Ancient Climate and Environmental History from Phytolith-Occluded Carbon
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ANCIENT CLIMATE AND ENVIRONMENTAL HISTORY FROM PHYTOLITH-OCCLUDED CARBON By JOHN ALEC CARTER A thesis submitted to the Victoria University of Wellington in fulfilment of the requirements for the degree of Doctor of Philosophy in Geology Victoria University of Wellington 2007 Panorama of the Wairarapa looking west, drawn by Samuel Charles Brees in the early 1840’s. From: Illustrations to Adventure in New Zealand by Edward J. Wakefield. “What happened under the low green tarred branch of the Ngaio tree cannot be clearly remembered but determined what we know now”-. James K. Baxter, The Ngaio Tree. i ABSTRACT Ancient Climate and Environmental History from Phytolith-occluded Carbon The best records of atmospheric change of glacial cycles are those from ice cores. 13 However, ice cores cannot provide estimates of changes in atmospheric CO2 because of as of yet unresolved technical problems. One of the least understood and important influences on the changes to the isotopic composition of atmospheric CO2 are that of vascular plants. While marine benthic δ13C records have been used to infer past changes in terrestrial vegetation, accurate estimation of changes in carbon storage on land during ice ages has proved elusive. Other estimates have been made from terrestrial biomes of pollen records but a large discrepancy between marine and land based estimates remains. 13 This thesis offers a new method of deriving an ancient atmospheric δ CO2 record using measurements of phytolith-occluded carbon as a proxy. The method is designed to measure 13 13 δ CO2 in ancient phytolith-occluded carbon and convert this signal into an atmospheric δ CO2 estimate for the atmosphere. Phytoliths are very small particles of silica (between 5 and 100 microns) that form distinctive and repeatable shapes in most plants. When phytoliths form within a plant, some of the host organic matter is trapped inside the phytolith. Phytoliths have been shown to contain occluded carbon and are present in most terrestrial sedimentary deposits. Moreover, because they survive well in most soils and sediments, the trapped carbon remains intact and preserved from contamination and alteration. Experiments were conducted to characterise and measure the natural variability of modern phytolith-occluded carbon. These included measurement of carbon isotopic fractionation effects between the atmosphere and whole plant material, measurement of carbon isotope fractionation between whole plant matter and phytolith-occluded carbon, and a determination of carbon compounds present in phytolith-occluded carbon. A formula was developed for 13 separating the plant physiological factors from the atmospheric CO2 value in the phytolith- 13 occluded carbon, thus providing a basis for estimating atmospheric CO2 values. Phytoliths were extracted and occluded carbon analysed from a 7.4m loess core. Changes in phytolith assemblages were used to create a direct record of changes to the local vegetation cover, and isotopic analyses of carbon in phytoliths to generate a record of 13 atmospheric CO2 for the last 120,000 years. The record exhibits a number of periods when the 13 atmosphere had very low δ CO2 values that correspond with CH4 peaks in the Vostok ice core. It is hypothesized here that these low values are a consequence of the release of large volumes of methane released from marine hydrate (clathrate) deposits into the atmosphere, thereby, diluting 13 atmospheric CO2. ii CONTENTS PAGE 1.0 Chapter One: 1 Introduction 1.1 Overview 1 1.2 The Purpose and Scope of this Thesis. 3 1.3 Field Location and Sampling of the Otaraia Loess Section. 4 1.4 A Brief History of Phytoliths. 6 1.5 Phytolith Formation. 6 1.6 Composition, Characteristics and Function. 9 1.7 Form and Preservation. 10 1.8 Terminology and Classification. 11 1.9 Paleoenvironmental Reconstruction. 13 1.10 Chronology. 16 1.11 Advances in Chemical Analysis. 17 1.12 Phytoliths in Archaeological Research. 17 1.13 Stable Isotope Analysis 19 1.13.1 Oxygen. 19 1.13.2 Carbon. 20 1.13.2.1 DNA 22 1.13.2.2 Stability of Phytolith-Occluded Carbon 22 1.14 Phytolith History in New Zealand. 24 1.15 Conclusions. 24 2.0 Chapter Two: 26 Paleoenvironmental Reconstruction using Phytolith Morphologies from Otaraia, Wairarapa 2.1 Introduction. 26 2.2 Previous work. 28 2.3 Study Area. 28 iii 2.3.1 Weather and Climate 29 2.3.2 Primeval, Recent Historic and Modern Vegetation. 29 2.4 Description and Age of the Loess Section at Otaraia. 30 2.5 Age Model. 31 2.6 Methods. 33 2.7 Results. 33 2.8 Discussion. 37 2.8.1 Chronology. 38 2.8.2 Sedimentology. 39 2.9 Paleoenvironmental Interpretation. 41 2.10 Conclusions. 51 3.0 Chapter Three: 53 Carbon Isotope Signatures in Modern Plant Phytolith and Whole Leaf Material 3.1 Introduction. 53 3.2 Fractionation of Atmospheric Carbon Dioxide in Whole Plant Material. 53 3.3 A History of Research into the use of C Isotopes in Phytoliths. 56 3.4 Methods. 61 3.4.1 Sample Preparation and Phytolith Extraction from Modern Plant Material. 63 3.4.2 Carbon Isotope Analysis. 63 3.5 Results: 64 3.5.1 Pilot study. 64 3.5.2 Main Study Results. - 65 3.5.3 Carbon Compounds Present within Phytolith-Occluded Carbon. 69 3.6 Discussion. 72 3.6.1 Comparison with Previous Work and Samples Grown Under Different CO2 Concentrations. 72 3.6.2 Possible Seasonal Fractionation Effects. 74 iv 13 3.6.3 Atmospheric δ CO2 for Carbon from Modern Plant Phytoliths. 77 3.7 Conclusions. 80 4.0 Chapter 4: 81 13 Atmospheric δ CO2 Signal from Ancient Phytolith-Occluded Carbon 4.1 Introduction 81 4.2 Results and Initial Assessment. 84 4.3 Influence of Vegetation Type. 86 13 4.4 Ancient Atmospheric δ CO2 Signal. 88 13 4.5 Comparison between Otaraia and Byrd Ice-Core δ CO2 Data. 93 4.5.1 Errors 95 4.6 Processes Affecting Carbon Sources and Sinks 96 4.7 Comparison between Otaraia and Vostok CO2 and CH4 Data. 99 5.0 Chapter Five: Summary and Conclusions 104 Acknowledgements 107 References 108 Appendix 1: Luminescence Dating of Three Samples 126 Appendix 2: Phytolith Counts from Otaraia Loess Sample 129 13 Appendix 3: Analysis of 125 ka Atmospheric δ CO2 Record 131 Appendix 4: Analysis of δ13C from Ancient Phytoliths 133 Appendix 5: Analysis of δ13C from Plants and Phytoliths from Pilot Study 138 Appendix 6: Analysis of δ13C from Plants from Main Study 140 Appendix 7: Analysis of δ13C from Phytoliths from Main Study 143 v LIST OF TABLES Table 1.1: List of publications where phytoliths have been used in research in New Zealand. 25 Table 2.1: Measured a-values and equivalent dose-rates and luminescence Ages 31 Table 3.1: Pilot study results from analyses of percentage carbon δ13C. 65 Table 3.2: Record of plant species from the main study. 67 Table 3.4: Seasonal differences in phytolith-occluded δ13C. 74 Table 4.1: Estimates and averages of changes in carbon stored in terrestrial biosphere since the LGM. 83 Table 4.2: Otaraia phytolith δ13C values and depths. 87 Table 4.3: Average mean and standard deviation values from the Otaraia core 97 13 Table 4.4: Correlation between low δ CO2 episodes at Otaraia and CH4 spikes in Vostok Ice Core. 100 vi LIST OF FIGURES Frontispiece: Panorama of the Wairarapa i Fig. 1.1: Photomicrograph showing a typical grassland phytolith assemblage extracted from a loess sample from South Wairarapa. 3 Fig. 1.2: Location of Otaraia drill site and other locations mentioned in this thesis. 5 Fig. 1.3: Photomicrograph of an anticlinal phytolith from a modern fern Blechnum sp. 7 Fig. 1.4: Fragile (polyhedral) tree phytoliths from modern native New Zealand Nothofagus solandri var solandri. 8 Fig. 1.5: Globular tree phytoliths from modern native New Zealand tree Knightia excelsa. 9 Fig. 1.6: Some of the internationally defined morphological phytolith shapes. 12 Fig. 1.7: Diagram showing the changes in phytolith assemblages from Bidwill Hill, Wairarapa, New Zealand. 14 Fig 1.8: Microprobe image showing occluded carbon within a phytolith from Festuca novae-zelandiae. 21 Fig. 2.1: Examples of phytoliths extracted from Otaraia loess core. 27 Fig. 2.2: Diagram of the Otaraia loess profile, mean grain size, sand, silt and clay percentages, major elements, iron and phosphorus and an age model showing the derived ages of Palmer (1982), the Kawakawa Tephra and OSL ages of this study. 32 Fig. 2.3: Phytolith stratigraphic frequency diagram. 34 Fig. 2.4: Correlation diagram of 18O SPAMAP data, and MIS of Martinson et al. (1987), Vostok temperature (ºC) data of Petit et al. (2000), and Otaraia phytolith data (this study). 42 Fig. 2.5: Correlation diagram of insolation values for 40º S and 65º N through last glacial cycle, pollen data form Okarito (Vandergoes et al. 2005), phytolith data from Otaraia (this study). 43 Fig. 2.6: Panorama of the Wairarapa. 52 Fig. 3.1: Absorption of Carbon Dioxide. 54 vii Fig. 3.2: Microprobe image of a cross-sectioned phytolith extracted from Festuca novae zelandiae. 57 Fig. 3.3: Carbon occlusion during phytolith formation. 58 Fig 3.4: Pilot study δ13C‰ fractionation between plant material and phytolith-occluded carbon extracted from the same material 64 Fig. 3.5: Combined carbon isotope analysis records from all New Zealand plant species. 68 Fig. 3.6: NMR spectra of Festuca novae-zelandiae (grass) and Nothofagus solandri (tree). 71 Fig. 4.1: Diagram of Otaraia core δ 13C values plotted against depth. 86 Fig. 4.2: Results from modelling show the possible influence of floral type on δ13C values. 88 Fig.