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Geochemistry Constrains Global Hydrology on Early Mars ∗ Edwin S

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Geochemistry Constrains Global Hydrology on Early Mars ∗ Edwin S Earth and Planetary Science Letters 524 (2019) 115718 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Geochemistry constrains global hydrology on Early Mars ∗ Edwin S. Kite , Mohit Melwani Daswani 1 University of Chicago, Chicago, IL, USA a r t i c l e i n f o a b s t r a c t Article history: Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks Received 28 December 2018 that formed during Mars’ Hesperian period are sulfate-rich rocks, explored by the Opportunity rover from Received in revised form 17 July 2019 2004–2012 and soon to be investigated by the Curiosity rover at Gale crater. A leading hypothesis for the Accepted 18 July 2019 origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, Available online xxxx as part of a global circulation of groundwater. Global groundwater circulation would imply sustained Editor: W.B. McKinnon warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water Keywords: initially in equilibrium with Mars’ CO2 atmosphere implies subsurface formation of carbonate. We find Mars that the CO2 sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich geochemistry rocks on Mars is 30–5000 bars if the Opportunity data are representative of Hesperian sulfate-rich rocks, hydrology which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This planetary science disfavors the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO2 sequestration is 0.3–400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. Curiosity will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change. © 2019 Elsevier B.V. All rights reserved. 1. Introduction global deep-groundwater flow could piston atmospheric CO2 into the deep subsurface and fix it as deep carbonates. Uncertainty Liquid water flowed over the surface of Mars billions of years in the size of the deep carbonate reservoir is a major uncer- ago, and aqueous minerals also formed kilometers below the tainty in Mars’ CO2 evolution (e.g. Jakosky and Edwards, 2018; surface (e.g., McLennan and Grotzinger, 2008; Ehlmann et al., Jakosky, 2019). 2011). However, the extent of hydrologic coupling between the Central to the question of vertical isolation versus vertical inte- surface/near-surface and deep subsurface on Early Mars is un- gration of Early Mars’ hydrosphere is the archive of ancient hydrol- known. In one view, continuous permafrost isolated the deep ogy contained within mostly Hesperian-aged (3.6–3.2 Ga, based on hydrosphere from the surface, with only local and transient ex- crater chronology) sulfate-bearing sedimentary rocks (Malin and ceptions (e.g., Fastook and Head, 2015; Fairén, 2010;Schwenzer Edgett, 2000;Bibring et al., 2007). This rock type was ground- et al., 2012). In another hypothesis, surface and deep-subsurface truthed by the Mars Exploration Rover Opportunity investigation waters repeatedly swapped places as part of a prolonged global of the Burns Formation at Meridiani (e.g. Squyres et al., 2006). At 7–8 groundwater cycle – vertical integration enabled by >10 yr Meridiani, texture and mineralogy record multiple stages of dia- of annual-averaged surface temperatures above the freezing point genesis – involving acid and oxidizing near-surface groundwater (Andrews-Hanna et al., 2010). The existence and extent of global (Fig. 1) (McLennan and Grotzinger, 2008). Burns Formation sand- deep-groundwater cycling are key unknowns for Early Mars cli- stones are ∼40 wt% {Mg,Fe,Ca}SO4. The cations for the sulfates mate and global hydrology (Wordsworth, 2016), water loss (Usui were initially interpreted to be derived from slow wicking-to-the- et al., 2015), and habitability (Onstott et al., 2019). Moreover, surface and evaporation of deep-sourced saline groundwater, as part of a global groundwater circulation that could also explain Corresponding author. the low elevations of the sulfate-bearing rocks (Andrews-Hanna et * = E-mail address: [email protected] (E.S. Kite). al., 2010). The rocks must have been altered in pH 2–4 waters 1 Now at: Jet Propulsion Laboratory, Caltech, Pasadena, CA, USA. in order to explain the detection of jarosite (Tosca et al., 2008). https://doi.org/10.1016/j.epsl.2019.115718 0012-821X/© 2019 Elsevier B.V. All rights reserved. 2 E.S. Kite, M. Melwani Daswani / Earth and Planetary Science Letters 524 (2019) 115718 Fig. 1. Evidence for shallow-diagenetic alteration of the Burns Formation, Meridiani, Mars. Image is ∼3 cm across. Laminae separated by <1 mm have different degrees of cementation. Halo girdles Fe2O3 concretion at right. Subframe of 1M130671710EFF0454P2953M2M1, Opportunity rover Microscopic Imager, sol 28. (After Grotzinger et al., 2005). 2+ The jarosite presumably formed near the surface. There, Fe → 2. Global groundwater circulation implies carbonate 3+ Fe oxidation (by UV photons, or by atmospheric O2), or vol- sequestration if recharge waters equilibrated with the canic H2SO4, could drive acidity (Tosca et al., 2008;Baldridge et atmosphere al., 2009;Hurowitz et al., 2010;Nie et al., 2017). Opportunity’s mapping of sulfate-bearing sedimentary rocks that experienced Here, we test the global deep-groundwater hypothesis for for- mation of the Hesperian sulfates. We do this by tracing the hy- acid and oxidizing alteration (driving the formation of jarosite and pothesis’ implications for carbon sequestration and thus global at- hematite) has been extended by orbital surveys. Sulfate-bearing 6 3 mospheric and climate evolution (Fig. 2). Our starting point is that sediments (>10 km in total) extend across much of Merid- ◦ ◦ ◦ CO -charged water is out-of-equilibrium with the basaltic crust iani (5 S–10 N); Valles Marineris and nearby chaos (0–15 S); 2 ◦ of Mars. As a result, initially atmospherically-equilibrated water and Gale crater (5 S), among other sites (Bibring et al., 2007; percolating along the long flow paths entailed by the global deep- Hynek and Phillips, 2008;Grotzinger and Milliken, 2012). Strati- groundwater hypothesis (>103 km; Andrews-Hanna et al., 2010) graphic thicknesses of >2km suggest millions of years of de- should deposit C as carbonate (e.g. Griffith and Shock, 1995; Niles position. Thus, if groundwater circulation was the engine of sul- et al., 2013; Tomkinson et al., 2013; Melwani Daswani et al., 2016). fate formation, then the sulfate-bearing sedimentary rocks are ev- Carbonate formation is inevitable for very long and deep flow idence for millions of years of warm Earthlike conditions on Early paths through basalt, and mechanisms that could inhibit carbon- Mars. ate formation in the surface/near-surface (e.g. Bullock and Moore, However, the Hesperian global deep-groundwater circulation 2007) do not apply here. (Even for relatively short and shallow hypothesis is disputed. For example, the sulfates themselves have groundwater flow-paths, carbonate precipitation from groundwater been proposed to form at 220–270 K by acid-rock reactions (Niles has occurred on Mars: e.g. van Berk et al., 2012; Ruff et al., 2014.) and Michalski, 2009;Niles et al., 2017). The textural evidence Once emplaced deep within basalt, Hesperian carbonates should for shallow flow of groundwater (Fig. 1) might be explained persist. That is because (unlike Earth) post-3.6 Ga Mars lacked a by a shallow, local source of water, such as seasonal meltwa- mechanism to heat carbonates to drive off CO2, such as plate tec- ter (Kite et al., 2013a). The survival (within sedimentary rocks) tonics or global volcanism (Ogawa and Manga, 2007), except locally of fragile minerals that would have been dissolved by high wa- (Glotch and Rogers, 2013). Therefore, deep groundwater circulation ter/rock ratio alteration at depth argues against persistent cir- between the atmosphere/surface and the basaltic crust of Mars im- culation (e.g. Dehouck et al., 2017; Phillips-Lander et al., 2019). plies one-way geologic sequestration of CO2 (Niles et al., 2013) The low variation of K/Th (5300 ± 220) on Mars’ surface at (Fig. 2). 300 km scales is an independent argument against global ground- By estimating the magnitude (Cseq) of the implied Hesperian CO sequestration, we can test the global groundwater hypothesis. water circulation (Taylor et al., 2006). Potassium is much more 2 Possible outcomes of our test are: mobile than thorium, and so extensive aqueous alteration of the crust would be expected to enhance K at evaporation zones. But – Cseq > 10 bar: If Cseq exceeds Mars’ total estimated outgassed this is not observed (Taylor et al., 2006). Therefore, the exis- CO2 inventory (1–10 bar; Stanley et al., 2011; Lammer et al., tence or otherwise of a global and deep groundwater circuit on 2013), then the deep-global groundwater hypothesis and our Early Mars remains an open question. Nevertheless, there is abun- understanding of Mars’ composition are not consistent with dant evidence for groundwater movement on Early Mars: miner- one another. alized fracture-fills including Ca-sulfate veins are widespread (e.g., – Cseq > 1bar: If Cseq exceeds the CO2 in the atmosphere at 3.6 Okubo and McEwen, 2007;Yen et al., 2017). Moreover, de-watering Ga plus the amount of CO2 outgassed 3.6–3.2 Ga (a subset drove sediment deformation during early diagenesis at some sites of Mars’ total estimated CO2 inventory, because of pre-3.6 Ga (Rubin et al., 2017), and water was released from the subsur- CO2 loss; Kite et al., 2014), then the deep-global groundwater face to form some chaos terrains and associated outflow chan- hypothesis predicts a very thin atmosphere.
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