Stream channels in the McMurdo Dry Valleys are characteristically wide,
incised, and stable. At typical flows, streams occupy a fraction of the
oversized channels, providing habitat for algal mats. In January 2012, we
discovered substantial channel erosion and subsurface thermomechanical
erosion undercutting banks of the Crescent Stream. We sampled stream water along
the impacted reach and compared concentrations of solutes to the long-term
data from this stream (
General circulation models predict a disproportionate increase in high-latitude air temperatures over the next century due to polar amplification mechanisms (Serreze and Barry, 2011a, b; Bekryaev et al., 2010). A potential consequence of increasing polar air temperatures is increased ground surface energy balance, and accelerated permafrost degradation (Chadburn et al., 2015) including surface subsidence, thermokarst formation, and mass wasting of the landscape, with significant implications for adjacent aquatic ecosystems. A growing body of literature has documented varying impacts of permafrost degradation on streams and lakes, with a primary focus on Arctic tundra landscapes. When degradation occurs at large scales or is hydrologically well connected to associated aquatic ecosystems, order-of-magnitude increases of sediment, solute, and nutrient loads to streams and lakes are common (Kokelj et al., 2005, 2009, 2013; Larouche et al., 2015; Bowden et al., 2008). Under these conditions impact can persist for decades (Kokelj et al., 2005, 2009). In contrast, when hydrologic connectivity linking degraded permafrost to aquatic systems is limited, or a small portion of a catchment is affected, the impacts to associated aquatic ecosystems are reduced and transient (Lafrenière and Lamoureux, 2013; Lewis et al., 2012; Larouche et al., 2015; Lamoureux and Lafrenière, 2009).
Permafrost degradation has received much less attention in polar desert
environments, which are common in Antarctica and also occur in the Arctic.
One potential mode of permafrost degradation is from enhanced flow of water
across hillslopes (i.e., non-channelized flow). Polar deserts receive little
to no rainfall and therefore have less potential for permafrost degradation
to be associated with shallow subsurface or overland flow outside of stream
channels. There is some evidence of ancient (Shaw and Healy, 1977) and
centuries-old thermokarst activity (Healy, 1975; Campbell and Claridge, 2003)
in Antarctic polar deserts, but there are limited examples of contemporary
thermokarst features. The largest polar desert region in Antarctica is the
McMurdo Dry Valleys (MDV), which occupy approximately 22 700 km
Map of
The streams and lakes in the MDV have unique characteristics that will likely affect their responses to, and the impacts of, permafrost degradation. Stream flow in the MDV originates from melt water emanating from alpine, piedmont, and terminal glaciers. Glacial melt occurs for up to 10 weeks during the austral summer (McKnight et al., 1999) with significant diurnal, monthly, and inter-annual variability driven by varying sun angle, insolation (McKnight et al., 1999) and air temperature (Doran et al., 2008). Thus the hydrographs in these streams are dynamic, with streamflow typically varying two to ten-fold on a diel basis for example. Once melt water enters stream channels it interacts with surrounding sediments through hyporheic exchange (Runkel et al., 1998; Gooseff et al., 2003) which alters stream chemistry (Gooseff et al., 2002; Welch et al., 2010; McKnight et al., 2004). These streams support an assemblage of cyanobacteria, chemotrophic bacteria, and diatoms (Esposito et al., 2006; Stanish et al., 2013), and supply closed-basin (endorheic) lakes with water and solutes (Lyons et al., 1998; Green et al., 1988). Due to the geomorphic stability of the MDV over the past several thousand years, nutrient and solute loads derived from weathering processes occurring in the hyporheic zone (Gooseff et al., 2002) have likely been fairly constant. Thus, the potential introduction of nutrient pulses from stream-side thermokarst activity represents a new input that may significantly impact the nutrient status and biological communities of MDV streams and contribute to downstream closed-basin lakes.
In January 2012 we found fresh permafrost degradation features along the channel margins of the West Fork of Crescent Stream in Taylor Valley, which is one of the central and most studied of the McMurdo Dry Valleys. Unlike the previously described dry valley thermokarst which resulted from the melting of buried ancient ice, these thermokarst features are contemporary examples of stream water interacting with and thawing extensive areas of permafrost soils adjacent to a stream channel. The goals of this study were to (1) describe the thermokarst as a basis for comparison for potential new thermokarst features in the MDV and other polar deserts in the future, (2) document the impacts of this permafrost degradation event on stream water sediment and solute concentrations and (3) compare these impacts to a long-term historical record. The impacts were evaluated by comparison of water quality above and below the thermokarst feature and by comparison of water quality of the impacted West Fork and the East Fork at their confluence.
The McMurdo Dry Valleys (77
On 19 January 2012 we observed major down cutting, sediment
deposition, and reworking of the stream channel at the long-term stream gauge
site on Crescent Stream (77.619064
Map of images captured along the West Fork of Crescent Stream, as
observed in January 2012, in which each black dot indicates a location at
which a picture was taken, and the green dots indicate images from a few
highlighted locations along the channel. Light blue highlighted image is of
the entrance to the thermokarst tunnel on the east bank of the West Fork.
Permafrost degradation features were observed along
On 21 January 2012, we conducted sampling to determine the impacts
of the thermokarst-affected reach, and in particular the thermokarst tunnel,
on stream water chemistry and sediment transport during a highly variable
portion of the diurnal hydrograph (a doubling of discharge occurred during
the sampling period). Prior to sampling, a Cutthroat Flume (Baski, Inc.,
Englewood, Colorado) was installed in an appropriate reach of the West Fork
to monitor discharge throughout the sampling. At the beginning and end of
the 4 h sampling we collected samples from the East and West Forks of
Crescent Stream immediately upstream of the confluence and at the stream
gauge location
During collection we measured water temperature and conductance of each
sample with a YSI 30 (Yellow Springs Instruments Inc., Yellow Springs,
Ohio). Samples for water chemistry were collected in ultra-pure water-rinsed
HDPE bottles and were carried to the laboratory and filtered within 24 h
of collection using 0.4
Samples were analyzed for major anions and cations by ion chromatography
using a Dionex DX-120 (Sunnyvale, CA) using methods described in Welch et
al. (2010). H
A paired
As briefly described in the site description section, permafrost degradation along the banks of the West Fork of Crescent Stream was observed for over 3 km of stream reach (Fig. 2). The observation of substantial sediment deposition at the gauge site initiated the upstream survey of the channel. It was obvious that the channel had been re-worked by substantial flows in the recent days with clear downcutting (up to 20 cm) in some sections and deposition of as much as 5 cm in low-gradient locations. Changes in sediment aggradation and degradation are the subject of other on-going studies of the field site, so we have chosen not to provide them here. Stream gauge records are insufficient to point to an exact moment that the sediment movement occurred at that location. Site visit notes indicate that the gauge was not in the observed condition even 7 days prior. The extent of degradation included many locations of undercutting of the banks on both sides of the channel. Frozen sediments within the banks provided enough cohesion that undercutting extended more than a meter laterally in some places, though the vertical gaps observed were on the order of 10–20 cm. At some meander bends on either side of the broad channel, the stream cut further into the banks, widening the channel by 1 m or less in a few isolated places. This may have occurred due to bank erosion and subsequent slumping of up-gradient sediments. It is possible that undercutting occurred at these locations first, and then slumping of overburden. The cause of the degradation is not immediately obvious, but likely is the result of ponded water backed up at some point in the channel, perhaps behind snowdrifts that often form in the winter along the western banks. No obvious water lines from ponded water were observed upstream, though they may have been modified by the impacts to the banks. Flows were great enough to mobilize sediment, causing degradation in some places and aggradation in others.
During the sampling, discharged on the West Fork ranged from 1.19 to 2.04 L s
Means and standard deviations of water chemistry parameters as
observed in three groups. Comparison significance reports the result of
paired
Stream-water electrical conductivity and total suspended solids (note log-scale axis) from historic data collected at stream gauge, above and below the thermokarst tunnel, observed at the mouths of the West and East Forks of Crescent Stream (just above their confluence), and at the stream gauge site in January 2012. See Fig. 3c for sampling location map. There are no historic TSS data available from Crescent Stream gauge.
At a broader spatial scale, comparisons of water chemistry between the East and West Forks of Crescent Stream provide an opportunity to assess the cumulative differences of the impacts of thermokarst development on the West Fork and the reference (no-impact) condition of the East Fork. We fully recognize that this is not as powerful as comparing before and after stream chemistry results from the West Fork. Because the pre-disturbance water chemistry data for the West Fork do not exist, we propose that this comparison is useful for characterizing the impact of these channel changes to the water quality of the West Fork.
West Fork stream water had higher EC and TSS than East Fork stream water
(differences in means of 142
In most cases the mean concentrations of the water flowing out of the
thermokarst tunnel were greater than those at the mouth of the West Fork.
These differences suggest that the tunnel was a strong modifier of stream
water chemistry locally, but is not indicative of the impacts of all
thermokarst impact on water chemistry along the
Streamflow and chemistry at the Crescent Stream gauge are made up of
contributions from the East and West Forks of the stream. Empirically, flows
in each channel appear to be comparable though no direct measurements have
been made. However, a shift in stream water chemistry at the stream gauge
would be evident if concentrations observed at the gauge after the
thermokarst development were above the range of concentrations observed
historically – over 22 years of data collection. Electrical conductivity is
measured every 15 min at the stream gauge (starting in December 1991), and
these measurements range from 1 to 1440
Mean (symbols) and ranges of concentrations of electrical
conductivity (
Historically, TSS samples are collected only when very high flows or other
abnormal events cause increased turbidity in the streams – any appreciable
suspended sediment is notable in MDV streams. In the case of Crescent
stream, there are no historic TSS measurements. For most of the major ions
(Na, K, Ca, Cl, and SO
The MDV make up < 0.3 % of Antarctica,
The glacial meltwater streams of the MDV are generally clear and do not
often transport significant, obvious sediment quantities, with the exception
of high flow events. The impact to Crescent Stream was along
One impact of the thermokarst features on stream ecosystems would be burial of the microbial mats which may be buried by sediments in the reach immediately below the thermokarst features, especially if the flows are low. Our observations along the channel in January 2012 indicate that a lot of fine sediment has been dispersed throughout the channel, and also indicate very little occurrence of algal mats. Stream discharge is quite cyclical in these streams. On a daily basis there is a flood pulse from enhanced glacier melt due to solar aspect, and across the season, streams generally start and end with fairly low flows and experience much higher flows in between. While the flow magnitude variability is unpredictable, the daily and seasonal pulses of stream flow are likely to transport deposited sediment through the next several years. The timescale of this legacy is not clear. Whether mats are likely to scour may also depend on how substantial the mats are (how extensive they are and how well they are attached to substrate), the magnitude of the flows during diurnal and seasonal cycles, and the extent to which mats can grow during low-flow (non-scour) conditions.
The substantial introduction of fine sediment associated with these
thermokarst features can be expected to have some influence of stream
ecosystem function also at high flows. Analysis of the long-term record
indicates that scour at high flows constrains the biomass of microbial mats
(Stanish et al., 2011; Kohler et al., 2015a). Kohler et al. (2015b)
specifically focused on epilithon responses from scour events, noting that
recovery times were generally weeks to months, potentially longer than a
single flow season. Furthermore, Cullis et al. (2014) showed that the daily
transport of particulate organic matter (POM) from sloughing driven by
fluvial shear stress was limited by the availability of “mobile biomass”
associated with the mats. However, at high flow the hysteretic pattern
associated with such a limitation was not observed and direct scour of the
mats appeared to be the dominant mechanism controlling POM transport. If
there is more abundant fine sediment in the channel, the magnitude of flow
required for a “re-setting” scouring event may be lower, e.g. potentially
lower than the 100 L s
Previous studies of nutrient uptake in these streams indicate that both the microbial mats in the channel and the hyporheic zone that occupies the sediments adjacent to the channels are important locations of uptake and processing (Gooseff et al., 2004; McKnight et al., 2004). In the water column, the reduction of algal mats due to either burial or scour would reduce the opportunity for biogeochemical processing of nutrients as these streams act as a filter of nutrients to the endorheic lakes at their termini. However, burial of algal mats may well fuel hyporheic biogeochemical cycling as the increased organic matter in the subsurface may help to stimulate microbiological transformation of nutrients that exchange through these sediments (Schindler and Krabbenhoft, 1998).
While substantial changes in major ion concentrations along a
It is surprising that the weathering solutes do not show a stronger response in the stream water downstream of the permafrost degradation. Weathering rates of the streambed materials in the MDVs has been reported to be among the highest in the world despite the cold temperatures (Lyons et al., 1997; Gooseff et al., 2002). The increase in major ion concentrations observed reflects a mobilization of readily soluble salts such as NaCl rather than an increase in chemical weathering (Si and Ca).
It is not yet clear how long the degradation will occur, and how long the fine sediment deposits in the stream channel and the elevated major ion and nutrient concentrations will persist in the West Fork of Crescent Stream. The degraded banks of the channel will slowly modify through annual and seasonal freeze-thaw cycles and associated slow cryoturbation, potentially “healing” the stark eroded surfaces observed in January 2012. Winter snow may accumulate in some of the new hollows insulating and stabilizing the banks. During the austral summer, a positive surface energy balance may cause further permafrost thaw and continued thermomechanical erosion. The channel will continue to respond to hydrology that is dynamic on several timescales (daily pulses of melt water; high and/or low flow seasons). As it does, the degraded sections of the channel may further erode due to shear stress associated with high flows. The fine sediment introduced by the thermokarst formation and algal communities in the channel will also respond, with high flows potentially moving the sediment further downstream and potentially scouring algal mats that are trying to re-establish and grow, and low flows promoting the persistence of fine sediment deposits and algal growth. Recent findings by Cozzetto et al. (2013) indicate that the hyporheic zones of MDV streams have a wide range of exchange timescales, some very short, and some long (Gooseff et al., 2003), and therefore strong heterogeneity in sediment size and hydraulic conductivity distributions likely exist in MDV streambeds.
The MDV endorheic lakes integrate all stream inputs and processes that
affect streamflow generation. Foreman et al. (2004) found that during the
very high-flow season of 2001–2002, the introduction of fine sediment from
primarily a single second-order sand-bed stream all of the streams to the
East Lobe of Lake Bonney reduced the incoming photosynthetically active
radiation (PAR), which decreased the chlorophyll
Extensive permafrost degradation on the banks of the West Fork of Crescent Stream in the McMurdo Dry Valleys, Antarctica has resulted in substantial input of fine sediment to the stream and increased solute concentrations, particularly nitrate. These streams have not been observed to experience large pulses of fine sediment except during very high flow events. This input of sediment related to permafrost degradation has the potential to bury and/or scour stream algal mats and provide turbidity to endorheic lake water columns. Increased nutrient concentrations are likely to promote algal mat re-establishment and growth. This pulse disturbance to this aquatic ecosystem may have persistent (several flow seasons) impacts to the channel as typical flows are fairly low and it may take years to flush the introduced sediment.
This research was funded by McMurdo LTER NSF OPP grant 1115245 (to MNG, DMM, and WBL). Additional support was provided by PLR 1246203 (to MNG) and 1245991 (to DVH). The authors wish to acknowledge Raytheon Polar Services Company, UNAVCO, and Petroleum Helicopters, Inc. for field support. Edited by: J. Vonk