P A R T II
STABLE ISOTOPE ANALYSIS
AND HUMAN DIET
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Isotope Deﬁnitions 131
History of Isotope Studies 132
Sample Preparation and Isotopic Analysis 135
Interpretation and Signiﬁcance of Carbon and Nitrogen
Isotope Data 136
Oxygen and Strontium Isotopes 138
Isotope Studies in this Volume 139
Glossary
Apatite (bone, tooth enamel) Inorganic portion reﬂecting
whole diet.
Carbon, nitrogen, and oxygen isotopes Stable isotopes
representing the main dietary components of skeletal
tissues.
Collagen Organic portion of bone mainly reﬂecting
dietary protein.
Fractionation The change in isotope ratios caused by
chemical processes such as photosynthesis and
metabolism.
Isotopes Elements with different numbers of neutrons in
the atomic nucleus.
Mass spectrometer Instrument used to produce precise
measurements of isotope ratios.
PDB International standard for C and O isotope analysis,
based on Belemnitella americana from the Peedee formation
in South Carolina.
Strontium isotopes Heavy stable isotope representing
geological area of food acquisition.
The contribution of isotope analyses to archaeology has signiﬁcantly
increased ever since the introduction of radiocarbon
dating by Willard Libby [60], with applications
of isotope analyses developing ever since. Today, stable
isotope analysis is regularly applied to address questions
concerning human diets around the world, and this is
illustrated in this book with the large number of studies on
the origins, importance, and spread of maize throughout the
Americas. In this introductory chapter, an overview of the
history and principles of isotope analyses is provided, while
the individual chapters in this section provide speciﬁc details
on the samples and procedures used in each study, and how
their results have contributed to our understanding the histories
of maize. In particular, “we are what we eat,” and
isotope analyses provide quantitative data that complement
ﬂoral, faunal, ethnohistoric, and other information about
dietary practices. The details that follow include the contributions
of methodological and interpretive isotope studies,
but like some of the previous synthetic publications on isotopic
analysis applied to archaeology, focus on relevance to
maize studies [1, 2, 6, 8, 23, 44, 45, 46, 54, 65, 81, 93, 94,
97, 99, 107, 108, 111, 112, 114].
ISOTOPE DEFINITIONS
Isotopes are deﬁned as one of the two or more forms of
an element (e.g., carbon) that have the same number of
protons in the nucleus (known as the atomic number) of an
atom but different numbers of neutrons in the nucleus, resulting
in different atomic weights. Radioactive isotopes (e.g.,
carbon-14) decay over time, whereas stable isotopes (e.g.,
carbon-12, carbon-13) do not. Carbon occurs in three isotopic
forms: 12
C, 13
C, and 14
C, where the superscript
number is the sum of the protons (6) and neutrons (6,
7, 8) in the carbon nucleus. Carbon dioxide (CO2) in the
atmosphere consists of about 98.9% 12
CO2 and 1.1% 13
CO2
(with radioactive 14
CO2 a tiny fraction of about 1 × 10−10
%).
The isotopic composition of atmospheric CO2 is established
by equilibration with the much more abundant pool of
dissolved inorganic carbon (mainly bicarbonate, HCO3−
) in
sea water. There is also considerable carbon in calcium
131
C H A P T E R
Isotope Analyses and the Histories of Maize
ROBERT H. TYKOT
Department of Anthropology, University of South Florida, Tampa, Florida
10
Histories of Maize
Copyright © 2006 by Academic Press.
All rights of reproduction in any form reserved.
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132 R. H. Tykot
carbonate (CaCO3), the main component of limestone and
marble. Atmospheric carbon dioxide is photosynthesized by
plants and metabolized into complex molecular compounds
that we categorize as carbohydrates, proteins, and lipids,
whereas many plants (wild and domesticated) are consumed
by organisms that then appropriate or convert, or both, these
compounds into their body tissues. In biosynthetic chemical
reactions, especially photosynthesis [76], the lighter weight
isotopic compounds react faster, using less energy. If, as in
the case of photosynthesis, the reaction is incomplete, the
product (e.g., maize) will be enriched in 12
C relative to
the substrate (e.g., atmospheric CO2), resulting in changes in
the 13
C/12
C ratio. This is called isotopic fractionation. When
plants are consumed by herbivorous animals, the metabolic
processes involved reverse the direction of fractionation,
increasing the proportion of the heavier carbon (and nitrogen)
isotope in the body tissues. Laboratory-based and other
isotope studies have speciﬁcally determined that bone collagen
is produced mainly from dietary protein, whereas bone
apatite and tooth enamel represent the whole diet [7, 106].
In addition, it is also clear that turnover rates for bone are
slow, so that the isotope values obtained represent at least the
last several years of an individual’s life.
Because of the small difference in 13
C/12
C ratios between
all samples, these and other isotope abundances, instead of
being reported as simple ratios (e.g., 13
C/12
C) they are converted
to ratios relative to international standards using the
delta notation:
The standard reference material, PDB, was originally derived
from a Cretaceous marine fossil sample, Belemnitella americana,
recovered from the Peedee formation in South
Carolina [19, 20]. Because the original material is no longer
available, sample carbon isotope ratios are now reported
relative toVienna Peedee belemnite (VPDB), with raw values
typically calibrated using the NBS National Bureau of
Standards (NBS) 19 reference sample so that results from different
laboratories may be reliably compared [18].
The same mathematical formula is used for reporting
nitrogen (15
N/14
N) and oxygen (18
O/16
O) isotope ratios, with
the standard reference materials being AIR for nitrogen and
VPDB for oxygen. Because the 13
C/12
C ratio of all photosynthesized
carbon is lower than that of the reference VPDB
material, the 13
C values for plants and animals are mostly
negative (see equation).
HISTORY OF ISOTOPE STUDIES
Variation in the relative proportions of stable carbon isotopes
was ﬁrst measured in 1939 by Nier and Gulbransen
δ13
13 12 13 12
C in ‰ or per mil
sample C C standar C C 1 1000
( )
= ( ) ( ){ }−[ ] ×
[71], and the general distribution of carbon isotopes in
nature was explored by Craig in 1953 [19]. It was less than
20 years later then that the archaeological signiﬁcance of
such analyses was recognized, both for studying prehistoric
human diets [110] and for sourcing marble [21, 41]. In
between, important advances had been made by Calvin and
Benson [16] in the understanding of the chemical pathway
for carbon during photosynthesis, followed by the realization
that there were multiple photosynthetic pathways,
commonly referred to as C3, C4, and crassulacean acid
metabolism (CAM) by Hatch and Slack [38] and Ransom
and Thomas [85]. At about the same time that it was determined
that maize followed the C4 photosynthetic pathway
[39], it was realized by archaeologists that radiocarbon dates
on preserved carbonized maize samples were consistently
about 200 years younger than expected [36], with the result
that stable isotope measurements were made and correction
factors were applied to radiocarbon dates being done on
maize and other nonwood charcoal samples [11]. A conference
held in Australia in 1970 led to the explicit realization
that plants following the C3 photosynthetic pathway had different
stable carbon isotope ratios (ave. δ13
C = −26.5‰) than
those following the C4 pathway (ave. δ13
C = −12.5‰), which
includes maize, sugarcane, millet, sorghum, and certain
species of amaranth and chenopodium [101]. These now
domesticated C4 plants, as well as a range of wild C4 grasses,
are originally native to areas of hot and dry climates,
whereas CAM plants switch between C3 and C4 depending
on their actual location and environmental circumstances.
The idea that carbon isotope values could be used for
dietary information was developed at least by 1964, when
Parker [77] published an article on carbon isotope analysis
of marine plants and animals, but it was not until 1971 when
a dietary study was done on humans. Van der Merwe and
Vogel [111] speciﬁcally tested an Iron Age Khoi skeleton
from the Transvaal of South Africa, whose bone collagen
(protein made of multiple amino acids) carbon isotope value
(δ13
C = −10.4) was interpreted then as indicating dependence
on sorghum (or other C4 plants) in the Transvaal
Lowveld. It should also be noted that although the earliest
radiocarbon dating of bone was done on whole bone
samples, by 1970 demineralization of bone and extraction
of humic and fulvic acids had been developed to produce
much more accurate dating results on bone collagen [51,
62], and this was the speciﬁc sample material tested by van
der Merwe and Vogel.
Shortly afterwards, discussions in van der Merwe’s [110]
1973 seminar at SUNY-Binghamton led to the isotopic
analysis of human skeletal remains from sites in New York
to address the question of when maize agriculture was introduced
to that region [110]. The results were ﬁrst published
in American Antiquity in 1977 [116]. This was followed by
a larger study of Archaic, Woodland, and Mississippian
period sites in North America, which achieved greater sciCh010-P369364.qxd
20/2/2006 4:52 PM Page 132
entiﬁc recognition with a publication in Nature in 1978
[115]. Both studies clearly showed that the introduction of
maize agriculture in North America dramatically changed
the isotope values on human bone collagen in measurable,
quantita-tive ways, demonstrating that isotope analyses
could strongly complement botanical and other evidence for
dietary interpretations. These early studies also demonstrated
that for collagen δ13
C values, “You are what you eat,
plus 5‰.” The percentage of C4 foods in the diet could also
be estimated by simple interpolation of the adjusted collagen
value between the C3 and C4 endpoints (e.g., a collagen
result of −14‰ suggests about 50% C4 in the diet) (Figure
10-1).
This initial work was quickly followed by other studies
by several scholars, with DeNiro and Epstein, who had
already studied isotopic variation among animals [24],
reporting results for early maize consumption at Tehuacan
in highland Mexico [25], at the same time that a study was
done on maize in the Hopewell Period of the midwestern
United States by Bender, Baerreis and Steventon [12], while
yet another study was done by van der Merwe and Vogel,
with Roosevelt on maize in the rain forests of the Orinoco
Valley of Venezuela [113]. This particular project emphasized
the importance when interpreting isotopic results for
humans of establishing baseline values for the plants and
animals in a given region, which in this case were especially
negative because of the recycling of CO2 under a dense
forest canopy [66]. Another early study speciﬁcally
addressed maize in the diet of domestic dogs in Peru and
Ecuador [15].
Stable isotope analysis of nitrogen also developed in the
early 1980s, with clear indications of differences caused by
trophic level effects, especially in marine systems [5, 25, 26,
90, 91, 96, 100]. More studies have added signiﬁcantly to
Isotope Analyses and the Histories of Maize 133
FIGURE 10-1 C3 and C4 pathways, showing isotopic differences passed on to skeletal tissues.
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134 R. H. Tykot
our understanding and interpretation of nitrogen isotope
ratios, in particular the effects of climate and environment
on both plant and animal values and trophic level increases
in both terrestrial and marine ecosystems [3, 14]. Nitrogen
isotope analysis has also been demonstrated to be useful for
investigation of ancient weaning practices [94, 122, 125]. In
general, nitrogen isotope ratios increase 2 to 3‰ with each
trophic level, with values for terrestrial plants and animals
generally much lower than for ﬁsh and mammals in most
freshwater or marine ecosystems (Figure 10-2).
Other major advances to bone chemistry studies came in
the 1980s regarding the preparation and carbon isotope
analysis of inorganic bone apatite (calcium hydroxyphosphate,
Ca5[PO4]3OH, with some carbonate substitution).
Because this mineral, which accounts for more than 75% of
whole bone, is inorganic, it is much more susceptible to
weathering and chemical reactions with soil deposits and,
thus, alteration (diagenesis) of its original isotopic signature.
This was quite clear from early radiocarbon dating of whole
bone samples, which produced unreliable results, so that
when stable isotope analysis of bone apatite for dietary purposes
was ﬁrst proposed by C. H. Sullivan and H. W.
Krueger in 1981 [53, 105], it was not accepted by other
researchers [89]. Since then, however, bone apatite sample
pretreatment procedures have matured [48, 52, 58], and
analysis of nonporous tooth enamel has been shown to be
reliable going back millions of years [49, 55, 56, 59, 102].
Increased understanding of fossilization pathways is also
now leading to better assessment of the isotopic integrity of
bone apatite samples [57, 117]; yet, there does seem to be
some systematic offsets in isotope values produced using
different sample preparation methods [34].
Overall, isotopic analysis of archaeological materials has
increased tremendously since the early studies in the 1970s,
when carbon isotope analysis of bone collagen were regularly
done by radiocarbon laboratories to properly correct
C14 dates, with both carbon and nitrogen isotope analysis
now done in many other laboratories to address dietary
FIGURE 10-2 AGulf Coast example of stable carbon versus nitrogen isotope ratios for plant and animal groups. Faunal
bone collagen carbon corrected −2% to simulate ﬂesh; modern samples corrected +1.5% for industrial effect. Although
values for plants and terrestrial consumers are similar in other regions, values for aquatic resources may be different.
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questions. Until recently, I have personally maintained a
detailed database of all published isotope studies in the New
World, but the feasibility of this is steadily decreasing as the
number of studies exponentially increase, not to mention
more studies being done by graduate students or for cultural
resource management (CRM) ﬁrms, without widespread
publication of the results.
SAMPLE PREPARATION AND
ISOTOPIC ANALYSIS
Archaeological isotope studies, in contrast to most forensic
studies of the recently deceased, must deal ﬁrst with
issues of preservation and contamination to produce reliable
scientiﬁc results with which to address ancient dietary questions.
Over time, slightly different laboratory preparation
procedures have been developed for preparing bone collagen
and bone apatite samples. In most cases, a single 1 gram
sample of reasonably preserved bone is sufﬁcient for the
preparation of both collagen and apatite (Figure 10-3); the
actual amount of sample analyzed by most mass spectrometers
is no more than a few milligrams, and as little as
several hundred micrograms. For preparation of collagen,
most techniques use an acidic solution to demineralize the
bone and other chemicals to remove contaminants and residual
lipids. For preparation of bone apatite samples, sodium
hypochlorite (bleach) is typically used to dissolve any preserved
organic components, and a weak acid solution is
employed to remove any nonbiogenic carbonates. The speciﬁc
procedures used are discussed in each of the chapters
on isotope studies, as well as the particular mass spectrometer
used and the precision of the measurements obtained.
The precision of carbon and nitrogen isotopic results is
almost always ≤0.3‰, and in many cases about 0.1‰, so
that actual dietary isotope differences of just a few percent
are noticeable.
Even more important is assessment of the reliability of
the isotope results produced. Although mass spectrometers
have signiﬁcantly changed over time, from manually operated
systems in which solid samples were converted to CO2
and N2 gas off-line (Figures 10-4A, 10-4B), to modern,
largely computer-operated instruments with automated
in-line sample introduction systems (CHN analyzers for
organic samples, common or individual acid baths for
apatite-enamel samples) (Figures 10-5A, 10-5B), the users
are always responsible for the type, quality, and purity of the
sample being tested. For bone collagen, this is typically dealt
with by measuring the percentage of collagen produced from
the original whole bone sample (<1% is usually considered
unreliable because bone originally is more than 20% collagen);
the amount of C and N measured by the mass specIsotope
Analyses and the Histories of Maize 135
FIGURE 10-3 Typical bone sample, and collagen and apatite produced in Tykot’s Laboratory for Archaeological
Science at the University of South Florida.
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136 R. H. Tykot
trometer, relative to the size of the sample introduced; and
the actual C:N ratio (which should be the same as in living
organisms). Problematic results mainly occur when preservation
has not been so poor that no collagen remains (and
thus no isotope data produced) but where degradation has
resulted in unequal breakdown and loss of the different
amino acids, which individually would have different
isotope values because of the different chemical reactions
involved in their initial production. For bone apatite and
tooth enamel samples, other than measures of sample loss
during the preparation procedures and CO2 yield during the
mass spectrometry analysis, more complex tests of sample
reliability, using Fourier transform infrared spectroscopy
(FTIR), have been developed [34, 48, 57, 69 70, 125]. One
logistical approach to producing reliable isotope results, in
addition to measures of sample preparation and analysis, is
the need for performing repeat isotope analyses for “outliers”
when testing a group of individuals thought to have
had similar diets.
INTERPRETATION AND
SIGNIFICANCE OF CARBON AND
NITROGEN ISOTOPE DATA
From the time of the earliest isotope studies in the 1970s,
it was realized that bone collagen has a slow turnover rate,
at least for adults, so that the isotope results obtained would
represent the average diet over at least the last several years
of an individual’s life. Although the exact rate of turnover
is still unknown, it is estimated to be at least 5 to 7 years if
not more and varies depending on the density of each bone.
In any case it has become evident that isotope studies generally
may be done on any available part of skeletal remains,
although microsampling of interior and exterior parts of long
bones could produce different values if diet isotopically
changed signiﬁcantly during the time of their recent production.
Although the same lengthy turnover time applies to
bone apatite as well, it is clear that tooth enamel, tooth roots
(dentin), hair, ﬁngernails, and ﬂesh are different. Teeth form
only once and, thus, represent at most a few years of diet,
FIGURE 10-4A Off-line gas sample preparation system for stable isotope samples, with Nikolaas J. van der Merwe.
University of Cape Town, Stable Light Isotope Laboratory. Photo by R. H. Tykot.
FIGURE 10-4B A venerable stable isotope ratio mass spectrometer
(VG602E), which requires manual input of CO2 and N2 gas samples. University
of Cape Town, Stable Light Isotope Laboratory. Photo by R. H.
Tykot.
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Isotope Analyses and the Histories of Maize 137
FIGURE 10-5A Example of a modern stable isotope ratio mass spectrometer (Finnigan MAT Delta Plus XL), run
by computer (far right), and connected with several input devices including a Carlo Erba CHN analyzer (upper right
corner) that converts organic samples into CO2 and N2 gas. University of South Florida, Paleolab. Photo by R. H. Tykot.
FIGURE 10-5B Another stable isotope ratio mass spectrometer, connected to an automated Kiel III individual acid
bath system (left) that converts inorganic carbonates into CO2. University of South Florida, Paleolab. Photo by
R. H. Tykot.
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138 R. H. Tykot
at the age when the tooth formed, with third molars the last
to form, between ages 10–12 for the enamel and by young
adulthood for the tooth dentin (roots). Analysis of other
permanent teeth can reveal the age of weaning, at which
point there is no longer a trophic level increase in isotope
values resulting from breast-feeding [125]. Analysis of
sequential samples in tooth dentin may be used to investigate
seasonal isotope variation in diet, as can microsampling
of tooth enamel layers [9, 10, 122]. When preserved, sequential
hair and ﬁngernail samples may also be used to assess
short-term dietary change [32, 64, 73–75, 88, 98].
As for speciﬁc aspects of diet that are revealed through
these different isotope studies, it became clear from studies
of animals raised on controlled diets that bone collagen is
produced primarily from dietary protein, whereas bone
apatite and tooth enamel are products of the whole diet
[7, 106]. For collagen, it appears that essential amino acids
are transferred directly from appropriate foods, whereas
nonessential amino acids may be transferred or produced
independently and, thus, represent the whole diet, including
maize [33, 37, 43]. Isotopic analysis of individual amino
acids would be especially useful for studies of the importance
of maize in coastal environments.
Controlled diet studies have also provided a more precise
assessment of the difference in δ13
C values between dietary
resources and bone apatite, with variation in fractionation
seemingly attributable to differences in body size, digestive
physiology, and amount of methanogenesis [9, 17, 43, 58,
78]. For humans (not directly tested with controlled diets),
the fractionation between diet and bone apatite is estimated
to be at least 9.5‰ [7] and as much as 11 to 12% [40].
Although it is currently possible to quantitatively compare
human individuals and groups of individuals, further studies
are necessary if we are to have better precision on the quantity
of maize and other C4 plants in the diet.
Overall, interpretations are currently as follows. For
herbivores, where the entire diet is based on carbohydrate
dominated plant foods, any mixtures of C3, C4, and CAM
plants would contribute equally to both collagen and apatite,
unless there were signiﬁcant differences in the percentage of
protein in those plants (e.g., beans with about 20% protein
versus maize with about 10% protein). For omnivores
(including humans), animal food—whether wild or domestic,
terrestrial or aquatic—would make a much greater contribution
to bone collagen. So for a hypothetical New World
inland society with maize agriculture, where dogs are the
only domesticated animals, it would be expected to have
isotope values for both collagen and apatite that indicate
maize consumption, with the percentage represented at least
somewhat higher for bone apatite than for bone collagen,
unless the dogs (or wild hunted animals) ate more maize
than the humans (one reason for testing the isotope values
of animals that were consumed). Differences between individuals
in the percentage of maize versus C3 plants in the
diet would be far more apparent in apatite than in collagen,
thus making analysis of bone and tooth apatite much more
effective than collagen for addressing questions about variation
within a population or at a single site; comparison of
individuals based on sex, status, and/or specialization; and
changes over time [e.g., 4, 35, 109].
For studies where C4 (or CAM) plants other than maize
existed or where seafood may have been consumed, it is
strongly recommended to test archaeological faunal remains
to learn whether they may have contributed more positive
carbon isotope values to human consumers, as well as to use
other archaeological and ethnohistoric information to interpret
the isotope results. Marine plants and their consumers
have more positive *13
C values because their original
sources of carbon include dissolved CO2 and carbonic acid,
as well as detritus from local terrestrial plants [93]. Because
of the many trophic levels for marine fauna, there are
continuous increases in both carbon and nitrogen isotope
ratios from level to level, resulting in many cases with large
ﬁsh and marine mammals with *13
C values similar to
C4 plants such as maize—but also enriched *15
N values.
Analysis of nitrogen isotopes in bone collagen is, thus, necessary
for looking at diets in coastal regions, and in conjunction
with bone carbonate carbon isotope analysis, it may
be used to assess the relative contributions of maize and
marine foods in the whole diet and in protein in particular.
Most freshwater aquatic ﬁsh from inland lakes and rivers
also have much higher *15
N values than terrestrial foods,
but *13
C values similar to C3 plants, so that consumption of
maize or other C4 plants is readily apparent. It should be
noted that some scholars have developed and used mathematical
mixing models to interpret isotope results [61, 68,
79, 80].
OXYGEN AND STRONTIUM
ISOTOPES
Even more signiﬁcant has been the expansion of isotope
analyses from carbon and nitrogen in bone collagen and
apatite to charred ceramic residues [e.g., 67], carbonates and
humic matter in soils [72, 118], and to carbon isotope analysis
of cholesterol preserved in bones and teeth, of animal
fats, and of lipid residues in ceramics [29, 30, 86, 87, 104].
Because the turnover time of cholesterol in bone samples is
much greater than that of collagen and apatite, cholesterol
analyses may be speciﬁcally used to address dietary change
during the time more immediately preceding death, whereas
analyses of residues and soils provide information on cultural
and spatial landscape practices.
Oxygen and strontium isotope analyses have also been
added to the isotope repertoire applied to ancient dietary
issues. Oxygen isotope values relate directly to local
climate, temperature, and humidity [50, 63] and, thus, have
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been used not only for determining the seasonality of shells
and presumably their consumers [22] but also for climate
studies [92, 103] and mobility [119–121], with appropriate
changes in dietary patterns. Isotope ratios of strontium,
which does not isotopically fractionate like biological C, N,
and O, directly represent the geographic area of food
production/acquisition and, thus, the mobility of dietary
resources or their consumers, or both [27, 28, 82]. Strontium
isotope analysis has, thus, been applied to signiﬁcant
migration periods in Europe [13, 83], immigration to
Mesoamerican sites such as Teotihuacan and Tikal [84, 42,
124], and migration and mortuary patterns in South America
[47].
ISOTOPE STUDIES IN THIS VOLUME
The chapters in the following part of this volume all focus
on isotope studies of maize acquisition and consumption. A
broad range of geographic areas are covered, including
White and colleagues (Chapter 11), Chisholm and Blake
(Chapter 12), and Mansell and colleagues (Chapter 13) on
Mesoamerica; Tykot and colleagues (Chapter 14) and Gil
and colleagues (Chapter 15) on parts of Andean South
America; and Greenlee (Chapter 16), Reber (Chapter 17),
Kelly and colleagues (Chapter 18), Katzenberg (Chapter
19), Coltrain and colleagues (Chapter 20), and Benson
(Chapter 21) on many areas throughout North America, followed
by a synthesis by Schwarcz (Chapter 22). Isotope
studies such as these continue to make signiﬁcant contributions
to our understanding of the histories of maize.
References Cited
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tooth collagen for isotopic analysis. Journal of Archaeological
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