Metal Enrichment of Wave-Rippled Sediments on Ancient Mars

Abstract

International audienceThe Curiosity rover is ascending a sedimentary-rock mountain, Mount Sharp, testing hypotheses about how and why Mars' surface dried out. Within the past year, Curiosity has investigated an apparently Mount-Sharp-spanning feature - the Marker Band, which frequently forms a topographic bench. The Marker Band is distinctive in its lateral extent, stratigraphic confinement, and nontrivial thickness. The Marker Band also shows a distinct metal-rich geochemistry unlike any other materials previously analyzed by the rover, and its lower part exhibits wave ripples extending across hundreds of meters (possibly kilometers). Thus, the Marker Band is a marker of a change in the environment within Gale crater from drier conditions that formed underlying sulfates to wetter conditions that formed wave ripples (Gupta et al. this conference). Wave ripples do not persist above the rippled Marker Band, but further clues regarding the evolution of Mars' carbon cycle and atmosphere are obtained from carbonate in drilled samples immediately above the rippled Marker Band (Tutolo et al., this conference), which is strongly elevated in δ13C (Burtt et al., this conference). APXS data for drill fines from ~1 cm depth within the rippled layers show >40 wt% FeO, ~2 wt% Zn, and >1 wt% MnO (Thompson et al., LPSC 2023); metal enrichment is also seen in ChemCam data, which also show highly variable MnO. Tentative, but reasonable extrapolation of these data to parts of the Marker Band not visited by the rover suggests an excess Fe mass of 0.2 Gton. Potential processes capable of transporting the metals include transport by chloride-rich brines, or (via interaction with CO) as metal carbonyls. Although post-lithification mechanisms for metal emplacement have not been ruled out, a possible pre-lithification mechanism involves Mn and Fe deposition in a shallow lake in oxidizing conditions. In this scenario, Fe and Mn oxide nodules form and scavenge trace metals (e.g. Zn) by adsorption. We will conclude by discussing remaining open questions about the formation and metal enrichment of the rippled Marker Band. For example, possible sources of water for metal transport include (but are not limited to) compaction water, or alternatively groundwater derived from precipitation inside the crater rim