Rapid oceanic and atmospheric changes during the

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Rapid oceanic and atmospheric changes during the
LETTERS
PUBLISHED ONLINE: 15 FEBRUARY 2009 DOI: 10.1038/NGEO439
Rapid oceanic and atmospheric changes during
the Younger Dryas cold period
Jostein Bakke1,2 *, Øyvind Lie2 , Einar Heegaard2,3 , Trond Dokken2 , Gerald H. Haug4,5 ,
Hilary H. Birks2,3 , Peter Dulski6 and Trygve Nilsen7
The Younger Dryas event, which began approximately
12,900 years ago, was a period of rapid cooling in the
Northern Hemisphere, driven by large-scale reorganizations
of patterns of atmospheric and oceanic circulation1–3 . Environmental changes during this period have been documented by
both proxy-based reconstructions3 and model simulations4 ,
but there is currently no consensus on the exact mechanisms
of onset, stabilization or termination of the Younger Dryas5–8 .
Here we present high-resolution records from two sediment
cores obtained from Lake Kråkenes in western Norway and the
Nordic seas. Multiple proxies from Lake Kråkenes are indicative
of rapid alternations between glacial growth and melting
during the later Younger Dryas. Meanwhile, reconstructed sea
surface temperature and salinity from the Nordic seas show
an alternation between sea-ice cover and the influx of warm,
salty North Atlantic waters. We suggest that the influx of warm
water enabled the westerly wind systems to drift northward,
closer to their present-day positions. The winds thus brought
relatively warm maritime air to Northern Europe, resulting in
rising temperatures and the melting of glaciers. Subsequent
input of this fresh meltwater into the ocean spurred the
formation of sea ice, which forced the westerly winds back to
the south, cooling Northern Europe. We conclude that rapid
alternations between these two states immediately preceded
the termination of the Younger Dryas and the permanent
transition to an interglacial state.
The impact of freshwater pulses released from Northern
Hemisphere ice sheets on the North Atlantic Meridional Overturning Circulation has been suggested as a likely mechanism for
the Younger Dryas cooling7–11 . In addition, a responsive sea-ice
cover has been proposed as a mechanism that could rapidly alter
the heat exchange between the ocean and atmosphere during the
last glacial period12,13 . The roles of the ocean and the atmosphere
in triggering the climate shifts around the Younger Dryas are
much discussed9 . These hypotheses are difficult to resolve, partly
because the temporal resolution of most palaeoclimate archives is
too coarse to enable the anatomy of the Younger Dryas stadial to be
adequately described14 . To address these questions we here present
a lake-sediment record at sub-annual resolution, together with
new data on the sea-surface conditions of the Norwegian Atlantic
Current (NwAC) throughout the Younger Dryas stadial (Fig. 1).
Lake Kråkenes, (62◦ 020 N,5◦ 00 E) located on the northwestern
side of Norway, contains sediments that have for decades been
recognized as a precise archive of the climate of the Younger
Dryas in Scandinavia14–17 (Fig. 2). Lake Kråkenes received glacial
meltwater from a cirque glacier in its catchment that formed
during the early Younger Dryas and melted rapidly during the
transition into the Holocene epoch16 . The northwest-facing cirque
that contained the glacier has two outer marginal moraines
and indications of a third16,17 . The reconstructed glacier had an
equilibrium-line altitude varying from 150 to 200 m a.s.l.17 It has
been calculated that snow accumulation in the cirque required
to maintain the glacier was at least three times higher than the
regional precipitation and was provided by leeward (wind-blown)
accumulation throughout the Younger Dryas17 .
A 1.4 m sediment core from Lake Kråkenes (see Supplementary
Fig. S1) spans the Allerød/Younger Dryas/Holocene transitions.
The age–depth relationship is anchored on three ‘event’ points
in the North Greenland Ice Core Project (NGRIP) chronology:
the transitions into (12.896 kyr; kyr = kilo yr bp b2k) and
out of the Younger Dryas (11.703 kyr) and the Vedde tephra
horizon (12.171 kyr) (ref. 18). The Younger Dryas boundaries are
visible in the lithostratigraphy. They were more precisely defined
independently by statistical optimization of the position of ‘knots’
in piecewise regressions of the X-ray fluorescence (XRF) data (see
Supplementary Information). The 96 radiocarbon dates previously
available from the lake sediments were recalibrated19 and an
age–depth relationship was established (details in Supplementary
Information). Geochemical analyses were carried out with the
micro XRF core scanner at GFZ, Potsdam. Eleven geochemical
elements (at every 60 µm) and four physical sediment parameters
(every 0.5 cm) were measured (see Supplementary Information).
Changes in sediment transfer rates are related to the mass
turnover gradient at the glacier, which is driven by accumulation
rates in winter and ablation-season temperatures20 . Dry bulk density (DBD; g cm−3 ) in lake sediments has been used as a measure
of glacial activity in glaciated catchments21 . Younger Dryas glacier
activity is reflected in the DBD of the glacio-lacustrine sediments at
Kråkenes (Fig. 1b). Other sediment parameters—grain-size distribution, loss-on-ignition and water content—also show immediate
responses to the presence or absence of the glacier (see Supplementary Fig. S7). As another proxy for glacier mass turnover, driven
by changes in grain size and DBD, we used the count rate of the
redox-insensitive element titanium (Ti) in the sediments. The main
source for Ti accumulation into the lake is glacial erosion of the
Precambrian bedrock (Fig. 1a). The Ti and DBD curves indicate a
relatively stable glacier during the first part of the Younger Dryas.
After 12.15 kyr, there is a marked increase in the Ti signal strength
1 Department of Geography, University of Bergen, Fosswinckelsgt 6, N-5020 Bergen, Norway, 2 Bjerknes Centre for Climate Research, Allégaten 55, N-5007
Bergen, Norway, 3 Department of Biology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway, 4 Geological Institute, Department of Earth Sciences,
ETH Zürich, CH-8092 Zürich, Switzerland, 5 DFG Leibniz Center for Earth Surface Process and Climate Studies, Institute for Geosciences, Potsdam
University, Potsdam D-14476, Germany, 6 Section 3.3., GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany, 7 Department of
Mathematics, University of Bergen, Johannes Brunsgate 12, N-5008 Bergen, Norway. *e-mail: [email protected]
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NATURE GEOSCIENCE DOI: 10.1038/NGEO439
LETTERS
Holocene/Younger
Dryas transition
11500
11700
11900
12100
Yr BP (b2k)
12300
12500
12700
Younger Dryas/
AllerØd transition
12900
13100
a
Ti¬X count rate
40
80
Lake Kråkenes
120
b
0
0.5
160
c
1.5
MD 99-2284
Vedde ash
Sea surface temperature (°C)
10
8
6
2.0
2.5
4
Meerfelder Maar
2
20
100
GISP2
200
e
0
40
Na GISP2 (p.p.b.)
Ca GISP2 (p.p.b.)
0
Varve thickness (mm)
1
GISP2
d
0
2
f
DBD (g cm¬3)
1.0
60
11500
11700
11900
12100
12300
12500
12700
12900
13100
Yr BP (b2k)
Figure 1 | Four sites around the North Atlantic with different proxies from the Younger Dryas time interval at high resolution. a, Ti count rate in Lake
Kråkenes (grey: 3 point running mean, red: 212 point running mean). b, DBD as a measure of glacigenic sediments released into Lake Kråkenes. c, SST from
core MD99-2284 located in the Faeroe–Shetland passage. d, Varve thickness from Meerfelder Maar in western Germany28 . e, Na concentration (blue) in
the GISP2 ice core, age scale adjusted according to NGRIP years18 . f, Ca concentration (red) in the GISP2 ice core, age scale adjusted as in e.
and numerous annual–decadal scale peaks in Ti counts. These are
interpreted as peaks in meltwater production from the glacier,
and give the appearance of ‘flickering’ (Fig. 1a). The pattern of a
stable early Younger Dryas and an unstable late Younger Dryas
is also seen in grain-size variability and DBD, but we have relied
mainly on the higher temporal resolution of the Ti count rates
as a proxy for mass turnover rates of the Kråkenes glacier (see
Supplementary Information).
To examine the composite geochemical record in the
Lake Kråkenes Younger Dryas sediment, with explicit focus on the
flickering, we have applied a local linear mixed model to eleven
geochemical elements (Al, Ca, Fe, invr-Rh(L-line), Rh(K-line),
2
K, log S, Mn, Si, Ti, V; ref. 22; see Supplementary Information).
The structure of variability in the XRF data through the Younger
Dryas is represented in Fig. 2. It shows an abrupt increase in
heterogeneity at 12.15 kyr.
To compare terrestrial and marine processes, we present new
data (Fig. 1c) from the marine core MD99-2284 (62◦ 220 4800 N,
0◦ 580 8100 W) from the middle of the modern NwAC in the
Faeroe–Shetland channel (Fig. 2). Its age–depth model is
well constrained and the rapid sedimentation rate enables
high-resolution sampling (see Supplementary Information). The
sea surface temperature (SST) is reconstructed from the planktonic
foraminiferal assemblages (Fig. 1c). A striking feature is the
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NATURE GEOSCIENCE DOI: 10.1038/NGEO439
Greenland
GISP2
60° N
50° N
Sea ice
Westerlies
LETTERS
Greenland
Easterlies
Norway
Kråkenes
Iceland
MD99-2284
GISP2
60° N
UK
Sea ice
Easterlies
Iceland
MD99-2284
UK
50° N Westerlies
Meerfelder Maar
Atlantic water
reach surface
Norway
Kråkenes
Meerfelder Maar
Arctic halocline
'Flickering'
Stable period
4
3
Ti count rate
40
2
1
80
XRF count-data heterogeneity
Warm salty
Atlantic water
Warm salty
Atlantic water
120
160
11600
11700
11800
11900
12000
12100
12200
12300 12400
Yr BP (b2k)
12500
12600
12700
12800
12900 13000
Figure 2 | Diagram illustrating the atmospheric and oceanic conditions during the Younger Dryas. The early Younger Dryas was kept in stadial
conditions by a prevailing halocline in the North Atlantic that resulted in a near-permanent sea-ice cover. These boundary climatic and oceanic conditions
over the high latitudes of Europe, Greenland and the North Atlantic displaced the westerly wind track southwards. The last 400 years were characterized
by a regime that repeatedly ‘flickered’ towards interstadial conditions, controlled by a highly dynamic sea-ice cover in the North Atlantic as the halocline
was periodically absent.
high-amplitude oscillations from 3 ◦ C to 6–9 ◦ C after the Vedde
ash horizon. The first, largest, SST peak occurs about 100 yrs
after the shift in Ti count rate at 12.15 kyr at Kråkenes. However,
uncertainty in the MD99-2284 age model prevents interpretation
of any lead–lag relationships. Associated with the large variability
in SST, we observe large variability in sea-surface δ 18 O, calculated from δ 18 O measurements on the planktonic foraminifer
Neogloboquadrina pachyderma sinistral, which may correspond to
periodic oscillations in salinity of over two practical salinity units
(PSU; see Supplementary Information). The δ 18 O and SST records
indicate the presence of cold, low-salinity polar water masses
until about 12.15 kyr ago coinciding with the low-amplitude and
low-frequency period at Kråkenes. The subsequent SST increase
with strong fluctuations coincides temporally with the intensification in frequency and amplitude of the glacier meltwater signal in
Kråkenes (Fig. 1a,c). The decadal to centennial periods of ocean
warming and increased salinity imply that the polar water residing
in the North Atlantic was periodically replaced with sub-polar
water masses after 12.15 kyr. The relationship between sea-ice
extent and a low-salinity ocean-surface layer (halocline) is well
established in the modern ocean23,24 . Moreover, sea-ice expansion
is a leading hypothesis for the large temperature anomalies around
the Younger Dryas12 . A strong salinity reduction is recognized at the
Allerød/Younger Dryas transition in the inflow area to the Nordic
seas25 , reflected in MD99-2284 as a shift of >1 PSU, as represented
in the δ 18 O SMOW (Standard Mean Ocean Water) record (see
Supplementary Information), that lasted about 800 years. This
freshening would have caused stratification of the upper ocean,
increasing the sea-ice extent. When the NwAC started to penetrate
northward (seen in proxy data as far north as 69◦ N; ref. 25)
during late Younger Dryas, seasonal sea-ice cover was reduced
enabling a rapid release of heat11 . A biotic response at Kråkenes
is demonstrated by an increase of about 1 ◦ C after 12.15 kyr in
reconstructed mean July temperatures from the pollen record
(see Supplementary Information).
To examine the atmospheric circulation mode throughout the
Younger Dryas, we compared our Ti record with data on Ca
and Na concentration from the GISP2 ice core26,27 (Fig. 1e,f) and
with varve thickness from a lake, Meerfelder Maar, in western
Germany2,28 (Figs 1d and 2). Calcium concentration in GISP2 is
mainly derived from circulation over land surfaces, whereas Na is
suggested to reflect changing transport efficiency26 (windiness). The
annual varves at Meerfelder Maar are thicker at the onset of the
Younger Dryas as a consequence of stronger winds transporting
more dust into the lake2,28 . This change can be explained as a
rapid southerly displacement of the storm tracks, driven by an
expansion of sea ice in the Nordic seas that placed Meerfelder Maar
in the centre of action of the atmospheric flow2 during the early
Younger Dryas (Figs 1d and 2). The reduced variability in varve
thickness during the later Younger Dryas can be attributed to an
apparent weakening of the westerlies over central Europe2 . We
notice the opposite patterns of variability between Meerfelder Maar
and Kråkenes, which suggests a fluctuating overall movement of
the atmospheric front systems towards Scandinavia during the late
Younger Dryas (see Supplementary Information). The transition
coincides with the intrusion of warmer and saltier waters into the
North Atlantic region (Fig. 1)25 . A similar shift in the mid-Younger
Dryas is also evident in the dust record and the snow accumulation
data from the GISP2 core1,26,29 as indicated by the Ca and Na records
(Fig. 1e,f and Supplementary Fig. S9).
The mid-Younger Dryas shift (around 12.15 kyr) illustrated
in Fig. 1 indicates that the freshwater-imposed cold reversal was
losing strength during the later part of the Younger Dryas, causing
periodic sea-ice break-up and enabling the zonal jet stream over
the North Atlantic region to move north. This effect has previously
been noticed in marine proxy data25 and model experiments1,8 .
The experiments show that, on suppression of freshwater discharge,
the modelled Atlantic Meridional Overturning Circulation exhibits
complex transient behaviour, with abrupt decadal oscillations of
SST followed by a gradual recovery towards the initial state4 . Model
experiments and also palaeodata suggest that the Younger Dryas
climate over northwest Europe could be divided into a first cold,
humid part followed by a less cold, drier period3 . From our palaeorecords, we propose the following mechanism of hemispheric
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NATURE GEOSCIENCE DOI: 10.1038/NGEO439
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change during the Younger Dryas, illustrated in Fig. 2. After
12.15 kyr, the imposed freshwater lid and sea-ice cover periodically
broke up as sub-polar North Atlantic water started to push
northwards, resulting in salinity increases of >2 PSU. The reduction
of sea ice released the westerly wind jet from its southern glacial
pattern30 , enabling it to move north into the eastern North Atlantic
with subsequent increased melting of marine and terrestrial ice. The
consequent increased flux of fresh meltwater to the ocean resulted
in the formation of more extensive sea ice that pushed the jet
south once more, thus re-establishing the stadial state. On the basis
of the interpretation of the high-frequency variability in Ti count
rates from Kråkenes and the above-mentioned complementary
archives, this happened repeatedly at decadal–centennial intervals.
Thus, we see the variability in the late Younger Dryas as rapid
oscillations between a persistent stadial state with North Atlantic
ice cover and brief incursions of the NwAC until the system finally
switched to the interglacial state at the onset of the Holocene.
The rapid ocean–atmosphere interactions during the late Younger
Dryas enable speculations about the reason for the observed abrupt
Younger Dryas/Holocene transition. The regime shift may have
been triggered ultimately by increased solar insolation reinforcing
the NwAC heat input that enabled a stable interstadial regime to
resist the meltwater feedback from the ice sheets occupying the
circum-Atlantic continents. We suggest that emphasis should be
put on retrieving more ultrahigh-resolution archives of stadials to
further study the behaviour of the climate system leading up to the
final transitions into interstadials and interglacials.
Received 3 December 2008; accepted 26 January 2009;
published online 15 February 2009
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Acknowledgements
Thanks to A. Nesje and S. O. Dahl for help with the fieldwork at Kråkenes. Thanks to
B. Kvisvik for help in the laboratory and to Ø. Paasche for discussions. This is a
contribution to X-LAKE and ARCTREC financially supported by the Norwegian
Research Council (NCR). The marine work has been financially supported by the Rapid
projects VAMOC and ORMEN, supported by the NCR, project number 169931 and
169932/S30. Thanks also to the IMAGES program and R/V Marion Dufresne for making
the marine core available for our work. This is publication number
A216 from the Bjerknes Centre for Climate Research.
Author contributions
J.B. and Ø.L. managed the sediment analyses of lake Kråkenes and developed the original
conceptual hypothesis. E.H. and T.N. built the statistical toolkits and carried out all
statistical analyses. T.D. contributed material, interpretation and analyses from
MD99-2284 and developed conceptual ideas. G.H. provided access to GFZ Potsdam’s
XRF laboratory and contributed to interpretation and building conceptual hypotheses.
H.H.B. provided data from Kråkenes. P.D. managed the XRF laboratory and
measurements. All authors collaborated on the text.
Additional information
Supplementary Information accompanies this paper on
www.nature.com/naturegeoscience. Reprints and permissions information is available
online at http://npg.nature.com/reprintsandpermissions. Correspondence and requests
for materials should be addressed to J.B.
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