A 3400 year paleolimnological record of prehispanic human–environment interactions in the Holmul region of the southern Maya lowlands

A 3400 year paleolimnological record of prehispanic human–environment interactions in the Holmul region of the southern Maya lowlands

Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31 Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeocli...

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Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31

Contents lists available at SciVerse ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

A 3400 year paleolimnological record of prehispanic human–environment interactions in the Holmul region of the southern Maya lowlands David Wahl a,⁎, Francisco Estrada-Belli b, Lysanna Anderson a a b

U.S. Geological Survey, Menlo Park, CA 94025, United States Archaeology Department, Boston University, Boston, MA 02215, United States

a r t i c l e

i n f o

Article history: Received 28 September 2012 Received in revised form 7 March 2013 Accepted 12 March 2013 Available online 20 March 2013 Keywords: Pollen Charcoal Climate Corn Agriculture Maya lowlands

a b s t r a c t The timing, magnitude and drivers of late Holocene environmental change in the Holmul region of the southern Maya lowlands are examined by combining paleoenvironmental and archeological data. Environmental proxy analyses on a ~3350 cal yr lacustrine sediment record include pollen, charcoal, loss on ignition, magnetic susceptibility, and elemental geochemistry. Archeological evidence is derived from extensive settlement surveys conducted near the study site. Results indicate nearby settlement and agricultural activity taking place in an environment characterized by open forest from around 3350 to 950 cal yr BP. The fire history shows a dramatic increase in burning during the Classic period, possibly reflecting changing agricultural strategies. A distinct band of carbonate deposited from 1270 to 1040 cal yr BP suggests decreased hydrologic input associated with drier conditions. Abrupt changes in proxy data around 940 cal yr BP indicate a cessation of human disturbance and local abandonment of the area. Published by Elsevier B.V.

1. Introduction Two abrupt decreases in population evident in the Lowland Maya archeological record (circa 1750 and 1050 BP 1) have led scholars to speculate that environmental degradation and climate change played an important role in these events. Many of the paleoecological and geochemical studies carried out in the Maya area over the last several decades suggest that agriculture and urbanization caused widespread forest clearance and soil erosion (Deevey et al., 1979; Binford et al., 1987; Jacob and Hallmark, 1996; Dunning et al., 2002; Hansen et al., 2002; Beach et al., 2006; Wahl et al., 2007a). Specifically, there is undisputed evidence of large-scale erosion into low-lying bajos during the Late Preclassic period (2350–1750 BP) across the southern Maya lowlands (Dunning et al., 2002; Hansen et al., 2002; Beach et al., 2006). Pollen studies show that this erosion was likely a result of forest clearance that began in the Early Preclassic period (2950–2350 BP) and intensified as populations grew through the Late Preclassic period (Jacob and Hallmark, 1996; Curtis et al., 1998; Wahl et al., 2007a). The dramatic ecological and hydrological shifts that occurred during these periods would have impacted local populations. Indeed, many sites in the Mirador Region were abandoned after 1750 BP, leading to the suggestion that subsistence strategies utilizing wetland

⁎ Corresponding author at: 345 Middlefield Rd. MS-975, Menlo Park CA 94025, United States. Tel.: +1 650 329 4533; fax: +1 650 329 4936. E-mail address: [email protected] (D. Wahl). 1 Unless otherwise noted, all dates reported as BP represent cal yr BP. 0031-0182/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.palaeo.2013.03.006

soils may have begun to fail as a result of forest clearance and associated erosion (Hansen et al., 2002). The Late Preclassic and Late Classic period abandonments (~1750 and ~1050 BP, respectively) show up clearly in pollen studies in the Mirador Region as dramatic decreases in weedy taxa coupled with the resurgence of forest taxa and attenuated erosion (Wahl et al., 2007a, 2007b). Not all regions exhibiting evidence of extensive prehispanic erosion were abandoned, however, and more data is needed to test the hypothesis that environmental degradation played an important role in site abandonment. Late Holocene climate of the Yucatan was characterized by centennial to millennial scale variability (Hodell et al., 1995; Curtis et al., 1996; Hodell et al., 2001; Hodell et al., 2005a, 2005b; Medina-Elizalde et al., 2010; Medina-Elizalde and Rohling, 2012). Reconstructions of relative moisture availability from the northern Maya lowlands reveal periods of increased aridity during the Terminal Classic period (1120–1000 BP), coinciding with population decline in the southern lowlands. These results have led researchers to theorize that drought may have been a factor in widespread abandonment of the southern lowlands (Hodell et al., 1995; Curtis et al., 1996; Hodell et al., 2001; Haug et al., 2003). A high-resolution speleothem based climate reconstruction from Belize indicates persistently dry conditions setting in after the Terminal Classic period, from ~930 to 850 BP (Kennett et al., 2012). Although the drought hypothesis has been accepted by a considerable number of scholars, it is not without problems. Importantly, a precipitation gradient exists between the southern and northern lowlands (Pérez et al., 2011); today the northern lowlands are much drier than the south. Unlike the widely abandoned southern lowlands, the northern region experienced unprecedented population growth during the

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Terminal Classic period. The apparent paradox of a drier northern lowlands flourishing during a drought that caused abandonment in the southern lowlands has not been resolved (Dahlin, 2002; Robichaux, 2002). A full picture of the timing, causes, and consequences of environmental change during the Maya period has yet to emerge. Dating problems have led to ambiguities regarding the timing of major shifts in environmental proxy data (Vaughan et al., 1985; Brenner et al., 2002; Leyden, 2002). These ambiguities have contributed to the diverse interpretations of the causes and impacts of reconstructed environmental changes. The use of AMS radiocarbon dating, and, more recently, high precision U–Th dating of a speleothem in Belize, has begun to constrain the timing of events and is allowing more secure comparisons (Curtis et al., 1998; Rosenmeier et al., 2002; Wahl et al., 2006; Mueller et al., 2010; Kennett et al., 2012). In spite of this, the paucity of sensitive paleoclimate records from the southern Maya lowlands has made it difficult to fully examine the theory that drought in the Late Classic resulted in widespread abandonment (Gill, 2000). Moreover, paleoenvironmental studies are lacking from many important cultural areas. The Maya lowlands are characterized by environmental heterogeneity, with distinct prehispanic cultural landscapes and local adaptations. As such, examination of these landscapes on a case-by-case basis is needed to provide the foundation for drawing broader conclusions about the dynamic human/environment relationship. Here we report on a high resolution, multi-proxy record of human occupation and environmental change from Laguna Yaloch in the Holmul region of the southern Maya lowlands. It includes the large Preclassic ceremonial center of Cival and the Classic period center of Holmul (Fig. 1). Archeological evidence suggests that local site abandonments correlate to broader, regional abandonments in the Late Preclassic and Terminal Classic periods. By using a wide array of proxy indicators, we can differentiate between ecological responses to human impacts versus climate change. Evidence of environmental change that is not only connected to local demographic shifts, but also temporally correlated to similar evidence from disjunct locations across the Maya Lowlands, offers strong support to models that suggest that environmental degradation and/or climate change had an effect on the cultural trajectory of the region. The data presented here build upon previous paleoenvironmental work in the Maya Lowlands in order to investigate how large-scale processes may have affected local settlement patterns.

A

2. Physical setting This study focuses on the area surrounding the Preclassic period center of Cival and the Classic period center of Holmul in northeastern Peten, Guatemala (Fig. 1). These sites are located in a karstic basin along the middle course of the Holmul River. This basin is bound on its north, west, and southwest sides, by a 200-m high escarpment bisected by several canyons and streams. The easternmost and southernmost portions of the region are characterized by hilly terrain delineating a basin though which various streams flow in a north–northeast direction. At the center of this basin is a small hilly upland plateau surrounded by wetlands and bisected by the Holmul River. Approximately two-thirds of the study area's 100 km 2 is occupied by seasonally flooded low-lying wooded swamps (bajos). The modern climate is characterized as tropical monsoon, with ~ 2000 mm of annual precipitation distributed over ten months. Rainfall is concentrated in the months of June, September/October and December/January. During the wet season, large parts of the bajos become inundated, although the water rarely rises above 1 m in most areas. Drainages receive high-energy flow for short periods of 2–3 weeks during the wettest periods and are often completely dry in the dry season. Variation of forest cover and ecotones is shown in the LANDSAT 7 image as differences in tones and textures (Fig. 2). Vegetation ranges from dense closed-canopy forest in the upland areas to shrubs and grasses in the lowest areas (Lundell, 1937; Fedick and Ford, 1990). Dominant arboreal species in the well-drained uplands include ramon (Brosimum alicastrum), cedar (Cedrela spp.), ficus (Ficus glaucensces), chico zapote (Manilkara zapota) and mahogany (Swietenia macrophylla) (Ford, 1986) (Fig. 2, green-blue areas). Patches of palm trees such as corozo (Orbignya cohune), escoba (Cryosophila argentea), and guano palm (Sabal spp.) are common in the transitional zones between uplands and bajos, particularly in connection with water courses (Fig. 2, light green areas). The bajos contain a variety of habitats. Less frequently inundated areas are dominated by leguminous scrub species, including the palo tinto (Haematoxylum campechianum). These areas are locally known as tintal bajos and are shown in Fig. 2 as areas of dark magenta mixed with blue. The lowest lying seasonally and perennially inundated areas (locally known as civales) support open herbaceous vegetation comprised of grasses, rushes and sedges (Fig. 2, light magenta areas). Riparian forests containing a mix of palms, upland trees, and thorny bamboo groves are common along the watercourses (Fig. 2, vibrant green areas).

B 88°

co

7

Me

xi

92°

of

20°

Cival

lf

Northern Lowlands

2

a

Gu

Southern Lowlands

1 Laguna Yaloch

Holmul

Se an Tikal

C ar i b b e

El Mirador

0

300 250 200 150 120 elevation (m)

Chichen Itza

4 3

5

6

Rio Holmul

100 km

Fig. 1. A. Map of Yucatan peninsula showing the Holmul region in the context of the northern and southern lowlands; B. Topography of the Holmul region derived from NASA's AIRSAR topographic mission of 1999 (courtesy of NASA). Numbers identify major and minor centers: 1. Hamontun, 2. Hahakab, 3. T'ot, 4. La Sufricaya 5. Riverona, 6. K'o, 7. Witz Na.

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19

Witzna'

Cival

Holmul T'ot

Yaloch K'o El Pilar

0

2.5

5

10 Kilometers

Fig. 2. Diversity of ecotones and topographic features in the Holmul region as visible LANDSAT ETM + image taken March 2001. False color composite with bands 7 (R), 5 (G), and 2 (B). Areas in magenta represent more flood prone areas of the bajos, yellow-green areas correspond with upland forest, light blue areas are mixed palm and riverine ecotones. Black text indicates ancient settlements; red text indicates coring site.

3. Cultural background The earliest archeological evidence for human occupation in the southern Maya lowlands dates to around 3050 BP and comes in the form of scant remains of ceramic containers for ritual and domestic use. The demographic trajectory of this region includes steady growth of settlement and monumental building activities during the Middle Preclassic (2950–2350 BP) and Late Preclassic (2350–1700 BP) periods. At the end of the Late Preclassic period, ca. 1750 BP some of the largest centers in this region became depopulated and/or major construction activity ceased. Such is the case for the largest centers in the lowlands, including most of the centers in the Mirador region, and a selection of sites around the Yucatan peninsula, including San Bartolo, Cival, Seibal, Cerros, and El Palmar. During the same period, some sites continued to grow, in some cases significantly, such as Tikal. Populations and monumental building activity continued to increase at an accelerated rate during the Early (1650–1350 BP) and Late (1350–1150 BP) Classic period only to enter a phase of diminished activity culminating in the widespread abandonment of most, but not all, of the cities in the southern Maya lowlands between 1150 and 950 BP. The causes of these periods of population decline have been the source of much debate among scholars (Demarest and Prudence, 2004; Sharer and Traxler, 2006; Estrada-Belli, 2011). Extensive archeological investigations have been carried out in the Holmul region proximate to Laguna Yaloch since the year 2000 (Estrada-Belli, 2001a, 2001b, 2002a, 2002b, 2003, 2005, 2006a, 2006b, 2011; Estrada-Belli et al., 2003a, 2003b, 2004, 2009; Estrada-Belli and Wahl, 2010). The large center of El Pilar, located 5 km to the south of Laguna Yaloch, is another nearby area of significant prehispanic occupation. This site was also first settled in the Middle Preclassic period (ca. 3050 BP), was heavily occupied in the Late Preclassic and Late Classic periods, and was abandoned by 950 BP (Ford, 1991).

Archeological surveys conducted on Holmul's upland plateau have located numerous sites of a broad range of scales, including the large Classic period city of Holmul and the Late Preclassic period center of Cival. Both are located on elevated terrain near the Holmul River. The landscape near Cival is largely dominated by seasonal swamps, and, in many respects, is similar to Preclassic period sites elsewhere in the southern Lowlands (i.e., the Mirador Region, Uaxactun, and San Bartolo). In contrast, the environment of Holmul, and other minor centers around it, is located on the highest elevation of upland ridges in connection with well-drained soils. This is consistent with (most) other Classic period sites in the southern lowlands, such as Tikal, Caracol, Naranjo, Rio Azul, and La Milpa. The archeological record currently available for these sites shows great depth and early development of complexity which correlate with cultural developments in the Mirador and other central Peten regions. Mapping and excavations carried out at Cival since 2001 have revealed a long occupation history beginning in the Middle Preclassic period ca. 3050 BP. The site's central plazas were first built with massive infilling and leveling of the natural hilltop. By the end of the Late Preclassic period the ceremonial area approximated 1 km2 (Estrada-Belli, 2005, 2006a). Cival's main plaza consists of an 18-m high western pyramid and a low eastern platform flanked by smaller buildings on the north and south. This “E-Group” architectural configuration is typical of Preclassic period sites in Peten (Fialko, 1988; Estrada-Belli et al., 2003a). Five tall pyramids are situated on the north, west, south and east sides of the ceremonial center. The tallest building is a west-facing 20-m high platform supporting a triadic group of temples (Group 1) located to the east of the E-Group plaza. Excavations in the E-Group plaza, along the main axis of its eastern platform, have recovered a number of important ritual deposits. The most striking (and earliest) is an offering of jade celts and jars in a cross pattern, which appears to be from the inaugural event of

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the first plaza. Radiocarbon results indicate this occurred between 2730 and 2500 BP (1-σ; 2520 ± 40 14C yr BP) and ceramic evidence further constrains the timing to the earlier end of this range. Other early finds at the site include a stela carving of a ruler from ca. 2250 to 2150 BP, the earliest so far documented in the lowlands (Estrada-Belli et al., 2003a). During the Late Preclassic period the site reached its maximum extent and included construction of a 33 m high triadic pyramid complex. The penultimate construction phase of this Triadic Group's eastern temple, dates to ca. 2300–2120 BP (1-σ; 2170 ± 40 14C yr BP) and is characterized by large high-relief stucco masks of a primordial sky/rain deity (Estrada-Belli, 2006a). Subsequent remodeling, although now largely eroded, was made with a massive investment of cut stone and stucco unrivaled in scale by any previous effort. Surprisingly, this temple, and much of the public architecture at the site, was not remodeled after ca. 1800 BP. The scale and symbolism associated with public architecture, ritual offerings, and stelae at Cival suggest that complex social institutions were in place since the Middle Preclassic period, increasing in size and complexity until reaching a climax in the Late Preclassic period (Estrada-Belli et al., 2003a; Estrada-Belli, 2006a). Other centers situated in the Holmul region, such as Holmul, K'o, Hamontun, Hahakab, and T'ot show extensive construction and settlement distribution during the Late Preclassic phase. These are large agglomerates of settlement around ceremonial centers. Their plazas and pyramid structures are smaller than those of Cival, suggesting that they form part of a network of secondary centers of ritual and power around the region's capital. Excavations at Cival indicate that the site was abandoned following the construction of a defensive wall during the Late Preclassic period, encircling the main hill. We tentatively place this event around 1700 BP based on ceramic dating. Small populations continued to occupy elite buildings during the first part of the Early Classic period without engaging in major construction until the site was completely abandoned by circa 1600 BP (Estrada-Belli, 2006b). Unlike Cival, all other sites in the region were continuously inhabited through the Early and Late Classic periods (1650–1120 BP). The Early Classic period (1650–1350 BP) also saw the establishment of a new but short-lived ceremonial center at La Sufricaya, which contained dynastic monuments and buildings with stylistic links to Tikal and Teotihuacan (Estrada-Belli et al., 2009). Overall, the Early Classic period is characterized by continued, although diminished, construction efforts at most sites. The Late and Terminal Classic periods (1350–1000 BP) see the resurgence of monumental platforms and building constructions at most sites. At this time, Holmul was the major local center with large palace, temple, and plaza complexes. During the Terminal Classic period (1120–1000 BP) twenty-six plain stone monuments were erected at Holmul, whereas T'ot, K'o, and Hamountun erected two or three. This Terminal Classic resurgence of monumental projects approximates levels not seen since the Late Preclassic period. 4. Methods 4.1. Paleoenvironmental methods A total of 3.97 m of sediment was recovered from Laguna Yaloch (17°18′35″N, 89°10′29″W), a shallow closed-basin lake 7 km east of the ceremonial center of K'o and ~13 km east of Holmul (Fig. 1). Cores were recovered in ~80-cm sections using a 5-cm Livingstone piston corer modified to accept butyrate tube liners; the sediment water interface was similarly captured in a butyrate liner tube. A replicate core, offset by 40 cm, was taken to ensure complete stratigraphic recovery. Water depth at time of coring was 1.15 m. Laguna Yaloch is bounded on its north side by a relatively steep scarp that rises ~100 m above the lake. An extensive seasonal wetland borders the lake to the south and west. Eleocharis sp. (spikerush, locally known as polol) dominates the lower marsh, nearest the lake. The presence of Graminoids increases

with distance from the lake and higher elevation. Other infrequent herbaceous taxa noted on the savanna include ferns and asters. Near the savanna/forest border, the herbaceous component is nearly entirely comprised of grasses. Occasional woody taxa found on the savanna include Crescentia cujete, Byrsonima crassifolia, Acoelorrhaphe wrightii, Cecropia sp., and Ceiba sp. The trunks of many of these trees are scarred by fire, and local workers reported that the savanna is usually burned during the dry season. Whole core magnetic susceptibility was measured in 1-cm increments using a Bartington MS2C Sensor. Due to the extremely low magnetic values, each core was measured twice and the results averaged together. Cores were then split longitudinally and imaged using a GEOTEK Multi-Sensor Core Logger (MSCL). Sediment water content was determined by drying 1.25 cm3 samples at 100 °C. Combustion of dried samples at 550 and 1000 °C determined total organic matter, carbonate, and non-carbonate inorganic (alumino-silicate) contents (Dean, 1974). Ten samples of macroscopic organic matter were isolated from the sediment matrix for AMS radiocarbon determination. To avoid hard-water error (Deevey et al., 1954), samples were limited to organisms that fixed atmospheric carbon and consisted of macroscopic plant material, wood, seeds, and insect fragments. Except when sample size was too small to provide a split, δ 13C was also measured and corrected for prior to calibration; a value of − 25‰ was assumed for samples that were not analyzed for 13C. Radiocarbon years were calibrated to calendar years using Calib 6.0 (Stuiver and Reimer, 1993) and the IntCal09 dataset (Reimer et al., 2009). The assigned age for each sample is derived from the median probability (as provided by Calib), rounded to the nearest 10 year. The age-depth model is a composite of two polynomial regressions, a third order from the base of the core to 197 cm and a fourth order from 197 to the surface. Isotopic composition of C was measured on sediment organic matter (SOM) using a VG Optima Mass Spectrometer. Samples were pretreated with a 2 N solution of HCl to remove inorganic carbon (Meyers and Teranes, 2001). Percent C and N were measured on all samples analyzed for isotopic composition using an inline Fisons EA1500 Elemental Analyzer. Isotopic results are presented in delta notation as per mil (‰) relative to V-PDB standard. Pollen was extracted using standard acetolysis processing (Faegri and Iverson, 1989) and sample residue was mounted in silicon oil. Known quantities of exotic Lycopodium spores were added prior to digestion to allow for the calculation of pollen concentration and influx rates (Stockmarr, 1971). Pollen grains and fern spores were identified to the lowest possible taxonomic level using the UC Berkeley Museum of Paleontology's collection of over 10,000 reference pollen samples, reference material collected in the field, and published pollen keys (Horn, 1986; Hansen, 1990a; Roubik and Moreno, 1991; Colinvaux et al., 1999). Cladium was separated from other Cyperaceae pollen based on its size and the presence of an elongated tip (Faegri and Iverson, 1989). A minimum of 350 grains and spores was counted at 500× magnification, except for two levels with extremely scarce pollen (61 and 149 cm, Σ = 297 and 317 respectively). To increase objectivity, the corresponding depth of each pollen sample was not known at the time of counting. To increase the probability of detecting Zea grains not encountered during standard counting, the entire cover slip of each pollen slide analyzed was scanned for Zea pollen at 100×. A second slide was scanned on levels lacking Zea pollen after the first scan. Size and long axis/pore ratio were used to differentiate Zea mays from other large Poaceae pollen grains. Further examination of a random selection of Zea grains (n = 5) was undertaken using Nomarski phase contrast light microscopy (Whitehead and Langham, 1965). The minimum long axis measurement of Zea pollen encountered was 58 μm (range = 58–86 μm, x = 67.2 μm) and the range of long axis/pore ratios was 4.25–7.82. The Zea grains examined with phase contrast microscopy exhibited uniform spacing of intertectile columella, a characteristic that distinguishes Zea pollen from other

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4.2. Settlement survey methods The standard methods for mapping of lowland Maya settlement uses transect lines radiating from the site center. These transects are typically 200 to 500 m wide and often follow the cardinal directions to facilitate orientation for the surveyors. Although settlement survey samples acquired through transects normally violate the assumption of randomness, they are well suited to delineate settlement continuity, density, and variability at selected distances from the center (Puleston, 1971; Tourtellot, 1988). Transects are also able to clarify the complex relationship between settlement and environment across ecotones. However, survey transects that correlate to the cardinal directions and/or intersect sparsely distributed settlement often fail to generate reliable data regarding variability of settlement types and environmental zones. To overcome the limitations and adverse effects of transect mapping, the Holmul Archaeological Project devised a multi-step GPSbased survey method. First, Infra-Red (LANDSAT) satellite imagery was analyzed to select areas of potential ancient settlement presence as well as landscape features such as water sources, hilly ground, etc. that are significant to human–environment interactions. Previous studies have shown that use of image classification in the forested Peten region can lead to prediction of Maya settlement with up to 90% accuracy (Estrada-Belli and Koch, 2007; Saturno et al., 2007). Tracts of land in connection with vegetation indicating high potential for ancient settlement were selected for mapping. Hiking routes leading to the target points are laid out, and the mapping crews inspected both the target points and the areas along the route for surface architecture and other relevant features. Settlement features were assigned a location control point using a GPS unit, and the feature was mapped to scale with tape and compass on gridded paper. In the lab, field maps were digitized and entered into the GIS database as feature-specific layers (e.g. settlement, reservoir, dams, creeks). In addition, surface artifact samples were collected from each settlement feature for chronological control. Finally, once the mapped settlement and landscape features were in the GIS database, a set of statistics was generated for each layer. These statistics were used to quantitatively analyze patterns in the environmental and settlement layers. Settlement features entered as GIS layers could also be matched to satellite imagery anomalies to assess the prediction's accuracy (Estrada-Belli, 2004; Estrada-Belli and Koch, 2007). Dating of ceremonial centers and surrounding residential zones is accomplished using a well-documented ceramic chronology with 14C determinations from sealed excavated contexts, such as burials, stuccolined sculptures and floors (Callaghan, 2008).

5. Results 5.1. Paleoenvironmental results Results of the AMS radiocarbon analyses show the core spans the period from ~ 3350 cal yr BP to present. (Fig. 3; Table 1). Core stratigraphy and proxy data show dramatic changes throughout and the record has been divided, based on distinct changes in stratigraphy and analytical results, into 6 stratigraphic zones (Figs. 4–8). 5.1.1. Zone 6 (3.97–3.43 m; 3350–2950 BP) The basal sediment is comprised of homogenous, heavy inorganic clay, which stopped the hand coring operation. Magnetic susceptibility and bulk density are at their highest values in this interval, and alumino-silicates are the dominant component of the sediment matrix (>80%). Total organic matter and CaCO3 percentages are relatively low throughout the zone. Sedimentation rate is relatively high during this period, averaging 1.3 mm/yr. Pollen percentages are dominated by grasses (Poaceae), which reach their highest values for the entire core at 68% of the total. The earliest Zea pollen is found at 2 cm above core base (3.95 m; 3330 BP). The percentages of the arboreal pollen taxa Urticales and Melastomataceae–Combretaceae (M–C) are relatively low throughout zone 6. C:N ratios of organic carbon are below 10 for most of the zone, before rising near the transition to zone 5. δ13CSOM values are relatively light, ranging from −28.30 to −24.90‰. Charcoal concentration and influx are low throughout. 5.1.2. Zone 5 (3.43–2.00 m; 2950–2280 BP) The transition to zone 5 is marked by a distinct change in core lithology. Most notably, the H2O content increases dramatically from an average of 43% in zone 6 to 66% in zone 5. Although the constituents of the sediment matrix do not vary across this zone boundary, the texture of the sediment changes from a dry, crumbly clay in zone 6 to a sticky, smooth homogenous clay in zone 5. Magnetic susceptibility values decrease in concert with bulk density. Alumino-silicate content reaches the highest values of the entire core (90.5%) at 2.01 m. δ13CSOM values increase in zone 5, while C:N ratios, although variable, remain at around 10. Charcoal influx increases at the base of zone 5 and remain relatively elevated from 3.43 to 2.58 m (x = 7.69), followed by decreased values for the remainder of zone 5 (x = 1.74).

0

100

Depth (cm)

large Poaceae pollen. It is unlikely that these grains represent wild Zea (teosinte), as the study area is well outside its known range (Doebley, 1990). Subfossil charcoal recovered from sediment cores has been shown to be a reliable recorder of both local and regional fire history. Both empirical (Clark and Royall, 1995; Long et al., 1998; Gavin et al., 2003; Lynch et al., 2004; Higuera et al., 2005; Brubaker et al., 2009; Higuera et al., 2010) and theoretical (Clark, 1988; Higuera et al., 2007; Peters and Higuera, 2007) studies have shown sediment charcoal particles >125 μm record fires that burned within a 10–20 km radius from the coring site. Macroscopic charcoal was isolated from 2.5 cm 3 samples taken at 1 cm intervals. Subsamples were extracted contiguously down core using a square spatula. Samples were soaked in 3% (NaPO3)6 for a minimum of 24 h, before wet sieving at 125 μm (Whitlock and Larsen, 2001). The remaining fraction was soaked for 1 h in 6% NaOCl in order to digest any remaining non-charcoal and partially burned organic matter (Ali et al., 2009). Samples were then sieved again at 125 μm, washed into 4 inch diameter single use Petri dishes, and dried at 50 °C overnight. Counts were made using a binocular scope at 10–15× magnifications. Charcoal counts were converted to concentration (particles/cm3) and, using calculated sedimentation rates, influx (particles/cm2/year).

21

200

300

400 0

500

1000 1500 2000 2500 3000 3500

Cal yr BP Fig. 3. Age-depth plots for Laguna Yaloch. Error bars represent 2-σ range. The age model is comprised of a composite polynomial (0.01–1.97 m, age = −0.00000336 ∗ depth4 + 0.0021734 ∗ depth3 − 0.44687502 ∗ depth2 + 40.97718868 ∗ depth − 59.37642607; 1.98–3.97 m, age = 0.00040035 ∗ depth3 − 0.3618141 ∗ depth2 + 110.57053193 ∗ depth − 8566.57978827).

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Table 1 AMS radiocarbon determinations from Laguna Yaloch. Samples that were not analyzed for δ13C were assigned an assumed value of −25‰.

Depth (cm)

Lab no.

δ13C

Radiocarbon age 14 C yr BP

11 38 52 100 107 142 199 259 333 358

CAMS148151 WW7098 WW6555 WW7655 WW6551 WW7099 WW6553 WW7100 WW6552 WW7765

−25‰ −25‰ −26‰ −28.19‰ −25‰ −15.24‰ −25‰ −25‰ −25‰ −29.07‰

320 1120 1152 1520 1603 1670 2258 2660 2801 2865

Poaceae pollen decreases dramatically throughout zone 5; percentages drop from > 56% in zone 6 to ~ 15% near the zone boundary, then further to ~ 3.5% for the remainder of zone 5. M–C, Byrsonima, Asteraceae, Pinus, Quercus, and Mimosa pollen percentages increase in zone 5. Urticales pollen remains at stable, low values throughout.

5.1.3. Zone 4 (2.00–1.32 m; 2280–1540 BP) An initial decrease in alumino-silicate content indicates the beginning of zone 4. Values decrease 13% between 2.01 and 1.96 m, while total organic matter and CaCO3 increase by 7% and 6%, respectively. The decrease in clay at the zone boundary is reflected in bulk density and magnetic susceptibility. Following this initial decline, clay content, bulk density, and magnetic susceptibility values increase throughout zone 4. Charcoal influx is low throughout zone 4. C:N ratios rise dramatically near the boundary with zone 3 and remain consistently high thereafter. δ 13CSOM values increase overall through zone 4.

Total Organic Matter

CaCO3

± ± ± ± ± ± ± ± ± ±

35 35 35 25 36 30 37 40 41 25

Age range 2σ (cal yr BP)

Median age (cal yr BP)

Calendar year (AD/BC)

305–475 938–1167 979–1173 1344–1516 1404–1563 1520–1692 2155–2346 2739–2849 2785–3000 2887–3072

390 1020 1075 1400 1480 1575 2235 2775 2905 2985

AD 1560 AD 930 AD 875 AD 550 AD 470 AD 375 285 BC 825 BC 955 BC 1035 BC

The most notable change in the pollen spectra is the nearly coeval rise in Potamogeton and Typha percentages at the base of zone 4; both then decrease at 1.49 m. Pollen from both taxa was nearly completely absent in zones 5 and 6. M–C and Byrsonima pollen decrease into zone 4 to consistently low percentages, save for one data point (1.76 m) with relatively high M–C pollen. Zea pollen is intermittently present through the zone. Asteraceae (including Ambrosia) and, to a lesser degree, Poaceae, pollen percentages increase steadily in zone 4. Mimosa pollen percentages decrease from the previous zone, yet remain elevated relative to the other zones in the core.

5.1.4. Zone 3 (1.32–0.68 m; 1540–1270 BP) Nearly all data sets exhibit dramatic changes at the onset of zone 3. Core stratigraphy shifts from homogenous, inorganic, sticky clay to a dark organic-rich clay. This change is quantified by the increase in total organic matter from an average of 12.5% in zone 4 to 33.5% in zone 3. Alumino-silicate content drops in concert with increased

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Fig. 4. Digital imagery, loss on ignition, H2O content, bulk density and magnetic susceptibility profiles from the Laguna Yaloch core.

D. Wahl et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31

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Fig. 5. Digital imagery, charcoal concentrations, charcoal influx, carbon isotopes and C:N ratios from the Laguna Yaloch core.

organics. Decreases in bulk density and magnetic susceptibility reflect the changing composition of the sediment matrix. δ 13CSOM values are stable and relatively light through zone 3. Charcoal influx shows an abrupt rise at the transition to zone 3. Values rise from near zero values in zone 4 to a broad peak of high values for the entirety of zone 3. Within this peak are the highest values of the entire record (85 particles/cm 2/yr). Pollen from Cyperaceae and Nymphaea (water-lily) both increase markedly at the transition to zone 3. Within the Cyperaceae curve, Cladium (sawgrass) pollen becomes a consistent component of the spectra. Poaceae and Asteraceae pollen percentages both increase in zone 3 as well, with Ambrosia pollen accounting for most of the Asteraceae increase. Zea pollen is consistently present through this zone. Urticales and M–C pollen percentages are at the lowest levels for the entire record. Arecaceae pollen appears at the base of zone 3, having been extremely rare or absent in lower zones. Monolete fern spores also increase into zone 3 from very low values in prior zones. Mimosa and Cecropia pollen decrease significantly from previous levels. 5.1.5. Zone 2 (0.68–0.43 m; 1270–1040 BP) The boundary between zones 2 and 3 derives from coeval changes in nearly all of the proxy data. Sediment CaCO3 increases in zone 2 to a broad distinct peak, ultimately comprising 55% of the matrix. Bulk density increases with CaCO3, while magnetic susceptibility shows a small peak at 0.52 m. A peak of heavier carbon isotopes in zone 2 parallels the increase in carbonate content. δ13CSOM values climb to −19.95‰ at 0.52 m. Influx of charcoal particles drops to sustained low values, averaging 3.7 particles/cm2/yr in zone 2. Percentages of several pollen taxa shift in concert with other proxy data in this interval. Most notably, Cyperaceae and Nymphaea pollen drop steeply; water-lily pollen is completely absent at 0.51 m, the only such level in the record. Cladium pollen percentages are at their lowest at the same level, save for one sample at the bottom of the core, where there was no Cladium present. Typha pollen also drops out of the record

at 0.51 and 0.46 m. Asteraceae (Ambrosia in particular), Arecaceae, Sesuvium-type, Potamogeton, and Quercus all increase through zone 2. Zea pollen was encountered at 0.51 m. 5.1.6. Zone 1 (0.43–0.00 m; 1040 BP–present) The uppermost section of the core, zone 1, is characterized by highly organic sediment. Total organic matter in zone 1 averages 54%, and increase from 16.3% and 33.4% in zones 2 and 3 respectively. CaCO3 content drops precipitously from peak levels of zone 2 to sustained, low levels in zone 1. Decreased alumino-silicate content is reflected in bulk density and magnetic susceptibility values. δ13CSOM becomes lighter in zone 1, dropping to the lowest values of the entire record (−28.87‰). The average sedimentation rate slows down dramatically at the zone 2/1 transition, from 1.1 mm/yr in zone 2 to 0.4 mm/yr in zone 1. The youngest radiocarbon determination, at 11 cm below the sediment water interface, indicates yet another decrease in sedimentation rate, down to 0.3 mm/yr for the uppermost sediment of the core. Charcoal influx decreases into zone 1 and continues to drop steadily toward the top of the core. Pollen from forest taxa (Urticales and M–C) increases steadily starting around 0.36 m. The rise in zone 1 is the only noticeable shift in Urticales pollen percentages through the entire record. M–C pollen percentages were greater in zone 5 and attenuated through zones 4, 3, and 2 prior to increasing in zone 1. Poaceae and Asteraceae pollen decreases into zone 1 and remains low throughout. Zea pollen is present at 0.38 and 0.36 m; it was not found above this level. Percentages of extra-local pollen types, namely Pinus and Quercus, decrease in zone 1. Amaranthaceae pollen and trilete spores exhibit peak percentages at 0.38 and 0.36 m respectively. Arecaceae pollen percentages are similar to zones 2 and 3 in the lower section of zone 1, then decreases to near zero above 0.31 m. Cyperaceae and Nymphaea pollen increase at the transition to zone 1, returning to percentages similar to zone 3. Potamogeton pollen declines into zone 1, and drops out of the record above 0.36 m, save for one grain at 0.16 m. Typha percentages peak briefly at 0.38 m before dropping to consistently low values.

D. Wahl et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31

a Ze

ha

A ( A ST m ER br A os C ia EA In E la AM y) AR AN TH AC C EA ( C YP E la ER di A um C In EAE la Po y) ta m og et on N ym ph ae a

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Fig. 6. Percentage diagram of selected pollen taxa from Laguna Yaloch. Ambrosia and Cladium percentages are shown as a white inlay on the Asteraceae and Cyperaceae profiles respectively. The Poaceae profile is truncated on the lower two levels, where it reaches 56% and 68% of the sum. * indicates Zea pollen identified during low power scanning. Note scale changes on x-axes.

5.2. Settlement survey results Field surveys and satellite imagery analyses conducted by the Holmul Archaeological Project in 2008 documented extensive residential structures on upland ridges and isolated hills up to a distance of 2.5 km from the Cival plaza (Fig. 2, yellow-and-red areas). Beyond this distance to the north and west the ground becomes flat and marshy and the imagery's spectral signature corresponding to ruined buildings underneath the forest canopy fades out (Estrada-Belli and Koch, 2007). This area lacks superficial signs of intensive farming and no evidence of settlement was encountered. Additional areas of settlement exist on the east bank of the Holmul River, particularly on the upland terrain facing the large flood plain to the north and west. Further west, the Cival settlement blends with that of another sprawling Preclassic period center, Hamontun. At the time of Cival's peak in ceremonial and settlement activity, ca. 1800 BP, Hamontun was a secondary regional center (Estrada-Belli, 2010; Paling, 2010). Around 0.5 km to the south of Cival's ceremonial center is a large, circular, sedimented-in sinkhole. This feature is currently an herbaceous wetland that occasionally receives inflow from the Holmul River, but is otherwise dry. The upland terrain surrounding this wetland supported some of the higher-status settlement groups associated with the Preclassic period ceremonial center. On the south side of this cival, a Preclassic period ceremonial group (Civalito) with two large platforms containing pyramids up to 15-m in height represents the largest ceremonial complex outside of the main center of Cival. Additional small ceremonial groups with plaza and temple architecture exist along the upland ridge, forming a network of peripheral ritual locations among residential groups up to a distance of about 2.5 km from Cival's center. Further from Cival, Preclassic period residential settlements were found around ceremonial centers within the Holmul upland areas.

These include Holmul, the second largest center in the region, and the smaller ceremonial centers T'ot, Riverona and K'o, located about 8–10 km from Cival on the southwest, south and southeast ends of the upland ridge, respectively. In addition, the center of Hahakab is located 4 km to the southwest on the upland ridge between Holmul and Cival, just outside of Cival's residential sprawl. Hamontun is located on the eastern bank of the Holmul River and was the region's third largest center in the Preclassic period, as evidenced by its extensive settlement area and ceremonial core. Hamontun features a sizable “E-Group” plaza and medium-sized pyramids up to 15 m in height. Holmul, located further to the south, may have been similar in size to the other minor Preclassic period ceremonial centers surrounding Cival, although the tall basal platforms of Groups I and III may hide yet undetected Preclassic period buildings, which, if found, would dramatically increase the Preclassic period built volume and perceived regional importance of this center. The Group II platform, which has been investigated continuously since 2005, was one of the earliest ceremonial complexes in the region. Ceramics from between 3050 and 2850 BP are the earliest archeological evidence of human occupation, predating any architectural manifestation in stone and plaster by about 200 years (Estrada-Belli, 2006b; Neivens de Estrada, 2006). These ceramics are among the earliest in the Maya lowlands and exhibit a style of incised decorations shared across the entire region, though documented at a limited number of sites so far, such as Tikal, Yaxha, Seibal, Altar de Sacrificios, and the Belize River Valley (Eb/Xe/Real Xe/Cunil areas; and Komchen) (Willey, 1970; Adams, 1971; Culbert, 1993; Laporte and Fialko, 1993; Clark and Cheetam, 2002). Archeological evidence indicates the abandonment of at least one center in the region, Cival, between 1800 and 1700 BP (AD 150–250). Most other locations show continuous occupation through that century and into the Classic period (1650–950 BP). During that period,

re Sp o

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D. Wahl et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31

Zone

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Percent Terrestrial Pollen and Spores Fig. 7. Pollen percentage diagram of extra-local and rare/less common pollen types from the Laguna Yaloch sediment core. Note x-axis scale changes.

especially the final two centuries, the site of Holmul was a major urban center with a residential zone extending up to 4 km from its center and merging with those of neighboring centers, T'ot, Riverona, Hamontun and K'o. The survey of K'o, carried out in connection with this study, documented an extensive network of Late Preclassic period residential groups in a 1 km radius around the ceremonial core (Tomasic, 2006, 2009). This demographic peak in the periphery occurred during a phase of major construction in the center, including erection of most of the temple pyramids. Following this, less ambitious building efforts in the Early Classic period (1650–1350 BP) reflect reduced population. A second increase in population and construction took place in the Late and Terminal Classic periods (1350–1000 BP). Similar to most other sites of the southern Maya lowlands, the K'o center and settlement area were finally abandoned at the end of the Classic period. Significantly, no archeological evidence has been found in excavations or surveyed areas for occupation post dating ca. 1000 BP in this region. Because of the proximity of K'o and the western edge of Laguna Yaloch (b3 km) we interpret the sediment sequence from Laguna Yaloch in the context of K'o's occupation history, which in turn follows the same patterns of occupation as neighboring centers mentioned above (with the exception of Cival). 6. Discussion The sediment stratigraphy comprises a record of the late Holocene, from around 3350 BP (1400 BC) to present. The heavy clays at the base of the Laguna Yaloch core plugged the core barrel and stopped the hand operated coring operation. It is likely that the sediment sequence, and therefore the age of the lake, is greater, though more robust coring equipment is necessary to retrieve longer cores. Unfortunately, the relatively short temporal record contained in the sediment core

precludes our ability to address research questions focused on arrival and impacts of early agriculture in the region. The high sedimentation rate, however, offers an opportunity for high-resolution analyses through the time interval spanned by the sediment core. 6.1. Forest cover The pollen record from Laguna Yaloch replicates a trend that has been found across the region: attenuated forest taxa values during the period of prehispanic Maya settlement followed by a distinct increase in the early Postclassic. Palynological studies in the Maya lowlands that date to the late Pleistocene/early Holocene show closed canopy forest developing around 10,250 BP and persisting through the middle Holocene (Leyden, 2002; Hillesheim et al., 2005). Initial onset of forest taxa decline ranges from ~5600 to 3000 BP; many records exhibit an initial or accelerated decline between 4000 and 3000 BP (Jones, 1991; Islebe et al., 1996; Pohl et al., 1996; Dunning et al., 1998; Leyden et al., 1998; Leyden, 2002; Rosenmeier et al., 2002; Wahl et al., 2006; Anselmetti et al., 2007). At Laguna Yaloch, the two major pollen groups representing arboreal taxa, Urticales and M–C, show lower percentages through the entire period of prehispanic settlement (Fig. 6). Urticales pollen is low throughout, averaging 7.8% from 3350 to 970 BP. M–C pollen begins to decline somewhat later, ~2620 BP, exhibiting low values (save for one level at 1935 BP) until ~970 BP. Thus, data from the basal levels of the core suggest that, similar to much of the southern Maya lowlands, the landscape around Laguna Yaloch had been largely opened by ~3350 BP. Although we are unable to ascertain when (indeed, if) the local forest was initially opened, the pollen data suggest that vegetation remained relatively open and/or disturbed throughout the period of prehispanic settlement. Evidence for this comes from primary and ancillary arboreal pollen data (Figs. 6 and 7). Lower percentages of

D. Wahl et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31

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Fig. 8. Diagram showing percentages of selected pollen profiles plotted with δ13CSOM, charcoal influx and loss on ignition from Laguna Yaloch. Note x-axis scale changes in pollen data.

Urticales and M–C pollen during the Preclassic and Classic periods are indicative of decreased forest cover. The Pinus (pine) and Quercus (oak) pollen percentages are elevated during this period. As these taxa are primarily found in the savannas of Belize and southern Peten, higher percentages in the Laguna Yaloch sediment are interpreted to reflect increased pollen dispersal due to decreased filtering by forest canopy. Byrsonima (nance), another tree associated with savannas in Peten and Belize (though present in small stands within bajo and upland forest area), also exhibits an out of phase relationship with forest taxa pollen. Cecropia and Mimosa, secondary forest elements, follow the same trend. Changes in pollen percentages from weedy herbaceous taxa, particularly Poaceae (grasses), Asteraceae (sunflower family, asters) and Amaranthaceae (goosefoot family), are often interpreted to indicate forest disturbance and/or agricultural activity in the lowland tropics. Indeed, decreased forest taxa pollen in the Preclassic and Classic periods is accompanied by elevated pollen percentages of grasses and asters (Fig. 6). Zea pollen, a clear indicator of nearby agricultural activity, is sporadically present throughout the period of prehispanic settlement. Amaranthaceae pollen rises to an abrupt peak at the transition to zone 1, just before forest taxa pollen increases (and coincident with the last occurrence of Zea pollen in the record). However, the relationship between disturbance taxa and upland forest at Laguna Yaloch is somewhat complicated. The lake is currently surrounded by a relatively large seasonally flooded herbaceous wetland that includes sedges, grasses, and asters. Any changes in lake level would have undoubtedly affected pollen percentages from these herbaceous taxa. One way to discern the relative importance of local (i.e., littoral) versus extra-local (i.e., upland) input of pollen from weedy annuals is to assess how they behave relative to other wetland indicators. Fig. 6 shows some coherence between Poaceae and Cyperaceae (sedges, including Cladium), suggesting that a portion of the grass pollen is coming from the nearby marsh. The extremely high levels of grass pollen in the lower levels of the pollen sequence (60 and 72%) diverge from sedge pollen and, in fact, reach percentages recorded in modern neotropical savannas (Bush, 2002; Bhattacharya et al., 2011). These data may indicate a more savannalike environment near the lake prior to 3000 BP. Asteraceae and Amaranthaceae pollen show little correlation with either grasses or sedges, appearing to represent disturbance in the surrounding uplands. Much of the Asteraceae abundance in the Classic period is comprised of ragweed (Ambrosia), a common agricultural weed. The

strong correlation between Ambrosia and Zea pollen throughout the pollen sequence exemplifies this relationship. Due to these characteristics in the data, it appears that Asteraceae pollen is a more useful indicator of agricultural activity at Laguna Yaloch than Poaceae. Increasing forest taxa pollen percentages at the end of the late Classic period, beginning ~930 BP (AD 1020), quickly reach levels equal to mean values for the Postclassic period (in ~160 years) (Fig. 6). Rapid afforestation in the Postclassic period has been noted elsewhere in the southern Maya lowlands (Wahl et al., 2006; Mueller et al., 2010), and is undoubtedly tied to the widespread abandonment of the region that occurred at the end of the Classic period (between AD 900 and 1000). Forest regeneration appears to have been relatively swift once anthropogenic pressures ceased in the region. Our conclusion that human activity is the primary driver of Maya lowland vegetation change in the late Holocene is drawn from the time transgressive nature of observed changes. Strong evidence against climatic forcing is presented by asynchronaiety in the onset of midHolocene forest decline reflected in pollen data from across the Maya lowlands. If decreasing precipitation was responsible for opening the forest in the Maya lowlands, one would expect temporal coherence in the signal across the region. As discussed above, this is not the case. Paleoecological studies across the lowlands do tend to align between 4000 and 3000 BP, showing a consistent signal of (initial or accelerated) forest decline and ecological disturbance. While there is evidence for drier conditions in the circum-Caribbean between 3500 and 3000 BP (Hodell et al., 1991; Haug et al., 2001; Mueller et al., 2009), the temporal covariance of Zea pollen, deforestation, increased disturbance taxa, and erosion leaves little doubt that the ecological signal, by this time, is primarily driven by human activity. Finally, the relatively synchronous afforestation of the Maya lowlands coincident with abandonment at the end of the Classic period once again connects forest cover to prehispanic populations rather than climate.

6.2. Development of surrounding wetland at Laguna Yaloch The major transition from zone 4 to zone 3 at Laguna Yaloch ~1540 BP is noticeable in much of the proxy data (Figs. 6 and 8). Sedimentological changes include a dramatic decrease in clay input with a concurrent increase in organic matter. Grass, sedge, and water lily (Nymphaea) pollen percentages rise significantly across this transition. Charcoal concentration and influx also exhibit a sudden increase. C:N ratios

D. Wahl et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 17–31

make a stepwise shift to higher values. Together, these data suggest a shallowing of the lake. The ratio of carbon to nitrogen (C:N) in sedimentary organic matter reflects the source material contributing carbon to the sediment matrix (Meyers and Ishiwatari, 1993; Meyers, 1997; Meyers and Teranes, 2001). Values below 10 are produced when lacustrine algae is the primary contributor and indicate the presence of a relatively deep-water lake; values above 15 indicate input from terrestrial plants, suggesting a shallow near shore environment. Lacustrine sediment usually contains organic matter from both sources with sediment organic matter C:N ratios varying along a spectrum. The lower C:N values from 3350 to 1700 BP indicate that the organic matter in the sediment was largely composed of autochthonous material, with relatively little terrestrial carbon reaching this part of the basin. These values suggest the presence of a deeper lake occupying the basin at this time. Given the low topography and extensive herbaceous wetland presently surrounding the lake, the implication here is important. An increase in water level of even 2 to 3 m above modern levels would result in the surface area of Laguna Yaloch increasing by orders of magnitude relative to today. Thus the data suggest that the lake was not only deeper, but much larger as well. Increasing organic content and C:N values between 1700 and 1550 BP may reflect the establishment of the surrounding wetland. Though available data do not provide conclusive evidence for the cause of this transition, it is likely that persistent erosional input of upland clay had an impact on local hydrology, slowly silting in the basin. The pollen data suggest that a tipping point was reached allowing herbaceous plants to become established in the littoral zone, augmenting sediment deposition and facilitating the expansion of terrestrial vegetation onto a shallow shelf. High C:N values persist to the present. Increased input of terrestrial carbon from 1550 BP to the present suggests that the coring site was, as it is today, in relatively shallow water. 6.3. Fire history The charcoal record from Laguna Yaloch represents the highest resolution fire reconstruction from the Maya lowlands to date. Concentration and influx values generally track each other, indicating that shifts in the accumulation rate of charcoal are not artifacts of the age-depth model (Fig. 5). The top centimeter of sediment at Laguna Yaloch represents the last ~40 years and reflects the relationship between modern burning and modern charcoal input. According to loggers living at Laguna Yaloch in 2005, the wetland surrounding the lake burns regularly. The composition of the present day plant community reflects this impact; woody vegetation on the wetland is characterized by pioneering, fire tolerant species. Charred trunks were present on nearly all arboreal individuals observed during the 2005 field season. Thus it is reasonable to conclude that the upper cm of sediment represents charcoal input from recent fires, including fires on the proximal wetland. The elevated levels of charcoal input during the Classic and early Postclassic periods therefore likely represent a signal of more extensive burning in the surrounding uplands, perhaps associated with an extreme dry interval at this time (Kennett et al., 2012). The veracity of this interpretation will be assessed though ongoing work on a network of sites in the area refining our understanding of local vs. regional signals in charcoal records from small to mid-sized lakes in the Maya lowlands. Changes in charcoal influx suggest dramatically different fire regimes near Laguna Yaloch over the past 3350 years. Influx data show low but persistent burning in the early Middle Preclassic period followed by a near absence of fire for the rest of the Preclassic and early Classic periods. Values rise dramatically at the zone 4/3 boundary, rapidly reaching significantly higher levels after 1550 BP. The highest sustained charcoal influx ( x = 39.8 particles/cm 2/yr) for the entire record is present in the Middle to Late Classic period from ~ 1530 until ~ 1275 BP (AD 420 to 675). Charcoal is nearly absent

27

during the Terminal Classic period ( x = 2.75 particles/cm 2/yr); influx then rises between 1000 and 900 BP before dropping to relatively low levels ( x = 8.1 particles/cm 2/yr) during the Postclassic period. During the Colonial period charcoal influx drops by a factor of 3 (x = 3.5 particles/cm 2/yr). The basal sediment of the Laguna Yaloch core terminates in the Early Preclassic period, making it impossible to assess whether burning was important during initial settlement. A longer record of microscopic charcoal and pollen from Cob Swamp, about 150 km northwest in Belize, shows a dramatic increase in charcoal associated with the onset of forest decline in the Archaic period, around 4400 BP (Pohl et al., 1996). Charcoal values decline at Cob Swamp into the Preclassic period, and data from the upper portion of the core are similar to those found at Laguna Yaloch. The Cob Swamp forest taxa pollen remains low and charcoal deposition decreases into the Middle Preclassic period and beyond, followed by a large peak of charcoal in the upper section. Unfortunately, chronology for the upper section of the Cob Swamp core is poorly constrained and it is difficult to compare the records beyond noting similar trends in the data. The extremely low charcoal deposition rates at Laguna Yaloch during the Preclassic period are of particular interest. There is ample evidence of nearby human activity: settlement data indicate a demographic peak at Cival, K'o and Hamontun during the Late Preclassic period (2350–1650 BP); Zea pollen is present as early at 2800 BP; high sedimentation rates coupled with elevated clay input indicate increased erosion during this period, likely tied to general disturbance in the watershed. Given the current understanding of Maya agricultural systems, we would expect to see contemporaneous use of frequent, extensive burning, yet charcoal influx values are consistently low compared to the Classic period. As noted above, the Cob Swamp data also show attenuated charcoal coupled with low forest taxa pollen percentages in the Preclassic period, so this trend is not unique. While no direct evidence exists, one possible explanation is that Preclassic agricultural strategies around Laguna Yaloch focused on intensive methods (i.e., raised/ditched fields, terraces) as opposed to swidden techniques. Fire was much more common and/or local during the Classic period than any other period of the record. Though settlement history for the Holmul region shows demographic shifts coincident with the Preclassic/Classic transition, major site centers maintained similar proximity to Laguna Yaloch. Large shifts in the charcoal influx data are correlated with changes in core lithology, suggesting a connection with changes in land use practices within the watershed. Modern charcoal deposition records annual fires on the seasonal wetland surrounding Laguna Yaloch as well as some input from burning to the east in Belize. The high influx rates during the Classic period, significantly above modern levels, suggest widespread burning of upland areas surrounding the lake and possibly more extensive burning in the nearby wetland. The negligible change in forest taxa pollen during the Classic period reflects the more regional extent of the forest taxa pollen signal. A forthcoming set of high-resolution macroscopic charcoal records from multiple sites in the southern Maya lowlands will enable us to further discern local vs. regional burning as well as how fire relates to forest cover, agricultural activity and climate. 6.4. Climate The carbonate horizon that comprises zone 2 (~1270–1040 BP) at Laguna Yaloch is of particular interest because it correlates temporally with evidence of drought in the northern Maya lowlands (Hodell et al., 1995; Curtis et al., 1996; Hodell et al., 2001; Hodell et al., 2005a; Medina-Elizalde et al., 2010; Medina-Elizalde and Rohling, 2012). Carbonate content in alkaline lakes comes primarily from two sources: it is washed in with surrounding soils and carried in aqueous solution as dissolved inorganic carbon. CaCO3 precipitation from the water column, and subsequent incorporation into the sediment matrix, can result from evaporative precipitation (usually, but not limited to, closed basin

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systems) and/or biogenic precipitation related to photosynthetic induced changes in pH (via CO2 depletion) near aquatic vegetation and algae. The latter process can be strongly influenced by turbidity and available nutrients, both of which are affected by nearby human activity. Although there is no sign of lake desiccation in the sediment during this period, the proxy evidence suggests that decreased hydrologic input was responsible for carbonate deposition at Laguna Yaloch (Figs. 6 and 8). Three of the rare/extra-local pollen types increase through this zone: Quercus, Arecaceae and Sesuvium-type. Increased oak pollen without a large shift in forest canopy may reflect expanding oak savannas to the east, possibly tied to drier conditions. Likewise, as many Arecaceae (palm) species are drought tolerant (Knox, 2005), increased Arecaceae pollen would not be surprising if drier conditions impacted lake hydrology. Finally, Sesuvium, a halophytic herb typically found in salt marshes (Lonard and Judd, 1997), exhibits increased percentages through zone 2; higher salinities may have provided a competitive advantage for this plant during this period. It is worth noting that the identification of this pollen type is currently tentative. The genus is present in Guatemala, and Yucatan in general (Lundell, 1934; Standley and Steyermark, 1946), though botanical descriptions are limited to coastal environments; more in-depth vegetation analysis near Laguna Yaloch will be needed to confirm this interpretation. δ13CSOM values become heavier during zone 2 and it appears that allochtonous organic matter is driving the signal. The alternative interpretation of increased lacustrine algae as the source of heavy carbon isotopes appears unlikely. If nutrient loading, replete with algal blooms, pH changes and biogenic carbonate precipitation, was involved, there would be a significant shift to lower C:N values. Moreover, Typha is well adapted to eutrophic conditions (Rejmankova et al., 1996) and would likely increase in abundance, rather than decrease. Therefore it appears that the relatively heavy δ13CSOM signal is reflecting an increase in nearby C4 vegetation. Drier conditions and/or increased agricultural activity could lead to a relative increase in C4 herbs. Recent work in neotropical lakes shows a positive relationship between δ13CSOM and evidence of nearby agricultural activity (Lane et al., 2004, 2008). δ13CSOM values from Laguna Yaloch exhibit similar trends and values as those reported in the above studies, linking heavy δ13CSOM in zone 2 to terrestrial, rather than aquatic, vegetation. Overall, the data from Laguna Yaloch suggest decreased precipitation impacting the system's carbonate flux, ultimately leading to a high rate of carbonate deposition in zone 2. The possibility that climatic drying caused the formation of the carbonate horizon around 1270 to 1040 BP is supported by comparing the Laguna Yaloch record with regional paleoclimate records (Fig. 9). Oxygen isotopes, bulk density, and gypsum deposition from lake sediments and speleothems in the Maya lowlands have been used to reconstruct late Holocene regional climate variability (Hodell et al., 1995; Curtis et al., 1996; Hodell et al., 2005a; Webster et al., 2007; Kennett et al., 2012). These records, from disjunct sites across the Yucatan peninsula, suggest drier conditions during the Terminal Classic and/or early Postclassic periods (~1120–850 BP), possibly associated with latitudinal changes in the ITCZ (Haug et al., 2003). The timing of this dry period, though variable due to different chronological methods, is important because it coincides, roughly, with widespread abandonment of the southern Maya lowlands. 6.5. Settlement and abandonment The combined archeological and paleoenvironmental data show evidence of settlement in the Holmul region from the Middle Preclassic through the Terminal Classic periods. Attenuated forest cover and high levels of clay input at the base of the Laguna Yaloch core suggest that settlement predates the current paleoenvironmental record. The first appearance of Zea pollen at 3330 BP provides the earliest direct evidence for agriculture in the region. Following its initial appearance, Zea pollen is intermittently present in the Preclassic periods and consistently present in the Classic period.

High clay and magnetic susceptibility values in zones 6–4 indicate erosional input during the Preclassic period. This thick clay horizon is analogous to the “Maya Clay” commonly found elsewhere in depositional environments in the Maya lowlands. Of note is the higher level of clay input, presumably from surrounding uplands, during the Preclassic period relative to the Classic period. High rates of erosion in the Early and Middle Preclassic period have been observed frequently throughout the southern Maya lowlands (Jacob, 1995; Dunning et al., 2002; Hansen et al., 2002; Wahl et al., 2007a; Beach et al., 2009), and may be connected to a greater abundance of easily eroded material present at the time of initial settlement. The shallowing of the lake, development of the nearby wetland, and expansion of littoral vegetation after 1560 BP also likely acted to trap/ inhibit delivery of eroded material to the center of the lake. Though there is some evidence of changes in local land use near the end of the Preclassic period, there is no clear signal of the Preclassic period abandonment apparent at many sites in the Maya lowlands (Howell and Copeland, 1989; Hansen, 1990b; Estrada-Belli, 2005). Zea pollen is absent in the record between ~1830 and 1600 BP (AD 120–350) and Urticales pollen percentages rise slightly, hinting at forest recovery. Many other proxies, however, remain constant at this time, making it difficult to conclude that the area near the lake underwent an abandonment. Results show clear evidence of anthropogenic activity near Laguna Yaloch during the Classic period. Elevated disturbance taxa, continued (though lowered) clay input, extremely high sedimentation rates, and persistently low forest taxa all point to human impacts. The high charcoal concentrations and influx at this time most likely reflect anthropogenic burning. During the Early Classic period, Zea pollen is consistently present and δ 13CSOM values relatively heavier, perhaps suggesting increased agricultural activity near the lake. The temporal association between apparent drought conditions in the Late Classic period and widespread abandonment in the southern Maya lowlands has led to the suggestion of a causal relationship (Hodell et al., 1995; Gill, 2000; Haug et al., 2003; Hodell et al., 2005a). The Laguna Yaloch record presents enticing evidence of drier conditions in the southern Maya lowlands during the Late Classic. Moreover, the multi-proxy approach used here allows us to compare the climate signal with evidence of local human activity to more fully understand causal relationships. Forest taxa pollen percentages remain low throughout this period, which would indicate persistent population pressure. Although anomalously dry conditions may have suppressed forest recovery, the continuity of low forest taxa pollen percentages from zone 6 to zone 2 makes such an interpretation tenuous. Peak Asteraceae and Ambrosia values in zone 2 point to sustained agricultural activity, as do the heavy δ13CSOM values. The near complete absence of charcoal deposition in zone 2, however, does indicate less burning. The pollen record at Laguna Yaloch has important implications for the relationship between drought and settlement in the region. Here we see the last occurrence of Zea pollen followed by forest regeneration after amelioration of Terminal Classic period dry conditions. If we assume that the forest taxa signal is somewhat regional, it appears that local populations persisted through the dry phase, only to decline after wetter conditions returned. Population decline may well have been underway during the dry phase, with smaller groups living near the lake after a collapse of political infrastructure. In any case, it is clear that permanent abandonment did occur, and available evidence shows that it happened after the anomalously dry conditions of the Terminal Classic period had ended. 7. Conclusions Our findings support and build upon the archeological record in the Holmul region and, in a broader context, the southern Maya lowlands. While we cannot speak to the timing and extent of initial impacts associated with the arrival of agriculture in the area, the data

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Laguna Yaloch

Lake Chichancanab

29

Punta Laguna

0

500

Cal yr B.P.

1000

1500

2000

2500

3000

3500 0

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Fig. 9. Paleolimnological evidence from Laguna Yaloch and other sites in the Maya lowlands indicating dry conditions at the end of the Late Classic period. Lake Chichancanab isotope and sulfur data from Hodell et al. (1995), GRA bulk density from Hodell et al. (2001). Punta Laguna data from Curtis et al. (1996).

presented here provide a youngest date for anthropogenically induced environmental change. Evidence of ecological disturbance at the base of the Laguna Yaloch core indicates impacts from nearby populations during the Early Preclassic period, as early as 3350 BP. The presence of Zea pollen at ~ 3330 BP provides the earliest direct evidence of agriculture in the area. The last appearance of Zea pollen, drop in disturbance taxa and rapid increase in forest taxa indicate that local abandonment occurred ~ 930 BP. The dispersed settlement of the Middle Preclassic period transitioned into a thickly settled landscape during the Late Preclassic period. At this time, settlement units of all sizes and complexity dotted the landscape, similar to the pattern of elite land tenure observed at Classic period sites such as Seibal, Tikal, and La Milpa. These data increasingly emphasize the complex pattern of resource management surrounding Preclassic period ceremonial centers. The subsequent Classic period occupation around Cival was low-density. Surrounding areas, however, particularly near Holmul, Hamontun and K'o, were densely occupied until final abandonment ~950 BP. Evidence for anomalously dry conditions from ~1270 to 1040 BP at Laguna Yaloch correspond with similar evidence for drought in lake sediments of the northern Maya lowlands. Our multi-proxy approach enables us to assess how this climate event may have affected local populations. Our findings show the presence of nearby burning and agriculture after a return to moister conditions, indicating that abandonment of the area did not occur during the dry phase in the Terminal Classic. While these results do not necessarily rule out the possibility that drought played a role in the breakdown of the socio-economic functioning of Classic period Maya society, they do raise questions about correlations between drought and abandonment in the southern Maya lowlands. Charcoal analysis shows that fire was less important in the Preclassic period than the Classic period. Changes in settlement patterns do not appear to be driving the fire signal, raising the possibility that a shift in agricultural strategies may be responsible. Charcoal input at the sediment water interface, reflecting recent fires, is low, yet anecdotal evidence indicates current regular burning of the surrounding wetland. It is reasonable to infer that high charcoal input values during the Classic period are likely due to more extensive burning in the uplands surrounding Laguna Yaloch. Low charcoal input values during the previous period suggest that the upland areas may have remained underutilized for agriculture

during the Preclassic period. Future work on a series of cores from across Peten will help constrain the nature of the charcoal signal at Laguna Yaloch (i.e., local vs. regional). This study establishes a chronology of environmental change and human settlement in a portion of the southern Maya lowlands that was occupied during the Preclassic and Classic periods. The use of multiple lines of evidence with firm chronological control clarifies the timing of major cultural events and furthers our understanding of prehispanic human/environment interactions in a fragile tropical environment. Acknowledgments This research was funded by the U.S. National Science Foundation (Research Grant #0647034), the National Geographic Society (Scientific Research Grant #8203-07), and the Alfawood Foundation. We wish to thank Roger Byrne and Thomas Schreiner for their insight and discussion. We also thank the Ministerio de Cultura y Deportes de Guatemala for cooperative support. We are grateful to John Barron, Tim Beach and Sally Horn for thoughtful feedback on the manuscript. References Adams, R.E.W., 1971. The ceramics of Altar de Sacrificios. Papers of the Peabody Museum of Archaeology and Ethnology 63 (1). Ali, A.A., Higuera, P.E., Bergeron, Y., Carcaillet, C., 2009. Comparing fire-history interpretations based on area, number and estimated volume of macroscopic charcoal in lake sediments. Quaternary Research 72, 462–468. Anselmetti, F., Hodell, D., Ariztegui, D., Brenner, M., Rosenmeier, M., 2007. Quantification of soil erosion rates related to ancient Maya deforestation. Geology 35, 915. Beach, T., Dunning, N.P., Luzzadder-Beach, S., Cook, D.E., 2006. Impacts of the ancient Maya on soils and soil erosion in the central Maya Lowlands. Catena 65, 166–178. Beach, T., Luzzadder-Beach, S., Dunning, N., Jones, J., Lohse, J., Guderjan, T., Bozarth, S., Millspaugh, S., Bhattacharya, T., 2009. A review of human and natural changes in Maya Lowland wetlands over the Holocene. Quaternary Science Reviews 28, 1710–1724. Bhattacharya, T., Beach, T., Wahl, D., 2011. An analysis of modern pollen rain from the Maya lowlands of northern Belize. Review of Palaeobotany and Palynology 164, 109–120. Binford, M.W., Brenner, M., Whitmore, T.J., Higuera-Gundy, A., Deevey, E.S., Leyden, B.W., 1987. Ecosystems, paleoecology and human disturbance in subtropical and tropical America. Quaternary Science Reviews 6, 115–128. Brenner, M., Rosenmeier, M.F., Hodell, D.A., Curtis, J.H., 2002. Paleolimnology of the Maya lowlands: long-term perspectives on interactions among climate, environment, and humans. Ancient Mesoamerica 13, 141–157.

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