Quaternary Dating Methodsby Mike Walker
- the concept of time in Quaternary Science and related fields
- the history of dating from lithostratigraphy and biostratigraphy
- the development and /li>/li>
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This introductory textbook introduces the basics of dating, the range of techniques available and the strengths and limitations of each of the principal methods.
- 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
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Quaternary Dating Methods
By Mike Walker
John Wiley & SonsCopyright © 2005 John Wiley & Sons, Ltd.
All right reserved.
Chapter OneDating 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
The Quaternary is the most recent period of the geological record. Spanning the last 2.5 million years or so of geological time and including the Pleistocene and Holocene epochs, 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, 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 Origin of 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 axis 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 184.108.40.206). 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-ring dating (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.
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.
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.
Excerpted from Quaternary Dating Methods by Mike Walker Copyright © 2005 by John Wiley & Sons, Ltd.. Excerpted by permission.
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