«by Carli Anne Arendt A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Earth and ...»
The Hydrologic Evolution of Glacial Meltwater:
Insights and Implications from Alpine and Arctic Glaciers
Carli Anne Arendt
A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
(Earth and Environmental Sciences)
in the University of Michigan
Assistant Professor Sarah M. Aciego, Chair
Assistant Professor Jeremy N. Bassis
Professor M. Clara Castro Associate Professor Eric A. Hetland Professor Kyger C. Lohmann
I would also like to thank the numerous others who supported me on this academic journey and encouraged me when I encountered challenges along the way. It would be impossible to name everyone who has helped contribute to my success but I would like to recognize: my fiancée and primary support system Ethan Hyland, my GIGL lab members and field companions Emily Stevenson, Yi-Wei Lui, Molly Blakowski and my roommate Sarah Aarons for being supportive of me for my entire journey here at the University of Michigan, the official and unofficial ii members of GEOFRAT who helped me grow along the way Rohit Warrier, Jen Cotton, Rich Fiorella, Tara Smiley, Petr Yakolvev, Clay Tabor, Say Yun Kwon, Katie Loughney, Lydia Staish, Laura Waters and many more, the UMich EARTH office staff especially Anne Hudon for all of their guidance and assistance over the years, my completely loving and supporting parental unit and sister, my undergraduate advisersDr. Tim Flood and Dr. Nelson Ham for encouraging me to pursue higher education and believe in myself, Dr. Henry Pollack for being a continued inspiration and role model, and the faculty, researchers, postdocs, and students in this department who have helped make the University of Michigan feel like home.
SPECIFIC CHAPTERSChapter 2 This research was funded by the University of Michigan Rackham Graduate School and the Turner Award from the Department of Earth and Environmental Sciences to CAA. I would like to thank Sarah Aarons for her assistance in the collection of samples used for the Athabasca Glacier case study. Datasets supporting this manuscript are available in Supporting Online Material Table S1 and Table S2. I would also like to thank Dr. Sarah Das, Dr. Maya Bhatia, and Dr. Oliver Chadwick for their cooperation and assistance in obtaining the isotopic values and fractional contribution estimations from the original studies of the data used in case studies two and three.
Chapter 3 The University of Michigan’s Rackham Graduate School and Department of Earth and Environmental Science provided funding for this project through grants to C.A.A. I thank Parks
Precision Isotope Laboratory personnel are acknowledged for providing support. I would like to wholeheartedly thank Dr. Don Porcelli for his insightful and extremely helpful insights regarding the derivation of the age-relationship equation used in this chapter.
Chapter 4 Funding for this project was provided by the University of Michigan’s Rackham Graduate School and Department of Earth and Environmental Science through research grants awarded to C.A.A. and through Packard funding awarded to S.M.A.; and by the Woods Hole Oceanographic Institution’s Ocean and Climate Change Institute Arctic Research Initiative research grant to S.B.D. I thank Air Greenland for assistance with field logistics and the WHOI field team for aiding in sample collection in Ilulissat. I thank Dr. Gideon Henderson for his generosity in allowing us access to the original box-model used in Henderson, 2002.
Many thanks to all those who contributed to my success in this thesis!
1.1 Transport pathways of glacial hydrology
1.2 Generalized map of sample locations
2.1 Detailed location map of the Athabasca Glacier
2.2 Dual isotope plot of end-members and Athabasca Glacier bulk meltwater samples.... 33
2.3 Athabasca Glacier meltwater source fraction contribution estimations
2.4 Covariance plots of Athabasca Glacier end-member source fraction estimations........ 38
2.5 Statistical histogram and covariance scatter plots for Athabasca Glacier sample 49.... 39
2.6 Dual isotope plot of end members and GrIS bulk meltwater samples
2.7 GrIS fraction contribution estimations with 2-isotope versus 3-isotope comparison.... 44
2.8 Comparison of GrIS contribution estimations from original study and this study........ 46
2.9 Dual isotope plot of end-members and Hawaiian soil samples
2.10 Source fraction contribution estimations for Hawaiian soil samples
3.1 Simplified fractal depiction of the seasonal evolution of subglacial hydrology............ 69
3.2 Seasonal measurements from the Athabasca Glacier
3.3 U-series isotopic measurements versus discharge
Changes in (234U/238U)activity measurements compared with 222Rn activities.................. 95 3.4
3.5 Changes in major cation concentrations (K, Ca, Na, and Mg)
3.6 Athabasca Glacier residence times versus Positive Degree Days
δ234U measurements of glacial meltwater versus corresponding proximal seawater.... 125 4.2
4.3 Uranium and salinity measurements of GrIS samples versus other Arctic samples..... 129
4.4 Seawater U-chemistry box model scenarios in this study
Box model seawater δ234U composition estimation scenarios
2.2 Athabasca Glacier end member isotopic compositions
2.3 Athabasca Glacier raw fraction contribution estimations
2.4 Isotopic end-member values from Bhatia et al.  GrIS samples............ 42
2.5 Isotopic end-member values from Chadwick et al. 
3.2 Athabasca Glacier traditional water chemistry measurements
3.3 Isotopic measurements and 2σ of Athabasca subglacial water samples.......... 93
3.4 Measurements of Athabasca Glacier meltwater major cations
4.1 Precise GPS coordinates and traditional water chemistry measurements........ 120
4.2 U-series measurements from all GrIS sample locations
4.3 Differing glacial melt U-flux scenarios addressed by our model
Appendix A. Supplementary Matlab Code
Appendix B. Field and Laboratory Methods
Glaciers are of critical importance in the global hydrological cycle and the implications of their recent rapid decline are still poorly understood. Subglacial processes impact glacial hydrology through nutrient production, water chemistry, and aquifer recharge, but are inaccessible to direct observation. Furthermore, understanding the impact current subglacial melt processes have on surrounding environments may provide insight to changes that likely occurred on glacialinterglacial timescales. In my work, I combine multiple elemental and isotopic systems to understand glacial hydrology, including the subglacial environment. First, I have developed a Bayesian Monte Carlo isotope-mixing model that incorporates stable isotope, δ18O and δD, measurements to extrapolate relative contributions of ice and snowmelt to the glacial system.
This model can also be applied to other earth surface systems with distinct end member isotopic compositions. Second, I have combined the melt fractions from my isotope-mixing model with a radioactive uranium-series (U-series) isotope age model to quantify the average residence time and storage length of subglacial melt. By combining these two isotopic systems, I provide unique insights on the size of the subglacial meltwater reservoir and its potential impacts on glacial sliding and meltwater nutrients. Third, I investigate the influence of the U chemistry of current glacial meltwater from the Greenland Ice Sheet on adjacent seawater as a proxy to reconstruct the potential influence of glacial melt on global seawater U chemistry over glacial-interglacial
to a broad geographic range to investigate the universality of climate-melt relationships and the impact of glacial melt on the chemistry of both freshwater and seawater reservoirs.
Approximately seventy-five percent of all the freshwater in the world is stored in glaciers and ice sheets, making them the largest freshwater reservoir on the planet. Glaciers have been shrinking significantly as a result of climate change (Dyurgerov, 2003; Lemke and Ren, 2007; Shea and Marshall, 2007). Consequently, freshwater reservoirs created and moderated by glaciers are also shrinking (Barnett et al., 2005; Coudrain et al., 2005; Bradley et al., 2006; Kehrwald and Thompson, 2008). The importance of glaciers in the global hydrological cycle and the implications of their rapid decline in modern years has only recently been studied. Undoubtedly river systems will suffer from lower water levels in late summer and early fall, and additional important water quality and productivity factors may also be affected; such as changes in the sediment load, nutrient production from glacial weathering, and aquifer recharge for human utilization (Smith et al., 2001; Brown, 2002; Milner et al., 2009). A key aspect to making predictions on the effects of the decline in ice mass is quantifying the interaction between climate and subsequent meltwater, which can be assessed through geochemical analyses. The primary objective of my thesis is to develop and use new geochemical and computational proxies to place constraints on the evolution of source fraction contributions to glacial discharge, the length of time meltwater is stored within/beneath the glacial system, and how glacial meltwater chemistry impacts river and ocean uranium chemistry over anthropogenic timescales and periods of glacialinterglacial climate change.
Glaciers and ice sheets are defined as perennial ice masses that flow under their own weight and are characteristically divided into zones of accumulation, where mass is gained by snow accrual, and zones of ablation, where mass is lost primarily through melting and iceberg calving. The balance between mass loss and mass gain results in either glacier advance or retreat for both high elevation alpine glaciers and polar ice sheets. Mass loss via melting has increased with increasing global temperatures, resulting in the retreat of the majority of ice masses in the world (Shea and Marshal, 2007). Glacial meltwater is a product of ablation (Braithwaite, 1995; Hock, 1999), and can lead to accelerated mass loss via positive feedbacks such as increased ice velocity associated with increased pressure from meltwater in the subglacial system (Anderson et al., 2004). Glacial meltwater also transports glacially derived nutrients (Aizen et al., 1996), which can significantly impact downstream productivity and ecosystems depending on total glacial flux. The scale of each of these processes depends on the magnitude of melt and the evolution of the glacial hydrological system. Meltwater is generated at the surface of the glacier (supraglacial), within the glacier (englacial) and at the base of the glacier (subglacial); the majority of meltwater is then transported to the base where it interacts with the bedrock and sediment located in the subglacial environment before exiting the system (Figure 1.1).
1.1.1. The Subglacial System Ice sheets and glaciers can be defined by their relationship to the bed: warm-based glaciers are lubricated at the bed by a layer of meltwater, whereas cold-based glaciers are frozen to the bed.
Most valley glaciers in mid-latitudes are warm based, while those at higher latitudes have warm based centers and cold margins (Wadham et al., 2000; Cuffey and Paterson, 2010). Warming
change in the ice sheet or glacier from cold to warm based. Ice sheets and ice caps are complex systems that may contain both cold and warm based regions (Siegert, 2001), with the majority of meltwater generated by surface melting within ~10 km of the ice margin (Zwally et al., 2002).
The glacier bed in this region is likely warm and allows water to penetrate to the bed through water-driven fracture propagation (Das et al., 2008). Therefore both glacier and ice sheet/ice cap systems show similar hydrologic networks that may evolve seasonally or, over longer timescales, as a result of climate change.
= Unsaturated snow = Ice = Unsaturated frn = Water = Saturated frn = Bedrock Figure 1: Transport pathways for water in a glacier. These pathways change with changing
Figure 1.1: Transport pathways of for glacial water during minimal melt evolvecold tempera- temperature:
temperature; A) shows pathways glacial hydrology. These pathways due to with changing
a) Depicts pathways for glacial for glacial water during peak melt due to warm temperatures.