Research Articles

Redox stratification of an ancient lake in Gale crater, Mars

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Science  02 Jun 2017:
Vol. 356, Issue 6341, eaah6849
DOI: 10.1126/science.aah6849
  • A hypothesized redox-stratified lake in Gale crater.

    Model of physical transport and geochemical processes occurring during deposition of the Murray formation. Fresh water and clastic materials are delivered by overland flow from fluvial systems; dissolved solutes enter the lake by groundwater seepage. Redox stratification results from differences in the mass balance of atmospheric oxidants and oxidizable cations, causing redox-sensitive mineral assemblages to vary as a function of lake water depth. Flow deceleration results in sediment fractionation into distinct sedimentological associations; coarser, denser clastic materials are deposited closer to shore (hematite-phyllosilicate facies), whereas finer, less dense clastics travel further into the lake (magnetite-silica facies). UV, ultraviolet.

  • Fig. 1 Stratigraphic column for the sedimentary rocks of Gale crater through sol 1300.

    Adapted from (10). Drill samples are denoted by black circles; the sandstone drill targets Windjana (WJ), Big Sky (BS), and Greenhorn (GH) are not discussed in this paper. The contact between the Bradbury and Mount Sharp groups is interfingering in nature. Vertical repetition of the HP and MS facies occurs in the stratigraphy, as indicated by the black and red column on the right. The MS facies at Hidden Valley (Bonanza King sample, APXS only, fig. S1) is at about the same stratigraphic level as the MJ and CH samples from the HP facies at Pahrump Hills. Khomas and Sperrgebiet are located near the top of the section, in the cross-stratified sandstones of the Stimson fm. Elevations are referenced to the zero-elevation datum on Mars defined in (86).

  • Fig. 2 Paleoweathering, provenance, and mineral addition in Gale crater mudstones.

    CIA (%) versus SiO2 (wt %) showing climate-induced variations in mudstone geochemistry. Dashed arrowhead lines are mixing vectors between the MS facies sample named Telegraph Peak (Tel. Pk., gray circles), SiO2, and CaSO4. The red arrowhead line shows the vectors generated by a two-stage mixing model where Tel. Pk. and SiO2 are mixed in 50:50 proportions, followed by the addition of CaSO4 to the mixed composition. Average martian crust (26) is plotted with a green diamond.

  • Fig. 3 Mn oxidation, metal scavenging, and dilution in the Murray fm.

    (A) Zn (ppm) versus MnO (wt %) showing a linear regression for the HP facies that results from Mn oxidation and scavenging of aqueous Zn by Mn oxides (the outlier samples Ricardo_Raster_1, Ricardo_DRT_1, Kleinberg, and Schwarzrand are excluded from the regression and labeled with arrowhead lines). (B) Ni (ppm) versus MnO (wt %) demonstrating dilution of MnO and trace metals in the MS facies by the addition of SiO2 and CaSO4, with data trending toward the origin. Green diamond is average Martian crust. The red square is target “Palmwag.”

  • Fig. 4 Magnesium and calcium sulfates in the Murray fm.

    MgO (wt %) versus SO3 (wt %), including HP facies analyses from diagenetic and high SO3 samples (red squares) and diagenetic targets in the Stimson fm. (Khomas and Sperrgebiet, yellow diamonds). HP facies and Stimson fm. targets exhibit linear correlations with slopes of 0.33 ± 0.05 and 0.31 ± 0.01, respectively. When low-MgO analyses of drill tailings from the target Buckskin are excluded, the MS facies also exhibits a positive correlation with slope 0.27 ± 0.11. The slopes of the regression lines are lower than 0.5, as expected for Mg sulfate, and consistent with a mixture of Mg- and Ca-sulfate. Average Gale soil is shown with a green triangle, the green diamond is average Martian crust, and Sheepbed member is plotted here with small blue circles for clarity.

  • Fig. 5 Mineralogical composition of mudstones in Gale crater.

    (A) Total abundance (wt %) of crystalline salts, amorphous material, crystalline clastic phases of igneous origin, and the combination of secondary phases and redox-sensitive minerals. (B) Detail of secondary and redox-sensitive–mineral abundances. Differences in the nature and abundance of these phases imply considerable variability in lake-water chemistry. Sheepbed member samples were collected at –4520-m elevation; the heights above that datum for Murray fm. samples are indicated by values between the bar charts.

  • Fig. 6 A hypothesized redox-stratified lake in Gale crater.

    Model of physical transport and geochemical processes occurring during deposition of the Murray fm. Fresh water and clastic materials are delivered by overland flow from fluvial systems; additional dissolved solutes enter the lake by groundwater seepage. Deceleration of the incoming river flow produces characteristic changes in bedforms and sedimentary textures. These are described at the bottom of the diagram with examples from Curiosity’s traverse where such bedforms have been observed (fig. S4, D to F). Flow deceleration also results in sediment fractionation; coarser, denser clastic materials are deposited closer to shore, whereas finer, less dense clastics travel further into the lake (also see fig. S5, A and B). Redox stratification results from differences in the mass balance of atmospheric oxidants (UV photons, O2) and oxidizable cations (Fe2+, Mn2+), causing redox-sensitive mineral assemblages to vary as a function of lake water depth.

Supplementary Materials

  • J. A. Hurowitz, J. P. Grotzinger, W. W. Fischer, S. M. McLennan, R. E. Milliken, N. Stein, A. R. Vasavada, D. F. Blake, E. Dehouck, J. L. Eigenbrode, A.G. Fairé n, J. Frydenvang, R. Gellert, J. A. Grant, S. Gupta, K. E. Herkenhoff, D. W. Ming, E. B. Rampe, M. E. Schmidt, K. L. Siebach, K. Stack-Morgan, D. Y. Sumner, R. C. Wiens

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    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Supplementary Text
    • Figs. S1 to S6
    • Table S1
    • Caption for Data File S1
    • References
    Data S1
    Geochemical Data from Gale Crater Mudstones and Soils

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