Journal of Paleolimnology 4: 239-252, 1990. �9 1990 Kluwer Academic Publishers. Printed in Belgium. 239

Recent sedimentary histories of shallow lakes in the Guatemalan savannas

Mark Brenner 1, *, Barbara Leyden 2 & Michael W. Binford 3 ~Department of Natural Sciences, Florida Museum of Natural History, Gainesville, FL 32611, USA ; 2Depart- ment of Geology, University of South Florida, Tampa, FL 33620, USA ; 3Graduate School of Design, Harvard University, 48 Quincy Street, Cambridge, MA 02138, USA; *Present address: Department of Fisheries and Aquaculture, University of Florida, 7922 N.W. 71st Street, Gainesville, FL 32606, USA

Received 20 February 1990; in revised form 25 April 1990; accepted 24 May 1990

Key words: Guatemala, paleolimnology, pollen, 21~ savannas, sediment geochemistry

Abstract

Shallow basins in the savannas of Peten, Guatemala filled with water after 305 _+ 55 BP (calibrated age = 1430-1660 AD). Aguadas Chimaj and Chilonche possess dilute waters and iron-rich, clayey sediments that are poor in Ca and Mg, reflecting the highly weathered nature of riparian soils. Low 21~ flux rates to Chimaj (0.085 pCi cm -2 yr - ~) and Chilonche (0.134 pCi cm -2 y r - 1) are attributed to low 222Rn emission rates from the nearby Caribbean Sea. Mean sediment accumulation rates in Chimaj and Chilonche for the past 150 years are 0.015 g cm -2 y r - ~ and 0.047 g cm -2 y r - ~ respectively. Forest expansion after 305 BP is documented in pollen profiles from the small aguadas and larger Lake Oquevix. Regional reforestation postdates the 9th century Classic Maya collapse and coincides with indigenous depopulation that was a consequence of European intrusion that began in the early 1500s. The timing of forest regrowth indicates the importance of historical anthropogenic factors in controlling Peten's vegetation. Nevertheless, other sedimentological lines of evidence (e.g. lithology, algal remains and charcoal particles) suggest that changing climate and/or local hydrology may have played a role in the reforestation process.

Introduction

It has been suggested that the anomalous grass- lands in Peten, Guatemala are Pleistocene relicts that remained after a more xeric epoch, but the lack of endemic flora (Lundell, 1937) and herpeto- fauna (Stuart, 1935) weakens this hypothesis. Alternatively, it has been proposed that they are human artifacts, resulting from Maya deforesta- tion. Data from preliminary archaeological surveys in the savannas run contrary to this notion, and suggest that Maya settlement was typically sparse and adapted to, rather than

created the grassland environment (Rice & Rice, 1979). Finally, edaphic factors are thought to account for the savannas that simply persist on hydromorphic, red-brown oxisols. The central Peten savannas are probably maintained today by anthropogenic fires and cattle grazing, but little is known of ttieir history.

Many small ponds (aguadas) and several larger lakes are scattered throughout the Peten savannas and offer the opportunity to explore the region's historical ecology. Earlier limnological work at Lake Oquevix (Deevey et al., 1980a), including palynological study of a short core (reported

240

here), as well as paleolimnological work at Aguada Santa Ana Vieja (Cowgill & Hutchinson, 1966) indicated the potential for further examina- tion of the region's ecological history.

We visited the Peten savannas in spring, 1985 and collected cores from two previously unstudied basins. This paper presents paleolimnological and paleoecological data from Lakes Oquevix, Chimaj and Chilonche (Fig. 1). Palynological and sedimentological data suggest that anthropogenic as well as climatic and/or hydrologic factors have influenced regional savanna and forest land- scapes over the past several centuries.

The study site

Guatemala's northern Department of Peten covers nearly 36000 km 2 and constitutes part of the area in which southern lowland Maya culture arose about 3000 ago, flourished and then cot- lapsed mysteriously in the 9th century AD. (Adams, 1977; Culbert, 1973). Following the Classic collapse, Maya populations persisted in the region and were finally defeated by the Spanish in 1697 (Jones, 1982). The area is charac- terized by karst terrain that lies between 100 and

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District showing Lake Oquevix and Aguadas Chilonche and Chimaj in the savannas south and west of Lake Peten-Itza. Stippling shows the loca- tion of the Peten Lake District in northern Guatemala (reset).

Fig. 1. Map of the Peten Lake

300 m above msl. Haystack hill topography has developed on limestones of Cretaceous and Tertiary age (Vinson, 1962; West, 1964). About 90~/o of the region is covered by forest soils (mollisols), but the remainder is characterized by clay-rich savanna soils (Stevens, 1964; Simmons et al., 1959). Peten receives 1600 mm of rainfall annually (Deevey, 1978), and lies within the tropical lowland dry forest life zone (Holdridge, i947). Dominant local vegetation has been refer- red to as 'quasi-rain-forest' (Lundell, 1937).

The Central Peten Lake District lies at 17 ~ N lat, and extends east-west for more than 100 km (Fig. 1). With the exception of Lake Perdida, most of the larger water bodies have been studied limnologically (Brezonik & Fox, 1974; Deevey et al., 1980a, 1980b). Paleolimnological work in several large Peten lakes yielded information about long-term Maya impact on tropical karst watersheds (Cowgill et al., 1966; Deevey et aL, 1979; Brenner, 1983a; Binford, 1983; Vaughan etal., 1985; Leyden, 1987; Binford etal., 1987) and Pleistocene/Holocene climate change at low elevation sites in the Neotropics (Deevey et al., 1983; Leyden, 1984).

The central Peten savannas lie south of Lake Peten-Itza and cover about 630 km 2 (Rice & Rice, 1979). The grasslands are large and open in some areas. Elsewhere they interdigitate with stands of forest or surround 'islands' of high vegetation (zukches) that possess a distinctive group of highly adaptive successional taxa. Several low-stature tree species are common throughout the grasslands, including 'nanze' (Byrsonima crassifolia), 'saha' (Curatella ameri- cana), 'cocoyol' (Acrocomia mexicana ), and 'cala- basa' (Crescentia cujete). There is an isolated oak savanna south of Lake Quexil.

Aguada Chilonche lies at 1 6 ~ lat, 90 ~ 02' W long, 9 km ENE of the town of La Libertad. Aguada Chimaj is located at 16 ~ 51' N lat, 90 ~ 08' W long, 6 km WNW of the settlement of La Esperanza (Fig, I). Both lakes are small ( < 2 ha) and shallow (zm~• < 2 m). L. Oquevix is considerably larger (,-~3km 2) and lies at 16 ~ 39' N lat, 89 ~ 46' W long, about 40 km ESE of La Libertad (Fig. 1). The aguadas are currently

subjected to some human disturbance (e.g. bathing and clothes washing), and Chimaj sup- ports a small apiary on its shore. All three basins serve as watering holes for cattle that graze the nearby grasslands.

The small aguadas are located where low- stature dry tropical fores t interdigitates with savanna. Chilonche's drainage is nearly all forested, whereas Chimaj possesses a forest fringe on its eastern shore, but is otherwise bordered by savanna. It is possible that Lundell visited the latter pond during his expedition of 1933. He mentions an 'Aguada Chimah Pequefio', about 8 km NW of Libertad, and noted that the small basin was nearly surrounded by scrubby bush. He also reported seeing alligators [sic] splashing in the shallow waters that were lowered by local drought conditions (Lundell, 1937). Oquevix is bordered by both high and low forests, in addition to savanna.

Regional soil maps have poor geographic reso- lution, but suggest that Chilonche is surrounded by forest soils of the shallow, well-drained Chacalte series, and deep, well-drained Sotz series. Chacalte soils border Aguada Chimaj, as do savanna soils of the deep, well-drained Chachaclun series. Riparian soils at Oquevix con- sist of the Chachaclun series as well as shallow, well-drained forest soils (Jolja series), and deep, poorly drained forest soils of the Yaloch series (Instituto Geografico Nacional, 1968; Simmons et al., 1959).

Previous limnological study demonstrated that Lake Oquevix possesses bicarbonate waters (TDS = 84.3 mg L - 1) typical of many central Peten lakes (Deevey et al., 1980a). Although its bathymetry is not known, depths of at least 3.5 m were recorded. It is improbable that the lake desiccated in the recent past because it supports substantial fish populations. Likewise, despite their shallow depth, it is unlikely that Aguadas Chilonche and Chimaj have dried recently. Seining at Chimaj yielded numerous cichlid fish ( Cichlasoma octofasciatum ), and crocodiles ( Cro- codylus moreleti) were observed during coring2

241

Methods

Surface waters were collected from the small aguadas by filling dark Nalgene bottles. Conduc- tivity was measured with a Lab-Line Inc. Lectro Mho-Meter. Cations in water were measured by atomic absorption. Chloride was determined using Technicon chloride color reagent. Sulfate measurements in water were performed using the BaC12 turbidimetric method (APHA, 1975).

Mud-water interface cores were taken from the aguadas with a 2-m long, 4-cm diameter, clear plastic piston corer. Short cores from Chilonche (104 cm) and Chimaj (137 cm) were extruded in the field in an upright position, and sectioned at 1-cm, 2-cm, or 4-cm intervals. Deeper sediments from the aguadas were taken with a steel-barrel, square-rod corer (Wright et aL, 1984). Section 2 from Chilonche contains sediments from 60-154cm depth, and data for depths below 70 cm are from section 2. The Oquevix core was collected in 1974 with a Livingstone piston corer (Deevey, 1965), and has only been analyzed for pollen, charcoal, dry weight and organic matter content. More complete analyses were run on the aguada sediments.

Percent dry weight was measured by weight loss on drying at 110 ~ Organic matter content was determined by weight loss on ignition at 550 ~ (HSkanson & Jansson, 1983). Carbonate content was evaluated by weight loss between 600 ~ and 990 ~ (Dean, 1974). Total carbon was measured coulometrically (Huffman, 1977) with a Coulometrics Inc. Model 5011 coulometer and a System 120 preparation line. Combustion tem- perature was set at 950 ~ C. Cations, phosphorus and sulfur were evaluated following ashing (550 ~ and digestion in 1N HC1 (Andersen, 1976). Cations were read by atomic absorption, and phosphorus was read after blue color develop- ment with ammonium molybdate-ascorbic acid (APHA, 1975). Sulfur content in the Chilonche digestate was too low to be measured accurately by turbidimetry (APHA, 1975), and was not evaluated for Chimaj.

Pollen was extracted from 1 cm 3 sediment samples (2 cm 3 from 0-2 cm in the Chilonche

242

core) using KOH, HC1, HF, and acetolysis. Pollen grains were stained with basic fuchsin, and suspended in tertiary butyl alcohol. Measured aliquots of suspension were mounted in silicone oil. The estimated area of charred particles per volume of sediment was calculated following Clark (1982).

21~ dating was done by modifying the method of Eakins & Morrison (1978). 21~ con- tent in sediments was measured by extraction and plating on copper planchettes, and dates were computed using the constant-rate-of-supply (CRS) model (Goldberg, 1963; Appleby & Oldfield, 1978, 1983).

Results

Water chemistry

Although Lake Oquevix waters are chemically similar to other bicarbonate lakes in Peten (Deevey et al., 1980a), Aguadas Chilonche and Chimaj have unusually fresh waters (Table 1), yielding conductivities of 45 #S c m - 1 and 25/~ S c m - 1 respectively. Total ionic charges in Chilonche (0.49 meq L - 1) and Chimaj (0.16 meq L - 1) waters were estimated by doubling measurable cation (Ca, Mg, K, Na) charges. Sulfate concentrations in waters were too low to be read accurately by BaC12 turbidi- metry, and only trace amounts of chloride were measured in Chimaj. Ionic strengths of the aguada waters are comparable to Peten rainfall (0.28 meq L - 1: Deevey et al., 1980a), and share

another similarity with local rainwater in that Na is the dominant cation (Table 1).

Chronology and sediment accumulation rates

Unsupported 21~ concentrations decline smoothly with depth in the Chimaj core (Fig. 2a). Large standard deviations about age estimates, particularly at deeper levels in the core (Fig. 2b), may contribute to substantial misestimates of net sediment accumulation rates (Fig. 2c). During the past century and a half, bulk sediment accumula- tion rates appear to have been slow and constant, varying between about 0.01 and 0 . 0 2 g c m - 2 y r -1 (Figs. 2b-c). 2.13 g c m -2 of sediment accumulated in the past 150 years, yielding a mean sediment accumulation rate of 0.015 g c m - 2 y r - 1. The total residual unsup- ported :~~ content of the Chimaj core is 2.74 pCi c m - : , which translates to a 21~ flux rate of 0.085 pCi c m - 2 yr - 1.

Sediment accumulation in Chilonche has been faster and more variable than in Chimaj (Fig. 3a-c). Unsupported :l~ activity does not decline smoothly with depth (Fig. 3a), indicating changing sedimentation rates (Figs. 3b-c). Again, large standard deviations about age deter- minations (Fig. 3b) may lead to misestimates of net sediment accumulation rates (Fig. 3c). Over the 21~ portion of the Chilonche core, net sediment accumulation has varied by more than an order of magnitude, from about 0.03 to 0.37 g cm -2 y r - 1 (Fig. 3c). 7.15 g cm -2 of sedi- ment accumulated during the last 1.5 centuries,

Table 1. Ionic content (meq L 1 ) of Guatemalan savanna lake waters and Peten rainwater. Lake Oquevix and rainwater data are from Deevey et al. (1980a). Bicarbonate concentrations in the aguadas are estimated assuming a balance between cation and anion charges,

S ample Ca Mg K Na H C O 3 S 04 C1 Sum

Oquevix 1.35 0.12 0.04 0.19 1.50 0.03 0.10 3.33 Chilonche 0,050 0.033 0,058 0,105 0.183 T 0.063 0.492 Chimaj 0,013 0.025 0.002 0.041 0.081 T T 0.162 Rainwater 0,048 0.006 0.004 0.072 0.075 0,040 0.032 0.277

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Fig. 3a. Unsupported 21~ activity versus depth in Chilonche core 2-V-85-D. The mean and standard deviation (horizontal bars) of the activity are plotted at the midpoint of each sample interval. Sample intervals are indicated by vertical lines. 3b. Age versus depth (solid line) and age versus cumulative mass (dotted line) in the core. Horizontal bars extending to the left or right of the age/depth plot indicate one standard deviation about the age estimate and were computed according to Binford (1990)

3c. Net bulk sediment accumulation rate plotted against age in the core.

yielding a mean sediment accumulation rate of 0.047 g cm- 2 yr - ~. The total residual unsup- ported 21~ load in Chilonche is 4.3 pCi cm-a , equivalent to a 21Opb flux rate of 0.134 pCi cm -2 yr - 1

A wood sample from 130 cm in the Chilonche core has an accelerator 14C age of 305 _+ 44 years BP (Unpublished date AA-3068). The calibrated age ranges from 1430 to 1660 AD (Klein et al., 1982), and the accumulated sediment mass at 130 cm is 73.6 g cm- 2. Using the extreme sidereal ages for calculations, the mean bulk sediment accumulation rate over the length of the section ranges from about 0.13 to 0.23 g cm -2 yr -

Sediment fithology and geochemistry

Physical and chemical analyses of Chimaj sedi- ments were run on close-interval samples (Fig. 4). Dry weight generally increases with greater depth in the core, except between 20 and 40 cm, where sediments with lower percent dry weight are asso- ciated with higher L.O.I. 550 ~ C. L.O.I. 550 ~ is typically higher at the top than the bottom of the cores. The highest L.O.I. 550 ~ values occur

between 25 and 35 cm. L.O.I. 550 ~ C is negatively correlated with the dry weight content of the sediments (r = -0.51, P < 0.01).

The total C plot parallels the L.O.I. 550 ~ profile (Fig. 4), and the two variables are highly correlated (r = 0.82, P < 0.01). L.O.I. 990 ~ and L.O.I. 550 ~ are negatively correlated (r = -0.66, P < 0.01) and L.O.I. 990 ~ is un- correlated with total C (P > 0.05).

Calcium content is weakly correlated with L.O.I. 990 ~ (r = 0.45, P < 0.05). The mean Ca concentration (1.26 + 0.41 mg g- l)for the whole core is similar to the mean Mg concentration (1.34 _+ 0.08 mg g- 1). Potassium exhibits mini- mum (0.30 mg g- 1 ) and maximum (0.63 mg g - t ) concentrations in the uppermost 18 cm of the profile. Manganese content varies little in the upper 51 cm of the core (0.09-0.11 mgg-1) , but is more variable and found in higher concentra- tions below 51cm (0.13-0.28mgg-1). Sedi- mentary iron content is high (42.4-59.6 mg g- 1) at all levels except 30-31 cm, where it drops to 29.9 mg g- 1 coincident with the peak in organic matter concentration. Iron is negatively correlated with L.O.I. 550 ~ (r = - 0.73, P < 0.05). Phos- phorus content is reasonably high throughout the

245

AGUADA CHIMAJ Core 3 - V - 8 5 - C

Dry L.O.L L.O.I. Weight 550~ 990~ Cto t Ca Mg K Mn Fe P

~: % ~< mg g-~dry > ~g g-ldry 2 0 6 0 1 0 2 0 2 4 6 8 2 4 6 8 1 0 1= 2 0 . 5 1 .5 0.4 0 .8 0.1 0 , 2 2 0 6 0 2 0 0 6 0 0

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Fig. 4. Chemical stratigraphy of Chimaj core 3-V-85-C. The date near the base of the section (305 _+ 55 BP) is approximate and is based on palynological correlation with the Chilonche core in which a 14C date was obtained on wood.

core (468 + 112 # g g - 1 ) and P concentration is correlated with L.O.I. 550 ~ (r = 0.57, P < 0.01).

Dry weight content in Chilonche shows a general increase with depth in the core, rising dramatically between 0 and 10 cm (Fig. 5). Below 10 cm, the dry weight content increases gradually to 119-120cm (55.2~/o). Below 120cm, dry weight again increases rapidly, reaching a maxi- mum of 74.9~o at 139-140 cm. L.O.I. 550 ~ generally declines with core depth, and is nega- tively correlated with dry weight content (r = - 0.88, P < 0.01). Total C tracks the organic matter curve (Fig. 5), and the two variables are highly correlated (r = 0.96, P < 0,01). The L.O.I. 990 ~ curve shows no obvious trend, and L.O.I. 990 ~ is uncorrelated with both L.O.I. 550 ~ and total C (P > 0.05).

Calcium concentrations tend to decline with

depth in the top 40 cm of the core, from a maxi- mum of 1 . 8 m g g - I at 2 - 4 c m . Below 40cm depth, Ca content stabilizes at about 1.0 mg g - 1

A high Ca concentration at 130-134cm (1.7 mg g - 1) is associated with the level in which terrestrial wood was found. Ca content is uncor- related with L.O.I. 990 ~ Mg concentrations show little change over the length of the core and the mean Mg content of the sediments (1.56 + 0.13 mgg -1) is slightly higher than the average Ca concentration (1.04 + 0.31 mg g-1).

Potassium concentration is highest in the uppermost 4 cm of the core (0.68 mg g - a), and varies between 0.34 and 0.56 mg g - ~ throughout the remainder of the section. Manganese content shows a very gradual increase with depth in the sediment profile, and generally ranges between 0.15 and 0 .52mgg -] . The peak Mn value (1.74 mg g - 1) was measured at 130-134 cm, and

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sos• BP

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is associated with the wood sample. Iron concen- trations in Chilonche muds are high ( 3 7 - 6 7 m g g -1) and generally increase with depth in the core. Iron is negatively correlated with L.O.I. 550 ~ (r = - 0.69, P < 0.05). Phos- phorus concentrations are highest in the topmost 6 cm of the core (746-862 #g g-1) , but decline thereafter to a mean of 600 _+ 36 #g g - 1. High P values in the uppermost sediments are associated with higher organic matter concentrations, and throughout the core, P is correlated with L.O.I. 550 ~ (r = 0.80, P < 0.01).

Pollen and charred particles

When sedimentary charred particle concentra- tions are expressed on a wet volume basis (cm 2 charcoal cm -3 wet sediment), all three lakes show similar trends. Charcoal content is high in the basal deposits and declines toward the surface of the cores. Suspecting that charcoal profiles expressed in this manner might simply reflect shifts in bulk density (g dry c m - 3 wet), charcoal concentrations were refigured on a dry weight basis. In all three sections, charcoal c o n t e n t ( c m 2 charcoal g - 1 dry sediment) decreases toward the top of the profile (Fig. 6).

Pollen types in Fig. 7 are loosely grouped from left to right as savanna, marginal forest, high forest, and long-distance transport indicators. The 'pinelands' grouping primarily consists of Pinus and Quercus, with varying amounts of Podo- carpus, Alnus, Ulmus, Ostrya-Carpinus, Liquidam- bar, Rapanea, Myrica, and Juniperus-type.

The base of the Chimaj core is dominated by pollen of grasses and sedges with some Moraceae, pineland taxa (predominantly pine and oak), and fern spores (Fig. 7). From 135 to 60 cm, per- centages for the herbaceous taxa decline as per- centages for the Moraceae, particularly Brosi- mum, increase. Nevertheless, there is a pro-

Chimaj Chilonche Oquevix

80

120

0 20 4 0 60 0 20 4O 6O 0 4 0 8 0 120

Charred Particles (cm 2 charcoal g-1 dry sediment)

Fig. 6. Charcoal content (cm 2 charcoal g 1 dry sediment) in cores from Aguadas Chimaj and Chilonche, and Lake Oquevix. A ~4C date of 305 _+ 55 BP (1430-1660 AD) was obtained on wood from 130 cm depth in the Chilonce core. Palynotogical correlation with the Chitonche core allows approximate placement of the 305 BP age on the Chimaj and

Oquevix stratigraphies (indicated by arrows).

nounced but temporary decline in the percentage of Brosimum at 80 cm. Between 60 and 30 cm, Brosimum and the herbs decline while trees and shrubs of the marginal forest (zukches) increase. The aquatic alga Botryococcus also begins to in- crease. Above 30 cm there is a marked increase then decline in percentages for grasses, aquatic macrophytes (primarily Potamogeton), and Bo- tryococcus. Percentages for marginal forest taxa, particularly palms, composites and Cecropia, con- tinue to increase.

The basal 20 cm of the Chilonche core are without pollen (Fig. 7). At 130cm, grasses, sedges and Byrsonima are the most abundant pol- len types. Pteridophytes are also prevalent. From

247

130 to 70 cm, percentages for grasses and sedges decline, while Brosimum, other Moraceae, and melastomes increase. Between 70 and 40 cm there is a general decline in the Moraceae and marginal forest taxa, initiated by a marked temporary de- cline in Brosimum and other Moraceae types. Above 40 cm there is a brief increase in the per- centage of sedges, followed by increases in melastomes, Cecropia, and Trema, with further declines in Brosimum. Botryococcus is prevalent at some levels near the top of the core.

Pollen of grasses, sedges, and Byrsonima pre- dominate at the base of the Oquevix core (Fig. 7). Palms and composites are also well represented, as are the algae Botryococcus and Pediastrum.

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r

_ 305• BP

�9 < 2 ~

Fig. 7. Pollen percentage diagram for Aguadas Chimaj and Chilonche, ~md Lake Oquevix. Note the break in the depth scale for the basal sample from A. Chimaj. Aquatics, algae, pteridophytes, and unidentifiable grains are excluded from the pollen sum. From left to right, principal savanna taxa include Gramineae, Cyperaceae, and Byrsonima; marginal forest types include Melastomataceae through Sapindaceae; high forest trees include Piperaceae, Alchornea, Brosirnum and other Moraceae. A wood sample from 130 cm in the Chilonche core provided the 14C age (305 + 55 BP). The 305 BP age was assigned to depths in the Chimaj and Oquevix cores by palynological correlation with the Chilonche stratigraphy, and is indicated by arrows at the extreme

right of the pollen plots.

248

................................................... ............................. I::r:,, M a y a Populat ion Densi ty ~ High

OB~O~O a~P

.............. .................. t High

Vegeta t ion Savanna

Fire F r e q u e n c y / I n t e n s i t y " " ' " - .~ / Low

....................... ~ Hig h W a t e r Level

.................................................... t =,., Cl imate ................... ' ............................... 1 ~ Brier

1400 ,(- . . . . . . . . . . . . . . . ~=. 2000 Age (Years AD)

Fig. 8. Summary of paleoecological inferences for Peten, Guatemala, based on study of sediment cores from savanna lakes Chilonche, Chimaj and Oquevix. Poor dating resolu- tion precludes establishing a detailed chronology, though stratigraphies are anchored by a ~4C date on wood (305 _+ 55 BP: 1430-1660 AD) from Chilonche. Maya popu- lation loss in the Colonial Period is inferred as a consequence of introduced disease, though the decline may have been offset to some degree by Maya migration into Peten

from northern Yucatan (Jones, 1982).

From 120 to 50 cm, percentages for grasses and Byrsonima decline, while palms, sedges, and the aquatic taxa remain unchanged. Ferns are more abundant in the middle of the zone. Percentages for Brosimum remain steady after an initial in- crease at 90 cm. Above 50 cm, the savanna taxa decline while Brosimum and marginal forest taxa, particularly Cecropia, Trema and Celtis, increase. The aquatic taxa continue unchanged.

Discussion

Lirnnology, chronology and geochemistry

Aguadas Chilonche (45/~S cm -1) and Chimaj (25/~S cm-1) contain the freshest lake waters sampled in Peten. For comparison, eight large lakes of the Central Peten Lake District have conductivities ranging from 240 to 5700/~S c m - 1, and ionic concentrations between 4.24 and 146.35 meq L - 1 (Deevey et al., 1980a). Unlike the shallow aguadas, cation charges in the large lakes are dominated by calcium and mag- nesium. Bicarbonate and sulfate are the principal anions. Calcium bicarbonate waters of Lake Oquevix (3 ,33meqL -1) are comparable to waters in the fresher central Peten lakes. The savanna aguadas are probably fresh because

water enters directly in rainfall and as runoff that flows over highly weathered soils. Nevertheless, evaporative water loss from the closed basins might be expected to concentrate dissolved salts. Their extreme freshness suggests that ions are removed by sedimentation and/or subterranean drainage (Cowgill & Hutchinson, 1966).

21~ flux rates at Chimaj (0.085 pCi cm -2 yr - l) and Chilonche (0.134 pCi cm-2 yr- 1) are low relative to the figure of < 0.6 pCi cm - 2 yr - 1 reported for continental regions with productive 222Rn output (E1-Daoushy, 1988). The probable source area for atmospherically-derived 21~ in Peten is the Caribbean Sea, that lies 150 km to the east. 222Rn emanation from the sea surface is probably low relative to land-injected rates. The small 21~ inventories recorded in the Peten savanna lakes are similar to the value measured at Lake Miragoane, on the Caribbean island of Hispaniola (2.97pCi cm-2 ; 0.09pCi cm 2 yr-1), where sea-surface exhalation of 22~Rn is probably the primary source of unsupported 21~ (Brenner & Binford, 1988).

Dry weight content of sediments in both Chimaj and Chilonche generally increases with depth in the profile, and at both sites lacustrine deposition appears to have commenced some time between 1430 and 1660 AD. In Chilonche, the percent dry weight shift between 140 and 120 cm marks the section of the core in which shallow lake sedi- ments overlie an indurated paleosol. Terrestrial wood found at 130 cm was probably from a small tree that drowned as water level rose in the pre- viously dry depression.

Mean sedimentation rates for the 2 lOpb_datable sections of the Chimaj (0.015 g c m -2 yr - 1) and Chilonche (0.047 g c m - 2 y r - 1) cores are com- parable to average post-Maya rates in central Peten Lakes Yaxha (0.038 g cm -2 yr - 1), Sacnab (0.045 g cm -2 yr-1) , and Quexil (0.016 g cm -2 yr - 1) for the last 420 years (Deevey et al., 1979). Estimates of bulk sediment accumulation in Chilonche (0.13-0.23 g cm -2 y r - l for the last 325-555 years) are based on the calibrated ~4C date on wood. They suggest that the bulk sedi- mentation rate in Chilonche was several-fold higher before 1840 AD. Higher sedimentation

rates in Chilonche may be associated with the initial filling of the basin that began between 1430 and 1660 AD.

High correlation between total carbon content and L.O.I. 550 ~ as well as low L.O.I. 990 ~ Ca, and Mg values in the aguada muds, suggest that most sedimentary carbon is bound in organic matter. Nevertheless, the low C content of the sedimentary organic matter is notable. Total C constitutes only 8-9~o of the organic matter in the basal, indurated soils of the Chilonche core. In the overlying muds, total C makes up 14-29~o of the organic matter. In Chimaj, total C/organic matter ratios are similarly low, generally ranging from 12-22~o. In three samples between 25 and 31 cm depth, total C comprises 39-41~o of the organic matter, approximating the expected percentage of organic C in carbohydrate (40~). Results from Aguada Santa Ana Vieja show that the organic C content of organic matter is consistently low (Cowgill & Hutchinson, 1966). The low computed total C/organic matter ratios in the savanna lake sediments may be an artifact of overestimating organic matter content by L.O.I. 550 ~

In Chimaj and Chilonche, carbonate equi- valents of Ca and Mg (CaCO3 + MgCO3) typically represent < 1.0 ~o of the dry weight and are considerably lower than carbonate values recorded in post-Maya deposits from Yaxha (5.1~o), Sacnab (6.9~o), and Quexil (19.4~o) [Deevey et al., 1979]. Limestone outcrops abound in the hills surrounding the basins of the major lake chain, and carbonate-rich sascab, the parent material for local mollisols, lies at shallow depth in the soil profiles. Limestone is deeply buried in the savanna region, and overlying, clay- rich soils are poor in exchangeable cations.

Whereas carbonate equivalents of Ca and Mg (CaCO3 + MgCO3)generally account for < 1~o of the sediment dry weight in the aguada sedi- ments, carbonate estimates based on L.O.I. 990 ~ are higher. Mean L.O.I. 990 ~ values in Chilonche and Chimaj are 1.88 + 0.47~o and 3.79 + 2.04~o respectively. Carbonate content, as CaCO3 and MgCO3, should be about twice the L.O.I. values. Loss of water of hydration from clays during high-temperature combustion may

249

account for overestimation of the carbonate con- tent (Dean, 1974).

Iron content of the sediments in both lakes is negatively correlated with L.O.I. 550 ~ and suggests that Fe is associated primarily with the inorganic (clay and hydroxide) fraction of the mud. Iron concentration in Chilonche and Chimaj muds is high, averaging 4.81 + 0.86~o and 4.96 + 0.70 ~o of the sediment dry weight respec- tively. Similar Fe concentrations (6-7 ~o Fe203 g-1 inorganic matter) are reported for Aguada Santa Ana Vieja (Cowgill & Hutchinson, 1966). Iron content in post-Maya sediments of the deeper, central Peten lakes is lower, generally ranging from 1 to 3 To of the dry weight. The high sedimentary iron values measured in the aguadas reflect high iron levels in riparian savanna soils. The chemistry of soils in the Chimaj and Chilonche drainages remains unstudied, but sur- ficial savanna soils (Exkixil series) from south of Lake Quexil contain 7.26 + 2.88 ~o iron, nearly an order of magnitude more Fe than is found in nearby forest soils (Brenner, 1983b).

High P concentrations in Chilonche (516-862 #g g - I ) and Chimaj (282-752#gg i) sediments were unexpected, as elsewhere in Peten savanna soils were shown to be P-deficient rela- tive to forest moUisols (Brenner, 1983b). Never- theless, comparable concentrations (,-~ 0.22-0.27~o P205 g 1 inorganic) are reported for Aguada Santa Ana Vieja (Cowgill & Hutchinson, 1966). Surface Exkixil soils from the savanna south of Lake Quexil contain only 164 + #g P g-1, whereas uppermost soils from forested settings at Lakes S alpeten and Macanche contain 598 + 271 #g P g- 1 and 594 + 318 #g P g- l , respectively. Forest soils do contact the aguada shores, are assumed to be nutrient-rich, and are probably the principal P source for the lakes. Correlation between P content of the mud and L.O.I. 550 ~ suggests that P is bound prin- cipally in sedimented organic matter.

250

Pollen and charred particles

Despite the small size of Aguadas Chimaj and Chilonche, the pollen and charcoal profiles for all three lakes record comparable regional trends, modified by distinctive local histories (Figs. 6 & 7). Peten savannas were more extensive and fires were more frequent and/or larger several centuries ago. Regionally, fires declined and high forests rapidly regrew coincident with the Spanish arrival and indigenous depopulation. The encroachment of marginal forest into the savannas appears to have proceeded more slowly.

Today, the plant associations and riparian soils surrounding Chimaj, Chilonche, and Oquevix differ, and have done so for the past 300 to 500 years. Chilonche has always had a more closed canopy than Chimaj. Percentages for savanna taxa and Brosimum are somewhat lower in Chilonche, and the number of unknown pollen types is very high. The percentage of unknown pollen types increases after a mid-core charcoal peak (Figs. 6 & 7), suggesting that many un- knowns represent non-arboreal and successional taxa. Lundell (1937) commented on the diversity of marginal forest associations in Peten savannas that are nevertheless collectively recognized as a group.

Oquevix lies closer than the small aguadas to pine forests in eastern Peten and Belize, and the pineland association is better represented in the Oquevix pollen record. The initial rise of Brosi- mum and decline of charcoal occurs at shallower depth in Oquevix compared to the small aguadas, perhaps because the Oquevix core represents a longer period of time. Persistence of poorly- drained forest soils near Oquevix is reflected in the high relative abundance of palms and sedges. The Brosimum curve is less influenced by the expansion of local marginal forest, because of the larger catchment area.

Historical limnological differences among the three lakes are detected in the microfossil record. Aquatic macrophytes are poorly represented throughout the Oquevix record, while Botryococ- cus and Pediastrum are common at all levels. The pollen of aquatic macrophytes is abundant in the

upper 40 cm of the Chimaj core, but sparse or absent below 40 cm. Likewise, Botryococcus is found in large numbers above 40 cm, but dis- appears completely below 80 cm. Pollen of aquatic macrophytes and algal remains are rare or absent throughout most of the Chilonche profile. These data indicate that Oquevix has always been larger and deeper than the aguadas during the last several centuries. Sparse or absent aquatic micro- fossils at deeper levels in the aguada cores suggest that low water levels prevailed and rose concur- rent with diminished fire intensities. Abundant unidentified algal remains at 30 cm in the Chimaj core probably account for the L.O.I. 550 ~ C peak at that level, as well as suppressed percentages of Botryococcus.

Geochemical and microfossil stratigraphies from Guatemalan savanna lakes provide insights into the regional historical ecology (Fig. 8). The calibrated 14C age on wood near the base of the Chilonche section anchors the chronology, and regional vegetation shifts allow intercore correla- tion by palynological comparison. Indurated soils near the bottom of the Chilonche profile, as well as microfossil evidence from all three cores sug- gest drier conditions and more human impact on vegetation prior to 305 BP (1430-1660 AD). The shallow aguadas filled with water after 305 BP, as indicated by the deposition of more organic lacustrine sediments above 130 cm in Chilonche, and the increased representation of aquatic pollen and algal remains near the top of the aguada sections.

Pollen stratigraphies from deep, central Peten lakes record a prolonged period of Maya de- forestation beginning about 3000 years ago (Leyden, 1987; Vanghan etal., 1985; Deevey et al., 1979). The subsequent forest regrowth was recognized in previous studies, but radiocarbon dates on sediments from the carbonate-rich basins were ambiguous regarding the timing of the reforestation event. Forest expansion has been variously attributed to cultural upheaval and attendant population decline following the Classic collapse (,~ 900 AD), or to drastic depopulation after European contact. Pollen evidence from the savanna lakes indicates that forest expanded at

the expense of savanna after 305 BP (1430-1660AD). Therefore, reforestation was associated temporally with the demise of Maya populations following European contact in the early 1500s, and the Itza defeat in 1697.

It is tempting to attribute historical vegetation changes in Peten to solely human factors. Never- theless, climate may have played a role. Sedi- mentological evidence indicates that reforestation and water-level rise in the savanna aguadas are roughly coincident with the Little Ice Age (1430-1850) that affected temperate regions (Gribbin & Lamb, 1978). Although vegetation cover, climate, and local hydrology are frequently linked (Lean & Warrilow, 1989), hydrology in karst terrain can fluctuate independently. High lake levels in Peten during the late 1930s and early 1980s (Anonymous, 1980) appear uncorrelated with precipitation, and are thought to be con- trolled by the regional water table. A goal for future paleoecological research in Peten will be to distinguish between natural and anthropogenic influences on historical vegetation.

Acknowledgments

We thank Nicolas Dorion, Jeff Kaufmann, Jean- Louis Trombetta, Hector Pefia and Bruce Mamel for field assistance. The Oquevix core was col- lected in 1974 by Don Rice and Hague Vaughan. Sr. Tono Ortiz and Sra. Aura Ortiz kindly pro- vided accommodations in Peten. The University of Florida IFAS Soils Lab assisted with cation and chloride analyses. Sr. Robert Dorion pro- vided financial assistance in the field. The project was supported by NSF grant EAR 84-19506 to E.S. Deevey.

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