in the Labrador Sea region - Swansea University

Terrestrial climate signal of the “8200 yr B.P. cold event” in the
Labrador Sea region
Timothy J. Daley 1,2* , F. Alayne Street-Perrott 1 , Neil J. Loader 1 , Keith E. Barber 2 , Paul D.M. Hughes 2 , Elizabeth H. Fisher 3 ,
and James D. Marshall 3
1
School of the Environment and Society, Swansea University, Swansea SA2 8PP, UK
2
Palaeoecology Laboratory (PLUS), School of Geography, University of Southampton, Southampton SO17 1BJ, UK
3
Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK
ABSTRACT
Accelerated melting of Greenland ice has raised concern about
the future impact of enhanced freshwater discharge on regional
climate through its effect on ocean circulation. An abrupt cooling
event ca. 8200 cal. yr B.P. has been linked to meltwater from
the decaying North American ice sheet. Oxygen isotopic analyses
of cellulose from subfossil Sphagnum mosses, isolated from a Newfoundland
peat core, reveal a pronounced anomaly ca. 8350 yr B.P.
with a duration of ~150 years. The maximum estimated δ 18 O precipitation
change, 4.53‰ ± 1.05‰ (Vienna standard mean ocean water), is the
largest observed in the circum-North Atlantic region. The magnitude
of change exceeds that predicted by recent paleoclimate simulations.
Comparisons with recent records of surface and deep ocean
proxies in the Labrador Sea and the wider North Atlantic region
suggest synchroneity. However, an ~200 year delay between the
responses of the Labrador Sea region and the Greenland Ice Sheet
to the effects of meltwater release remains to be explained.
INTRODUCTION
The largest climatic event recorded in the Northern Hemisphere during
the past 10 ka (Alley et al., 1997; Klitgaard-Kristensen et al., 1998; von
Grafenstein et al., 1998) has been attributed to an outburst of meltwater
from glacial Lake Agassiz, 8470 ± 270 years ago (Barber et al., 1999),
with a discharge of ~5.2 Sv (1 Sv = 10 6 m 3 s –1 ) over ~1 year (Teller et al.,
2002; Clarke et al., 2004). In the western Labrador Sea, this megaflood is
represented by a double layer of carbonate-rich turbidites, dated between
ca. 8500 and ca. 8350 yr B.P. (Hillaire-Marcel et al., 2007). It was followed
by a sustained increase in meltwater flux through the Hudson Strait (Teller
et al., 2002; Clarke et al., 2004) that created a freshwater anomaly sufficiently
large to slow the Meridional Overturning Circulation of the Atlantic
Ocean (AMOC) (Renssen et al., 2001; Wiersma and Renssen, 2006), and
resulted in an abrupt cooling of the circum-North Atlantic region (Klitgaard-Kristensen
et al., 1998; von Grafenstein et al., 1998; Rohling and
Pälike, 2005; Thomas et al., 2007; Marshall et al., 2007). An increasing
meltwater flux from the Greenland Ice Sheet (Chen et al., 2006) into the
Nordic and Labrador Seas during the twenty-first century could potentially
impede deep convection and northward heat transport by the AMOC
(Intergovernmental Panel on Climate Change, 2007). The “8200 yr B.P.”
event, therefore, provides a means of validating the response of numerical
climate models to a large meltwater perturbation (Alley and Ágústsdóttir,
2005; Wiersma and Renssen, 2006; LeGrande et al., 2006). While significant
cooling at that time has been reported from Europe and Greenland
(Alley et al., 1997; Klitgaard-Kristensen et al., 1998; von Grafenstein et al.,
1998; Thomas et al., 2007; Marshall et al., 2007), quantitative estimates of
change from northeastern North America are scarce. Paleoecological data
from peat cores have traditionally been used to reconstruct past moisture
balance (Barber et al., 1998; Hughes et al., 2006). Here we use a novel
extraction method to derive estimates of variation in the oxygen isotopic
composition of precipitation over northeastern North America from analyses
of the cellulose fraction of subfossil Sphagnum leaves. These results
*E-mail: t.j.daley@swansea.ac.uk.
challenge present understanding of the climatic sensitivity of this region to
the postulated forcing mechanisms for the 8200 yr B.P. event.
SITE SETTING AND THE ADVANTAGES OF SPHAGNUM
Nordans Pond Bog is a precipitation-fed mire located 1.5 km inland
from the present-day Atlantic coast of Newfoundland (49.150°N, 53.583°W;
60 m altitude) (Fig. 1). Excellent Sphagnum preservation throughout the
8700 year period captured in peat core NDN02/1, taken from the center
of the mire, allowed us to construct a Sphagnum-specific time series of
isotopic variations. Sphagnum moss is an excellent indicator of the isotopic
composition of growing-season precipitation, owing to the simplicity of the
pathway by which it incorporates meteoric water into cellulose. Given its
lack of roots and functioning guard cells, all fractionation of the plant water
is environmentally controlled prior to assimilation and cellulose synthesis
(Ménot-Combes et al., 2002). Isotopic fractionation during cellulose synthesis
is independent of temperature (DeNiro and Epstein, 1979; Sternberg
et al., 1986). Previous studies have reported the preservation of an evaporative-enrichment
signal in the cellulose of surface Sphagnum relative to peat
pore waters, based on samples collected on a single day or month during
summer months (Brenninkmeijer et al., 1982; Aravena and Warner, 1992).
Subsequent isotopic analyses of surface samples of Sphagnum moss, modern
precipitation, and modern bog waters taken regularly throughout an
70ºN
60ºN
NGRIP1
Goose Bay
Ocean
50ºN
Nordans
Pond Bog
Truro
50ºW
MD03-2665
L.C.
E.G.C.
L.C.
40ºW
GRIP
GREENLAND
North
Atlantic
G.S.
I.C.
E.G.C.
MD99-
2251
N.A.D.
0 km 1500
Figure 1. Map of northwest Atlantic region showing modern oceansurface
currents, glacial Lake Agassiz outflow, and site locations.
Also shown are core locations (solid circles), Global Network of Isotopes
in Precipitation (GNIP) (International Atomic Energy Agency–
World Meteorological Organization, 2004) monitoring stations at
Goose Bay, Labrador, and Truro, Nova Scotia (open circles), modern
ocean-surface currents (dark lines—warm currents, gray lines—cold
currents, L.C.—Labrador Current, G.S.—Gulf Stream, N.A.D.—North
Atlantic Drift, E.G.C.—East Greenland Current, I.C.—Irminger Current),
and routing of final drainage from glacial Lake Agassiz ca. 8470
years ago (Barber et al., 1999) (thick gray arrow). GRIP—Greenland
Ice Core Project; NGRIP—North Greenland Ice Core Project.
30ºW
© 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY, Geology, September 2009; 2009 v. 37; no. 9; p. 831–834; doi: 10.1130/G30043A.1; 4 figures; Data Repository item 2009203.
831

entire year have revealed that such evaporative effects are not maintained
(Daley, 2007). Instead, these analyses yielded a constant cellulose-precipitation
fractionation factor (1.0274 ± 0.0010; Daley, 2007), statistically
identical to the biochemical enrichment of oxygen isotopes during photosynthesis
as determined by laboratory experiments (Sternberg et al., 1986).
While we cannot exclude entirely the possibility that leaf water in Sphagnum
is subject to some evaporative enrichment prior to cellulose synthesis,
the data reported by Daley (2007) suggest that it is minimal. Measured
δ 18 O and δD of summer bog-surface water samples from Nordans Pond
Bog (n = 8) are within the scatter of regional precipitation values, showing
that the isotopic impact of evaporation at this site is negligible (Fig. 2A).
Furthermore, in the modern day, δ 18 O values of precipitation (δ 18 O precipitation
)
are strongly and consistently related to air temperature across the Atlantic
provinces of Canada (Fig. 2B).
δD (‰)
0
-50
-100
Goose Bay Autumn
Truro Autumn
Goose Bay Spring
Truro Spring
Goose Bay Summer
Truro Summer
Goose Bay Winter
Truro Winter
Bog waters - June 2004
A
The east coast of Newfoundland is proximal to the hypothesized route
of meltwater from glacial Lake Agassiz through the Hudson Strait and the
western Labrador Sea (Keigwin et al., 2005). It also is close to the oceanic
surface front between subarctic waters flowing south from the Labrador
Sea and the warm waters of the Gulf Stream (Fig. 1) (Minobe et al., 2008).
Recent model simulations predict the greatest response of surface air temperature
(SAT) to meltwater forcing in the area overlying Newfoundland
and the southern Labrador Sea region (LeGrande et al., 2006). Nordans
Pond Bog, therefore, is an ideally located, sensitive archive for tracking
atmospheric variations forced by past changes in ocean circulation.
NEWFOUNDLAND RECORD
We measured 23 accelerator mass spectrometer (AMS) 14 C ages on
Newfoundland core NDN02/1. A cubic polynomial age-depth model provides
a good fit to the distribution of dates (r 2 = 0.996; Fig. DR1 and Methods
in the GSA Data Repository 1 ). The accumulation rate varies from ~5
a/cm (2 mm/a) in the base of the core to ~17 a/cm (0.6 mm/a) through the
central section of the core.
The δ 18 O precipitation
values estimated from the δ 18 O Sphagnum
record of
core NDN02/1 (Data Repository) exhibit an abrupt isotopic decrease
in the early Holocene, followed by higher values from ca. 7000 to
4000 yr B.P. (Fig. 3). Lower average values of δ 18 O precipitation
, with considerable
superimposed centennial-scale variability, were registered from
ca. 4000 yr B.P. to present.
Air temperature (ºC)
-150
-200
30
20
10
0
-10
-20
-30 -25 -20 -15 -10 -5 0
Goose Bay, Labrador
y = 2.284x + 35
δ 18 O (‰)
-30
-30 -25 -20 -15 -10 -5 0
δ 18 O precipitation (‰)
Truro, Nova Scotia
y = 2.283x + 28
Figure 2. Modern isotopic and climate data from Global Network
of Isotopes in Precipitation (GNIP) [International Atomic Energy
Agency–World Meteorological Organization (IAEA/WMO), 2004] stations
at Goose Bay, Labrador, and Truro, Nova Scotia. A: Variation
in isotopic composition of meteoric waters by season. Open symbols—Truro.
Closed symbols—Goose Bay. Samples of bog-surface
water taken in June 2004 are overlaid as inverted gray triangles. B:
Monthly air-temperature (T) values and oxygen isotopic composition
of monthly precipitation (IAEA/WMO, 2004). Open circles—Truro.
Closed circles—Goose Bay. Linear regressions of these data generate
slopes, with 95% confidence bandings (dashed lines), for both
stations that are identical to within 0.001. R 2 values for Goose Bay
and Truro are 0.62 and 0.60, respectively (p < 0.0001). Slope is used
to define a ΔT – Δδ 18 O precipitation
relationship for the Atlantic provinces
of Canada. Data are expressed as per mil deviations from Vienna
standard mean ocean water.
B
18 Oprecipitation (‰)
δ
-4
-5
-6
-7
-8
-9
-10
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Age (cal. yr B.P.)
Figure 3. δ 18 O precipitation
reconstructed from δ 18 O Sphagnum
values of core
samples from NDN02/1. Precipitation estimates were derived using
measured cellulose-water enrichment factor α = 1.0274 ± 0.0010
(Daley, 2007).
Closer examination of the large negative excursion in δ 18 O precipitation
during the early Holocene reveals a complex paleoclimate signal. A primary
isotopic decline beginning ca. 8450 ± 90 yr B.P. was followed by
a smaller negative excursion ~200 years later (Fig. 4). From higher than
modern values before 8450 ± 90 yr B.P., δ 18 O precipitation
fell abruptly by 3‰
in ≤55 years. The isotopic minimum was centered on 8350 ± 90 yr B.P.,
representing a total decrease of 4.53‰ in ~100 years. As defined by the
midpoints of the rising and falling limbs, the duration of the primary anomaly
was ~150 years. A rapid recovery to δ 18 O precipitation
values similar to the
average for the past 4 ka preceded a smaller, secondary, negative excursion
with an amplitude of just under 2‰, beginning at 8250 ± 90 yr B.P. and
culminating at 8170 ± 90 yr B.P. Recovery to values similar to those of the
mid- to late Holocene average was complete by ca. 8060 ± 110 yr B.P.,
giving a total duration for the secondary isotopic excursion of ~200 years.
A gap in the record follows due to the disappearance of Sphagnum taxa,
although three coregistered biological and physical proxies from the core
indicate that warmer conditions prevailed (Hughes et al., 2006).
1
GSA Data Repository item 2009203, methods and chronology, is available
online at www.geosociety.org/pubs/ft2009.htm, or on request from editing@geosociety.org
or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
832 GEOLOGY, September 2009

A
B
C
D
E
δ 18 O precipitation
(‰ VSMOW)
% N. pachyderma (s)
(Ellison et al., 2006)
C. wuellerstorfi
δ 13 C (‰ VPDB)
(Kleiven et al., 2008)
F
-4
-6
-8
-10
Age (cal. yr B.P.)
7600 7800 8000 8200 8400 8600 8800 9000
0
1
2
3
4
5
6
7
1.5
1.0
0.5
0.0
-0.5
MD99-2251
MD03-2665
Terminal outburst of
glacial Lake Agassiz
-37
7600 7800 8000 8200 8400 8600 8800 9000
Age (cal. yr B.P.) GICC05
NDN02/1
HWLC1
MD03-2665
GRIP and NGRIP
δ 18 O ice (‰)
INTERPRETATION OF THE STABLE ISOTOPE RECORD
Comparison of the estimated values of δ 18 O precipitation
with measured
modern δ 18 O precipitation
and temperature data from the Global Network for
Isotopes in Precipitation (GNIP; International Atomic Energy Agency–
World Meteorological Organization, 2004) site at Truro, Nova Scotia
-5.0
-5.2
-5.4
-5.6
-5.8
-6.0
-6.2
-6.4
-6.6
2.0
2.2
2.4
2.6
-33
-34
-35
-36
δ 18 O c (‰ VPDB)
(Marshall et al., 2007)
N. pachyderma (s)
δ 18 O (‰ VPDB)
(Kleiven et al., 2008)
Isotopically
depleted
precipitation/
cooler
surface air
temperatures
Cooler SSTs
Reduced
LNADW
Figure 4. Atmospheric response over Newfoundland compared with
terrestrial record from northern England, records of surface and
deep ocean variability from Labrador Sea and North Atlantic, and
oxygen isotopic records from Greenland. A: δ 18 O precipitation reconstruction
from core NDN02/1. Error bars show 2σ calibrated range from
accelerator mass spectrometry 14 C dates bracketing primary isotopic
event. B: Oxygen isotopic variation in lake-sediment calcite from
Hawes Water, northern England (Marshall et al., 2007). C: Variation
in percent Neogloboquadrina pachyderma (s) in ocean core MD99–
2251, south of Iceland (Ellison et al., 2006). D: Variation in oxygen
isotopic composition of N. pachyderma (s) in core MD03–2665 from
the Eirik Drift (Kleiven et al., 2008). E: Benthic Cibicidoides wuellerstorfi
δ 13 C values from core MD03–2665 (Kleiven et al., 2008). F: Two
recently synchronized δ 18 O ice
records from Greenland ice cores GRIP
(dashed line) and NGRIP1 (Vinther et al., 2006; Rasmussen et al.,
2006) covering period 9000–7600 yr B.P. Chronological uncertainty
in F assumes layer-counting error of 47 years throughout this time
period (Vinther et al., 2006). SST—sea surface temperature; GRIP—
Greenland Ice Core Project; NGRIP—North Greenland Ice Core Project;
VSMOW—Vienna standard mean ocean water; VPDB—Vienna
Peedee belemnite; LNADW—Lower North Atlantic Deep Water.
(Fig. 2A), suggests that during the primary anomaly, SATs during the
Sphagnum growing season cooled from values warmer than modern Nova
Scotian summers to ones more similar to Nova Scotian autumn or early
spring. If the modern temperature-isotope relationship for the Atlantic
provinces of Canada, ΔT = 2.284(Δδ 18 O precipitation
), is applied (Fig. 2B), the
apparent cooling during the primary anomaly was ~10.3 °C. This implausibly
large estimate must be corrected for the reduction in δ 18 O values
of surface ocean water in the moisture source region by the meltwater
released from Lake Agassiz, which may reduce its magnitude significantly.
However, we are not aware of any suitably located marine cores
with sufficient temporal resolution to calculate the maximum isotope
anomaly and its subsequent evolution through time. Numerical modeling
studies (Winsor et al., 2005; LeGrande et al., 2006) suggest that the meltwater
plume passed the study site and mixed throughout the wider North
Atlantic within ~10–20 years, implying that the δ 18 O precipitation
anomaly
observed in NDN02/1 primarily reflects a centennial-scale change in the
thermal gradient between the moisture source and the site of precipitation
and/or a change in the prevailing origin of that moisture (a change in the
modern ΔT/Δδ 18 O precipitation
relationship).
Strong atmospheric cooling and decreased δ 18 O precipitation
values over
the study area were observed in simulations of the climate response to
catastrophic drainage of glacial Lake Agassiz (Wiersma and Renssen,
2006; LeGrande et al., 2006). The most pronounced response of both
δ 18 O precipitation
and surface air temperature was found over the Labrador Sea
south of Greenland and northern Newfoundland (LeGrande et al., 2006).
However, the maximum isotopic change recorded in NDN02/1 exceeds
that simulated by numerical models (Renssen et al., 2001; Wiersma and
Renssen, 2006; LeGrande et al., 2006). Moreover, modeled decadal-average
changes consistently underestimate those observed in paleoclimate
records around the North Atlantic region (LeGrande et al., 2006).
The complexity of the terrestrial climate record in Newfoundland
during the period ca. 8500 to ca. 8000 yr B.P. is attested to by the presence
of the secondary isotopic depletion in NDN02/1 between ca. 8250
and ca. 8170 yr B.P. The causes for this oscillation are not clear, but may
reflect nonlinearity in the AMOC response to drainage of glacial Lake
Agassiz and associated northward heat transport (LeGrande et al., 2006),
the influence of atmospheric reorganization in association with the collapse
of the Laurentide Ice Sheet (Hillaire-Marcel et al., 2007; Carlson et
al., 2008), or a local response to one or more of the ~30 meltwater pulses
that affected the Labrador Sea region in the period 8400–7000 yr B.P.
(Jansson and Kleman, 2004).
COHERENT TERRESTRIAL AND OCEANIC CLIMATE SIGNALS
Core NDN02/1 provides evidence for a terrestrial response to meltwater
forcing that is coherent with oceanic proxies in the Labrador Sea
region. The primary anomaly in NDN02/1 appears synchronous with evidence
for lower benthic δ 13 C values, reflecting reduced deep water transport,
and coinciding with lower sea surface temperatures in a marine core
from the Eirik Drift southwest of Greenland (MD03–2665; 57°26.56N,
48°36.60W) (Kleiven et al., 2008) (Figs. 4D and 4E). Both records provide
evidence for a dramatic cooling that lasted ~150 years and occurred
within 1σ of the date for the final drainage of Lake Agassiz (Fig. 4) (Barber
et al., 1999; Hillaire-Marcel et al., 2007). Kleiven et al. (2008) argued
that the benthic δ 13 C record in core MD03–2665 represents the combined
effects of changes in overflow of Iceland Scotland Overflow Water and
Denmark Strait Overflow Water, integrated in the Deep Western Boundary
Current. The timing of the variations observed in the Labrador Sea region
also corresponds closely to those found in an independently dated record
from the Gardar Drift (southwest of Iceland) (Ellison et al., 2006) and
a U/Th dated lacustrine record from the coastal fringes of northwestern
Europe (Marshall et al., 2007) (Figs. 4B and 4C). In contrast, records of
variations in δ 18 O precipitation
over the summit of Greenland, and synchronized
on the GICC05 (Greenland Ice Core Chronology 2005) (Svesson et al.,
GEOLOGY, September 2009 833

2006) time scale, provide evidence for an anomaly starting ca. 8250 yr
B.P. and lasting 160 years (Thomas et al., 2007). The onset of reduced
δ 18 O precipitation
values over Greenland occurred ~200 years after the isotope
anomalies recorded in NDN02/1 and MD03–2665, and is outside the 2σ
range of the calibrated AMS 14 C age of this event in NDN02/1 (Fig. 4).
CONCLUSIONS
Core NDN02/1 demonstrates that the northeastern seaboard of North
America is highly sensitive to the climatic effects of freshwater discharge
into the Labrador Sea and its subsequent impact on the AMOC. While
paleoclimate-model simulations reproduce this sensitivity in part and
indicate that the resulting changes in air temperature and δ 18 O precipitation
values were substantial, they nevertheless underestimate the magnitude
of variation. The final drainage of glacial Lake Agassiz and the subsequent
increase in freshwater flux through the Hudson Strait resulted in a
complex response of δ 18 O precipitation
values over Newfoundland. The primary
isotopic anomaly in core NDN02/1 is consistent with changes in surface
and deep ocean proxies from the Labrador Sea region. It remains to be
explained why the signal of the 8200 yr B.P. event in Greenland ice cores
occurred ~200 yr later than in NDN02/1 and the other highly resolved,
independently dated, terrestrial and oceanic records discussed here.
ACKNOWLEDGMENTS
We thank J. Woodman-Ralph, P. Stanillo, S.F. Crowley, and J.D. Ball for technical
assistance, and the UK Natural Environment Research Council (NERC), including
the NERC RAPID Programme, for financial support. Accelerator mass spectrometer
14
C dates were provided by the NERC Radiocarbon Laboratory, Beta-Analytic Inc.,
and Groningen Radiocarbon Laboratory.
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Manuscript received 20 January 2009
Revised manuscript received 15 April 2009
Manuscript accepted 28 April 2009
Printed in USA
834 GEOLOGY, September 2009