Paleolimnology of the Peten Lake district, GuatemalaII. Mayan population density and sediment and nutrient loading of Lake Quexil

Mark BrennerFlorida State Museum, University of Florida, Gainesville, FL 32611, U.S.A.

Keywords: paleolimnology, colluvium, soil chemistry, clay, phosphorus loading, tropical lakes

Abstract

The long-term impact of Maya culture on a lowland tropical watershed is assessed, using data from a 9.2 msediment core taken from deep water (28 m) in Lake Quexil. Human population growth, estimated by the1980 archaeological survey, is associated with a shift in the composition of the sediment to a dominance byinorganic material, the Maya clay formation, beginning ca. 3500 B.P. Increasing settlement densities arecorrelated with accelerated influxes of phosphorus, carbonates, and siliceous sediment. However, chemicaldata do not track short-term population fluctuations closely. Because much of the sediment is delivered ascolluvium, and not by running water, there is a lag between terrestrial disturbance and impact on the aquaticsystem. As an indication of this lag, contemporary high sedimentation rates are a residual of Maya activitythat virtually ceased some 300-400 years B.P. Comparison of the deep-water core with a shallow-water (7 m)section, based on palynological correlation, reveals only minor differences in proximate chemical compo-sition. Chemical influxes are much higher at the deep-water site, however, as a consequence of sedimentfocusing in this hyperconical basin. Chemical analyses of soil samples from 21 test pits in the Quexil basinsupport the principal conclusion that bulk soil movement was the mode of nutrient transfer to the lake,following forest clearance by the Maya.

Introduction

Northern Guatemala's Peten lake district (Fig. 1)provides ideal study sites for a paleolimnologicalinvestigation of the long-term interactions betweenhuman populations and aquatic systems of thetropical lowlands. The karsted, central Peten re-gion covers some 20 000 km 2 and archaeological in-vestigations there reveal that the area was oncedensely settled by the Maya. Ceramic evidencefrom the sites of Altar de Sacrificios and Ceibaldates the earliest human presence in Peten at 3000B.P. (Rice 1976), although earlier occupation isknown in Belize (Hammond et al. 1977). By classictimes (A.D. 250-850), regional Maya populationdensity exceeded 20 persons · km-2 (Sanders 1973),and Late Classic (A.D. 550-850) urban centres

supported 200 persons . km- 2, with levels of600-700 persons · km- 2 plausible at some sites likeTikal (Culbert 1973).

Tsukada's (1966) study of Lake Petenxil sedi-ments employed palynology to document a longhistory of human disturbance. Subsequent studiesat Lakes YaxhA, Sacnab and Quexil (Brenner 1978;Deevey et al. 1979; Vaughan 1979) have used sedi-ment chemistry, pollen, and microfossils to assessMaya perturbations in the watersheds and conse-quent effects on the lakes. Archaeological examina-tion of the YaxhA and Sacnab watersheds revealedthat populations in the basins increased exponen-tially from the Middle Preclassic (1000-250 B.C.)until the Late Classic (A.D. 550-850) and one en-vironmental response to the population growth wasan increase in the delivery rate of phosphorus to the

Hydrobiologia 103, 205-210 (1983).@ Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

206

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Fig. 1. Map of the Peten lake district and Lake Quexil showing locations of archaeological sampling transects, soil pits, and core sites.

lakes by colluviation (Deevey et al. 1979).With the completion of the 1980 archaeological

field season in the Quexil watershed, it is nowpossible to extend our research to a third lake,exploring correlations between Maya populationfluctuations (Rice & Rice, unpublished) and thepaleolimnological record. Quexil is particularly in-teresting because populations did not undergo theslow steady growth revealed at Yaxhd and Sacnab,but instead displayed a Terminal Preclassic-EarlyClassic (100 B.C.-A.D. 550) population decline be-fore rising once again in the Late Classic (A.D.550-850) prior to the as yet unexplained regionalpopulation collapse around A.D. 900.

This study employs both a shallow-water (7 m)and a deep-water (28 m) core (the latter designatedQuexil H) to examine the sedimentary history ofLake Quexil. Both cores contain nearly completeHolocene profiles, document initial vegetation dis-

turbance at about 5000 B.P., and demonstrate theprofound effects of sediment focusing in this hyper-conical basin (Lehman 1975; Deevey et al. 1977).Chemical analyses of basin soils substantiate ourclaim that bulk soil movement was the major modeof nutrient transport from watershed to lake fol-lowing Maya forest clearance.

Results and discussion

Because radiocarbon dating fails to provide reli-able results as a consequence of the hard-water lakeeffect (Deevey & Stuiver 1964) and colluviation, weare compelled to zone our cores by identifying sec-tions consisting of discrete pollen assemblages andassigning archaeologically correlated dates to them(Vaughan & Deevey, 1981). Thus, as at Yaxhiand Sacnab, we rely on the changing regional

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pollen spectrum to date sections of the Quexilcores, estimates proximate sediment compositionwithin zones, and calculate chemical influxes dur-ing the various archaeological periods.

The percentage chemical composition of equi-valent zones in the shallow and deep-water cores isnot dissimilar, though some differences are appar-ent (Table 1). The deep-water section is dominatedby silica while the shallow core can be characterizedas more organic. However, proceeding from thedeepest, pre-Maya sediments to the modern muds,both cores display similar zone-to-zone shifts in theorganic-inorganic ratio.

Human occupation of the watershed caused ashift toward inorganic domination of the sedimentsand depopulation has resulted in a partial return tomore organic muds like those of the deepest, pre-Maya and low population zones. In view of thisinterpretation, Early Classic sediments are anom-alous as peak inorganic percentages are associatedwith this low population period. However, modernsediments of the now-forested watershed containless organic matter than pre-disturbance mud, and it appears that material transfer has not achievedequilibrium in the 400 years since the abandonmentof the catchment. Apparently, equilibrium was notattained during the Terminal Preclassic-EarlyClassic population hiatus either.

Influx rates

Chemical delivery rates to Quexil are now exam-ined as they may have controlled the trophic state ofthe lake. This chemical transfer implies removal ofnutrients from the drainage basin, a process thatperhaps reduced soil fertility.

Despite the problem of non-equivalent time

correlations, total phosphorus influxes can becompared roughly to changing population densitiesas expressed by mound occupations (Fig. 2). Inboth cores it can be seen that human settlement ofthe basin was accompanied by higher phosphorusinfluxes. High Early Classic influxes (normal atYaxha and Sacnab) calculated for the Quexil coresare unexpected considering the low population ofthe period. With respect to this anomaly, it is note-worthy that post-Maya influxes at the shallow anddeep sites are much higher than their respectivebaseline rates, indicating once again that long peri-ods of time (>400 years) are necessary for equilibri-um to be achieved.

Comparing equivalent zonal influx rates betweenthe two cores (Fig. 2), it is apparent that morephosphorus is deposited per unit time in deep wa-ter. This stems from the fact that more bulk sedi-ment is deposited in deep water and deep-site sedi-ments contain more dry weight per unit volume.While some delivery rates calculated for the deep-water site exceed tolerable levels (Vollenweider1968), it has been argued elsewhere (Brenner 1978;Deevey et al. 1979) that little of the sedimentedphosphorus was available for eutrophication, thusthe deep-water data are biased indicators of pasttrophic state and are simply presented as evidenceof the strong sediment focusing that occurs in thehyperconical Quexil basin (Fig. 3).

Like phosphorus, other sediment components(Corg, carbonates, silicates) were transferred to thelake at higher rates as a consequence of humanactivity in the watershed (Table 2), but augmenta-tion of carbonate and silicate influx rates exceededchanges in organic carbon deposition as expressedby the inorganic nature of disturbance-zone sedi-ments. Diatoms and other microfossils are extreme-

Table 1. Percent composition of Lake Quexil shallow-water and deep-water core sediments.

Zone Organic matter CaCO3 SiO2 Fe2 03

Shallow Deep Shallow Deep Shallow Deep Shallow Deep

Post-Maya 42 36 26 5 30 54 2 5Late & Postclassic 18 9 30 5 48 81 4 5Early Classic 14 8 6 6 75 81 5 5Late Preclassic 66 40 5 5 27 51 2 4Middle Preclassic 69 3 25 3Early Preclassic 59 3 36 2Pre-Maya 69 48 3 6 24 41 4 5

208

PHOSPHORUS INFLUX (4g P cmZ.yr-')

ARCHAEOLOGICALPERIODS

POST- MAYA

POSTC LASSIC

TERMINAL CLASSILATE CLASSICEARLY CLASSIC

TERMINAL PRECLASSIC

MIDDLE AND

LATE PRECLASSIC

EARLY PRECLASSIC

PRE- MAYA

OCCUPIEDMOUNDS

4

528O0

15

SHALLOWWATERCORE

- 4 0

DEEPWATERCORE

8 16 24 32 40I . . . . . . . .

POST- MAYA

LATE AND

POSTCLASSIC

EARLY CLASSIC

LATE PRECLASSIC I

-- - MIDDLE PRECLASSIC

---EARLY PRECLASSIC

- -- PRE- MAYA

.L

Fig. 2. Time correlation of Maya population levels and phosphorus influxes.

ly scarce in disturbance-level sediments and it issuspected that the bulk of the organic matter, car-bonates and silicates deposited following vegeta-tion removal was derived allochthonously. In fact,zone-to-zone phosphorus influx rates are positivelycorrelated with deposition rates of carbonates andsilicates in both cores, suggesting that a common

delivery mechanism may be shared by all threechemical constituents.

Soils

Since Peten rainfall contributes only a small frac-tion of the annual phosphorus income to the lakes

Table 2. Influxes to the Lake Quexil sediments at the shallow-water and deep-water sites (amount · cm -2 · a-l).

Zone Corg (mg) CaCO3 (mg) SiO2 (mg) Ptot (ig)

Shallow Deep Shallow Deep Shallow Deep Shallow Deep

Post-Maya 2.2 4.8 3.1 1.8 4 20 4.4 18.0Late & Postclassic 1.3 1.6 5.0 2.9 8 47 5.9 16.6Early Classic 1.7 4.0 1.6 7.8 20 II11 5.8 36.7Late Preclassic 4.8 7.8 0.7 3.0 4 32 4.2 18.9Middle Preclassic 1.8 0.2 0.4 1.7Early Preclassic 3.3 0.3 4 2.7Pre-Maya 1.3 1.4 0.2 0.4 1 3 0.7 2.6

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SHALLOW WATERCORE

POST-MAYA

LATE ANDPOSTCLASSIC

EARLY CLASSIC

LATEPRECLASSIC

MIDDLEPRECLA SSIC

EARLYPRECLASSIC

PRE- MAYA

DEEP WATERCORE

…-- - -- --

POST- MAYA

LATE ANDPOSTCLASSIC

EARLYCLASSIC

LATEPRECLASSIC

MIDDLE ANDEARLY PRECLASSIC

PRE- MAYA

Fig. 3. Palynological zone correlation between the Lake Quexilshallow-water and deep-water cores.

(Deevey et al. 1979), it is likely that basin soils arethe principal reservoir from which this nutrient, aswell as carbonates and silicates were derived duringMaya times. Leaching of phosphorus is held to beunlikely as surface soil (0-10 cm) concentrationsfrom the 21 pits (Fig. 1) are about 2.5 times deep-sample values. Aluminum, calcium and sodium al-so display gradients in the profiles, but are richer indeep soils.

Though perhaps coincidental, total phosphorusconcentrations in surface soils (0-10 cm) are sur-prisingly close to levels found in the lake sediments.In fact. Quexil surface soil concentrations (278 ±156 g g- ) are not statistically different from val-ues measured in the shallowcore (289 ± 156Ag . g- 1)and deep core (358 ± 122/gg · g 1) sediments. Like-wise, surface soil iron concentrations are no differ-ent from sediment iron concentrations. These dataconfirm our original conclusions about phosphorus

movement that were based on earlier soil samplingat Yaxha and Sacnab: bulk soil movement was theprincipal means of nutrient transfer to the lakefollowing Maya deforestation.

Conclusions

Human-induced deforestation of the Quexil wa-tershed resulted in accelerated deliveries of phos-phorus, carbonates, and silica to the lake, primarilyvia soil translocation. The environmental impactwas sustained not only during the more than 3millennia of Maya occupation, but continues todayin the form of high modern influxes. Clearly, we canidentify changes in sedimentary phenomena thatresulted from human disturbance. However, we feelthat using data from Quexil to refine our per capitaphosphorus loading model developed at YaxhA andSacnab would be premature at this time. This is dueto our inability to track short-term population fluc-tuations in the Lake Quexil cores. This difficultyresults from several possible factors: 1) We are un-able to zone the cores with the same archaeologicaltime zone designations used by the social scientists.Hence, we present a single Late and Postclassicsediment zone that covers three identifiable ceramicperiods possessing very different population levels,and in fact, includes the famous collapse. Likewise,our Late Preclassic zone overlaps two archaeologi-cal zones of very different population densities; 2)Long periods necessary to establish equilibrium forland-water nutrient transfers may in fact make itimpossible to see responses to short-term popula-tion changes; 3) It may be that the environmentalresponse we measure paleolimnologically reflectschanges in land use rather than population num-bersper se; 4) Finally, using a regional pollen strati-graphy to zone the Quexil core may produce ar-chaeologically correlated palynological zonationwhich does not accurately reflect deforestationevents in the Quexil watershed proper. Attempts atusing magnetic susceptibility to establish interbasinstratigraphic correlations proved futile due to thelack of sedimented, magnetic particles. However,recent work with particle size analysis (Binford1983) indicates that granulometry may be an ap-propriate technique for assessing basin-specific dis-turbance.

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Acknowledgments

This study was supported by National ScienceFoundation grants nos. BMS-72-01859, DEB-77-06629, and EAR-79-26330 awarded to E. S. Dee-vey. Grateful acknowledgment is made to E. S.Deevey, M. S. Flannery, M. W. Binford, D. S. andP. M. Rice.

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