Sea ice variations in the central Canadian Arctic Archipelago during the Holocene

13 Pages·2009·0.46 MB·English

QuaternaryScienceReviews28(2009)1354–1366
ContentslistsavailableatScienceDirect
Quaternary Science Reviews
journal homepage: www.elsevier.com/locate/quascirev
Sea ice variations in the central Canadian Arctic Archipelago during the Holocene
Lindsay L. Varea,b, Guillaume Masse´ a,b,1, Thomas R. Gregoryb, Christopher W. Smartb, SimonT. Belta,b,*
aPetroleumandEnvironmentalGeochemistryGroup,UniversityofPlymouth,DrakeCircus,PlymouthPL48AA,UK
bSchoolofEarth,OceanandEnvironmentalSciences,UniversityofPlymouth,DrakeCircus,PlymouthPL48AA,UK
a r t i c l e i n f o a b s t r a c t
Articlehistory: AseaicerecordforBarrowStraitintheCanadianArcticArchipelago(CAA)ispresentedfortheinterval
Received22October2008 10.0–0.4cal.kyrBP.ThisHolocenerecordisbasedprimarilyontheoccurrenceofaseaicebiomarker
Receivedinrevisedform chemical, IP25, isolated from a marine sediment core obtained from Barrow Strait in 2005. A core
27January2009 chronologyisbasedon14CAMSdatingofmolluscshellsobtainedfromtenhorizonswithinthecore.The
Accepted28January2009
primaryIP25dataarecomparedwithcomplementaryproxydataobtainedfromanalysisofotherorganic
biomarkers, stable isotope composition of bulk organic matter, benthic foraminifera, particle size
distributionsandratiosofinorganicelements.Thecombinedproxydatashowthatthepalaeo-seaice
recordcanbegroupedaccordingtofourintervals,andthesecanbecontextualisedfurtherwithrespect
totheHoloceneThermalMaximum(HTM).Springseaiceoccurrencewaslowestduringtheearly–mid
Holocene (10.0–6.0cal. kyr BP) and this was followed bya second phase (6.0–4.0cal. kyr BP) where
spring sea ice occurrence showed a small increase. Between 4.0 and 3.0cal. kyr BP, spring sea ice
occurrenceincreasedabruptlytoabovethemedianandweassociatethisintervalwiththeterminationof
theHTM.Elevatedspringseaiceoccurrencescontinuedfrom3.0to0.4cal.kyrBP,althoughtheywere
morevariableonshortertimescales.Withinthisfourthinterval,wealsoprovideevidenceforslightly
lowerandsubsequentlyhigherspringseaiceoccurrenceduringtheMediaevalWarmPeriodandthe
Little Ice Age respectively. Comparisons are made between our proxy datawith those obtained from
otherpalaeo-climateandseaicestudiesfortheCAA.
(cid:2)2009ElsevierLtd.Allrightsreserved.
1. Introduction theseandclimatevariabilitycontinuestogaininmomentum,with
the relative contributions that natural forcing and anthropogenic
The sensitivity of the Arctic to climatic changes is most effectshaveontheseassociations,attractinganadditionalfocus.
convincingly demonstrated by the recent acute and dramatic In order to gain a greater appreciation of the significance of
changes to its permafrost and sea ice cover (Stroeve et al., 2007, anthropogenicinputsonclimaticchanges,itisvitaltounderstand
2008;Perovichetal.,2008).Thisobservationofrapiddecreaseinsea natural climate variability both in modern times and throughout
icecoveroverthelast30yearshasreceivedconsiderableattention history, including during the Holocene. Gaining a greater knowl-
fromarangeofclimatescientists(Johannessenetal.,1999;Vinnikov edgeofrecentandlonger-termseaicevariabilityshouldcontribute
etal.,1999;ComisoandParkinson,2004;Serrezeetal.,2007;Stroeve fundamentally to this understanding since sea ice is such a key
et al., 2008) and it has been speculated that continued sea ice componentof the Earth’s climate system(Comisoand Parkinson,
reduction may open the North West Passage within the Canadian 2004). Satellite-based passive microwave methodology has
ArcticArchipelago(CAA)tointernationalshipping,withthepassage providedadetailedrecordofseaicecoverageforthelast30years,
able to sustain a prolonged shipping season (Howell and Yackel, but extended historical datasets are either scarce or lacking in
2004;Schelletal.,2008).Givenanarrayofsuchmotivatingfactors, detailduetothelimitednumberofdirectobservationalrecordsor
researchintoseaice,seaiceextent,andtherelationshipsbetween indirectaccountsfromproxystudies.
Recently,weproposedtheuseofanovelbiomarker(IP25;Fig.1;
compound1)producedbyseaicediatomsasanewproxyforArctic
* Corresponding author. Petroleum and Environmental Geochemistry Group, sea ice (Belt et al., 2007). IP25 is a mono-unsaturated highly
University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK. Tel.: þ44 1752 branchedisoprenoid(HBI)thatisbiosynthesisedspecificallybysea
584799;fax:þ441752584709.
icediatomsandwehaveshownittobestableinsedimentsbelow
E-mailaddress:[email protected](S.T.Belt).
1 Present address: UMR 7159 LOCEAN, Institut Pierre-Simon Laplace, 4 Place Arcticseaice.Asaresultofthisstability,webelievedthatquanti-
Jussieu,ParisCedex05,France. fication of IP25 in sediments had the potential to provide
0277-3791/$–seefrontmatter(cid:2)2009ElsevierLtd.Allrightsreserved.
doi:10.1016/j.quascirev.2009.01.013
L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366 1355
inBarrowStrait,whichconnectsBaffinBaywiththeArcticOcean
viaLancasterSoundandViscountMelvilleSound(Fig.2).Barrow
Strait is bounded to the west bya sill and gradually increases in
1 2 depthintheeasttowardsLancasterSound(PrisenbergandBennett,
1987).Thesillinfluenceswatercirculationthroughthestrait,and
watersareamixofsouthwardflowfromtheArcticOcean,eastward
Fig.1. StructuresofIP25(1)andC25:2(2). flowfromtheCanadaBasinandwestwardflowfromtheBaffinBay
area. Jones et al. (2003) report a predominantly Pacific origin,
a continuous, high-resolution sea ice record for the Holocene (at althoughthishasnotalwaysbeenthecase,withamajorchangein
least).Giventhispotential,wequantifiedtheabundancesofIP25in the marine circulation involving a switch in the dominance of
a marine sediment core obtained from the North Icelandic Shelf AtlanticandArcticwaterswithintheNorthernCanadianArcticat
(Masse´ et al., 2008); a location where comprehensive historical approximately6.0cal.kyrBPfortheeasternBaffinIslandcoast,but
records for sea ice variability existed and could be used for aslateas2–4kyrBPforthecentralregionincludingBarrowStrait
comparative purposes. Significantly, IP25 abundances in the core (Williamsetal.,1995).
demonstrated numerous correlations with historical sea ice
records, and these were supported further by diatom-based sea
surfacetemperaturereconstructions(Masse´ etal.,2008). 2.2. SeaiceintheCAA
Forthecurrentstudy,wedecidedtofocusonalocationinthe
CAA(BarrowStrait;Fig.2)forwhichprevioushistoricalseaicedata During the Last Glacial Maximum (LGM), the CAAwas totally
were relatively scarce (Gajewski and Atkinson, 2003; Ledu et al., covered by the Laurentide and Innuitian ice sheets (Dyke et al.,
2008). This paper provides a long-term (ca 10.0–0.4cal. kyr BP) 2002).TherecessionoftheicemarginincloseproximitytoDevon
record of sea ice conditions for Barrow Strait within the central Island,SomersetIslandandnorthernBaffinIsland(north,westand
CAA,basedmainlyonvariationsintheabundanceandfluxofIP25in south of Barrow Strait, respectively; Fig. 2) began approximately
morethan 600 individual horizonsobtained froma marine sedi- 10kyr BP (Dyke et al.,1996a,b; Dyke,1999, 2008), with the final
mentcore.SincetheIP25biomarkerisbiosynthesisedselectivelyby remnants of ice disappearing approximately 8kyr BP, following
seaicediatomswhichbloomtypicallyinApril–May,theunderlying arapidincreaseintemperature(Dykeetal.,1996a,b;Dyke,1999).
sedimentaryrecordcanbemorespecificallyinterpretedintermsof Inmoderntimes,theCAAhasbeencharacterisedbythepres-
reflectingspringseaiceconditions,andweusethisinterpretation ence of sea ice throughout the year (Melling, 2002). During the
hereafter. The data are interpreted further with relation to the winter months, between November and May, the entire CAA is
climatic event commonly referred to as the Holocene Thermal completelycoveredbyice,withmaximumiceextentoccurringat
Maximum(HTM).TheprimaryIP25proxydataarecomplemented thebeginningofMay.Inlatespring,thebreak-upoftheseaiceis
by other chemical, biological and physical analyses including initiated, with the emergence of a number of polynyas. The
distributionsofotherorganicbiomarkers,stableisotopecomposi- minimum ice extentoccurs in September, following which, rapid
tion(d13C)ofbulkorganicmatter(OM),abundancesofarangeof growth in sea ice generally occurs. More locally, Barrow Strait is
geochemical elements, occurrence and accumulation rates of characterised by dense land-fast ice from November until early
benthicforaminifera,andsedimentparticlesizedistributions. June, with a relatively short ice-free season in August and
September(PrisenbergandBennett,1987).
2. Regionalsetting AlthoughitisrecognisedthatmodernArcticseaicecoverhas
been declining over the last few decades (Serreze et al., 2007;
2.1. TheCanadianArcticArchipelago(CAA)andBarrowStrait Perovichetal.,2008),withadecreaseinSeptemberseaiceextentof
7.8% per decade over the last 30 years (Stroeve et al., 2007),
The CAA is a complex array of islands, with narrow channels decliningseaiceextentwithintheCAAbetween1978and1996has
interconnectinglargerbasins.Thisstudyconcentratesonalocation been reported to be statistically insignificant (Parkinson et al.,
Fig.2. Mapofthesamplinglocation(ARC-3)andtheCanadianArcticArchipelago.
1356 L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366
1999). Parkinson et al. (1999) noted that there are a number of times and mass spectra (full scan mode recorded every 8–16
regionalfactorsinfluencingthechangestoseaicecover,withthe samples).RelativeabundancesofIP25andanHBIdiene(C25:2)were
unique characteristics of the CAA responsible for a slower than calculatedonthebasisofthemagnitudesoftheirGC–MSresponses
average response in this region. Only relatively few studies, compared with those of the internal standard, with these ratios
however,haveattemptedtoconstructaseaicehistoryforBarrow corrected according tothe mass of sediment analysed. Individual
Strait between the timing of the ice margin retreat in the early n-alkaneswerealsoquantifiedusingthismethodandbycompar-
Holocene and these contemporary observations (e.g. Ledu et al., isons of GC–MS responses with those of standards of known
2008). concentrations. Analytical reproducibility was checked using
a standard sediment with known abundances of biomarkers and
3. Methods alkanes for every 8–16 sediment sample extractions (analytical
error<5%,n¼8).
3.1. Fieldmethods AnITRAXcorescannerusingnon-destructiveX-rayfluorescence
(XRF) detection was used to determine the geochemical charac-
Thecurrentstudyisbasedontheanalysisofapistoncore(ARC- teristics of the archived core sections. For the current study,
3,99.2mminternaldiameter),collectedduringtheCCGSAmund- a 0.5cmstepsize resolutionwas selected, with a 40s XRFcount
sen ArcticNet cruise (August/September 2005). The 641-cm core time.Theanalysisprovidedgeochemicaldataforawiderangeof
wastakenfromawaterdepthof347m(Lat:74(cid:2)16.050;Long:91(cid:2) elements, including calcium (Ca), iron (Fe), titanium (Ti) and
06.380). The core was divided into six 100-cm sections plus rubidium (Rb). Down-core profiles were created using either
ashortersectionatthetopofthecore.Eachsectionwaslongitu- absolute peak area integrals for individual elements or ratios of
dinallysplit,withonehalfarchivedattheBritishOceanSediment these integrals for different elements (e.g. Ca/Fe). The peak area
CoreResearchFacility(BOSCORF,UniversityofSouthampton,UK). integralswereconsideredtobeproportionaltotheabundanceof
Theremaininghalfwassub-sectionedata1cmresolutionandeach each element (Rothwell et al., 2006). Since the abundances of
horizonwasfurtherdivided,withonehalffreeze–dried,theother aluminium(Al)werebelowthelimitofdetectionoftheITRAXcore
storedinacoldroom(þ4(cid:2)C). scanner, an alternative method of analysis was employed. Evenly
spaced (100cm) horizons were selected and subjected to
3.2. Datingofthesedimentsequence acombinednitric(HNO3),hydrochloric(HCl)andhydrofluoric(HF)
acidtotaldigest,followedbyquantitativeanalysisofAlbyinduc-
Dating Arctic sedimentary archives can pose difficulties that tivelycoupledplasma–opticalemissionspectroscopy(ICP–OES).
arisemainlyfromthepaucityofmicro-andmacrofossilswithinthe Particle size analyses were undertaken using a Malvern Mas-
cores. For the core used in the current study, we previously tersizer2000laserparticlesizer.Samplesoffreeze–driedsediment
reportedaconventionalradiocarbonageof9.15(cid:3)0.2kyrBPforthe (ca 0.5g) were taken at 10cm intervals throughout the core and
bottomofthecore(641cm),whichwasobtainedby14CAMSdating material was dispersed ultrasonically in 0.1% sodium meta-
of foraminifera (Neogloboquadrina pachyderma (s)) (Belt et al., phosphate solution (5mL; 1min). Particle size measurements
2007). Due to the extremely low abundance of these and other (5replicates)weremadeusingaredandbluelaserfor30s,with
foraminiferapresentthroughoutthecore,wechosetodetermine a particle refractive indexof 1.53 and a light absorption value of
the chronology of the core using 14C AMS analyses of bivalve, 0.005. Individual distributions of particle sizes were calculated
gastropodandscaphopodshells.IntactbivalveshellsofPortlandica usingpreviouslyreportedmethods(FriedmanandSanders,1978)
arctica,previouslyreportedbyDykeetal.(1996a),wereidentified according to the following classifications – clay: <4mm; silt:
insomehorizonsandusedfor14CAMSdating.Insomecases,intact 4–60mm; sand (coarse): 60mm–2mm. Dry sediment densities
shells were supplemented by shell fragments of gastropods, sca- weredeterminedforthesamehorizonsbyconsiderationof%water
phopods and unidentified bivalve species, though some were contentandfixedsedimentspecificgravity(2.65gcm(cid:5)3).
thoughttobeofHiatellaarctica(Dykeetal.,1996a).Shellmaterial Stableisotopecomposition(d13C)measurementsweremadeon
from ten horizons were 14C AMS-dated (Beta Analytic, USA), and sub-samplesoffreeze–driedsediment(100mg)at10cmintervals
conventional radiocarbon ages converted to calibrated ages (cal. throughout the core using an elemental analyser coupled with
kyrBP)with theCALIB 5.0.1 program(Stuiverand Reimer,1993), a dual inlet mass spectrometer (Finnigan, DeltaplusXP). Sediment
usingthecalibrationdatasetMarine04,whichincorporatesaglobal samples were decarbonated with 10% HCl (1mL) prior to d13C
oceanreservoircorrectionof400yr.Anadditionalregionalmarine analysis.Standarderrorswere(cid:3)0.1&.
reservoir correction (DR) of 290(cid:3)40yr was also applied, taken Benthic foraminifera (>63mm) were picked from sediment
fromArcticBay,NWBaffinIsland(McNeelyetal.,2006).TypicalDR samples at 10cm intervals down-core. Oven-dried sediment
values for the eastern CAA, taken from the marine reservoir samples were disaggregated in 10% sodium hexametaphosphate
correction database (http://intcal.qub.ac.uk/marine/), range from solution (50mL), thenwet-sievedthrough a 63mm mesh. Where
180(cid:3)80yrto435(cid:3)56yr. possible,>250specimensofbenthicforaminiferawerepickedfrom
each sample. However, this was not possible in samples from
3.3. Laboratorymethods 450cm to 0cm since there were very few specimens. Benthic
foraminiferal accumulation rates (BFARs, number of specimens
For biomarker analyses, an internal standard (7-hex- cm(cid:5)2kyr(cid:5)1)werecalculatedas:numberofspecimenspergramof
ylnonadecane;0.1mg)wasaddedtoaliquotsoffreeze–driedsedi- dry bulk sediment(cid:4)linear sedimentation rate (cmkyr(cid:5)1)(cid:4)dry
ment material (ca 1g), which weresubsequentlyextracted using bulkdensity(gcm(cid:5)3).
dichloromethane/methanol(3(cid:4)3mL;2:1v/v)andultrasonication.
Solventwasremovedfromthecombinedextractsusingnitrogen. 4. Results
The resultant total organicextract (TOE)waspurified usingopen
column chromatography (silica), with hydrocarbons (hexane; 4.1. Corechronology
6mL) collected as a single fraction before being analysed by gas
chromatography–mass spectrometry (GC–MS). All biomarkers The14CAMSdataobtainedfromanalysisofmolluscshellsiso-
wereidentifiedonthebasisoftheircharacteristicGC–MSretention lated from ten horizons from the ARC-3 core are summarised in
L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366 1357
Table1andinFig.3.Notably,therearenoreversalsintheprofile calibrated radiocarbon date (kyr BP)
and a date corresponding to the lowest horizon measured 0 2 4 6 8 10
(623.5cm;9684cal.yrBP)isconsistentwithacalibrateddateof 0
9680cal.yrBPobtainedpreviouslyforN.pachyderma(s)obtained
from the bottom of the core (641cm). For the purposes of the
currentstudy,wehaveconstructedtwocontinuouscorechronol- 100 DBD (g cm-3)
0.5 1.0 1.5 2.0
ogies that are bound mainly by the upper (56.5cm) and lower
(623.5cm)14CAMS-datedhorizons.Forconsistency,onlytheAMS 100
dates obtained from mollusc shells have been used to construct
200 200
theseprofiles.Thetwochronologies(Fig.3)havebeenobtainedby
linear interpolation and third-order polynomial fitting of the ten cm) 300
AlinMeSardaitnetse.rFpoorlathtieonlowweasstseexctteionndeodfthuesicnogret(h6e235.57–76.54–16c2m3.)5,tchme cm) 300 epth ( 400
section.Differencesbetweenthetwoprofilesareca(cid:3)50yrforthe h ( D 500
pt
upper (<350cm) section of the core and ca (cid:3)100–150yr for the e
D 600
lowersection.Sinceourdataareinterpretedintermsoffourbroad 400
intervalswhichrangebetween1.0and4.0kyr(seefurtherSections
4and5forspecificintervals),weconsiderthesesmalldifferences
to be insignificant, and we have opted to use the polynomial
500
interpolation model for definition of intervals throughout.
Samplingat1cmintervalsresultsinaneffectiveageresolutionof
6–40yr. The higher sedimentation rates observed for the upper
sectionofthecoremostlikelyreflectthelowerconsolidationatthe 600
core-topasindicatedbylowersedimentdensitiesforthissection
(Fig.3).However,sinceconsolidationcannotsatisfactorilyexplain
theenhancedsedimentationrateatthebaseofthecore,wesuggest Fig.3.14CAMSdatesoftenhorizonsfromtheARC-3coretogetherwithdepth/age
that the latter may possibly be attributed to some glaciomarine modelscorrespondingtolinearinterpolation(dashedline)andthird-orderpolynomial
(solid line). The sampling interval is defined by the ‘core-top/bottom’ boundaries.
sedimentinputfollowingicesheetretreatatthebeginningofthe
Inset:sedimentdrybulkdensities(DBD).
Holocene.Indeed,anincreasein%coarseatthebaseofthecoreis
consistent with this hypothesis, although other parameters
(e.g.BFARs,Ca/Fe,Ti/Rb)suggestthatthisisnottheonlycontrib-
indicativeofvariationsinspringseaiceoccurrence,andthesecan
utor to increased average particle size (see later). Finally, in the
be grouped into relatively broad intervals as follows. The IP25
presentstudy,biomarkerandotherproxydataarepresentedand
abundances are lowest in the first or bottom section of the core
interpreted for the interval defined by the age–depth model
(10.0–8.0cal. kyr BP), before increasing slightly in the second
(10.0–0.4cal.kyrBP)anddoesnotincludetheupperca50cmof
section(ca8.0–3.2cal.kyrBP).Thereisanabruptenhancementin
thecore.
theIP25abundanceprofileatca3.2cal.kyrBP,afterwhich,abun-
dancesremainhigherthanthosefoundforthelowersectionsofthe
4.2. IP25andotherbiomarkers core, and a greater variability exists between adjacent horizons.
Given the variability in the sediment accumulation rates, we
InspectionoftheabundanceprofileoftheseaicebiomarkerIP25 calculated IP25 fluxes from the abundance, sediment density and
alongtheARC-3core(Fig.4(a))revealsanumberoffeatures.Firstly, sedimentationrateprofilesinordertogetabetterappreciationof
itisclearthatIP25ispresentineachoftheca600horizonsana- changes in IP25 delivery to the sediments on a more consistent
lysed, resulting in a continuous, high-resolution record. Since temporalscale.TheIP25fluxprofile(Fig.4(b))isslightlydifferent
inverse accumulation rates range from ca 6 to 40yrcm(cid:5)1, the fromthatdescribedforIP25abundancesandtemporalchangesto
uninterrupted presence of IP25 in each of the horizons provides fluxesarebestgroupedintofourintervals.Firstly,theIP25fluxesare
evidenceforthecontinuousoccurrenceofseaicebetweenca10.0 lowestfortheinterval10.0–6.0cal.kyrBPconsistentwithrelatively
and0.4cal.kyrBPonsub-tomulti-decadaltimescales.Secondly, lowspring sea ice occurrence throughout this time. Between 6.0
thereareanumberoftrendsintheIP25abundanceprofilethatare and4.0cal.kyrBP,theIP25fluxesincreaseslowlybeforeanabrupt
Table1
SummaryofradiocarbonagedeterminationsfortheARC-3core.Themediancalibratedagesareusedfortheconstructionofthedepth/ageprofiles(Fig.3).
RadiocarbonagedeterminationsforARC-3
Depth Materialdated Laboratorysample Measuredradiocarbon Conventional 2-sigmacalibrated Mid-pointcalibrated
(cm) number(Beta) age(yrBP) radiocarbonage(yrBP) agerange(cal.kyrBP) age(cal.kyrBP)
56.5 Bivalvefragments–possiblyP.arctica 247356 660(cid:3)40 1060(cid:3)40 0.29–0.50 0.4
82.5 Bivalvefragments 247357 960(cid:3)40 1380(cid:3)40 0.54–0.75 0.65
180.5 Largebivalvefragments 247358 1640(cid:3)40 2080(cid:3)40 1.24–1.48 1.36
368.5 P.arcticafragments 247359 3590(cid:3)40 4010(cid:3)40 3.48–3.81 3.65
403.5 Bivalvefragments 247360 3860(cid:3)40 4270(cid:3)40 3.82–4.14 3.98
477.5 Gastropodandbivalvefragments 247361 5680(cid:3)40 6060(cid:3)40 6.02–6.30 6.16
515.5 P.arcticavalveandscaphopodfragments 247363 7930(cid:3)40 8330(cid:3)40 8.38–8.63 8.5
555.5 P.arcticaandbivalvefragments 247364 8370(cid:3)60 8780(cid:3)60 8.95–9.33 9.14
577.5 P.arcticaandbivalvefragments 247365 8450(cid:3)40 8880(cid:3)40 9.07–9.41 9.24
623.5 P.arcticaandbivalvefragments 247366 8840(cid:3)40 9270(cid:3)40 9.52–9.85 9.68
640.5 N.pachyderma 210008 9150(cid:3)200 10.31–9.05 9.68
1358 L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366
Fig.4. TemporalprofilesofbiomarkersintheARC-3core(ca600samplingpoints):(a)IP25abundances;(b)IP25fluxes;(c)differencesbetweenIP25fluxesandthemedian.
increaseisseenduringathirdintervalfrom4.0to3.0cal.kyrBP.In 2providesstrongevidencethat,inthiscase,thesetwocompounds
thefourthphasefrom3.0to0.4cal.kyrBP,thefluxesremainhigh, originatefromacommonsource(seaicediatoms).Thisisfurther
but with great inter-horizon variability, consistent with the IP25 supported by the similar and distinctive stable isotope composi-
abundance profile. These changes to the IP25 fluxes are further tion (d13C¼ca (cid:5)19& to (cid:5)21&) of these two biomarkers in the
illustratedbyconsiderationofthedifferencesbetweenindividual ARC-3 core (Belt et al., 2008). In order to investigate any differ-
fluxesandthemedian.Inparticular,thisrepresentation(Fig.4(c)) encesbetweenthesetwoprofiles,were-expressedtheabundance
showsthattheIP25fluxesarehigherthanthemedianafter3.0cal. of the diene as a percentage of the combined abundances of the
kyrBPwithnonegativedeparturesandsomefurtherincreasein twoHBIs(Fig.5(c)).Thisprofiledemonstratestheclosecorrelation
thelastsection(ca0.8–0.4cal.kyrBP). betweentheabundancesofIP25andthedieneintheuppersection
Analysis of the GC–MS chromatograms of the purified hydro- of the core (3.0–0.4cal. kyr BP), but also reveals that, prior to
carbonextractsrevealed,inadditiontoIP25,thepresenceofanHBI 3.0cal. kyr BP, there is a small, but steady enhancement of the
diene(C25:2),identifiedascompound2(Fig.1)onthebasisofits dienerelativetoIP25tothebaseofthecore.
characteristic mass spectrum and GC retention index. Since this GC–MSanalysisofthehydrocarbonextractsfromeachsediment
isomer has been reported in marine sediments from both the horizon from ARC-3 also revealed the presence of a suite of
Arcticandmoretemperateregions(Johns,1999;Beltetal.,2007), n-alkanes,inadditiontothetwoHBIbiomarkers.Giventheubiq-
itisnotconsideredtobeasdefinitiveasaniceproxybiomarkeras uityof n-alkanes and the difficulties associated with interpreting
IP25,butitisworthnotingtheextremelyclosesimilaritybetween their abundances, we chose not to focus on these biomarkers in
the flux profile of this isomer compared to that of IP25 for the detail. However, we quantified the accumulated abundances and
ARC-3core(Fig.5).ThesimilarityintheprofilesofIP25anddiene fluxesofthehighmolecularweightn-alkanes(C27þC29þC31)to
L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366 1359
Fig.5. ComparisonbetweentemporalprofilesofIP25andC25:2:(a)differencesbetweenIP25fluxesandthemedian(b)differencesbetweenC25:2fluxesandthemedian;(c)%C25:2
determinedfromthecombinedabundancesofC25:2andIP25.
trytomakeanassessmentofhigherplantorterrigenousinputsto 4.3. Benthicforaminifera
thesediments(Yunkeretal.,1995;FernandesandSicre,2000;Stein
and Macdonald, 2004). Analysis of the n-alkane flux profile Benthic foraminiferal accumulation rates (BFARs; number of
(Fig.6(b))doesnotrevealanysignificantenhancements,andfluxes specimenscm(cid:5)2kyr(cid:5)1)arecommonlyusedasarecordofpalaeo-
are below the median for the majority of the bottom–middle productivity(e.g.HergueraandBerger,1991;Smart,2002).Inthe
sectionsofthecore(ca8.6–3.4cal.kyrBP).Themuchmorenotable present study, our analysis focused on the identification of indi-
variations seen in the fluxes of the IP25 and C25:2 HBIs are not vidual species together with combined abundances and fluxes,
observed for these n-alkanes, especially the substantial enhance- rather than on down-core variations between species (a more
mentsintheHBIfluxesafter3.0cal.kyrBP. detailed species-level analysis will appear elsewhere; Gregory
Inadditiontothesebiomarkeranalyses,wedeterminedthestable et al., submitted for publication). Islandiella norcrossi, Islandiella
isotope(13C)compositionofthebulkorganicmatter(OM)inorderto islandica,Elphidiumclavatum,CibicideslobatulusandBuccellafrig-
assessforsignificantchanges(ifany)toitssource.d13Cvaluesrange ida,whicharefoundinshelfareasoverlainbyseasonallyice-free
fromca(cid:5)22&intheuppersectionofthecoretoca(cid:5)25&atthecore waters (Murray, 1991), were identified as the major species,
bottom,withthemostabruptchangeoccurringatca4.0cal.kyrBP accountingfor>85%ofthetotalabundances.BFARs(Fig.6(d))were
(Fig.6(c)).Thesevaluesliewithinthetypicalrangefoundformarine- highest between ca 10.0 and 8.4cal. kyr BP before diminishing
derivedOM(e.g.Go˜nietal.,2000;SteinandMacdonald,2004;Go˜ni rapidlyfromca8.4to4.0cal.kyrBP.After4.0cal.kyr,foraminifera
etal.,2005;Beltetal.,2008). were virtually absent. These reductions in the BFARs were
1360 L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366
Fig.6. Temporalprofilesof(a)differencesbetweenIP25fluxesandthemedian;(b)differencesbetweenlongchainn-alkanefluxesandthemedian;(c)d13CforbulkOM;(d)BFARs
(totalbenthicforaminiferalaccumulationrates;numberofspecimenscm(cid:5)2kyr(cid:5)1);(e)%coarsefraction(>63mm).
accompanied bya concomitant increase in the % benthic forami- (Fig.7(b))showsauniformdistributionthroughouttheARC-3core,
niferalfragments,indicativeofcarbonatedissolution. consistent with a continuous deposition of geochemically similar
material.Timineralshaveahighresistancetochemicalweathering
4.4. Bulksedimentgeochemistry within sediments (Rothwellet al., 2006) and, assuming that Tiis
derived predominantly by the aluminosilicate phases, it can
As a complement to the organic biomarker analysis, we also provide information on any changes in the detrital or lithogenic
analysedvariousinorganicelementsandparticlesizedistributions component. Aluminium content can provide an independent
intheARC-3coreinordertoprovideadditionalinformationonthe measureofthedetritalconstituentofthesediment.AlthoughanAl
lithogenicandbiogenicproperties. profile was not achievable using the XRF methodology, the Al
Firstly, we measured Ti/Rb ratios to investigate any possible content of the core was measured (albeit on a relatively small
lithogenic changes to the source of the sedimentary material number of samples) via dissolution of selected horizon sub-
deposited(Croudaceetal.,2006).AprofileoftheTi/Rbratiodata samplesandanalysisbyICP–OES.ConsistentwiththeTi/Rbprofile
L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366 1361
Fig.7. Temporalprofilesof(a)differencesbetweenIP25fluxesandthemedian;(b)Ti/Rb;(c)%Al;(d)Ca/Fe.
andinterpretation,theAlcontentofthecoreshowslittlevariation 5. Discussion
(Fig.7(c))withanaverageabundanceof3.9%wt,withintherange
typicalofmarinesediments(Chester,1990;O’Brienetal.,2006). The majority of palaeo-reconstruction studies for the CAA are
Secondly,weassessedthecontributionofbiogenicmaterialto basedonarchivalrecordscorrespondingtochangesinambientand
thesedimentbydeterminationofCa/Feratiosasapotentialpalaeo- seasurfacetemperatures.Theseclimatereconstructionscomefrom
productivity indicator (Croudace et al., 2006). The Ca/Fe ratios a variety of lacustrine, marine and ice core records, and include
(Fig.7(d))arelargelyinvariantintheuppersectionoftheARC-3core evidence from the Agassiz ice cores (Koerner and Fisher, 1990),
(4.0–0.4cal.kyrBP),whereasbefore4.0cal.kyrBP,theCa/Feratio various microfossil assemblage distributions including diatoms
increasesandreachesaplateauatca7.0cal.kyrBP.Anexceptionto (Smith, 2002; Podritske and Gajewski, 2007), calcareous micro-
thisplateauregionoccursbetweenca9.2and8.8cal.kyrBPwhere fossils, dinoflagellate cysts(Levac et al., 2001; Mudie et al., 2001,
thereisanabrupt,thoughtransientincreaseintheratio. 2005,2006),andpollenrecords(Gajewski,1995).Evidenceforsea
Finally, particle size analysis of the ARC-3 core shows silt iceconditionswithintheCAAcomesfromrelativelyfewandnon-
materialtobethemajorsizefraction,rangingbetween72and95%. continuousarchivalrecordssuchasbowheadwhaleremains(Dyke
Themostsignificantchanges,however,areobservedinthecoarse et al., 1996b; Savelle et al., 2000), marine mollusc assemblages
fraction (dry sediment >63mm). The coarse fraction shows an (Dyke et al.,1996a), the presence of driftwood located on raised
overalldecreaseup-corefromca5%atthebaseto<1%betweenca beaches (Dyke et al., 1997) and dinoflagellate cyst assemblages
2.8and0.4cal.kyrBP(Fig.6(e)).Ithasbeensuggestedthatthe% (Leduetal.,2008).Suchstudieshavefocussedonnumberofice-
coarsefractionisareliableproxyofcarbonatedissolutionindeep- free months during the year and whether permanent sea ice
seacarbonates(e.g.,VolbersandHenrich,2002),withareduction conditions prevail. For example, Dyke et al. (1996b) and Savelle
indicative of the breaking up of foraminiferal tests into smaller et al. (2000) used the postglacial remains of bowhead whales to
fragments(e.g.,WuandBerger,1991). propose a period of maximum postglacial warmth between 10.5
1362 L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366
and8.5cal.kyrBP,andtheprevalenceofsevereiceconditionsat termsofagreaterterrigenousinput.However,theabsenceofany
8.5–5.0cal. kyr BP, with most of the channels within the CAA major enhancement in the higher molecular weight n-alkanes
containingicethroughouttheyear.Bowheadwhaleremainswere (Fig.6(b)),oftenassociatedwithterrigenousinputs(Drenzeketal.,
again found in some areas within the CAA after 5.0cal. kyr BP, 2007),arguesagainstthisand,forthemiddlesectionofthecore(ca
indicating a slightly warmer period, with a further period of 8.6–3.4cal.kyrBP),theoppositeistrue(viz.arelativedepletionin
increased sea ice after 3.0cal. kyr BP. Analysis of dinoflagellate then-alkaneflux).Instead,thehigherd13Cvaluesassociatedwith
cysts,foraminiferaanddiatomsinArcticsedimentshavebeenused thetopsectionofthecore,inparticular,mostlikelyrepresentsan
to infer sea surface temperature and sea ice conditions, but this enhancementinseaice-derivedOMrelativetothelowersections.
approach has been limited to Baffin Bay, Lancaster Sound and Recently,wedemonstratedthatd13CvaluesforIP25andothersea
furthereastintheCanadianArctic(e.g.Levacetal.,2001;Mudie ice-derivedbiomarkers,lietowardsthehigheror‘heavier’endof
etal.,2005;Leduetal.,2008). the marine OM range, typically between (cid:5)19& and (cid:5)21& (Belt
The data presented in this study are therefore interpreted etal.,2008).Assuch,thed13Cdataareconsistentwithapredomi-
mainlyintermsofimprovingourunderstandingofseaicevaria- nantly marine-derivedorigin for the bulk OM with an increasing
tions within a region of the CAA (Barrow Strait) throughout the contributionfromseaiceOM,especiallysinceca4.0cal.kyrBP.
Holocene.Theinterpretationsarebasedprimarilyonthevariations Thus far, we have interpreted changes in IP25 abundances in
inabundanceandfluxofaseaicebiomarker(IP25)withinamarine sedimenthorizonsasbeingreflectiveofvariationsinspringseaice
sedimentcore.Arangeofotherproxydataisalsopresentedandthe occurrenceatthedifferenttimesofdeposition.Potentially,theIP25
extenttowhichtheycomplementtheIP25dataisdiscussed. abundance profile may be influenced during certain intervals by
TheIP25abundanceandfluxprofiles(Fig.4(a–c))suggestvari- sedimentologicalfactors,suchasachangeinprovenance.Logically,
ablespringseaicecoverforBarrowStraitduringtheHolocenewith such influences would be most apparent for intervals where the
no periods of ice-free conditions during the spring–summer, at IP25abundanceiseither‘low’orundergoesanabruptchange,since
least for the temporal resolution (ca 6–40yr) afforded by the the‘true’abundancecouldhavebeenalteredbyanexcesssupplyof
sediment accumulation rates.The lowIP25 fluxes fromca 10.0 to e.g.terrestrialsediment,icerafteddebrisoraturbidite(Scheffler
6.0cal. kyr BP provide evidence for relatively low spring sea ice etal.,2006;Marshetal.,2007).FortheARC-3core,bothTi/Rband%
occurrence during this interval. This was followed by a cooling Al profiles show uniform distributions down-core, with no inter-
phasewithprogressivelymoreseaicebetween6.0and4.0cal.kyr ruptions,providingstrongevidencethattherehasbeencontinuous
BP,although IP25 fluxes remained below the median level during deposition of geochemically similar material, with no significant
thisinterval(Fig.4(c)).ArelativelyrapidincreaseinIP25fluxesin changesinthesourceofsedimentdeposited(Croudaceetal.,2006).
athirdphase(4.0–3.0cal.kyrBP),indicatesamajorchangetothe Thus,weattributechangesinIP25abundanceswithinthesediment
spring sea ice occurrence at this time, and this was followed by horizonstotemporalvariationsinspringseaicecover.
a final interval, where IP25 fluxes are highest. The fluxes in this We investigated foraminiferal fluxes, % coarse (sediment
fourthintervalaresignificantlymorevariablethanthoseobserved particle size) and Ca/Fe to give additional proxy data to the IP25
for the early to mid Holocene, indicative of greater variations in recordand,inparticular,provideevidenceforchangestoprimary
spring sea ice occurrence on shorter timescales, although fluxes production as a result of temporal sea ice variations. Previously,
remainedabovethemedianthroughoutthistime,symptomaticof Levac et al. (2001) analysed abundances and distributions of
aprolongedperiodofcoldconditions. dinoflagellatecystsinnorthernBaffinBaytogetherwithchangesin
TheoccurrenceandprofileofasecondHBIbiomarker(diene2), diatomfluxesclosetonorthwesternBaffinIsland(Shortetal.,1994)
alsoprobablyderivedfromseaicediatomssinceithasanextremely and the coasts of Baffin and Devon Islands (Williams,1990) and
similar stable isotope (d13C) signature to that of IP25 (Belt et al., concluded that these could be influenced by sea ice cover with
2008),providestrongsupporttotheseprimaryconclusions.Theflux higherproductionunderlowiceorwarmerconditions.Leduetal.
profileofthisdiene(Fig.5(b))closelymatchesthatofIP25(Fig.5(a)) (2008) also analysed dinoflagellate cysts to infer, consecutively,
and can be described by the same four time intervals. However, warmer and colder intervals in the mid- and late Holocene in
whentherelationshipbetweenthetwoisomersisconsideredmore LancasterSound,ca300kmeastofthecurrentcoresite(Leduetal.,
closelybylookingatthe%contributionfromthediene(Fig.5(c)), 2008).Assuch,weanticipatedthattheforaminiferalfluxprofilein
arelativeenhancementinthismoreunsaturatedisomerisnoted thecurrentstudywouldbeinverselycorrelatedwiththatofIP25,
priortoca3.0cal.kyrBPtowardsthebaseofthecore.Thischangeis butthiswasnotstrictlythecase.BFARsweresignificantlyhigherin
subtle, but is consistent with the preferential formation of more theca10.0–8.0cal.kyrBPinterval,beforediminishingrapidlytoca
unsaturated HBI isomers at higher diatom growth temperatures 4.0cal.kyrBP,afterwhich,veryfewspecimensweredetectedatall.
(Rowlandet al., 2001) and this, in turn, endorses the proposal of ThelowBFARsbetween8.0and4.0cal.kyrBParenotatfirstsight
awarmerperiodintheearlytomidHolocene. consistentwithlowspringseaiceoccurrencesindicatedbytheIP25
ThestableisotopiccompositionofthebulkOMinthesediments fluxes, although the higher BFARs in the 10.0–8.0cal. kyr BP
further supports the biomarker data, particularly for the warmer intervalare.Weattributethisdifferencetotheonsetofadissolu-
early–mid Holocene. Interpreting d13C data for bulk OM in Arctic tion event starting at ca 8.0cal. kyr BP and this is even more
sedimentsisnotstraightforwarddue,inpart,tothemixednatureof significantafter4.0cal.kyrBP.Theproposalofadissolutioneventis
the OM, together with difficulties in assigning well-defined end furthercorroboratedbyreductionsinthe%coarseandbiogenicCa
members for OM from various sources. However, it is generally (Ca/Fe)composition of thesedimentsat8.0cal.kyrBPandespe-
agreedthatd13Cvaluesformarine-derivedArcticOMliewithinthe cially after 4.0cal. kyr BP. Carbonate dissolution is further evi-
range(cid:5)19&to(cid:5)24&(e.g.Go˜nietal.,2000,2005;Beltetal.,2008) dencedbytheverylowplanktonicforaminiferal abundancesand
althoughawiderrange(ca(cid:5)17&to(cid:5)30&)hasalsobeenreported enhancedcorrosionandfragmentationofmicro-andmacrofossils,
(SteinandMacdonald,2004).TerrigenousOMisgenerally‘lighter’ particularlyintheuppersectionsofthecore(<4.0kyrBP).Amore
thanmarineOMwithad13Crangeof(cid:5)26&to(cid:5)28&(Stein and detailed account of the up-core dissolution based largely on
Macdonald,2004).Assuch,thed13CvaluesforbulkOMfromthe changes in distributions and preservation of individual forami-
ARC-3core((cid:5)22&to(cid:5)25&)liefirmlywithintherangeofmarine- nifera and macrofauna can be found elsewhere (Gregory et al.,
derivedOM.Thegeneraltrendtowards‘lighter’ormorenegative submitted for publication). Dissolution is likely controlled by
d13Cvaluestowardsthebottomofthecoremightbeinterpretedin anumberoffactorsincludingCO2levelswhichareinfluencedby,
L.L.Vareetal./QuaternaryScienceReviews28(2009)1354–1366 1363
amongstotherthings,originofwatermassesandseaicecover.For Our conclusions can be compared with others drawn from
example,thedissolutionofforaminiferaobservedatca8.0kyrBP previous palaeo-climate studies, including those that address sea
in Baffin Bay (Aksu, 1983) has been attributed to CO2-enriched icecover.Firstly,thecontinuousoccurrenceoftheIP25biomarker,
Arctic water entering through the Arctic channels (Dyke et al., andthereforeofseasonalseaiceconditionsbackto10cal.kyrBP,
1997), while changes in distributions of foraminifera in Barrow confirmstheretreatofthemainicemarginswithinBarrowStraitby
Strait suggest a transition from Atlantic to Arctic water masses, this time (Dyke, 1999; Dyke et al., 2002; Kaufman et al., 2004).
notablyat4.0–2.0kyrBP(Williamsetal.,1995).Withrespecttosea Secondly,thefourmaindiscreteintervalsofspringseaiceoccur-
ice, Wollenburg and Kuhnt (2000) demonstrated that dissolution rence presented in this study compare very favourably with the
was higher under seasonal ice cover which aligns well with our summericemeltrecordoftheAgassizIceCap(KoernerandFisher,
ownfindingsofhigherdissolutionup-corewithincreasingspring 1990;Fisheretal.,1995),withrelativelylowspringseaicecondi-
sea ice occurrence. As such, it is the occurrence of carbonate tions aligning well with high summer ice melt during the early–
dissolution within the core, particularly after ca 4.0cal. kyr BP, midHolocene(Fig.8).Theabruptandpersistentincreasestoseaice
ratherthantheoccurrencesofforaminiferaasaproductivityindi- duringthemid–lateHolocene(<ca4.0cal.kyrBP)arereflectedin
catorthatprovidesevidencefortheproposedtemporalchangesto the low ice melt phase for the Agassiz Ice Cap. Thirdly, our
seaice.However,sincethereareanumberofalternativeexplana- conclusions are consistent with those of Levac et al. (2001) who
tions for dissolution including the influence of different water constructedaseaicedurationprofilefornorthernBaffinBayusing
masses,weconsiderthisobservationtobesupportiveratherthan transfer functions of dinocyst assemblages. These authors
definitive. concludedthatseaicecoverwaslow(oratleastlowerthanpresent
In summary, these proxy data can be interpreted in terms of daylevels)priortoca4.5kyrBP,followedbysignificantly higher
providingevidenceforthevariableoccurrenceofspringseaicein sea ice duration between this point and modern times. For Lan-
four intervals: (1) A period in the early–mid Holocene (ca 10.0– caster Sound, east of Barrow Strait, the prolonged early to mid
6.0cal. kyr BP) where the spring sea ice occurrence was at its Holocene warm period, as predicted from dinocyst assemblages
lowest;(2)Ashorterinterval(ca6.0–4.0cal.kyrBP)wherespring (Ledu et al., 2008), terminated around 2.9kyr BP, with a shift
sea ice occurrence steadily increased, but remained below the towardscoolersummertemperatures.Thislatterobservationisin
medianfortheHolocene;(3)Abriefinterval(ca4.0–3.0cal.kyrBP) closeagreementwiththeseasonalseaicerecordforBarrowStrait,
wherespringseaiceoccurrencesincreasedrapidly;(4)Aperiodin derivedfromthebiomarker(andotherproxy)datapresentedhere.
the late Holocene (ca 3.0–0.4cal. kyr BP) where spring sea ice Probably the most extensive previous proxy record of palaeo-
occurrencewasat its highest. There is some evidencefora short seaicereconstructionintheCAAhasbeenmadebyconsideration
termdecreaseinspringseaicetowardstheendofthislateHolo- of the occurrences of bowhead whale remains in raised beach
ceneinterval(ca1.2–0.8cal.kyrBP),followedbyafinalincreasein deposits throughout the archipelago (Dyke et al., 1996b; Savelle
thelast0.4kyr(0.8–0.4cal.kyrBP). et al., 2000). The basis of this proxy is the argument that the
Fig.8. Temporalprofilesof(a)IP25fluxes;(b)summericemeltrecordfortheAgassizIceCap(datafromhttp://www.ncdc.noaa.gov/paleo/icecore/polar/agassiz/melt.html).