Paleoecological evidence of an Early Postclassic occupation in the southwestern Maya lowlands:...

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Paleoecological Evidence of an Early Postclassic Occupation in the Southwestern Maya Lowlands: Laguna Las Pozas, Guatemala Author(s): Kevin J. Johnston, Andrew J. Breckenridge and Barbara C. Hansen Source: Latin American Antiquity, Vol. 12, No. 2 (Jun., 2001), pp. 149-166Published by: Society for American ArchaeologyStable URL: http://www.jstor.org/stable/972053Accessed: 20-05-2015 12:45 UTC

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Magnetic, palynological, and paleoecological data indicate that in the Rio de la Pasion drainage, one of the most thoroughly investigated areas of the southern Maya lowlands, a refugee population remained in the Laguna Las Pozas basin long after the Classic Maya collapse and the Terminal Classic period, previously identified by archaeologists as eras of near-total regional abandonment. During the Early Postclassic period, ca. A.D. 900 to 1200, agriculturalists colonized and deforested the Laguna Las Pozas basin for agriculture while adjacent, abandoned terrain was undergoing reforestation. After discussing the archaeological utility of magnetic analyses, we conclude thatfollowing the Maya collapse, some refugee populations migrated to geographically marginal non-degraded landscapes within the southern lowlands not previously occupied by the Classic Maya.

Los resultados de analisis magneticos, palinologicos y paleoecologicos muestran que en el Valle del Rio de la Pasion, una de las zonas ma's estudiadas de las Tierras Bajas del sur del a'rea Maya, una poblacion de refugiados permanecio en la cuenca de la Laguna Las Pozas hasta mucho despue's del colapso de la sociedad maya del Clasico y del periodo Clasico Terminal, hasta ahora considerados por los arqueologos como periodos de Abandono casi total de la zona. Durante el Poscla'sico Temprano, c. 900- 1200 d. C., Ia cuenca de la Laguna Las Pozas fue colonizada por campesinos y desforestada para el cultivo, mientras que el terreno abandonado adyacente experimentaba un proceso de reforestacion. La aplicacion paralela de te'cnicas de ana'lisis magne'ticos y palinologicos constituye una herramienta de investigacion muy poderosa para el estudio de los cambios ambientales ecoge'nicos y antropoge'nicos. Los ana'lisis magne'ticos revelan tendencias de erosion y modificaciones del terreno local, mientras que los ana'lisis de polen ayudan fundamentalmente a determinar los cambios en la vegetacion a nivel regional. En este trabajo, tras una discusion sobre la utilidad arqueologica de los ana'lisis magne'ticos, llegamos a la conclusion de que despue's del colapso Maya, algunos grupos de refugiados emigraron a terrenos geogrdficamente marginales no degradados, dentro de los limites de las Tie- rras Bajas del sur (por ejemplo, a la cuenca de la Laguna Las Pozas), que no habian sido ocupados intensivamente por los mayas durante el periodo Clasico.

Kevin J. Johnston * Department of Anthropology, Ohio State University, 244 Lord Hall 124 W. 17th Ave., Columbus, OH 43210-1364 Andrew J. Breckenridge * Large Lakes Observatory, 109 RLB, University of Minnesota at Duluth. 10 University Drive, Duluth, MN 55812 Barbara C. Hansen * Limnological Research Center, University of Minnesota, 220 Pillsbury Hall, Minneapolis, MN 55455-0219

Latin American Antiquity, 12(2), 2001, pp. 149- 166 Copyright(D 2001 by the Society for American Archaeology

A mong the collapses of ancient complex soci- eties investigated by archaeologists, that of the southern lowland Classic Maya is dis-

tinctive because of its alleged catastrophic character (Yoffee 1988). Following centuries of continuous population growth and increasing cultural complexity, Maya society in the southern lowlands suffered during the ninth century A.D. a severe cultural, political, and economic decline, from which it did not recover. According to the model endorsed by many archaeologists, the Maya collapse throughout much of the southern lowlands was a demographic collapse precipitated by an anthropogenic environmental cri-

sis, itself set in motion by agricultural intensification necessitated by population growth (Abrams and Rue 1988; Abrams et al. 1996; Culbert 1988; D. Rice 1993; D. Rice et al.1985; Sanders 1973; Sanders and Webster 1994; Santley et al. 1986; Webster et al. 1992; Willey and Shimkin 1973; Wingard 1996), and accompanied by political crisis (Adams 1999; Martin and Grube 2000; Sharer 1994).1 Turner and his colleagues (Turner 1989, 1990; Whitmore et al. 1990:35) propose that during a 200-year period marking the collapse and its immediate aftermath (ca. A.D.800to 1000), the southernMayapopulation suffered a remarkable 80 to 90 percent decline.

149

PALEOECOLOGICAL EVIDENEE OF AN EARLY POSTCLASSIC OCCUPATION IN 1 HE SOU 1 HWESTERN MAYA LOWLANDS:

LAGUNA LAS POZAS, GUATEMALA

Kevin J. Johnston, Andrew J. Breckenridge, and Barbara C. Hansen



150 LATIN AMERICAN ANTIQUITY [Vol. 12, No. 2, 2001

' * Archaeological Site

g Central Peten Lakes District _

_

Figure 1. Map of Maya lowlands showing locations mentioned in the text.

Advocates of the model propose that by A.D. 950, most Maya had abandoned all but a few areas of the southern lowlands (e.g., Culbert 1988:87-89).

The empincal foundation for this model of the collapse and its aftermath is settlement data collected at centers and their suwounding rural sectors where Maya

populations reached maximum densities during the period that preceded the collapse. In the southern lowlands, archaeologists have collected these data primarily in three general areas (Figure 1): (1) in and around the large centers of Tikal, Uaxactun, andYaxha (Carr and Hazard 1961; Ford 1986; Puleston 1974



Johnston et al.] PALEOECOLOGICAL EVIDENCE OF AN EARLY POSTCLASSIC OCCUPATION 151

1983; Ricketson and Ricketson 1937; Wauchope 1934); (2) in the central Peten lakes district (P. Rice and D. Rice 1985; D. Rice and P. Rice 1990); and (3) in the Belize Rivervalley (A. Chase 1985; Fedick and Ford 1990; Ford andFedick 1992). Only in these areas have archaeologists intensively sampled rural settlement at large spatial scales. We define as "rural" all settlement located outside polity capitals, secondary administrative centers, their sustaining areas, and minor centers. Beyond these three areas rural settlement patterns remain poorly known.2 In the Rio de la Pasion drainage, for example, archaeologists have surveyed and excavated mostly non-rural settlement (Demarest 1997; Palka 1997; Willey 1973, 1990).

From these settlement data, archaeologists reconstruct post-collapse southern lowland cultural trends. Yet the data from which the trends have been reconstructed were collected primarily at sites whose populations peaked prior to the collapse and plummeted during or after the collapse. Are the post-collapse cultural trends that occurred at sites largely abandoned during the collapse likely to be representative of southern lowland post-collapse cultural trends as a whole? To reconstruct broad southern lowland trends from these data one must assume that the question can be answered aff1rmatively. Yet this assumption is likely to be problematic if a substantial portioFn of the post-collapse population lived in areas not

densely settled and intensively exploited by pre-collapse peoples. Do such areas exist; that is, did the Classic Maya not intensively occupy and exploit all areas of the southern lowlands? Archaeologists do not know the answer to this question because vast areas of the southern lowlands remain unexplored and unsurveyed. As Sharer (1994:341) observes, when survey of these areas is completed, archaeologists' views of southern lowland post-collapse cultural dynamics may change.

This article examines evidence inconsistent with the model of the aftermath of the collapse developed from these settlement data. From this new evidence we conclude that in at least part of the vast southern lowland area that archaeologists have yet to survey, a post-collapse population was present during the Early Post- classic period, ca. A.D.900 to 1200.18his discovery at Laguna Las Pozas, in the southwestern section of the Maya lowlands, within the Rio de la Pasion drainage (Figure 2), is significant for several reasons. First, from settlement data collected atAltar de Sacrificios, Seibal, Itzan, Dos Pilas, Aguateca, Arroyo de Piedra (near Dos

Pilas), Tamarandito, Punta de Chimino, and other sites within the same RIo de la Pasion drainage (Figures 1 and 2), archaeologists have concluded thatbyA.D. 950 to 1000 only a very small, widely dispersed population remained within the drainage. The Laguna Las Pozas data suggest that this conclusion must be modified. Second, the evidence of an Early Postclassic occupation is entirely paleoecological. That is, it was collected without the benefit of pedestrian survey, which is the traditional means of gathering data about ancient Maya occupations. We employ a method of paleoecological reconstruction-magnetic analysis- notwidely used in the southem lowlands (Balser 1995; Curtisetal. 1998). Third, thepaleoecologicaldataindi- cate that farmers occupied the Las Pozas basin in significant numbers during the Early Postclassic period only. In contrast to most of the Pasion drainage, the Las Pozas basin was not intensively occupied during earlier periods. From this we conclude that at least some post-collapse peoples (we do not describe them as "Maya" because we do not yet know their ethnic identity) preferred to occupy terrain not inhabited by their pre-collapse predecessors. This preference is perhaps best explained in tenns of the traditional model of the collapse, which postulates that the collapse tran- spired because the Maya significantly degraded their environment through agricultural intensification. Assuming that the model is reliable, would post-collapse peoples have occupied terrain that earlier peoples abandoned because it was degraded? Might some post-collapse peoples, as the Laguna Las Pozas dat suggest, have preferred terrain that today remains unsurveyed because in antiquity it was not intensively occupied by the pre-collapse Maya?

We begin by reviewing the traditional model of post-collapse (Tenninal Classic and Early Postclas- sic) Maya settlement in the southern lowlands. Most archaeologists characterize the post-collapse occupation as small in scale and widely dispersed. Next we review paleoecological data indicating that a farming population was present during the Early Postclassic period in the Laguna Las Pozas basin. We conclude by examining the archaeological sig- nificance of the Laguna Las Pozas discovery.

Previous Research: The Classic Maya Collapse and Its Aftermath

The ninth-century collapse affected primarily the Maya inhabiting the southern lowland region. Archaeologists divide the eras of southern lowland




Figure 2. Map of the Laguna Las Pozas and vicinity.

cultural history relevant to this paper into three periods: (1) the Late Classic (A.D. 550 to 800), a period of great cultural florescence that predates the collapse; (2) the Terminal Classic (A.D. 800 to 900), which marks the collapse and its immediate aftermath; and (3) the Early Postclassic (A.D. 900 to 1200), which postdates the collapse and, according to the traditional collapse model, was a period of intense regional abandonment, particularly along the Rio de la Pasion. Several archaeologists propose that

prior to the collapse, the southern lowlands were inhabited by at least several million Maya (D. Rice andCulbert l990;Whitmore etal. 1990), greatnum- bers of who occupied large administrative centers, their dependencies, and the rural hinterlands that sur- rounded them (Ashmore 1981). Within a broad area defined by Rio Bec to the north and Tikal to the south (Figure 1), Classic-period rural population densities were high, averaging 180 persons per km2 (D. Rice and Culbert 1990:26). Urban population densities



PALEOECOLOGICAL EVIDENCE OF AN EARLY POSTCLASSIC OCCUPATION 153 Johnston et al.]

were considerably higher, ranging from 500 to 800 persons per km2 at many sites (D. Rice and Culbert 1990:20) to more than 1,000 persons per km2 at a few (A. Chase and D. Chase 1998:62; Webster and Freter 1990:51). Some archaeologists conclude that prior to the collapse the southern lowlands was one of the most densely occupied regions of the preindustrial world (D. Rice and Culbert 1990:26).

According to the traditional collapse model (Abrams and Rue 1988; Abrams et al.1996; Culbert 1988; D. Rice 1993; D. Rice et al.1985; Sanders 1973; Sanders andWebster 1994; Santleyetal.1986; Sharer 1994; Websteret al.1992; Willey and Shimkin 1973), Late Classic agricultural intensification necessitated by population growth transformed large areas of the southern lowland environment in a manner that eventually reduced agricultural productivity. When the southern lowland agriculture system collapsed, so did the population and the society that it supported. Important political, economic, and ideolog- ical processes contributed to the collapse (Sharer 1994:341; Webster 1999). During the ninth century, the distinctive elite-sector cultural achievements for which the Classic Maya are well known-dynastic rulership, the construction of massive public archi- tecture, the production of hieroglyphic texts and royal art all but ceased in the southern lowlands (Sharer 1994:338-356). By the mid-tenth century, only sparse, widely scattered remnant populations remained at once massive, now largely abandoned centers (Culbert 1973).

The traditional model also states that as the collapse completed its sweep, politically centralized, densely clustered populations survived in only two areas of the interior southern lowlands (not including the Caribbean coast and northern Belize): the central Peten lakes district (P. Rice and D. Rice 1985; Willey 1986), and, in the extreme southwestern cor- ner of the lowlands, at Seibal and Altar de Sacrifi- cios, located along the Rio de laPasion (Willey 1973, 1990) (Figure 1). Some archaeologists attribute the unique cultural and political renaissance of Altar de Sacrificios and Seibal to an invasion of Putun (or Chontal) Maya from the Gulf Coast of Mexico (Adams 1973; Andrews and Sabloff 1986; D. Rice 1986; Sabloff 1973,1977; SabloffandWilley 1967; Sharer 1994; Willey 1986). Terminal Classic activity within the Pasion drainage (of which Laguna Las Pozas is a part) was widespread (Figures 1 and 2): modest occupation dating to this period has been

documented at Itzan (Johnston 1994), Punta de Chimino, Tamarandito, and Dos Pilas (Foias 1996). According to research carried out at many locations within the drainage, by the Early Postclassic the Pasion was so thoroughly depopulated as to be rea- sonably characterized as abandoned. The last archaeologically discernible settlement at Seibal dates to about A.D. 930 (Sabloff 1973: 110) and at Altar de Sacrificios to about A.D. 1000 (Adams 1973: 143).

The period of interest to this paper the Early Postclassic was a nadirofcultural activity and population density in most of the southern lowlands (Willey 1986:31). Remnantpopulations shiftedeast toward the Caribbean coast of Belize and north toward the Yucatan Peninsula (Andrews and Sabloff 1986; A. Chase and P. Rice 1985; Sabloff 1977). In Belize, Early Postclassic populations remained at the riverine sites of Barton Ramie (P. Rice 1986:253; Willey 1986; Willey et al. 1965), Colha (Willey 1990:23), Lamanai (Pendergast 1986), and Nohmul (A. Chase and C. Chase 1980; Hammond 1974), at the coastal center of Santa Rita Corozal (D. Chase 1990), and at numerous small coastal and interior set- tlements (Guderjan and Garber 1995; Masson 1999). Inland and well to the west, the southern lowlands' largest archaeologically documented population in the central Peten lakes district continued to decline, although densities in some areas remained substantial (more than 60 persons per square kilometer [D. Rice and P. Rice 1990:145]). Elsewhere in the rainforest interior small groups squatted in the remains of ruined centers. At Tikal, for example, about 1,000 Maya inhabited a central 120-km2 zone (population density of 8.1 persons per km2), which formerly had supported many tens of thousands (Culbert et al. 1990:116; Fry 1990:297).

Research at Altar de Sacrificios (Willey 1973), Seibal (Willey et al. 1975), and in the Petexbatun region (Demarest 1997) indicates that Early Post- classic settlement in the Pasion drainage was small in scale and widely scattered. Very small quantities of Tohil Plumbate, a distinctive Early Postclassic diagnostic (Willey 1973:5), have been found along most of the river's length: at Altar de Sacrificios (Adams 1971: Figure 74c), Seibal (Willey 1990:260), Aguateca and Punta de Chimino (Foias 1996), and two island sites located near the limits of navigabil- ity on the upper Rio de la Pasion (Brown 1958). At Itzan, evidence of Early Postclassic occupation includes Tohil Plumbate sherds found atop a rural Late




Classic housemound and a Paxcaman Red, or possibly Augustine Red, modeled effigy foot recovered in the site's Nori Plaza (see A. Chase and D. Chase 1985:15). These data are consistent with the traditional model's hypothesized pattern of regional cultural decline, depopulation, and virtual abandonment. Judging from the limnological data presented in this paper, that southern lowland "abandonment" was more regionally variable and in some cases less com- plete than the traditional model suggests.

The Laguna Las Pozas Paleoecological Data

The Laguna Las Pozas research is one component of a larger study of the paleoecology of the southwestern Maya lowlands (Breckenridge 2000; John- ston and Breckenridge 1999). In 1997 the two senior authors cored lagoons near the site of Itzan (Figure 1), where Johnston has been conducting archaeological settlement research (Johnston 1994). To assess its suitability for future, more intensive pale- oclimate studies, we cored the Laguna Las Pozas, located in a small, closed basin approximately 7 km southeast of Aguateca (Figure 2). Although the Las Pozas basin is adjacent to areas extensively investigated by Harvard axld Vanderbilt University projects (Demarest 1997; Willey 1973, 1990; Willey et al. 1975), archaeologists have not yet conducted research within the basin, and so its settlement history remains unknown. Our objective in coring Laguna Las Pozas was to (a) obtain a sediment record from a closed basin with no evidence of intensive Late Classic Maya settlement and (b) compare it with the Itzan records, all obtained from an open basin (connected to the Rio de la Pasion) intensively occupied by Preclassic and Classic Maya. Our discovery at Laguna Las Pozas of an Early Postclassic colonization was wholly unexpected.

Field and Laboratory Methods

Laguna Las Pozas (Figure 2), which lies at 16° 20' N, 90° 10'W, is 150mabove sealevel andhas amea- sured maximum depth of 23 m. The lake is approximately 2 km long (north-south) and .75 km wide (east-west). Terrain flanking the north, east, and south shores of the lake recently has been deforested for agriculture and cattle raxlching. Located about .5 km east of the lake are the small village of Las Pozas and a military outpost of the same name.

At the north end of the lake we retrieved two cores totaling 3.1 meters of sediment from 9.4 meters of

water. The first core penetrated sediment from 0 to 69 cm; the second core penetrated sediment from 68 to 310 cm. (Ihe uppermost sediment of the second core [0 to 68 cm] was lost during shipping from Guatemala to the United States.) The coring instrument was a square-rod piston corer fitted with a special collar to accommodate 2.75-inch transparent polycarbonate tubing (Fisher et al. 1992; Wright et al. 1984). We cored from the side of an anchored rowboat.

Like all lake sediments, those of the Laguna Las Pozas reflect dynamic interactions between the watershed, groundwater, the local atmosphere, and the lake environment (Binford et al. 1983). Although the Laguna sediments presumably are characterized by some spatial heterogeneity, those sampled in our cores were recovered at depths probably too great to have been affected by wind-driven surface water cur- rents, and so we assume they are representative of general patterns within the basin's sediments. No doubt additional coring of the lake at various water depths would enhance understanding of sediment- focusing patterns and other basin dynamics.

All cores were sealed and packed in their polycarbonate tubing in the field, and they were shipped to the laboratory. Breckenridge conducted sediment analyses of the cores at the University of Minnesota Limnological Research Center (Dr. Kerry Kelts, Director) and the Institute of Rock Magnetism (Dr. Subir Banerjee, Director). Laboratory analysis focused on comparing and correlating multiple records of Maya impact on the local and regional environment. These records include sediment magnetic parameters (magnetic susceptibility, Saturation Isothermal Remanent Magnetization [SIRM], and Anhysteretic Isothermal Remanent Magnetization [ARM]), total organic and inorganic carbon, and bulk sediment elemental composition.

Barbara Hansen analyzed the cores' pollen contents. Radiocarbon dating of four samples from the core was done by Accelerator Mass Spectrometry (AMS) at the National Science Foundation-Univer- sity of ArizonaAMS Laboratory and at the National Ocean Sciences AMS Facility at Woods Hole Oceanographic Institution, Massachusetts (Table 1).

Core Lithology

Visual and smear slide inspection of the core reveals that its sediments are divided into three non-lami- natedunits. UnitA (Oto 110cm) is acalcareous mud containing fine silt-sized (.008-.016 mm) carbonates



PALEOECOLOGICAL EVIDENCE OF AN EARLY POSTCLASSIC OCCUPATION 155 Johnston et al.]

and moderate amounts of fine-grained organic matter (sapropel). Sediment color grades from light gray- brown at the top to dark gray-brown at the bottom. UnitA grades into Unit B ( 1 10 to 150 cm), suggesting that the two are lithogically continuous. Unit B, also a sapropelic, carbonate mud, is characterized by several distinctive features. First, its organic matter content is higher than, and thus its color is considerably darker (very dark gray) than, that of Units A and C. Second, its carbonate crystals are on average larger and exhibit a greater range of size variation (from fine silt to silt-sized [.008-.062 mm]) than those of the other two units. Third, smear slide analysis indicates that the silt-sized rhombohedral carbonate crystals of Unit B are similar in size to those found in lake shore soils a feature that Unit A and C sediments do not share. Fourth, X-ray diffraction reveals that dolomite-to-calcite ratios in Unit B sediments (1:1) are higher than those in Units A and C. Dolomite and calcite crystals are differently sized. The small, fine silt-sized sediments of Units A, B, and C appear to be calcite (CaC03), which originates in lakes from bio-induced precipitation (Whitmore et al. 1996:283). The larger, silt-sized grains, found primarily in Unit B, appear to be dolomite, a material that does not originate from bio-induced precipitation and would enter the Laguna Las Pozas only by being washed in from the surrounding terrain. These features suggest that Unit B accumulated during a period of increased detrital input.

Whereas UnitA grades into Unit B, the transition from Unit B to Unit C is marked by a 10-cm (140 to 150 cm) sediment zone that is mottled, suggesting that Units B and C are interbedded. It is possible that this transitional zone is the product of sediment disturbance. Unit C (150 to 230 cm), which resembles Unit A, is a light gray-brown calcareous mud (fine silt-sized) containing moderate amounts of organic matter.

Magnetic, geochemical, and pollen analyses were undertaken to investigate the sedimentary history represented by the three stratigraphic units.

Paleomagnetic Analysis

Magnetic analysis, when used in conjunction with the analysis of other lake sediment records, enables paleoecologists to measure and reconstruct ancient landscape changes, including the intensity and dura- tion of past erosion episodes. Magnetic analyses measure the capacity of sediment particles to hold a

magnetic charge under a variety of conditions (King and Hunt 1991; Moskowitz et al. 1993; Thompson and Oldfield 1986). The capacity, or "susceptibility," of sediments varies as a function of the concentration, grain size, and mineralogy of the sediment. Sed- iments with high concentrations of magnetic minerals produce strong susceptibility readings, while those with low mineral concentrations produce weak readings. Although magnetic materials can form in lake sediment through in situ chemical processes, most of those found in lake sediments originate outside the lake (Thompson and Oldfield 1986).

As is the case in temperate environments, the principal source of magnetic minerals in the rain- forested tropics is soil, and the source of soil magnetic minerals is subsoil and bedrock (Thompson and Oldfield 1986:101-123). In the Laguna Las Pozas basin and throughout much of the Pasion drainage, the bedrock is Tertiary (Paleocene-Eocene) marine carbonates, sometimes interbedded with evaporates (Weyl 1980). The- greater the exposure of bedrock and the subsoil clay layers that cover it, the greater the magnetic content of the rainforest soils. Miner- als move from land surfaces into lake basins through the action of streams, wind, and, most importantly, surface runoff. In closed lake systems, in other words, the magnetic content of sediment is determined largely by erosion of the soils that surround the lake. Thus lake sediment magnetism reflects processes that occurred at the local level. Sediments produced during periods of intense soil erosion should have high susceptibility readings; those produced when the land lake is forested, and thus when soil erosion is minimal, should have lower readings.

The ease with which a sediment material can be magnetized or maintain a magnetic field is determined by its magnetic content. X-ray diffraction analysis indicates that the principal ferrimagnetic minerals in the Laguna Las Pozas lake sediments and soils are magnetite and maghemite. The two processes most likely to be responsible for the pres- ence of these minerals in the Las Pozas sediments are (a) natural climate change and (b) human modification of the landscape (Thompson and Oldfield 1986). From the magnetic and other geological analyses, we conclude that the process responsible for several major changes in the Las Pozas magnetic record is anthropogenic landscape modification.

Laboratory Methods. To begin magnetic analy-




volume susceptibility (k)

mass normalized susceptibility (c) ca/c ARM SIRM ARM/SIRM

A

E 100

o 150

200

e - 250

B

C

0 6 xlO 14 xlO O 3 xlO 6 XlO O

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0.003 0

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40 1 00 1 60 0.08 0.22

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Am2/kg

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Figure 3. The Laguna Las Pozas magnetic data. Dashed lines separate major lithologic units.

sis, the cores first were scanned in the tube using a Geotek multi-sensor core logger (MSCL). This mea- sures gamma ray attenuation, or wet bulk density. "Wet" volume susceptibility (measured while the core is in its tube) was determined by passing each core through a Bartington MS2 C sensor loop (Thompson and Oldfield 1986). The loop's weak magnetic field induces in the sediments a magnetBc field, facilitating measurement of the magnetic character of sediments. Thereafter the cores were split lengthwise, described, and photographed. Half of each core was set aside for analysis and half was archived in the Limnological Research Center stor- age facility. Samples for mass-normalized susceptibility, ARM, and SIRM analyses were removed from the core at close intervals (5 cm) at lithologic bound- aries and at coarser intervals (10 to 20 cm) where sediment had a more uniform character. Samples were stored in 5.28 cm3 plastic boxes, freeze-dried, and weighed, and magnetic parameters were calculated from their dry weights. Volume susceptibility and mass normalized magnetic susceptibility of boxed samples were measured with a Geofyzika KLY-2 (KappaBridge) AC Susceptibility Bridge, which determines the magnetic response of samples to a small applied magnetic field, or the ease with which a material can be magnetized (Thompson and Oldfield 1986:25, 280).

During ARM analysis, samples were exposed to a 100 mT peak alternating field combined with a

superimposed .05 mT DC field. The alternating and DC fields were applied with a modified Schonstedt alternating field demagnetizer, and the remanent magnetization was measured with a 2-G Supercon- ducting Rock Magnetometer. SIRM measurements were obtained by applying a very strong magnetic field (1 T) to the samples, and theirremanence magnetization was measured with the Superconducting Rock Magnetometer.

Results. Magnetic analyses reveal that between 100 to 145 cm (Unit B), and peaking at 130 cm, the core contains a pronounced "spike" in magnetic mineral concentration (Figure 3). The parameters, ARM, SIRM are sensitive to mineralogy, magnetic mineral concentration, and, significantly, magnetic mineral grain size. To determine whether the magnetic spike simply is an expression of (non-anthropogenic) stratigraphic variations in grain size, ratios between the parameters, ARM, SIRM were examined (Fig- ures 3e and f) (Banerjee et al. 1981; King et al. 1982). Throughout the Las Pozas core the three parameters show little variation in magnitude, suggesting little stratigraphic variation in grain size. We conclude that the magnetic spike is not an expression of grain . . .

slze varlatlon.

We then hypothesized that the spike reflects ancient cultural modifications of the surrounding landscape and, judging from its shape, concluded that it reveals several sequential cultural and environmental processes: (a) colonization of the Laguna Las




Sediment Lithology from Coulometry

Carbon Coulometry

o

so

100

150

200

A

B

c

250 + s l l T l ' l, l i, l l f

0 5 10 15 20

Percent TIC: Circles Percent TOC: Squares

Figure 4. Carbon coulometry and sediment lithology of the 1 units.

Pozas basin the beginning of the spike; (b) deforestation for agriculture the apex of the spike; and (c) abandonment of the basin and reforestation the decline of the spike. To test this hypothesis, two additional analyses were performed.

Elemental Chemistry

First, Breckenridge measured the organic and inorganic content of the cores through carbon coulometry. If the magnetic spike is the product of topsoil erosion, it should be accompanied by a pronounced lithogical change because detritus would have been carried into the lake by the same erosion processes that transported the magnetic mineral-bearing sediments. Sediment was sampled for organic content at 5-to- 10 cm intervals, and total organic carbon (TOC) and total inorganic carbon (TIC) mass were measured with aUIC model 501 1 CO2 coulometer, which detects carbon by automatic, coulometric titration (Engleman et al. 1985). On the assumption that the inorganic carbon is calcite authigenically produced in the lake or in detrital limestone during or after sediment deposition, TIC measurements were converted to calcium carbonate percentages. TOC measurements were converted to percent organic matter. By summing the values for calcite and organic matter,

0 20 40 60 80 100 Percent Mass of Bulk Sediment

Laguna Las Pozas core. Dashed lines separate major lithologic

and subtracting this from the total mass of sediment, approximations of the total mass of inorganic, non- carbonate, and clastic material were calculated. The results, illustrated in Figure 4, reveal that between 100 and 140 cm, organic carbon spikes in a manner reminiscent of magnetism. Carbon coulometry analysis is consistent with our hypothesis.

Elsewhere in the southern lowlands, paleoecologists have demonstrated that Maya land-clearance episodes are marked lithologically by major influxes of phosphorus, silica, and other elements (Curtis et al. 1998: Figure 5; Deevey et al. 1979; D. Rice et al. 1985: Figure 7.2). Breckenridge next tested our interpretation of the magnetic data by measuring nine bulk

: sediment, major element concentrations on a Perkin/Elmer/Ealn 5000 Inductively Coupled Plasma Mass Spectrometer (ICPMS) following pre-

v treatment by lithium metaborate infusion (Figure 5). L Between 1 10 and 150 cm (Unit B) concentrations of

iron, titanium, aluminum, and silica substantially L increase, almost certainly as the result of an increase

in soil colluviation. Elsewhere in the core aluminum L and silica ratios are stable. Phosphorus influxes of

which in the central Peten lakes are attributed to soil denudation (D. Rice et al. 1985; Tsukada 1966) peaks at 100 cm, as does magnesium, most of which



[Vol. 12, No. 2, 2001 158 LATIN AMERICAN ANTIQUITY

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Figure 5. Major element concentrations in the Laguna Las Pozas core. Dashed lines separate major lithologic units.

probably was washed in as dolomite from surrounding bedrock and soils. Smear slide inspection reveals the same trend. The elementary geochemistry data are consistent with our interpretation of the magnetic data.

We conclude from the magnetic susceptibility, the ARM, SIRM, the organic carbon analyses, and the elementary geochemistry analysis that the pronounced magnetic spike in the Laguna Las Pozas core corresponds to an episode of basin colonization, population expansion and agricultural deforestation, and abandonment.

Pollen

To gain a more detailed picture of paleoecological change in and around the Las Pozas basin, eight 1 cc samples were removed at 40-cm intervals from the cores for pollen analysis. Samples were spiked with .5 ml of microspheres (6.955 + 1.5X104) to determine pollen concentrations per cc of sediment. Breckenridge prepared the samples at the Limno- logical Research Center using standard laboratory procedures: 10% KOH, acetolysis, HF, and nitexing (Cwynar et al. 1979; Faegri and Iversen 1989). Hansen performed the pollen analysis. This portion of the study was a preliminary effort to test the fea- sibility of a palynological analyses of these lake sediments and therefore some pollen sums are lower than 300. Charcoal fragments were counted in pollen preparations and assigned to one of two categories: fragments over. 100, and those between ca. 30 andlO0. Charcoal fragment counts are plotted as concentrations per cc of sediment (MacDonald et al. 1991). Pollen results are summarized in a traditional

relative percentage diagram (Figure 6) and as summary curves included with sediment analyses. Zon- ing for pollen interpretation is based on visual inspection.

The Pozas pollen record suggests dramatic changes in vegetation in the Rio de la Pasion basin during the last 3,000 years. Three pollen zones within the core are recognized. None correspond strati- graphically to the core's three major lithologic units.

The bottom zone of the pollen record (235 cm and lower, in Unit C) is dominated by Urticales pollen, primarily Moraceae (e.g., Brosimum t. and Cercropia spp.) and Urticaceae. Both of these families are important components of the tropical semideciduous forest. Pollen of savanna species or disturbance and agricultural plants (e.g. Poaceae, Ambrosia sp., Tubu- liflorae) is nearly absent. This was a pre-clearance period during which closed forest occupied the Rio Pasion drainage area.

The middle pollen zone (195 to 1 15 cm), encom- passing portions of Units C and B, is dominated by Poaceae, Tubuliflorae, and Byrsonima sp., a tree of savanna landscapes. Rapidly declining Moraceae and Urticaceae pollen below the magnetic spike indicate that during this period forest coverage was reduced because of clearance for agriculture or perhaps climate change. The expansion of savanna (grasses, Poaceae, Tubuliflorae and Byrsonima sp.) also reflects either agricultural practices or climate aridity. Elsewhere in the Peten, comparable pollen ratios have been interpreted as evidence of a replace- ment of forests with agricultural fields (Brenner et al. 1990; D. Rice 1993). The one grain of Zea (maize) from the column was recovered at 195 cm, in Unit

Ca Mg

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Johnston et al.l PALEOECOLOGICAL EVIDENCE OF AN EARLY POSTCLASSIC OCCUPATION 159

Laguna Pozas, Yucatan, Mexico, pollen percentage diagram

Charcoal PineSoak foresrt Savanna ., ?]g,concentrations

Dlsturbance andSor agriculturalbplants ,zey Scc sediment

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Included In pollen sum -- f [

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Figure 6. Pollen diagram for Laguna Las Pozas. Pollen analysis by B. C. S. Hansen. The three lithologic units discussed in the text (designated A, B, and C) appear on the right side of the diagram separated by dashed lines. The open curves are XS exaggerations. Graphics software Psidium vers. 2 by E. J. Cushing (1993).

Pozas (greater than 1 km2) pollen records reveal changes at the regional level (Bradley 1999:362). Magnetic and geochemical records, in contrast, reveal changes that occurred in the immediate vicinity of the lake. ThusX assuming that agriculture was the cause of the deforestation that is reflected in low arboreal pollen percentages, we conclude that regional vegetation within the Pasion drainage was most heavily impacted by clearance for agriculture during the period encompassed by the sediments between 235 and 115 cm a period that encompasses parts of lithologic Units C and B. Assuming also that agricultural intensity and population density are correlated, we conclude that this was the period of maximum regional population density, which archaeological research dates to the Late Classic period. General abandonment of the drainage was completed at about 150 cm (at the base of Unit B).

In contrast, according to the magnetic and geochemical records, colonization of the Laguna Las Pozas basin did not begin until about the 150 cm mark, and by the 100 cm mark the basin had been abandoned. From these data we conclude that farmers first colonized the Laguna Las Pozas at a time when populations elsewhere in the Pasion drainage were abandoning farmsteads, villages, and administrative centers. Thus, agricultural activity within the basin intensified while surrounding, abandoned areas reforested. To what time periods do lithologic Units B and C date?

C. The zone also contains high concentrations of charcoal fragments, which could reflect slash-and- burn agriculture or, alternatively, increased ecogenic fire. Judging from the magnetic record (which shows no evidence of erosion [i.e., agricultural activity] below Unit B), if the charcoal concentrations indicate agricultural activity, that activity almost certainly occurred outside the Las Pozas basin. The 155 cm sample, in Unit C, just below the base of the magnetic spike, marks the point of minimum Urticales (primarily Moraceae) pollen and therefore minimum forest coverage in the region that surrounds the Las Pozas basin.

Above 155 cm, and still within the middle pollen zone (in sediment that encompasses portions of Units B and A), arboreal pollen dramatically increases, while pollen associated with land disturbance or savanna habitat declines. This suggests either reforestation and a progressive reduction in forest clearance or a moister, moderating climate favorable to forest expansion.

The top zone of the pollen record (above about 90 cm) falls entirely within the upper half of Unit A. No pollen of agricultural or disturbance taxa are present in this zone. Indeed, the topmost pollen sample at 35 cm is dominated by high forest taxa. This rep- resents a period of regional forest regrowth.

Significantly, the pollen record reveals environmental changes that occurred at spatial scales different from those presented by the magnetic and geochemical records. In lakes the size of Laguna Las




AMS 14C Dates

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Figure 7. Sedimentology, magnetic data, 14C dates, and reconstructed chronology of the Laguna Las Pozas core.

Core Chronology

When the Unit B magnetic spike was first detected (prior to completing the pollen analysis), we hypothesized that it marks a Late Classic expansion into and a Terminal Classic abandonment of the Laguna Las Pozas basin a hypothesis consistent with archaeologists' traditional model of the collapse (described above). To test this hypothesis we AMS-dated a burned leaf fragment recovered from the core at 100.5 cm (Date 2), a depth that corresponds roughly to the upper termination point of the Unit B magnetic spike (Figure 7). Contrary to our expectations, the leaf dates to 840 + 40 14C yr B.P., placing the upper terminus of the spike not in the Terminal Clas- sic but at the end of the Early Postclassic period. The date is surprisingly late. Decades of research carried out at more than ten large centers and in their rural hinterlands along the Rio de la Pasion indicate that by the Early Postclassic the Maya had virtually abandoned the entire region. The Laguna Las Pozas paleoecological data reveal that this conclusion must be modified.

To resolve the issue of chronology three additional samples of plant macrofossils (mostly stem and carbonized leaf fragments) were dated using the

AMS method. The dates are summarized in Table 1. One sample was removed above the spike, at 52 cm (UnitA), and two were removed below it, at 147 and 242 cm. Dates 1, 3, and 4 were obtained from aggre- gated small stem and carbonized leaf fragments, each recovered from a small, discrete stratigraphic sediment sample. Each sample was screened and shell fragments and other contaminants were picked out. Date 1 corresponds to the Late Postclassic period (460 + 90 14C yr BP), the two lowermost dates (3 and 4) to the Early Preclassic and Archaic periods, respectively (Figure 7).

Dating Lithologic Units B and C

To identify the time period represented by the Unit B magnetic and geochemical spikes we interpolated betweenAMS l4CDates 1 and2, assuming linearsed- imentation rates between dated horizons. This is jus- tified because sedimentary, magnetic, and geochemical data indicate that Units A and B are lithologically continuous. From this interpolation we date Unit B to ca. A.D. 900 to 1200, the Early Post- classic period. That is, Unit B dates to the period that immediately followed political-demographic collapse and abandonment of the Rio de la Pasion drainage.

Magnesium Aluminum

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Table 1: Laguna Las Pozas Radiocarbon Dates.

Lab No. Radiocarbon Age B.P. 613C Calibrated at 2 Sigma Depth (cm)

1. AA33224 460 + 90 -28.9 calA.D. 1300-1370 (p = 11.0) 52 Plant macrofossils cal A.D. 1380-1640 (p = 84.4)

2. OS15623 840 + 40 -25.00 cal A.D. 1040-1090 (p = 8.2) 100.5 Carbonized Leaf cal A.D. 1120-1140 (p = 4.6)

cal A.D. 1150-1280 p = 82.5)

3. AA33223 3240 + 55 -29.9 1690-1650 cal B.C. (p = 2.3) 147 Plant macrofossils 1640-1400 cal B.C. (p = 93.1)

4. AA33222 3815+55 -30.5 2470-2130calB.C.(93.3) 242 Plant macrofossils 2080-2050 cal B.C. (p = 2.1) Note: All samples were AMS-dated, and calibrated with OxCal 3.5 (Bronk Ramsey 1994, 1995).


In contrast, Unit C cannot currently be dated with certainty. If Unit C dates to the Archaic and Early Preclassic periods (as indicated by the radiometric dates), the Laguna Las Pozas cores are missing all sediments deposited during the Middle Preclassic, Late Preclassic, Early Classic, Late Classic, and Ter- minal Classic periods a span of approximately 2,400 years (i.e., 3200 l4C BP [Date 3] to 800 l4C BP [Date 2]). However, a comparison of the Las Pozas pollen profile and profiles from other lowland Maya lakes suggests that the Unit C sediments date to a later time span.

Palynological evidence of Archaic-period forest clearing for agnculture in the southern lowlands has been recovered only in northern Belize (Pohl et al. 1996). But the severity of early deforestation represented by the northern Belize pollen data is difficult to evaluate. Elsewhere in the southern lowlands the earliest evidence of agriculture dates to later periods (Figure 1). For example, in the central Peten lakes (Lakes Peten-Itza, Quexil, Salpeten, and Sacnab), the first palynological evidence of forest clearing for agriculture dates to either the Early Preclassic or the Middle Preclassic period (Brenner 1994: Figure 2; Curtis et al. 1998:153-154, Figure 4; Deevey et al. 1979: Figure 4; Islebe et al.1996: Figure 3; Tsukada 1966: Figure 2; Vaughan et al.1985: Figures 6.1 and 6.2), although Leyden (1984: Figure 2; 1987; Fig- ure 2) andVaughan et al. (1985: Figure 6.4) propose that near Lakes Quexil and Salpeten clearing for agriculture did not commence until the Late Pre- classic. Also dating to the Early Preclassic or Mid- dle Preclassic period is palynological evidence of early agriculture at Coba, Quintana Roo, Mexico (Leyden.et al. 1998), Laguna de Cocos in northern Belize (Hansen 1990; Jones 1994; Pohl et al.1996), and the Laguna Petexbatun (Dunning et al.1998: 150,

Figure 6), located only 7 km north of the Laguna Las Pozas.

While these data might seem to support the dating of Las Pozas Unit C to the Archaic and Early Preclassic periods, inspection of the pollen profiles suggests otherwise. In the Laguna Las Pozas pollen sample from 155 cm (Unit C)-dated to 3200 14C yr B.P. (the Early Preclassic period)-disturbance and savanna taxa clearly predominate. Indeed, at 158 cm the ratio of high forest to disturbance and savanna taxa is roughly 1 :3. Yet in the central Peten lakes and elsewhere, disturbance and savanna taxa do not dom- inate pollen assemblages in comparable percentages until the much later Late Preclassic through Late Classic periods. Nowhere else in the southern lowlands do pollen data suggest that agriculture had a comparable impact on forests during the Archaic or Early Preclassic periods. Moreover, given that the pollen contents of lake sediments reflect regional processes (Bradley 1999:362), had severe deforestation occurred during these early periods in the vicinity of the Laguna Las Pozas, some traces of that process ought to be evident in the Laguna Petexbatun pollen record (Dunning et al. 1998), but none have been recognized. We conclude that it is unlikely that in the southern lowlands, Early Preclassic and Archaic farming severely impacted the forest only in the vicinity of the Laguna Las Pozas.

Equally unlikely is the possibility that deforestation of ecogenic, or natural, origin affected the Las Pozas region during the Early Preclassic. Data recovered elsewhere in the Maya lowlands indicate that from approximately 3000 B.P, (the Early Preclassic period) through 1300 B.P. (the end of the Late Clas- sic period), climate throughout the Maya region was stable (Curtis et al.1996; Hodell et al. l995; Leyden et al. 1993).




deforestation, or erosion prior to that colonization. Because settlement survey and testing have not been carried out within the Las Pozas basin, the spatial extent of the colonization is not known, and we do not know whether the colonizers were Maya "refugees"-collapse survivors who were the direct cultural descendants of the Classic Maya. Indepen- dent settlement survey data from the basin are needed to further test the hypotheses presented here. If the basin was affected by drought during the Late Clas- sic period (see Curtis et al. 1996; Gill 2000; Hodell et al. 1995) a possibility that we view cautiously but can neither confirm or rule out-our interpretation of the Early Postclassic sediments would not be affected.

Magnetic and lithogical analyses reveal what pollen analysis (the traditional means of reconstructing lowland Maya paleoecology and land use) does not: landscape transformations, and by impli- cation, cultural practices at the local level. Had magnetic analysis not been undertaken, the Early Postclassic occupation of the Laguna Las Pozas would have remained unknown. That pollen record betrays no evidence of local Early Postclassic occupation. Pollen and magnetic analyses are compati- ble and complementary research tools. To detect changes and transformations at the local level, magnetic analyses are critical.

The Laguna Las Pozas study is significant on several counts. First, it demonstrates the potential archaeological utility of magnetic analysis. Second, it suggests that the post-collapse population of the southern lowlands may have been somewhat larger and more territorially widespread than archaeologists have previously suspected. Third, it supports the hypothesis posed at the start of this paper: that to develop a fuller understanding of post-collapse cultural and ecological processes in the southern lowlands, archaeologists must conduct investigations in some of the vast southern areas that remain unknown. Many of those areas have not been surveyed because they lack substantial pre-collapse centers, and pre- cisely because they lack such centers post-collapse peoples may have favored them.

Acknowledgments. Fieldwork at Laguna Las Pozas was supported by a National Science Foundation Grant-in-Aid of Research administered through the University of Minnesota Paleo-records of Global Change Research Training Group, the JFM Foundation, and the Department of Anthropology and the College of Social and Behavioral Sciences of The Ohio State

For these reasons, we conclude that Dates 3 and 4 are in error, perhaps because of the hard-water effect (Deevey and Struiver 1964), produced when l4C-depleted lake water is incorporated into plant tis- sues. (The carbon in lake water is l4C-depleted when it comes from dissolved, l4C-free carbonate bedrock.) The hard-water effect has plagued other paleoecologists' attempts to date lake sediment in the Maya lowlands (e.g., Curtis et al. 1996:39-40; Hodell et al.1995:392). We propose that Unit C sediments date from the Late Preclassic through the Ter- minal Classic periods, but we caution that the possibility that Unit C dates to theArchaic and Early Preclassic periods cannot currently be eliminated.

Conclusions

In the Rio de la Pasion drainage, one of the most thoroughly investigated areas of the southern lowlands, a possibly substantial, geographically localized population remained long after the date proposed by archaeologists to mark the region's abandonment. This discovery lends credence to the claims of some archaeologists that as our survey coverage of the southern lowlands improves so will our understanding of the cultural and ecological aftermath of the collapse (e.g., Sharer 1994:341). More than a decade ago, Culbert (1988:89) reflected that "There have not been enough rural surveys to rule out the possibility that large Postclassic populations lurked in the hinterlands in areas yet to be discovered by archaeologists," yet he dismissed as unlikely the possibility that there are "large [Postclassic] populations hidden 'somewhere out there."' While archaeologists cannot determine from paleoecological data alone the size of Postclassic populations at Laguna Las Pozas or other southern lowland locations, the Las Pozas data raise the possibility that some, and possibly many, post-collapse peoples colonized terrain not heavily degraded by their predecessors. To develop a clear and accurate picture of post-collapse cultural processes, archaeologists may need to conduct settlement surveys in areas outside those preferred (and most densely inhabited) by the pre-collapse Maya.

That the Laguna Las Pozas magnetic, organic carbon, and geochemical spikes match one another in general form and stratigraphic position indicates that the basin was colonized and deforested, occupied by agriculturalists, and abandoned and reforested all during the Early Postclassic period. There are no limnological indications of substantial occupation,




University. AMS-dating of the radiocarbon samples was funded by a National Science Foundation grant administered through the University of Minnesota Paleo-records of Global Change Research Training Group (grant no. 1717-4416112).

With the kind permission of Drs. Kerry Kelts and Subir Banerjee, Breckenridge completed the geological research at the University of Minnesota Limnological Research Center and the Institute of Rock Magnetism while a graduate student in the Department of Geology and Geophysics. Jarrod Burks prepared figures 1 and 2. We dedicate this paper to the memory of our col- league and friend, Kerry Kelts.

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