Amapari Marker Band Metal-Enrichments: Potential Mechanisms and Implications for Surface and Subsurface Water and Weathering in Gale crater

NASA’s Curiosity rover is exploring a 5 km tall sedimentary mound that is hypothesized to record thetransition from a warm and wet (phyllosilicate-rich) to a cold and drier (sulfate-rich) Mars. Evidence ofmagnesium sulfate-bearing rock has shown that Curiosity has crossed through this phyllosilicate-sulfate transition. Recently, Curiosity arrived at the Amapari Marker Band, a darker, indurated unit that can betraced laterally for tens of kilometers in orbiter images. Here, Curiosity found evidence for a very broadlake, and bedforms interpreted as wave-ripple laminated sedimentary rock that likely was deposited inshallow water in the explored location, before becoming a deeper lake. These rocks are enriched in Fe,Mn, and Zn which has major implications for groundwater paleohydrology in Gale crater. Threeformation hypotheses are considered: concretion formation during early diagenetic alteration of shallowlake sediments, laterization or leaching of the sediments, and addition of Fe, Mn, and Zn by a mildlyacidic and reducing groundwater interacting with a redox and/or pH front in a stratified lake. Thepreferred interpretation of the metal enrichments within the Amapari Marker band sedimentary rocks isthat they formed in a shallow water environment at a redox and/or pH front within the ripple unit, whichdrove precipitation and concentration of metals. If the enrichments are due to groundwater alteration,these processes could link subsurface and surface environments. Water and the presence of highamounts of redox sensitive elements and other metals are favorable indicators for habitability.