UNIVERSITY OF EXETER COLLEGE OF LIFE AND ENVIRONMENTAL SCIENCES GEOGRAPHY Does Vegetation Mediate The Effects Of Climatic Change On Lake Catchment Dynamics at Lake Mugecuo, Southwest China? Submitted by Sarah-Jane Phelan to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Geography In August 2013. 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 acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signature: ……………………………………………………………….. 1 Abstract Lake sediment archives are widely used in palaeoenvironmental reconstructions on the basis that lake ecosystems are sensitive to environmental changes and they integrate changes in the surrounding landscape and atmosphere (Birks & Birks 2006; Carpenter et al., 2007; Pham et al., 2008; Williamson et al., 2008).The primary aim of this research is to investigate the links between long-term changes in climate and changes in plant community dynamics, and the potential impacts of these interactions on catchment palaeo- erosion rates during the Holocene in a high relief lake catchment. There is a distinct lack of palaeo-record coverage for the Hengduan Mountains and surrounding areas including the south eastern edge of the Tibetan Plateau (Zhang & Mischke, 2009). The Lake Mugecuo MG1 record analysed here is one of the few high resolution Holocene palaeoenvironmental and palaeoecological records available for Southwestern China and one of the first for this part of the Hengduan Mountains in Sichuan Province, extending back to ~12.6 kcal.yr BP. The long-term relationship between major climatic change, vegetation change, and denudation events is complex and poorly understood (Willis et al., 1997). A PFT (plant functional type) approach was adopted to provide a method of classifying pollen taxa into plant communities in order to determine whether plant functional diversity mediates the effects of climate on lake catchment dynamics (including long-term erosion rates) at Lake Mugecuo. 10Be concentrations in bedrock and fluvial sediment samples were used as an independent proxy to estimate surface erosion rates, along with lacustrine deposits of quartz from two shallow water cores (MG3 & MG4) which are used to estimate palaeo-erosion rates for the Lake Mugecuo catchment. A published speleothem record (Wang et al., 2005; Dykoski et al., 2005) from Dongge cave (25°17′N, 108°5′E; 680 m), is used as an indicator of the general pattern of effective moisture change in this region during the Holocene in order to determine if there is a direct link between climate change and the lake deposition record and whether factoring in vegetation improves our understanding of the link between climate change and the lake deposition archive. The MG1 record provides a unique insight into the environmental and vegetation changes that took place during the Late-Glacial/Early Holocene transition. Statistical analysis of the data reveals that plant functional diversity and lake catchment dynamics (using particle-size analysis as an insight into catchment stability) respond to long-term changes (>2,800yr) in climate on in the Lake Mugecuo catchment whilst interactions between plant functional diversity and lake catchment dynamics are found to vary on shorter timescales (<2,800yr). It is argued here that apparent environmental shifts captured in the Lake Mugecuo record are primarily driven/mediated by changes in plant functional diversity and that an array of different types of species and/or functional groups play an important role in modulating and landscape evolution across millennial timescales (approximately 1,000-3,000 years). Vegetation is found to have a filtering effect on the signals of the proxies that are traditionally used as indicators for changes in erosion and deposition in palaeoenvironmental reconstructions (e.g. particle size). 10Be cosmogenic measurements provided supplementary data on past changes in landscape dynamics in the Mugecuo catchment which the conventional proxies used in palaeoenvironmental studies were unable to achieve. 2 Acknowledgments I would like to thank the following people for their help with this research; My supervisors Dr Liam Reinhardt, Dr Richard Jones and Dr Mike Ellis for their contributions to this research Dr Enlou Zhang and colleagues at the Nanjing Institute of Geogrpahy and Limnology for logistical and technical support whilst on fieldwork in China Laboratory staff in the Geography Department at Exeter University for their assistance and guidance The Quaternary Research Association (QRA), the Royal Geographical Society (RGS) and the NERC NRCF-Steering Committee for providing additional funding for laboratory and fieldwork costs A special thanks must also go to my family and friends for their unconditional love, support and enthusiasm throughout this process. 3 Table of Contents 1) Chapter 1: Introduction .............................................................................................................15 1.1 Research Context ............................................................................................................................15 1.1.1 Geomorphology and lake catchment dynamics ................................................................. 16 1.1.2 A multi-proxy analysis of palaeoenvironmental records ..................................................... 18 1.2 Geographical context.......................................................................................................................20 1.2.1 Site Selection Sichuan ...................................................................................................... 20 1.2.2 Hengduan Mountain region ............................................................................................... 23 1.2.3 Mountain vegetation Sichuan ............................................................................................ 24 1.2.4 Sichuan climate- what is known ........................................................................................ 25 1.3 Research hypotheses, aims and objectives .....................................................................................28 1.4 Thesis outline/structure ...................................................................................................................30 1.5 Project Partners:..............................................................................................................................30 2) Chapter 2: Materials & Methods.........................................................................................................31 2.1 Site Description ...............................................................................................................................31 2.2 Reasons for site selection ................................................................................................................35 2.3 Lake Sediment Coring .....................................................................................................................37 2.3.1 Core site selection ............................................................................................................ 37 2.3.2 Extraction of cores from Lake Mugecuo ............................................................................ 39 2.3.3 Deep water core MG1 ....................................................................................................... 40 2.3.4 Shallow water cores MG3 and MG4 .................................................................................. 41 2.4 Core chronologies ...........................................................................................................................41 2.4.1 AMS 14C radiocarbon dating ............................................................................................. 41 2.4.2 Calibration of 14C dates and the construction of 14C age models ........................................ 42 2.4.3 Unified core chronologies ................................................................................................. 43 2.4.4 Sedimentation rates .......................................................................................................... 43 2.5 Organic and lithological analyses .....................................................................................................43 2.5.1 Physical properties and organic analysis ........................................................................... 43 2.5.2 Loss-on-ignition ................................................................................................................ 44 2.5.3 Total organic carbon, total organic nitrogen and C/N ......................................................... 44 4 2.5.4 δ13C .................................................................................................................................. 45 2.5.5 Particle size ...................................................................................................................... 46 2.5.6 X-ray diffraction (XRD) mineralogical analysis ................................................................... 47 2.6 Cosmogenic Isotope Analysis ..........................................................................................................48