Study area

The McMurdo Dry Valley's landscape is a mosaic of bedrock, soils, glaciers, ice-covered lakes and ephemeral streams in southern Victoria Land between 77–78°S latitude. The climate is considered a polar desert with mean annual temperatures on the valley bottom between -14.8 to -30.0°C (Doran et al. Reference Doran, McKay, Clow, Dana, Fountain, Nylen and Lyons2002) and precipitation ∼ 5 cm a^-1 (Witherow et al. Reference Witherow, Bertler, Welch, Lyons, Mayewski, Sneed, Nylen, Handley and Fountain2006). The upland ponds under investigation here consist of two separate complexes in the south-central portion of Taylor Valley, one above the Nussbaum Riegel, a 700 m ridge (Marr ponds), and one c. 4 km to the east of the Nussbaum Riegel in a topographic depression at 350 m (Parera ponds). The geology of this region consists of two major series of meta sedimentary rocks, one rich in carbonate and one not, including schists, quartzites and marbles, which are in turn in contact to the west with a granodiorite; these formations are cut by lamprophyric dykes (Haskell et al. Reference Haskell, Kennett, Prebble, Smith and Willis1965). The western portion of Nussbaum Riegel is dotted with recent McMurdo Volcanic basaltic cones. The glacial drift and morainal materials overlying portions of the metamorphic and granodiorite rocks is thought to be derived from the advance of the Taylor Glacier (i.e. East Antarctic Ice Sheet) from the west, and is termed Taylor IV drift. The surface on which the Parera ponds are located is between 1.5 and 2.55 Ka old (Wilch et al. Reference Wilch, Denton, Lux and McIntosh1993). The surfaces occupied by the Marr ponds at 750 m a.s.l. investigated here are thought to be the oldest surfaces in Taylor Valley (Marchant & Denton Reference Marchant and Denton1996).

The Nussbaum Riegel serves as a climatic delineation, as the altitude of the glacier equilibrium line (i.e. the separation between mass gain and mass loss), changes abruptly at the Riegel, shifting from ∼ 400 m on the east side, closer to the coast, to ∼ 1200 m inland on the west side of the Riegel (Fountain et al. Reference Fountain, Lyons, Burkins, Dana, Doran, Lewis, McKnight, Moorhead, Parsons, Priscu, Wall, Wharton and Virginia1999). Local meteorological data from the Nussbaum Riegel are not available. The MCM-LTER program has maintained a series of automated weather stations in the valleys since 1993, with the closest stations being on the Howard Glacier ∼ 8 km to the east of the Riegel at 473 m elevation and at Lake Bonney, ∼ 7 km west of the Reigel at 64 m elevation (Fig. 1). Since 1993, the mean annual temperatures at these locations are -17.2°C and -16.9°C respectively.

There is a strong spatial gradient of snowfall in Taylor Valley, where less snow falls inland because the Nussbaum Riegel acts as a barrier, blocking low-level clouds that move moisture inland from the ocean (Fountain et al. Reference Fountain, Nylen, Monaghan, Basagic and Bromwich2010). Precipitation measured at the nearby Howard Glacier, the closest site to the upland ponds area, ranged from 2–74 mm water equivalent (weq) from 2004–06 (only data available). However, net accumulations can be higher due to wind drifting events (20–87 mm weq) (Fountain et al. Reference Fountain, Nylen, Monaghan, Basagic and Bromwich2010). Our observation during the period of the MCM-LTER project is that much, if not all, of the spring and summer precipitation is rapidly sublimed and unavailable as runoff, especially at these elevations.

The location of the Taylor Valley upland ponds is shown in Fig. 1. Table I provides a brief description of the ponds and their hydrologic properties. The Marr ponds (unofficial name – officially known as the Kaki Ponds) are at an elevation of c. 750 m a.s.l. and consist of five ponds. Marr ponds 1, 2, 3, and 5 receive direct meltwater input from the Marr Glacier. Marr ponds 1–4 are also connected hydrologically: Pond 1 flows into Pond 2 which flows into Pond 3, which finally flows into Pond 4, the terminal pond. Marr pond 5 is on a bedrock ridge and volcanic cone to the west of the other Marr ponds, and receives melt from the Marr Glacier but is not hydrologically connected to the other ponds.

Table I Pond sizes and hydrologic properties.

^aMarchant & Denton 1996

The Parera Ponds complex is at c. 350 m, and includes associated wetlands and extended hyporheic zones in the down slope areas of the drainage. This pond complex consists of one large pond (Parera Pond also referred to as “Highland Pond” in Moorhead Reference Moorhead2007) and two smaller more ephemeral ponds referred to here as Parera west and Parera south (Fig. 1). These ponds receive meltwaters from the Goldman, Moa and the Marr glaciers (Fig. 1). The water in these ponds has extensive interaction with their inflow channels, showing large “wetting zones” or hyporheic interactions many meters away from their primary channels. Water occasionally ponds along the edges of these channels in late December/early January and “wetland” regions can be extensive.

Results and discussion

Major ion (both cation and anion) data and stable isotope data of water (where available) for the ponds are tabulated in Appendix A. The 2001–02 summer was abnormally warm and produced a large amount of stream discharge (Foreman et al. Reference Foreman, Wolf and Priscu2004), as well as flow features, such as seeps, which had not previously been observed in Taylor Valley (e.g. Lyons et al. Reference Lyons, Welch, Carey, Doran, Wall, Virginia, Fountain, Csatho and Tremper2005). Since 2001–02, stream flows have generally remained substantially lower, with the exception of the 2007–08 season, which was slightly higher than others (www.mcmlter.org) were.

In general, the cation chemistry of the Marr ponds and most of the Parera ponds clusters together on a ternary diagram (Fig. 2a). A few of the Parera Pond samples contain higher relative Mg²⁺ than the others, with one having Mg²⁺ as the major cation and much lower Na⁺ + K⁺ concentrations. The explanation for this is not obvious, although there are abundant cones of the McMurdo volcanics that could be undergoing weathering to produce these different cation variations. The higher Mg²⁺ concentrations could possibly reflect a higher degree of evaporation history of the pond water at these times, as Mg²⁺ is concentrated relative to Ca²⁺ during the evapoconcentration process (Eugster & Jones Reference Eugster and Jones1979).

Fig. 2a Ternary diagram for major cations. Ca²⁺, Mg²⁺ and Na⁺ + K⁺ are shown as percent by equivalents.

The Marr pond 5 samples are enriched in Ca²⁺ compared to the others (Fig. 2a) and closely resemble the cationic composition of the ponds in Victoria Valley as noted by Healy et al. (Reference Healy, Webster-Brown, Brown and Lane2006). The Marr pond data represent a transition of compositions from Wright Valley to Victoria Valley cationic concentrations (Healy et al. Reference Healy, Webster-Brown, Brown and Lane2006). The anionic diagram (Fig. 2b) shows a wide range of HCO₃^- and Cl^-, but a much smaller range of SO₄^2- values. Only three of the Parera Pond samples have SO₄^2- > 20%, with relative Cl^- concentrations varying between 95 and ∼50%. The Marr pond 5 samples contain the highest relative HCO₃^- concentrations. The Ca-Na-Cl Marr pond 5 waters are very unusual in the MCM region; the Parera Pond waters are Na-Cl and Mg-Cl. Clearly this is a very wide diversity of chemical composition within a close proximity. The Na-Cl rich waters reflect the primary precipitation signal of the marine aerosol, as the majority of samples have Na:Cl slightly less than 1:1 (Fig. 3). The enrichment of Ca²⁺, Mg²⁺ and/or HCO₃^- clearly suggests that the chemical weathering of alkaline earth-rich minerals within the catchments is a major process even at these low mean annual temperatures. This enrichment may also be due to the dissolution of CaCO₃-rich dust introduced by Aeolian processes, especially in the winter. The source of the CaCO₃-rich dust is unknown, but it is found on the alpine glaciers in Taylor Valley with some of the highest values on the nearby Howard Glacier (Witherow et al. Reference Witherow, Bertler, Welch, Lyons, Mayewski, Sneed, Nylen, Handley and Fountain2006).

Fig. 3 Ternary diagrams for major anions. Cl^-, SO₄^2- and alkalinity are shown as percent by equivalents.

Fig. 3 Sodium versus chloride in millimolar concentrations for the pond samples. The seawater line represents the ionic ratio of Na to Cl in seawater.

Because Ca²⁺ and CO₃^2- binary salts are the least soluble in these types of environments (Healy et al. Reference Healy, Webster-Brown, Brown and Lane2006), the enrichment of these ions in the pond waters is not due to selective loss of the other major ions in the solution. In the Marr pond 5 region, as noted above, the abundance of McMurdo volcanic rocks must be a major source of solute acquisition via weathering. Unlike previous work in both the Labyrinth ponds in Wright Valley and the ponds in Victoria Valley, the enrichment of HCO₃^- over SO₄^2- makes the Marr pond 5 pond waters unusual. Also unlike the Wright Valley ponds, these Taylor Valley ponds have very low NO₃^- concentrations (Appendix A), with the highest values being only a few μM. Because our samples were collected in December and January and because all these ponds have abundant algal mats associated with them (Moorhead et al. Reference Moorhead, Barrett, Virginia, Wall and Porazinska2003), we speculate that most of the input of N into these systems is rapidly taken up by the benthic mats. Total N along the edges of these ponds has been determined to be ∼ 11 μM with C:N molar ratios of 9:1 (Moorhead et al. Reference Moorhead, Barrett, Virginia, Wall and Porazinska2003).

Cl^- concentrations in these ponds range over three orders of magnitude (Fig. 3), demonstrating the significance of evapoconcentration via freezing and sublimation (i.e. water loss) and salt dissolution (i.e. salt gain) as major processes controlling their geochemistry, supporting the earlier work of Torii et al. (Reference Torii, Nakaya, Matsubaya, Matsumoto, Masuda, Kawano and Murayama1989), Webster et al. (Reference Webster, Brown and Vincent1994), Timperley (Reference Timperley1997), Healy et al. (Reference Healy, Webster-Brown, Brown and Lane2006) and Wait et al. (Reference Wait, Webster-Brown, Brown, Healy and Hawes2006). Although the Na:Cl ratio is similar to seawater, suggesting a marine aerosol source as indicated by Torii et al. (Reference Torii, Nakaya, Matsubaya, Matsumoto, Masuda, Kawano and Murayama1989) for the Labyrinth ponds in Wright Valley and the upland ponds in Victorian Valley (Healy et al. Reference Healy, Webster-Brown, Brown and Lane2006), most of the ponds are slightly enriched in Cl^- relative to seawater (Fig. 3). At Na⁺ concentrations above 2 mM, the Cl^- enrichment becomes greater, perhaps suggesting loss of Na⁺ via Na₂SO₄ precipitation or the dissolution of more soluble Cl^- rich salts over time. The highest Cl^- and Na⁺ values occur in the lowest elevation pond with the longest flow path: the Parera Pond complex.

The Ca:Cl ratio of all the ponds is greater than that of seawater, but approaches the seawater value at the highest Cl^- concentrations (Fig. 4). We interpret this to mean that Ca²⁺ is gained through rock/soil weathering (above marine aerosol values) and is lost via CaCO₃ precipitation as the pond waters are concentrated via freezing and/or evaporation, as suggested by Webster et al. (Reference Webster, Brown and Vincent1994). The highest Ca²⁺ enrichment to Cl^- (∼1:1) relative to seawater is in Marr pond 5 where abundant basalt outcroppings occur and where chemical weathering may be more extensive.

Fig. 4 Calcium versus chloride in millimolar concentrations for the pond samples. The seawater line represents the ionic ratio of Ca to Cl in seawater.

Figure 5 shows the Mg²⁺ + Ca²⁺ data plotted vs 2 x HCO₃^- (alkalinity) + SO₄^2-. Points close to the 1:1 line reflect either the dissolution of Mg + Ca:CO₃ + SO₄ salts or the initial marine aerosol input (this is particularly true at the lower concentrations). Points with excess HCO₃ + SO₄ (above 1:1) reflect chemical weathering of minerals and/or dissolution of salts with cations other than Ca²⁺ or Mg²⁺, while those data falling below the line indicate samples enriched in Ca²⁺ and Mg²⁺ relative to HCO₃^- and SO₄^2-. These latter samples may represent what Healy et al. (Reference Healy, Webster-Brown, Brown and Lane2006) have termed “suprapermafrost fluids” enriched in CaCl₂ brines. These fluids have lost Na through Na₂SO₄ precipitation as the waters are concentrated. It is possible that these few samples do contain a fraction of hypersaline Ca-Cl fluid, but the Marr ponds samples also have higher concentrations of HCO₃^- + SO₄^2-, suggesting that they are a mix of this suprapermafrost brine plus slightly evaporated surface waters containing weathering products. Because we have little knowledge of the hydrology and sub-surface water movement in these regions, we cannot definitively determine the sources of water or the solutes in these ponds. Clearly, our data suggest that these ponds are more complex geochemically than many of the other ponds in the McMurdo region. The complexity of these ponds could be a result of their relative elevation, long flow paths and large catchments.

Fig. 5 The sum of bicarbonate and sulphate versus the sum of magnesium and calcium as milliequivalents per litre for pond samples.

Dissolved Si concentrations in the ponds range from 0.022–0.123 mM with no geographic pattern (Appendix A). These values are within the range observed in the streams on the Taylor Valley floor (Welch et al. Reference Welch, Lyons, Whisner, Gardner, Gooseff, McKnight and Priscu2010). Because dissolved Si can only be derived from the chemical weathering of aluminosilicate minerals, it is apparent that even at extremely low mean annual temperatures, chemical weathering is occurring in the stream channels going to and from these ponds.

Role of evaporation in concentrating solutes

We analysed a number of pond water samples for their stable isotopic composition (Fig. 6). The initial glacier melt should reflect the δ¹⁸O value of the snow/ice of the glaciers. Moving inland from the ocean in Taylor Valley, the δ¹⁸O of the precipitation becomes lighter, with Howard Glacier (east of Marr Glacier) having a δ¹⁸O value of -27‰ and Hughes Glacier (to the west of Marr Glacier) having a value of -30.8‰ (Gooseff et al. Reference Gooseff, Lyons, McKnight, Vaughn, Fountain and Dowling2006). Although we have not analysed any samples from the Marr Glacier, based on our previous work on the glaciers throughout Taylor Valley, the Marr Glacier should have values between -28 and -29‰ (Gooseff et al. Reference Gooseff, Lyons, McKnight, Vaughn, Fountain and Dowling2006). A water sample collected in 2004 from a stream flowing directly from the Marr Glacier yielded a δ¹⁸O value of -29.2‰, suggesting that our δ¹⁸O isotopic composition estimate for the Marr Glacier melt is appropriate. As water evaporates, the ¹⁶O is preferentially lost and the liquid remaining becomes enriched in ¹⁸O. The freshest pond waters have δ¹⁸O of -28.2‰ and -27.2‰ (Fig. 6), perhaps suggesting that even these lowest Cl^- waters have experienced some degree of evaporation. All but one of the samples above 5 mM Cl^- have δ¹⁸O more enriched than -26‰, with one Parera Pond sample having a value of -18.3‰ and a Marr pond sample having a value of -17.3‰. Water is lost from these ponds from sublimation of their ice covers, especially during colder years. This loss is replaced by the freeze-on of ice at the ice-water interface. The production of ice from liquid water at the surfaces of these ponds also fractionates the isotopes leading to ice covers with more enriched isotopic values (Horita Reference Horita2009). The isotopic enrichment of ice with respect to water ranges from 1.00291–1.0048 for ¹⁸O/¹⁶O with 1.00291 thought to be the best estimate (Horita Reference Horita2009). Therefore, the more enriched stable isotope values of the water cannot be produced from ice development as the fractionation is in the wrong direction. Both evaporative loss of ¹⁶O and the sublimative loss of ¹⁸O complicate the interpretation of the isotope data. Clearly, many of the samples are greatly enriched in ¹⁸O relative to the Marr Glacier melt. These samples indicate evaporative processes have occurred in the ponds or in streams in transit to the ponds, or both. With one exception, the most ¹⁸O-enriched samples are in the Parera ponds, which are at the lowest elevations (i.e. warmer locations) and have the longest stream lengths (Table I; Fig. 1). Samples with glacier-like δ¹⁸O signatures but higher dissolved salt concentrations may reflect recent unevaporated glacier melt that has redissolved previously precipitated salt or mixed with “flushed out” preconcentrated salt from the upflow ponds (Timperley Reference Timperley1997). This process has explained similar chemistries in hot desert ecosystems (Drever & Smith Reference Drever and Smith1978).

Fig. 6 δ¹⁸O‰ versus chloride for pond samples. Not all of the pond samples have been analysed for oxygen isotopic ratio.

Role of climate variation on pond evolution

The size, chemistry and even existence of these ponds depend primarily on the amount of glacier melt produced during the summer. The MCM-LTER has done extensive work since 1993 to document the linkage between changing meteorological conditions, primarily temperature, glacier melt and stream flow (e.g. McKnight et al. Reference McKnight, Niyogi, Alger, Bomblies, Conovitz and Tate1999, Ebnet et al. Reference Ebnet, Fountain and Nylen2005). The geochemistry of the surface waters of the large lakes in Taylor Valley (i.e. lakes Bonney, Fryxell and Hoare) has responded to changes in meltwater inflows. For example, the Cl^- concentration in Lake Hoare, the freshest of these lakes, increased by a factor of c. 2 during the low flow years between 1993–94 and 1997–98 (Welch et al. Reference Welch, Neumann, McKnight, Fountain and Lyons2000). In addition to glacial melt, substantial melt can occur from multi annual snowpacks and subsurface melting of permafrost during extremely warm years, although these contributions to the overall hydrology of the valley are probably small (Lyons et al. Reference Lyons, Welch, Carey, Doran, Wall, Virginia, Fountain, Csatho and Tremper2005, Harris et al. Reference Harris, Carey, Welch, Lyons and Fountain2007). Temperature data from the LTER's network of automatic weather stations have been used to model the generation of glacier melt, and these models have been validated using the LTER's stream gauge records (Jaros Reference Jaros2002, Ebnet et al. Reference Ebnet, Fountain and Nylen2005). We have used the model developed by Jaros (Reference Jaros2002) to predict meltwater generation from both the Marr and the Goldman glaciers from the 1990/91–2008/09 seasons (Table II). This model uses temperature data collected at the Taylor Valley floor (Lake Fryxell station) and an adiabatic lapse rate calculation to predict temperatures at the elevations of these glaciers. Although a simple approach, this model has worked very successfully at predicting the flows from the lower elevation glaciers in the eastern portion of Taylor Valley including Commonwealth, Canada and Howard glaciers (Jaros Reference Jaros2002). During this period, according to our model, melt from the Goldman Glacier occurred in all but five years, although flow volume varied by approximately a factor of 9 during the 14 summers when flow did occur. Conversely, for the Marr Glacier, flow occurred only in three summers during this period, according to our model simulations. These summers were the warmest of this period, and only one with substantial melt (i.e. > 1000 m³) took place since the establishment of the MCM-LTER (i.e. 2001–02).

In previous work, Moorhead et al. (Reference Moorhead, Barrett, Virginia, Wall and Porazinska2003) noted that Marr 3 pond was almost dry and Marr ponds 1, 2 and 4 had extensive algal mat distribution beyond their shorelines and had “thick ice with high albedo” but with melted out margins. During 2000–01, there was stream flow into Marr 2 pond and from the eastern portion of the Marr Glacier, but it did not reach the Parera ponds as it disappeared 50 m down slope (Moorhead et al. Reference Moorhead, Barrett, Virginia, Wall and Porazinska2003). There was also no flow observed between the ponds as normally observed (Fig. 1, Table I). These earlier observations by Moorhead et al. (Reference Moorhead, Barrett, Virginia, Wall and Porazinska2003) suggest that our model simulations may underestimate melt from these glaciers into these ponds. There is no doubt that the 2001–02 summer produced extensive melt and surface water flow through both Taylor and Wright valleys (Foreman et al. Reference Foreman, Wolf and Priscu2004). This increase in melt also occurred at higher elevations in the valley as shown in the measured pond areas in Table II, and suggests that the hydrology of the valley is extremely sensitive to the short, intense warming events (Doran et al. Reference Doran, McKay, Clow, Dana, Fountain, Nylen and Lyons2002, Lyons et al. Reference Lyons, Welch, Carey, Doran, Wall, Virginia, Fountain, Csatho and Tremper2005).

Our modelled flows can be compared to the actual measurement of the pond sizes during this time period (Table II). In general, the Marr ponds and the large Parera Pond are relatively small from 1992/93–2000/01, and Parera south and Parera west were not present in 1992–93. Most of the summers in this period were very cool with medium to very low flows in the valley streams (Doran et al. Reference Doran, McKay, Clow, Dana, Fountain, Nylen and Lyons2002). From 2000/01–2002/03, the ponds increased in size. The 2001–02 summer was an extremely high flow season (deemed the “flood year”) when the valley lakes increased greatly in volume (Foreman et al. Reference Foreman, Wolf and Priscu2004). There was another increase in size in the large Parera Pond during 2003–04, with only slight but significant fluctuations in size after this time. The measured Cl^- concentrations in Marr pond 1 and Marr pond 2 reflect the pond sizes at least generally. Cl^- concentrations decrease from 2001/02–2002/03, increase in 2003–04, change little during 2005–06 with a slight decrease in 2006–07 and increases in 2008–09 and 2009–10 (Table II, Fig. 7). In general, variations over this period fit the picture of our knowledge of austral summer temperature variations and the subsequent hydrologic response on the Taylor Valley floor (www.mcmlter.org). There are years when all the Marr ponds and the Parera Pond complex have similar Cl^- values (2009–10, 2001–02, 2002–03), and years when the two groups of ponds vary greatly in their Cl^- concentrations (2007–08 and 2008–09). In 2002–03, it appears that the Parera system was diluted while the Marr system was also diluted, but the concentrations between Marr ponds varied by almost an order of magnitude. This difference in the Marr ponds’ Cl^- concentration may be due to lack of sufficient water to “flush” the system (this is similar to the fractionation of soluble salts as proposed in the Timperley (Reference Timperley1997) model), or the preferential loss of ice-cover and more enhanced evaporative loss in some of the ponds. The lowest Cl^- concentrations were in the uppermost ponds (Marr pond 1 and Marr pond 2) suggesting Cl^- was flushed from these ponds downstream into the lower elevation ponds. These data demonstrate that depending on the year, the amount of flow and surface solute concentrations can vary by at least an order of magnitude.

Fig. 7 Chloride concentrations for pond samples plotted versus time.

Our original hypothesis was that during “warm” summers when abundant melt is produced from the high elevation glaciers in Taylor Valley such as the Marr and Goldman, the upper set of nested ponds would “flush” salt that was previously concentrated during cooler summers to the terminal pond. Between 2003–04 and 2006–07, Parera Pond was at its largest area and in January 2006 it also had the highest measured solute concentrations (Cl^- = 225 mM). This suggests that the high flow event in 2001–02 and perhaps another in 2002–03 (Table II) transported large amounts of salt into the pond. The anion composition is dominated by Cl^- with low SO₄^2- and extremely low HCO3^- concentrations as proposed by Timperley (Reference Timperley1997) for flow-through pond systems. Two years after the maximum solute concentrations in Parera Pond, the Cl^- decreased by a factor of 45 times. Where had all this salt gone? We propose that during these accumulations of salt, the terminal ponds (such as Parera Pond) act similar to a “recharge playa” in warm, arid regions of the world. The brackish water accumulated by the flush of meltwater through the upstream pond is lost through sinkage at the bottom of the pond. We also propose that this salt water could flow down-slope at the bottom of the active layer (i.e. top of the permafrost) and become a potential liquid water source lower in the valley. Recent work by Levy et al. (in press) suggests that “water tracks” found in the Lake Hoare region could originate from similar processes. Solute transport by this mechanism may play an important role in transporting both salt and water in the lower valley soils (Levy et al. in press).