GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 6, PAGES 679-682, MARCH 15, 1997
Alkenone sea surface temperature in the Southern Ocean
for the last two deglaciations
Minoru Ikehara 1, Kimitaka Kawamura 2, Naohiko Ohkouchi 2, Katsunori Kimoto 1
Masafumi Murayama 3, Toshio Nakamura4, TadamichiOba 3, and Asahiko Taira 1
Abstract. A pistoncore takenfrom the TasmanPlateauin
on zooplanktonicmicrofossilscontainsuncertainty, because
the diversity of foraminifera is much lower and their living
reconstruct the late Pleistocene marine environments.
Here we
depth rangeswidely.
report paleo sea surface temperature (SST) for the last two
A geochemicalthermometer,
referredas U k37[Brassellet
deglaciations using the unsaturation degree of alkenones al., 1986], is basedon the analysisof long-chainalkenones
(Uk37') preservedin the marinesediments.The Uk37' record (C37) synthesizedby certainhaptophytemicroalgae[Marlow
indicates that the SST was at least 4øC lower than the present et al., 1990], and is characterizedby a linear relationshipto
SST at the last glacial maximum(LGM), and the amplitudeof its growthtemperature[Prahl and Wakeham, 1987]. This ratio
paleo-SST is at most 5.2øC from the penultimateglacial (MISis also unalteredthroughthe food web in the water columnand
6) to the last interglacialwarm period(the Eemian).Our results early diagenesisin the sediments [Prahl et al., 1989, 1993].
also demonstratethat the Eemian warm period in the Southern The Uk37ratioshavesofar successfully
beenusedto revealthe
Ocean lasted only 3,000 years followed by a sharpcooling at past-SST variations in the equatorial Pacific [Prahl et al.,
around 120 kyr BP. The sharpcoolingin the SouthernOcean 1989; Ohkouchi et al., 1994], North Pacific [Prahl et al.,
seemed to occur a few millennia (2-3 kyrs) before the 1995; Herbert et al., 1995], equatorialAtlantic [Brassellet al.,
beginningof continentalice-sheetgrowth.
1986; Jasperand Gagosian, 1989; Sikesand Keigwin, 1994;
Schneideret al., 1995], and North Atlantic [Eglinton et al.,
1992; Zhao et al., 1993, 1995; Sikes and Keigwin, 1996].
Introduction
Here we report for the first time alkenone SST recordsfor the
The Southern Ocean occupies a significant position of
last two deglaciationperiodsin the SouthernOcean.
world ocean and is characterizedby several unique features:
high-nutrient and low-chlorophyll (HNLC) conditions [e.g.
Coale et al., 1996], development of polar zonation Samplesand methods
surroundingAntarctica, deep convectionto intermediateand
A piston core (KH94-4
TSP-2PC;
48ø08.18'$,
deep waters. Reconstructionof the late Pleistocenesea surface
146ø52.45'E,2321m water depth)and a multiple corer sample
temperature (SST) of the Southern Ocean is important for
(TSP-2MC; 48ø07.60'S, 146ø53.99'E, 2283m water depth)
understandingpaleoceanographic
conditionssuchas the shift were collected from the Tasman Plateau in the Southern Ocean
of the polar frontal zonationand its link to the Earth'sclimate
duringthe R/V Hakuho-Marucruise[Ikeharaet al., 1996; Fig.
system.In the early 1980s,the CLIMAP ProjectestimatedSST
1]. Sedimentsare mainly composedof calcareousforaminifera
for the last glacial maximum (LGM) and the last interglacial
-nanno ooze (CaCO3 content: 87-94%). It is verified that the
period (the Eemian) based on the planktonic microfossil
multiple core TSP-2MC corresponds to the very surface
assemblages
in a large numberof deepseasediments[CLIMAP
sedimentsof piston core TSP-2PC basedon comparisonwith
Project Members, 1981, 1984]. One of the significantfindings
magnetic susceptibility, color data, and oxygen isotopic
was that the estimated SST in the low- to mid-latitudes
at the
the SouthernOcean has been studiedfor organic compoundsto
LGM was not much different (<2øC) from the present SST.
Further,the differencein the SST for the high-latitudeswas not
large (2-3øC) except in the North Atlantic and northwest
Pacific. CLIMAP Project Members [1984] also concludedthat
SST of the last interglacial ocean was not significantly
different from that of the modern ocean. However, the paleoSST reconstructedat high-latitudesbased
i OceanResearchInstitute,Universityof Tokyo, Nakano-ku,Tokyo,
Japan
2 Instituteof Low TemperatureScience,HokkaidoUniversity,Sapporo,
Hokkaido, Japan
3 Graduate School of Environmental Earth Science, Hokkaido
University,Sapporo,Hokkaido,Japan
4 DatingandMaterialResearch
Institute,NagoyaUniversity,Chikusaku, Nagoya, Japan
values of these two core sediments.
A •5180 stratigraphywas establishedfor the subsurface
dwelling planktonic foraminifera Globigerina hulloides (250
to 300 gm in test size; 40 specimens).These testswere reacted
with 100% H 3PO4at 60øCandanalyzedin a FinniganMAT 251
massspectrometer.
The •5180resultsare expressed
as per rail
deviations from PDB (PeeDee Belemnite) standard. The core
chronologywas determinedbasedon graphiccorrelationto the
SPECMAP •5180 record [Imbrie et al., 1984] and AMS-14C
dating of planktonic foraminifera G. hulloides (see Fig. 3a).
The AMS-14C dates were obtainedusing the graphitization
method[Nakamuraet al., 1987; Kitagawaet al., 1993] andwere
correctedfor a constantsurfacewater age of 400 years [Bard,
1988]. The averagedsedimentationrate was estimatedto be 1.5
cm/kyr.
The Uk37valueswereobtainedby the followingprocedure.
Copyright1997by theAmericanGeophysical
Union.
Papernumber97GL00429.
The frozen sediment samples were cut at 2 cm intervals.
Approximately20 g of wet sedimentswere extractedfor lipids
0094-8534/97/97 GL-00429505.00
with
679
methanol/dichloromethane
(3:1)
and
then
680
IKEHARA ET AL.: SEA SURFACE TEMPERATURE IN THE SOUTHERN OCEAN
that derivedfrom three calibrationsof the Uk37 method(Fig.
2) showsthat the SST calculatedusingthe calibrationof Prahl
30øS
' ugtralia
40ø
.•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•
.................
'•j• ...........;.:.:.:
STC
I
..........
'"":':':'"::::i:!!::i::.;.......::i:i::::-;..
.::::2:'"'
et al. [1988] is similar to the summer SST near the sampling
site. Therefore, we decidedto use the following equationfor
the calculationof the past SST, whose precisionis about 0.5
øC [Prahl et al., 1988].
......
'":"'"
'":'"
":"
........
"::::"'"'•:
:'•••:'•'"••'-::•'
.'•••!':-:.-..:....
'
APF
'"-:4:.
70 .ø .
.
120øE 130 ø 140 ø 150 ø 180 ø 170 ø
Figure 1. Location map of the deep sea coresTSP-2PC and
collected
from the Tasman
Plateau
in the Southern
Ocean and hydrographic conditions. STC ß Subtropical
Convergencezone, APF ß Antarctic Polar Front zone, AD ß
Antarctic Divergence zone. Subantarctic Water properties
betweenSTC and APF are characterizedby relatively warm and
saline waters. In contrast, Antarctic Surface Water between
APF and AD are characterizedby cool and lesssalinewaters.
dichloromethane/methanol
(10:1) using an ultrasonic
homogenizer.The extracts were isolated by a centrifuge and
washed with 50 ml of 0.15M
A slightdifferencebetweenthe alkenoneSST (12.0øC) and the
summerLevitus SST (11.1øC; see Fig. 2) may be causedby a
difference of the data source:the former representsan average
SST for a few millennia whereasthe latter representsat best
few decades.
Antarctica
TSP-2MC
T (øC) = (Uk37'- 0.039) / 0.034
HC1 to remove
salts contained in
the sediments,and then saponifiedwith 0.5M KOH/methanol
under a reflux. Neutral components were separated by
extraction with dichloromethane/n-hexane(10:1). They were
further divided into four sub fractions on a Pasteur pipette
column packed with silica gel (BIO-SIL A, 200-400 mesh)
which was deactivatedwith 1% water. The fraction containing
long-chainalkenoneswas analyzedby a Carlo Erba 5160 gas
chromatograph(GC) equippedwith a cold on-columninjector,
a HP-5 fused silica capillary column (30 m x 0.32 mm i.d.;
0.25 pm film thickness),and a flame ionization detector(FID).
Hydrogen was used as a carrier gas, and the column oven
temperaturewas programmedfrom 70øC to 120øCat 30øC/min.
and then to 320øC (30 min.) at 6øC/min. The GC peakswere
processedusing a Shimadzu ChromatopacC-R7A integrator.
Identification of alkenoneswas conductedby comparing gas
chromatographic retention times with previous work
[Ohkouchi et al., 1994]. Uk37' values were obtainedwith the
following definition [Prahl et al., 1988]:
Glacial-interglacial
variation
of
alkenone
SST
OurUk37' values rangedfrom 0.281 to 0.465 during the
last deglaciation (last 30 ka; MIS-3 to Holocene) and from
0.385 to 0.563 during the penultimate deglaciation (110-145
ka; MIS-6 to MIS-5d). Figure 3 demonstrateschangesin the
alkenone SST and planktonic •5180 [Ikehara, 1997]. The
alkenone SST between LGM
and Holocene
varied from 8.1 to
12.5øC (Fig. 3b), whereasthose between MIS-6 and MIS-5e
ranged from 10.2 to 15.4øC (Fig. 3c). The SST difference
between LGM
and Holocene is 4.4øC, and that for the
penultimatedeglaciationis 5.2øC. Thus, we concludethat the
alkenone SST shifted by ca. 5øC during the last two
glacial/interglacialclimate variations in the Tasman Plateau
region. These results suggest that SST variations in this
region were greater by ca. 2øC than the CLIMAP results
[CLIMAP Project Members, 1981]. This larger amplitude of
glacial/interglacialSST changeagreeswith the SST changes
reconstructedfrom diatom assemblagesin the sedimentsfrom
the Indian
sector Southern Ocean, which showed 5-6øC
difference for the glacial/interglacialperiods [Pichon et al.,
1992]. The SST decreasein the glacial SouthernOcean may be
MonthlyMean SST (48øS,147øE)
I
I
I
I
I
I
I
I
I
I
I
(a) 14.6øC•
(b)14.3øC
•
(• 12
(c) 12.0øC •
E
• 10
Uk37'= [C37:2]/ ([C37:2]+ [C37:3])
•
where[Cx:y]denotesthe concentration
of the alkenonewith x
carbon number and y double bonds. Triplicate analyses of
compositesedimentsshowed that the analytical errors in the
8
summer
winter
i
' '; ' '
i
i
spring
i
i
I
v, g-g
Month
procedureswere +_0.001for Uk37' values(i.e. _+0.03øC).
C37:4
alkenone has been detected in some sediment samples,
however, its concentrationswere generally low.
autumn
Figure 2. Annual surface temperature cycle of a location
(48øS, 147øE) close to TSP-2PC and TSP-2MC [Levitus, 1982].
Alkenone SST for the surface sediments (TSP-2MC, 0-1cm)
Results
and Discussion
Core top alkenone SST
calculatedby three calibrationsare shown,as follows; (a) the
linear field data calibration of Sikes and Volkman [1993] in
the Southern Ocean (Uk37' = 0.0414T- 0.156), (b)the
calibration of Brassell [1993] based on global combinedfield
Long-chain C37-C39 alkenones detected in the surface
sediments (TSP-2MC, 0-1 cm) had a Uk37' value of 0.448.
data(Uk37'= 0.037T - 0.083), and(c) the calibrationof Prahl
et al. [1988] basedon culturesof Emiliania huxleyi(Uk37'=
Comparison
of theannual
SSTcyclefromLevitus
[1982]With
0.034T + 0.039).
(a•
681
KH94-4 TSP-2PC
•, .i.i
il.......i' ' ':....._•
,- 0
",50
,,'
00'1•,
'-,
150"--.
200
Age
(ka),
(b)•
•
'--...
...
(c)•
•
14G
•
12•
m
3
0
10
20
30
8•
].........
•o
110
Age (ka)
120
130
140
'•8•
Age (ka)
Figure 3. (a) Changesof theplanktonic
foraminiferal
5180 in the sediment
core(TSP-2PC)collectedfromthe TasmanPlateau
in theSouthern
Ocean.Oddnumbers
meaninterglacial
periods
andshaded
evennumbers
indicate
glacialperiods.
AMS-]4Cage
levelsare shownby bold markson the top of the figure. (b) Changesin the planktonicforaminiferal5180 and sea surface
temperature
(SST) reconstructed
fromthe Uk37' indexduringthelastdeglaciation
(MIS-3 to Holocene;0-30 ka). (c) Sameas (b)
but for penultimatedeglaciation(MIS-6 to MIS-5d; 110-145ka). For the definitionof Uk37', seethe text. EstimatedSST was
calculated
by usingthe followingequation:T (øC)= (Uk37'- 0.039)/ 0.034 [Prahlet al., 1988], whereT denotesa haptophyte
growth temperature.
causedby northward shift of the Antarctic Polar Front (APF) at
that time.
Although the alkenoneSST curve from LGM to Holoceneis
similar to that from MIS-6 to MIS-5e, the reconstructed SST
this abruptcoolingtook place entirely in the SouthernOcean,
because similar cooling of mid-Eemian have been found in
sedimentcoresrecoveredfrom other regionsin the Southern
Ocean [e.g. Martinson et al., 1987; Pichon et al., 1992].
However, the cooling event is unlikely explainedby any
significant decrease in the continental ice volume, because
valuesof the las.tdeglaciationare systematicallylower by ca. 3
øC compared to those for the penultimate deglaciation. In
contrast, the foraminiferal results did not show a systematic planktonic•]80 valuesdid not showa positivepeakduring
shift of 5•80 values,whichhasbeenfluctuatedmainlyby ice the mid-Eemian (Fig. 3c). These results suggest that the
volume. Thus, the systematic shift of alkenone SST between cooling of the Southern Ocean preceded the significant
the last and penultimate deglaciations suggeststhat main increaseof the continentalice sheetsby 2-3 kyrs and that the
contributor (co.ccolithophorid) to the C37 alkenones was global coolingfrom the last interglacialto glacial, which is
differentin the SouthernOceanfor the last two deglaciations. recordedin the oxygen isotope signal of foraminifera, was
In fact, the abundanceof E. huxleyi decreasedduring the triggered by an abrupt cooling of the high latitudes. We
penultimate
deg•aciati0n
in comparison
withthose
of thelast
believe that this documented offsets in time between SST and
deglaciation[M. Horikoshiand H. Okada, pers. commun., •80
1997]. Al.ternatively,an excursionof the polar frontal zone
and salinity change probably caused by evaporation to
precipitationbalance may contributeto the systematicshift of
the alkenoneSST betweentwo deglaciations.
Eemian
SST changes
shiftsare real recordsand cannotbe explainedby a
bioturbation
process, which
could cause a selective
redistributionof foraminiferal fossils and coccolithophorid
grains [Hutson, 1980], because the abundance of both G.
bulloidesandalkenones
wereobservedto stayalmostconstant
throughoutthe sedimentsequence
from 110 to 130 kyr BP.
Conclusions
Southern
Oceansediment
coreswerestudied
for long-chain
The h.ighestSST (15.4øC)in the Eemianwas recordedat alkenones
to reconstruct
theSSTfor thelasttwodeglaciation
around122 kyrs BP when the •80 valuesbecameminimum, periodsin relationto the globalglacial/interglacial
cycles.
and the .alkenoneSST droppedabruptlyby 4.5øC (similar to Our results demonstrate that:
glacial SST) in the middleof the Eemian(Fig. 3c). Basedon the
(1) The alkenoneSST differencebetweenglacial and
an.alysesof grain size and compositionof sediments,no interglacial in the Southern Ocean was found to be
significantchangewas suggestedin sedimentaryprocesses
at approximately
5øC,indicatingthat the amplitudeof Southern
thi.õperiod.Assuming
a constant
sedimentation
rateduringthe OceanSSTfluctuation
wastwicegreaterthanthatreported
by
Edmian,we estimated
thatthewarmestperiodhigherthan15øC the CLIMAP projects.
lastedonly ca. 3,000 yearsandendedat around120 kyrsBP.
(2) TheSSTfor theEemianappearto be higherthanthatof
The climateof the Eemianwas thoughtto be stableand similar
to thatof the Holocene[e.g. CLIMAP ProjectMembers, 1984].
However, our results suggestthat the Eemian SST in the
the Holoceneby ca. 3øC.
SouthernOceanis not stablethroughout
this period,and that
rapidcoolingoccurredat around120 kyrsBP. It is likely that
cooling
eventoccurred
a fewmillennia
before
thebeginning
of
(3) The Eemianwarmestperiodlastedonly 3,000 years
followedby a sharpcoolingat around120 kyrs BP. This
a largecontinental
ice-sheet
growth.
682
IKEHARA ET AL.' SEA SURFACE TEMPERATURE IN THE SOUTHERN OCEAN
on Accelerator Mass Spectrometry,Nuclear Instruments and
Acknowledgments: We thank the on-boardscientistsand crew of the
R/V Hakuho-MarucruiseKH94-4 for their cooperation
in collectingthe
samples.We also acknowledgeF. Prahl and one anonymousreviewer
for their critical and useful comments, T. Nakatsuka, P. Hesse, and N.
Methodsin Physics
Research
B29, editedby H. E. Cove, A.E.
Litherland,
andD.Elmore,
pp.335-360,
1987.
Ohkouchi, N., K. Kawamura, T. Nakamura, and A. Taira, Small
Ahagon
forfruitfuldiscussions,
Y. ImaiandY. Ishimura
foranalytical
help.ThisstudywasPartlysupported
by a grantfromtheJSPS(Japan
Societyfor the Promotionof Science)for JuniorScientiststo M. I.
changes
in theseasurfacetemperature
duringthelast20,000years'
Molecularevidencefrom the westerntropicalPacific, Geophys.
Res.Lett.,21,2207-22
i 0, 1994.
Pichon,J. -J., L. D. Labeyrie,G. Bareille,M. Lagracherie,
J. Duprat,
and J. Jouzel, Surfacewater temperaturechangesin the high
latitudes of the southern hemisphere over the last glacial-
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)
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Nakano•-ku,
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