Supplementary Figure 1. Images from sampling locations. a) and b): Field observations of meltwater ponds, Tear Drop Lake and Boulder Lake,

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1 Supplementary Figure 1. Images from sampling locations. a) and b): Field observations of meltwater ponds, Tear Drop Lake and Boulder Lake, respectively. The ponds are situated at the depressions of the LCIT moraine ridges, and are small and shallow; c) and d): Field observations of evaporite mounds and beds, respectively. The mirabilites are water-clear crystals in the core part of evaporites, covered by chalky minerals of thenardite and minor nahcolites, trona and borax.

2 Supplementary Figure 2. Time-resolved X-ray diffraction patterns of the clear core of the evaporite mounds. All measurements were made at room temperature. The beginning pattern (bottom) contained primarily mirabilite, with minor thenardite, and thenardite gradually increased in abundance at the expense of mirabilite with time, as a result of dehydration. The initial sample also contained minor nahcolite.

3 Supplementary Figure 3. δ 18 O vs. δ D of snow, glacial ice, secondary glacial ice, and melt-water ponds from LCIT. The thick black line is defined by the equation for meteoric precipitation (δ D = 8(δ 18 O + 10). All data fall to the left side of the GMWL, indicating those waters experienced slight evaporation. Symbol size is larger than error bars.

4 Supplementary Table 1. Major ion concentrations in the melt-water ponds at the Lewis Cliff Ice Tongue, units are weight ppm (mg/kg). F - Cl - SO 4 2- NO 3 - Na + Ca 2+ Kip Kettle Tear Lake Drop Lake Monroe

5 Supplementary Table 2. Sample location and type, stable isotope data, and ph for samples collected from Lewis Cliff Ice Tongue during the field season, and for samples from Ace Lake. SAMPLE LOCATION SAMPLE Name SAMPLE TYPE (VSMOW) D (VSMOW) (VSMOW) (water) S VCDT (VSMOW) (sulfate) (VSMOW) (sulfate) ph Water T( C) Eli Ledges ice lens IL-01 IL Chimney A ice lens IL-02 IL Fairy Castle F ice lens IL-03 IL Chimney B ice lens IL-04 IL Lower Chimney B ice lens IL-05 IL Site C Ice lens IL-06 IL Below Chimney A ice lens Moraine ice north of camp Snow at lower Chimney B (12/28/05) Snow at Big Boy Lake (12/27/05) IL-07 IL IL-09 IL S-01 S S-02 S Camp snow (12/29/05) S-03 S Snow at site C (12/29/05) Snow at site C (12/25/05) S-04 S S-05 S Camp snow (12/29/05) S-06 S Snow at Fairy Castle F (12/30/09) S-07 S

6 Camp snow (1/2/06) S-08 S Snow at Chimney A (12/28/05) Snow at Eli Ledges (12/29/05) Camp snow collected 12/ /05 Glacial ice south of camp Moraine ice near Fairy Castle F S-09 S S-10 S S-11 S G-02 I G-03 I Lemon Lake (12/26/05) L-01 L Boulder Lake (12/26/05) L-02 L Lake Monroe (12/26/05) Lower Chimney Lake (12/26/05) L-04 L D 1.36 D D L-05 L Big Boy Lake (12/27/05) L-06 L Kip Kettle (12/28/05) L-07 L D 2.10 D D Lake Okoboji (12/26/05) L-08 L Lower Chimney Lake (1/2/06) L-09 L Lake Monroe (1/2/06) L-10 L Tear Drop Lake (1/2/06) L-11 L D 2.35 D D Lake Okoboji (1/2/06) L-12 L Mirabilite from Chimney A-1 Mirabilite from Chimney A M M Mirabilite at Eli Ledges M Mirabilite at Chimney B M

7 Mirabilite at Fairy Castle F M Ace Lake 5.5m depth* D Ace Lake 15.5m depth* D Sample type key: IL = Secondary glacial ice lenses (re-frozen periglacial ice lenses), S = Precipitation (snow), I = Glacial ice, L = Melt-water pond, M = Mirabilite, D=Dissolved Sulfate 2. All δ are in *Sample information is described in detail by ref 2

8 Supplementary Discussion: δ 18 O and δd values of precipitation (snow), glacial ice, secondary glacial ice (ice lenses), and lake water range from to -29.7, and to , respectively (Supplementary Table 2 and Fig. 3). Nearly all samples fall on or to the right of the Global Meteoric Water Line (GMWL) defined by the equation δd = 8 δ 18 O δ 18 O and δd values of snow (local precipitation) are always higher than those of glacial ice. We interpret this as an indication that glacial ice originated at higher latitudes or higher altitudes and migrated to its present location, consistent with the geography of ice flow at the LCIT, whereas snow originated from more local precipitation sources. Previous work at LCIT identified geographic ice origins based on oxygen isotope compositions and suggested two sources for the ice at LCIT, one from a comparatively warm, more-local source, and the other from a colder, probably more-inland source. Our data suggest that the eastern edge of the ice tongue, where the mirabilite mounds are found, corresponds to the colder, more inland ice source previously identified by Grootes. Glacial ice that we measured at the edge of the LCIT tends to fall on the GMWL, whereas secondary glacial ices (ice lenses or periglacial ice) tend to fall to the right of the MWL, indicating an evaporation trend for these ices (Supplementary Fig.3). A similar evaporation trend was observed in glacial ice from the Beacon Valley, Antarctica. Likewise, our data suggest that pond water is a product of melted glacial water mixed with some local precipitation. Furthermore, the ponds do not appear to have experienced severe evaporation. Based on these data, we term these water bodies Melt-Water Ponds.

9 Supplementary References 1 Craig, H. Isotopic variations in meteoric waters. Science 133, 1702-&, doi: /science (1961). 2 Lauro, F. M. et al. An integrative study of a meromictic lake ecosystem in Antarctica. Isme Journal 5, , doi: /ismej (2011). 3 Grootes, P. in Workshop on Antarctic Meteorite Stranding Surfaces. (ed W. Cassidy and I. Whillans) Marchant, D. R. et al. Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica. Geological Society of America Bulletin 114, , doi: / (2002)114<0718:fopgas>2.0.co;2 (2002).