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Modelling the Liquid-Water Vein System Within Polar Ice Sheets As a Potential Microbial Habitat

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Modelling the Liquid-Water Vein System Within Polar Ice Sheets As a Potential Microbial Habitat Earth and Planetary Science Letters 333–334 (2012) 238–249 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Modelling the liquid-water vein system within polar ice sheets as a potential microbial habitat K.G. Srikanta Dani a,1, Heidy M. Mader a,n, Eric W. Wolff b, Jemma L. Wadham c a School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK b British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK c School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK article info abstract Article history: Based on the fundamental and distinctive physical properties of polycrystalline ice Ih, the chemical and Received 12 October 2011 temperature profiles within the polar ice sheets, and the observed selective partitioning of bacteria into Received in revised form liquid water filled veins in the ice, we consider the possibility that microbial life could survive and be 21 March 2012 sustained within glacial systems. Here, we present a set of modelled vertical profiles of vein diameter, Accepted 6 April 2012 vein chemical concentration, and vein water volume variability across a range of polar ice sheets using Editor: G. Henderson Available online 22 May 2012 their ice core chemical profiles. A sensitivity analysis of VeinsInIce1.0, the numerical model used in this study shows that the ice grain size and the local borehole temperature are the most significant factors Keywords: that influence the intergranular liquid vein size and the amount of freeze-concentrated impurities polar ice cores partitioned into the veins respectively. Model results estimate the concentration and characteristics of polycrystalline ice the chemical broth in the veins to be a potential extremophilic microbial medium. The vein sizes are psychrophilic bacterial metabolism vein system estimated to vary between 0.3 mmto8mm across the vertical length of many polar ice sheets and they temperature depression may contain up to 2 mL of liquid water per litre of solid ice. The results suggest that these veins in polar ice sheets could accommodate populations of psychrophilic and hyperacidophilic ultra-small bacteria and in some regions even support the habitation of unicellular eukaryotes. This highlights the importance of understanding the potential impact of englacial microbial metabolism on polar ice core chemical profiles and provides a model for similar extreme habitats elsewhere in the universe. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Fig. 1(a)–(e) shows the geometry of the intergranular vein system found in natural polycrystalline ice Ih (hexagonal ice) that The presence of bacteria and archaea in glaciers and ice sheets makes up the bulk of temperate glaciers and polar ice sheets on has been reported by numerous researchers (Abyzov, 1993; Karl Earth (Nye and Frank, 1973). The liquid water phase exists in solid et al., 1999; Siegert et al., 2001; Foght et al., 2004; Gaidos et al., ice because the ice lattice tends to reject foreign ions (impurities) 2004; Abyzov et al., 2005; Kastovska et al., 2007; Hodson et al., as water is frozen. In other words, the solubility of compounds in 2008). For many years it was thought that the extremely low the individual ice grains is generally very low and the expelled temperatures within natural ice deposits on Earth and the lack of foreign ions from the growing grains remain in the liquid water liquid water, light and nutrients in them presented too harsh an and become more concentrated with decreasing temperature. environment to sustain any form of life. More recently however it Ultimately, an ice polycrystal is formed which contains an inter- has been proposed that the intergranular aqueous vein system connected network of highly concentrated water-filled veins found in ice could provide a habitat capable of sustaining around ice grains and films on grain boundaries. microbial life (Price, 2000; Mader et al., 2006; Rohde and Price, Experiments have demonstrated that microorganisms partition 2007) on Earth and that similar habitats might exist on other icy preferentially to the veins during grain growth (Fig. 1(f)–(i), Mader heavenly bodies such as Mars and some icy moons of Jupiter and et al., 2006; Amato et al., 2009). Even at extreme sub-zero tempera- Saturn (Price, 2002, 2007; Parkinson et al., 2008; Newman et al., tures in the veins, bacteria can find both liquid water and much 2009). higher concentrations of nutrients (impurities) than the average impurity concentration estimated in bulk ice. Moreover, some psychrophilic microbes show active growth at temperatures as low n Corresponding author. Tel.: þ44 0117 9545445. as À12 1C(Breezee et al., 2004). Some are shown to be metabolically E-mail address: [email protected] (H.M. Mader). 1 Present address: Department of Biological Sciences, Macquarie University, active down to À20 1C(Gilichinsky, 2002; Price and Sowers, 2004) North Ryde, Sydney, NSW 2109, Australia. and À39 1C(Bakermans, 2008). However, the mechanisms that 0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.04.009 K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 239 Fig. 1. Ice vein geometry and partitioning of microbes into liquid veins in ice (for a detailed figure caption with explained notations, please see supplementary material 1. (a) SEM of a typical ice vein cross section at a triple junction (crystal edge). (b) Diagram of a vein cross-section. (c) Semi-regular truncated octahedron (represents an ice grain) and a sketch of vein network surrounding it (after Price, 2000). (d and e) Transmitted white lightphotographs of the vein system in laboratory-grown ice (from Mader, 1992a); veins are 100 mm across. It is a curious point to note that tetrahedral geometry of nodes complements the inherent structural tendency towards stability of water molecules that cluster to form hydrogen bonds in a tetrahedral geometry. (f) Light and (g) fluorescence micrographs showing 1.9 mm fluorescent beads (size equivalent of bacteria) lined up along an ice vein running into a node (from Mader et al., 2006). (h) Bright field and (i) 510–560 nm epifluorescent micrographs showing yeast cells (44 mm) within an ice vein triple junction of size 410 mm (reproduced with permission from Amato et al., 2009). The scale on (f) applies to all the 4 photographs. enable maintenance of microbial intracellular fluidity at subzero A sensitivity analysis of the numerical model (VeinsInIce1.0) temperatures remain uncertain (Russel, 2006). There are very limited allows us to draw inferences about the relative impact of the experimental and/or modelled data on the interaction of ice bio- parameters tested in the model on the features of the habitat. For chemistry and the microbial activities at extremely low temperatures ice veins to develop in polycrystalline ice, the deposited snow has (below À20 1C) within glacier ice (Rivkina et al., 2000; Junge et al., to be preserved and compressed over several hundred years, 2004; Bakermans, 2008). which is not observed in shallow perennial snowpacks (up to Inorganic impurities from polar ice cores have been studied in 50 m deep) and some temperate glaciers, which are mostly detail in the scientific literature because they can act as proxies characterised as either superficial fresh snow or firn ice. The for palaeoclimate reconstructions (e.g., Wolff et al., 2006, 2010). polar ice sheets are volumetrically the most significant on earth. It is known that temperature and chemical concentrations in For these reasons we consider only deep (100–3000 m) polar ice natural polycrystalline ice regulate the intergranular vein size sheets for our analysis to establish the conditions in the veins. (Mader, 1992b; Paterson, 1994) and hence control their water volume and potential microbial population carrying capacities. Furthermore the chemical concentrations control the diffusion rates of chemical impurities through grain boundaries and veins 2. Definitions and ultimately govern the microbial metabolic reactions, which in turn could impact on vein chemistry. Anomalies observed in polar In the following sections, it is important to distinguish care- ice core gas records (e.g. N2O, CH4) have been attributed to fully between various symbols and terms that are used to refer to potential in situ microbial activity (Sowers, 2001; Tung et al., impurity concentration. 2005). In addition, some models suggest that the rate of down- The bulk impurity concentration C is given by the total mass of ward diffusion of impurities along the liquid veins within ice impurity (usually given in moles) divided by the total ice volume sheets could be significantly more than the rate of rheological ice (grainsþveinsþgrain boundary films). In this paper the term bulk is flow in ice sheets and this can lead to a major displacement of always used for concentrations averaged across the total ice volume climate signals trapped in ice cores (Hubbard et al., 2003; Rempel in this way. The vein impurity concentration is cv and is simply the et al., 2001; Rempel and Wettlaufer, 2003). total mass of impurity that partitions to the veins divided by the We utilise selected site-specific data sets on temperature and total vein volume. It is assumed that this is also the impurity chemical impurity profiles of polar ice cores and combine these concentration in the grain boundary films. We can define an datasets with a numerical model (VeinsInIce1.0) that encapsulates associated bulk veinþfilm impurity concentration Cb which is the our knowledge of the physical properties of the vein system to total mass of impurity that partitions to the veins and films divided calculate vertical profiles of vein diameter, vein impurity con- by the total ice volume.
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