Elemental Composition and Isochemical Characteristics of the Clay-Sulfate Transition in Gale Crater, Mars: APXS Results From Mont Mercou to the Marker Band Valley

A major mission goal for the Mars Science Laboratory’s rover, Curiosity, is to investigate the transition from clay-bearing to hydrated-Mg-sulfate-bearing sedimentary strata hypothesized to record a transition from a wet to a dry paleoclimate. Alpha Particle X-ray Spectrometer (APXS) results from this region, named the Clay-Sulfate Transition (CST), indicate an overall ∼5% increase in Ca-sulfate, but Mg-sulfate enrichment is limited to diagenetic nodules. Sulfates in the CST change sharply at the contact with the overlying Mg-sulfate unit, which has ∼5% Ca-sulfate and ∼10% Mg-sulfate in the bedrock matrix. Despite this change in sulfate assemblage, and the transition from fluvial-lacustrine to drier aeolian sedimentary deposits, the bulk chemical composition of the aeolian sandstone (sulfate-free basis) effectively has the same altered basalt chemical fingerprint as the underlying fluvial-lacustrine mudstone. That is, the composition of rocks that record the transition from a wet to a dry paleoclimate is isochemical. It is remarkable that the aeolian sandstone has the same altered bulk chemical characteristics as the fluvial-lacustrine mudstone, notwithstanding very different inferred geochemical regimes. We propose a simplified model wherein older basaltic sediment was aqueously altered in a fluvial-lacustrine regime and reworked, likely during cycles of alteration, salt formation, and reworking. This process led to an averaging of the bulk chemical composition of the Mt. Sharp group sediment, resulting in the isochemical characteristics of the paleoenvironment change.