Our AEM survey produced regional-scale resistivity data that confirms and expands the overall extent of permafrost and reveals two extensive subsurface brine systems in the MDV. The AEM data correlate well with conductivity profiles from the MDV lakes (Table 1). For example, the hypersaline bottom waters of the west lobe of Lake Bonney (30–35 m depth) have a resistivity of 0.13–0.12 Ωm, and the AEM returned a resistivity value of 0.42 Ωm for this depth interval. The AEM sensor was flown over the site of the DVDP ground-based resistivity survey and boreholes in the Fryxell Basin (Fig. 1) and consistently recorded resistivity around 100 Ωm or less at depths below 185 m, indicating the presence of brine in sediments. Although not directly comparable, our results are consistent with previous DVDP geophysical measurements26 (Table 1; Supplementary Fig. 2). For example, when DVDP Borehole 10 (Fig. 1a; near the McMurdo Sound) penetrated the permafrost layer at 183 m below sea level (b.s.l.), liquid entered the borehole rising to ∼125 m (ref. 26). This borehole fluid was twice the salinity of seawater with an in situ temperature of −4 °C (ref. 26). Further inland at DVDP 11, drilling fluid drained from the borehole at ∼248 m b.s.l. The loss of drilling fluid suggested that the drill penetrated the confining layer (described by Cartwright and Harris26 as ‘the interface between frozen ground and liquid groundwater’) before entering an aquifer. Temperature measurements collected from boreholes (DVDP 10–12) in the Taylor Valley were above −10 °C at depths greater than 100 m from the surface39. Given the salinities of the sediments at these depths40, porewater would remain liquid. Collectively these earlier observations and the low-resistivity values detected with AEM (<100 Ωm) support our interpretation of the presence of two distinct brine groundwater systems in the Taylor Valley.

Taylor Glacier is a well-studied polar outlet glacier representative of ice drainage pathways in the cold margins of the Antarctic ice sheet, see, for example, ref. 16. Thermodynamic models of ice temperature41 distribution in Taylor Glacier indicates that basal temperatures are well below the pressure melting point of ice and therefore it had been considered predominantly cold based, moving through internal ice deformation. The area of lowest resistivity below Taylor Glacier corresponded with a topographic overdeepening of 80 m b.s.l. at ∼5.75 km up-glacier from the terminus at Lake Bonney (Fig. 3). Hubbard et al.41 measured high bed reflectance at this same location with ice-penetrating radar; both this radar data and our AEM measurements are indicative of subglacial hypersaline liquid. The radar survey, however, did not detect evidence of saturated sediments at the glacier snout in the vicinity of Blood Falls nor did it reveal the deep connectivity with Lake Bonney that AEM was able to resolve (Figs 3 and 4c). AEM confirms the presence of unfrozen water at the base of Taylor Glacier (Figs 3 and 4), likely because its freezing point is depressed by salts, and to a much smaller degree, pressure from the overlying ice. From the AEM data, we estimate a volume of 1.5 km3 for subglacial brine-saturated sediments below Taylor Glacier (Fig. 4). Porosity in glacial sediments varies but reported values are typically in the range of 20% or higher42. The ANDRILL AND-2A core drilled near the mouth of Taylor Valley yielded ∼20–30% porosity in Late Quaternary glacigenic sediments43. In the saturated sediments below Taylor Glacier, a porosity of just 12% is required for a subglacial brine volume equivalent to the water column volumes of Lakes Bonney, Fryxell and Hoare combined (∼0.18 km3).

Geologic evidence indicates that the MDV was a fjord ecosystem during the Miocene when seawater intruded Taylor Valley beyond the current extent of the Taylor Glacier44,45. Subsequent climatic cooling may have led to a build-up of salts through freezing (cryoconcentation) of the saline water46 creating dense brines. Our data indicate that this brine still exists beneath the Taylor Glacier (Figs 3 and 4) an inference that is further supported by the presence of Blood Falls (Fig. 4a). Low-resistivity (<0.17 Ωm) subglacial water discharges from Blood Falls intermittently and is saline enough to remain liquid to temperatures as low as −6 °C at atmospheric pressure7. Multiple lines of evidence support a marine origin of this subglacial effluent: the major ions were present in marine ratios (Na:Cl=0.88 in Blood Falls; Seawater=0.86)46, the δ37Cl signature was marine (∼0.0‰)46 and genomic material and bacterial isolates recovered from the brine were phylogenetically related to marine lineages47. Our AEM results suggest that discharges at Blood Falls are sourced from a more regionally extensive body of subglacial brine and not a small-scale feature confined to the terminus of Taylor Glacier. Such cryogenically concentrated fluids may underlie other parts of the Antarctic ice sheet margins. Findings presented here suggest that other parts of the ice sheets with beds below the pressure melting point of freshwater ice may contain liquid water and may move through basal sliding48, rather than internal deformation alone.

The unfrozen brines under the surveyed lakes (Figs 3 and 5) could be accounted for by solute concentration due to freezing and/or evaporation events of a large paleolake, see, for example, ref. 49. Models based on radiocarbon chronology of perched deltas, shorelines and other lake deposits suggest that Glacial Lake Washburn occupied much of Taylor Valley during the Last Glacial Maximum up to an elevation of ∼300 m above sea level (a.s.l.)17,50,51. However, soluble salt accumulation in MDV soils suggests that Lake Washburn only occupied the west end of the valley up to the same elevation49. Following retreat of the Ross Sea Ice Sheet, smaller lakes occupied Taylor Valley in both ends up to ∼120 m a.s.l. as controlled by lake sills or spill points. Geochemical profiles in the current water columns46 indicate that, within the past 1,000 years, lake levels in the Taylor Valley were lower than present day. Thus, the current lakes appear to be remnants of these larger paleolakes following periods of major drawdowns to small ponds or even complete evaporation, with subsequent refilling with less saline waters to modern day levels52,53. As lakes in the Taylor Valley lowered and concentrated, dense bottom brine would have infiltrated the highly permeable glacial till in the basin, sinking within the subsurface, similar to the above proposed formation of the brine below Taylor Glacier. Alternatively, these subsurface brines could be a legacy of much older marine deposition. The presence of unfrozen soil extending beyond the current lake margins to elevations approximating the estimates of a high stand Glacial Lake Washburn (Fig. 5) supports the large lake hypothesis of Hall and Denton51.

Previous to our study, the MDV lakes were viewed as being isolated from one another. From the surface, Canada Glacier appears to be preventing communication between the surface waters of Lakes Hoare and Fryxell (Fig. 1a). However, our data suggest that there is flow from the bottom of Lake Hoare into Lake Fryxell (Fig. 3 and Fig. 6). The implication of this to the geochemistry of the lakes is profound. It was previously thought that Lake Hoare completely evaporated around 1,200 years ago and its salts blew away. In this model, the relatively fresh, modern Lake Hoare resulted from a subsequent refilling with Canada Glacier melt waters52. An alternative hypothesis for dilute Lake Hoare water is that Lake Hoare is a headwater lake in our groundwater system. Lake Fryxell on the other hand is more brackish as it is receiving some portion of its bottom water from the groundwater flow system. Lake Bonney has the most saline bottom water in the valleys, which similarly may be related to its position as a terminal lake in a separate groundwater system receiving contributions from the saline subglacial marine brines from beneath Taylor Glacier (Fig. 3).

Figure 6: Conceptual diagram depicting predicted hydrological connectivity. Two distinct regions of subsurface brine were identified in the MDV. The ‘?’ indicates the zone between Lake Bonney and Lake Hoare where no connectivity was identified with our survey. Full size image

The weight of Canada Glacier could cause subsurface discharge at the glacier terminus and/or into Lakes Hoare and Fryxell. Our AEM data indicate that Canada Glacier has over-ridden what we interpret as lake water and brine-saturated sediments however a surface discharge feature has not formed. Discharge sourced from beneath Canada Glacier would involve squeezing of local groundwater or recycling of proglacial lake water. Thus, the lack of a Blood Falls like feature at Canada Glacier supports the model for Taylor Glacier and Blood Falls, where discharge requires an upstream brine supply.

Blood Falls is the only known surface manifestation of these deep brine systems and has been shown to contain a viable ecosystem with numerable microbial cells (∼6 × 104 ml−1). These numbers are typical for groundwater (∼1 × 103–1 × 104 cells ml−1)54 and other subglacial environments (∼1 × 104–1 × 107 cells ml−1)2. Previous work has shown that some of the energy needed to support cellular biosynthesis in this microbial community is gained from oxidation–reduction reactions that involve iron and sulfur, resulting in the liberation of iron as Fe (II)7. Silica concentrations in Blood Falls effluent are also high relative to other streams in the MDV55, suggesting a high degree of weathering below Taylor Glacier, which is likely enhanced by microorganisms10,28. If Blood Falls brine is representative of the subsurface fluid observed with AEM, an extensive ecosystem exists below the Taylor Glacier and much of Taylor Valley (Figs 3 and 6). DVDP borehole temperature logs indicate that in situ temperatures at depths where resistivity is indicative of liquid range between −3 to −20 °C (ref. 39), temperatures considered within the range suitable for microbial life56. Thus, the relative frequency of resistivity measurements across the Lower Taylor Valley (Fig. 2) shows the prevalence of potential habitats where temperature, salinity and liquid water might combine to support life.

Brine systems within and below permafrost along Antarctica’s coastal margins may influence surface ecosystem processes. Blood Falls reveals how microbial metabolism can release iron from underlying bedrock, which is ultimately discharged to the surface or below ground to Lake Bonney. Two major contributions of bioavailable iron to the Southern Ocean include aeolian dust (0.01–0.13 Tg per year) and nanoparticulate iron (0.06–0.12 Tg per year) in iceberg entrained sediments57. Submarine groundwater discharge, is another unaccounted for, and potentially vital source of iron and silica to a micronutrient limited Southern Ocean11. Release events at Blood Falls are episodic. We calculate, based on a surface discharge estimate of ∼2,000 m3 in volume58 with Fe and Si concentrations in outflow of 3.2 mM (ref. 7) and 264 μM (ref. 55), respectively, that a release event can deliver ∼420 kg of bioavailable Fe and 13.5 kg of Si to proglacial Lake Bonney. While similar subglacial outflow events of coastal glaciers might represent small, episodic releases of growth-limiting micronutrients, these pulses could still significantly enhance lake or near-shore marine productivity. Discharge events like at Blood Falls would represent only a small fraction of the subsurface groundwater discharge possible along coastal margins. The total flux of these nutrients remains poorly resolved; however, a recent report estimates iron flux from ice sheet meltwaters at 0.06–0.17 Tg per year, which is comparable to aeolian fluxes to polar waters59. If Antarctic submarine groundwater discharge is relatively rich in dissolved iron, for instance, if it has the concentration of iron comparable to that in Blood Falls brines, then it would only take a modest discharge of approximately 0.3–0.9 km3 to supply 0.06–0.17 Tg per year of Fe to the Southern Ocean. This represents about 0.5–1.5% of the total annual subglacial meltwater production estimated for Antarctica (∼60 km3)60. On other continents, submarine groundwater discharge represents a much higher fraction of their total surface water inputs, 6–10% (ref. 61). The paucity of constraints on groundwater pressure gradients and hydraulic conductivity distribution in Taylor Valley prohibits us from estimating the specific regional contribution of submarine groundwater discharge.

The subpermafrost brines in the MDV provide an important terrestrial analogue for future exploration of a subsurface Martian habitat. Briny groundwater has been suggested as supporting a deep biosphere on Mars62. Recent mineralogical analysis of Gale Crater supports the notion that previous fluvio-lacustrine environments may have hosted chemoautotrophic microorganisms63. On Mars, as we observe in the dry valleys, connectivity between lacustrine systems and groundwater would be important in sustaining ecosystems through drastic climate change, such as lake dry-down events63.

On the basis of the first AEM study of the MDV region, we conclude that a deep briny groundwater system exists beneath glaciers, lakes and permafrost in Taylor Valley (Fig. 6). These brines appear related to the long-term geological history of the MDV and may represent ancient changes in sea level and subsequent marine intrusion and the draw-down of paleolakes linked to the Last Glacial Maximum and recent climate variation. We observed geophysical evidence of hydrological connectivity between lakes, which were previously assumed to be isolated from one another. This finding has significant implications for interpreting past geochemical models of the evolution of dry valley lake chemistry and biology. The subsurface deep brines contain an active microbial community, as evidenced by the surface release of brine at Blood Falls, Taylor Glacier. Our results also suggest that brine flows towards the coast from ∼18 km inland where it must become submarine discharge. Microbial weathering of mineral substrates in subsurface groundwater discharge may be a significant source of solutes to the Southern Ocean. The subpermafrost brines in the MDV may provide an important terrestrial analogue for future exploration of a subsurface Martian habitat.