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Feasibility of Field-Portable XRF to Identify Obsidian Sources in Central Petén, Guatemala 1

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1

Leslie G. Cecil , Matthew D. Moriarty , Robert J. Speakman , and Michael D. Glascock 1

1

Research Reactor Center, University of Missouri, Columbia, M O 65211 Department of Anthropology, Tulane University, New Orleans, L A 70118

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Recent research concerning the Postclassic (A.D. 1000-1524) period in central Petén has focused on defining changes in architectural features and pottery manufacturing techniques. Obsidian is frequently excavated from these structures and occurs as offerings in cache vessels. Field-portable X R F can be used to identify obsidian sources based on artifacts excavated from Trinidad de Nosotros. These data will be compared to previous analyses from other archaeological sites in the Petén lakes region. In addition to identifying obsidian sources, obsidian trade in the Petén lakes region will be better understood because of the samples analyzed from the trading port (Trinidad de Nosotros) on the north shore of Lake Petén Itzá.

Archaeological research and artifact analysis involves obtaining any number of permits and following regulations as to the extent to which one can export artifacts for analyses that are not feasible in the field. Many times the permits for exportation of artifacts limit the number of artifacts that can be used for a study as well as the extent to which the artifact can be damaged in the analysis before it must be returned. With the development of portable technology, such as the field-portable X R F , that does not damage the artifact, it is possible to analyze artifacts in the field without obtaining the often elusive export permits. Additionally, because the technique is rapid, it is possible to greatly increase the 506

© 2007 American Chemical Society

507 sample size of any analysis allowing the archaeologist to examine more artifacts and to obtain more robust data sets. This is especially needed in the analysis of artifacts from inland trade ports and/or trans-shipment locations given the scarcity of information known about these archaeological contexts.


Background To test the feasibility of the portable X R F and to create a robust obsidian data set from an inland trade port/trans-shipment locale, we analyzed a number of Postclassic obsidian artifacts from Trinidad de Nosotros in the central Petén lakes region (Figure 1). Obsidian is ubiquitous in the archaeological record, and can be easily traced to its source area because each volcanic eruption that produces volcanic glass has a unique chemical composition. In Mesoamerica, most obsidian artifacts originate from about 40 known sources of which only about ten were heavily used by the Maya (7). Although obsidian sourcing studies have focused on the Preclassic and Classic periods in the Maya region, more recent research is concentrating on the Early Postclassic period coastal sites. Even though this research is advancing our knowledge concerning coastal trade, it rarely examines the inland trade that must have occurred in conjunction with the coastal trade routes. With the excavations at Trinidad de Nosotros and the previous analysis of obsidian at other sites in the region, it is possible to examine the extent to which inland trade routes differed from coastal trade routes during the Early Postclassic period. Trinidad de Nosotros (or Sik'u' in Itzaj Maya) is located on the north shore of Guatemala's Lake Petén Itzâ, 2.6 km southeast of the Late Classic site of Motul de San José and approximately 35 km southwest of the major center of Tikal. Two seasons of investigations at Trinidad de Nosotros have defined it as a medium-sized center, covering an area of about 1 km and including approximately 150 structures. Its occupation extends from the Middle Preclassic (ca. 600 B.C.) period to the Historical era, with major peaks in settlement during the Late Preclassic (300 B . C . - A . D . 250) and Late Classic periods (A.D. 600830) (2). The central portion of the site is characterized by elite residences, several small temples, a ballcourt, and a series of five public plazas (Figure 2). Soil chemical analyses in the largest of these plazas have suggested that it may have served as a setting for periodic markets (5). 2

Trinidad de Nosotros's location situates the site at one of the best natural portages on the north shore of the lake. Along most of this shore, steep natural terraces step down to narrow rocky beaches. At Trinidad de Nosotros, however, the discharge from a seasonal drainage has created a wide beach ideal for landing and loading small boats (2). Further, a series of artificial and modified natural features provide a small harbor. At the base of the slope running down


508

Figure 1. Map of archaeological sites in the central Petén lakes area, El Petén, Guatemala.

from the center of the site, a low platform with a cut stone outer retaining wall extends around the inner circumference of the harbor and provides a level surface for loading and unloading canoes. On the outer edge of the harbor, a small hillock or island was modified and extended to create a 70m long outer harbor wall. Finally, within the harbor, a 7x15 m platform served as a dock. Excavations within these features have delineated a long history of construction and modification beginning in the Preclassic and particularly heavy during the Early Postclassic period (4). The combination of Trinidad de Nosotros's location, the presence of a harbor, and high densities of exotic trade goods recovered during excavations provide multiple lines of evidence for identifying the site as an ancient Maya port (2). If this was the case, then Trinidad de Nosotros was well situated to have served several pre-Columbian trade routes. First, the central Petén lakes form a natural transportation route for waterborne travel between the east-flowing rivers of Belize and the northwest flowing rivers of western Petén (5). The recent identification of port facilities at Nixtun-Ch'ich' on the west end of Lake Petén Itzâ, canalized rivers on the east end, and a series of possible inter-lacustrine canals provide some confirmation for this possibility (2 ,4, 6). Second, Trinidad de Nosotros may also have served as a point of trans-shipment between the lake and a northwestern trade route utilizing the Rio K'ânte't'u'ul, a small river that starts 5 km north of Trinidad de Nosotros and leads to the Rio San Pedro Martir, a major pre-Columbian transportation artery. Access from the lake to the river

509

Trinidad de Nosotros Sun Jose, Pvtcn. CuaU-»utla


MSI PtofxX,

Lagt> Petén Itzâ

Figure 2. Map of Trinidad de Nosotros, Petén, Guatemala. The Postclassic portion (and harbor) of the site is indicated with a dashed rectangle.


510 and other points north was facilitated by a natural path of least resistance through the rolling topography north of the lake (2). One of the many classes of artifacts that may have been traded through Trinidad de Nosotros was obsidian. Because there are no obsidian sources in the Maya lowlands, the material had to be transported from Mexico and highland Guatemala. The three main Guatemalan obsidian sources utilized by the lowland Maya were San Martin Jilotepeque, E l Chayal, and Ixtepeque (7, 2, 7, 8) (Figure 3). In addition, obsidian from several sources in the Mexican highlands was also imported into the Maya lowlands (7).

Figure 3. Regional map showing locations of Guatemalan (l=San Martin Jilotepeque; 2= El Chayal; 3=Ixtepeque) and Mexican (4=Pachuca; 5-Zaragoza) obsidian sources used in the study.

San M a r t i n Jilotepeque This obsidian source zone is located within the department of Chimaltenango, west of the Valley of Guatemala (ca. 275 km from Trinidad de Nosotros, as the crow flies) (7). It was the dominant import into the Maya lowlands during the Preclassic period (8, 9), however, it continued to be imported to the lowlands throughout the Late Classic and Postclassic periods (9, 70). San Martin Jilotepeque obsidian was not under the control of any one sociopolitical group, as the architecture and lack of dominant workshops suggest an egalitarian society (1,9). Braswell (9) suggests that this obsidian reached the


511 Maya lowlands through multiple down-the-line exchanges. During the Classic period, society became more complex and stratified and the archaeological record indicates the presence of an obsidian lithic industry (9). San Martin Jilotepeque obsidian is more commonly excavated from archaeological sites in central Maya lowlands than from sites in Belize and northern Yucatan as well as coastal sites such as Wilde Cane Cay and Moho Cay. This presence suggests overland and riverine routes for trade of obsidian from this source (77). During the Late Classic period and with the increasing dominance of Kaminaljuyu in the Guatemala highlands (and the E l Chayal obsidian source), San Martin Jilotepeque was traded west to the Pacific coast, north to the Chiapas, and the central Maya lowlands (72).

E l Chayal This obsidian source is located in the upland flanks of the Motagua Valley in the highlands of Guatemala. It is approximately 40 km east of the San Martin Jilotepeque source area and 70 km northwest of the Ixtepeque source area (ca. 250 km from Trinidad de Nosotros, as the crow flies). During the Late Classic period, E l Chayal was the dominant obsidian source traded throughout the Maya region. However, obsidian from E l Chayal was being traded to E l Mirador as early as the Late Preclassic period (70) and occurs in Postclassic deposits in central Petén (75). Kaminaljuyu, one of the larger sites in highland Guatemala during the Late Classic period, controlled the technology of blade production of obsidian from the E l Chayal source (20 km from Kaminaljuyu) (14) . Obsidian artifacts sourced to E l Chayal have been excavated from archaeological sites around the Usumacinta River basin, in northeastern Petén, in the Belize Valley, and the Toledo District of southern Belize (72). Hammond (75) suggests that highland Maya transported and traded obsidian to the lowland communities by inland routes through the Usumacinta and Sarstoon River basins or it could have been transported down the Rio Pasion to Seibal and then north toTikal(77, 16).

Ixtepeque The Ixtepeque obsidian source is located 85 km from the E l Chayal source zone (ca. 300 km from Trinidad de Nosotros, as the crow flies). It is found at archaeological sites east and north of the source, along the coast of Belize, northeastern Petén, the Belize Valley, and northern Yucatan (7, 14). Obsidian artifacts dating to the Late Preclassic, Terminal Classic and Postclassic periods are predominately from this source and it was the main source during the Postclassic period (77, 15, 17).

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Ixtepeque obsidian most likely reached the lowlands by being transported overland during the Preclassic period (77) or down the Rio Motagua and north along the Caribbean coast and then inland (7, 77, 75) during the Terminal and Postclassic periods. This route is supported by the large quantities of Ixtepeque obsidian at Copân and Quirigua (72), as well as the plethora of it at coastal sites that may have served as trans-shipment nodes (7, 16, 17, 18).

Pachuca Green obsidian originates from the Pachuca obsidian source. This source is located between the modern cities of Pachuca and Tulancingo, Hidalgo, Mexico (ca. 900 km from Trinidad de Nosotros, as the crow flies). The nearest prehistoric centers of population were Pachuca, Teotihuacân (50 km from the source) and Tula (70 km from the source). During the Classic period, Pachuca workshops were located at Teotihuacân (19, 20). Artifacts made from this source excavated from Classic period archaeological sites in the Maya region may be the result of gifts among elites (20, 21). During the Terminal Classic and Early Postclassic periods, Tollan workshops appear to have used the Pachuca resource (22). Trade of Pachuca obsidian during the Classic Period appears to have been through Kaminalyuju and followed the E l Chayal pattern to the Maya lowlands. On the other hand, during the Postclassic period Pachuca obsidian trade routes may have included previously established overland routes, as well as routes along the coast of the Yucatan Peninsula (77) and then inland.

Zaragoza, Puebla, Mexico The Zaragoza-Oyameles obsidian source is approximately 100 km southeast of the Pachuca obsidian source (ca. 800 km from Trinidad de Nosotros, as the crow flies). It was used from the Preclassic to the Aztec Periods in central Mexico. During the Classic Period, the Veracruz region dominated the Zaragoza prismatic blade obsidian network (23). Within this region, Cantona may have had more control of the source (23). After the fall of Teotihuacân, the distribution of Zaragoza obsidian was not disrupted thus suggesting that Teotihuacân did not have control of this obsidian source (23). The most likely path of distribution into the Maya lowlands from A . D . 850—1050 was along the Gulf Coast around the Yucatan Peninsula and inland through trans-shipment points. This is supported by the presence of blades made of Zaragoza obsidian found at Isla Cerritos (24), Cozumel (25), and Mayapân (7).

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Analytical Methods Two seasons of excavations at Trinidad de Nosotros produced approximately 1800 obsidian artifacts. O f these, approximately one-quarter (450) were recovered from Postclassic contexts within the harbor area and other parts of the site. To examine patterns in obsidian source procurement at Trinidad de Nosotros during the Early Postclassic period, a sample of artifacts from Postclassic contexts were selected for analysis. Samples less than 3 cm in length were excluded from consideration and only artifacts from chronologically secure contexts were analyzed. With this limitation and employing a stratified random sampling technique, a total of 70 obsidian artifacts were selected for analysis. These samples came from Postclassic contexts within the site's harbor area including a residence, the inner harbor platform, the outer harbor wall, and a high-density midden within the harbor itself. Nondestructive elemental analysis of the obsidian samples was conducted at M U R R using an ElvaX desktop energy-dispersive x-ray fluorescence (EDXRF) spectrometer. The instrument consists of an x-ray generator, an x-ray detector, and a multi-channel analyzer ( M C A ) . The detector is a solid state Si-pin-diode with a resolution of 180 eV at 5.9 keV (at 1000 counts per second) with an area of 30 mm . The output signal of the detector is formed by a time-variant time processor with pile-up rejector, base line restorer, and automatic adaptation of shaping time to the input count rate. The M C A consists of a fast shaping amplifier (FSA) and a 4K-channel spectrometric analog-to-digital converter (SADC), built as a successive approximations A D C with conversion time of 2 μ 8 , 4096 channels, a 32-bit per channel buffer R A M , "sliding scale" linearization of differential nonlinearity, and dead time correction circuit (26). The x-ray tube is air-cooled and low powered with a tungsten anode and 140 μπι beryllium end-window.


1

2

The analyses were conducted at 30 k V with a tube current of 45 μΑ and a 400 second live time using a 0.8 mm primary aluminum filter. The following elements were measured: titanium (Ti), manganese (Mn), iron (Fe), zinc (Zn), gallium (Ga), rubidium (Rb), strontium (Sr), yttrium (Y), zirconium (Zr), and niobium (Nb). Concentration values (in parts per million) were determined using ElvaX Regression—a program based on the quadratic regression model using data derived from ten reference samples (obsidian from Guatemala and Mexico) of similar composition. The quadratic regression model of the form 5

*=o describes the relationship between the set of analytical intensities and analyte concentrations where C, is the concentration of analyte / in the sample, 1 = 1, 7, 0

514 and I are the analytical intensities of analytes j and k respectively, and S is the number of analytes in the product. The regression coefficients A =A are determined from calibration by a set of reference samples. As a rule, as their number exceeds the number of reference samples at hand, not all of the coefficients can be estimated, and only the most significant ones are taken into account (26). The number of significant coefficients present in the model for each analyte i is referred to as the number of degrees of freedom of the regression model. Elemental concentration values of the obsidian artifacts were correlated to known obsidian sources in Guatemala (El Chayal, Ixtepeque, and San Martin Jilotepeque), and Mexico (Pachuca, Ucareo, and Zaragoza) using the ElvaX software comparison spectra feature. To verify matches made with the comparison spectra, elemental concentrations were transformed using base-10 logarithms, plotted, and grouped according to statistical group membership. Use of log concentrations rather than raw data compensates for differences in magnitude between the major and minor elements. Transformation to base-10 logarithms also yields a more normal distribution for many elements. The interpretation of compositional data obtained from the analysis of archaeological materials is discussed in detail elsewhere (e.g., 27-32). The main goal of data analysis is to identify distinct homogeneous groups within the analytical database. Based on the provenance postulate, different chemical groups may be assumed to represent geographically restricted sources (33). For obsidian, raw material samples are frequently collected from known outcrops or secondary deposits and the compositional data obtained on the samples is used to define the source localities or boundaries. Obsidian sources tend to be more localized and compositionally homogeneous, making artifact-source comparisons straightforward. Groups are characterized by the locations of their centroids and the unique relationships (i.e., correlations) between the elements. Decisions about whether to assign a specimen to a particular compositional group are based on the overall probability that the measured concentrations for the specimen could have been obtained from that group. k


iJk

ikJ

A l l obsidian samples were analyzed as unmodified samples; they were washed in the field. Each sample was placed in the sample chamber with the flattest part of the surface facing the x-ray beam. A l l samples were at least 3 cm in length with varying widths and thicknesses. The width of the sample did not produce errors when comparing obsidian artifact to potential obsidian source. Accuracy errors result from inaccuracies of the regression model, statistical error of the calibration spectra, inaccuracy of the intensity of the calibration curve and the energy calibration. When the error is taken into account, the relative analytical uncertainty for this project is less than seven percent with this portable X R F unit (26)?

515


Results and Discussion Source determination of all 70 Postclassic obsidian samples was possible using the field-portable X R F instrument. Although the majority of the samples (56%, n=39) represent the Ixtepeque source, 29% (n=20) of the sample comes from El Chayal, and 11% (n=8) can be sourced to San Martin Jilotepeque. In addition to these Guatemalan sources, two different Mexican sources were identified in the Postclassic sample: Pachuca (3%, n=2) and Zaragoza (1%, n=l) (Figure 4). The high percentage of Ixtepeque samples is consistent with the change in source dominance for the Postclassic period as noted elsewhere in the Maya lowlands (7, 16, 17, 34, 35, 36). This is different from the frequency of obsidian during the Late Classic period (primarily E l Chayal with smaller amounts from the other highland Guatemala sources) (Figure 5). Although there is a marked change, it is not the caliber of change documented at the coastal sites. The coastal trans-shipment points (such as Wild Cane Cay, Moho Cay, and San Gervasio) had a much higher frequency (approximately 84%) of Ixtepeque obsidian and much lower frequencies (under 10% each) of E l Chayal and San Martin Jilotepeque obsidian (77, 75, 36). On the other hand, the percentage and sources of Mexican obsidian at Trinidad de Nosotros and coastal trading sites is consistent with other inland Postclassic sites. When examining the frequency and sources of obsidian found at Trinidad de Nosotros with those at other archaeological sites in the Petén lakes region, similarities exist suggesting that Trinidad de Nosotros may have served as a trans-shipment port for the region (Table I). As a trans-shipment port, Trinidad de Nosotros most likely participated in overland and coastal/riverine trade routes. During the Early Postclassic period, Ixtepeque was transported down the Motagua River to places like Wild Cane Cay in the Bay of Honduras and around the Yucatan Peninsula (7, 5,15, 17, 18, 36). In all likelihood, Ixtepeque obsidian was transported to central Petén via a number of rivers (Mopan, Mojo, Belize, and New) and overland routes. In addition to this route, E l Çhayal and San Martin Jilotepeque traders may have followed the Rio Pasion to Seibal and then used natural overland routes similar to those used in the Preclassic and Classic periods (7, 77, 75). This route easily could have included Trinidad de Nosotros as it is in direct line on the route to Tikal further north. Because of the higher frequencies of these two obsidian sources at the sites in the Petén lakes region, it appears that the trade route did not cease with the end of the Classic period. Mexican obsidian came from a third trade route that originated in central Mexico and Veracruz and was transported east along the Gulf of Tuantepec and the Yucatan Peninsula. There were many possible entry points of obsidian into central Petén that included Isla Cerritos, San Gervasio, Moho Caye, and Cerros (7, 77, 36, 37).

516

Ι.β

2.0

2.2

2.4

2.6

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J.O

Zirconium (log base-10 ppm)

Figure 4. Plot of zirconium and rubidium base-10 logged concentrations showing the separation of the Early Postclassic period obsidian excavatedfrom Trinidad de Nosotros. Ellipses represent 90% confidence interval for group membership.

to

c\i El Chayal

-

10

Ε CL α Ο

2 . 1 4 2.18

CN CN oj

2.06

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2.02

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Ixtepeque

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San Martin Jilotepeque

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1.98

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Zirconium (log base-10 ppm)

Figure 5. Plot of zirconium and rubidium base-10 logged concentrations showing the separation of the Classic period obsidian excavated from Trinidad de Nosotros. Ellipses represent 90% confidence interval for group membership.

517 Table I. Frequency of Postclassic Period Guatemalan and Mexican Obsidian Artifacts from Selected Archaeological Sites


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Archaeological Site

San Martin Jilotepeque

El Chayal

Ixtepeque

Mexican Sources

Trinidad de Nosotros Lake Macanché (38) Lake Salpetén (38) Lake Quexil (38) Topoxté (39) Tipuj (34) Wild Cane Cay (17) Colha (16, 35) Isla de Ceritos (24) San Gervasio (/) Laguna de On *(40)

8(11%)

20 (29%)

39 (56%)

3 (4%)

2 (8%)

10(38%)

12 (46%)

2 (8%)

7 (20%) 5 (29%) 8 (17%) 15 (9%) 1 (2%) 0 (0%) 0 (0%) 1(3%) 21(3%)

7 (20%) 5 (29%) 18(38%) 19(11%) 6 (8%) 0 (0%) 6 (34%) 0 (0%) 178(27%)

19 (58%) 7 (42%) 21(45%) 135 (79%) 63 (85%) 10(100%) 0 (0%) 28 (91%) 441 (67%)

1 (2%) 0 (0%) 0 (0%) 2(1%) 4 (5%) 0 (0%) 12 (66%) 2 (6%) 0 (0%)

* Twenty samples (3%) could not be sourced.

The obsidian obtained and analyzed from Lakes Macanché, Yaxhâ, Salpetén, and Quexil as part of the Central Petén Historical Ecology Project (CPHEP) reflects a similar distribution of sources as seen at Trinidad de Nosotros (13, 38). Archaeological sites in central Petén with Postclassic occupation (including those tested by the CPHEP) reflect a dynamic sociopolitical milieu. The Early Postclassic period is typified by an introduction of new architectural and pottery styles that may indicate the influx of new sociopolitical groups in the region (41). On the other hand, some ceramic types seem to mimic Late Classic gloss wares suggesting the presence of an extant local population (42). The material culture distribution during the Early Postclassic period is similar throughout the region (from Nixtun Ch'ich' to Tipuj) suggesting ease of movement in the area. During the Middle and Late Postclassic periods a distinctive east-west dichotomy of cultural patterns is observable, reflecting historical alliance shifts, changing dominance relations, and repeated migrations to and from the northern Yucatan peninsula was established by the Itzâ (western lake sites) and the Kowoj (eastern lake sites) (41, 42). These distinctive characteristics resulted from the presence of various socio-political groups that established and maintained principal provinces headed by a leader at a provincial capital. Each controlled different subprovinces and each had a distinct origin and migration myths. As a result, east-west boundaries were created and maintained thus restricting movement between Itza (west) and Kowoj (east) territories/archaeological sites. If Trinidad de Nosotros was the central Petén


518 lakes trans-shipment point of obsidian (and presumably other goods) during the Early Postclassic period, obsidian frequencies at Trinidad de Nosotros should be similar to those at other central Petén archaeological sites. As a trans-shipment port, Trinidad de Nosotros would have received obsidian from a number of trade routes that may have been very similar to those used during the Classic Period. After receiving the obsidian, its inhabitants may have spread equivalent amounts to other archaeological sites in the region. This may be the case since similar quantities and kinds of obsidian are found throughout the central Petén lakes region (with the exception of Tipuj, see below) during the Early Postclassic period, further suggesting that the socio-political distinctions (and boundary maintenance) that occurred in the Late Postclassic period were not yet strong (4L 42). Unlike the other central Petén Postclassic archaeological sites, Tipuj appears to be a participant in another or an additional trade alliance that brought them almost twice the amount of Ixtepeque obsidian. This may be the result of Tipuj being the end point of the Belize River transportation network that was part of a coast trans-shipment route involving Moho Cay. However, obsidian from Tipuj presents a different pattern of distribution. Tipuj, the eastern most archaeological site that is considered part of the central Petén lakes region, has frequencies of obsidian that are more similar to those of the coastal sites than those of the Petén lakes region (34). Tipuj may have functioned as the eastern outpost site for the central Petén Maya during the Early Postclassic period as the Maya migrated from northern Yucatan to the central Petén occupation via the east coast of the Yucatan peninsula, down the New and Mopan Rivers to Tipuj. Additionally, it is from here that Fuensalida, Orbita, and Avendafio began their efforts to conquer and convert the Petén Itza in the 17 century (43). Therefore, there is a history of Maya using coastal and inland river routes to reach Tipuj suggesting multiple paths by which central Petén Postclassic Maya acquired obsidian. th

Conclusion The obsidian excavated from Trinidad de Nosotros demonstrates that obsidian trade during the Postclassic period was as complex as other periods. And, inland trade, as seen, through evidence of a trans-shipment port, was different than coastal trade. In addition to the cultural aspect of this study, we have demonstrated that it is possible to successfully use a field-portable X R F to correlate obsidian artifacts to their sources. Not only were sources identified, but the process was rapid (4-6 minutes per sample), cost-effective, and as accurate and precise as more traditional methods of analysis. Above all, and perhaps most importantly, when working with artifacts from museum collections and foreign countries, the process was non-destructive to the artifacts.

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Acknowledgements The authors wish to acknowledge the Archaeometry Laboratory at the University of Missouri Research Reactor (funded by NSF Grant No. 0504015 and US Department of Energy Office of Nuclear Energy, Science and Technology Award No. DE-FG07-03ID14531) for the X R F analyses. Fieldwork at Trinidad de Nosotros was funded by grants from F A M S I , the Middle American Research Institute, the Amherst College Fellowship Program, and Williams College. We would also like to thank I D E A H for the permission to export the samples from Guatemala. A l l errors and omissions are our own.

Notes 1

Dating of the Terminal Classic and Early/Middle Postclassic periods in the central Petén Lakes region is difficult because there is no abrupt and absolute distinct change in pottery (Late Classic to Postclassic) or obsidian frequencies. Some of the pottery types have characteristics of Terminal Classic forms and paste constituents while using a different clay base. For example, the most prominent ceramic grouping in the area during the Postclassic period, Paxcamân, is characterized by an early version that contains a snail inclusion clay (Postclassic) with ash temper (Late/Terminal Classic) whereas Late Postclassic Paxcamân pottery lacks an ash temper. The pottery from Postclassic contexts at Trinidad de Nosotros is most closely associated with these earlier Postclassic ceramic types, suggesting that the contexts reflect Postclassic occupation and not Terminal Classic occupation.

2

Given the error rate, two samples from each obsidian compositional group determined with X R F were analyzed with I N A A to determine the veracity of the X R F groups. There were no differences between I N A A and X R F data with regard to source identification. For the most complete counts of all known sites with obsidian in the Postclassic period see reference (7).

3

References 1.

2.

Braswell, G . E. In The Postclassic Mesoamerican World; Smith, M . E . ; Berdan, F. F., Eds; University of Utah Press: Salt Lake City, UT., 2003; pp 131-158. Moriarty, M. D.; Lawton, C.; Spensley, E. Paper Presented at the Annual Meeting of the Society for Archaeology, San Juan, 2006.

520 3. 4.


5.

6.

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