Post-Columbian environmental history of Lago Petén Itzá ... · del Lago Petén Itzá indica en...
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490 Post-Columbian environmental history of Lago Petén Itzá, Guatemala Liseth Pérez 1,* , Rita Bugja 1 , Julieta Massaferro 2 , Philip Steeb 1 , Robert van Geldern 3 , Peter Frenzel 4 , Mark Brenner 5 , Burkhard Scharf 1 , and Antje Schwalb 1 1 Institut für Umweltgeologie, Technische Universität Braunschweig, Langer Kamp 19c, 38106 Braunschweig, Germany. 2 CENAC-APN, CONICET, San Marín 24, Bariloche, 8400, Argentina. 3 GeoZentrum Nordbayern, Angewandte Geowissenschaften, Friedrich-Alexander-Universität Erlangen-Nürnberg, 90518 Erlangen, Germany. 4 Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749, Jena, Germany. 5 Department of Geological Sciences & Land Use and Environmental Change Institute, University of Florida, Gainesville, Florida, 32611, USA. * [email protected] ABSTRACT Two ~40-cm-long sediment cores, PI-SC-1-10m and PI-SC-2-40m, were recovered at 10 and 40 m water depth, respectively, from Lago Petén Itzá, in the Department of Petén, northern Guatemala. The cores span the last ~525 years of sediment accumulation in the basin. This study explores lake level and trophic state changes that Lago Petén Itzá has experienced since European contact in the early 1500s. We inferred past environmental variability using changes in sediment geochemistry and fluctuations in relative species abundances of ostracode and chironomid fossil assemblages. Changes in concentrations of organic matter (OM), carbonate, total carbon (TC), total nitrogen (TN), C/N ratios, bromine (Br), and faunal relative abundances were used to infer changes in the trophic status of the lake. Cultural eutrophication began in the 1930s, and anthropogenic impact increased significantly after ~1970. Higher linear sedimentation rates, up to 9.1 mm yr -1 , began recently. They are attributed to increases in riparian settlement, deforestation and associated land use changes, which led to accelerated soil erosion. An ostracode-based transfer function was applied to assemblages in core PI-SC-1-10m, which enabled us to identify periods when lake level fluctuated. Such historical fluctuations in lake levels were driven primarily by changes in rainfall. Past lake levels can be summarized as follows: (1) fluctuating, high lake levels from ~1550s to the 1730s and from the early 1940s to 2005, and (2) stable, lower lake levels from about 1750 to the early 1900s. Higher relative abundance of the ostracode Physocypria globula and higher rubidium (Rb) concentrations indicate higher lake levels than today. Chironomids also show sharp fluctuations along the cores that could be related to water level changes. The presence of chironomid assemblages Chironomus, Procladius, and Einfeldia from 1960–2000 AD shows high productivity levels. The Lago Petén Itzá sediment record indicates a generally arid Little Ice Age (LIA), with exceptions around 1580 and 1650 when high lake levels similar to those of the 20 th century, i.e. ~5 m higher than today, indicate more humid conditions. Key words: freshwater ostracodes, chironomids, lake level, geochemistry, eutrophication, Lago Petén Itzá, Guatemala. Revista Mexicana de Ciencias Geológicas, v. 27, núm. 3, 2010, p. 490-507 Pérez, L., Bugja, R., Massaferro, J., Steeb, P., van Geldern, R., Frenzel,. P, Brenner, M., Scharf, B., Schwalb,A., 2010, Post-Columbian environmental history of Lago Petén Itzá, Guatemala: Revista Mexicana de Ciencias Geológicas, v. 27, núm. 3, p. 490-507.
Transcript of Post-Columbian environmental history of Lago Petén Itzá ... · del Lago Petén Itzá indica en...
Pérez et al.490
Post-ColumbianenvironmentalhistoryofLagoPeténItzá,Guatemala
Liseth Pérez1,*, Rita Bugja1, Julieta Massaferro2, Philip Steeb1, Robert van Geldern3, Peter Frenzel4, Mark Brenner5, Burkhard Scharf 1, and Antje Schwalb1
1 Institut für Umweltgeologie, Technische Universität Braunschweig, Langer Kamp 19c, 38106 Braunschweig, Germany.
2 CENAC-APN, CONICET, San Marín 24, Bariloche, 8400, Argentina.3 GeoZentrum Nordbayern, Angewandte Geowissenschaften, Friedrich-Alexander-Universität Erlangen-Nürnberg,
90518 Erlangen, Germany.4 Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749, Jena, Germany.
5 Department of Geological Sciences & Land Use and Environmental Change Institute, University of Florida, Gainesville, Florida, 32611, USA.
ABSTRACT
Two ~40-cm-long sediment cores, PI-SC-1-10m and PI-SC-2-40m, were recovered at 10 and 40 m water depth, respectively, from Lago Petén Itzá, in the Department of Petén, northern Guatemala. The cores span the last ~525 years of sediment accumulation in the basin. This study explores lake level and trophic state changes that Lago Petén Itzá has experienced since European contact in the early 1500s. We inferred past environmental variability using changes in sediment geochemistry and fluctuations in relative species abundances of ostracode and chironomid fossil assemblages. Changes in concentrations of organic matter (OM), carbonate, total carbon (TC), total nitrogen (TN), C/N ratios, bromine (Br), and faunal relative abundances were used to infer changes in the trophic status of the lake. Cultural eutrophication began in the 1930s, and anthropogenic impact increased significantly after ~1970. Higher linear sedimentation rates, up to 9.1 mm yr-1, began recently. They are attributed to increases in riparian settlement, deforestation and associated land use changes, which led to accelerated soil erosion. An ostracode-based transfer function was applied to assemblages in core PI-SC-1-10m, which enabled us to identify periods when lake level fluctuated. Such historical fluctuations in lake levels were driven primarily by changes in rainfall. Past lake levels can be summarized as follows: (1) fluctuating, high lake levels from ~1550s to the 1730s and from the early 1940s to 2005, and (2) stable, lower lake levels from about 1750 to the early 1900s. Higher relative abundance of the ostracode Physocypria globula and higher rubidium (Rb) concentrations indicate higher lake levels than today. Chironomids also show sharp fluctuations along the cores that could be related to water level changes. The presence of chironomid assemblages Chironomus, Procladius, and Einfeldia from 1960–2000 AD shows high productivity levels. The Lago Petén Itzá sediment record indicates a generally arid Little Ice Age (LIA), with exceptions around 1580 and 1650 when high lake levels similar to those of the 20th century, i.e. ~5 m higher than today, indicate more humid conditions.
Key words: freshwater ostracodes, chironomids, lake level, geochemistry, eutrophication, Lago Petén Itzá, Guatemala.
Revista Mexicana de Ciencias Geológicas, v. 27, núm. 3, 2010, p. 490-507
Pérez,L.,Bugja,R.,Massaferro,J.,Steeb,P.,vanGeldern,R.,Frenzel,.P,Brenner,M.,Scharf,B.,Schwalb,A.,2010,Post-ColumbianenvironmentalhistoryofLagoPeténItzá,Guatemala:RevistaMexicanadeCienciasGeológicas,v.27,núm.3,p.490-507.
Post-Columbian environmental history of Lago Petén Itzá, Guatemala 491
RESUMEN
Dos núcleos de sedimentos PI-SC-1-10m y PI-SC-2-40m de 40 cm de largo fueron extraídos bajo un tirante de agua de 10 y 40 m en el Lago Petén Itzá, Departamento de Petén, en el norte de Guatemala. Los núcleos abarcan los pasados ~525 años de acumulación de sedimentos en el lago. Este estudio explora los cambios en los niveles del lago y cambios en el estado trófico que el Lago Petén Itzá ha sufrido desde el contacto Europeo a inicios del siglo XVI. Hemos inferido la variabilidad ambiental del pasado utilizando cambios en la geoquímica de los sedimentos y en las fluctuaciones de las abundancias relativas de las especies de comunidades de ostrácodos y quironómidos. Cambios en las concentraciones de materia orgánica (MO), carbonato, carbono total (CT), nitrógeno total (NT), bromo (Br), en la proporción C/N y cambios en la abundancia relativa faunística fueron utilizadas para inferir cambios en el estado trófico del lago. La eutroficación cultural inició desde la década de 1930, y el impacto antropogénico ha aumentado significativamente desde ~1970. Una alta tasa de sedimentación linear de hasta 9.1 mm yr-1 ha iniciado recientemente. Esto se atribuye a un incremento en la ocupación ribereña del lago durante las últimas décadas, la cual está asociada a cambios de uso de la tierra y ha llevado a una acelerada deforestación y erosión de los suelos. Una función de transferencia basada en ostrácodos fue aplicada a las comunidades del núcleo PI-SC-1-10m pudiendo identificar períodos durante los cuales el nivel del lago ha fluctuado unos ~5 m. Estas fluctuaciones históricas en el nivel del lago fueron causadas principalmente por cambios en la precipitación pluvial. Los niveles pasados del lago se resumen de la siguiente forma: (1) niveles altos fluctuantes entre las décadas de 1550 y 1730 y de los primeros años de la década de 1940 a 2005, y (2) niveles bajos estables de 1750 a la década de 1900. Una abundancia relativa alta de Physocypria globula y altas concentraciones de rubidio (Rb) indican niveles del lago más altos a los actuales.
Quironómidos indican fluctuaciones a lo largo de los núcleos que pueden estar relacionadas con cambios en los niveles del lago. La presencia de las comunidades de quironómidos Chironomus, Procladius, y Einfeldia de 1960–2000 AD indica niveles altos de productividad. El registro sedimentario del Lago Petén Itzá indica en general una árida Pequeña Edad de Hielo (PEH), con excepción de los años alrededor de 1580 y 1650, cuando los niveles del lago fueron altos y similares a los del siglo XX, i.e., unos ~5 m más altos que los actuales, lo que indica condiciones más húmedas.
Palabras clave: ostrácodos lacustres, quironómidos, nivel del lago, geoquímica, eutroficación, Lago Petén Itzá, Guatemala.
INTRODUCTION
Climatechangeandanthropogenicimpactshaveaf-fectedneotropicalfreshwaterenvironmentsintheMayaLowlandsfor thousandsofyears(Curtiset al.,1998;Brenneret al.,2002;Rosenmeieret al.,2002,2004),mak-ingtheregionespeciallyinterestingforpaleolimnologists.PaleolimnologicalstudiesontheYucatánPeninsulahavefocusedonHolocene(Hodellet al.,1995;Curtiset al.,1996;Leydenet al.,1998)andPleistocenedeposits(Hodellet al.,2008),withrecentspecialattentiononthelatedegla-cial-earlyHolocenetransition(Hillesheimet al.,2005).Anumberofproxyvariableshavebeenutilizedtoinferpastclimateandenvironmentalchangesinthelowlandneotropi-calregion,includingpollen(Leyden,1987;Leydenet al.,1998;Islebeet al.,1996;Wahlet al.,2006;Hodellet al.,2008),gastropods,foraminifers,ostracodes(Hodellet al.,1995,2005;Curtiset al.,1996,1998;Rosenmeieret al.,2002),anddiatoms(Rosenmeieret al.,2004).Theoxygenandcarbonisotopiccompositionofcarbonateshellsofsedi-mentedgastropods,foraminifersandostracodescanbeusedtoinferpastlakeproductivityandprecipitation/evaporationratios(Brenneret al.,2002;Schwalb,2003).
Ostracodes(Crustacea:Ostracoda)arebivalvedcrus-
taceanswithlow-Mgcalciteshellsthatinhabitavarietyofaquaticenvironments.Ondeath,theirshellsaredepositedandcanbewellpreservedinlacustrinesediments(Smith,1993;Curtiset al.,1998;Holmes,1998;Dole-Olivieret al.,2000). Ostracode taxa have specific ecological requirements andtheknowledgeofmodernecologicalrequirementscanbeusedtoreconstructpastenvironmentalconditionswherefossilsarefound(ForesterandSmith,1994;Schwalb,2003;Scharfet al.,2005).RecentlimnologicalstudiesinMesoamericashowthattheostracodefaunaisabundant,diverse,andinfluencedbywaterdepthandassociatedphysico-chemicalcharacteristics(Pérezet al.,2010a,2010b,inpress).
Similar toostracodesanddiatoms,chironomids(Insecta:Diptera),ornon-bitingmidges,alsorespondrapidlytoenvironmentalchange,makingthempowerfulbioindicators.Chironomidsarepartofthefreshwater,ben-thicmacroinvertebratefauna.Theirlarvaehavechitinizedheadcapsules,whicharedepositedandpreservedinlakesediments(Massaferroet al.,2004;Verschurenet al.,2000).Therehavebeenanumberoftaxonomicandpaleolimno-logicalstudiesofchironomidsinNorthAmerica(Heinrichset al.,1999;LittleandSmol,2000;Woolleret al.,2004)andSouthAmerica(MassaferroandBrooks,2002;Massaferro
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numberofstudieshaveexploredthelake’smorphometryandlimnology(BrezonikandFox,1974;Deeveyet al.1980;Hillesheimet al.,2005;Anselmettiet al.2006;MARN-AMPI,2008;Pérezet al.,2010b).
FollowingEuropeancontactintheearly1500s,humanpopulationdensityinnorthernGuatemalaremainedlowforcenturies.In1964,thereportedpopulationinPeténwas~26,720inhabitants.Thereafter,therewasanabruptincreaseinpopulationthroughimmigrationdrivenbyagovernmentpolicythatprovidedlandandencouragedsettlementintheregion(PainterandDurham,1995;Rosenmeieret al.,2004).Acensusdonein2002(XICensoNacionaldePoblaciónyVIdeHabitación)reportedatotalpopulationof11,237,196inhabitantsinGuatemala,ofwhom366,735livedinPetén.ThepopulationofFlores,theripariancapitalcityoftheDepartmentofPetén,was30,807(INE,2002).Populationgrowthintownsaroundthelakeappearstobechangingthelake’strophicstate.
Inadditiontoeutrophication,documentedrapidlakelevelchangesareanotherchallengeforlakesidedwellers.Stage fluctuations in Petén Itzá have evidently been a part ofthelake’shistorysincethelatePleistocene(Anselmettiet al., 2006). Recently, high lake level stands have flooded shorelineproperties,forcingsomeresidentstoabandontheirhomes.HighprecipitationisthemaincauseofincreasedlakelevelstandsinLagoPeténItzá.InJune2008,Peténreceivedheavyrainfallandseveraltowns,includingSanBenito and Santa Ana, experienced severe flooding that causedtemporaryevacuationofinhabitants(Escobar,2008).1938wasapparentlytheyearofhighestannualprecipita-tion(2600mm)inthelastcentury,andtheconsequentriseinlakeleveliscommemoratedbyaplaqueonthewallof
et al.,2005),however,thechironomidfaunaofthenorth-ernlowlandneotropicsremainslargelyunexplored.OnlyafewtaxonomicandecologicalstudieshavebeendoneinCentralAmericaandtheCaribbeanregion(Coffmanet al.,1992;WatsonandHeyn,1992;VinogradovaandRiss,2007;Woolleret al.,2007).
Ostracodeandchironomidcommunitiesreactrapidlytoclimateandenvironmentalchanges,makingthemreliableindicatorsofpastlimnologicalconditions(MourguiartandCarbonel,1994).Theycanbeusedtoinferpastwaterqualityandtrophicstatus(Smith,1993;Nazarovaet al.,2008),changesinprecipitationandsedimentlithology(Scharf,1998;MassaferroandBrooks,2002),andothercharacteristicsofthewatercolumn,includingpH,temperature,conductivity,depth(BrooksandBirks,2004;Mischkeet al.,2010),andsalinity(Heinrichset al.,1999).
STUDY AREA
LagoPeténItzáliesabout110ma.s.l.,hasasurfacearea~100km2,andamaximumdepth~160m.ThelakeislocatedintheCentralPeténLakeDistrictofnorthernGuatemala(16°55’N,89°50’W)(Figure1).TheCentralPeténLakeDistrictislocatedinthesedimentarybasinoftheMayablockontheNorthAmericanplate.ThesouthernmostextentoftheYucatánPeninsulaisunderlainbyCretaceouscarbonates(Anselmettiet al.,2006).LagoPeténItzáisoligo-mesotrophic,butbegantoexperienceanthropogenicimpactsinthelastfewdecades.Today,lakewatershaveadissolvedionconcentrationof~11meqL-1andaredomi-natedbycalcium,magnesium,sulfate,andbicarbonate.A
Figure 1. Study area in central Petén Lake District (black scale), northern Guatemala (modified from Google Earth 2010), and Lago Petén Itzá (indicated bythewhitearrowandwithcorrespondingblackandwhitescalebar)showingthelocationwhereshortcoresPI-SC-1-10mandPI-SC-2-40mwereretrieved.
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Description PI-SC-1-10m PI-SC-2-40m
CoordinatesLatitudeN 16°59.29’ 16°59.17’LongitudeW 89°50.46’ 89°43.32’
Water depth 10m 40m
Description of site of recovery
Southern part of thelake.Shallowseasonalswampyzone.
Easternpartofthelake.Close to villages, andtributaryriverIxlú.
Core lengthDating 38cm 38cmGeochemistryanalyses 38cm 40cmBioindicatoranalyses
-Ostracodes 36cm 40cm-Chironomids 36cm 42cm
Estimated age 488years 135years
Mean sedimentation rate
0.89mmyr-1 4.3mmyr-1
Sediment description Lightolivegrayclayeysilts
Darkolivegrayclayeysilts
Type of analysisBioindicators
-Ostracodes + +Speciesincore 7 6
-Chironomids + +Speciesincore 8 37
Geochemistry-LOI + +-C:Nratio + +-Mainelements + +-Traceelements + +
Table1.Descriptionofthetwoshortcoresanalyzedinthisstudy.abuildinginFloresthatindicatesthehigh-watermark.Anecdotalaccountsoflakelevelchangeforthelastcen-turysuggestithasrangedbyasmuchas8m.Thereare,however,nopublishedrecordsoflakelevelchangeinPeténItzáspanningaperiodlongerthanthreeyears(MARN-AMPI,2008).TheInstitutoNacionaldeVulcanología,Sismología,MetereologíaeHidrología(INSIVUMEH,1985)reportedmonthlyrainfallandstagechangeinLagoPeténItzáandotherregionalwaterbodiesfortheperiod1982-1985.Priortothattime,rainfallandlakelevelrecordsareevenscarcer(Deeveyet al.,1980).Alternativesourcesofinformationarerequiredtoreconstructpastlakelevelandclimatehistory.
Waterlevelchangesinlakesareoftencausedbyclimatechanges,i.e.shiftsintherelativeamountofprecipitationtoevaporation.Inturn,suchalterationsinlakeleveltriggerchangesinostracodeandchironomidcommunities.Thesefaunalchangesarefrequentlypreserved in lacustrinesediments.LagoPeténItzáisatropicalclosed-basinlakeanditssedimentrecordhasproventobeahighlysensitiverecorderofchangesinthebalancebetweenprecipitationandevaporation(Hodellet al.,2008).Inthisstudy,weusedfaunalchangesofostracodeandchironomidassemblages,aswellasshiftsingeochemistryinsedimentcores,toinferpastchangesinthetrophicstateandwaterlevelinthebasinoverthepast~525years.
METHODS
Twoshortsedimentcores,PI-SC-1-10mandPI-SC-2-40m,wereretrievedfrom10mand40mwaterdepth,respectively,inLagoPeténItzá(Figure1).ThecoreswerecollectedwithanUWITEC-gravitycorerusing60-cm-long,transparentPVCtubes.Locationofcollectionsites,corelengths,age,sedimentationrates,andvariablesanalyzedarefoundinTable1.Chironomidsandostracodeswereanalyzedinbothcores.Inadditiontothecorescollectedforfaunalremains,twoparallelcoreswereretrievedfor210Pbdating,andanothertwocoresweretakenforgeochemicalanalysis.Shortcoresweresectionedat2-cmintervalsforbioindicatoranalysis.Shortcorestakenforgeochemicalanalysisanddatingweresectionedat0.5-cmintervalsabove20cmdepth,andat1-cmintervalsbelowthatdepth.Bydoingthisweachievedgooddatingresolutionintheupperpartofbothshortcores.AnUWITECcorecutterwasusedforsectioningsedimentcores.AllsampleswerepackedinWhirl-pack®bagsandkeptunderrefrigeration.
Initialsedimentdescriptionwasmadeonwholecores.Color,texture,odorandothercharacteristicsweredescribedfromsedimentsamples.ColorwasdeterminedwithaRockColorChart(GeologicalSocietyofAmerica)withMunsellcolorchips.Mülleret al.(2009)reportedonthelithologiccompositionofsurfacesedimentstakenalongawater-depthtransectinLagoPeténItzá.
Priortogeochemicalandchronologicalmeasure-
ments,wetsedimentsampleswerehomogenizedusingasmallspatula.Watercontentwasdeterminedbyweighingsamplesbeforeandafterovendryingat105°Cfor24hours(DIN/18121-1;DIN,1998).Organicmatter,carbonate(CO2weightloss)andnon-carbonateinorganicmatter(clastics)weremeasuredbyloss-on-ignition(LOI)asdescribedinHeiriet al. (2001). Dried samples were pulverized to a fine powderwithanelectronicballmill.WeusedX-raydiffrac-tion(XRD)todeterminequalitativelythemainmineralsintheshortcoresediments.TheelementsFe,Sr,Br,Cu,Zn,Rb, Zr, Y, and Pb were quantified using energy-dispersive X-ray fluorescence (XRF) spectrometry (Cheburkin et al.,1997).ZrandPbcouldnotbedeterminedincorePI-SC-1-10m.TotalC,N,andSmeasurementsweremadewithanelementalanalyzer(HEKAtechGmbH,EuroEA3000)byoxidativecombustionat1000°C,gas-chromicsepara-tionofgasesandthermalconductivitydetection.OrganiccarbonandtotalnitrogencontentwereusedforcalculationofC/Nratiosinshortcores.AllgeochemicalanalysesweredoneattheInstitutfürUmweltgeologie(IUG),TechnischeUniversitätBraunschweig,Germany.
For210Pbdating, dry,weighedpulverizedsampleswerestoredinsealedvialsatroomtemperatureforfourweeks.Analysisforradioisotopes(210Pb,214Pb,and137Cs)
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wasmadebydirectgammaspectroscopyusingalow-backgroundgammaspectrometer(AmetekORTECandCanberraEurisys)withahigh-puritygermaniumwell-de-tector.MeasurementsweremadeattheGeochronologie&IsotopenhydrologieSektion,Leibniz-InstitutfürAngewandteGeophysik(LIAG),Hannover,Germany.Unsupported210Pbactivitywascalculatedforallmeasureddepthsthroughouttheshortcores.TheConstantSedimentationRateModel(CSR)wasusedtodeterminethesedimentages.Themodelassumesthatinitialconcentrationandsedimentationratearebothconstant,andnopost-depositionalmixingoccurred(Robbins,1978;Laissaouiet al.,2008).210Pbdatingcanonlybeappliedreliablytodatesedimentsofthelast~110years,thuswecalculatedinbothshortcoresthemeanmassaccumulationrate(MAR,kgm-2yr-1) for the first 100 yearsandusedittoinferagesdeeperinthecore.MARwasestimatedbyalinearregressionofthelogexcess210Pbactivitiesateachdepthagainstlinearaccumulatedsedimentmass/area(kgm-2).Theanthropogenicradioisotope137Cs,whichcansometimesbeusedtoidentifythe1963atmo-sphericbomb-testingpeak,wasalsomeasuredandusedasacomplementarychronologicalmarker.
Weenumeratedvalvesandcarapacesofadultos-tracodesinthe2-cmwetsedimentslices.Two1g(wet)aliquots were separated from each sample. The first aliquot waswashedintoasmall(10cmdiameter),63µmmeshsieve.Allostracodeswereseparatedandcountedinthissample.Thesecondaliquotwasanalyzedforwatercontent,usingthesamemethodappliedtosamplesforgeochemicalanalysis. Ostracodes were separated using fine brushes and enumeratedinagriddeddishusingaLeicaMZ7.5stereo-microscope.Ostracodeabundanceispresentedasvalvesorcarapaces per dry mass sediment. Identification followed Pérezet al.(2010a),andpaleoenvironmentalinterpretationreliedonmodernostracodeecologicaldatareportedbyPérezet al.(2010b).
Subfossilchironomidswereanalyzedinwet,2cmsedimentslices.Asmallsubsample(<3g)ofwetsedimentfrom each sample was deflocculated in 10 % KOH, heated to70°Cfor10min,andlaterheatedinwaterto90°Cfor20min.Sedimentswerethensievedusing212µmand95µm mesh sieves. A Bogorov sorting tray and fine forceps wereusedtoseparatelarvalchironomidheadcapsules(HC)fromsamples.Astereo-microscopewith25–40xmagnification was used for identifications. Head capsules weremountedas indicatedinMassaferroandBrooks(2002).Giventhepaucityofliteratureonchironomidsinthe lowland Neotropics, most specimens could be identified onlytogenuslevel.OstracodeandchironomidstratigraphicdiagramswereproducedusingGrapher7.ChironomidandostracodespecimensarestoredincollectionsoftheauthorsattheLaboratoriodeBiodiversidad,INIBIOMA,UniversidaddeComahue,Bariloche,Argentina,andIUG,TechnischeUniversitätBraunschweig,Germany.
FaunaldiversitywascalculatedinsamplesfromshortcoresusingtheShannonWienerIndex“H”(Krebs,1989).
ModernspatialdistributionofspeciesinLagoPeténItzáwaspresentedinPérezet al.(2010b),andshowedastrongrelationbetweenostracodespeciesassemblagesandwaterdepth. This relation justified the development of a transfer functiontoinferwaterdepthanditsapplicationtoassem-blagesintheshortcores.Allsamplesinthecalibrationdatasetwereincludedinthestatisticalanalysisbecauseostracodeswereabundant(>50valvesg-1)insediments.Forstatisticalanalysisandthesetupofourtransferfunction,weusedtheprogrampackagesPAST(Hammeret al.2001)andC2(Juggins,2003).Waterdataforcomparisonwiththe ostracode samples came from water profiles taken at thedeepestpointofthelake(17°00’N,89°51’W)(Pérezet al.,2010b).Sedimentconsistencywasrepresentedbythreeclasses drawn from sediment sample description in the field. Themoderntrainingsetincludessevenostracodespecies,awaterdepthrangefrom0.1to160m(median20m),and23surfacesedimentsamples.Abinomialcorrelation(BC)model(Wrozynaet al.,2009;Frenzelet al.,2010)wasusedforthereconstructionofwaterdepthinshortcoresoverthelast~525years.
RESULTS
Chronology
Core PI-SC-1-10mTotal210Pbactivitywas186Bqkg-1inthe0–2cm
section(Figure2).Maximumactivitywasfoundinthe2–4cmsection.Belowthat,sedimentsshowedaconsistentdeclinein210Pbactivitywithdepth.Minimumactivity(10Bqkg-1)wasreportedforthe36–38cmsection.Theactivityof214Pb,i.e.supported210Pbactivity,rangedfrom25to9.5Bqkg-1.Unsupported(excess)210Pbactivityateachlevelwascalculatedasthedifferencebetweentotal210Pband214Pbactivity,anddisplayedagenerallyexponentialdecaywithdepth.AccordingtotheCSRmodel,thetotalageofthesedimentcoreisabout525years,i.e.,thedateatthebaseis~1480AD.Thelong-term,meanlinearsedimentationrate is0.89mmyr-1.Agesolder than110yearswereestimatedroughlyusingthemeanmassaccumulationrateofthepast100years.Maximum137CsactivityinPI-SC-1-10m(21.6and20.5Bqkg-1)occurredat0.5and3cminthecore,correspondingto210Pbdatesof2003and1987,respectively.
PI-SC-2-40mTotal210Pbactivitydisplayedamaximumvalue(525
Bqkg-1)intheuppermost,0–2cmsectionanddeclinedwithdepth(Figure2).Theminimumactivity(36Bqkg-1)wasreportedatadepthof35cm.Theactivityof214PbwashigherthaninPI-SC-1-10mandrangedfrom41to23Bqkg-1.Unsupported210Pbrangedfrom484to13.5Bqkg-1.Theagemodelindicatedamaximumageat37.5cmof119years(1886)andthusahighermeanlinearsedimentation
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Total Pb [Bq kg ]210 -1
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epth[kgm
]-2
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Figure2.AgemodelsofshortcoresPI-SC-1-10mandPI-SC-2-40m.
rate(4.3mmyr-1)thaninPI-SC-1-10m.Maximum137CsactivityinPI-SC-2-40mwas76.8Bqkg-1andwasmeasuredat1cmdepth,correspondingtoa210Pbdateof2004.Thereisdetectable137Csactivitybelowtheleveldatedto1954by210Pb(~20cmcoredepth),indicatingpossibletransferofsoluble137Cstodeepersediments,whichhasbeendocu-mented in other lake sediment profiles (Longmore et al.,1983;Hermanson,1990).
Sediment lithology
SedimentsthroughoutshortcorePI-SC-1-10marehomogenousandcharacterizedbylightolive-graysiltyclays.Twocoresectionspossesseddarkersediments,at0–8.5cmand22–24cmdepth(Figure3a).PI-SC-2-40mwascomposedofdarkolive-graysiltyclays.Theuppermost15cmofthecoreareslightlydarkerthanthebottomsedi-ments(Figure3b).Bothcoresweredominatedbyinorganicmatter(Figures3aand3b).Thehighestorganicmatter(OM)contentincoresPI-SC-1-10mandPI-SC-2-40mwasat3cmdepth(Figures3a,b).OMvariedgenerallyfrom7.3to 21.7 % of dry weight in PI-SC-1-10m (Figure 3a) and from 16 to 22.5 % in PI-SC-2-40m (Figure 3b). Short core PI-SC-1-10mdisplayedanincreaseinOMandadecreaseinthecarbonatecontentintheuppermost8.5cm,i.e.,since1944.ThehighestOMconcentrationinPI-SC-2-40m(17.8%) was detected at 27 cm depth. Carbonate content was higher in PI-SC-1-10m (the highest at 31 cm depth, 35.8 %)thaninPI-SC-2-40m(Figure3a).Thelowestcarbonatecontent (26.3 %) in PI-SC-1-10m was found at 1 cm depth (Figure 3a), and in PI-SC-2-40m (10.4 %) at 7 cm depth
(Figure3b).RapidhumansettlementofPeténbeginning~1960,correlateswellwiththetimingofhigherOMandlowercarbonatecontentinbothshortcores.XRDresults(notshown)indicatedthatcorePI-SC-1-10mwascomposedmainlyofcalcite.Montmorillonite,calcite,dolomiteandquartzweredetectedincorePI-SC-2-40m.
Sediment geochemistry
C/N ratio Sincetheearly1930s, totalcarbonandnitrogen
concentrationsinLagoPeténItzásedimentshavebeenincreasing,andtheorganiccarbon/totalnitrogen(C/N)ratiohasdecreased,atrendthatisclearsincearound1960(Figure4a).TotalC(organicandinorganic)rangedinPI-SC-1-10 m from 12.1 to 16.7 %, and in PI-SC-2-40 m from 9.4 to 10.4 %. Nitrogen (N) content was low in both cores. Total N in PI-SC-1-10m ranged from 0.2 to 1.0 %, and in PI-SC-2-40m from 0.4 to 0.7 %. Sections II, III, IV, and V (Figure4a)havelowernitrogenconcentrationsandhigherC/Nratio.Nitrogenconcentrationsincreasedat11,23,andat37cmdepthinshortcorePI-SC-2-40(Figure4b).C/NratiovariedthroughoutPI-SC-1-10mfrom13.0(1cmdeep)to48.2(33cmdeep).PI-SC-2-40mhadanarrowerrange of C/N ratio, which fluctuated from 12.0 to 20.3. The highest C/N ratios of this core were identified at 23 and 39 cm(sectionsIIIandV,Figure4b).
Major and trace elementsPI-SC-1-10m. Iron (Fe) concentration varied from
0.50 to 0.58 % (Figure 5a). Maximum values were found
Pérez et al.496
lowerlimitofdetection(LLD).PI-SC-2-40m. SectionsIandIIIinFigure5bdisplay
thehighestvaluesofFethroughoutthecore.Concentrationsvaried from 1.75 to 2.3 %. Br varied from 16.8 to 36.8 ppm and the profile displayed an increase from a depth of 10 cm to the top. The Sr profile exhibited high concentrations in sectionsIIandIII.Inbothcores,Srwastheelementthatpresented the highest concentrations, and in this core itranged from262 to368ppm.Thehighestconcentrationof Cu (54.6 ppm) was found at 37 cm, and the lowest(26.5ppm)valueat27cm(Figure5b).ZnandRbhavecomparable profiles, and their concentrations show a slight increaseatabout1880AD.Ingeneral,concentrationswerehighin theupper10cm,exceptfora lowZnvalueat3cm.Znvariedfrom36.4to63.6ppm,andRbfrom22.2to30.4 ppm. The Y profile fluctuated from 11.7 to 15.8 ppm throughouttheshortcore.Zrvariedfrom113to138ppm,andPbfrom15.2 to22.8ppm.Bothelementsdisplayedlowestconcentrationsat1cm.ThemaximumconcentrationofZrwasreachedat37cmandthemaximumconcentrationofPbat17cm.
atadepthof23to25cm(sectionIII).SectionsIIandIVdisplaythelowestvalues.Strontium(Sr)rangesfrom554to670ppm.Srconcentrationsstartedtodecreaseat11cm,i.e.,sincetheearly20thcentury.Therehasbeenanincreaseinbromine(Br)sincethe1950s,from10.5to27.5ppm.Thehighestconcentrationsweredetectedintheuppermost5 cm, since the mid 1970s. The copper (Cu) profile had highest concentrations at 1 cm (22.8 %, section I, Figure 5a),andfrom11 cm to 29 cm (max. conc. 24.2 %, sections II-IV). The zinc (Zn) profile displayed substantial variation, ranging from 1.4 to 20.1 ppm. Peaks in Zn concentration were identified at1cm,at7cm(sectionI),at19cm(sectionII),at27cm(sectionIII),andat33cmdepth(sectionIV).Rubidium(Rb) ranges from 7.8 to 12.0 ppm. The Rb profile displays aslightincreaseforthelast~190years,i.e.,theuppermost17cm.ThehighestconcentrationofRbwaspresentat1cm(12.0ppm).Yttrium(Y)rangedfrom0.1to1.7ppm.Themaximumvalueoccurredatasedimentdepthof25cm(1.7 ppm). Another peak was identified at 35 cm (1.0 ppm). Zirconium(Zr)andlead(Pb)concentrationswerebelowthe
Figure3.Organicmatterandcarbonatecontentinshortcoresa)PI-SC-1-10mandb)PI-SC-2-40m.Boldblacksquaresshowsectionsofdarkersedimentsgenerallydepositedsincethe1970s.
Post-Columbian environmental history of Lago Petén Itzá, Guatemala 497
I
II
b)
PI-SC-1-10m PI-SC-2-40m
I
II
III
IV
V
III
IV
Age [Years AD]
2005
1975
1930
1845
2005
Age [Years AD]
V
1900
1780
1480
1980
1954
1945
1885
1955
1680
12 13 14 15 16 17C (%)
40
35
30
25
20
15
10
5
0
Dep
th (c
m)
0 0.2 0.4 0.6 0.8 1 1.2N [%]
10 20 30 40 50C/N (mass)
40
35
30
25
20
15
10
5
09 9.5 10 10.5 11
40
35
30
25
20
15
10
5
0
0.4 0.5 0.6 0.7 0.8N [%]
12 14 16 18 20 22
40
35
30
25
20
15
10
5
0a)
Dep
th (c
m)
C (%) C/N (mass)
Bioindicators: Ostracode and chironomid species assemblages
OstracodesSevenostracodespecieswere identified incore
PI-SC-1-10m(Figure6a),andsixincorePI-SC-2-40m(Table2,Figure6b).TheostracodefaunaconsistsofsomespeciesendemictothePeténLakeDistrictandtheYucatánPeninsulaandcloselyresembles themodernspeciesassemblagesfromLagoPeténItzá.Physocypria globula isanektonicspecies,Cypridopsis okeechobei,andStrandesia intrepidaarenektobenthicspeciesandCytheridella ilosvayi,Darwinula stevensoni,Limnocythere opesta,andPseudocandona sp.arebenthicspecies(Pérezet al.,2010a,inpress).Deep-water-corePI-SC-2-40mwasnotasostracode-richasshallow-water-corePI-SC-1-10m,wherespeciesabundancesanddiversitieswerehigher.
PI-SC-1-10m. ThedominantspeciesofthecorewasC. okeechobei, with a relative abundance of 23 to 54 %, and L. opesta (11–39 %). Cytheridella ilosvayi (7-37%), P. globula (2–13 %), D. stevensoni (1–8 %) and Pseudocandona sp.(1–15 %) were also abundant. Strandesia intrepidawastheleast abundant species (<1%), and was found only in the uppercentimetersofthecore.Diversitywashigher(H=1.62)intheshallow-watercorethaninthedeep-watercore,PI-SC-2-40m (H=0.22).Ostracodediversity andabundance
fluctuated throughout the core. Sections I and III (Figure 6a)displaypeaksofhighostracodediversity.Thehighestvalueswerereachedat1and5cm,i.e.,duringthelast~30years. The profile shows an increase in diversity in the upper 10 cm. A period with low abundances was identified from 13to19cm,i.e.,~1780–1880AD.Thelowestquantityofvalveswasreportedat3cm(814valvesg-1,sectionI,Figure6a)and31cm(298valvesg-1,sectionIII,Figure6a).
PI-SC-2-40m. PI-SC-2-40mwasdominatedprimarilybyP. globula,whichwas theonly speciespresent inallsedimentsamples(sectionsI-III,Figure6b).Thisspecieshas a relative abundance from 95 to 100 % throughout the core.P. globulaprefersawaterdepthof<40m(Pérezet al.,2010b).Abundanceofthisspecieshasincreasedsincetheearly1990s(9cmdepth),exceptinthelate1990swhenabundancedecreasedto228valvesg-1.VeryhighabundanceofP. globulaisaccompaniedbylowabundanceoftheotherostracodespecies,forinstanceat3and7cm.Cypridopsis okeechobei,L. opesta,D. stevensoni,andPseudocandona sp. were less abundant. Cytheridella ilosvayi was foundonlyintheuppermostcentimeters(sectionI,Figure6b).Ingeneral,diversityinthecorewaslow.Depthswithhigherdiversity were identified at 5 cm, from 13 to 17 cm, from 21to23cm,at31andat37cm.Thetotalnumberofvalvesvariedfrom79to856valvesg-1.
Figure4.TotalcarbonandnitrogenconcentrationsandorganicC/totalNratioofshortcores(a)PI-SC-1-10mand(b)PI-SC-2-40m.Graybarsindicatethedifferentsectionsusedforinterpretation.LowC/Nratiossuggestgreaterrelativecontributionfromphytoplanktonandthuseutrophication.
Pérez et al.498
I
II
III
IV
I
III
II
0.48 0.52 0.56 0.6Fe (%)
40
35
30
25
20
15
10
5
0
Depth(cm)
8 16 24 32Cu (ppm)
0 6 12 18 24Zn (ppm)
0 10 20 30Br (ppm)
4 8 12 16Rb (ppm)
480 560 640 720Sr (ppm)
0 0.6 1.2 1.8Y (ppm)
40
35
30
25
20
15
10
5
0
1.6 1.8 2 2.2 2.4Fe (%)
50
45
40
35
30
25
20
15
10
5
0
Depth(cm)
20 30 40 50 60Cu (ppm)
30 40 50 60 70Zn (ppm)
16 24 32 40Br (ppm)
20 24 28 32Rb (ppm)
240 300 360 420Sr (ppm)
10 12 14 16Y (ppm)
110 120 130 140Zr (ppm)
12 16 20 24Pb (ppm)
50454035302520151050
2005 AD19751930
1480
a)
1680
1880
2005 AD
19851970
b)
1890
ChironomidsChironomidsinbothshortcoreshadhigherspe-
ciesrichnessthanostracodes(Tables2and3).Table3presentsallchironomidtaxafoundinbothshortcores.Figure7displaysonlythemostdominantspecies.TotalabundancesweremuchlowerinPI-SC-1-10m(totalcounts≤10) than in PI-SC-2-40m (total counts ≤113). Since the minimumreliablenumberofHCpersampleis50,weareonlygivingageneraldescriptionofdiversitycompositionforthePI-SC-1-10m.Fivezones(sectionsI-V,Figure7)weredistinguishedinbothshortcores.Chironomidsweredescribedasmorphotypesbecausethelownumberoftaxonomic characteristics present in the HC is insufficient tomakedeterminationstospecieslevel.Inseveralcases,anumberorletterwasdesignatedalongwiththegenustoidentifydifferentmorphospecies(e.g.Tanytarsus1A,Cladotanytarsussp.2).
PI-SC-1-10mwasdominatedbyAblabesmiya sp.(≤100 %), Stempellinasp.andChironomus sp. (≤80 %), followedbyCryptochironomus sp.,TanytarsusD,orthocla-diinaeindet.,andParakiefferiella sp. (≤40 %, Figure 7a). Tanytarsus 1A was the least abundant species (≤16 %). From thebottomtoca.25cm(sectionV),aperiodofastableabundance was identified. The dominant taxa in section V andIIIwereAblabesmiyasp.,Stempellinasp.,Chironomussp.andCryptochironomussp.Nochironomidswerefoundat23,19,and7cm(sectionsIV,IIIandII).Chironomidsshowedagradualincreaseinabundanceanddiversityfrom
19to9cm(SectionIII),whereabundancedroppedsharply.Highestabundanceofchironomidswasfoundat5cm(13individuals,sectionI,Figure7a),at9cm(7individuals,sectionIII),andat21cm(6individuals,sectionIII).Theuppermost5cmofthecoreshowedaspeciesabundancedecrease.TheabundanceofChironomussp.,andTanytarsusDshowedaslightincreaseintheuppermost5cm,i.e.,sincethe late 1970s. The profile presents other peaks with higher abundancesat9,13,21,27,31,and33cm.
PI-SC-2-40m(Figure7b)possesses37chironomidspecies.Literatureregardingchironomidtaxonomyinthestudyareaisstillscantandthussomespecimenscouldnotbeidentified to species level. Only the most abundant species wereincludedinFigure7b.TheshortcorewasdominantedbyTanytarsus 1A (≤67 %), Cladotanytarsus sp.1andCladotanytarsus sp.2,Stempellinasp., Polypedilumsp.,andCoelotanypus/Clinotanypus. Less dominant species (≤20 %) were TanytarsusD,Labrundinia sp.,Cryptochironomussp., Paratanytarsus sp., Ablabesmiya sp.and, Cladopelma sp.Theremainingspeciesinthecoredisplayedrelativeabundances <10%.
Fromthebottomtoca.31cm(sectionV),theas-semblage was dominated (>20 %) by Tanytarsus1A,Cladotanytarsus sp. 1, Cladotanytarsus sp. 2, andStempellinasp.SectionsIVandIIIweremainlydomi-natedbyTanytarsus1A,andStempellinasp.Highestoverallabundancewasfoundatacoredepthof11cm(113individuals,sectionII,Figure7b).SectionIIshowed
Figure5.Elementconcentrationsinshortcore(a)PI-SC-1-10mand(b)PI-SC-2-40m.Graybarsindicatethedifferentsectionsusedforinterpretation.Lowconcentrations of Fe and high concentrations of Br indicate an increase in eutrophication. High Cu and Zn reflect an increase in anthropogenic impact. Pb isanindicatorofpollution.HigherRbandZn,andlowerSrconcentrationssuggesthighlakelevels.Yisindicativeofdistantaeoliansupply.
Post-Columbian environmental history of Lago Petén Itzá, Guatemala 499
0
a)
b)
I
II
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I
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IIIN NB B B B B
Cypridopsisokeechobei
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Dep
th(c
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Cytheridellailosvayi
Limnocythereopesta
Darwinulastevensoni
Pseudo-candona sp.
Strandesiaintrepida
Physocypriaglobula
Totalvalves g-1
Diversity(H)
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0 4 8 12 0 4 8 12 0 2 4 6 0 2 4 6 0 0.2 0.4 0.6 0 400 800 0 0.1 0.2 0.3
Physocypriaglobula
Cypridopsisokeechobei
Limnocythereopesta
Darwinulastevensoni
Pseudo-candona sp.
Cytheridellailosvayi
Totalvalves g-1
Diversity(H)
NB B B N B B NB
2005 AD1987197519151880
1780
15801520
2005 AD19971990
1880
Figure6.Ostracodespeciesassemblages,totalnumberofvalvespergramofdrysediment,anddiversityinshortcore(a)PI-SC-1-10mand(b)PI-SC-2-40m.N:nektonic,NB:nektobenthic,B:benthic.
adecreaseintheabundanceofStempellinasp,andanincreaseofCladotanytarsussp.1,andParatanytarsussp.Atacoredepthof3cm,nochironomidswerefound(sec-tionI).Theupper2cmweredominatedbyTanytarsus1A,Coelotanypussp./Clinotanypussp.,andCladotanytarsussp.2.Thecoreshowedarelativelystableabundanceofchironomids,exceptforseveraldepthswithlowconcentra-tions,i.e.,23,31,and41cm.PeaksofChironomussp.wereidentified at 7, 15-17 and 33 cm.
Water depth transfer function
Todevelopanostracode-basedtransferfunction,weselectedsurfacesampleswithaminimumof100ostra-codespecimensandusedostracodespeciesdominance,ostracodediversityandtotalabundance,aswellaswaterdepth,organicmatterandCaconcentrationofthesediment,sedimentconsistency,andtheabundanceofgastropodsforCanonicalCorrespondenceAnalysis(CCA)(Mülleret al.,2009;Pérezet al.,2010).BecauseO2concentrationinthewaterhasaveryhighnegativecorrelationwithwaterdepth(Spearman’sr=-0.99)wedecidedtosubstitutewaterdepth
forO2concentrationintheanalysis.Watertemperatureandchemicaldata(conductivity,pH,ionconcentrations)wereexcludedfromanalysisbecauseoftheirverynarrowrangealongthedepthgradient(25.6–27.6°C;529–544µS/cm;pH8.1–8.5;seePerezet al.,2010b).Weassumedtheyarenotimportantindeterminingostracodedistribution.Twoostracodespecieswereexcludedbecausetheyoccurinless
Family CandonidaePseudocandonasp.Physocypria globula
Family CyprididaeCypridopsis okeechobeiStrandesia intrepida
Family DarwinulidaeDarwinula stevensoni
Family LimnocytheridaeCytheridella ilosvayiLimnocythere opesta
Table 2. List of ostracode species identified in both short cores.
Pérez et al.500
thanthreesamples(Cyprettasp.,Heterocypris punctata).TheCCAshowsthatwaterdepth(O2concentration)isthemain factor with a high loading on the first axis, and is significantly linked to organic matter (LOI 550 °C) and the abundanceofgastropods.Thesecondaxisislinkedmainlytosedimentconsistency(Table4).
Threeostracode-basedwater-depthtransferfunctionsweredevelopedusingourtrainingsetofrecentostracodedata:(1)abinomialcorrelation(BC)usingtheprogramEXCEL(cf.Wrozynaet al.,2009andFrenzelet al.,2010),(2)weightedaveraging(WA)withtolerancedownweighting
usingC2(cf.Juggins2003),and(3)amodernanalogue(MA)techniqueusingweightedthree-pointcorrelationofsamples.BecausetheperformanceoftheBCtransferfunctionwasthebestandonlyreliableone(Table5),weusedthismodelforlaterreconstruction.Below,wepresentthebinomialequationsthatwereusedtorelatewaterdepthtorelativeabundanceforalltaxathatdisplayedhighcorrelation(r)with water depth and contributed significantly to the transfer function(x = water depth in m). The correlation coefficient (r)ofthemodelwasvalidatedby“leaveoneout”crossvalidation(bootstrapping)(Figure8).Physocypria globula (%) = -0.002 x2+0.696x+5.517;
r =0.397Cypridopsis okeechobei (%) = 0.006 x2-0.964x+42.94;
r =0.276Cytheridella ilosvayi (%) = -0.003 x2+0.353x+12.55;
r =0.183Strandesia intrepida (%) = -0.039 x+1.731;
r =0.172
Water depth reconstruction
WeappliedthetransferfunctiontocorePI-SC-1-10m(Figure9).WedidnotapplyittocorePI-SC-2-40mbe-causea“non-modernanaloguesituation”provideduswithunrealisticwaterdepthresults.Thisanomalousresultwasreproducedbyallthreemodels.Thereconstructedwaterdepthduringthelast500yearsisshowninFigure9.Higherlake levels than today were identified in the late 1580s and 1640s.Alowerstagecharacterizedthetimefromaboutthemid-1700stotheearly20thcentury.Onceagain,higherlakelevelsareindicatedfortheperiodfromthelate1930stotheearly1990s,withtheexceptionoftheearly1970swhenlakelevelwaslow(MARN-AMPI,2008).
DISCUSSION
Recent eutrophication (1930s-2005 AD)
GeochemicaldataandbioindicatoranalysesofshortcoresPI-SC-1-10mandPI-SC-2-40mrevealedthatmodernculturaleutrophicationinLagoPeténItzástartedinthe20thcentury. Our results confirm the nutrient enrichment in the lakesince~1930AD,reportedbyRosenmeieret al.(2004).Thepreviousstudyusedgeochemicaldataanddiatomsinasedimentcoreasindicatorsforeutrophication,andcomple-mentsourstudybasedonostracodes,chironomids,andothergeochemicalvariables.
Loss-on-ignitionvaluesinbothshortcoresrevealedaslowincreaseinorganicmatterconcentrationandadecreaseincarbonatecontentsincethelate1930s,whensedimentsbecameslightlydarker.Totalcarbonandnitrogenhavealsoincreased since the late 1930s, confirming LOI results. High inputoforganicmatterduringthelastcenturymayexplain
Subfamily ChironominaeTribe Tanytarsini
Tanytarsus1ATanytarsus1CTanytarsusBTanytarsusCTanytarsusDTanytarsiniind.Micropsectrasp.Cladotanytarsussp.1Cladotanytarsussp.2Paratanytarsussp.Lauternboniellasp.
Tribe ChironominiChironomussp.Goeldochironomussp.Einfeldiasp.Endotribelossp.Apedilumsp.Cryptochironomussp.Paracladopelmasp.Cladopelmasp.Nilothauma/PegastiellaDicrotendipessp.Stempellinasp.Polypedilumsp.Pseudochironomussp.Chironominiindet.
Subfamily OrthocladiinaeParakiefferiellasp.Georthocladiussp.Corynoneurasp.Cricotopussp.Orthocladiinaeindet.
Subfamily TanypodinaeLabrundiniasp.Macropelopiasp.Procladiussp.Ablabesmiyasp.Alotanypussp.Coelotanypus/ClinotanypusFittkauimyasp.
Table 3. List of chironomid species identified in both short cores.
Post-Columbian environmental history of Lago Petén Itzá, Guatemala 501
a)
Tanyt
arsus
1A
Parakie
fferie
llasp.
Tanytars
usD
Cryptoc
hirono
mussp.
Chirono
mussp.
Stempel
linasp.
Orthocl
adiina
eindet.
Totalco
unts 4035302520151050
Ablabes
miyasp.
4035302520151050
Depth(cm)
I II
III
IV V
020
600
4080
040
800
2040
020
400
2040
020
400
816
04
8
2005
AD
1975
1955
1930
1780
1750
1715
1520
b)
Total co
unts
403020100
Cladota
nytars
ussp.
1
Tanytars
usD
Tanyta
rsusC
Tanytars
iniind
.
Tanytar
susB
Tanyta
rsus1C
Tanyta
rsus1A
403020100
Depth(cm)
Cladota
nytars
ussp.
2
Paratan
ytarsu
ssp.
Chirono
mussp.
Einfeldia
sp.
Cryptoc
hiron
omus
sp.
Paracla
dopel
masp.
Cladope
lmasp.
Stempel
linasp. Poly
pedilum
sp.
Pseudo
chiron
omus
sp.
Labrund
iniasp.
Proclad
iussp.
I
IV VIIIII
Coelot
anyp
ussp
./
Clinota
nypu
ssp.
Ablabe
smiya
sp.
040
80
020
40
030
030
030
030
010
20
010
20
010
20
010
20
010
20
010
20
05
10
05
10
03
6
03
602
4
03
602
4 01
230
12
3 060
120
2005
AD
2000
1980
1950
1910
1880
810
26
100
Figu
re7
.Chi
rono
mid
spec
iesa
ssem
blag
esa
ndto
talc
ount
sin
shor
tcor
e(a
)PI-
SC-1
-10m
and
(b)P
I-SC
-2-4
0m.
Pérez et al.502
therelativedecreaseincarbonatecontent.C/Nratioswereusedtoidentifytheoriginoftheorganicmatter(terrestrialversusaquatic)inthelakesediments.LowestC/Nratiosweremeasuredintheuppermostcentimetersoftheshortcores,andsuggestagreaterrelativecontributionfromphy-toplankton.Ratios<10aretypicalofalgae(Meyers,1994;KaushalandBinford,1999;Vaalgamaa,2007),whereashighervaluesindicatesomecontributionfrommacrophytesorterrestrialplants.Theshallow-watercore(PI-SC-1-10m)displayedhigherC/Nratiosthanthedeep-watercore(PI-SC-2-40m).Thisiscausedbyahighercontributionfromterrestrialorganicmattertothesesedimentsclosertotheshoreline. Terrestrial flora is high in carbon-rich support tissueslikecelluloseandlignin,whichleadstorelativelyhigherC/N.PI-SC-2-40mhadahighermeansedimentationrate(4.3mmyr-1)thanPI-SC-1-10m(0.9mmyr-1),probablyindicatingfocusingofsedimenttothedeeper-watersite,nearthebaseofthemetalimnion.ThehighersedimentationrateatthePI-SC-2-40mcoresitemayalsobeaconsequenceofproximitytolocalvillagesandsomesedimentloadfromtheRíoIxlú.
Othergeochemicaldata(Fe,Br,Cu,andZnvalues)alsopointtoaperiodofanthropogenicimpactinthe20thcentury.Ironconcentrationwasgenerallylowinbothshort
cores (≤ 2.3 %). PI-SC-2-40m yielded lower concentrations sincethelate1980s.Ironisassociatedwiththeinorganicfractionofthesediment(Brenneret al.,1990),whichwaslikelydilutedasprimaryproductivityandorganicsedimen-tationaccelerated.
Furthermore,organicmattercanalter theredoxenvironmentfromoxidizingtoreducing,atwhichpointreducedionicspeciesofFearereleasedintosolution.The presence or high abundance of the chironomidChironomus sp.inshortcoreswasaccompaniedbylowerironconcentrations,suggestinganoxicenvironments.Chironomuspossesshaemoglobin,whichallowsthemtosurviveunderhypoxicconditions(Walker,1995).Thisshowsthatchironomidscanbeusedas indicatorsforchangesinredoxconditions.SectionsofshortcorePI-SC-1-10malsodisplayedlowerabundancesofchironomids,whichwebelievearerelatedtoallochthonousinputsofsediment.Bromineisgenerallyboundtoorganicmatter,andresultsofthisstudysuggestitcanbeusedasanindicatorofeutrophication.OtherstudieshaveshownthatBrcanbeusedasanindicatorofdiatomproduction(Kerfootet al.,1999).Copperandzincaretwoofthebestindicatorsforpollutionandurbandevelopment.Thedeep-watercore,whichwascollectedclosertovillages,hadthehighestvaluesofCuandZn.Zninaquaticenvironmentscomesmainlyfrom industrial effluents, municipal wastewaters, emission fromsmeltingoperationsandfossil fuelcombustion(Vaalgamaa,2007).Highestzincconcentrationsweremeasured in theuppermostcentimetersofbothshortcores, indicating an increase in anthropogenic influence in recentyears.Copperalsoshowsanincreasesincethelate1930sinshortcorePI-SC-1-10m.Copperconcentrationispositivelycorrelatedwithorganicmattercontent,becauselike bromine, Cu shows strong affinity to organic matter. LeadvaluesinshortcorePI-SC-2-40mwerelow(<23ppm).AccordingtotheGeoaccumulationindex(Igeo)(Müller,1979),theseconcentrationscorrespondtoclassordegree“0,”whichisindicativeofalakewithoutPbpollution.SurfacesedimentsfromLagoPeténItzácontain>30ppmPb(unpublisheddata),indicatingmoderatelevelsofpollution.FuelcombustionanddecompositionoforganicmattercouldincreasePbconcentrationinsediments.Monitoringheavymetalconcentrationsisimportantbecausetheyaretoxic,non-degradable and have serious ecological ramifications inaquaticecosystems(JumbeandNandini,2009).
Weusedchironomidstoinferchangesintrophicsta-
Parameter axis 1 axis 2 axis 3eigenvalue 0.028 0.021 0.014
Ostracoda valves/mL -0.082 -0.124 -0.049Cypridopsis okeechobei 6.672 5.850 3.477Cytheridella ilosvayi -4.999 5.732 3.743Darwinula stevensoni 5.733 10.967 -18.673Pseudocandonasp. 1.376 9.281 -8.724Limnocythere opesta -4.927 8.636 7.740Physocypria globula 7.599 -0.097 3.045Strandesia intrepida 2.583 14.340 -18.123diversity 2.874 6.717 -1.578
Samples PI-5mN 0.022 0.349 -0.297PI-5mS 0.161 0.015 0.046PI-15mN -0.102 -0.078 -0.026PI-15mS 0.055 -0.031 -0.022PI-30mN -0.470 0.854 0.673PI-30mS 0.365 1.055 -0.261PI-50mS -0.017 -0.117 -0.021PI-100mN 0.272 0.165 0.189PI-100mS 0.224 0.198 0.155PI-160m 1.334 0.317 0.578
Environ-mental variable
waterdepth 0.756 -0.003 0.569organicmatter(LOI550°C) 0.683 -0.073 0.150Ca -0.675 0.132 -0.129consistency 0.155 -0.655 0.006gastropods -0.598 0.292 0.347
Table4.CanonicalCorrespondenceAnalysisforostracodeandenviron-mental data. The first axis is best explained by water depth (highlighted). Thesecondaxisisnegativelycorrelatedtosedimentconsistency(high-lighted).Wecannotexplainthethirdaxis.
Transfer function
Standard error (m)
Correlation coefficient
Binomialcorrelation 14.9 0.895Weightedaveraging 29.8 0.279Modernanaloguetechnique 37.0 0.277
Table5.Performanceofwater-depthtransferfunctionsforLagoPeténItzá.
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20
40
60
80
100
120
140
160
180
-50
-40
-30
-20
-10
0
10
20b)In
ferre
d w
ater
dept
h (m
)
Resid
uals
(m)
0
a)
Measured water depth (m) Measured water depth (m)0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180
y = 0.8639x + 14.817r = 0.8952
tusbecausespeciesexhibitawiderangeofenvironmentaloptimaandtolerances.InLagoPeténItzá,oppositetowhatweexpected,thedeep-watershortcorehadhigherdiversityandabundanceofchironomidsthantheshallow-watercore,whichdisplayedlowerconcentrationsoforganiccarbon.Duetothelowabundanceofchironomidsinshallow-watercorePI-SC-2-10m,thechironomidinterpretationisbasedonPI-SC-2-40m.ThechironomidrecordofLagoPeténItzáshowsagradualincreaseofabundancefromthebaseofthecoreupto~10cmdepth(ca.1980AD.),whenabundancestartstodecreasegradually.Atca.30–31cm,i.e.,thelate1910s,apronounceddecreaseinchironomidabundanceanddiversityisrecorded.Atthattime,onlyafewtaxawerepresent(Polypedilum, Tanytarsus, Stempellina y Cladotanytarsus).Thisreductioninnumbersoftaxaandabundancecouldhavebeenrelatedtowaterlevelchanges.Althoughtheabundanceoscillatesalongthewholesedimentsequence,from20cm(early1960s)to5cm(late1990s),thepresenceofChironomus, Procladius, Einfeldia reflects moreproductiveconditions(BrodersenandQuinlan2006),i.e.,highertrophicstatusinthelake.Theoveralldecreaseofchironomidabundanceintheuppermost10cmcouldberelatedeithertohighlevelsofeutrophicationthatimpactthefaunanegatively,ortorapidinputofbulksedimentduetoerosionorwaterlevelchange.
Incontrast,ostracodeabundanceanddiversityshowanincreaseduringthelast~20years.Ostracodesseemtobecorrelatedwithhigherorganicmatterandnutrientavail-abilityinthelake.However,itisimportanttoknowtheupperandlowerlimitsoftheirtolerance(Külköylüoglu,2004).Therearefewstudies(Scharf,1993;Mezquitaet al.,1999) about the influence of eutrophication on ostracodes, andthusabouttheirpotentialaseutrophicationindicators.Theutilityofostracodesasindicatorsofeutrophicationmaybeconfoundedbyseveralfactors.Forinstance,highorganicmattercontentinsedimentscouldleadtolowpH
andpoorpreservationofostracodesoftandespeciallyhardparts.Darwinula stevensoni,Physocypria globulaandPseudocandonasp.,however,seemtotolerateeutrophi-cationandcouldbeindicativeofachangeinthetrophicconditionofLagoPetenItzá.
EutrophicationinLagoPeténItzáwasprobablytrig-geredbyrapidhumanimmigrationintoPeténthatbegan~1960.Oneconsequencewasarapidincreaseinagriculture,fisheries, tourism and deforestation. Before 1960, >85 % of Petén was covered with tropical forest, but by 1990, 40 % ofPeténwascompletelydeforested(PainterandDurham,1995).Humanpopulationgrowth,urbanization,deforesta-tion,changinglanduse,andotherhumanactivitiesthatincreasenutrientinputratestothelake,arealteringthetrophicstatusofLagoPeténItzá,whichnowisconsideredoligo-mesotrophic. This is reflected in the declining water clarityofthelakeandgreateralgaeabundanceinthelit-toralzone.Thehumanpopulationcontinuestoincreaseandeutrophicationisemergingasacriticalissue.OtherlakesinGuatemalahavealsoexperiencedprofoundanthropogenicimpact.LakeAmatitlánhasreceivedmassiveamountsofsewageandindustrialwastefromGuatemalaCityandistodayhypereutrophic.OncepristineLakeAtitlánrecentlyexperiencedlargecyanobacteriablooms.Bothlakesareimportanttouristdestinations,anddeclinesinwaterqualitythreatentonegativelyimpactthelocaleconomies.
Lake level history of the last 500 years
High lake levels (1550-1730 and 1940-2005 AD)Theostracode-basedlakelevelreconstructionrevealed
two periods characterized by fluctuating, high lake levels (maximumrelativelakelevelof5m):(1)about1550-1730sand(2)about1940-2005AD,withtheexceptionoftheearly1970swhenannualmeanprecipitation(~1260mmyr-1)and
Figure8.a:Correlationbetweenmeasuredandinferredwaterdepths;b:residualsofourostracode-basedwater-depthtransferfunction(BC).
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1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
Relative lake level (m)
Yea
rsA
D
wetdry
0 5 10 15 20
Little Ice Age
lakelevelwerelow(MARN-AMPI,2008).Ourreconstruc-tiondoesnotshowthislakelevellowering.Forostracodeanalysisinshortcores,weused2-cmwetsedimentslices,whichdoesnotprovidesufficientlyhighresolutiontocaptureshort-termevents(~2years),andratherindicatesgeneral,longer-termchanges.Becausethestandarderrorisaboutequaltotheamplitudeofrelativelakelevelchangesindicatedbyourreconstruction,wecannotestimateexactlythehistoricalrelativelakelevels.Highlakelevelperiods,however,areclear.Theseinferencesaresupportedbygeo-chemicalanalysesandchironomidspeciesassemblages.Furtherevidenceisprovidedbymaximumrelativelakelevelsof+5mdocumentedfor1938(MARN-AMPI,2008).KaushalandBinford(1999)reportedthatC/Ninsedimentsincreasesafterdeforestationandlandclearance.Weidenti-fied high C/N ratios, approaching 50, in both cores during theperiods~1780-1840,~1940s,andintheearly1980s.ThesehighC/Nvaluesareprobablyrelatedtoheavyrainsthat caused flooding in the area and transported terrestrial organicmatterintothelake.Nevertheless,lakelevelin1938wasclearlymuchhigher.
RatiosofelementsSr,Rb,andZrmayalsoprovideinformationaboutmaterialinputintothelakeduringpe-riodsofhighprecipitation,andthushighlakelevels.Thestrontium profile is negatively related to the profiles for rubidiumandzirconium.RbandZrareelementsoriginatingmainlyfromsilicates.Silicateweatheringinthecatchment,enhancedbyprecipitation,couldaccountforhighercon-centrationsofRbandZr,andthuslowerSrinsediments.Aslightincreaseinashcontentintheuppermostcentimetersof the short cores confirms this, and is correlated with a periodofhighprecipitation.Similarconditionsprevailedaround1938.AccordingtotheInstitutodeSismología,Vulcanología,MeteorologíaeHidrología(INSIVUMEH),1938wastheyearwiththehighestannualaverageprecipita-tionduringthe20thcentury,andlakelevelrose~5m.ShortcoresdisplayedpeaksinconcentrationofRbandZratthattime,whileSrdecreased.Friedmanet al.(2000)reportsimilarresultsinTurkey,whereRbwasusedasaproxyforwetenvironments.
ScientistsandlocalinhabitantsinPeténobservedthatlevelsofLagoPeténItzároseduringthelastfewdecades.AccordingtoRice(1997),therisebeganafter1976andwasrelatedtoincreasedannualrainfall.Unfortunately,thedramaticriseofthelate1970stoearly1990sisnotfullydocumentedbymeteorologicaldata.WaterlevelvariationsinLagoPeténItzá,however,appeartohaveshiftedincon-certwithstagechangesinotherPeténwaterbodiessuchasSacpuy,SanDiego,Perdida,NaranjoandLasCuaches,stronglysuggestingaregionalclimaticcauseforlake-levelvariations.Deeveyet al.(1980)andINSIVUMEH(1985)studiedtheeffectthatprecipitationhasonlakelevelchangesinLagoPeténItzáandotheraquaticenvironmentsinthePetén Lake District. Their findings indicate that water level ofLagoPeténItzárisesabouttwomonthsafteraperiodofhighprecipitation.Regionalsubsoilsaresaturated,ac-
countingfortherelationshipbetweenlakelevelsandpre-cipitation.Whenheavyrainsoccurinthearea,thereisnocapacitytoabsorbincomingwaterthatentersthesystem,leading to rapid runoff and flooding.
Stable lake levels (1750s-1900s AD)Ourlakelevelreconstructionshowsthattheperiod
fromthelate1750stotheearly1900swascharacterizedbylakelevelssimilartothoseoftoday,probablycausedbyarelativelydryandcoolclimate.RbandFeconcentrationswereslightlylowerandstrontiumvalueshigher,suggest-inglessprecipitationthanduringtheperiodswithhighlakelevels.Ironisanindicatoroferosionofland-derivedmaterials(Hauget al.,2001)andlowvaluesindicatedrierclimate.C/Nratiosgraduallydecreasedfrom~45to35indicatinglesstransportofterrestrialorganicmattertothelakeduringthisperiod.Interestingly,ostracodeabundanceanddiversitydisplayedasharpdecreaseinthisperiod,perhapstriggeredbylowertemperatures.Forinstance,theostracodesC. ilosvayi,andC. okeechobei,whichpreferwarmerwaters(Pérezet al.,2010b),displayedadecreaseintheirabundancesduringthisperiod.
Figure9.Ostracode-basedwater-depthreconstructionforshortcorePI-SC-1-10m.DashedsquareshowstheperiodoftheLittleIceAge.Graylineindicatesinferredrelativelakelevelsandtheblacklineshowsafive-point smoothed line.
Post-Columbian environmental history of Lago Petén Itzá, Guatemala 505
The late Little Ice Age (~1500-1850 AD)ShortcorePI-SC-1-10mcontainssedimentsthatwere
depositedduringthelateLittleIceAge(LIA;1500-1850AD).AdditionalsedimentcoresfromLagoPeténItzá(A.Müller,personalcommunication)andotherstudiesfromnearbyregions(Hauget al.,2001;Hodellet al.,2005)suggestthatclimateinthenorthernlowlandneotropicsofCentralAmericaduringtheLIAwasdry,probablycausedbyamoresoutherlypositionoftheIntertropicalConvergenceZone(ITCZ).OurstudysuggeststhattheendoftheLIA(1750s-1850)wasdry,andlakelevelwasconsistentlylow.Ostracodediversitywasslightlyhigherduringthisperiodduetothefactthatlittoralzonesgenerallypresenthigherdiversityandabundanceofostracodes.Incontrast,theearly1580sandaround1650wererelativelywetandthelakelevelwashigh.δ18OvaluesfromostracodevalvesinsedimentcoresfromnearbyLakeSalpetén(Rosenmeieret al.,2002)showaslightdecreaseduringtheseyears,thusindicatinghighlakelevelandmoistconditions.AstudyinLagoVerde,alongthecoastoftheGulfofMexico,Mexico(Lozano-Garcíaet al.,2007)concludedthatlakelevelroseduringthe17thcentury.Theauthorssuggestedthatlakelevelfluctuations in Lago Verde were a response to solar forcing. Bothlakes,LagoVerdeandPeténItzá,lieinrelativelymoistareas,withannualmeanprecipitationof2500mmyr-1and1665mmyr-1,respectively.Thewetperiodduringtheearly1580sand~1650seemtohavebeenrelatedtoanincreaseinwinterprecipitation,butreducedsummermoisturesupply.DrierareasinthenorthernYucatánPeninsuladisplayedverydry conditions during these periods, as reflected in enriched δ18OvaluesinostracodevalvesandgastropodsextractedfromsedimentsofAguadaX’caamalandPuntaLaguna(Hodellet al.,2005;2007).
Yttrium and iron profiles of short core PI-SC-1-10m displayedtheirhighestvaluesat25cm(~1680AD)whichcoincidedwithperiodsofhigherprecipitationandlakelevels.Yttriumconcentrationsindicateadistantaeoliansupplyandcanbeusedasaproxyforwind-borneinputs(Minyuket al.,2007).
CONCLUSIONS
SedimentsfromLagoPeténItzá,Guatemalacontaingeochemicalandbiologicalevidenceforrecenteutrophi-cationandlakelevelchangesoverthelast~500years.Anincrease in eutrophication since ~1930s was identified in shallow-anddeep-watersedimentcores.Eutrophicationduringthisperiodwascharacterizedby:(1)highercon-centrationsoforganicmatter,andBrvalues,(2)lowerC/Nratios,carbonatecontent,andFevalues,(3)higherostracodeabundanceanddiversity,and(4)presenceofthehigh-pro-ductivitychironomidtaxaChironomus,Procladius,andEinfeldiafrom1960-2000AD.IncreasedCuandZncon-centrations in uppermost deposits reflect a recent increase inanthropogenicimpact.ThelakelevelhistoryofLago
PeténItzáwasreconstructedusingchangesintherelativeabundanceofostracodespecies,andshiftsinRb,Zr,andSrconcentrationsinshortcorePI-SC-1-10m.Higherlakelevels,upto+5m,from~1550tothe1730sandfromthe1940sto2005ADdisplayedhigherRbandZrvalues,lowerSrvalues,andhigherrelativeabundanceofPhysocypria globula.Theperiodfrom~1750totheearly1900swasatimeoflowerandmorestablelakelevelswhenSrconcentra-tionswerehigher.ThelatterpartoftheLittleIceAgewascharacterizedmainlybyadrierandcoolerclimate,andlowlakelevels,withtheexceptionofthelate16thcentury,whichhadhigherlakelevelsthatcoincidedwithhigherstandsatLagoSalpetén,GuatemalaandLagoVerde,México.ThissuggestsanincreaseinwinterprecipitationandamoresoutherlypositionoftheITCZ.Ourmulti-proxysedimentcorestudyofpastlakelevelisinagreementwithhistoricalflood and meteorological data from the 20thcentury.
ACKNOWLEDGMENTS
We are grateful to the Universidad del ValledeGuatemala(UVG)andConsejoNacionaldeÁreasProtegidas(CONAP)forsupportprovidedduringthefieldtrip and to the Asociación de Manejo de la Cuenca PeténItzá(AMPI),andInstitutoNacionaldeSismología,Vulcanología,MetereologíaeHidrología(INSIVUMEH),InstitutoNacionaldeEstadística(INE)thatkindlypro-videduswithvaluabledata.SpecialthankstoMargaritaPalmieri,MargaretDix,RobertoMoreno,andEleonordeTott;RaúlCalderónforhisassistancemakingthemaps.EvgeniaVinogradova,WolfgangRiss,MalteLorenzandJulia Lorenschat for their help during the field campaigns andlaboratorywork.YvonneHermannsandBenjaminGilfedder(TechnischeUniversitätBraunschweig)fortheirassistanceduringgeochemicaldatainterpretation.DavidHodell(UniversityofCambridge),thestaffofthelabora-toryofGeochronologie&IsotopenhydrologieSektion,Leibniz-InstitutfürAngewandteGeophysik(LIAG),twoanonymousreviewersfordetailedcomments,andtheDeutscheForschungsgemeinschaft(DFG),whichkindlyprovidedfunding(GrantSchw671/3).
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Manuscriptreceived:April6,2010Correctedmanuscriptreceived:July29,2010Manuscriptaccepted:August5,2010
