Redox stratification of an ancient lake in Gale crater, Mars

Abstract

In 2012, NASA's Curiosity rover landed on Mars to assess its potential as a habitat for past life and investigate the paleoclimate record preserved by sedimentary rocks inside the ~150-kilometer-diameter Gale impact crater. Geological reconstructions from Curiosity rover data have revealed an ancient, habitable lake environment fed by rivers draining into the crater. We synthesize geochemical and mineralogical data from lake-bed mudstones collected during the first 1300 martian solar days of rover operations in Gale. We present evidence for lake redox stratification, established by depth-dependent variations in atmospheric oxidant and dissolved-solute concentrations. Paleoclimate proxy data indicate that a transition from colder to warmer climate conditions is preserved in the stratigraphy. Finally, a late phase of geochemical modification by saline fluids is recognized.

Large sulfur isotope fractionations in Martian sediments at Gale crater

“…These mineral trends may indicate changes in the pH and redox characteristics of lake waters, as well as some differences in sediment source (e.g., a more silicic sediment source for BK), through time 54,55 . Another important feature of these samples is that the amounts of key trace elements, such as Zn and Ni, decrease with increase in elevation (from CH to BK).…”

Mineral paragenesis on Mars: The roles of reactive surface area and diffusion

“…We modeled how pore spaces in the sedimentary column become occupied by authigenic minerals. Consequently, the whole set of sedimentary processes resulting from our models are better understood under the global term of “early diagenetic processes.” Although the term “early diagenesis” and the chemical sedimentary processes associated with it have only recently been applied to sedimentary deposits Mars, some fresh papers do describe the sedimentary context at Gale crater in terms of a lake where diagenetic process took place at the water‐sediment interface, inducing stratification (meaning chemical stratification) of redox gradients and authigenic phases by sequential precipitation [ Hurowitz et al ., 2017; Rampe et al ., 2017; Frydenvang et al ., 2017]. …”

“…Indeed, subsequent interactions between water and the central mound could have remobilized/reactivated older deposits. Upcoming data from the Curiosity rover will be useful to test our model results, as the rover exploration has already provided direct evidence for different lakes coming and going over Gale crater during up to tens of millions of years [ Grotzinger et al ., 2015; Hurowitz et al ., 2017]. …”

“…In our models the system is open to the atmosphere, and therefore, the loss of water through evaporation is regarded as the main factor inducing supersaturation (as multiple indicators for evaporation have been reported on Mars) [see e.g., Squyres et al ., 2004; Sears and Moore , 2005; Tosca et al ., 2005; Osterloo et al ., 2008; Fairén et al ., 2009; Arvidson et al ., 2014; Toner et al ., 2015; Schwenzer et al ., 2016; Hurowitz et al ., 2017]; and because the system is likewise open at the water‐sediment interface, reactive transport was also considered (Figure 1). In addition, we considered that dissolved volcanic gases (H 2 O, CO 2 , Cl 2 , and SO 2 ) [e.g., Halevy and Head , 2014] are the main anion source to the aqueous solutions, as well as drivers of redox processes and alkalinity, modifying the dissolution kinetics of primary minerals, which, in turn, are largely dependent of pH, temperature, and the reactive surface.…”

“…And second, because equilibrium models do not take into account the role of reactive surface area in determining the differences on crystallization pathways from small‐to‐large sediment particle sizes (kinetic models are needed to accomplish this objective). To help solve this problem, we have applied a reactive‐transport approach to model the formation of mineral sequences known to exist on Mars, as described from lander and orbiter data [e.g., Hamilton and Christensen , 1997; Bandfield et al ., 2000; Mustard et al ., 2008; McSween et al ., 2009; Ehlmann et al ., 2011; Vaniman et al ., 2014; Rampe et al ., 2017; Hurowitz et al ., 2017], considering open system conditions both at the atmosphere‐water and water‐rock interfaces and implementing a kinetic approach for the dissolution and precipitation of solid phases. We present a suite of models representing aqueous environments in systems under an early Mars atmosphere, to analyze the role of (i) the reactive surface area of primary minerals; (ii) the reactive transport processes through the basalt interface; and (iii) the cation adsorption by the surface of clays.…”