Stratigraphic Partitioning of Evaporites Through the Clay-Sulfate Transition in Gale Crater, Mars

Abstract

Changing surface conditions on Mars during the Hesperian eon (∼3.6–3.0 billion years ago) produced widespread stratal sequences transitioning from clay to sulfate-bearing according to orbital data. Using data collected in situ at Gale crater by the Curiosity rover, we examined both the sedimentology and geochemistry of a 210-m-thick succession across a reference clay-sulfate transition. Four intervals (A–D) are defined based on both sedimentary structures and nodule distribution and composition, reflecting changes in depositional setting and diagenetic parameters. Large trough cross-beds, which become prevalent within interval A and dominate intervals B and C, are interpreted to indicate eolian processes, whereas dark-toned lens-shaped sandstone bodies in interval C point to recurring fluvial incursions. Planar bedding in uppermost interval D suggests damp or wet interdune conditions. ChemCam analyses show nodules are enriched in calcium and magnesium sulfates. The highest observed nodule densities coincides with strata interpreted to reflect damp or wet depositional environments (A, C, D), whereas their near-absence corresponds to the interval interpreted as the driest (B). We propose that this stratigraphic partitioning of the type, density, and sulfate-content of nodules indicates that they mostly formed during early diagenesis via capillary evaporation in the shallow subsurface, with only subordinate late-stage remobilization. The preserved strata record fluctuations in water availability and sediment supply, driven by repeated changes in climate, rather than by a single drying trend. These observations suggest new constraints on the fate of sulfur on early Mars and sulfate evaporite formation within sedimentary basins.