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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

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Abstract

Aim Within the intensively‐studied, well‐documented latitudinal diversity gradient, the deep‐sea biodiversity of the present‐day Norwegian Sea stands out with its notably low diversity, constituting a steep latitudinal diversity gradient in the North Atlantic. The reason behind this has long been a topic of debate and speculation. Most prominently, it is explained by the deep‐sea glacial disturbance hypothesis, which states that harsh environmental glacial conditions negatively impacted Norwegian Sea diversities, 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 glacials than during interglacials? ( 2) Was there any faunal shift at the Mid‐Brunhes Event (MBE) when the mode of glacial–interglacial climatic change was altered? Location Norwegian Sea, deep sea (1819–2800 m), coring sites MD992277, PS1243, and M23352. Time period 620.7–1.4 ka (Middle Pleistocene–Late Holocene). Taxa studied Ostracoda (Crustacea). Methods We empirically test the deep‐sea glacial disturbance hypothesis by investigating whether diversity in glacial periods is consistently lower than diversity in interglacial periods. Additionally, we apply comparative analyses to determine a potential faunal shift at the MBE, a Pleistocene event describing a fundamental shift in global climate. Results The deep Norwegian Sea diversity was not lower during glacial periods compared 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 Norwegian Sea ecosystem was established by the MBE, more specifically by MBE‐induced changes in global climate, which has led to more dynamic post‐MBE conditions. In a broader context, this implies that the MBE has played an important role in the establishment of the modern polar deep‐sea ecosystem and biodiversity in general.
Global Ecol Biogeogr. 2024;00:e13844. 
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https://doi.org/10.1111/geb.13844
wileyonlinelibrary.com/journal/geb
Received:8Januar y2023 
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Revised:5M arch2024 
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Accepted :21March2024
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 tmentofGeosci ences,PrincetonUniversity,Princeton ,NewJer sey,USA
4SchoolofBiologicalSciences,AreaofEcologyandBiodiversit y,SwireInstituteofM arineScience,Instit uteforClimateandCarbo nNeutr alityandMuske teers
Foundat ionInstituteofDataScie nce,TheUniversityofHongKong,H ongKongSA R,Chi na
5StateKeyLaboratoryofMarinePollution,CityUniversityofHongKong,H ongKongSA R,Chi na
6Institute of Oceanography, National Taiwan University, Taipei, Taiwan
7Alfred-Wegener-Inst itutHelmholt zCente rforPolarandOcea nResearch,c/oGEOMA RHelmholtzCentreforOceanResearchK iel,Ki el,Ger many
8SimonF.S.LiMarineS cienceL abor atory,SchoolofLifeSciences,TheChines eUniver sityofH ongKong,HongKongSAR,China
9FlorenceBascomG eoscienceCenter,U.S.Ge ologic alSur vey,Reston ,Virginia,USA
Correspondence
AnnaB.Jöst,Kor eaInstituteofOcean
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
MoriakiYasuhara,SchoolofBiological
Sciences, The University of Hong Kong,
Kadoorie Biological Science Building,
PokfulamRoad,HongKongS AR,Ch ina.
Email:moriakiyasuhara@gmail.com
Funding information
U.S. Geologic al Sur vey Climate Research
and Development Program; Peter
BuckPostdocFellowship,S mithsonian
Instit ution;Ministr yofScie nceandICT,
SouthKor ea,Gr ant/AwardNumber:
2019H1D3A1A01070922;t heEcology
and Biodiversity Division Fund, Grant/
AwardNumber:5594129;Facult yof
ScienceR AEIm proveme ntFundofthe
University of Hong Kong; Seed Funding
Program for Basic Research of the
UniversityofHo ngKong,G rant/Award
Number :20121015904 3,201411159017,
Abstract
Aim: Wit hint heintensively-studied, well-documented latitudinal diversity gradient,
the deep- sea biodiversity of the present- day Norwegian Sea stands out with its notably
lowdiversity,constitutinga steeplatitudinal diversitygradientin the NorthAtlantic.
Thereasonbehindthishaslongbeenatopicofdebateandspeculation.Mostpromi-
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)WasthereanyfaunalshiftattheMid-BrunhesEvent
(MBE)whenthemodeofglacial–interglacialclimaticchangewasaltered?
Location: NorwegianSea,deepsea(1819–2800 m),coringsitesMD992277,PS1243,
andM23352.
Time period: 620.7–1.4 ka(MiddlePleistocene–LateHolocene).
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 CreativeCommonsAttribution-NonCommercial-NoDerivs License, which permits use and distribution in
anymedium,providedtheoriginalworkisproperlycited,theuseisno n-commercialandnomodi ficat ionsoradaptat ionsaremade.
©2024TheAut hors.Global Ecology and Biogeographypublis hedbyJoh nWiley&SonsLtd.
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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-Llodraetal.,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;Jöstetal.,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
AtlanticGateway(Figure 1;alsoseeJöstetal.,2019),ischaracterized
by a low deep- sea biodiversity that constitutes an important part of
theNorthAtlanticdeep-sealatitudinaldiversitygradient.Notablylow
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 (Jöstetal., 2019; Yasuhara,Hunt, et al., 2009; Yasuhara,Hunt,
Dowsett, et al., 2012), resulting in a low regional benthic diversity.
ManyresearchersattributethislowNor wegianSeabenthicdeep-sea
biod ivers ity to ,a tl eas ti npart, “Q uat er nar y gl aciat ion”.A ss 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),whereasothersgeneralizebysaying“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 iceage,70,00 0to11,700 yearsBP)ormultiple 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, andarethebest representedmarine metazoans, ec-
dysozoans a nd arthropo ds in sediment core -base d fossil research
(Schellenberg, 2007;Yasuharaetal.,2022;Yasuhara&Cronin,2008).
Theirwell-calcifiedfossilizedremainsrepresentamajorpartoftheir
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,
ifnotall,othermetazoaninvertebrates, asthey eitherhave limited
fossilizationpotential(duetoalackofhardparts)orlowabundance
(becauseoftheirlargesize).Thus, ostracodsareanexcellent 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
theclimatetransitionfromlow-amplitudetohigh-amplitudeglacial–
interglacial climatic variability (Cronin et al., 2010, 2017; DeNinno
et al., 2015; Yin & Berger,2010). This fundamental shift in climate
201511159075,202011159122and
2202100581;theResearchGrantsCouncil
oftheHongKongSpecialAdministr ative
Region,C hina,G rant/AwardNumber:
HKU 17301818, HKU 17311316 and
RFS2223- 7S02; the Korea Institute of
Ocean Science and Technology, Grant/
AwardNumber:PO01471andPEA0205
Handling Editor:AdamTomasovych
apotential faunalshift at the MBE,aPleistocene 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. Inabroadercontext, this impliesthat the MBE has playedanimportantrolein
the establishment of the modern polar deep- sea ecosystem and biodiversity in general.
KEYWORDS
deep-seadiversity,faunalturnover,macroecologicalpatterns,Mid-BrunhesEvent,North
Atlantic,Ostracoda
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.
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
andextinctionshaverecentlybeenrepor tedacrosstheMBEinvari-
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 pNo rwegianSeabiodi versity,ashi ftint hemod eofgla 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-
codsin thesedimentcoreMD992277andpublishedostracodcensus
datafromtwo additionalsedimentcores(M23352andPS1243)from
thedeepNor wegian Sea. Specificresearch questionsare:(1)Wasthe
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
ofglacial–interglacialclimaticchangewasaltered?Ourpaleobiological
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.EspeciallytheHolocene diversityismuchhigherthanthatofthe
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,allostracodspecimenswerepickedfromsedi-
ment of >125 μm,thatismeshsizeeffec tcanbedisregarded.
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 LocalitymapdepictingthethreecoringsiteswithintheNordicSeasandtheJanMayenfracturezone.Ridges:whitelinesbased
on1000,2000,and350 0 mdepthcontour,respectively.Coringlocationofstudiedcoreisindicatedbybluestar(MD992277);coringlocations
ofreferencedcoresindicatedbyreddot(M23352)andblacksquare(PS1243),respectively.MD992277:Pistoncore(Labeyrieetal.,1999);
M23352:KastencoreM23352(-3)(Hirschleberetal.,198 8),PS1243:CompositecoreofPS1243(-1)andPS1243(-2)(Augsteinetal.,198 4).
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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the French R/V Marion- Dufresnein1999aspartoftheIMAGES
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,alongtheeasternflankoftheJanMayenRidgeat2800 m
waterdepth(69°15.01N, 06°19.75W) (Figure 1; Table S1, given
asAppendix1 in Online Supplement S1).Atotallengthof33.66 m
ofsedimentcorewithadiameterof12 cmwasretrieved.Thecore
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 2and3grainsizefractions, respectively,dependingon
core section. The core section of this study contained 397 sam-
plesthatweresplitinto3grainsizefractions:125–250,250–500,
and >500 μm,and35samplesthatweresplitinto2grainsizefrac-
tions:150–500and >500 μm. Fractions were merged for ostracod
censusdatatogetcompletesamplesofaspecificage.Effectively,
all sediment of >125 μmwaspickedforostracods.Ostracodspeci-
menswe repi ck ed ,s orted,an di denti fiedattheUn iv er sit yofH ong
Kong(HKSAR) andHanyang University (ROK).Taxonomicidenti-
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),Yasuharaetal.(2014),YasuharaandOkahashi(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 7expedition(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 of1819 m(70° 00.4N,12°25.8W)(Figure 1; Table S1, given
asAppendix1 in Online Supplement S1). The sampling half- core was
cutinto1-cmsedimentsamples,every1–3 cm,fromtheuppermost
layer down to 350 cm, whereas the uppermostlayers werespliced
together with an additional trigger boxcore to ensure undisturbed
topsedimentlayers(Didié&Bauch,2002). Sediment samples were
dividedintograinsizefractionsof125–250,250–500,and >500 μm.
Ostracodswerepicked,countedandidentifiedfromtheseseparate
sizefractions, although theresultswere later merged for ostracod
censusdatatogetcompletesamplesofaspecificage.Effectively,all
sediment of >125 μmwaspickedfor ostracods. Thisstudyincluded
143sediment samplesof1 cm thickness ofa3.48 m-longcoreseg-
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
asAppendix1 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(ARKII/5)togetherwiththetriggerboxcorePS1243(-2)ob-
tained with aBoxCorer(Augsteinetal.,198 4). The coring site was
theIcelandSeaattheeasternslopeoftheIcelandPlateauat2710 m
[PS1243(-1)] and at 2716 m [PS1243(-2)] water depths, respec-
tively, along the eastern flankofthe Jan Mayen Ridge [PS1243(-1)
at 69°22.3N, 06°32.1W; PS1243(-2) at 69°22.5N, 06°32.4W]
(Kandiano, 2003) (Figure 1; Table S1,givenas Appendix1 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 cmoriginal core depth, equivalentto339.9–2.7 ka)
treated with a sieve of >125 μmmeshsize,yieldinga totalof 2853
ostracod valves, were included in this data set (Appendix 1 in
Online Supplement S1). The core hada 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 cmcore segment(1.5–38.5 cmoriginalcoredepth,equivalentto
10.9–1.4 ka), yielding a total of 119ostracod valves,were included
inthisdataset (Appendix 1 in Online Supplement S1). The boxcore
hadasizeof50× 50× 50 cmandwassampledevery2 cm,withaddi-
tionalsamplingby10 mLsyringesforaccumulationratecalculations
(Bauch,Struck,&Thiede,2001). Sediment samples were sieved into
63–125 and >125 μm size frac tions for foramin iferal assemblages,
whereasthesmallersizefractionwasnotusedforpickingostracods.
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)andBauch,Struck,
and Thiede (2001),respectively.Allsedimentof>125 μmwaspicked
for ostr acods. A tota l of 3072 ostraco d valves from P S1243we re
included in this study (S2).
2.2  | Chronology
Age models (i.e. age/depthrelationships)and linearsedimentation
rates of cores M23352 (from the northwestern flank of the Jan
MayenRidge) and PS1243andMD992277 (from theeasternflank
oftheJanMayenRidge)areplottedinFigure 2.
2.2.1  |  ForthecoreMD992277
The chronology applied was published in Helmke, Bauch, and
Erlenkeuser(2003), Helmke, Bauch, andMazaud(2003). Magnetic
inclination, sediment density, lightness and carbonate content were
usedassedimentaryclocks.Theadoptedageschemeretrievedfrom
thesesedimentologicalparameterswascalibratedandstandardized
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by correlation to the benchmark δ
18O chronology records of
SPECMAPbylinearinterpolationbetweenthepointsofcorrelation.
Surfacelayers(uppermost50 cm ofsediment) werecorrelatedwith
PS1243duetocorer-relateddisturbanceinsedimentlayers(Helmke,
Bauch,&Mazaud,2003).Well-documentedcharacteristictrendsin
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
isotopestages(MIS)(seeBauch&Kandiano,2007;Elliotetal.,1998;
Helmke,Bauch,&Mazaud,2003).MISboundariesweresetaccord-
ingtoLisieck iandRay mo(2005). The core section used in this study
wasdeterminedto span from thebeginningof MIS 15(620.7 ka)to
thebeginningofMIS10(361.6 ka)(seeTable S1,givenasAppendix1
in Online Supplement S1). Further details regarding core chronology
aregiveninAppendix2 in Online Supplement S1.
2.2.2  |  ForthecoreM23352
ThechronologyappliedwaspublishedinBauchandHelmke(1999),
Didié and B auch (2002), a n dHe l m k eand B a u c h(2003). Foram iniferal
oxygen isotope analysiswas basedonplanktonic and benthic spe-
cies, whereas for isotope analyses on ostracods, specimens of two
genera with near- continuous time- record were used. Overall, the
agemodelisbased onthesynchronizationofplanktic foraminiferal
δ18Oandthe sediment lightness recordtothestandard SPECMAP
chronology after applying a smooth filter with a 14- point least
squares r unning average (Helmke & Bauch, 2003). This study in-
cludesM23352sedimentsbetween194.5–4.2 ka(Table S1, given as
Appendix1 in Online Supplement S1). Further details regarding core
chronologyaregiveninAppendix2 in Online Supplement S1.
2.2.3  |  ForthecorePS1243
The chronology applied was partially based on published records
in Bauch (1997),Bauch andHelmke(1999), and Bauch et al. (2000).
Planktic foraminaferal δ18O records were correlated to carbon-
ate content to establish the downcore positions ofMIS (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
chronologyrecordsofSPECMAP(Bauchetal.,2000). The age/depth
curve correlates well with the age/depth curve from the overlapping
FIGURE 2 Agemodelsand
sedimentationratethroughtime.Black
linesrefertoPS1243data,redtoM23352
data,andbluedepictscoreMD992277.
MIS:MarineIsotopeStage;grey
highlighted areas indicate glacial periods
(evenMIS);Mid-BrunhesEventindicated
bybisquecolourbarfrom350to430 ka
(accordingtoYinandBerger[2010] and
Cronin et al. [2017]). Top: sedimentation
rates through time given in cm sediment
perkabasedonrawdata.Bottom:age
models depicted as depth/age relationship
line graph.
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   JÖST et al.
agesections ofcoreM23352,signifyingcomparable 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
inAppendix2 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,givenasAppendix1 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
sedimentsamplesfrombetween194.5–4.2 ka(allpost-MBE;Table 1;
Figure 2; Appendix 1).Allochthonous taxa,thatisdepositedonsite
bydo wn-sl ope tra n sp o r to ric e- raf t ing ev ent s(k n ow nsh all ow -ma rin e
species; see Table S2,givenasAppendix3 in Online Supplement S1
forMD992277 and M23352data), aswellasspecimenswithout,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-andpost-MBE)asameasureforalessbiaseddiversityby
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)wasusedtoinvestigateglacialversus inter-
glacialandpre- versuspost-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 atedHillnumbers(i.e.st an dardizedto85%s amplecover age)
arereported.A sresult,11samples(~2.2%oftotal)wereremoved
before analysis due to low sample coverage (< 85%).PERMANOVA
(Permutational MultivariateAnalysisofVariance) 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:Averagerelativeabundancevaluesofostracodtaxaandtheirst andarddeviationswithinthe
totalpre-MBEandpost-MBEassemblagesofNorwegianSeacoresMD992277(N= 213),M23352
(N= 143),andPS1243(N= 61).Pre-MBEsamplesizeisN= 219(213fromMD992277;6from
PS1243);post-MBEsamplesizeisN= 198(143fromM23352;55fromPS1243).Taxagiveninbold
aresubjecttobiasintaxonomichandling,thatisabsentinM23352aftertheMBE,hencesample
sizeforcalculationsislower(N=55).
TAB LE 1  Comparisonofrelative
abundancesbeforeandaftertheMid-
BruhnesEvent.
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toquantifycomparisonsof 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 5in OnlineSupplementS1),
as PERMANOVA works better for balanced designs (Anderson
&Walsh,2013) and our focus lies on the differences before and
after the MBE,rather thanontheMBEitself. PERMANOVAwas
computedwiththeRpackage“vegan”(Oksanenetal.,2018). The
diversit ycalculationswerecomputedwiththeRpackage“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 usedtounderstandtherelationshipsamongsamples
and taxonomic variables, generating a two- dimensional configu-
ration of the faunal assemblages,while preservingtheir 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 Abundancecomparison
to global benthic δ18O changes from
~630 katopresentday.Blacksquares
denote ostracod data obtained from
PS1243 (Cronin et al., 2002); red dots
denote ostracod data obtained from
M23352(Didiéetal.,2002); blue stars
denote ostracod data obtained from
MD992277(thisstudy).Lowabundance
samplesomittedfromthenMDSanalysis
are indicated by open symbols. Different
scales for x-axesapplied:MD992277data
plotted along blue scale at the bottom,
M23352(redscale)andPS1243(black
scale) data plotted along scales shown at
thetop.MIS:MarineIsotopeStage;grey
highlighted areas indicate glacial periods
(evenMIS);whitehighlightedareas
indicateinterglacialperiods(oddMIS);
Mid-BrunhesEventindicatedbybisque
colourbarfrom350to43 0 ka(according
toYinandBerger[2010] and Cronin
et al. [2017 ]).Abundancegivenintotal
raw counts.
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was applied, resultingin 52 samples (9 low-countsamples omit-
ted);forM23352,a ≥ 150specimen-cut-offwasapplied,resulting
in58samples(85low-countsamplesomitted);andforMD992277,
a ≥ 100specimen-cut-offwasapplied,resultingin58samples(374
low- count samples omitted). Omitted samples are indicated by
open symbols in Figure 3.WeappliedBray–Curtisdissimilarityon
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
doneusingtheRpackage“vegan”(Oksanenetal.,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
MISs2–4,10and14(atleastforq = 0,i.e.rarespeciesdiversit y)
FIGURE 4 EstimatedHillnumbers(qD) of order q= 0(0D), q= 1(1D) and q= 2(2D)basedon85%samplecoverageasfunctionofage.The
strippedrectanglesindicateinterglacialperiods.ThebisquecolourbackgroundshowstheMBEperiod(~350–430 ka).Thetopx- axis shows
MarineIsotopeStages.Thehorizontallinesshowthemeanandtheshadedareashow95%confidenceinterval(mean ± standarderror*
1.96).NotethatblackdotsfortheleftpanelsindicatetheM23352samplesandthosefortherightpanelsindicateMD992277samples.
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(Figure 5, upper panel). Glacial gamma diversity also tends be
higher than interglacialgamma diversity,especiallyinMISs2–4,
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 AlphaandgammadiversitybyMarineIsotopeStage.Top:Alphadiversity.MeanHillnumbersbasedon85%samplecoverage
as function of order q.ThecurvesshowthemeanHillnumbers(i.e.meanalphadiversity)andtheshadedareashowsthe95%confidence
interval(mean ± standarderror*1.96).Bottom:Gammadiversit y.ThecurvesshowthepooledHillnumbers(i.e.gammadiversity)basedon
99%samplecoverage.Non-overlapofshadedareasindicatessignificantdifferenceindiversity.ColoursindicateMISstagesasshowninthe
legendsonthefigures.InterglacialMISdenotedbydashedlines(pre-MBEMIS13,15;MBEMIS11;post-MBEMIS1,5),whileglacialMIS
denotedbysolidlines(pre-MBEMIS14,12;MBEMIS10;post-MBEMIS6,2–4).
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post-MBEabundance rates, although the respectivestandardde-
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-MBEabundance ofaround 7.3%compared toamean
relativepost-MBEabundanceofnearly32%(Table 1). Polycope was
mainly ab sent prior to the M BE, except for a fe w sudden spikes
inabundanceduring MIS 15, in which it is a veryabundant taxon
within the ostracod assemblage, and often even the sole dominant
taxon (Figure 6).AftertheMBE,Polycope occurred commonly from
MIS5onwards(Figure 6). Krithe dominates assemblages before the
MBE and up untilaround 260 ka(Figure 6). From MIS 7 onwards,
they remain a very abundant component of the ostracod assem-
blage,butwithlowerrelativeabundancevalues.AlthoughKrithe in
general appears very abundant within the assemblages before the
MBE,therearespecies-specificdifferences.WhereasKrithe hunti
showsamean relativeabundanceof above60%before the MBE,
its post-MBEmean relative abundance reaches onlyroughly 16%
(Table 1). Figure 7 illustrates this drop, which occurred at around
260 ka onwards. Before 260 ka, Krithe hunti appears frequently
with ar elati ve ab und anc eofu pt o100 %ofth et ot als pe cie sa sse m-
blages.After260 kahowever,itdecreasestolessthan40%ofrela-
tive abundance, occurring primarily during glacials (Figure 7). Krithe
minima,ontheotherhand,isextremelyrarebeforetheMBE,with
amean 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
lowerpriortotheMBEwithanaveragerawcountpersampleof67,
asopposedto143 afterthe MBE (Table 1). The above- mentioned
faunal compositionalchanges 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 Timeserieschangesin
abundance of major ostracod genera
from ~630 katopresentday.Taxa
are annotated by their abbreviations:
Cytheropteron (Cyt), Eucythere(Euc),
Henryhowella (Hen), Krithe (Kri),
Paracytherois (Par), Polycope (Pol),
Propontocypris (Pro) and Pseudocythere
(Pse).Blacksquaresdenoteostracod
data obtained from core PS1243 (Cronin
et al., 2002); red dots denote ostracod
dataobtainedfromcoreM23352(Didié
et al., 2002); blue stars denote ostracod
dataobtainedfromcoreMD992277
(thisstudy).Abundancegivenasraw
specimen counts (including all samples,
nospecimencut-offapplied).MIS:Marine
Isotope Stage; grey highlighted areas
indicateglacialperiods(evenMIS);white
highlighted areas indicate interglacial
periods(oddMIS);Mid-BrunhesEvent
indicatedbybisquecolourbarfrom350to
430 ka(accordingtoYinandBerger[2010]
and Cronin et al. [2017 ]). Global climate
curve (i.e. deep- sea oxygen isotope
record)fromLisieckiandRaymo(2005).
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p< 0.001).Henryhowella has the highest value on the negative end
ofnMDS2,ch ar acter izingpost-MBEsamples(Figure 8). Several un-
common genera thatexclusively occurin 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,
morecloselylocatedsites,PS1243andMD992277.
3.3  | Diversity dependence on
abundance, and diversity and abundance
dependencies on sedimentation rate
Abundanceversus diversity crossplots revealedthat, compared to
raw diversity measures, estimated Hill numbers substantially reduce
the abundance dependency of diversity (see R2 values on Figure S2,
givenasAppendix6 in Online Supplement S1).Wealso 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
Appendix6 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 Comparisonofmajor
species of Krithe to global benthic δ18O
changes from ~630 katopresentday.
Blacksquaresdenoteostracoddata
obtained from core PS1243 (Cronin
et al., 2002); red dots denote ostracod
dataobtainedfromcoreM23352(only
Krithe genus level data available) (Did
et al., 2002); blue stars denote ostracod
dataobtainedfromcoreMD992277
(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).M23352hasaKrithe record
with uncertain species information,
hence is excluded from the individual
speciesgraphs.Abundancegivenas
relative abundance within the assemblage
[%].MIS:MarineIsotopeStage;grey
highlighted areas indicate glacial periods
(evenMIS);whitehighlightedareas
indicateinterglacialperiods(oddMIS);
Mid-BrunhesEventindicatedbybisque
colourbarfrom350to43 0 ka(according
toYinandBerger[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)andgammadiversitiesarehigh(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,MIS6wasconsiderablycoolerandshowslower
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-
spitethe lackofhightemperatureindications(Cronin etal.,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)ishigh,whileduringMIS10and14,respectively,thediversity
of dominant species (q= 2) ishigh(especiallyforMIS14,it ishigher
thanduringMIS2–4)(Figure 5).Apossi bleexplanationfortheseno-
table differences could be species- specific sensitivities to changes in
surface production. Low or changing surface production may allow
thecoexistenceofrelativelymanydominantspecies,as itisknown
that high productivity often leads to the dominance by only few op-
portunisticspecies(asseeninthecaseofeutrophication;Yasuhara,
Hunt, Breitburg, et al., 2012). Changes in abundance and sedimenta-
tion rate should not have significantly distorted the general results
ofthisstudy.EstimatedHillnumberssubstantiallyreducetheabun-
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,givenasAppendix6
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
NorwegianSeaarecharacterizedbylower surfaceproduc tivit ythat
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 ds 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-
tivityishigh(Lutzetal.,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
itisknownthatdeep-seadiversity actuallyincreasedduringperiods
FIGURE 8 Non-metric
multidimensionalscaling(nMDS)plotof
the faunal assemblages. PS1243 samples
areindicatedbydots,M23352samples
bytriangles,andMD992277samplesby
plus-signs.Pre-MBEsamples(olderthan
430 ka)areindicatedbybluesymbols;
MBEsamples(bet ween43 0and350 ka)
are indicated by grey symbols, and post-
MBEsamples(youngerthan350 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),150forM23352
(number of samples N= 58),100for
MD992277(numberofsamplesN= 58).
The stress value of ~0.1 indicates that
the preservation of multivariate distance
inthenMDSconfigurationiswithin
good to acceptable range, quantitatively
supporting the observed compositional
changes.
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JÖST et al.
ofintensiveIRDdeposition,aphenomenonknownasHeinrichevents
(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 foraminiferansto 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,etal.,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
wasestablishedbyanMBE-inducedshiftinclimate.Thedirecttrig-
geroftheMBEfaunalshiftandchangesinpre-MBEandpost-MBE
diversity is difficult to explain, and probably complex. There are sev-
eralpossiblereasons,first,theexchangebetweentheNorthAtlantic
proper waters and the Nordic Seas: Stronger post-MBE glacials
haveledtoamuchlowersealevelaftertheMBE,whichmighthave
caused a s tronger isolati on of the Nordic-Arctic regio n. This may
also explainthe pre-MBE presence ofAtlantict axa and their post-
MBEdisappearance(DeNinnoetal.,2015).Asecond reasoncould
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 reasonfor higher post-MBE diversity, although
the gamm a diversity is lower i n the post-MB E period (Figure S1,
givenasAppendix4 in Online Supplement S1).Athirdpossiblerea-
son is chang es in sea ice. Th ep re-an d post-MBEs ea ice regimes
weredifferent,withthepre-MBEArcticregionhavingexperienced
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
Appendix4 in Online Supplement S1), we found a subst antial faunal sh ift
acrosstheMBE(Figure 6). For example, Henryhowella, abundant during
and afterthe MBE, is primarily absent in pre-MBE samples (Figure 6).
Propontocyprisshowsasimilartrend,beingmorecommoninpost-MBE
samples (Figure 6). In contrast, Eucythere and Paracytherois are abun-
dantinmanypre-MBEsamples,butareveryrare inpost-MBEsamples
(Figure 6). Krithe is dominant almost throughout the record, but compara-
tivelymor eab undantinpre-MBEsamplesthaninpos t-MBEones.Sin ce
Eucythereisknownasani ndicatorofseasonalsur faceproduct ion(Didié
et al., 2002), and Henryhowella and Krithe are probably sensitive and tol-
eranttolowoxygenconditions,respectively(Yasuhara&Cronin,2008),
wespeculatethattheMBEfaunal shiftintheNorwegianSeaisrelated
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
NorthAtlanticproper(i.e.onthesouthsideofIceland)isabsentinpost-
MBE samples, butabundantinMBE and pre-MBE samples (Figure 7).
Krithe hunti and Krithe minima on the other hand, both extant species
intheNorwegianSeaandtheArcticOcean(Jöstetal.,2022;Yasuhara,
Grimm, et al., 2014),showcontrastingabundancepatterns.WhileKrithe
huntiis moreabundant inpre-MBE samples, Krithe minima tends to be
moreabundantin post-MBE samples (Figure 7). Generic faunal shifts
andfaunal differencesacrosstheMBEareclearlyshowninthenMDS
plot (Figure 8).A similarstrongfaunalshift isalsoknownfromthe high
Arctic (Cronin et al., 2017; DeNinno et al., 2015). Acetabulastoma and
PolycopeareabundantaftertheMBEbutveryrarebeforetheMBE
(Cronin et al., 2017 ). Several taxa, such as Echinocythereis, Arcacythere,
and several species of Krithe (including Krithe dolichodeira), are absent in
thepresent-dayand post-MBEArctic, butthey were abundant before
theMBE(DeNinno etal.,2015).Whilepartsofthefaunal shiftaredif-
ferentbetweentheNorwegianSeaandArcticOcean(e.g.Polycope does
not show a clear trend in the Norwegian Sea; Henryhowella shows op-
pos itepatternb et weenth eNor we gianSeaan dA rc ti cO cean)(Figure 6),
thislargelyconsistentfaunalshiftindicatesthattheMBEplayedanim-
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 sonbiodiversity.Ecological causesofthever ylow,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
withlimiteddispersalabilities(Jöstetal.,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  |  Taxonomicbias
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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14 of 18 
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   JÖST et al.
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, givenasAppendix7 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,
althoughitapp earsmorelikelythatthesedifferencesrefle ctsam-
plesizeef fectandvaryingcompletenessofrawdata.Sampleef-
fort islikely the causefortheappearanceofadditionalraretaxa
in M23352 that are lacking in PS1243, as more samples were
picked (143vs.61),coveringashortertime frame(190.3k yrs vs
530.7kyrs)(Table S1,given asAppendix1 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
andM23352,wedonothaveanyrecordofostracod-barrensam-
ples, whereasforour owncore, MD992277,wedohave thatre-
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  |  Discontinuoustimerecord
Given the very limited amount of data immediately following the
MBE(b as ic allynoda tainMI S9,8 ,7,exce pt fo rasin gl eP S1243sam-
pleduringMIS8),thereisthepossibilit ythatsomeeventduringthis
period,otherthantheMBE, could havehadsome influenceonthe
observed faunal shift . However, faunal turnovers of microfossil com-
munitiesrecordedbydeep-seasedimentlong-coresfromtheArctic
Ocean have shown to be driven by fundamental shifts in sea- ice
covervariability,sur faceprimaryprodu ct ion,andArct icO ceante m-
peratu re as consequences of M BE-enhan ced Arctic Amp lification
(i.e.amplifiedwarming inArcticregionsrelative totheglobalmean
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,wasset 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 resultingsteep latitudinal diversitygradient. 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
concludethat 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 andtaxonomically identified MD992277 specimens. A.B.J.,
H. H .M.H .,Y. H .a n dC .-L .W.p erfo rme dth eda tah and lin gan dst a tis ti-
calanalysesandgeneratedthefigures.H.A .B.providedMD992277
sedimentsamples.T.M.C.providedostracodassemblagerawdataof
coresPS1243andM23352.H.B.providedtherawdataforthenew
agemodelofcorePS1243.B.T.providedcriticalfeedback.A.B.J.and
M.Y.wrotethepaperincollaborationwithallauthors.
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-
ingthis manuscript.The work describedinthis studywas partially
supported by grants from the Research Grants Council of the
Hong Kong Special Administrative Region, China (project codes:
RFS2223-7S02toM.Y.,HKU17311316to M.Y.,HKU 17301818to
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
HongKong(toM.Y.),andtheEcologyandBiodiversityDivisionFund
(referencecode:5594129toA.B. J.).Thewritingandfigure-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
   
|
15 of 18
JÖST et al.
process was financially supported by the Brain Pool Program
through NRF fundedby the Ministry ofScience and ICT (reference
code:2019H1D3A1A01070922toA .B.J.).H.H.M.H.wassupported
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
DevelopmentProgram.Anyuse oftrade,firm,orproductnamesis
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).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. Global Ecology and
Biogeography, 00, e13844. https://doi.org /10.1111/
geb.13844
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Presented here is an illustrated checklist of benthic marine Ostracoda (Crustacea) recorded from Recent surface sediments of the sub-polar North Atlantic Ocean (SPNA). It presents 142 species (and species groups) belonging to 62 genera from 41 sampling sites collected from the water depths of 144–2749 m.We provide census data with scanning electron microscope images of representative specimens of most species, as well as geographical and bathymetrical distribution maps of selected species and genera. Samples from the Nordic seas (i.e., Greenland Sea, Norwegian Sea), as well as North Atlantic proper waters (i.e., Irminger Sea, Iceland Sea) are included. The bathymetry covers shelf to continental rise surface sediments from the Irminger Basin, the Iceland Basin, the Iceland Plateau, the Denmark Strait, the Faeroe-Shetland Channel, the Iceland-Faeroe Ridge, the Reykjanes Ridge, and the Faeroe Plateau. The data presented here is an important taxonomic and biogeographical baseline of the SPNA benthic ostracod fauna.
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Chapter
The biodiversity of many ecosystems is under threat and although seas cover the majority of our planet's surface, far less is known about the biodiversity of marine environments than that of terrestrial systems. It is also not clear whether many of the patterns known to occur on land also occur in the sea. Until we have a firmer idea of the diversity of a wide range of marine habitats and what controls it, we have little hope of conserving biodiversity, or determining the impact of human activities such as mariculture, fishing, dumping of waste and pollution. This book brings together key studies from the deep sea and open ocean, to tropical shores and polar regions to consider how comparable the patterns and processes underlying diversity are in these different ecosystems. Marine Biodiversity will be a major resource for all those interested in biodiversity and its conservation.