Bitterfeld amber is not Baltic amber

12 Pages·2015·3.48 MB·English

by  Alexander P. Wolfe

ReviewofPalaeobotanyandPalynology225(2016)21–32
ContentslistsavailableatScienceDirect
Review of Palaeobotany and Palynology
journal homepage: www.elsevier.com/locate/revpalbo
BitterfeldamberisnotBalticamber:Threegeochemicaltestsandfurther
constraints on the botanical affinities of succinite
AlexanderP.Wolfea,⁎,RyanC.McKellarb,RalfTappertc,RanaN.S.Sodhid,KarlisMuehlenbachsc
aDepartmentofBiologicalSciences,UniversityofAlberta,Edmonton,ABT6G2E9,Canada
bRoyalSaskatchewanMuseum,2445AlbertSt.,Regina,SKS4P4W7,Canada
cDepartmentofEarthandAtmosphericSciences,UniversityofAlberta,Edmonton,ABT6G2E3,Canada
dDepartmentofChemicalEngineeringandAppliedChemistry,UniversityofToronto,Toronto,ONM5S3E5,Canada
a r t i c l e i n f o a b s t r a c t
Articlehistory: BalticandBitterfeldambersareimportantdepositsofpolymerizedconiferresinthatarewidelyrecognizedfor
Received10October2014 theirexquisitefossilinclusions,especiallyinsects.Becauseofover-archingsimilaritieswithrespecttovisualap-
Receivedinrevisedform12November2015 pearance,organicgeochemistry,arthropodassemblages,andproximitytoforestsofthePaleogeneNorthSea
Accepted16November2015 margin,thesetwoambershavenotyetbeendifferentiateddefinitively,leadingtoongoingdebateastowhether
Availableonline23November2015
ornotthey(andtheirrespectiveinclusions)aretrulyequivalent.Wecombinemicro-Fouriertransforminfrared
spectroscopy(FTIR),timeofflight-secondaryionmassspectrometry(ToF-SIMS),andstableisotopes(δ13Candδ2H)
Keywords:
toestablishthatBalticandBitterfeldambersdifferconsistentlyintheirgeochemicalproperties,andthuscapturedis-
Amber
Baltic tinctdepositionalepisodesinspace,butnotnecessarilyintime.Balticamberhasmoresuccinicacid,succinicanhy-
Bitterfeld dride,andcommunicacidrelativetoBitterfeldamber,butlessdehydroabieticacid.Althoughbothambersproduce
FTIR overlappingδ13Cvalues,supportingasimilarageofformation,δ2Hismarkedlydepleted(by~20‰)inBalticamber
ToF-SIMS relativetoBitterfeldamber.Thehydrogenisotopicresultsconferpaleolatitudinaldifferencesinamberprovenance,
Stableisotopegeochemistry thatis,acleardifferentiationbetweensourcesoriginatingfromthenorthern(Baltic)andsouthern(Bitterfeld)
marginsofthePaleogeneNorthSea.Weconcludethatthetwodepositsaregeologicallydistinctinorigin,butthat
similaritiesintheirrespectivefaunalrecordsarisebecausetheyarebroadlycoevalintime.Wealsopresentnew
ToF-SIMSresultsthatimplyonlyresinsfrommodernconifersofthefamiliesPinaceaeandSciadopityaceaebegin
tosatisfytheexpandedgeochemicalprofilespresentedforBalticandBitterfeldambers.
©2015ElsevierB.V.Allrightsreserved.
1.Introduction (Millsetal.,1984;MosiniandSamperi,1985;Wolfeetal.,2009;
Dolezychetal.,2011).
Balticamberistheworld'sbestknowndepositoffossilplantresin, Bitterfeldamberoriginatesfromamuchmorerestrictedgeographi-
and by far the single largest repository of fossil insects of any age calarea,thesiltsandsands,or“Bernsteinschluff”,oftheCottbusForma-
(WeitschatandWichard,2002,2010).Unlikeinsitufossilresinsthat tion nearthetownof Bitterfeld in UpperSaxony (Sachsen-Anhalt;
aredirectlyassociatedwithlignite,coal,orotherplant-richstrata,Baltic hence the synonym Saxonian amber). Although once assigned a
amberisasecondarydepositfoundmainlyinglauconiticmarinesedi- Mioceneage(BarthelandHetzer,1982),morerecentgeochronological
mentsofmiddleEoceneage(LutetianStage;41.3–47.8Ma),deposited efforts(Knuthetal.,2002)placethesesedimentsinthelateOligocene
alongthepaleo-NorthSeamargin.Theblueearth(orBlaueErde)in (Chattian;23.0–28.1Ma).AswithBalticamber,Bitterfeldamberis
whichBalticamberisprincipallyhostedoccursinRussia(Kaliningrad a secondary deposit that preserves an exceptional record of fossil
Oblast),Poland,andGermany,butdetritalBalticamber,redeposited arthropods.BitterfeldamberwasactivelyminedatthesiteofGoitzsche
byQuaternaryglacialandfluvialprocesses,reachesScandinavia,the between1975–1993,yieldingagemqualityresourceandthousandsof
Balticrepublics,andtheBritishisles.Balticamberhasbeenexploited arthropodinclusions(Dunlop,2010).Carefulgeologicalmappingof
formillennia,andiswidelydisseminatedinEuropeanarchaeological the Bitterfeld amber complex shows that amber is concentrated
contexts(Becketal.,1965).ThebotanicaloriginofBalticamberis in low-energy lagoonal facies associated with a deltaic system
atopicofintensescrutinyandlongstandingdebate,forwhichthe dischargingintotheNorthSeafromthesouth(Wimmeretal.,2006;
only firm conclusion is that source trees were extinct conifers Fuhrmann,2008).
BitterfeldamberissimilartoBalticamberwithrespecttohardness
andvisual appearance(Fig. 1), the ubiquitous presenceof succinic
⁎ Correspondingauthor.Tel.:[phone];fax:[phone]. acid(botharereferredtoassuccinites;AndersonandBotto,1993),sev-
E-mailaddress:[email protected](A.P.Wolfe). eralelementsoftheirrespectivearthropodassemblages,andthe
http://dx.doi.org/10.1016/j.revpalbo.2015.11.002
0034-6667/©2015ElsevierB.V.Allrightsreserved.
22 A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32
generalizedgeographyofEuropeanamberdistribution.Forthisrea-
son,somehavearguedthattheyarenecessarilycoeval,Bitterfeld
amber being merely a younger redeposited fraction of primary
EoceneBalticamber.Inthismodel,bothambersshareacommon
botanicalorigin.Thisviewissupportedbysimilaritiesbetweenthefau-
nalinclusionsofbothdepositswithrespecttoArachnida(harvestmen:
DunlopandMitov,2009;spiders:Wunderlich,1993,2004),Coleoptera
(dermestids: Háva and Alekseev, 2015); Diptera (acalyptrates:
vonTschirnhaus andHoffeins,2009;anthomyzids: Roháček,2013;
ceratopogonids:Szadziewski,1993;SontagandSzadziewski,2001;
limoniids: Kopeć and Kania, 2013; nymphomyiids: Wagner et al.,
2000),andHymenoptera(apoidbees:Engel,2001;andwasps:Ohl
andBennett,2009).Indeed,theviewpointthatBalticandBitterfeld
ambershaveanidenticalprovenanceisheldstrongly,andhasbeen
particularlywellarticulatedbyWeitschat(2008,pp.94),whosetrans-
latedstatementreads:
“NorthernEuropeanamberproductionbeganduringwarmcondi-
tionsoftheearlyEocene,andterminatedbytheendofthemiddle
Eocene.Thecoolingtrendoverthisintervalresultedinirreversible
changesinthefloraandfaunaofnorthernEurope:tropicaland
subtropicalelementswereprogressivelyreplacedbyboreal‘arcto-
Tertiary’assemblages.Theamberforestsrecordedthistransition,
giventhatBalticandBitterfelddepositsbothcontaintaxabelonging
totropicalaswellasborealecotypes,attimestheverysamespecies.
Thecaseisespeciallyconvincingwithregardtospiders,suggesting
thatBalticandBitterfeldambersbothoriginatedfromasingleforest
ecosysteminwesternScandinavia,whichpersistedforupto10
millionyearsunderasustainedwarmclimateregime.”
Morerecently,evenstrongerstatementstothesameeffecthave
beenissuedfromthepaleoentomologicalcommunity(Szwedoand
Sontag,2013,pp.380):
“Atpresent,thereisnodoubtthatamberfromBitterfeld(Saxonian
amber)iscontemporaneouswithBalticamber,i.e.thatitoriginated
intheEoceneandthatitbelongstotheBalticambergroup.”
However,abalancedandthoroughreviewofthesubject(Dunlop,
2010)leavesunresolvedthequestionastowhetherBalticandBitterfeld
ambersaretrulyidenticalinageandorigin.Argumentsbasedonstra-
tigraphy(Knuthetal.,2002)andorganicgeochemistry(Yamamoto
etal.,2006)challengetheviewthatBalticandBitterfeldambersare
equivalent, as do paleobiological studies that nuance the rate and
tempo of evolutionary processes among and between organismal
groups(BarthelandHetzer,1982;DunlopandGiribet,2003;Schmidt
andDörfeldt,2007;DlusskyandRasnitsyn,2009).Theresolutionof
thisdilemmaconstitutestheimpetusforthepresentstudy,inthefoot-
stepsofimportantyetinconclusiveregionalsymposiaonthisexact
topic(Ganzelewskietal.,1997;Rascheretal.,2008).Wereportresults
fromthreeparallelsuitesofgeochemicalanalysesthatbeardirectlyon
thedifferencesandsimilaritiesbetweenBalticandBitterfeldambers,
andconcludethattheyarecompositionallydistinctfromeachother
anddonotsharethesamegeographicalprovenance,whileremaining
largelycontemporaneousintheirageofformation.
2.Materialsandmethods
Samples of Baltic and Bitterfeld ambers have been collected,
Fig.1.PhotographsofBaltic(A–F)andBitterfeld(G–N)amberspecimens.Polished(A–B) purchased,andobtainedthroughexchangewithcolleagues.Asizeable
andunpolished(C)clearBalticambers(“honey”),thelatterwithsurfacedesiccation collectionofBalticamberspecimensfromGermany,Lithuania,Poland,
cracks.(D)Internalzonationbetweenclearandpartiallyopaque“butterscotch”ambers.
Russia,andsouthernSwedenwasamassedduringpreviousinvestiga-
(E–F)Outerandinternalviewsofcompletelyopaque“bone”amberwithmultiplegener-
ationsofflowlines,orschlaube.(G–L)Bitterfeldamberrangingfromclearyellowtodark tions(Wolfeetal.,2009).Balticamberspecimensweresub-sampled
reddish-brown.Thedarknearlyspecimen(M)isclassifiedas“glessite”,thenamegivento from this collection for the geochemical analyses described below.
thisvariant,whichoccursinbothBalticandBitterfelddeposits.(M–N)Bitterfeldbone Bitterfeldamberspecimensincludesamplesconfirmedtooriginate
amber. fromtheGoitzschemineandofferedforstudybyAlexanderSchmidt
A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32 23
(UniversityofGöttingen).Forbothambers,weanalyzedintriplicatethe representativeofreplicatedanalyses;allfeaturesillustratedanddiscussed
fourmostcommoncolorvariants(“honey”,“butterscotch”,“bone”,and inthetextaremanifestedreproducibly.
“glessite”;Fig.1),whichrangefromclearyellowtodarkred,withvari- Carbon(δ13C)andhydrogen(δ2H)stableisotopicratiosfrom
ousdegreesofopacity. amberprovideusefulancillaryinformationforunderstandingthe
Fouriertransforminfrared(FTIR)spectroscopyhasbeenamainstay genesisofamberdeposits,inpartbecauselittleisotopicexchangeoc-
inamberchemicalfingerprintingforhalfacentury(Becketal.,1964; cursbetweenpolymerizedresinsandtheirsurroundingenvironment
Langenheim and Beck, 1965). FTIR remains an important tool in afterburial(Murrayetal.,1989;NissenbaumandYakir,1995).Thisis
amber research, in part because new technologies coupled to IR becausetheisoprene(C H )buildingblocksofterpenoidcyclichydro-
5 8
microscopesobviatetheneedforanembeddingmedium(typically carbons are especially recalcitrant towards diagenetic isotopic ex-
KBr,whichishygroscopic),facilitatingtheanalysisofmuchsmaller changewithrespecttobothCandH.Inthepresentstudy,δ13Cwas
specimens(e.g.,mg-scale;Tappertetal.,2011;Seyfullahetal.,2015). measuredfrom77Balticamberspecimensandanadditional68speci-
We conducted FTIR micro-spectroscopy on untreated amber flakes mensofBitterfeldamber.δ2Hwasmeasuredfrom34and33specimens
chippedfromfreshsurfacesfreeofinclusions.WealsoobtainedFTIR ofBalticandBitterfeldamber,respectively.Allsampleswerefragments
spectrafrommonomethylsuccinate(99%,Sigma-Aldrich)andthree fromfreshlybrokensurfacescleanedwithdistilledwaterandair-dried,
diterpeneresinacidsthatareimportantconstituentsofEuropeanPaleo- butnotchemicallyorthermallypretreatedinanyotherway.Samplesof
geneambers:abieticacid,dehydroabieticacid,andcommunicacid.The 2–7mgwerecombustedat800°Cfor12hinvacuum-sealedquartz
latterwereisolatedfromnaturalpinaceousresinstoN95%purityatthe glasswithCuO(1g)astheoxygensource.EvolvedCO wasmeasured
2
CanSynInc.Laboratory,Toronto(http://www.cansyn.com/index.html). directlyforδ13C,whereasH OwasreducedtoH withZn(100mg)
2 2
Allspecimensweremountedoninfrared-transparentNaCldiscsandkept priortoδ2Hanalysis.Bothgasesweremeasuredisotopicallywitha
tothicknesses≤10μminordertominimizeoversaturation.Absorption FinniganMAT-252dual-inletisotope-ratiomassspectrometer.Results
spectra were collected over the 700–4000 cm−1 (wavenumber) areexpressedinδnotationas‰relativetoViennaPeeDeeBelemnite
interval (i.e., wavelengths of 2.5–14.0 μm) with a Thermo Nicolet (VPDB)forδ13CandViennaStandardMeanOceanWater(VSMOW)
Nexus470FTIRspectrometerequippedwithaNicoletContinuumIR forδ2H.Analyticprecisionis±0.1‰forδ13Cand±3‰forδ2H.
microscope.Spectralresolutionwas4cm−1andbeamsizewassetbe-
tween50and100μm.Noadditionalmanipulations,suchascontinuum 3.Resultsanddiscussion
removalorsmoothing,wereappliedtothespectra.Furtherdetailsof
ourFTIRmethodologyhavebeenpresentedelsewhere(Wolfeetal., 3.1.Micro-FTIR
2009;Tappertetal.,2011).
Timeofflight-secondaryionmassspectrometry(ToF-SIMS)isan TheFTIRspectraofBalticamberareverysimilartoeachother,irre-
emergingtechnologyingeobiology,largelybecauseitisamenabletoor- spectiveofthecolororexternaltextureofthespecimeninquestion
ganicmoleculesfromavarietyofgeologicalcontexts,andfurthermore (Fig.2A–B).Thisremarkablestabilityhasbeennotedrepeatedlysince
highlyeffectiveincapturingabroadrangeofionizedproductswith thepioneeringinvestigationsofBecketal.(1965),andsuggeststhat
extremelyhighmassresolution(ThielandSjövall,2011).InToF-SIMS all Baltic amber (sensu stricto) shares a common botanical origin
analysis,samplesarebombardedwithahighenergyprimaryion (Wolfeetal.,2009).Thecharacteristicfeatureofthesespectraisthe
stream, producing secondary ions from the analyte surface that “Balticshoulder”situatedbetween1190–1280cm−1,andflankedbya
enterthedetectorandconsequentlyformamassspectrum.Inapply- strongabsorbancepeakat1170cm−1.Thisfeaturereflectsthesucci-
ingToF-SIMStoamber,specimenswerecutwithadiamondblade natecontentoftheamberspecifically,asitisstronglyexpressedin
ultracryomicrotome(LeicaEMUC6)immediatelypriortotheirin- thespectrumofpuremonomethylsuccinate(Fig.2D).WhileBitterfeld
troductionintotheloadlockwithnofurthersamplepreparation. amberalsodisplaysthisspectroscopicfeature,inkeepingwithprior
Threefragmentsofeachambertypewereanalyzedintriplicate.In FTIRanalysesofEuropeansuccinites(Kosmowska-Ceranowicz,1999),
ordertobetterconstraintheionfragmentationpatternsobservedin itsexpressionisfarmoresubdued.Furthermore,theensembleofFTIR
amber,wealsoobtainedduplicateToF-SIMSspectrafrommonomethyl spectrafromBitterfeldamberismorevariablethanthoseobtained
succinate,abieticacid,dehydroabieticacid,andcommunicacid.These fromBalticamber,potentiallyreflectinggreatervariabilitywithregards
standardsweredissolvedinchloroform,driedunderUVandO for toeitherbotanicalaffinityordiagenetichistory.
3
severalminutes,thenspin-coatedontoSiwafersimmediatelyprior Thespectroscopicdifferencebetweenconsensusspectraderivedfrom
toanalysis.WealsoreportexploratoryToF-SIMSanalysesofmodern bothambers(Fig.2C)indicatesthreeregionswhereBalticamberhas
resinsfromexemplarspeciesofthedominantresin-producingconifer consistentlystrongerabsorbancerelativetoBitterfeldamber:(a)thecar-
families:Araucariaceae(Agathisaustralis),Cupressaceae(Metasequoia bonyl(C=O)bandassociatedwithCOOHandhencetotalcarboxylic
glyptostroboides),Pinaceae(Pinuscontorta),Podocarpaceae(Podocarpus acids(1700–1800cm−1);(b)thesuccinateband(1170–1280cm−1);
totara), and Sciadopityacae (Sciadopitys verticillata). These materials and(c)theout-of-planearomaticC–Hband(870–900m−1).Thelatter
originate from our extensive collection (Wolfe et al., 2009; Tappert featureissupportedbythesecondaryC–Hbandat3080cm−1,which
etal.,2011)andwerepreparedforToF-SIMSinmuchthesamewayas isalsobetterdevelopedinBalticamber.Ofthethreediterpeneresin
amberspecimens.Itisimportantnottousechloroformatanystageof acidsanalyzed,thesetwoaromaticC–Hbandsonlyfigurestronglyin
preparation of amber and resin samples, as this induces a range of the spectrum of communic acid. The bands from the abietic and
artefactsowingtopartialsolubilityinthissolvent.ToF-SIMSspectra dehydroabieticacidstandardsthatarebestexpressedinamberspeci-
wereobtainedwithanION-TOFGmbHToF-SIMSIVequippedwithabis- mensarethoseassociatedwithC–HinCH andCH (1380–1400cm−1
2 3
muth(Bi)liquidmetaliongun.TheBi++clusterwasusedastheprimary and1440–1460cm−1,Fig.2),butthesedonotcontributetoanypro-
3
ionbeamoperatedinhighmassresolutionbunchedmode(Sodhi,2004). nouncedspectroscopicdifferencesbetweenBalticandBitterfeldamber.
Theionbeamwasrasteredoveranareaof500×500μmfor60–120sin FromthesynthesisofFTIRresults,weareabletosurmisethatBaltic
ordertoremainbelowstaticlimits.Chargeneutralizationwasachieved amber, on the whole, contains more succinate relative to Bitterfeld
usinglowenergy(b20eV)electronssuppliedbytheinstrument'spulsed amber,andlikelymorecommunicacid.
electronfloodgun.Althoughbothpositiveandnegativepolarityspectra
wereobtained,wereportonlyresultsobtainedinnegativemode,which 3.2.ToF-SIMS
have proven more interpretable in pilot studies (Sodhi et al., 2013,
2014).ThereproducibilityofToF-SIMSspectrawasexcellentforbotham- BuildingonresultsfrompreliminarystudiesaddressingtheToF-
bers,resins,andstandards.TheIllustratedToF-SIMSspectraareentirely SIMS spectra of ambers obtained in negative polarity (Sodhi et al.,
24 A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32
Fig.2.FTIRspectraof(A)Baltic(red)and(B)Bitterfeldamber(blue)specimens.Illustratedspectrafrombothambershavebeencombinedwithadditionalmeasurements(n=8ineach
case)toyieldconsensusspectra(boldlines),averagedfromindividualspectrarescaledtoacommonrangeof0–1relativeabsorbanceunits.(C)ThedifferencebetweenBalticand
Bitterfeldconsensusspectraisshownwithshadedareasindicatingonestandarddeviationfromthemean.(D)FTIRspectraobtainedfrompurifiedmonomethylsuccinateandditerpene
resinacids.Verticalgraybarsindicatesalientspectroscopicfeaturesdiscussedinthetext.
2013),wefocusontwomassintervalsofparticularinterest:the“succi- massspectrainthesuccinateregiondeviatebyb0.02ufromthethe-
nateregion”(70–120u;Fig.3)andthe“diterpeneresinacidregion” oreticalmonoisotopicmassesof[M–H]−ionsproducedbytheiden-
(250–350u;Fig.4).Theseareaddressedsequentiallybelow.Realized tifiedparentmolecules,andbyb0.05uinthediterpeneresinacid
A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32 25
Fig.3.NegativepolarityToF-SIMSspectraofthe70–120uregionfor(A)Balticamber,(B)Bitterfeldamber,and(C)monomethylsuccinate.Theentirerangeisshowntotheleft,alongside
expansionsoftheshadedregionsthatincludepeaksinthe72.8–73.2u,98.8–99.2u,and116.8–117.2uranges,primarilyassociatedwithionsfromtheillustratedparentmolecules.Black
arrowsindicatethetheoreticalmonoisotopicmassofnegativeionsofpropanoic(=propionic)acid,immediatelyadjacentpeaksattributedtoC6H−,showningray.
region.Suchlevelsofaccuracyandprecisionareconsistentwith providenoconclusiveevidenceforthepresenceofnativepropanoic
thosereportedincurrentgeobiological applicationsofToF-SIMS acidinamber.Moreover,theToF-SIMSpeaksobtainedinthe73ure-
(Leefmannetal.,2013). gionarenotascleanandunimodalasthoseattributedtosuccinicacid
Negativeionsthatoriginatefromsuccinicacid(C H O−)producea (117u)andsuccinicanhydride(99u),oftenyieldingasecondpeakat
4 5 4
clear[M–H]−peakat117uinToF-SIMSspectrafrombothBalticand slightlylowermass(Fig.3).Theexceptionalresolutionaffordedby
Bitterfeld ambers (Fig. 3A–B). This peak is much stronger in Baltic ToF-SIMSallowscleardifferentiationofpeaksoccurringat73.02uand
amber relative to Bitterfeld amber, consistent with a greater total 74.04u,forwhichonlythelattercanbeattributedtopropanoicacid.
succinate content and the results from FTIR. Not surprisingly, the The73.02upeakisattributedtoC H−,forwhichtheparentmolecule
6
117upeakisevenmorepronouncedinthemassspectrumofpure maybehexatriyne.Becausethispeakisbetterexpressedintheambers
monomethylsuccinate(Fig.3C).Relatedpeaksarethoseassociated relativetothemonomethylsuccinatestandard,itisprovisionallyasso-
withdehydration anddecarboxylation of succinic acid,namelythe ciatedwithphotodissociationduringresinpolymerization.However,
ionsoccurringat[(M–H)–H O]−=99uand[(M–H)–CO ]−=73u, untilfurtherstudyisconducted,werestrictallsubsequentconsider-
2 2
whichreflecttheparentmoleculessuccinicanhydrideandpropanoic ationstothe73.04upeakproducedbythe[M–H]−ionofpropanoic
(= propionic) acid, respectively. The acid anhydride is not well acid.
expressedinthemonomethylsuccinatemassspectrum,implyingthat Despitethesecomplexities,therecognitionofthreedistinctpeaks
itspresenceinamberrelatestoprocessesassociatedwithgeological associatedwithsuccinicacid,theacidanhydride,andsecondaryfrag-
maturation.Thisisimportantbecausethesuccinicanhydridepeakis mentsofparentmoleculescontainingsuccinate,whichmayinclude
greater in both ambers than that of succinic acid, and particularly an assortment of fenchyl and bornyl succinates known to occur in
intenseinthecaseofBalticamber(Fig.3A).Wethusconsidersuccinic both Baltic and Bitterfeld ambers (Yamamoto et al., 2006), should
anhydridetoreflectthedegreesofdehydrationduringmaturationof beconsideredapowerfuldiagnostictoolandanovelapplicationof
thetwoambers. ToF-SIMS.Moreover,thesuccinateregionofamberToF-SIMSspectra
Thecaseofpropanoicacidismorecomplicatedforseveralreasons. alsoincludespeaksat80,85and97u(Fig.3),whichcorrespondto
AlthoughnegativeionsofthismoleculehavebeenidentifiedinBaltic the[M–H]−ionsC H O−,C H O−,andC H O−,respectively.While
5 4 4 5 2 5 5 2
amberbefore(Tonidandeletal.,2009),innaturepropanoicacidresults the85upeakexistsinbothambers,andlikelyrepresentsvinylacetate,
from microbially-assisted decarboxylation of succinic acid esters the80and97upeaksaremuchbetterexpressedintheBitterfeldmate-
(e.g.,Whiteley,1953).Becausepropanoicacidisequallywellrepresent- rial.Althoughtheparentmoleculesforthesepeaksremainelusive,they
edinthemassspectraofmonomethylsuccinateasinthoseofamber nonethelessassistindifferentiatingtheambersgeochemically.
samples,weinferthatitoriginatesfromsecondaryfragmentationof Withrespecttothediterpeneresinacids,theavailabilityofpurified
ionizedsuccinicacidduringToF-SIMSanalysis,suchthatourresults crystallinestandardsofkeychemicalspeciesprovesinvaluablewith
26 A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32
Fig.4.NegativepolarityToF-SIMSspectraofthe250–350uregion(left)andexpansionofthe295–305uregion(right)for(A)Balticamber,(B)Bitterfeldamber,(C)dehydroabietic
acid,(D)abieticacid,and(E)communicacid.Structuresaregivenforparentmolecules,whereasblackarrowsindicatepeaksat317and333ucorrespondingtooxygenadditionsto
the[M–H]−=301upeak.Shadedzones(leftpanels)indicatetheregionmagnifiedatright,inwhichshadedlinesindicatethemassesofprimarymolecularionsassociatedwithditerpene
resinacids.
respecttounderstandingtheirdistributioninamber(Fig.4).Further- molecular ions are commonly preserved in mass spectra (Dethlefs
more,thetricyclicstructureofditerpeneresinacidsconfersconsider- etal.,1996;Diefendorfetal.,2012).NegativepolarityToF-SIMSspectra
ablestabilityofthesemoleculestowardsfragmentation,implyingthat ofBalticandBitterfeldambersbothexhibitstrong[M–H]−peaksat299
A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32 27
Fig.5.StableisotopicratiosofBaltic(red)andBitterfeld(blue)ambers.Rawdata(AandC)andprobabilitydensityfunctions(BandD)areshownsequentiallyfortheδ13Candδ2H
valuesobtainedfrombothmaterials.In(B),thedashedlinesandshadedareasaremeanand2S.D.rangesofvaluescompiledvaluesfromTappertetal.(2013)forcompilationsof
Miocene–Oligoceneambers(fromGermancoal,Mexico,DominicanRepublic,MalaysiaandBorneo)comparedtoEoceneambers(fromWashingtonState,theCanadianHighArctic
andkimberlite-hostedsediments,aswellasBalticamberbutexcludingBitterfeld).In(D),additionalaxeshavebeenaddedforinferredδ2Hplantwaterusingaconstantfractionationof
−229‰betweenamberandenvironmentalwater(Wolfeetal.,2012),andaprovisionaltemperaturescalebasedonthemodernrelationshipbetweenδ2HandairtemperatureatKraków,
Poland.Thedashedlinein(D)separatesmesothermalfromtropicalclimatesinKöppen–Geigerclassification.
28 A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32
and 301 u. The former corresponds to primary molecular ions of amber(Eocene)ashypothesized,itshouldproducemeanδ13Cvalues
dehydroabieticacid(C H O ),whilethelattermaybeattributableto thatareabout2‰moredepletedrelativetoBalticamber,whichisnot
20 28 2
eitherabieticorcommunicacids(bothC H O ),oracombinationof observedintherawdata(Fig.5B).Thesimplestinterpretationofthe
20 30 2
both.Thepeaksobservedat303uareattributabletoC H O diter- closesimilaritybetweenBitterfeldandBalticamberδ13Cvaluesisthat
20 32 2
pene resin acids having a molecular mass of 304 u, for example they are equivalent in age, despite the compositional differences
dihydroisopimaric acid, 8-abietenic acid, and 8(14)-abietenic acid borneoutoftheirorganicgeochemicalcharacterization.
(Sodhietal.,2014).The301upeakalsoproducescorresponding Withrespecttoamberhydrogenstableisotopes(Fig.5C–D),thesit-
[(M–H)+O]−and[(M–H)+O ]−peaksobservedinbothambers uationdiffersmarkedlyfromthecarbonresults,notingthatallδ2H
2
(i.e.,317and333u).Ofthediterpeneresinacidsconsideredhere, valueswereobtainedfromasub-setoftheexactsamesamplesanalyzed
onlyabieticacidproducessecondarypeaksassociatedwithoxygen- forδ13C.Balticamber(meanδ2H=−277±22‰)isconsistentlymore
ation, and thus it clearly represents an important component of depletedrelativetoBitterfeldamber(meanδ2H=−256±9‰).This
bothambers.Ontheotherhand,the299upeakassociatedwith differenceishighlysignificant(Pb0.0001;t=5.28;d.f.=99).Because
dehydroabieticacidisonaveragethree-foldstrongerinBitterfeld amber δ2H is ultimately modulated by the isotopic composition of
amberrelativetoBalticamber,implyingconsiderablygreaterconcen- source waters accessed by trees at the time of resin biosynthesis
trations of this diterpene resin acid. Although ToF-SIMS spectra of (McKellaretal.,2008;Wolfeetal.,2012),thisresultimpliesthat,onav-
abieticandcommunicacidsareindistinguishablefromeachotherin erage,forestsresponsiblefortheformationofBalticamberexploited
the295–305urange(Fig.4),wehavealreadyshown,onthebasisof watersthatwere20‰lighterthanthoseinvolvedinthegenesisof
FTIR,thatcommunicacidismoreabundantinBalticamber. Bitterfeldamber.Themostparsimoniousinterpretationofthisdiffer-
WhatemergesfromthecombinedFTIRandToF-SIMSresultsisa enceisthat,whereasthedrainagesourceforBitterfeldamberlaywell
geochemicaldifferentiationbetweenBalticandBitterfeldambersthat tothesouthofthedeposit,thatofBalticamberwassituatedtothe
isconsistentbetweenanalyticalplatforms:Balticamberhasagreater northinScandinavia.Suchascenarioisreadilyaccommodatedbythe
apportionmentof succinic andcommunic acids, whereasBitterfeld paleogeography of regions bordering the Eocene North Sea basin
ambercontainsmoredehydroabieticacid.Thelatterobservationis (Wimmeretal.,2009;Fig.6).
entirelyconsistentwithpriorresultsobtainedoncomparablematerials Modernenvironmentsprovideadditionalcontextwithwhichthe
usinggaschromatography–massspectrometry(GC–MS;Yamamoto δ2HdifferencebetweenBalticandBitterfeldamberscanbeappreciated.
et al., 2006). Thus, from the perspective of organic geochemistry Forexample,inmodernPinusresinssampledbetweenScotlandand
alone,BalticandBitterfeldambersappeartobecompositionallydistinct Cyprus,a20‰differenceinδ2Hcorrespondsto~7°oflatitude(Stern
inanumberofsubtleyetreproducibleways.Inmakingthisstatement, etal.,2008).WithinthemodernisoscapeofcentralEurope,adifference
weadvocatestronglytheadvantagesofFTIRandToF-SIMSasanalyses of20‰inleaf-waterδ2Hrepresentsapproximately800kmoflatitude
conducted on amber in the solid state, thereby eliminating issues (Westetal.,2008).Bothoftheseexamplesareentirelyconsistents
associatedwiththesematerialsbeingonlypartiallysolubleinorganic withtheenvisagedpaleogeographyatthetimeofamberformation
solvents(Millsetal.,1984).Forthisreason,wedisagreewiththecom- (Fig.6).Weconcludefromtheamberδ2Hresultsthatforestsresponsi-
mentofAndersonandBotto(1993,pp.1037)that,withrespectto blefortheproductionofBalticandBitterfeldambersaccessedsource
glessitefromBitterfeld(e.g.,Fig.1J)andgenuineBalticamber:“theex- watersoriginatingfromfundamentallydifferentsectorsoftheEocene
tentofsimilarityoftheseresinitesissuch,thatthevalidityofcontinued NorthSeadrainage:theScandinavianhighlandsinthefirstcase,and
distinctionbetweenthemis,onchemicalgroundsatleast,unjustified.” theParatethyansectorofcentralEuropeinthesecond.
Fromourperspective,werestrictthecompositionalsimilaritybetween AsperhapsthemostsalientgeochemicaldifferencebetweenBaltic
BalticandBitterfeldamberstostatingthatbothclearlybelongtoClassIa andBitterfeldamber,thehydrogenisotopicmeasurementsmeritfur-
resinites,i.e.,thosecontainingditerpenesbasedonlabdanoidskeletal ther discussion.First,wenotethattherange of results fromBaltic
structuresinthepresenceofsuccinicacid,andarangeofassociated amberisconsiderablylargerthanthatobtainedfromBitterfeldamber
alcoholsandesters(Andersonetal.,1992). (Fig.5C).ThissuggeststhattheclimateenveloperealizedduringBaltic
amberformationwasbroader,andbyinferencelonger,thanthatasso-
3.3.Stableisotopes ciatedwithBitterfeldamber.Second,althoughBitterfeldamberpro-
ducesamoreenrichedmeanδ2Hsignature,Balticambernonetheless
The carbon stable isotopic compositions of Baltic and Bitterfeld producesnumerousmeasurementsthatareequallyenriched,resulting
ambers are virtually identical, yielding means of δ13C = −23.6 ±
1.0‰and−23.9±1.7‰,respectively(Fig.5A).Inageneralsense,the
δ13Cofamberreflectsthelocalizeddegreeoftreeecophysiological
stressatthetimeofresinproduction,withgreaterstressresultingin
lesseffectiveisotopicdiscriminationagainst13C(i.e.,higherδ13Cvalues;
McKellar et al., 2011). When probability distribution functions are
appliedtotheδ13CresultsfromBalticandBitterfeldambers(Fig.5B),
aprincipalmodeisapparent,flankedbytwoshouldersthatpresumably
reflectoccasionsofresinproductionunderalternatelyluxuriant(most
depletedδ13Cvalues)andstressed(enriched)environmentalcondi-
tionsthatdeviatefromthecentralmodesexpressedinbothdeposits.
Thesesimilaritiesaresuchthatδ13Cvaluesdonotdifferentiatethe
twoambers.Indeed,thepopulationsofBalticandBitterfeldambers
δ13Cvaluesdonotyieldstatisticallysignificantdifferencesunderanun-
pairedt-test(P=0.27;t=1.11;d.f.=109).
Moreover,attheglobalscale,amberδ13Cisalsoinfluencedbyatmo-
sphericcompositionwithrespecttoCO andO partialpressures,yield-
2 2
ingarobustseculartrendof~5‰towardsmoredepletedvaluesover
the past 50 Ma that is superposed upon the localized influences Fig.6.AproposedpaleogeographicscenariofortheprovenanceofBalticandBitterfeld
ambersduringthemiddleEocene.ShadedareasrepresenttheEocenelandmass,whereas
discussedabove(Tappertetal.,2013).Thisimpliesthat,ifBitterfeld
dashedlinesaremoderncoastlines,followingScoteseetal.(1989)andWimmeretal.
amber were truly younger (e.g., Miocene–Oligocene) than Baltic (2009).Darkgrayregionsapproximatethelociofamberdeposition.
A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32 29
inasecondarymodeintheprobabilitydistributionofisotopicvalues Weawaitunbiased,abundance-weightedcensusesatsufficienttaxo-
(Fig.5D).Thisobservationisinkeepingwiththeentomologicalrecord nomicresolutiontotestthishypothesis,acknowledgingthatseveral
ofBalticamber,whichcontainsadmixturesoftaxawithalternately collectionsaresufficientlyrichtoundertakethisactivitysystematically.
tropical and boreal ecological affinities (Larsson, 1978; Weitschat, Wefurtherexplorepaleoclimaticsignificanceofamberδ2Hresults
1997;WeitschatandWichard,2002;WeitschatandWichard,2010). byderivingprovisionaltemperatureestimatesasfollows.First,values
However,ifourisotopicresultsarebroadlyrepresentativeofbothde- ofamberδ2Hwereconvertedtoδ2H usingaconstantfraction-
plantwater
posits,wepredictthatBitterfeldambershouldcontainagreateroverall ationof−229‰betweenamberandenvironmentalwater(Chikaraishi
proportionofwarmstenothermousarthropodsrelativetoBalticamber. etal.,2004;Wolfeetal.,2012).Then,usinganappropriatemodern
Fig.7.NegativepolarityToF-SIMSspectraspanningthesuccinateregion(70–120u)forBalticandBitterfeldambers(A–B)andmodernresinsrepresentativeofvariousconiferfamilies
(C–G).Verticalshadinghighlightsdominantions,andarrowsindicateionsderivedfromsuccinicacidandsuccinicanhydridepeaks,asdiscussedinthetext.Duetobetween-sample
differencesinabsoluteintensities,allspectrahavebeenrescaledtothemaximumpeakheightinthe70–120urange.
30 A.P.Wolfeetal./ReviewofPalaeobotanyandPalynology225(2016)21–32
temperature–δ2HrelationshipfromcentralEurope,weareabletore- Botto,1993),thisisanimportantconfirmatoryresult.Indeed,conifers
scaleδ2H asafirstapproximationofcorrespondingambient ofthefamilySciadopityaceaehavebeenproposedasapotentialsource
plantwater
temperatures.Weusethestrongrelationshipbetweenairtemperature forBalticamberbasedonextensivecomparativeanalysesofFTIRspec-
andprecipitationδ2HatKraków,Poland(50.1°N,19.9°E),whichyields tra(Wolfeetal.,2009),whereasvariousPinaceaehavebeeninvokedin
δ2H=2.6(temperature)–91.8(R2=0.59;n=406).Thisisamong thissameregardforwelloveracentury,largelyonthebasisoftheanat-
themostintenselysampledEuropeanstationsintheGlobalNetwork omyofwoodyinclusions(GoeppertandMenge,1883;Dolezychetal.,
ofIsotopesinPrecipitation(GNIP)program(IAEA/WMO,2014).Results 2011).FossilsbelongingtobothfamiliesarepresentinBalticamber,al-
usingotherdenselysampledstations,suchasLeipzig(R2=0.35; thoughpinaceouselementsappearmoreconspicuous(Weitschatand
n=341)andBerlin(R2=0.32;n=273),yieldsimilarresultsbut Wichard,2002).AdditionalToF-SIMS[M–H]−peaksthatareconsistent-
sufferfromlowercoefficientsofdeterminationbetweenδ2Hand lyexpressedinthesuccinateregionofmodernconiferresinsinclude
temperature.Resultingδ2H andtemperatureestimatesare thoseat73u(C H O−,whichmustbedifferentiatedfromC H−as
plantwater 3 5 2 6
portrayedasadditionalaxesontheamberδ2Hprobabilitydistributions discussedearlier),80u(C H O−),85u(C H O−),and97u(C H O−).
5 4 4 5 2 5 5 2
(Fig.5D).Inthismodel,themeantemperaturesassociatedwithBaltic Two peaks are specific to individual species: 79 u (C H O−) in
5 3
andBitterfeldambersare17°Cand25°C,respectively,flankingthe Sciadopitysverticillataand112u(C H O−)inAgathisaustralis.Further
6 8 2
18°Cthresholdofmeanannualtemperaturethatdistinguishestropical differentiationbetweenthemodernresinscanbegleanedfromthe
and mesothermal climate regimes (Peel et al., 2007). While both diterpene resin acid region of their ToF-SIMS spectra (Table 1).
ambers produce a large number of samples with tropical isotopic AlthoughweareonlybeginningtoexploitToF-SIMSasastrategyin
signatures,asdiscussedabove,onlyBalticamberproducesvaluescon- resinchemotaxonomy,andclearlyadditionalanalysesaredesirable,
sistentwithtemperaturesb15°C.Ofcourse,wedonotconsiderthese theinitialresultspresentedhereportendconsiderablepotentialfor
reconstructionstobedefinitive;theyarepresentedasanexploratory thisapproach.
tool grounded in the premise that the systematics of isotopes in The relationships between amber and modern resin ToF-SIMS
precipitationduringthePaleogeneweremechanisticallysimilartothe spectra can be formalized by hierarchical cluster analysis of the
modernworld,despitedifferencesinprecipitationquantity,ambient dominantrecurrentpeakslistedinTable1.Weappliedclusteringby
temperature,andseasonality(Wolfeetal.,2012).Nonetheless,the centroidlinkagetomatricesofEuclideandistancesusingClusterand
results appear realistic in the sense of their consistency with the TreeViewsoftwarepackages(Page,1996;deHoonetal.,2004).Raw
paleoentomologicalrecord,particularlythatofBalticamber.Having data(i.e.,intensitiesnormalizedtoarangeof0–100)werenottrans-
nowestablishedongeochemicalgroundsnumerousdistinctionsbe- formedinanyotherwaypriortoclustering.Whenpeaksrestrictedto
tweenBalticandBitterfeldamber,thelargestremainingquestionper- thesuccinateregionareconsidered(Fig.8A),onlySciadopitysverticillata
tainstothebotanicaloriginofthesedeposits. resinclusterswiththeambersamples,revealinganespeciallyclosesim-
ilaritytotheBitterfeldmaterial.ResinsfromAgathis,Pinus,Metasequoia
4.ToF-SIMSspectraofmodernresinsandthebotanicaloriginof andPodocarpusformasecondhighordergrouping.Inasecondand
succinites moreinclusiveanalysis,peaksrepresentingmolecularionsofditerpene
resinacids(i.e.,299–303u)wereincludedinadditiontothosefromthe
NegativepolarityToF-SIMSspectraofmodernconiferresinsassistin succinateregion,yieldingadendrogramwherePinusjoinsSciadopitysas
elucidatingthebotanicalprovenanceofBalticandBitterfeldamber. themodernresinswiththehighestdegreeofsimilaritytotheambers
Of the modern taxa analyzed, only Pinus contorta and Sciadopitys (Fig.8B).Collectively,theseanalyseseffectivelyeliminateconifersof
verticillatapreservepronouncedpeaksassociatedwithnegativeions thefamiliesAraucariaceae,Cupressaceae,andPodocarpaceaeaspoten-
fromsuccinicacidanditsacidanhydride(Fig.7).Assuccinicacidis tialsourcesforEuropeansuccinites,leavingeitherextinctpinaceousor
thedefiningbiomarkerforsucciniteambervarietalsincludingboth sciadopityaceoustreesasthemostviablecandidatesourceplants.Al-
Baltic and Bitterfeld ambers (i.e., Class Ia resinites, Anderson and though the clustering exercises (Fig. 8) illustrate the longstanding
Table1
Relativeintensitiesofdominant[M–H]−peaksobservedinthesuccinateandditerpeneresinacidregionsofToF-SIMSspectrafrommonomethylsuccinate,BalticandBitterfeldambers,
andvariousmodernconiferresins.Valuesarescaledtothemaximumpeakintensitywithineachregion.
Succinateregion
Relativeintensitiesofdominantpeaks(maximumpeakin70–120urange=100)
Analyte n 73.04u S.D. 85.09u S.D. 97.04u S.D. 99.02u S.D. 117.03u S.D.
Monomethylsuccinate 2 25.94 3.43 2.28 0.52 7.90 3.19 9.59 1.33 100.00 0.00
Balticamber 3 41.83 9.10 22.71 0.93 14.93 4.20 100.00 0.00 25.00 3.29
Bitterfeldamber 3 19.40 7.02 24.47 3.32 78.84 25.63 39.14 9.34 6.48 0.64
Agathisaustralisresin 3 73.06 0.03 79.81 17.63 27.79 6.52 14.24 6.21 5.58 1.56
Metasequoiaglyprostroboidesresin 3 25.07 5.66 100.00 0.00 18.06 3.12 7.50 1.11 1.72 0.20
Pinuscontortaresin 3 59.59 3.62 95.01 8.65 36.10 4.40 24.31 5.40 8.28 0.51
Podocarpustotararesin 3 25.15 2.23 100.00 0.00 17.03 6.03 11.72 3.74 1.99 0.54
Sciadopitysverticillataresin 3 24.46 2.53 20.14 8.02 46.19 13.10 26.83 8.64 18.12 9.51
Diterpeneregion
Relativeintensitiesofdominantpeaks(maximumpeakin298–304urange=100)
Analyte n 299.21u S.D. 300.17u S.D. 301.22u S.D. 302.22u S.D. 303.24u S.D.
Monomethylsuccinate 2 5.19 0.58 1.84 0.69 100.00 0.00 9.10 0.05 4.17 1.21
Balticamber 3 35.76 9.12 8.79 0.53 100.00 0.00 28.42 3.96 46.34 14.30
Bitterfeldamber 3 98.83 2.03 26.52 2.37 94.63 5.83 29.20 2.70 56.50 3.29
Agathisaustralisresin 3 12.12 7.26 4.42 1.91 100.00 0.00 25.84 2.09 11.89 6.54
Metasequoiaglyprostroboidesresin 3 7.49 1.17 3.06 0.44 100.00 0.00 22.68 2.40 14.19 3.95
Pinuscontortaresin 3 90.75 9.46 25.24 1.25 88.02 20.74 21.68 3.37 50.87 7.69
Podocarpustotararesin 3 4.17 0.59 1.85 0.57 100.00 0.00 22.67 3.16 10.70 3.83
Sciadopitysverticillataresin 3 95.96 7.00 50.37 2.28 79.69 2.32 45.70 3.24 91.54 12.18