Evidence for Shallow, Oxic Waters in the Gale Crater Lake.

Results from the Curiosity rover have revealed that Gale crater once hosted a long-duration, habitable, stratified lake fed by rivers and streams. Multiple episodes of groundwater circulated through fractures within the lithified lakebed deposits (informally called the "Murray" formation), transporting and redepositing vein-filling materials. Recent observations by ChemCam show highly enriched deposits of Mn, Mg, Fe, and P. Manganese is a particularly redox sensitive element; its concentration in sediments suggest a strongly oxidizing environment at the time of its emplacement. Overall, the Murray has a low abundance of Mn. However, there is a substantial increase in Mn found in bedrock at elevations -4275 - -4340 m, often in dark-toned sandstones and nodules. Nearby light-toned materials display low-angle cross stratification that is suggestive of traction transport. Possible lenticular bedding of high Mn targets may indicate oscillatory flow of a near-shore environment. These chemical and sedimentological observations suggest that Mn deposition occurred in shallow, oxic waters. In a redox stratified lake on Earth, the upper section of the water column becomes oxidized due to interaction with the (oxygen-bearing) atmosphere, and both Mn and Fe oxides may precipitate. The water column may be oxygenated above any shallow shelf. In deeper waters, the water column contains less dissolved oxygen and can precipitate Fe oxides, but not Mn oxides. Oscillations in lake level also promote the redeposition and concentration of Mn oxides on to the shallowest near-shore surfaces and can lead to multiple layers of Mn oxides among heterolithic and spatially discontinuous beds of mudstone and sandstone materials, consistent with our observations on Mars. We hypothesize that the high abundance of Mn observed in the study area represents deposition of Mn oxides by precipitation in an oxidizing, shallow water near-shore environment, although a diagenetic origin cannot be ruled out. This suggests that the martian atmosphere contained more oxygen in the past than it does now. Regardless of emplacement model, the presence of high Mn abundances suggests strongly oxidizing conditions, either in lake or groundwaters, and provides additional support for a long-lived, habitable, redox stratified lake in Gale crater.