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Denudation Rates Derived from Spatially-Averaged Cosmogenic Nuclide Analysis in Nelson/Tasman Catchments, South Island, New Zealand

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Denudation Rates Derived from Spatially-Averaged Cosmogenic Nuclide Analysis in Nelson/Tasman Catchments, South Island, New Zealand Denudation rates derived from spatially-averaged cosmogenic nuclide analysis in Nelson/Tasman catchments, South Island, New Zealand Abby Jade Burdis 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 2014 Abstract New Zealand’s tectonically and climatically dynamic environment generates erosion rates that outstrip global averages by up to ten times in some locations. In order to assess recent changes in erosion rate, and also to predict future erosion dynamics, it is important to quantify long-term, background erosion. Current research on erosion in New Zealand predominantly covers short-term (100 yrs) erosion dynamics and Myr dynamics from thermochronological proxy data. Without competent medium-term denudation data for New Zealand, it is uncertain which variables (climate, anthropogenic disturbance of the landscape, tectonic uplift, lithological, or geomorphic characteristics) exert the dominant control on denudation in New Zealand. Spatially- averaged cosmogenic nuclide analysis can effectively offer this information by providing averaged rates of denudation on millennial timescales without the biases and limitations of short-term erosion methods. Basin-averaged denudation rates were obtained in the Nelson/Tasman region, New Zealand, from analysis of concentrations of meteoric 10Be in silt and in-situ produced 10Be in quartz. The measured denudation rates integrate over ~2750 yrs (in-situ) and ~1200 yrs (meteoric). Not only do the 10Be records produce erosion rates that are remarkably consistent with each other, but they are also independent of topographic metrics. Denudation rates range from ~112 – 298 t km-2 yr-1, with the exception of one basin which is eroding at 600 - 800 t km-2 yr-1. The homogeneity of rates and absence of a significant correlation with geomorphic or lithological characteristics could indicate that the Nelson/Tasman landscape is in (or approaching) a topographic steady state. Millennial term (10Be-derived) denudation rates are more rapid than those inferred from other conventional methods in the same region (~50 – 200 t km-2 yr-1). This is likely the result of the significant contribution of low frequency, high magnitude erosive events to overall erosion of the region. Both in-situ and meteoric 10Be analyses have the potential to provide competent millennial term estimates of natural background rates of erosion. This will allow for the assessment of geomorphic-scale impacts such as topography, tectonics, climate, and lithology on rates of denudation for the country where many conventional methods do not. Cosmogenic nuclides offer the ability to understand the response of the landscape to these factors in order to make confident erosion predictions for the future. i Acknowledgements Firstly the biggest thanks to my supervisor Kevin Norton. I’m so thankful for the opportunity to embark on this project, the experience has been amazing. Thank you for your patience in reading my thesis (so very many times) and for your enthusiasm for science, which has always been infectious. And thanks to Kevin and also Bethanna Jackson for the opportunity to go the AGU conference. That was certainly an experience I won’t forget and I’m so grateful for it. Thanks also to Steve Tims for all the help with sample collection and for sharing your expertise in meteoric method. Thanks to Richard for helping with field and lab work, your fabulous photography, and general support when I was experiencing cosmo-related explosions (both physical and mental). Thanks to Bob Ditchburn and Albert Zondervan for completing the preparation of my samples out at GNS and processing them in the AMS, I appreciate it hugely. Thanks so much to my office mates for the welcomed distractions as we ploughed (sometimes crawled and fumbled) our way through the Masters journey. Kirsten and Melody you guys have been a great support and heaps of fun. And finally thanks to my friends and family, without you I wouldn’t be where I am today. Sam, thanks for being my rock, it has meant the world to me. Mum thanks for your constant support, and Dad for your belief in me and your guidance - thank you both for making me who I am today. ii Contents Abstract………………………………………………………………………….. ..…i Acknowledgements………………………………………………………………......ii Table of contents……………………………………………………………….….....iii List of figures……………………………………………………………………….viii List of tables………………………………………………………………………….x Table of Contents 1. Introduction and Background ......................................................... 1 1.1 Introduction ................................................................................................... 1 1.2 Erosion in New Zealand ............................................................................... 2 1.2.1 Sediment yields ................................................................................ 3 1.2.2 Limitations ....................................................................................... 4 1.2.3 Micro-erosion meter ......................................................................... 5 1.2.4 Limitations ....................................................................................... 6 1.2.5 Thermochrononology ....................................................................... 7 1.2.6 Limitations ....................................................................................... 7 1.2.7 Sedimentation rates .......................................................................... 8 1.2.8 Limitations ....................................................................................... 9 1.2.9 Longitudinal profiling of fluvial terraces ......................................... 9 1.2.10 Limitations ................................................................................... 10 1.2.11 Landslide inventories ................................................................... 10 1.2.12 Limitations ................................................................................... 11 1.2.13 Modelling ..................................................................................... 11 1.2.14 Limitations ................................................................................... 14 1.3 Controls on erosion ..................................................................................... 14 1.3.1 Geomorphic controls on erosion .................................................... 15 1.3.2 Tectonic controls on erosion .......................................................... 15 1.3.3 Climatic controls on erosion .......................................................... 16 1.3.4 Lithological controls on erosion..................................................... 16 iii 1.3.5 Steady state..................................................................................... 17 1.3.6 Current theories of controls on erosion in New Zealand ............... 19 1.4 Discussion ..................................................................................................... 21 1.5 Cosmogenic nuclides ................................................................................... 21 1.6 Objectives ..................................................................................................... 23 1.7 Thesis structure ........................................................................................... 23 2. Cosmogenic nuclide methodology ................................................ 24 2.1 Cosmic rays .................................................................................................. 24 2.1.1 Cosmic ray origins and cascades.................................................... 24 2.1.2 Cosmic ray intensity and interactions with the magnetic field ..... 24 2.2 Cosmogenic nuclide production ................................................................ 26 2.2.1 Production of meteoric nuclides .................................................... 26 2.2.2 Production of in-situ nuclides ........................................................ 26 2.2.3 Production of 10Be ......................................................................... 27 2.3 Application of 10Be ...................................................................................... 31 2.3.1 Applications ................................................................................... 31 2.3.2 Denudation rates derived from in-situ 10Be analysis ..................... 31 2.3.3 Erosion rates derived from meteoric 10Be analysis ....................... 32 2.3.4 Apparent ages and denudational timescales .................................. 33 2.4 Basin-averaged denudation ........................................................................ 35 2.4.1 Basin-averaged theory .................................................................... 35 2.4.2 Basin-averaged denudation from analysis of in-situ 10Be .............. 36 2.4.3 Assumptions, considerations, and requisites .................................. 37 2.4.3.1 Isotopic steady state/uniform erosion ……………….…37 2.4.3.2 Lithology……………………………………….………38 2.4.3.3 Storage and transport ……………………………….….38 2.4.3.4 Grainsize ………………………………………….……38 2.4.3.5 Shielding……………………………………….…….…39 2.4.3.6 Quartz enrichment …………………………………...39 2.4.4 Basin-averaged denudation from analysis of meteoric 10Be .......... 39 2.4.3 Assumptions, considerations, and requisites ................................
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