Searching for Redox Stratification in the Jezero Crater Delta

In Gale crater, elevated Mn deposits have been investigated, including veins at Kimberley and Hutton, nodules at Groken, and sandstones at Sutton Island. Mn requires more strongly oxidizing conditions to precipitate from solution than Fe. In a redox stratified lake (Hurowitz et al., 2017), Mn(III, IV) and Fe(II, III) insoluble oxides form at the lake surface (oxidizing), and only Mn(II, aq) and Fe(II, aq) are stable in the deepest parts of the lake (reducing). Fe and especially Mn, will tend to become concentrated along shorelines and Mn will redissolve in the reduced zone while Fe oxide can remain stable depending on depth. On Earth, the redox can be controlled by organics, by input of reducing groundwater, and atmospheric O2. On Mars, the oxidant is likely O2 or UV light and the reductant on Mars may be reducing groundwater or H2 (e.g., from serpentinization or magnetite formation; Wordsworth et al., 2021). While it is difficult to interpret these features within Gale because there are no obvious deltaic or shoreline features, traction cross-stratification and possible wave ripple deposits suggest that the Groken and Sutton Island materials are potentially shallow water deposits. Thin beds of Fe-Mn nodules can be found along shorelines of lakes on Earth, which form along the sharp redox boundary just below the lakebed surface. The thin beds of nodules at Groken could represent a similar depositional location. Jezero crater contains a delta, and it should be more straightforward to find and investigate redox stratification between the deep and shallow deposits. If Jezero's lake was redox stratified, then the Fe-Mn oxidation would indicate this, and Fe-Mn should be elevated in the topsets compared to the bottomsets or foresets. If elevated Fe-Mn is not found in the topsets it could indicate Jezero is not redox stratified. However, most Mn deposits are concentrated in nodules, and these can be small and spatially heterogeneous (e.g., Groken). We propose searching for Mn enrichments for sampling as they would provide constraints on oxidation on Mars in the Noachian and are likely to contain organics based on Earth analogs. Shorelines are important habitats on Earth and Mn is a signature of a redox gradient, which is important for life, thus it is important to find Fe-Mn nodules in Jezero topsets to collect a sample for Mars Sample Return.