Precipitated minerals, including salts, are primary tracers of atmospheric conditions and water chemistry in lake basins. Ongoing in situ exploration by the Curiosity rover of Hesperian (around 3.3–3.7 Gyr old) sedimentary rocks within Gale crater on Mars has revealed clay-bearing fluvio-lacustrine deposits with sporadic occurrences of sulfate minerals, primarily as late-stage diagenetic veins and concretions. Here we report bulk enrichments, disseminated in the bedrock, of 30–50 wt% calcium sulfate intermittently over about 150 m of stratigraphy, and of 26–36 wt% hydrated magnesium sulfate within a thinner section of strata. We use geochemical analysis, primarily from the ChemCam laser-induced breakdown spectrometer, combined with results from other rover instruments, to characterize the enrichments and their lithology. The deposits are consistent with early diagenetic, pre-compaction salt precipitation from brines concentrated by evaporation, including magnesium sulfate-rich brines from extreme evaporative concentration. This saline interval represents a substantial hydrological perturbation of the lake basin, which may reflect variations in Mars’ obliquity and orbital parameters. Our findings support stepwise changes in Martian climate during the Hesperian, leading to more arid and sulfate-dominated environments as previously inferred from orbital observations.
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All in situ and orbital data used in this study are available in the NASA Planetary Data System (https://pds-geosciences.wustl.edu). Other supplementary data that support the findings of this study are available from the corresponding author on reasonable request.
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Thanks to the MSL operations team for their dedication in generating this dataset, and to the LANL team for collection of data to support sulfur calibration. Thanks to J. Grotzinger, C. Fedo, K. Siebach, L. Edgar and other members of the informal MSL Sed-Strat group for discussions that helped improve this work. The authors also thank S. Clegg and other members of the ChemCam team for discussions on sulfur signal calibration. W.R. and B.L.E. were funded by a MSL Participating Scientist grant NNN12AA01C. The work of G.D. was supported by the CNES through the ChemCam Program. This work and the MSL project are supported by the NASA Mars Exploration Program.
The authors declare no competing interests.
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Geophysical Research Letters (2019)