und Museum, Christian-Albrechts Universitaet, ... [Wust, 1936; Worthington, 1976; Reid, 1981;. Lacombe ... formation in the Norwegian Sea [Reid, 1979], that.
PALEOCEANOGRAPHY, VOL. 2, NO. 6, PAGES543-559, DECEMBER 1987
BENTHIC ISOTOPE EVIDENCE FOR CHANGES OF THE MEDITERRANEAN OUTFLOW
DURING THE LATE QUATERNARY
RainerZahn1 andMichael Sarnthein Geologisch-Palaeontologisches Institut und Museum, Christian-AlbrechtsUniversitaet, Olshausenstrasse 40-60, Kiel,
FederalRepublicof Germany Helmut
Erlenkeuser
Institutfuer Reineund Angewandte Kernphysik,Christian-AlbrechtsUniversitaet, Olshausenstrasse 40-60, Kiel,
FederalRepublicof Germany
Abstract. A suit of sediment cores close to and
southof theStraitof Gibraltar(12ø-36øN,5002800 m waterdepth)wereanalyzedfor stable isotopesin epibenthicforaminifersCibicidoides
climaticchangeof the carbonisotopesignalsof bothwatermasses indicates,thatthesupplyof salineMOW to thenorthAtlanticmay be less
wuellerstorfi andPlanulin•a ariminensi•s. During
importantfor the formation of NADW than previouslyassumed.
valuesof up to 1.6"/ooat intermediate depths
INTRODUCTION
butalsoto thenorthasrevealedby dataof intermediate depthcoresnorthof 38øN(in
pronounced highsalinity(>38"/00) component to the modernhydrographyof the AtlanticOcean [Wust, 1936;Worthington,1976; Reid, 1981;
peakglacial times,..the dataexhibit higher •5•3C neartheStraitof Gibraltar(36øN).Thevalues decrease to thesouthasevidenced by ourdata,
Duplessyet al. [ 1987]). Thus, the distribution
pattern of •5•3C provides crucial evidence foran increased influenceof nutrientdepleted MediterraneanOutflow Water (MOW) on the glacialnortheast Atlantichydrography. During oxygenisotopeTerminationsI and II, the
meridionalcarbonisotopegradientindicatesa significantlydecreasedbut still activeMOW. As
deduced fromthe•5•80fluctuations, the temperaturesof the MOW in the Atlantic were
lowerduringglacialtimesby asmuchas5øC. Duringglaci. aj timesandduringTerminationI the maximum •St•C values of the MOW correlate with
minimumvaluesof theNorthAtlanticDeepWater (NADW) andviceversa.Thisinverseresponse to
•NowatCollege of Oceanography, Oregon State University, Corvallis, OR 97331.
Copyright1987 by theAmericanGeophysical Union. Papernumber7PO834. 0883- 8305/87/007P-0834510.00
Mediterranean OutflowWater .. (MOW) provides a
Lacombeand Richez, 1985; Washburnand Kaese, 1987]. Upon leavingthe Straitof Gibraltar,it sinksto water depthsbetween800 and 1500 m [Zenk, 1971; Armi and Farmer, 1985; Gascard and
Richez, 1985] and spreadsto the west and southwest[Kawase and Sarmiento, 1986; Kaese and
Zenk, 1987]. Along the northwestAfrican continentalmargin, the MOW can be tracedsouth-
wardsto CapeBlancat21øN[e.g.,Mittelstaedt et al., 1975; Huber et al., 1977]. To the south of the
Greenland-Scottland Ridge, a northernbranchof the MOW
mixes with overflow
waters from the
Norwegian-Greenlandseasandwith deepwaters from the LabradorSea [Dooley andMeincke, 1981; Talley and McCartney, 1982; Kaeseet al., 1986; Kawase and Sarmiento, 1986]. As a result, a
relatively_warm (7.7< TøC< 11.6)andsaline
(35.0 < S•'/oo < 36.1) water massforms, which buildsthe upperlayer of the North Atlantic Deep Water (NADW) and can be easily traced throughoutthe Atlantic and even into the Pacific Ocean [Wust, 1936]. Thus the advection of MOW
into the Atlantic is likely to have oceanwide implications.It has alsobeen suggestedthat the
544
MOW may directlycontributeto thedeepwater formationin the NorwegianSea[Reid, 1979], that is, to the formation of the NADW itself, although this model is still under debate.
The advectionof the modemMOW dependson the deepwaterformationin the Mediterranean, whichis closelytied to the climaticconditionsof this area [Bethoux, 1979a; Lacombeet al., 1981].
In thisadjacentsea,deepandintermediate water formationresultsfrom excessevaporationof surfacewatersandwinterly deepconvection, particularlyin the northernBalearenandthe Levantineseas[Medoc Group, 1970; Lacombeet al., 1981;BrydenandStommel,1982;Millot, 1987]. A numberof studiespresentedgeochemical, micropaleontological, andstableisotopeevidence for markedchangesof bothclimaticand hydrographic conditions in theMediterranean duringthelate Quaternary[e.g.,Thunellet al., 1977, 1984; Williams et al., 1978; Thiede, 1980; Nesteroffet al., 1981; Cita andGrignani, 1983; Rossignol-Strick et al., 1982;Rossignol-Strick, 1985;VergnaudGrazziniet al., 1986;Ganssen and Troelstra, 1987; ten Haven et al., 1987]. These studiesrevealedthat the glacialwinterly sea
surfacetemperatures (SST) of the Mediterranean
Seawereconsiderably colder byabout 7øCand
that the seasonalSST variationswere higher than
today.Furthermore, theytestifyto a high freshwaterinputinto theeasternMediterranean duringthelastdeglacial.Thissituation ledto a highlystratifiedwatercolumnand,in turn,to intermittent periodsof deepwaterstagnation as evidenced by theformationof sapropels (details in the work of Thunell et al. [ 1984]). These
changes of theMediterranean hydrography canbe expectedto alsoaffectthe intermediate water andpossiblyevendeepwaterhydrography of the Atlantic Ocean.It will closelyrespondto possible variationsin both the advectionrate and density of the MOW, as well as to its nutrient com-
position.The mostextrememodelis a totalshutoff of MOW advectionduringglacialand/or deglacialtimesasa resultof theenhanced freshwatersupplyinto the MediterraneanSea [e.g.,Huanget al., 1972;Diester-Haass, 1973; Bethoux, 1979a].
In thisstudywe presentstableisotoperecords of epibenthic foraminifersfrom 13 sediment cores, which have been obtained from the northeast
Atlantic continentalmargin off and to the south of the Straitof Gibraltar(Figure 1, Table 1) (the
isotopedataareavailableuponrequestfromthe seniorauthor).The coresform threebathymetric
transects between 20ø and35øNranging from500 to 2800 rn waterdepth,thatis, crossingthe modemflow pathof the MOW (Figure1). For comparative purposes, additional isotoperecords
Zahnet al.: Late QuaternaryMediterranean Outflow convincingevidencethattheintermediate depth levels of the northeast Atlantic Ocean were well
oxygenated duringthepastglacial-interglacial cycle.By analogywith themodemcirculation pattern,we concludethatthedataindicatea continuousadvectionof nutrient-depletedMOW to the Atlanticduringthe last 140,000yearsB.P. METHODS
StableIsotopeAnalysis
Stableisotopemeasurements wereperformedat theC 14 Laboratoryof Kiel Universityon botha MicromassVG 602D anda FinniganMAT 251 massspectrometer, thelatterwith theCARBOKIEL automatedpreparationsystem.Both instruments were calibrated to the Pee Dee
Belemnite(PDB) scalethroughNationalBureauof Standards(NBS) 19 andNBS 20 carbonate standards. Additionally,intercalibrationwas checkedwith a laboratorystandard(Solnhofen limestone),thusallowingfor comparison of values measuredwith bothsystems.Externalreproducibilities(includingcarbonate r•reparation and
spec[ometric ana12/sis) for•51•O and•513C were
0.10•'/oo and 0.05"/00 for the VG 602D, and
0.070/00and0.040/00for theMAT 251, respectively. Measurements weremadeon monospecificbenthicforaminiferalsamples of 1-10 specimens, whichwerepickedfromthe250-to 400-gm-sizefraction.Sampletreatment and preparation followedstandard procedures [e.g., Ganssen,1983; Zahn, 1986].
Mostisotopicdatahavebeenobtainedfrom Cibicidoides wuellerstorfi. The carbonate of this
species exhibits •5•Cvalues whichlieveryclose to those of the 2;CO.. of the ambient bottom
water [Belanger eta•.,1981; Graham etal.,1981; Duplessyet al., 1984;Zahnet al., 1986]because of its habitat,which is well elevatedabovethe sedimentsurface[Lutze and Thiel, 1987]. This
species, however,livesalongthecontinental marginof northwestAfrica mainly at water depthsbelow3000 rn anddoesnotoccurat
depthsshallower thanabout1000rn [Lutze,1980]. To determinethepastcirculationpatternsalsoin theupperdepthrangeof themodernMOW, that is, above 1000 m, isotopemeasurements were carriedout on Planulinaariminensisd'Orbigny
[ 1826].This species livesoff northwestAfrica at waterdepthsbetween300 and 1000rn [Lutze, 1980].Comparison of its oxygenandcarbon isotopedatawith thoseof C. wuellerstorfi show highcorrelations of 0.93 and0.91,respectively (Figure2). Slightshiftsin theC. wuellerstorfi
versus P. ariminensis isotot•e valuesof-0.3ø/00
and+0.20/00occur for•5•8'O and•5•3C. Larger
of six northeastAtlantic deep seacoreswere
offsets of some1ø/ooaremainlyrestricted to
used(Figure1, Table1). Ourisotopedataprovide
TerminationsI and II, wherethey obviouslyresult
Zahnet al.- Late QuaternaryMediterraneanOutflow degrees 40
"'•( •S•
'LY-II-13A•
545
northern
latitude
30
20
I
I
10
I
15666
I
-- / ++•5669 ,•1 0 .•
lO
o
.s.
• • •'
20 -
+ + 603013289 +12347
12379 •
•
-
12328
20
/ degrees 40
northern
latitude
30
20
I
I
lO
b
+15663 +1600•
+ 15666
LY-II-13A
MOW
+
+ 15670
11944
+ 15627 •
16017 +
•
12345
+ 16004
-1
+ 16030
+
-2
+ 15669
NADW +
+
12379 13289
15672
+ 12392
+ 12347
3'
1230F
12328
-3
Fig. 1. Positionsof sedimentcores.(a) Locationmap. Isohalinesat 1000 m water depth indicatemajor flow path of the MOW (adoptedfrom Kaeseand Zenk [ 1987]). (b) Depth distributionof the coresand of major modernwater masses. from bioturbationalmixing of oxygenisotopically heavyshells,that is, of glacialshellsof C. wuellerstorfiinto interglacialsectionsof the records(Figure 2a, core 15666). Thus testsof P. ariminensisappearto providea valuablebasis
Since most of our sediment cores lack the
modern sediment surface, benthos versus water
isotopicadjustments were basedon a separateset of stableisotopedataof surfacesedimentsamples
[Ganssen, 1983].The/5•80 equilibrium values for
for/5•3C-based studies onthepaleohydrography of
calcite were calculated after O'Neil et al. [ 1969]
upperintermediatewater massessuchas the
from a te_rnperature and salinityprofile at
of the bottom water ZCO• and thus, also those of
profile from the samelocationwas availablefrom Duplessy[ 1972]. Largely oxygenisotopedata from Uvigerina were usedto deducethe oxygenisotope stratigraphyand to infer possibletemperature/ salinity(TS-) fluctuationsin the MOW. Oxygen isotopevaluesof this genusare consideredas
MOW.Based onKroopnick [1971],the/5•3C values ep•benth•cforarmmferacanbe depletedby as
muchas0.350/00wherethefluxof organic carbonreachesa maximum near the seafloor(see alsoMcCorkle [ 1987]), which, however, is not the case at our core sites.
36øN/09øW [Stramma, 1981].A hydrographic •13C
546
Zahn et al.: Late QuaternaryMediterraneanOutflow TABLE 1. Locationsand Depthsof SedimentCores Average
Core Number
Northern Latitude
Western Longitude
Depth, m
Sedimentation Rates
Ambient Water Mass
Profile A
MOW a
LY-II-13A
35o58.0'
07o49.0'
1201
22.8
11944
35ø39.1'
08o03.7
1765
5.7
15663
34055.2'
06050.7'
500
16.6
NACW
15666
34057.6'
07ø07.1'
803
2.3
MOW
15669
34053.5'
07048.9'
2022
5.1
NADW
15670
34054.5'
07034.6'
1482
4.1
NADW
15672
34051.6'
08007.6'
2455
4.4
NADW
Profile
MOW/NADW
B
15627
29ø10.0'
12ø05.2'
1021
2.9
MOW
16004
29o58.7'
10o38.8'
1512
4.0
NADW
16006
29ø14.8'
11029.8'
796
4.0
MOW/NADW
Profile C
12379
23o08.1'
17ø44.7'
2136
11.1
NADW b
16017
21ø14.7'
17ø48.2'
812
17.4
EUC/NADW
16030
21ø14.1'
18ø03.3'
1500
6.0
NADW
Reference Cores
12309
26o50.3'
15o06.6'
2820
10.4
NADW
12328
21o08.7'
18o34.4'
2778
14.0
NADW
12345
15028.8'
17021.6'
945
29.8
SACW
12347
15049.5'
17051.7'
2576
22.1
NADW
12392
25010.3'
16050.7'
2575
8.5
NADWC,d
13289
18004.4'
18000.6'
2490
6.6
NADW
SU 81-21
38015.0'
09033.0'
1260
MOW e
SU 81-44
44015.0'
02042.0'
1173
MOW e
SU 81-45
44006.0'
02029.0'
994
MOW e
EUC Equatorward UnderCurrent;MOW Mediterranean OutflowWater;NACW North AtlanticCentralWater;NADW NorthAtlanticDeepWater;SACW SouthAtlanticCentralWater. a Samplesprovidedby L. Diester-Haass.
bStratigraphy rearranged afterKoopmann [1979]. c Data from Shackleton [ 1977].
dDatafromZahnet al. [ 1986]. e Data from Duplessyet al. [ 1987] beingcloseto equilibriumwith the ambient seawatervalues [Shackleton,1974]. For the rest
equivalents by applyinga conversion factorof
of the cores the data of C. wuellerstorfi/
closelymatchesthevalueof 0.69"/00, whichwas
P. ariminensiswere normalizedto Uvigerina
obtained from 423 double measurements on the
0.640/00 [Shackleton etal.,1984]c. Thisfactor
Zahn et al.: Late QuaternaryMediterraneanOutflow
5q?
16006
6180 vs
PDB
ß
i
ß
i
ß
ß
i
ß
ß
ß
-
-a- P.arlmlnensls -e- C.wuellerstorfi 15666 1
6180 vs
PDB 2
i
0
:20
i
i
i
40
60
80
i
i
100
ß
120
i
140
ago ( 1,000 yr)
b
4-
•180 PDB
R=0.93/
vs
v S PDB
•
I
ß
,
2
ß
!
3
o
4
I
1
•
2
C. wuellorstorfl
Fig. 2. Comparisonof stableisotopedataobtainedfrom C. wuellerstorfiandP. ariminensis.(a) down-coreoxygenisotopeprofilesand (b) correlationgraphs,withoutvaluesfrom oxygenisotopeTerminationsI andII (for discussion seetext).
respectivespeciesalong 19 sedimentcoresfrom
fraction> 125 gm. The calculationof the radio-
the northeastAtlantic [Zahn, 1986].
carbondata,(Table2) is basedon 95% of theNBS
Radio CarbonAges
oxalic acid •'•C standardacitvity and on the conventional(Libby) half-life of 5568 yearsfor
•4C.Corrections forisotope fractionation were notapplied since the6•3Cvalues ofthedated
For a more precisechronostratigraphical control of the sedimentsyoungerthan30,000 years, radiocarbonageswere measuredon six cores
effectofisotope fractionation onthe•4Cages
for cores12379 and 12392 (in Koopmann[ 1979], andunpublished data).Samplescontainedbetween 10 and30 g of carbonateandwere from the size
will closelybalancethe reservoirage of about400 yearsof the ocean'ssurfacelayer. Time controlof the oxygenisotoperecords older than stage2 was adoptedfrom the
(Table2). A fewfurther•4Cdatawereavailable
carbonates werecloseto 0ø/ooPDB,thatis, the
5•48
Zahn et al.: Late QuaternaryMediterraneanOutflow PROFILE A (35øN) oxygen isotope stages i
ß
i
20
&]80 2
vs PDB
ß
!
,
i
oxygen isotope stages
,
.
ß
,;o
&180
15663
3
.!,
ß
40 60 8; 1;0 1:•0 140kyr B.P. vs
11944
PDB
1,765 m
500m
4
11 2
15666
3
803m
15669 2,022 m
4
3
2
15672
3i!iii: ..• LY-II-13A 4
2,455m
1,201 m
5
4
1
6
3 ' '•
15670
4
1,482 m
,
0
20
40
60
80
1 0
, 120! ! 140!
age (1,000 yr B.P.)
,
0
40 age (1,000 yr B.P.)
Fig. 3. Oxygenisotopestratigraphies of coresat profilesA-C. Valuesof C. wuellerstorfiand P. ariminensis arenormalized to Uvi[erinavalues.Curveof core15666basedonUvigerina
values. Arrows markposition of ]4C•tges listed inTable2. Forchronostratigraphy seetext. CARTUNE time scale(modified from CARPOR and STUNE time scales;Herterich and Sarnthein [1984]).
thoseexpectedfrom the respectiveoxygen isotopiccurve structure.In core 16017, maximum
offsets between expected andmeasured 14Cages reach14,000 years.This benthicisotoperecord
Stratigraphy
exhibits an unusual scatter,which is also observed
in threeplanktonoxygenisotopeprofilesfrom the Most of our oxygenisotoperecordsfrom the upperand middle northeastAtlantic continental margin(Figure3) closelyresemblethe established oxygenisotopehistoryof theworld ocean[e.g., Pisias et al., 1984]. Most of them contain the climatic transitionof Termination I; somealso
containTerminationII (Figure3). Only core 15627 lackssediments youngerthan15,000yearsB.P., possiblydueto a cotingloss.Core 15666might lack the sediments of the oxygenisotopesubstages5c and5d asrevealedby the anomalously smoothcourseof the up.per stage5 curve,aswell
asbytherather sharp/51•O peakofsubstage 5e.
Despitethe generallywell-definedstructureof
ourO-isotope curves, the14Cages of cores
15666, 16006, and 16017 from shallower water
depthsappearto be too old whencomparedwith
samecore[Zahn,1986].Furthermore, the
data which have been obtained from the smaller
sizefraction(125-200 gm) reveala significantly older age than thosefrom the coarsersize fractions(>200 gm; sample211-222 cm; Table 2).
Hence thediscrepancies between 14Cages and expected 5180curveagesandthe14Coffsets
betweensamplesof differentsizefractionsfrom the samesamplemay be relatedto downslope redeposition of older,fine-grainedcarbonate
particles. Thusthe14Cdataof these shallow site coreswere rejectedand stratigraphic controlwas setby analogywith the oxygenisotopeprofilesof coresfrom the deepsea. Averagesedimentation ratesof the coresrange from 3 crn/1000yearsto >20 cm/1000yearsalong the northwestAfrican continentalmargin(Table
Zahn et al.' Late QuaternaryMediterraneanOutflow
5119
PROFILEB (29øN)
PROFILE C (22øN) oxygen isotope stages
oxygen Isotope stages
s
', '
'
'
'
I
I
',;o
vs PD8
16006
5180 2
ß
i
ß
ß
i
ß
ß
i
ß
I
,
•180
vs PDB 3
16017
3
796
i
20 40 60 8; 100 1•0 140kyr B.P. 812m
m 4
4
3
3 •
16030
15627 1,021 rn
1,500 m
4
5
11 3
16004 4
12379
1,512 rn
4 0
20
40
60
80
100
120
140
2,136 rn
5
age (1,000 yr 6
ß
0
2O
age (1,000 yr B.P.)
Fig. 3. (continued) 1). Thus the time resolutionof our stratigraphic recordsreaches250-1200 yearsfor sampling intervals of 5-10 cm.
RESULTS
AND
DISCUSSION
watermass ofheavy•5•3C values of about +1.30ø/oo(Figure 5b,> recent recent recent l) tlldop aele•
i
t •)
(re>l) qldep aoIet•
55•
Zahn et al.: Late QuaternaryMediterraneanOutflow age
(1,000 yr B.P.)
10
I
20
i >1.2
•
och-6 carbonshift:A changein 13 I2 the oceans C/ C ratio 6.2 million yearsago, Mar. Micropaleontol.,5, 185-203, 1980. Washburn, L., and R. H. Kaese, Double diffusion
and the distributionof the densityratio in the Mediterranean
water front southeast of the
Zahn et al.: Late QuaternaryMediterraneanOutflow Acores,J. Phys.Oceanogr.,17, 12-25, 1987. Williams, D. F., R. C. Thunell, and J.P. Kennett,
Periodicfreshwaterflooding and stagnationof the easternMediterraneanSea duringthe late Quaternary,Science,201,252-254, 1978. Worthington,L. V., On the North Atlantic circulation,JohnHopkinsOceanogr.Stud.,6, 110 pp., 1976. Wust, G., Schichtungund Zirkulationdes AtlantischenOzeans,Wiss, Ergeb.Dtsch.Aft. Exped.Forsch,Schiff "Meteor" 1.925-1927,6 (1/2), 1936. Zahn, R., Spatquartare Entwicklungyon Kustenauftrieb und Tiefenwasserzirkulation
im
Nordost-Atlantik, Rekonstruktion anhand stabiler
Isotopekalkschaliger Foraminiferen,Ph.D. thesis,111 pp., Kiel University,Kiel, Federal Republicof Germany, 1986. Zahn, R., K. Winn. and M. Sarnthein, Benthic
foraminiferal•13C andaccumulation ratesof
559
organiccarbon(UvigerinaperegrinaGroupand Cibicidoideswuellerstorfi),Paleoceanography, _1, 27-42, 1986.
Zenk, W., Zur SchichtungdesMittelmeerwassers westlichyon Gibraltar,"Meteor"-Forschungsergeb.,Reihe A, 9, 1-30, 1971. H. Erlenkeuser, Institut fuer Reine und
AngewandteKemphysik,C-14 Labor, ChristianAlbrechts Universitaet, Olshausenstrasse40-60,
D-2300 Kiel, FederalRepublicof Germany. M. SarntheinandR. Zahn, GeologischPalaeontologisches Institutund Museum,ChristianAlbrechts Universitaet, Olshausenstrasse 40-60,
D-2300 Kiel, FederalRepublicof Germany. (ReceivedJuly 15, 1987; revised October 20, 1987;
acceptedOctober22, 1987)