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Asensationaldiscovery:TracesofrainforestsinWestAntarctica

ClimateHistory:NewstudyledbyAWIandincooperationwithKielUniversityprovidesunexpectedevidenceforexceptionallywarmclimateneartheSouthPoleduringtheCretaceous

AninternationalteamofresearchersledbygeoscientistsfromtheAlfredWegenerInstitute,HelmholtzCentreforPolarandMarineResearch,havenowprovidedanewandunprecedentedperspectiveontheclimatehistoryofAntarctica.InasedimentcorecollectedintheAmundsenSea,WestAntarctica,inFebruary2017,theteamdiscoveredpristinelypreservedforestsoilfromtheCretaceous,includingawealthofplantpollenandsporesandadensenetworkofroots.Theseplantremainsconfirmthat,roughly90millionyearsago,thecoastofWestAntarcticawashometotemperate,swampyrainforestswheretheannualmeantemperaturewasca.12degreesCelsiusanexceptionallywarmclimateforalocationneartheSouthPole.TheresearcherssurmisethatthiswarmthwasonlypossiblebecausetherewasnoAntarcticicesheetandbecausetheatmosphericcarbondioxideconcentrationwassignificantlyhigherthanindicatedbyclimatemodelstodate.Thestudy,whichprovidesthesouthernmostdirectlyassessableclimateandenvironmentaldatafromtheCretaceousandposesnewchallengesforclimatemodellersaroundtheglobe,wasreleasedtodayinthejournalNATURE.

Themid-Cretaceoustimeinterval,fromca.115millionto80millionyearsago,isnotonlyconsideredtheageofthedinosaurs,butwasalsothewarmestperiodinthepast140millionyears.Seasurfacetemperaturesinthetropicsatthistimewerelikelyashighasca.35degreesCelsius,andsealevelwas170metreshigherthantoday.YetwestillknowverylittleaboutenvironmentalconditionsintheCretaceoussouthofthepolarcircle,sincetherearevirtuallynoreliableclimatearchivesthatextendthatfarbackintime.ThenewsedimentcoreofferstheteamofexpertsthefirstchancetoreconstructtheWestAntarcticclimateduringthewarmestintervaloftheCretaceous,thankstotheuniqueevidenceitcontains.

Inthesedimentcore,whichtheteamcollectedwiththeUniversityofBremensseafloordrillrigMARUM-MeBo70nearthePineIslandGlacieronanRVPolarsternexpedition,theyfoundpristinelypreservedforestsoilfromtheCretaceous.Duringtheinitialshipboardassessments,theunusualcolourationofthesedimentlayerquicklycaughtourattention;itclearlydifferedfromthelayersaboveit.Moreover,thefirstanalysesindicatedthat,atadepthof27to30metresbelowtheoceanfloor,wehadfoundalayeroriginallyformedonland,notintheocean,reportsfirstauthorDrJohannKlages,ageologistattheAWI.

Evidenceofaswampylandscaperichinvegetation

YetitdidnotbecomeclearjusthowuniquetheclimatearchivetrulywasuntilthesedimentcorewassubjectedtoX-raycomputedtomography(CT)scans.TheCTimagesrevealedadensenetworkofrootsthatspreadthroughtheentiresoillayeroffine-grainedclayandsilt,andwhichwassowell-preservedthattheresearcherscouldmakeoutindividualcellstructures.Inaddition,thesoilsamplecontainscountlesstracesofpollenandsporesfromvariousvascularplants,includingthefirstremnantsoffloweringplantseverfoundatthesehighAntarcticlatitudes.

Thenumerousplantremainsindicatethat93to83millionyearsagothecoastofWestAntarcticawasaswampylandscapeinwhichtemperaterainforestsgrewsimilartotheforeststhatcanstillbefound,say,onNewZealandsSouthIsland,explainsco-authorProfUlrichSalzmann,apalaeoecologistatNorthumbriaUniversityinNewcastleuponTyne.

Theresultsofthevegetationanalysispuzzledtheresearchers:underwhatclimaticconditionscouldtemperaterainforestshaveformedbackthenatageographiclatitudeofroughly82degreesSouth?EvenduringtheCretaceous,theAntarcticcontinentwasattheSouthPole,whichmeanstheregionwheretheforestsoiloriginatedwassubjecttoafour-monthpolarnight;forathirdofeveryyear,therewasnolife-givingsunlightatall.

TogetabetterideaofwhattheclimatewaslikeinthiswarmestphaseoftheCretaceous,wefirstassessedtheclimaticconditionsunderwhichtheplantsmoderndescendantslive,saysJohannKlages.Theresearcherssubsequentlysearchedforbiologicalandgeochemicaltemperatureandprecipitationindicatorsinthesoilsample,onthebasisofwhichtheycouldreconstructtheairandwatertemperatureintheWestAntarcticrainforests,aswellastheamountofprecipitationtheyreceived.

Numerousanalyses,oneresult:IntheCretaceous,Antarcticawasice-freeandextremelywarm

Theoutcomesofthevariousanalysesfittogetherlikethepiecesofapuzzle:Roughly90millionyearsago,therewasatemperateclimatejust900kmfromtheSouthPole.TheamountandintensityofrainfallinWestAntarcticaweresimilartothoseintodaysWales.Theannualmeanairtemperaturewascirca12degreesCelsius;inotherwords,backintheCretaceous,theaveragetemperatureneartheSouthPolewasroughlytwodegreeswarmerthanthemeantemperatureinGermanytoday.Summertemperaturesreachedabout19degreesCelsiusonaverage.Watertemperaturesintheriversandswampswereupto20degreesCelsius.OurresultsshowthatthisregionofAntarcticawasabout25degreesCelsiuswarmerinsummerthantoday,saysco-authorDrThorstenBauersachs,organicgeochemistandgeo-microbiologistfromtheInstituteofGeosciencesatKielUniversity.ThegeoscientistcontributedtoreconstructingthetemperaturesofWestAntarcticaonthebasisofmolecularproxyinvestigationsonsedimentsfromthedrillcore,whichwasobtainedduringtheexpeditionwiththeresearchvesselPolarstern.

Theresearchersthenusedthisnewvegetation,temperature,andprecipitationdatafromWestAntarcticaastargetvaluesforsimulationsofthemid-Cretaceousclimate.Theircalculationswithapalaeoclimatemodelrevealedthatthereconstructedconditionscouldonlybeachievedwhen(1)theAntarcticcontinentwascoveredwithdensevegetation,(2)therewerenoland-icemassesonthescaleofanicesheetintheSouthPoleregion,and(3)thecarbondioxideconcentrationintheatmospherewasfarhigherthanpreviouslyassumedfortheCretaceous.Beforeourstudy,thegeneralassumptionwasthattheglobalcarbondioxideconcentrationintheCretaceouswasroughly1000ppm.Butinourmodel-basedexperiments,ittookconcentrationlevelsof1120to1680ppmtoreachtheaveragetemperaturesbackthenintheAntarctic,saysco-authorandAWIclimatemodellerProfGerritLohmann.

Accordingly,thestudyshowsboththeenormouspotencyofthegreenhousegascarbondioxide,andhowessentialthecoolingeffectsoftodaysicesheetsare.WenowknowthattherecouldeasilybefourstraightmonthswithoutsunlightintheCretaceous.Butbecausethecarbondioxideconcentrationwassohigh,theclimateattheSouthPolewasneverthelesstemperate,withouticemasses,explainsco-authorDrTorstenBickert,ageoscientistattheUniversityofBremensMARUMresearchcentre.

Thebigquestionnowis:ifitbecamesowarmintheAntarcticbackthen,whatcausedtheclimatetosubsequentlycoolsodramaticallytoformicesheetsagain?Ourclimatesimulationshaventyetprovidedasatisfactoryanswer,saysGerritLohmann.Findingthecausesofthesetippingpointsisnowakeychallengefortheinternationalclimateresearchcommunity.

Informationformembersofthepress:


ThesedimentcoreanalysedinthestudywascollectedintheAmundsenSeaduringtheGermanpolarresearchvesselPolarsternsexpeditionPS104(6February19March2017).ThiswasaccomplishedwiththeseafloordrillrigMARUM-MeBo70,whichwasdevelopedatMARUMinBremenandusedforthefirsttimeintheAntarctic.TheX-raycomputedtomography(CT)scanswereconductedattheKlinikum-MittehospitalinBremen.TheresearchprojectandPolarsternexpeditionPS104asawholewerejointlyfinancedbytheAWI,MARUM,theBritishAntarcticSurveyandtheUKdivisionoftheInternationalOceanDiscoveryProgram(UK-IODP).

Expertsfromthefollowinginstitutionsparticipatedinthestudy:

  • AlfredWegenerInstitute,HelmholtzCentreforPolarandMarineResearch(AWI),Bremerhaven
  • NorthumbriaUniversity,NewcastleuponTyne,GreatBritain
  • MARUMCenterforMarineEnvironmentalSciences,UniversityofBremen
  • BritishAntarcticSurvey,Cambridge,GreatBritain
  • FacultyofGeosciences,UniversityofBremen
  • ImperialCollegeLondon,GreatBritain
  • UniversityofSouthampton,GreatBritain
  • InstituteofGeosciences,KielUniversity
  • InstituteforGeophysicsandGeology,LeipzigUniversity
  • PolishAcademyofSciences,Krakow,Poland
  • DepartmentofGeology,TrinityCollegeDublin,Ireland
Reconstruction of West Antarctica about 90 million years ago.
AlfredWegenerInstitute/J.McKay(CreativeCommonslicenceCC-BY4.0)

ReconstructionofWestAntarcticaabout90millionyearsago.Researchersfoundevidenceofatemperate,swampyrainforestwheretheAntarcticicesheetnowcoverstheground.

View on ice floes from the ship bridge of the research vessel Polarstern.
ThomasRonge,AWI

TheGermanresearchvesselPolarsternunderthedirectionoftheAlfredWegenerInstitute(AWI)intheAmundsenSeaofWestAntarctica.

Originalpublication

ThestudywasreleasedtodayinthejournalNATURE,underthefollowingoriginaltitle:Klages,J.P.etal:TemperaterainforestsneartheSouthPoleduringpeakCretaceouswarmth,Nature,DOI:10.1038/s41586-020-2148-5
 www.nature.com/articles/s41586-020-2148-5

AcademiccontactattheAlfredWegenerInstitute:

DrJohannKlages
Geologistandfirstauthor
+49(0)471/4831-1574
Johann.Klages@awi.de

DrKarstenGohl
Geophysicist,expeditionleaderandco-author 
+49(0)471/4831-1361
Karsten.Gohl@awi.de

ProfGerritLohmann
Palaeoclimatemodellerandco-author
+49(0)471/4831-1758
Gerrit.Lohmann@awi.de

AcademiccontactatKielUniversity:

DrThorstenBauersachs
InstituteofGeosciences/OrganicGeochemistry
+49(0)431/880-3694
thorsten.bauersachs@ifg.uni-kiel.de

Presscontact

FriederikeBalzereit
ScienceCommunication
KielMarineScience/KielUniversity
+49(0)431/880-3032
fbalzereit@uv.uni-kiel.de