Oxygen in Mars' Gale Crater. Is this evidence for life?

In a recent published Science article by Hurowitz et al (2017), Redox stratification of an ancient lake in Gale crater, Mars, it is suggested that there was atmospheric oxygen around in enough quantities to cause the oxidation of transition metals like iron and manganese, about 3.5 billion years ago. I was left with many questions regarding the questions, so I left the following commentary in the eLetters section:

The authors write: “The recognition of a stable redox-stratified water body adds important detail to our understanding of the potential for microbial chemoautotrophy within the ~3.8- to 3.1-billion-year-old Gale crater lake system.”

I am just wondering on what assumptions photoautotrophy is excluded as a possibility. It is well known that, at the very least, anoxygenic photosynthesis was ongoing 3.8 billion years ago (Nisbet & Fowler, 2014). While the exact date for the origin of oxygenic photosynthesis on earth is debated, there are many reports for the presence of biogenic oxygen hundreds of millions of years before the Great Oxidation Event (Lyons et al., 2014). So it is not unreasonable to think that some forms of biological water oxidation to oxygen already existed before 3.0 billion years ago.

The authors write: “The model depends on the depth of penetration of ultraviolet (UV) light and low levels of photochemically generated atmospheric O₂ into the water column to establish a depth-dependent boundary between oxidized and anoxic zones”.

How much oxygen can be produced photochemically on Mars? I am not a geochemist, but from discussions regarding the oxygenation of Earth, I understand that it was an almost negligible contribution. With the levels of O₂ being only a maximum of 10^-8 of the current level by photochemistry alone (Kasting & Walker, 1981). That is on Earth, Mars is farther from the Sun and the young was fainter back then.

Unfortunately, I do not know the literature on Mars early atmosphere, but is photochemical produced O₂ really a valid alternative?

What concentrations of oxygen do you need to account for the level of iron and manganese oxides that you see?

Based on this work, can you put a minimum constrain on the amount of oxygen present in the atmosphere of Mars at this time?

Kasting, J. F., & Walker, J. C. G. (1981). Limits on oxygen concentration in the prebiological atmosphere and the rate of abiotic fixation of nitrogen. Journal of Geophysical Research-Oceans and Atmospheres, 86(Nc2), 1147-1158. doi:DOI 10.1029/JC086iC02p01147

Lyons, T. W., Reinhard, C. T., & Planavsky, N. J. (2014). The rise of oxygen in earth's early ocean and atmosphere. Nature, 506(7488), 307-315. doi:10.1038/nature13068

Nisbet, E. G., & Fowler, C. F. R. (2014). The early history of life. In K. D. M. & W. H. Schlesinger (Eds.), Treatise on geochemistry (2nd ed., Vol. 10, pp. 1-42). Amsterdam: Elsevier Science.

Gale Crater, Mars. Was there ever life thriving in here?