The Holocene Glacial History of Dart Glacier, Southern Alps, New Zealand Lisa Holly Dowling A thesis submitted to Victoria University of Wellington in partial fulfilment of requirements for the degree of Master of Science in Physical Geography School of Geography, Environment and Earth Sciences Victoria University of Wellington 2019 Abstract ABSTRACT Mountain glaciers are sensitive climate indicators, as climate variability drives mass changes that are expressed in glacier length fluctuations. These length changes are preserved in the geological record, thus offering the potential to generate new palaeoclimate proxy data that can be used to extend instrumental climate records. This study presents geomorphological mapping and cosmogenic 10Be surface exposure dating of the Holocene moraines at Dart Glacier, New Zealand. These findings show that an early Holocene advance (~6 km longer than present-day) took place ~7817 ± 336 years ago. Moraine ages also show that a more restricted glacier readvance (~4 km longer than present-day) occurred ~321 ± 44 years ago. Through better constraining the timing and magnitude of Holocene glacier length changes, we extend the ~100-year history of observational records in the upper Dart valley. Net retreat of Dart Glacier during the Holocene is consistent with other moraine chronologies from New Zealand, which supports existing hypotheses that suggest summer insolation was the dominant driver of multi-millennial climate change at southern mid-latitudes during the current interglacial. Individual moraine forming events at Dart Glacier also coincide with moraine ages from several other catchments in the Southern Alps and likely reflect shorter-term (decadal-centennial-scale) climatic changes. The new geological record constraints of length changes at Dart Glacier offer the opportunity to test such hypotheses more formally using physics-based modelling. Connecting Holocene moraine records to historical glacier observations using 10Be surface exposure dating requires consistently low background levels of this rare isotope. Systematic blank experiments show that concentrated analytical grade hydrofluoric acid and reused beakers are likely the largest contributors of 10Be to the average process blank in the VUW Cosmogenic Laboratory. Based on these findings I recommend small methodological improvements that could be implemented to lower process blank ratios for routine application of 10Be surface exposure dating to near-historic glacial landforms. i ii Acknowledgements ACKNOWLEDGEMENTS Undertaking this research has been an incredible journey that I have shared with many people. Firstly, I would like to thank my supervisors, Shaun Eaves and Kevin Norton. Shaun, you have been an excellent mentor over the past three years. Thank you for always being so generous with your time and expertise, and for allowing me to be a part of this exciting project. Kevin, thank you for introducing me to the technique of cosmogenic nuclide dating, and for your encouragement and positivity throughout this project. I would also like to thank Lauren Vargo and Matt Ryan for their hard work during our unforgettable field expedition to Dart Glacier in 2017. Never again will we underestimate the intelligence and strength of kea! Thank you to the members of the VUW Cosmo group, particularly Claire Lukens, Ross Whitmore, Jamey Stutz, and our technicians Cassandra Trinh-Le and Luisa Ashworth - you were all immensely helpful and supportive during my time spent in the lab. Special thanks to my family, Linda, Deanna, Sam and Dave for their emotional support over the past year and for making all those delicious dinners! Thank you to my wonderful flatmates, Alice, Wynn and Rose, for their kindness and understanding during the weeks leading up to my hand in. I am also grateful to my office mates and peers for supporting me through the challenging but rewarding experience of completing this thesis. Finally, I would like to acknowledge Andrew Mackintosh, Drew Lorrey and Steve Tims for their involvement in the initial stages of designing this project. All sample analyses were carried out at Lawrence Livermore National Laboratory (LLNL) Center for Accelerator Mass Spectrometry (CAMS) by Alan Hidy and Susan Zimmerman. This research was funded by a National Geographic Waitt Grant. iii iv Table of Contents TABLE OF CONTENTS ABSTRACT ...................................................................................................................... i ACKNOWLEDGEMENTS ........................................................................................... iii TABLE OF CONTENTS ................................................................................................ v LIST OF FIGURES ...................................................................................................... vii LIST OF TABLES ......................................................................................................... ix CHAPTER 1: INTRODUCTION .................................................................................. 1 CHAPTER 2: BACKGROUND .................................................................................... 5 2.1 Holocene palaeoclimate ......................................................................................... 5 2.2 Mountain glaciers as palaeoclimate proxies ....................................................... 8 2.2.1 Glacier-climate interactions .............................................................................. 8 2.2.2 Controls on moraine formation and preservation ........................................... 11 2.2.3 Paleoclimatic significance of moraines .......................................................... 13 2.2.4 Dating methods ............................................................................................... 15 2.3 Cosmogenic 10Be surface exposure dating in glacial geomorphology ............. 16 2.3.1 Sources of cosmic rays ................................................................................... 16 2.3.2 Cosmic ray interactions with Earth’s geomagnetic field ................................ 16 2.3.3 Cosmic ray interactions with Earth’s atmosphere .......................................... 18 2.3.4 Cosmic ray interactions with Earth’s surface and nuclide production ........... 21 2.3.5 Production rates .............................................................................................. 22 2.3.6 Scaling models ................................................................................................ 23 2.2.7 Geological scatter in 10Be sample datasets ..................................................... 24 2.3.8 State-of-the-art of cosmogenic 10Be surface exposure dating ........................ 26 2.4 Existing 10Be moraine chronologies in New Zealand ....................................... 28 2.4.1 Pre-Holocene 10Be moraine chronologies....................................................... 28 2.4.2 Holocene 10Be moraine chronologies ............................................................. 29 2.5 Study site .............................................................................................................. 34 2.5.1 Geological and climatic setting....................................................................... 34 2.5.2 Dart Glacier catchment ................................................................................... 36 2.5.3 Observational record of glacier length changes at Dart Glacier ..................... 38 2.5.4 Geomorphology – previous work ................................................................... 42 CHAPTER 3: METHODS ........................................................................................... 45 3.1 Geomorphological mapping of glacial landforms at Dart Glacier ................. 45 v Table of Contents 3.1.1 General approach to geomorphological mapping ........................................... 45 3.1.2 Compilation of mapping resources and landform identification .................... 49 3.2 Cosmogenic 10Be surface exposure dating......................................................... 50 3.2.1 Sample Collection ........................................................................................... 50 3.2.2 Physical quartz separation and cosmogenic 10Be preparation ........................ 51 3.2.3 10Be measurement and exposure age calculation ............................................ 57 3.3 10Be sensitivity experiment ................................................................................. 58 3.3.1 Airborne contamination .................................................................................. 59 3.3.2 Contamination from reagents and the 9Be carrier ........................................... 59 3.3.3 Labware contamination .................................................................................. 61 CHAPTER 4: RESULTS.............................................................................................. 63 4.1 Glacial geomorphology of the upper Dart valley ............................................. 63 4.1.1 Upper study site geomorphology .................................................................... 63 4.1.2 Lower study site and Dart Hut site geomorphology ....................................... 69 4.2 10Be surface exposure ages .................................................................................. 74 4.3 10Be sensitivity experiment results ..................................................................... 83 4.3.1