CAVE SEDIMENTS AND PALEOCLIMATE


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CAVE SEDIMENTS AND PALEOCLIMATE

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CAVE SEDIMENTS AND PALEOCLIMATE
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Journal of Cave and Karst Studies
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White, William B.
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Cave Sediments ( local )
Caves ( local )
Paleoclimate ( local )
Clastic Sediments ( local )
Chemical Sediments ( local )
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This paper is a review of cave sediments: their characteristics and their application as paleoclimate archives. Cave sediments can be separated into two broad categories, clastic sediments and chemical sediments. Of these, stream-transported clastic sediments and calcite speleothems are both the most common and also the most useful as climatic records. Techniques for dating cave sediments include radiocarbon and U/Th dating of speleothems and paleomagnetic reversals and cosmogenic isotope dating of clastic sediments. Cosmogenic isotope dating of clastic sediments in caves with multiple levels or which occur at different elevations provide a geomorphic record of cave ages and river system evolution over the past 5 Ma. Isotope profiles, trace element profiles, color banding and luminescence profiles of speleothems, mainly stalagmites, produce a detailed paleoclimate record with very high time resolution over the past several hundred thousand years. There is potential application of these methods to late Holocene climates with implications for evaluation of current concern over global warming.
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Journal of Cave and Karst Studies, Vol. 69, no. 1 (2007-04-01).

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CAVESEDIMENTSANDPALEOCLIMATE W ILLIAM B.W HITE MaterialsResearchInstituteandDepartmentofGeosciences,ThePennsyl vaniaStateUniversity,UniversityPark,PA16802USAwbw2@psu.edu Abstract: Thispaperisareviewofcavesediments:theircharacteristicsandtheir applicationaspaleoclimatearchives.Cavesedimentscanbeseparatedin totwobroad categories,clasticsedimentsandchemicalsediments.Ofthese,streamtransportedclastic sedimentsandcalcitespeleothemsareboththemostcommonandalsothemos tusefulas climaticrecords.Techniquesfordatingcavesedimentsincluderadiocar bonandU/Th datingofspeleothemsandpaleomagneticreversalsandcosmogenicisotop edatingof clasticsediments.Cosmogenicisotopedatingofclasticsedimentsincav eswithmultiple levelsorwhichoccuratdifferentelevationsprovideageomorphicrecord ofcaveages andriversystemevolutionoverthepast5Ma.Isotopeprofiles,traceelem entprofiles, colorbandingandluminescenceprofilesofspeleothems,mainlystalagmi tes,produce adetailedpaleoclimaterecordwithveryhightimeresolutionoverthepas tseveral hundredthousandyears.Thereispotentialapplicationofthesemethodst olateHolocene climateswithimplicationsforevaluationofcurrentconcernoverglobal warming. I NTRODUCTION Cavesareopencavitiesintheearth.Assuchtheyare naturalsedimenttraps.Nocaverneedstoberemindedthat cavesaremuddy.Closerinspection,however,showsthat cavedepositsareremarkablycomplicatedasarethe transportmechanismsthatcarrythesedimentsintothe caves.Cavesedimentshavebeenrecognizedforaslongas therehasbeenscientificinterestincaves.However,onlyin thepastseveraldecadeshasitbeenrecognizedthatcave sedimentscontainbothhydrogeologicalandpaleoclimatologicalrecords.Inpart,thislaterecognitionisduetothe recentdevelopmentoftechniquesforassigningdatesto cavesediments.Withanaccuratechronology,thecave archivescanbecorrelatedwitheventsonthelandsurface above.Theinvestigationofcavesedimentshasmovedfrom anobscurecornerofkarstsciencetooneoftoday’shottest topics(e.g.,SasowskyandMylroie,2004).Thereisthevery realpossibilitythatcavesedimentswilltaketheirplace alongsideofoceansedimentcoresandcontinentalice cores(GreenlandandAntarctica)asthemostimportant recordsofhowEarth’sclimatehasevolvedoverthepast severalmillionyears. Theobjectiveofthepresentpaperistosummarizesome ofthecurrentstateofknowledgeofcavesediments.Aswill beseen,thesubjecthasgrownmuchbroaderthanitwasin 1966ontheoccasionofthe25 th anniversaryvolume.The literaturehasbecomeverylarge.Whatfollowsare examplestogivesomefeelforwhathasbeenaccomplished, particularlyinthepast10–20years.Itisnotacomprehensivereview. S OME H ISTORICAL B ACKGROUND Therehasalwaysbeenadistinctionbetweenspeleothemsandclasticsedimentsincaves.Speleothemsare aestheticallypleasingandtheirbizarreshapesgivecaves muchoftheircharm.Ittakesaspecialpointofviewtosee thesamescientificvalueinamudbankasinaclusterof stalagmites. Thefirstdescriptionsofspeleothemsarelostinthe mistsofantiquity.Shaw’s(Shaw,1992)monumental treatiseoncavesciencepriorto1900devotes13chapters tospeleothemsincludingaccountsofearlyandsomewhat fancifulattemptstoexplainthem.Manydescriptionsofthe variousformsofspeleothemshaveappeared,andoneof thebestandmostdetaileddescriptionsoftheircrystal structureswaswrittennearlyacenturyago(Prinz,1908). Thecorrectchemicalreactionforthedepositionofcalcite incaveswasdescribedasearlyas1812byCuvierandin 1820byBenjaminSillimantheelder(Shaw,1992).Inanow classicpaper,Hollandetal.(1964)setforthadetailed chemicalmodelforcalcitedepositionthatremainsthe acceptedexplanationtothepresenttime. Clasticsedimentsarerarelymentionedpriorto1900 (Shaw,1992).Thetwomaincategories,breakdownand streamdeposits,areonlybrieflymentionedinthetwomost importantearly19 th Centurytextbooks(Kyrle,1923; Trombe,1952).Clasticsedimentsdidplayacentralrole intheBretz(1942)modelforcavedevelopment.According toBretz,cavesformeddeepbelowthewatertableandthen filledwithredunctuousclaywhichfiltereddownfrom overlayingsoils.Laterdissectionofpeneplainsand drainingofthecavesallowedthesedimentstowashout, leavingbehindtheopencavepassagesweseetoday.Many ofBretz’sfieldobservationswereinMissouricaveswhere thestickymudsareparticularlycommon.Inresponse, Reams(1968)devotedanentirePh.D.dissertationto demonstratingthatmanyoftheMissouricavesediments are,infact,riversedimentscarriedinthroughsinkholes andbysinkingstreams.Incontrast,Davies(1960)used thesandandgravelsedimentsinAppalachiancavesto WilliamB.White–Cavesedimentsandpaleoclimate. JournalofCaveandKarstStudies, v.69,no.1,p.76–93. 76 N JournalofCaveandKarstStudies, April2007

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demonstratethatthecaveswereformedbyfast-moving waterclosetolocalbaselevelsandnotbyslowpercolation deepbelowthewatertable. Cavessedimentscameintotheirownasasignificantpart ofcavescienceinthe1960swithanimportantsymposium onbothchemicalandclasticsediments(Dell’Oca,1961)and thecomprehensiveresearchofRenault(1967–1969).The paleoclimaticsignificanceofclasticsedimentswasrecognizedinEuropeanalpinecavesbySchmid(1958).Thefirst twoEnglishlanguagetextbooksoncaves(Jennings,1971; Sweeting,1972)hadchaptersdevotedtocavesediments althoughthecoveragewaspredominantlyonchemical sediments.ClasticsedimentresearchintheUnitedStatesgot underwayinthemid-1960swiththeworkofFrank(1965) onthecavesofTexasandlaterworkinAustralia(Frank, 1969,1971).Manyoftheseearlystudiestreatedclastic sedimentsincavesasapeculiarsortofsedimentaryrock withemphasisonin-cavestratigraphyandprovenanceof thesediment.Oneofthemostcomprehensivestudiesof sedimentsourceanddepositionwasanunpublishedPh.D. thesis(Wolfe,1973)describingcavesedimentsinthe GreenbrierkarstofWestVirginia. C LASSIFICATIONOF C AVE S EDIMENTS Thereisnogenerallyacceptedclassificationschemefor cavesediments.Eachofthemorerecenttextbooksoncaves andkarst(Bo ¨gli,1980;White,1988;FordandWilliams, 1989,Gillieson,1996)presentsaclassificationofsediments. Thesehavemanypointsincommonbutalsosignificant differences.TheclassificationinFigure1isacompromise. Itliststhemaincategoriesofsediment,butdoesnot attempttoprovideapigeonholeforeverypossible materialthatmightaccumulateinacave. Sedimentsaredividedintotwobroadcategories:clastic sedimentsandchemicalsediments.Clasticsedimentsare movedmechanicallywhereaschemicalsedimentsare formedinplace,precipitatedfromsolutioninseeping, dripping,orflowingwater.Clasticsedimentscanthenbe subdividedagainintomaterialsthatarederivedlocally withinthecaveandmaterialsthataretransportedintothe cavefromtheoutside.Theseareknownrespectivelyas autochthonoussedimentsandallochthonoussediments. Chemicalsedimentsaresubdividedintocategoriesbased ontheircomposition. Locallyderivedclasticmaterialconsistsofweathering detritus,breakdown,andguano.Weatheringdetritusisthe insolublecomponentofthebedrock,leftbehindwhenthe bedrockdissolved.Weatheringdetritusisonesourceofthe stickyclaysfoundinsomecavesthathavenoevidenceof streamaction.Weatheringdetrituscanincludesand, silicifiedfossilfragments,andchert,thelatterinsome limestonesbeingamajorcomponentoftheclastic sediment.Breakdownconsistsoffragmentsofbroken bedrockinawiderangeofsizes.Theclassificationof breakdownandthegeologicprocessesresponsibleforcave passagecollapseismorecomplexthanexpected(Whiteand White,1969;Jameson,1991).Guanoisthefecalmaterial depositedincavesbybatsandbirds.Incaveswithlargebat populations,guanoispresentinsufficientquantitiestobe classifiedasasediment. Thecompositionofallochthonoussedimentvaries dependingontherocktypesandothermaterialsavailable inthedrainagebasininwhichthecaveisembedded.Cave entrancesareusuallysitesofintensiveweatheringandthe combinationofbreakdown,downslopemovementfrom otherrockunitsabovethecave,soilslumping,and incorporatedplantmaterialcreatesacharacteristicpileof roughlystratifieddebrisknownasentrancetalus.Entrance talusisofimportancebecauseitoftenhousespaleontologicalorarchaeologicaldeposits.Infiltratesaresediments thatmigrateintothecavefromthelandsurfaceabove. Theysubdivideintosoilwashdownwhichismainlysoil fromtheepikarstthatiswashedintothecavethrough solutionally-widenedfractures,andgravitationaldebris whichiscoarser-grainedmaterialthatfallsdownopen shafts.Sinkingstreamscarryagreatvarietyofmaterials intocavesincludingalluvialsediment,glacialtills,volcanic ashandanyotherunconsolidatedmaterialthatmaybe pickedupbythestream.Debrisflowsareessentially avalanchesthatflowunderground.Theseproviderarebut Figure1.Aclassificationofcavesediments. W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 77

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dramaticcavedeposits,forexamplethosefoundinthe cavesoftheNewGuineahighlands(Gillieson,1986). Finally,quantitiesofsandandloesscanbeblowninto cavesdirectlybywindactiontoproduceaeoliandeposits. Mostcommonofthechemicalsedimentsarethe travertines.Thewordtravertineishereusedasablanket labelforfresh-watercarbonatedepositsofalltypes. Travertineisusedasarocknamewhereasspeleothemrefers tothespecificexternalmorphologyofthedeposits.Cave travertinesarechemicallysimilartothecalcareoustufas foundasspringandfreshwaterdepositsthroughoutthe worldandtothegeothermaltravertines(FordandPedley, 1996).Tufasgenerallyareporousandcontainagreatdealof plantmaterialincomparisontobothcaveandgeothermal travertines,whichareusuallydenseandcompact.Tufasalso containpaleoclimaticrecords(Andrews,2006). Thetermevaporiteisusedforcavesedimentsinmuch thesamewaythatitisusedforsurfacesediments; assemblagesofmineralsformedbyevaporatingwater. Mostcommonincavesisgypsum,CaSO 4 2H 2 O,butthere existaselectionofothersulfatemineralsaswellas occasionalhalitedeposits.Theremainingcategoriesof chemicalsedimentslistedinFigure1arevolumetrically ratherminor.Phosphates,mainlyhydroxyapatite,are associatedwitholdanddecomposedguanodeposits.The resistates,ironandmanganeseoxides,areusuallylimited tocoatings.Perennialiceoccursinalpinecavesandsocan reasonablybeconsideredacavesediment. C LASTIC S EDIMENTS Figure1providesasystematiccategorizationofclastic sedimentsintermsofsourcearea.However,theconduit systemactsasamixingchambersothatthesediments observedincavesoremergingfromkarstspringscontain componentsfromallofthesources.Figure2givesthe overallconcept.Thekeyparametersareflowvelocities, flowvolumes,particledensity,andparticlesize.Although theremaybesomecontributionofrelativelylightorganic material,mostclay,silt,sand,cobblesandbouldershave densitiesnotgreatlydifferentfromthatofquartz(2.65g cm 3 ).Densityvariationsarenotaverysignificant variable.Thesizeofclasticparticlesvariesoversixto eightordersofmagnitudefromsub-micrometercolloidal andclayparticlestobouldersapproachingonemeter.The clasticsedimentsremainingincavesandthefluxofclastic sedimentsthroughtheaquiferaredeterminedmainlyby stormflowandbyparticlesize.Clasticparticlesflushed throughthesystemarepartofthehydrology;clastic particlesremainingafterthecavesaredrainedbecomethe sedimentrecord. T RANSPORT M ECHANISMS Sedimenttransportinopenchannelsoccursbytwo mechanisms:suspendedloadandbedload.Thesuspended loadconsistsofparticlestakeninsuspensionbythe turbulenceofstreamflow.Suchparticlestendtosettleout, butareheldinsuspensionbytheturbulence.Theenergy requiredtoholdparticlesinsuspensionandtherateat whichtheysettleoutofsuspensiondependonparticlesize. Ifflowvelocitiesdecrease,forexamplebyanexpansionof thepassagecross-sectionorbypondingofwaterbehindan obstruction,coarsesedimentsdropoutrapidlybutvery fine-grainedsedimentsmayremaininsuspension.Flowing watermovesbedloadbydraggingitalongthebedbecause oftheboundaryshearbetweenthemovingwaterandthe bedmaterial.Itisnotrequiredthattheseflowsbeturbulent althoughtheyusuallyare.Theboundaryshearnecessaryto putbedparticlesinmotionincreasesalmostlinearlywith theparticlesizeandwiththesquareoftheflowvelocity. Thereisawiderangeinparticlesizesforthematerial injectedintotheconduitsystemsfromthesourcessketched inFigure2.Velocitiesvaryalongtheconduitdependingon whetheraparticularsegmentispipe-fullorflowinginan openchannel,dependingonpassagecross-sectionalarea, anddependingonobstaclesorblockageswithinthe conduit.Velocitiesalsovarydependingonwhetherthe systemisatbasefloworstormflow,andifatstormflow, onthemagnitudeofthestorm.Asaresultthereisavery complexmixingandpartitioningofsedimentwithinthe conduitsystem.Smallparticlesinthecolloidalorfineclay sizerangemayremaininsuspensionandprovide acontinuousfluxofsedimentatthespringevenduring baseflowconditions.Clayandsiltsizedparticlesmaysettle intotemporarystorageduringbaseflowbutareresuspendedandcarriedtothespringduringstormflow, thuscausingthespringtobecomemuddy.Verycoarse pebbletocobblesizesedimentsmaybesweptintothe conduitsystemduringlargestormsandremaininthe systeminlong-termstorageuntiltheyaresweptonthrough byanotherexceptionalstorm.Thereisacontinuous interchangeofsedimentdependingonflowconditions. Muchrecentresearchhasbeenconcernedwiththe hydraulicsofsedimenttransportandwiththeappearance ofsuspendedsedimentsatkarstsprings.Becauseofthe openconduits,manyspringsdischargeverysmallparticles Figure2.Sketchshowingclasticsedimentsourcesandthe roleoftheconduitsystemasamixingchamber. C AVESEDIMENTSANDPALEOCLIMATE 78 N JournalofCaveandKarstStudies, April2007

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evenunderbase-flowconditions(Atteiaetal.,1998).The fine-grainedparticlesareofimportancebecausethey provideatransportmechanismforbacteria(Mahleret al.,2000)andheavymetals(VesperandWhite,2003). Turbidormuddyspringwatermakesthespringsunsuitableaswatersuppliesandasaresulttherehavebeenmany studiesofsuspendedsedimenttransporttosprings(Mahler andLynch,1999;Drysdaleetal.,2001;Amraouietal., 2003;Masseietal.,2003;DogwilerandWicks,2004). Therehasbeenrelativelylittlerecentworkonthe mechanismsoftransportofthecoarsefractionthatmakes upmuchofthesedimentseenincaves.Gale(1984)was abletousetheobservedsedimentparticlesizestoback calculatethehydrauliccharacteristicsoftheconduitin whichtheywerefound.Onerecentstudy(Hartand Schurger,2005)estimatedtherateofsedimentinjection throughsinkholestobe111Mgkm 2 yr 1 forawatershed incentralTennessee. L ITHOFACIES Ultimately,ofcourse,thecomposition,mineralogy,and lithologyofcaveclasticsedimentsdependonthematerial availableinthesourceareas.Withinthisconstraint, however,thesedimentarydepositsdependontheinternal dynamicsoftheconduitsystem.Thus,caveclastic sedimentsoccurinfaciesthatreflectthewayinwhich injectedsedimenthasbeenrearrangedanddeposited. Therehavebeenanumberoffaciesschemesproposed dependingonthecriteriausedforidentifyingthefacies (e.g.,Gillieson,1986;SpringerandKite,1997).Thescheme sketchedinFigure3(BoschandWhite,2004)describes faciesaccordingtoparticlesizeandthedegreeofsorting.It willbenotedthattheaxesinFigure3havenoscale. Further,theboundariesbetweenthefaciesdomainsare exceedinglyfuzzy. Thewidestrangeofparticlesizeandparticlesorting occursinthemostcommonstreamdepositsincaves,the channelfacies.Theyoccurasroughlystratifiedlayersof sands,silts,andcobbledepositsmosteasilyobservedwhere morerecentstreamshavecutthroughearliersediments thusexposingthelayeringinthestreambank.Anygiven exposureofchannelfaciesislikelytoshowdistinct bedding,butthisstratigraphyisrarelycontinuousalong thecavepassage.Thethalwegfaciesisaderivativeofthe channelfacies.Thalwegfaciesarethecoarsecobble armoringthatformsthebedsofmanyactivecavestreams. Thecoarsematerialisaresidualdepositformedbystream actionwinnowingoutthefine-grainedmaterialfromthe channelfacies.TheotherthreefaciessketchedinFigure3 eachinvolvedifferenttransportprocesses. Slackwaterfaciesarethethinlayersoffinegrainedsilt andclaythatoftenformthefinaltoplayerofclastic deposits.Thisfaciesisoftenfoundinblindsidepassages, pockets,andothernichesincavesunlikelytobereachedby flowingstreams.Theslackwaterfaciesappeartobe depositedfromsuspendedsedimentthathassettledout offloodedcavepassages.Risingfloodwatersfillall availablespacesandarethenpondedforextendedperiods oftimeuntilthefloodrecedes.Duringthetimewhenthe passageiswater-filled,suspendedsedimentshavetimeto settleoutandformasedimentlayer.Aninvestigationof thisfaciesinAgenAllweddCaveintheUK(Bull,1981) revealeddistinctlaminaewhichwereinterpretedtobethe resultofarhythmicpulsingofsediment-ladenwaterinto subterraneanlakesinresponsetosurfaceclimate.Careful examinationofthismicrostratigraphyrevealedasequence ofclimaticeventsextendingback17,000years(Bull,1980). Diamictonfaciesaretheresultofdebrisflows.These arechaoticdepositscontainingparticlesrangingfromclays tocobblestossedtogetherwithnobeddingandnosorting. Specialcircumstances,mainlyextremestormintensities andhighgradientcavepassages,allowentiresediment depositstobetakenintosuspensionandsweptdownacave passagetolaterbedepositedinanundifferentiatedmass. Diamictonfaciesweredescribedfromthehighgradient cavesinNewGuinea(Gillieson,1986). Insomecaves,particularlynetworkmazes,flow velocitiesneverreachthresholdvaluesforsedimenttransport.Thesedimentsinthesecavestendtobelocally derived,eithertheresidualinsolublematerialfrom dissolutionofthelimestoneorinfiltratingsoilfromthe landsurfaceabove.Theterm‘‘backswampfacies’’was chosenbecause,hydraulically,networkmazesandother slow-flowcavestendtobehaveastheunderground equivalentofswamps. C HEMICAL S EDIMENTS M INERALOGY Caves,wheretemperature,watervaporpartialpressure, carbondioxidepartialpressureandotherenvironmental Figure3.Faciesofclasticsediment.Adaptedroughlyfrom BoschandWhite(2004). W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 79

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parameterseitherremainconstantorcyclethroughfixed patternsoverlongperiodsoftime,areeffectivedeposition sitesforasurprisingrangeofminerals.Hill(1976) described74mineralsmainlyfromcavesintheUnited States.By1997whentheexpanded CaveMineralsofthe World reachedasecondedition(HillandForti,1997),the numberofidentifiedcavemineralshadreached255.Very fewoftheseoccurinsufficientvolumetobeimportantas cavesediments. Ionsthatoccurcommonlyincarbonategroundwaters andwouldbeavailablefortheformationofcaveminerals areCa 2 ,Mg 2 ,Na ,HCO 3 ,andSO 4 2 ,plus,ofcourse, water.Becausecaves,withsomeinterestingexceptions,are oxidizingenvironments,theresultingmineralscanbe describedintermsoftheircomponentoxides.Thesystem wouldbeCaO–MgO–Na 2 O–CO 2 –SO 3 –H 2 O.Because anymaterialobjectcontains100percentstuff,thetotal concentrationsofallcomponentsmustaddupto100 percent.Therefore,thenumberofindependentcompositionvariablesisalwaysonelessthanthenumberof components.Thechemicalsystemofsixoxidesrequires afive-dimensionalspacetoplotthecompositionswhichis difficultonatwo-dimensionalsheetofpaper.Figure4 showsthetopologicalrelations.Ifthecomponentsare selectedfouratatime,thecompositionsofanymineralsin thesesub-systemscanberepresentedinthreedimensions, namelybyplottinganycombinationoffourcomponents ontheapicesofaregulartetrahedron.TheprincipalCaMg-carbonatemineralscanbedisplayedinthisfashion (Fig.5). OfthemineralsplottedonFigure5,onlycalciteandto alesserextentaragonitearesignificantconstituentsofcave sediments.Inspiteofthecommonoccurrenceofcavesin dolomiterocks,dolomiterarelyprecipitatesfromkarst waters.Magnesiumappearseitherasasolidsolutionin calciteorasthefine-grainedhydratedmagnesiumcarbonatemineralsknownasmoonmilk. Thesulfatemineralsrequirethefive-componentsubsystemCaO–MgO–Na 2 O–SO 3 –H 2 Otocoverthe commonminerals.Gypsum,CaSO 4 2H 2 Oisthesecond mostcommoncavemineraland,intermsofsediment volume,isbyfarthemostimportantsulfatemineral. Othersincludeepsomite,MgSO 4 7H 2 Oandmirabilite, Na 2 SO 4 10H 2 O.Botharehighlywater-solubleandare foundonlyinextremelydrycaves.Thestabilityofthese mineralscanbedescribedintermsofthetemperatureand watervaporpartialpressure(White,1997). D EPOSITIONOF C ALCITEAND A RAGONITE Thedepositionofbothcalciteandaragoniteincavesis describedbythedeceptivelysimplechemicalreaction Ca 2 2HCO 3 ? CaCO 3 CO 2 H 2 O 1 Thesamereactionwritteninreversedescribesthe dissolutionoflimestone.Theequilibriumconcentration ofdissolvedcarbonatedependsonthepartialpressureof CO 2 ,thetemperature,reactionsamongthevarious carbonatespecies,andalsoontheionicstrength,other ionsinsolution,andvariouscomplexesthatmayform. Carbonatechemistryisnowunderstoodinconsiderable detailandappearsinavarietyoftextbooks(e.g., Langmuir,1997).TheequilibriumconcentrationofdissolvedCa 2 inasystemopentoCO 2 isgivenby m Ca 2 P 1 3 CO 2 K 1 K C K CO 2 4 K 2 c Ca 2 c 2 HCO 3 1 3 2 Inthisequation, K 1 and K 2 arethefirstandsecond dissociationconstantsforcarbonicacid, K CO 2 isthe Henry’slawconstantforthesolubilityofCO 2 andthe c ’saretheactivitycoefficientsfortheCa 2 andHCO 3 ions.Thesameequationdescribesthesolubilityof aragoniteif K C ,thesolubilityproductconstantforcalcite, isreplacedby K A ,thesolubilityproductconstantfor Figure4.Interconnectionsofthesixoxidecomponentsthat makeupmostcommoncaveminerals. Figure5.Compositionsofthecavecarbonateminerals representedonaregulartetrahedron. C AVESEDIMENTSANDPALEOCLIMATE 80 N JournalofCaveandKarstStudies, April2007

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aragonite.Numericalvaluesfortheconstantsatvarious temperaturesaregivenbyWhite(1988).Calculated solubilitiesforcalciteandaragoniteareplottedinFigure6. ThesolubilitycurvesinFigure6provideamodelfor thechemistryofcarbonatemineraldeposition.Rainwater containsCO 2 attheconcentrationof0.037volumepercent ( logP CO 2 3 43)butessentiallyzerodissolvedcarbonate.Whenrainwaterpercolatesthroughorganic-richsoils, theCO 2 concentrationmayriseashighas10% ( logP CO 2 1).WhentheCO 2 -richwaterreachesthe bedrockitdissolvesthelimestonetakingCa 2 ionsinto solution.Ca 2 -ionconcentrationsmayreachvaluesinthe rangeof400mgL 1 asCaCO 3 asgivenbyequation(2). TheseCO 2 -andCa 2 -richpercolationwatersseepdownwardalongjointsuntiltheyemergefromtheceilingsand wallsofcavepassages.TheCO 2 concentrationincave atmospheresistypicallyabouttentimesthatofthesurface atmosphere( logP CO 2 2 5)sointhisenvironment,the drippingwatersarehighlysupersaturated.CO 2 isdegassed intothecaveatmosphereandCaCO 3 isprecipitated,until (atequilibrium)theconcentrationfallsbacktoabout 100mgL 1 .Becausethecaveenvironmentisusually water-saturated,thesurfacesofthegrowingspeleothems remainwet.Eachsuccessivedropdepositsitssmallloadof CaCO 3 inregisterwiththeionsalreadypresentonthe growingcrystalsurface.Thusindividualcalcitecrystalsin speleothemsinsealedcavestendtobelarge,oftenmuch largerthananythathavebeengrowninthelaboratory. Therateatwhichcalcitedissolveshasbeenthesubject ofahugenumberofinvestigations(forreviewseeMorse andArvidson,2002).Dissolutionratestudiesarepertinent totheinterpretationofcavedevelopment(speleogenesis). Thereverseratesforcalciteoraragoniteprecipitationand thusofspeleothemgrowthatsaturationconditionsfar fromequilibriumtendtofollowthesamelawsascalcite dissolution(withsomesignsreversed)(Reddyetal.,1981). Nearequilibrium,whichistheconditioninmostcave depositionalenvironments,themechanismsaremuchmore complicated.Inorderforcalcitecrystalstogrow,thefeed solutionsmustbesupersaturated.However,ifthesupersaturationistoolarge,newcalcitecrystalswillbenucleated and,insteadofonelargecrystal,thespeleothemwill consistofmanysmallcrystals.CaCO 3 growthrateshave beenstudiedinthelaboratorybycontrollingsupersaturationsandmeasuringgrowthrates(Gutjahretal.,1996). Moreinsightintothedetailsoftheprocessatthe atomicscalehavebeenobtainedbyusingtheatomicforce microscopetoactuallymeasuretherateofgrowthof individuallayerswithinthecrystal(Tengetal.,2000).The agreementisgenerallygoodbetweenlaboratorymeasurements,calculationsofgrowthrate,andgrowthrates directlymeasuredincaves(Bakeretal.,1998).Thetradeoffbetweengrowthratesofindividualcrystalsandthe nucleationandgrowthofmultiplecrystalsresultsin avarietyofmicrostructures(alsocalledfabricsortextures) inspeleothemswhichcanbeexaminedbypolarizedlight microscopyorbyscanningelectronmicroscopy(Frisiaet al.,2000).TracequantitiesofMn 2 orrareearthelements fluoresceundercathode-rayactivation,providingabetter visualimageofgrowthpatternsthanthosethatcanbeseen bywhitelightmicroscopy.Cathodoluminescencemicroscopyhasbeenusedtoobservegrowthpatternsin speleothemsandproduceapaleoclimaticinterpretation (Richteretal.,2004). Ithaslongbeenestablishedthatcalciteisthestable polymorphofCaCO 3 atalltemperaturesunderambient pressures(Carlson,1983).Pressuresof300–500MPaare requiredtostabilizetheorthorhombicaragonitestructure. Inwhatseemstobeadirectviolationofthermodynamics, aragoniteoccurscommonlyincavesforminganthodites, frostwork,aragonitebushes,andbulkstalactitesand stalagmites.Theliteratureonthearagoniteproblemis verylarge(seereviewbyCarlson,1983).Twofactorsseem toberesponsible.Oneisthesensitivityofcalcitenucleation andgrowthtothepresenceofimpurities.Theotheristhe solubilityofaragonitewhich,asametastablephase,must belargerthanthatofcalcite.However,asshownbythe Figure6.Solubilityofcalciteandaragoniteasafunctionof CO 2 pressure.Calculatedfromequation(1)for10 u Cand 20 u C. W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 81

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curvesinFigure6,thesolubilityofaragoniteisonlyabout 10%greaterthanthatofcalciteoverawiderangeofCO 2 pressuresatthesametemperature.Indeed,thearagonite curveat20 u Cisalmostcoincidentwiththecalcitecurveat 10 u C.Unlikecalcite,aragonitenucleateseasilyandits growthisrelativelyinsensitivetoimpurities.Itisonly necessarytoinhibitcalcitenucleationsufficientlytoallow thesupersaturationtobuildpastthearagonitesolubility curveandaragonitewillprecipitate.Sr 2 enhances aragonitegrowthbyprecipitatingaSr-richnucleuswith thearagonitestructure.Thisactsasatemplateonwhich aragonitecangrow.Mg 2 haslongbeenknownasacalcite growthinhibitor(Berner,1975),aneffectconfirmedby morerecentdirectmeasurementswiththeatomicforce microscope(Davisetal.,2000). Growthofbulkspeleothems,ofwhichstalagmiteshave beenofgreatestinterest,combinesconsiderationofthe growthofindividualcalcitecrystalswithconsiderationsof precipitation,flowpathsfromthesurfacetothecave,and carbondioxideconcentrationcontrastsbetweenthesoil andthecaveatmosphere.Afurtherconsiderationfornearentranceenvironmentsistheroleofevaporationas acompetitiveprocesswithCO 2 loss.Becausedrip-rate (relatedtoprecipitation)andCO 2 pressurevaryseasonally inmostlocations,speleothemsdevelopanannuallayering withlayersthatvaryintextureandalsosometimesin calcite/aragonitecontent.Therateofgrowthcanbe determinedbydatingthestalagmitelayers,eitherby countingthelayersincontemporarystalagmitesorbyU/ Thdatingofsamplestakenalongthestalagmiteaxis. Typicalvaluesareintherangeof0.01to0.1mmy 1 .The relationshipbetweengrowthrateandstalagmiteshapehas beenmodeledbyKaufmann(2003). Thethickness,texture,andmineralogydependprimarilyonprecipitationasthemostimportantexternal parameter,thusmeasurementofthesequantitiesprovides apaleoclimateindicator.Growthlayeringhasprovided informationfromthepastseveralmilleniawhereconfirmationfromhistoricalrecordsispossible.Suchmeasurementgavearainfallproxyrecordforthepastfive centuriesinMadagascar(Brooketal.,1999).Several thousandyearsofrecordoftheintensityofIndian monsoonswasextractedfromastalagmitefromNepal (Dennistonetal.,2000a).Spectralanalysisofthelayering patternintheShihuaCave,Chinaappearedtotrack climaticcycles(Qinetal.,1999).Olderrecordsarealso possible. D EPOSITIONOF G YPSUM Gypsumdepositionisamatteroftheevaporationof sulfate-bearingsolutions.Nochemistryisinvolved.The reactionis: Ca 2 SO 2 4 2H 2 O ? CaSO 4 2H 2 O 3 Theconcentrationofdissolvedgypsumexactlyinequilibriumwithcrystallinegypsumisgivenby C gyp M gyp K gyp c Ca 2 c SO 2 4 1 2 4 C gyp istheconcentrationofdissolvedgypsuming/L,M gyp isthemolecularweightofCaSO 4 2H 2 O, K gyp isthe solubilityconstantforgypsumandthegammasarethe activitycoefficientsforcalciumandsulfateions. M gyp 172.17atomicmassunits.Thesolubilityofgypsumdoes notdependonpHorCO 2 pressurebutdoesvarywith temperature.Otherionsinsolutionaffectgypsumsolubilitythroughtheactivitycoefficients. K gyp asafunctionoftemperatureisgivenby log K gyp 68 2401 3221 51 T 25 0627log T 5 Equation(5)istakenfromLangmuirandMelchior(1985). Inthisequation, T istheabsolutetemperatureinkelvins. Atthestandardreferencetemperatureof25 u C,the calculatedvalueislog K g yp 4 581,avaluethathas beenacceptedbyseveralcontemporarytextbooks(e.g., Langmuir,1997;Drever,1997).Thisnumberisbasedon directsolubilityexperimentsandisrecommendedagainst othervaluesthathavebeencalculatedfromthermodynamicdata. Thesourcesofgypsumincavesvarydependingonthe specificcave.Sourcesthathavebeenidentifiedinclude: N Dissolution,transport,andredepositionofgypsumbeds thatoccurinterbeddedwiththecarbonateunits. N OxidationofH 2 Stosulfuricacidfollowedbyreaction oftheacidwiththelimestonetoformgypsum.Thisis thesourceofgypsuminsulfuricacidcaves. N Oxidationofpyrite,FeS 2 ,thatoccursdispersedinthe limestonefollowedbyreactionwiththelimestoneto formgypsum. N Oxidationofpyritefromoverlyingstratafollowedby transportofsulfate-bearingsolutionstothecavewhere reactiontogypsumtakesplace. Residualgypsumisakeypartofthemechanismforthe formationofsulfuric-acidcaves(Hill,1987,1990).Thus far,gypsumhasnotbeenfoundtocontainanysignificant paleoclimaticrecord.Anothersulfatemineral,alunite, KAl 3 (SO 4 ) 2 (OH) 6 ,hasbeenfoundtoretain 40 Arfrom thedecayof 40 Ksothat 40 Ar/ 39 Ardatingofthealunite fromCarlsbadCavernsandotherGuadalupeMountain caveswaspossible(Polyaketal.,1998).Agesrangedfrom 3.89MainCarlsbadCaverntotheoldestandhighestcave examined,CottonwoodCave,at12.26Ma. I CE Seasonaliceformsinmanycaveswherewinter temperaturesfallbelowfreezing.Oftentheicetakesthe formofspectacular,iftransient,speleothemsnearcave entrances.Caveswithperennialicearelesscommonand C AVESEDIMENTSANDPALEOCLIMATE 82 N JournalofCaveandKarstStudies, April2007

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usuallyoccuronlyathighaltitudes.FossilMountainIce CaveinWyomingisanexampleintheUnitedStates.Best knownarethebigicecavesoftheEuropeanAlpssuchas theEisriesenweltandtheRieseneisho ¨hleinAustria.Some ofthebestpaleoclimaticrecordshavebeenrecoveredfrom icecoresdrilledinthecontinentalglaciersofGreenlandor Antarctica.Itwouldseemthatdrillingtheperennialicein anicecavewouldbevaluable.StudieshavebeenundertakenintheSca ris ¸oaraicecaveintheApuseni MountainsofRomania(Racovita andOnac,2000) (Fig.7). R ESISTATES :I RONAND M ANGANESE O XIDES Amongthemorewidespreadminorcavesedimentsare theoxidesandhydroxidesofironandmanganese.These compoundsarehighlyinsolubleinneutralpHwaterandso areknownasresistates. Goethite(FeOOH),ferrihydrite(Fe(OH) 3 )andother hydratedandhydroxylatedironoxidesoccurwidelyin caves.GoethiteisusuallycrystallineattheX-raydiffractionscalebuttheotherironoxidesandhydroxidesare usuallynon-crystallineattheX-rayscale.Speleothemsof ironhydrateshavebeenfoundandarewelldevelopedin Rohrer’sCave,LancasterCounty,Pennsylvania(Whiteet al.,1985)(Fig.8). Blackcoatingsthatoccurwidelyonstreamsediments areusuallydescribedasmanganeseoxide.Usuallythe manganeseoxidesappearonlyasthin( 1mm)coatings althoughthickerandmoremassivedepositsoccur. Althoughatleastninemanganeseoxidemineralshave beenreportedfromcaves(HillandForti,1997),mostof theidentifieddepositsarecomposedofbirnessite,the d polymorphofMnO 2 ,butwithacompositionapproximately(Na,Ca,Mn 2 )Mn 7 O 14 2.8H 2 O.Themanganese oxidesactasscavengersforheavymetals.Concentrations frompartspermilliontomorethanonepercentofZn,Cu, Ni,Co,V,andCrhavebeenfound.Likewise,rareearth elementshavebeenfoundinmanganeseoxidedeposits (Onacetal.,1997).Depositionofthemanganeseoxide mineralsrequiresoxidationofMn 2 inthecavewaterto Mn 4 ,aprocessthatislikelymicrobiallymediated(Teboet al.,1997;NorthupandLavoie,2001). Figure7.Icespeleothems,Sca ùris üoaraIceCave,Romania. Figure8.Ferrihydritespeleothems,RohrersCave,Pennsylvania. W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 83

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A GE M EASUREMENTSON C AVE S EDIMENTS Thewidespreadinterestincavedepositsaspaleoclimate archivesarisesbecauseofwell-developedandreliable methodsforassigningdatestothedeposits.Different sedimentsprovidedifferentinformationanddifferent methodsareusedtodatethem.Thesectionsthatfollow giveashortsummaryofthemostimportantofthedating techniques. C ARBON 14 Spallationof 14 Nbycosmicraysintheupper atmosphereproducestheradioactiveisotopeofcarbon, 14 C.The 14 Cisoxidizedto 14 CO 2 whichthenmixeswith other 12 CO 2 intheloweratmospherethusproducing aconstantradioactivecomponent.Thesameconstant radioactivebackgroundispresentinplantswhichgrowby extractingCO 2 fromtheatmosphere.ThebasisforC-14 datingisthatwhenplantsdie,theexchangeofCO 2 with theatmosphereceases,theincorporated 14 Cbeginsto decayaccordingtoits5730yearhalf-life.Measurementof theresidual 14 Cinwoodandcharcoalthenallowsthe calculationofthetimethathaselapsedsincethewoodor charcoalwaspartofagrowingplant.Theusualtimelimit forC-14datingisabouttenhalf-livesor50,000years,but therearemanycorrectionsandcaveatstothecalculationof dates(Tayloretal.,1992). Thecalciteinspeleothemsisderivedfromlimestone dissolvedatthesoil/bedrockcontactabovethecavefor whichthechemicalreactionis CaCO 3 CO 2 H 2 O ? Ca 2 HCO 3 6 Inthecavethecalciumandbicarbonateionsare recombinedtoprecipitatecalciteinthegrowingspeleothem.Halfofthecarbonintheprecipitatedcalcite comesfromCO 2 andtheotherhalffromtheCaCO 3 inthe limestone.TheCO 2 isderivedpartlyfromtheatmosphere andpartlyfromthedecompositionoforganicmaterialin thesoil.Theorganicmaterialoriginatedaslivingplants andthusalsocontainsthebackgroundconcentrationof 14 C.Onemightexpect,therefore,thatthecarboninthe bicarbonateionsthatformspeleothemsshouldconsistof abouthalfyoungcarbonfromtheatmosphereandsoil CO 2 andhalfold,dead,carbonfromthelimestone.C-14 datingofspeleothemsshouldthereforebepossibleby takingaccountofthefractionofyoung,zeroage,carbonin thecalcite. Theisotopicchemistryofcalcitedepositionismore complicated.Thereiscarbon-isotopeexchangeinthesoil andinthedissolutionanddepositionprocesses.Asaresult, speleothemstypicallycontainabout85%youngcarbon, acircumstancethatactuallymakesC-14datingalittle easier.TherehavebeenrelativelyfewC-14datesof speleothems.Onecriticalcomparisononspeleothemsin theLobatseIICave(Holmgrenetal.,1994)showedthatC14datescomparedwellwithU/Thdatesbackto20,000 years. U/T H D ATINGOF S PELEOTHEMS U/Th-datinghasbecomethegoldstandardforspeleothemdating(Doraleetal.,2004;RichardsandDorale, 2003).Thecommonuraniumisotope, 238 U,undergoes alongandcomplicateddecaychainbeforereachingthe stableisotope 206 Pb(seeFigure1inField,2007).Along thatchainofmostlyshort-livedintermediatesaretwolonglivedisotopes, 234 U(halflife 248,000years)and 230 Th (half-life 75,200years).Measurementoftheratioof 234 U/ 238 Uand 230 Th/ 234 U,assumingnothoriuminthe initialsample,allowsthecalculationofanage.What makesU/Thisotopedatingparticularlyusefulforcalcite speleothemsisaquirkinthegeochemistryofthesetwo elements.Bothuraniumandthoriumhave4 valence stateswhichproducecompoundsthatarehighlyinsoluble. Uraniumalsohasa6 valencestatewhichusuallyappears astheUO 2 2 ion.Thoriumdoesnot.Theuranylionfurther complexesincarbonatewatersandbecomeshighlymobile (Langmuir,1997).Asaresult,speleothemsoftencontain tenstohundredsofpartspermillionuraniumbutno thorium.Theradiogenicthoriumthataccumulatesinthe calciteisadirectmeasureofthetimeelapsedsincethe calcitewasdeposited. Uranium/thoriumdatingofspeleothemswaspioneered byDerekFord,HenrySchwarcz,andtheirstudentsat McMasterUniversity.Agedatescombinedwithoxygen isotoperatiosanddeuteriumratiosproducedpaleoclimatic recordsforcavesinWestVirginia(Thompsonetal.,1976). Theseearlymeasurementsusedalphaparticlespectroscopy tomeasureuraniumandthoriumisotopeconcentrations, atechniquethatrequiredtensofgramsofsample(Harmon etal.,1975).Manydateswereobtainedbymultiple laboratoriesandapicturewasframedfornorthernNorth AmericaandnorthernEuropeofabundantspeleothem growthduringtheHoloceneandduringtheEemian (Sangamon)interglacial(isotopestage5;100,000–130,000 yearsB.P.).Relativelylittlegrowthoccurredduringthe periodsofglacieradvancement(Hennigetal.,1983; Gascoyne,1992).AlthoughsomeU/Thdatingbyalphacountingcontinues,forexample,thedemonstrationoffour episodesofspeleothemdepositioninlevelIVofthe DemanovaCavesinSlovakia(Hercmanetal.,1997), betterresolutionisnecessarytoprobethepaleoclimate recordsindetail.Thebreakthroughwastheuseofheavy ionmassspectroscopytomeasureisotopeconcentrations (Lietal.,1989).Morepreciseisotopeconcentrations allowedextensionofthedatingrangefrom350,000years foralphacountingtechniquestoapproachingthetheoreticallimitofabout600,000years.Heavyionmass spectroscopyandlateracceleratormassspectroscopyalso reducedtherequiredsamplesizesothatdatescouldbe obtainedfrommilligramsratherthantensofgram quantitiesofspeleothem.Thusastalagmitecouldbe C AVESEDIMENTSANDPALEOCLIMATE 84 N JournalofCaveandKarstStudies, April2007

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sectionedalongitslongaxis,thensampledatclose intervalsalongtheaxistoobtainacompletechronology ofthestalagmiteÂ’sgrowthhistory. P ALEOMAGNETIC C HRONOLOGIES TheEarthÂ’smagneticfieldreversesatirregularintervals duringwhichthemagneticnorthpoleandthemagnetic southpoleexchangeplaces.Intervalswherethenorthpole isnearitspresentlocationarecallednormal;theother intervalsarecalledreversed.Thegeophysicsofthe magneticfieldhavebeenextensivelyinvestigated(e.g., Channelletal.,2004)andageomagneticstratigraphy establishedwellbackintogeologictime.ThepaleomagnetictimescaleshowninFigure9hasbeendrawnonlyfor thepastfivemillionyearssincethisisthetimeintervalof mostinterestinkarstprocesses. Fine-grainedclasticsedimentscontainsmallamountsof magneticminerals.Asthesesedimentssettle,themagnetic grainsrotatetoorientwiththeEarthÂ’smagneticfieldasit wasatthetimeoftheevent.Oncethegrainshavesettled, theirmagneticorientationislockedinplaceandcanbe determinedbycarefulmeasurementofthemagnetic propertiesoforientedsamples.Themeasurementsrequire veryhighsensitivityequipmentbecausethereareonly asmallnumberofmagneticgrainsandmostofthesehave onlyweakmagneticmoments Ifpaleomagneticreversalscanbeidentifiedincave sediments,theyprovidetimemarkersandthusdatesfor sedimentsandthepassagesinwhichtheyoccur.Becauseof thelongtimespansbetweenreversals,themethodisbest appliedtotieredcaveswherethetotalsequenceofcave passagesandtheirsedimentsrepresentequallylongtimes. Withinasinglecavepassage,sedimentsareexpectedtobe stackedintheusualstratigraphicfashionwiththeoldest sedimentsonthebottom.Cavepassages,however,are arrangedintheoppositeorder,withtheyoungestpassages nearpresentdaybaselevelsandolderpassageshigheron theridges(Fig.10).Thisarrangementmustbetakeninto accountintheinterpretationofpaleomagneticdata.The bestsamplesareobtainedfromtheslackwaterfacieswhere thesedimentgrainssettleoutinquietwater.Oftenwhatis availablearethechannelfacies,andthesemayverywell havebeenreworkedbylaterfloodeventssothatthe paleomagneticsignalsarescrambled.Ifacompletesedimentarysequenceisnotavailable,identificationofareversalinasedimentpileinahigh-lyingcaveleavesopenthe questionofwhichreversalisbeingobserved. Paleomagneticstratigraphyhasbeendeterminedfor clasticsedimentsinMammothCave(Schmidt,1982),the cavesoftheObeyRiverGorgeinthewesternCumberland EscarpmentofTennessee(Sasowskyetal.,1995),thecaves oftheCheatRiverGorgeinWestVirginia(Springeretal., 1997),andKookenCave,Pennsylvania(Sasowskyetal., 2004).Becauseoflimitedavailabilityofsensitivemagnetic measuringapparatus,andbecausethereversalsprovide onlywidelyspacedtimemarkers,thismethodofsediment datinghasseenlimitedapplication. C OSMOGENIC I SOTOPE D ATING TheEarthisunderconstantbombardmentbycosmic rays,whichareextremelyenergeticparticlesfromspace. Theseparticlestreamsandtheirspallationproductsfrom collisionsintheupperatmosphere,raindownonthe surfacewheretheycaninducenuclearreactionsinsurface Figure9.GeomagnetictimescalethroughthePliocene. DrawnfromdataofCandeandKent(1995),Berggrenetal. (1995)andSingeretal.(2004).Theprecisedatesforsomeof thereversaleventsarenotincompleteagreementbetween literaturesources.Labelingatbottomgivesthemain paleomagneticperiods(orchrons).Attoparelabeledsome secondaryevents. Figure10.Sketchshowingsedimentstratigraphicrelations intieredcaves.Theschematicsedimentlayersarenumbers fromyoungesttooldest,1to10. W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 85

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materials.Thesereactionproductsareknownascosmogenicisotopesandcanbeusedasaninterpretativetool (GosseandPhillips,2001).Theisotopesofimportancefor cosmogenicisotopedatingofcavesedimentsare 10 Be(halflife 2.18Ma)and 26 Al(half-life 1.02Ma).Bothare formedfromthesecondarycosmicrayneutronandmuon bombardmentofquartz, 10 Befromoxygenand 26 Alfrom silicon. Ifquartz,eitheraspebblesorsand,isleftexposed onornearthelandsurface,acertainverytinyconcentrationof 10 Beand 26 Alwillbuildup.Ifsinkingstreamsthen washthequartzsedimentintoacave,thequartzwillbe shieldedfromcosmicraysandtheaccumulated 10 Beand 26 Alwillbegintodecay.Althoughtheinitialconcentrationsofthecosmogenicisotopesarenotknown,theratio oftheconcentrationswillchangesothatthetimesince burialcanbeextracted(GrangerandMuzikar,2001). Carefulsamplepreparationisneededand,because concentrationsaremeasuredinmillionsofatoms,an acceleratormassspectrometerisneededtodeterminethe isotoperatios. Cosmogenicisotopedatingisoneofthemostpromising recenttechniques.Itcoversatimescalebackto5Ma whichisthetimescaleformostactivekarstsystems.The quartzthatisdatedispartofthechannelfaciessediment loadandthesematerialsareusuallydepositedwhenthe cavepassageispartoftheactivedrainagesystem.To areasonableapproximation,thecosmogenicdateofthe sedimentisalsoameasureoftheageofthecavepassage. Byusingcosmogenicisotopedatingofcavesediments,it waspossibletoestablisharateofdowncuttingforriversin theSierraNevada(Stocketal.,2004). T HE P ALEOCLIMATIC R ECORD A VAILABLE A RCHIVES Thestudyofcavesediments,ashasbeendemonstrated above,hascomealongway.Fromabeginningofsimply tryingtounderstandwhyspeleothemshavetheirobserved shapesandmineralogy,investigationsofcavesediments arenowdelvingintosedimentsasarchivesofcritically importantpaleoclimaticinformation.Cavesedimentshave comeintotheirownastheunopenedhistorybookofthe Pleistocene. OfthemainclassesofcavesedimentslistedinFigure1, carbonatespeleothemshavethegreatestinformation content.Clasticsedimentsappearasfacieswhichreflect thehydraulicsoftheflowsystemsthatdepositedthem. Previousepisodesofintensivefloodingcansometimesbe recognized,butclasticsedimentsdonotrecordmuchdetail anddonotprovidemuchtimeresolution.Speleothems,in contrast,aredepositedfromsolutionandthuscarry arecordoftexture,mineralogy,traceelementcontent, andisotopecontent.Datedspeleothemsprovideamicrostratigraphywithaveryhightimeresolution(Perrette, 1999). Cylindrical(broomhandle)stalagmitesarethespeleothemofchoice(Fig.11a).Stalagmitesgrow,layerby layer,aswaterdripsontotheirtops.Astalagmiteof uniformdiameterisevidenceofuniformgrowthrateover longperiodsoftime.Uniformgrowthalsoimpliesuniform dripratesanduniformchemistry.Stalagmitegrowthrates mostlyfallintherangeof0.01to0.1mmy 1 .Ifthereare nobreaksinthegrowth,suchstalagmitesprovidetime spansof100,000yearsto10,000yearspermeterof stalagmite.Moststalagmitesexaminedthusfarhavebeen lessthantwometersinlength.Muchlargerstalagmites exist(Fig.11b).Measurementsonthesestalagmitespose thetechnicalproblemofremovingandlongitudinally sawingthespecimen.Speleothemworkalsoraisesavery seriousethicalquestion:Isthescientificinformation obtainedfromthestalagmiteworththedamagedoneto thecaveinordertocollectit? Becauseannualgrowthlayersinatypicalstalagmiteare fractionsofamillimeterthick,hightimeresolution Figure11.Cylindricalstalagmites.(a)Typeofstalagmites typicallysampledforpaleoclimaticstudies.SitesCave, PendletonCounty,W.Va.(b)LargestalagmitesinFurong Cave,ChongqingProvince,China. C AVESEDIMENTSANDPALEOCLIMATE 86 N JournalofCaveandKarstStudies, April2007

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dependsonmicro-samplesandmicro-analysis.With sufficientmicro-samplingitispossibletoconstruct parameterprofilesalongthestalagmiteaxis,which,of course,isalsoatimeaxis.Quantitiesthathavebeen measuredinclude: N Oxygenisotoperatios N Carbonisotoperatios N Deuterium/hydrogenratios N Traceelements–typicallyMg,SrandBa,andmany others N Layer-to-layercolorvariations(colorbanding) N Layer-to-layervariationsinluminescenceintensity (luminescencebanding) Mostwidelymeasuredaretheisotoperatiosfor 18 O/ 16 O and 13 C/ 12 C.Theoxygenisotoperatioisrelatedto temperatureandiswidelyinterpretedasapaleoclimate thermometer.However,atemperaturecalibrationdepends onspeleothemdepositionunderconditionsofisotopic equilibriumwhichmaynotbeachieved(Hendy,1971; Mickleretal.,2006).Thecarbonisotoperatioisrelatedto thetypeofvegetationonthelandsurfaceaboveandhas beenusedtodistinguishforestfromgrassland. Calcitespeleothemsappearinavarietyofcolorsmostly rangingfromwhitetoshadesoftan,orange,andbrown. Themostcommonsourcesofcolorarehumicsubstances derivedfromtheoverlyingsoil(White,1981).Likewise, speleothemcalciteisusuallystronglyphosphorescent underlongwaveUVexcitationduemostlytothefulvic acidfractionofthehumicsubstances(vanBeynenetal., 2001).Animportantdiscoverymadeinthe1980sbyYavor Shopov(Shopovetal.,1994)isthattheluminescenceis bandedonamicrometerscaleandthattheindividual bandsrepresentanannualcycle.Therehasbeenatremendousamountofinterestinluminescenceandcolor banding.Theseappeartobeameasureofprecipitation, butcalibrationintermsofspecificclimaticvariableshas beendifficult. Therearearangeofpertinenttimescalesandassociated questions.Thesearedealtwithindividuallyinthe followingsections.Foramuchmorecompletediscussion oftheotherpaleoclimaticdataintowhichthespeleothem resultsaremerged,seeBradley(1999). T HE P LIOCENE /E ARLYTO M IDDLE P LEISTOCENE :5M A TO 300ka Muchoftoday’slandscape,includingmostofthecaves andsurfacekarst,hasbeensculpturedduringthepast 5Ma,thatisthePliocene,thePleistoceneandthe Holocene.Nearthebeginningofthisperiodthemild climateoftheEoceneandMiocenegavewaytoaclimatic cyclingfromwarmtocoldandbacktowarmperiods.In highnorthernandsouthernlatitudesandathighaltitudes thesecycleswereaccompaniedbyadvancesandretreatsof glaciers.Sealevelroseandfellassignificantvolumesof waterweresequesteredincontinentalicesheetsandlater releasedastheicemelted.Theclimaticoscillationsoccurin regularcycles:41,000yearsatthebeginningoftheperiod ofoscillationsandswitchingtoa100,000yearcycleabout 800,000yearsago.Thecyclesareknowninconsiderable detailbecauseofdataextractedfromthelongicecores drilledinGreenlandandAntarctica(Fig.12). Eventspriortoabout500,000yearsarebeyondthe rangeofU/Thdating.InformationfromthePlioceneand EarlytoMiddlePleistocenemustbeextractedfromclastic sedimentswhichcanbedatedbycosmogenicisotope methodsandbypaleomagneticreversals.Ataboutthetime thatthe25 th anniversaryissueofthisjournalwasbeing published,Davies’(1960)viewthatmastertrunkcaves wereformedclosetoregionalbaselevelswasbecoming accepted(WhiteandSchmidt,1966).Fossilcavescould,in principle,bedatedbytheirrelationtonearbyriver terraces.However,theevolutionofsurfacevalleysis destructive,withearliervalleyfloorsandchannelsbeing dissectedandlostassurfacestreamscontinuetodowncut. Asaresult,thedatingofterraceswasalwayshighly uncertain.Inrecentyears,thesituationisreversed. Cosmogenicisotopedatingofclasticsedimentinmaster trunkcavesgivesveryaccuratedateswhichcanthenbe appliedtotheterraceleveltowardwhichthecavepassage isgraded. Cosmogenicisotopedatinghasbeenusedtoestablish achronologyforthemaintrunkpassagesinMammoth Cave(Grangeretal.,2001)andformajorcavesinthe westernCumberlandPlateauofKentuckyandTennessee (AnthonyandGranger,2004).Thecaveschosenfordating spantheentiretimeperiodofcaveandkarstdevelopment inthesouth-centralKentuckykarstandtheAppalachian plateaus.Themainconclusionsfromtheseinvestigations aresummarizedinFigure13.Thesestudieshavelockedin atimescaleformostcavedevelopmentintheeastern Figure12.Climaticcyclesoverthepast1.5Mabasedon icecorerecords.FromBrooketal.(2006).Oxygenisotope dataarethebenthicrecord.VostokandEpicaDomeCare icecoresfromAntarctica. W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 87

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UnitedStates.ByrelatingtheoldestCumberlandPlateau cavestothedissectionoftheHighlandRimsurfaceat5.7 to3.5Ma,byimplication,thesametimescaleshouldapply tocavesrelatedtotheHarrisburgSurfaceintheValleyand RidgeProvinceoftheAppalachians(WhiteandWhite, 1991). OneoftheeventscommontobothMammothCaveand theCumberlandPlateauwasamassivesedimentin-filling atabout800,000yearsB.P.Suchasedimentationevent implieshighprecipitationandfloodingonthelandsurface above.Thistimeintervalalsocorrespondstothetransition from41,000yearto100,000yearclimaticcycles.Whether thisismerelyacoincidenceorwhetherthecavesedimentationisrecordingtheglobalshiftinclimaticcyclesshould befurtherinvestigated. T HE L ATE P LEISTOCENEAND T HE E EMIAN I NTERGLACIAL TheyoungerendofthePleistoceneliesinthetimerange forU/Thdatingsothatfullusecanbemadeofthe speleothemrecords.Forreference,Figure14givesthe usualtimedivisions.Itwasaperiodofextensiveclimatic variationwithbothglacialandinterglacialperiods. Inareasofcoastalkarst,loweringsealevelsduringthe glacialmaximaallowedthedevelopmentofair-filledcaves thatwerethenfloodedwhentheglacialicemelted.These cavesoftencontainspeleothemswhichcanbeU/Thdated andtheremaybeovergrowthsonthespeleothemsthat depositedafterthecavesre-flooded.Oneofthefirst accomplishmentsofU/Thdatingofspeleothemswasto identifysea-levelloweringduringtheIllinoianglaciation (Gascoyneetal.,1979).Sealevelminimacanbetrackedby datingspeleothemsfromsubmergedcaves(Lundbergand Ford,1994;Richardsetal.,1994).Likewisesealevelstands higherthanpresentsealevelcanbemeasuredbydatingthe overgrowthsthatformunderphreaticconditions(Vesicaet al.,2000). OxygenisotopeprofilesalongTasmanianstalagmites producedanoxygenisotopeprofilefromtheEemian interglacialwellintotheWisconsinglacialperiod(Goedeet al.,1986;Goedeetal.,1990).Theprofileswereinterpreted intermsoftemperaturechangesduringtheperiodof record.Deuterium/hydrogenprofilesobtainedbyextractingfluidinclusionsindatedspelethemstracedtemperature overthepast140kaintheeasternMediterranean (McGarryetal.,2004).Ithaslongbeenarguedthatthe aridregionsoftheNear-Eastweremuchwetterduring interglacialperiods.AspeleothemrecordfromOman showsrapidcalcitegrowthandanoxygenisotoperecord consistentwithhighrainfallat6.5–10.5,78–82,120–135, 180–200,and300–325ka(Burnsetal.,2001).Anoxygen andcarbon-isotoperecordinthreestalagmitesfrom Missouriclearlyshowstransitionsfromforesttosavanna toprairietoforestthroughtheWisconsinglacialperiod,75 to25ka(Doraleetal.,1998). T HE E NDOF T HE L AST I CE A GE :E XTREME C LIMATIC O SCILLATIONS TheendoftheWisconsiniceagewasatumultuoustime withsomeextremeclimaticexcursionsbeforetheclimate settledintothemorestableperiodoftheHolocene. Figure14namestheseperiodsandgivestheirapproximate timeintervals.Itshouldbenotedthatnotallsourcesgive thesametimeboundariesfortheseperiods.Thecold glacialclimateoftheOldestDryasgavewaytoaperiodof warming,butoscillatingclimateintheBølling/Older Dryas/Allerød.Atabout12,900yearsB.P.,accordingto icecoredata(Alleyetal.,1993),thenorthernclimate plungedintoa1300yearcoldperiod,theYoungerDryas, fromwhichitemergedwithanabruptwarmingmarking thebeginningoftheHoloceneat11,640yearsB.P.The transitionfromglacialcoldtopre-borealclimateapparFigure13.Chronologiesforcavesanderosionsurfacesin theMammothCaveareaandtheCumberlandPlateau. InformationcompiledfromGrangeretal.(2001)and AnthonyandGranger(2004). Figure14.Sketchshowingclimateperiodsofthelate PleistoceneandthePleistocene-Holocenetransitionalperiod, theperiodmosteasilyaccessibletoU/Thdating. C AVESEDIMENTSANDPALEOCLIMATE 88 N JournalofCaveandKarstStudies, April2007

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entlyrequirednomorethanadecade.Thelastgaspofthe iceagewasaneventat8200yearsB.P.,anabruptcoldsnap thatlastedforabout200years.Itwasidentifiedinicecores amongotherrecords(Alleyetal.,1997). Muchofthedataonclimaticeventsatthecloseofthe lastglaciationcomefromstudiesatnorthernlatitudes. Thereisalwaysthequestionofhowtheseclimatic excursionsappearedintemperateortropicalclimatesor inthesouthernhemisphere.Theintensecoolingperiodof theYoungerDryasappearedinaspeleothemrecordfrom Australia(Goedeetal.,1996).Climaticfluctuations appearedinspeleothemrecordsfromNewZealand (Williamsetal.,2004).Speleothemtexturecombinedwith traceelementconcentrationsandheavymetalisotopes wereusedtosortoutwet/dryandwarm/coolperiodsinthe SoreqCave,Israel(Ayalonetal.,1999). IntheAmericanSouthwest,therewastheonsetofawet climateat12,500yearsB.P.nearthebeginningofthe YoungerDryas.Basedonstalagmitegrowthasdatedby U/Thmethods,thewetperiodpersistedabout2000years takingitintobeginningoftheHoloceneandendingat 10,500yearsB.P.whentheclimateshiftedtothepresent aridregime(Polyaketal.,2004).The8200yearB.P.event appearedasaweakenedmonsoonintropicalCostaRica accordingtospeleothemdata(Lachnietetal.,2004).Itwas alsofoundasananomalyintraceelementprofilesin astalagmitefromCragCave,southwesternIreland (Baldinietal.,2002) T HE H OLOCENE: 11,650Y EARSTO P RESENT TherewereclimaticfluctuationsduringtheHolocene, buttheseweregenerallylesspronouncedthanthoseatthe endoftheglacialperiod.TheHoloceneisusually consideredtoextendfromabeginningat11,500to 11,700yearsB.P.(variablystatedindifferentsources)to thepresent.ThebeginningdategivenaboveisfromAlley etal.(1993).Shiftsinclimateasreflectedintheoxygen isotopeprofilesofdatedspeleothemshavebeenmeasured forEurope(McDermottetal.,1999),Israel(Frumkinet al.,1999),southernAfrica(Lee-Thorpeetal.,2001),and China(Wangetal.,2005;Zhangetal.,2004). Aseriesofstudiesofcarbonisotopesfromavarietyof cavesintheAmericanMidwesthasallowedthetrackingof vegetationoverthecourseoftheHolocene(R.G.Bakeret al.,1998;Dennistonetal.,1999a;Dennistonetal.,1999b; Dennistonetal.,2000b).Nearlyalltrees,shrubs,andcoolseasongrassespreferentiallyareenrichedinthe 12 Cisotope (C 3 vegetation)whereasgrasslands(C 4 vegetation)contain moreofthe 13 Cisotope.Apatternofshiftsfromforestto grasslandandbacktoforesthasbeendocumented,butthe timingoftheshiftsvarieswithlocation.Generally,thereis goodagreementwithpollenrecords. Measurementof 87 Sr/ 86 Srratiosinaspeleothemfrom Harrison’sCave,Barbados,showedasystemicchange throughtheHolocene(Banneretal.,1996).Thisrecord correlateswithrainfall. T HE H ISTORIC P ERIOD :T HE P AST S EVERAL T HOUSAND Y EARSOF T HE H OLOCENE Anissueofnationalandinternationalimportanceatthe timeofthiswritingisthatofglobalwarming.Itseemsthat ontheaverage,wintershavebecomewarmer,summers hotter,glaciersareretreating,hurricaneshaveincreased intensity,andsealevelsarerising.Ononesidearethose whoassignthesechangestoagreenhouseeffectbroughton bytheanthropogenicincreaseincarbondioxideandother greenhousegasesintheatmosphere.Ontheothersideare somewhoassertthatthewholeideaofhuman-induced globalwarmingisamythandthatthesefluctuationsare partofanaturalcycle.Theimplicationsareenormous. ProlongedwarmingsufficienttomelttheGreenlandand Antarcticicecaps(aswellasotherglacialice)wouldraise sealevels,shiftoceancurrents,drowncoastalcities,and changeatmosphericcirculationpatternssothatproductive farmlandsmightbecomedustbowls.Arehumanbeings creatingadisasterorarewesimplyridingyetanother climaticcycle,thisoneonacenturiestimescale? Climatehasindeedoscillatedoverthepastseveral millenniawithaperiodofoscillationofseveralcenturies (Fig.15).Thewarmclimatesduringthefloweringofthe RomanEmpiregavewaytocoldclimatesinEuropeduring whatisknownastheDarkAges.Climatewarmedagain duringtheMedievalperiodandthenshiftedintowhat historicallyhasbeencalledtheLittleIceAge.Sincethelate 19 th Century,climateinNorthAmericaandEuropehas beenwarming.The‘‘grandfatherwinters’’arefadingfrom memory. Thereissomeaccumulatingevidencethatthesemore subtlerecentclimaticeventscanbeteasedfromthe speleothemrecord.Spikesinthe 13 Crecordinastalagmite fromBelizeovera30yearperiodfrom1970to2000 correlatedwiththesouthernoscillationindex–ameasure ofElNin ˜oevents(Frappieretal.,2002).Oxygenisotope profilestakenfromthreestalagmitesfromacavein southernOmanprovideda780yearrecordofmonsoon activity.Therecordrevealedthetransitionfromthe MedievalWarmPeriodtotheLittleIceAgeatAD1320 (Fleitmannetal.,2004).SpeleothemrecordsfromSouth AfricaindicatethattheLittleIceAge,whichextended fromroughlyAD1300to1800,wasabout1 u Ccoolerthan presentwhiletheMedievalWarmPeriodmayhavebeen 3 u Cwarmer(Tysonetal.,2000).Theboldestproposalhas Figure15.Sketchshowingclimateperiodsforthehistorical periodasrecordedinEurope.Theboundariesarenotsharp andvarysomewhatbetweenregions. W ILLIAM B.W HITE JournalofCaveandKarstStudies, April2007 N 89

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beentocalculatea3000yeartemperaturerecordfrom stalagmitecolorbandingfromSouthAfrica(Holmgrenet al.,2001).Whatisneeded,andwhatmoreaccurately calibratedspeleothemrecordsmayprovide,isanassessmentofwhetherthepresentdaywarmingiscomparableto otherwarmperiodsofthepastseveralmillennia,or whetherweareenteringanew,andpossiblyquite dangerous,climaticregime. C ONCLUSIONS Cavesedimentshavethepotentialforproviding detailedpaleoclimatearchiveswithvaluecomparableto icecoresanddeepseasedimentcores.Techniquesfor establishingaccuratedatesonhighresolutionrecordsin speleothemsareinplaceforthetimescalefromthelate Pleistocenetothepresent.Cosmogenicisotopedatingof clasticsedimentsextendsthemeasurabletimescalebackto theearlyPliocenebutwithmuchlesstimeresolution.The problemremainsofestablishingreliablerelationships betweenmeasurableparameters,traceelementprofiles, isotopeprofiles,colorandluminescenceprofiles,andthe actualclimaticvariablesforwhichthemeasurementsare aproxy(McDermott,2004). Climate,asrecordedincavesediments,isverymuch amatterofthelocalclimateintheimmediatevicinityofthe cave.Inordertoobtainabroaderregionalorevenglobal pictureofclimateatsometimeinterval,manymoredata areneeded.Cavesediment–paleoclimatestudiesarein theirinfancy. A CKNOWLEDGEMENTS Thispaperisareviewandsodrawsontheworkofmany investigatorsasindicatedbytheextensivereferencelist. SpecificcavesedimentresearchatPennStatewas supportedbytheNationalScienceFoundationandby theArmyResearchOffice.ElizabethKnappandEvan Hartarethankedfortheircarefulandthoughtfulreviews. R EFERENCES Alley,R.B.,Meese,D.A.,Shuman,C.A.,Gow,A.J.,Taylor,K.C., Grootes,P.M.,White,J.W.C.,Ram,M.,Waddingon,E.D., Mayewski,P.A.,andZielinski,G.A.,1993,Abruptincreasein GreenlandsnowaccumulationattheendoftheYoungerDryas event:Nature,v.362,p.527–529. Alley,R.B.,Mayewski,P.A.,Sowers,T.,Stuiver,M.,Taylor,K.C.,and Clark,P.U.,1997,Holoceneclimaticstability:Aprominentwidespreadevent8200yrago:Geology,v.25,p.483–486. 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