International Journal of Osteoarchaeology
Int. J. Osteoarchaeol. 13: 37–45 (2003)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/oa.654
Sulphur Isotopes in Palaeodietary
Studies: a Review and Results from a
Controlled Feeding Experiment
M. P. RICHARDS,a * B. T. FULLER,b,c M. SPONHEIMER,d,e T. ROBINSONf
AND L. AYLIFFEe
a
Department of Archaeological Sciences, University of Bradford, Bradford, West
Yorkshire, UK
b
Research Laboratory for Archaeology and the History of Art, University of Oxford,
Oxford, UK
c
Department of Biochemistry, University of Oxford, Oxford, UK
d
Department of Biology, University of Utah, Salt Lake City, Utah, USA
e
Department of Geology & Geophysics, University of Utah, Salt Lake City, Utah, USA
f
Department of Animal and Veterinary Sciences, Brigham Young University, Provo,
Utah, USA
ABSTRACT
Recent advances in mass spectrometry now allow relatively routine measurements of sulphur
isotopes (δ 34 S) in small samples (>10 mg) of tissue from archaeological human, plant, and faunal
samples. δ 34 S values of human and faunal bone collagen can indicate residence or migration
and can provide palaeodietary information. Here we present a review of applications of sulphur
isotopes to archaeological materials, and we also present preliminary results from one of the
few controlled feeding experiments undertaken for sulphur isotopes. This study indicates that
there is relatively little fractionation (−1‰) between diet and body protein (keratin) δ 34 S values
for modern horses on a protein adequate C3 plant diet. In contrast, horses fed a possible low
protein C4 feed have a diet to hair fractionation of +4‰ that could be the result of the input of
endogenous sulphur from the recycling of body proteins. Copyright  2003 John Wiley & Sons,
Ltd.
Key words: stable isotopes; sulphur; migration; palaeodiet; collagen; keratin
Introduction
There have been relatively few applications of
sulphur isotopes to archaeological organic material, such as bone collagen or hair keratin. This
is due to the time-consuming laboratory preparation and large sample sizes traditionally required.
Recent advances in mass spectrometry now allow
measurement of sulphur isotope values in relatively small organic samples with much simpler
sample preparation methods, permitting routine
* Correspondence to: Department of Archaeological Sciences,
University of Bradford, Bradford, West Yorkshire BD7 1DP, UK.
e-mail: [email protected]
Copyright  2003 John Wiley & Sons, Ltd.
sulphur isotope analysis in conjunction with analysis of carbon and nitrogen isotopes (Richards
et al., 2001).
The stable isotope composition of sulphur
34
(δ S) in human and faunal tissues reflects the δ 34 S
values of foods consumed. Local food web δ 34 S
values are controlled by the δ 34 S values of underlying local bedrock, atmospheric deposition, and
microbial processes active in soils. Therefore, like
strontium and lead isotopes in bone mineral (Sealy
et al., 1995; Price et al., 2000), human and faunal
sulphur isotope values can be used to establish
the local food web sulphur isotope values. These
Accepted 25 September 2002
38
values can then be used to identify local and extralocal (migrants) individuals in a region, providing
the extra-local individuals were from a locality
with sufficiently different environmental δ 34 S values. Additionally, δ 34 S values have the potential to
provide palaeodietary information supplementary
to δ 13 C and δ 15 N measurements perhaps indicating the consumption of freshwater resources.
Below we review the environmental factors that
influence δ 34 S values, as well as review the few
measurements of δ 34 S in human tissue.
There are few published controlled feeding
experiments that have measured δ 34 S fractionation between diet and consumer tissue δ 34 S
values for mammals. Clearly, this fractionation
must be established before δ 34 S can be used to
track migration, residential mobility or diet. We
report here preliminary results from a controlled
feeding experiment where δ 34 S values of horse
hair and diet were measured for two individuals
that were fed C3 and C4 diets.
Sulphur isotopes in plants and animals
There are four stable isotopes of sulphur:
32 S(95.02%),
33 S(0.75%),
34 S(4.21%),
and
36
S(0.02%) (Trust & Fry, 1992). The ratio between
the two most abundant isotopes, 32 S and 34 S
is defined as the δ 34 S value which is measured
relative to the meteorite standard Canyon Diablo
Troilite (now the Vienna CDT) (Coplen &
Krouse, 1998). Sulphur is an abundant element.
There are two significant isotopically uniform S
reservoirs: the earth’s metallic core, with a δ 34 S
value near 0‰, and oceanic sulphate (SO4 − ),
with a δ 34 S value near +20‰. Sedimentary rocks
are the largest reservoir of sulphur near the
earth’s surface, but isotopic values for sedimentary
sulphur are highly variable depending on rock
type and age (Faure, 1977). Plants receive the
majority of their sulphur through their roots as
sulphate, which is derived from the weathering
of local geological formations. Plants can also
obtain sulphur from the atmosphere by wet or dry
deposition. Wet deposition is the incorporation
of sulphur falling to earth in water droplets from
sea spray or acid rain (H2 SO4 ), whereas dry
deposition results from the uptake of SO2 gas.
The amount of atmospheric sulphur absorbed
Copyright  2003 John Wiley & Sons, Ltd.
M. P. Richards et al.
varies depending upon location and plant species,
but in some cases 25 to 35% of the plant’s sulphur
can be obtained in this way even if there are
adequate amounts of soil sulphate (Brady & Weil,
1999). Once obtained by the plant, most sulphur
is stored in organic molecules such as amino acids
and sulphate esters. The δ 34 S value of plants is
variable depending upon location and geology,
with values falling between the extremes of −22
to +22‰ (Peterson & Fry, 1987).
In animals, sulphur is an essential element for
growth and survival that must be obtained from
the diet. It is predominantly found in the amino
acids of cysteine, methionine, and taurine as
well as in various vitamins and cofactors such
as thiamine, vitamin B, biotin, and coenzyme A.
In modern and archaeological bone, sulphur is
distributed throughout the inorganic matrix as
calcium sulphate (CaSO4 ) and within the protein
collagen as methionine with a frequency of five
residues per 1000 (Eastoe, 1955). The sulphur
in hair is primarily derived from the amino acid
cysteine (112 residues per 1000) although there
is a small contribution from methionine (five
residues per 1000) (Valkovic, 1977).
There are significant differences among the
δ 34 S values of plant and animal specimens
from freshwater and marine ecosystems. Modern
marine organisms have δ 34 S values close to +20‰
whereas freshwater organisms can have a wide
range of δ 34 S values, between −22 to +22‰
(Peterson & Fry, 1987; Mekhtiyeva et al., 1976).
The wide range of freshwater δ 34 S values is largely
due to the reduction of sulphate ions (SO4 − ) to
hydrogen sulphide (H2 S) by anaerobic bacteria
that dwell in the sediments of rivers and lakes
(Faure, 1977). These anaerobic bacteria generate
energy for survival by using sulphate in place
of molecular oxygen as an electron acceptor
during the oxidation of organic matter. Since
it is thermodynamically easier to break a 32 S–O
bond versus a 34 S–O bond, the final H2 S that is
excreted is significantly depleted in 34 S. In some
cases this 34 S depletion can reach 50‰ depending
upon season and environmental conditions such
as moisture, aeration, temperature, and pH
(Faure, 1977).
This difference between freshwater and marine
δ 34 S values has been used in a number of modern
ecological studies, for example to discriminate
Int. J. Osteoarchaeol. 13: 37–45 (2003)
Sulphur Isotopes in Palaeodietary Studies
between lake dwelling and migratory ocean fish
that co-existed in freshwater lakes in northern
Canada (Hesslein et al., 1991). A food web
analysis of modern fauna from the Canadian
Arctic was able to distinguish between continental
(terrestrial) and coastal (marine) based diets using
solely δ 34 S values (Krouse & Herbert, 1988).
Terrestrial animals and birds had values less
than +10‰ whereas mammals from more marine
environments, such as polar bears, had values
ranging between +16 and +18‰ reflecting their
consumption of seals.
While these studies show the potential of
using sulphur isotopic analysis in (palaeo)dietary
research, it should be noted that δ 34 S values
do not necessarily reflect consumption of marine
protein, as they can also register the proximity
of the dietary protein source to the sea. This is
a result of the so-called ‘sea spray effect’, which
can carry sulphur particles inland and cause the
coastal soil δ 34 S values to be similar to those of the
ocean (Wadleigh et al., 1994). In some cases this
sea spray effect may only extend a few kilometers
inland from the ocean (Robinson, 1987). In other
cases, entire islands (e.g., New Zealand) can have
soil δ 34 S values related to those of marine water
(Kusakabe et al., 1976). In order to help distinguish
between the consumption of marine resources
and proximity to the ocean in an archaeological
population, δ 13 C and δ 15 N measurements must
be made in conjunction with δ 34 S values.
The field of study where sulphur isotopic
analysis may have its greatest impact is in the
detection of residence and migration within a
population. Geochemical research indicates that
δ 34 S values are heavily dependent on geographical
location, which is a reflection of the local
geology and atmospheric sulphur composition
of the area (Faure, 1977; Krouse & Herbert 1988;
Brownlow, 1996). This geographically distinct
δ 34 S isotopic signature was used in conjunction
with δ 15 N measurements to assign the origin
of milk samples from alpine regions in Europe
(Rossmann et al., 1998). The study found that
the δ 34 S values obtained from the milk protein
casein were similar to the δ 34 S values of soils
from the area in which the cattle were grazed. In
addition, cattle that had grazed on soils that were
similar in geological age had similar milk δ 34 S
values. Krouse & Herbert (1988) also observed
Copyright  2003 John Wiley & Sons, Ltd.
39
a large variation in δ 34 S from migratory moose
in the Canadian Arctic, which they attributed to
variations in the local geology along the migration
routes. Katzenberg & Krouse (1989) conducted a
study of the potential of δ 34 S and δ 13 C isotopes
to discriminate between modern humans from
various geographical locations. They obtained
human hair samples from five different countries
(Brazil, India, Japan, Canada, and Australia) and
plotted the δ 34 S values versus the δ 13 C values.
While there was some overlap in the values, it was
possible to distinguish among the individuals from
different geographic regions, and they argued
that this type of analysis held promise as a
tool for identifying geographic origin in human
forensic studies.
Applications of sulphur isotopes to
archaeological material
There have only been a handful of applications
of sulphur isotope analysis in archaeology as
compared to the large body of data from
carbon and nitrogen isotope analysis. While
the importance of the information obtained
(or obtainable) from δ 34 S measurements was
realized by H.R. Krouse and others 15 years
ago (Krouse et al., 1987), the large sample size
needed and laborious methods employed for
organic samples limited the use of sulphur in
archaeological studies. In the past few years,
many of these problems have been solved and
it is now possible to conduct isotopic sulphur
analysis by continuous flow isotope ratio mass
spectrometry (CF-IRMS) (Giesemann et al., 1994;
Morrison et al., 2000). The method entails minimal
preparation (samples are placed in tin boats and
combusted) and the procedure is automated so
that many measurements can be made in a single
run. In addition, due to the decrease in the
number of extraction procedures, there is less
chance for fractionation during pretreatment. The
greatest advantage for archaeological research is
the reduction in sample size to ca. 10 mg of
purified bone collagen for a single measurement.
Using ‘traditional’ methods of sulphur isotope
analysis, Macko et al. (1999) measured δ 34 S values
of hair (which has a much higher S content
than bone) from Egyptian and South American
Int. J. Osteoarchaeol. 13: 37–45 (2003)
40
mummies. They found significant differences in
δ 34 S values from inland Egyptian and coastal
South American mummies. The majority of the
coastal South American mummy samples had high
δ 34 S values (ca. 15‰), which could reflect both a
marine diet, as well as a coastal location (due to
the sea spray effect). Analysis of δ 13 C and δ 15 N
values confirmed that high δ 34 S values were due at
least in part to consumption of marine food. The
inland Egyptian samples had δ 34 S values less than
10‰, which they argue is indicative of their more
inland location. A similar pattern of inland/coastal
differences in South American mummy samples
was found by Iverson et al. (1992).
For archaeological bone collagen (rather than
hair), the first δ 34 S values were reported by Leach
et al. (1996) on a collection of human and animal
remains from several South Pacific archaeological
sites spanning the last 1000 years in age. This
pioneering study found a definite marine δ 34 S
signal in humans and fauna that subsisted on
marine protein and resided at coastal locations.
They also used δ 34 S values to distinguish between
a European who lived and died on the South Island
in the 19th century (+2‰), and a local Morori
who lived and died on the Chatham Islands (+14
to +17‰), thereby confirming the possibility
of detecting immigrants. In addition, this study
demonstrated that there is sufficient sulphur in
archaeological bone collagen for isotopic analysis,
and that the loss and degradation of methionine
had not compromised the integrity of the sulphur
isotopic composition.
The first measurements conducted on small
samples of ancient bone collagen from European
archaeological sites were by Richards et al. (2001).
Using a Europa continuous flow isotope ratio mass
spectrometer, 27 collagen samples ranging in age
from ca. 6500 BC to 1300 AD were analysed
from nine archaeological sites. The samples were
selected to determine how bone δ 34 S values were
related to archaeological and isotopic indications
of marine food consumption. The results showed
that all of the collagen samples obtained from
coastal regions had a clear δ 34 S marine signature
although only some of these had a marine δ 13 C
signal (indicating marine food diets). These data
further support the notion that for individuals
living close to the ocean, δ 34 S values alone are not
reliable indicators of marine protein consumption
Copyright  2003 John Wiley & Sons, Ltd.
M. P. Richards et al.
(unless coupled with δ 13 C or δ 15 N measurements)
as a result of the sea spray effect. Measurements of
human bone collagen samples from inland regions
of England and the Ukraine did not have δ 34 S
marine signals but were consistent with predicted
local sedimentary sulphate values.
In addition to archaeological human collagen
measurements, δ 34 S values were obtained from
modern faunal collagen collected from around
the UK. The modern samples from England had
low δ 34 S values that were attributed to anthropogenic sulphur pollution. The only modern δ 34 S
measurements that resembled those from archaeological materials were from less polluted areas of
coastal North Wales and the interior of northern
Scotland. Thus, it seems that archaeological material could provide an effective source from which
to obtain baseline δ 34 S measurements for the
determination of the amount of sulphur pollution
within a region.
Sulphur isotope fractionation in
mammals
It is necessary to establish the degree, if any,
of fractionation between dietary and body tissue
δ 34 S values, as well as an understanding of the
fractionation when geological sulphur enters the
biosphere. In marine, freshwater, and terrestrial
plants, there is only a slight isotopic fractionation
during sulphate incorporation and reduction.
Plants are typically depleted in 34 S by ca.
1.5‰ relative to their sulphate sources (Trust
& Fry, 1992). Like plants, there seems to be
little isotopic fractionation of sulphur in animals,
although there are few published studies on
this topic. Feeding experiments conducted on
gypsy moth caterpillars found a trophic level
shift in δ 34 S of +1.3‰, and brook trout had
a similar δ 34 S enrichment of +1.2 to +1.4‰
(Peterson et al., 1985). In a study of muscle tissue
and hair from bears and other animals from
Yellowstone National Park and British Columbia,
Kester et al. (2001) observed a slight depletion in
δ 34 S between consumer and food source. To date
there has only been one controlled feeding study
that has measured the δ 34 S isotopic fractionation
between the diet and tissues of mammals. This
study used pigs fed isotopically known diets
Int. J. Osteoarchaeol. 13: 37–45 (2003)
Sulphur Isotopes in Palaeodietary Studies
(acorns or feed), and measured the liver δ 34 S
values from these animals (González-Martín et al.,
2001). The results indicated that there was little
difference between dietary and liver δ 34 S values.
Katzenberg & Krouse (1989) have published the
only study that attempts to examine the amount of
fractionation between diet and tissue in humans.
They measured animal feed, meat, milk, eggs
and human hair from a Hutterite community in
Calgary, Canada. Since Hutterites tend to make
their own food rather than consume processed
food, they were viewed as an approximate closed
system. The δ 34 S values from animal feed and
human food items ranged from 0 to +5‰, whereas
the human hair samples had values near +3‰.
While the exact magnitude of the fractionation
was not obtained, this study demonstrated that it
was small in comparison to the δ 34 S values of the
diet. In addition, analysis of kidney stones and
other tissue samples from humans such as hair,
nails, blood and urine found that there was little
δ 34 S variation (1 to 2‰) among tissues of the
same individual (Krouse et al., 1987).
Sulphur isotope fractionation in
modern horses on a controlled diet
Here, we present the first results from our ongoing study to measure the degree of diet-tissue
δ 34 S fractionation in mammals in a controlled
feeding experiment, undertaken as part of the
larger Stable Isotope Biology (SIB) project, which
is a joint venture of Brigham Young University
and the University of Utah. For this study we
fed two adult horses (the male Dandy and the
female Sassy) three different controlled hay diets
over a period of nine months. The horses were
housed in a covered enclosure and had no access
to other food sources during the period of the
experiment. Temperature changed dramatically
during the experiment, but was not correlated
with stable carbon, nitrogen, or sulphur isotope
values (Sponheimer, unpublished data). Neither
horse was reproductively active during the course
of the experiment. Hay and water were provided
ad libitum. The study began with both horses on a
local Utah grass hay diet (Bromus inermis), a C3 plant
(δ 34 S ≈ 10.8‰) with 9% crude protein. They
only stayed on these initial diets for seven weeks,
Copyright  2003 John Wiley & Sons, Ltd.
41
however, as they had been consuming a similar
local hay for the previous several years, so they
were largely equilibrated with the diet before the
advent of the study. After this acclimation period,
the horses were switched to an isonitrogenous
grass hay from near the California/Mexico border
(Cynodon dactylon), a C4 plant, that had a different
δ 34 S value (δ 34 S ≈ −1.9‰). They remained on
this diet for a period of 21 weeks, which was more
than ample time for diet-hair isotope equilibration
in nitrogen (Sponheimer et al., 2002). Both horses
were then switched to a high-protein (19% crude
protein) local Utah alfalfa hay (Medicago sativa),
a C3 plant, with a sulphur isotope composition
that was nearly identical to that of the initial local
hay (δ 34 S ≈ 10.5‰). They remained on this diet
for another 19 weeks until the completion of the
experiment, at which point tail hair was obtained
from each individual.
While many strands of tail hair were obtained
from each individual, we only present data from
one strand for each individual in this paper. The
tail hairs of Dandy and Sassy were pre-treated
with a 2 : 1 chloroform:methanol soak for 6 h
at room temperature, after which the samples
were rinsed in deionized water and then dried at
approximately 40 ° C overnight. Individual hairs
were cut into 1.5 cm sections starting at the skin
end (sample number 1) for δ 34 S measurement
(Table 1). Sample pretreatment was undertaken
at the stable isotope laboratory at the Department
of Archaeological Sciences, University of Bradford, UK, and isotope measurements were made
at Iso-Analytical, Cheshire, UK. Each hair strand
had approximately 4–5% sulphur, and the reproducibility on seven measurements of the standard
‘IAEA-S’ and seven measurements of the standard
‘NBS 1577A’ were between ±0.2–0.3‰(1σ ).
As can been seen in Figure 1, there are clear
trends in both horses. The data from Dandy are
clearer, because Dandy’s hair grew more slowly
and all three diets are in evidence. In contrast,
the 18 cm analysed for Sassy were not sufficient
to capture the initial experimental diet, thus only
the final two diets are in evidence. For Dandy, for
hair sections older than section nine (section ten
and greater), when the experiment began, we can
see a clear equilibration with local hay δ 34 S. After
this section there is a sharp decrease in δ 34 S,
indicative of the change from the local Bromus hay
Int. J. Osteoarchaeol. 13: 37–45 (2003)
42
M. P. Richards et al.
Table 1. δ 34 S and %S values for tail hair segments of two horses, Dandy and Sassy as well as three types of feed.
Errors on the δ 34 S measurements are ±0.3‰(1σ )
Sulphur content
(%)
δ 34 S
(‰)
Sample number
MSS Dandy 1
MSS Dandy 2
MSS Dandy 3
MSS Dandy 4
MSS Dandy 5
MSS Dandy 6
MSS Dandy 7
MSS Dandy 8
MSS Dandy 9
MSS Dandy 10
MSS Dandy 11
MSS Dandy 12
MSS Dandy 13
MSS Dandy 14
MSS Dandy 15
MSS Dandy 16
MSS Dandy 17
MSS Dandy 18
MSS Dandy 19
MSS Dandy 20
MSS Dandy 21
MSS Dandy 22
MSS Dandy 23
MSS Dandy 24
MSS Dandy 25
MSS Dandy 26
MSS Dandy 27
MSS Dandy 28
MSS Dandy 29
4.74
4.78
4.45
4.20
4.40
4.68
4.88
4.65
4.69
4.89
4.64
3.73
4.50
4.34
4.06
4.12
4.10
4.34
4.27
4.42
4.56
4.45
4.22
4.50
4.49
4.29
4.47
4.44
4.28
6.00
9.15
8.97
8.50
7.37
3.05
1.90
2.34
3.60
3.46
10.31
11.02
10.33
9.95
10.07
10.06
10.41
9.85
9.56
9.18
8.72
8.62
9.03
11.36
10.89
10.52
9.39
8.97
8.62
MSS Dandy 30
MSS Dandy 31
MSS Dandy 32
MSS Dandy 33
MSS Dandy 34
MSS Sassy 1
MSS Sassy 2
MSS Sassy 3
MSS Sassy 4
MSS Sassy 5
MSS Sassy 6
MSS Sassy 7
MSS Sassy 8
MSS Sassy 9
MSS Sassy 10
MSS Sassy 11
MSS Sassy 12
MSS Sassy 13
MSS Sassy 14
Medicago sativa (C3 )
Medicago sativa (C3 )
Medicago sativa (C3 )
Cynodon dactylon (C4 )
Cynodon dactylon (C4 )
Cynodon dactylon (C4 )
Bromus inermis (C3 )
Bromus inermis (C3 )
Bromus inermis (C3 )
δ34S
Sample number
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
Sulphur content
(%)
δ 34 S
(‰)
4.26
4.31
4.29
4.37
4.52
4.34
4.60
4.54
3.98
4.46
4.23
4.06
4.74
4.29
4.38
4.39
4.39
4.46
4.71
0.15
0.15
0.15
0.58
0.91
0.70
0.28
0.28
0.26
8.45
8.62
8.72
9.18
9.04
6.29
5.19
8.69
9.47
8.68
8.25
7.93
9.28
5.45
1.96
1.28
1.52
1.56
2.34
10.71
10.76
10.02
−2.00
−1.82
−1.79
10.58
11.18
10.51
Dandy
Bromus inermis C3
Sassy
Dietary change reflected in hair
Medicago sativa C3
Cynodon dactylon C4
Time
Most recent hair
0
2
4
6
8
10
12
Scalp
Older hair
14
16
18
20
22
24
26
28
30
32
34
36
Hair section (1.5 cm each)
Figure 1. δ 34 S values of tail hair segments from the horses Dandy and Sassy. δ 34 S values of diets are indicated on the graph.
Errors on the δ 34 S measurements are ±0.3‰ (1σ ).
Copyright  2003 John Wiley & Sons, Ltd.
Int. J. Osteoarchaeol. 13: 37–45 (2003)
Sulphur Isotopes in Palaeodietary Studies
to the California/Mexico Cynodon hay. The hair
δ 34 S reaches its lowest point at segment 6, after
which it dramatically increases when the diet was
changed to local Utah Medicago hay. Eventually,
the hair δ 34 S values became close to those of the
Medicago hay, but were still depleted by about
1‰. An almost identical pattern is observed
for Sassy, although the hair records only the
Cynodon hay diet and the subsequent switch to the
Medicago hay.
Both data sets clearly demonstrate large changes
in hair δ 34 S values as the horses’ feeds were
switched through time. Interestingly, however,
these data seem to indicate that diet-hair fractionation is not constant on all diets. Most
conspicuously, it appears that when the diets
were switched to the Cynodon hay, their hair was
enriched in 34 S by nearly 4‰ (but see discussion below). When switched to the local Medicago
hay, however, hair values approached, but never
reached dietary δ 34 S values, suggesting a diet hair
fractionation of about −1‰ for these horses. We
only have data from Dandy on the local Bromus hay, and hair values and dietary δ 34 S values
appear nearly identical when on this hay. Thus,
there appears to be a large diet-hair fractionation
when horses are consuming Cynodon hay, but relatively little fractionation when they are on either
of the local hays. This change in fractionation
is not associated with temperature changes during the course of the experiment (Sponheimer,
unpublished data). One possible explanation for
this patterning is that, even though the Cynodon
and Bromus hays were isonitrogenous, digestible
protein was lower for the Cynodon hay than either
of the local hays. There is some support for this
as both horses lost body mass when on this diet.
This might have led to increased recycling of their
body proteins, which had been formed while on
a local diet.
The sulphur in hair is primarily derived from
cysteine and methionine. As cysteine is a nonessential amino acid, it can be synthesized from
other amino acids, serine and methionine. The
unusually high diet-hair fractionation seen during
the consumption of Cynodon hay could have been
the result of contributions from endogenous 34 Senriched sulphur atoms from methionine to form
cysteine. If this explanation is correct, it means
that the true diet-hair equilibrium would have only
Copyright  2003 John Wiley & Sons, Ltd.
43
been obtained once all of the horses’ metabolically
active tissues had turned over. In contrast, when
on nutritionally adequate diets there appears to
be a direct routing of the sulphur in cysteine and
methionine from the diet to hair protein.
While this study provides new experimental
data for the magnitude of diet-hair fractionation of
sulphur isotopes in large mammalian herbivores,
it is apparent that it only begins to address this
question for mammalian herbivores in general,
much less for omnivorous or carnivorous species.
Future studies are needed in which multiple
taxa are raised on isotopically homogenous
diets from birth, which would eliminate the
protein recycling that has proven problematic
here. Ideally, such studies would also test the
potential impact of feeding animals diets with
differing amounts of protein, or more specifically,
different amounts of sulphur-containing amino
acids such as cysteine and methionine. It is
likely that diets that are deficient in sulphur
containing amino acids will have very different
fractionation patterns (endogenous synthesis)
compared to diets that have adequate levels of
cysteine and methionine (direct routing from diet
to tissue).
Discussion and conclusions
We have presented a review of sulphur isotopes
in the environment and their application to
archaeological studies. In addition, we have
argued that δ 34 S measurements can provide
supplementary palaeodietary evidence to δ 13 C
and δ 15 N measurements, and have the potential
to identify migration and residence locality in the
archaeological record. Finally, we have presented
the results of one of the few controlled mammal
feeding experiments and observed that there is
minimal (−1‰) δ 34 S fractionation between diet
and consumer hair on a nutritionally adequate
C3 diet. On a C4 diet that is likely to be
low in digestible protein we observed a δ 34 S
fractionation of +4‰, which could be the result of
sulphur recycling from body proteins in addition
to dietary sulphur intake. Future studies on animals
that are raised on known δ 34 S diets from birth are
needed to support or challenge the fractionation
patterns seen in this study.
Int. J. Osteoarchaeol. 13: 37–45 (2003)
44
Acknowledgements
We would like to thank Gundula Müldner for
help with sample preparation. We are indebted
to Steve Brookes and Ian Begley at Isoanalytical
for measuring the δ 34 S values and to Paul Koch
for helpful suggestions and editing. We also want
to thank Thure Cerling, Denise Dearing, Jim
Ehleringer, Jordan Hammer, Yasmin Rahman, and
Bev Roeder.
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