Radiocarbon Dating

An Archaeological Perspective

R.E. Taylor, Ofer Bar-Yosef

• Verlag: Routledge
• Jahr: 2016
• ISBN:
  9781315421193
• Auflage: 2
• Thema: Sozialwissenschaften
• Thema: Archäologie

This volume is a major revision and expansion of Taylor's seminal book Radiocarbon Dating: An Archaeological Perspective. It covers the major advances and accomplishments of the 14C method in archaeology and analyzes factors that affect the accuracy and precision of 14C-based age estimates. In addition to reviewing the basic principles of the method, it examines 14C dating anomalies and means to resolve them, and considers the critical application of 14C data as a dating isotope with special emphasis on issues in Old and New World archaeology and late Quaternary paleoanthropology. This volume, again a benchmark for 14C dating, critically reflects on the method and data that underpins, in so many cases, the validity of the chronologies used to understand the prehistoric archaeological record.

Chapter 1
Basic Elements

1.1. Fundamentals

In the second decade of the 21st century, radiocarbon (¹⁴C) dating continues to be the most securely established and widely used scientific method of obtaining chronometric age determinations for terminal Pleistocene and Holocene carbonaceous (carbon-containing or organic) materials.¹ The types of organics recovered from archaeological and archaeologically related geological or paleontological contexts employed most often as ¹⁴C samples are wood, charcoal, marine shell, and bone (Section 3.2). The routine lower limit of the method is ~250 ¹⁴C years which is equivalent to ~300 calendar years (Sections 1.1.2, 2.5). Depending on instrument configurations and capabilities in different laboratories, the routine upper limit ranges between 40,000 and 60,000 years (Section 1.1.3). Under some circumstances, the lower limit can be reduced and the upper limit extended (Section 5.6).

Beginning in the late 1970s, the development of accelerator mass spectrometry (AMS) technology for direct or ion counting–based ¹⁴C measurements initiated an expansion in many areas of ¹⁴C research that previously had not been feasible to pursue. Over the ensuing three decades, AMS-based ¹⁴C measurements have both continued to be applied to new topics and permitted more focused investigations to address, and sometimes resolve, long-standing problems (Section 4.5). These developments are primarily due to the ability of AMS-based ¹⁴C measurements to be carried out on a routine basis on milligram, and, with additional efforts in some cases, on submilligram (microgram) amounts of carbon. Future research in AMS technology may also permit an extension of the ¹⁴C time frame in excess of 50,000 to 60,000 years if routine strategies can be developed and consistently employed to exclude trace amounts of contamination in milligram-size samples.²

Critically evaluated suites of calibrated ¹⁴C age determinations (Section 5.5.2) continue to constitute the “gold standard” for assigning accurate temporal frameworks for organic compounds from terminal Pleistocene archaeological, geological, and, for the middle and late Holocene, historical contexts.³ In addition to its widespread use in many strictly scientific applications, the ¹⁴C method has also been applied in efforts, sometimes widely cited in the popular media, to provide a definitive means of authenticating or disconfirming proposed ages of artifacts of potentially significant historical and/or cultural interest. Illustrations of the use of this method for such purposes include the Dead Sea Scrolls (Section 1.5.4), the Shroud of Turin (Sections 1.5.5, 5.7), and the Vinland map (Section 7.4). It has even been applied to pseudo-artifacts such as wood allegedly recovered from a site on Mount Ararat in northeastern Turkey purportedly taken from the remains of a mythical structure known only through documents produced by one culture in the ancient Near East as “Noah’s Ark” (Section 8.9).

The research that ushered in the first “radiocarbon revolution”⁴ was initiated in 1946 at the University of Chicago by Willard Libby (1908–1980), his research collaborator, James Arnold (1923–2012), and Libby’s first Chicago graduate student, Ernest Anderson (1920–2013). Libby received the 1960 Nobel Prize in chemistry for his development of the ¹⁴C method. Chapter 8 reviews the history of the development of the technique. A brief section in Chapter 8 (Section 8.10) includes a summary over view of the lingering objections to the overall validity of ¹⁴C dating. Surprisingly, these current objections reflect almost entirely a non-scientific set of concerns born out of what are essentially theologically inspired arguments. These arguments are raised by fundamentalist-oriented elements within several Western religious communities and traditions. This section also explains the basis of the misunderstandings and misinformation about ¹⁴C dating occasionally encountered in the contemporary popular media.

The principal purposes of this volume are to (1) review the basic principles underlying the ¹⁴C dating method, (2) examine the nature of and means of resolving ¹⁴C dating anomalies, and (3) consider relevant elements involved in the critical application of ¹⁴C data as a dating isotope with special emphasis on topics and issues in Old and New World archaeology and, in some cases, late Quaternary paleoanthropology. In addition, we will, when appropriate, comment on ¹⁴C data employed in the late Quaternary geological and paleoenvironmental sciences as they interface with archaeological and paleoanthropo-logical research efforts. Some of the discussions of topics in the text and a number of the citations included in the references are designed to provide historical contexts and a general background to some of the current problems, issues, and controversies.

In addition to its use as a strictly dating isotope, the ¹⁴C method functions as one of the principal means of providing a time scale for proxy records for late Quaternary environmental histories, including the critical role that variations in atmospheric CO₂ and ocean circulation patterns played in late glacial and Holocene temperature and climate.⁵ As a tracer isotope, ¹⁴C continues to be widely employed in the biological and biomedical sciences, with recently expanded areas of applications made possible by the application of AMS-based technology.⁶ AMS-based measurements of the in situ production of ¹⁴C in surface rocks has made feasible detailed studies of the evolution of various types of land forms, including the determination of erosion rates and rates of formation of alluvial fans.⁷ Other examples of tracer applications include basic research on the terrestrial and marine carbon cycle and providing data used to calculate recharge rates in hydrological (groundwater) systems.⁸

Still other applications involve recurrence data used by paleoseismologists for studying surface-rupturing earthquakes,⁹ the age of feed stock ingredients used in the manufacture of foods and beverages,¹⁰ and the forensic characterization of undocumented human skeletal remains to identify those of individuals who had died relatively recently and thus whose death might be of concern to law enforcement agencies.¹¹ Other forensic medicine applications have employed ¹⁴C measurements of hair and lipid samples of human subjects with death dates known to have occurred during the period of “bomb ¹⁴C” (see below, and Section 2.5.3) to more closely determine the date of death to ~±2 years.¹² These examples illustrate just a few of the continually increasing range of scientific disciplines and types of investigations utilizing ¹⁴C measurements in their research.

We should note that, in this volume, discussions of physical and chemical processes, including the geophysics and nuclear physics mechanisms involved in ¹⁴C production, distribution, and decay, together with the biochemistry and geochemistry of its distribution in natural environments, will employ primarily descriptively oriented illustrations, examples, and terminology. Detailed technical and/or specialized discussions, including approaches that define, describe, and model in mathematical terms physical or chemical processes as typically expressed in contemporary nuclear physics, physical chemistry, biochemistry, and oceanography, can be found in the standard literature of those fields.

For an introductory, largely descriptive, overview of some relevant foundational physical science concepts and terminologies for those individuals who may not have had an undergraduate-level background in the physical sciences, the “Basic Science” chapters in Malainey¹³ can be profitably consulted. A more detailed introduction to the basic processes and foundation principles involved in the measurement of radioactivity can be found in L’Annunziata.¹⁴

1.1.1. Dating Model

On this planet, all living organisms are carbon based. Carbon-containing or organic compounds are distributed throughout the earth’s atmospheric, terrestrial, and hydrological (primarily marine but also freshwater) environments in many forms (Section 1.4.2). These compounds are cycled through various carbon reservoirs on different time scales by a variety of physical and chemical mechanisms. However, these carbon cycle processes are dominated by the operation of two intersecting and interacting geophysical and geochemical systems.¹⁵

One system involves the cycling of various chemical species primarily between the atmosphere and marine environments. These chemical species include dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the form of carbon dioxide, carbonates, and bicarbonates. The other system is centered on terrestrial and marine photosynthetic cycles utilizing several biochemically complex pathways involving the fixation of atmospheric carbon dioxide (CO₂) in plant materials and the incorporation of a small part of this plant biomass into animal tissue. The subsequent death and decomposition of this plant and animal biomass results in the release of CO₂ and methane (CH₄) back into the atmosphere. Various other chemical and physical processes, including the deposition of carbonates in terrestrial and oceanic sediments and volcanic activity, are also involved in the operation of the carbon cycle on much longer geologic time scales.¹⁶

Carbon that becomes an active part of the earth’s carbon cycles contains three naturally occurring isotopes, two of which are stable (¹²C, ¹³C) and one (¹⁴C) that is, at the nuclear level, naturally unstable or radioactive. This natural radioactive isotope of carbon, or radiocarbon, decays with a half-life of ~5700 years, providing the fundamental basis of the ¹⁴C dating “clock.” This half-life is greatly in excess of what would be expected from a comparison with isotopes with similar decay energy characteristics (Section 4.3).

The basis of the use of ¹⁴C as a dating isotope can be simply illustrated (Figure 1.1) in terms of the production (Section 1.4.1), distribution (Section 1.4.2), and decay (Section 1.4.3) of ¹⁴C. The production of ¹⁴C is a secondary effect of cosmic-ray interactions with th...