Beryllium isotope signatures of ice shelves and sub-ice shelf circulation Earth and Planetary Science Letters

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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Beryllium isotope signatures of ice shelves and sub-ice shelf circulation

Duanne A. White

^a,∗

, David Fink

^b

, Alexandra L. Post

^c

, Krista Simon

^b

, Ben Galton-Fenzi

^d,e

, Simon Foster

^a

, Toshiyuki Fujioka

^b

, Matthew R. Jeromson

^a

, Marcello Blaxell

^a

,

Yusuke Yokoyama

^f

aInstituteforAppliedEcology,UniversityofCanberra,ACT,2617,Australia

bAustralianNuclearScienceandTechnologyOrganisation(ANSTO),PMB1,Sydney,NSW,2234,Australia cGeoscienceAustralia,Symonston,ACT,2609,Australia

dAntarcticClimateEvolutionCooperativeResearchCentre,PrivateBag80,Hobart,TAS,7001,Australia eAustralianAntarcticDivision,Kingston,Tasmania,Australia

fUnivesityofTokyo,Tokyo,Japan

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received14June2018

Receivedinrevisedform1October2018 Accepted5October2018

Availableonline25October2018 Editor:D.Vance

Keywords:

meteoric¹⁰Be marinesediments Beisotopemixing sub-iceshelfcurrents sedimentadvection

Be isotopesare ausefultracer ofsediment sourceand transport pathwaysbuthave not beenwidely testedinglacio-marineenvironments.WemeasuredBeisotopesinarangeofdepositionalenvironments fromopenmarine,sub-iceshelfandsubglacialsettingsthroughoutPrydzBay,oneofAntarctica’slargest ice drainage systems. We find that strong sub-ice shelf and bottom current circulations can advect 10Be-rich open marine sediments into an ice shelf cavity, and ¹⁰Be-poor terrestrial sediments onto the continental shelf atthe ice shelf outflow, meaning that ¹⁰Be concentrationsreflect sub-ice shelf circulationpatternsratherthandepositionalenvironment.However,HCl-extractable¹⁰Be/⁹Beratioscan provideamorerobustdiscrimination ofsedimentdepositedinopenmarineand sub-iceshelfsettings.

Thus,Beisotopesareausefultracerofbothenvironmentalsettingandsub-iceshelfcirculationstrength inbothmodernandpaleo-icesheetmargins.

CrownCopyright©^{2018PublishedbyElsevierB.V.Allrightsreserved.}

1. Introduction

Berylliumisotopesdisplayarangeofpropertiesthatmakethem usefultracersofgeochemicalprocessesandsedimentsources.¹⁰Be is produced in the atmosphere (termed meteoric ¹⁰Be) by in- teraction of cosmic rays with oxygen nuclei, and decays with a 1.387-million-yrhalf-life (Chmeleff etal., 2010). Cosmicrays also interact withEarth surface rocks (termed in-situ ¹⁰Be), although production rates are orders of magnitude lower. Stable ⁹Be is a naturallyoccurringtraceelementincrustalrocksandaccumulates, together with meteoric ¹⁰Be, in many sediment archives in ei- thertheprimarymineralsorsecondaryweatheringproducts (von Blanckenburgetal.,2012).

Muchofthedevelopmentandapplicationofberylliumisotopes in sediment cycling to date has focused ontemperate terrestrial andmarineenvironments.Intemperateareas,meteoric¹⁰Berains down onto the land and ocean surfaces, where it is adsorbed to theoutsideof particles orincorporatedinto secondary phases suchasMn andFe-oxideswhereit mixeswith⁹Besourced from

*

Correspondingauthor.

E-mailaddress:duanne.white@canberra.edu.au(D.A. White).

weatheringoftheprimary minerals(WillenbringandvonBlanck- enburg, 2010). The relatively constant flux of¹⁰Be to the earth’s surface atanyone location overtime meansthat ¹⁰Be/⁹Be ratios and ¹⁰Be concentrations provide useful measures of soil forma- tion (e.g. Pavich et al., 1984), sediment transport pathways and provenanceinriverineenvironments(Brown,1987).Inthemarine environment meteoric ¹⁰Be is efficiently andquickly transported fromsurfacewatersviabiologicalpathwaystotheseabed(Frank etal.,1997),apropertywhichhasbeenusedtomonitorweather- ingandsequestrationofCO₂ onglacial/interglacialcycles(e.g.von Blanckenburgetal.,2015).

Ice sheetstransport and disperseberyllium isotopes ina way thatdiffersmarkedlytothatintemperateortropicalsystems.The sources andtransport pathwaysof ⁹Be and ¹⁰Be across the ma- jor iceoutletsofcontinentalscaleicesheetsarelargelyseparated (Graly et al., 2018). ⁹Be is sourced primarily by glacial erosion andistransportedinsedimentsthatareconcentratedinthebasal zone (lower few meters and the deforming bed; Fig. 1). Mete- oric¹⁰Beproducedintheatmosphereisdepositedwithsnowand then found throughout theice column(Beer etal., 1987), which is largely debris-free. During seaward transport by the ice sheet there is limited or no mixing of these two components, unlike riverinesystemswheremixingoccursbothatthesourceanddur- https://doi.org/10.1016/j.epsl.2018.10.004

0012-821X/CrownCopyright©^{2018PublishedbyElsevierB.V.Allrightsreserved.}

Fig. 1.Conceptualmodelsoftheinfluenceofdifferentglacialandmarineprocesses onthedistributionofBeisotopesinmarginalmarineandice-shelfenvironments.

Top(afterYokoyamaetal.,2016);ificebergcalvingisthedominantprocessofice loss,anycontinentalsedimentreleasedunderneathiceshelveswillhaveverylow 10Beconcentrations,as allterrestriallysourced¹⁰Beistransportedseaward into theopenmarineenvironment.Middle;ifsub-iceshelfmeltisactive(dottedline), 10Befrommeltingicewillbedepositedinsedimentsinthesub-iceshelfcavity, althoughthehighsedimentgenerationratesnearthegroundinglinecauses¹⁰Be concentrationstobelowerthanintheopenmarine settings,especiallycloseto theicesheet.Bottom;ifstrongbottomcurrentsareactive,sedimentsfromopen marineenvironmentshighin¹⁰Be,organicmatteranddiatomsaretransportedinto thesub-iceshelfcavity,reworkingandmixing¹⁰Beconcentrationsand ¹⁰Be/⁹Be ratiosinsedimentthroughoutthemarineportionofthesystem.

ingtransport.Thus,icesheetscandelivertheirisotopicberyllium fluxesfromcontinentalinteriors tocoastal marginsin a spatially segregatedmanner,withlimitedinteractionduringtransport.This means there is little opportunity for sediments entrained in the ice to mix their Be isotope fluxes and set a locally representa- tive ¹⁰Be/⁹Be ratio. The independent pathways in Be isotopes in glacial environments resultsin generally low-meteoric ¹⁰Be con- centrationsinsedimentsproducedbyglacialtransport,whichhas successfullybeenappliedasamarkerforpresenceandabsenceof icesheets(Schereretal.,1998).

Therehavebeenrelativelyfewstudiestodeterminehowdiffer- entmodes ofberyllium transport inice affects their distribution onglaciatedcontinentalandmarinemargins.Existingstudiesfrom theRossSeasectorofAntarctica(Sjunneskogetal.,2007) suggesta strongpositive¹⁰Be concentrationgradientbetweentheicesheet andthe continentalshelf.There are indicationsthatsub-ice shelf

sediments,traditionally challenging to identify insediment cores usingexisting proxies(Postetal., 2014),mayberecognisedfrom their relatively low ¹⁰Be concentrations (Yokoyama et al., 2016).

However, the utility of ¹⁰Be as an iceshelf proxy relieson calv- ing of icebergsat the iceshelf front transportingmeteoric ¹⁰Be deposited on the ice shelf and sheet to the open marine envi- ronment, thus preventing ¹⁰Be reaching sub-ice shelf sediments in the ocean cavity (Fig. 1, top). Other marine-margin processes such as basal ice-shelf melt and sub-ice shelf ocean circulation mayalsoinfluencespatial distributionofBe isotopes(Fig.1,mid- dleandbottom),buttherateandroleoftheseprocessesispoorly quantified.The influenceofsub-iceshelf processesmaylimit our abilitytocorrelateBeisotopesinsedimenttothedynamicsofice sheet retreat and advance, and understand the effect of glacial–

interglacialcycleson berylliumcyclinginthe globalocean.Thus, moreinformationisrequiredtobetterunderstandthedepositional environments and processeswhich control Be isotopeconcentra- tions.

In thispaperwe investigate thedistribution of berylliumiso- topes and their range in concentration in sediment from Prydz Bay, the outlet for the Lambert Glacier–Amery Ice Shelf system.

Thisareapresentsausefulcasestudy,astheprocessesofoceanic circulation(Galton-Fenzietal.,2012),sediment transport(Postet al., 2014), and ice shelf (Hemer and Harris,2003) and ice sheet history(e.g. Domacketal., 1998; Whiteetal., 2011) inthissec- toroftheAntarcticcoastalmarginarerelativelywelldefined.Itis alsoan area wherestrongoceaniccirculationsonthe continental shelfandinthesub-iceshelfcavityblurthephysicalandbiological characteristics ofdifferentglacio-marineenvironments (Hemer et al.,2007; Postetal.,2014) andthusposechallengestotraditional sedimentologicalmodels ofproximal glacio-marinesedimentation (e.g.DomackandHarris,1998) usedtoidentifypasticeshelf ex- tents.

Here,weaimtodistinguishhowsub-iceshelfcirculationaffects theBeisotopiccompositionofsedimentsindifferingenvironmen- tal settings andrelate theseto oceanicand glacial environments andprocesses.We investigatewhetherBe isotopes can begener- ally applied asa tracer ofdifferent depositional settings along a transect froma continentalicesheet, to the groundinglinezone (where groundedicebeginsto floatandinteractwiththe ocean) alongthesub-iceshelfsettingtoanopenmarinecontinentalshelf environment.

2. Environmentalsetting

At the northern extension of the Lambert Glacier–Amery Ice Shelf System, the Amery IceShelf is the drainagepoint ofeight tributary glacial basinsthe largestofwhich are theLambert and MellorBasins(Fig.2).Thesystemdrains16%oftheEastAntarctic IceSheet,transmitting88.2±^{2.9Gtayear,makingitasignificant} contributortotheoverallicemassbalanceofAntarctica(Yuetal., 2010; Postetal.,2014).

The Amery IceShelf isapproximately 450 kmlong and thins fromamaximumofover 2500matthesouthern groundingline to around 200m atthe shelf front. The thicknessof thesub-ice cavity rangesfrom 250m to 1600m (Galton-Fenzi etal., 2008), witha highbasal melt ratecomparedto other EastAntarctic ice shelves(Pritchardetal.,2012).

TheLambert–AmerysystemdrainsintoPrydzBay,wheresteep bathymetryandtopographyallowsforcurrentstoformthestrong clockwise circulationof the Prydz Bay Gyre. ModifiedCircumpo- larDeepWaterflowsfromtheeastoverFourLadiesBank, where it intersects the East Wind Drift current, carrying fresh iceshelf water from West Ice Shelf and highly saline water produced in thesea-icefreeareaoftheBarrierPolynya(Williamsetal.,2016).

Here,thecurrentsplits,withpartcontinuingwestwardacrossthe

Fig. 2.MapofLambertGlacier–AmeryBasin,PrydzBay,moderniceshelfedge(blueline)andlocationofcurrentgroundingline.Sitesdiscussedinthetext,showinglocation ofkeyiceshelves,openmarineandsub-iceshelfbottomcurrents.Notethestronggyre-likecirculationunderneathAmeryIceShelf(afterGalton-Fenzietal.,2012),andthe differingcharacteristicsofwatermassesinPrydzBay,andtheeasternandwesternsidesofthesub-iceshelfcavity.CDP,MP,DPandBPindicateCapeDarnley,Mackenzie, DavisandBarrierPolynyasrespectively.Redboxininsetonlowerrightindicateslocationofmainmap.Sea-bedelevation(metresbelowsealevel)fromIBSCOcompilation shownforareasseawardofthegroundingline.(Forinterpretationofthecoloursinthefigure(s),thereaderisreferredtothewebversionofthisarticle.)

ice shelf front, while the remainder flows south into the sub- iceshelf cavityof theAmery IceShelf (Fig. 2). Belowthe Amery IceShelf, thesetwo currentsmix andforma highly saline water mass,whichthensinks.Theback-slopingbathymetryoftheAmery IceShelf cavityandCoriolis effectdrives thesecurrentsalong an eastern channel belowthe AmeryIce Shelf towards the ground- ingline,causingsignificantbasalmelt.Here,thenowfreshermelt waterrisesandfollowstheclockwisegyrealongthewesternmar- gin of the Amery Ice Shelf, andis ejected into Prydz Bay as Ice Shelf Water, completing the ‘ice pump’ driven circulation typical of Antarcticsub-shelf circulation systems (i.e.Herraiz-Borreguero etal., 2015). IceShelf Waterfrom theAmery IceShelf is mostly recirculatedthroughthePrydzBayGyreback undertheice-shelf, butsomeflowswestwardsoutofthebay(Fig.2).

In the open marine environment, in particular on the west- ern flank of the Prydz Channel depression, currents can exceed 1cm/s (Nunes Vaz andLennon,1996). Flow speedscan be even greaterundertheAmeryIceShelf,whichhasastrongsub-iceshelf current system (∼^{5–10 cm/s; Fig. 2). Sub-ice shelf currents un-} der the Amery are substantially more vigorous than other large ice shelves (e.g. Ross Ice Shelf, 1 cm/s; Jenkins et al., 2003;

Post et al., 2014), and is capable of depositing sediment from a marineenvironmentgreatdistancesbeyondtheicefront.

3. Methods

The PrydzBayregionhasbeenthefocusofseveraldecadesof geophysical mapping andsediment coring.Sediment grabs, grav- ityandpistoncoreshavebeencollected oncruisesby theAurora Australis(186&901),JoidesResolution(Leg188,2000),Nathaniel B. Palmer(NBP01,2001) andPolarstern (XXIIV,2007).Coreshave alsobeencollected fromunderneath theAmeryIceShelf through the AMISOR program (Allison, 2003). For this paper, we inves- tigated material from AuroraAustralis cruises 186, 901 and the AMISOR program,which hasbeen preservedat4^◦C sincecollec- tion.

To investigate the modern distribution of ¹⁰Be in Prydz Bay sediments, we selected samples from core top (surficial) sedi- ments representative of different modern depositional environ- ments.Sevencore-topsamplesfromopen marine(labelledasGC) andsixsea-bedsamplesfromsub-iceshelf(labelledasAM01–06) environments were obtained from a wide geographic area (Ta- ble 1). Open marine sediment samples were restricted to areas where iceberg turbation is absent, or in the case of GC18, po- tentially limited. Core top GC27 was sampled by ship in open marineconditionsadjacenttotheiceshelfmarginbutisnowcov- eredbytheiceshelf.Observationsoftheshelf-frontpositionsince

Table 1

Berylliumisotopeandcorelocationdata.

Core name Cruise Water

depth^a (m)

Core depth^b (cm)

Latitude Longitude Sample mass (g)

9Be total^c (μg/g)

9BeHCl extracted (μg/g)

10BeHCl extract^d (10⁶at/g)

10Be/9 HCl ext.

(10⁻⁸at/at) Open marine core tops

GC22 901 766 0 68.065 72.2760 0.41 – 0.22 785.1±18.4 5.5

GC27 901 776 0 68.947 73.5858 0.68 – 0.25 985.6±22.7 5.8

GC29 901 789 4 68.663 76.6955 0.34 – 0.18 847.1±^{19.8 7.1}

GC24 901 705 0 68.094 73.1893 0.46 – 0.21 892.1±^{20.7 6.2}

GC15 186 1050 1 68.518 70.4075 0.86 – 0.12 173.2±^{4.5 2.2}

GC16 186 726 4 68.375 71.3070 0.70 – 0.13 347.7±^{10.4 4.1}

Sub-ice shelf core tops

AM01B AMISOR 840 0 69.431 71.4462 0.62 2.01 0.49 810±19.6 2.5

AM02 AMISOR 843 0 69.713 72.6400 0.44 – 0.64 1110±28.7 2.6

AM03 AMISOR 1339 0 70.561 70.3322 1.35 2.58 0.53 838.3±^{19.2 2.4}

AM04 AMISOR 1002 0 69.900 70.2903 1.36 2.61 0.50 457.5±^{10.5 1.4}

AM05 AMISOR 979 1 70.230 69.6967 1.79 2.73 0.44 248.3±^{5.7 0.8}

AM06 AMISOR 902 0 70.250 71.3500 2.11 2.41 0.27 508±^{11.8 2.8}

Basal diamicts (subglacial)

AM01b AMISOR 840 45 69.431 71.4462 2.75 – 0.32 31.1±1.1 0.1

AM04 AMISOR 1002 115 69.900 70.2903 2.51 – 0.24 36.2±^{0.9 0.2}

GC16 186 726 227 68.375 71.3070 2.13 – 0.42 160.9±^{4.1 0.6}

GC24 901 705 431 68.094 73.1893 2.32 – 0.42 86.3±^{2.2 0.3}

GC29 901 789 351 68.663 76.6955 2.27 – 0.17 100.7±^{2.4 0.9}

Shallow water

GC18 901 320 1 67.283 76.5703 0.71 – 0.35 776±18.2 3.3

Down-core

AM01b^* AMISOR 840 0 69.431 71.4462 0.62 2.01 0.49 810±^{19.6 2.5}

AM01b AMISOR 840 4 69.431 71.4462 0.77 – 0.45 766.1±17.9 2.5

AM01b AMISOR 840 11 69.431 71.4462 1.13 2.22 0.48 813.8±18.7 2.5

GC22^* 901 766 0 68.065 72.2760 0.41 – 0.22 785.1±^{18.4 5.5}

GC22 901 766 13 68.065 72.2760 0.57 – 0.29 757.6±18.3 3.9

GC22 901 766 22 68.065 72.2760 0.57 – 0.54 703.8±16.4 2.0

Repeat analyses

GC24-1^e 901 705 0 68.094 73.1893 0.39 – 0.24 834.8±^{19.7 5.1}

GC24-1b^f 901 705 0 68.094 73.1893 N/A – – 47.1±1.4

AM05-2^e AMISOR 979 1 70.230 69.6967 1.34 2.73 0.45 252.4±6 0.8

AM05-2b^f AMISOR 979 1 70.230 69.6967 N/A 2.73 – 14.5±^0.5

a Waterdepthforopenmarinecores,depthbelowgroundediceshelfsurfaceforsub-iceshelfcores.

b Sedimentsub-sampledepthbelowcoretop.

c Total⁹BecontentfromcompleteHFdissolutionofdrysedimentaliquot.

d 10Beuncertaintiesare1σstandarderrors.

e Repeatfromasecondsedimentaliquotandfullprocessing.

f RepeatleachingofleachedsedimentusedinfirstGC24-1andAM05-1.

* Duplicateddataentries(initalics)fromthemarineandsub-iceshelfcoretopsforeaseofcapturingthedowncoretrends.

1936suggests thissite is likely open marine for atleast 80% of the 60–70 yr calving cycle (Fricker et al., 2002). Core top sedi- ments from open marine andsub-ice shelf environments are all finegrained,withvarying siliceouscontent inopen marine envi- ronmentsof 1to 50%, andof diatom abundancein sub-iceshelf sedimentsof0to160×^{106 valves/g(Postetal.,2014).}

Thenatureofgravitycoringmeansthesurficialsedimentsmay havebeendisturbed ornot retained,andthus there issome po- tentialoursamplesmaynotrepresentthemostrecentlydeposited sediment.Availableradiocarbondating (e.g.Postetal.,2014)and comparisonwith ages of known surface samples (Domack et al., 1991) indicatesthat ifthishasoccurred, oursedimentsarelatest Holoceneinage,likelythelast∼^{1ka.Tounderstandthetemporal} variationsthatmayhaveinfluencedtheBeisotopeconcentrations, wesupplemented the coretopsediments withalimitednumber ofdown-coresamples.Holocenesiliceousmarineooze(SMO)from open marine (GC22, depths of 13 and 22 cm) andsub-ice shelf (AMO1B,4 and11 cm, Table1). The limitedvariation presentin BeisotopeconcentrationacrossHolocenetimescalesprovidescon- fidencethatthedifferencein¹⁰Beconcentrationsinourcoretops across the region reflect geographic rather than temporal varia- tions.

Depthsamples were also takenfrombasal sands and diamict facies interpretedasgrounding lineorsubglacial sedimentsfrom a range of sub-ice shelf and open marine cores (AMO1B [depth 45 cm],AMO4[115 cm],GC16[227 cm],GC24[431 cm]andGC29 [351 cm]).

WeusedaHClextractantchemistryprocesstoreleaseboth⁹Be and¹⁰Befromthesediments(Knudsenetal.,2008).Thisapproach effectivelyremovesthefullauthigenicmeteoric¹⁰Befractionwith- outaccessingin-situ¹⁰Be.Sedimentwascollectedfrom1cmthick sections,dried overnight at100^◦C, andthen reactedwith10 ml of6 MHClfor3h atroom temperature.Thesolutionswerethen splitintotwoaliquotstoseparatelymeasureconcentrationsof⁹Be by ICP-OESandICP-MSatthe UniversityofCanberraandcosmo- genic¹⁰Be byAcceleratorMassSpectroscopy.Thealiquotremoved for⁹Bewas∼^{20%oftotalextractedsolutionandwassufficientto} achieve arepeatabilityof<5%for⁹Be assaysintherange100to 500ppb.

FortheAMSanalysis,thesecond aliquotwasspikedwith0.22 or 0.45 mg of ⁹Be prepared from beryl crystal with a negligi- ble¹⁰Becontent.Berylliumwas isolatedfromthesolutions using methods described in Child et al. (2000), oxidised, mixed with Nb powder and pressed into AMS targets (Fink et al., 2000). Fi-

nal ¹⁰Be/⁹Be ratios were corrected by full chemistry procedural blanks andnormalized to the NIST-4325 ¹⁰Be/⁹Be AMS standard usinga nominal ratioof27,900 × 10⁻¹⁵ (FinkandSmith, 2007;

Nishiizumi et al., 2007). Full procedural blanks gave a mean 10Be/⁹Be ratio of 20 ± ¹⁰ × ¹⁰⁻¹⁵ (n=²), which was equiva- lentto 0.5% the ratiomeasured forthe lowest sediment sample.

Finalanalyticalerrorinconcentrations(atoms/gram)werederived fromaquadraturesumofthelargerofthetotalstatisticalerroror standardmeanerrorfromrepeatAMSratios(typically1%),2%for AMSstandardreproducibilityand1%inBespike.

Toassessthat our chemistrymethods were reproducible with respect to equivalent extraction efficiencies, full replicate proce- dural measurements were carried out on two core-top samples (GC24, AM05) starting from unprocessed sediment sub-samples resulting in ¹⁰Be concentrations consistent within 1 and 5% re- spectively.While6 MHClhasbeenshowntoquantitativelyextract themeteoric¹⁰Becomponentinmarinesediment(Knudsenetal., 2008),wefurthertestedthequantitativenatureoftheHClextrac- tionby conductinga second6 MHCl leachonthe sameleached sedimentaliquotsofGC24-1andAMO5-1usedabove.Thesecond repeat leach extracted a further ∼^{1–3% of the 10Be extracted in} thefirstleach.Asimilarresultwasgivenforwhole-sedimentcon- centrationsofmostmajor elements,although thiswashigher for themorerefractoryelements,suggestingmostofthemoremobile fractions of the sediment had been dissolved by the first leach.

Thesetests suggest that the HClextraction issufficiently aggres- sivetoquantitativelyremovethemeteoric(mobile)componentof thesediment.

To assess that fraction of element concentrations leached via our chemistry procedures to their total whole sediment concen- tration, regardlessofaffinity, we comparedextractedsolutions to assaysinHF-basedtotaldigest (Bourlesetal., 1989).Between10 and30%(meanof23%)ofthetotalBepresentinthesedimentwas extractedbythe 6 M HClleachon freshsediment,valuessimilar to that obtained via weaker extractiontechniques used to target

‘reactive’ ⁹Be in Antarcticsediments (Vallettaet al., 2018). There werenoobvioustrendsbetweenBeextractionefficiencyandsam- ple characteristics such as mineralogy or total Be concentration.

The first HClleach typically extracted∼^{30% ofthe more soluble} major elements (Fe, Mn, Na, K), and 10% of more refractory el- ements(Ti,Al),consistentwithexistingstudies (e.g.Larneretal., 2007)indicatingitiscapableofextractingtheadsorbed,oxide,car- bonateandorganicfractionsofthesediment.

Lastly,we comparedourberylliumisotopemeasurements toa range of sediment characteristics to better understand the pro- cesses that influence isotopeconcentrations andratios. Data was sourcedfromavailablecore-topanalysesofgrainsize,diatomabun- dance, opalandpercentmodern radiocarbon (fromcore-topbulk organic radiocarbon ages) previously analysed from Prydz Bay (O’Brienetal.,1995; Harrisetal.,1998; TaylorandLeventer,2003) andunder the ice shelf (Post et al., 2014). Holocene sedimenta- tionratesateachsitewereestimatedbasedonradiocarbondating whereavailable (Domack et al., 1998; Taylor andMcMinn, 2002;

Post etal., 2014), orthe assumption that biogenic sedimentation began shortly after ice decoupled from the bed across the re- gionaround thebeginningofthe Holocene(Domack etal.,1998;

HemerandHarris,2003).

4. Results

In the Prydz Bay region, we observed distinct differences in 10Be and ⁹Be concentrationsofsediments depositedin thethree majorice-marginaldepositional environments(open marine,sub- ice shelf andinferred subglacial/grounding line; Fig. 3, Table 1).

Be-isotope concentrations and ratios were generally within the range of those reported elsewhere on the Antarctic continental

Fig. 3.¹⁰BevsHClextractable⁹BeinPrydzBay.Openmarineandsub-iceshelfare core-topsedimentsintheirrespectiveenvironmentsinPrydzBayandunderthe AmeryIceShelf,while‘subglacial’sedimentsarebasaldiamictsinselectedcores bothundertheiceshelfandinPrydzBay.Notethatbothisotopesareneededto discriminatesamplesfromthethreeenvironments,thoughallsubglacialsediments canlargely bedifferentiatedby¹⁰Bealone.Errorbarsrepresent 1σ standarderrors.

shelfandslope(Franketal.,1995; Sjunneskogetal.,2007; Valletta etal., 2018).However, therelationshipbetweenenvironmentand Be isotope concentrations was substantively more complex than previously reported fromthecontinentalshelf (Sjunneskog etal., 2007), with isotope concentrations varying by up to a factor of five within each environment. Apart fromlower ¹⁰Be concentra- tionsinsubglacialenvironments,nosingle concentrationprovided a unique signature ofdepositional environment.Spatial trends in theberylliumisotope concentrationsinPrydz Bayprovideinsight into the different sources andprocesses that influence ¹⁰Be and 9Beconcentrationsinicemarginalmarinesediments.

4.1. ¹⁰Be

Modern, core-top¹⁰Be concentrationsinopen marine (∼^200–

1000 × ^{106 at/g) and sub-ice shelf} environments (250–1100 × 10⁶ at/g) displayed substantial variation within, andoverlap be- tween,thetwogeographicallydifferentdepositionalenvironments.

Biogenic poor, basal diamicts inferred to have been deposited in subglacialsettings(30–160×^{106 at/g)weredistinctlylowerthan} modern(coretop)sediments.

Thespatialvariationincore-top¹⁰Beconcentrationsfollowsthe pattern ofopen marine and sub-iceshelf circulation. Higher val- uesarepresentinthecentralandeastern partsofPrydzBay,and the‘inflow’sectorunderneaththeiceshelf.Muchlowervaluesare presentonthewestern,‘outflow’sideoftheiceshelf,andtheopen marine immediatelynorth ofthe calvingmargin of theice shelf, wheresub-iceshelfwaterdischargesintoPrydzBay(Fig.4).

VariationoverHolocenetimes(i.e.down-core)withintheSMO horizons was limited, confirming the environmental conditions duringthisperiodwerebroadlystable(Table1).Thisconfirmsthat core-topsedimentsprovidean accuraterepresentationofmodern depositional processes forboth open marine (GC22) and sub-ice shelfsettings(AM01B).

4.2. ⁹Be

In contrast, HCl extractable ⁹Be variability is more subdued, with intra and inter-group variability halved that seen in ¹⁰Be.

HClextractable⁹Be wasgenerallyhigherunderneaththeiceshelf

Fig. 4.Corelocations(left)andcore-topresultsforHClextractable¹⁰Be(10⁶ at/g),⁹Be(μg/g)and¹⁰Be/⁹Beratio(10⁻⁷at/at)concentrations.Ineachplot,thesamples dominatedbyaterrestrialsedimentsourceofBeisotopeconcentrationsorratiosareshownaswhitedots,andsedimentsdominatedbyanopenoceansourceofBeisotopes areshownasblackdots,witha3levelgrey-scaleforvaluesbetweenthetwolimits.NotethattheiceshelfhasexpandedovercoresiteGC27sincecollection,aspartofa 60–70yr calvingcycle(Frickeretal.,2002).

Fig. 5.RelationshipbetweenpercentModernCarbonand¹⁰Beconcentration(leftpanel;RossSea,Sjunneskogetal.,2007; Yokoyamaetal.,2016andPrydzBay),and<63 μm mudfractiontoBeisotopesvalues(rightpanel;PrydzBay)fromcoretop(i.e.modern)sediments.VerticaldottedlineinlefthandpanelrepresentsthepercentModern Carbon(pMC)ofpre-bombwatersandtypicalcarbonateshellsontheAntarcticcontinentalshelf(BerkmanandForman,1996),includingunderthemodernAmeryIceShelf (Postetal.,2014).

(range = 0.27–0.64 μg/g) than in open marine (0.12–0.25 μg/g), withsubglacialsedimentsshowingintermediatevalues(0.17–0.42 μg/g,Fig.3).

4.3.¹⁰Be/⁹Beratios

The ¹⁰Be/⁹Be ratio inthe HClextractable portion ofthe sedi- mentprovidedimproveddiscriminationbetweenthethreedeposi- tional environments than either of these values alone. The ratio also allowed discrimination of additional geographic trends that werenotobservableintherawconcentrations.

The lowest ¹⁰Be/⁹Be ratios were observed in subglacial sedi- ments(0.1–0.9×¹⁰⁻⁸atoms/atom),whilethehighestvalueswere foundintheopen marine environmentofeastPrydz Bay(5–7 × 10⁻⁸). Sediment in the eastern, inflow side of the iceshelf pro- ducedanarrowrangeof¹⁰Be/⁹Beratios(2.5–2.8×^{10−8),despite} widevariationintherawconcentrationsofboth.Sedimentonthe western,outflowsideoficeshelfcavity(<1×^{10−8)andinopen} marineenvironmentsimmediatelynorthofcalvingmarginonthe outflow(2.2–4.1×^{10−8)providedlowervaluesthantheirequiva-} lentsintheeastern/inflowingsideofthecirculation.

4.4. Comparisontoothersedimentcomponents

Our sampling was biased toward the mud-rich surface sed- iments, namely siliceous muddy ooze. However, within the ge- ographic range that we have covered, there is little observed correlation between grainsize and either ¹⁰Be concentrations or 10Be/⁹Beratios.HClextractable¹⁰Beand⁹Bearepoorlycorrelated with abundance of the mud fraction (R²=⁰.1 and 0.3 respec- tively,Fig.5).Inaddition,inallthreemeasures(⁹Be,¹⁰Be,andthe 10Be/⁹Be ratio), sampleswith 100% mud fraction exhibit the full range of values observed in the dataset. The limited correlation between Be isotope values and the sediment grainsize, suggests that althoughsome partitioningmaybe present(e.g.Wittman et al., 2012), the delivery and deposition of Be isotopes in sub-ice shelfandopenmarineenvironmentsinthissectoroftheAntarctic continentalshelfarenotcontrolledprimarily bygrainsize.Inturn thisimpliesthattheBeisotopesextractedfrombulksediment(i.e.

the HClextractable or‘reactive’ Be) via ourHCl leachingprocess were well equilibrated within the water column prior to being incorporated in the authigenic phase of the sediment. Similarly, theabsenceofcorrelation(R² of0.1orless)betweensedimenta- tionrateandeitheroftheBeisotopeconcentrationsindicatesthat

Fig. 6.Sedimentadvectionandmixingalongabottomcurrentflowlineasinferredfromatwo-componentmixingmodelusingboth¹⁰Beand¹⁴Casproxiesforsediment source.FlowlinedistancefollowsthetrajectoryofbottomcurrentsinFig.2,startingfromoffthecontinentalshelf,movingclockwisetothesoutherngroundinglineunder theiceshelf,andthenreturningnorthalongthewesternboundaryoftheiceshelf.Theoverallreductionintheopenmarinesedimentfractionalongtheflowlinemoving frominflowtooutflowzones(i.e.withincreasingdistanceunderneaththeshelf)suggests¹⁴Cand¹⁰Beconcentrationsareprimarilycontrolledbysedimentadvection,andan increasingadmixtureofcontinentalsediment.Secondaryprocesses(suchasverticalmixing,storage,crosscurrentflows)mayexplaintheoffsetfromasimpletwo-member linearmixingmodel.

core-sitesedimentationratesare nottheprimarycontrol onlocal 9Beor¹⁰Beconcentrations.

Thecorrelation betweenthe¹⁰Be concentrationswas explored against several measures of the biogenic componentof the sed- iment, including diatom abundance, opal and radiocarbon reser- voir ages. Diatom varve counts are available for most sub-ice shelf (Post et al., 2014) and a few open marine sites (Taylor and Leventer, 2003), while measurements of biogenic opal are only available for open marine sediments (O’Brien et al., 1995;

Harris et al., 1998). Both measures (diatom count and biogenic opal) correlate only weakly with both ¹⁰Be concentration (R²= 0.2)and¹⁰Be/⁹Beratios(R²=⁰.3).Berylliumconcentrationsand ratios are much more strongly correlated with the radiocarbon

‘age’, perhaps best expressed by the linear correlation between percentModernCarbon(pMC;definedasthemeasuredsamplera- diocarbonnormalisedtoatmosphericradiocarboncontentat1950 referencedtotheOxalic acid-IIstandard)and¹⁰Be concentrations (R²=⁰.5)and¹⁰Be/⁹Beratios(R²=⁰.8;Fig.5).

Tellingly,thelineartrendbetween¹⁰BeconcentrationandpMC insurfacesedimentsintersectstheorigin(Fig.5).Thisresultwould be expectedif ¹⁰Be concentrationsare a product of two compo- nent mixing, withendmembers beingdominatedby terrigenous sediments(low/nearzero¹⁰BeandpMC)andbiogenicmarinesed- iments(high¹⁰BeandpMC)respectively.Withinthesub-iceshelf cavity,both the ¹⁰Be concentrationsandpMCdecrease alongthe flow-pathofsub-iceshelfcirculation(Fig.1,Postetal.,2014),with thelowestvaluesofbothfoundatAM05ontheoutflowsideofthe cavity.

5. Discussion

5.1. Whatdeterminessediment¹⁰BeconcentrationsinAntarcticshelf environments?

Thehigh¹⁰BeconcentrationsfromunderneathAmeryIceShelf (AM01–6corestops)whichoverlapwithopenmarinevaluesare incompatible with a ‘calving dominant’ process of ¹⁰Be distribu- tionasshowninFig.1(top).Thiscontrastswiththatobservedby Yokoyamaetal. (2016) inastudyofRossIceShelfretreat.Here,we considerthatallthreeprocessesdepictedinFig.1–calving,sub- glacialmelt, andstrongbottom(sub-surface cavity)currents may allhavearoleininfluencing¹⁰BeconcentrationsinthePrydzBay sub-iceshelfregion.

Toaidinterpretationofthe¹⁰Bedistribution,weuseatwoend- membermixingmodel,where sedimentis derived fromtwopri- marysources:(1)openmarinesettingsand(2)continentalsources derivedfromiceerodingbedrockandoldersediments.Thepropor- tion of sediment mixing is estimated using end-member source values for ¹⁰Be and ¹⁴C as proxies along the clockwise current gyre commencing atthe continental shelf break to the southern groundingline,returningnorthwardstotheshelfbreakforatotal distance of about 1400km. The model successfully captures the first-order mixingbehaviour ofthe two isotopesseen in theraw dataofFig.6,suggestingatwo-componentmixingistheprimary controlonthe¹⁴Cand¹⁰Beconcentrationsinthesediments.How- ever,giventhepotentialcomplexityinthesystemwealsoidentify instanceswhereother factorsmayhaveinfluencedconcentrations oftheseisotopesinsedimentsinPrydzBay.

Sedimentoriginatingfromtheopenmarineenvironmentispri- marily biogenic, with thepotential fora mineral component de- rivedfromicerafteddebrisanddust.¹⁰Beconcentrationsinthese environments are a function ofthe ¹⁰Be deposition rate(derived from direct atmospheric deposition and ice melt) and the sedi- ment mass flux. The maximum core top ¹⁰Be concentrations in open marine settings in Prydz Bay (∼^800–1000 × ^{106 at/g) are} likethosedepositedinthemodernopenocean(vonBlanckenburg etal.,2015).However,theaverageconcentrationinPrydzBaysed- imentsarelowerthanthoseobservedintheRosssea(Sjunneskog etal.,2007) (Fig.5).Thus, thetrue¹⁰Beconcentration ofrecently producedbiogenicsedimentinPrydzBaymaybepartiallymasked bybottomcurrentsredepositingreworkedsedimentfromthecon- tinental shelfwithlower ¹⁰Beconcentrations.However, thesimi- larityinthecore-topsedimentsacrosseastern PrydzBaysuggests thatifthisisthecase,thesedimentspresentlybeingdepositedon the continental shelf are relatively well mixed, eitherin the wa- tercolumnorbypost-depositionalreworkingbybottomcurrents.

Basedonthisinterpretation,we assigntheendmembervaluesin our mixingmodelforopen marinesedimentsof 74%modern for 14Cand877×^{106 at/gfor10Be,calculatedastheaverageofcore} top-sedimentsfromPrydzChannel(GC22–27).

Sedimentderivedfromprimary continentalsourcesisassumed to be dominantly lithogenic, sourced by glacial erosion of pre- Cenozoic (and largely Precambrian) bedrock that lies underneath most of the modern Lambert Glacier–Amery Ice Shelf drainage system (Mikhalsky et al., 2001). The subglacially derived sedi- mentisexpectedtohaveverylow ¹⁰Be concentrations,reflecting the lithogenic sources, although subglacial melt of meteoric ice

may contribute some ¹⁰Be to the sediment (Graly et al., 2018).

Thesesourcesedimentsarealsoexpectedtohaveabiogeniccom- ponent derived from Cenozoic and Mesozoic sediments present underneath the modern outlet glaciers (e.g. Mishra et al., 1999;

Whiteetal., 2011),which wouldincludean organiccarbon frac- tionwitheffectivelyzeropMC.Sub-glacialbasaldiamictssampled fromthePrydz Baycontinentalshelf have¹⁰Be concentrationsof

∼¹⁰⁰ × ^{106 at/g (Table 1, GC cores) withlittle spatial variation} ortemporalvariationduring formerglacialadvances acrossPrydz Channel (Guitard, 2015). Concentrations from the basal diamicts further south underneath the Amery Ice Shelf (∼³⁰ × ^{106; Ta-} ble 1, AM0 cores) are lower than in Prydz Bay, likely reflective of an increased continental component, which dilutes the open marine signal or alternatively a reducedinput ofreworked open marine material. None of the subglacial sediments we measure frombeneath theAmery Ice Shelf have¹⁰Be concentrations that areaslowasthosemeasuredinmodernsubglacialsettings(∼⁸× 10⁶atoms/g,Sjunneskogetal.,2007),suggestingthatwemaynot havesampledthisprimary source.Forourcontinentalendmem- berinourmixingmodel,weutilizethebasaldiamictsfromunder theiceshelf(AM01&4,¹⁰Beof34×^{106 at/g,assumedradiocar-} bondead).

Weinterpret ¹⁰Be and ¹⁴C concentrationsatour samplinglo- cationsalong theclockwisebottomcurrentflow-linethatfall be- tweenthesetwocomponentsources (i.e.subglaciallithogenic,34

×^{106 at/g,andzeropMCwithopen marine877} ×^{106 atoms/g} and74%pMC)tobeprimarilytheresultofatwo-sourcesediment mixing process (see Fig. 6). Mixing could occur either by (hori- zontal)mixingofprimarysourcesinthemodern-dayenvironment viaoceancirculation,orby(vertical)mixingofsedimentbyglacial deformationofunderlying,andthuschronologicallydifferinghori- zons.

Both the ¹⁰Be and ¹⁴C concentrations incore topsunder and nearthe moderniceshelf provideevidenceforhorizontalmixing ofthe two primary sediment sources on length scales ofseveral hundredkilometres(Fig.6).Someofoursub-iceshelfsitesonthe eastern,inflow side ofthe shelf (AM01, 2and3) have ¹⁰Be con- centrations identical to those deposited in modern open marine environments in Prydz Bay (GC24, 27 and 29), and thus are in- ferred to have sourced almost all their sediment fromadvection fromtheopenmarineenvironment.Thesehighconcentrationsare primarilytowardthefrontoftheiceshelf,butalsointhecentral portion(AM03)wheremodelledsub-iceshelfcirculationisdirectly fromPrydz Bay(Galton-Fenzi et al., 2012) with little interaction withthe ice shelf margins. Other proxies in these sites such as diatomconcentrations (Postet al., 2014) indicatethat sediments deposited at AM01, 2 and 3 are also compatible with a wholly openmarinecomponentofthesediment.

Mixing-modelvaluesusingtheradiocarbonconcentrationssug- gestalower openmarinecomponentinsomeofthesub-iceshelf core-topsthan¹⁰Be(AM01 & 3,Fig.6), whichmaybe afunction ofeitherdifferentialtransportpathwaysfor¹⁰Beand¹⁴C,orsome reworking of late Quaternary open marine sediment fromwhich 14Chasdecayed.

Furthersupportforthebottomcurrentadvectionmodelispro- vided by the ¹⁰Be and ¹⁴C concentrations at AM05, which sits on a modelled outflowzone (∼^{1200km along the mixing line).}

Whileconcentrationsarelow(∼250×10⁶at/g),theyneverreach theverylowconcentrationsseeninmodernsubglacialconditions in west Antarctic outlet glaciers (Sjunneskog et al., 2007). There are two possibilities for thelack of a dominantcontinental ¹⁰Be signature in the sub-ice shelf sediments atthis location;(1) ad- vection ofsediment fromtheopen marine settingextends asfar south as∼ ^{200km from themodern iceshelf edge against the} outflow zone, so none of our sites are truly dominated by sub- glacialsediment(2)¹⁰Beisreleasedfrommeltofthebaseofthe

iceshelf,andattachestocontinental-derivedsedimentdeliveredat thegroundinglineandmovedbyoutflowcavitycurrentsinquan- tities sufficient to provide a ‘mixed’ ¹⁰Be signal (i.e. the sub-ice shelfmelt processofFig.1).Weconsiderthereissupportingevi- denceforbothprocesses.AM05haslittletonodiatomabundance, suggestinglimitedadvectivetransportofclayorsiltsizedparticles fromopenmarinesettings,whichwouldmeanthe¹⁰Beatthissite isderivedfromlocalsources(i.e.sub-iceshelfmelt).However,ra- diocarbonconcentrationsatthissiteimplyasmallfractionofthe organiccomponentofthesediment (∼^{10%)isderivedfrommod-} ernsources,supportingadvection(Postetal.,2014).Ineithercase, themeasured¹⁰Beconcentrationsintherangeof∼²⁰⁰×^106at/g observed atAM05inthe outflowarea, andthecore-topforRISP 79–14at the rear ofthe RossIce Shelf (Sjunneskog etal., 2007) mayapproximatethelowerrangeof¹⁰Beconcentrationsbeingde- positedundermodernAntarcticiceshelves.

Thisdistributionof ¹⁰Be, incombinationwithsimilar patterns for other proxies of sediment transport (diatoms & radiocarbon), providesupportfortheconceptualmodelthat strong-bottomcur- rentsareakeyprocessindetermining¹⁰Beconcentrationsinsub- iceshelfenvironments.

5.2. Beisotopesastracersofpaleo-iceshelvesandsub-iceshelf circulation

For ice-shelveswith morelimited sub-ice shelf current circu- lationandwherecurrentvelocitiesarelowerthanneededforsilt suspension and transport, the dilution of the ice shelf ¹⁰Be sig- nalmaynot beassignificant aswe observeinPrydz Bay.Hence, in those regions where advection is low and/or circulation does nottransfersedimentfromtheiceshelfcalvingmargintowardthe groundingline,shiftsin¹⁰Be concentrationsthroughtimemaybe representative of,andthus interpreted as, a changefrom sub-ice shelftoopenmarinesettings(e.g.Yokoyamaetal.,2016).

In contrast to the above, in regions withstrong sub-ice shelf currentsandsediment advection, itmaynot be possibletoiden- tifyaspecific¹⁰Beconcentration(orrange)thatisuniquetoeither openmarine orsub-iceshelfsettings.Thisisthecasewe observe in theAmery IceShelf – Prydz Baydata.In thiscase, the distri- butionof ¹⁰Be isprimarily affectedbythe strengthanddirection ofsub-iceshelfcirculation.Thus,thereispotential forusing¹⁰Be to identify whether or not changes in sub-shelf circulation have influencedpaleo-icesheetfluctuations.

Where strongsub-shelf circulations arepresent, the combina- tion ofboth Be isotopes, inthe Prydz Baydata, i.e.the ¹⁰Be/⁹Be ratio, appears to provide a better distinction between the three environments than ¹⁰Be in isolation. Due to the lower ‘reactive’

9Be concentrations (i.e.HCl extractable)inopen marine coretop sediments, corresponding ¹⁰Be/⁹Be ratios are twice that of sub- iceshelfsediments,whichinturnarefivetimeslargerthanratios insubglacialbasalsediments.Moreover,the¹⁰Be/⁹Be displayless variability within each environment than either of the isotopes alone. We attribute both the spatial pattern and relatively low variability of ¹⁰Be/⁹Be to (a) input of ⁹Be into the sub-ice shelf cavityfromaterrestrialsource(b)thelongresidencetimeofbot- tomwaters (2 yrinthecavity)andpotential foreddycirculation (Galton-Fenziet al.,2012), whichcan thusproduce a well-mixed watermassinthecavity,and(c)equilibrationofthe¹⁰Be/⁹Beratio characterizing the sediment source withthe(dissolved) ¹⁰Be/⁹Be of the water column while the sediments are transported (e.g.

Brown etal., 1992). Thus, the combination ofthe two process – mixingandequilibration–resultsinsediment¹⁰Be/⁹Beratiosthat aremorehomogeneous withineachenvironmentthanacrossenvi- ronments.Thisexplainstheconsistencyof¹⁰Be/⁹Bewithineachof thethreeenvironmentsdespiteclearlyfarlargervariabilityin¹⁰Be abundances. Thus, we conclude that ¹⁰Be/⁹Be ratios are a more