PALEOCEANOGRAPHY, VOL. 20, PA4009, doi:10.1029/2005PA001146, 2005
A 70-kyr sea surface temperature record off southern Chile
(Ocean Drilling Program Site 1233)
J. Kaiser
Deutsche Forschung Gemeinschaft Research Center Ocean Margins, University of Bremen, Bremen, Germany
F. Lamy
GeoForschungsZentrum-Potsdam, Potsdam, Germany
D. Hebbeln
Deutsche Forschung Gemeinschaft Research Center Ocean Margins, University of Bremen, Bremen, Germany
Received 15 February 2005; revised 27 May 2005; accepted 13 July 2005; published 29 October 2005.
[1] We present the first high-resolution alkenone-derived sea surface temperature (SST) reconstruction in the
southeast Pacific (Ocean Drilling Program Site 1233) covering the major part of the last glacial period and the
Holocene. The record shows a clear millennial-scale pattern that is very similar to climate fluctuations observed
in Antarctic ice cores, suggesting that the Southern Hemisphere high-latitude climate changes extended into the
midlatitudes, involving simultaneous changes in air temperatures over Antarctica, sea ice extent, extension of the
Antarctic Circumpolar Current, and westerly atmospheric circulation. A comparison to other midlatitude surface
ocean records suggests that this ‘‘Antarctic’’ millennial-scale pattern was probably a hemisphere-wide
phenomenon. In addition, we performed SST gradient reconstructions over the complete latitudinal range of the
Pacific Eastern Boundary Current System for different time intervals during the last 70 kyr. The main results
suggest an equatorward displaced subtropical gyre circulation during marine isotope stages 2 and 4.
Citation: Kaiser, J., F. Lamy, and D. Hebbeln (2005), A 70-kyr sea surface temperature record off southern Chile (Ocean Drilling
Program Site 1233), Paleoceanography, 20, PA4009, doi:10.1029/2005PA001146.
1. Introduction
[2] While climate changes at millennial timescales during
the last glacial period (marine isotope stages (MIS) 4 to 2)
are relatively well known in the Northern Hemisphere (NH),
there is a clear lack of records in the Southern Hemisphere
(SH). Nevertheless, a number of new proxy records have
been published over the last years and begin to provide a
clearer picture of paleoceanographic pattern in the SH
midlatitudes to high latitudes [e.g., Charles et al., 1996;
Ninnemann et al., 1999; Kanfoush et al., 2000; Pahnke et
al., 2003; Lamy et al., 2004]. In the most widely accepted
view, the primary trigger of millennial-scale changes on a
global scale is located in the NH and involves abrupt
changes in the global thermohaline circulation (THC), in
particular in the formation of North Atlantic Deep Water
(NADW) [e.g., Rahmstorf, 2002]. These changes result in
large and abrupt climate shifts most clearly observed in the
North Atlantic realm but also in other parts of the NH [e.g.,
Voelker, 2002]. The synchronization of ice cores from
Antarctica and Greenland using methane concentrations
provides strong evidence for a climatic seesaw pattern of
the temperature changes in the NH and SH polar regions
and the reconstruction suggests that the onset of seven
major millennial-scale warmings in Antarctica preceded
Copyright 2005 by the American Geophysical Union.
the onset of Greenland warmings [Blunier and Brook,
2001]. Many modeling studies provide evidence that this
interhemispheric seesaw pattern could be largely induced by
changes in NADW formation [e.g., Rahmstorf, 2002]. Other
models suggest on the other hand that changes in temperature, sea ice extent and/or salinity around Antarctica could
influence the strength of the North Atlantic THC [Shin et
al., 2003b] and possibly trigger abrupt events in the North
Atlantic region as well [Knorr and Lohmann, 2003; Weaver
et al., 2003], which would support a more prominent role of
the SH in abrupt climate changes. These different modeling
results show that the ultimate mechanism behind short-term
climate variability during the last glacial and the seesaw
pattern still remains uncertain and high-resolution paleoceanographic records from the SH can greatly contribute to
solving some of the open questions.
[3] Wherever the ultimate origin of millennial-scale climate and ocean variability is located, the surface eastern
boundary currents along the western margins of the major
continental landmasses act as conduits for the exchange of
heat from the cold, high latitudes to the warm, low latitudes.
The Peru-Chile Current (PCC), or Humboldt Current, is one
of the largest and most productive Eastern Boundary
Current systems in the world. Up to date, its spreading
and functioning during the last glacial period is not well
known because of the lack of records with sufficient
resolution, especially in its southernmost part. Near the
equator, recent studies suggest that during glacial maxima,
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Figure 1. Modern annual mean sea surface temperature
(SST) distribution (C) [after Levitus and Boyer, 1994] in
the southeast Pacific and the location of Ocean Drilling
Program (ODP) Site 1233 (41000S, 74270W), as well as
sediment cores discussed in the text: GeoB3302-1
(72020W, 33130S) [Kim et al., 2002], GIK17748-2
(72020W, 32450S) [Kim et al., 2002], TG-7 (78600W,
17140S) [Calvo et al., 2001], ODP846B (90490W, 3050S)
[Martinez et al., 2003], RC13-110 (95650W, 0090N)
[Feldberg and Mix, 2003], and TR163-19 (86260W,
2120N) [Lea et al., 2000]. Abbreviations are ACC,
Antarctic Circumpolar Current; CHC, Cape Horn Current;
PCC, Peru-Chile Current; SEC, South Equatorial Current;
STH, subtropical high pressure; and L, low-pressure belt
associated with the westerlies. Inset shows location of the
cores in the Southern Hemisphere midlatitudes to high
latitudes used in the study (see Table 2). PF is Polar Front;
SAF is Subantarctic Front.
the Pacific Eastern Boundary Current System (PEBCS)
flow was stronger than today and thus substantially contributed to the cooling in the equatorial region [Feldberg
and Mix, 2003; Martinez et al., 2003].
[4] Southernmost South America is ideally located to
reconstruct climate variability in the SH as it is situated
under the influence of the dominant oceanographic (Antarctic Circumpolar Current, ACC) and atmospheric (westerly winds) circulation members. On land, extensive climate
reconstructions mainly based on glaciological and palynological studies have shown the sensitivity of southernmost
South America to climate changes since the Last Glacial
Maximum (LGM) and during the Holocene [e.g., Lowell et
al., 1995; Denton et al., 1999a, 1999b; Moreno et al.,
2001]. Some terrestrial records suggest a link to NH
millennial-scale climate changes, such as a cooling during
the NH Younger Dryas (YD) cold event [e.g., Moreno et al.,
2001], whereas others show no response to the YD [e.g.,
Bennett et al., 2000]. Recently, Lamy et al. [2004] have
presented high-resolution marine records in the southeast
Pacific spanning the interval from 8 to 50 kyr. Contrary to
reconstructions on the adjacent land, the record of past sea
surface temperatures (SST) shows a clear millennial-scale
‘‘Antarctic timing’’ suggesting a close connection to SH
high-latitude climate changes. On the basis of the record of
iron concentrations of the same core, Lamy et al. proposed
that the inertia of the Patagonian ice sheet to respond to
rapid climate changes could partly explain the disagreements between land and ocean records.
[5] Here we present a high-resolution alkenone-based
SST record from the SE Pacific midlatitudes off southern
Chile (Ocean Drilling Project (ODP) Leg 202 Site 1233)
covering the last 70 kyr. SST and age model data are
available at the World Data Center1. The study contains five
main points: (1) the prolongation and improvement of the
alkenone-based SST reconstruction published in the work of
Lamy et al. [2004], which confirms our previous interpretations; (2) the improvement of the age model by tuning the
SST to the oxygen isotope record of the Antarctic Byrd ice
core between 40 and 70 kyr; (3) the regional implications
of the SST reconstruction for the southern PCC and adjacent
southern South America; (4) a zonal comparison to other
paleoceanographic records in the SH midlatitudes and a
discussion of the possible forcing mechanisms at millennial
to submillennial timescales during MIS 4 to 2; and (5) a
latitudinal SST gradient reconstruction covering the complete PEBCS in order to better understand the different SST
patterns and their implications for the atmospheric and
oceanic circulations at different time intervals during the
last glacial period and the Holocene.
2. Oceanographic and Atmospheric Settings
[6] The study area (Figure 1) is located at the northern
margin of the ACC under the influence of cold, subantarctic
surface waters and steep latitudinal SST gradients (5C
between 43S and 39S) [from Levitus and Boyer, 1994].
Auxiliary data are available electronically at the World Data Center for
Marine and Environmental Sciences, Centre for Marine Environmental
Sciences (MARUM), Klagenfurter Strasse, D-28359 Bremen, Germany
(http://www.wdc-mare.org/[email protected]).
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Mean annual SSTs are around 14C and the seasonal
amplitude is 5C. The northern part of the ACC splits
around 43S into the PCC flowing northward and the
Cape Horn Current (CHC) turning toward the south [Strub
et al., 1998]. Subsurface currents at the core site include the
southward flowing Gunther Undercurrent near the shelf
edge, at depths of 100 – 300 m [Fonseca, 1989]. Between
400 and 1000 m, Antarctic Intermediate Water flows
northward along the Chilean continental margin [Strub et
al., 1998].
[7] In the SE Pacific midlatitudes to high latitudes, the
steepest SST gradient within the ACC is strongly related to
the main atmospheric circulation member of the Southern
Hemisphere, the westerly wind belt [Streten and Zillman,
1984]. This intense and powerful circulation, annually
centered around 50S, results from the strong thermal
gradient and atmospheric pressure difference between cold
air masses over Antarctica and the warmer air and water
masses in the subtropical regions [Cerveny, 1998]. In
southernmost South America, the westerlies and associated
storm tracks bring heavy rainfalls, e.g., an annual mean
greater than 2000 mm in Puerto Montt (41S), and prevent
upwelling south of 42S [Strub et al., 1998].
[8] Trenberth [1991] has described the modern seasonal
fluctuation of the storm tracks associated to the westerlies in
the Southern Hemisphere. In summer, the storm track
activity can be as strong as in winter, but is located slightly
equatorward of its winter position, and is concentrated in a
tight band centered around 49– 50S. In winter, storm track
activity extends over a broader range of latitudes and is
centered only 2 poleward from its summer position. The
strong SST gradients associated with the ACC are marked
by a northward latitudinal shift of 5 in winter. The
seasonal shifts of this coupled system are apparently controlled by seasonal changes in sea ice extent around Antarctica [Markgraf et al., 1992], which has been estimated to
range between 4 million km2 in summer and 19 million km2
in winter [Comiso, 2003].
[9] Site 1233 (41S) is located at a key position to
investigate the meridional oceanic heat exchanges in the
southeast Pacific, i.e., at the origin of the PCC, the latitudinally most extensive Eastern Boundary Current system in
the world, driven by south easterly winds along the Pacific
coast of South America [Strub et al., 1998]. The resulting
offshore Ekman flow drives perennial upwelling of cool,
nutrient-rich waters that produces one of the biologically
most productive regions in the oceans [Berger et al., 1987].
At about 5S, the PCC is deflected offshore, feeds the South
Equatorial Current (SEC) and flows westward as the equatorial cold tongue between 10S and 4N [Wyrtki, 1965].
North of the SEC, the Equatorial Front separates the cold,
salty waters of the Peru-Chile Current from warmer and
fresher tropical waters from the Northern Hemisphere. Thus
the PCC acts as a conduit for exchange of heat and nutrients
between high and low latitudes in the eastern Pacific.
3. Material and Methods
[10] Site 1233 was drilled during ODP Leg 202 off
southern Chile (410.010S, 7426.990W; 40 km offshore;
838 m water depth) in a small basin on the upper continental
slope away from the pathway of major turbidity currents
[Mix et al., 2003]. Five Advanced Piston Corer holes were
drilled at Site 1233 to ensure a complete stratigraphic
overlap between cores from different holes. Detailed comparisons between high-resolution core-logging data performed shipboard demonstrated that the entire sedimentary
sequence to 116.4 m below surface (mbsf) was recovered.
On the basis of these data, a composite sequence (the socalled splice) was constructed representing 135.65 m composite depth (mcd).
[11] Sediments at Site 1233 are dominated by terrigenous
components (clay and silty clay) with varying but generally
small amounts of calcareous components (primarily nannofossils and foraminifera). Calcium carbonate concentrations
and TOC contents range from 1 to 11 wt % (average =
5.4 wt %) and from 0.4 to 2.5 wt % (average = 0.9 wt %)
[Mix et al., 2003]. The TOC contents are substantially lower
between 30 and 136 mcd (0.4 to 1 wt %) in comparison to
the top of the core (up to 2.5 wt %). Samples for alkenone
measurements were taken with intervals ranging from 12 to
149 cm (average = 61 cm) from the splice. Samples for 14C
accelerator mass spectrometry (AMS) dating were taken
from outside the splice [Lamy et al., 2004].
[12] To determine past SST variations off Chile, we have
measured the alkenone unsaturation index UK037 as defined
by Prahl and Wakeham [1987] on 1– 3 g freeze-dried and
homogenized sediment samples. After the addition of internal standards [squalane (C30H62) and 2-nonadecanone
(C19H38O)], alkenones were extracted using mixtures of
methanol and methylene chloride with decreasing polarity
(MeOH, MeOH/CH2Cl2 1:1, CH2Cl2) by ultrasonication
(UP 200H sonic disruptor probe, Hielscher GmbH, 200W,
105mm amplitude, 0.5s pulse). After centrifuging, the
extracts were combined, desalted with deionized water,
dried with Na2SO4 and evaporated to dryness. The concentrated residue was dissolved in CH2Cl2. To avoid interferences with coeluting C 36 -fatty acid methyl esters,
saponification was performed using 0.1 N KOH in methanol
(90/10 CH3OH/H2O) at 80C for 2 hours followed by
partitioning of the neutral fraction containing the alkenones
into hexane. The extracts were finally concentrated under
N2 and taken up in 25 mL MeOH/CH2Cl2 (1:1).
[13] The extracts were analyzed by capillary gas chromatography using a HP 5890 series II Plus gas chromatograph
equipped with a 60 m 0.32 mm fused silica column (DB5 MS, J&W) using split/splitless injection and a flame
ionization detection. Helium was used as carrier gas with
a constant pressure of 150 kPa. After injection at 50C, the
oven temperature was programmed to 250C at a rate of
25C/min, then to 290C at a rate of 1C/min, held for
26 min, and finally to 310 at a rate of 30C/min, where the
final temperature was maintained for 10 min.
[14] Quantification of the alkenones was achieved using
HPGC ChemStation as analytical software. UK037 was
calculated from UK037 = (C37:2)/(C37:3 + C37:2), where
C37:2 and C37:3 are the di- and tri-unsaturated C37 methyl
alkenones. For conversion into temperature values, we used
the culture calibration of Prahl et al. [1988] (UK037 =
0.034T + 0.039), which has been validated by core top
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compilations [Müller et al., 1998]. The analytical precision
was estimated to be around ±0.5C. The SST estimate for
the uppermost sample (14C) matches the modern annual
mean SST value for the core site [Levitus and Boyer, 1994].
This is in agreement with other alkenone temperature
analyses of surface sediments recovered north of our study
site [Kim et al., 2002]. We thus consider that alkenonederived SSTs correspond to the annual average of the ocean
seawater surface. The alkenone content (defined as the sum
of the C37:3 and C37:2 alkenones) ranges from 2600 ng/g
(between 0 and 10 mcd) to 700– 1000 ng/g (for the rest
of the core).
[15] It has been recently observed that alkenones may be
substantially older than co-occurring planktic foraminifera
[Mollenhauer et al., 2005]. Holocene age differences measured on the Site 1233 survey core GeoB 3313-1 showed
rather constant age offsets of 1000 years. Mollenhauer et
al. [2005] explained this offset as most likely resulting from
continuous resuspension/redeposition cycles induced by
internal tides and sediment focusing in morphologic depressions such as the small basin at Site 1233. By comparing the
age offsets in different continental margin settings, they
further noted that age offsets were largest where TOC
contents and alkenone concentrations are highest. Therefore
we expect that the age offsets if they are indeed induced by
resuspension/redeposition cycles should be much smaller
for the glacial section where both TOC and alkenone
concentrations are significantly lower. We also note that
grain-size data suggest constant and rather undisturbed finegrained hemipelagic sedimentation (at least during the
Holocene [see Lamy et al., 2001]). Available oceanographic
data show that bottom water circulation at the depth of Site
1233 (within the Antarctic Intermediate Water [e.g., Shaffer
et al., 2004]) is rather too sluggish for the resuspension of
sediments and internal waves have not been described at the
Chilean margin. Therefore it is likewise conceivable that a
constant admixture of older material would affect the 14C
ages but not significantly the reconstructed alkenone temperatures, a possibility that Mollenhauer et al. [2005] did
not exclude either.
[16] In some samples of the core (30 on a total of 223
samples), mainly located between 30 and 80 mcd, the
presence of coeluting organic compounds altered the peaks
of the long-chain alkenones and thus the UK037 values
(Figure 2a top). To improve the measurements, liquid
chromatography was applied to all the samples. The extracts
were separated into three fractions by elution through a
Bond silica column (Bond Elute column, Varian): (1) 4 mL
of Hexane (apolar fraction), (2) 4 mL of a mixture of hexane
and methylene chloride (3:1) (ketone fraction, including the
alkenones) and (3) 2 mL of methylene chloride (alcohol
fraction). Finally, all fractions were concentrated under N2
and taken up in 25 mL MeOH/CH2Cl2 (1:1). The method
was first tested on a reference sample and the UK037 values
obtained were within the estimated error bar (±0.056
UK037, or 0.5C). Figure 2a shows the gas chromatograms
of the fractions containing the C37:3 and C37:2 alkenones and
the organic compounds before (top plot) and after (bottom
plot) liquid chromatography. A precise identification of the
coeluting organic compounds using GC-MS is still unre-
solved (M. Elvert, personal communication, 2005) and is
beyond the topic of the present study.
[17] The SSTs determined with this additional analytical
step result in very similar UK370 values as those obtained by
Lamy et al. [2004] on the 10– 90 mcd interval of the core.
The correlation between both data sets is r = 0.98
(Figure 2b), and the highest SST differences occur at
relatively low temperatures (8 – 10C). Only 13% of the
data have significantly different SSTs (i.e., >0.5C relative
to the estimated methodological error; Figure 2 C), and
mainly result in a shrinking of the extreme SST values.
Therefore the interpretation of the previous published SST
record is not significantly affected by our new data.
4. Stratigraphy
[18] The age model of the 135.7 mcd-long composite
sequence at Site 1233 (Figure 3 and Table 1) has been
constructed as follows. (1) The uppermost 9 mcd have
been correlated to the AMS 14C dated gravity core GeoB
3313-1 from the same location [Lamy et al., 2001] using
magnetic susceptibility and Ca relative concentration
records (Figure 3a). This correlation allowed us to transfer
the age model of core GeoB 3313-1 (based on 7 AMS 14C
datings) to Site 1233. (2) Age control for the 10 to
70 mcd interval is provided by 17 AMS 14C dates on
mixed planktonic foraminifera samples [Lamy et al., 2004]
and the record of the Laschamp magnetic field excursion
[Lund et al., 2005]. All AMS 14C dates were calibrated with
the CALPAL software (available at www.calpal.de) using
the CALPAL 2004 January calibration curve. We assume no
regional deviation from the global reservoir effect of
400 years because the core position lies significantly
south of the Chilean upwelling zone and north of the
southern polar front. Therefore the assumption of a
400 years reservoir age which is also the mean reservoir
age for the Pacific Ocean at 40S appears to be the most
reasonable assumption. For more details the reader can refer
to the supporting online material of Lamy et al. [2004].
(3) As d18O data on planktic and benthic foraminifera are
not yet available farther down hole, we propose here an
updated and better constrained age model than that published by Lamy et al. [2004] for the older part of the record,
i.e., below the Laschamp excursion, based on visual tuning.
The millennial-scale SST variations in the AMS 14C-dated
part of the record closely follow Antarctic temperature
fluctuations as recorded in the Byrd ice core [Lamy et al.,
2004] and the SST pattern farther down core shows a clear
visible resemblance to the Antarctic record as well. Therefore we decided to tune our alkenone SST records to the
Antarctic record using a minimum number of correlation
points between our SST data set and the d18O record of the
Byrd ice core (Figure 3b). For our purposes, the Byrd ice
core has presently the most suitable age model for the last
glaciation as it is linked to the Greenland GISP2 ice core
record as well as our 14C age calendar year conversion.
[19] The 135.7 mcd-long core covers the last 70 kyr.
The resulting mean sedimentation rates range between
1.4 m/kyr in the Holocene to an average of 2.2 m/kyr
during MIS 4 to 2. These high sedimentation rates are
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Figure 2. Effects of applying liquid chromatography (LC) to the extracts and comparison with previous
published results. (a) Gas chromatogram (GC) of the GC window containing the C37 and C38 alkenones:
whole fraction (top) without LC and (bottom) after LC (ketone fraction). (b) Correlation of SSTs based on
measurements with (this study) and without LC (as published by Lamy et al. [2004]). (c) SST difference
between the individual measurements. Only 13% of the data have significantly different SSTs (i.e.,
>0.5C relative to the estimated methodological error).
consistent with strong fluvial discharge in response to heavy
continental rainfall in southern Chile during the Holocene
[Lamy et al., 2001]. During the last glacial (MIS 2 to 4), the
continental hinterland of Site 1233 was extensively glaciated
as the Patagonian ice shield advanced toward the north
[Denton et al., 1999b] explaining even higher terrestrial
input through glacial erosion processes [Lamy et al., 2004].
5. Results and Discussion
5.1. SST Changes off Chile During the last 70 kyr:
Results and Regional Aspects
[20] The alkenone-based sea surface temperature reconstruction at Site 1233 covers the last 70 kyr with a mean
resolution of 320 years (Figure 3b) and thus provides the
longest high-resolution SST record in the SE Pacific pres-
ently available. After a maximum of 14.7C, probably
corresponding to MIS 5.1, SSTs decrease to 8C in MIS
4, the coldest temperatures of the record. Temperatures rise
again up to 12C in early MIS 3 and display a general
long-term cooling trend until 45 kyr B.P. Superimposed
on this trend, the major Antarctic warm events A2 to A4
[Blunier and Brook, 2001] are characterized by SST
increases of up to 3C. From 45 to 19 kyr B.P., the SSTs
show millennial-scale variability of 2– 3C around a mean
temperature of 9.5C. The LGM is not clearly defined in the
record. Denton et al. [1999a] have reconstructed the summer mean air temperature in the adjacent Lake District
region based on a combination of glacier fluctuations and
pollen records spanning the last 60 kyr (not shown). Despite
an apparent disagreement in the details of the timing of the
terrestrial compared to the marine record in the SE Pacific
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Figure 3. Age model of ODP Site 1233. (a) Correlation of Site 1233 to the 14C-AMS dated core
GeoB3313-3 [Lamy et al., 2001] based on the magnetic susceptibility and Ca relative concentration
records in the middle and late Holocene. (b) Alkenone-based SST reconstruction at Site 1233 compared
to the Byrd d18O record over the last 70 kyr. The open arrows represent the correlation points to the core
GeoB3313-1, the black arrows represent the 14C-AMS datings and the Lashamp event at 41 kyr [Lund et
al., 2005], and the gray arrows show the tuning points to the oxygen isotope record of the Byrd ice core.
A1 to A5 are Antarctic warm events after Blunier and Brook [2001]. ACR is Antarctic Cold Reversal;
MIS is marine isotope stages 1 to 4.
[see Lamy et al., 2004], their results are similar to our SST
reconstruction in terms of main tendencies and amplitudes:
8C around 60 kyr B.P., an abrupt increase to 12C at
57 kyr B.P., decreasing values toward the LGM with
temperatures around 8C.
[21] A 6C SST warming over Termination I is similar to
results obtained from land, other marine records and modeling studies in the region. First, the previously cited
summer air temperature reconstruction in the Lake District
shows a 6C warming over the last deglaciation. Second,
two alkenone-based SST reconstruction farther to the north
at 35S and 33S have a 6C – 7C SST increase between 19
and 12.5 kyr B.P. [Kim et al., 2002; O. Romero, preprint,
2005]. Third, in a recent modeling study of the changes in
the Patagonian ice sheet extent during the LGM and the
deglaciation, Hulton et al. [2002] have shown a good
agreement between modeled ice extent and empirical evidence at the LGM by applying a temperature decrease of
6C relative to present day. In our record termination I is
interrupted by one major cooling event (0.8C) between
14.8 and 13.3 kyr B.P., followed by a plateau until 12.7 kyr
B.P. (Figure 3b). This cooling matches the Antarctic Cold
Reversal (ACR; 14 to 12.5 kyr B.P.) [e.g., Jouzel et al.,
1995] and clearly precedes the NH Younger Dryas (YD)
(13 – 11.5 kyr B.P.) [e.g., Rutter et al., 2000]. Instead, we
observe a SST increase of 2.1C during the early part of the
YD (12.7 and 12.1 kyr B.P.) and early Holocene values
thereafter. This pattern seems to be confirmed by two other
alkenone-based SST records situated at 35S and 30S
along the Chilean coast (O. Romero, preprint, 2005;
J. Kaiser, unpublished data, 2005). On the basis of terrestrial
records, the presence or absence of the NH YD in southern
South America is discussed controversially. A cooling
during the YD has been for example proposed from pollen
records in NW Patagonia close to Site 1233 (Chilean Lake
District region and Isla Grande de Chiloé) [e.g., Denton et
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Table 1. Age-Depth Relation for ODP Site 1233a
Age, kyr
Error, kyr
Age, kyr cal. B.P.
Dating Method
assumption 0 mcd = 2000 A.D.
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
correlation to core GeoB 3313-1b
paleomagnetic age
(Laschamp event)
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
SST tuned on Byrd d18O
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All radiocarbon ages are calibrated using the CALPAL 2004 January calibration curve (available at www.calpal.de) and a constant reservoir age
correction of 400 years (see section 4 for details); mcd is meters composite depth.
Correlation to the 14C AMS-dated core GeoB 3313-1 from the same location [Lamy et al., 2001] using the magnetic susceptibility and Ca relative
concentration records.
al., 1999a; Moreno et al., 2001] and seems to be present in
subantarctic Patagonia as well [Heusser et al., 2000;
Massaferro and Brooks, 2002]. However, it has been
recently suggested that the deglacial cold reversal in NW
Patagonia started earlier (at 14.7 to 13.4 kyr B.P.), and that
the YD interval is rather characterized by fire disturbances
[Hajdas et al., 2003; Moreno, 2004] that may not necessarily imply cooling. In addition, other paleoenvironmental
reconstructions in southern Chile (40S to 48S) based on
pollen, glacial morphology, and beetle assemblages did not
find evidence of a cooling during the YD epoch either [e.g.,
Ashworth and Hoganson, 1993; Bennett et al., 2000;
Glasser et al., 2004].
[22] The SSTs reach a maximum of 15.6C in the early
Holocene (11 to 9 kyr B.P.) and generally decrease
thereafter, reaching the modern SST (14C) in the late
Holocene (Figure 3b). A warmer and drier-than-today
climate over southwestern South America in the early
Holocene was also recorded on the adjacent land [e.g.,
Massaferro and Brooks, 2002; Moreno and León, 2003;
Abarzúa et al., 2004], and even in the low latitudes, e.g., in
the Huascaràn ice core [Thompson et al., 1995]. Furthermore, most Antarctic ice core records show a widespread
early Holocene optimum between 11.5 and 9 kyr B.P.
[Masson et al., 2000]. This early Holocene optimum was
not documented in the earlier SST record based on the short
core (GeoB 3313-1 [Lamy et al., 2001]) drilled at the same
location as Site 1233 that only covers the last 8 kyr.
Details on millennial to multicentennial-scale variations
during the middle and late Holocene can be found in the
work of Lamy et al. [2002].
[23] Holocene and glacial climate fluctuations in the SE
Pacific region and adjacent South America have often been
related to changes in the latitudinal position of the SH
westerlies and a northward shift of this wind belt has been
proposed for this region based on a number of terrestrial and
marine archives [e.g., Heusser, 1989; Lamy et al., 1998;
Benn and Clapperton, 2000; Moreno and León, 2003].
7 of 15
1992] as well as with modeling results [Wyrwoll et al.,
2000; Wardle, 2003] that suggest little latitudinal change or
even a slight poleward shift. Nevertheless, Wyrwoll et al.
[2000] point out that associated with the southward shift
and increase westerly flow around 60S, there was a distinct
widening of the zone of strong westerlies, similar to the
modern winter conditions (see section 2). Taken together,
the overall regional proxy evidences strongly suggest that the
SST variability at Site 1233 was predominantly caused by
latitudinal shifts of the ACC and the southern westerlies (or
their northern boundary), i.e., cold temperatures associated
with a northward shift of the coupled system and vice versa.
Figure 4. Comparison of the Site 1233 SST record in the
SE Pacific with other SST proxy records from the Southern
Hemisphere, and the Antarctic air temperature records of
Byrd and European Programme for Ice Coring in Antarctica
(EPICA) Dome C ice cores, during the past 70 kyr.
(a) Oxygen isotope record on planktic foraminifera in the
South Atlantic [Ninnemann et al., 1999]. (b) Mg/Ca SST
reconstruction in the southwest Pacific [Pahnke et al., 2003].
(c) Mg/Ca SST reconstruction in the South Pacific [Mashiotta
et al., 1999]. (d) Alkenone SST reconstruction at ODP Site
1233 (this study). (e) Oxygen isotope record of the Byrd ice
core [Blunier and Brook, 2001]. (f) Deuterium profile from
the EPICA Dome C ice core [EPICA Community Members,
2004]. ACR is Antarctic cold reversal; A1 to A5 indicate
Antarctic warm events after Blunier and Brook [2001]. See
Figure 1 for the location of the cores.
Together with the westerlies, the ACC was apparently
displaced northward at the LGM as well. High paleoproductivity at 33S off Chile during the LGM suggests that
an equatorward shift of the ACC would have brought the
main nutrient source closer to the core sites resulting in
increased productivity [Hebbeln et al., 2002; Mohtadi and
Hebbeln, 2004]. We note that a northward displacement of
the westerlies during the LGM is in disagreement with some
other paleoenvironmental records [e.g., Markgraf et al.,
5.2. Surface Water Changes in the SH Midlatitudes:
Toward a Common Millennial-Scale Pattern and
Associated Forcing Mechanisms
[24] The close resemblance of SST changes at Site 1233
to Antarctic temperature changes within the 14C-AMS dated
section and the very similar temperature pattern in the
earlier part of the records, allowed us to transfer the Byrd
age model to Site 1233 for the older interval (see section 3).
Our new results are thus prolonging the previously discussed pattern of close climate linkages between the SH
midlatitudes and high latitudes [Lamy et al., 2004] into late
MIS 5. In Figure 4, our 70 kyr SST record off southern
Chile is compared with other records of changes in surface
water properties in the SH midlatitudes (see Figure 1 for site
locations): an oxygen isotope record of Globigerina bulloides in the South Atlantic [Ninnemann et al., 1999] and
two Mg/Ca SST records, one from the southwest Pacific
[Pahnke et al., 2003] and a second from the central south
Pacific [Mashiotta et al., 1999]. The 14C-based parts of the
age models were recalculated with the CALPAL January
2004 calibration curve. A recalibration was not possible for
the record in the central South Pacific [Mashiotta et al.,
1999], that is based on tuning (see Table 2). Considering the
errors of dating and the differences in the mean resolutions
(between 350 and 920 years), the records present a common
pattern which is very similar to the Antarctic Byrd d18O and
the recent EPICA d deuterium records (Figures 4e – 4f):
warmer SSTs during the Antarctic warm events A1 to A4
(as defined by Blunier and Brook, [2001]), a 4C to 6C
warming over termination I, beginning simultaneously
around 19 kyr B.P., the record of the Antarctic Cold
Reversal (except in the south Pacific, most likely because
of the low resolution of the record), and finally a Holocene
climatic optimum between 9 and 11 kyr B.P. This
common pattern suggests that Antarctic climate changes
extended into the SH midlatitudes involving changes in the
ACC and the westerlies. The most likely mechanism is a
common latitudinal shift of the coupled system, i.e., equatorward during cold phases and poleward during warm
intervals. Assuming a global mean LGM cooling of 2C
as predicted by a coupled ocean-atmosphere GCM model
for the intertropical oceans [Ganopolski et al., 1998] and
that the remaining SST changes in the SE Pacific were
mainly driven by coeval shifts of the westerlies (or their
northern boundary) and the ACC during the last 70 kyr, it is
possible to give a rough estimation of the latitudinal shifts
when extrapolating from the modern SST pattern [after
8 of 15
Table 2. Modifications Made to the Published Age Models From Some Records Used in Figures 4 and 5a
Ninnemann et al. [1999]
41360S, 9480E
45320S, 174550E
Pahnke et al. [2003]
Mashiotta et al. [1999]
56040S, 115050E
Shemesh et al. [2002]
53200S, 5100E
Original Age Model
C AMS dates (40 kyr to present)
MIS 5/4 boundary based on SPECMAP chronology
C AMS dates (35 kyr to present)
benthic d18O tuned to benthic d18O of core MD952042 (72 – 35 kyr)
planktonic d18O correlated to planktonic d18O of
core RC11-83 (40 kyr to present) and core
RC11-120 (110 – 40 kyr) [after Ninnemann and
Charles, 1997]
C AMS dates
Modification for This Study
new calibration with CALPAL
2004 January
no modification
new calibration with CALPAL
2004 January
no modification
no modification
new calibration with CALPAL
2004 January
All other records coming from the literature presented in this study are plotted on their original age models.
Levitus and Boyer, 1994]. Our results suggest that the whole
coupled system might have shifted northward by 4– 5 of
latitude during the LGM. Applying the same assumptions to
MIS 4 would result in a 5 – 6 northward shift of the
system. The estimated displacement for the LGM is consistent with the 5– 10 northward latitudinal expansion of
Antarctic cold waters as recently suggested by Gersonde et
al. [2005] and the 5 latitudinal shift of the westerlies as
proposed by works in South America [Heusser, 1989; Lamy
et al., 1998; Mohtadi and Hebbeln, 2004]. On the other
hand, a warmer-than-today climate during the early Holocene as suggested by the records of surface water changes
would imply a southward shift of the coupled system. This
is in agreement with most of the climate reconstructions on
terrestrial and marine archives of the SH midlatitudes [e.g.,
Brathauer and Abelmann, 1999; Moreno and León, 2003;
Haberle and Bennett, 2004; Shulmeister et al., 2004].
[25] A dust content record from Antarctica provides
additional evidence for a close coupling of atmospheric
circulation around Antarctica and temperature changes in
the SH midlatitudes at millennial timescales. In Figures 5c
and 5d the SST record at Site 1233 is plotted against the
dust content as measured on EPICA Dome C ice core
[Delmonte et al., 2002; EPICA Community Members,
2004]. The main origin of Antarctic dust is the Patagonian
region in southernmost South America [Grousset et al.,
1992; Basile et al., 1997]. The general increase in the dust
contents during the coldest periods, i.e., MIS 4 and 2 in our
case, has been explained by expanded source regions linked
to the global sea level drop, a decrease of the vegetation and
a general dryer climate in Patagonia [Petit et al., 1981;
Delmonte et al., 2002]. Despite these pronounced long-term
signal in the dust record, short-term variations on millennial
and submillennial timescales appear to parallel the shortterm variations in our SST record. Considering the offsets
between 70 and 41 kyr B.P. induced by age model discrepancies between the EPICA Dome C and Byrd ice cores
(Figures 5a and 5b) and the limitations in dating accuracy of
our record (Figure 5d) as well as in the age models of the ice
cores, we observe a reasonable correlation of high dust
contents to millennial-scale SST cold peaks and conversely.
Environmental changes in the source areas of the dust might
have played a role in dust variability on millennial timescales [Rothlisberger et al., 2002]. High-resolution palyno-
logical studies from Patagonia are unfortunately not yet
available in order to proof this hypothesis. On the other
hand, dust input maxima on millennial timescales could
have been induced by intensified circumpolar winds probably linked to a strengthening of the polar vortex by a
steeper latitudinal thermal gradient, generated by the northward extension of sea ice [COHMAP, 1988; Delmonte et al.,
2002]. A faster dust transport from Patagonia to Dome C
during the LGM has also been proposed based on GCM
simulations [Krinner and Genthon, 2003]. In addition,
intensified SH westerlies during the LGM are suspected
(but not proofed) in records of dust/loess and glacier
advances in New Zealand (for a review see Shulmeister et
al. [2004]) and are often proposed in simulations of a global
LGM climate [e.g., Wyrwoll et al., 2000; Shin et al., 2003a].
It is thus conceivable that the millennial-scale SST variability in the SE Pacific was likewise linked to changes in the
intensity of the westerlies in addition to the latitudinal shifts
as discussed above.
[26] Recent modeling studies on the last deglaciation
suggest that changes in sea ice extent and/or salinity in
the Southern Ocean may have had large consequences for
millennial-scale changes on a global scale [Knorr and
Lohmann, 2003; Weaver et al., 2003]. The involvement of
sea ice extent changes in SH millennial-scale climate
variability is consistent with the comparison of our SST
record to an index of the sea ice presence during the last
45 kyr in the South Atlantic [Shemesh et al., 2002] (core
TN057-13, location in Figure 1). The sea ice record suggests millennial-scale variations that imply, within the errors
of dating, extended sea ice duration during the cold intervals
in the southeast Pacific as shown by our data (Figures 5d
and 5e). Therefore we suggest that the extended sea ice in
the Southern Ocean displaced the ACC equatorward and
triggered enhanced advection of cold, subantarctic surface
waters into our study area. Furthermore, Kanfoush et al.
[2003] improved the chronology of the ice-rafted detritus
(IRD) record in the SE Atlantic [Kanfoush et al., 2000] and
concluded that major IRD events occurred during the cooling phases following the A1 to A4 warm events (not
shown). It has been argued that the IRD events were
associated with increased NADW production during major
NH interstadials [Kanfoush et al., 2000] what would be
consistent with the bipolar seesaw mechanism [Broecker,
9 of 15
record [Lamy et al., 2004], it is equally conceivable that the
millennial-scale changes are of SH origin.
[27] The emerging pattern of common millennial-scale
climate and ocean changes in the SH midlatitudes and high
latitudes suggests similar SST changes in the midlatitudes,
most likely controlled by changes in the strength and
latitudinal position (or extension) of the westerlies and the
ACC. These variations appear to be closely linked to
changes in Antarctic temperatures and the extent of sea
ice as well. The paleorecords suggest a scenario that largely
resembles the coupled atmosphere-ocean mechanisms
linked to the Southern Annular Mode (SAM), an important
modern mode of interannual to decadal-scale climate variability in the SH [Thompson and Wallace, 2000; Thompson
et al., 2000]. During a positive SAM, the surface temperature over Antarctica cools and the strength of the westerlies
over the subpolar Southern Ocean increases [Thompson and
Wallace, 2000]. Enhanced westerly winds could generate a
northward Ekman flow advecting sea ice farther north, as
proposed by a coarse resolution model [Hall and Visbeck,
2002], that would act as a positive feedback mechanism.
Therefore we speculate that long-term changes in the
interannual SH climate modes, such as the SAM, may
ultimately be involved in millennial-scale ocean and atmosphere changes in the SH midlatitudes and high latitudes
during the last glaciation.
Figure 5. Relationship of the SE Pacific SST reconstruction and Southern Hemisphere high latitudes. (a) Oxygen
isotope record of the Byrd ice core [Blunier and Brook,
2001]. (b) Deuterium record from the EPICA Dome C ice
core [EPICA Community Members, 2004]. (c) EPICA
Dome C dust content (plotted inverse) [EPICA Community
Members, 2004; Delmonte et al., 2002]. Original data (gray
line) and 3-point moving average data between 15 and
35 kyr (black line). (d) Alkenone SST at Site 1233 (this
study), plotted with the age control points of the core (see
Table 1). (e) Sea ice presence reconstruction based on
diatoms in the South Atlantic [Shemesh et al., 2002] (core
TN057-13, see Figure 1 for location). A1 to A5 are
Antarctic warm events after Blunier and Brook [2001]. Note
that the offsets between SST and EPICA Dome C dust
records from 70 to 41 kyr are primarily due to the offset
between Byrd d18O and EPICA dD records.
1998; Stocker, 1998; Blunier and Brook, 2001]. On the
other hand, the middle to high southern latitude – wide
millennial-scale pattern of surface ocean changes with
Antarctic timing during the last 70 kyr in all ocean basins
(see Figure 4) is inconsistent with most modeling studies on
the SH response to THC throttling or shutdown [Ganopolski
and Rahmstorf, 2001; Schmittner et al., 2003]. Therefore, as
already discussed in relation to the 14C-dated part of our
5.3. SST Gradient Changes Along the Pacific Eastern
Boundary Current System During the Last Glacial
Period (MIS 4 to 2) and the Early Holocene
[28] On the basis of coupled ocean-atmosphere models,
Liu et al. [2002] and Shin et al. [2003a] have proposed that
the upper ocean circulation in the southern midlatitudes and
high latitudes plays a key role in explaining tropical cooling
at the LGM. A similar line of evidence comes from a
number of paleoenvironmental reconstructions in the tropical eastern Pacific low latitudes. On the basis of a Mg/Ca
SST reconstruction near the equator, Lea et al. [2000] have
hypothesized a link between polar and tropical temperature
changes because of some similarities with isotopic changes
in Antarctic ice cores. High southern latitude foraminifera
species were present even north of the equator suggesting an
intensification of the Peru-Chile Current during the LGM
[e.g., Feldberg and Mix, 2003; Martinez et al., 2003]. Sea
surface temperature reconstructions based on foraminiferal
faunal assemblages have shown that the meridional SST
gradient in the Eastern Equatorial Pacific (EEP) was stronger during the LGM, suggesting ‘‘La Niña-like’’ conditions
[Martinez et al., 2003]. Likewise, a reconstruction of zonal
gradients in the tropical Pacific Ocean suggests a shallower
thermocline with a steeper east-west slope resulting in an
intensified Walker circulation during the LGM [Andreasen
and Ravelo, 1997], as presently occurring during La Niña
events. However, a reversed pattern for the LGM, i.e., ‘‘El
Niño-like’’ conditions, has been proposed by other authors
[Koutavas et al., 2002].
[29] Our new SST record off southern Chile provides
the opportunity to reconstruct paleoceanographic changes
in the PEBCS in its middle- to low-latitude section during
the last 70 kyr, based on a SST gradient reconstruction.
10 of 15
Figure 6. Latitudinal distribution of SST within the Pacific
Eastern Boundary Current between 2N and 41S. (a) Mg/
Ca SST (core TR163-19) [Lea et al., 2000]. (b) SST
reconstruction based on planktonic foraminifera fauna
distribution (core RC13-110) [Feldberg and Mix, 2003].
(c) SST estimation from the planktonic foraminifera
assemblage data using the modern analog technique MAT
(core ODP846B) [Martinez et al., 2003]. (d) Alkenonebased SST reconstruction (core TG7) [Calvo et al., 2001].
(e) Alkenone-based SST (cores GIK17748-2 and
GeoB3302-1) [Kim et al., 2002]. (f) Alkenone-based SST
(core ODP1233, this study). The gray bars mark the time
intervals as defined for Figure 7 and Table 3: the modern
(SST values from Levitus and Boyer [1994]), 8 – 12 kyr
(Holocene Climatic Optimum), 19– 23 kyr (Last Glacial
Maximum), 51–60 kyr (early MIS 3), and 63– 68 kyr (MIS
For this purpose, we used our alkenone record at 41S,
two alkenone-based SST reconstructions at 33S [Kim et
al., 2002] and 17140S [Calvo et al., 2001], two SST
reconstructions based on foraminiferal fauna assemblages
at 3050S [Martinez et al., 2003] and 0090N [Feldberg
and Mix, 2003], and a Mg/Ca SST reconstruction at
2150N [Lea et al., 2000] (see Figure 1 for the location
of the cores).
[30] As the records have very different time resolutions,
age models (the original age models were used here) and SST
proxies, we have focused on the meridional SST gradients in
five time intervals defined as follows (Figure 6): the present
day [from Levitus and Boyer, 1994], the Holocene Climatic
Optimum (HCO) (8– 12 kyr), the Last Glacial Maximum
(LGM) (19 – 23 kyr), early MIS 3 (51 – 60 kyr) and MIS 4
(63 – 68 kyr). In Table 3, we report the values of the present
and reconstructed SST gradients, calculated from the mean
SSTs for each record and each time interval, and Figure 7
shows the reconstructed SST gradients in the PEBCS for the
different time intervals. As the records used here have
different SST proxies (alkenone, Mg/Ca and foraminiferal
fauna assemblages) with various analytical errors (±0.3 to
±0.5C, ±0.6C and ±1 to ±1.8C respectively; details in
publications aforementioned), small SST gradient changes
should be considered with caution.
[31] During the coldest intervals, the overall midlatitudeto-equator gradient (2N–41S) was around 2.8C (MIS 2)
and 3.7C (MIS 4) higher than today. Conversely, we observe
similar to slightly reduced gradients during the HCO and
slightly increased gradients during early MIS 3. The increase
in meridional SST gradients during the LGM and MIS 4 is
mainly derived from a relatively strong cooling in the
southern and central part of the PEBCS (41– 17S), with
particularly enhanced gradients in the southernmost part
between 33S and 41S. This is in agreement with modeling results suggesting stronger SST gradients in the midlatitudes at the LGM [Shin et al., 2003a]. In terms of oceanic
surface circulation (and linked atmospheric circulation) during the last glacial cold periods (MIS 4 and 2), our SST
gradient reconstruction suggests a northward shift of the
strong SST gradient area (linked to the ACC) of which the
northern limit is nowadays located around 40S. On
the basis of Figure 7, this limit could have been situated
around or south of 33S at the LGM. This would be in the
same order of magnitude as the aforementioned ACC
northward shift of 5 in latitude at the LGM in comparison to the present day (see section 5.2). An enhanced
northward influence of the ACC up to 33S during MIS
2 has also been suggested in paleoproductivity reconstructions along the Chilean coast [Hebbeln et al., 2002;
Mohtadi and Hebbeln, 2004]. In terms of atmospheric
Table 3. Latitudinal SST Gradients Reconstruction in the Pacific
Eastern Boundary Current System for the Five Time Intervals as
Defined in Figure 6a
Early MIS 3
2N – 41S
0 – 17S
17S – 41S
0 – 41S
2N – 0
0 – 3S
3S – 17S
17S – 33S
33S – 41S
no data
no data
no data
no data
no data
no data
no data
no data
Latitudinal SST gradients reconstruction in the Pacific Eastern Boundary
Current system for the five time intervals as defined in Figure 6. The values
represent the SST differences (in C) between the different latitudes (left
column). Abbreviations are HCO, Holocene Climatic Optimum; LGM, Last
Glacial Maximum; and MIS, marine isotope stage.
11 of 15
the low latitudes during MIS 4 and 2, typically a ‘‘La
Niña-like’’ pattern as proposed by Martinez et al. [2003].
[33] Our SST gradients reconstruction suggests also some
interesting pattern for the relatively warm periods (HCO and
early MIS 3). During a warmer-than-today climate as the
HCO the SST gradients along the whole coast of South
America were slightly weaker, especially in the midlatitudes. Consequently the ACC influence through the PCC
was weaker, and the associated westerly winds probably
situated more poleward. A stronger gradient between 17S
and 3S could eventually reflect a more southward influence
of the equatorial warm waters. Nevertheless, a weaker SST
gradient in the south agrees with the previous suggestion of
a southward shift of the ACC and the westerlies during the
early Holocene (see section 5.2). Finally the SST gradient
distribution during early MIS 3 shows an intermediate
pattern. Whereas the whole gradient was stronger than
present, the latitudinal distribution was different than during
MIS 4 and 2, and closer to the HCO distribution.
Figure 7. Latitudinal reconstruction of the SST gradients
in the Pacific Eastern Boundary Current System during the
five time intervals as defined in Figure 6. Figure 7 is based
on the mean SSTs for each record and for each period. See
Table 3 for the SST gradient values and text for details.
circulation, reconstructions of glaciers movements
[Ammann et al., 2001] and humidity changes [Stuut and
Lamy, 2004] have shown that 27S should have been the
northernmost limit of the westerlies influence during
winter at the LGM. Ultimately, a northward displacement
of the westerlies (or their northern boundary), as implied
by our results, would be in agreement with the proposed
northward shift of the STH at the LGM [e.g., Andreasen
and Ravelo, 1997; Mohtadi and Hebbeln, 2004].
[32] In the low latitudes, previous studies have mentioned an influence of the PCC in the EEP during MIS 2
as MIS 4 [e.g., Mix et al., 1999; Feldberg and Mix, 2003;
Martinez et al., 2003]. Furthermore, following Hostetler
and Mix [1999], who have modeled the LGM tropical
climate based on a SST field in which lower tropical
SSTs than those of CLIMAP are prescribed in the eastern
tropical Pacific and equatorial Atlantic Oceans [see Mix et
al., 1999], the westward wind flow over northern South
America into the Pacific and the tropical Walker circulation were enhanced at the LGM. During cold intervals,
we observe similar-than-today SST gradients between
17S and the equator as well as around the equator
(2N–3S), within the Equatorial Front, (Figure 7 and
Table 3). With this kind of SST gradient configuration, it
is not possible to conclude on a stronger (or weaker)
circulation in the PEBCS. In comparison to the present
day, however, the whole SST distribution was similar to
the modern winter pattern (a northward shift of the strong
SST gradient area in the south and the presence of the
Equatorial Front near the Equator), while the PCC flow is
enhanced [Strub et al., 1998]. Therefore our reconstruction would be consistent with a stronger Peru-Chile
Current and a cold water tongue extended westward in
6. Summary and Conclusions
[34] 1. At a regional scale, the general trends of our SST
record are in agreement with paleoenvironmental records
from the adjacent continent and previous SST reconstructions that cover the last 30 kyr, in particular the SST
increase of 6C over termination I. Other features like a
cooling that matches the ACR as defined in Antarctic ice
cores are partly inconsistent with land records. Some of
these latter results provided evidences for a cooling synchronous with the NH YD cold event whereas our new SST
record suggests a pronounced warming of 2C during this
time interval similar to the Antarctic ice core records.
[35] 2. Our SST record reveals a clear ‘‘Antarctic
timing’’ of millennial-scale temperature changes during
the last 70 kyr. A comparison to other paleoceanographic
records from the SH midlatitudes suggests that these
changes occurred quasi hemisphere wide. In addition,
coeval changes can be observed in Antarctic dust and
sea ice extent records that would be consistent with the
following scenario. For SH cold periods, the westerly
wind circulation around Antarctica was enhanced and the
northern boundary of the westerlies moved equatorward
resulting in sea ice export away from Antarctica through
an enhanced Ekman drift. Linked to a northward widening of the westerlies, the ACC was displaced northward
and advected cold, subantarctic water into the SH midlatitudes. Conversely, during SH warm phases wind
intensities decreased, the westerlies and the ACC were
more poleward confined and eventually decreased the
advection of cold water masses into the midlatitudes.
Our scenario for SH millennial-scale changes resembles
the observed pattern related to interannual to decadalscale SH climate modes such as the SAM, suggesting that
long-term changes in these modes may ultimately be
involved as well. Finally, the clear ‘‘Antarctic timing’’
pattern is consistent with the seesaw mechanism often
discussed in relation to NH versus SH millennial-scale
climate pattern. However, the consistent temperature pattern around Antarctica in different ocean basins could
12 of 15
also imply a larger involvement or even a source of
millennial-scale climate variability in the Southern
[36] 3. A paleo-SST gradient reconstruction covering the
complete latitudinal range of the PEBCS suggests an
equatorward displaced subtropical gyre circulation during
MIS 2 and 4, similar to the modern winter pattern. This
configuration would be mainly linked to an equatorward
shift of the northernmost boundary of the ACC resulting in
enhanced SST gradients in the southern part of the PEBCS,
and to the presence of the Equatorial Front near the equator.
Therefore we might suggest that the PCC flow was stronger,
that more cold waters of a southern high-latitude origin
entered the southeast equatorial Pacific and that the cold
tongue was extended westward. Conversely, the oceanic
circulation in the PEBCS was probably weakened and the
ACC, and associated westerly wind belt, moved southward
during relatively warm periods (early MIS 3 and HCO).
[37] Acknowledgments. We would like to thank Peter Müller and
Ralph Kreutz for help in the laboratory and Marcus Elvert and Enno
Schefuss for helpful suggestions on the alkenone preparation and analysis.
We further acknowledge Helge Arz and Emmanuel Chapron for helpful
discussions and Eva Calvo as well as Xavier Crosta for contributing data.
Marco Mohtadi is thanked for suggestions on an earlier version of the
manuscript. Two anonymous reviewers provided helpful comments that
improved the manuscript. The study was funded by the German Science
Foundation through the grant DFG-He-3412-1-3 and was technically
supported by the Research Center Ocean Margins (RCOM) in Bremen.
This research used samples provided by the Ocean Drilling Program (ODP).
The ODP is sponsored by the U.S. National Science Foundation (NSF) and
participating countries under management of Joint Oceanographic Institutions (JOI), Inc. This is RCOM publication 0305.
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