SNOW DYNAMICS IN A POLAR DESERT, MCMURDO DRY VALLEYS, ANTARCTICA

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SNOW DYNAMICS IN A POLAR DESERT, MCMURDO DRY VALLEYS, ANTARCTICA

ABSTRACT

Snow in the McMurdo Dry Valleys is rare source of moisture for subnivian soils (beneath snow) in a cold desert ecosystem.  While sublimation dominates the ablation process, measurable increases in soil moisture are expected to provide more favorable conditions for subnivian soil communities.  In addition, snow cover insulates the underlying soil from temperature extremes.  Quantifying the spatial distribution and ablation patterns of seasonal snow is necessary to understand these dynamics.  Annual snowfall varies spatially ranging from 3 to 50 mm of snow water equivalent, with greater amounts occurring at the coast.  Despite receiving very little precipitation, significant amounts of snow can accumulate (via aeolian redistribution) in topographic lees at the valley bottoms, forming thousands of discontinuous patches (typically 1100 m2 in area).  These patches have the potential to act as fertility islands, controlling the landscape distribution of microbial communities, and biogeochemical cycling. High resolution imagery acquired during the 2009-2010 and 2010-2011 austral summers was used to quantify the distribution of snow across Taylor and Wright Valleys.  An objectbased classification was used to extract snow-covered area from the imagery.  Coupled with topographic parameters, unique distribution patterns were characterized for 5 regions within the neighboring valleys.  Time lapses of snow distribution during each season in each region provide insight into spatially characterizing the aerial ablation rates (change in area of landscape covered by snow) across the valleys.  The distribution of snow-covered area during the 2009-2010 austral summer is used as a baseline for seasonal comparison.  The surrounding regions of Lake Fryxell, Lake Hoare, Lake Bonney, Lake Brownworth, and Lake Vanda exhibited losses of snow-covered area of 9.61 km2 (-93%), 1.63 km2 (-72%), 1.07 km2 (-97%), 2.60 km2 (-82%), and 0.25 km2 (96%) respectively, as measured from peak accumulation in October to mid-January during the 2009-2010 season.  Differences in aerial ablation rates within and across local regions suggest that both topographic variation and regional microclimates influence the ablation of seasonal snow cover.  Elevation has shown to be the strongest control over aerial ablation.  Fifteen 1 km2 plots (3 in each region) were selected to assess the prevalence of snow cover at smaller scales.  Results confirm that snow patches form in the same locations each year with some minor deviations observed.  Stable isotopes from snow patches also provide insights into temporal and spatial processes associated with ablation.  At the snow patch scale, neighboring patches often exhibit considerable differences in aerial ablation rates, presumably controlled by snow depth.  This highlights the importance of both the landscape and snow patch scales in assessing the effects of snow cover on biogeochemical cycling and microbial communities.  

TABLE OF CONTENTS

  LIST OF FIGURES ……………………………………………………………………………………………………. vii LIST OF TABLES ……………………………………………………………………………………………………… x ACKNOWLEDGEMENTS …………………………………………………………………………………………. xi Chapter 1 Introduction …………………………………………………………………………………………………1 Chapter 2 Spatial and Temporal Patterns of Snow Accumulation and Aerial Ablation ………… 4 2.1  Introduction ………………………………………………………………………………………………. 4 2.2  Methods for Quantifying Snow-Covered Area ………………………………………………. 5 2.3  Snow Accumulation Patterns ………………………………………………………………………. 12 2.4  Aerial Ablation Patterns ……………………………………………………………………………… 17 2.5  Implications of Snow Distribution Analysis ………………………………………………….. 24 2.6  Conclusions ………………………………………………………………………………………………. 28 Chapter 3 Seasonal Controls on Aerial Ablation and the Influence of Scale ………………………. 30 3.1  Introduction ………………………………………………………………………………………………. 30 3.2  Seasonal Comparison of Accumulation Patterns ……………………………………………. 31 3.2  Seasonal Controls on Snow Accumulation and Aerial Ablation ………………………. 40 3.3  Snow Ablation Rates and Modeled Snow Water Equivalent …………………………… 46 3.4  Coupling of Landscape and Snow-Patch Processes………………………………………… 52 3.5  Conclusions ………………………………………………………………………………………………. 55 Chapter 4 Stable Isotopic Analysis of Snow…………………………………………………………………… 57 4.1  Introduction ………………………………………………………………………………………………. 57 4.2  Description of Sampling Scheme and Sample Analysis ………………………………….. 59 4.3  Results and Discussion of Stable Isotopic Analysis ……………………………………….. 64 4.4  Conclusions ………………………………………………………………………………………………. 82 Chapter 5 Conclusions ………………………………………………………………………………………………… 84 4.1  Snow Accumulation Patterns ………………………………………………………………………. 84 4.2  Aerial Ablation of Seasonal Snow ……………………………………………………………….. 85 4.3  Snow-Patch Dynamics ……………………………………………………………………………….. 86 REFERENCES ………………………………………………………………………………………………………….. 87 Appendix A UEB Model Parameters …………………………………………………………………………….. 93 Appendix B Snow Patch Outlines…………………………………………………………………………………. 95 Appendix C Depth Profile Measurements ……………………………………………………………………… 102 Appendix D Linear Regression Analysis ……………………………………………………………………….. 106 Chapter 1 Introduction   The McMurdo Dry Valleys (MDV) of Antarctica are a hyper-arid polar desert, receiving less than 10 cm of snow water equivalent annually (Keys, 1980; Fountain et al. 2009).  Situated within the Victoria Land region of Antarctica, west of the McMurdo Sound, the MDV are a unique ice-free environment on a continent that is otherwise covered 98% in area by ice (Drewery et al., 1982).  The Transantarctic Mountains prevent the East Antarctic Ice Sheet from covering the region (Chinn, 1990).  Alpine glaciers flow from the surrounding mountains, reaching the valley bottom and supplying melt water for ephemeral streams (Fountain et al., 1999b).  Endorheic lakes at the valley bottoms, which only maintain partially open water during the austral summer, are balanced by gains from stream flow and losses due to sublimation and evaporation (during the summer) at the surface.  The mean annual temperature ranges from -15 °C to -30 °C and is strongly dependent on the frequency of katabatic winds during the winter months (Doran et al., 2002).  Katabatic winds originate at the Polar Plateau where net radiation losses produce cool dense air that flows downslope toward the coast reaching speeds up to 38 m/s (Nylen et al., 2004; Doran et al., 2002).  More prevalent during the winter months, these winds can raise local temperatures by 30 °C (Nylen et al., 2004).  The valleys are characterized by soils devoid of vascular vegetation with very low biological activity and very simple food chains relative to temperate regions (Wall and Virginia, 1999).   The biota of soils within the MDV contains several invertebrate species, but is microbially dominated, and the availability of water has been shown to be a primary biogeographical control on the MDV system (Kennedy, 1993).  Because of the scarcity of water, the interface between seasonal snow cover and subnivian soil is a critical component of the overall ecosystem.  While stream and lake margins provide geochemical and microbial hot spots (Zeglin et al., 2009), seasonal snow cover is the only potential source of moisture beyond soils adjacent to water bodies. Annual snowfall within the MDV varies spatially ranging from 3 to 50 mm of snow water equivalent, with greater amounts occurring at the coast.  In areas with little snow accumulation there are no seasonal patterns of snowfall, but snow accumulation for coastal areas is greatest during winter months (Fountain et al., 2009).  Despite receiving very little precipitation, significant amounts of snow can accumulate at the valley bottoms in discontinuous patches (typically 1-100m2 in area).  Wind transport of snow from the Polar Plateau and adjacent valley walls greatly increases the amount of snow reaching the valley bottoms (Fountain et al., 2009).  Snow is further redistributed within the valleys into topographic depressions allowing for the formation of discontinuous snow patches with appreciable depths (Gooseff et al., 2003a).  Fountain et al. (2009) reported the fraction of accumulation that is actual precipitation as ranging from 36% to 80% depending on location. Very few studies have focused on the role of snow in the hydrology or ecology of the MDV system.  During the 1960s and 1970s, precipitation was measured continuously at a field camp at Lake Vanda (Keys, 1980; Bromley, 1985).  Measured snowfall varied greatly during the 3 years of occupation at Lake Vanda and on average was almost an order of magnitude lower than recent precipitation measurements, highlighting the variability of snowfall from year to year (Keys, 1980; Fountain et al., 2009).  Other studies have made inferences about the spatial and temporal patterns of snowfall based on point observations (Bertler et al., 2004; Witherow et al., 2006), but accumulation and ablation patterns of snow across the landscape have yet to be characterized.  Because there is minimal interaction at the snow-soil interface, snow has been dismissed in the past as hydrologically insignificant (Chinn, 1981), and therefore it has not received much attention with respect to landscape processes.  While snow is a negligible component of the overall water balance of the valleys, it is a potential moisture source and insulator for subnivian soils (Campbell et al., 1998; Gooseff et al., 2003a; Ayres et al., 2010). In a highly water limited environment, volumes of water that usually seem insignificant may be an important control in the structuring of subnivian communities.  Despite sublimation dominating the ablation of seasonal snow, increases of soil moisture in subnivian soils relative to the neighboring dry soils have been observed (Gooseff et al., 2003a; Ayres et al., 2010).  In addition to providing moisture, snow cover reduces temperature extremes in the subnivian soil (Walker et al., 1999; Schimel et al., 2004), and controls biogeochemical cycling and microbial activity in subnivian soils for alpine and Arctic environments (Brooks et al., 1995; Brooks et al., 1996; Brooks and Williams, 1999; Walker et al., 1999; Schimel et al., 2004; Edwards, 2007).

SNOW DYNAMICS IN A POLAR DESERT, MCMURDO DRY VALLEYS, ANTARCTICA

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