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Anomalously mild Younger Dryas summer conditions in southern Greenland

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Svante Björck at Lund University
Svante Björck
Ole Bennike at Geological Survey of Denmark and Greenland
Ole Bennike
Peter Rosén
Peter Rosén
Peter Rosén
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Camilla S. Andresen at Geological Survey of Denmark and Greenland
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Abstract and Figures

The first late-glacial lake sediments found in Greenland were analyzed with respect to a variety of environmental variables. The analyzed sequence covers the time span between 14400 and 10500 calendar yr B.P., and the data imply that the conditions in southernmost Greenland during the Younger Dryas stadial, 12800 11550 calendar yr B.P., were characterized by an arid climate with cold winters and mild summers, preceded by humid conditions with cooler summers. Climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and Föhn effects. It shows that regional and local variations of Younger Dryas summer conditions in the North Atlantic region may have been larger than previously found from proxy data and modeling experiments.
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Geology; May 2002; v. 30; no. 5; p. 427–430; 4 figures; 1 table. 427
Anomalously mild Younger Dryas summer conditions in southern
Greenland
Svante Bjo¨rck* Department of Geology, Quaternary Geology, Lund University, Tornavagen 13, SE-223 63 Lund, Sweden
Ole Bennike Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark
Peter Rose´n Department of Ecology and Environmental Science, Umea˚ University, SE-901 87 Umea˚, Sweden
Camilla S. Andresen Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
Sjoerd Bohncke Department of Quaternary Geology and Geomorphology, Vrije University, De Boelelaan 1085, 1081 HV
Amsterdam, Netherlands
Eigil Kaas Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen, Denmark
Daniel Conley Department of Marine Ecology, Danish National Environmental Research Institute (DMU), P.O. Box 358, DK-
4000 Roskilde, Denmark
ABSTRACT
The first late-glacial lake sediments found in Greenland were analyzed with respect to
a variety of environmental variables. The analyzed sequence covers the time span between
14400 and 10 500 calendar yr B.P., and the data imply that the conditions in southernmost
Greenland during the Younger Dryas stadial, 12800–11 550 calendar yr B.P., were char-
acterized by an arid climate with cold winters and mild summers, preceded by humid
conditions with cooler summers. Climate models imply that such an anomaly may be
explained by local climatic phenomenon caused by high insolation and Fo¨hn effects. It
shows that regional and local variations of Younger Dryas summer conditions in the North
Atlantic region may have been larger than previously found from proxy data and mod-
eling experiments.
Keywords: Southern Greenland, lake sediments, paleoclimatic proxy records, Younger Dryas.
Figure 1. Location map. A: Greenland’scen-
tral position within North Atlantic region. B:
Nanortalik–Kap Farvel area in southern-
most Greenland with N14 site indicated.
INTRODUCTION
The interest in the so-called Younger Dryas
stadial—which terminates the last glacial
stage and precedes the present interglacial, the
Holocene—has grown strong during the last
few decades of the twentieth century, and the
general implications of this climatic event
have also been avidly discussed (e.g., Alley
and Clark, 1999).
Although originally discovered and de-
scribed in detail for a century from Scandi-
navian lacustrine sediments (Hartz and Milth-
ers, 1901), the records from the deep
Greenland Summit ice cores have become a
template of this cool event, defined as the
Greenland stadial 1 (GS-1) event (Bjo¨rck et
al., 1998) and dated to 12 650–11500 calen-
dar (cal.) yr B.P. in the GRIP (Greenland Ice
Core Project) ice core. The d
18
O values of the
ice as well as other techniques imply that cen-
tral Greenland was subject to mean annual
temperature drops and rises at the beginning
and end of the GS-1 event on the order of 10–
20 8C (Cuffey et al., 1995; Severinghaus et
al., 1998).
LATE-GLACIAL LACUSTRINE
RECORD FROM SOUTHERN
GREENLAND
Five lakes with pre-Holocene records were
cored on islands south and southeast of Na-
nortalik, southwesternmost Greenland. The
oldest of these records was found on the island
*E-mail: svante.bjorck@geol.lu.se.
of Angissoq (Fig. 1), and was retrieved with
a Russian corer (chamber 1 m long, 7.5 cm
diameter) from a lake named N14
(59858.999N, 44810.819W, 33 m above sea lev-
el, 3.3 m water depth). Because of glacial un-
loading, crustal rebound exceeded the global
sea-level rise following the deglaciation, and
the island rose above sea level. This process
turned former marine basins into lakes, and
the
14
C age of the isolation of N14 from the
sea was calibrated to 13 800 cal. yr B.P. (Ben-
nike and Bjo¨rck, 2000). At the Younger Dryas
onset, the lake had thus been a proper lake for
;1000 yr, but the seashore was only a few
hundred meters away and the Greenland ice-
sheet margin was possibly situated ,20 km
from the site.
The sediments were analyzed with a mul-
tistratigraphic-data approach (organic and
mineral matter; biogenic silica [bioSi], sulfur
content; magnetic susceptibility; diatom, pol-
len, and macrofossil content; and annual dry-
mass accumulation rate [DMAR] of organic
and mineral matter and of bioSi), including
high-resolution
14
C dating. Here we report
from the lower part of the sequence, which
begins at 780 cm below the lake surface with
a marine silty clay and gyttja clay (Fig. 2). At
771 cm, the sediments become lacustrine, ow-
ing to the isolation from the sea. Clay gyttjas
alternating with moss gyttjas thereafter dom-
inate the sequence (Fig. 2).
The 18
14
C dates of this soft-water lake
were calibrated (Table 1) to create a calendar
year–based time scale. Apart from the well-
known
14
C plateau at 9900–10 100
14
Cyr
B.P., where calibration is almost meaningless,
the calibrated ages display two main sedimen-
tation rates, one below and one above the pla-
teau. If the two sedimentation rates are ex-
trapolated over the plateau, the two curves
meet at 742 cm. Because accumulation rates
usually alter in connection with sedimentary
changes and the only such change during the
time of the plateau occurs at 741.5 cm, this is
most likely the point at which the sedimen-
tation rate changed markedly. This results in
an age of 11 550 cal. yr B.P. for the 741.5 cm
level and produces the proper time span, 700
yr, for the plateau (Stuiver et al., 1998). Be-
cause of the well-established chronology, all
data presented here are related to time (Figs.
2–4).
The boundary between the Allerød intersta-
dial (GI-1a) and the onset of the Younger
Dryas stadial (GS-1) is characterized by a dis-
tinct shift in
14
C ages (e.g., Bjo¨rck et al.,
1996; Hughen et al., 1998); ages of 11 000–
428 GEOLOGY, May 2002
Figure 2. Left diagram shows dry weight percentage of three main sediment components.
Sediment lithology is displayed in center; abbreviations: cl—clay, si—silt, sa (mo)—sand
and mosses. Gyttja is organic sediment consisting of plant and animal remains (detritus)
and with .30% organic material by dry weight. Clay gyttja contains 6%–30% and gyttja clay
contains 3%–6% organic material. Right diagram shows annual influx of three components
per square centimeter. Note that age scale to right is linear, whereas depth scale to left
changes as consequence of changing sedimentation rate at 741.5 cm (see text).
TABLE 1. THE RADIOCARBON DATING SERIES FROM N14
Depth (cm) Dated
material*
d
13
C
(‰)
14
C age
(
14
C yr B.P.)
Calibrated age
(cal. yr B.P.)
Lab no.
708.75–708.25 Aqm 220.4 9335 6 60 10 600–10 420 AAR-5805
712.75–712.25 Aqm 219.6 9445 6 55 10 750–10 560 AAR-5804
719.25–718.75 Aqm 218.9 9690 6 70 11 200–10 860 AAR-5803
727.25–726.75 Aqm 217.5 9810 6 60 11 235–11 170 AAR-5801
730.2–729.8 Aqm 220.2 10 025 6 80 11 650–11 290 Ua-15411
736.2–735.8 Aqm 221.1 10 005 6 95 11 640–11 250 Ua-15410
738.1–737.9 Aqm 222.3 9955 6 85 11 450–11 220 Ua-15409
740.1–739.9 Aqm 222.7 10 100 6 100 11 950–11 300 Ua-14925
747.1–746.9 Aqm 222.0 10 040 6 95 11 700–11 290 Ua-15407
748.9–748.7 Aqm 222.6 10 330 6 90 12 350–11 900 Ua-15406
751.2–750.8 Aqm 223.5 10 430 6 85 12 650–12 100 Ua-15405
754.1–753.9 Aqm 223.9 10 585 6 85 12 860–12 350 Ua-15404
757.25–756.75 Bulk 219.9 10 780 6 95 12 980–12 800 Ua-15883
759.75–759.25 Bulk 222.0 11 030 6 95 13 150–12 940 Ua-15884
761.25–760.75 Bulk 220.7 11 355 6 95 13 430–13 170 Ua-15885
762.75–762.25 Bulk 220.7 11 510 6 100 13 550–13 150 Ua-15886
764.25–763.75 Bulk 218.3 11 600 6 95 13 700–13 430 Ua-15887
771–765 Tem 222.3 11 665 6 125 13 830–13 450 Ua-14844
Note
: Calibrated ages are shown with age ranges of more than 50% probability according to Oxcal 3.5 (Bronk
Ramsey, 1998), based on the INTCAL98 calibration data set (Stuiver et al., 1998). Datings were performed at
the AMS laboratories in A
˚
rhus and Uppsala.
*Abbreviations: Aqm 5 aquatic mosses, Bulk 5 bulk sediment, Tem 5 terrestrial mosses.
10 900
14
C yr B.P. below the boundary sud-
denly shift to ages of 10 700
14
C yr B.P. This
shift is situated at 758 cm (Table 1), corre-
sponding to 12800 cal. yr B.P. in our age
model, and is between the two established
Greenland Summit ice-core ages (Johnsen et
al., 1992; Alley et al., 1993) for the onset of
the Younger Dryas cooling.
Apart from the lithologic change at the iso-
lation of the lake from the sea, the clearest
sedimentary change occurs at 11 550 cal. yr
B.P. (Fig. 2), which possibly constitutes a re-
sponse to altered limnic conditions at the on-
set of the Holocene. The sedimentary changes
between the isolation at 13 800 cal. yr B.P. and
11 550 cal. yr B.P. are, however, very subtle,
and the clear lithologic shift at the start of the
Younger Dryas cooling, usually seen in north-
west European lake records, is clearly lacking.
By plotting the main sediment components
as a percentage of dry weight (Fig. 2), a de-
creasing trend in mineral matter is seen be-
tween 14 400 and 11550 cal. yr B.P (Fig. 2).
This trend is partly balanced by increasing or-
ganic matter, especially from 12 600 cal. yr
B.P. onward, and by bioSi percentages, es-
pecially between 12 700 and 11 600 cal. yr
B.P. If, however, these components are ex-
pressed as annual DMAR (expressed as
mg·yr
2
1
·cm
2
2
), a somewhat different picture
emerges. The DMAR of organic matter is sta-
ble after isolation, followed by an abrupt rise
at 11550 cal. yr B.P., whereas the DMAR of
mineral matter shows a decreasing trend, but
with no distinct change in connection with the
lithologic shift at 11 550 cal. yr B.P. The
DMAR of bioSi reaches a peak during the iso-
lation phase, followed by fairly stable values
until 13 000 cal. yr B.P. Between 13 000 and
12 200 cal. yr B.P., higher values are attained,
followed by slightly lower but varying values.
It is noteworthy that higher bioSi values are
usually found in connection with the moss-
barren sediments (Fig. 2), i.e., the clay gyttjas,
and that these sediments after 13 000 cal. yr
B.P. have considerably higher contents and
DMARs of bioSi than corresponding sedi-
ments before 13 000 cal. yr B.P.
We conclude that pre-Holocene lake pro-
ductivity was fairly constant, but with possi-
bly higher productivity and less surface runoff
(decreased DMAR of mineral matter), after
13 000 cal. yr B.P., i.e., during Younger Dryas
time. The increase in DMAR of organic mat-
ter at 11 550 cal. yr B.P. is explained by the
sudden moss dominance. This implies better
light conditions at the lake bottom due to, e.g.,
decreased phytoplankton biomass in the upper
water column, but possibly not because of a
decreased amount of suspended mineral mat-
ter; the previously decreasing trend of DMAR
of mineral matter is interrupted by slightly in-
creasing values after 11600 cal. yr B.P. (Fig.
2). Instead, the latter implies increased surface
runoff.
The sediments are extremely poor in pollen
grains. With the exception of the 13 150 cal.
yr B.P. level, pollen concentrations vary be-
tween 0 and 5700 grains per cubic centimeter.
Because of the low pollen sums, we only pre-
sent concentration and influx curves (Fig. 3).
Apart from the 13 150 cal. yr B.P. level, pollen
concentrations and influx values are very low
until 11 400 cal. yr B.P., followed by a gradual
rise. This occurs a few hundred years into the
Holocene, beginning at 11 550 cal. yr B.P., im-
plying a gradual establishment of higher veg-
etation. The appearance of, e.g., Empetrum ni-
grum and Vaccinum pollen types in the core
at 11 500 cal. yr B.P. suggests that these plants
were present on the island already during the
Younger Dryas. Other types present between
12 900 and 11 500 cal. yr B.P. are, e.g., Po-
aceae, Caryophyllaceae of Sagina type, Chen-
opodiaceae, and Saxifraga caespitosa type.
The latter may suggest arid conditions, partly
in contrast to the more wet-demanding Saxi-
GEOLOGY, May 2002 429
Figure 3. Magnetic susceptibility and sulfur content of sediments, as well as
total pollen concentrations, pollen influx values, and flux of
Pediastrum
(green
algae) colonies related to calendar years before present.
Figure 4. Most abundant diatom species, related to calendar years B.P., as well as diatom-inferred pH and
diatom diversity. Between 330 and 400 diatom valves per sample were counted; methods followed Rose´net
al. (2000). For pH reconstruction, weighted-average model was employed with inverse deshrinking (Birks et
al., 1990) used on 50-lake training set from northern Sweden (Rose´n et al., 2000), where 5 0.61 and root-
2
R
jack
mean-squared error of prediction (RMSEP) 5 0.30 units.
R
2
and RMSEP are based on jackknifing. Diatom
diversity was calculated as Shannon index (Zar, 1996). To right is shown July insolation curve for 608N (Berger,
1978) and inferred local climatic development and correlations to North Atlantic climatic events, with both
traditional terminology and new event stratigraphy (Bjo¨rck et al., 1998; Walker et al., 1999).
fraga stellaris found before 12 800 and after
11 550 cal. yr B.P. More than half of the pol-
len grains at 13 150 cal. yr B.P. are of the Car-
yophyllaceae Sagina type, which indicates
that the plant must have grown close to the
lake. The only macroscopic remains of vas-
cular plants were a few seeds of Minuartia sp.
(Caryophyllaceae) and Saxifraga cf. opposi-
tifolia in a sample between 13 800 and 13 400
cal. yr B.P., and the former was also found
between 12 800 and 12 600 cal. yr B.P.
The sediments are rich in colonies of green
alga Pediastrum. The Pediastrum flux curve
shows a maximum between 12 900 and 12500
cal. yr B.P., followed by lower, but still high
values (Fig. 3). The lower frequencies after
11 500 cal. yr B.P., causing better light con-
ditions at the bottom of the lake, can partly
explain the abundance of aquatic mosses dur-
ing the early Holocene.
The diatom flora, with 53 different species,
is dominated by a few pioneer species (Fig.
4). The lower part of the sequence is domi-
nated by Achnanthes conspicua and Fragilar-
ia virescens var. exigua. The former is a
brackish-water–tolerant species, indicating,
e.g., influence from sea spray, whereas the lat-
ter often occurs early in lake successions. At
12 800 cal. yr B.P., the flora became dominat-
ed by Achnanthes [minutissima agg.] and Ach-
nanthes pusilla, but F. virescens var. exigua
was still common. The two former taxa are
cosmopolitan, and especially A. [minutissima
agg.] is common in early Holocene profiles
(Bradshaw et al., 2000; Rose´n et al., 2001),
following disturbances of different types. The
lake was thus dominated by three pioneer spe-
cies during the Younger Dryas stadial, but the
increasing dominance of the two Achnanthes
taxa is best explained by increased pH; A.
[minutissima agg.] often occurs after liming
(Rhodes, 1991). The pH reached a maximum
between 12 500 and 11 500 cal. yr B.P. (Fig.
4), indicating increased weathering and leak-
age of base cations from the drainage area or
430 GEOLOGY, May 2002
increased concentration of nutrients. The si-
multaneous decreasing flux of mineral matter
(Fig. 2) implies that the latter explanation is
the most likely one. This is also supported by
high Pediastrum fluxes (Fig. 3), the maximum
in diatom diversity (Fig. 4), and the fairly high
content and fluxes of bioSi (Fig. 2).
LATE-GLACIAL PALEOCLIMATIC
SCENARIO
Our data suggest that the time period for
the Younger Dryas stadial was preceded and
followed by more unstable and humid condi-
tions. They also imply that lake productivity
during the Younger Dryas was high and that
the pH of the lake water increased, possibly
because of an increased nutrient load during
the growing seasons. This suggests that Youn-
ger Dryas summers, with high insolation,
were fairly warm and dry, causing lake levels
to drop, concentrating the nutrients, and in-
creasing lake-water temperatures. The pres-
ence of the diatom Synedra tenera, with an
abundance optimum at temperatures of 14 8C
(Rose´n et al., 2000), during the Younger
Dryas and after 11 000 cal. yr B.P. (Fig. 4)
also implies fairly warm lake temperatures.
The seemingly stable conditions and the lack
of vegetation succession imply that winters
were cold and dry. A thin snow cover reduced
spring melt effects and the possibility for most
plants to survive harsh winter conditions.
Our data therefore suggest that the Younger
Dryas climate in southernmost Greenland was
characterized by fairly arid conditions. Fur-
thermore, the Younger Dryas minimum of the
sea-spray–indicating diatom Achnanthes con-
spicua (Fig. 4), together with the sulfur min-
imum (Fig. 3), suggests that the surrounding
ocean was ice covered during most of the year,
preventing a wind- and wave-induced sea-
spray influence.
At 11 550 cal. yr B.P., the conditions rap-
idly changed into a humid climate with sig-
nificantly warmer winters, and the previously
ice-covered sea opened. However, a period of
less favorable conditions occurred a few hun-
dred years into the Holocene (Figs. 3 and 4),
which may correspond to the Preboreal oscil-
lation (Bjo¨rck et al., 1996) or the GH-11.2
event (Walker et al., 1999).
DISCUSSION
A reduction of the Atlantic thermohaline
circulation, triggered by increased freshwater
fluxes, is an important hypothesis to explain
the Younger Dryas cooling. We therefore
compare our results with the output from
global coupled atmosphere ocean models
forced to simulate the consequences of such a
process. Schiller et al. (1997) forced the
ECHAM3/LSG coupled general circulation
model (GCM) with increased meltwater dis-
charge into the North Atlantic to directly sim-
ulate the isolated effect of a reduction in the
North Atlantic deepwater formation. Aiming
at simulating the rapid warming terminating
the Younger Dryas, Fawcett et al. (1997) used
the GENESIS model—an atmospheric GCM
coupled to a mixed-layer ocean model with
specified meridional heat transport—and in-
directly specified a change in the deep-water
formation by varying the heat convergence in
the Nordic Seas. The simulation responses are
consistent with our reconstruction: a south-
eastward extension of the North Atlantic sea-
ice margin with an associated winter cooling
spreading over most of the northern North At-
lantic region, resulting in winter temperature
drops between 20 8C (Schiller et al., 1997) and
15 8C (Fawcett et al., 1997) in southern
Greenland. Both models show reduced Youn-
ger Dryas precipitation as compared to the sit-
uation with nonreduced deep-water formation.
Regarding seasonality, both models (although
not shown in Schiller et al., 1997) show by
far the largest anomalies in winter, in contrast
to small or even close to zero temperature and
precipitation anomalies in summer. None of
the simulations, however, indicate anomalous-
ly warm summers as suggested by our data
set. This discrepancy could be attributed to lo-
cal weather conditions, which cannot be cap-
tured at the coarse horizontal resolution of the
two models. In particular, this may relate to
locally very dry lee conditions with high in-
solation, i.e., a so-called Fo¨hn effect, devel-
oping in southernmost Greenland if the anom-
alous wind has a northerly direction. It is
likely that such local phenomena could offset
larger scale patterns and lead to anomalous lo-
cal energy inputs during summer—an effect
that, compared to present day, could be en-
hanced by the high summer insolation during
the Younger Dryas.
The study demonstrates the necessity of a
dense network of paleoclimatic sites in com-
bination with fine-resolution modeling, to ad-
vance our knowledge about the complexities
of seemingly stable climatic scenarios, e.g.,
the Younger Dryas cooling in the North At-
lantic region.
ACKNOWLEDGMENTS
Supported by the Commission for Scientific Research in
Greenland, the Danish Research Council, and the Carlsberg
Foundation.
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Printed in USA
... One parameter probably underestimated or overlooked in DO cycle studies is the seasonal contrast from winter to summer (Bjorck et al., 2002;Denton et al., 2005;Carré and Cheddadi, 2017), although some authors highlighted more pronounced winter than summer cooling during GSs (e.g., Sánchez-Goñi et al., 2000). Investigation of mismatches between mean annual temperature oscillations from Greenland ice cores and snowline changes in East Greenland have led some to conclude that DO cycles were mainly a winter phenomenon and that mild summer temperatures might have occurred even during GSs (see Bjorck et al., 2002;Denton et al., 2005). ...
... One parameter probably underestimated or overlooked in DO cycle studies is the seasonal contrast from winter to summer (Bjorck et al., 2002;Denton et al., 2005;Carré and Cheddadi, 2017), although some authors highlighted more pronounced winter than summer cooling during GSs (e.g., Sánchez-Goñi et al., 2000). Investigation of mismatches between mean annual temperature oscillations from Greenland ice cores and snowline changes in East Greenland have led some to conclude that DO cycles were mainly a winter phenomenon and that mild summer temperatures might have occurred even during GSs (see Bjorck et al., 2002;Denton et al., 2005). The pronounced winter signature of DO cycles has since been illustrated in several records from the North Atlantic region and adjacent lands (e.g., Flückiger et al., 2008;Datema et al., 2019) where it has been associated with variations in sea-ice cover extent in the northern North Atlantic (Denton et al., 2005(Denton et al., , 2022Flückiger et al., 2008). ...
Decoupled winter and summer climate changes in southern Europe during the Dansgaard-Oeschger cycles
Article
Full-text available
  • Jul 2025
  • QUATERNARY SCI REV
The marine isotope stage 3 (MIS 3) (~60-30 ka) exhibits large amplitude millennial climatic oscillations, also named Dansgaard-Oeschger (DO) cycles, which are marked by alternation of warm (GIs-Greenland in-terstadials) and cold phases (GSs-Greenland stadials). Here, we explore the seasonal nature of the DO cycle climate signal in southern Europe using 12 pollen records that allowed us to reconstruct the vegetation types and key climatic parameters such as seasonal temperatures and precipitation. Our results show the development of arboreal vegetation during GIs and non-arboreal vegetation during GSs. They also indicate that vegetation changes were mainly driven by winter precipitation and temperatures, with high and low values during GIs and GSs, respectively. The results tend to demonstrate a decoupling between winter and summer conditions, with relatively stable and warm summer temperatures throughout MIS 3, but large amplitude variations in winter. We infer that the climate over southern Europe was mainly influenced by surface conditions over the North Atlantic driven by stratification due to meltwater discharge, and also possibly by extension/contraction of the subpolar gyre (SPG) through changes in the westerlies' strength, thus impacting the extent of winter sea-ice cover, temperature and moisture availability over southern Europe.
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... Yet in Ireland, as elsewhere in maritime Europe, HS1 was also characterised by severe wintertime cooling; abundant periglacial features dating to that postglacial period (Bryant, 1974;Dardis, 1986;Lewis, 1979;Wilson, 2017) require that mean annual air temperatures at HS1 sea level were no higher than − 1 • C (Smith and Riseborough, 2002). The coincidence, therefore, of deglaciation (i.e., summertime warmth) indicated by the Connemara dataset and Irish permafrost expansion (i.e., wintertime cooling) supports a growing body of evidence from the mid-and high-latitude North Atlantic for anomalous thermal seasonality under stadial conditions (Björck et al., 2002;Bromley et al., 2014Bromley et al., , 2018Bromley et al., , 2023Denton et al., 2005;Flückiger et al., 2008;Funder et al., 2021;Putnam et al., 2023;Schenk et al., 2020;Tóth et al., 2012;Wittmeier et al., 2020). If true, deposition of the prominent Corcóg moraines potentially reflects a brief interruption of this stadial climatic configuration, possibly accompanied by a short-lived return to more maritime climate and higher wintertime precipitation. ...
... This scenario is included in IPCC assessments as a high-magnitude, albeit low-probability, consequence of modern anthropogenic climate warming (IPCC, 2022). If our interpretation of the Connemara deglacial record is correct, however, stadial cooling was strongly biased towards winter, as proposed by numerous prior researchers (Björck et al., 2002;Denton et al., 2005;Renssen and Isarin, 1997;Buizert et al., 2018;Schenk et al., 2020), whereas stadial summers were characterised by atypical warmth more akin to continental zones than to maritime ones. Should this conceptual model be substantiated (e.g., by further direct dating of North Atlantic glacial records), it will require reappraisal of the climatic parameters used currently to calibrate our frontline climate models. ...
Thinning and retreat of the temperate Connemara ice centre, Ireland, during Heinrich Stadial 1 constrained with cosmogenic 10Be dating
Article
Full-text available
  • Feb 2025
  • GEOMORPHOLOGY
Within the North Atlantic region, climatic perturbations such as Heinrich Stadial 1 (HS1) and the Younger Dryas stadial (YD) have traditionally been viewed as anomalous periods of extreme cooling linked to abrupt changes in the poleward oceanic transport of heat. While there is considerable geologic data to support strong cooling during stadial winters, recent work in mid- and high-latitude regions fringing the North Atlantic suggests that this thermal signal did not extend to stadial summers, contrary to previously inferred palaeoecological and ice core proxies. Some directly dated glacial records from Britain, Ireland, and Scandinavia, for instance, document the large-scale retreat of terrestrial ice masses during both HS1 and the YD, coincident with meltwater pulses from the European continent and consequent weakening of Atlantic meridional overturning circulation. Similar patterns of stadial deglaciation have been reported from Greenland and north-east North America. Together, these datasets support the emerging model of stadials as periods of anomalous seasonality imposed on the strongly maritime North Atlantic climate. In this study, we present a geologic record of ice sheet behaviour from the Maumturk Mountains in western Ireland during the last glacial termination, constrained with cosmogenic 10Be surface-exposure dating. Coupled with glacio-geomorphologic and sedimentologic characterisation of moraine landforms, our record describes a temperate ice mass undergoing sustained active retreat during the first half of HS1, synchronous with increased European meltwater discharge into the North Atlantic.
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... A better understanding of the seasonal patterns of Younger Dryas climate changes is also important to understanding mechanisms. For instance, moraine and snowline records in Greenland (Denton et al., 2005), plant-indicator temperature assessments in Europe (Bjorck et al., 2002;Isarin and Bohncke, 1999), and fossilbeetle assemblages (Atkinson et al., 1987) have been used to infer little change in summer temperatures and pronounced winter cooling during the Younger Dryas. European summer warming has been attributed to atmospheric blocking by the Fennoscandia Ice Sheet from ESM simulations (Schenk et al., 2018) while winter cooling in Greenland may be linked to wintertime sea ice expansion (Atkinson et al., 1987;Buizert et al., 2014). ...
... difference between winter and summer temperature) increased in the northeastern United States and decreased in the southeastern United States from a meltwater-induced slowdown of the AMOC (Fig. 3). Enhanced seasonality aligns with model and proxy evidence in Europe that the Younger Dryas cooling was likely isolated to the winter season (Bjorck et al., 2002;Isarin and Bohncke, 1999;Schenk et al., 2018), while decreased seasonality may be restricted to the southeastern United States. Temperate tree taxa are sensitive to wintertime low temperatures (Sakai and Weiser, 1973), a likely control on the decline of Bølling-Allerød mesic taxa throughout the Great Lakes region (e.g. ...
Spatial fingerprints and mechanisms of precipitation and temperature changes during the Younger Dryas in eastern North America
Article
  • Oct 2022
  • QUATERNARY SCI REV
Here we seek to establish the spatial fingerprints of precipitation and temperature changes in eastern North America during the Younger Dryas and explore the role of meltwater forcing in producing this pattern. Our analyses integrate a network of 42 fossil pollen records and 27 other hydroclimate proxy records, three AOGCM experiments with an imposed freshwater forcing, and the TraCE-21ka transient deglacial simulation. A recent synthesis of proxy-based temperature reconstructions suggests that Younger Dryas temperature reversals were limited to sites north of ∼35°N, while southern sites experienced either steady warming or a temperature maximum. Proxy records suggest a tripole precipitation pattern during the Younger Dryas and earlier Heinrich events, with the northeastern United States and Florida wetting, while sites from the Great Lakes Region to the Carolinas were dry. Of the AOGCMs analyzed, TraCE-21ka simulates Younger Dryas mean-field temperature and precipitation changes with the most skill but does not simulate warming in the southeastern United States. The likely role of meltwater forcing in producing subregional warming is indicated by the hosing-experiment AOGCMs, which consistently indicate localized warming and tripole precipitation anomalies, but the reconstructed and simulated patterns are poorly aligned, resulting in moderate model skill of mean-field temperature anomalies and negative skill in simulating past fingerprints. The reconstructed tripole helps reconcile prior apparent discrepancies in proxy records about whether eastern North America was wetter or dryer during the Younger Dryas. Although the hosing-experiment AOGCMs simulate the fingerprints poorly, they show that meltwater forcing can produce subregional warming and wetting in the eastern United States. This is due to enhanced northward heat transport from the Gulf of Mexico induced by a geostrophic adjustment of the midlatitude jet to cooling in the North Atlantic.
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... Minimum-limiting 14 C ages just above these transitions (1-4 cm above; N = 3; Fig. 4; Table 2) suggest they occurred before ∼ 11 700 cal yr BP. In general, this is consistent with existing deglacial estimates from sites closer to the coast, where deglaciation happened earlier: ∼ 14 800 cal yr BP at Nanortalik (Levy et al., 2020), ∼ 13 600 cal yr BP from Lake N14 at the southern tip of Greenland (Björck et al., 2002;Puleo et al., 2022), ∼ 13 400 cal yr BP at Pamiagdluk (Levy et al., 2020), and ∼ 12 300 cal yr BP at Qaqortoq (Levy et al., 2020). However, our local deglaciation ages are earlier than suggested by 10 Be dates on boulders and bedrock near Kiattuut Sermiat and closest to our sites (∼ 6.5 km southeast; Carlson et al., 2014;Nelson et al., 2014). ...
Duration and ice thickness of a Late Holocene outlet glacier advance near Narsarsuaq, southern Greenland
Article
Full-text available
Greenland Ice Sheet (GrIS) outlet glaciers are currently losing mass, leading to sea level rise. Reconstructions of past outlet glacier behavior through the Holocene help us better understand how they respond to climate change. Kiattuut Sermiat, a southern Greenland outlet glacier near Narsarsuaq, is known to have experienced an unusually large Late Holocene advance that culminated at ∼1600 cal yr BP and exceeded the glacier's Little Ice Age extent. We report sedimentary records from two lakes at slightly different elevations in an upland valley adjacent to Kiattuut Sermiat. These reveal when the outlet glacier's surface elevation was higher than during the Little Ice Age and constrain the associated outlet glacier surface elevation. We use bulk sediment geochemistry, magnetic susceptibility, color, texture, and the presence of aquatic plant macrofossils to distinguish between till, glaciolacustrine sediments, and organic lake sediments. Our 14C results above basal till recording regional deglaciation skew slightly old due to a reservoir effect but are generally consistent with regional deglaciation occurring ∼ 11 000 cal yr BP. Neoglacial advance of Kiattuut Sermiat is recorded by deposition of glaciolacustrine sediments in the lower-elevation lake, which we infer was subsumed by an ice-dammed lake that formed along the glacier's margin just after ∼ 3900 cal yr BP. This timing is consistent with several other glacial records in Greenland showing neoglacial cooling driving advance between ∼ 4500–3000 cal yr BP. Given that glaciolacustrine sediments were deposited only in the lower-elevation lake, combined with glacial geomorphological evidence in the valley containing these lakes, we estimate the former ice margin's elevation to have been ∼ 670 m a.s.l., compared with ∼ 420 m a.s.l. today. The ice-dammed lake persisted until the glacier surface fell below this elevation at ∼ 1600 cal yr BP. The retreat timing contrasts with overall evidence for cooling and glacier advance in the region at that time, so we infer that Kiattuut Sermiat's retreat may have resulted from reduced snowfall amounts and/or local glaciological complexity. High sensitivity to precipitation changes could also explain the relatively limited Little Ice Age advance of Kiattuut Sermiat compared with the earlier neoglacial advance.
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... Minimum-limiting 14 C ages just above these transitions (1-4 cm above; N=3; Fig. 4; Table 2) suggest they occurred before ~11,700 cal yr BP. In general, this is consistent with existing deglacial estimates from sites closer to the coast, where 250 deglaciation happened earlier: ~14,800 cal yr BP at Nanortalik (Levy et al., 2020), ~13,600 cal yr BP from Lake N14 at the southern tip of Greenland (Björck et al., 2002;Puleo et al., 2022), ~13,400 cal yr BP at Pamiagdluk (Levy et al., 2020), and ~12,300 cal yr BP at Qaqortoq (Levy et al., 2020). However, our local deglaciation ages are earlier than suggested by 10 Be dates on boulders and bedrock near Kiattuut Sermiat and closest to our sites (~6.5 km southeast; Carlson et al., 2014;Nelson et al., 2014). ...
Duration and Ice Thickness of a Late Holocene Outlet Glacier Advance near Narsarsuaq, South Greenland
Preprint
Full-text available
  • Mar 2023
Greenland Ice Sheet (GrIS) outlet glaciers are currently losing mass leading to sea level rise. Reconstructions of past outlet glaciers through the Holocene help us better understand how they respond to climate change. Kiattuut Sermiat, a south Greenland outlet glacier near Narsarsuaq, is known to have experienced an anomalously small Little Ice Age advance compared with a larger Holocene advance that culminated at ~1600 cal yr BP. We report sedimentary records from two lakes at slightly different elevations in an upland valley adjacent to Kiattuut Sermiat, which reveal when the outlet glacier was significantly larger than its Little Ice Age size and constrain the associated outlet glacier surface elevation. We use bulk sediment geochemistry, magnetic susceptibility, color, texture, and the presence of aquatic plant macrofossils to distinguish between till, glaciolacustrine sediments, and organic lake sediments. Our 14C results above basal till recording regional deglaciation skew slightly old due to a hard water effect but are generally consistent with regional deglaciation occurring ~11,000 cal yr BP. Neoglacial advance of Kiattuut Sermiat is recorded by deposition of glaciolacustrine sediments in the lower-elevation lake, which was subsumed by an ice-dammed lake that formed along the glacier’s margin just after ~3900 cal yr BP. This timing is consistent with several other glacial records in Greenland showing Neoglacial cooling driving advance between ~4500–3000 cal yr BP. Given that glaciolacustrine sediments were deposited only in the lower-elevation lake, combined with glacial geomorphological evidence in the valley containing these lakes, we estimate the former ice margin’s elevation to have been ~670 m, compared with ~420 m today. The ice-dammed lake persisted until glacier retreat at ~1600 cal yr BP. That retreat timing contrasts with overall evidence for cooling and glacier advance in the region at that time, so we infer that retreat may have resulted from reduced precipitation amounts and/or local glaciological complexity. High sensitivity to precipitation changes could also explain the limited Little Ice Age advance of Kiattuut Sermiat compared to other Greenland glaciers.
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Reconstruction and palaeoclimatic implications of Loch Lomond Readvance glaciers in the Southeast Grampians, Scotland
Article
  • Apr 2025
  • J QUATERNARY SCI
The glaciation of Scotland during the Loch Lomond Stadial (LLS; 12.9–11.7 ka BP) left a distinct and well‐preserved geomorphological signature, permitting detailed reconstructions of former glaciers and palaeoclimatic conditions. However, the extent and style of glaciation in the Southeast Grampians remains poorly constrained. This study employs geomorphological mapping and glacier surface profile modelling to reconstruct a ∼60 km ² icefield covering much of the Mounth Plateau. The results suggest that plateau ice was thicker and more extensive than previously considered, with a polythermal regime inferred from the geomorphological evidence. Based on contrasting landform assemblages inside and outside of reconstructed glacial limits and basal radiocarbon dates, the icefield is attributed to the LLS. The geomorphological evidence and superimposed streamlining of recessional moraines within the study site suggest that deglaciation was active and interrupted by periods of re‐advance occurring during fluctuating climatic conditions. An equilibrium line altitude of 738 m and a precipitation value at sea level of 845 ± 150 mm a ⁻¹ are calculated for the Mounth Icefield, suggesting a slightly more arid climate than present. Comparison with published glacier‐derived palaeoclimatic variables supports a strong west–east precipitation gradient across Scotland during the LLS.
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Lateglacial Shifts in Seasonality Reconcile Conflicting North Atlantic Temperature Signals
Article
Full-text available
  • Jan 2023
The accelerating flux of glacial meltwater to the oceans due to global warming is a potential trigger for future climate disturbance. Past disruption of Atlantic Ocean circulation, driven by melting of land‐based ice, is linked in models to reduced ocean‐atmosphere heat transfer and abrupt cooling during stadial events. The most recent stadial, the Younger Dryas (YD), is traditionally viewed as a severe cooling centered on the North Atlantic but with hemispheric influence. However, indications of summer warmth question whether YD cooling was truly year‐round or restricted to winter. Here, we present a beryllium‐10‐dated glacier record from the north‐east North Atlantic, coupled with 2‐D glacier‐climate modeling, to reconstruct Lateglacial summer air temperature patterns. Our record reveals that, contrary to the prevailing model, the last glacial advance in Scotland did not occur during the YD but predated the stadial, while the YD itself was characterized by warming‐driven deglaciation. We argue that these apparently paradoxical findings can be reconciled with regional and global climate events by invoking enhanced North Atlantic seasonality—with anomalously cold winters but warming summers—as an intrinsic response to globally increased poleward heat fluxes.
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Younger Dryas and early Holocene climate in south Greenland inferred from oxygen isotopes of chironomids, aquatic Moss, and Moss cellulose
Article
  • Nov 2022
  • QUATERNARY SCI REV
Ice core records have long indicated that the Younger Dryas began and ended with large, abrupt climate shifts over Greenland. Key climatic features remain unknown, including the magnitude of warming during the Younger Dryas-Holocene transition along with the seasonality and spatial variability of Younger Dryas climate changes across Greenland. Here, we use geochemical and paleoecological proxies from lake sediments at Lake N14 in south Greenland to address these outstanding questions. Radiocarbon dating and diatom assemblages confirm early deglaciation and isolation of Lake N14 before ∼13,600 cal yr BP, consistent with previous work. Oxygen isotope ratios (δ¹⁸O) of chironomid head capsules, bulk aquatic moss, and aquatic moss-derived cellulose are used to reconstruct oxygen isotopes of past lake water and annual precipitation. Oxygen isotope proxies indicate annual precipitation δ¹⁸O values increased by 5.9–7.7‰ at the end of the Younger Dryas. Following the Younger Dryas, moss and cellulose δ¹⁸O values show a clear decline in precipitation δ¹⁸O values of 2–3‰ from ∼11,540–11,340 cal yr BP that may correspond with the Preboreal Oscillation. Reconstructed precipitation δ¹⁸O values then gradually increased from 11,300–10,100 cal yr BP. All three aquatic organic materials register similar shifts in precipitation δ¹⁸O values over time, and they closely parallel the δ¹⁸O shifts observed in ice cores. This evidence strongly supports the utility of these methods for reconstructing lake water δ¹⁸O, and furthermore precipitation δ¹⁸O values where lake water reflects precipitation. The relatively large shift in isotopic composition of precipitation at Lake N14 suggests that shifts in temperature, precipitation seasonality, and/or moisture sources at the end of the Younger Dryas were even larger in south Greenland than they were in central Greenland, most likely because of the proximity to major changes in North Atlantic Ocean circulation. The annual air temperature change estimated at Lake N14 at the end of the Younger Dryas is also very large (∼18 ± 7 °C) compared to the summer warming previously inferred from chironomid species assemblages there (∼6 °C). This indicates that the strongest warming at the end of the Younger Dryas occurred in the winter season, consistent with past observations of intensified Younger Dryas seasonality at Lake N14 and elsewhere in Greenland.
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Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice
Article
Full-text available
  • Jan 1998
Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 +/- 3°C colder than today.
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Lake sediment coring in South Greenland in 1999
Article
Full-text available
  • Jan 2000
NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Bennike, O., & Björck, S. (2000). Lake sediment coring in South Greenland in 1999. Geology of Greenland Survey Bulletin, 186, 60-64. https://doi.org/10.34194/ggub.v186.5216 _______________ The transition from the last ice age to the Holocene was a period of extremely rapid and large climatic changes (Björck et al. 1998). Because of this, the period has attracted much attention by Quaternary workers since these fluctuations were first demonstrated by Danish scientists (Hartz & Milthers 1901; Iversen 1934, 1954). In the ice-free parts of Greenland, many attempts have been made over the past few decades to find sediments from this transitional period. Some radiocarbon dates on marine molluscs from the late-glacial have been published, but most are based on conventional dating of several shells that might represent a mixture of Holocene and interglacial material. Conventional radiocarbon dating of lake sediments has also produced a number of ‘late-glacial’ dates, but where checked by accelerator mass spectrometry (AMS) radiocarbon dating, the sediments have proved to be Holocene (Björck et al. 1994a, b). These sediments contain ‘old carbon’ in the form of coal fragments and reworked interglacial organic detritus. In 1999 we tried a new approach to locate late-glacial lake sediments in Greenland. In southernmost Greenland, the shelf is narrow and the land area relatively small. Therefore the amount of glacierization during the Quaternary glacial stages must have been limited. In addition, this region is situated so far south in the North Atlantic that it must have been much influenced by the warming at 14,700 GRIP years BP (Björck et al. 1998). The southern location also means that the temperature conditions would allow a fairly rich plant and animal life to have become established rather early after recession of the ice. Sediment records from lakes located near sea-level at some distance from the outer coast extend back to the earliest Holocene (Fredskild 1973). Lakes situated at higher elevations might have become deglaciated earlier, when the Inland Ice thinned over the coast towards the end of the last ice age. Thus, in the 1999 programme we have sampled high-elevation basins, situated at 350–720 m above sea level (see Table 1). Basins situated in cirque valleys were avoided because it is possible that glaciers would have been present in such basins during the Little Ice Age. However, it turned out that most of the high-elevation basins investigated were devoid of sediments. Even at water depths over several tens of metres, the bottom consisted of stones and boulders and a good sedimentary sequence was only found in a single lake. For this reason, low-elevation basins as far away as possible from the present ice margin were also cored. In addition, it was decided to core a series of isolation basins at different elevations below the marine limit in order to establish a securely constrained curve for the relative shore-level change after the last deglaciation. Many such curves have been published from different parts of Greenland, but they are mainly based on mollusc shell dates which are much more uncertain than dates from isolation basins. The dated molluscs lived at various depths below sea-level and their relationship to the former sea-level is always uncertain. The locations of the cored basins are shown in Fig. 1 and short notes on the lakes are given in Table 1. This work is a continuation of the studies of recent years on lake sediments in South and West Greenland by the Geological Survey of Denmark and Greenland (Anderson & Bennike 1997; Overpeck et al. 1998; Anderson et al. 1999; 2000, this volume; Bennike 2000; Brodersen & Anderson 2000, this volume).
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Holocene climatic change reconstructed from diatoms, chironomids, pollen and near-infrared spectroscopy at an alpine lake (Sjuodjljaure) in northern Sweden
Article
Full-text available
  • May 2001
The results of a multiproxy study reconstructing the climate history of the last 9300 years in northern Sweden are presented. It is based on diatom, chironomid and pollen analyses, as well as near-infrared spec troscopy (NIRS), of a radiocarbon dated sediment core from Sjuodjijaure (67°22N, 18°04E), situated 100 m above tree-line in the Scandes mountains. Mean July air temperature was reconstructed using transfer functions established for the region. The biological proxies show significant changes in composition during the Holocene and the inferred temperatures all follow the same general trend. For the period between about 9300 to 7300 cal. BP the reconstructions should be interpreted with caution due to the lack of convincing modern analogues in the training set. However the reconstruction suggest that July temperature was on average about the same as today, with several rapid short-term cold and warm periods. Cold periods were dated to about 8500, 8200 and 7600 cal. years BP and a warm period to about 7700 cal. BP. About 7300 cal. BP, a major shift to a warmer climate occurred. Pine migrated into the area, which was previously covered with birch forest. From the mid-Holocene until today the sediment record suggests a descending tree-limit and a gradual lowering of July temperature.
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An event stratigraphy for the Last Termination in the North Atlantic region based on the Greenland ice-core record: a proposal by the INTIMATE group
Article
Full-text available
  • Jul 1998
It is suggested that the GRIP Greenland ice-core should constitute the stratotype for the Last Termination. Based on the oxygen isotope signal in that core, a new event stratigraphy spanning the time interval from ca. 22.0 to 11.5 k GRIP yr BP (ca. 19.0–10.0 k 14C yr BP) is proposed for the North Atlantic region. This covers the period from the Last Glacial Maximum, through Termination 1 of the deep-ocean record, to the Pleistocene–Holocene boundary, and encompasses the Last Glacial Late-glacial of the traditional northwest European stratigraphy. The isotopic record for this period is divided into two stadial episodes, Greenland Stadials 1 (GS-1) and 2 (GS-2), and two interstadial events, Greenland Interstadials 1 (GI-1) and2 (GI-2). In addition, GI-1 and GS-2 are further subdivided into shorter episodes. The event stratigraphy is equally applicable to ice-core, marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon-dated chronostratigraphy that has been used hitherto. © 1998 John Wiley & Sons, Ltd.
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Deglacial changes in ocean circulation from an extended radiocarbon calibration
Article
Full-text available
  • Jan 1998
Temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age. This problem is particularly severe beyond the limit of the high-resolution radiocarbon calibration based on tree-ring data, which stretches back only to about 11.8kyr before present (BP), near the termination of the Younger Dryas cold period. If a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation, extending the well-calibrated radiocarbon timescale back further in time is crucial. Several studies attempting such an extension, using uranium/thorium-dated corals and laminae counts in varved sediments, show conflicting results. Here we use radiocarbon data from varved sediments in the Cariaco basin, in the southern Caribbean Sea, to construct an accurate and continuous radiocarbon calibration for the period 9 to 14.5 kyr BP, nearly 3,000 years beyond the tree-ring-based calibration. A simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that North Atlantic Deep Water formation shut down during the Younger Dryas period, but was gradually replaced by an alternative mode of convection, possibly via the formation of North Atlantic Intermediate Water.
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The Younger Dryas Termination and North Atlantic Deep Water Formation: Insights from climate model simulations and Greenland Ice Cores
Article
Full-text available
  • Feb 1997
Results from the GISP2 and GRIP ice cores show that the termination of the Younger Dryas (YD) climate event in Greenland was a large and extremely fast climate change. A reinitiation of North Atlantic Deep Water formation following a shutdown, and its associated winter release of heat to the atmosphere, has been suggested as the most likely cause of this climate transition. To test this idea, two general circulation model experiments using GENESIS have been completed for YD time (12,000 calendar years ago): one with low heat flux in the Nordic Seas (10 W/m(2), deep water shutdown) and one with high Nordic Sea heat flux (300 W/m(2), active deep water formation). Comparison of Greenland climate differences between these experiments with the ice core records shows that when deep water is turned on, much of the YD termination warming is achieved. The increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation, which obscures the dominantly wintertime response to the specified forcing. The winter storm track shift toward Greenland contributes much of the climate change at the YD termination.
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Isotopic 'events' in the GRIP ice core: A stratotype for the Late Pleistocene
Article
  • Aug 1999
An event stratigraphy for the Last Termination, based on the stratotype of the GRIP ice-core record, has been outlined for the North Atlantic region. It is suggested that such an approach to stratigraphic subdivision may offer a more satisfactory alternative to conventional stratigraphical procedures for those parts of the recent Quaternary record that are characterised by rapid and/or short-term climatic fluctuations.
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Irregular glacial interstadials recorded in a new Greenland ice core
Article
  • Sep 1992
  • NATURE
Profiles of O-18/O-16 ratios along three previous deep Greenland ice cores seemed to reveal irregular but well-defined episodes of relatively mild climate conditions (interstadials) during the mid and late parts of the last glaciation. Results are presented from a new deep ice core drilled at the summit of the Greenland ice sheet, where the depositional environment and the flow pattern of the ice are close to ideal for core recovery and analysis. The results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt. According to a preliminary timescale based on stratigraphic studies, the interstadials lasted from 500 to 2000 yrs; their irregular occurrence suggests complexity in the behavior of the North Atlantic ocean circulation.
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