The Importance of Fe-redox Processes in Groundwater Chemistry on Earth

Introduction: For nearly a decade, the international program of Mars exploration has been guided by a remarkably useful framework that describes how Mars' environmental conditions have evolved in the context of time-dependent changes in the mineralogy of rock and regolith deposits at the Martian surface [1]. Built on the basis of observations by the OMEGA spectrometer onboard ESA's Mars Express Orbiter, this framework describes an early era of neutral-pH environments in which clay mineral formation was important. At around the Noachian-Hesperian boundary (~3.5-3.8 Ga), these clay forming environments give way to sulfate minerals formed under evaporative, low-pH conditions that are thought to reflect volcanic input of S-bearing gases to surface waters. Ongoing observations by Mars Express and the higher-resolution instruments onboard NASA's Mars Reconnaisance Orbiter have continued to refine and extend this paradigm [e.g., 2-4], but as is the case on Earth, the greatest insight into the evolution of surface environments comes from close-up examination of the sedimentary rock record. In-situ observations of sedi-mentary strata by NASA's Opportunity and Curiosity rovers enable evaluation of hypotheses developed from orbit, and reveal that iron-based redox processes have played a critical role in the mineralogic evolution of the Martian surface [e.g., 5]. Here, we describe some important aspects of redox processes in Fe-bearing waters, using the chemistry of basaltic groundwaters on Earth as a guide [6-8]. We then discuss examples from Meridiani Planum and Gale Crater on Mars that highlight the power of Fe-based redox processes to affect water chemistry and sedimentary mineralogy. On the basis of these examples , we suggest that the observed transition from neutral -pH, clay forming environments to those in which acid-sulfate minerals were important reflects a change in the availability of oxidants, rather than a change in the rate of volcanic outgassing. Dissolved Fe-chemistry on Earth: Examples from Iceland and Siberia: On Fig.1, we plot pH versus the activity of dissolved Fe, showing the speciation of dissolved Fe 3+ (blue fields) and the field of Fe(OH) 3 stability (tan field). Owing to the presence of significant O 2 in the terrestrial atmosphere, meteoric waters will evolve along a pathway described by the black arrow as they enter a basaltic aquifer and undergo neu-tralization via water-rock reaction. As a result of the insoluble nature of the Fe(OH) 3 formed under these