Portenga, Eric W. (2015) Assessment of Human Land Use, Erosion, and Sediment Deposition in the Southeastern Australian Tablelands
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Portenga, Eric W. (2015) Assessment of human land use, erosion, and sediment deposition in the Southeastern Australian Tablelands. PhD thesis. https://theses.gla.ac.uk/6319/ Copyright and moral rights for this work are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This work cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Enlighten: Theses https://theses.gla.ac.uk/ [email protected] ASSESSMENTS OF HUMAN LAND USE, EROSION, AND SEDIMENT DEPOSITION IN THE SOUTHEASTERN AUSTRALIAN TABLELANDS Eric W. Portenga University of Vermont, MS University of Michigan, BS Submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy School of Geographical and Earth Sciences College of Science and Engineering University of Glasgow & Department of Environmental Sciences Faculty of Science and Engineering Macquarie University February, 2015 ABSTRACT Humans have interacted with their surroundings for over one million years, and researchers have only recently been able to assess the geomorphic impacts indigenous peoples had on their landscapes prior to the onset of European colonialism. The history of human occupation of Australia is noteworthy in that Aboriginal Australians arrived ~50 ka and remained relatively isolated from the rest of the world until the AD 1788 when Europeans established a permanent settlement in Sydney, New South Wales. The southeastern Australian Tablelands landscape, west of Sydney, has seemingly undergone drastic geomorphic change since European arrival. The introduction of European grazing practices reportedly led to the occurrence of deep erosional incisions, gullies, into valley bottoms and hillslopes, releasing sediment, which is subsequently deposited over downstream wetland environments – swampy meadows. This sediment is often called post-settlement alluvium (PSA); however, the age and genesis of PSA in Australia are debated. Questions regarding the geomorphic features and processes in the Tablelands remain unanswered because few studies quantify the timing of gully incision, PSA deposition, or the pre- human rate of landscape change. Erosion rates inferred from concentrations of in situ 10Be measured in fluvial sediment (n = 11) and bedrock outcrops (n = 6) range from 2.9–11.9 mm/kyr and 5.2–13.8 mm/kyr, respectively. The two sample populations are statistically indistinguishable, suggesting no relief has been generated since 600–110 ka. The overall erosion rate in the Tablelands is 7.5 mm/kyr, equal to long-term denudation rates integrated since ~20 Ma. Aboriginal Australians have been present in the Tablelands for at least 30 kyr, ~12–26% of the cosmogenic integration time, yet widespread Aboriginal fire use did not measurably affect landscape erosion until ~5.5 ka, in sync with increased charcoal in the sediment record. Portable optically stimulated luminescence (OSL) reader data from poly-mineral and poly- grain size samples collected from gully wall profiles of PSA and swampy meadow sediment show that swampy meadow environments were buried by PSA and that PSA is alluvium derived from upstream gully erosion. No relationships between bulk OSL and sample grain size or mineralogy exist, and inferences about bulk sediment mineralogy or ii grain size cannot be determined from portable OSL reader data. Large variability in adjacent PSA sample replicates, however, reveals incomplete sediment bleaching conditions during PSA deposition during floods. Greater bleaching efficiency is inferred from the small variability of bulk OSL data in the uppermost 10s of cm of PSA profiles. Measured concentrations of meteoric 10Be and bulk OSL in two PSA deposits in Birchams Creek show that initial gully incision eroded into weathered sandstone regolith and not swampy meadow environments, as previously believed. Initial gully incision was shallow (<15 cm) and PSA filled ponds in the lower reaches of the catchment. Continued erosion upstream led to a second depositional episode of PSA before headward gully incision from the mouth of Birchams Creek eroded through PSA deposits. Headward erosion of this gully created the continuous gully present at the site today. Initial gully incision was likely the result of livestock trampling in valley bottoms during droughts, creating localised slopes greater than the critical slope threshold required to erode the valley bottom. OSL burial ages of six PSA deposits collected throughout the Tablelands range from 195.1 ± 17.8 to 90.4 ± 8.9 a, corresponding to AD 1800–1932. The OSL burial ages are younger than European arrival in the Tablelands, and the term, PSA, is redefined as post-European settlement alluvium in Australia, recognising the earlier settlement of the region by Aboriginal Australians whose land use did not lead to PSA deposition. PSA burial ages agree with existing quantitative and anecdotal gully incision data. Contrary to previous assertions that gully incision began asynchronously in the Tablelands, three periods of synchronous gully erosion in localised areas within the Tablelands are recognized: 185 a, 158 a, and 94 a (AD 1828, 1855, and 1919, respectively) – in the southern, northern, and central Goulburn Plains, respectively. The AD 1828 and AD 1919 periods of gully incision correspond to the transition from drought-dominated climate regimes to flood-dominated regimes, and the AD 1855 period of gullying corresponds to a flood-dominated regime. Gully incision in the Tablelands is thus a result of European-introduced grazing practices, which primed the landscape for further erosion and degradation during climatic shifts. PSA deposits in the southeastern Australian Tablelands are some of the most recent examples of anthropogenic sedimentation in human history. The earliest preserved examples of PSA-type sediments are ~8,000 years old and found throughout the world. The establishment of an onset date for the Anthropocene is currently debated, and I believe the oldest PSA and PSA-type sediments around the world can define this modern epoch. iii DEDICATION Living so far away from family is not an easy thing to do. Dad, Lizzie, Ryan, Britta, Sarah, and Rob, without your unconditional support, I would never have been able to leave Michigan in pursuit of geomorphic curiosity. In particular, I dedicate this Thesis to my nephew and niece, Baker and Aubrey, with whom I have not had nearly enough Uncle-time! I also dedicate this Thesis in loving memory of my mother, Ann, and my grandparents who are always with me in spirit. To my extended family: Thank you for the on-line Christmas “Hellos!” I promise I will be home for the holiday next year. To my second family: Brennen, Barb, Steve, Alissa, and Cyndy. I wish I could see you more often. iv TABLE OF CONTENTS Abstract .................................................................................................................................. ii Dedication ............................................................................................................................. iv List of Figures ....................................................................................................................... ix List of Tables...................................................................................................................... xiii Acknowledgements .............................................................................................................. xv Declaration of Originality ................................................................................................ xviii Definitions & Abbreviations ............................................................................................... xix Time Notation in this Thesis ............................................................................................... xxi 1.0 Introduction .................................................................................................... 1 1.1 A Brief History of the Global Rise and Spread of Agriculture ...................... 1 1.2 Anthropogenic Geomorphology ..................................................................... 3 1.2.1 Loss of Natural Land ...................................................................................... 3 1.2.2 Soil Erosion and Post-Settlement Alluvium ................................................... 3 1.2.3 The Australia Conundrum .............................................................................. 6 1.3 Thesis Format and Structure ........................................................................... 7 2.0 The Tablelands of Southeastern Australia ...................................................... 9 2.1 Geological Setting .......................................................................................... 9 2.2 Late Quaternary Climatic and Glacial History ............................................. 11 2.3 Modern Tablelands Geography .................................................................... 12 2.4 Human Habitation of the Tablelands ............................................................ 14 3.0 What is the Background Erosion Rate in the Tablelands? ..........................