appendix 1

Calibrated Radiocarbon Dates for the Late Classic and Postclassic Periods in the Valley of Oaxaca, Mexico

ROBERT MARKENS, MARCUS WINTER, AND CIRA MARTÍNEZ

One of many challenges archaeologists face in the Valley of Oaxaca is explaining the decline of Monte Albán at the end of the Classic period and defining the political, economic, and social changes that ensued. Monte Albán emerged soon after 500 BCE as one of Mesoamerica’s earliest urban centers, and for more than a millennium this hilltop metropolis occupied a position of regional political dominance, integrating for a time dozens of smaller urban centers and hundreds of villages in the valley into a single political, economic, and social system known as a state. Then at approximately 800–900 CE the center was largely abandoned. Although this time of collapse and transformation has long commanded the attention of archaeologists, research on the matter has been frustrated by a problematic segment of what is otherwise a well-defined regional ceramic chronology. Several generations of researchers, beginning with Alfonso Caso, have grappled with the chronological problem at length, yet its resolution has been slow and fitful and remains to this day controversial.

Recently the authors of this appendix concluded a five-year salvage project in the Valley of Oaxaca known as the Proyecto Salvamento Carretera Oaxaca-Istmo, Tramo Oaxaca-Mitla 2002–2006, or by its initials as SACOI. Directed by Marcus Winter, SACOI personnel excavated portions of five public buildings and nearly thirty noble and commoner residences at three key Prehispanic settlements spanning the Late Classic–Postclassic periods: Macuilxóchitl, Lambityeco, and the dispersed community of Xaagá, located in the hinterland of Mitla. In this appendix we present twelve radiocarbon dates resulting from these investigations. We believe that these new dates considered together with existing radiocarbon dates for the valley clarify the problematic segment of the regional ceramic chronology. The new and previously published dates are presented in two formats: first as uncalibrated dates (Table A1.3) and second as calibrated dates (Table A1.4). Mr. Christopher Patrick, Deputy Director of the Beta Analytic Radiocarbon Dating Laboratories of Miami, Florida, undertook the calibration of the Late Classic–Postclassic period dates from the valley that had been published decades ago (e.g., Drennan 1983).

It is fitting that this appendix accompany a monograph concerning Lambityeco, since that site’s ceramic assemblage has occupied center stage in efforts aimed at clarifying the Classic–Early Postclassic segment of the regional ceramic chronology.

A BRIEF HISTORY OF THE PROBLEM OF DEFINING THE LATE CLASSIC–EARLY POSTCLASSIC PERIOD CERAMIC SEQUENCE

Although the difficulties with the Classic–Early Postclassic portion of the regional ceramic chronology are well-known to Oaxaca specialists, a review of recent general works on ancient Mesoamerica (cf. Coe and Koontz 2002; Evans 2004; Smith and Berdan 2003) suggests that researchers working in neighboring regions are unaware of the matter. For this group as well as for the general reader, we provide a brief background to the problem. Other treatments of the issue can be found in the following sources: Flannery and Marcus 1983; Kowalewski et al. 1989:251–254; Lind 1991–1992, 1994, 2008; Markens 2004:12–105; 2009; Martínez et al. 2000; Winter 1989a, 1990, 1997, 2006, 2007b.

Beginning in the 1930s, Caso and his collaborators formulated a ceramic chronology for the Valley of Oaxaca that partitions the history of settlement at Monte Albán into five major intervals. These are called épocas in Spanish, and although they are ceramic phases, they have been translated as periods in English. Each phase was named for the ancient Zapotec metropolis and was assigned a Roman numeral or numerals, and in some instances a letter as well. From earliest to most recent, the principal ceramic phases were Monte Albán I, Monte Albán II, Monte Albán IIIA, Monte Albán IIIB-IV, and Monte Albán V (Caso, Bernal, and Acosta 1967:17) (Table A1.1: see column marked Blanton et al. 1993). Caso and his colleagues also recognized two transitional phases, Monte Albán Transition II-IIIA (Caso, Bernal, and Acosta 1967:279–308) and Monte Albán Transition IIIA-IIIB (Ibid. 363–378), although these last two have seldom been used by subsequent generations of archaeologists. These transitional phases were defined on the basis of a small number of contexts, each of which was considered the product of a single depositional event in which ceramic vessels diagnostic of two sequential phases were found together.

By the time they published their definitive ceramic study (Caso, Bernal, and Acosta 1967), Caso and his associates conceived Phase IIIB-IV at Monte Albán as a single, unusually long ceramic phase, although they found evidence suggesting that with excavation at other valley sites, it might be possible some day to divide this interval into two separate and consecutive phases: IIIB corresponding to the Late Classic period and IV to the Postclassic (Bernal 1965:804–808; Caso and Bernal 1952:372; Caso, Bernal, and Acosta 1967:84–86, 381–382). They believed that during Phase IIIB-IV, Monte Albán reached its apogee as a political and urban center and then subsequently declined, as made evident by the two kinds of contexts from which pottery from this interval was recovered. Ceramics found in association with the construction and use of buildings in Monte Albán’s Main Plaza were considered indicative of a flourishing urban center and were classified as IIIB, whereas pottery caches found in the debris of ruined structures were thought to have been left there as offerings, as the center fell into decline or after it had been abandoned. These deposits were assigned to IV.

Caso, Bernal, and Acosta believed that an event (or process) as far-reaching as the decline of a great urban center would surely manifest itself in changes in ceramic style (Bernal 1965:804; Caso and Bernal 1965:889; Caso, Bernal, and Acosta 1967:381–385). Yet in spite of their efforts to separate the pottery from these two kinds of contexts into distinct, temporally consecutive assemblages, they were ultimately unable to define systematic differences.

With their monumental program of excavation all but complete at Monte Albán by the early 1950s, Caso’s colleagues Ignacio Bernal and Jorge R. Acosta set out to clarify the Postclassic period segment of the regional ceramic sequence (Bernal and Gamio 1974:85; Paddock 1989:43). Toward this end they undertook excavations at a number of large sites located on the valley floor that presumably continued to be occupied after the collapse of Monte Albán. These included Cuilapan de Guerrero (Bernal 1958a), Etla (Bernal 1965:804), Macuilxóchitl (see Oliveros 1992:85), Mitla (Bernal 1963:223), Noriega (Bernal 1960), San Luis Beltrán (Bernal 1965:804), and Yagul (Bernal and Gamio 1974:9). Caso and his coauthors concluded that they had no recourse but to present Phase IIIB-IV as a single inseparable interval in their definitive monograph on the ceramic sequence for Monte Albán and the Valley of Oaxaca, La Cerámica de Monte Albán, which they published in 1967.

Of all the attempts to divide Phase IIIB-IV into two separate and successive ceramic phases on the basis of detectable differences within the IIIB-IV phase pottery, the efforts of John Paddock (Paddock 1978, 1983a, 1983b; Paddock, Mogor, and Lind 1968) have had the greatest impact on the course of research in the Valley of Oaxaca. The ceramic assemblage that he characterized as Phase IV has been widely accepted and employed most importantly to identify Early Postclassic period settlements during the settlement survey of the valley (Blanton et al. 1982:103; Kowalewski et al. 1989:251–252). Other research dealing either wholly or in part with the Early Postclassic period as defined by Paddock addresses a variety of issues. These include culture history (Whitecotton 1990:7–9), diet (Flannery, Moser, and Maranca 1985:89–91; Flannery and Smith 1983) and its relationship to morbidity (Hodges 1989), and economic specialization (Feinman 1982; Finsten 1995; Peterson 1976).

During his excavations at Lambityeco from 1961 to 1974, Paddock came to believe that he had found the pottery assemblage that had eluded Bernal and Acosta for years (Paddock 1978:48–49). Considered as a whole, the material remains at Lambityeco were remarkably similar to those of Monte Albán during Phase IIIB-IV (see Paddock 1978:49, 56–57n4), yet subtly distinct. Paddock was convinced that here at last was a site with ceramics that fulfilled the expectations of Caso and his colleagues for the Postclassic period (Bernal 1965:804; Caso and Bernal 1952:372; Caso, Bernal, and Acosta 1967:381).

Paddock obtained an unusually consistent set of radiocarbon and archaeomagnetic dates (Drennan 1983:370; Lind 1991–1992:fig. 3; 2008b:fig. 2; Paddock, Mogor, and Lind 1968:23; Rabin 1970:14–15; Wolfman 1973, 1990:table 15.1) from organic remains and fired-clay features recovered or exposed during the excavation of two contiguous elite residences (Structures 190 and 195) that served as the focus of his investigations at the site. The most recent of several construction episodes within the two elite residences were dated to 640–755 CE. The center was apparently abandoned soon after that. No other absolute dates existed at that time from other Phase IIIB-IV components in the valley, so the radiocarbon and archaeomagnetic assays from Lambityeco were perfectly acceptable, even though they were unexpectedly several centuries earlier than what Caso, Bernal, and Acosta had predicted for the end of Phase IIIB and the beginning of Phase IV (Paddock 1978:49).

Since Paddock’s explorations ended more than thirty years ago, radiocarbon dates from Late Classic period valley sites have begun to accumulate, casting doubt on Lambityeco’s temporal placement in the Early Postclassic period. It should be a straightforward matter of deciding to which period Lambityeco’s Phase IV ceramic assemblage corresponds, since Paddock’s suite of highly consistent radiocarbon dates from Lambityeco falls in the Late Classic and not the Early Postclassic period. Nonetheless, the issue has been clouded by the practice common among Oaxaca specialists, considered both individually and collectively, of using a number of terms to refer to the divisions of the regional ceramic sequence; these include “period,” “phase,” “stage,” and the Spanish “época.” In some instances the terms are used in ways that are not conceptually equivalent, confounding criteria of ceramic chronology with those of cultural development. The terminological ambiguity that characterizes Paddock’s writings in particular, as well as those of many Oaxaca specialists, has obscured the problems generated by research treating Lambityeco’s Phase IV as an Early Postclassic period interval (see Markens 2004:12–105 for a detailed discussion).

Beginning in the late 1980s a small number of researchers began to question the temporal assignment of Lambityeco to the Early Postclassic period (Lind 1991–1992; Winter 1989a, 1990). The first radiocarbon sample recovered from Phase IIIB-IV Late Classic period contexts at Monte Albán in the 1970s proved to be contemporary with Phase IV materials from Lambityeco. This raised the possibility that Phase IIIB-IV at Monte Albán, and for that matter throughout the valley, might be a single comparatively short interval. If this was the case, it implies that the novel varieties of pottery and building techniques documented at Lambityeco were nothing more than the expression of local styles within the same valley-wide ceramic phase. This insight has served as inspiration to define an alternate and hopefully viable ceramic sequence for the Late Classic and Postclassic periods (Herrera 2000; Martínez et al. 2000; Markens 2004; Winter 1997, 2006, 2007b).

Some years ago Lind (1991–1992) proposed a series of phase names to replace the phase numbers that Caso, Bernal, and Acosta (1967) established for the Monte Albán ceramic sequence. Lind observed that what began as five phases numbered consecutively I, II, III, IV, and V (Caso 1938:32) had degraded through revision into a series of awkward compound and fraction-like designations; Monte Albán Transition II-IIIA, Monte Albán IIIA, and Monte Albán IIIB-IV are cases in point. As a remedy to this terminological muddle, Lind recommended that the phase numbers be replaced by names, noting that a ceramic sequence denoted in this way readily lends itself to modification as research continues. When a new phase is defined, its name is simply inserted into the existing order at the appropriate place, whereas new numbers disrupt the sequence.

We have followed this lead in our work of the last several years and have defined the following ceramic phases. For the Late Classic period we define the Peche and Xoo phases (Martínez et al. 2000). The Peche phase corresponds to and builds on the Monte Albán Transition IIIA-IIIB phase originally formulated by Caso and his colleagues (1967:365–378). The Xoo phase (Martínez et al. 2000), which follows, corresponds to the Monte Albán IIIB-IV phase of Caso and colleagues (1967:381–444), but this too has been expanded by incorporating ceramic materials from other valley sites. For the Early Postclassic period we have defined the Liobaa phase (Herrera 2000; Markens 2004) based on ceramic criteria entirely different from those Paddock (1983c) and others (Kowalewski et al. 1989; Balkansky 2002) use to define Phase IV. Finally, the Chila phase, corresponding to the Late Postclassic period, has been expanded by incorporating pottery from other valley sites (Markens 2004, 2008) and represents a much shorter temporal interval than Monte Albán Phase V as defined by Paddock and others (Blanton et al. 1993). The phase equivalents appear in Table A1.1 (see columns marked Blanton et al. 1993 and Urcid 2003). We now turn to a consideration of the radiocarbon dates.

RADIOCARBON DATES FOR THE LATE CLASSIC AND POSTCLASSIC PERIODS IN THE VALLEY OF OAXACA

Table A1.2 provides a list of the thirty-six radiocarbon samples presented in this appendix. Each sample is identified by its laboratory number, the site and context of discovery, and the phase of any pottery found in association with the sample. The sample dates are provided in two formats: as uncalibrated and calibrated dates. Finally, references are provided to indicate the source of information about the sample. The initials “SACOI” in the table’s reference column accompany each of the twelve samples recovered during the SACOI project.

Table A1.3 presents an array of the thirty-six uncalibrated dates spanning the Late Classic–Postclassic periods. The SACOI dates are highlighted by light gray shading. Perhaps the most striking feature of the table as a whole is the fact that all the sample dates can be arranged to form a smooth incremental progression through time, beginning in the upper lefthand corner with the earliest Peche and Xoo phase dates and continuing down to the bottom right of the page to the Late Postclassic Chila phase dates. This suggests that samples are well distributed throughout this temporal interval. The only slight break in the series occurs between the last two Liobaa phase dates: samples Beta-205986 and Beta-205990.

A second important observation is that the operation of ordering the sample dates from early to late gathers together those carbon samples found with pottery from the same phase. This indicates that the ceramic attributes used to define each of the pottery phases Peche, Xoo, Liobaa, and Chila are in fact good temporal markers. Although this is the case in general, it is also evident that the Peche phase sample (Beta-63236) overlaps with the earliest Xoo phase date (Beta-20592) from Lambityeco. This suggests that the Peche phase cannot yet be isolated on the basis of this set of radiocarbon dates. We note, however, that there is at present only one Peche phase radiocarbon sample. The placement of the Peche phase date at the earliest part of the temporal continuum makes sense intuitively, for Peche phase pottery corresponds to the Monte Albán Transition phase IIIA-IIIB initially defined by Caso and his colleagues. Pottery of this phase shares attributes with the earlier Monte Albán IIIA phase and the later Monte Albán IIIB-IV or what we call the Xoo phase.

Turning next to the set of Xoo or IIIB-IV phase dates, we note that there is complete temporal overlap between the ranges of the three Monte Albán samples (I-15921, TX-1815, and I-15920) and the Lambityeco samples. This implies that the Phase IV defined by Paddock (Paddock, Mogor, and Lind 1968) is contemporary with the IIIB-IV or Xoo phase of Monte Albán and should therefore be placed in the Late Classic period. As mentioned above, the subtle differences between the Monte Albán IIIB-IV or Xoo phase ceramic assemblage and the Phase IV assemblage of Lambityeco are the result of local preferences within the same ceramic phase.

We now consider the Liobaa phase dates corresponding to the Early Postclassic period. Two Early Postclassic Liobaa samples (Beta-205984 and Beta-205986) were recovered from Lambityeco during the SACOI project. One of the samples (Beta-205986) 920±30 CE comes from an oven dug through the floor in the room of an abandoned Xoo phase house whose tomb is dated to 520±70 CE (SACOI Beta-205983). No pottery was found in the feature, which may have been used to process salt. The second date, Beta-205984, corresponds to an outdoor open-air ceramic oven dating to 940±60 CE and is associated with Liobaa phase pottery. Liobaa phase pottery is wholly unlike the ceramics Paddock used to define Phase IV (Paddock, Mogor, and Lind 1968) at Lambityeco. The Liobaa assemblage is made from coarse brown paste and consists of jars or ollas, boot-shaped jars known as patojos, conical/hemispherical bowls, miniature vessels, and poorly made handled censers or sahumadores. The pottery occurs as a widespread but light scatter in the plow zone of the site. Not a single Liobaa sherd was recovered in any of the nine tombs, thirty-four burials, and other sealed features of the seventeen Xoo phase Lambityeco residences explored during the SACOI project.

Table A1.4 presents the radiocarbon dates after calibration. Calibrated dates are generally preferred for two reasons. The intervals for calibrated dates are associated with a 95 percent probability that the computed date falls within the specified interval whereas uncalibrated dates are associated with a lower 68 percent probability. It is also known that uncalibrated dates contain a systematic error factor for they do not correct for fluctuations in the carbon content of the atmosphere through time. Calibrated dates are sometimes associated with two or more non-overlapping intervals depending where on the calibration curve the sample falls. Multiple intervals for a single sample are indicated in Table A1.4.¹ Finally the SACOI dates are once again highlighted in gray.

In general terms the calibrated dates do not change the overall interpretation offered for the set of uncalibrated dates presented in Table A1.3. The calibrated series still forms a steady progression of overlapping dates from the Late Classic through the Postclassic period, and samples associated with pottery from the same phase are positioned adjacent to one another after arranging the dates in a temporal series from oldest to most recent. The calibrated dates are useful because they indicate in some instances how the temporal limits of a phase may be fine-tuned. For example, one of the uncalibrated dates from Lambityeco SACOI, Beta-205982, places the tomb of a residence in the Peche phase, yet only Xoo phase pottery was found. The calibrated sample interval, however, falls well within the range of other Xoo phase dates.

A second insight concerns the terminal date for the Xoo phase. Whereas the calibrated dates of Xoo or IIIB-IV samples collected years ago present intervals on the order of 500 years, the SACOI dates from the succeeding Liobaa phase are much more manageable (samples Beta-205987, Beta-205989, Beta-205984, Beta-205986, and Beta-205990) and present a cluster of Early Postclassic dates associated with Liobaa phase pottery spanning the interval from 860 CE to 1300 CE. These dates suggest that the terminal date for the Late Classic Xoo phase may fall between 800–900 CE, somewhat later than previously proposed (Markens 2008; Winter 2006). This adjustment would explain the presence of the two Tohil Plumbate vessels that Caso and his colleagues recovered from Monte Albán Tombs 47 and 52, which otherwise contained Xoo phase pottery (Caso, Bernal, and Acosta 1967:431; E. W. Andrews V, personal communication, 2009). Tohil Plumbate is a pan-Mesoamerican diagnostic of the Early Postclassic period (see Neff 1989a, 1989b).

That said, there also are some very obvious limitations inherent in this set of calibrated dates. Several of the carbon samples, especially those acquired decades ago, display temporal intervals far too long to be useful. Samples such as M-2096 from Guilá Naquitz or M-1151 from Yagul display ranges 600 years or more in duration. These extremely long ranges may result from smaller sample size or shorter counting time in the detector. On a more general level, the Late Classic period is simply not one of the intervals for which radiocarbon dating is very precise (C. Patrick, personal communication, 2009). We see this especially in the series of Late Classic Xoo phase dates where the average range is about 500 years in length.

To assign calendar dates to the phases in Table A1.4 we divide the area of overlap between the later dates of a phase and the earlier dates of the succeeding one. We place the Late Classic Peche phase from 550 to 650 CE, the Xoo phase from 650 to 850 CE, the Early Postclassic Liobaa phase from 850 to 1200 CE, and the Late Postclassic Chila phase from 1200 to 1521 CE.

Finally, we note that reassigning the Phase IV ceramic assemblage of Lambityeco to the Late Classic period entails two important implications. First, other sites identified as Early Postclassic components during the settlement pattern survey of the valley (Blanton et al. 1982; Finsten 1995; Kowalewski et al. 1989) based on similarities of pottery to the Lambityeco assemblage should also be reassigned to the Late Classic. Doing so suggests that not only Monte Albán but most, if not all, of the major urban centers throughout the valley experienced a similarly sudden population decline. These include Jalieza, Macuilxóchitl, Cuilapan, El Palmillo, Santa Ana del Valle, and El Cerro de la Campana, among others. Second, we need to reconsider existing models of political, economic, and social reorganization that took place in the valley during the Early Postclassic period.

Acknowledgments. The authors are most grateful to Mr. Christopher Patrick, Deputy Director of Beta Analytic Radiocarbon Dating Laboratory of Miami, Florida, for his calibration of radiocarbon samples acquired decades ago and for his attentive response to the many questions asked of him. We also acknowledge with gratitude the laboratory’s president, Mr. Darden Hood, who kindly offered sound advice regarding the appropriate format for presenting the radiocarbon samples in Table A1.2. Michael Lind and Javier Urcid made many helpful suggestions during the preparation of this appendix for which we extend our sincere thanks. The authors assume responsibility for any errors.

NOTE

1. The two most recent ranges for two radiocarbon samples were omitted from Table A1.4 to economize on space. (1) Sample Beta-205992 from Xaagá, Area C, Structure C, Feature 17: ranges 1720–1820 CE and 1920–1950 CE are omitted from the table. (2) Sample M-1250 from Yagul, Patio A, North Room: ranges 1770–1800 CE and 1940–1950 CE are omitted from the table as well.

TABLE A1.1. Chronological chart for the State of Oaxaca

TABLE A1.2. Radiocarbon dates from the Valley of Oaxaca for the Late Classic and Postclassic periods

TABLE A1.3. Uncalibrated radiocarbon dates from the Valley of Oaxaca

TABLE A1.4. Calibrated radiocarbon dates from the Valley of Oaxaca