Quaternary Dating Methods E-Book

Mike Walker

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 580

Veröffentlichungsjahr: 2013

Beschreibung

This introductory textbook introduces the basics of dating, the range of techniques available and the strengths and limitations of each of the principal methods.

Coverage includes:

• the concept of time in Quaternary Science and related fields
• the history of dating from lithostratigraphy and biostratigraphy
• the development and application of radiometric methods
• different methods in dating: radiometric dating, incremental dating, relative dating and age equivalence

Presented in a clear and straightforward manner with the minimum of technical detail, this text is a great introduction for both students and practitioners in the Earth, Environmental and Archaeological Sciences.

Praise from the reviews:

"This book is a must for any Quaternary scientist." SOUTH AFRICAN GEOGRAPHICAL JOURNAL, September 2006

“…very well organized, clearly and straightforwardly written and provides a good overview on the wide field of Quaternary dating methods…” JOURNAL OF QUATERNARY SCIENCE, January 2007

Leseprobe

Contents

Preface

1 Dating Methods and the Quaternary

1.1 Introduction

1.2 The Development of Quaternary Dating

1.3 Precision and Accuracy in Dating

1.4 Atomic Structure, Radioactivity and Radiometric Dating

1.5 The Quaternary: Stratigraphic Framework and Terminology

1.6 The Scope and Content of the Book

Notes

2 Radiometric Dating 1: Radiocarbon Dating

2.1 Introduction

2.2 Basic Principles

2.3 Radiocarbon Measurement

2.4 Sources of Error in Radiocarbon Dating

2.5 Some Problematic Dating Materials

2.6 Calibration of the Radiocarbon Timescale

2.7 Applications of Radiocarbon Dating

Notes

3 Radiometric Dating 2: Dating Using Long-Lived and Short-Lived Radioactive Isotopes

3.1 Introduction

3.2 Argon-Isotope Dating

3.3 Uranium-Series Dating

3.4 Cosmogenic Nuclide Dating

3.5 Dating Using Short-Lived Isotopes

Notes

4 Radiometric Dating 3: Radiation Exposure Dating

4.1 Introduction

4.2 Luminescence Dating

4.3 Electron Spin Resonance Dating

4.4 Fission Track Dating

Notes

5 Dating Using Annually Banded Records

5.1 Introduction

5.2 Dendrochronology

5.3 Varve Chronology

5.4 Lichenometry

5.5 Annual Layers in Glacier Ice

5.6 Other Media Dated by Annual Banding

Notes

6 Relative Dating Methods

6.1 Introduction

6.2 Rock Surface Weathering

6.3 Obsidian Hydration Dating

6.4 Pedogenesis

6.5 Relative Dating of Fossil Bone

6.6 Amino Acid Geochronology

Notes

7 Techniques for Establishing Age Equivalence

7.1 Introduction

7.2 Oxygen Isotope Chronostratigraphy

7.3 Tephrochronology

7.4 Palaeomagnetism

7.5 Palaeosols

Notes

8 Dating the Future

8.1 Introduction

8.2 Radiometric Dating

8.3 Annually Banded Records

8.4 Age Equivalence

8.5 Biomolecular Dating

Notes

References

Index

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Library of Congress Cataloging in Publication Data

Walker, M.J.C. (Mike J.C.), 1947–

Quaternary dating methods / Mike Walker.

p.   cm.

Includes bibliographical references and index.

ISBN 0-470-86926-7 (hb : acid-free paper) — ISBN 0-470-86927-5 (pbk. : acid-free paper)

1. Geochronometry. 2. Geology, Stratigraphic—Quaternary. 3. Radioactive dating. I. Title.

QE508.W348 2005

551.7′01—dc22

2004029171

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN-13 978-0-470-86926-0 (HB)     978-0-470-86927-7 (PB)

ISBN-10 0-470-86926-7 (HB)     0-470-86927-5 (PB)

Preface

In a letter to Thomas Manning in 1810, Charles Lamb wrote: ‘Nothing puzzles me more than time and space; and yet nothing troubles me less, as I never think about them.’ All of us working in the field of Quaternary science would, I suspect, tend to agree with the first part of this statement but take issue over the second. I for one have always been fascinated by time and, in particular, by the way in which we are able to assign ages to events in the distant past. My family and friends have been amused and intrigued in equal measure by me talking, with apparent confidence and authority, about the earth being formed 4.5 billion years ago, or the present warm period within which we live lasting 11 500 yrs. ‘But how can you be so sure?’ is the usual question. One of my aims in writing this book is to show them that there are indeed ways in which we can date the past and, moreover, that we can do so with an ever-increasing sense of assurance. My principal purpose, however, is to describe the various dating techniques that are routinely employed in Quaternary science in a way that is comprehensible to both undergraduate students and interested lay-people alike. I have therefore tried to avoid using mathematical formulae, although in the first chapter I felt it necessary to cover some of the basics of chemistry in order to provide the groundwork for what comes later. I have also orientated the book towards the practical aspects of dating by basing it around specific examples. Hopefully, this approach will appeal to students and others with a non-scientific background but, at the same time, will not appear to those who are fortunate in possessing a stronger scientific pedigree to be ‘dumbing down’. Above all, however, my aim is to encourage readers (unlike Charles Lamb) to think a little more about the past and to recognise the importance of being able to frame the momentous events of recent earth and human history within a reasonably secure temporal framework.

Throughout the book I have drawn on a previous volume that I wrote with John Lowe (Reconstructing Quaternary Environments, 1997, Addison-Wesley-Longman, London). I make no apologies for this because I know that book has been, and continues to be, widely used at undergraduate and postgraduate levels in both Britain and abroad. I hope that this new book on Quaternary Dating Methods will find an equally wide readership. John and I are about to embark on the third edition of Reconstructing Quaternary Environments (due 2006), and during the course of preparing that revision, I hope I will be able to reciprocate and that some of the material contained in the following pages will find its way into Lowe and Walker Mark III. The text also includes a large number of references. Some might find that this disrupts the flow of the narrative, but I felt that it was important not only to acknowledge the sources of material upon which I have drawn but, equally importantly, to point the reader in the direction of this work so that those who might be interested in taking matters further will be able to do so.

It is customary in a Preface to express thanks to those who have assisted either directly or indirectly in the production of the book, and I do not intend to depart from that practice. Over the last 15 years or so, I have enjoyed the national and international collaboration, and friendship, of many colleagues, first through the North Atlantic Seaboard Programme of IGCP-253, and more recently through the INTIMATE (Integration of ice-core, marine and terrestrial records) Programme of INQUA (International Quaternary Union). I am particularly appreciative of the time that I have spent at a number of different meetings with, amongst others, Hilary Birks, Sjoerd Bohncke, Svante Björck, Russell Coope, Les Cwynar, Irka Hajdas, Jan Heinemeir, Wim Hoek, Konrad Hughen, Sigfus Johnsen, Karen-Luise Knudsen, Nalan Koç, Thomas Litt, Jørgen Peder Steffensen, Chris Turney, Bas van Geel and Barbara Wohlfarth. My work with the Natural Environmental Research Council, formerly as a member and subsequently as chairman of the NERC Radiocarbon Facilities Committee, and latterly as chairman of the NERC AMS (Accelerator Mass Spectrometry) Strategy Group, has brought me into contact with colleagues at the East Kilbride and Oxford Radiocarbon Dating Laboratories, notably Chris Bronk-Ramsay, Charlotte Bryant, Doug Harkness, Robert Hedges and Tony Fallick, whose company I have enjoyed and from whom I have learned a great deal. I should also like to thank Lin Kay and Chris Franklin at NERC for supporting me in my role as Committee Chairman. Finally, I am grateful to my colleagues in the Department of Archaeology and Anthropology, University of Wales, Lampeter, especially David Austin and John Crowther, for providing such a congenial working environment over the past four years, and to the university itself for allowing me a period of study leave during which much of the first draft of the book was completed.

In writing this book, I have constantly been aware of the fact that I am approaching the material as a member of the user community. I am not an expert in the technical aspects of dating, and hence I have prevailed upon colleagues who know far more about these matters than I ever will to read what I have written and to show me where I have gone wrong. I am deeply indebted to Tim Atkinson, Simon Blockley, Charlotte Bryant, Tony Fallick, Rob Kemp, Olav Lian, Danny McCarroll, James Scourse, Mike Summerfield, Chris Turney and John Westgate for their careful scrutiny and constructive critical appraisal of various sections of the text; I simply could not have completed this book without their assistance. It goes without saying, however, that any remaining errors are my own. Several friends and colleagues have provided me with photographs, for which I am most grateful, and Phil Gibbard and Richard Preece helped considerably in the compilation of Figure 1.4. I should also like to thank Sally Wilkinson, Keily Larkins, Lynette James and the staff in the production department of John Wiley. Last, and by no means least, I would like to express my gratitude to my wife, Gro-Mette, who has not only been a constant source of encouragement, but who has also read the draft text from cover to cover, and has provided many valuable inputs along the way.

One name is missing from the above list. As colleagues within the Quaternary community will know, for more than 30 years I have worked in collaboration with John Lowe. We first met as postgraduate students in the University of Edinburgh and since then we have produced more than 50 joint publications. I have no doubt whatsoever that John could have written this book and, I suspect, he might well have made a better fist of it. Nevertheless, I hope he will find some of the material in the following pages of interest and that he will enjoy reading it. Not only have John and I been close academic colleagues, but we have also remained firm friends, and in acknowledgement of this I would like to dedicate the book to him.

Mike WalkerOctober, 2004

1

Dating Methods and the Quaternary

Whatever withdraws us from the power of our senses; whatever makes the past, the distant or the future, predominate over the present, advances us in the dignity of thinking beings.

Samuel Johnson

1.1 Introduction

The Quaternary is the most recent period of the geological record. Spanning the last 2.5 million years or so of geological time1 and including the Pleistocene and Holocene epochs,2 it is often considered to be synonymous with the ‘Ice Age’. Indeed, for much of the Quaternary, the earth’s land surface has been covered by greatly expanded ice sheets and glaciers, and temperatures during these glacial periods were significantly lower than those of the present. But the Quaternary has also seen episodes, albeit much shorter in duration, of markedly warmer conditions, and in these interglacials the temperatures in the mid- and high-latitude regions may have exceeded those of the present day. Indeed, rather than being a period of unremitting cold, the hallmark of the Quaternary is the repeated oscillation of the earth’s global climate system between glacial and interglacial states.

Establishing the timing of these climatic changes, and of their effects on the earth’s environment, is a key element in Quaternary research. Whether it is to date a particular climatic episode, to estimate the rate of operation of past geological or geomorphological processes, or to determine the age of an artefact or cultural assemblage, we need to be able to establish a chronology of events. The aim of this book is to describe, evaluate and exemplify the different dating techniques that are applicable within the field of Quaternary science. It is not, however, a dating manual. Rather, it is a book that is written from the perspective of the user community as opposed to that of the laboratory expert. It is, above all, a book that lays emphasis on the practical side of Quaternary dating, for the principal focus is on examples or case studies. To paraphrase the words of the actor John Cleese, it is intended to show just what Quaternary dating can do for us!

In this chapter, we examine the development of ideas relating to geological time and, in particular, to Quaternary dating. We then move on to consider the ways in which the quality of a date can be evaluated, and to discuss some basic principles of radioactive decay as these apply to Quaternary dating. Finally, we return to the Quaternary with a brief overview of the Quaternary stratigraphic record, and of Quaternary nomenclature and terminology. These sections provide important background information, and both a chronological and stratigraphic context for the remainder of the book.

1.2 The Development of Quaternary Dating

Early approaches to dating the past were closely associated with attempts to establish the age of the earth. Some of the oldest writings on this topic are to be found in the classical literature where the leitmotif of much of the Greek writings is the concept of an infinite time, equivalent in many ways to modern day requirements for steady-state theories of the universe (Tinkler, 1985). This position contrasts markedly with that in post-Renaissance Europe where biblical thinking placed the creation of the world around 6000 years ago, and when the end of the universe was predicted within a few hundred years. This restricted chronology for earth history derives from the biblical researches of James Ussher, Archbishop of Armagh, who in 1654 published his considered conclusion, based on Old Testament genealogical sources, that the earth was created on Sunday 23 October 4004 BC, with ‘man and other living creatures’ appearing on the following Friday. Another momentous event in the Old Testament, the ‘great flood’, occurred 1656 years after the creation, between 2349 and 2348 BC.

In his magisterial review of the history of earth science, Davies (1969) has observed that although modern researchers have tended to scoff at Ussher’s chronology he was, in fact, no fanatical fundamentalist but rather a brilliant and highly respected scholar of his day. It is perhaps for this reason that his chronology had such a pervasive influence on scientific thought, although it is perhaps less clear to modern geologists why it still forms a cornerstone of contemporary creationist ‘science’! During the eighteenth and nineteenth centuries, however, with the development of uniformitarianist thinking in geology,3 the pendulum began to swing once more towards longer timescales for the formation of the earth and for the longevity of operation of geological processes, a view encapsulated by James Hutton’s famous observation in his Theory of the Earth (1788) that ‘… we find no vestige of a beginning, no prospect of an end’.

The difficulty was, of course, that pre-twentieth-century scientists had no bases for determining the passage of geological time. One of the earliest attempts to tackle the problem was William McClay’s work in 1790 on the retreat of the Niagara Falls escarpment, which led him to propose an age of 55 440 years for the earth (Tinkler, 1985). Others tried a different tack. The nineteenth-century scientist John Joly, for example, calculated the quantity of sodium salt in the world’s oceans, as well as the amount added every year from rock erosion, and arrived at a figure of 100 million years for the age of the earth. Increasingly, however, came an awareness that even this extended time frame was simply not long enough to account for the entire history of the earth and, moreover, for organic evolution, a view that was underscored by the publication of Darwin’s seminal work Originof Species in 1859. Further challenges to the Ussher timescale and to its successors came from the field of archaeology, with noted antiquarians such as John Evans (and his geological colleague Joseph Prestwich) arguing, on the basis of finds of ancient handaxes, for a protracted period of human occupation extending into a period of antiquity ‘. . . . remote beyond any of which we have hitherto found traces’ (Renfrew, 1973).

It was into this atmosphere of chronological uncertainty that Louis Agassiz introduced his revolutionary idea of a ‘Great Ice Period’, which arguably marks the birth of modern Quaternary science. This notion, first propounded in 1837, was initially received with a degree of scepticism by the geological establishment, but the idea not only of a single glaciation but, indeed, of multiple glaciations rapidly gained ground. By the beginning of the twentieth century, most geologists were subscribing to the view that four major glacial episodes had affected the landscapes of both Europe and North America, although the basis for dating these events remained uncertain. An early attempt at establishing a glacial– interglacial chronology was made by the German geologist Albrecht Penck, using the depth of weathering and ‘intensity of erosion’ in the northern Alpine region of Europe to estimate the duration of interglacial periods. On this basis, an age of 60 000 years was assigned to the Last Interglacial and 240 000 years to the Penultimate Interglacial, the duration of the Quaternary being estimated at 600 000 years (Penck and Bruckner, 1909). An alternative approach using the astronomical timescale based on observed variations in the earth’s orbit and axis4 again arrived at a similar figure, although if older glaciations recorded in the Alpine region were included, the time span of the Quaternary was extended to around 1 million years (Zeuner, 1959). This figure has since been widely quoted and, for the first half of the twentieth century at least, was generally regarded as the best estimate of age for the Quaternary.

At about the time that the Quaternary glacial chronology was being worked out for the European Alps, the first attempts were being made to develop a timescale for the last deglaciation, using laminated or layered sediment sequences which were interpreted as reflecting annual sedimentation cycles. These are known as varves, and are still employed as a basis for Quaternary chronology at the present day (section 5.3). Some of the earliest studies were made on the sediments in Swiss lakes and produced estimates of between 16 000 and 20 000 years since the last glacial maximum (Zeuner, 1959), results that are not markedly different from those derived from more recent dating programmes. The seminal work on varved sequences, however, was carried out in Scandinavia where Gerard de Geer (Figure 1.1) developed the world’s first high-resolution deglacial chronology in relation to the wasting Fennoscandian ice sheet (section 5.3.3.1). This approach was subsequently applied in North America to date glacial retreat along parts of the southern margin of the last (Laurentide) ice sheet (Antevs, 1931).

The early years of the twentieth century saw the development of another dating technique which is still widely used in Quaternary science, namely dendrochronology or tree-ringdating (section 5.2). Research on tree rings has a long history, and the relationship between tree rings and climate (a field of study known as dendroclimatology) has intrigued scientists since the Middle Ages. Indeed, some of the earliest writings on this subject can be found in the papers of Leonardo da Vinci (Stallings, 1937). The basics of modern dendrochronology, however, were formulated by the American astronomer Andrew Douglass, who was the first to link simple dendrochronological principles to historical research and to climatology (Schweingruber, 1988). Together with Edmund Schulmann, he founded the world-famous Laboratory for Tree-Ring Research at the University of Arizona in 1937. In Europe, it was not until the end of the 1930s that dendrochronology began to gain a foothold, largely through the work of the German botanist, Bruno Huber. His research laid the foundation for the modern school of German dendrochronology which has remained at the forefront of tree-ring research in Europe to the present day.

Figure 1.1Gerard de Geer measuring varves at Beckomberga, Stockholm, in 1931. Varve chronology was the first dating technique to provide a realistic estimate of Quaternary time (photo: Ebba Hult de Geer, courtesy of Lars Brunnberg and Stefan Wastegård)

The most significant advance in Quaternary chronology, however, came during and immediately after the Second World War, with the discovery that the decay of certain radioactive elements could form a basis for dating. Although measurements had been made more than 30 years earlier on radioactive minerals of supposedly Pleistocene age (Holmes, 1915), it was the pioneering work of Willard Libby and his colleagues that led to the development of radiocarbon dating, and to the establishment of the world’s first radiocarbon dating laboratory at the University of Chicago in 1948. During the 1950s and 1960s, other radiometric methods were developed that built on technological advances (increasingly sophisticated instrumentation) and an increasing understanding of the nuclear decay process. These included uranium-series and potassium–argon dating (Chapter 3), while a growing appreciation of the effects on minerals and other materials of exposure to radiation led to the development of another family of techniques which includes thermoluminescence, fission track and electron spin resonance dating (Chapter 4). In the late 1960s and 1970s, advances in molecular biology enabled post-mortem changes in protein structures to be used as a basis for dating (amino acid geochronology), while remarkable developments in coring technology led to the recovery of long-core sequences from ocean sediments and from polar ice sheets, out of which came the first marine and ice-core chronologies. The last two decades of the twentieth century have been characterised by a series of technological innovations that led not only to a further expansion in the range of Quaternary dating techniques, but also to significant improvements in analytical precision. A major advance was the development of accelerator mass spectrometry (AMS), which not only revolutionised radiocarbon dating (Chapter 2), but also made possible the technique of cosmogenic nuclide dating (section 3.4). The last decade has also witnessed the creation of the high-resolution chronologies from the GRIP and GISP2 Greenland ice cores, and from the Vostok and EPICA cores in Antarctica (section 5.5).

These various developments and innovations mean that Quaternary scientists now have at their disposal a portfolio of dating methods that could not have been dreamed of only a generation ago, and which are capable of dating events on timescales ranging from single years to millions of years. The year 2004 sees the 350th anniversary of the publication of the second edition of Ussher’s ground-breaking volume on the age of the earth. How he would have reconciled the recent advances in Quaternary dating technology with his 6000-year estimate for the age of the earth is difficult to imagine!

1.3 Precision and Accuracy in Dating

Before going further, it is important to say something about how we can judge the quality of an age determination. Two principal criteria reflect the quality of a date, namely accuracy and precision, and these apply not only to dates on Quaternary events, but to all age determinations made within the earth, environmental and archaeological sciences. For dating practitioners and for interpreting dates, it is important to understand the meaning and significance of these terms. Accuracy refers to the degree of correspondence between the true age of a sample and that obtained by the dating process. In other words, it refers to the degree of bias in an age measurement. Precision relates to the statistical uncertainty that is associated with any physical or chemical analysis that is used as a basis for determining age. As we shall see, all dating methods have their own distinctive set of problems, and hence each age measurement will have an element of uncertainty associated with it. These uncertainties tend to be expressed in statistical terms and provide us with an indication of the level of precision of each age determination (Chapter 2).

An example of the distinction between accuracy and precision in the context of a dated sequence is shown in Figure 1.2. In sample A, there is close agreement in terms of mean age between the four dated samples, and the standard errors (indicated by the range bars) are small; however, the dates are 2000–2500 years younger than the ‘true age’. These dates are therefore precise, but inaccurate. In sample B, the reverse obtains; the dates cluster around the true age but have wide error bars. Hence they are accurate but imprecise. In sample C, however, the dates are of similar age and have narrow error bars. These age determinations are both accurate and precise, which is the optimal situation in dating.

Figure 1.2Accuracy and precision in a dated sequence (modified after Lowe and Walker, 1997); see text for details

1.4 Atomic Structure, Radioactivity and Radiometric Dating

Radiometric dating methods form a significant component of the Quaternary scientist’s dating portfolio. Indeed, half of the chapters in this book that deal specifically with dating methods are concerned with radiometric dating. All radiometric techniques are based on the fact that certain naturally occurring elements are unstable and undergo spontaneous changes in their structure and organisation in order to achieve more stable atomic forms. This process, known as radioactive decay, is time-dependent, and if the rate of decay for a given element can be determined, then the ages of the host rocks and fossils can be established.

In order to understand the basics of radiometric dating, it is necessary to know something about atomic structure and the radioactive process. Matter is composed of minute particles known as atoms, the nuclei of which contain positively charged particles (protons), and particles with no electrical charge (neutrons), which together make up most of the mass of an atom. Third elements are electrons, which are tiny particles of negative charge and negligible mass that spin around the nucleus. Collectively, protons, neutrons and electrons are referred to as elementary or sub-atomic particles, and for many years were considered to be the fundamental building blocks of matter. With the development of large particle accelerators however, machines that are capable of accelerating samples to such high speeds that matter breaks down into its constituent parts, dozens of new sub-atomic particles have been discovered and current research suggests that atomic matter is made up of elementary particles from two families, quarks and leptons. Our understanding of electrons (which are members of the lepton family of particles) and their behaviour has also changed. At one time it was believed that electrons orbited the nucleus in shells (or orbitals), similar to the way in which the planets orbit the sun, and that in each of these orbits they had certain energy. However, the situation now appears to be more complex, as modern physics has shown that it is not possible to determine both the location and the velocity of a sub-atomic particle.5 More recent work on atomic structure therefore envisages electrons with a particular energy existing in volumes of space around the nucleus, even though their exact location cannot be established. These volumes are known as atomic orbitals. The build-up of electrons in atomic orbitals allows scientists to explain many of the physical and chemical properties of elements, and lies at the heart of our modern understanding of chemistry.

The atoms of each chemical element have a specific atomic number and atomic mass number. The former refers to the number of protons contained in the nucleus of an atom, while the latter is the number of protons plus neutrons. In other words, the mass number is the total number of particles (nucleons) in the nucleus. The atomic number is usually written in subscript on the left-hand side of the symbol for the chemical element (e.g. oxygen – 8O; uranium – 92U), while the atomic mass number is shown in superscript (e.g. 16O; 238U). In some elements, although the number of protons in the nucleus remains the same, the number of neutrons may vary. Elements that possess the same number of protons but different numbers of neutrons are referred to as isotopes. The number of electrons is constant for isotopes of each element, and hence they have the same chemical properties, but the isotopes differ in mass, and this will be reflected in change in the atomic mass number. Examples include carbon (12C, 13C, 14C) and oxygen (16O, 17O, 18O). Individual isotopes of an element are referred to as nuclides. Most of these are stable; in other words the binding forces created by the electrical charges are sufficient to keep the atomic particles together. In some cases, however, where there are too many or too few neutrons in the nucleus, for example, the nuclides are unstable and this results in a spontaneous emission of particles or energy to achieve a stable state. This is the process of radiation (or radioactive ‘decay’), and such isotopes are known as radioactive nuclides.

Unstable nuclei can rid themselves of excess energy in a variety of ways, but the three most common forms are alpha, beta and gamma decay. In alpha (α) decay, a nucleus emits an alpha particle consisting of two protons and two neutrons, which is a nucleus of helium. Nuclides that emit alpha particles lose both mass and positive charge. The atomic mass number changes to reflect this, and the result is that one chemical element can be created by the decay of others. In beta (β) decay, a different kind of particle is ejected – an electron. The emission of a negatively charged electron does not alter mass, hence there is no change in atomic mass number. There is, however, a change in atomic number because the reason for the ejection of the electron is that as the nucleus decays, a neutron transmutes into a proton, and the nucleus must rid itself of some energy and increase its electrical charge. The emission of an electron, with its negative charge and small amount of excess energy, enables this to be achieved. The third common form of radioactivity is gamma (γ) decay. Here, the nucleus does not emit a particle, but rather a highly energetic form of electromagnetic radiation. Gamma radiation does not change the number of protons and neutrons in the nucleus, but it does reduce the energy of the nucleus. Gamma rays are not important in most forms of radiometric dating (with the exception of some short-lived isotopes: Chapter 3), but they do contribute to the build-up of luminescent properties in minerals (Chapter 4). In addition, the cosmic rays from deep space that constantly bombard the earth’s upper atmosphere, and which initiate the chemical reaction that leads to the formation of radiocarbon (Chapter 2) and other cosmogenic isotopes (Chapter 3), are largely composed of gamma radiation.

An atom that undergoes radioactive decay is termed a parent nuclide and the decay product is often referred to as a daughter nuclide. Some parent–daughter transformations are accomplished in a single stage, a process known as simple decay. Others involve a more complex reaction in which the nuclide with the highest atomic number decays to a stable form through the production of a series of intermediate nuclides, each of which is unstable. This is known as chain decay and occurs, for example, in uranium series (section 3.3). The intermediate nuclides that are formed during the course of decay are therefore both the products (or daughters) of previous nuclear transformations and the parents in subsequent radioactive decay. Such nuclides are referred to as supported. Where the decay process involves a nuclide that has not, in itself, been created by the decay process, or where that nuclide has been separated from earlier nuclides in the chain through the operation of physical, chemical or biological processes, this is known as unsupported decay. The distinction between supported and unsupported decay is considered further in the context of 210Pb (lead-210) dating (section 3.5.1).

Radioactive decay processes are governed by atomic constants. The number of transformations per unit of time is proportional to the number of atoms present in the sample and for each decay pathway there is a decay constant. This represents the probability that an atom will decay in a given period of time. Although the radioactive decay of an individual atom is an irregular (stochastic) process, in a large sample of atoms it is possible to establish, within certain statistical limits, the rate at which overall distintegration proceeds. In all radioactive nuclides, the decay is not linear but exponential (e.g. Figure 2.1) and is usually considered in terms of the half-life, i.e. the length of time that is required to reduce a given quantity of a parent nuclide to one half. For example, if 1 gm of a parent nuclide is left to decay, after t½ only 0.5 gm of that parent will remain. It will then take the same period of time to reduce that 0.5 gm to 0.25 gm, and to reduce the 0.25 gm to 0.125 gm, and so on. The half-life concept is fundamental to all forms of radiometric dating.

1.5 The Quaternary: Stratigraphic Framework and Terminology

As we saw above, the Quaternary is conventionally subdivided into glacial (cold) and interglacial (temperate) stages, with further subdivisions into stadial (cool) and interstadial (warm) episodes. The distinction between glacials and stadials on the one hand, and interglacials and interstadials on the other, is often blurred, but glacials are generally considered to be cold periods of extended duration (spanning tens of thousands of years) during which temperatures in the mid- and high-latitude regions were low enough to promote extensive glaciation. Stadials are cold episodes of lesser duration (perhaps 10 000 years or less) when cold conditions obtained and when short-lived glacial readvances occurred. Interglacials, on the other hand, were warm periods when temperatures in the mid- and high latitudes were comparable with, or may even have exceeded, those of the present, and whose duration may have been 10 000 years or more. Interstadials, by contrast, were short-lived (typically less than 5000 years) warmer episodes within a glacial stage, during which temperatures did not reach those of the present day. This type of categorisation, which is based on inferred climatic characteristics, is known as climatostratigraphy (Lowe and Walker, 1997).

Evidence for former glacial and interglacial conditions (as well as stadial and interstadial environments) has long been recognised in the terrestrial stratigraphic record. Former cold episodes are represented by glacial deposits, by periglacial sediments and structures, and by biological evidence (such as pollen or vertebrate remains) which are indicative of a cold-climate régime. Interglacial and interstadial phases are reflected primarily in the fossil record (pollen, plant macrofossils, fossil insect remains, etc.), or in biogenic sediments that have accumulated in lakes or ponds during a period of warmer climatic conditions. However, because of the effects of erosion, especially glacial erosion, the Quaternary terrestrial stratigraphic record is highly fragmented and, apart from some unusual contexts such as deep lakes in areas that have escaped the direct effects of glaciation, long and continuous sediment records are rarely preserved. During the later twentieth century, therefore, Quaternary scientists turned to the deep oceans of the world, where sedimentation has been taking place continuously over hundreds of thousands of years. Indeed, many ocean sediment records extend in an uninterrupted fashion back through the Quaternary and into the preceding Tertiary period. One of the great technological breakthroughs of the twentieth century was the development of coring equipment mounted on specially designed ships (Figure 1.3) which enabled complete sediment cores to be obtained from the deep ocean floor, sometimes from water depths in excess of 3 km!

What these cores revealed was a remarkable long-term record of oceanographic and, by implication, climatic change. This is reflected in the oxygen isotope ‘signal’ (or trace) in marine microfossils contained within the ocean floor sediments. The variations in the ratio between two isotopes of oxygen, the more common and ‘lighter’ oxygen-16 (16O) and the rarer ‘heavier’ oxygen-18 (18O), are indications of the changing isotopic composition of ocean waters between glacial and interglacial stages. As the balance between the two oxygen isotopes in sea water is largely controlled by fluctuations in land ice volume,6 downcore variations in the oxygen isotope ratio (δ18O) can be read as a record of glacial/ interglacial climatic oscillations, working on the principle that ice sheets and glaciers would have been greatly expanded during glacial times but much less extensive during interglacials (Shackleton and Opdyke, 1973). The sequence can therefore be divided into a series of isotopic stages (marine oxygen isotope or MOI stages) and these are numbered from the top down, interglacial (temperate) stages being assigned odd numbers, while even numbers denote glacial (cold) stages. The record shows that over the course of the past 800 000 years or so, there have been around ten interglacial and ten glacial stages, while over the course of the entire Quaternary, back to 2.5 million years or so, more than 100 isotopic stages have been identified (Shackleton et al., 1990; Figure 1.4, left). This is many more temperate and cold stages than has been recognised in terrestrial sequences, and hence the deep-ocean isotope signal provides a unique proxy record7 of global climate change. It also constitutes an independent climatostratigraphic scheme against which terrestrial sequences can be compared. This approach is exemplified in a number of case studies discussed in the following pages, while the use of oxygen isotope stratigraphy as a basis for dating Quaternary events is considered in Chapter 7.

Figure 1.3The Joides Resolution, a specially commissioned ocean-going drilling ship for coring deep-sea sediments (photo Bill Austin)

Figure 1.4The MOI record based on a composite of deep-ocean cores (V19–30; ODP-677 and ODP-846) (left) and the Quaternary stratigraphy of the northern hemisphere set against this record (right). The marine isotope signal shows the oxygen isotope stages back to 2.6 million years BP. In the correlation table, temperate (interglacial) stages are shown in upper case, while cold (glacial) stages are shown in lower case. Complexes which include both temperate and cold stages are in italics (based on Gibbard et al., 2004)

The Quaternary terrestrial stratigraphic sequence in different areas of the northern hemisphere, and possible correlatives with the MOI record, is shown on the right-hand side of Figure 1.4. Broadly speaking, the Quaternary can be divided into Early, Middle and Late periods. The Late Quaternary, which includes the present interglacial, last cold stage and last interglacial (ca. 0–125 000 years ago), is readily correlated between the various regions, and this warm–cold–warm sequence can be equated with the MOI stratigraphy (MOI stages 1–5). Prior to that, however, the various regional records are less easily correlated. During the Middle Quaternary, which encompasses the period from ca. 125 000 to 780 000 years ago, a number of glacial and interglacial episodes are reflected in the various terrestrial stratigraphic records, but several of these have no formal designation. Moreover, some designated warm and cold periods appear to contain both warm and cold stages (often several), while there are clearly gaps or hiatuses in the stratigraphic sequences. As a result, correlation not only between each of the regional sequences but also between these and the MOI ‘template’ becomes increasingly uncertain. These problems are even more acute during the Early Quaternary (prior to ca. 780 000 years ago) where the number of designated stages is even fewer, and both regional correlations and links with the MOI sequence become increasingly speculative. In many ways, Figure 1.4 exemplifies one of the principal difficulties in Quaternary science, namely the lack of a universal dating technique that is applicable to the entire Quaternary time range and to all stratigraphic contexts. The figure is, nevertheless, a useful aide-memoir, and the reader will find it helpful to refer back to it when working through some of the case studies later in the book.

One part of the Quaternary record where there is a broad measure of agreement and where, moreover, there is also closer dating control is the climatic oscillation that occurred at the end of the Last Cold Stage (ca. 15 000 and 11 500 years ago) and which is most clearly reflected in proxy climate records from around the North Atlantic region. This episode is referred to as the Weichselian Lateglacial in northern Europe and the Devensian Lateglacial in Britain (Figure 1.5, right). It is characterised by rapid warming around 14 800 years ago (the Bølling-Allerød Interstadial in Europe; Lateglacial or Windermere Interstadial in Britain), a significant cooling (Younger Dryas or Loch Lomond Stadial) around 12 900 years ago, and finally an abrupt climatic amelioration at the onset of the present (Holocene) interglacial at ca. 11 500 years ago. In the Greenland (GRIP) ice core (Chapter 5), this climatic oscillation is reflected in a series of clearly defined ‘events’ in the oxygen isotope record (GS-2; GI-1; GS-1:Figure 1.5, left). Greenland Interstadial 1 is further divided into a series of sub-events, with GI-1a, GI-1c and GI-1e representing warmer intervals, and GI-1b and GI-1d reflecting cooler episodes (Björck et al., 1998; Walker et al., 1999). Whereas the timescale for terrestrial sequences from Britain and northern Europe is based on calibrated radiocarbon years (section 2.6), the Greenland (GRIP) record is in ice-core years (section 5.5). Again, the reader may find it useful to cross-reference some of the later case studies with Figure 1.5.

1.6 The Scope and Content of the Book

In the following chapters, the various dating techniques that are available to Quaternary scientists (Figure 1.6) are introduced, explained and evaluated. The last element is especially important because it is important to understand not only how each method works, but also where and why errors are likely to occur. Some of these may arise from the nature of the sample; others from analytical limitations. Whatever the cause, these will impact on the resultant age determinations. Each section concludes with a number of examples or case studies. These have been carefully selected to show how the different techniques can be employed in Quaternary science and to give an indication of the range of applications of each method.

Figure 1.5The δ18O record from the GRIP Greenland ice core showing the Lateglacial event stratigraphy (left), and the stratigraphic subdivision of the Lateglacial in northwest Europe and the British Isles. The isotopic record is based on the GRIP ss08c chronology, and the colder stadial episodes are indicated by dark shading. The radiocarbon ages should be regarded as indicative ages only (partly after Lowe et al., 2001)

Chapters 2–5 deal with techniques that enable ages to be determined in years before the present (years BP). In other words, they allow estimates of age to be obtained. Chapters 2–4 describe radiometric dating techniques, where age is determined from measurements either of radioactive decay of some unstable chemical elements (Chapters 2 and 3), or of the effects of radioactive decay on the crystal structure of certain minerals or fossils (Chapter 4). Chapter 5 reviews a group of methods based on the regular accumulation of sediment or biological material through time, and which form annually banded records. All of the techniques that enable estimates of age to be made have sometimes been referred to as absolute dating methods. This term has not been used here; indeed it has been deliberately avoided because it implies a level of accuracy and precision that can seldom, if ever, be achieved in reality. As we saw above, and as will be amplified in the following discussions, where age estimates are being obtained, errors are unavoidable and hence there will inevitably be an element of uncertainty associated with each age determination. There is, therefore, nothing ‘absolute’ about a date, and it should not be referred to as such.

Figure 1.6The effective dating ranges of the different techniques discussed in this book

In Chapters 6 and 7, two further groups of dating techniques are considered. The first involves the grouping of fossils or sedimentary units which are then ranked in relative order of antiquity; hence, these are known as relative dating methods. Some are based on the principles of stratigraphy where relative age can be determined by the position of stratigraphic units in a geological sequence; others use the degree of degradation or chemical alteration (both of which may be time-dependent) on rock surfaces, in soils or in fossils, to establish relative order of age. Chapter 7 considers methods that enable age equivalence to be determined, based on the presence of contemporaneous horizons in separate and often quite different stratigraphic sequences. With respect to both relative dating and age-equivalence techniques, where the various stratigraphic units or fossil materials can be dated by one of the age-estimate methods described in Chapters 2–5, it may prove possible to fix the relative or age-equivalent chronologies in time. In other words, they can be calibrated to an independently derived timescale.

Notes

1. The duration of the Quaternary is still a matter for debate, with some authorities arguing for a ‘shorter’ timescale of around 1.6–1.8 million years, while others subscribe to the view that a longer timescale of 2.5–2.6 million years is more appropriate. There is, perhaps, a majority in favour of the longer chronology and this interpretation has been followed here.

2. The Pleistocene epoch ended around 11 500 years ago and was succeeded by the Holocene, the warm period in which we live. As the present temperate period is simply the most recent of a number of temperate episodes that form part of a long-term climatic cycle, the last 11 500 years can be seen as part of the Pleistocene (West, 1977). Hence, the terms ‘Quaternary’ and ‘Pleistocene’ are often used interchangeably.

3. Uniformitarian reasoning, as initially developed by the Scottish geologist James Hutton in the later eighteenth century, emphasises the continuity of geological processes through time. Hence contemporary processes (modern analogues) can be used as a basis for interpreting past events. Uniformitarianism is often described by the dictum ‘the present is the key to the past’.

4. The Astronomical Theory of Climate Change is based on the assumption that surface temperatures of the earth vary in response to regular and predictable changes in the earth’s orbit and axis. The three principal components are the precession of the equinoxes (apparent movement of the seasons around the sun) with a periodicity of ca. 21 000 years, the obliquity of the ecliptic (variations in the tilt of the earth’s axis) with a periodicity of ca. 41 000 years, and the eccentricityof the orbit (changes in the shape of the earth’s orbit) with a periodicity of ca. 96 000 years. Collectively these govern the amount of heat received by the earth and the distribution of this heat around the globe. First developed in its modern form by the Scottish scientist James Croll in the nineteenth century, the theory was subsequently elaborated by the Serbian geophysicist Milutin Milankovitch in the 1930s. The radiation balance curves that he produced can be calibrated to the orbital parameters and used to provide an astronomical timescale for glacial–interglacial cycles (Chapter 5). Further explanation of the astronomical theory can be found in standard Quaternary texts, such as those by Lowe and Walker (1997), Roberts (1998), Williams et al. (1998), Wilson et al. (2000) and Bell and Walker (2005).

5. A major discovery in physics during the early twentieth century was that sub-atomic particles sometimes behave as if they are waves, a concept that lies at the heart of the science of quantum physics. One consequence is that it is impossible to measure both the position of a sub-atomic particle and its velocity, an idea that was first proposed by the German physicist Werner Heisenberg in his Uncertainty Principle. This indeterminacy in the sub-atomic world can be seen very clearly whenever a single atomic event can be observed, such as in radioactivity. Although quantum physics is a highly complex field, there are a number of accessible texts that deal with this subject or that include sections on it. Ones that I have found particularly informative (and enjoyable!) are by Gribbin (1984), Close et al. (1987), Barrow (1988), Gribbin (1995), Penrose (1999), Rees (2000) and, of course, Bryson (2003).

6. During evaporation from the free ocean surface, a fractionation (or separation) occurs so that more of the lighter oxygen isotope, 16O, is drawn into the atmosphere than the heavier isotope, 18O. In the cold stages of Quaternary, therefore, large amounts of the lighter isotope would have been transported poleward by moisture-bearing winds and locked into the greatly expanded ice sheets. As a consequence, ocean water would have been relatively ‘enriched’ in the heavier isotope 18O. The reverse would obtain during interglacial stages for, with reduced land ice cover, more 16O would have been returned to the oceans where water would have become relatively ‘depleted’ in 18O. Accordingly, the δ18O trace provides a record of changing volumes of land ice, and hence of glacial/interglacial climatic fluctuations.

7. A ‘proxy climatic record’ is one based on an indirect measure of climate. In other words it is based on inferential evidence (pollen, plant macrofossils, etc.), as opposed to direct evidence obtained using a thermometer or rain gauge.

2

Radiometric Dating 1: Radiocarbon Dating

Life exists in the universe only because the carbon atom possesses certain exceptional properties.

Sir James Jeans

2.1 Introduction

Radiocarbon dating was one of the first radiometric techniques to be developed and, despite the fact that it is applicable to only a relatively short span of Quaternary time (50 000 years or so; see below), it is perhaps the most widely used of all the radiometric techniques. It owes its origin to a remarkable American chemistry professor, Willard Libby, who, in the years immediately following the end of the Second World War, was investigating the possibility that radiocarbon might exist in biological materials. Along with a group of colleagues, Libby was able to demonstrate that radiocarbon could be detected in samples from the Baltimore sewage works, and from these seemingly unprepossessing beginnings, a technique was born that was to revolutionise our view of Late Quaternary time. Not only is the carbon atom the building block of life, therefore (see above), it also provides us with a means of dating life. The first radiocarbon measurements were published in 1949, and since then hundreds of thousands of dates have been produced by more than 100 laboratories all over the world. The story of the development of radiocarbon can be found in Libby’s book Radiocarbon Dating (1952; 2nd edition 1955), and he was awarded the Nobel Prize for chemistry in 1960. Good overviews of radiocarbon dating can be found in the volumes by Taylor (1987), Bowman (1990) and Taylor et al. (1992), and there are shorter accounts in, inter alia, Aitken (1990), Lowe and Walker (1997) and Taylor (1997; 2001). The journal Radiocarbon, along with its website and associated links, is a valuable source of information on recent developments in radiocarbon dating and on applications of the technique.

2.2 Basic Principles

Radiocarbon (14C) is one of three isotopes of carbon, the others being 12C and 13C. By far the most abundant of these is 12C which comprises around 98.9% of all naturally occurring carbon. 13C forms around 1.1% and 14C one part in 1010%. In other words, only about one in a million million atoms of carbon is 14C. Both 12C and 13C are stable isotopes, but 14C is not and it ‘decays’ to a stable form of nitrogen, 14N, through the emission of beta (β) particles. One β particle is released from the nucleus for every atom of 14C that decays. It is this instability, or radioactivity, which gives us the name ‘radiocarbon’.

Atoms of 14C are formed in the upper atmosphere through the interaction between cosmic ray neutrons, which reach the earth’s atmosphere from deep space, and nitrogen. This involves neutron capture by the nitrogen (14N) atom, and the loss of a proton, to create 14C. The 14C atoms produced by this process combine with oxygen to form a particular form of carbon dioxide (14CO2) which mixes with the non-radiocarbon containing molecules of CO2. In this way, 14C becomes part of the global carbon cycle and is assimilated by plants through the photosynthetic process, and by animals through the ingestion of plant tissue. The majority of 14C (more than 95%) is absorbed into the oceans as dissolved carbonate, which means that organisms that live in sea water (corals, molluscs, etc.) will also take up 14C during the course of their life cycle. Although the 14C in the terrestrial biosphere and in the oceans is constantly decaying, it is continually replenished from the atmosphere. Hence the amount of 14C that is stored in plant and animal tissue and in the world’s oceans, the global carbon reservoir, remains approximately constant through time. In effect, a position has been reached where the carbon that is used to build plant and animal tissue is in isotopic equilibrium with the atmosphere; in other words the levels of 14C activity in plants and animals are the same as that in the atmosphere. The 14C reservoir can therefore be likened to a car with a drip-feed to the fuel tank; the car will use fuel as the engine runs, but as the tank is being constantly topped-up, the fuel will remain at more or less the same level.

Once an organism dies, however, it becomes isolated from the 14C source, no further replenishment of 14C can take place, and the ‘radiocarbon clock’ runs down by radioactive decay, which occurs at a constant rate. Hence, by measuring the amount of 14C that remains in a sample of fossil material (the residual 14C content) and comparing this to modern 14C in standard material, an age can be inferred for the death of the organism. In order to be able to do this, however, we need to know the rate at which 14C decays. Experimental results have shown that the decay rate of 14C is 1% every 83 years. This might imply that after 8300 years, all residual activity in a sample will have ceased. However, as with all radioactive isotopes, the decay curve for 14C is not linear but exponential (Figure 2.1), and this means that materials significantly older than this can still be dated. The half-life of a 14C atom is 5730 years,1 and under normal circumstances, the limit of measurement of 14C activity (i.e. decay rate) is eight half-lives. This translates into an upper age limit of around 45 000 years (but see following sections). Samples older than this are usually described as being of ‘infinite age’, and are expressed, for example, as >45 000 years.

Figure 2.1The decay curve for radiocarbon is exponential, not linear. This means that the percentage decrease in number of atoms in a given unit of time is constant. Hence, after each half-life the number of atoms remaining is halved. If there are A0 atoms of radiocarbon at the beginning of the decay process, then after one half-life there will be A0/2 atoms remaining; after two half-lives, there will be A0/4, after three, A0/8, and so on (after Bowman, 1990). Reproduced by permission of The British Museum Press

2.3 Radiocarbon Measurement

Radiocarbon dates can be obtained on a range of biogenic materials. These include, interalia, wood, peat, organic lake sediment, plant remains, charcoal, shell and coral. More problematic are bone and soil, while radiocarbon dates have also been obtained on more unconventional materials, such as cloth, metalwork or fossil pigment. Two approaches are employed to measure residual 14C activity in samples of these materials relative to modern standards: beta counting, which involves the detection and counting of β emissions from 14C atoms over a period of time, working on the principle that the rate of emissions will reflect the residual level of 14C activity within the sample, and accelerator mass spectrometry (AMS), which employs particle accelerators as mass spectrometers to count the relative number of 14C atoms in a sample, as opposed to the decay products.

2.3.1 Beta Counting

Beta counting can be carried out in two ways. In gas proportional counting, the sample is converted to a gas (carbon dioxide, ethylene or methane) and injected into a counting chamber where each β emission is detected by a charged wire that runs down the centre of the chamber. In liquid scintillation counting, the sample is converted to benzene to which a ‘scintillant’ (usually a phosphoric substance) is added, and each β particle emission stimulates a pulse of light which can be counted photoelectrically. In the early days of radiocarbon dating, laboratories employed gas proportional counting, but the majority of radiocarbon facilities today now use liquid scintillation counting. This is because when converted to benzene, the majority of the sample is carbon, whereas in gas counting, the carbon dioxide is mostly oxygen. Hence significantly more sample material is required for gas proportional than for liquid scintillation counting. In order to ensure comparability between dates, laboratories compare sample activity to a modern reference standard, which is the modern activity of NBS (National Bureau of Standards) oxalic acid held by the American Bureau of Standards. Radiocarbon dates are always measured with respect to this standard and are expressed in years BP (before present), where ‘present’ is the standard year AD 1950 (van der Plicht, 2002).

In beta counting, it is important to bear in mind that age is not the quantity that is being measured; rather it is the 14C activity