Sorting and Weathering Trends of Soil at Gale Crater, Mars: Implications for Regional Pedological Processes

Detailed soil characterization at Gale crater based on in situ observations has revealed compositional trends within soils, while the physical and chemical processes underlying the compositional trends remain to be evaluated. Here we use the grain-morphometrical and geochemical trends across the Wentworth-classes of 48 in situ soil targets at Gale crater to evaluate underlying pedological processes and potential chemical weathering signatures. The concentration of olivine minerals within the ∼250 to ∼500 μm size range indicates the prevalence of heavy mineral sorting in a granulometric sense in Gale soils that surpasses the possible effect of the cratering-induced mixing processes. The extent of olivine sorting in soils varies spatially and is influenced by the targets' aeolian setting. The finest portion of Gale soils (<125 μm) is likely a mixture of allochthonous sediment, globally sourced from atmospheric suspension, and autochthonous counterparts from the weathering of local rocks, while the coarser soils (>125 μm) are mostly sourced from local rocks, with possible inputs from both the unaltered parent rock of the Stimson formation and the bedrocks that have undergone diagenetic alteration. If applicable globally, this would reinforce prior inferences that even dust-mantled regions are geochemically heterogeneous owing to a substantial fraction of soils derived from underlying lithology. The low chemical weathering intensity and coupling of mobile elements in soils suggest localized, low pH and low water-to-rock ratio aqueous weathering conditions under predominantly cold and arid climates, which may occur either during post-pedogenetic alteration in soils or during the acidic alteration of source rocks.