chapter 15 archaeological chemistry

CHAPTER 15ARCHAEOLOGICAL CHEMISTRY

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INTRODUCTION: ARCHAEOLOGY IN THE LABORATORY

WHY IS ARCHAEOLOGICAL CHEMISTRY IMPORTANT?

CHEMISTRY IN ARCHAEOLOGY

INSTRUMENTATION

PROTECTING THE PAST: THE ETHICS OF DESTRUCTIVE ANALYSIS

NEUTRON ACTIVATION ANALYSIS (NAA)INDUCTIVELY COUPLED PLASMA-MASS

SPECTROMETRY (ICP-MS)

SCIENCE IN ARCHAEOLOGY: THE ARCHAEOLOGICAL CHEMISTRY LABORATORY AT ARIZONA STATE UNIVERSITY

GAS CHROMATOGRAPHY-MASS SPECTROMETRY (GC-MS)

X-RAY DIFFRACTION (XRD)

ELEMENTAL ANALYSESLITHIC ANALYSIS

IN FOCUS: OBSIDIAN SOURCES AND TRADE IN THE ANCIENT NEAR EAST

CERAMIC ANALYSISMETAL ANALYSIS

ISOTOPIC ANALYSESPREHISTORIC DIET AND ISOTOPES

IN ARCHAEOLOGY

ARCHAEOLOGICAL THINKING: CLIMATE, ISOTOPES, AND VIKINGS IN GREENLAND

ANCIENT MIGRATION AND ISOTOPES IN ARCHAEOLOGY

IN FOCUS: THE FIRST KING OF COPAN, A CLASSIC MAYA CENTER IN HONDURAS

ORGANIC RESIDUES IN ARCHAEOLOGY

IN FOCUS: TRACES OF CHOCOLATE IN CERAMIC VESSELS FROM THE NORTH AMERICAN SOUTHWEST

IN FOCUS: ZOOLOGY BY MASS SPECTROMETRY

CONCLUSIONS

ARCHAEOLOGY PROJECT: ISOTOPES AND PREHISTORIC DIET

STUDY QUESTIONS

FURTHER READING

CHAPTER 15 ARCHAEOLOGICAL CHEMISTRY / 341

INTRODUCTION: ARCHAEOLOGY IN THE LABORATORY

Archaeological chemistry takes a variety of direc-tions in the study of the past. Two important applications are the authentication of ancient materials and the determination of place of origin (provenience). A number of elements and iso-topes have been used in the investigation of provenience. Lead isotopes, for example, have been used for many years in Europe to determine the origins of bronze artifacts. Copper and tin, the major components of bronze, occur in limited areas and do not appear together in most cases. Both contain small amounts of lead, of which the isotopes vary in the diff erent sources for copper and tin, allowing them to be identifi ed. Th e trade and transport of these raw materials and fi nished products across the European con-tinent is reasonably well documented, thanks to archaeological chemistry. Th ere are many other uses of archaeological chemistry as tools to study the past, and new applications appear regularly. Th e investigation of the chemistry of the living fl oors at Keatley Creek, for example, was discussed in Chapter 9 (see p. 220), while the use of carbon and nitrogen isotopes has been mentioned in several previous chapters. In the following section we describe one of the more famous cases of authentication in archaeology: the Getty kouros.

A kouros (Greek: “youth”) is a stone statue of a nude, muscular young male, carved during the Classic period of Greek civilization between the sixth and third centuries bc. A kouros [KURH-oss] was an artistic manifestation of the Greek worldview that emphasized youth and male beauty. Th e poet Simonides (c. 556–468 bc) may have been referring to a kouros in the late sixth century bc when he wrote, “In hand and foot and mind alike foursquare/ fashioned without fl aw.”

Th e statue in the photograph is 2.25 meters (7 ft 4 in.) in height and has several features that are characteristic of kouros statues, which have variously identifi ed as gods, warriors, or victorious athletes. Th e hands, for example, are balled into fi sts and are held along the body. Th e hair is arranged in a regular grid of vertical and horizontal lines. Th e feet are placed with the left foot forward. Th e faces of these statues are very distinctive and appear to depict individuals in life-like portraits, as opposed to a generic human. Th e eyes are wide open and the mouth is formed in a serene, closed-lip smile.

Th ere are only a dozen examples of such fi gures in good condition in existence. When

the opportunity to acquire this piece came to the attention of the J. Paul Getty Museum in California in the late 1980s, excitement was high. Th e statue was accompanied by documents that indicated its origin and authenticity. Th e museum checked with the governments of Greece and Italy to ensure that the statue had been legally obtained. Th e Getty also requested samples of stone from the statue for analysis. Preliminary studies pointed to the island of Th asos [THOSS-os], an ancient quarry site, as the source of the marble. Moreover, the surface chemistry of the stone revealed a calcite crust that was thought to require a long period of time to form.

Th e Getty purchased the kouros for approxi-mately $8,000,000 and put it on display while experts—art historians, conservators, archaeo-logists, and archaeometrists—studied the piece. Opinions regarding its authenticity were divided. Why was it in such good condition, and why so white? Th e styles of depicting the hair and feet were diff erent: the wig-like hair is normally found around 600 bc, while the arrangement of the feet should date to 525 bc. Would an ancient sculptor combine several styles in a single piece? Was the quarry at Th asos in operation when this statue was purported to have been made?

Th en, in the early 1990s, evidence came forth that the authenticating documents were forgeries. Moreover, a clearly fake marble torso very similar to the Getty kouros was found. Th e debate regarding the statue intensifi ed. New tests were performed on the museum piece as well as the forged torso, but were not conclusive. Scientists were able to show, however, that the marble torso had been treated in an acid bath to simulate aging. Analysis revealed that the surface of the Getty kouros was a complex compound (calcium oxalate monohydrate), not a simple calcite (calcium carbonate), with characteristics that could not be duplicated in the laboratory.

Furthermore, the Getty kouros and the torso had diff erent surfaces.

Th e story of the Getty kouros is a classic example of archaeometry in action, where art and science meet. Th ere is no doubt that the Getty statue is a kouros. To most viewers, it is beautiful: it is art. Th e question, however, is whether it is ancient art or a more recent forgery. While most art historians and archae-ologists believe the statue is a fake, the scientists involved believe it to be authentic. Th e status of the Getty kouros remains a mystery. Th e infor-mation plaque with the statue at the museum today reads “Greek, 530 bc, or modern forgery.” Th e story of the kouros also raises an important issue of ethics in archaeology. Should museums purchase artifacts and monuments that are part of the heritage of other nations or peoples? Th is question of who owns the past is addressed further in Chapter 16.

Th is chapter is an introduction to archaeo-logical chemistry, the application of chemical and physical methods—hard science—to the study of archaeological materials; archaeo metry is another term for such investigations. In addi-tion to marble statues, archaeological chemists study a wide variety of materials including ceramics, bone, lithics, soils, dyes, and organic residues. Th is chapter off ers some information on instrumentation and archaeological chemistry laboratories, the questions asked, and exam-ples of important studies. Various aspects of archaeological chemistry are described in sections on elemental analyses, isotopic analyses, and organic analyses. Several examples are included, such as obsidian sourcing in the Near East, the use of isotopes to document the diets of early Greenlanders, and the birthplace of a Maya king. Various kinds of residues are the focus of most organic chemistry in archaeological chemistry, and are discussed toward the end of the chapter.

WHY IS ARCHAEOLOGICAL CHEMISTRY IMPORTANT?

Archaeological chemistry, and archaeometry more generally, is primarily concerned with (1) identifi cation (determining the original material of an unknown item); (2) authentication (verifying the antiquity of an item, often associated with works of art); and (3) characterization (meas-uring the chemical composition of a variety of

prehistoric materials). Archaeometric studies can tell us about subsistence and diet, exchange and trade, residence, demography, status, and many other aspects of prehistoric human behavior and organization. The goal of archaeological chemistry, in common with all analysis in archaeology, is to learn more about the human past.

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CHEMISTRY IN ARCHAEOLOGY

Archaeologists are often found in the labora-tory. As we have seen in previous chapters, there are laboratories for studying faunal remains, laboratories for archaeobotany, and laboratories for spreading out artifacts for analysis. Th ere are also laboratories where archaeologists and physical scientists investigate the chemical prop-erties of remains from the past. Th ese are wet labs with chemical fume hoods and a variety of scientifi c instrumentation.

Archaeological science is a general term that includes non-instrumental areas, such as faunal analysis, paleoethnobotany, and human osteo logy. Archaeometry is the measurement of the chemical or physical properties of an arti-fact, and technically includes dating methods, remote sensing, and ancient DNA (aDNA), but these fi elds tend to pursue a separate identity. (Physicists, for example, are usually responsible for dating laboratories, while geneticists are the experts on aDNA.) Archaeological chemistry, a part of archaeometry, involves the investigation of the inorganic and organic composition—elements and isotopes, molecules and compounds—of archaeological materials. Th e phrase molecular archaeology is sometimes used to refer to the organic component of archaeological chemistry, and particularly to the investigation of aDNA in plant and animal remains (including humans, as we saw in Chapter 14).

To appreciate how archaeological chemistry works, it is necessary to understand the basic tenets of how matter is constituted. All matter is composed of atoms. Atoms have three major components: neutrons, protons, and electrons (fi g. 15.1). Neutrons and protons make up the

core of an atom and have about the same weight. Neutrons have no electrical charge; protons have a positive charge. Electrons spin around the core of neutrons and protons with a negative electrical charge and a very small mass. Atoms vary in the number of protons and neutrons they contain, resulting in diff erent atomic weights; these dif-ferent weights make up the ninety-four chemical elements in nature.

Th e atomic number of an element is the number of protons in the nucleus. Isotopes of an element are slightly diff erent atoms of the same element; they have the same atomic number, but diff erent numbers of neutrons. Ions are electrically charged atoms that have lost or gained electrons.

Every substance on earth is made up of combi-nations of these ninety-four elements. A m olecule is a combination of atoms held together by bonds (for example, water—H2O—is a combination of two hydrogen atoms and one oxygen atom). C ompounds are combinations of elements in either organic or inorganic molecules in nature. O rganic compounds make up the tissues of living organisms and have the element carbon as a base. Inorganic compounds do not normally contain carbon.

Over the last sixty years, many new ideas, instruments, and procedures have been added to the tool chest of what is now called archaeological chemistry. Th e evolution of both methodo logy and instrumentation has permitted more detailed descriptions of the composition of a variety of materials, be they geological, biological, or archaeological. Today, a number of innovative approaches and techniques provide exciting new information about the past.

INSTRUMENTATION

Archaeological chemistry (or archaeometric) labo-ratories utilize a wide range of instruments and equipment. Four commonly used instruments are described in this chapter, each based on diff erent scientifi c principles: neutron activation analysis (NAA), inductively coupled plasma-mass spec-trometry (ICP-MS), x-ray diff raction (XRD), and gas chromatography-mass spectrometry (GC-MS). (Another important instrument, the scanning electron microscope (SEM), was discussed in Chapter 12.) Th ese instruments measure the composition of various kinds of materials. Each technique has advantages and disadvantages for diff erent archaeological materi-als, as described below.

NEUTRON ACTIVATION ANALYSIS (NAA)

Neutron activation analysis is an instrumental method for measuring elemental concentrations in a wide variety of samples. Ceramics and various kinds of stone are common archaeological materi-als analysed using NAA, but since samples must be ground into powder for analysis, this method is destructive. Neutron activation involves exposing a sample to a burst of neutrons that causes many elements in the sample to become temporarily radioactive. Th ese radioactive elements decay into stable ones by giving off gamma rays, which have energy levels specifi c to diff erent elements, allowing many elements to be identifi ed and measured simultaneously.

P ROTECTING THE PAST

PROTECTING THE PAST: THE ETHICS OF DESTRUCTIVE ANALYSIS

An important concern in instrumental analysis is the condition of the sample, requirements for preparation, and whether the technique is destructive or non-destructive. Many instru-ments require destructive sample preparation in the form of a powder or liquid. For example, samples must be converted into powders for NAA and XRD, into liquids or solids for ICP-MS, and gases for GC-MS. This is an important consideration to bear in mind, as very rare or valuable artifacts and materials should not be subjected to damaging analytical methods.

In archaeological chemistry, scientists must balance the importance of protecting and preserving the past for the future with the importance of learning as much as we can about our common human heritage. New developments in instrumenta-tion are allowing archaeologists to extract information about the past from smaller and smaller samples, which mini-mizes the destruction of priceless artifacts and archaeological materials. In addition, some instruments with larger sample chambers can perform non-destructive analyses.

Th e basic requirements for NAA are a source of neutrons, instrumentation for detecting gamma rays, and information about the reactions that occur when neutrons interact with nuclei. Although there are several possible sources for neutrons (reactors, accelerators, and radioisotope neutron emitters), nuclear reactors with high fl uxes of neutrons from uranium fi ssion off er the best sensitivities for most elements. For this reason, facilities for NAA are somewhat limited in number and accessibility. In addition, problems with waste disposal are resulting in the closing of some research reactors.

INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (ICP-MS)

ICP-MS is a widely used technology that employs a superheated plasma source to generate ions and a mass spectrometer to determine the mass of the atoms that are carried to the target (fi g. 15.2). Th e combination provides an effi cient, general-purpose instrument for the analysis of a wide variety of materials. ICP-MS is a standard tech-nique for the measurement of trace elements, and can record elemental concentrations to ppb (parts per billion). Th e method is destructive, but almost anything that can be put into solution can be analysed by ICP-MS. Th e addition of a laser as an ion source allows the analysis of solids.

A wide range of archaeological materials have been analysed by ICP-MS including bone, ceram-ics, stone, metals, and glass. In a typical application, samples are placed in solution by digestion in acid. Th e solution is sprayed into fl owing stream of inert argon gas and carried to a torch that is heated to

Exit slit

Electrostatic analyser

15.2Basic components of ICP-MS. Samples are ionized in the plasma and moved through the entrance slit and toward the detector by a magnetic fi eld that separates the atoms by weight. The detector counts the atoms of different weights that arrive.

15.1Basic diagram of an atom with neutrons and protons in the nucleus and an electron moving around the nucleus.

Electron

Proton

Neutron

Nucleus containing protons and neutrons

Electrons moving around nucleus

Plasma

Detector

Magnetic sector

Entrance slit

Slide valve

Ion optics

Interface

CHAPTER 15 ARCHAEOLOGICAL CHEMISTRY / 345344 / PART 3 ANALYSIS AND INTERPRETATION

6,000 degrees Celsius (10,800°F; the temperature of the surface of the sun). In this plasma, the gas and sample are ionized into their atomic constitu-ents. In the ICP-MS instrument, positive ions in the plasma are magnetically focused through a mass spectrometer to a collector that records

the mass. Th e amount of most elements present in the original material can be measured in just seconds, even at low concentrations (fi g. 15.3).

By combining other instruments with an ICP-MS, even smaller archaeological samples can be analysed and even smaller parts of an atom can be measured. For example, a multi-collector ICP-MS (MC-ICP-MS) can be used to measure the isotopes in a sample, as discussed later in this chapter.

15.3The ICP-MS in the Laboratory for Archaeological Chemistry, University of Wisconsin-Madison, operated by T. Douglas Price (standing) and James Burton.

SCIENCE IN ARCHAEOLOGY

THE ARCHAEOLOGICAL CHEMISTRY LABORATORY AT ARIZONA STATE UNIVERSITY

The Archaeological Chemistry Laboratory at Arizona State University is one of a growing number of archaeometric labo-ratories in the United States. Founded in 2005 and directed by Kelly J. Knudson, the laboratory is a center for research and training in chemical analysis of archaeological materials.

Research in the Archaeological Chemistry Laboratory usually involves studying the composition and source of dif-ferent kinds of materials to answer archaeological questions about past human behavior. Research methods involve the elemental and isotopic analyses of teeth, bones, ceramics, and sediments. Research questions include past diet, human migration, interaction and trade, and the identifi cation of activity areas on prehistoric living fl oors. The Archaeologi-cal Chemistry Laboratory is involved with research projects on fi ve continents, and hundreds of new samples arrive each year from all over the world. Collaborative activi-ties include projects in Argentina, Armenia, Bolivia, Chile,

Honduras, Ireland, Mexico, Niger, Peru, Spain, Turkey, and the United States.

The Archaeological Chemistry Laboratory has one large room for research and teaching. This is a wet lab where archaeological bone, enamel, soil, plant, and rock samples are prepared before analysis (fi g. 15.4). The sample prepa-ration area is dedicated to processing samples and includes a fume hood, furnaces, a system for deionized water, light microscopes to examine samples microscopically, balances for weighing small samples, drills and grinding equipment, and necessary glassware and chemical supplies. Funding for the laboratory comes largely from the National Science Foundation and other grants.

Student training is one of the most important things that takes place in the Archaeological Chemistry Laboratory. Each semester, up to twenty undergraduate and graduate students work in the laboratory on their own research projects. While

most of these students attend Arizona State University, some come from other universities in the United States and around the world. The Archaeological Chemistry Laboratory is also where Knudson teaches hands-on courses in laboratory techniques.

Samples are analysed using mass spectrometers in the W. M. Keck Foun-dation Laboratory for Environmental Biogeochemistry. In this laboratory,

15.5Research scientist Gwyneth Gordon prepares to put a sample on the MC-ICP-MS to measure strontium isotopes.

15.4In the Archaeological Chemistry Laboratory, Kelly J. Knudson (left) works with undergraduate student Kate Spencer. Light microscopes are used to examine archaeological bone samples.

there are three ICP-MS instruments for elemental analysis and the isotopic analysis of heavy isotopes (such as strontium), as well as two mass spec-trometers that can be used to analyse light isotopes, such as carbon, nitrogen, and oxygen (fi g. 15.5).

SCIENCE IN ARCHAEOLOGY (CONTINUED)

GAS CHROMATOGRAPHY-MASS SPECTROMETRY (GC-MS)

Th e use of a gas chromatograph-mass spectro-meter (GC-MS) has become standard practice in the analysis of organic compounds (fi g. 15.6). Th e liquid sample is fi rst converted to a gas. Th e gas chromatograph separates the hundred of molecules present in the sample in a long column containing a solid that slows some of the molecules more than others. Th e molecules then exit sequentially from the chromatograph and pass into a mass spectrometer with a detector that registers a peak for each type of molecule. Th e GC-MS produces a spectrum of the weight and amount of the various molecules present. Th ese spectra are compared with known materials in order to make identifi cations.

X-RAY DIFFRACTION (XRD)

X-ray diff raction is used to obtain structural and compositional information from crystalline materials, and is an important technique in the fi eld of material characterization. (Solid matter can be either amorphous, with the atoms arranged in a random way, or crystalline, where the atoms are arranged in a regular pattern; about 95 percent of all solids are crystalline.) In archaeology, XRD has been used mostly to identify the minerals present in ceramics, rock, and sediment samples.

15.6The basic components of a gas chromatograph-mass spectrometer (GC-MS). An archaeological sample is converted to gas and introduced into a gas chromatograph, which separates molecules by weight. Then, molecules are ionized and sent through a magnetic fi eld to separate by weight, which is measured by a detector. Output graphs (below) show the results of the gas chromatography and the mass spectrometry.

Detector

Time Mass

Gas chromatograph

Gas fl ow

GAS CHROMATOGRAPH

MASS SPECTROMETER

Mass spectrum

Sample introduction

Magnetic fi eld

Ionization


When X-rays are directed at crystalline materi-als, they are scattered in a systematic pattern by the regular arrangement of the atoms (fi g. 15.7). Diff erent kinds of material with diff erent arrange-ments of atoms produce distinctive scatter, or diff raction, patterns. XRD directs an X-ray source at a powdered sample and measures the pattern of diff raction that results. Th at pattern is compared to a large database of patterns from known materials to identify the sample. Using XRD, scientists can determine the crystal struc-tures of metals and alloys, minerals, inorganic compounds, polymers, and organic materials, as well as crystal size and orientation and chemical composition. Finally, one of the advantages of XRD is that portable instruments are available, making studies possible in museums and the fi eld (fi g. 15.8).

ELEMENTAL ANALYSES

Elemental analysis is a major part of archaeomet-ric research and is used for a variety of studies, including authentication and characterization. Th e elemental composition of a material is often a specifi c signature that can be repeatedly recognized, so the elemental composition of archaeological items has been used for years as a tool to determine provenience. Th e elemental composition of a variety of materials has been studied, including lithic, metal, ceramics, and sediments. Lithic, metal, and ceramic studies are described below, while soil chemistry studies are described in Chapter 9.

LITHIC ANALYSIS

Th e geological sources of a variety of stone materials have distinctive elemental signatures. Finds of these materials at sites some distance from the original sources can help archaeolo-gists study trade and interaction if the sources can be determined. An example of such a study involves obsidian sources and trade in the ancient Near East.

Measuring circle

15.7The diffractometer beam path and detector.

15.8Scientists can use the Terra Portable XRD Analyzer to achieve rapid results in the fi eld.

X-ray tube

Focus

Aperture diaphragm

Sample

Scattered-radiation

diaphragms

Detector diaphragm

Detector

IN FOCUS

OBSIDIAN SOURCES AND TRADE IN THE ANCIENT NEAR EAST

Obsidian is a translucent, hard, black or dark green glass that is produced during volcanic eruptions when molten silica fl ows out of a volcanic core and hardens into this material. It is therefore available from only a few sources, limited by proximity to volcanic terrain and the chance formation of a silica fl ow. In the past, obsidian was highly sought by prehistoric makers of stone tools, and was often traded or exchanged over long distances, sometimes hundreds of kilometers or more. Obsidian, in common with glass and fl int, fractures easily and regularly, creating very sharp edges (fi g. 15.9).

Archaeologists can fi ngerprint different fl ows of obsidian by analysing minor differences in the chemical composition of the material, allowing pieces found elsewhere to be traced to the places where they originated. This procedure relies on what is known as the provenience postulate, which states that the chemical differences within a single source of material must be less than the differences between two or more sources of the material. In principle, this means that if a source is chemically distinct, pieces removed some distance from the original source share that same chemical signature and can be identifi ed; that is to say, the provenience or place of origin of the piece can be determined. The provenience postulate must be tested by analyzing the composition of the source. The chemical composition of some materials, such as chert in North America, varies greatly within a single source and different sources cannot be distinguished. Chert (and other materials that do not comply with the provenience postulate) therefore cannot be used for studies of place of origin. The principle, however, has been applied to a variety of archaeo-logical materials including obsidian, pottery, turquoise, tin, and many others.

Neutron activation analysis (NAA) is commonly used in provenience studies of obsidian. The sources of obsidian in Southwest Asia, the Mediterranean, North America, Mexico, and elsewhere have been examined using NAA. Most of the obsidian in Southwest Asia comes from sources either in the mountains of Turkey or in northern Iran, both outside the Fertile Crescent.

The identity of the sources of obsidian found at early Neo-lithic sites in the Near East provides information on both the direction and intensity of trade (fi g. 15.10). Sites along Mediterranean coast generally obtained obsidian from Ana-tolia, while sites in the eastern part of the region used the Armenian obsidian. The percentage of obsidian in the total fl aked stone assemblage at these sites indicates that places closest to the sources used a great deal of obsidian, while those farthest away had only a small amount available. At the site of Jericho, for example, 700 kilometers (435 miles) from the Turkish sources, only about 1 percent of the stone tools were made from obsidian.

15.10The location of obsidian sources and samples in the early Neolithic of Southwest Asia. Major rivers shown on the map are the Nile, Tigres, and Euphrates. Two major obsidian sources are shown in Anatolia and two in Armenia. The distribution of obsidian from these sources is seen at settlements across the area. The distributions are largely separate with the exception of one site where obsidian from both source areas is found. Note also the Anatolian obsidian found on the island of Cyprus.

15.9Two obsidian cores and two blades. This glass-like stone produces very sharp edges and was a highly desired raw material in prehistory.

M E D I T E R R A N E A N S E A

B L A C K S E A

ANATOLIAN ARMENIAN

500 km

SourceSettlement with obsidian

N


CERAMIC ANALYSIS

In addition to obsidian and other stones, a wide variety of archaeological materials can be studied using elemental analyses, including ceramics, metals, bone, and many more. Ceramic analysis often involves determining elemental concentra-tions to examine the composition and potential sources of raw material for the pottery. Usually, NAA or ICP-MS is used for these analyses. Using the chemical characterization of ceramic com-position, archaeologists can learn more about manufacturing locations, trade and exchange, and more general economic patterns.

A simple example can provide an introduction to ceramic analysis in archaeological chemistry. To evaluate the utility of elemental analyses for the study variation in ceramics, James Burton of the Laboratory for Archaeological Chemistry at the University of Wisconsin-Madison under-took an experiment using modern pottery from three Mexican villages. Each village used diff er-ent sources of clay for raw material and diff erent recipes for their paste. Potsherds were obtained from each village and analysed by ICP-MS. A plot of several elements in the pottery, combined using a statistical technique known as discri-minant analysis, produced results that almost exactly matched the three sets of pottery made in the three diff erent villages (fi g. 15.11); in this case, the archaeological chemistry data recorded human behavior. Another ceramic provenience study can be found in Chapter 11 in the example involving the Salado polychromes.

METAL ANALY SIS

Archaeological metal artifacts and ores can also be “sourced” using elemental analyses. For example, Heather Lechtman (born 1935) of the Mass-achusetts Institute of Technology, who won the prestigious MacArthur Fellowship (often called the “Genius Grant”) for her work on prehis-toric metallurgy in South America, has used the elemental concentrations of diff erent ores to reconstruct the sources used for Andean bronze artifacts. In addition to identifying the sources and mines used, Lechtman can also trace the development of new metallurgical technologies.

More recently, scientists moved beyond ele-mental analysis of metal artifacts and ores to look more broadly at the environmental impact of mining in the past and present. For example, using ICP-MS and other methods to examine elemental concentrations, a group of scientists has identifi ed shifts in pollution due to mining. Sediment samples from cores collected from three diff erent lakes in Peru and Bolivia showed lead concentrations from pollution over the last 1,500 years. While we often think of environmental pol-lution as a modern problem, the scientists showed that smelting activities at diff erent mining centers released lead into the lake water. Lead pollution was high after the Spanish arrived in the Andes in the Colonial period (fi g. 15.12), and also in the Industrial age. Some lakes, however, showed lead pollution as early as ad 400, which was related to intensive mining operations by the Wari, Tiwan-aku, and Inca empires. Unfortunately, pollution is not simply a modern problem.

ISOTOPIC ANALYSES

Isotopes are atoms of the same element that have diff erent atomic masses. (Th ey are alternate states of the element with the same number of protons, but a diff erent number of neutrons.) Th ree isotopes of carbon, 12C, 13C, and 14C, have already been encountered in Chapter 8, along with isotopes of potassium (40K) and argon (40Ar). Radiocarbon dating relies on the ratio of 14C to 12C to determine the age of archaeological mate-rials. 14C is a radioactive isotope, unstable and subject to decay within a known period of time, meaning that the ratio of 14C to 12C will change over time. Th e majority of isotopes, however, are stable and not subject to radioactive decay.

An important distinction is also made between light and heavy isotopes. Th e lighter elements (primarily carbon, nitrogen, and oxygen in

archaeological studies) fractionate. Th at means that the proportion of diff erent isotopes present can be changed by processes in nature involving heat, photosynthesis, enzymes, and the like. Heavier isotopes (with a mass greater than 40) do not fractionate to nearly the same extent as lighter elements. Isotope analyses are normally reported in ratios of one isotope to another in order to standardize the results for diff erent kinds of materials and varying original isotope amounts.

Isotopes of several elements are used in other applications in archaeology aside from the dating of archaeological materials. Lead isotopes have been used to study the sources of silver, lead, and other ore deposits. Carbon isotopes have been used to determine the sources for marble and other stone composed of marine sediments (an example of this testing is the Getty kouros statue, described at the beginning of this chapter). Carbon, hydro-gen, and oxygen isotopes have been measured in phytoliths to assess the environmental condi-tions when these plant silicates were formed. Th e isotopes of carbon and nitrogen have been used

extensively to study human diets in the past and are described in the next section. Heavy isotopes of strontium and lead, along with the light isotopes of oxygen, have been used to look at human migra-tion and provenience. Th is subject is discussed in the following pages, along with an example from the Maya site of Copan in Honduras.

PREHISTORIC DIET AND ISOTOPES IN ARCHAEOLOGY

Th e primary use of isotopes in archaeology, outside of dating, has been in research on past diet. A basic principle of such studies is that “we are what we eat.” Carbon and nitrogen iso-topes from the food we eat are deposited in our skele ton. Human bone is a remarkable material, composed of organic and mineral compounds and water. Isotopic studies of the composition of bone can utilize both the organic portion, primarily the protein known as collagen, and the inorganic or mineral portion of the bone.

15.11Plot of two discriminant functions (a statistical summary of several different elements) that separate the modern pottery from Mexico into three distinct groups belonging to the three potters in different villages. 15.12

This Colonial-period watercolor shows the silver mine at Potosi, Bolivia, which was one of the most important silver mines in the world until AD 1800.

Dis

crim

inan

t fun

ctio

n 2

Discriminant function 1

4

0

-4

-8 -4 0 4 8


species, such as corn, and temperate species, such as wheat. Analysis of carbon isotopes from human bone from Mexico indicates that a dependence on corn began sometime before 4000 bc.

Nitrogen isotopes are used in much the same way as carbon isotopes, but they provide dif-ferent information about diet. Th e ratio of 15N (0.37 percent of all nitrogen in nature) to 14N (99.63 percent of all nitrogen in nature) is used in paleodiet studies. Th is nitrogen ratio is meas-ured in bone collagen using a mass spectrometer, usually at the same time as carbon isotope ratios. Th is allows archaeologists to obtain two kinds of information to understand diet in the past from a single sample of bone.

Figure 15.14 provides a summary look at carbon and nitrogen isotope ratios in nature. Variations in nitrogen isotope ratios are largely due to the role of leguminous plants in diet and the trophic level (position in the food chain) of the organism. Atmospheric nitrogen is isotopi-cally lighter than plant tissues; values in soil tend to be even higher. Non-nitrogen-fi xing plants, which derive nitrogen solely from soil nitrates, can thus be expected to be isotopically heavier than nitrogen-fi xing plants, which derive some nitrogen directly from the atmosphere.

Th ese values in plants are passed through the food chain accompanied by an approximately 2–3‰ higher shift for each tro phic level, includ-ing between mother and nursing infant. Grazing animals exhibit 15N enrichment, and higher δ15N values, compared to the plants they eat; predators show enrichment relative to their prey species. Th ere are also diff erences in nitrogen isotope ratios between marine and terrestrial sources of food that can be used in the study of past diets. Human consumers of terrestrial plants and animals typically have δ15N values of 6–10‰, while consumers of freshwater or marine fi sh, seals and sea lions usually have δ15N values of 15–20‰. Nitrogen isotope ratios may also vary with rainfall, altitude and other factors.

Th e composition of past human diet is one of the most important questions in prehistoric research. Th e quest for food directly aff ects many aspects of human behavior and society, includ-ing group size and social organization, residence patterns, technology, and transportation. Th e use of carbon and nitrogen isotopes in tandem provides a powerful means for determining the sources of food in the human past. An example of the application of isotopes to human diet is provided in the next section on the Vikings in Greenland.

Th e amounts of diff erent isotopes of carbon (reported as a ratio of 13C/12C) and nitrogen (ditto, 15N/14N) are measured in collagen using a mass spectrometer. While the level of these ele-ments in bone is under strict metabolic control, the ratio of stable isotopes refl ects the ratio in the diet. Th ese isotope ratios are reported in parts per thousand (‰) and as a diff erence (delta or δ) between the measured ratio in the sample and a known standard. Convention dictates that carbon is reported as δ13C and nitrogen as δ15N.

Th ere are two primary sources of variation in 13C in human diet and bone collagen: diff erent ratios in the kinds of plants we eat and diff erent ratios between terrestrial and marine foods. In certain kinds of tropical plants, such as corn, 13C is more abundant because of the ways that the plants produce carbon during photosynthesis. People who eat these kinds of tropical grasses therefore have higher amounts of 13C isotopes in their bones. Changes in this isotope ratio in prehistoric bone can indicate when corn became an important part of the diet. Figure 15.13 shows the results of a diet involving both a tropical

ARCHAEOLOGICAL THINKING

CLIMATE, ISOTOPES, AND VIKINGS IN GREENLAND

As we all learned in elementary school, Christopher Columbus arrived in the Caribbean islands in 1492. But the fi rst Euro-peans to set foot in the Americas came almost fi ve hundred years earlier and were not Italian but Scandinavian: the Vikings.

Their daring voyages across the North Atlantic were made in a series of shorter trips from Norway, Sweden, and Denmark to Great Britain and Ireland, and then to the Faeroe Islands, Iceland, Greenland, and eventually to Newfoundland in eastern Canada (fi g. 15.15, p. 352). The Vikings conquered parts of Britain and Ireland by AD 800 and occupied many of the island groups in the northern British Isles, including the Shetlands, Orkneys, and Hebrides. As explorers and colonists, they were the fi rst people to settle on the Faeroe Islands and Iceland in AD 874. On Greenland and in North America, the Inuit and American Indians had been present for thousands of years.

The story of the Viking exploration of the North Atlantic is an incredible saga of many tales. One chapter concerns the colonies on Greenland and tells of their initial success and subsequent failure in the light of major climatic change. A group of Icelandic farmers led by Erik the Red founded the Eastern Settlement in southwest Greenland around AD 985.

Another group from Iceland went further north along the west coast of Greenland and colonized the Western Settle-ment. These Viking groups took domesticated cereals and animals with them and successfully cultivated these crops and fed their herds. The North Atlantic climate had entered a particularly good phase, known as the Medieval Warm Period, around AD 900. Temperatures were 1 to 2 degrees Celsius warmer than today on average and the growing season was longer. The Viking population of Greenland soon expanded to between 4,000 and 5,000 people. After AD 1300, however, those numbers began to decline, and by about AD 1450 Greenland was completely abandoned by the Norse.

The present icecap on Greenland is more than 2 kilo-meters (1.25 miles) thick, made up of layer upon layer of ice and frozen snow in a stratigraphy of the last several hundred thousand years. Ice cores from this deep ice sheet provide information on past climate on Greenland and show a steady decline in maximum temperature during the fi ve hundred years of the Viking occupation. This cold period between AD 1300 and 1850 is called the Little Ice Age, and its effects were dramatic in the North Atlantic.

15.13Schematic representation of human diet involving both maize, a tropical grass with a higher δ13C value, and wheat, with a lower δ13C value.

15.14A schematic view of carbon and nitrogen isotope ratios in marine and terrestrial systems. (Schoeninger and Nelson, 1991.)

Wolfδ15N = +8.0‰δ13C = -18.3‰

White-tailed deerδ15N = +5.3‰δ13C = -18.9‰

Tree leavesδ15N = +3.0‰δ13C = -26.0‰

C4 grassδ15N = +3.0‰δ13C = -13.0‰

C3 grassδ15N = +3.0‰δ13C = -26.0‰

Legumeδ15N = +1.0‰δ13C = -26.0‰

Rabbitδ15N = +5.0‰δ13C = ?‰

Pilot whaleδ15N = +16.7‰δ13C = -12.8‰

Walrusδ15N = +13.3‰δ13C = -11.8‰

Mollusksδ15N = +12.5‰δ13C = -14.0‰

Plankton & Krillδ15N = +7.0‰δ13C = -14.0‰

Blue whaleδ15N = +13.8‰δ13C = -14.5‰

Kelpδ15N = +7.0‰δ13C = -14.0‰

Small fi shδ15N = +10.0‰δ13C = -13.0‰

Atmospheric carbon dioxide δ13C = -7 parts per thousand

Human food based on 50% of

each kind of plant

Photosynthesis in most plants

Photosynthesis in corn and some tropical plants

Wheat Corn

Average δ13C = -26.5 parts per thousand

Average δ13C = -12.5 parts per thousand

Human food chain based on other plants only

Human food chain based on corn

Bone collagen δ13C = -21.4 parts per thousand




ANCIENT MIGRATION AND ISOTOPES IN ARCHAEOLOGY

One of the core questions in archaeology con-cerns changes in material culture. New things are innovations and they represent what is called culture change; for example, a new kind of projectile point appears, domestic plants and animals are found for the fi rst time at a site, or a new burial practice spreads across a region. Th e question then arises; from where did these new things come? Did new people coming to the area introduce culture change, or did local people independently invent these things or simply borrow foreign ideas and artifacts? Th is is a question of invention versus diff usion, and is one of the more problematic questions for archaeologists to answer.

Th e diffi cult part is determining if people moved around in the past. We can usually identify exotic artifacts, but cannot determine how the artifacts came to be in that place. For example, iron and horses were absent in North America until the Europeans arrived. But, after the arrival of Europeans in eastern North America, iron hatchets and domesticated horses quickly spread across western North America, where they were found long before the Europeans who introduced them reached that part of the continent. Th e answers to this conundrum are in fact known—axes were popular trade items among Native American groups; the horses came from the earlier Spanish colonists in the Southwest—but the example illustrates the complexity of tracing the origins of cultural change. Artifacts and other objects are often used as markers of groups of people in archaeology, but such objects can be traded, borrowed, or stolen, and are not necessarily carried directly by the people who made them.

Until recently, archaeologists have not been able to determine directly if people themselves

moved. Th e application of isotopic tracers, however, has made it possible to identify human migration in the people themselves. Th e prin-ciple is straightforward. Tooth enamel forms during infancy and does not change during life. Bone, on the other hand, is constantly rebuilding itself as part of the body’s mainte-nance plan, as anyone who has broken a bone knows. Th e composition of tooth enamel then is composed of the things an individual (and his or her mother) ate during infancy. Th e composi-tion of bone is a product of the foods consumed during the last years of life.

Certain isotopes in the foods we eat are geo-graphically distinctive. Isotopes of strontium and oxygen are particularly good signatures for the place of residence. Strontium isotopes vary among diff erent types of rock and go into the body through the food chain, from rock to soil to plant to animal to human. Oxygen isotopes enter the body in drinking water, which ulti-mately comes from rainfall. Isotopes in rainfall vary with temperature and latitude. Rain that falls in warm areas close to the sea has a high oxygen isotope ratio, while rain that falls more inland and at higher elevations and latitudes has a lower ratio.

Th ese isotopes can be measured in the enamel (fi g. 15.17) and bone of a human skeleton using a mass spectrometer. Th e isotope ratios in tooth enamel come from the place of birth; the isotope ratios in bone come from the place of death. If the ratios in the enamel and the bone of the same individual are diff erent, that person must have changed their place of residence, i.e. she or he must have moved. In some cases, it is possible to determine not just that a person moved, but also to suggest from where they may have come.

Isotopic studies of the tooth enamel from Norse burials in Greenland doc-ument these changes in climate. Oxygen isotopes—a refl ection of atmospheric temperature—in tooth enamel show a clear increase in cold during the period of settlement (fi g. 15.16). Carbon iso-topes, on the other hand, indicate a marked increase in the proportion of marine foods in the diet over time, from approximately 20 percent when the Vikings arrived to approximately 80 percent at the end of their time on Greenland. Based on the isotopic data, as temperatures declined and the growing season shortened, the Viking inhab-itants of Greenland ate more seals and fi sh. A long summer was necessary to grow hay to store and feed their cows and sheep through the spring, when the Norse could hunt seals

on the ice. If the crop failed, the cattle and sheep died before the seals arrived and the Norse would starve.

Archaeological evidence corroborates this scenario. Excava-tions at Norse houses from the later period of the settlement have revealed the skeletons of cattle that died in their stalls during the winter. Other bioarchaeological information sug-

gests a clear decline in human nutrition. There are indications of a decline in stature in the Greenland Vikings over time and a number of the later skeletons exhibit bioarchaeological evidence of disease and malnutrition.

ARCHAEOLOGICAL THINKING (CONTINUED) ARCHAEOLOGICAL THINKING (CONTINUED)

15.16Climatic changes over the last fourteen hundred years, revealed in Greenland ice cores, document periods of warmer and colder conditions than today. The temperature curve on the bottom of the diagram is based on oxygen isotopes, a proxy for atmospheric temperature. The Medieval Warm Period witnessed the expansion of the Vikings across the North Atlantic while the Little Ice Age documents a time of cooler conditions and declining harvests. The carbon isotope evidence from human tooth enamel shows a shift from terrestrial to marine diet during this period. (Data from Dansgaard et al. [1975] and Arneborg et al. [1999].)

15.17Kelly J. Knudson taking a sample of dental enamel from a prehistoric tooth using a dental drill.

WarmerCooler

Car

bon

isot

ope

ratio

Mar

ine

food

die

t (pe

rcen

tage

)

-12.0

-14.0

-16.0

-18.0

-20.0

100

80

60

40

20

0

Years AD

600 800 1000 1200 1400 1600 1800 2000

Medieval Warm Period Little Ice Age

Settlement declines

Norse arrive Norse disappear

Changes in climate played a major role in Viking history. The Medieval Warm Period provided better growing conditions in Scandinavia and supported more people, which may have forced the exploration and colonization of new land. Several hundred years later, as the Little Ice Age took hold and the

growing season declined, the Viking way of life collapsed. At the same time, colder conditions on Greenland increased the herds of seal and reindeer, which meant that the Inuit hunters further north on Greenland fl ourished.

15.15The homelands, settlements, and routes of the Vikings in the North Atlantic (adapted from McGovern and Pedarkis, 2000).

A T L A N T I C O C E A N

Viking homelands

Viking settlement areas

L’Anse aux Meadows

LABRADOR

GREENLAND

ORKNEY ISLANDS

ICELAND

BAFFIN ISLAND SHETLAND

ISLANDS

Labrador Current

Arctic Circle

c. 870

c. 860c. 800


IN FOCUS (CONTINUED)

IN FOCUS

THE FIRST KING OF COPAN, A CLASSIC MAYA CENTER IN HONDURAS

One example of identifying human migration from isotopes in the skeleton comes from the Maya area in Mesoamerica. The site of Copan [CO-pahn] is located in Honduras, in the southwestern corner of the Maya region. The central part of this huge site is dominated by an acropolis covered with temples, buildings, and inscribed stone stelae and altars (fi g. 15.18). This was the civic and ceremonial focus of the site, and the residence and burial place of its rulers.

In the 1990s, Robert Sharer (1940–2012) from the University of Pennsylvania and a team of archaeologists began to tunnel into the artifi cial hill of the acropolis to see what lay inside. This large terraced mound had been built in stages, each new level burying the previous architecture under layers of clay and gravel. The tunnel of the archaeologists revealed earlier temples and altars that had been left intact and were preserved almost perfectly with their brightly painted facades. At the bottom of the earliest level of the acropolis they uncovered a series of graves and human burials around a large central tomb (fi g. 15.19). It seems that the mound was initially built to mark the burial place of one of the early rulers, but which one? There were sixteen kings listed in the dynasty at Copan.

Ancient Maya hieroglyphic inscriptions abound at Copan and often refer to the important events in the lives of these rulers, such as birth, marriage, conquests, and death. A number of inscriptions described Yax Kuk M’o [YASCH kook moe], the

fi rst king of Copan. He was apparently quite a warrior, having won a number of major battles. He was said to have come to Copan from the north in AD 427 to found the dynasty at what was then a simple village. Was this deep tomb at the base of the Copan acropolis the grave of Yax Kuk M’o? This fi rst king was also sometimes depicted wearing a costume typical of the major Mexican center of Teotihuacán, almost 1,200 kilometers (745 miles) to the northwest near modern-day Mexico City. Did he come from that distant center?

Isotopic analysis was used to answer these questions. Bones and teeth from the individual buried in the central tomb and several of the adjacent graves were analysed. This infor-mation was compared with isotope ratios from modern-day local animals and human bone from other parts of the Maya region. The combination of strontium and oxygen isotopes in the tooth enamel from the central tomb burial points to a place of birth to the north, perhaps somewhere around the site of Tikal, in modern-day Guatemala, meaning that this individual most defi nitely did not come from Teotihuacán. The regalia he wore only emphasized symbolic connections with that important center.

15.18A computer reconstruction of the acropolis at Copan, Honduras.

15.19The primary burial under the acropolis at Copan, Honduras, probably the tomb of Yax Kuk M’o.

ORGANIC RESIDUES IN ARCHAEOLOGY

Th e food and many raw materials that humans use are organic—meat, fi sh, fowl, vegetables, fruits, wood, hide, bone, antler, thatch, fur, and more—and come from living things. Th ese mate-rials were once abundant at the living places of prehistoric people. Unfortunately, in most cases this “biological” component of the past is very susceptible to decomposition and does not survive to the present. Fortunately, however, biological materials sometimes leave traces in

and on artifacts and sediments that can survive for thousands of years.

Trace organic compounds are distinguished from visible organic residues, such as charred food on pottery or other macroscopic organic remains. Analysis of trace organic compounds (which have adhered to or been absorbed into the structural matrix of archaeological materi-als) can provide information about past artifact function, diet, and other aspects of prehistoric

This individual in the central tomb was an older male, between fi fty and sixty years of age, and his skeleton showed a number of old breaks and lesions that were probably the result of confl ict and warfare. Certainly the age of the

individual, the wounds he had suffered, and the probable place of birth correspond with what we know of Yax Kuk M’o. This is likely his tomb and skeleton at the base of the Copan acropolis.


societies. This branch of archaeometry is sometimes referred to as molecular or biomo-lecular archaeology.

A variety of archaeological materials may contain trace organic compounds, including ceramics, stone tools, grinding stones, cooking slabs, plaster, fecal material, soil, and sediments. Th e best preservation seems to be in such arti-facts as pottery in which the structural matrix of the material has absorbed trace organic com-pounds, thus preventing, or at least reducing, the introduction of contaminants from handling or diagenesis (physical and chemical changes after deposition or burial), and the oxygen-induced degradation that can interfere with the identifi ca-tion of the original parent material.

Although the analysis of trace organic com-pounds has great potential in archaeological research, a number of problems remain, largely due to post-depositional changes in the molecules

CONCLUSIONS

Archaeological chemistry uses the elemental and isotopic characterization of archaeological mate-rials and organic analyses to learn more about the past. Studies using archaeological chemistry can identify unknown materials, authenticate the antiquity of pieces of questionable origin, and characterize the chemical composition of various raw materials and fi nished artifacts. A large part of archaeometric research has to do with determining provenience, or place of origin, to investigate questions of exchange or move-ment in the past.

Archaeological chemistry involves many more methods and materials than are discussed here. In this chapter, we have presented some informa-tion about the goals of archaeological chemistry, about instrumentation, and about laboratories. We have focused on the major techniques of ele-mental and isotopic analyses and archaeological chemistry. Examples have included the elemental characterization of obsidian for information on source and exchange, and isotopic studies of prehistoric Greenland that revealed changes in diet associated with climatic deterioration. Isotopic investigation of human mobility in the Maya region documents the origin of the fi rst king of Copan to the north, probably in the Peten [PAY-ten] region of Guatemala.

Archaeological chemistry has expanded dra-matically in the last three decades. One way to consider this is to look at the meetings and pub-lications that appear each year in this fi eld. Th e number of symposia that discuss archaeological science at the annual meetings of American archaeologists increases year-on-year, the number of journals focused on archaeological science also

either through contamination—the addition of new materials to the matrix—or the breakdown of the original molecules into small, unidentifi -able components. Th ere are in fact a relatively small number of both reliable and useful studies that have been conducted to date.

Most of the eff ort, and success, in the organic analysis of archaeological residues has been in characterizing specifi c organic molecules retained in potsherds. Highly specifi c residues from fruits, milk, wine, olive oil, and cedar wood oil have been identifi ed in various studies. Other poten-tially diagnostic compounds recovered from potsherds include proteins, some of which are diagnostic of certain animal foods. No doubt as scientifi c instrumentation becomes more sensitive to the identifi cation of ancient compounds, and as these instruments become more accessible to archaeology, studies of trace organic compounds will become more common.

IN FOCUS

TRACES OF CHOCOLATE IN CERAMIC VESSELS FROM THE NORTH AMERICAN SOUTHWEST

Archaeological sites in the Chaco Canyon region of New Mexico frequently provide evidence of many different exotic goods that were imported from Mesoamerica, such as copper bells and scarlet macaws. The archaeologist Patricia Crown (born 1953) wondered if other exotic goods were also being transported thousands of kilometers from Mesoamerica, and focused her attention on ceramics. Certain ceramic vessels were decorated in a Puebloan black-and-white style typical of Chaco Canyon, yet the cylinder shape was unusual (fi g. 15.20). The shape of the ceramic vessels resembled Mesoamerican cylinder jars that were used for a chocolate drink made with cacao beans, the key ingredient in chocolate. A team of scientists used GC-MS and other techniques to see if the organic residues in the ceramic vessels contained theobromine, which is a chemical found in large quantities only in the cacao plant. Many of the vessels did indeed contain theobromine, showing that ancient peoples in the Southwest United States imported and consumed cacao that had been grown thousands of kilo-meters away in the jungles of Central America.

15.20Ceramic vessels from Chaco Canyon, New Mexico that were used as containers for a chocolate drink.

IN FOCUS

ZOOLOGY BY MASS SPECTROMETRY

Zoology by Mass Spectrometry (ZooMS) is a new technique for identifying species of animal from otherwise unidentifi able bone fragments in archaeozoology. A small extract of collagen protein is taken from the bone, usually without damage to the specimen. That protein is digested into peptides, a compound of two or more amino acids in a chain. These peptides are placed in a mass spectrometer and the isotopic signature is recorded and compared to a reference library with the known

signature for many species. In this way the identity of the animal becomes available.

There are many uses for this new method. Zooarchaeolo-gists are normally unable to distinguish sheep and goats because of the similarity of their skeletons. These two species, however, can be distinguished by ZooMS primarily through a single peptide showing a mass difference.

has grown steadily to more than twenty-fi ve, and there are an increasing number of archaeologi-cal science laboratories and undergraduate and graduate degree programs all over the world.

Along with this expansion have come new and exciting developments that are revolutionizing the way archaeology is conducted. Archaeologi-cal chemistry, and archaeometry more generally, is a particularly exciting branch of archaeology because there are so many things to be learned and new approaches are coming so quickly. Archaeological chemistry is also a fascinating area of archaeology because it intriguingly combines the sciences and humanities, the quantitative and qualitative, and the objective and subjective in solving problems concerning our human past.


ARCHAEOLOGY PROJECT

ISOTOPES AND PREHISTORIC DIET

Now it is your turn. You are an archaeologist specializing in archaeological chemistry, and you are investigating what people ate at the ancient Peruvian site of Huaca Blanca. The site was on the coast, so the site’s inhabitants could have had access to fi sh and seafood from the Pacifi c Ocean, but Huaca Blanca was also surrounded by agricultural fi elds that could have been used to grow corn, beans, and cotton. After receiv-ing permission from the Peruvian government to export the samples and analyse them in your laboratory, you prepared the archaeological human bone samples and analysed the stable carbon and stable nitrogen ratios in the collagen extracted from the bone samples using a mass. Now, after months of preparing and analysing the samples, it is time to interpret your data, which are shown in Table 15.1.

First, you need to decide if the samples were well pre-served, or if they were contaminated by groundwater or the burial environment (diagenesis). When you excavated the bone samples, you noted that they resembled fresh, healthy bone, rather than fossilized bone. The carbon:nitrogen (C:N) ratio of pure, fresh collagen is 3.2. A carbon:nitrogen value between 2.9 and 3.6 is considered necessary for a sample to be reliable.

Now it is time to analyse the carbon and nitrogen isotopic data. One way to do this is to make a scatterplot so you can compare two variables at the same time (the carbon isotope data, or δ13C, and the nitrogen isotope data, or δ15N). To make a scatterplot, draw two lines on the graph paper, one hori-zontal and one vertical. The vertical line should rise from the left end of the horizontal line. The horizontal line will be the carbon isotope values. Mark the left end of the line with the minimum value in the dataset and the right end of the line with the maximum value. You will need to indicate some of the values for the grid lines between the minimum and maximum values to make it easier to plot the bone samples. Now do the same thing for the nitrogen isotope values along the vertical line. Minimum value at the bottom; maximum value at the top. Put values on some of the grid lines in between.

Now you are ready to make a scatterplot. Look at the fi rst sample; read the value for the carbon isotope value (δ13C) and fi nd this point on your horizontal line. Now read the value for the nitrogen isotope value (δ15N) for the same sample. Find this point on your vertical line. Now draw an imaginary horizontal line from your nitrogen isotope value and an imaginary vertical line from your carbon isotope value. Mark a dot or small circle at the point where those two imaginary lines cross. Now you have reduced two numerical values to a single point on the graph. Continue this process for the rest of the bone samples. You should end up with one dot on the graph for each sample. If you want to add another variable to your scatterplot, you

Table 15.1 Measurements of bone-collagen isotope ratios for carbon and nitrogen and the carbon:nitrogen ratio for twenty-four samples of bone from the archaeological site of Huaca Blanca. In the table, M=male, F=female, and PM=possible male, based on the skeletal sex of the individuals, who were all adults when they died.

Sample Number Sex C:N δ13C δ15N

1 F 3.2 -14.9 15.6

2 M 3.2 -14.6 4.9

3 F 3.6 -16.8 14.7

4 F 3.4 -10.2 15.7

5 F 3.2 -17.9 18.0

6 F 3.5 -12.0 14.3

7 PM 3.8 -17.4 13.4

8 F 2.9 -17.8 12.7

9 F 3.1 -23.0 16.0

10 F 2.8 -17.6 15.5

11 M 3.2 -22.8 6.3

12 M 3.3 -17.0 6.5

13 F 3.2 -19.0 14.7

14 M 3.2 -15.6 5.6

15 M 3.1 -16.5 4.3

16 M 3.0 -14.9 7.4

17 F 3.2 -19.7 16.2

18 F 3.4 -19.8 7.4

19 F 3.2 -18.9 8.3

20 M 3.2 -8.9 4.3

21 M 3.2 -22.7 5.2

22 M 3.3 -22.9 5.7

23 M 3.2 -21.4 4.0

24 F 3.4 -21.4 6.6

can use different shapes or colors for the dots for males and the dots for females.

What does this scatterplot of isotope data tell you? Do you see distinct clusters or groups of data points? If so, what do these mean? Based on your readings, you know that eating seafood and fi sh from the Pacifi c Ocean would lead to higher nitrogen isotope values, and that eating corn from the neigh-boring agricultural fi elds would lead to higher carbon isotope values. What did people who were buried at Huaca Blanca eat? Did females and males have different diets?

Given your analysis of the isotope data and the archaeological

information on the site of Huaca Blanca, write a brief essay about diet at the site. Please answer each of the questions below in a brief paragraph.1. What, if any, evidence did you fi nd for diagenesis, or for

contamination in the bone samples? Describe how you decided that the bone samples were well preserved or not.

2. Did you fi nd any clusters or groups in your scatterplot? How did you interpret the scatterplot of your data?

3. What did the people buried at Huaca Blanca eat? Did you fi nd one or more groups in the dataset, based on diet and/or sex? How did you interpret these groups?

ARCHAEOLOGY PROJECT (CONTINUED)

D. R . Brothwell and A. M. Pollard (eds.), Handbook of Archaeological Sciences (Chichester, England: Wiley, 2001).

M. Buckley, M. J. Collins, J. Thomas-Oates, and J. C. Wilson, “Species identifi cation by analysis of bone collagen using matrix-assistedlaser desorption/ionisation time-of-fl ight mass spectrometry,” Rapid Communications in Mass Spectrometry 23 (2009): 3,843–54.

Julian Henderson, The Science and Archaeology of Materials: An Investigation of Inorganic Materials (London: Routledge, 2000).

Joseph B. Lambert, Traces of the Past: Unraveling the Secrets of Archaeology through Chemistry (New York: Addison Wesley Longman, 1997).

A. Mark Pollard, Catherine Batt, Ben Stern, andSuzanne M. M. Young, Analytical Chemistry in

Archaeology (Cambridge: Cambridge University Press, 2007).

A. Mark Pollard and Carl Heron, Archaeological Chemistry, 2nd edn (Cambridge: Royal Society of Chemistry, 2008).

T. Douglas Price and James H. Burton, An Introduction to Archaeological Chemistry (New York: Springer, 2011).

J. Thomas-Oates and M. J. Collins, “Distinguishing between archaeological sheep and goat bones using a single collagen peptide,” Journal of Archaeological Science 37 (2010): 13–20.

N. L. Van Doorn, “Zooarchaeology by Mass Spectrometry (ZooMS),” Encyclopedia of Global Archaeology (New York: Springer, 2014), pp. 7,998–8,000.

FURTHER READING

STUDY QUESTIONS

1. What kinds of materials are studied in archaeological chemistry laboratories?

2. Elemental analysis is one of the primary methods of archaeometric research. What methods are available to measure elemental composition in archaeological samples?

3. How do archaeologists and archaeological chemists learn about past human diets?

4. Discuss three isotope ratios and how they are used in archaeological chemistry.

5. Organic residues are a potentially important but diffi cult to analyse source of information about the past. Discuss these materials and techniques for analysis.

chapter 15 archaeological chemistry

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