Earth and Planetary Science Letters 393 (2014) 14–25 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Efflorescence as a source of hydrated sulfate minerals in valley settings on Mars Anna Szynkiewicz a,b,∗ , David M. Borrok c,b , David T. Vaniman d a Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, USA b Geological Sciences, University of Texas at El Paso, 500 W University Ave., El Paso, TX 79968, USA c University of Louisiana at Lafayette, 611 McKinley Street, Lafayette, LA 70504, USA d Planetary Science Institute, 1700 E Fort Lowell, Tucson, AZ 85719, USA article info abstract Article history: Received 27 April 2013 Received in revised form 10 December 2013 Accepted 14 February 2014 Available online xxxx Editor: C. Sotin Keywords: sulfate efflorescence water arid climate Mars A distinctive sulfur cycle dominates many geological processes on Mars and hydrated sulfate minerals are found in numerous topographic settings with widespread occurrences on the Martian surface. However, many of the key processes controlling the hydrological transport of sulfur, including sulfur sources, climate and the depositional history that led to precipitation of these minerals, remain unclear. In this paper, we use a model for the formation of sulfate efflorescent salts (Mg–Ca–Na sulfates) in the Rio Puerco watershed of New Mexico, a terrestrial analog site from the semiarid Southwest U.S., to assess the origin and environmental conditions that may have controlled deposition of hydrated sulfates in Valles Marineris on Mars. Our terrestrial geochemical results (δ 34 S of −36.0 to +11.1❤) show that an ephemeral arid hydrological cycle that mobilizes sulfur present in the bedrock as sulfides, sulfate minerals, and dry/wet atmospheric deposition can lead to widespread surface accumulations of hydrated sulfate efflorescences. Repeating cycles of salt dissolution and reprecipitation appear to be major processes that migrate sulfate efflorescences to sites of surface deposition and ultimately increase the aqueous SO 2− 4 flux along the watershed (average 41,273 metric tons/yr). We suggest that similar shallow processes may explain the occurrence of hydrated sulfates detected on the scarps and valley floors of Valles Marineris on Mars. Our estimates of salt mass and distribution are in accord with studies that suggest a rather short-lived process of sulfate formation (minimum rough estimate ∼100 to 1000 years) and restriction by prevailing arid conditions on Mars.  2014 Elsevier B.V. All rights reserved. 1. Introduction One of the prominent geochemical features on Mars related to past water activity is the widespread occurrence of sulfate- bearing minerals on its surface. In Valles Marineris (Fig. 1), the monohydrated sulfate mineral kieserite (MgSO 4 · H 2 O), the fully- hydrated Ca sulfate mineral gypsum (CaSO 4 · 2H 2 O), and poly- hydrated sulfate minerals of unknown composition (likely Mg-, Ca-, Na-, or Fe-SO 4 · nH 2 O) have been identified by the OMEGA and CRISM orbital spectrometers (Gendrin et al., 2005; Murchie et al., 2009a). These sulfates usually occur along scarps with varied erosion, in interior layered deposits, in bright-toned mounds, and in topographic depressions within valley floor materials (Mangold et al., 2008; Murchie et al., 2009a, 2009b; Bishop et al., 2009; * Corresponding author at: University of Tennessee, Earth and Planetary Sciences, 1412 Circle Drive, Knoxville, TN 37996, USA. E-mail address: aszynkie@utk.edu (A. Szynkiewicz). Lichtenberg et al., 2009; Fueten et al., 2011). The origin of sul- fate minerals is poorly understood and scientists have suggested a variety explanations such as hydrothermal circulation, oxida- tion of sulfide minerals, and/or evaporation of groundwater fed by regional-scale recharge (Mangold et al., 2008; Murchie et al., 2009b; Lichtenberg et al., 2009; Dehouck et al., 2012). Compounding these uncertainties in the Martian sulfur cycle is the fact that it is unknown how long water may have persisted on the Martian surface. The equatorial valley network (Hynek and Phillips, 2003; Hoke et al., 2011) and widespread occurrences of sulfate minerals (Gendrin et al., 2005; Murchie et al., 2009a) sug- gest that an early hydrological cycle likely sustained precipitation (rain and/or snow) and might have lasted in near-surface environ- ments up to thousands of years (Hoke et al., 2011). In this paper, we propose that the origin of hydrated sulfates in Valles Marineris can be satisfactorily explained by intermittent shallow water circu- lation and the formation of sulfate efflorescent salts during short, ephemeral, episodes of near-surface water activity. Consequently, this process does not require deeper groundwater flow on Mars http://dx.doi.org/10.1016/j.epsl.2014.02.035 0012-821X/ 2014 Elsevier B.V. All rights reserved.