RECENT ICE SHEET SNOW ACCUMULATION AND FIRN
RECENT ICE SHEET SNOW ACCUMULATION AND FIRN STORAGE OFMELTWATER INFERRED BY GROUND AND AIRBORNE RADARSbyClement MiegeA dissertation submitted to the faculty ofThe University of Utahin partial fulfillment of the requirements for the degree ofDoctor of PhilosophyDepartment of GeographyThe University of UtahAugust 2015
Copyright Clement Miege 2015All Rights Reserved
Th e U n i v e r s i t y o f U t a h G r a d u a t e S c h o o lSTATEMENT OF DISSERTATION APPROVALThe dissertation ofClement Miegehas been approved by the following supervisory committee members:Richard R. ForsterChairFeb 2nd, 2015Date ApprovedPhilip E. DennisonMemberFeb 2nd, 2015Date ApprovedSimon C. BrewerMemberFeb 2nd, 2015Date ApprovedMemberLora S. KoenigFeb 2nd, 2015Date ApprovedJason E. BoxMemberFeb 2nd, 2015Date Approvedand bythe Department ofAndrea R. BrunelleGeographyand by David B. Kieda, Dean of The Graduate School.Chair of
ABSTRACTRecent surface mass balance changes in space and time over the polar ice sheetsneed to be better constrained in order to estimate the ice-sheet contribution to sea-levelrise. The mass balance of any ice body is obtained by subtracting mass losses from massgains. In response to climate changes of the recent decades, ice-sheet mass losses haveincreased, making ice-sheet mass balance negative and raising sea level. In this work, Ibetter quantify the mass gained by snowfall across the polar ice sheets; I target specificregions over both Greenland and West Antarctica where snow accumulation changes areoccurring due to rising air temperature.Southeast Greenland receives 30% of the total snow accumulation of theGreenland ice sheet. In this work, I combine internal layers observed in ice-penetratingradar data with firn cores to derive the last 30 years of accumulation and to measure thespatial pattern of accumulation toward the southeast coastline. Below 1800 m elevation,in the percolation zone, significant surface melt is observed in the summer, whichchallenges both firn-core dating and internal-layer tracing. While firn-core drilling at1500 m elevation, liquid water was found at 20-m depth in a firn aquifer that persistedover the winter. The presence of this water filling deeper pore space in the firn wasunexpected, and has a significant impact on the ice sheet thermal state and the estimate ofmass balance made using satellite altimeters. Using a 400-MHz ice-penetrating radar, theextent of this widespread aquifer was mapped on the ground, and also more extensively
from the air with a 750-MHz airborne radar as part of the NASA Operation IceBridgemission. Over three IceBridge flight campaigns (2011-2013), based on radar data, the firnaquifer is estimated to cover 30,000 km2 area within the wet-snow zone of the ice sheet.I use repeated flightlines to understand the temporal variability of the water trapped in thefirn aquifer and to simulate its lateral flow, following the gentle surface slope ( 1 ) andundulated topography of the ice sheet surface toward the ablation zone of the ice sheet.The fate of this water is currently unknown; water drainage into crevasses and at leastpartial runoff is inferred based on the analysis of radar profiles from different years.I also present results from a field expedition in West Antarctica, where datacollection combined high-frequency (2-18 GHz) radar data and shallow ( 20 m) firncores from Central West Antarctica, crossing the ice divide toward the Amundsen Sea.The radar-derived accumulation rates show a 75% increase ( 0.20 m w.eq. y-1) of netsnow accumulation from the ice divide, toward the Amundsen Sea for a 70-km transect,assuming annual isochrones being detected in the radar profile. On the Ross Sea side ofthe divide, with accumulation rates less than 0.25 m w.eq. y-1 and significant windredistribution, only a multi-annual stratigraphy is detected in the radar profile. Usingradar, I investigated the small-scale variability within a radius of 1.5 km of one firn-coresite, and I find that the averaged variation in accumulation-rate in this area is 0.1 m w.eq.y-1 in the upper 25-m of the firn column, which is 20% of the average accumulation rate.iv
I dedicate this dissertation to my two grand-fathers, Rene and Georges.Merci pour tout ce que vous m ’avez appris.
TABLE OF CONTENTSABSTRACT. iiiLIST OF TABLES. viiiLIST OF FIGURES. ixACKNOWLEDGEMENTS.xiChapters1 INTRODUCTION . 11.1 Motivations and objectives. 31.2 Radar background. 71.3 Accumulation variability and uncertainties. 131.4 References.182 SOUTHEAST GREENLAND HIGH ACCUMULATION RATES DERIVED FROMFIRN CORES AND GROUND-PENETRATING R A D A R . 222.1 Abstract.222.2 Introduction.232.3 Arctic Circle Traverse 2010. 272.4 Methods.292.5 Results.342.6 Discussion.392.7 Conclusions. 452.8 Acknowledgements.462.9 References.553 SPATIAL EXTENT AND TEMPORAL VARIABILITY OF THE GREENLANDFIRN AQUIFER DETECTED BY GROUND AND AIRBORNE RADARS .613.1 Abstract.613.2 Introduction.623.3 Field site.683.4 D ata. 693.5 Methods.72
3.6 Results.793.7 Discussion.863.8 Conclusions. 933.9 Acknowledgements.953.10 References. 1134 SNOW-ACCUMULATION SPATIAL VARIABILITY DETECTED FROM 2-18GHZ RADARS ACROSS THE WESTERN ICE DIVIDE, CENTRAL WESTANTARCTICA.1194.1 Abstract.1194.2 Introduction. 1204.3 Satellite Era Accumulation Traverses. 1244.4 D ata. 1254.5 Methods.1284.6 Results.1314.7 Discussion.1364.8 Conclusions. 1394.9 Acknowledgements.1404.10 References. 1515 CONCLUSIONS. 1555.1 Summary.1555.2 Broader impacts of this work. 1585.3 Future research directions. 1605.4 References. 166vii
LIST OF TABLESTablePage2.1 Characteristics of the three extracted ACT-10 firn cores. 473.1 Flightline characteristics for three airborne campaigns. 1113.2 Stratigraphy of the three firn cores extracted above the firn aquifer. 1124.1 SEAT-10 firn core locations and specifications.141
LIST OF FIGURESFigurePage1.1 Electromagnetic wave interaction with a medium.172.1 Study site in Southeast Greenland Ice Sheet. 482.2 Ground-penetrating radar profile between the firn cores. 492.3 Accumulation rates derived over the radar profile.502.4 Density profiles for the three firn cores.512.5 Comparison between radar-derived and simulated accumulation rates.522.6 Influence of the topography on the accumulation spatial pattern. 532.7 Temporal variation of accumulation for firn cores and m odel.543.1 Field area in Southeast sector of the Greenland Ice Sheet.963.2 Density profiles for five firn cores extracted in Southeast Greenland . 973.3 Two-way-travel time to depth conversions. 983.4 400 MHz ground-based radar compared with 750 MHz airborne radar .993.5 750 MHz airborne radar compared with 195 MHz airborne radar. 1003.6 Map of the firn aquifer surface and depth to w ater. 1013.7 Water table depth and surface elevation distributions. 1023.8 Airborne radar profile imaging the water table, corrected for topography. 1033.9 Temporal evolution of the water table at the field location.1043.10 Missing bed echoes in depth-sounder radar for the last 2 decades.105
3.11 Yearly evolution of the water table for 2011-2013 for Helheim G lacier. 1063.12 Firn aquifer connecting with crevasses at K 0 ge B u g t.1073.13 Firn aquifer connecting with crevasses at Helheim Glacier.1083.14 Water table lateral flow for the upper part of Helheim Glacier. 1093.15 Perched water table above the aquifer surface.1104.1 Study site in Central West Antarctic Ice Sheet.1424.2 Firn core density profile and uncertainty associated to the mixture model.1434.3 Comparison between the ground Ku-band and the airborne Snow Radar.1444.4 Ku-band radar profile evolution on both sides of the modern ice divide.1454.5 Spatial variability of the radar-derived accumulation rates.1464.6 Comparison between the firn-core and the radar depth-age scales. 1474.7 Interannual variability of the accumulation rates from firn-core and radar. 1484.8 Small-scale accumulation variability at SEAT10-4. 1494.9 Accumulation-rate variations and isochrone-depth differences at SEAT10-4. 150x
ACKNOWLEDGEMENTSThe work accomplished during these five years of research was made possible byfunding from the National Science Foundation Office of Polar Programs and a graduatefellowship from the National Aeronautics and Space Administration Earth and SpaceSciences Program. I am grateful for support from a Chateaubriand Fellowship thatsponsored my visit to the Centre d’Etude de la Neige/Meteo France in Grenoble for fivemonths.I would like to thank my advisor Rick Forster for his patience, support, andexpertise to guide me through the obstacles of graduate school. Rick also gave meresponsibilities to lead research and fieldwork, allowing me to grow as an independentresearcher. I am thankful to Jason Box who gave me the opportunity to come study in theU.S. and for teaching me all about fieldwork, Greenland weather, and climate variability.I am thankful to Lora Koenig for letting me join two Antarctic field expeditions, forbeing so responsive each time I needed help, and for the friendship that we builtthroughout the years. Finally, thanks to Simon Brewer and Phil Dennison, my Universityof Utah committee members, for their time to follow my PhD work, to evaluate myresearch skills during the qualifying exams, and later to evaluate this PhD dissertation.The results presented in this work are due to the tremendous help of valuablecollaborators and friends: Ludovic Brucker at NASA Goddard, Joe McConnell and DanPasteris at DRI, Summer Rupper and Landon Burgener at BYU, Susan Zager at CPS,
Laurent Mingo at BSI, Blue Spikes at ESA, and Kip Solomon at the University of Utah,Geology.Many thanks to Samuel Morin and Matthieu Lafaysse at Meteo France for theirsupport with the snow model CROCUS and an extra thanks to the entire CEN team fortheir warm welcoming during my stay in fall 2012.With major fieldwork components during this PhD work, I would like to thank allmy other teammates who made the field such a happy place: Evan Burgess, Terry Gacke,Brian Ballard, Jay Kyne, Mike Atkinson, Randy Skinner, Jeff VanLooy, and GregVandeberg.Thanks to Ed Waddington and Twit Conway who gave me the great opportunityto finish my PhD research at the Department of Earth and Space Sciences at theUniversity of Washington.I got the chance to have amazing officemates and great friends both at Univ. ofUtah and Univ. of Washington: Evan, Annie, Kat, Al, Lisbeth, and others: thank you.During grad school, it was great to have supportive friends. In Salt Lake City,Sebastian, Sylvain, Marc, Julie, Corinne, Clement, Stanley, and others were part of greatSouthern Utah adventures, Wasatch hikes, and backcountry skiing. In Seattle, the rainyweather was not bad at all with the very warm Tuesday-dinner group! During my visits inFrance, I had the chance to see my old friends and their families.I am saving a few words in French for my family: Elsa, Guillaume, Maman, Papa,Mamie et Grand-mere: Un grand merci d’etre toujours la pour moi.Finally, thanks to Michelle for your incredible generosity, endless patience, andunconditional support along the way.xii
CHAPTER 1INTRODUCTIONHuman interference with our climate is occurring, which has raised the level ofCO2 in the atmosphere and caused the Earth surface to warm since the mid-20th century(IPCC, 2013). Recent climate changes affect ecosystems, ice reservoirs, weatherextremes, and already have resulted in consequences for our society. The cryosphericresponse to climate change has been observed worldwide, in particular, glaciers areshrinking, Arctic sea ice extent is decreasing, drainage of ice from the ice sheets isincreasing, permafrost is degrading, and the amount of water from spring snowpack isreducing for western North America (IPCC, 2013). In many regions of our planet,melting snow and ice have direct consequences on hydrological systems and on thequality and quantity of water resources (IPCC 2013).The contribution of glaciers and ice sheets to sea level rise is significant. For the21st century, mountain glaciers and ice caps contribute to more than half of the sea levelrise (SLR) coming from ice loss (Meier et al., 2007). For the last two decades (1992 2010), the Greenland and Antarctic ice sheets contributed 0.43 0.11 mm yr"1 and0.27 0.11 mm yr"1 to SLR, respectively, based on reconciled observations andsimulations (Shepherd et al., 2012; IPCC 2013). From 2003-2009, mountain glacierscontributed 0.76 0.37 mm yr"1to SLR (Gardner et al., 2013; IPCC, 2013).
2Mountain glaciers are rapidly shrinking, dominating the cryospheric contributionto SLR today, and their total contribution is estimated to be 0.41 m of vertical SLR(IPCC, 2013). The ice sheets’ total ice mass corresponds to a SLR of 65.7 m, with 7.4m for Greenland and 58.3 m for Antarctica. Reducing the uncertainties in sea-level risecontributions from Greenland and Antarctica is crucial for policy and decision makers,since coastal cities will be at a very likely risk of flooding by the end of the 21st century(IPCC, 2013).Ice loss from the ice sheets can be partitioned between ice discharge fromAntarctica and from Greenland due to ice calving at the edges and due to surface meltrunoff to the ocean (occurs primarily for Greenland and the Antarctic Peninsula). Tooffset the mass loss, the ice sheet is gaining mass by snowfall, which is not equallydistributed over the ice sheets. The mass balance of each ice sheet is established byadding mass losses and mass gains for each year.For Greenland, surface melt contributed to 50% of the total ice loss for the lastdecades (van den Broeke et al., 2009). For the 2009-2012 period, the surface meltcontribution to ice loss increased to 70% (Enderlin et al., 2014) since it includes theexceptional and widespread melt during summer 2012 (Nghiem et al., 2012).Compensating for some of the mass loss, and correlated with increasing air temperatur
Greenland ice sheet. In this work, I combine internal layers observed in ice-penetrating radar data with firn cores to derive the last 30 years of accumulation and to measure the spatial pattern of accumulation toward the southeast coastline. Below 1800 m elevation, in the percolation zone,
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relatively flat land. Alpine glaciers form on top of high mountains. 4. Snow falls on an area but does not melt over the course of the year. Over time, more snow builds up in the area. The weight of the snow above pushes down on the snow below, causing the snow at the bottom to turn into ice. This ice is a glacier. 5.
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