Redox stratification of an ancient lake in Gale crater, Mars

INTRODUCTION The primary goal of NASA’s Curiosity rover mission is to explore and quantitatively assess a local region on Mars’ surface as a potential habitat for past or present life. A necessary component of that assessment involves an investigation of the surface chemical conditions and paleoclimate of ancient Mars. Gale crater was selected as the landing site for Curiosity; it hosts a ~5-km-tall mountain of layered sedimentary rock. The rocks of Mount Sharp preserve a long-duration record of martian environmental conditions. Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment that was sustained for tens of thousands to tens of millions of years by rivers draining into the crater. RATIONALE We seek to constrain the chemical environment within the lake in Gale crater, as well as short- and long-term climate variations in and around Gale crater. We focus on fine-grained sedimentary rocks that carry information about sediment provenance, the environment of deposition, the conversion of sediment to rock during burial (i.e., lithification), and the chemical conditions of later modification (i.e., diagenesis). These were investigated during the first 1300 martian solar days (sols) of rover operations in Gale crater using bulk geochemical and mineralogical analysis techniques, combined with high-resolution color imagery at a variety of scales. RESULTS Two mudstone units have been recognized, both deposited in lakes: the Sheepbed member of the Yellowknife Bay formation, an older set of strata defining the base of the stratigraphic section; and the Murray formation, of relatively younger age and positioned higher in the stratigraphic section. The chemical index of alteration (CIA) paleoclimate proxy increases by up to ~10 to 20 CIA units (expressed in %) from the Sheepbed member to the Murray formation. On the basis of mineralogy, geochemistry, textural properties, and stratigraphic relationships, the Murray formation can be subdivided into two sedimentary associations, or facies: the hematite-phyllosilicate (HP) facies and the magnetite-silica (MS) facies. The HP facies is characterized by abundant Fe 3+ oxides accompanied by phyllosilicates, as well as indications of Mn oxidation and trace metal concentration. These properties are consistent with deposition in an oxidizing environment. The MS facies is recognized by a near-complete absence of pure Fe 3+ minerals, and high concentrations of silica accompanied by magnetite, consistent with deposition in an anoxic environment. Both facies were affected by a saline overprint after burial and lithification. CONCLUSION The observed variations in CIA are consistent with modest short-term fluctuations in the ancient climate between cold, dry conditions and relatively warmer, wetter conditions. These changes occurred during the deposition of lake-bed mudstones in an environment that was conducive to the presence of a long-lived lake in Gale crater. We propose that the distinct properties of the two Murray facies were developed as a result of (i) fractionation of river-borne detritus into coarser, denser materials in shallow water close to shore and finer, lower density materials offshore in deeper water as a result of deceleration of river flow as it entered the lake; and (ii) redox stratification of the lake water body, caused by depth-dependent variations in the concentration of atmospheric oxidants and dissolved, groundwater-derived solutes, resulting in oxidizing conditions in shallow water and anoxia in deeper water. The addition of saline minerals during a later phase of brine migration through the section records longer-term changes in martian climate at Gale crater, perhaps driven by global atmospheric escape processes. The recognition of redox stratification in the lake in Gale crater adds new detail to our understanding of ancient martian aquatic environments. Previously reported detections of organic carbon compounds, nitrogen, phosphate minerals, and Fe and S minerals in a variety of redox states, combined with the evidence presented here for relatively stable climate conditions and gradients in fluid oxidation state, provide compelling evidence that all of the physical, chemical, and energetic conditions necessary to establish a habitable environment were present on Mars between ~3.8 billion and 3.1 billion years ago.