Chemical, paleontological, and mineralogical analyses of a 7.5-meter core from the middle of Lake Valencia, Venezuela, have provided information on the paleoclimatic history of this low-elevation, low-latitude site for the last 13,000 years. The data show that dry climates existed in this region from 13,000 years before present (B.P.) until about 10,000 years B.P. The Lake Valencia Basin was occupied by intermittent saline marshes at that time. About 10,000 years B.P., a permanent lake of fluctuating salinity formed and arboreal plant communities replaced the earlier dominant xeric herbaceous vegetation and marsh plants. By 8500 years B.P., Lake Valencia reached moderate to low salinities and discharged water; the modern vegetation became established at that time. After 8500 years B.P., the lake twice ceased discharging as a result of reduced watershed moisture. The second of these drying episodes is still in progress and has been aggravated by human activities in the watershed.

The transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition (~11,250 to 7,500 cal yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ~11,250 cal yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage.

Sediments from two lakes in the Peten Department, Guatemala, provide palynological evidence from Central America of late Pleistocene aridity and subsequent synthesis of mesic forests. Late Glacial vegetation consisted of marsh, savanna, and juniper scrub. An early Holocene temperate forest preceded a mesic tropical forest with Brosimum (ramon). Thus "primeval" rain forests of Guatemala are no older than 10,000 to 11,000 years and are considerably younger Holocene (10,[0][1][2][3][4][5][6][7][8]500 B.P.) (3, 4) but are unsubstantiated by palynological data from Central America. Evidence of late Glacial (15,000-10,000 B.P.) aridity in the Neotropics comes from pollen studies in the savannas of northern South America (5), the Galapagos Islands (where the lowest sections are devoid of pollen) (6), an undated diagram from the Amazon Basin (7), and Lake Valencia, Venezuela (8, 9, 36). Supporting data for the Neotropics come from geomorphic studies (10, 11) and deep sea cores (12)(13)(14). Much of equatorial Africa and Indo-Malaysia were also arid during the late Glacial (15-17). In Central America, montane lakes have been either too youthful (18) or uninformative with regard to lowland paleoclimates (19,20). Shallow lakes in the karst lowlands were dry when water tables responded to the lower sea levels of glacial episodes, and they provide incomplete records (21). Sediments from Lakes Quexil and Salpeten, Department of Peten, Guatemala (Fig. 1), provide an opportunity to obtain paleoecological data covering the Pleistocene-Holocene transition for the Central American lowlands. The pollen data complement interpretations based on sedimentology (22).The low-lying Peten (200-400 m) receives between 900 and 2500 mm of annual rainfall, with a pronounced dry season between January and May. Mean monthly temperatures range between 220C and 290C (23). Internal drainage predominates within the Peten Lake District. Edaphic factors make Salpeten (area = 2.6 kM2, maximal depth = 32 m, average depth = 7.8 m, elevation = 104 m) considerably more saline than Quexil (area = 2.1 kM2, maximal depth = 32 m, average depth = 7.2 m, elevation = 110 m) although the lakes are only 20 km apart (23,24). Lake Quexil lies 1 km from Lake Petenxil, which was studied by Cowgill et al. (25) (Fig. 1). The vegetation is not a true rain forest, but rather a semievergreen seasonal forest. According to Lundell (26), mature forests of the northern Peten are composed of three major tree associations characterized by Swietenia (mahogany), Manilkara (chicle), or Brosimum alicastrum (ramon). Brosimum frequently dominates associations covering archaeological ruins, fostering speculation that these trees were nurtured by the Maya for economic purposes (26). Other common arboreal genera (26, 27) include Calophyllum, Pouteria, Cecropia, Bursera, Spondias, Cryosophila, Ficus, and Piper. METHODSPrevious studies of Lake Quexil and other lakes in the Peten have described the Holocene record (28). The Quexil H core was believed to have terminated just short...

Palynology provides a record of past environmental change in the Maya Lowlands. The underlying principles are simple, but, as with all proxies, there are limitations. During the late glacial period, environmental change was governed by climate, which was cooler and much drier and supported sparse temperate vegetation. The early Holocene epoch was warmer and very wet in the southern Lowlands when mesic tropical forests predominated, while the northern Yucatan Peninsula was edaphically dry until eustatic sea level rose. The modern distributions of plant associations and climatic gradients were established at the end of the early Holocene. Climatic variability continued throughout the Holocene. However, the ability of palynology to identify climatic events is hindered after the Maya became numerous in the Lowlands. Then, multidisciplinary studies provide a better interpretation of events, especially during the late Holocene. Pollen records poorly reflect cyclical droughts seen in isotopic records, as natural vegetation has adapted to these fluctuations.