The Timing, Dynamics and Palaeoclimatic Significance of Ice Sheet Deglaciation in Central Patagonia, Southern South America
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The timing, dynamics and palaeoclimatic significance of ice sheet deglaciation in central Patagonia, southern South America Jacob Martin Bendle Department of Geography Royal Holloway, University of London Thesis submitted for the degree of Doctor of Philosophy (PhD), Royal Holloway, University of London September, 2017 1 Declaration I, Jacob Martin Bendle, hereby declare that this thesis and the work presented in it are entirely my own unless otherwise stated. Chapters 3-7 of this thesis form a series of research papers, which are either published, accepted or prepared for publication. I am responsible for all data collection, analysis, and primary authorship of Chapters 3, 5, 6 and 7. For Chapter 4, I contributed datasets, and co-authored the paper, which was led by Thorndycraft. Detailed statements of contribution are given in Chapter 1 of this thesis, for each research paper. I wrote the introductory (Chapters 1 and 2), synthesis (Chapter 8) and concluding (Chapter 9) chapters of the thesis. Signed: ..................................................................................... Date:.............................. (Candidate) Signed: ..................................................................................... Date:.............................. (Supervisor) 2 Acknowledgements First and foremost, I am very grateful to my supervisors Varyl Thorndycraft, Adrian Palmer, and Ian Matthews, whose tireless support, guidance and, most of all, enthusiasm, have made this project great fun. Through their company in the field they have contributed greatly to this thesis, and provided much needed humour along the way. For always giving me the freedom to explore, but wisely guiding me when required, I am very thankful. I am indebted to my brother, Aaron Bendle, who helped me for five weeks as a field assistant in Patagonia, and who tirelessly, and without complaint, dug hundreds of sections – thank you for your hard work and great company. Chris Hayward and Christina Manning helped me analyse tephra samples, and Andy Breckenridge gave up much of his time responding to email queries about lake reconstruction in ArcGIS. A huge thank you is also owed to the Royal Holloway technical staff, Katy Flowers, Iñaki Valcarcel, Jenny Kynaston, Raymond Aung, and Marta Perez-Fernandez, who have always been approachable and helpful, and have solved more than a few issues. Many of the Department of Geography staff and PhD community have made the last four years so enjoyable. There are too many people to name individually, but a special mention goes to Paul Lincoln, Jenni Sherriff, Rhys Timms, Ash Abrook, Angharad Jones, Rachel Squire and Dan Webb, who were there throughout, and all helped, listened and provided a steady, and much needed flow of humour. And to those who came later, Julian, Nicola, and the other Quaternarists, your company in the office has made the whole process bearable, and even quite enjoyable! Finally, to my family – Mum, Dad, Aaron and Gus – thank you for your endless encouragement and for always believing in me. Last, and by no means least, Helen, your positivity, motivation and unwavering support has kept me going – I wouldn’t have got through it without you. This thesis was supported by a Natural Environment Research Council (NERC) studentship [grant number: NE/L501803/1]. In addition, several other funding bodies supported fieldwork and/or laboratory costs, namely: an Explorers Club Exploration Grant; a Quaternary Research Association (QRA) New Research Workers' Award; and a University of London PGR Grant. 3 The timing, dynamics and palaeoclimatic significance of ice sheet deglaciation in central Patagonia, southern South America Jacob Martin Bendle Abstract Robust reconstructions of glacial retreat are required to elucidate controls on ice-sheet deglaciation and (a)synchronicity in the climate system. Such reconstructions can be derived through detailed geological investigations of glacial landforms and sediments. Patagonia, in southern South America, contains a well-preserved glacial record and occupies an important location regarding major Southern Hemisphere climate systems. However, resolving the history of ice-sheet deglaciation has proven problematic, owing to generally low-resolution geomorphological mapping, and a paucity of high-precision dating control. This thesis explores the deglacial history of the Lago General Carrera– Buenos Aires (LGC–BA) and Lago Cochrane–Pueyrredón (LC–P) ice lobes (46–48°S) of the former Patagonian Ice Sheet (PIS). The region is chosen because there are uncertainties associated with: (i) the deglacial configuration and evolution of retreating ice lobes and proglacial lake systems; and (ii) the sub-millennial timing of deglacial events. A new glacial geomorphological map is produced to reconstruct the spatio-temporal evolution of ice lobes and proglacial lakes. The elevation(s) of relict lake shorelines are examined to quantify glacio-isostatic adjustment and accurately assign former lake levels, lake outflows, and ice-margin positions, through the last deglaciation. These new reconstructions are used to target sites for high-resolution dating. A ~1000-year varve chronology is developed at LGC–BA after examining the sedimentary properties of laminated glaciolacustrine sequences. Tephrochronology anchors this record to the calendar-year timescale. The chronology indicates that PIS retreat commenced at 18,086 ± 214 cal a BP, and accelerated from 5 to 18 m yr-1 over the next ~1000 years. The new timing for deglaciation at LGC–BA allows an exploration of climate forcing mechanisms. Comparisons with palaeoclimatic records reveal a hemisphere-wide warming influence on LGC–BA deglaciation. Moreover, spectral analyses of varve thickness time-series demonstrate a likely teleconnection with tropical Pacific climate oscillations, and PIS sensitivity to short-term variations in melt-season temperature. 4 Contents Declaration……….. ..………………………………………………………………………….2 Acknowledgements ..………………………………………………………………………….3 Abstract ...……………………………………………………………………………………....4 Contents .....…………………………………………………………………………………….5 List of figures …………………………………………………………………………………11 List of tables …………………………………………………………………………………14 Chapter 1. Introduction……………………………………………………...……………....15 1.1 Rationale ………………………………………………………………………………15 1.2 Study area ……………………………………………………………………………..17 1.2.1 Central Patagonian Andes ………………………………………………………17 1.2.2 Río Fenix Chico valley ……………………………………………………………19 1.3 Previous work …………………………………………………………………………21 1.3.1 Glacial extent ...…………………………………………………………………...21 1.3.2 Glacial chronology ………………………………………………………………..24 1.3.3 Proglacial lakes …………………………………………………………………...27 1.3.4 Glacial dynamics …………………………………………………………………29 1.3.5 Numerical modelling ……………………………………………………………..30 1.3.6 Summary …………………………………………………………………………..34 1.4 Aims and objectives …………………………………………………………………..36 1.4.1 Aims ………………………………………………………………………………..36 1.4.2 Objectives …………………………………………………………………………36 1.5 Thesis structure and results …………………………………………………………37 1.5.1 Chapter 3 …………………………………………………………………………37 1.5.2 Chapter 4 …………………………………………………………………………37 1.5.3 Chapter 5 …………………………………………………………………………38 1.5.4 Chapter 6 …………………………………………………………………………39 1.5.5 Chapter 7 …………………………………………………………………………39 Chapter 2. Methods …………………………………………………………………………41 2.1 Research framework …………………………………………………………………41 2.2 Phase 1: Mapping the glacial geomorphology of the LGC–BA and LC–P ice lobes ………………………………………………………………………………………………….41 2.2.1 Rationale ………………………………………………………………………….41 2.2.2 Glacial geomorphological mapping ……………………………………………42 5 2.2.3 Glacial sedimentology and stratigraphy ……………………………………….44 2.3 Phase 2: Reconstructing glacio-isostatic rebound and proglacial lakes ..………45 2.3.1 Rationale ………………………………………………………………………….45 2.3.2 Mapping criteria for shoreline identification …………………………………..45 2.3.3 Determining rebound isobases and former lake levels ……………………...48 2.4 Phase 3a: Investigating the sedimentology of laminated glaciolacustrine deposits ………………………………………………………………………………………………….49 2.4.1 Rationale …………………………………………………………………………49 2.4.2 Macroscale analysis of laminated sediments ….………………………………50 2.4.3 Sampling and thin section production ………………………………………...51 2.4.4 Microscale analysis of laminated sediments …………………………………52 2.4.5 Particle-size analysis of laminated sediments ..………………………………52 2.5 Phase 3b: Constructing a varve chronology ……………………………………….52 2.5.1 Rationale ………………………………………………………………………….52 2.5.2 Site selection …………………………………………………………………….53 2.5.3 Varve counting procedure ………………………………………………………53 2.5.4 Macroscale varve counting ……………………………………………………..56 2.5.5 Microscale varve counting ………………………………………………………56 2.5.6 Constructing site varve chronologies and evaluating counting uncertainties ………………………………………………………………………………………………….58 2.5.7 Cross-dating to construct a composite varve chronology …………………...58 2.5.8 Error estimation ………………………………………………………………….59 2.6 Phase 3c: Dating the floating varve chronology using tephrochronology ………59 2.6.1 Rationale ………………………………………………………………………….59 2.6.2 Field sampling ……………………………………………………………………60 2.6.3 Electron microprobe analysis of individual glass shards ……………………60 2.6.4 X-ray fluorescence spectrometer analysis of bulk tephra …………………..62 2.6.5 Assigning an eruption age ………………………………………………………62 2.7 Phase 3d: Bayesian modelling of ice lobe deglaciation ………………………….62 2.7.1 Rationale ………………………………………………………………………….62 2.7.2 Modelling of ice lobe retreat dynamics ……………………………….............63 2.8 Phase 4: variations