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Earth and Planetary Science Letters 339–340 (2012) 95–102
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Earth and Planetary Science Letters
journal homepage: www.elsevier.com/locate/epsl
Letters
Pronounced subsurface cooling of North Atlantic waters off Northwest Africa
during Dansgaard–Oeschger interstadials
Jung-Hyun Kim a,n, Oscar E. Romero b, Gerrit Lohmann c, Barbara Donner d, Thomas Laepple c,
Eddie Haam e, Jaap S. Sinninghe Damsté a
a
Royal Netherlands Institute for Sea Research (NIOZ), Department of Marine Organic Biogeochemistry, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands
Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Avenida de las Palmeras 4, 18100 Armilla-Granada, Spain
c
Alfred Wegener Institute for Polar and Marine Research, Bussestrasse 24, D-27570, Bremerhaven, Germany
d
MARUM—Center for Marine Environmental Sciences, University of Bremen, P.O. Box 330440, D-28334 Bremen, Germany
e
Applied Mathematics, Harvard University, Cambridge, MA, USA
b
a r t i c l e i n f o
abstract
Article history:
Received 20 November 2011
Received in revised form
11 May 2012
Accepted 15 May 2012
Editor: P. DeMenocal
Millennial-scale Atlantic meridional overturning circulation (AMOC) variability has often been invoked
to explain the Dansgaard–Oeschger (DO) events. However, the underlying causes responsible for
0
H
millennial-scale AMOC variability are still debated. High-resolution UK
37 and TEX86 temperature records
for the last 50 kyr obtained from the tropical Northeast (NE) Atlantic (core GeoB7926-2, 201130 N,
181270 W, 2500 m water depth) show that distinctive DO-type subsurface (i.e. below the mixed layer:
4 20 m water depth) temperature oscillations occurred with amplitudes of up to 8 1C in the tropical NE
Atlantic during Marine Isotope Stage 3 (MIS3). Statistical analyses reveal a positive relationship
between the reconstructed substantial cooling of subsurface waters and prominent surface warming
over Greenland during DO interstadials. General circulation model (GCM) simulations without external
freshwater forcing, the mechanism often invoked in explaining DO events, demonstrate similar
anti-phase correlations between AMOC and pronounced NE Atlantic subsurface temperatures under
glacial climate conditions. Together with our paleoproxy dataset, this suggests that the vertical
temperature structure and associated changes in AMOC were key elements governing DO events
during the last glacial.
& 2012 Elsevier B.V. All rights reserved.
Keywords:
Dansgaard–Oeschger events
0
UK
37
TEXH
86
Atlantic meridional overturning circulation
Ocean relaxation oscillations
1. Introduction
Ice core studies have revealed that air temperatures over
Greenland varied between relatively cold and warm stages during
the last glacial period, the so-called Dansgaard–Oeschger (DO)
events (e.g., Dansgaard et al., 1984; NGRIP members, 2004). DO
events, characterized by rapid warming (interstadial) and more
gradual cooling (stadial) stages, are apparent in many highresolution Quaternary paleoclimatic records worldwide and are
particularly pronounced during MIS3 of the last glacial period
(e.g., Voelker et al., 2002). Although it is generally accepted that
density-driven variations in the Atlantic meridional overturning
circulation (AMOC) were responsible for this millennial-scale
climatic variability (e.g., Knutti et al., 2004; Rahmstorf, 2002), it
is still debated how these density changes were triggered.
n
Corresponding author. Tel.: þ31 222 369567; fax: þ31 222 319674.
E-mail addresses: [email protected] (J.-H. Kim),
[email protected] (O.E. Romero), [email protected] (G. Lohmann),
[email protected] (B. Donner), [email protected] (T. Laepple),
[email protected] (E. Haam), [email protected] (J.S. Sinninghe Damsté).
0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.epsl.2012.05.018
For instance, some authors suggested an internal ocean mechanism (so-called ocean relaxation oscillations) to explain the AMOC
variability (e.g., Munk and Wunsch, 1998; Rial and Yang, 2007;
Shaffer et al., 2004), while others proposed an external freshwater
forcing (e.g., Ganopolski and Rahmstorf, 2001; McManus et al.,
2004; Stocker et al., 1992). Atmospheric processes in low-latitude
regions, such as changes in the atmospheric transport of heat and
moisture to higher latitudes, may also have played a key role in
changes in the MOC variability and, thus, in the generation and
subsequent transfer of the DO signal (e.g., Cane and Clement,
1999; Schmidt et al., 2006). Hence, the key to elucidate forcing
mechanisms that govern millennial-scale AMOC variability during
the last glacial is the acquisition of high-resolution temperature
records in the tropics.
While evidence regarding the millennial-scale climatic variability in the low-latitude Northeast (NE) Atlantic is growing (e.g.,
Itambi et al., 2009; Mulitza et al., 2008; Tjallingii et al., 2008;
Romero et al., 2008; Niedermeyer et al., 2009; Zarriess et al.,
2011), the determination of phase relationship between Greenland ice core records and marine sediment records for DO-type
temperature variations still presents a challenge due to limited
96
J.-H. Kim et al. / Earth and Planetary Science Letters 339–340 (2012) 95–102
age constraints for the last glacial. This thus hampers the disentanglement of the physical mechanisms connecting changes in
tropical NE Atlantic temperature to AMOC variability on DO
timescales.
Here, we investigated a high resolution sediment core from the
tropical NE Atlantic and examined phase relationships between
low-latitude NE Atlantic temperatures and air temperatures over
Greenland. Our results revealed a possible link between the
tropical NE Atlantic temperatures and AMOC variability, and
potential mechanisms responsible for DO events during the last
glacial.
2. Material and methods
2.1. Geochemical analyses
Marine sediment core GeoB7926-2 (201130 N, 181270 W, 2500 m
water depth, 1328 cm core length, Fig. 1) was recovered in the
major upwelling region along the northwestern African continent
(Romero et al., 2008 and references therein) . Site GeoB7926-2 is
presently situated in a region that is almost year-around under
the influence of large upwelling filaments streaming offshore. The
0
d18O and UK37 records of up to 25 kyr BP (upper 820 cm of the
core) were previously published by Romero et al. (2008). We
extended these records until 48 kyr BP, analyzing the lower
section (508 cm) of the core.
Core GeoB7926-2 was sampled at 5 cm intervals for determination of the stable oxygen isotopes (d18O) record (n¼265). For
each analysis, twenty individual shells of the planktic foraminifer
Globigerina bulloides were picked from the 250–350 mm size
fraction. The d18O analysis was performed with a Finnigan MAT
251 mass spectrometer (Isotope Laboratory, MARUM, Germany)
as described previously (Romero et al., 2008). The analytical
standard deviation for d18O values is ca. 70.07% VPDB.
Core GeoB7926-2 was sampled at 1–10 cm intervals for
alkenone and glycerol dialkyl glycerol tetraethers (GDGT) analysis
(n¼262). The sample preparation and analytical procedures for
0
H
UK
37 (lower 508 cm of the core, i.e. 25–48 kyr BP) and TEX86 were
performed at NIOZ (The Netherlands) following previous studies
(Schouten et al., 2007; Kim et al., 2010; Lopes dos Santos et al.,
0
2010). Briefly, UK
37 values were determined by capillary gas
chromatography (Agilent Technologies 6890N) and TEXH
86 were
determined using high performance liquid chromatography/
atmospheric pressure positive ion chemical ionization-mass spec0
H
trometry (Agilent Technologies 1100 series). UK
37 and TEX86 values
were converted into temperature values using the global core-top
calibrations (Müller et al., 1998; Kim et al., 2010). The analytical
0
precision of these methods is ca. 0.3 1C for UK
37 and 0.2 1C for
H
TEX86.
2.2. Chronology
The age control for gravity core GeoB7926-2 is based on 16
Accelerator Mass Spectrometry (AMS) 14C dates determined on
the planktonic foraminifera Globigerina inflata ( 4150 mm fraction; Table 1) and the d18O record of the planktonic foraminifera
Globigerina bulloides, exhibiting a typical glacial–interglacial
pattern (Fig. 2). The 14C ages were newly converted into calendar
years using the CALIB REV6.0.0 program with the marine calibration dataset (MARINE09) (Stuiver and Reimer, 1993; Reimer et al.,
2009). Since no information on regional reservoir age is available
for this region, we used the mean ocean reservoir age (Bard,
1988). Regional reservoir ages (DR), however, might be larger
than the mean ocean reservoir age of 400 yr due to the upwelling
of older subsurface waters (deMenocal et al., 2000a). We
arbitrarily assumed that changes in radiocarbon reservoir age
have been similar throughout the last 50 kyr, but it might have
changed through time. For instance, the marine 14C reservoir age
in the Younger Dryas (YD) was nearly twice as large as the
modern value due to changes in deep ocean circulation and
related changes in ocean atmosphere radiocarbon partitioning
(Bard et al., 1994). In addition, large increases in the intensity of
Mauritanian upwelling would have sufficed to increase the DR
during the DO cold periods similar to Heinrich Stadial 1 (HS1) and
the YD with a minimum value of ca. 800–900 yr (deMenocal et al.,
2000a, 2000b; Staubwasser et al., 2002). Calendar ages between
dated levels were obtained by linear interpolation between the
nearest AMS dating points (Table 1).
2.3. Tuning and significance test
The millennial-scale comparison of the GeoB7926-2 records
with other climate records is complicated by the uncertainty of
the 14C derived chronology (see Section 2.2 and Supplementary
data Figs. S1 and S2). A classical solution for this problem is to rely
on wiggle-matching to visually identify a relationship between
data sets (Peterson et al., 2000). However, it is known that a visual
Fig. 1. (A) Map showing schematically the North Atlantic surface ocean circulation (black lines), surface air temperature pattern (red shading) over Greenland, and
subsurface temperature pattern (blue shading) off NW Africa during a strong AMOC period. GS, NAC, AC, PC, and LC denote Gulf Stream, North Atlantic Current, Azores
Current, Portuguese Current, and Labrador Current, respectively. (B) Detailed regional oceanographic setting with the location of site GeoB7926-2 off Mauritania. Arrows
represent present-day surface water circulation along the NW African continental margin. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
J.-H. Kim et al. / Earth and Planetary Science Letters 339–340 (2012) 95–102
97
Table 1
Age control points for core GeoB7926-2. Radiocarbon (14C) accelerator mass spectrometry (AMS) analyses were performed at the Kiel Leibniz Laboratory for Radiometric
Dating and Stable Isotope Research.
Lab no.
Depth in
core
(cm)
Uncorrected
AMS 14C ages
(yr BP)
Analytical
error (7 1s)
(yr)
Agesa
(DR¼ 0 yr) ( 72s))
(cal yr BP)
Ages
(cal yr BP)
Analyzed
material
Referenceb
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
KIA
8
48
98
173
248
293
378
418
483
508
553
688
803
868
1053
1203
1323
1170
4455
8970
10830
11020
10050
12220
13050
13720
14290
14670
16600
20450
23650
29600
36440
45410
35
35
45
70
70
70
70
70
100
60
80
130
160
220–230
360
1400
360
649–787
4519–4779
9515–9798
12054–12560
12357–12677
10772–11190
13450–13824
14487–15247
15568–16813
16777–17166
16990–17686
18903–19571
23484–24388
27562–28622
32949–34654
37740–43136
46857–49503
718.0
4649.0
9656.5
12307.0
12517.0
10981.0
13637.0
14867.0
16190.5
16971.5
17338.0
19237.0
23936.0
28092.0
33801.5
40438.0
48180.0
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
G.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
Romero et al.
This study
This study
This study
24287
25812
25810
24286
22417
27311c
24285
29030
27310
29029
22416
27309
29028
24283
22415
27308
24282
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
inflata
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
(2008)
Note that BP indicates Before Present (0 cal yr BP¼ AD 1950).
a
b
c
Uncorrected 14C ages were converted to calendar ages using the CALIB REV6.0.0 program (Stuiver and Reimer, 1993).
The original 14C data from Romero et al. (2008) were re-converted using the new calibration dataset (MARINE09) of Reimer et al. (2009).
Not used data for the age model.
Fig. 2. (A) d18O of NGRIP (NGRIP Members, 2004) with the Greenland Ice Core Chronology 2005 (GICC05) time scale (Svensson et al., 2008). Note that 50 yr were
subtracted from the original GICC05 time scale (before year AD 2000) in order to be comparable with the 14C calibrated time scale (i.e. before year AD 1950). (B) d18O of
0
H
Globigerina bulloides from core GeoB7926-2. (C) Temperature records of core GeoB7926-2: UK
37 (red line) and TEX86 using the calibration model based on the satellite SSTs
(Eq. 1, Kim et al., 2010, black line) and the 0–200 m core top calibration (Eq. 2 in Fig. 4B, this study, blue line). Light blue bars indicate the approximate occurrence of
Heinrich Stadials. Numbers and filled triangles indicate DO interstadials and the 14C dating points, respectively. YD denotes the Younger Dryas and HS Heinrich Stadials.
Timing and duration of HSs were adopted from Sánchez Goñi and Harrison (2010) (For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.).
correlation can often be misleading, especially when comparing
time series with similar frequency spectra (Wunsch, 2006;
Blaauw, 2012). For an objective view on the relation of
GeoB7926-2 climatic records with the North Greenland Ice Core
Project (NGRIP) d18O record (NGRIP Members, 2004), we made
use of a modified version of the Maximum Covariance between
Time Uncertain Series Test (MCTEST, Haam and Huybers, 2010).
This test was designed to estimate the significance of the
covariance between time uncertain paleoclimatic records. The
0
method, applied to our study, consists of (1) tuning the UK
37,
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J.-H. Kim et al. / Earth and Planetary Science Letters 339–340 (2012) 95–102
0
H
K
TEXH
86, and DT (temperature difference between TEX86 and U37)
time series of GeoB7926-2 to the NGRIP record to maximize the
correlation and (2) assessing the significance of this ‘‘tuned
correlation’’ by performing the same process on a large number
of surrogate time series. The maximum correlation obtained,
using the surrogate time series, provides the distribution of the
null-hypothesis that can be compared with the maximum correlation obtained with the GeoB7926-2 records. To implement this
process, the NGRIP and GeoB7916-2 time series were re-sampled
in 100 yr steps and detrended. We used a high frequency pass
filter to remove the long-term trend by applying the butterworth
algorithm from the MATLAB software package with the cut off
frequency of 1/2500 yr. In the tuning process we allowed for a
chronology uncertainty of 300 yr (one standard deviation) for
each data point. This uncertainty assumption accounted for the
variable reservoir age, which was likely higher than the modern
estimate of 400 yr, and for the 14C dating uncertainty (see Section
2.2). The results were insensitive to this choice; similar results
were obtained using different age uncertainty assumptions.
2.4. Model simulations
The GCM is based on the Max-Planck Institute ocean model
(MPIOM; Marsland et al., 2003) and a one-dimensional energy
balance model of the atmosphere similar to an earlier version
(Lohmann et al., 1996). The ocean model configuration has 40
vertical levels with 20 in the upper 600 m. The horizontal
resolution gradually varies between 12 km close to Greenland
and 150 km in the tropical Pacific. The mean climate background
conditions of monthly near-surface temperature, wind stress, and
the hydrological cycle are taken from ECHAM3 atmosphere
simulations for present and glacial setups (Lohmann and Lorenz,
2000). A simple runoff-bucket model over land surfaces closes the
hydrological cycle. The simple energy balance model allows a
temperature feedback (Rahmstorf and Willebrand, 1995;
Lohmann et al., 1996). The atmospheric simulations were driven
by reconstructed sea surface temperature (SST) and sea ice (see
Sarnthein et al., 2003 and references therein) gridded on the
ECHAM3 T42 grid (approx. 2.8 horizontal resolution; Lohmann
and Lorenz, 2000). The model was run over 1500 yr after a spin up
of 500 yr of model integration. The model shows different
behaviors for the mean and variability under glacial and interglacial conditions.
tropical NE Atlantic under glacial conditions. It has been demon0
strated that the UK
37 ratio off Northwest (NW) Africa represents
principally the annual average of the mixed-layer temperature,
i.e. SST (Müller and Fischer, 2001). In a global perspective the
TEXH
86 proxy is considered to reflect annual mean SSTs as well
0
H
(Kim et al., 2010) and, in principle, UK
37 and TEX86 records should
reflect the same thermal history. However, especially during
K0
MIS3, TEXH
86 temperatures were lower than U37 temperatures.
This may have been caused by different seasons for production of
alkenones and GDGTs, long-distance transport of alkenones or the
varying depth of production of Thaumarchaeota, which produce
the GDGTs used for TEXH
86.
A recent study based in the area of our core site revealed that
the seasonal variation alone cannot fully explain the reconH
0
structed SST difference using the UK
37 and TEX86 proxies in the
tropical NE Atlantic (Lopes dos Santos et al., 2010). This is
consistent with our records that show differences in reconstructed temperatures of up to 6 1C, which is substantially larger
than the present-day seasonal variation in SST (4 1C, Fig. 3).
Therefore, it seems unlikely that a shift in the season of GDGT
production would solely account for stronger cooling of TEXH
86
0
than UK
37 during MIS3.
Several earlier studies (e.g., Benthien and Müller, 2000;
0
Ohkouchi et al., 2002) have shown that UK
37 SST records may be
affected by laterally advected allochtonous input of alkenones.
0
However, UK
37 -reconstructed SST values in the surface sediments
in the tropical NE Atlantic fit well to annual mean SST (Lopes dos
0
Santos et al., 2010). UK
37 reconstructed temperature from the
shallowest sediment of our core (2–4 cm core depth, 0.23 ka BP)
is 20.8 1C, which is within the calibration error identical to the
present-day annual mean SST of 20.9 1C (Levitus and Boyer,
1994), in good agreement with previous studies (Müller and
Fischer, 2001; Kim et al., 2010). This argues against a substantial
0
effect of lateral transport on alkenone UK
37 values and, thus,
K0
cannot explain the large difference in TEXH
86- and U37-reconstructed temperatures during MIS3.
Thaumarchaeota occur throughout the water column (Karner
et al., 2001) and thus TEXH
86 may potentially record the temperature of a deeper water layer than that of the mixed layer (cf. Kim
et al., 2008). A water column study performed in the Benguela
3. Results and discussion
3.1. Sub-millennial scale temperature variations in the tropical NE
Atlantic
To estimate oceanic temperature variations, we applied two
0
organic paleothermometers: the UK
37 index based on the degree of
unsaturation of C37 alkenones (Brassell et al., 1986; Prahl and
Wakeham, 1987) and the TEXH
86 based on archaeal GDGT lipids
(Kim et al., 2010), a modified version of TEX86 (Schouten et al.,
0
2002). UK
37 temperature estimates varied between 18 and 22 1C
over the last 50 kyr (Fig. 2C, red line). The most prominent cooling
occurred during the YD, HS1, and HS3, with less distinct cooling
phases during HS2 and HS4. The TEXH
86 record (Fig. 2C, black line,
see also Supplementary data Fig. S3) revealed a wider range of
temperature variation, from 16 to 23 1C, and the most striking
feature is the distinct, abrupt warming and cooling intervals
0
during MIS3, which are not observed in the UK
37 record.
0
The different temperature amplitudes observed in our UK
37 and
H
K0
TEX86 records during MIS3 (Fig. 2C) indicate that the U37 and
TEXH
86 proxies must reflect different regimes of temperature in the
Fig. 3. Annual mean (T-AM) and seasonal temperature profiles for winter (T-Win)
and summer (T-Sum) of the upper 200 m of the water column (Levitus and Boyer,
0
1994) at the core site. Also indicated are the temperatures for UK
37 (20.8 1C, red bar)
and TEXH
86 using the 0–200 m core top calibration (Eq. 2 in Fig. 4B, 16.91C, blue bar)
at 3 cm core depth. The depth-integrated annual mean temperature values are
20.7 1C for the surface mixed-layer (0–20 m water depth) and 17.1 1C for 0–200 m
water depth (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.).
J.-H. Kim et al. / Earth and Planetary Science Letters 339–340 (2012) 95–102
upwelling system in the SE Atlantic (Lee et al., 2008) indeed
showed that TEXH
86 reconstructed temperatures were lower than
0
those obtained by the UK
37 and more representative of subsurface
(thermocline) water depths rather than the surface mixed-layer.
Huguet et al. (2007) also observed that TEXH
86 temperature
estimates in sediment traps and a sediment core in the Santa
Barbara basin reflected subsurface temperatures (100–150 m)
rather than SSTs. These studies thus indicate that TEXH
86, on a
regional scale, may reflect different water depth temperatures
(surface or subsurface) due to the varying depth of Thaumarchaeota production in the different oceanic provinces. Importantly,
Lopes dos Santos et al. (2010) have shown that in our study area,
the TEXH
86 record covering the last 200 kyr reflected subsurface
0
temperatures (below the mixed-layer), whilst the UK
37 record
represented surface mixed-layer temperatures. The present-day
vertical temperature profile in the study area shows a 20 m thick
surface mixed layer with an average annual mean temperature of
20.7 1C (Levitus and Boyer, 1994, Fig. 3). The temperature difference between the surface mixed layer (0–20 m) and the euphotic
zone (0–200 m) is 4 1C.
Following the interpretation of the TEXH
86 record as a more
subsurface signal (Lopes dos Santos et al., 2010), we established a
new global calibration of TEXH
86 with depth-integrated annual
mean temperatures from 0 to 200 m water depth (Fig. 4B). The
residual standard error is 72.2 1C. The reconstructed TEXH
86
temperature using Eq. 2 from the shallowest sample at 3 cm core
depth derived was 16.9 1C (Fig. 2C, blue line), which is in good
agreement with the depth-integrated (0–200 m) annual mean
temperature value of 17.1 1C (Fig. 3, Levitus and Boyer, 1994). In
this way, a reconstruction of subsurface temperature was made
(Fig. 2C, blue line), which allowed to reconstruct past differences
0
H
in temperature (DT) between UK
37-derived (0–20 m) and TEX86derived (0–200 m) temperatures (Fig. 5A). Strikingly, DT during
MIS3 exceeded the present-day depth-derived temperature difference of up to 4 1C (Fig. 5A), suggesting that temperature
differences between the mixed-layer and the thermocline were
larger at that time. To the best of our knowledge, such large
temperature shifts of up to 8 1C have not previously been
observed in other tropical NE Atlantic records.
3.2. Phase relationship for DO events between Greenland and the
tropical NE Atlantic
Interestingly, the pattern of temperature differences between
surface and subsurface water masses (DT) during the last glacial
period strongly resembles that of DO events documented in the
Greenland NGRIP record (Fig. 5A). No direct relationship between
DT and surface air temperatures over Greenland was detected
(R¼0.0, p 40.1), based on the 14C chronology of our core and the
established chronology of the NGRIP record. However, a substantial degree of uncertainty in our 14C derived chronology due to
high analytical errors and variable regional reservoir ages in the
time period of 30–50 kyr BP (see Table 1) complicates this direct
comparison of potential DO events in both records.
For an alternative method to explore the relationship between
DT and NGRIP d18O records, we used a modified version of the
MCTEST (Haam and Huybers, 2010). We tuned the chronology of
GeoB7926-2 by maximizing the correlation between DT and d18O
records for MIS3. As the uncertainty in 14C age along the NW
African upwelling system has a similar magnitude as the millennial-scale climatic signals, we tested both positive and negative
correlations. The tuned chronology always remained within the
uncertainty of the raw chronology determined by the 14C measurements and regional reservoir age uncertainties (Supplementary
data Fig. S4). Increased maximum covariance and significance level
(0.77, p¼0.12) were obtained with the assumption of a positive
99
Fig. 4. Comparison of TEXH
86 sediment core-top calibration models. (A) Cross plots
of satellite SST with TEXH
86 values for the global calibration set (Eq. 1) of Kim et al.
(2010)). (B) Cross plots of depth-integrated annual mean WOA09 temperature for
0–200 m water depth (Locarnini et al., 2010) with TEXH
86 values (Eq. 2).
relationship (Fig. 5B), whereas the assumption of a negative
relationship revealed weaker maximum covariance and significance level (0.63, p¼0.41, Supplementary data Fig. S5). Hence, it is
more probable that DT and air temperatures over Greenland varied
0
in-phase given the dating uncertainty. The comparisons of UK
37 and
H
TEX86 records with the NGRIP time series demonstrate that a
negative relationship is more likely than a positive one. Taken
together, when air temperatures over Greenland were relatively
high, subsurface temperatures in the tropical NE Atlantic, which
were probably associated with millennial changes in AMOC intensity, were relatively low and vice versa.
3.3. Potential underlying mechanism for DO events
To assess underlying mechanisms for subsurface temperature
fluctuations that occurred at site GeoB7926-2, we performed GCM
simulations. Surface temperatures and velocities of both experiments showed characteristic patterns; a northeastward structure
in the present and a zonal structure in the glacial run (Fig. 6A and
B). In our GCM simulations, for the present day situation, a quasisteady state was observed whereas for the glacial climate setup,
the NADW transport was characterized by self-sustained oscillations. In the upper 150 m, these states quite substantially differed
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J.-H. Kim et al. / Earth and Planetary Science Letters 339–340 (2012) 95–102
0
Fig. 5. (A) Detailed comparison of d18O of NGRIP (NGRIP Members, 2004), UK
37,
TEXH
86, and DT records for MIS3. DT is the temperature anomaly calculated as the
0
H
difference between UK
37 and TEX86 (0–200 m) temperatures and filled green parts
indicate anomalies larger than the value of 4 1C. Dashed black lines stand for
corresponding timing of DO interstadials in the DT record under the assumption of
a positive relationship between the DT and NGRIP records. (B) The DT time series
(red line) and the NGRIP record (black line) on the tuned chronology under the
assumption of a positive relationship of DT and d18O of NGRIP. Comparisons of the
0
H
UK
37 and TEX86 records (red lines) with the NGRIP record on a tuned chronology for
GeoB7926-2 were also performed under the assumption of a negative relationship
0
18
H
between UK
37 (or TEX86) and d O of NGRIP. Numbers and HS stand for DO
interstadials and Heinrich Stadials (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.).
(Fig. 6C). The simulations showed oscillations with a period of
416 yr (Fig. 7A). The amplitudes in the maximal overturning in
the North Atlantic varied from 12 Sv for model stadials to
25–60 Sv for model interstadials (Fig. 7A). The model results
revealed that subsurface warming occurred off NW Africa, while
the northern North Atlantic experienced NADW convection weakening. Subsequently, an abrupt onset of subsurface cooling off
NW Africa accompanied an intensification of NADW transport.
Our model simulation, without external freshwater forcing, can
be interpreted as a template of DO features: pronounced subsurface temperature fluctuations due to variations in NADW and
Fig. 6. (A) Simulated temperature and velocity in the upper 150 m with present day
boundary conditions. (B) Simulated glacial temperature and velocity in the upper
150 m for phases when the overturning is in a weak mode (model stadial).
(C) Anomalous glacial velocity and temperature for the upper 150 m between
maximum (model interstadials) and minimum (model stadials) overturning phases.
thus AMOC. A direct comparison is, however, not possible since
the time scale of the modeled oscillation is 416 yr (instead of
1470 yr). Interestingly, we find that surface cooling during DO
interstadials was less pronounced than the subsurface cooling
J.-H. Kim et al. / Earth and Planetary Science Letters 339–340 (2012) 95–102
101
4. Conclusions
Our high-resolution TEXH
86 proxy record reveals pronounced
DO-type events in subsurface ocean water temperatures in the
tropical NE Atlantic during the last glacial period, with an antiphase relationship with Greenland air temperatures. This is
consistent with earlier findings that subsurface temperatures do
indeed provide a distinct fingerprint of AMOC variability in the
tropical NE Atlantic (Lohmann et al., 2008; Zhang, 2007). Consequently, this study shows that the TEXH
86 temperature proxy can
depict abrupt and large-amplitude variations in subsurface water
temperature that have not been previously documented, and may
be useful in determining possible mechanisms for the pronounced
DO-type climate variability. Our study provides new insights into
linkages between paleoclimatic data and climate models, and
thus may contribute to reconciling different views between
paleoclimatic data and climate model communities to better
understand the underlying mechanisms for DO events.
Acknowledgments
Fig. 7. (A) NADW transport in the Atlantic (black) and averaged subsurface
temperature (30–80 m) at the position 201N, 181W (red). (B) Temperature
evolution over 1500 yr at this site for different water depths indicating a stronger
response in subsurface water compared to the surface (black). During a weak
NADW convection, the AMOC weakens and shoals as a halocline develops.
Consequently, the ocean interior slowly warms by a downward diffusion of heat
at low latitudes. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.).
(30–80 m water depth) in both model simulations and proxy
records (Figs. 5A and 7B).
The anti-phase behavior between high latitude surface water
temperatures and deeper water layers is a common feature of
AMOC-related variability (Rühlemann et al., 2004; Winton, 1993)
and also appears in our simulation (Fig. 7, see also Supplementary
data Fig. S6). The self-sustained oscillation is inherently linked to
the vertical stratification in the ocean: a reduction in NADW
formation provides higher subsurface temperatures until the
vertical density gradient becomes unstable. As ocean stratification
becomes unstable, a pronounced halocline is removed and salty
and warm water is brought to the surface, providing a strong heat
release to the atmosphere. An abrupt resumption of NADW
formation (Fig. 7) leads to a pronounced northward oceanic heat
transport until a high latitude halocline redevelops. The AMOC
gradually weakens as warmer water from the low latitude thermocline moves poleward and is replaced by cooler water from below
the halocline and a decrease in poleward salt transport during the
phase of weak AMOC results in a gradual halocline build-up and,
finally, shutdown of NADW convection. The oscillator mechanism
for self-sustained oscillations has previously been described in
two-dimensional (2D) models (Wang and Mysak, 2006; Winton,
1993) and here we show evidence that such spatio-temporal
features can be traced in a full 3D ocean setup. Furthermore, the
model shows a strong heterogenic pattern. Interstadials and strong
AMOC correspond to large subsurface cooling off tropical NW
Africa, whereas they are in-phase with surface temperatures off
Portugal (Fig. 6). The different periodicity of the model and proxy
data can have multiple factors. It is likely that the model’s response
does depend on model parameterizations and parameters like
vertical mixing rates affecting the time scale of the halocline. A
detailed study of the sensitivity of the model periodicity is,
however, beyond the scope of the present paper.
We thank S. Schouten, I.S. Castañeda, and L. Handley for
helpful discussions and H. Buschof, M. Klann, B. Meyer, and
M. Segl at MARUM and J. Ossebaar. M. Kienhuis, and E. Hopmans
at NIOZ for analytical supports. M. Schröder is acknowledged for
his help with the chronology tuning. We thank H. Haak and P.
Scholz for support in setting up the model. The research leading
to these results has received funding from the European Research
Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. [226600].
O.E.R. was partially supported by the Spanish Council of Scientific
Research. E.H. was supported by the National Science Foundation
under grant DMS 0940342. Two anonymous reviewers are
thanked for their insightful comments.
Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.epsl.2012.05.018.
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