Environmental and Cultural Changes in Highland Guatemala Inferred from Lake Amatitlán Sediments
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GEA_20352.qxd 3/24/11 12:45 PM Page 1 Environmental and Cultural Changes in Highland Guatemala Inferred from Lake Amatitlán Sediments Maria I. Velez,1,* Jason H. Curtis,2 Mark Brenner,2 Jaime Escobar,2, 3 Barbara W. Leyden,4 and Marion Popenoe de Hatch5 1Department of Geology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada 2Department of Geological Sciences, and Land Use and Environmental Change Institute (LUECI), University of Florida, Gainesville, FL 32611 3Center for Tropical Paleoecology and Archaeology, Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Panama 4Deceased 5Departamento Arqueología, Universidad del Valle de Guatemala, Apartado Postal No. 82, Guatemala 01901, Guatemala We inferred the late Holocene environmental history of the Guatemala highlands from multi- ple lines of evidence in a sediment core from Lake Amatitlán. Inferred environmental changes are generally synchronous with archaeologically documented highland Maya cultural shifts. Population increases in the Middle Preclassic, Early Classic, and Late Postclassic are associ- ated with deforestation and soil erosion. Land abandonment in the Late Preclassic, Late Classic, and Early Postclassic is associated with evidence for reforestation and soil stabilization. Diatoms indicate relatively lower lake level and greater trophic status at times of reduced human impact, from ca. 250 B.C. to A.D. 125 and from ca. A.D. 875 to 1375. Decreased water levels were probably due to drier climate, to reforestation, or both. Lake eutrophication was caused by reduced water volume combined with a legacy of long-term agricultural activity. Our data contribute to the understanding of relations among ancient Maya culture, climate, and envi- ronment. © 2011 Wiley Periodicals, Inc. INTRODUCTION Investigations of lake sediment cores have been conducted in the Maya lowlands since the late 1950s to shed light on relationships between paleoclimate, past envi- ronmental conditions, and ancient Maya culture (Brenner et al., 2002; Neff et al., 2006). Early studies on lake sediments explored the impact of Maya agricultural and engineering activities on lowland karst watersheds of the Yucatán Peninsula, from ca. 3000 to 1000 cal yr B.P., with a focus on human-induced deforestation and soil *Corresponding author; E-mail: [email protected]. Geoarchaeology: An International Journal © 2011 Wiley Periodicals, Inc. Published online in Wiley Online Library (wileyonlinelibrary.com). DOI:10.1002/gea.20352 GEA_20352.qxd 3/24/11 12:45 PM Page 2 VELEZ ET AL. erosion (Cowgill et al., 1966; Deevey et al., 1979; Brenner, 1983; Vaughan, Deevey, & Garret-Jones, 1985). By the 1970s, attention had turned to inferring past climate in the Maya Lowlands (Covich & Stuiver, 1974). High-resolution stable oxygen isotope and geochemical records from lakes in the Mexican region of the Yucatán Peninsula indicated that moisture availability fluctuated appreciably throughout the late Holocene (Hodell, Curtis, & Brenner, 1995; Curtis, Hodell, & Brenner, 1996; Hodell, Brenner, & Curtis, 2005). Pronounced droughts, identified paleolimnologically (Hodell, Curtis, & Brenner, 1995; Curtis, Hodell, & Brenner, 1996; Hodell et al., 2001) and by speleothem studies (Webster et al., 2007), were invoked as a possible con- tributing factor to the Terminal Classic Maya cultural collapse of the 9th century A.D. More recent studies of regional lake sediment cores addressed quantitative anthro- pogenic soil loss from a catchment in Petén, Guatemala (Anselmetti et al., 2007), as well as climate and environmental conditions leading up to, during, and following the protracted period of sedentary Maya settlement in the lowlands (Müller et al., 2009, 2010). Whereas studies at low-elevation sites in Guatemala and other selected regions of Central America have yielded abundant information about past climate and envi- ronmental conditions in the Maya region, little has been done in the Guatemalan highlands (Popenoe de Hatch et al. 2002; Marcus 2003). There have, however, been several attempts to obtain lake sediment cores from high-elevation areas for paleo- environmental study. Newhall et al. (1987) retrieved cores up to 9.8 m long from Lake Atitlán, but serious concerns about “14C reservoir effects” in this caldera lake made radiocarbon ages suspect, compromising the sediment chronology. Sediments were ultimately “dated” by palynological correlation with records from lowland sites (Higuera-Diaz, 1983), assuming that vegetation changes in both regions were con- temporaneous. Other issues also made paleolimnological study of Lake Atitlán chal- ϳ 2 ϭ lenging. First, the lake is large ( 130 km ) and deep (zmax 340 m), with some 70% of its area (ϳ91 km2) deeper than 100 m (Deevey, 1957), posing logistical challenges for recovering long cores. Furthermore, sediment pore waters are gas-rich and the stratigraphy of deep-water cores is locally complicated by massive turbidity flows and slumping (Newhall et al., 1987). Poppe et al. (1985) studied Lake Ayarza, another ϭ caldera system that also presents problems. It too is very deep (zmax 240 m) and the uppermost 9 m of sediment consist mainly of turbidites. The best candidate for paleolimnological study in the Guatemalan highlands is Lake Amatitlán. With an area of 15.35 km2 and a maximum depth of ϳ33 m, suitable sediment cores can be obtained with hand-coring technology. The lake’s morphom- etry, and the well-documented archaeological history of the watershed, which extends back Ͼ3000 years (Borhegyi, 1959) prompted Tsukada and Deevey (1967) to retrieve a core from a relatively shallow area (ϳ16 m) of the lake. The undated 4.25-m-long sediment column was studied palynologically. The base of the section was inferred to be ca. 3000 years old by pollen correlation with lowland records, suggesting that settlement in the catchment dated back to the earliest Formative period, 1500 B.C.–600 B.C. The bottom ϳ2.0 m of the core contained pollen evidence of sub- stantial human disturbance, with low arboreal and high non-arboreal percentages, 2 GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 26, NO. 3 GEA_20352.qxd 3/24/11 12:45 PM Page 3 ENVIRONMENTAL AND CULTURAL CHANGES IN HIGHLAND GUATEMALA abundant Ambrosia and other Compositae, large amounts of chenopod, amaranth, and Zea pollen, and abundant carbonized fragments. From approximately 2.0 m to 0.75 m depth, there was evidence for declining human impact, with only spotty pres- ence of maize, declines in relative abundance of composites and “cheno-ams,” greater proportions of tree pollen, and lower abundance of carbonized fragments. The upper- most ϳ75 cm reflected a new cycle of human disturbance that continues to the pres- ent, with abundant charcoal, high relative abundance of weedy species and low forest pollen types, and large amounts of maize pollen. The work of Tsukada and Deevey (1967) suggested that the sediments from Lake Amatitlán could yield valuable paleoenvironmental information about the Guatemalan highlands. In 2000, a collaborative project between the University of Florida and Universidad del Valle de Guatemala was initiated to understand the past dynamics among climate, environment, and Maya culture in the highlands (Popenoe de Hatch et al., 2002). Several cores were recovered from extinct Lake Miraflores and from extant Lake Amatitlán. Lake Miraflores was of particular interest because it dried out completely by ϳA.D. 100 and it is not known if its desiccation was due to human impact or to natural climatic variability (Popenoe de Hatch et al., 2002). The core from Lake Amatitlán was dated using accelerator mass spectrometry 14C measurements on terrestrial macrofossils. The core was analysed for pollen, diatoms, and sediment geochemical variables (magnetic susceptibility, C/N ratio, and d15N) to infer past environmental changes in the lake and its watershed. Overall, our goal was to pro- vide a paleoenvironmental context for archaeologists, against which they could inter- pret cultural development in the Guatemalan highlands over the last three millennia. Here we present the results of that investigation. STUDY SITE Lake Amatitlán (14°27'23''N, 90°33'58''W) lies at 1186 meters above sea level (masl), ca. 25 km southwest of Guatemala City in the mountains of southern Guatemala (Figure 1). The area contains four major active volcanoes that lie within 30 km of Lake Amatitlán: Agua (3760 masl), Acatenango (3976 masl), Fuego (3766 masl), and Pacaya (2544 masl). Volcanic activity contributed to the formation and fertility of soils in the area (Sanders & Murdy, 1982). The lake owes its origin to combined volcanic and tectonic processes. Mean direct rainfall on the lake is ϳ1220 mm/yr. The main hydrologic input to the lake is the Rio Villalobos, which drains a watershed of ϳ313 km2 and today carries a substantial suspended sediment load and pollutants from Guatemala City. Other smaller inlets include surface streams and groundwater inflow from hot springs. Excess water exits the lake via the Rio Michatoya. Combined annual hydrologic inputs exceed yearly evapotranspiration (ϳ830 mm/yr). A railroad bridge constructed across a nar- row constriction now divides the lake into two subbasins (Figure 1). Lake Amatitlán has displayed high nutrient concentrations since at least 1950 (Deevey, 1957), when total phosphorus concentrations in surface waters approached 60 mg L–1. By 1988, total phosphorus concentration displayed a mean annual value of GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 26, NO. 3 3 GEA_20352.qxd 3/24/11