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Global Ecol Biogeogr. 2024;00:e13844.
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1 of 18
https://doi.org/10.1111/geb.13844
wileyonlinelibrary.com/journal/geb
Received:8Januar y2023
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Revised:5M arch2024
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Accepted :21March2024
DOI : 10.1111/geb .138 44
RESEARCH ARTICLE
Testing the deep- sea glacial disturbance hypothesis as a cause
of low, present- day Norwegian Sea diversity and resulting
steep latitudinal diversity gradient, using fossil records
Anna B. Jöst1,2 | Huai- Hsuan M. Huang3 | Yuanyuan Hong4,5 | Chih- Lin Wei6 |
Henning A. Bauch7 | Benoit Thibodeau8 | Thomas M. Cronin9 | Hisayo Okahashi4,5 |
Moriaki Yasuhara4,5
1Korea Institute of Ocean Science and Technolog y, Tropical and Subtropical Research Center, Jeju- si, Republic of Korea
2Depar tment of Life Science, College of Natural Sciences, Hanyang University, Seoul, Republic of Korea
3Depar tmentofGeosci ences,PrincetonUniversity,Princeton ,NewJer sey,USA
4SchoolofBiologicalSciences,AreaofEcologyandBiodiversit y,SwireInstituteofM arineScience,Instit uteforClimateandCarbo nNeutr alityandMuske teers
Foundat ionInstituteofDataScie nce,TheUniversityofHongKong,H ongKongSA R,Chi na
5StateKeyLaboratoryofMarinePollution,CityUniversityofHongKong,H ongKongSA R,Chi na
6Institute of Oceanography, National Taiwan University, Taipei, Taiwan
7Alfred-Wegener-Inst itutHelmholt zCente rforPolarandOcea nResearch,c/oGEOMA RHelmholtzCentreforOceanResearchK iel,Ki el,Ger many
8SimonF.S.LiMarineS cienceL abor atory,SchoolofLifeSciences,TheChines eUniver sityofH ongKong,HongKongSAR,China
9FlorenceBascomG eoscienceCenter,U.S.Ge ologic alSur vey,Reston ,Virginia,USA
Correspondence
AnnaB.Jöst,Kor eaInstituteofOcean
Science and Technology, Tropical and
Subtropical Research Center, Iljudong- ro
2670, Gujwa- eup, Jeju- si, Jeju- do 63349,
Republic of Korea.
Email:annajoest@outlook.com
MoriakiYasuhara,SchoolofBiological
Sciences, The University of Hong Kong,
Kadoorie Biological Science Building,
PokfulamRoad,HongKongS AR,Ch ina.
Email:moriakiyasuhara@gmail.com
Funding information
U.S. Geologic al Sur vey Climate Research
and Development Program; Peter
BuckPostdocFellowship,S mithsonian
Instit ution;Ministr yofScie nceandICT,
SouthKor ea,Gr ant/AwardNumber:
2019H1D3A1A01070922;t heEcology
and Biodiversity Division Fund, Grant/
AwardNumber:5594129;Facult yof
ScienceR AEIm proveme ntFundofthe
University of Hong Kong; Seed Funding
Program for Basic Research of the
UniversityofHo ngKong,G rant/Award
Number :20121015904 3,201411159017,
Abstract
Aim: Wit hint heintensively-studied, well-documented latitudinal diversity gradient,
the deep- sea biodiversity of the present- day Norwegian Sea stands out with its notably
lowdiversity,constitutinga steeplatitudinal diversitygradientin the NorthAtlantic.
Thereasonbehindthishaslongbeenatopicofdebateandspeculation.Mostpromi-
nently, it is explained by the deep- sea glacial disturbance hypothesis, which states that
harsh environmental glacial conditions negatively impacted Norwegian Sea diversi-
ties, which have not yet fully recovered. Our aim is to empirically test this hypothesis.
Specific research questions are: (1) Has deep- sea biodiversity been lower during gla-
cials than during interglacials? (2)WasthereanyfaunalshiftattheMid-BrunhesEvent
(MBE)whenthemodeofglacial–interglacialclimaticchangewasaltered?
Location: NorwegianSea,deepsea(1819–2800 m),coringsitesMD992277,PS1243,
andM23352.
Time period: 620.7–1.4 ka(MiddlePleistocene–LateHolocene).
Taxa studied: Ostracoda (Crustacea).
Methods: We empirica lly test the dee p-sea glac ial disturbance hy pothesis by in-
vestigating whether diversity in glacial periods is consistently lower than diversity
in interglacia l periods. Addit ionally, we apply comparati ve analyses to determi ne
This is an open access article under the terms of the CreativeCommonsAttribution-NonCommercial-NoDerivs License, which permits use and distribution in
anymedium,providedtheoriginalworkisproperlycited,theuseisno n-commercialandnomodi ficat ionsoradaptat ionsaremade.
©2024TheAut hors.Global Ecology and Biogeographypublis hedbyJoh nWiley&SonsLtd.
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1 | INTRODUCTION
Macroecological patterns, such as the classic deep-sea biodiversity
patterns, namely the latitudinal diversity gradient and the depth di-
versit y gradient, wer e first discovere d in the North At lantic Ocean
(Ramirez-Llodraetal.,2010; Rex&Etter,2010). The deep- sea latitu-
dinal diversity gradient, which defines a decrease in deep- sea species
diversity with increasing latitude, has since been intensively studied in
that area (Corliss et al., 2009;Jöstetal.,2019; Lambshead et al., 2000;
Rex et al., 1993, 2000; Tittensor et al., 2011; Yasuhara,Okahashi,&
Cronin, 2009). The Norwegian Sea, which is situated within the North
AtlanticGateway(Figure 1;alsoseeJöstetal.,2019),ischaracterized
by a low deep- sea biodiversity that constitutes an important part of
theNorthAtlanticdeep-sealatitudinaldiversitygradient.Notablylow
benthic alpha diversities in the Norwegian Sea have been reported
across many taxonomic groups, for example in gastropods, bivalves
and isopods (Rex et al., 1993 , 2000; Svavarsson, 1997; Svavarsson
et al., 1993), amphipods (Dahl, 1979), nematodes (Lambshead
et al., 2000), foraminiferans (Culver & Buzas, 2000) and ostra-
cods (Jöstetal., 2019; Yasuhara,Hunt, et al., 2009; Yasuhara,Hunt,
Dowsett, et al., 2012), resulting in a low regional benthic diversity.
ManyresearchersattributethislowNor wegianSeabenthicdeep-sea
biod ivers ity to ,a tl eas ti npart, “Q uat er nar y gl aciat ion”.A ss um mar iz ed
in Rex et al. (1997) an d Culver and B uzas (2000), it is often implied
that a glacial disturbance (or the effect of glaciation: Rex et al., 1993)
had, at the time, negatively affected the Norwegian Sea deep- sea
benthic diversity to an extent that it has not yet fully recovered from
the disturbance. However, there are two uncertainties in their argu-
ments, which entail the exact meaning of (1) “Quaternary glaciation”,
and (2) “glacial disturbance”. Some papers specify the time of the dis-
turbance, estimating it to have occurred during the last glacial period
(Svavarsson, 1997),whereasothersgeneralizebysaying“Quaternary
glaciation”, or “Pleistocene glaciations”, etc. (Bodil et al., 2011;
Lambshead et al., 2000; Rex et al., 1993, 1997, 2005). Hence, the
meaning of “Quaternary glaciation” should be either the last glacial
period (a.k.a.last iceage,70,00 0to11,700 yearsBP)ormultiple gla-
cial periods during the Pleistocene. The term “glacial disturbance”
may imply that harsh environmental glacial conditions eradicated the
deep- sea fauna and negatively affected the deep- sea diversity in the
Norwegian Sea at that time, although the responsible environmental
parameter(s) has (have) never been specified (Bodil et al., 2011; Culver
&Buzas,2000; Rex et al., 1993, 2005;Stuar t&Rex,2009).
Paleobiology uses microfossils in sediment cores as a tool to
reconstruct past ecosystem and biodiversity dynamics with high
time resolution and accuracy, also known as “time machine bi-
ology” (Yasuhara, 2018; Yasuhara et al., 2015, 2020; Yasuhara
& Deutsch, 2022). Marine ostracods (Crustacea, Ostracoda,
Podocopida) are a large group of small benthic (i.e. meiobenthic)
invertebrates, andarethebest representedmarine metazoans, ec-
dysozoans a nd arthropo ds in sediment core -base d fossil research
(Schellenberg, 2007;Yasuharaetal.,2022;Yasuhara&Cronin,2008).
Theirwell-calcifiedfossilizedremainsrepresentamajorpartoftheir
body (carapace) and are preserved in small- volume samples from
sediment cores abundantly enough for rigorous statistical analyses
of their diversity and faunal composition. This is in contrast to most,
ifnotall,othermetazoaninvertebrates, asthey eitherhave limited
fossilizationpotential(duetoalackofhardparts)orlowabundance
(becauseoftheirlargesize).Thus, ostracodsareanexcellent model
system representing, especially, small marine invertebrates (Chiu
et al., 2020) that account for about two thirds of marine biodiversity
(Leray&Knowlton,2015).
The Mid-Brunhes Event (MBE) refers to a major shift in
Pleistocene climate (at ~430–350 ka) t hat marks the f inal stage of
theclimatetransitionfromlow-amplitudetohigh-amplitudeglacial–
interglacial climatic variability (Cronin et al., 2010, 2017; DeNinno
et al., 2015; Yin & Berger,2010). This fundamental shift in climate
201511159075,202011159122and
2202100581;theResearchGrantsCouncil
oftheHongKongSpecialAdministr ative
Region,C hina,G rant/AwardNumber:
HKU 17301818, HKU 17311316 and
RFS2223- 7S02; the Korea Institute of
Ocean Science and Technology, Grant/
AwardNumber:PO01471andPEA0205
Handling Editor:AdamTomasovych
apotential faunalshift at the MBE,aPleistocene event describing a fundamental
shift in global climate.
Results: The deep Norwegian Sea diversity was not lower during glacial periods com-
pared to interglacial periods. Holocene diversity was exceedingly lower than that of
the last glacial period. Faunal composition changed substantially between pre- and
post-MBE.
Main conclusions: These results reject the glacial disturbance hypothesis, since the
low glacial diversity is the important precondition here. The present- day- style deep
Norwegia n Sea ecosystem was es tablished by the MBE , more specificall y by MBE-
induced cha nges in global climate, w hich has led to more dyn amic post-MBE co ndi-
tions. Inabroadercontext, this impliesthat the MBE has playedanimportantrolein
the establishment of the modern polar deep- sea ecosystem and biodiversity in general.
KEYWORDS
deep-seadiversity,faunalturnover,macroecologicalpatterns,Mid-BrunhesEvent,North
Atlantic,Ostracoda
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JÖST et al.
involves lar ge- scale changes in ice shelf de velopment, sea ice vo lume,
global ocean circulation and potentially marginal marine systems that
are sensitive to changes in climate (Huang et al., 2018). From analy-
ses of microfossils in deep- sea sediment cores, major faunal shifts
andextinctionshaverecentlybeenrepor tedacrosstheMBEinvari-
ous places (Cronin et al., 2014, 2017; DeNinno et al., 2015; Hay ward
et al., 2007, 2012; Huang et al., 2018, 2019; Polyak e t al., 2013;
Zariki an et al., 2022). If “Quaternary glaciation” plays a role in the
dee pNo rwegianSeabiodi versity,ashi ftint hemod eofgla cia l–inter-
glacial climatic variability may affect it to a certain degree.
The above- mentioned glacial disturbance hypothesis as reason for
the low deep- sea benthic biodiversity in the present- day Norwegian
Sea has never been rigorously tested. Here, we empirically test this
hypothesis by using an alpha- scale diversity dataset of fossil ostra-
codsin thesedimentcoreMD992277andpublishedostracodcensus
datafromtwo additionalsedimentcores(M23352andPS1243)from
thedeepNor wegian Sea. Specificresearch questionsare:(1)Wasthe
benthic deep- sea biodiversity lower during glacials than during inter-
glacials? (2) Was th ere any faunal sh ift at the MBE wh en the mode
ofglacial–interglacialclimaticchangewasaltered?Ourpaleobiological
result shows that the actual long- term benthic deep- sea biodiversity
trend in the Norwegian Sea was not consistent with the trend pre-
sumed by the glacial disturbance hypothesis, thus rejecting this hy-
pothesis. Benthic deep- sea biodiversity in the glacial Norwegian Sea
was not lower than that of interglacial periods, including the present
day.EspeciallytheHolocene diversityismuchhigherthanthatofthe
last glacial period, therefore, the present- day deep Norwegian Sea
benthic diversity cannot be in the middle of its recovery process. This
result highlights the importance of paleobiological data for empirically
testing hypotheses to explain present- day diversity patterns that often
involve speculations about historical processes.
2 | METHODS
2.1 | Coring location and sample treatment
Althou gh the sediment of e ach core was sieved into va rying size
fractions,ultimately,allostracodspecimenswerepickedfromsedi-
ment of >125 μm,thatismeshsizeeffec tcanbedisregarded.
Piston cor e MD992277 (in the follow ing referred to as jus t
MD992277) w as obtaine d with a Caly pso Piston C orer onboa rd
FIGURE 1 LocalitymapdepictingthethreecoringsiteswithintheNordicSeasandtheJanMayenfracturezone.Ridges:whitelinesbased
on1000,2000,and350 0 mdepthcontour,respectively.Coringlocationofstudiedcoreisindicatedbybluestar(MD992277);coringlocations
ofreferencedcoresindicatedbyreddot(M23352)andblacksquare(PS1243),respectively.MD992277:Pistoncore(Labeyrieetal.,1999);
M23352:KastencoreM23352(-3)(Hirschleberetal.,198 8),PS1243:CompositecoreofPS1243(-1)andPS1243(-2)(Augsteinetal.,198 4).
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JÖST et al.
the French R/V Marion- Dufresnein1999aspartoftheIMAGES
program (02.08.1999; 5th cruise, leg 3: Reykjavik, Iceland to
Tromsø, Norway; Labeyrie et al., 1999). The coring site was the
western Norwegian Basin at the eastern slope of the Iceland
Plateau,alongtheeasternflankoftheJanMayenRidgeat2800 m
waterdepth(69°15.01′N, 06°19.75′W) (Figure 1; Table S1, given
asAppendix1 in Online Supplement S1).Atotallengthof33.66 m
ofsedimentcorewithadiameterof12 cmwasretrieved.Thecore
was cut lengthwise, in half. Only one of the half- cores was sam-
pled; the other one was archived. The sampling half- core was cut
into 1-cm s ediment sam ples, ever y 1, or altern ating 2 and 3 cm,
depending on core section. Roughly ¾ of the half- core was sam-
pled. The sediment volume was ~27 cm3 per each 1- cm sediment
layer. This stud y investigated 1-cm thick sedim ent samples of a
7.11 m long core segment (10.45–17.56 m original core depth,
equival ent to 620.7–361.6 ka; Table S1, given as A ppendix 1 in
Online Supplement S1). A tot al of 432 sample s was studied fo r
ostracods (see raw census file supplied as S2). Each sample was
split into 2and3grainsizefractions, respectively,dependingon
core section. The core section of this study contained 397 sam-
plesthatweresplitinto3grainsizefractions:125–250,250–500,
and >500 μm,and35samplesthatweresplitinto2grainsizefrac-
tions:150–500and >500 μm. Fractions were merged for ostracod
censusdatatogetcompletesamplesofaspecificage.Effectively,
all sediment of >125 μmwaspickedforostracods.Ostracodspeci-
menswe repi ck ed ,s orted,an di denti fiedattheUn iv er sit yofH ong
Kong(HKSAR) andHanyang University (ROK).Taxonomicidenti-
fication was based mainly on Sylvester- Bradley (1973), Whatley
and Coles (1987), Cronin (1989), Coles et al. (1994), Whatley
et al. (1996, 1998), Stepanova et al. (2004),Wood(2005),Jellinek
et al. (2006), Yasuhar a, Okahashi, and Cronin (2009), Yasuhara
et al. (2013),Yasuharaetal.(2014),YasuharaandOkahashi(2014,
2015), Gemer y et al. (2015).
Kasten core M23352(-3) (in the following referred to as sim-
ply M23352) was obtained with a Kastenlot Corer onboard the
German R/V Meteor in 1988 as part of the Meteor 7expedition(M7;
Hirschleber et al., 1988). The coring site was the western part of the
southe rn Norwegian B asin at the easter n slope of the Icela nd Plateau,
along the n orthwester n flank of the Jan Maye n Ridge at a water
depth of1819 m(70° 00.4′N,12°25.8′W)(Figure 1; Table S1, given
asAppendix1 in Online Supplement S1). The sampling half- core was
cutinto1-cmsedimentsamples,every1–3 cm,fromtheuppermost
layer down to 350 cm, whereas the uppermostlayers werespliced
together with an additional trigger boxcore to ensure undisturbed
topsedimentlayers(Didié&Bauch,2002). Sediment samples were
dividedintograinsizefractionsof125–250,250–500,and >500 μm.
Ostracodswerepicked,countedandidentifiedfromtheseseparate
sizefractions, although theresultswere later merged for ostracod
censusdatatogetcompletesamplesofaspecificage.Effectively,all
sediment of >125 μmwaspickedfor ostracods. Thisstudyincluded
143sediment samplesof1 cm thickness ofa3.48 m-longcoreseg-
ment (2–350 cm original core depth, equivalent to 194.5–4.2 ka),
yielding a total of 26,138 ostracod valves (Figure 2; Table S1, given
asAppendix1 in Online Supplement S1). Ostracod census data (S2)
and the age model (Figure 2) are from Didié et al. (2002).
Gravity core PS1243(- 1) was obtained with a Gravity Corer on-
board the German R/V Polarstern i n 1984 as part of the A rktis 2
program(ARKII/5)togetherwiththetriggerboxcorePS1243(-2)ob-
tained with aBoxCorer(Augsteinetal.,198 4). The coring site was
theIcelandSeaattheeasternslopeoftheIcelandPlateauat2710 m
[PS1243(-1)] and at 2716 m [PS1243(-2)] water depths, respec-
tively, along the eastern flankofthe Jan Mayen Ridge [PS1243(-1)
at 69°22.3′N, 06°32.1′W; PS1243(-2) at 69°22.5′N, 06°32.4′W]
(Kandiano, 2003) (Figure 1; Table S1,givenas Appendix1 in Online
Supplement S1). In case of the main core PS1243(- 1), fifty- five
sedimen t samples of 1 cm t hickness of a rou ghly 7.47 m-long cor e
segment (8–755 cmoriginal core depth, equivalentto339.9–2.7 ka)
treated with a sieve of >125 μmmeshsize,yieldinga totalof 2853
ostracod valves, were included in this data set (Appendix 1 in
Online Supplement S1). The core hada diameter of 10 cm and the
samplin g half-core was s ampled every 1 c m throughout. For l ow-
specime n-co unt samples w ithin the upp ermost 50 cm of s ea floor
sediment, additional sediment from the trigger boxcore was used
to increase specimen counts to >10 valves. From this additional
boxcore PS1243(−2), six sedime nt samples of 1 cm thic kness of a
37 cmcore segment(1.5–38.5 cmoriginalcoredepth,equivalentto
10.9–1.4 ka), yielding a total of 119ostracod valves,were included
inthisdataset (Appendix 1 in Online Supplement S1). The boxcore
hadasizeof50× 50× 50 cmandwassampledevery2 cm,withaddi-
tionalsamplingby10 mLsyringesforaccumulationratecalculations
(Bauch,Struck,&Thiede,2001). Sediment samples were sieved into
63–125 and >125 μm size frac tions for foramin iferal assemblages,
whereasthesmallersizefractionwasnotusedforpickingostracods.
Ostracod census data and the age model for composite core PS1243
[i.e. merged cores PS1243(- 1) and (- 2); in the following referred to
as simply PS1243] are from Cronin et al. (2002)andBauch,Struck,
and Thiede (2001),respectively.Allsedimentof>125 μmwaspicked
for ostr acods. A tota l of 3072 ostraco d valves from P S1243we re
included in this study (S2).
2.2 | Chronology
Age models (i.e. age/depthrelationships)and linearsedimentation
rates of cores M23352 (from the northwestern flank of the Jan
MayenRidge) and PS1243andMD992277 (from theeasternflank
oftheJanMayenRidge)areplottedinFigure 2.
2.2.1 | ForthecoreMD992277
The chronology applied was published in Helmke, Bauch, and
Erlenkeuser(2003), Helmke, Bauch, andMazaud(2003). Magnetic
inclination, sediment density, lightness and carbonate content were
usedassedimentaryclocks.Theadoptedageschemeretrievedfrom
thesesedimentologicalparameterswascalibratedandstandardized
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JÖST et al.
by correlation to the benchmark δ
18O chronology records of
SPECMAPbylinearinterpolationbetweenthepointsofcorrelation.
Surfacelayers(uppermost50 cm ofsediment) werecorrelatedwith
PS1243duetocorer-relateddisturbanceinsedimentlayers(Helmke,
Bauch,&Mazaud,2003).Well-documentedcharacteristictrendsin
sediment density, lightness, carbonate content, foraminiferal fluxes,
and the amount of coarse lithic fraction in respect to Pleistocene
Nordic Sea sediments, were used to identify cold and warm marine
isotopestages(MIS)(seeBauch&Kandiano,2007;Elliotetal.,1998;
Helmke,Bauch,&Mazaud,2003).MISboundariesweresetaccord-
ingtoLisieck iandRay mo(2005). The core section used in this study
wasdeterminedto span from thebeginningof MIS 15(620.7 ka)to
thebeginningofMIS10(361.6 ka)(seeTable S1,givenasAppendix1
in Online Supplement S1). Further details regarding core chronology
aregiveninAppendix2 in Online Supplement S1.
2.2.2 | ForthecoreM23352
ThechronologyappliedwaspublishedinBauchandHelmke(1999),
Didié and B auch (2002), a n dHe l m k eand B a u c h(2003). Foram iniferal
oxygen isotope analysiswas basedonplanktonic and benthic spe-
cies, whereas for isotope analyses on ostracods, specimens of two
genera with near- continuous time- record were used. Overall, the
agemodelisbased onthesynchronizationofplanktic foraminiferal
δ18Oandthe sediment lightness recordtothestandard SPECMAP
chronology after applying a smooth filter with a 14- point least
squares r unning average (Helmke & Bauch, 2003). This study in-
cludesM23352sedimentsbetween194.5–4.2 ka(Table S1, given as
Appendix1 in Online Supplement S1). Further details regarding core
chronologyaregiveninAppendix2 in Online Supplement S1.
2.2.3 | ForthecorePS1243
The chronology applied was partially based on published records
in Bauch (1997),Bauch andHelmke(1999), and Bauch et al. (2000).
Planktic foraminaferal δ18O records were correlated to carbon-
ate content to establish the downcore positions ofMIS (Kandiano
et al., 2016). Additionally, the reflectance of sediments was meas-
ured across glacial and interglacial sections of the core and com-
pared to the results of other Norwegain Sea cores, including
M23352 (Bauch & Helmke, 1999). The adopted age scheme was
calibr ated and sta ndardized by cor relation to th e benchmark δ
18O
chronologyrecordsofSPECMAP(Bauchetal.,2000). The age/depth
curve correlates well with the age/depth curve from the overlapping
FIGURE 2 Agemodelsand
sedimentationratethroughtime.Black
linesrefertoPS1243data,redtoM23352
data,andbluedepictscoreMD992277.
MIS:MarineIsotopeStage;grey
highlighted areas indicate glacial periods
(evenMIS);Mid-BrunhesEventindicated
bybisquecolourbarfrom350to430 ka
(accordingtoYinandBerger[2010] and
Cronin et al. [2017]). Top: sedimentation
rates through time given in cm sediment
perkabasedonrawdata.Bottom:age
models depicted as depth/age relationship
line graph.
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6 of 18
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JÖST et al.
agesections ofcoreM23352,signifyingcomparable sedimentation
rates, and therefore supporting the comparability and compatibil-
ity of both cores (Figure 2). This study includes PS1243 sediments
between 339.9–1.4 ka (Table S1, given as Appendix 1 in Online
Supplement S1). Further details regarding core chronology are given
inAppendix2 in Online Supplement S1.
2.3 | Data analyses
MD992277 dataset consists of 15, 293 ostracod specimens col-
lected from 297 ostracod- bearing sediment samples spanning
620.7–361.6 ka(Table S1,givenasAppendix1 in Online Supplement
S1). PS1243 yielded 3072 ostracod specimens collected from 61
sediment samples corresponding to ~532.1–1.23 ka(Table S1, given
as Appen dix 1 in Online Supplement S1; census file, given as S2).
M23352 containe d 26,019 ostracod s pecimens obt ained from 143
sedimentsamplesfrombetween194.5–4.2 ka(allpost-MBE;Table 1;
Figure 2; Appendix 1).Allochthonous taxa,thatisdepositedonsite
bydo wn-sl ope tra n sp o r to ric e- raf t ing ev ent s(k n ow nsh all ow -ma rin e
species; see Table S2,givenasAppendix3 in Online Supplement S1
forMD992277 and M23352data), aswellasspecimenswithout,at
least, genus level identification were omitted from analyses.
To determine the general trend of abundance fluctuations over
time, autochthonous ostracod specimen counts (i.e. raw counts) of
the three cores were plotted along the total age scale of ~640–0 ka
(Figure 3). Similarly, autochthonous ostracod diversity was plotted
as function of age along the benthic δ18O paleoclimatic trend
(Figure 4). Ostracod diversity was given as Hill number, a mea-
sure of the effective number of species in a hypothetical commu-
nity (Chao et al., 2014). The Hill number (qD) is calculated based
on the qth power sum of the relative species abundance; there-
fore, the q value determines its sensitivity to the relative species
abundance. In this study, we employed the three most commonly
adopted Hill numbers: q = 0 (0D or species richness), q = 1 (1D or
Shannon diversity), and q = 2(2D or Simpson diversity) to evalu-
ate the species richness and diversity emphasis on abundant (i.e.
1D) and highly abundant (i.e. 2D) species.Alpha(Figure 5, upper
panel), as well as gamma diversities (Figure 5, lower panel) were
calculated, while we consider pooled diversity (MISs and pre-,
during-andpost-MBE)asameasureforalessbiaseddiversityby
abundance and sedimentation rate. Gamma diversity (i.e. pooled
diversity of each bin, as opposed to average of each individual
sample per,e.g.MIS)wasusedtoinvestigateglacialversus inter-
glacialandpre- versuspost-MBE trends(Figure 5 and Figure S1,
given as Appendix 4 in Online Supplement S1). To standardize
the sampling efforts, we performed coverage- based rarefaction
and extrapolation (sample coverage = 85% for alpha diversity,
and 99% for g amma diversi ty) using 100 0 bootst rap resam pling
(Chao et al., 2020). Both the o bserved (i .e. unstand ardized) and
estim atedHillnumbers(i.e.st an dardizedto85%s amplecover age)
arereported.A sresult,11samples(~2.2%oftotal)wereremoved
before analysis due to low sample coverage (< 85%).PERMANOVA
(Permutational MultivariateAnalysisofVariance) was performed
Tax on
Before MBE (N = 219) Af ter MBE (N = 198)
Relative abundance (%) Relative abundance (%)
Average Standard deviation Average Standard deviation
Allochthonous 1.80 8.04 5. 27 6.3
Cytheropteron 13.73 18.57 3 0.15 22.45
Eucythere 5.86 13.46 1.38 3.25
Henryhowella 7.3 7.41 31.70 26.21
Krithe (all) 68.97 31.03 40.48 20.30
Krithe hunti 62.44 32.45 16. 27 25.32
Krithe minima 0.83 7. 5 4 18.92 19.94
Paracytherois 2.33 9.3 5 0.37 1.13
Polycope 3.11 13.0 0 4.39 10.32
Propontocypris 0.006 0.87 0.06 0.43
Pseudocythere 1.44 10.21 2.38 5.36
Raw count per sample Raw count per sample
Average Standard deviation Average Standard deviation
Total abundance 67 90 143 184
Note:Averagerelativeabundancevaluesofostracodtaxaandtheirst andarddeviationswithinthe
totalpre-MBEandpost-MBEassemblagesofNorwegianSeacoresMD992277(N = 213),M23352
(N = 143),andPS1243(N = 61).Pre-MBEsamplesizeisN = 219(213fromMD992277;6from
PS1243);post-MBEsamplesizeisN = 198(143fromM23352;55fromPS1243).Taxagiveninbold
aresubjecttobiasintaxonomichandling,thatisabsentinM23352aftertheMBE,hencesample
sizeforcalculationsislower(N = 55).
TAB LE 1 Comparisonofrelative
abundancesbeforeandaftertheMid-
BruhnesEvent.
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toquantifycomparisonsof pre-and post-MBE diversity.Barren
samples were removed prior to PERMANOVA computation. The
84 ostracod-bearing MBE samples (350–430 ka) were not in-
cluded (Table S3,given as Appendix 5in OnlineSupplementS1),
as PERMANOVA works better for balanced designs (Anderson
&Walsh,2013) and our focus lies on the differences before and
after the MBE,rather thanontheMBEitself. PERMANOVAwas
computedwiththeRpackage“vegan”(Oksanenetal.,2018). The
diversit ycalculationswerecomputedwiththeRpackage“iNEXT”
(Hsieh et al., 2020). Core localities were mapped with the soft-
ware QGIS (version 3.16.8 Hannover; 1989, 1991, Free Software
Foundation, Inc.). Continent shape files were acquired through
open sources at https:// www. igism ap. com, country specific
shape files through https:// gadm. org/ downl oad_ count ry_ v3. html.
Faunal diagrams were generated with the software SigmaPlot
(version 10.0; 2006, Systat Software, Inc.) and edited with vec-
tor graphics software Inkscape (0.92.1 version 3; 2007, Free
Software Foundation, Inc.). Non- metric multidimensional scaling
(nMDS)was usedtounderstandtherelationshipsamongsamples
and taxonomic variables, generating a two- dimensional configu-
ration of the faunal assemblages,while preservingtheir ranks of
differences (Borcard et al., 2011;Legendre&Legendre,2012). To
reduce bias caused by the differences in numbers of samples avail-
able for th e three core s, nMDS was ru n using uneven sp ecimen
cut- off thresholds, so that each core has roughly the same number
of sample s (i.e. aroun d 50). For PS1243, a ≥ 20 sp ecimen-cut-of f
FIGURE 3 Abundancecomparison
to global benthic δ18O changes from
~630 katopresentday.Blacksquares
denote ostracod data obtained from
PS1243 (Cronin et al., 2002); red dots
denote ostracod data obtained from
M23352(Didiéetal.,2002); blue stars
denote ostracod data obtained from
MD992277(thisstudy).Lowabundance
samplesomittedfromthenMDSanalysis
are indicated by open symbols. Different
scales for x-axesapplied:MD992277data
plotted along blue scale at the bottom,
M23352(redscale)andPS1243(black
scale) data plotted along scales shown at
thetop.MIS:MarineIsotopeStage;grey
highlighted areas indicate glacial periods
(evenMIS);whitehighlightedareas
indicateinterglacialperiods(oddMIS);
Mid-BrunhesEventindicatedbybisque
colourbarfrom350to43 0 ka(according
toYinandBerger[2010] and Cronin
et al. [2017 ]).Abundancegivenintotal
raw counts.
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was applied, resultingin 52 samples (9 low-countsamples omit-
ted);forM23352,a ≥ 150specimen-cut-offwasapplied,resulting
in58samples(85low-countsamplesomitted);andforMD992277,
a ≥ 100specimen-cut-offwasapplied,resultingin58samples(374
low- count samples omitted). Omitted samples are indicated by
open symbols in Figure 3.WeappliedBray–Curtisdissimilarityon
relative abundances of the allochthonous taxa and all 16 genera
that are pr esent in this subse t for the nMDS. Th e analysis was
doneusingtheRpackage“vegan”(Oksanenetal.,2018).
3 | RESULTS
3.1 | Diversity
Diversity as calculated by Hill numbers (0D, 1D, 2D) (Chao et al.,
2014) shows substantial glacial–interglacial and shorter time-
scale variations (Figure 4). Generally, glacial alpha diversity
tends to be higher than interglacial alpha diversity, especially in
MISs2–4,10and14(atleastforq = 0,i.e.rarespeciesdiversit y)
FIGURE 4 EstimatedHillnumbers(qD) of order q = 0(0D), q = 1(1D) and q = 2(2D)basedon85%samplecoverageasfunctionofage.The
strippedrectanglesindicateinterglacialperiods.ThebisquecolourbackgroundshowstheMBEperiod(~350–430 ka).Thetopx- axis shows
MarineIsotopeStages.Thehorizontallinesshowthemeanandtheshadedareashow95%confidenceinterval(mean ± standarderror*
1.96).NotethatblackdotsfortheleftpanelsindicatetheM23352samplesandthosefortherightpanelsindicateMD992277samples.
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JÖST et al.
(Figure 5, upper panel). Glacial gamma diversity also tends be
higher than interglacialgamma diversity,especiallyinMISs2–4,
and 14 (Figure 5, lower pan el). Post-M BE diversit y tends to be
higher than pre-MBE diversity in alpha diversity, but it is the
opposite in gamma diversity (Figure S1, gi ven as Appen dix 4 in
Online Supplement S1).
3.2 | Relative and total abundance
Looking a t the relative a bundance da ta of ostra cod taxa fro m the
three cores, the following patterns are revealed:
Prior to the MBE, Eucythere, Krithe and Paracytherois show
higher mean relative abundances when compared to their
FIGURE 5 AlphaandgammadiversitybyMarineIsotopeStage.Top:Alphadiversity.MeanHillnumbersbasedon85%samplecoverage
as function of order q.ThecurvesshowthemeanHillnumbers(i.e.meanalphadiversity)andtheshadedareashowsthe95%confidence
interval(mean ± standarderror*1.96).Bottom:Gammadiversit y.ThecurvesshowthepooledHillnumbers(i.e.gammadiversity)basedon
99%samplecoverage.Non-overlapofshadedareasindicatessignificantdifferenceindiversity.ColoursindicateMISstagesasshowninthe
legendsonthefigures.InterglacialMISdenotedbydashedlines(pre-MBEMIS13,15;MBEMIS11;post-MBEMIS1,5),whileglacialMIS
denotedbysolidlines(pre-MBEMIS14,12;MBEMIS10;post-MBEMIS6,2–4).
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post-MBEabundance rates, although the respectivestandardde-
viations are substantial (Table 1). On the contrary, Cytheropteron,
Henryhowella and Propontocypris show lower mean relative abun-
dances pr ior to the MBE (Table 1). Henryhowella shows a mean
relative pre-MBEabundance ofaround 7.3%compared toamean
relativepost-MBEabundanceofnearly32%(Table 1). Polycope was
mainly ab sent prior to the M BE, except for a fe w sudden spikes
inabundanceduring MIS 15, in which it is a veryabundant taxon
within the ostracod assemblage, and often even the sole dominant
taxon (Figure 6).AftertheMBE,Polycope occurred commonly from
MIS5onwards(Figure 6). Krithe dominates assemblages before the
MBE and up untilaround 260 ka(Figure 6). From MIS 7 onwards,
they remain a very abundant component of the ostracod assem-
blage,butwithlowerrelativeabundancevalues.AlthoughKrithe in
general appears very abundant within the assemblages before the
MBE,therearespecies-specificdifferences.WhereasKrithe hunti
showsamean relativeabundanceof above60%before the MBE,
its post-MBEmean relative abundance reaches onlyroughly 16%
(Table 1). Figure 7 illustrates this drop, which occurred at around
260 ka onwards. Before 260 ka, Krithe hunti appears frequently
with ar elati ve ab und anc eofu pt o100 %ofth et ot als pe cie sa sse m-
blages.After260 kahowever,itdecreasestolessthan40%ofrela-
tive abundance, occurring primarily during glacials (Figure 7). Krithe
minima,ontheotherhand,isextremelyrarebeforetheMBE,with
amean relative abundance of around 0.8% as opposed to nearly
19% after the M BE (Table 1). In the present- day Nordic seas, K.
minima is a fairly common inhabitant (Figure 7). Total abundance is
lowerpriortotheMBEwithanaveragerawcountpersampleof67,
asopposedto143 afterthe MBE (Table 1). The above- mentioned
faunal compositionalchanges are well reflected in the nMDS plot
(Figure 8), showing a c lear separati on of pre- a nd post-MBE a s-
semblages(with some overlaps though) (PERMANOVA:F = 4 4. 61,
FIGURE 6 Timeserieschangesin
abundance of major ostracod genera
from ~630 katopresentday.Taxa
are annotated by their abbreviations:
Cytheropteron (Cyt), Eucythere(Euc),
Henryhowella (Hen), Krithe (Kri),
Paracytherois (Par), Polycope (Pol),
Propontocypris (Pro) and Pseudocythere
(Pse).Blacksquaresdenoteostracod
data obtained from core PS1243 (Cronin
et al., 2002); red dots denote ostracod
dataobtainedfromcoreM23352(Didié
et al., 2002); blue stars denote ostracod
dataobtainedfromcoreMD992277
(thisstudy).Abundancegivenasraw
specimen counts (including all samples,
nospecimencut-offapplied).MIS:Marine
Isotope Stage; grey highlighted areas
indicateglacialperiods(evenMIS);white
highlighted areas indicate interglacial
periods(oddMIS);Mid-BrunhesEvent
indicatedbybisquecolourbarfrom350to
430 ka(accordingtoYinandBerger[2010]
and Cronin et al. [2017 ]). Global climate
curve (i.e. deep- sea oxygen isotope
record)fromLisieckiandRaymo(2005).
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JÖST et al.
p < 0.001).Henryhowella has the highest value on the negative end
ofnMDS2,ch ar acter izingpost-MBEsamples(Figure 8). Several un-
common genera thatexclusively occurin pre-MBE samples, such
as Rosaliella and Thaerocythere, have the highest positive values of
nMDS2.Pseudocythere and Polycope have the highest values on the
positive end of nMDS1, characterizing some samples of the two,
morecloselylocatedsites,PS1243andMD992277.
3.3 | Diversity dependence on
abundance, and diversity and abundance
dependencies on sedimentation rate
Abundanceversus diversity crossplots revealedthat, compared to
raw diversity measures, estimated Hill numbers substantially reduce
the abundance dependency of diversity (see R2 values on Figure S2,
givenasAppendix6 in Online Supplement S1).Wealso contrasted
sedimentation rate with diversity to illuminate potential temporal
grain bias affecting the time averaging. The respective plot revealed
that there is no significant relationship between diversity and sedi-
mentation rate (see p- values on bottom panel of Figure S2, given as
Appendix6 in Online Supplement S1).
4 | DISCUSSION
4.1 | Has the deep Norwegian Sea diversity been
suppressed by quaternary glaciation?
Our results show that deep Norwegian Sea diversity was not sup-
pressed by Quaternary glaciation, with post-MBE glacial diver-
sity tending to be notably higher than that of interglacial periods
FIGURE 7 Comparisonofmajor
species of Krithe to global benthic δ18O
changes from ~630 katopresentday.
Blacksquaresdenoteostracoddata
obtained from core PS1243 (Cronin
et al., 2002); red dots denote ostracod
dataobtainedfromcoreM23352(only
Krithe genus level data available) (Didié
et al., 2002); blue stars denote ostracod
dataobtainedfromcoreMD992277
(this study). Krithe spp. here shows
the complete Krithe record from each
core, that is specimens of K. hunti and
K. minima (PS1243), and K. hunti, K.
minima, K. dolichodeira, and specimens
with unclear species identification
(MD992277).M23352hasaKrithe record
with uncertain species information,
hence is excluded from the individual
speciesgraphs.Abundancegivenas
relative abundance within the assemblage
[%].MIS:MarineIsotopeStage;grey
highlighted areas indicate glacial periods
(evenMIS);whitehighlightedareas
indicateinterglacialperiods(oddMIS);
Mid-BrunhesEventindicatedbybisque
colourbarfrom350to43 0 ka(according
toYinandBerger[2010] and Cronin
et al. [2017 ]).
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(Figures 4 and 5). Es pecially fo r MIS 2–4, alpha (par ticular ly in re-
spect to rare species; q = 0)andgammadiversitiesarehigh(Figure 5).
Therefore, the fundamental precondition of the glacial disturbance
hypothesis is not supported.
The high MIS 2–4 diversities may be caused by the compa-
rably high intermediate water temperature at that time (Cronin
et al., 2017).Indeed,MIS6wasconsiderablycoolerandshowslower
diversities according to our data (Figures 4 and 5). On the other
hand, dive rsities du ring MIS 10 and 14, resp ectivel y,are h igh, de-
spitethe lackofhightemperatureindications(Cronin etal.,2017),
which could point to other mechanisms operating diversity changes.
Our data s how that during M IS 2–4,t he diversit y of rare specie s
(q = 0)ishigh,whileduringMIS10and14,respectively,thediversity
of dominant species (q = 2) ishigh(especiallyforMIS14,it ishigher
thanduringMIS2–4)(Figure 5).Apossi bleexplanationfortheseno-
table differences could be species- specific sensitivities to changes in
surface production. Low or changing surface production may allow
thecoexistenceofrelativelymanydominantspecies,as itisknown
that high productivity often leads to the dominance by only few op-
portunisticspecies(asseeninthecaseofeutrophication;Yasuhara,
Hunt, Breitburg, et al., 2012). Changes in abundance and sedimenta-
tion rate should not have significantly distorted the general results
ofthisstudy.EstimatedHillnumberssubstantiallyreducetheabun-
dance dependency of diversity compared to raw diversity measures
(i.e. observed Hill numbers), and there is no statistical relationship
between estimated Hill numbers and averaged sedimentation rates
that is higher diversities are not caused merely by lower sedimenta-
tion rates (i.e. higher time averaging) (Figure S2,givenasAppendix6
in Online Supplement S1).
Our results, based on alpha and gamma diversities, are reasonably
comparable to deep- sea biological studies reporting low Norwegian
Sea diver sity (Culver & B uzas, 2000; Dahl, 1979; Jöst et a l., 2019;
Lambshead et al., 2000; Rex et al., 1993, 2000; Svavarsson, 1997;
Svavarsson et al., 1993; Yasuhara, Hunt, et al., 2009; Yasuhara,
Hunt, Cronin, et al., 2012). Indeed, glacial environments in the deep
NorwegianSeaarecharacterizedbylower surfaceproduc tivit ythat
positively affected deep- sea diversity (Didié et al., 2002). Since it is
k no w n t ha t d iv e r si t y a n ds u r f a ce p r od u c t i vi t y (t h e m ai n f oo d s o ur c e of
deep- sea benthos as particulate organic carbon flux) have a unimodal
relationship (Tittensor et al., 2011) and the Norwegian Sea produc-
tivityishigh(Lutzetal.,2007), it is reasonable that we see a negative
relationship between diversity and productivity, as it represents the
descending rim of this unimodal relationship with increasing domi-
nance of few opportunistic species. The glacial Norwegian Sea was
covered by sea ice, but only seasonally (Pflaumann et al., 2003). This
ice cover probably decreased surface productivity, as mentioned
above. The seasonal nature of sea ice caused the deposition of ice-
rafted debris (IRD), lithic grains of terrestrial origin brought in by drift-
ice when the ice melted (Bond et al., 1997, 2001; Heinrich, 198 8). But
itisknownthatdeep-seadiversity actuallyincreasedduringperiods
FIGURE 8 Non-metric
multidimensionalscaling(nMDS)plotof
the faunal assemblages. PS1243 samples
areindicatedbydots,M23352samples
bytriangles,andMD992277samplesby
plus-signs.Pre-MBEsamples(olderthan
430 ka)areindicatedbybluesymbols;
MBEsamples(bet ween43 0and350 ka)
are indicated by grey symbols, and post-
MBEsamples(youngerthan350 ka)are
indicated by red symbols. Uneven cut- off
thresholds were applied to have a similar
number of samples from each core: at
least 20 specimens for PS1243 (number
of samples N = 52),150forM23352
(number of samples N = 58),100for
MD992277(numberofsamplesN = 58).
The stress value of ~0.1 indicates that
the preservation of multivariate distance
inthenMDSconfigurationiswithin
good to acceptable range, quantitatively
supporting the observed compositional
changes.
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JÖST et al.
ofintensiveIRDdeposition,aphenomenonknownasHeinrichevents
(Heinrich, 1988), probably because (1) surface productivit y was lower
during these events and/or (2) the soft sediment habitat was dis-
turbed or habitat heterogeneity was increased by the IRD deposi-
tion (Yasuha ra & Cronin, 2008). Similarly, ostracod abundance also
does not show a clear trend of low glacial values (Figure 3). However,
benthic foraminiferans reportedly show lower glacial abundance
in the dee p Norwegian S ea (Struck, 1995). This probably indicates
the well-known sensitivity of benthic foraminiferansto food sup-
ply (Gooday, 2003; Herguera, 2000; Nees et al., 1999; Rasmussen
et al., 2003; Yasuhara, Hun t, Cronin, et al., 2012). In addition, re-
cent paleoceanographic studies indicate that, in the subpolar North
Atlantic and Arctic, glacial and stadial deep-sea (especially inter-
mediate water) temperature was warmer than during interglacial
and interstadial periods (Cronin et al., 2012; Mar cott et al., 2011;
Rasmussen & Thomsen, 2004; Yasuhara et al., 2019; Yasuhara,
Okahashi,etal.,2014). In sum, our result and also results from other
paleoecological and paleoceanographic studies, consistently show
that the glacial deep Norwegian Sea environment was not particu-
larly harsh for deep- sea benthos and their biodiversity.
Rather, the present- day- style deep Norwegian Sea ecosystem
wasestablishedbyanMBE-inducedshiftinclimate.Thedirecttrig-
geroftheMBEfaunalshiftandchangesinpre-MBEandpost-MBE
diversity is difficult to explain, and probably complex. There are sev-
eralpossiblereasons,first,theexchangebetweentheNorthAtlantic
proper waters and the Nordic Seas: Stronger post-MBE glacials
haveledtoamuchlowersealevelaftertheMBE,whichmighthave
caused a s tronger isolati on of the Nordic-Arctic regio n. This may
also explainthe pre-MBE presence ofAtlantict axa and their post-
MBEdisappearance(DeNinnoetal.,2015).Asecond reasoncould
be the generally much warmer intermediate water temperature in
the region (except Deglacial- Holocene; see Cronin et al., 2017). As
the temperature- diversity relationship is significantly positive in os-
tracods(at least in lower-temperature geographic ranges; see Jöst
et al., 2019), warmer post-MB E intermediate water temperatures
could be another reasonfor higher post-MBE diversity, although
the gamm a diversity is lower i n the post-MB E period (Figure S1,
givenasAppendix4 in Online Supplement S1).Athirdpossiblerea-
son is chang es in sea ice. Th ep re-an d post-MBEs ea ice regimes
weredifferent,withthepre-MBEArcticregionhavingexperienced
ice- free conditions more frequently (Cronin et al., 2017). This might
have affected surface productivity and POC flux, resulting in the ob-
served ostracod faunal composition. Overall however, more data are
needed to reach a better, more comprehensive conclusion.
4.2 | Did the MBE play a role in establishing the
deep Norwegian Sea ecosystem, including faunal
composition and biodiversity?
While we did not see a consistent difference between pre-and post-
MBE diversities in alpha and gamma diversities (Figure S1, given as
Appendix4 in Online Supplement S1), we found a subst antial faunal sh ift
acrosstheMBE(Figure 6). For example, Henryhowella, abundant during
and afterthe MBE, is primarily absent in pre-MBE samples (Figure 6).
Propontocyprisshowsasimilartrend,beingmorecommoninpost-MBE
samples (Figure 6). In contrast, Eucythere and Paracytherois are abun-
dantinmanypre-MBEsamples,butareveryrare inpost-MBEsamples
(Figure 6). Krithe is dominant almost throughout the record, but compara-
tivelymor eab undantinpre-MBEsamplesthaninpos t-MBEones.Sin ce
Eucythereisknownasani ndicatorofseasonalsur faceproduct ion(Didié
et al., 2002), and Henryhowella and Krithe are probably sensitive and tol-
eranttolowoxygenconditions,respectively(Yasuhara&Cronin,2008),
wespeculatethattheMBEfaunal shiftintheNorwegianSeaisrelated
to a shift in the surface productivity mode, and thus in food supply and
the oxygen state in the deep sea, while further details remain elusive.
Krithe dolichodeira, a species with a modern distribution limited to the
NorthAtlanticproper(i.e.onthesouthsideofIceland)isabsentinpost-
MBE samples, butabundantinMBE and pre-MBE samples (Figure 7).
Krithe hunti and Krithe minima on the other hand, both extant species
intheNorwegianSeaandtheArcticOcean(Jöstetal.,2022;Yasuhara,
Grimm, et al., 2014),showcontrastingabundancepatterns.WhileKrithe
huntiis moreabundant inpre-MBE samples, Krithe minima tends to be
moreabundantin post-MBE samples (Figure 7). Generic faunal shifts
andfaunal differencesacrosstheMBEareclearlyshowninthenMDS
plot (Figure 8).A similarstrongfaunalshift isalsoknownfromthe high
Arctic (Cronin et al., 2017; DeNinno et al., 2015). Acetabulastoma and
PolycopeareabundantaftertheMBEbutveryrarebeforetheMBE
(Cronin et al., 2017 ). Several taxa, such as Echinocythereis, Arcacythere,
and several species of Krithe (including Krithe dolichodeira), are absent in
thepresent-dayand post-MBEArctic, butthey were abundant before
theMBE(DeNinno etal.,2015).Whilepartsofthefaunal shiftaredif-
ferentbetweentheNorwegianSeaandArcticOcean(e.g.Polycope does
not show a clear trend in the Norwegian Sea; Henryhowella shows op-
pos itepatternb et weenth eNor we gianSeaan dA rc ti cO cean)(Figure 6),
thislargelyconsistentfaunalshiftindicatesthattheMBEplayedanim-
port ant role in the establishme nt of the present- day polar deep- sea eco-
system by initiation of compositional changes with possible consecutive
impact sonbiodiversity.Ecological causesofthever ylow,present-day
Norwegian Sea diversity were tested in a previous macroecological
study, resulting in the conclusion that the barrier effect caused by the
Greenland- Iceland- Faeroe Ridge (i.e. physical and hydrochemical barrier
through strong temperature and productivity gradients along the ridge)
in combination with the present- day very low water temperature, are
responsible for the Nordic Seas' low diver sity in, especially meiobenthos
withlimiteddispersalabilities(Jöstetal.,2019). But it is still elusive, why
such diversity decline has occurred in the Holocene, but not in the many
pre- Holocene interglacials.
4.3 | Limitations of study
4.3.1 | Taxonomicbias
Our study is based on a combined data set of alpha diversities
from three coring sites of different locations and depths, obtained
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by three types of corers. These heterogeneities in sampling, as
well as taxonomic bias based on species identification by several,
individual taxonomists, could potentially account for some of the
observed differences in diversity and composition, unrelated to
ecological causes. However, all three cores generally show a solid
data comparability by examining published scanning electron
microscope (SEM) images from the cores M23352 and PS1243
(Cronin et al., 2002; Didié et al., 2002; Didié & Bauch, 2002).
Generally, faunal compositions of both cores are very similar, es-
pecially on genus level. Both show a high presence of Krithe and
Cytheropteron specimens, and other major taxa (Henryhowella,
Polycope, Eucythere, Propontocypris) are present in both cores
(Table S4, givenasAppendix7 in Online Supplement S1). The re-
cord of M23352 list s specimens of Swainocythere, Nannocythere
and Microcythere?, all of which do not occur in the PS1243 record
(Table S4, gi ven as Appendix 7 in Online Supplement S1). This
could indicate ecological differences at the two coring locations,
althoughitapp earsmorelikelythatthesedifferencesrefle ctsam-
plesizeef fectandvaryingcompletenessofrawdata.Sampleef-
fort islikely the causefortheappearanceofadditionalraretaxa
in M23352 that are lacking in PS1243, as more samples were
picked (143vs.61),coveringashortertime frame(190.3k yrs vs
530.7kyrs)(Table S1,given asAppendix1 in Online Supplement
S1). Ther efore, to furt her reduce ta xonomic bias, wh ile keeping
full insight into our interpretative basis and avoiding statistical
misinterpretation, for nMDS, we deliberately used genus-level
counts only. Genus- level taxonomy was confirmed prior to analy-
ses based o n the SEMs provid ed in the respe ctive publi cations,
hence taxonomic identification on genus level is considered unaf-
fected by taxonomic bias. In respect to census data, for PS1243
andM23352,wedonothaveanyrecordofostracod-barrensam-
ples, whereasforour owncore, MD992277,wedohave thatre-
cord. This discrepancy only plays a role when plotting abundance
over time, as t he plots lac king barre n samples ap pear smooth er
(see Figures 6 and 7).
4.3.2 | Discontinuoustimerecord
Given the very limited amount of data immediately following the
MBE(b as ic allynoda tainMI S9,8 ,7,exce pt fo rasin gl eP S1243sam-
pleduringMIS8),thereisthepossibilit ythatsomeeventduringthis
period,otherthantheMBE, could havehadsome influenceonthe
observed faunal shift . However, faunal turnovers of microfossil com-
munitiesrecordedbydeep-seasedimentlong-coresfromtheArctic
Ocean have shown to be driven by fundamental shifts in sea- ice
covervariability,sur faceprimaryprodu ct ion,andArct icO ceante m-
peratu re as consequences of M BE-enhan ced Arctic Amp lification
(i.e.amplifiedwarming inArcticregionsrelative totheglobalmean
temperature) (Cronin et al., 2017; DeNinno et al., 2015). Therefore,
it appears plausible, that our faunal turnover in the Norwegian Sea,
which is adjacent to the Arctic Ocean, similarly,wasset off during
and because of the MBE,and was not caused by a later event for
which we lack data. Further limitations are described in detail in
Appendix.
5 | CONCLUSION
Our fossil study enabled the testing of the deep- sea glacial distur-
bance hypothesis as a cause of the low, present- day Norwegian Sea
diversity and the resultingsteep latitudinal diversitygradient. We
found that Norwegian Sea deep- sea diversity based on ostracod
fossils was higher during the last glacial than during the present in-
terglacial. Hence, our findings did not support the deep- sea glacial
disturbance hypothesis. Instead, the present- day deep Norwegian
Sea ecosystem was likely est ablished by the Mid- Brunh es Event,
and the major faunal shifts and, perhaps, extinctions, resulting from
this Pleistocene climate transition. In a broader context, we may
concludethat the MBE has played an important role in the estab-
lishment of the present- day- style polar deep- sea ecosystem and
biodiversity in general.
AUTHOR CONTRIBUTIONS
A.B.J. and M.Y. designed the research. A.B.J., H.O. and M.Y.
picked andtaxonomically identified MD992277 specimens. A.B.J.,
H. H .M.H .,Y. H .a n dC .-L .W.p erfo rme dth eda tah and lin gan dst a tis ti-
calanalysesandgeneratedthefigures.H.A .B.providedMD992277
sedimentsamples.T.M.C.providedostracodassemblagerawdataof
coresPS1243andM23352.H.B.providedtherawdataforthenew
agemodelofcorePS1243.B.T.providedcriticalfeedback.A.B.J.and
M.Y.wrotethepaperincollaborationwithallauthors.
ACKNOWLEDGMENTS
We thank Lau ra Wong, Cecil y Law and Mar ia Lo for assist ance in
laboratory-related mat ters, Jan P. Helmke from the GEOMAR
Helmholtz Center for Ocean Research Kiel (Germany) for pro-
viding chronological raw data of core MD992277, and Prof. Dr.
Alan Lord from the Senckenberg Research Institute in Frankfur t
am Main (Ger many) for hosting A .B.J. as gue st researcher in t he
Micropalaeontology department to work on sections of core
MD992277, and Laura Gemer y and Marci M. Robinson for their
pre- submission comments in fulfilment with U.S. Geological Survey
regulations. We also highly appreciate the quality and depth of
comment s by the handling e ditor, Dr.Ad am To mašových and t he
two anonymous reviewers, which contributed to sincerely improv-
ingthis manuscript.The work describedinthis studywas partially
supported by grants from the Research Grants Council of the
Hong Kong Special Administrative Region, China (project codes:
RFS2223-7S02toM.Y.,HKU17311316to M.Y.,HKU 17301818to
B.T.), the Seed Funding Program for Basic Research of the University
of Hong Kong (reference codes: 2202100581, 202,011,159,122,
201,210,159,043, 201,511,159,075, 201,411,159,017 to M.Y.), the
Faculty of Science R AE Improvement Fund of the University of
HongKong(toM.Y.),andtheEcologyandBiodiversityDivisionFund
(referencecode:5594129toA.B. J.).Thewritingandfigure-plotting
14668238, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/geb.13844 by HGF GEOMAR Helmholtz Centre of Ocean, Wiley Online Library on [24/04/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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JÖST et al.
process was financially supported by the Brain Pool Program
through NRF fundedby the Ministry ofScience and ICT (reference
code:2019H1D3A1A01070922toA .B.J.).H.H.M.H.wassupported
by Peter Buck P ostdoc Fellows hip, Smithsonia n Institutio n. T.M.C
was funded by the U.S. Geological Survey Climate Research and
DevelopmentProgram.Anyuse oftrade,firm,orproductnamesis
for descriptive purposes only and does not imply endorsement by
the U.S. Government.
CONFLICT OF INTEREST STATEMENT
The authors declare that there is no conflict of interest regarding the
publication of this article.
DATA AVAILAB ILITY STATE MEN T
The data that supports the findings of this study are available in the
supplementary material of this article.
ORCID
Anna B. Jöst https://orcid.org/0000-0002-1289-3630
Chih- Lin Wei https://orcid.org/0000-0001-9430-0060
Moriaki Yasuhara https://orcid.org/0000-0003-0990-1764
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h t t p s : / / d o i . o r g / 1 0 . 1 0 1 6 / j . m a r m i c r o . 2 0 2 2 . 1 0 2 1 0 5
BIOSKETCH
Anna B. Jöst is curr ently working as pos t-docto ral researcher
at the Tropical and Subtropical Research Center of the Korea
Institute of Ocean Science and Technology (KIOST) on Jeju
Island, Republic of Korea. Her main focus of research are the
controlling factors of spatial and temporal patterns of marine
biodiversity and abundance distribution, and the role of anthro-
pogenic disturbances and ecological processes in shaping these
patterns, from deep- sea to shallow marine environments.
SUPPORTING INFORMATION
Additional supporting information can be found online in the
Supporting Information section at the end of this article.
How to cite this article: Jöst,A.B.,Huang,H.-H.,Hong,Y.,
Wei,C.-L.,Bauch,H.A.,Thibodeau,B.,Cronin,T.M.,
Okahashi,H.,&Yasuhara,M.(2024).Testingthedeep-sea
glacial disturbance hypothesis as a cause of low, present- day
Norwegian Sea diversity and resulting steep latitudinal
diversity gradient, using fossil records. Global Ecology and
Biogeography, 00, e13844. https://doi.org /10.1111/
geb.13844
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