PIIAEO
ELSEVIER
Palaeogeography, Palaeoclimatology,Palaeoecology 126 (1996) 1-14
The isotopic composition and diagenesis of human bone from
Teotihuacan and Oaxaca, Mexico
Hilary Le Q. Stuart-Williams ", Henry P. Schwarcz a, Christine D. White b,
Michael W. Spence b
"Department of Geology, McMaster University, 1280 Main Street West, Hamilton, Ontario, LSS 4M1, Canada
b Department of Anthropology, University of Western Ontario, London, Ontario, N6A 5C2, Canada
Received 24 July 1995; revision 20 September 1995; accepted 2 May 1996
Abstract
We analyzed archaeological human bone from Teotihuacan and Oaxaca, dating from about 300 BC to 750 AD to
distinguish ethnic groups within Teotihuacan using oxygen isotopes. Sixty-eight analyses of bone phosphate 6180 were
made of 64 individuals. In addition to oxygen isotopic analysis, the bones were examined using FTIR spectra, with
some additional DNAA and ICP-MS analyses. Little change occurs in the bone apatite until the amount of collagen
(as combustible organics) has been reduced considerably, when the bone becomes softer and FTIR crystallinity
increases. The 6180 of the phosphate (~p) appears to be unaltered even after extensive diagenesis and, probably,
solution. On FTIR plots the relative area of the carbonate peak to the main phosphate peak decreases with diagenetic
level. The bones absorb some metals rapidly after burial, for example uranium, which then leach out as diagenesis of
the bone apatite progresses. Other metallic elements increase irregularly in concentration as alteration proceeds.
Keywords." stable isotopes; O-18; burial diagenesis; phosphate composition; Mexico Oaxaca
I. Introduction
The isotopic composition of a m a m m a l ' s body
fluids is controlled by its total water intake
(Longinelli and Peretti Padalino, 1980; Luz et al.,
1984; Luz and Kolodny, 1985; Z i m m e r m a n and
Cegla, 1973) and the animal's phosphate reservoir
is equilibrated with the fluids at body temperature
during energy transfer processes (such as ATP
synthesis and destruction) and bone mineral precipitation (Levinson et al., 1987; Longinelli, 1984;
Longinelli and Peretti Padalino, 1983; Luz et al.,
1984). Once the phosphate is fixed as carbonate
hydroxylapatite (dahllite) in bone the isotopic
0031-0182/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved
PII S0031-0182 (96)00066-1
values remain relatively unchanged except for the
effects of remodelling.
The oxygen isotopic composition of h u m a n
body fluids is dominantly controlled by the 6180
of drinking water derived from meteoric water,
with smaller contributions from food water and
food metabolism. The composition of meteoric
water reflects the cooling history of the air mass
which carried the moisture (Dansgaard, 1954,
1964), modified by an amount effect and topographic effects, such as the distance travelled over
land and passage over ranges of mountains. In
most areas a good relationship exists between the
mean annual temperature and the 6180 of local
2
ILL. Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1 14
meteoric water (Smw)(Dansgaard, 1964). This isotopic signature is passed on to plants and animals
that directly or indirectly take up the water
(Longinelli, 1973, 1984; Longinelli and Peretti
Padalino, 1983; Luz et al., 1984) although the
moisture may be enriched in 1sO by evaporation
(Ayliffe and Chivas, 1990; D'Angela and
Longinelli, 1990; Luz et al., 1990).
In a population of mixed origin such as at
Teotihuacan it may be possible to identify immigrant groups using isotopic variation in their
bones, if the isotopic signature of their area of
origin has not been destroyed by remodelling. We
analyzed sixty-four humans from Teotihuacan and
Oaxaca to identify separate groups living in
Teotihuacan and provide comparative material
(Table 1).
The ruins of Teotihuacan are on a relatively flat
plain about 30 km north of Mexico City (Fig. 1).
At the height of Teotihuacan's development ca.
500 AD it was the largest city in pre-Columbian
America. From about 100 BC to 200 AD it
expanded very rapidly (Cowgill, 1992), drawing
people from areas as distant as the Oaxaca valley,
450 km to the south. Some of the immigrant
groups maintained an ethnic identity for a considerable period: the Oaxacan barrio (Tlailotlacan)
lasted as a distinct entity from 200 to 750 AD. In
addition other groups were buried in selected areas,
for example 200 victims of unknown ethnic affiliations were sacrificed over a very brief period of
time and interred, early in the history of
Teotihuacan, in the environs of the Pyramid of
Quetzalcoatl. Teotihuacan samples for this study
came from the Merchant's Barrio, mass burials
around the Temple of Quetzalcoatl, a burial from
the Tlamimilolpa site just north of Merchant's
Barrio, a group of Tlajinga burials and
Tlailotlacan. The Temple of Quetzalcoatl samples
are from the east and south sides of the pyramid,
where they were covered by concrete floors, and
from burials under the structure itself. Samples
were also obtained from Monte Alban in the
Valley of Oaxaca, as the inhabitants of Tlailotlacan
are believed to have come from somewhere in the
Valley of Oaxaca. The soils of both areas are
similar, being composed of Cenozoic lavas and
pyroclastic deposits decayed in situ.
2. Analytical procedures
Samples of about 100 mg of cortical bone (primarily ribs from Teotihuacan, a variety of bones
from Oaxaca) (Table 1) were processed by either
of two in-house methods (Stuart-Williams and
Schwarcz, in prep.): the first uses lead phosphate
and barium phosphate intermediate products to
produce silver orthophosphate; the second method
uses only a lead phosphate intermediate. The bone
is dissolved in 3 molar acetic acid and phosphate
is initially extracted as lead orthophosphate.
Organics are destroyed by heating to 95°C in a
water bath with 6 molar nitric acid and 30%
hydrogen peroxide. The final solution (after potentially interfering cations have been removed)
contains only ammonium, some sulphate
and phosphate in 0.1 M nitric acid. The acid is
neutralised with potassium hydroxide, and silver
orthophosphate is precipitated by ammonia volatilization (Firsching, 1961). These processing
methods do not decrease the precision of the
phosphate analyses when applied to soluble standards, although improper silver orthophosphate
precipitation can produce errors of 2-3%0 (StuartWilliams and Schwarcz, 1995).
Oxygen was liberated from the silver orthophosphate using a technique of polymerization at high
temperature in a bromine atmosphere (StuartWilliams and Schwarcz, 1995) or by fluorination
in bromine pentafluoride (Tudge, 1960). The
results are reported as ~lSO values with respect to
the SMOW scale. When the whole of a precipitate
is analyzed the precision (cy) is 0.06%0 using the
bromine method but when aliquots of a precipitate
are used the best precision (cy) falls to 0.12%o, as
a result of heterogeneity within the silver phosphate. The analytical precision using bromine
pentafluoride to liberate oxygen from the phosphate is similar. There is heterogeneity within
single bones which was found to be as great as
0.4%o. The Oxygen was converted to carbon dioxide with a heated carbon rod and analyzed using
a VG SIRA isotope ratio mass spectrometer.
Fourier transform infrared spectra were made
using a Bio-Rad FTS 40 infrared photospectrometer. Pellets for analysis consisted of 2 mg of bone
with 200mg of thoroughly dried potassium
H. L. Q. Stuart- Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1 14
3
Table 1
Isotope, crystallininity and site data for samples
Sample
no.
Burial
01
542
02
576
04
578
05
585
10
153-155
23
25
28
30
31
34
36
39
42
45
46
47
49
50
34
6
61
39
21a
11
17b
28a
42a
5
46a
56
1
2-G
51
2-N
52
4-A
53
4-L
54
54
55
56
57
58
59
61
62
63
64
65
66
67
68
69
73
74
75
76
79
5-f
5-f
5-H
6-1
6-D
10-A
10-D
ll-C
12
13
14-C
14-F
15
7-P
7-M
F322
F345N
F133
F133
F130
F408
Description
Merchants' barrio,
adult
Merchants' barrio,
adult
Merchants' barrio,
adult
Merchants' barrio,
adult male
Merchants' barrio,
child
Tlamimilolpa, adult
Tlajinga, female
Tlajinga, male
Tlajinga, 6 month old
Tlajinga, 2 yr old
Tlajinga, male
Tlajinga, female
Tlajinga, 3-4 yr old
Tlajinga, male
Tlajinga, perinatal
Tlajinga, female
Tlajinga, male
Quetzal', south, male
Quetzal', interior,
female
Quetzal', interior,
female
Quetzal', interior,
male
Quetzal', interior,
male
Quetzal', east, male
Quetzal', east, male
Quetzal', east, male
Quetzal', east, male
Quetzal', east, male
Quetzal', east, female
Quetzal', east, female
Quetzal', east, female
Quetzal', interior, male
Quetzal', interior
Quetzal', interior, male
Quetzal', interior, male
Quetzal', interior, male
Quetzal', south, male
Quetzal', south, male
Tlailotlacan
Tlailotlacan, child
Tlailotlacan, child
Tlailotlacan, child
Tlailotlacan, child
Tlailotlacan, infant
Period
51so
FTIR
crystallinity
CO3/PO 4
ratio
EX
13.9
3.2
0.61
"
14.2
3.7
0.37
"
14.7
3.5
0.39
"
14.4
3.9
0.24
LX
16.6
3.1
0.27
EX
ET1
LX
EX
LX
LT1
LX
LT1
LTI
Met
EX
ET1
Ca. 200 AD
"
15.7
14.4
15.2
14.8
14.6
15.0
14.7
14.6
14,3
15.1
14.9
14.6
15.8
16.3
3.1
4.3
3.8
2.9
3.6
3.9
3.5
3.8
3.7
3.6
3.4
4.1
4.3
4.6
0.58
0.25
0.52
0.35
0.33
0.25
0.25
0.28
0.31
0.47
0.30
0.34
0.24
0.18
"
16.2
4.1
0.19
15.6
4.6
0.20
"
15.5
4.4
0.16
"
15.4
15.4
14.2
13.4
14.1
14.3
14.1
13.6
15.9
16.4
17.7
16.6
16.3
14.6
14.2
14.4
16.4
15.5
16.5
15.3
15.2
3.9
3.9
4.1
3.8
3.7
3.6
4.0
3.7
4.5
4.7
4.3
4.5
3.6
3.7
4.0
4.8
3.5
3.4
4.3
4.2
3.8
0.31
0.31
0.29
0.35
0.38
0.48
0.26
0.39
0.19
0.10
0.23
0.23
0.26
0.32
0.28
0.35
0.45
0.35
0.42
0.56
0.44
Pre 200 AD
Ca. 200 A D
"
4
6
15
20?
19
4
4
H.L.Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
Table 1 (continued)
Sample
no.
Burial
Description
80
83
88
F409
F381
F 113
89
90
oax1949
oax1959
oax1960
oax1965
oax1968
oax1969
oax1973
oax1975
oax1999
oax2011
oax2021
oax2025
oax2034
oax2045
oax 2048
oax2148
South tomb
F372
Tomb 120
Tomb 113
Tomb 111
Bur. 4
Tomb 2
Bur. 17
Tomb 63
Tomb 121
Bur. Vi-7
Bur. IV-32
Tomb 21
Bur. IV-36
Tomb 77
Bur. V-20
Tomb 58
Tomb 153
Tlailotlacan
Tlailotlacan
Tlailotlacan,
young female
Tlailotlacan
Tlailotlacan
OAX 1949
OAX 1959
OAX 1960
OAX 1965
OAX 1968
OAX 1969
OAX 1973
OAX 1975
OAX 1999
OAX 2011
OAX 2021
OAX 2025
OAX 2034
OAX 2045
OAX 2048
OAX 2148
Site or period
Mean
~180
Merchant's Barrio
Tlamimilolpa
Tlajinga
Quetzalcoatl (interior)
Quetzalcoatl (east side)
Quetzalcoatl (south side)
Tlailotlacan
Monte Alban (Oaxaca)
14.76
15.7
14.75
16.3
14.2
14.4
15.2
13.0
8180
FTIR
crystallinity
CO3/PO 4
ratio
20
15.0
15.6
14.1
4.7
4.9
3.9
0.27
0.27
0.30
20
?
IV
II
I
Ii
I
IIIa
V
IV
IIIa
V
lIIa
IIIb
II
V
IV
IlIb
15.1
14.2
13.2
13.1
13.4
12.5
12.2
14.0
13.9
13.7
12.4
12.5
12.5
12.9
12.7
13.2
13.9
12.1
4.1
4.7
4.5
3.3
3.0
3.9
3.l
3.6
4.4
4.3
3.4
3.4
4.1
3.3
4.4
3.7
4.8
5.3
0.48
0.43
0.21
0.53
0.82
0.40
0.40
0.43
0.17
0.24
0.49
0.33
0.49
0.37
0.24
0.30
0.31
0.27
Standard
deviation
Identifier
Period
Date
_+1.1%0
ET1
TL
LT1
EX
LX
Met
I
II
IIIa
IIIb
IV
V
Early Tlamimilolpa
Tlamimilolpa
Late Tlamimilolpa
Early Xolalpan
Late Xolalpan
Metepec
Monte Alban I
Monte Alban II
Monte Albanm IIIa
Monte Alban IIIb
Monte Alban IV
Monte Alban V
200-350 AD
200 450 AD
350 450 AD
450-550 AD
550-650 AD
650-750 AD
500 200 BC
200 BC 300 AD
300 500 AD
500-750 AD
750 1000 AD
1000 1520 AD
_+0.3%0
_+0.7%0
_+0.6%0
+_0.8%o
_+0.6%0
bromide. The crystallinity index (CI) was calcul a t e d u s i n g p e a k s p l i t t i n g o f t h e PO34 - p e a k s a t
605 a n d 565 c m x ( S h e m e s h , 1 9 9 0 ) . D e l a y e d n e u t ron activation analysis was undertaken using the
McMaster
nuclear
reactor.
Induction-coupled
plasma mass spectrometric analyses were perf o r m e d o n a S C I E X E l a n m o d e l 250. T h e I C P - M S
a n a l y s e s w e r e all n o r m a l i z e d t o c o n v e r t t h e m i n i m u m v a l u e f o r e a c h e l e m e n t i n t h e d a t a set t o z e r o
a n d t h e m a x i m u m t o 10.
Period
4
Bone hardness was estimated from 1 (the hardest
bone) to 5 (the softest bone) on the basis of the
compressive
and
tensile strength of broken
fragments.
B o n e d e n s i t y w a s d e t e r m i n e d o n 75 300 m g
combusted fragments. Water saturated fragments
of bone were floated in glycerine and the weight
reduction
proportional
to their displacement
recorded, assuming a relative density for glycerine
o f 1.27.
tt. L. Q. Stuart- Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
500 Km
Fig. 1. Locationmap of sites studied. T= Teotihuacan, O=
Oaxaca.
The total organic content of the bone, which
ranged from 4.4 to 11.2%, was determined by
weighing a dried sample before and after combustion in air at 450°C for 4 hours to oxidize the
organics in the highly permeable bone but not
decompose any carbonates, although some polymerization and mass loss of the phosphate may
occur (LeGeros and LeGeros, 1984).
The carbonate and phosphate infrared peak
ratios were calculated from the heights of the
carbonate peak at 1405 cm -1 and the phosphate
peak at 1035 cm -1. The area of each peak was
estimated as the product of the height and the
half-height width. The CO ]- area was divided by
the PO~- area to produce a dimensionless ratio
which removes the majority of analytical variation.
This ratio has been found to correlate with the
CO2 wt% yield obtained by manometry in this
laboratory (Wright and Schwarcz, submitted).
3. Results
3.1. Isotopic analyses
Each archaeologically defined group of burials
had a distinct oxygen isotopic composition
although some overlap was present between the
groups (Table 1, Fig. 2). The Oaxacan samples in
5
particular formed a distinct population which was
isotopically lighter than almost all samples from
the Teotihuacan area apart from the isotopically
lightest Pyramid of Quetzalcoatl sacrifices. There
was no isotopic overlap between the Tlailotlacan
group and the Oaxacan samples. If the samples
from the Temple of Quetzalcoatl are regarded as
two discrete groups then the most scattered populations are the Tlailotlacan and Merchant's
Barrio groups with spreads of about 2.5%0 which
largely result in both cases from isotopically heavy
results from weaned children's bones; the least
scattered are the Tlajinga burials with a scatter of
less than l%0 (Table 1, Fig. 3). There are few
studies of variance of 6p in bone of modern human
populations (Longinelli, 1984; Luz et al., 1984)
but analyses of teeth of known origin (Levinson
et al., 1987) show minimum scatters of 1.7%o in 3
populations of 4 or more samples and a maximum
of 2.3%0. A total variation of about 1.1%0 exists
between different samples from the same bone of
individual laboratory rats (Luz and Kolodny,
1985). From this it would appear that the archaeological data set examined here is rather homogeneous but not essentially dissimilar to a modern
population. Variance in the 6p of humans may
tend to be higher than in many other animals as
a result of the more varied water sources exploited.
As an example of 6p variation in other mammals,
white-tailed deer collected over much of Oklahoma
showed only a 1.7%o spread in 6p, and deer from
southern Ontario only 0.7%o (Luz et al., 1990) but
in the southern half of Alberta, deer show a larger
variance of 4.3%o which may result from very
strong topographic effects and isotopic heterogeneity on the eastern slopes of the Cordillera.
3.2. Crystallinity, softness, density and organic
content
Bone is highly prone to isotopic alteration after
burial due to its small croystallite size (platelets
250-350 ~, wide by 25-50 A thick) and very high
surface area (100-200 m 2 g-l) (Posner et al.,
1984). Oxygen isotopic change is commonly
mirrored by changes in one or more measures
of diagenetic alteration, such as the crystallinity of
the mineral portion. Modern bone typically has
6
H.L.Q. Stuart- Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
18
t
17
_ 4L_____ •
16
0~
-%
15
V
14
v
o
~
v I
v
•
13
D
12
i o
2.5
3.0
i
v
I
D
=
o
[
i
i
3.5
4.0
4.5
FTIR crystallinity index
m
5.0
5.5
Fig. 2. Isotopic distribution of all samples. Bars at right show isotopic spread of groups. No errors are shown as populational diversity
is uncertain. Analytical error is 0.12%o(see text). Maximum inter- and intra-bone variability noted is 0.4%~.
18
18
17
• Child (10)
:¢
17
Tlamimilolpa
~
7~
66-
16
• Merchants'Barrio
o
x
E a s t side of pyramid
•
Pyramid interior
15
7,o 14
14
A
13
i
12
2.5
3.0
i
i
I
3.5
4.0
4.5
FTIR crystallinity index
13
i
5.0
,
12
5.5
i
i
i
3.5
4.0
4.5
FTIR crystallinity i n d e x
3.0
2.5
5.0
5.5
18
17
Oaxaca
=16
•
0 15
c,C
14
--:
•
.
=
''
3,0
C
"
:
Child
16
Tlajinga
(73) •
I Oaxaca
* Child (75)
•
Tlailotlaca n
15
=
13
12
2.5
° "
17
O ~ 18
~
14
13
:
3.5
4.0
4.5
FTIR crystallinity index
12
:
5.0
. . . . .
-
5.5
2.5
-
~-~
'=
3.0
=
=
=
"••
{'
'
:
3.5
4.0
4.5
FTIR crystallinity i n d e x
5.0
5.5
Fig. 3. lsotopic composition of groups. A. Merchant's barrio with outlying value. B. Temple of Quetzalcoatl. Arrow shows possible
diagenetic trend. C. Comparison of indigenous Tlajinga group and Oaxacan samples. D. Comparison of Oaxacan and Tlailotlacan
samples. Key samples are identified.
an initial CI of about 2.8 to 3.0 while this value
tends to increase in ancient bone and most of the
archaeological samples examined had values
between 3.5-4.8 (Fig. 2). As the CI increases the
probability of isotopic alteration becomes greater
but not certain. Apparent crystallinity m a y
increase due to dissolution of the more disordered
outer surfaces of the bone crystallites or recrystallization in a closed system without alteration of the
isotopic value. Despite these reservations, C] is a
useful measure of diagenetic change to compare
with other variables. Sub adult bones (for example
samples 10 and 30) with a high collagen content
were the least recrystallized, which is compatible
with the findings of a number of workers that the
combination of collagen and bone mineral
strengthens and stabilises the bone synergistically
(e.g. Tuross et al., 1989). There is no significant
correlation of changing 6p with increasing C! in
any of the groups of bone samples (Fig. 3), with
the possible exception of the bones from the
Temple of Quetzalcoatl. The correlation between
increasing softness/friability and CI is good
(Fig. 4a) which parallels findings from skeletons
in more arid regions (Sali6ge et al., 1995). The
softest bones with the highest CI have a powdery
appearance compatible with advanced collagen
loss.
H.L.Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoeeology 126 (1996) 1-14
~
h
-'°
o
, ~
2.5
Q
..Q
_Q
E
8
3.0
=
,~
t
I~
3.5
4.0
FTIR Crystallinity Index
A
4.5
11
58 o
"
67
10
9
- ~66
8
64e
7
6
5
4
3.4
3.6
3.8
4.0
4.2
4.4
FTIR crystallinity index
5.0
I
B
4.6
5
7
between the CI and 6p of the south/east side and
interior samples, possibly of diagenetic origin,
make it difficult to identify more minor effects.
Bone density was determined on the same subset
of 8 samples. The results varied from a bulk
relative density of 0.55-1.14. g mL -1 for the
cortical bone, indicating very substantial mass loss
during burial. No meaningful correlations with
other variables were detected in the results, perhaps
indicating that initial density variations in the bone
were much greater than any effects resulting from
diagenesis.
The carbonate:phosphate peak ratio analyses of
all 64 samples (Fig. 5a) show that the CO Icontent decreases as recrystallization of the bone
proceeds. Carbonate is flushed from the bones by
groundwater as the volcanic ash-like subsoil is
carbonate deficient. A similar effect is found in
weathering marine phosphorites in low-carbonate
settings (Flicoteaux and Lucas, 1984).
4
4~
C
3
2
1
i
4
5
i
i
i
i
6
7
8
9
% combustible organics
i
10
11
Fig. 4. Physical alteration of bone. A. Bone crystallinity increase
related to loss of strength, r = 0.59 with sample 10, increasing
to r = 0.81 without. B. Crystallinity change as collagen
is removed. Note that sample 68 appears to be an outlier.
C. Structural weakening as collagen is removed. All samples
are from the Temple of Quetzalcoatl, Teotihuacan.
Eight samples were selected from the Temple of
Quetzalcoatl group on the basis of diversity of CI
and 6p to obtain a wide spectrum of possible
diagenetic alteration. Bones from the east side of
the pyramid are different in their CI and 6p from
samples from the south/interior of the pyramid.
The correlation between mass lost (indicating the
percentage of organics remaining from the ancient
bone) and CI (Fig. 4b) is good, r--- 0.59, rising to
0.81 if sample 68 is removed from the data set. In
addition there is a positive correlation between the
ratio of mass lost during combustion and softness
(Fig. 4c) of r = 0.53. Comparisons using 6p were
not meaningful because the great differences
3.3. Elemental analyses
The sample of 8 bones used for density and
organic ratio determinations was also analyzed for
its uranium content using delayed neutron activation analysis (DNAA). Uranium was selected in
particular as being a mobile element very susceptible to changes in pH and Eh. In addition the
analysis could be made rapidly and precisely as
this laboratory routinely performs uranium measurements for dating purposes (Table 2). Samples
from the south side/interior of the pyramid had
lower U contents of 0.21 to 8.03 ppm while values
for samples from the east sides ranged from 15.76
to 24.20 ppm. The uranium content is negatively
correlated with CI when the group is viewed as a
whole: less altered bone contains more uranium.
This is probably not an artefact of the groupings
as pyramid interior sample 66, which antedates
the pyramid, has a CI similar to the east side
samples and a high uranium content. In most of
the samples uranium content is positively correlated with the total organic content (Fig. 5b). The
exception is sample 68 which has the highest
uranium content but a low organic ratio and an
intermediate CI. This sample is anomalous in most
of the correlations of organic content and may
8
H.L.Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1 14
1.0
0.8
0.6
d 0.4
0
x
Oaxaca
o
Teotihuacan
o o
o
-
.
.*x..--
x
IJ.*~
.
0.2
o
0.0
I
2.5
3.0
I
I
I
3.5
4.0
4.5
A
5.0
FTIR crystallinity i n d e x
25
= Sample68
20
E
en
~_
15
B
10
5
5
6
7
8
9
10
% combustible organics
11
12
Fig. 5. A. The amount of carbonate decreases as bone crystallite change progresses. B. Uranium appears to leach out as the organics
are removed.
Table 2
Physical properties, isotopic composition and uranium content of selected samples from the Temple of Quetzalcoatl
Sample
no.
Burial
81+O
FTIR crystallinity
index
% combustible
organics
Bulk
density
Uranium
(ppm)
Softness
50
53
58
62
64
66
67
68
Q2-G
Q4-L
Q10-A
Q12
Q14-C
Q15
Q7-P
Q7-M
16.13
15.40
14.28
15.86
17.70
16.12
14.59
14.24
4.59
4.44
3.55
4.52
4.30
3.61
3.68
4.04
5.8
7.9
10.6
8.7
7.4
9.0
11.2
4.4
0.96
0.67
0.91
0.55
1.14
0.83
1,02
0.64
0.34
0.28
18.33
0.57
0.21
8.03
15.76
24.20
3
4
2
5
5
2
2
4
have been altered u n d e r u n u s u a l c o n d i t i o n s ,
a l t h o u g h s a m p l e 67 f r o m a similar setting is similar
in its p r o p e r t i e s to o t h e r b o n e s in the study.
F o u r o f the eight b o n e s (samples 50, 53 a n d 64
f r o m the p y r a m i d i n t e r i o r a n d s a m p l e 68 f r o m the
s o u t h side) f r o m the subset used for D N A A were
also e l e m e n t a l l y a n a l y z e d b y I C P - M S . This
was also i n t e n d e d as a p i l o t s t u d y f r o m w h i c h
o n l y tentative conclusions can be d r a w n .
C a l c i u m : p h o s p h o r u s r a t i o s a p p e a r to fall as recrys-
tallization proceeds. A n a l y s e s o f some o f the elem e n t s are p r e s e n t e d g r a p h i c a l l y b e l o w ( F i g . 6),
m a n y o f which show no clear r e l a t i o n s h i p between
c o n c e n t r a t i o n in the b o n e a n d crystallinity as a
p r o x y m e a s u r e o f diagenesis. S a m p l e 68 f r o m the
east side o f the p y r a m i d has the lowest C I a n d is
d e p l e t e d in all elements a p a r t f r o m u r a n i u m ,
b a r i u m , tin a n d tellurium. F r o m this limited invest i g a t i o n it was a p p a r e n t t h a t a t t e m p t s to correlate
degree o f b o n e diagenesis m o n o t o n i c a l l y with trace
H.L.Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
11
I3-
u
S a m p l e 64 I I
9
S a m p l e 50
7
v-
5
3
•
Sample 68
•
I
4.0
4.1
I
I
I
I
4.2
4.3
4.4
FTIR crystallinity index
==MnoC0
eNi
'VRb o Y
~La
•Ce
11
7
5
Sample 5 3 ~
I
4.1
I
I
B
I
I
4.2
4.3
4.4
FI-IR crystallinity index
• Fe o Cu • M o v
• Dy = Y b
N
APt
Sample 64 •
4.0
CL
4.6
m
o Sample 68
3
11
4.5
• Sample 5 0 •
i
9
O
Z
_u
A
Sample 53
O
Z
~u_
.N
9
zLu
4.5
4.6
Gd • Cd o Nd
•TI
* Th
o Sample 68
9'
Sample 50
7
5
3¸
Sample 53 •
C
Sample 64 •
O
Z
I
4.0
4.1
I
I
z
I
4.2
4.3
4.4
FTIR crystallinity index
o B a v S n roTe z u
I
4.5
4.6
Fig. 6. Elemental analyses of Teotihuacan bones. Sample 68 is from south of the Temple of Quetzalcoatl and is unlike the other
samples from the pyramid interior. It is depleted in the elements from diagram A, has comparable amounts to B and is enriched
relative to elements in C, including uranium. Plots are normalized with the maximum for each element equal to 10 and the minimum
to zero.
element concentrations at this site would be
unsuccessful.
4. Discussion
4.1. Climate and regional variability
The 8180 analyses from Teotihuacan and
Oaxaca show little overlap of values: Teotihuacan
sample groups show 6p ranging from about 14 to
17%0, while 5p for samples from Monte Alban in
the Oaxaca valley ranges from 12 to 14%o. The
isotopic data for Teotihuacan and Oaxaca are
relatively opposite to initial expectations based on
altitude and proximity to the ocean: 6p for Oaxaca
is <6p for higher and cooler Teotihuacan. The
average for the Oaxacan bones with a mean annual
temperature of 20.5°C at an altitude of about
1900 m is 13%o, whereas Teotihuacan is about
10
ILL. Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1 14
500 m higher with a mean annual temperature of
15.6°C but mean 8p for the Tlajinga sample is
approximately 14.75%0. The climate of Oaxaca is
hot and dry with an annual rainfall of about
650mm with semi-desert vegetation (Tamayo,
1962), whereas Teotihuacan is temperatehumid with a mean annual rainfall of 746 mm.
Oaxacan 8p should be substantially greater than
Teotihuacan 8p, due to temperature, amount and
altitude effects (Dansgaard, 1954, 1964). Analyses
of well water from the two areas are in general
agreement with the ~p results, although well water
may not be isotopically equivalent to meteoric
water. 8w=-9.8%~, for samples from wells at
Teotihuacan; 8w = - 8 . 9 to - 9 . 8 for well water
samples from the Valley of Oaxaca around Monte
Alban, while 8w = - 1 1 . 5 for the fringes of the
valley. The lighter isotopic value from the elevated
edges of the valley is almost certainly more similar
to local 8mw for Monte Alban as the water in the
valley bottom is more influenced by isotopically
fractionated water from greater distances, including evaporatively enriched river water. Effects of
climate change can probably be ruled out as the
Oaxacan samples come from periods I V , which
more than span the period of the Teotihuacan
samples, with no clear isotopic separation of bones
from different periods. The differences between the
sites must result from the combination of topography and atmospheric circulation patterns. At both
sites the majority of the rain falls during the middle
of the year when the winds are blowing from the
east or northeast (Newell et al., 1972). Water
vapour is transported from the Gulf of Mexico
over the mountains, cooling and condensing as it
travels. Oaxaca is therefore in the rain-shadow of
the Cordillera and receives only precipitation from
clouds already depleted in 180. This is comparable
to the contrast between Holsteinsborg and Sdr.
Stromsfjord in Greenland noted by Dansgaard
(1964): these sites have similar temperatures but
the latter receives precipitation about 10%0 lighter.
This emphasizes the importance of atmospheric
circulation in the control of oxygen isotopic values:
if the dominant weather pattern shifted to westerlies both Teotihuacan and Oaxaca would have very
much increased 8w and their relative isotopic positions would be reversed.
4.2. Examination of possible diagenesis and
diagenetic models
Although the bones do not appear to have
undergone isotopic alteration, as shown by the
lack of a significant correlation between 8p and
CI, a further check was made by calculating the
8p that might be expected for diagenetic phosphate.
Inorganic exchange of oxygen between phosphate
and water is negligible under common burial conditions of archaeological material (Brodskii and
Sulima, 1953) but biological activity and kinetic
effects during solution and reprecipitation may
alter the isotopic signature (Stuart-Williams and
Schwarcz, in prep.), although kinetic isotope effects
may not move 8p toward an equilibrium value. An
average 8so, ,~aterof about --10%0 can be estimated
from the well water data for both sites. The 8180
of phosphate biologically equilibrated with that
water at soil temperature is determined using Eq. 1
(Longinelli and Nuti, 1973a, modified by Friedman
and O'Neil, 1977).
t~"C= 111.4 - 4.3(6p- 8,~ + 0.5)
( 1)
A soil temperature of 15°C for Teotihuacan is
equivalent to the mean annual temperature at
Mexico City 30 km away. This results in diagenetic
phosphate with By=approximately 12%0. The
average temperature at Oaxaca is approximately
20°C, resulting in 8p for equilibrated phosphate of
about 10.8%0. In both cases the 8180 of diagenetic
phosphate equilibrated by biological activity would
be very close to the tested values of the bones. As
a result of the proximity of the diagenetic and
original 6p, equilibration effects (if they occurred)
on the phosphate are expected to be small with a
tendency for the 8p to move toward lower values.
This effect is not statistically verifiable in any of
the groups and isotopic equilibration is unlikely
to be present.
Data from the Temple of Quetzalcoatl when
viewed as a whole appear to display a rapid
increase in 6p as the CI increases, with an initial
8p of about 12%o, determined using the intersection
of a regression line through the two groups of
samples with the crystallinity index at 2.8 (Fig. 3).
This interpretation is possible but seems unlikely
as no similar effect is visible in the Tlajinga or
H.L. Q. Stuart- Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
Tlailotlacan data sets. The material from the east
side of the pyramid was buried about 4 metres
deeper than the bones from the south side and
might have been exposed to less flushing by soil
water. The interior samples were even more deeply
buried, but the south and east side burials were
covered by concrete floors with a substantial carbonate content which must have had an effect on
soil water chemistry. It is difficult to devise a model
in which groups of bone buried under such disparate conditions would define the same diagenetic
path. Further, for the data to fit an apparently
straight line with a steep slope, ultimate equilibration with water with an unrealistically high ~180
is required. Precedents for this do exist, for example the data from Byzantine and modem human
bones from Rehevoth, Israel (Luz, 1992)(Fig. 7).
The most altered bones from Rehevoth have
6p=22.5%0 with no indication of a lessening in the
rate of isotopic alteration versus the crystallinity
index, implying that 6p at equilibrium with burial
conditions must be >>23%o. 8w for modern humans
in the sample is -4.5%o but at a soil temperature
of 20°C, 6sw must be >2.25%0 which implies an
improbably great evaporative enrichment of 6.75%0
(Cerling, 1984; Allison and Hughes, 1983). As
inorganic equilibration of the phosphate is very
improbable, equilibration if present must occur by
enzymatic activity of soil organisms, particularly
bacteria and fungi. Eq. 1 is based upon studies of
organic phosphates, with all known organisms
equilibrating their phosphates with their body
fluids according to that relationship. It is unlikely
that bacteria or fungi equilibrate phosphate
differently; the mitochondria of higher organisms
which are responsible for a considerable portion
of the phosphate equilibration and manipulation
(Lowenstam and Weiner, 1989) are themselves of
bacterial origin. Highly isotopically fractionated
body fluids in the soil organisms would result in
highly fractionated phosphates but all "water
breathing" aquatic organisms examined have body
fluid compositions identical to the surrounding
medium (e.g. Kolodny et al., 1983; Longinelli and
Nuti, 1973a, b) due to the need for efficient
exchange of gas and nutrients. Other considerations also indicate that a simple re-equilibration
with groundwater is not a probable process. As
bone exchanges with soil water it should asymptotically approach a 8180 in dynamic equilibrium
with burial conditions (Fig. 8). Neither the Temple
of Quetzalcoatl data in Fig. 3 nor the Rehevoth
data show any such decrease in slope with increasing ~180, which would require that the region of
decreasing slope is encountered at 6180 values
much greater than the values observed here. This
implies that the water with which the phosphates
were exchanging was anomalously heavy when
compared with local meteoric waters.
Bone diagenesis and recrystallization occur most
rapidly during the final stages of the removal of
the organic portions (Fig. 4b) when bacterial activity probably reaches a minimum. Conversely collagen is known to adsorb phosphate strongly
(Koutsoukos and Nancollas, 1986) and to be
associated with the initial formation of bone crystallites (Lowenstam and Weiner, 1989) and the
removal of it destabilises the bone matrix.
Collectively this indicates that the processes altering the isotopic composition of the bone phosphate
23
o
4aul, ~
,or
d21
~o
-*r
20
z
Living bones
•
Good preservation
u Poor preservation
v
19
x
I
2
3
I
4
I
5
11
I
6
I
7
8
FTIR C r y s t a l l i n i t y I n d e x
Fig. 7. Byzantineand modernbones from Rehovoth,Israel (Luz, 1992).
12
H.L.Q. Stuart-Williams et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
24
Groundwatere q u i l i b r i u m ~ v a l u e _~'~
~li
0
o0
i i
l i
~
li
22
21
20
19
~'~'-~ Fresh bone
8
I
2
3
I
I
I
I
4
5
6
7
FTIRcrystallinityindex
I
8
9
Fig. 8. Phosphate alteration curves. A. Bone phosphate being equilibrated to 23%,, at 10% exchange for 0.2 change in crystallinity
index. B. Phosphate undergoing selective solution/recrystallization to reach 23%o at crystallinity index 8.
are inorganic and result from effects associated
with the dissolution, reprecipitation and recrystallization of the bone mineral, most probably with
entire PO] ions being fractionated by kinetic
isotope effects. Oxygen isotopic offsets of 2-3%0
have been shown to exist between P O ] - ions in
solution and in a slowly precipitated solid phase,
providing a mechanism for the alteration of Sv
(Stuart-Williams and Schwarcz, 1995 and StuartWilliams and Schwarcz, in prep.).
The uranium contents of the bones may indicate
burial conditions soon after interment, as the
conditions under which uranium is mobilised are
known from geological studies of uranium ores.
Uranium is mobile as the uranyl ion, UO 2+ under
oxidizing conditions (Guilbert and Park, 1986;
Hostetler and Garrels, 1962). In its reduced form
as U 4 +, uranium is highly insoluble in soil waters.
This combination of factors may explain the higher
concentration of uranium in the samples with less
recrystallization and a higher organic content.
Initially the body decays and releases organic
compounds and CO2 into the surrounding oxic
soil under slightly basic conditions. Uranyl in the
soil water is then fixed in proximity to the bones
as U 4+ by decreasing pH and strongly reducing
conditions, in a manner similar to a roll-front
uranium deposit (Guilbert and Park, 1986). Once
the soft tissues have completely decayed and the
bone collagen breaks down, the uranium is oxi-
dised by molecular 02 in the soil water and transported away in solution. The anomalous
composition of sample 68 suggests that it was
perhaps sheltered from flushing by soil water after
the soft tissues had decayed or that conditions
remained rather more reducing in the neighbourhood of this bone.
In conclusion to this section, it is unlikely that
the Temple of Quetzalcoatl samples have suffered
isotopic alteration as they were all buried at the
same time under very different conditions and
could not have followed a similar diagenetic pathway. In addition it is improbable that ~sw was
sufficiently high for equilibration to be a possible
mechanism but enrichment by fractionation of
entire PO 3- ions is possible. The balance of the
evidence suggests that the isotopic separation
between the two Temple of Quetzalcoatl sample
groups results from in vivo differences. The retention or uptake of uranium in the bone samples
does not follow a monotonic pathway and cannot
be interpreted to give a concise description of
burial conditions.
5. Summary and conclusions
The oxygen isotopes of the Oaxacan burials
form a distinct and well defined group with little
overlap with the Teotihuacano data (Fig. 2). While
H.L. Q. Stuart- Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14
quite considerable recrystallization and/or solution
has occurred, there is surprisingly no evidence that
it has affected the 5180 of the bone. Both the
degree of recrystallization and 5p appear to be
uncorrelated with the relative ages of the samples,
with gp remaining approximately constant and
recrystallization depending on burial conditions.
An average 6p for the Oaxacan population is
13.0 +0.6%0. The narrow spread of values implies
that climate has been relatively constant over the
period studied.
The distribution of values from Teotihuacan is
rather greater, perhaps representing a mixture of
peoples of different origins. There is little indication of isotopic alteration after burial, perhaps for
the same reasons as at Oaxaca. The Tlajinga group
has a low variance, suggesting a regionally discrete
population with a very constant weighted mean
annual 8w. The Merchant's Barrio and Tlailotlacan
groups have rather greater isotopic spread while
the Temple of Quetzalcoatl samples appear to form
two discrete groups. The single Tlamimilolpa
sample has a value similar to the mean of the
Tlailotlacan group.
It is possible that some of the isotopically heavier
groups may have been derived from the Gulf coast,
while isotopically lighter samples are from people
who are native to the highlands and the rain
shadow areas or have been there long enough that
substantial remodelling of their bone has occurred.
Collagen decay exposed the bone crystallites to
attack by oxidizing, slightly acid fluids, and the
bone crystallites were progressively dissolved,
removing the more soluble defect- and carbonaterich outer layers. Little recrystallization took place
and the flushing conditions did not favour kinetic
effects that might produce an isotopic fractionation, so that the removed phosphate had a composition close to ~p and the composition of the
remaining bone is apparently unchanged.
It must be stressed that diagenetic alteration of
bone is entirely a product of local burial conditions
and that results from one site are not indiscriminately applicable to others. For example, increased
pH or altered sediment permeability at the sites
examined might have encouraged reprecipitation
of the hydroxylapatite with a composition altered
by kinetic isotope effects.
13
Acknowledgements
Delayed neutron
activation analysis
Fourier transform
infrared analyses
ICP-MS analyses
Isotope ratio mass
spectrometry,
vacuum line and
laboratory help
and water analyses
Sample contributors
Karen Goodger
Trish Duthie, Danica
Popic
Robert Bowins
Martin Knyf
Jennifer Blitz (Oaxaca
samples)
Rub6n Cabrera Castro
(INAH Mexico)
George Cowgill (Arizona
State University)
Saburo Sugiyama
(Arizona State
University)
Rebecca Storey
(University of Houston)
Evelyn Rattray ( U N A M
Mexico)
Lorraine Mclntosh
Sample processing
assistant
Improvements in this paper resulting from the
contributions of two reviewers, H. Bocherens and
E. Reinhard, and the editor, A. Longinelli, are
gratefully acknowledged. This work was supported
by NSERC funding to HPS.
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