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

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Earth and Planetary Science Letters

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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 12 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 20 1C(Gilichinsky, 2002; Price and Sowers, 2004) North Ryde, Sydney, NSW 2109, Australia. and 39 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 20 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. Cb therefore represents the contribution to centration and total vein water content for these ice cores in order the bulk impurity concentration made by the impurities resident in to assess the vein system as a possible habitat for microbial life. the veins and films. Cb and cv are simply related via the total water 240 K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 volume fraction (veinsþfilms) in the polycrystal denoted by f. measurements respectively. For GISP2 D and Dome C the number of data points per ice core is reduced by taking averages within each Cb f ¼ ð1Þ 10 m depth interval (with varying number of observations per depth cv interval). The ice core data from GRIP is discontinuous in some It is expected that the process of impurity partitioning requires regions of the ice core with occasional gaps of 20 m. The resolution of that impurity molecules form liquid or quasi-liquid layers at the ice the source data is 10 m and so no averaging across the depth interval grain boundaries as they are excluded from the solidifying ice is done in this case and data gaps are preserved. In some depth grain. We distinguish between the bulk film impurity concentration intervals, individual ion values that lie far from the main trend (more Cf (the total mass of impurities resident in the grain boundary films than about 0.5 standard deviations) are excluded from the analysis. divided by the total ice volume) and the bulk vein impurity We find that the 10 m depth interval successfully preserves the concentration Cv (the total mass of impurities resident in the veins observed trends in all the data sets with no noticeable loss of divided by the total ice volume). Clearly, there is then the following information. The data from Vostok is extracted from published simple relationship between the bulk impurity concentrations graphs; a value is extracted at every 25 m and that data is used to represent its respective 25 m depth interval. The original data set Cv ¼ CbCf ð2Þ fromSipleDomecoreisdifferentfromtheothersinthatithas and we can now also work out the vein water volume fraction from chemical profiles for the first 10 m of every 100 m depth interval (100–110 m, 200–210 m and so on). The average of those 10 points is Cv fv ¼ ð3Þ used to represent each 100 m depth interval. The Siple Dome dataset cv is used only to test for significant differences (if any) between the two calibration options available in the model. 3. Ice core data Grain sizes reported from these ice cores show complex variations with depth. However, in general it is seen that grains The ice core data used in this study, including depth range, become larger and coarser with increasing depth. The grain size number of measurements, their resolution (average distance between also shows consistent oscillating vertical trends for some ice cores measurements), bulk chemistry profiles, grain size profiles and their (Fig. 2(b)). Following such trends in grain size variation in original reference sources, are summarised in Table 1. Ice core available data sets, we assume a reasonable grain size (through chemical impurity profiles are available from the NOAA archive interpolation) wherever the information is unavailable. repository for Antarctic (Vostok, Dome C, Siple Dome) and Greenland We seek to determine the chemistry of the solution in the veins (GRIP, GISP2 D) ice cores. The original data sets available in the public and films and the vein size as a function of depth along the core. domain comprise chemical profiles that list concentration of indivi- No direct measurements of vein chemistry or size exist. Indeed, dual inorganic ions with depth (available online: the web link is given whilst the size of the veins within a natural ice sample at its in Table 1). Most of the ice core inorganic ion data sets, both at the original in situ temperature could in principle be measured in the levels of individual ions and the bulk concentrations examined and laboratory, no technique currently exists that would allow the used in the study are distributed normally about their means with or direct determination of the vein chemistry. Therefore, it is neces- without extreme values making positive skewness in sulphate a sary to rely on our knowledge of the bulk chemistry of the ice and noteworthy exception (Fig. 2(a)). The ice core data sets from GISP2 D make reasonable assumptions on how the different solutes parti- and EPICA Dome C have the maximum original resolutions (0.2– tion between ice grains, grain boundaries, and veins and couple 0.5 m and 0.55–1 m respectively) and produce 12,000 and 3000 this with our knowledge of the vein system.

Table 1 Polar ice core data sets used in this studya.

Ice core Location Depth Grain size Borehole Bulk ion impurity Resolution range range temperature range concentration

Cbi range Original Depth interval (m) (mm) (1C) (lM) (m) (m)

Vostokb Antarctica 781 28’S 1061 48’E 50–2080 1.6–18c 58 to 35d 5–30e –25 EPICA Dome C 751 06’S 1231 24’E 100–3200 1.8–14f 54 to 3g 5.6–15h 0.55–1 10 Siple Dome 811 65’S 1481 81’W 10–977 1–4 25 to 6i 1.9–4.5j – 100

GRIP Greenland 721 35’N 371 38’W 100–3021 1.6–26k 31 to 8l 2.2–35.6m 10 10 GISP2 D 721 35’N 381 28’W 110–3040 0.6–10n 32 to 9o 2.2–33p 0.2–0.5 10

a Sourced from the online ice core gateway maintained by NOAA at http://www.ncdc.noaa.gov/paleo/icecore.html. b Although Vostok is a much deeper ice core (3400 m), the available information is limited to 2080 m and, unlike other data sets, the values are extracted from graphs published in the cited peer-reviewed literature (Obbard and Baker, 2007; Vostretsov et al., 1984; Legrand et al., 1988). c Obbard and Baker, 2007. d Vostretsov et al., 1984. e Legrand et al., 1988. f Durand and Weiss, 2004. g Ritz et al., unpublished. h Wolff et al., 2006. i MacGregor et al., 2007. j Kreutz et al., 1999. k Thorsteinsson et al., 1997. l Johnsen et al., 1995. m Legrand et al., 1993. n Alley and Woods, 1996. o Clow et al., 1996. p Mayewski et al., 1997. K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 241

Fig. 2. Features of ice core data. (a) Frequency distribution of sulphate ion concentration from EPICA Dome C (data from Wolff et al., 2006). Note the positive skewness and the most frequent concentration ranging between 2 and 3 mM. The distribution of bulk ion concentration Cbi is given in the inset showing the most frequent bulk value to be between 8 and 10 mM. (b) The consistent increasing trend and oscillatory behaviour of grain size with depth observed in EPICA Dome C (data from Durand and Weiss, 2004).

þ Current available evidence suggests that NH4 and Cl are cations are assume to be available for partitioning into veins. In such þ favourably partitioned to the ice grains where they can be incorpo- cases, the Cl trapped in grains is balanced by NH4 and, if needed, rated substitutionally in the ice lattice whereas Naþ ,Ca2þ ,Mg2þ , then by Naþ . In certain regions within some ice cores (see example 2 2þ SO4 ,andNO3 are expected to partition favourably to grain from GISP2 D) the cation concentration (especially Ca ) is several boundaries and veins (Mulvaney et al., 1988; Cullen and Baker, times more than the available anions. In such situations, after 2000, 2001; Barnes and Wolff, 2004). The ion partitioning is not balancing it with Cl trapped in grains we assume all the excess perfect and trace amounts of ions are expected in the regions where cations to be available in veins (see footnote in Table 2). Finally, the the ions are not assumed to be favourably partitioned. A further bulk veinþfilm impurity concentration of ions Cbi in mMiscalculatedby complication is that chemical data for a specific field setting is adding the concentrations of all cations and anions that partition to invariably incomplete with only the concentrations of certain ion the veins and films (5.032 mMþ2.759 mM¼7.791 mM). Then the bulk species measured. We can infer some of the impurity load that has veinþfilm impurity concentration Cb is calculated by dividing Cbi by 3 not been measured by requiring that both the grains and veins are (Cb¼2.597). This is done since the calibration used in the model is for separately charge neutral. To account for overall ionic charge balance, molecular H2SO4 (a single entity), which is made up of three molar þ þ þ þ 2 a certain concentration of protons (H ) is assumed. H ,NH4 equivalents of two ions (2H and 1 SO4 ). Note that we have þ (substitutional), and Na (assumed interstitial) concentrations are calculated Cbi assuming that all compounds are fully dissociated. Thus utilised in that order to balance Cl in the grain lattice. The ion we have not taken into account that some salts may precipitate from concentrations reported in parts per billion (ppb) are converted the solution as the temperature is lowered. Establishing salt pre- into mM. cipitation from the mixtures would involve detailed thermodynami- From these considerations we first calculate the bulk cal calculations, which go beyond the scope of this research. However, veinþfilm concentration of ions expressed as Cbi, which is then in cases, where much of the material consists of compounds (such as used to compute the bulk veinþfilm impurity concentration Cb as H2SO4) which individually have a low eutectic point, it is likely that a function of depth from the raw data using the method explained precipitation will be minimal, and our calculations of vein size will below. See Table 2 for three examples of the calculations for polar therefore be close to valid. By contrast, in conditions where most salts ice core chemical profiles. The values given in parenthesis would precipitate, the veins could be much smaller than we estimate.

(italicised) below relate to the example data set from EPICA Dome Finally, we have also treated H2SO4 as fully dissociated, even though C given in Table 2, unless stated otherwise. it is likely that the second proton is not dissociated in the strong acid ThechargeinmeqL1 is calculated, taking account of the valency solutions that are experienced in the veins. This is implicitly taken 2þ of the relevant ions (e.g. [Ca ] 0.092 mMresultsinachargeof into account by our use of the actual H2SO4 freezing point curve. 0.184 meqL1). The overall charge difference is calculated by subtract- ing the total cationic charge ([Na þ]þ[Ca2þ]þ[Mg2þ ]¼1.718 meqL1) 2 fromthetotalanioniccharge([Cl ]þ[SO4 ]þ[NO3]þ[other 4. The numerical model VeinsInIce1.0 anions]¼5.852 meqL1)([anions][cations]¼4.134 meqL1). If anions are in excess (which is true in general), the positive charge deﬁcit is We used a numerical model called VeinsInIce1.0,whichwas assumedtobesuppliedbyHþ .ApartofthetotalHþ needed for developed to estimate some of the basic physical and chemical pro- 2 overall charge balance is transferred to SO4 to form HSO4 perties of the vein environment. The equations utilised by the pro- (2.518 meqL1 of [Hþ ] will form bisulphate) and the remaining Hþ gramme have been reported and discussed in earlier studies (Mader, (1.616 meqL1)isbalancedagainstCl (0.576 meqL1)trappedin 1992a; Barnes et al., 2003). The programme takes the inputs as: grains. Any excess Hþ (1.040 meqL1) is assumed to be in the veins. This way, the overall charge balancing is observed in both the grains depth h of the ice from the surface in [m], as well as the veins. If cations are in excess (which is rare), the excess the local temperature depression y below 0 1Cin[1C], 242 K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249

Table 2

Example calculations for bulk veinþﬁlm impurity concentrations Cb and their chemical breakdown. Individual ion impurity concentrations are given in mM and their charge is given in meqL1 in parenthesis. Shaded ions (italicised) are assumed to partition favourably into ice grains.

Note: The conditions within veins are expected to be highly acidic (pHo3, Price, 2002) and the excess Hþ concentration in most cases is consistent with that requirement. Although in general, charge equivalence is observed after assuming H þ to compensate for positive charge deﬁcit, it is important to note that in certain regions along the depth range of ice cores (e.g., GRIP and GISP2 D, 41500 m), certain cations (Ca2þ ,Mg2þ ) are in great excess making the charge deﬁcit negative. It is possible that these ions might form micro inclusion bodies stuck within ice grains (Ohno et al., 2005). However, we assume that the excess cations are available for partitioning into veins. þ Bearing in mind that cations are better mapped within ice cores than anions, the actual charge difference is most likely to be less than the computed one.NH4 concentration is not available for EPICA Dome C, which otherwise offers the most comprehensive data set. Wherever available, it is included in the calculations. Trace ions such as formate and acetate are assumed to be in their native forms as weak organic acids and their concentrations are added to Cb wherever available (GRIP).

bulk impurity concentration in the ice with a potential to A suitable expression for the yp term, the temperature depres- 1 partition into veins Cb in [molL ], and sion due to pressure, can be obtained by using the comprehen- grain size a in [mm]. sive experimental data for the freezing curve of ice Ih from Bridgman (1912) (Fig. 3(a)). The best ﬁt to the data is given by the equation The model subsequently calculates the vein size, vein impurity concentration and other derived parameters (e.g. water content). 16 2 2 8 yp ¼ð1:32 10 1C=Pa Þp þð7:61 10 1C=PaÞp ð5Þ As polycrystalline ice contains a water phase in contact with the solid of the same chemical species, it is formally a ‘freezing mixture’. Comparison of laboratory conditions, where the pressure term The equilibrium temperature of such a mixture is the freezing point is zero, with those of natural ice at depth can be easily achieved of water which will in general be depressed relative to the freezing by considering temperature depressions relative to the pressure point of pure water (0 1C) by some amount y called the temperature melting point; in other words by calculating the pressures in depression. Note that this deﬁnition follows the convention of 3 terms of depth h from p¼righ where ri¼916 kgm is now the treating y as a positive quantity (e.g. y¼0.1 means that water freezes density of ice and g¼9.81 ms 2 and hence at 0.1 1C). y is a function of the pressure p, impurity concentration in the liquid phase c , and radius of vein wall curvature (r ). v v y ¼ 1:066 108½degm2h2 þ6:838 104½degm1h ð6Þ p 1 y ¼ yp þyc þyr ¼ f ðpÞþf ðcvÞþf ð4Þ Fig. 3(b) shows the temperature depression in a freezing rv mixture of liquid water and water ice as a function of solute

This holds down to the eutectic temperature below which the concentration cv in the liquid water for several compounds. We residual, highly concentrated vein solution will completely freeze. note that for small temperatures depressions yco10 1C, the The eutectic temperature is speciﬁc to a chemical impurity. Some curves for different compounds follow the same linear relation- of the typical chemical impurities found in polar ice cores such as ship. For large temperature depressions the behaviour is com-

H2SO4 and HCl can maintain a liquid environment within ice pound dependent. The vein chemical environment will be veins even at temperature as low as 55 1C(CRC Handbook of heterogeneous and the eutectic behaviour of the complex mixture Chemistry and Physics, 2009). will be difﬁcult to gauge (also see Section 6.2). However, to

A clear implication of Eq. (4) is that changing the temperature simplify the modelled system we use the data for H2SO4 (homo- of a polycrystal involves both latent and speciﬁc heats; for geneous), which is a common acid found in ice sheets. example, if the temperature of a polycrystal at a particular depth An expression for the yc term, the temperature depression due (pressure) is lowered (i.e., y increases), the curvature (1/rv) and to concentration, is obtained by the best ﬁt for freezing point concentration (cv) must increase. This occurs by freezing on the depression curve for H2SO4, vein walls so as to make the vein cross-section smaller and 3 2 concentrate the impurities within it. yc ¼ 0:685cv1:188cv þ4:7971cv ð7Þ We now derive expressions for the separate terms in Eq. (4) in terms of known physical parameters. where the units of the numeric constants are mol, litre, 1C. K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 243

Fig. 3. Temperature depression as a function of (a) pressure and (b) impurity concentration. (a) The freezing curve of Ice Ih. The black squares are measured data from Bridgman (1912) and the solid line is the best ﬁt curve through the data, given by Eq (5) (Section 4) with regression coefﬁcient R2¼0.9996. The dotted line shows the

Clausius-Clapeyron relationship for comparison. (b) Temperature depression vs. concentration calibration curves for H2SO4 and HCl (Data from CRC Handbook of 2 Chemistry and Physics, 2009). For temperature depressions o10 1C the behaviour is approximately linear. If yc¼kc, then the constants k and regression coefﬁcients R 1 2 1 2 associated with best ﬁt lines are given by H2SO4: k¼4.53 1Clmol and R ¼0.977; HCl: 4.36 1Clmol and R ¼0.988.

The temperature depression due to the curvature of the vein polycrystal. To make the problem tractable we follow Nye and faces yr is derived from the Gibbs–Thompson relation Frank (1973) and assume that the grains are uniform semi-regular truncated octahedra (Fig. 1(c)). gslTo 1 yr ¼ ð8Þ Inserting Eqs. (11), (7) and (6) into (4) gives us an expression Lf rv for the temperature depression as a function of depth h, grain size 7 2 where gsl ¼(0.033 0.003)Jm is the solid–liquid interfacial energy a, bulk veinþﬁlm impurity concentration Cb and the vein impur- (Ketcham and Hobbs, 1969), To¼273 K is the freezing point of pure ity concentration cv. The temperature depression is simply the 8 3 water, and Lf¼3.06 10 Jm is the latent heat of pure water. local temperature at the depth of interest h and a and Cb are The radius of curvature rv cannot be measured directly. We determined from the ice core data. The theory presented here need to use our knowledge of the geometry of the vein system does not rely on any assumed relationship between temperature (see e.g. Nye and Frank, 1973; Mader, 1992a) to ﬁnd an expres- and crystal size and can be applied for any dataset where these sion for rv in terms of measurable parameters and cv which we two variables are measured separately, as they are in ice core seek to determine. If ‘ is the length of veins per unit volume of data. With these inputs, the Eq. (4) in principle allows us to 2 bulk ice such that fv ¼ ‘arv (see Fig. 1 and detailed caption in calculate the vein concentration and then from this the vein size Supplementary Material), then combining this with Eqs. (2) and and other derived quantities. However, the equation cannot be

(3) and the equation given for the vein cross-section gives analytically solved to give an expression cv ¼ f ðh,a,Cb,yÞ.We sffiffiffiffiffiffiffiffiffiffiffiffiffiffi therefore use a Newton–Raphson scheme to calculate cv. 1 CbCf rv ¼ pffiffiffiffiffiffi ð9Þ ‘a cv It has been suggested that the solution on the grain boundaries 5. Model sensitivity analysis enters as a monolayer (Barnes and Wolff, 2004). This monolayer will contain 1019 molecules/m2 of solution (i.e. including the The ion impurity data reported from Siple Dome ice core in water). Pure water has 55 M. We assume that the concentration of West Antarctica are utilised initially to test the sensitivity of the impurities in the monolayer is the same as in the veins. So, for numerical model VeinsInIce1.0 (Table 1). Since the local bore hole example, if the concentration in the veins was found to be temperature range (25 1Cto6 1C) of the Siple Dome core falls within the range of all the three freezing mixture calibration cv ¼5.5 M, then the impurities make up 10% of the molecules in the grain boundary monolayer, i.e. 1018 molecules of impurity at curves available in the model, its data is ideally suited to test the grain boundaries per m2. In general, the bulk film impurity significance of difference, if any between the estimated results concentration is given by based on the two separate calibration curves. We test the significance of impact of each observable at a cv 19 molecules S moles 10 m2 wider scale using a standardized set of input parameters (Table 3) 55 litre 10 Cf ¼ ¼ Scv3:03 10 m ð10Þ that cover the range of values observed in the data sets. A general NA full factorial Analysis of Variance design (ANOVA, 4 factors at where S is the grain boundary area per unit volume of bulk ice 23 4 levels, 256 combinations) is followed (Mitra, 1998). Treating ¼ and NA 6 10 is Avogadro’s number. vein impurity concentration (mol L 1) and vein diameter (mm) as We use (10) and (9) in (8) to obtain sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi response variables, we formulate a null hypothesis, which can be g T ‘ac stated as follows: ‘‘The difference between the estimated vein ¼ sl o v ð Þ yr 10 11 diameters (similarly vein impurity concentrations) due to the Lf C 3:03 10 ½mSc b v observed variation in grain size (and/or), bulk impurity concentration Finally, we need expressions for S and ‘. Strictly speaking we (and/or), depth (and/or), local bore-hole temperature is not signifi- would need to know the grain size and shape for all grains in the cant’’. The alternative hypothesis states that the observed 244 K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249

difference is significant. The sensitivity analysis also accounts for Table 3 any significant effect on the measured response due to either A standardized set of input parameters for VeinsInIce1.0, used to test the magnitude of possible variations in modelled vein properties due to each input synergistic or antagonistic interaction between the major input parameter (256 combinations of input parameters are generated using these factors. Note that we have used the terms and/or in our null values). hypothesis, which indicates potential interactions between the input factors. The results are analysed using the statistical Input parameter Level 1 Level 2 Level 3 Level 4 package Minitab 15. Depth (m) 30 300 1000 3000 Grain size (mm) 1 2.5 5 10 Bulk veinþfilm impurity 1.5 3 6 12 6. Results concentration (mM) Local temperature (1C) 5 20 35 50 6.1. Sensitivity of VeinsInIce1.0

The Siple Dome core dataset from Antarctica, used to test the difference in results between the two freezing point depression calibration curves available to the model, shows no signiﬁcant difference is observed between the estimates of vein size (Fig. 4)

made based on the calibration curves of H2SO4 and HCl at temperatures 4 12 1C, as is to be expected on the basis of the curves given in Fig. 3(b) which overlap in this region. Since, the chemical proﬁles of polar ice cores are predominantly sulphate rich, all model interpreta-

tions are therefore based on the H2SO4 calibration (Fig. 3(b)). The procedure used to calculate Cb treats all ions as equivalent in terms of freezing point depression. This is an approximation that imposes an uncertainty in terms of concentration and hence vein diameter. The results of the sensitivity analyses based on the set of input parameters in Table 3 substantiate that grain size is the most Fig. 4. The difference between vein diameter estimates for Siple Dome based on the significant factor influencing vein size, and local borehole tem- two possible freezing point depression calibrations (that of H2SO4 and HCl) available in the numerical model VeinsInIce1.0. These estimates are derived for perature is the most significant factor determining the amount of the local bore hole temperatures from the Siple Dome ice core (Antarctica) keeping freeze-concentrated impurities partitioned into the veins the remaining parameters of the model viz., grain size a (3 mm), depth (500 m), and (Po0.0001). Although the remaining input factors are also sig- bulk ion impurity concentration Cb (1.1 mM) as constants, which represent their nificant within their respective observed variation range, their respective mean values for Siple Dome. The depth of 500 m represents a midpoint of the depth range (10–977 m) for Siple Dome. Please see references in Table 1 and respective impact magnitudes (Po0.001) are smaller than grain interpretation in Section 6.1. size and vein impurity concentration. A ten-fold increase in the

Fig. 5. Model sensitivity plots. (a, b) Graphs representing the impacts of grain size, bulk veinþﬁlm impurity concentration Cb on vein diameter do (c, d) The impacts of sample depth and borehole temperature on vein impurity concentration cv. The error bars encompass and account for 95% of possible observable variations due to other input factors in the programme VeinsInIce1.0 (Table 3). K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 245 grain size results in an average four-fold increase in the vein 6.2. Vein impurity concentration diameter (Fig. 5(a)). Similarly, a six-fold increase in the bulk impurity load in the ice sample results in a three-fold increase in In Fig. 6, the modelled vein impurity concentration cv of the vein diameter (Fig. 5(b)). Compared to all these factors the selected polar ice sheets is plotted as a function of depth. The impact of sample depth on the ice veins has the least magnitude vein impurity concentration cv is determined primarily by its ﬁrst of difference across the length of ice sheets on Earth (Fig. 5(c)). order relationship with the local borehole temperature (tempera- However, at greater depths (43000 m), pressure could prove to ture depression Fig. 3(b), Fig. 5(d)). As a result, in all ice cores, the be a critical factor that decides the physical properties of a ‘vein concentration of solutes within veins gradually decreases from like’ habitat, especially in known extra-terrestrial settings. These the surface to the bottom as the local temperature increases with results effectively captured the magnitude of possible variation in depth. Note that the term for temperature depression due to the the model response. radius of vein wall curvature (Eq. (11)) is much smaller than the

Fig. 6. Modelled vein diameter do (solid) and vein impurity concentration cv (dotted) proﬁles. (a) Vostok (b) EPICA Dome C in Antarctica and (c) GRIP (d) GISP2 D in

Greenland. Note that both do and cv have the same scale on the x-axis. 246 K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 concentration term (Eq. (7)). As a result, the main impact of 6.4. Vein water volume fraction varying Cb and grain size is primarily to alter the size of the veins and hence the volume of water in the veins with cv fixed largely The profiles of total water volume fraction show complex by the local temperature. behaviour with depth, as these changes are modulated by varia- The concentration of impurities within veins show a range of tion in grain size and hence vein size. In general, the volume of values from 4 M at the surface (100 m, r 50 1C) to 0.8 M in water varies between 1 mL and 2 mL per litre of ice. However, not depth intervals closer to the base (42500 m, 4 12 1C) across all all of this water resides in the veins throughout the depth of all the ice cores. Average local temperature within most ice sheets ice sheets. Fig. 7 shows that from surface down to depths of demands for the vein impurity concentration to be from 2.5 to 1600 m (local borehole temperatureso 30 1C) as much as half 3.5 M over much of their depth (up to 2500 m). Antarctic ice of the available liquid water can be located at the grain bound- sheets in general, because of their lower local temperatures than aries as quasi-liquid layers, assuming that impurities at the grain the ice sheets in Greenland, require higher vein impurity concen- boundaries partition into veins as monolayers; this is most trations to maintain liquid condition within their vein network. pronounced for GRIP and GISP2 (see Figs. 7(c) and (d)). However, at greater depths (42000 m), with increasing temperature, the fraction of water as quasi-liquid layers at the grain boundaries 6.3. Vein size tends to zero. As the grain size tends to increase at greater depths, the grain boundary area decreases accordingly and hence the The plots in the Fig. 6 also depict the vertical profile of vein size, proportion of water at the grain boundary monolayers also decreases and nearly all water resides within the veins. modelled as vein diameter do, for the polar ice cores as predicted by the model. Since, the vein impurity concentration cv and the vein diameter do share the same linear scale on the x-axis, they are plotted together on the same graph. Whereas vein concentration 7. Discussion decreases gradually with depth, mapping the gradual increase in temperature, there are large fluctuations in the vein size. This is a The modelled physicochemical conditions within water-filled result of local fluctuations in grain size a and the bulk veinþfilm veins of polar ice sheets indicate that microbial habitation could impurity concentration Cb.Therelativeimpactsofa and Cb on vein be potentially supported across the entire length of polar ice diameter do have different magnitudes under different local condi- sheets. There is already observational evidence for localisation of tions found in polar ice sheets (Fig. 5). For ice cores with conditions microbes in the ice veins (Mader et al., 2006; Rohde and Price, similar to that of EPICA Dome C (Fig. 6(b)), which show a steady 2007; Amato et al., 2009). The model results presented here increase in grain size with depth (Fig. 2(b)) and minimal variation in demonstrate that the veins are large enough to accommodate

Cb, the vertical vein size profile is governed mostly by the grain size. microbes and that the acidic vein water is chemically enriched. However, for GISP2 D and GRIP cores in Greenland, since their grain Based on the modelled plots, it is predicted that in most polar size profiles remain more or less homogenous throughout, the bulk ice sheets, the top 500–1500 m of ice with temperatures of veinþfilm impurity concentration Cb plays a more important role in r 30 1C would have an average vein diameter of 0.4–1.2 mm. determining vein dimensions than grain size. This is in line with SEM studies on both laboratory grown and

Fig. 7. Modelled total water volume fraction f (dotted) and fraction available in veins fv (solid) profiles. (a) Vostok (b) EPICA Dome C in Antarctica and (c) GRIP (d) GISP2 D in Greenland. Total water volume is calculated as a volume fraction per unit volume of bulk ice. The fraction within veins is calculated as a ratio between the impurity concentrations attached to the grain boundaries and the concentrated impurity in veins. Note that the two lines run very close to each other and the difference between 6 them tends to zero with increasing depth. From the water volume fraction (volume of water/volume of ice), we can infer the volume of water in veins. e.g., fv ¼ 1 10 translates to 1 mL of liquid water per litre of solid ice. K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249 247 natural polycrystalline ice samples that suggest that at extremely microbes are likely to have access into the intergranular vein low temperatures (below 30 1C), observed veins can be less than system and potentially may undergo slow decay to provide organic 0.5 mm in diameter (Barnes et al., 2002). Although evidence-based nutrients to the englacial anaerobes. Although not tested by the predictions have previously suggested that the vein sizes within ice model, some high altitude-deep temperate glaciers (E.g., 150 m sheets could range from 0.5 to 10 mm, the hypothesis of englacial deep glaciers on the summit of the Andes mountains at 45000 m microbial habitation and metabolism in ice veins has so far AMSL) have the potential to develop ice vein systems. Due to their restricted itself to relatively warm and deep (10 to 2 1C, warmer temperature some temperate glaciers could prove to be 42500 m) ice samples (Price, 2000; Rohde and Price, 2007). The better habitats for englacial microbes than polar ice sheets. fact that psychrophilic microorganisms isolated and cultured from The question of microbial metabolism and reproduction within Vostok accretion ice samples and Greenland deep ice cores are ice veins remains unresolved. Although surface temperatures on shown to be ultra-small (0.1–0.4 mm), much smaller than an Antarctic ice sheets can go below 50 1C such temperatures are average bacterium of size 1–2 mm(Karl et al., 1999; Miteva and not lethal. In addition, at such temperatures, as the model results Brenchley, 2005; D’Elia et al., 2008) opens the possibility of an suggest, the concentration of impurities can be as high as 4–5 M, a extensive microbial population that spreads the entire length of good five to ten times more concentrated than an enriched vein network within polar ice sheets. We do not know whether any microbial culture medium used in a laboratory. Water activity at microbes if they are enclosed in the narrow veins are able to move, such high concentration of solute and low temperature is expected either actively or passively. If they could move, then the vein to be close to 0.62, which is the observed lower threshold value networks could potentially be acting as tiny (2 mm) yet lengthy that could support life (MEPAG SR-SAG, 2006). It is interesting to (2000 m) corridors of microbial traffic within polar ice sheets. infer the chemical broth in the veins using the example chemical The characteristic vein size profiles with depth in polar ice profile from EPICA Dome C given in Table 2. We use this sheets may prevent the larger eukaryotes from inhabiting narrow information to derive a simplified chemical profile and the bulk vein systems (Mader et al., 2006), especially in the shallow ice ion concentration (Cbi¼7.791 mM) is approximated to 8 mM such samples. However, there could be regions of englacial vein net- that it also includes some of the missing ions (Table 4). For a works within polar ice sheets, as estimated by our model (Fig. 6), borehole temperature equal to 42 1C at 1200 m for Dome C, the which show vein sizes large enough (43 mm) to support uni- resulting vein impurity concentration will be cv¼4M(Fig. 6(b)). In cellular eukaryotic habitation. Like the ultra-small bacteria iso- Table 4, we see for example that 75 mM (1.875%) of cv is made up lated from icy habitats, these eukaryotes are most likely to be of magnesium (derived salts). We utilise equal amount of sulphate smaller (2–3 mm) than an average yeast cell (4 mm). (75 mM) to form 150 mM of MgSO4 in veins. Charge equivalence is The absence of veins in the surface region of certain ice cores also observed. Likewise, sulphate is utilised sequentially to form due to either extremely low temperatures (Antarctic ice cores such salts of calcium and potassium. Since Naþ is the second most as Vostok) or small grain sizes (Greenland ice cores, GISP2 D) may abundant cation (after Hþ), it is utilised to form sodium nitrate. act as a barrier for microbial and chemical diffusion into the Sodium bisulphate is readily formed in presence of sodium englacial vein system. However, it should be noted that the annual sulphate and sulphuric acid. Similarly the rest of the constituents polar mean surface temperature measurements show a great of the vein chemical broth are obtained through simple propor- seasonal variation. Not all ice cores in Antarctica are as elevated tional concentration balancing procedures (Table 5). As a result, and deep as Dome C (surface at 3280 m AMSL) and Vostok (surface the vein composition is approximated to be K2SO4 (37.5 mM), at 3488 m AMSL). Therefore surface temperatures of many other MgSO4 (150 mM), CaSO4 (100 mM) HNO3 (50 mM), H2SO4 places in Antarctica will be relatively warmer than Dome C and (2437.5 mM), Na2SO4 (450 mM) NaHSO4 (450 mM), NaNO3 Vostok making the development of a continuous vein network (100 mM), acetic acid and formic acid with traces of their sodium from the surface to the bottom more likely. The Greenland ice salt derivatives (125 mM), and methyl sulphonic acid (100 mM). sheets are shown to develop huge areas of shallow (up to 5 m) They together constitute a 4 M solution (pH close to zero). surface melt water lakes during northern summers (McMillan The freezing behaviour of such complex mixtures is not easy to et al., 2007), making temporary fresh water habitats to allow a estimate but is most likely to follow the behaviour of its dominant range for microflora to flourish. With a relative expansion of ingredient. We therefore assume that the freezing mixture will surface vein networks under seasonal warm conditions, these remain liquid above the eutectic of H2SO4, which is the major

Table 4 A possible characterisation of the chemical broth present in the veins with a numerically simpliﬁed ionic concentration proﬁle from EPICA Dome C.

Ions Example profile from Modified, simplified Percentage share Vein impurity a b EPICA Dome C from Table 2 example profile concentration cv at 42 1C (lM) (lM) (mM)

Na þ 1.230 1.20 12.5 500 Ca2þ 0.092 0.10 1.25 50 Mg2þ 0.152 0.15 1.875 75 Kþ – 0.05c 0.625 25 NO3 0.160 0.15 1.875 75 2 SO4 2.518 2.50 31.25 1250 CH3SO3 0.081 0.10 1.25 50 Other organic acids – 0.25c 3.125 125 H þ 3.568 3.50 43.75 1750

Cbi 7.791 lM 8.00 lM 100% cv ¼4M

a The modified concentrations represent a numerically simplified profile that includes some of the missing ions in the example set from EPICA Dome C and is meant to help formulate a reference composition for the chemical medium within veins. b The concentration of the individual ions in the veins is calculated by applying the same percentage share deduced from the bulk ion impurity concentration Cbi to the vein impurity concentration cv. c The concentrations of Kþ and other organic acids are derived from observed overall average values for GRIP (Legrand et al., 1993). 248 K.G.S. Dani et al. / Earth and Planetary Science Letters 333–334 (2012) 238–249

Table 5 Approximation of chemical broth composition within veins using the information from Table 4.

2 Impurities Anions NO3 SO4 CH3SO3 Total

Cations Concentration [mM] 75 1250 50 þ Na 500 NaNO3 (50þ50) Na2SO4 (300þ150) – 1000 þ NaHSO4 (150þ 150 of [H ]þ150) 2þ Ca 50 – CaSO4 (50þ50) – 100 2þ Mg 75 – MgSO4 (75þ75) – 150 þ K 25 – K2SO4 (25þ12.5) – 37.5 þ H 1750 HNO3 (25þ25) H2SO4 (1625þ812.5) MSA (50þ50) 2587.5

Total 150 3625 100 3875 mM

Other organic acids 125 mM

Vein impurity concentration cv 4M component within the veins. This demands that the englacial Commission (UK) for sponsoring his fellowship during this study. microbes capable of inhabiting ice veins must be both acidophilic We also thank four anonymous reviewers, whose constructive and halophilic (Price, 2000). Attempts to monitor them in situ critique helped to improve the manuscript. using scanning spectrofluorometry through protein bound trypto- phan autoflourescence have improved the estimates of microbial abundance in icy ecosystems (Tung et al., 2005; Rohde and Price, Appendix A. Supporting information 2007). 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