PALEOCEANOGRAPHY,
VOL. 10, NO. 4, PAGES 763-773, AUGUST
1995
Assessment of sea-surface temperature at 42øN in the
California Current over the last 30,000 years
FredrickG. Prahl, NicklasPisias,MargaretA. Sparrow,and AnneSabin
Collegeof Oceanicand Atmospheric
Sciences,
OregonStateUniversity,Corvallis
Abstract. Assessment
of changes
in surface
oceanconditions,
in particular,
sea-surface
temperature
(SST),is essential
to understand
long-term
changes
in
climateespecially
in regions
wherecontinental
climateis strongly
influenced
by
oceanographic
processes.
To evaluatechanges
in SST in the northeastPacific,we
haveanalyzedlong-chain
alkenones
of prymnesiophyte
originat 38 depthsin a
pistonand associated
triggercorecollected
beneaththe contemporary
coreof the
California
CurrentSystem
at 42øN,-0270km offthecoastof Oregon/California.
The
samples
span30,000yearsofdeposition
at thislocation.Unsaturation
patterns
k•
(U37)in thealkenone
series
display
a statistically
significant
difference
(p((0.001)
between
interglacial
(0.44q-0.02,n- 11)andglacial(0.29q-0.04,n- 20)intervals
of thecores.Detailedexamination
of othercompositional
features
of the C37,C38,
C39alkenone
series
and
a
related
C36
alkenoate
series
measured
downcore
suggests
k•
thepublished
U37- temperature
calibration
(u3k•?
-- 0.034x T q-0.039),
defined
for
culturesof a strainof Emilianiahuxleyiisolatedfromthesubarctic
Pacific,provides
bestestimates
of winterSST at ourstudysite. Thisinference
is purelystatistical
and doesnot imply, however,that the phytoplanktonsourceof thesebiomarkers
iskmost
productivein winter or at the oceansurface.The temperaturerecordfor
•
U37 implies (1) an -04øCshift occurredin winter SST from -0 7.5 4- 1.1øCat the
lastglacialmaximum
to -0 11.74-0.7øCin thepresent
interglacial
period,and(2)
thiswarmingtrendwasconfined
to the time frame14-10Ka withinthe glacialto
interglacial
transitionperiod.Theseconclusions
arecorroborated
entirelyby results
froman independent
SSTtransformation
of radiolarian
species
assemblage
data
obtained from the same core materials.
Introduction
coincidentwith the point where wind stressdiverges
[Hickey,1979]. Wind stressdivergence
beginsin the
mid-ocean(•-160øW) at •-40øN in summerand •-30øN
The modernCaliforniaCurrentis a well-studied
hydrographic
featurein the NorthPacificOcean[Hickey, in winter. Separation of the wind field continuesto the
1979].Temporalmeasurements
of the physicalandbio- east but at progressivelyhigher latitudes as the North
(50øNin summer;
logicalcharacteristics
of this currentsystemhavebeen Americancontinentis approached
40øN
in
winter).
The
northward
and
southward
flowing
the focusof the CaliforniaCooperative
Fisheries
(CalCOFI) Programstarted in the early 1950s. CalCOFI
currentsseparatein a broad transitionalzone (Figure
data reveal variations in the character of the California
Current on seasonal as well as interannual timescales.
while that flowing southward becomesthe core of the
Detailed studiesof the physicsof this systemand consequences
of physicson regionalpatterns of primary
productivityhaveevolvedin morerecentyearsthrough
otherprograms
[Strubet al., 1990;Hoodet al., 1991].
The California Current derives from the West Wind
Drift (Figurela). The WestWind Drift separates
into
northwardand southwardflowingcomponents
roughly
l a). The northwardcomponent
feedsthe AlaskanGyre,
CaliforniaCurrent. North of 40øN and nearshore,the
CaliforniaCurrentislargelySubarctic
in type(lowtemperature and salinity,high dissolvedO•. and phosphate
content).The percentage
of Subtropical
water(warmer
and saltier,O•. and phosphate-poorer)
mixedinto this
system increases to the south and toward the west. The
core of flow is located --150-300 km off the coast from
the Oregon/California
border(Figurelb). Flowintensity is always maintained southward but varies season-
Copyright 1995 by the American GeophysicalUnion.
Paper number 95PA01394.
0883-8305/95/95PA-01394510.00
ally, being fully developedand situated most nearshore
in late summerto early fall and diminishedand situated
most offshore in winter.
Changes in the structure of the California Current
betweenthe last glacialand the presentinterglacialpe-
764
PRAHL ET AL.' PALEOTEMPERATURES IN THE CALIFORNIA CURRENT (42øN)
60øN
riod are known only vaguely at best. Some insight has
been gleaned from paleoceanographicstudy of microfossil records in sediment cores from the region. The
Subarctic boundary appearsto have shifted northward
(Figurela) sincethe last glacialperiod.Moore[1973a]
showedthe radiolarian assemblagenow representative
of the Alaskan Gyre withdrew from the region -o41ø to
43ø30'N and 126025' to 127øW off Oregon/northern
50øN
California and was replaced successivelyby a central
Subarcticassemblageand finally a TransitionalZoneas-
semblage.The samepattern of biotic chan•ehasbeen
40øN
documented subsequently in a wider range of northeast Pacific cores spanning as far west as 132øW off
Cape Blanco(-043øN)and as far southas 39øNoff the
RussianRiver [Sabin,1995]. Theseobservations
argue
180øW
160øW
140øW
120øW
that the contemporary positions of divergencefor the
West
46øN
....................
44ON
.................
Wind
Drift
into northward
and southward
flow-
ing currents[Hickey,1989]and of coastalupwellingoff
the coast of the northwesternUnited States [Huyer,
1983]are significantlydifferenttoday fromthosewhich
prevailed during the last glacial period. In glacial
times, either the intensity of coastal upwelling diminished markedly in this region by an increasedinfluence
of the Alaskan Low-PressureSystem or the whole area
of episodiccoastalupwellingshiftedsouthward[Moore,
1973a]. This oceanographic
implicationfrom the mi-
...................
crofossil record
42øN
in sediments
is consistent
with
model
predictionsfor large-scalechangesin atmosphericcirculation in the northeast Pacific resulting from deglacia-
tion in North America[Cooperative
HoloceneMapping
Project(COHMAP), 1988,and references
therein].
In this paper, we examine sedimentaryevidencefor a
glacial to interglacial changein sea-surfacetemperature
_•4ooo
•Mendocino
FZ
I i::i::!iiiiiiiii!i!::i::
CA::iii::iiiii::
40øN
(SST) within the CaliforniaCurrentat a siteoff north-
ern California.
The site is located
-0270 km offshore at
-042øNwithin the contemporarycoreof this current sys-
tem (Figurelb). Mooreet al. [1980]analyzedmicrofosi
i
i
130oW
i
i
i
120ow
38ON
Figure 1. (a) Map identifyingthe major surfacecurrents in the North Pacific and showingthe relationship
of the California Current systemto the West Wind Drift
(adaptedfromHickey[1989]).(b) Expandedmapindicating the coring site for W8709A-8TC and W8709A-
8PC examinedin this study (seestar; 3111 m water
depth,42ø32.5'N127ø40.7'W).The maprelatesthe coring site to the longitudinalrange of the contemporary
CaliforniaCurrentSystemoff Oregon/northernCalifornia (seeareaenclosed
by bolddashedlines).For refer-
sil speciesassemblages
in a wide range of sedimentcores
and reconstructeda map of SST for the entire glacial
Pacific Ocean. Their resultssuggested<2øC colderwaters existed in this portion of the California Current
during the last glacial period than at present,although
SST estimatesalong the eastern limb of the North Pacific Current were not well constrainedby sampling. We
employ
indep,
endent
analyses
of alkenone
unsaturation
patterns(Uak7)
andradiolarian
species
assemblages
on
more recently collectedcorematerials to reconfirmprior
assessmentof colder temperatures in the glacial California
Current.
Our
results
define winter
SST in the
northeast Pacific over the past 30,000 years and show
ence purposes,three sediment trap sites for the MUL-
(1) an -•4øC differencebetweenglacialand interglacial
TITRACERS project (NS, MW, and G [Lyle et al., estimatesand (2) confinementof the shift from colder
1992]) and the collectionsite for two additionalcores to warmer conditions to the time frame of 14-10 Ka
are alsoindicated(seesolidcircles;Y6604-10:3002 m
water depth, 43ø16'N 126ø24'W; Y6910-2:2615 m wa-
during the glacial to interglacial transition.
ter depth,41ø16'N127ø01'W).Mooreet al. [1980]pre-
Methods
viously examined the microfossilrecords of the latter
coresto reconstruct sea-surfacetemperatures at 18 Ka
in this region of the northeast Pacific Ocean.
Sample collection. A pistoncore(W8709A-8PC;
875 cm long, 10 cm diameter) and its associated
trig-
PRAHL ET AL.' PALEOTEMPERATURESIN THE CALIFORNIA CURRENT (42øN)
765
gercore(W8709A-8TC,190cm long)werecollectedin
et al., 1988].ME/Ka7 measures
the relativeabundance
1987 aboard RV Wecomafrom 3111 m water depth at
of alkenoatesto alkenonesin a given biomarker series
and is calculated from the abundance ratio of total Ca6
42ø32.5'N127ø40.7'W(Figurelb). Bothcoresweresec-
tionedimmediately
onboard
shipinto-01.5-mlengths. methyl alkenoatesto total C37 alkenones[Prahl et al.,
Each length was capped,sealed,and returned to Corval-
1988]. The reproducibilityof measurements
for each
lis underrefrigeration(4øC) for storagein the Oregon
State University (OSU) corerepository.At OSU, the
compositional ratio is better than +5% in duplicate
core sectionswere each split lengthwise to produce a
working and archivalhalf. One-centimeter-thick,quarter round subsampleswere then taken as needed from
samples.
Radiolarian
analysis.
Radiolarian speciescensus
data (Table 2) were collectedon thesecoresas part of
a larger study of the northeastPacific[Sabin,1995].
the working half for purposesof organicgeochemical Cores 8TC and 8PC were subsampleddowncoreevery
and microfossilanalysis.
10 cm yielding a total of 34 samples. Radiolarian miComparisonof detailed magnetic susceptibilityand croscopeslides were prepared using the technique of
calcium carbonate records for 8TC and 8PC indicated
Moore[1973b]asmodifiedby RoelofsandPisias[1986].
the piston coreoverpenetratedby 140 cm. So, all sam- After chemicalcleaning,sampleswere sievedwith a 63pling depthsfor the 8PC are reported +140 cm to cor- /•m screenand radiolarian fossilswere randomly settled
onto microscopeslides. QUantitative countsof radiorect for overpenetration. A timescalefor these coreswas
establishedusing combineddata for acceleratormass larian specieswere completed based on a total count of
spectrometry
(AMS) 14Cdatesonbulkorganiccarbon 800 radiolarian tests. The proportion of total radiolarand/or calciumcarbonateand oxygenisotope(5180) ian counted for 41 specieswas used in the estimation of
mean annual SST.
stratigraphy on benthie foraminifera. Sedimentation
rate variedfrom 10 to 12 cm/kyr overthe 3.2 m of core
examined in this study with lower values estimated at
greater core depth. All data and the method used to
establishchronostratigraphyfor $TC and 8PC are dis-
cussedelsewhere[Lyleet al., 1992].
Alkenone analysis. Total extractablelipids(TEL)
were recovered ultrasonically from 3 to 4 g of wet
sediment using a 1:3 solvent mixture of toluene and
methanol. Lipid fractions enriched in C37, C38, and
C39 alkenonesand structurally related C36 alkenoates
of prymnesiophyte
origin [Marloweet al., 1990]were
isolatedfrom TEL by column chromatographyand analyzedby capillarygaschromatography
(GC). Further
details of the extraction and analysisprocedureare dis-
Mean
annual
SST
were estimated
from
radiolarian
population studies by a paleoecologicaltransfer function developed using the technique of Imbrie and Kipp
r•a•
•;•'•;•
population data from l•n surface
sediment samplesfrom the Pacific basin were analyzed
usingQ modefactoranalysis(N. PisiasandA. Mix, unpublisheddata, 1994). This approachidentifiedseven
radiolarian assemblages
with discretegeographicdistributions and thereby allowed statistical distinction between surface waters of specificoceanographicsettings
in the Pacific: Subtropical, Antarctic, Arctic, Transitional, Eastern Boundary Current, Gyre, and the Warm
Western
Basin.
The relative
abundance
of these seven
assemblageswas used to develop a paleotemperature
transfer
function
for mean annual
SST.
cussedelsewhere
[Prahlet al., 1989,1993].
The transfer function was developedusing nonlinear
Alkenone/alkenoate signatures were measured multiple regressionanalysiswhere the assemblagedata
throughoutthe triggercore (8TC) and in all intervals were the independent variable and mean annual SST at
examinedfrom the top 1.8 m of the pistoncore(8PC). the location for each surfacesediment sample was used
Various
indices
(U3k'7,
K37:4/K37,
K37/K38,
U• e, and
ME/Ka7; Table 1) are utilized to describethe overall prymnesiophyte
biomarkercomposition
identifiedby
k•
GC in each sample(Figure 2). U37, Ka7:4/Ka7,and
U• e measurethe degreeof unsaturationin a given
k•
alkenone/alkenoate
series.U37 is calculatedfrom the
abundance ratio of the diunsaturated C37 ketone to
the combineddiunsaturatedand triunsaturated C37 ketone.
The latter
two unsaturation
indices are new to
as the dependent variable. Mean annual SST was estimated for each sample location using data in the atlas
of Levitus[1982].The standarderrorof the regression
equation is +1.5øC. Residuals of the regressionfit for
surfacesediment samplesfrom the northeast Pacific are
less than +IøC. Thus we feel that this paleotemperature transfer function provides estimates for past seasurface conditions
q- 1.5 ø C.
in the northeast
Pacific
accurate
to
within
the literature. K37:4/K37is calculatedfrom the abundance ratio of tetraunsaturated C37 ketone to the combined diunsaturated, triunsaturated, and tetraunsaturated C37 ketone. And U• e is calculatedfrom the
abundanceratio of diunsaturatedC36methyl alkenoate
to the combineddiunsaturated and triunsaturated C36
methylalkenonate.K37/K38measures
the overallchain
length in a given alkenoneseriesand is calculatedfrom
the abundance
ratioof total C37to C38alkenones
[Prahl
Results
and
Discussion
Alkenone/alkenoate compositions. Figures2a
and 2b illustrate typical gas chromatogramsof the
alkenone/alkenoate
seriescontainedin sedimentintervals of interglacial and glacial age, respectively. The
most conspicuouscompositional differenceevident from
this comparison is the virtual absenceof a tetraunsat-
766
PRAHL ET AL.' PALEOTEMPERATURES IN THE CALIFORNIA CURRENT (42øN)
Table la. CompositionalProperties of Alkenoneand Alkenoate SeriesAnalyzed With Depth in a Trigger Core
(W8709A-8TC) and PistonCore (W8709A-8PC)Fromthe NortheastPacificOcean
Core
Depth,
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
See text
Ka
2.5
4.5
23.5
26.0
47.5
65.5
67.5
88.5
89.5
97.5
107.5
117.5
127.5
128.5
134.5
137.5
140.5
141.5
144.5
146.5
149.5
152.5
157.0
163.5
166.5
175.0
183.5
185.5
159.5
178.5
199.5
219.5
239.5
259.5
259.5
279.5
299.5
319.5
0.2
0.4
2.1
2.3
4.5
6.4
6.6
8.9
9.0
9.9
10.9
12.0
13.1
13.2
13.9
14.2
14.6
14.7
15.0
15.2
15.5
15.8
16.3
17.0
17.4
18.1
19.0
19.2
16.6
18.5
20.5
22.2
23.7
25.1
25.1
26.5
28.0
29.4
for further
Table lb.
Age,
cm
U•;
SST,
U• e
Ka7/K3s
ME/K37
Ka•:4/Ka•
øC
0.438
0.435
0.484
0.414
0.421
0.427
0.427
0.428
0.4 77
0.448
0.417
0.364
0.391
0.369
0.308
0.309
0.317
0.331
0.339
0.328
0.326
0.337
0.308
0.323
0.333
0.315
0.319
0.301
0.271
0.323
0.249
0.265
0.226
0.227
0.235
0.294
0.249
0.311
11.7
11.6
13.1
11.0
11.2
11.4
11.4
11.4
12.9
12.0
11.1
9.6
10.4
9.7
7.9
7.9
8.2
8.6
8.8
8.5
8.4
8.8
7.9
8.4
8.6
8.1
8.2
7.7
6.8
8.4
6.2
6.6
5.5
5.5
5.8
7.5
6.2
8.0
0.77
0.80
0.87
0.72
0.75
0.74
0.76
0.80
0.83
0.77
0.74
0.64
0.69
0.67
0.63
0.68
0.64
0.62
0.61
0.61
0.62
0.61
0.59
0.62
0.62
0.61
0.61
0.63
0.61
0.70
0.66
0.60
0.54
0.56
0.57
0.74
0.56
0.53
1.06
1.09
1.02
1.11
1.18
1.05
1.11
1.14
1.07
1.21
1.18
1.19
1.15
1.17
1.13
1.16
1.04
0.97
0.95
0.90
0.86
0.88
1.00
0.90
0.96
0.87
1.02
0.99
0.79
0.92
0.89
0.82
0.82
0.89
0.89
0.88
0.76
0.97
0.24
0.24
0.22
0.27
0.24
0.27
0.23
0.21
0.23
0.22
0.24
0.24
0.30
0.26
0.23
0.22
0.24
0.26
0.28
0.28
0.25
0.28
0.28
0.26
0.26
0.24
0.23
0.24
0.28
0.20
0.21
0.30
0.26
0.28
0.29
0.20
0.29
0.21
0.02
0.00
0.02
0.01
0.00
0.00
0.02
0.00
0.01
0.02
0.03
0.06
0.03
0.06
0.08
0.08
0.08
0.09
0.09
0.10
0.10
0.10
0.11
0.10
0.10
0.10
0.11
0.11
0.09
0.11
0.10
0.13
0.13
0.14
0.13
0.04
0.15
0.08
details.
k!
AverageAlkenoneCompositional
Propertiesand U37-Be•sed
Sea-Surface
TemperatureEstimates
Calculated for Interglacial and Glacial SedimentIntervals of Cores W8709A-8TC and 8PC From the Northeast
Pacific
Ocean
Va•;
Interglacial
SSTøC
VTM
,
36
Ka•/Kas
ME/Ka•
KaT:4/Ka•
n
Ave
Std
Ave
Std
Ave
Std
Ave
Std
Ave
Std
Ave
Std
12
0.438
0.022
11.7
0.7
0.78
0.04
1.11
0.06
0.24
0.02
0.01
0.01
20
0.294
0.038
7.5
1.1
0.61
0.05
0.90
0.07
0.26
0.03
0.11
0.02
34
0.344
0.070
9.0
2.1
0.67
0.08
1.00
0.13
0.25
0.03
0.07
0.05
(0-10 Ka)
Glacial
(15-30 Ka)
Overall
(0-30 Ka)
Ave is average,Std is standarddeviation,and n is the total numberof samples
in eachagecategory.
PRAHLET AL.-PALEOTEMPERATURES
IN THE CALIFORNIA
CURRENT(42øN)
• •E
(A)
E
e _w•
767
A differenceof high statistical significanceis also observedwhen the same time averagesare comparedfor
the alkenonechainlengthindex (K37/K3s,p < 0.001)
•
but not for the index of alkenoate to alkenone abun-
dance(ME/Ks•, p • 0.1). Notably,all compositional
changes
occurduringthe periodof glacialto interglacial
transition(i.e., •14-10 kyr B.P.).
Alkenone
ature.
estimates
of surface water
ternperk•
The alkenone unsaturation index U37 varies
with phytoplankton
growthtemperature[Brassellet al.,
(B)
I
I
1986]. For a singlestrainof Emilianiahuxleyi(55a,
Northeast Pacific Culture Collection), isolated from
k•
the subarcticPacificOcean,U37changes
quitelinearly
I
59
60
61
RetentionTime (minutes)
withgrowthtemperature
(u3k'7
-- 0.034X T + 0.039,
r 2 = 0.994[Prahlet al., 1988]). Prior fieldtestsink•
Figure
2.
Partial capillary gas chromatograms of
alkenone/alkenoatefractionsisolatedfrom (a) interglacial and (b) glacial intervalsof W8709A-8TC and
W8709A-8PC. The alkenonesare depicted by the code
K followed by 37, 38, or 39 indicating carbon chain
length; :4, :3, or :2 indicating the number of double
bonds; and m or e indicating methyl or ethyl ketone.
The alkenoates are depicted by the code A followed
by 36 indicating carbon chain length, :3 or :2 indicating number of double bonds, and m or e indicating methyl or ethyl ester. The observed chromatographic separation was accomplishedusing on-column
injection, a 0.25-mm i.d. x 30-m-long DB-1 fused silica capillary column, hydrogen carrier gas maintained
dicatethe applicabilityof the U37 -T calibrationfor
strain 55a of E. huxleyi to Pacific waters ranging in
Table 2a. Sea-Surface Temperatures Predicted Using
a Transform Function for Radiolarian SpeciesAssemblage Data Obtained With Depth in a Trigger Core
(W8709A-STC) and PistonCore (W8709A-SPC)From
the Northeast
Core
Pacific
Ocean
Depth•
Age,
cm
Ka
øC
TC8
1
0.1
10.8
TC8
7
0.6
11.7
at constantheadpressure
(0.35 kg/cm2) throughout
TC8
16
1.3
10.5
the analysis,and temperatureprogramming(5øC/min
from 100-300øCthen isothermal). As illustrated, a
TC8
26
2.3
11.1
TC8
36
3.2
12.6
coelution problem existed between the ethyl alkenone
K38:3e and the ethyl alkenoate A36:2e present in all
TC8
TC8
46
56
4.2
5.3
12.1
11.5
samples.Quantitativemeasurement
of the ME/K37 in-
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
TC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
PC8
65
6.3
76
86
96
105
125
135
145
155
165
175
185
190
200
210
220
230
240
251
261
270
280
290
300
310
320
330
7.4
8.5
9.6
10.7
12.9
14.0
15.0
16.1
17.1
18.1
19.1
19.6
20.5
21.5
22.2
23.0
23.7
24.5
25.2
25.9
26.6
27.3
28.0
28.7
29.4
30.1
11.6
10.8
11.0
11.5
11.9
11.1
10.4
9.3
8.5
9.0
7.2
6.5
6.7
7.5
8.3
7.2
7.4
7.3
7.2
7.6
7.1
6.9
6.7
6.6
7.3
6.5
8.6
dex in selected glacial and interglacial samples before
and after transesterification
with 14% boron trifluoride
in methanol[Prahl et aL, 1989]showedthe concentration of A36:2e was consistently60-70% of the concen-
tration for A36:2m. All reportedvaluesfor K37/K•s
have been correctedfor this coelutionproblem by mathematically subtracting away the A36:2e contributionto
K38:3e usingthe followingexpression:K37/K38 (cor-
rected)= [(K37/K38)
-1 (measured)-0.65 x U•6e x
ME/K37]-1.
urated C37 ketone (K37:4m) in interglacialsediments
and its clear occurrence in glacial sediments. Besides
this feature, all analyzed sedimentintervalscontain the
sameset of alkenone/alkenoate
components.
Table I displays complete data for the various atk•
tributes(U37, K3?:q/K37,K3?/K3s,U• e, and ME/K3?)
of the alkenone/alkenoate
compositions
measuredat 38
depths in the cores. Comparison of data averaged for
the interglacial(0-10 Ka, n = 11) and glacial(15-30
Ka, n - 20) samplingintervalsshowssignificantdifferences(one-tailedStudent-ttest) for all threeunsaturak', K37:4/K37, andu•e).
tion indices(p • 0.001 for U37
See text
for further
details.
768
PRAHL ET AL.' PALEOTEMPERATURES
IN THE CALIFORNIACURRENT (42øN)
Table 2b.
Average Sea-SurfaceTemperature Esti-
mates
on a Transform
Based
Function
for Radiolar-
JanSpeciesAssemblagesCalculated for Interglacial and
Glacial Sediment Intervals of Cores W8709A-8TC
8PC From the Northeast
Pacific Ocean
and
k!
inferredfromthe U37data and followsconsistently
with
the temperatureresponseof K3•/K3s observedin cultures of strain 555 (Figure 35). Second,valuesof the
alkenoneunsaturationparameter,K3•:q/K3•, increase
frominterglacialto glacialsedimentintervals(Table 1).
SST, øC
2.0
n
Ave
Std
(A)
1.8
Interglacial
12
11.4
0.6
20
7.5
0.8
34
9.1
2.1
(0-10 Ka)
1.6
Glacial
1.4
(15-30 Ka)
Overall
1.2
(0-30 Ka)
Ave is average, Std is standard deviation, and n is the
total number of samples in each age category.
1.0
Strain
55a
8TCand8PC
0.8
,
0.6
,
temperaturefrom -.•4 to >_25øC[Prahl and Wakeham,
1987; $ikes and Volkman,1993]. With this findingas
precedent,the laboratory calibration equationwas used
k•
I
,
,
,
10
,
I
,
,
,
15
,
I
Pacific into estimates
i
I
i
Temperature
(øC)
0.20
to convertdowncoremeasurements
of U37at our study
site in the northeast
i
20
(B)
of absolute
surfacewater temperature(Table 1). Estimatesaver-
0.15
age 11.7 + 0.7øC and 7.5 + 1.1øC for sediment intervals
ofinterglacial
andglacial
age,respectively.
The steplikedowncore
decrease
in u3k7
(Table1) al-
most certainly depicts lower glacial relative to interglacial surface water temperatures. But we must now
question the accuracy of the absolute water temperatures estimated above for two reasons. First, more re-
0.10
0.05
k•
cent culturestudiesshowthe specificU37 -T relationship definedfor strain 555 of E. huxleyiis not applicable
to all alkenone-producingprymnesiophytespotentially
contributingto the marine sedimentaryrecord [Marloweet al., 1990]. Most notably,significantdeviations
are evident betweencultured strainsof E. huxleyiisolated from variousmarine watersincludingoff the coast
ofBermuda
(u3k'7
-- 0.053x T- 0.55,calibration
range:
15-25øC),
theGulfofMaine(u3k'7
-- 0.016x T - 0.028,
calibration
range:10-20øC),
a Norwegian
fjord(u3k'7
=
0.033x T- 0.009, calibrationrange:10-20øC),and the
0.00
10
15
20
25
Temperature
(øC)
0.30
(c)
AA
A&
ß
0.25
&ß
ß
ß
ß AA
ß
ß
ß
0.20
ß
0.15
0.10
Sargasso
Sea(u3k'7
= 0.013x T-0.12, calibration
range:
15-25øC)(K. Amthor,unpublished
results,1994). Seck•
ond, anomalousrelationshipsbetweenU37 and water
0.05
,
temperature now seem apparent in the North Atlantic
[Conteet al., 1992],the BlackSea[Freemanand Wakeham, 1992], and in very cold (<4øC) watersfrom the
SouthernOcean[Sikesand Volkman,1993].
However, despite such concern,our alkenone-derived
estimatesof absolutewater temperature are judged as
reasonableby comparisonof various compositionalfea-
turesof the alkenone/alkenoate
signatures
preserved
in
the cores with
culture
data for strain 555 of E. hux-
leyi. First, valuesof K3•/K3s, an indexof carbonchain
length in the alkenoneseries,decreasefrom interglacial
to glacialintervalsof the core (Table 1). This trend
accompaniesthe -.•4øCdecreasein growth temperature
,
0.00
5
10
15
20
25
Temperature
(øC)
3. (a) K37/K38, an index of carbonchain
length, (b) K37:4/K37,an index of alkenoneunsaturation, and (c) ME/K37, an index of the relativeabunFigure
dance of alkenoatesto alkenonesin a given biomarker
series plotted versus temperature for strain 55a of E.
huxleyigrown in culture [Prahl et al., 1988]. For
reference purposes, downcore data for each parameter are also plotted at growth temperatures predicted
k'
from U37 measurements
usingthe calibrationequation
k'
U37- 0.034x T + 0.039 [Prahlet al., 1988].
PRAHL ET AL.- PALEOTEMPERATURESIN THE CALIFORNIA CURRENT (42øN)
In cultures of strain 55a, this precisetrend is observed
for K37:4/K37asgrowthtemperaturedecreases
overthe
inferredrangeof 11.7øto 7.5øC(Figure3b). So,a strain
of E. huxleyi behaving like 55a is a probable, major
alkenonecontributorto sedimentsin this regionof the
northeastPacificoverthe last 30,000yearsof deposition
and providesan appropriatetemperature calibrationfor
k•
U37.
Evidence for selective degradation of prymnesiophyte biomarkers. Comparisonof culture data
for strain 55a of E. huxleyi with core data reveals
one noteworthy compositional difference that warrants
769
et al., 1988, and references
therein]. A varietyof field
k'
observations
now bolsterour view that Us7 is insensitive to early diageneticalteration [Prahl et al., 1989;
McCaffreyet al., 1990]. Growthtemperatureremains
the key factor recognizedto affect alkenone unsaturation patterns recorded in sedimentary samples. But, as
describedabove, determination of the absolutequantitative relationship between alkenoneunsaturation patterns and growth temperature still requires a means of
recognizing the specific algal source contributing the
biomarker signal to a particular sedimentary setting.
Current evidencefor the resistanceto diagenetic alter-
discussion. The alkenoate to alkenonecomposition ation of indices based on the abundance ratio of two
(ME/Ka7) of this straindisplaysconsiderable
tempera- biosynthetically related but chemically distinct comture dependence
with valuesof 0.10-0.15 expectedwhen poundclasses(e.g.,AA36, ME/Ks7) is tenuousat best
growthoccursat _<10øC(Figure3c). But ME/Ka7 val- by comparison
(see,for example,Conteet al. [1992]).
uesshowlittle significantdifference(p -0 0.1) between
Both diunsaturated and triunsaturated C•6 methyl
interglacial(0.24q-0.02) andglacial(0.26q-0.03) sed- alkenoatesoccur in all depth intervals of 8TC and 8PC
iment intervals(Table 1) and are --2-fold higherthan examinedin this study(Figure2). U• e, an indexanalo-
expectedif growthoccurredin the inferredtemperature
range of 7.5-11.7øC (Figure 3c). Assuminga strain
of E. huxleyilike 55a representsthe major biomarker
contributorto thesesediments,selectivedegradationof
alkenonesrelativeto alkenoateswouldexplainthe dis-
goustOU'37,k' isnow
to quantify
downcore
variaß defined
tion in alkenoateunsaturationpatterns(Table 1). U• •
k'
showssignificant,positivecorrelationwith U37 in the
k'
complete
dataset(U• e -- 1.00xUa7+0.33,r 2 -- 0.74).
parity.
Alkenone/alkenoate
data for sedimenttrap time series and surfacesedimentsfrom this regionof the north-
may, in fact, be higher if analytical conditions were
optimized to allow baseline peak resolution of the mi-
eastPacific[Prahlet al., 1993]supporta selective
degra- nor triunsaturatedC• methylester(A36:3m)and the
dation hypothesis. ME/Ks7 measuredin three sur- major diunsaturatedC•7 ketone(K37:2m) (Figure2).
face sedimentsbetween120, 270, and 630 km offshore Nonetheless,the observeddegreeof correlation supports
along42øN (NS, MW, and G; Figurelb) are uniform our view that C•6 alkenoates and C•7, C•8, and
(0.25q- 0.01), while averageparticulatematerialsset- alkenones derive from the same source and unsaturation
patterns in thesetwo biosyntheticallyrelated compound
lower and vary widely (0.09, 0.06, and 0.02, respec- classesconveythe same information about growth temtively). Flux comparisons
showonly a smallfractionof peratures for E. huxleyi in the northeast Pacific. More
the alkenonestransportedthrough 1000 m water depth significantly, however, if alkenonesare degraded selectively over alkenoatesin the sedimentary process,this
survivesburial in sediment. If slight differencesin the
degreeof degradation of alkenonesrelative to alkenoates correlation suggeststhat alkenoate degradation operoccurred,the dissimilaritybetweenME/Ks7 valuespre- ates nonselectivelyon a compound specificbasis, an inservedin sedimentsand expectedfor the presumedalgal ference completely consistent with our current view of
alkenone geochemistry.
source would be accountable.
Relationship of biomarker estimates to seaThe possibilityof selectivedegradationof alkenones
Using sedimenttraps deployrelative to alkenoatesin the sedimentaryprocessis rel- surface temperature.
tling through 1000 m water depth above each site are
ed in time series,Prahl et al. [1993]examinedthe com-
evant to recentwork of Conte et al. [1992]. These
investigatorscoinedan index (AA36) to quantifythe
relativeabundance
of alkenoates
to alkenones
in a given
sampleand proposedits useeither independently
or in
k'
conjunctionwith U37 to assesssurfacewater temperatures. If thesetwoclasses
of biochemicals
(estersof fatty
acidsand ketones)are derivedfrom a commonsource
[Marloweet al., 1990]but degraded
selectively
to any
significantextent in the early stagesof sedimentation
gardlessof samplingperiod, U37 ws•sremarkablyuniform throughoutthe year (0.41 q- 0.04) and compared
wellwith valuesrecordedin underlyingsediment(0.43).
k'
Again, averageU37 valuesfor sedimenttrap materials
as our data might suggest,the utility of AA36 as an
unequivocaltool for paleotemperaturereconstructionis
translated into growth temperatures of 10.9 q- 1.2øC using the existing calibration for strain 55a of E. huxleyi
position and flux of alkenonesthrough 1000 m water
depth at the coringsite for the presentstudy. Alkenone
flux was evident in all samplingperiodsimplying some
level of prymnesiophyteproductivity year round. Rek'
compromised.
[Prahlet al., 1988].This watertemperatureis observed
Similar concernfor selectivedegradationof the more
unsaturatedalkenoneisomerswas raised in the early
at the sea surface along 42øN in the northeast Pacific
throughout the winter and during summer periods of
upwelling along the coast. But such cold water tern-
k'
development
of Us7asa paleotemperature
index[Prahl
770
PRAHL ET AL.: PALEOTEMPERATURES IN THE CALIFORNIA CURRENT (42øN)
Radiolarian Assemblages
perature occurs only within the upper thermocline at
more offshoresites in the northeast Pacific during sum-
mer to fall periodsof stratification[Prahlet al., 1993].
Interestingly, the inferred subsurfacedepth of alkenone
production invariably correspondsto a zone of chloro-
phyll maximumin theseoffshoreregions[Prahl et al.,
1993]. Consequently,
we surmisethat the alkenoneunsaturation patterns preservedin sedimentsfrom study
regionssuchas ours beneaththe core of the contemporary California Current provide a proxy of winter SST
even though prymnesiophyte productivity is not constrained to or maximal in this season. And, on this
basis, we concludethat winter SST in the glacial California Current
situated
at 42øN in the northeast
Pacific
was at least 4øC colder than at present.
Radiolarian species compositions. Figure 4 illustrates downcorechangesin radiolarian assemblages
_
•80
.• 0.70
•
0.60
•
0.•
observedin 8TC/8PC. Here we plot the four dominant
factors found for modern and glacial sedimentsfrom
the northeast Pacific, i.e., the Subtropical,Transitional,
Eastern Boundary Current, and Arctic assemblages.
The other three assemblagesidentified in the study of
minor importance in this region. A marked changein
radiolarian assemblagespreservedin these sedimentsis
evident. The radiolarian population in glacial age sediments is dominated by a mixture of Arctic and Transitional assemblages.The speciesassociatedwith the
Arctic assemblageare not found in modern sediments
of the northeast Pacific but are presently found in sediment from the far North Pacific and Bering Sea. At
0,2
(18 to 14 Ka), the Arctic assemblage
is replacedfirst by
the return of the Transitional assemblage.Assemblages
associatedwith the Subtropical Pacific and the Eastern Boundary Current subsequentlyincreasein importance. The reduced importance of the Eastern Boundary Current assemblages,and associatedreduction in
the importance of upwellingrelated species,are consistent with inferred reduction of upwellingin the north-
east Pacificduringthe last glacialepisode[Lyle et al.,
1992]. This pattern suggests
that the subpolarfront of
the northeast
Pacific
was located
much farther
south
t0
t5
20
25
•
0
5
tO
t5
20
25
50
5
10
15
20
25
50
0.4
0,5
prevails as the Transitional assemblagebecomesmuch
reduced in importance.
At the onset of the glacial to interglacialtransition
5
0,5
N. Pisiasand A. Mix (unpublisheddata, 1994) are of
the glacial maximum (18 Ka), the Arctic assemblage
0
0,4
0.5
0.2
0.1
[
0.0
0
Age (kyr)
Figure 4. Downcoreplotsof radiolarianassemblages
derived from a Q Mode factor analysisof radiolarian
populationsfrom 170 surfacesedimentsamplesspanningthe PacificBasin. Factorsshownare the Subtropical, Transitional,Arctic and EasternBoundaryCurrent
factorsdefinedby N. Pisiasand A. Mix (unpublished
results,1994).
tions(Table2). The concordance
of thesetwo,indepen-
during the last glacial event. The observedchangesin
the radiolarian assemblagesalso suggestthat deglacia-
dent estimatesof past sea-surfaceconditionsis striking.
Thus the conclusionfrom alkenoneanalysisseemsjusti-
tion in the northeast
fied that SST was at least 4øC colder at this site in the
Pacific
is related
to a northward
migration of the subpolar front acrossthe region. The
pattern observedin 8TC/8PC is consistentwith the
overall regional responseto deglaciation based on a
northeastPacificduringthe last glacialmaximumthan
at present. But why should a paleotemperatureequation, calibrated to mean annual SST, yield the same
north-southtransectof coresfrom55øNto 30øN[Sabin, estimate as that provided by alkenoneunsaturation patterns reflecting winter SST?
1995].
Radiolarian species assemblages and SST estimates. In Figure 5, we plot versussedimentage both
k'
the SST estimatesbasedon U37 measurements
(Table 1) and radiolarianpaleotemperature
transferfunc-
The radiolarian-based paleotemperature equation
used in the presentwork was derivedthrough study
of basinwide sediments from the Pacific and has been
appliedto sedimentrecordsfrom the central and east-
PRAHLET AL.. PALEOTEMPERATURES
IN THE CALIFORNIACURRENT(42øN)
16
771
2.5
0
Radiolarian
• uk'37
14
3.0
5180
X 14Cdates
12
3.5
10
4.0
o
4.5
6
5.0
I i
0
5
10
15
20
25
30
35
SedimentAge (kyr)
k!
Figure 5. Comparison
of downcore
SST predictions
basedon Us7datawith thoseestimated
usinga transformfunctionfor radiolarianspeciesassemblages.
For purposes
of stratigraphic
reference,a downcoreprofile is shownfor 5180 measuredin tests of the benthic foraminifera
Uvigerinaspp.,andcalendaragescalculated
fromAMS 14Cdateson bulkorganiccarbonand
calciumcarbonate
samples
are indicated(datafromLyle et al. [1992]).Seetext for further
details.
ern equatorial Pacific as well as the northeast Pacific
(N. PisiasandA. Mix, unpublished
results,
1994).The
strategy of developingpaleoecological
equationsfor
bothmeansea-surface
temperature
andseasonality
was
chosenfor two reasons.First, prior approaches
used
paleotemperatureequationsto estimate a summerand
winter SST. However,the correlationbetweenSST dur-
returning to the question of why mean SST estimates
derived from radiolarian assemblagedata shouldlook
similar to winter SST estimates derived from alkenone
data, we comparethe mean SST with water temperatures at the pycnoclinein the summerseason(Figure
6) where both parametershave been calculatedfrom
the hydrographic
data setof Levitus[1982].We assume
that the temperature at the summerpycnoc!inepro-
ingwarmandcoldseasons
in thePacific(datafromLevitus[1982])isveryhigh(r = 0.93;N. PisiasandA. Mix, vides an estimate of oceantemperaturesat the baseof
unpublished
results,1994)andthusindependent
regres- the euphoticzone which seemsto be the real physiosion equationscannot be developed,whereasthe cor-
logical growth temperature reflectedby the alkenones
relationbetweenmeanSSTandseasonality
(datafrom
Levitus
[1982])ismuchreduced
(r = -0.44) andthusindependent
equationscanbe developed.Second,
the definitionof winter and summerseasons
nearthe equator
becomes
complicated
because
the thermalequatordoes
not coincidenecessarily
with the geographic
equator.
[Prahlet al., 1993].The correlationbetweenthesetwo
temperatureparametersis very high (r - 0.90; Figure 6), and the two parametersare equivalentoverthe
If the positionof the thermal equatoralsovarieswith
pastclimate,then reconstructing
pastseasonal
temperatures is much more complicated at low latitudes. So
temperature range of 0 to 15øC. The alkenonetemperature estimates seem to reflect winter SST or subsurface
temperatures
in summer[Prahlet al., 1993],andthe radiolarian transfer function, while calibrated to annual
mean sea-surfacetemperatures, also provides a reasonable estimate of subsurfacetemperatures in summer.
772
PRAHL ET AL' PALEOTEMPERATURES IN THE CALIFORNIA CURRENT (42øN)
30-
/
Conclusions
1. Examination of various compositional properties of
r=O.9
25
ß
-**
ß
..o/.
ß
%**ø ß
%*'*
ß,/?Y*
.. 0
20
ß
o
o
t-
ß
ß
e/O
15
the alkenone/alkenoate
seriespreserved
in glacial/interglacial intervals of sedimentsfrom the northeast Pacific
k/
showsthat the U37 - temperature calibration established for E. huxleyistrain 55a isolatedfrom the subarc-
ticPacific
(u3k'7
-- 0.034x T + 0.039[Prahl
etal.,19881)
providesrealistic estimates of absolutewater tempera-
ß
ture.
•o
lO
ß
2. Early diageneticprocessespotentially alter the relative abundanceof alkenonesto alkenoatespreservedin
oo
k'
ß
ß
o/
o
I
I
I
I
I
lO
16
20
26
30
MeanAnnualSST (TøC)
sedimentseventhoughvaluesfor U37 and other compositionalindices(e.g., U•6, K37:4/Ks7,Ks7/Kss) based
on ratios of compoundswithin a given chemical class
appear faithfully translated from algal sourceto sediments. Given this possiblediagenetic effect, paleotemperature assessmentsbased on indices such as AA36
[Conteet al., 1992]shouldnowbe interpretedwith caution.
k'
3. UsT-basedpaleotemperatureestimates for the
Figure 6.
Observed mean sea-surfacetemperatures
(estimatedfrom averageof winter,spring,summer,and
fall data) comparedto the mean temperatureat the
northeast Pacific correspond to winter SST. This correspondenceis purely statistical and does not imply
alkenone producers are most productive in the winter
periodsor at the oceansurface[Prahlet al., 1993].
4. Ocean temperature estimatesbasedon U3• and
las of Levitus[1982].Data are basedon the geographic
radiolaria transfer functions follow essentiallyidentical
locationsof the 170 surfacesedimentsthroughoutthe
summer pycnocline as determined from data in the at-
Pacific Basin used to developthe sea- surfacetemperature transfer function for radiolarian speciesassem-
blages(N. Pisiasand A. Mix, unpublished
data, 1994).
Thus it is not too surprisingthat our two independent
estimatesof glacial/interglacialdifferencein oceantemperature are of the same magnitude in this core from
the northeast
Pacific.
Our estimateof an •-4øC oceantemperaturechange
since the last glacial maximum is higher than previ-
ousestimates[Moore,1973a]. As in this study,Moore
[1973a]usedradiolarian-based
transferfunctions
to estimate past sea-surfaceconditions and estimated a _<2øC
temperature changeduring the last deglaciation.There
are two factors which should be considered in evaluat-
patterns of changeduring the last deglaciationand show
that the northeast Pacific warmed by •-4øC since the
glacial maximum.
5. Convergenceof the alkenoneand transfer functionderived temperature estimates supportsthe hypothesis
that the oceanographicparameter being reconstructed
by these techniquesin the northeast Pacific is a subsurface water temperature within the summer pycnocline.
Acknowledgments.
This manuscript benefited from
thoughtful comments by Liz Sikes, Robert Thunell, and
an anonymous reviewer. This research was supported by
fundsfrom NSF grantsOCE-9000517(FGP), OCE-9203292
(FGP), and USGS-92-9460-1026(NP). We are also grateful to NSF for support of the MULTITRACERS
project
(OCE-8609366and OCE-8919956)and the corerepository
at OregonState University (OCE-9402298). Graduatestudent assistantshipfor Anne Sabin was provided with support
from the U.S. Geological Survey.
ingthis difference:(1) the standarderrorof the transfer
functionanalysisis of the orderof 1.5-2øCand (2) the References
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fprahl@oce.orst.edu)
(e-mail:
(ReceivedSeptember28, 1994;revisedMay 1, 1995;
acceptedMay 1, 1995.)