E L S E V I E R Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14

PIIAEO

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 composit ion of a mammal ' s body fluids is controlled by its total water intake (Longinelli and Peretti Padalino, 1980; Luz et al., 1984; Luz and Kolodny, 1985; Z immerman 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 pre- cipitation (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 human body fluids is dominantly controlled by the 6180 of drinking water derived from meteoric water, with smaller contributions f rom 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 topo- graphic 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 iso- topic 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 immi- grant 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 con- siderable 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 affilia- tions 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 (pri- marily 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 poten- tially 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 vola- tilization (Firsching, 1961 ). These processing methods do not decrease the precision of the phosphate analyses when applied to soluble stan- dards, although improper silver orthophosphate precipitation can produce errors of 2-3%0 (Stuart- Williams and Schwarcz, 1995).

Oxygen was liberated from the silver orthophos- phate using a technique of polymerization at high temperature in a bromine atmosphere (Stuart- Williams 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 phos- phate. The analytical precision using bromine pentafluoride to liberate oxygen from the phos- phate 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 diox- ide 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 photospectrome- ter. 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 Burial Description Period 51s o FTIR CO3/PO 4 no. crystallinity ratio

01 542 Merchants ' barrio, EX 13.9 3.2 0.61 adult

02 576 Merchants ' barrio, " 14.2 3.7 0.37 adult

04 578 Merchants ' barrio, " 14.7 3.5 0.39 adult

05 585 Merchants ' barrio, " 14.4 3.9 0.24 adult male Merchants ' barrio, LX child

23 34 Tlamimilolpa, adult EX 15.7 3.1 0.58 25 6 Tlajinga, female ET1 14.4 4.3 0.25 28 61 Tlajinga, male LX 15.2 3.8 0.52 30 39 Tlajinga, 6 month old EX 14.8 2.9 0.35 31 21a Tlajinga, 2 yr old LX 14.6 3.6 0.33 34 11 Tlajinga, male LT1 15.0 3.9 0.25 36 17b Tlajinga, female LX 14.7 3.5 0.25 39 28a Tlajinga, 3-4 yr old LT1 14.6 3.8 0.28 42 42a Tlajinga, male LTI 14,3 3.7 0.31 45 5 Tlajinga, perinatal Met 15.1 3.6 0.47 46 46a Tlajinga, female EX 14.9 3.4 0.30 47 56 Tlajinga, male ET1 14.6 4.1 0.34 49 1 Quetzal', south, male Ca. 200 AD 15.8 4.3 0.24 50 2-G Quetzal', interior, " 16.3 4.6 0.18

10 153-155 16.6 3.1 0.27

female Quetzal', interior, " female Quetzal', interior, male Quetzal', interior, " male Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', Quetzal', south, male " Tlailotlacan 4 Tlailotlacan, child 6 Tlailotlacan, child 15 Tlailotlacan, child 20? Tlailotlacan, child 19 Tlailotlacan, infant 4

51 2-N 16.2 4.1 0.19

52 4-A 15.6 4.6 0.20

53 4-L 15.5 4.4 0.16

54 5-f 54 5-f 55 5-H 56 6-1 57 6-D 58 10-A 59 10-D 61 l l -C 62 12 63 13 64 14-C 65 14-F 66 15 67 7-P 68 7-M 69 F322 73 F345N 74 F133 75 F133 76 F130 79 F408

east, male " east, male east, male east, male east, male east, female east, female east, female interior, male interior interior, male interior, male interior, male Pre 200 AD south, male Ca. 200 AD

15.4 3.9 0.31 15.4 3.9 0.31 14.2 4.1 0.29 13.4 3.8 0.35 14.1 3.7 0.38 14.3 3.6 0.48 14.1 4.0 0.26 13.6 3.7 0.39 15.9 4.5 0.19 16.4 4.7 0.10 17.7 4.3 0.23 16.6 4.5 0.23 16.3 3.6 0.26 14.6 3.7 0.32 14.2 4.0 0.28 14.4 4.8 0.35 16.4 3.5 0.45 15.5 3.4 0.35 16.5 4.3 0.42 15.3 4.2 0.56 15.2 3.8 0.44

4 H.L.Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14

Table 1 (continued)

Sample Burial Description Period 8180 FTIR CO3/PO 4 no. crystallinity ratio

80 F409 Tlailotlacan 4 15.0 4.7 0.27 83 F381 Tlailotlacan 15.6 4.9 0.27 88 F 113 Tlailotlacan, 20 14.1 3.9 0.30

young female 89 South tomb Tlailotlacan 20 15.1 4.1 0.48 90 F372 Tlailotlacan ? 14.2 4.7 0.43 oax1949 Tomb 120 OAX 1949 IV 13.2 4.5 0.21 oax1959 Tomb 113 OAX 1959 II 13.1 3.3 0.53 oax1960 Tomb 111 OAX 1960 I 13.4 3.0 0.82 oax1965 Bur. 4 OAX 1965 Ii 12.5 3.9 0.40 oax1968 Tomb 2 OAX 1968 I 12.2 3.l 0.40 oax1969 Bur. 17 OAX 1969 IIIa 14.0 3.6 0.43 oax1973 Tomb 63 OAX 1973 V 13.9 4.4 0.17 oax1975 Tomb 121 OAX 1975 IV 13.7 4.3 0.24 oax1999 Bur. Vi-7 OAX 1999 IIIa 12.4 3.4 0.49 oax2011 Bur. IV-32 OAX 2011 V 12.5 3.4 0.33 oax2021 Tomb 21 OAX 2021 lIIa 12.5 4.1 0.49 oax2025 Bur. IV-36 OAX 2025 IIIb 12.9 3.3 0.37 oax2034 Tomb 77 OAX 2034 II 12.7 4.4 0.24 oax2045 Bur. V-20 OAX 2045 V 13.2 3.7 0.30 oax 2048 Tomb 58 OAX 2048 IV 13.9 4.8 0.31 oax2148 Tomb 153 OAX 2148 IlIb 12.1 5.3 0.27

Site or period Mean Standard Identifier Period Date ~180 deviation

Merchant's Barrio 14.76 _+ 1.1%0 ET1 Early Tlamimilolpa 200-350 AD Tlamimilolpa 15.7 TL Tlamimilolpa 200 450 AD Tlajinga 14.75 _+0.3%0 LT1 Late Tlamimilolpa 350 450 AD Quetzalcoatl (interior) 16.3 _+0.7%0 EX Early Xolalpan 450-550 AD Quetzalcoatl (east side) 14.2 _+0.6%0 LX Late Xolalpan 550-650 AD Quetzalcoatl (south side) 14.4 Met Metepec 650-750 AD Tlailotlacan 15.2 +_0.8%o I Monte Alban I 500 200 BC Monte Alban (Oaxaca) 13.0 _+0.6%0 II Monte Alban II 200 BC 300 AD

IIIa Monte Albanm IIIa 300 500 AD IIIb Monte Alban IIIb 500-750 AD IV Monte Alban IV 750 1000 AD V Monte Alban V 1000 1520 AD

b r o m i d e . T h e c r y s t a l l i n i t y i n d e x ( C I ) w a s ca l cu - l 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 , 1990) . D e l a y e d n e u t - r o n a c t i v a t i o n a n a l y s i s w a s u n d e r t a k e n u s i n g t he M c M a s t e r n u c l e a r r e a c t o r . I n d u c t i o n - c o u p l e d p l a s m a m a s s s p e c t r o m e t r i c a n a l y s e s w e r e pe r - f 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 we re all n o r m a l i z e d to c o n v e r t t he m i n i - m u m v a l u e fo r e a c h e l e m e n t in t h e d a t a se t to z e r o a n d t h e m a x i m u m to 10.

B o n e h a r d n e s s w a s e s t i m a t e d f r o m 1 ( t h e h a r d e s t b o n e ) to 5 ( t h e so f t e s t b o n e ) o n t h e b a s i s o f t he c o m p r e s s i v e a n d t ens i l e s t r e n g t h o f b r o k e n f r a g m e n t s .

B o n e d e n s i t y was d e t e r m i n e d o n 75 300 m g c o m b u s t e d f r a g m e n t s . W a t e r s a t u r a t e d f r a g m e n t s o f b o n e w e r e f l o a t e d in g lyce r ine a n d t h e w e i g h t r e d u c t i o n p r o p o r t i o n a l to t h e i r d i s p l a c e m e n t r e c o r d e d , a s s u m i n g a r e l a t i ve d e n s i t y f o r g lyce r ine o f 1.27.

tt. L. Q. Stuart- Williams et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14 5

500 Km

Fig. 1. Location map 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 combus- tion in air at 450°C for 4 hours to oxidize the organics in the highly permeable bone but not decompose any carbonates, although some poly- merization 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).

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 pop- ulations 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 archaeo- logical data set examined here is rather homogen- eous 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 hetero- geneity on the eastern slopes of the Cordillera.

3.2. Crystallinity, softness, density and organic content

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

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 17

16 0 ~ 15 -%

14

13 12

2.5

V

t

_ 4L_____ •

v ~ v I v v I

o • D

D = o i o i [ i i m

3.0 3.5 4.0 4.5 5.0 5.5 FTIR crystallinity index

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%~.

1 8

17

~ 15 7~

14 13 12

2.5

• Child (10)

i i i I

3.0 3.5 4.0 4.5 3.5 4.0 4.5 5.0 5.5 FTIR crystallinity index FTIR crystallinity i n d e x

:¢ Tlamimilolpa

• Merchants' Barrio

A i

5.0 5.5

18

17

16

7,o 14

13 12

2.5

66- o

, i i i

3.0

x E a s t s ide of pyramid

• Pyramid interior

1 8

17 = 1 6

0 15 c,C 14 =

13 12 ' '

2.5 3,0

O ~ 18 17 16

- - : ° " • 1 5

C ~ 1 4 . . . . . ~ - - = " • - . = 1 3 - - ~ • = =

" : : : 12 ' = { ' ' : 3.5 4.0 4.5 5.0 5.5 2.5 3.0 3.5 4.0 4.5

FTIR crystallinity index FTIR crystall inity i n d e x

Oaxaca

• Tlaj inga

C h i l d (73) • * Child (75)

5.0 5.5

I Oaxaca

• Tlailotlaca n

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 may increase due to dissolution of the more disordered outer surfaces of the bone crystallites or recrystalli- zation 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 7

~ h

- ' ° o =

, ~ , ~ I ~ t 2.5 3.0 3.5 4.0 4.5 5.0

FTIR Crystallinity Index

Q

..Q

_Q E 8

11 10 9 8 7 6 5 4

3.4

58 o " 67 - I

- ~66 64e

3.6 3.8 4.0 4.2 4.4 4.6 FTIR crystallinity index

A

B

4~

5

4

3

2

1 4

i i i i i i

5 6 7 8 9 10 11 % combustible organics

C

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

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 I- content 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).

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 activa- tion analysis (DNAA). Uranium was selected in particular as being a mobile element very suscepti- ble to changes in pH and Eh. In addition the analysis could be made rapidly and precisely as this laboratory routinely performs uranium meas- urements 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 corre- lated 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 0.2

0.0 2.5

o o

o - . * x . . - - x . IJ.*~ .

o

I I I I

3.0 3.5 4.0 4.5 5.0 FTIR crystallinity index

x Oaxaca

o Teotihuacan

A

2 5

2 0

1 5 E en ~_ 1 0

5

= Sample 68

5 6 7 8 9 10 11 12 % combustible organics

B

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 Burial 81+O FTIR crystallinity % combustible Bulk Uranium Softness no. index organics density (ppm)

50 Q2-G 16.13 4.59 5.8 0.96 0.34 3 53 Q4-L 15.40 4.44 7.9 0.67 0.28 4 58 Q10-A 14.28 3.55 10.6 0.91 18.33 2 62 Q12 15.86 4.52 8.7 0.55 0.57 5 64 Q14-C 17.70 4.30 7.4 1.14 0.21 5 66 Q15 16.12 3.61 9.0 0.83 8.03 2 67 Q7-P 14.59 3.68 11.2 1,02 15.76 2 68 Q7-M 14.24 4.04 4.4 0.64 24.20 4

have been a l tered under unusua l condi t ions , a l t hough sample 67 f rom a s imilar set t ing is s imilar in its p roper t i e s to o ther bones in the s tudy.

Fou r o f the eight bones (samples 50, 53 and 64 f rom the p y r a m i d in ter ior and sample 68 f rom the south side) f rom the subset used for D N A A were also e lementa l ly ana lyzed by ICP-MS. This was also in tended as a p i lo t s tudy f rom which only tenta t ive conclus ions can be drawn. C a l c i u m : p h o s p h o r u s ra t ios a p p e a r to fall as recrys-

ta l l iza t ion proceeds. Ana lyses o f some o f the ele- ments are presented graphica l ly be low (Fig . 6), m a n y o f which show no clear re la t ionship between concen t ra t ion in the bone and crystal l ini ty as a p roxy measure o f diagenesis. Sample 68 f rom the east side o f the p y r a m i d has the lowest CI and is deple ted in all e lements a p a r t f rom uran ium, ba r ium, tin and tel lur ium. F r o m this l imited inves- t iga t ion it was a p p a r e n t tha t a t t empts to corre la te degree o f bone diagenesis mono ton i c a l l y with trace

H.L.Q. Stuart-Williams et al./Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14 9

11 9

I3- u 7

5 3

O Z

4.0

S a m p l e 64 I I

S a m p l e 50

v -

S a m p l e 53

• S a m p l e 68 • I I I I I

4.1 4.2 4.3 4.4 4.5 FTIR crystallinity index

= = M n o C 0 e N i 'VRb o Y ~ L a •Ce A P t

4.6

A

11 ~- 9 u_ 7

5 .N 3

O Z

• Sample 5 0 • i

m

1 1 9 ' CL

_u 7 5

N 3 ¸

O Z

o Sample 68 Sample 53~ Sample 64 •

I I I I I

4.1 4.2 4.3 4.4 4.5 FI-IR crystallinity index

• Fe o Cu • M o v Gd • Cd o Nd • Dy = Y b z L u • T I * Th

4.0 4.6

o Sample 68

B

Sample 53 •

Sample 50

Sample 64 • z I I I I I

4.0 4.1 4.2 4.3 4.4 4.5 4.6 FTIR crystallinity index

o B a v S n roTe z u

C

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 temperate- humid 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, includ- ing 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 topogra- phy 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 wester- lies both Teotihuacan and Oaxaca would have very much increased 8w and their relative isotopic posi- tions 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 con- ditions 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, ,~ater of 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 11

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 car- bonate 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 dispa- rate conditions would define the same diagenetic path. Further, for the data to fit an apparently straight line with a steep slope, ultimate equilibra- tion with water with an unrealistically high ~180 is required. Precedents for this do exist, for exam- ple 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 considera- tions also indicate that a simple re-equilibration with groundwater is not a probable process. As bone exchanges with soil water it should asymptot- ically 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 increas- ing ~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 activ- ity probably reaches a minimum. Conversely colla- gen is known to adsorb phosphate strongly (Koutsoukos and Nancollas, 1986) and to be associated with the initial formation of bone crys- tallites (Lowenstam and Weiner, 1989) and the removal of it destabilises the bone matrix. Collectively this indicates that the processes alter- ing the isotopic composition of the bone phosphate

2 3

d 2 1 ~o 2 0

1 9

2

o

4aul, ~

, o r -*r

v

x

I

3 I I I I

4 5 6 7 FTIR C rys ta l l i n i t y I ndex

z Living bones

• Good preservation

u Poor preservation

8

Fig. 7. Byzantine and modern bones from Rehovoth, Israel (Luz, 1992).

12 H.L.Q. Stuart-Williams et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 1-14

0 o0

24

22 21 20 19

Groundwater e q u i l i b r i u m ~ v a l u e _~'~ ~ l i i i l i ~ l i

~'~'-~ Fresh bone 8 I I I I I I

2 3 4 5 6 7 8 9 FTIR crystallinity index

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 recrystalli- zation 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 PO]- ions in solution and in a slowly precipitated solid phase, providing a mechanism for the alteration of Sv (Stuart-Williams and Schwarcz, 1995 and Stuart- Williams 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 trans- ported 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 neighbour- hood 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 path- way. 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 reten- tion 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 13

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 indica- tion 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 carbonate- rich outer layers. Little recrystallization took place and the flushing conditions did not favour kinetic effects that might produce an isotopic fraction- ation, so that the removed phosphate had a com- position 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 indiscrimi- nately 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.

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 (UNAM Mexico)

Sample processing Lorraine Mclntosh 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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