Palaeosalinity Change in the Taw Estuary, South-West England: Response to Late Holocene River Discharge and Relative Sea-Level Change
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Palaeosalinity change in the Taw Estuary, south-west England: response to late Holocene river discharge and relative sea-level change submitted by Glenn Michael Havelock to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Geography, January 2009. This thesis is available for Library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgment. I certify that all material in this thesis which is not my own work has been identified and that no material is included for which a degree has previously been conferred upon me. Glenn Havelock ……………………………….…. ABSTRACT Present models of Holocene estuary evolution are driven largely by changes in relative sea-level (RSL) with little reference to long-term changes in fluvial regime and regional climate. Recent US studies of estuarine sequences have shown that decadal-centennial scale fluctuations in river discharge and freshwater inflow can be inferred by changes in estuarine palaeosalinity and that the timing of these events reflect changes in regional precipitation. It is therefore becoming apparent that estuarine sequences may hold an archive of mid-late Holocene climate change information, as well as being recorders of RSL change. The principal aim of this study is to produce a palaeosalinity-based climate record for southern England during the late Holocene, based on changes in climate-driven freshwater influx into the estuarine environment. The late Holocene palaeosalinity record of the inner Taw Estuary will be reconstructed using diatom salinity index as a proxy for salinity. Nine periods of below-average or above-average palaeosalinity have been recognised in the Taw Estuary since 300 cal.yr.BC. Four intervals of high river discharge are identified at 520-780, 850-1030, 1215-1315, and 1420-1900 cal.yr.AD. Five intervals of low river discharge are identified at 300-520, 780-850, 1030-1215, 1315-1420, and 1900-2000 cal.yr.AD. This shows that there has been significant climatic variation in southern Britain since c.300 cal.yr.AD, with climatic shifts evident in the estuarine record. In order to validate this record, the fluvial geomorphic history of the lower Taw valley was also investigated. There is a strong correspondence between the dry and wet climatic periods identified in the estuary and the geomorphic fluvial history and flood record of the lower Taw valley. Comparisons with other proxy climate records in the UK and Europe show a high degree of correspondence with the Taw Estuary palaeosalinity-based climate record. As the inner estuary environment will also be influenced by RSL change during the late Holocene, RSL change since c.6600 cal.yr.BP was successfully reconstructed in the Taw Estuary, with eleven new validated SLIPs providing evidence of former MSL. The magnitude and rates of RSL rise in north Devon are compared with other RSL records in southern Britain, suggesting that the isostatic history is similar to other areas bordering the Bristol Channel and with the central south coast of England. Fluctuations in palaeosalinity in the late Holocene are seen to be mainly controlled by centennial- scale changes in climate-driven river discharge, rather than RSL change. 2 ACKNOWLEDGMENTS My research at the University of Exeter (Department of Geography) could not have happened without the very considerable support of many individuals, in so many ways. My supervisors Prof Tony Brown and Dr Jason Jordan gave valuable professional help and encouragement throughout for which I am truly grateful. Personal advice and helpful discussions came from Prof John Allen (University of Reading) and Prof Antony Long (University of Durham). I would like to thank the technical staff in the Department of Geography at Exeter, for their valuable advice and technical assistance, especially from Jim Grapes, Sue Frankling, Angela Elliot, Neville England, Mike Rouillard (Department of Archaeology) and Terry Bacon. I would also like to thank Helen Jones and Tracey Reeves for there administrative support throughout my time at Exeter. I would like to thank the University of Exeter for the provision of a three-year departmental scholarship, for which I am truly grateful. Much appreciated funding for the OSL dating programme came via grants awarded from the QRA-RLAHA Luminescence Dating Award (this award is run collaboratively by the Quaternary Research Association and the Research Laboratory for Archaeology and the History of Art at Oxford University) and the British Society for Geomorphology Postgraduate Research Fund. Much appreciated funding for the radiocarbon dating programme came via the substantial support from a NERC Radiocarbon Dating grant (Allocation Number: 1213.0207) and from funding by Devon County Council Archaeological Service (this fund was used for rangefinder dates; many thanks to Frances Griffith, County Archaeologist, and her colleague Bill Horner). Special thanks go to Dr Jean-Luc Schwenninger, at the University of Oxford RLAHA Luminescence Dating Laboratory, for his support, encouragement and provision of additional OSL dates. I also thank Jean-Luc and his technician David Peat for the training given in OSL sample preparation and for there help in OSL sample recovery from the UH1 field site. I would like to thank Dr Kevin White (Department of Geography, University of Reading) for the use of their Mineral Magnetics Laboratory and for the practical training, advice and support that he gave. Thanks again go to Frances Griffith for access to the Devon County Council Archaeological Service aerial photograph collection 3 (invaluable in the early desk study) and the Devon Sites and Monuments Register. I thank Edina Digimap/Ordnance Survey for supply of digitised map data and I am also grateful to the NERC British Isles GPS Archive Facility (BIGF) for supply of archive GPS RINEX data (used in the modern estuary survey). In terms of field work, I am very grateful for the generous time and help (coring in all weather!) given by so many Exeter colleagues. Many thanks go to Jason Jordan, Mark Dinnin, Laura Basell, Chris Carey, Rob Stroud, Phil Allen, Ben Thomas, Jenny Bennett, Paul Clark, Naomi Holmes, Lucy Clarke, Steve Davis, and Lynda Yorke (University of Aberystwyth). Of these, I thank Laura Basell, Chris Carey and also Chris Smart (Department of Archaeology) for help with the differential GPS survey and post- processing, and thank Chris Carey and Phil Allen for the help with the Geoprobe (much fun!). And most especially, my thanks go to the various landowners who generously allowed me access to their land in the lower Taw Valley, especially Colonel Maxse who took particular interest in the project. I would also like to thank Dr Klaus Kuhn for taking me on as his teaching assistant for so many years, and for his general support and good humour; and I thank my work colleagues at the University of Exeter Library (the shelving team!) for the many good hours together. I owe a particular debt and thanks to my parents, for their invaluable support throughout my education. And last but by no means least, my heartfelt gratitude to my wife Caryn, who together with our sons Samuel and Jacob, was so patient, supportive and enduring throughout. G.M.H. 4 CONTENTS LIST Page ABSTRACT ………………………………………………………………………. 2 ACKNOWLEDGMENTS ………………………………………………………... 3 LIST OF CONTENTS ……………………………………………………………. 5 LIST OF FIGURES ………………………………………………………………. 12 LIST OF TABLES ………………………………………………………………... 17 1. INTRODUCTION……………………………………………………............ 18 1.1 BACKGROUND TO RESEARCH ……………………………………........ 18 1.2 AIMS AND OBJECTIVES ………………………………………………… 19 1.3 STUDY AREA ……………………………………………………………... 22 2. ESTUARINE EVOLUTION AND PALAEOSALINITY ………………… 26 3. METHODOLOGY …………………………………………………………. 32 3.1 GEOMORPHOLOGICAL MAPPING ……………………………............... 32 3.2 STRATIGRAPHIC TRANSECTS ………………………………................. 32 3.3 CORE RECOVERY AND FIELD SAMPLING ……………………….…... 34 3.4 CORE ANALYSIS ……………………………………………………..…... 34 3.4.1 Core description ……………………..………………………….…... 34 3.4.2 Sediment sub-sampling ……………………………………………... 35 3.4.3 Organic matter and carbonate content ………………………………. 35 3.4.4 Magnetic susceptibility ……………………………………………... 36 3.4.5 Magnetic remanence ………………………………………………... 39 3.4.6 Diatom analysis ……………………………………………………... 42 3.5 CHRONOLOGICAL FRAMEWORK ……………………………………... 43 3.5.1 Radiocarbon dating …………………………………………………. 43 3.5.2 Optically stimulated luminescence (OSL) dating …………………... 46 3.6 RELATIVE SEA-LEVEL RECONSTRUCTION …………………………. 50 3.7 CONTEMPORARY DIATOM DISTRIBUTION …………………………. 53 3.8 DATA ANALYSIS …………………………………………………………. 56 3.8.1 Analysis of the modern dataset ………………………...…………… 56 3.8.2 Analysis of the Holocene dataset …………………………...………. 56 5 4 CONTEMPORARY DIATOM DISTRIBUTION IN THE TAW ESTUARY AND THE LOWER TAW VALLEY ………………………………………. 58 4.1 INTRODUCTION ………………………………………………………...... 58 4.1.1 Taw Estuary survey ………………………………………................. 58 4.1.2 River Taw survey …………………………………………………… 60 4.2 TAW ESTUARY …………………………………………………………… 61 4.2.1 Taw Estuary transect ………………………………………………... 61 4.2.2 Contemporary diatom distribution and environmental variables …… 69 4.2.3 Altitude and diatom salinity index ………………………….............. 80 4.3 RIVER TAW ……………………………………………………………...... 82 4.3.1 River Taw transect ………………………………………………...... 82 4.3.2 Salinity …………………………………………………………........ 89 4.3.3 Sedimentary environmental variables ………………………………. 91 4.3.4 Contemporary diatom distribution ………………………………….. 94 4.3.5 Limitations