the model; accordingly, the effeciency of oxidant production is highly sensitive to atmospheric and water compositions.

Our results suggest that when Fe²⁺ is supplied from the top of a lake, acidic pH of 4–5, high Fe²⁺ influx (0.1–100 mol m^–2 yr^–1), and thick atmosphere (~1 bar CO₂) are required for redox stratification. In the case of Fe²⁺ input from the top, Fe²⁺ needs to be transported to the deep before being fully photo-oxidized for redox stratification. The required conditions can achieve low efficiency of Fe²⁺ photo-oxidation near the surface sufficient to transport to the deep. When Fe²⁺ is supplied only from the bottom of a lake, required conditions for redox stratification are circum-neutral pH (6–7) and moderate Fe²⁺ influx (0.01–1 mol m^–2 yr^–1). At acidic pH, water became ferruginous throughout a lake due to inefficient photo-oxidation and low levels of Fe²⁺ near the surface. High input flux of Fe²⁺ results in abundance of carbonate formation, which is inconsistent with the finding of little carbonates in Gale’s lacustrine sediments. Considering the recent estimate of circum-neutral pH of early Gale lake (Fukushi et al., 2019), our results prefer groundwater upwelling as the major source of Fe²⁺ on the lakes. In addition, low CO₂ in the atmosphere would be prefered to avoid Fe-carbonate formation and to promote efficient Fe²⁺ photo-oxidation for redox stratification, which implies the need of abundance of other efficient greenhouse effect gases to warm the surface on early Mars.