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The sensitivity of oceanic thermohaline circulation to freshwater perturbations is a critical issue for understanding abrupt climate change[1]. Abrupt climate fluctuations that occurred during both Holocene and Late Pleistocene times have been linked to changes in ocean circulation [2, 3, 4, 5, 6], but their causes remain uncertain. One of the largest such events in the Holocene occurred between 8,400 and 8,000 calendar years ago [2,7,8] (7,650–7,200 14C years ago), when the temperature dropped by 4–8 °C in central Greenland2 and 1.5–3 °C at marine [4,7] and terrestrial [7,8] sites around the northeastern North Atlantic Ocean. The pattern of cooling implies that heat transfer from the ocean to the atmosphere was reduced in the North Atlantic. Here we argue that this cooling event was forced by a massive outflow of fresh water from the Hudson Strait. This conclusion is based on our estimates of the marine 14C reservoir for Hudson Bay which, in combination with other regional data, indicate that the glacial lakes Agassiz and Ojibway [9, 10, 11, 12] (originally dammed by a remnant of the Laurentide ice sheet) drained catastrophically approx 8,470 calendar years ago; this would have released >10^14 m^3 of fresh water into the Labrador Sea. This finding supports the hypothesis [2,7,8] that a sudden increase in freshwater flux from the waning Laurentide ice sheet reduced sea surface salinity and altered ocean circulation, thereby initiating the most abrupt and widespread cold event to have occurred in the past 10,000 years.
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discrete Bragg peaks. This continuous pattern can therefore be
sampled on a finer scale. That sufficient oversampling can lead to a
reconstruction was pointed out by Bates
4
. To perform such a
reconstruction, Chapman
2
devised a Fienup-type
17
iterative algo-
rithm. Using a strengthened form of this, Miao et al.
5
were able not
only to perform reconstructions of model data in two and three
dimensions, but also to show that the degree of oversampling called
for by Bates
4
can be relaxed somewhat for the higher-dimensional
cases.
In our experiment we made use of this reconstruction algorithm.
The reconstruction from the diffraction pattern of Fig. 2 is shown in
Fig. 4. Our phasing algorithm uses knowledge of a finite support
which is defined as an enclosing boundary of the specimen. In this
reconstruction, we chose a 5:7 mm 3 5:7 mm square as the finite
support which is larger than the size of the image itself. The initial
input to the iterative algorithm was a random phase set and, after
about 1,000 iterations, a good reconstruction (Fig. 4) was obtained.
The computing time of 1,000 iterations is , 30 min on a 450-MHz
Pentium II workstation. Details of the reconstruction procedure are
given elsewhere
5,16
. The reconstructed image is consistent with the
resolution limit, ,75 nm, set by the angular extent of the CCD
detector. The inner portion of the diffraction pattern could also be
filled by Fourier processing of a moderate-resolution image of the
specimen made with a scanning transmission X-ray microscope
1
,
whereupon a reconstruction with an almost perfectly clean back-
ground was obtained.
We believe that the successful recording and reconstruction of the
test pattern reported here is the critical step that will open the way
to high-resolution three-dimensional imaging of such structures as
small whole cells, or large sub-cellular structures, in cell biology.
Extension from two to three dimensions requires that a series of
diffraction patterns be recorded as the specimen is rotated around an
axis perpendicular to the beam. We have take the first steps in this
direction.Modelcalculations indicatethattheiterativealgorithm used
in this work is able to reconstruct such a data set
5
. To be able to collect
the data set from a biological (or other radiation-sensitive) specimen,
it would be necessary to keep the specimen near the temperature of
liquid nitrogen. Experiments show that specimens at this tempera-
ture can withstand a radiation dose up to 10
10
Gy without observable
morphological damage
18,19
. Finally, to improve the resolution with-
out sacrificing specimen size, a CCD detector with more pixelswould
be needed: such detectors are now commercially available.
M
Received 24 March; accepted 8 June 1999.
1. Kirz, J., Jacobsen, C. & Howells, M. Soft X-ray microscopes and their biological applications. Q. Rev.
Biophys. 28, 33130 (1995).
2. Sayre, D. & Chapman, H. N. X-ray microscopy. Acta Crystallogr. A 51, 237252 (1995).
3. Millane, R. P. Phase retrieval in crystallography and optics. J. Opt. Soc. Am. A 7, 394411 (1990).
4. Bates, R. H. T. Fourier phase problems are uniquely solvable in more than one dimension. I:
underlying theory. Optik 61, 247262 (1982).
5. Miao, J., Sayre, D. & Chapman, H. N. Phase retrieval from the magnitude of the Fourier transforms of
non-periodic objects. J. Opt. Soc. Am. A 15, 16621669 (1998).
6. Sayre, D., Kirz, J., Feder, R., Kim, D. M. & Spiller, E. Potential operating region for ultrasoft X-ray
microscopy of biological specimens. Science 196, 13391340 (1977).
7. Jacobsen, C. & Kirz, J. X-ray microscopy with synchrotron radiation. Nature Struct. Biol. 5,
(synchrotron suppl.), 650653 (1998).
8. Jacobsen, C., Kirz, J. & Williams, S. Resolution in soft X-ray microscopes. Ultramicroscopy 47, 5579
(1992).
9. Thieme, J., Schmahl, G., Umbach, E. & Rudolph, D. (eds) X-ray Microscopy and Spectromicroscopy
(Springer, Berlin, 1998).
10. Haddad, W. S. et al. Ultra high resolution x-ray tomography. Science 266, 12131215 (1994).
11. Lehr, L. 3D x-ray microscopy: tomographic imaging of mineral sheaths of bacteria Leptothrix ochracea
with the Go
¨
ttingen x-ray microscope at BESSY. Optik 104, 166170 (1997).
12. Wang, Y., Jacobsen, C., Maser, J. & Osanna, A. Soft x-ray microscopy with cryo STXM: II.
Tomography. J. Microsc. (in the press).
13. Howells, M. et al. X-ray holograms at improved resolution: a study of zymogen granules. Science 238,
514517 (1987).
14. Lindaas, S., Howells, M., Jacobsen, C. & Kalinovsky, A. X-ray holographic microscopy by means of
photoresist recording and atomic-force microscope readout. J. Opt. Soc. Am. A 13, 1788–1800 (1996).
15. Sayre, D. in Imaging Processes and Coherence in Physics (eds Schlenker, M. et al.) 229235 (Springer,
Berlin, 1980).
16. Sayre, D., Chapman, H. N. & Miao, J. On the extendibility of X-ray crystallography to noncrystals.
Acta Crystallogr. A 54, 233239 (1998).
17. Fienup, J. R. Phase retrieval algorithm: a comparison. Appl. Opt. 21, 27582769 (1982).
18. Schneider, G. & Niemann, B. in X-ray Microscopy and Spectromicroscopy (eds Thieme, J., Schmahl, G.,
Rudolph, D. & Umbach, E.) 2534 (Springer, Berlin, 1998).
19. Maser, J. et al.inX-ray Microscopy and Spectromicroscopy (eds Thieme, J., Schmahl, G., Rudolph, D. &
Umbach, E.) 3544 (Springer, Berlin, 1998).
20. Lindaas, S. et al.inX-ray Microscopy and Spectromicroscopy (eds Thieme, J., Schmahl, G., Rudolph, D.
& Umbach, E.) 7586 (Springer, Berlin, 1998).
Acknowledgements. The decision to try oversampling as a phasing technique was arrived at in a
conversation in the late 1980s with G. Bricogne. W. Yun and H. N. Chapman also participated in early
parts of this experiment. We thank C. Jacobsen for help and advice, especially with the numerical
reconstruction, and we thank him and M. Howells for use of the apparatus
20
in which the exposures were
made; we also thank S. Wirick for help with data acquisition. P.C. thanks the Leverhulme Trust Great
Britain for supporting the nanofabrication programme at King’s College, London. This work was
performed at the National Synchrotron Light Source, which is supported by the US Department of
Energy. Our work was supported in part by the US Department of Energy.
Correspondence and requests for materials should be addressed to J.M. (e-mail: miao@xray1.physics.
sunysb.edu).
Forcing of the cold event of
8,200 years ago by
catastrophic drainage
of Laurentide lakes
D. C. Barber*, A. Dyke
, C. Hillaire-Marcel
, A. E. Jennings*,
J. T. Andrews*, M. W. Kerwin*, G. Bilodeau
, R. McNeely
,
J. Southon§, M. D. Morehead* & J.-M. Gagnonk
* Institute for Arctic & Alpine Research, and Department of Geological Sciences,
University of Colorado, Boulder, Colorado 80309, USA
Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada
Centre de recherche en ge
´
ochimie isotopique et en ge
´
ochronologie,
Universite
´
du Que
´
bec a
`
Montreal, Que
´
bec H3C 3P8, Canada
§ Center for Accelerator Mass Spectrometry, L-397, Lawrence Livermore National
Laboratory, PO Box 808, Livermore, California 94551, USA
k Canadian Museum of Nature, PO Box 3443, Station D, Ottawa,
Ontario K1P 6P4, Canada
.........................................................................................................................
The sensitivity of oceanic thermohaline circulation to freshwater
perturbations is a critical issue for understanding abrupt climate
change
1
. Abrupt climate fluctuations that occurred during both
Holocene and Late Pleistocene times have been linked to changes
in ocean circulation
2–6
, but their causes remain uncertain. One of
the largest such events in the Holocene occurred between 8,400
and 8,000 calendar years ago
2,7,8
(7,6507,200
14
C years ago), when
the temperature dropped by 48 8C in central Greenland
2
and
1.53 8C at marine
4,7
and terrestrial
7,8
sites around the north-
eastern North Atlantic Ocean. The pattern of cooling implies that
heat transfer from the ocean to the atmosphere was reduced in the
North Atlantic. Here we argue that this cooling event was forced
by a massive outflow of fresh water from the Hudson Strait. This
conclusion is based on our estimates of the marine
14
C reservoir
for Hudson Bay which, in combination with other regional data,
indicate that the glacial lakes Agassiz and Ojibway
9–11
(originally
dammed by a remnant of the Laurentide ice sheet) drained
catastrophically ,8,470 calendar years ago; this would have
released .10
14
m
3
of fresh water into the Labrador Sea. This
finding supports the hypothesis
2,7,8
that a sudden increase in
freshwater flux from the waning Laurentide ice sheet reduced
sea surface salinity and altered ocean circulation, thereby initiat-
ing the most abrupt and widespread cold event to have occurred in
the past 10,000 years.
During the period of deglaciation that preceded the abrupt
climate event of 8,4008,000 calendar years (cal. yr) ago (the ‘8.2-
kyr event’), a remnant Laurentide ice mass occupied Hudson Bay
and served as an ice dam for glacial lakes Agassiz and Ojibway
9–12
(Fig. 1). The rapid collapse of ice in Hudson Bay allowed lakes
Agassiz and Ojibway, which had previously discharged over spill-
ways southeastwards to the St Lawrence estuary, to drain swiftly
northwards through the Hudson Strait to the Labrador Sea
10,1315
.
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Before its demise, the Hudson Bay ice mass and the associated
proglacial lakes contained a combined volume
11
estimated at 5 ×
10
14
m
3
; however, .50% of this was ice that could not have left the
Hudson Strait as rapidly as water from the lakes. So to calculate the
peak freshwater flux to the Labrador Sea, we consider only the water
in lakes Agassiz and Ojibway. The approximate volume of Lake
Ojibway
9,11
before its abrupt northward drainage was 10
14
m
3
. The
volume of Lake Agassiz at that time is not well constrained, but we
follow Veillette
11
in assuming that the volume approximately equalled
that of Ojibway. Thus the total freshwater volume released by
drainage of both lakes is ,2 × 10
14
m
3
. Before drainage, the lake
surfaces stood >175 m above the contemporaneous sea level
9,11
,
providing a large initial hydraulic head that drove the outburst once
a conduit to the sea opened.
The stratigraphy of the Hudson and James Bay lowlands
9,11,16
(Fig. 1) ubiquitously records the abrupt lake drainage: glacial-
marine sediments lie directly above the proglacial lake sediments.
Additional evidence for the lake outburst is the 580-cm-thick, red-
coloured, haematite-rich sediment layer traceable for 700 km in
cores from the western to the eastern reaches of the Hudson Strait
14,15
(Fig. 1). Regional stratigraphic correlations and provenance studies
15
suggest that the Hudson Strait ‘red bed’ shared a common source
with red glacial deposits in north-central Hudson Bay and red-
brown glaciolacustrine sediments in the former Agassiz and
Ojibway basins
911,16
. The simultaneous deposition of the red bed
throughout the Hudson Strait at a time in the glacial period when
the strait was free of ice
14,17
required an extraordinary sediment
transport mechanism; the catastrophic outburst flood released from
lakes Agassiz and Ojibway probably provided this mechanism.
We evaluated the outburst drainage model, which would have
resulted in high freshwater flux and long-range sediment dispersal,
by running numerical simulations of plume deposition in the
Hudson Strait. The discharge resulting from instantaneous removal
of the ice dam was estimated, and we then used an oceanic plume
sedimentation model
18
to predict the distribution of sediment
Table 1 Dates on drainage of lakes Agassiz and Ojibway
Sample site Interval
(cm)
Laboratory no. Ref.* Lat., lon.†
(8N, 8W)
Radiocarbon age‡
(
14
CyrBP 6 1j)
DR§
(yr)
Cal. agek
(cal. yr BP)
1j range
(cal. yr BP)
...................................................................................................................................................................................................................................................................................................................................................................
SE Hudson Bay
Post-dates drainage; dates marine incursion
SE James Bay Qu 122 9 53.35, 77.57 8,280 6 160
SE James Bay Qu 124 9 53.35, 77.57 8,150 6 180
SW James Bay GSC 897 16 50.22, 84.30 8,160 6 160#
SW James Bay GSC 880 16 51.93, 84.53 8,120 6 140#
(8,150 6 50)
310 8,280 8,3308,160
...................................................................................................................................................................................................................................................................................................................................................................
West Hudson Strait
Post-dates drainage; above red bed
90023-085 98100 TO 3265 14 62.62, 76.38 8,170 6 140 130 8,420 8,6008,320
Pre-dates drainage; below red bed
90023-101 365367 AA 12888 15 63.05, 74.30 8,260 6 60
90023-099 320325 AA 12887 14 63.07, 74.57 8,270 6 70
85027-068 989996 TO 751 21 63.08, 74.31 8,310 6 70
(8,280 6 40) 130 8,550 8,6508,490
...................................................................................................................................................................................................................................................................................................................................................................
East Hudson Strait
Post-dates drainage; above red bed
93034-004 1921 CAMS 25762 17 61.22, 66.43 8,030 6 60
90023-045 480483 AA 17380 15 60.95, 66.14 8,155 6 130
90023-064 460462 TO 3263 14 61.13, 70.58 8,160 6 150
(8,065 6 50) 85 8,380 8,4408,325
Pre-dates drainage; below red bed
85027-057 814822 TO 749 21 61.07, 66.43 8,140 6 70
93034-004 78–80 AA 13055 17 61.22, 66.43 8,395 6 70
90023-045 777779 AA 11879 15 60.95, 66.14 8,490 6 200
(9,280 6 50) 85 8,610 8,7408,520
...................................................................................................................................................................................................................................................................................................................................................................
* References cited provide additional sample information.
Positions in decimal degrees; also see Fig. 1.
Radiocarbon dates given as conventional, d
13
C-normalized
14
C ages (no reservoir correction).
§ See Table 2, Fig.1 and text for derivation of local DR values.
k Calibrated date (cal. yr) converted
26
from weighted mean
14
C age using local DR values. Mean calibrated age between bounding dates on lake drainage is 8,470 cal. yr BP.
Note that calibrated age ranges are asymmetrical due to the nonlinear
14
C calibration curve.
# In accord with laboratory protocol, GSC dates are reported here with 2j errors; corresponding 1j errors were used when calculating weighted means from these dates.
Parentheses enclose weighted averages of preceding
14
C dates.
50°N
60°N
50°W70°W90°W
Hudson Strait
Greenland
Hudson
Bay
LSW
Labrador
Sea
70°N
Baffin
Bay
2
3
4
5
1
James
Bay
130
85
310
Lakes
Agassiz & Ojibway
8.9 cal kyr
ice margin
8.2 cal kyr
ice margin
Lowlands
Labrador
1
Figure 1 Northeastern Canada and adjacent seas. Former ice-sheet margins
12
are shown for ,8.9 cal. kyr ago (vertical-hatched line) and for ,8.2 cal. kyr ago
(thick grey line), before and after disintegration of ice in central Hudson Bay,
respectively. Horizontal hatching shows lakes Agassiz and Ojibway
9–12
. North-
ward drainage through Hudson Bay and Hudson Strait (dark grey arrows)
occurred as the Hudson Bay ice mass disintegrated. Arrows with dashed lines
show Labrador Sea current patterns and the area of Labrador Sea Intermediate
Water (LSW) formation. Numbers in boxes are regional mean DR values (years),
based on radiocarbon analyses of mollusc shells collected alive before 1955
(DR
local
=
14
C age
meas.
contemporaneous surface ocean
14
C age; Table 2). Sites
discussed in text and referred to in Table 1 (filled circles) are as follows: site 1, SW
and east of James Bay, marine deposits post-dating drainage of glacial lakes
Agassiz and Ojibway
9–16
; site 2, west Hudson Strait cores HU85027-068
21
, 90023-
085
14
, -099
14
,-101
15
; site 3, east Hudson Strait cores HU85027-057
21
, 90023-
045
15
, -064
14
, 93034-004
17
; site 4, Cartwright saddle
19
cores HU87033-017, -018; and
site 5, Orphan knoll
20
cores HU91045-094 and MD95-2024.
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resulting from an event of this magnitude. The simulation predicted
deposit thicknesses of ,50 cm in the western Hudson Strait,
thinning to ,30 cm in the eastern Hudson Strait. The approximate
agreement of modelled deposit thicknesses with the observed red
bed
14,15,17
supports the interpretation that final drainage of lakes
Agassiz and Ojibway produced a massive freshwater pulse to the
Labrador Sea (Fig. 1).
Evidence for a freshwater pulse is also found beyond the Hudson
Strait. Cores from Cartwright saddle on the Labrador shelf (Fig. 1)
exhibit a 0.6 reduction in the d
18
O of planktonic foraminifera
between 8.5 and 8.3 cal. kyr ago (,7.87.6
14
Ckyr)
19
. Farther
southeast, piston cores from the flank of Orphan knoll (Fig. 1) show
increased offsets between the d
18
O compositions of planktonic
foraminifera Globigerina bulloides and left-coiling Neogloboquadrina
pachyderma (shallow- and deep-dwelling species, respectively)
20
.
The offset increases from 0.8 before the freshwater pulse to 1.3
during the pulse, due to a 0.5 reduction in the G. bulloides values.
This isotopic shift is comparable to that at Cartwright saddle and
indicates lower sea surface salinities and increased water-mass
stratification throughout the western Labrador Sea
20
. Thus data
from various sites support a large freshwater plume entering the
Labrador Sea in the early Holocene. We determined the calendar age
of the Laurentide lake outburst to ascertain whether this freshwater
input coincided with onset of the ‘8.2-kyr’ cold event.
The published radiocarbon dates constraining the final Agassiz
Ojibway drainage are on marine carbonates from the Hudson and
James Bay lowlands
9,12,16
and the Hudson Strait
14,15,17,21
(Table 1;
Fig. 1). Because of the pattern of ice marginal recession along the
southern margin of Hudson Bay, the post-glacial sea reached its
highstand throughout that region simultaneously. Dates on fossil
marine bivalves from uplifted basal post-glacial marine sediments
in the Hudson Bay lowlands constrain the time of initial marine
incursion, and thus also provide limiting ages that post-date
drainage of lakes Agassiz and Ojibway (Table 1; Fig. 1)
9,16
. In the
Hudson Strait (Fig. 1), the radiocarbon ages bounding the fresh-
water pulse are on bivalves and foraminifera collected above and
below the red bed in marine sediment cores (Table 1; Fig. 1). In
earlier work on the overall chronostratigraphy of the Hudson
Strait
14,15,17,21
, many more dates were obtained than we include in
Table 1. We excluded many dates because sediment reworking
obfuscated their stratigraphic context. Of the remaining dates, we
consider only those that most closely constrain deposition of the red
bed.
Conversion of
14
C dates to the calendar-year timescale allows
comparison with events in ice-core chronologies
2,6
produced by
counting of annual layers, but precise calibration cannot be per-
formed without a correction for the local marine
14
C reservoir
effect
22
. The local reservoir correction is the sum, in radiocarbon
years, of the mean global surface ocean reservoir age (R) plus any
local deviation (DR) from the contemporaneous global-mean ocean
age
22
. Detailed comparisons between radiocarbon dates from dif-
ferent regions or reservoirs (for example, the atmosphere) require
an estimate of DR. In the past, DR values were not well established;
so in earlier work, reservoir corrections of 400450 years (that is,
DR = 0) were applied to shallow marine dates. Here we estimate
local DR values using radiocarbon dates on 34 museum shell
Table 2
14
C ages of live-collected Hudson Bay shells
Laboratory
no.
Collection site
(lat. 8N, Ion. 8W)*
Collection year (AD) Radiocarbon age
(years 6 1j)†
Model age
(years)‡
DR
(years)§
...................................................................................................................................................................................................................................................................................................................................................................
East Hudson Strait and Ungava Bay
CAMS-33148 60.41, 64.83 1948 630 6 50 480 150
CAMS-34644 59.22, 65.75 1947 540 6 50 480 60
CAMS-34654 59.48, 65.25 1950 650 6 50 480 170
CAMS-46546 61.63, 71.97 1920 500 6 50 460 40
CAMS-46550 60.83, 69.93 1950 620 6 40 480 140
CAMS-46555 60.07, 69.43 1949 430 6 40 480 50
GSC-6107 59.22, 65.75 1947 480 6 40 480 0
TO-5980 59.22, 65.75 1947 650 6 40 480 170
n = 8 Mean: 560 85
...................................................................................................................................................................................................................................................................................................................................................................
West Hudson Strait and North Hudson Bay
CAMS-33144 64.40, 77.93 1953 760 6 50 480 280
CAMS-33146 66.47, 86.20 1955 690 6 50 480 210
CAMS-33149 63.00, 82.65 1954 690 6 50 480 210
CAMS-34647 63.60, 82.00 1953 480 6 50 480 0
CAMS-34648 64.33, 75.58 1954 600 6 50 480 120
CAMS-46547 62.95, 81.84 1954 510 6 40 480 30
CAMS-46549 63.00, 82.65 1954 590 6 40 480 110
CAMS-46551 64.23, 76.55 1954 430 6 40 480 50
CAMS-46552 63.68, 80.20 1953 560 6 40 480 80
CAMS-46556 64.23, 76.55 1954 590 6 40 480 110
CAMS-46557 63.00, 82.65 1954 530 6 50 480 50
CAMS-46559 64.23, 76.55 1954 520 6 50 480 40
CAMS-46560 63.60, 82.00 1953 670 6 50 480 190
CAMS-47241 66.92, 81.33 1955 810 6 40 480 330
CAMS-47244 62.98, 82.69 1954 610 6 40 480 130
TO-5977 64.40, 77.93 1953 690 6 50 480 210
n = 16 Mean: 610 130
...................................................................................................................................................................................................................................................................................................................................................................
SE Hudson Bay and James Bay
CAMS-46545 56.50, 77.00 1920 630 6 40 460 170
CAMS-46561 56.25, 76.33 1920 580 6 50 460 120
CAMS-46755 52.00, 79.50 1941 970 6 40 475 495
CAMS-46757 52.95, 79.00 1920 720 6 40 460 260
CAMS-46759 52.00, 79.50 1920 1,050 6 40 460 590
CAMS-46760 52.95, 79.00 1920 740 6 40 460 280
CAMS-46761 52.60, 78.75 1920 810 6 40 460 350
CAMS-47247 55.28, 77.75 1949 560 6 50 480 80
CAMS-48978 53.12, 79.86 1920 740 6 40 460 280
CAMS-48979 52.00, 79.50 1920 940 6 40 460 480
n = 10 Mean: 775 210
...................................................................................................................................................................................................................................................................................................................................................................
* Live shell sample locations in decimal degrees.
Laboratory-reported, conventional, d
13
C-normalized
14
C ages.
Model-derived mean surface ocean ages for the year of collection
22
.
§ DR = measured shell radiocarbon age minus modelled surface ocean age. Regional means of the DR values are used in calibration
26
of ages for the lake outburst event (Table 1).
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specimens that were collected alive in the Hudson Bay region from
AD 1920 to 1955, before significant contamination of the atmo-
sphere with bomb radiocarbon (Table 2). The ages of these shells
range from 430 to 1,050
14
Cyr(1j errors range from 40 to 50 yr).
Despite the observed variability, the ages typically exceed 550 yr and
the means of ages from each region increase consistently with
distance from the open Labrador Sea (Fig. 1). By comparing the
shell dates with modelled mean-ocean reservoir age data
22
, we derive
DR values for the southeastern Hudson Bay and James Bay of 310 6
50 yr, for the northern Hudson Bay and western Hudson Strait of
130 6 50 yr, and for the eastern Hudson Strait of 85 6 50 yr (Table 2;
Fig. 1).
We identify two possible causes of the higher DR values (that is,
lower initial
14
C activities) for the marine carbon pools in the
Hudson Bay and Hudson Strait: (1) runoff draining Palaeozoic
limestone bedrock and carbonate-rich glacial sediments in the
region provides dissolved
14
C-free bicarbonate
23
; (2) persistent
sea-ice coverage inhibits airsea
14
C equilibration. Modelling
24
suggests that the 78 months of seasonal sea ice in the Hudson
Strait
25
could cause an apparent ageing of the marine
14
C reservoir
by 150200 yr, slightly more than observed (Table 2; Fig. 1).
However, despite a shorter sea-ice season, James Bay shells have
the highest DR values in the region (Table 2; Fig. 1). Significant
runoff from carbonate-rich areas enters James Bay, thus both sea ice
and inputs of ancient carbon seem to contribute to the observed
pattern in DR values. To calibrate
14
C dates from the deglacial period
8.87.5 cal. kyr ago , we must evaluate possible differences in the
factors influencing DR. During the time of interest, high inputs of
melt water from the residual ice sheet
12
(Fig. 1) probably lengthened
the sea-ice season
25
. Additionally,
14
C-free bicarbonate input was
probably higher due to the abundance of fresh, glacially abraded
Palaeozoic carbonate
23
. The effects on DR of differing water masses
and current patterns during deglaciation are not known, although
the lower percentage of Labrador Sea surface water (pre-bomb DR =
0) in the mixed Hudson Bay water mass
25
may have produced larger
DR values during deglaciation. Taken together, these effects imply
that the regional DR values in Table 2 are conservative (that is, low)
with respect to those ,8,000 years ago.
The DR values derived here facilitate conversion of radiocarbon
ages for the final AgassizOjibway drainage into calendar ages using
the marine calibration scheme in CALIB 3.03A
26
. We calculated the
age of the freshwater pulse (8,470 cal. yr before present,
BP) as the
midpoint between means of both the younger and older event-
bounding calibrated ages (Table 1). Within the limits of annual ice-
core layer counting, radiocarbon dating, DR estimates, and
14
C-to-
calendar year conversion, the age derived here for final northward
drainage of lakes Agassiz and Ojibway coincides with the 8,400 6
100 cal. yr
BP onset of climate cooling in Greenland and elsewhere
2–8
(Fig. 2).
The cataclysmic release of 2 ×10
14
m
3
of lake water over 1, 10 or
100 years would have increased the freshwater flux to the Labrador
Sea by 6, 0.6 or 0.06 Sv (1 Sv = 10
6
m
3
s
1
), respectively. Numerical
simulation of the Hudson Strait redbed deposit suggests that
drainage occurred in less than one year, but the available chronology
does not yield a precise duration. Results from ocean circulation
models
2729
suggest that excess freshwater discharges of 0.060.12 Sv
can reduce the formation rates of Labrador Sea Intermediate Water
(LSW) and North Atlantic Deep Water (NADW), thereby strongly
affecting ocean heat transport. These simulations do not specifically
apply to the lake outburst case, however, because the excess
discharges were prescribed for periods of .500 years in the
models
2729
. Although the ocean freshening due to AgassizOjibway
drainage was of shorter duration, the lakewater pulse was preceded
by an interval (600900 years) of somewhat reduced sea surface
salinity in the Labrador Sea. This previous low-salinity interval,
recorded by reduced d
18
O values and increased ice-rafted detritus at
both Cartwright saddle
19
and Orphan knoll
20
(Fig. 1), apparently
resulted from the advance and subsequent breakup of a partly
marine-based northern Labrador ice sheet
17
.
The low sea surface salinities resulting from the AgassizOjibway
outburst propagated southeast from Hudson Strait, producing
more pronounced freshening in the region of LSW formation
(Fig. 1) than at the more distant NADW formation sites
7
. The
present northward ocean heat transport associated with formation
of LSW is 0.3 PW (1 PW = 10
15
W), half that due to formation of
NADW (0.6 PW)
30
. If the formation of both LSW and NADW
ceased during the Younger Dryas cold event, but only LSW forma-
tion was disrupted during the ‘8.2-kyr’ event, then for the latter
event we might expect regional atmospheric cooling of one-third
the magnitude as that during the Younger Dryas. This scenario
freshwater pulse
from lake outburst
185
190
195
200
-36
-35
-34
58769
14
C kyr
8
109765
Calendar kyr
BP
Cariaco Basin
sediment greyscale
Summit, Greenland
GISP2
δ
18
O
ice
(‰)
8.2 cal kyr
Cold Event
Figure 2 Climate proxy records of the ‘8.2-kyr’ cold event. Both
14
C (top) and
calendar (lower) timescales are given. Upper curve shows Cariaco basin
greyscale record; reduced greyscale values indicate increased zonal wind
speed due to high-latitude cooling
5
. Timescale of greyscale variations differs from
that in ref. 5 due to subsequent work by those authors; data on the revised
timescale are available from the World Data Center-A for Paleoclimatology (http://
www.ngdc.noaa.gov/paleo). Lower curve shows bidecadal d
18
O values of ice
from the GISP2 ice core
6
, interpreted to reflect primarily the temperature of
precipitation over Summit, Greenland; more negative values indicate colder
temperatures. Also shown is age for the lake drainage event: 8,470 cal. yr
BP or
,7.7
14
C kyr (vertical dashed line); extremes of the 1j cal. age ranges on the
bounding dates (Table 1) give an error range of 8,1608,740 cal. yr
BP (shaded).
© 1999 Macmillan Magazines Ltd
letters to nature
348 NATURE
|
VOL 400
|
22 JULY 1999
|
www.nature.com
undoubtedly oversimplifies ocean circulation and climate boundary
conditions, but the resulting prediction of the relative amplitudes of
the two cold events approximates the relative cooling observed in
proxy records that contain both events
2–6
.
Evidence presented hereof a large freshwater pulse from the
final outburst drainage of lakes Agassiz and Ojibway, together with
the revised timing of this pulse (,8,470 cal. yr
BP,or,7.7
14
C kyr
BP)directly supports the hypothesis
3,7,8
that an increase in fresh-
water flux modified ocean circulation, thereby causing the observed
‘8.2-kyr’ climate cooling. This result provides perspective both on
the sensitivity of ocean circulation to freshwater inputs and on the
climate oscillations of the present interglacial period. Specifically,
our findings suggest that in the case of the ‘8.2-kyr’ event, the
thermohaline circulation responded to exceptionally strong forcing:
initiation of the most abrupt and widespread climate shift known
from the past 10,000 (calendar) years required a massive, albeit
short-lived, perturbation of the North Atlantic freshwater
balance.
M
Received 26 March; accepted 8 June 1999.
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Acknowledgements. We thank the Canadian Museum of Nature for providing archived live-collected
shells, and G. Bond, D. Fisher, B. MacLean and J. Teller for comments on the manuscript. This work was
supported by the Terrain Sciences Division, Geological Survey of Canada, and the US NSF (A.E.J. and
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Correspondence and requests for materials should be addressed to D.B. (e-mail: barberdc@ucsub.
colorado.edu)
Asynchronous deposition of
ice-rafted layers in the Nordic
seas and North Atlantic Ocean
J. A. Dowdeswell*, A. Elverhøi
, J. T. Andrews
& D. Hebbeln§
* Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol,
Bristol BS8 1SS, UK
Department of Geology, University of Oslo, Postboks 1047, Blindern,
N-0316 Oslo, Norway
Institute of Arctic and Alpine Research and Department of Geological Sciences,
University of Colorado, Boulder, Colorado 80309, USA
§ FB Geowissenschaften, University of Bremen, Postfach 330440,
D-28344 Bremen, Germany
.........................................................................................................................
Instabilities in ice-stream flow within the North American
Laurentide Ice Sheet, leading to the periodic release of armadas
of icebergs into the North Atlantic Ocean over the past 60,000
years, have produced extensive layers of coarse-grained iceberg-
rafted debris (Heinrich layers) in North Atlantic sediments
1,2
.
Correlation of these layers with iceberg-discharge events from the
ice sheets on Greenland, Iceland and Scandinavia, suggested in
previous studies for some Heinrich layers and in some areas
3–5
,
would imply that ice-sheet instability had been synchronous
across the North Atlantic, presumably in response to a common
environmental cause. Here we show a lack of widespread systematic
correlations, both between ice-rafted debris layers in different
sediment cores from the Nordic seas, and between the Nordic
layers and the North Atlantic Heinrich layers. This suggests that
the full-glacial Nordic ice sheets did not exhibit unstable behav-
iour coincident with iceberg discharge from the vast Hudson Bay
drainage basin of the Laurentide Ice Sheet
6,7
. Off the Hudson
Strait, significant ice-sheet discharge of melt water is indicated by
size-sorted sandy and muddy turbidite sediments, different from
the poorly sorted debris flows which dominate sedimentation on
the margins of the Nordic seas
8–10
. Together, these results suggest
that the dynamics of Quaternary ice sheets surrounding the
Nordic seas were different from the outlet glacier draining the
Hudson Bay basin, and they provide evidence against a common
circum-North-Atlantic mechanism driving the discharge of
icebergs.
A number of marine geological studies have demonstrated that
huge numbers of icebergs, derived mainly from the Hudson Strait,
produced a series of six rapidly deposited
11
Heinrich layers of
iceberg-rafted debris (IRD). These distinctive layers range from
about one metre to a few centimetres in thickness, and can be traced
for more than 3,000 km across the North Atlantic
1,2
. However,
although many sediment cores have been examined from the
Nordic seas (Fig. 1), coarse-grained layers of the thickness and
spatial continuity of the Heinrich layers have not been identified.
Correlative IRD events have so far been found only in some areas
and for some of the Heinrich events
35,12,13
(Fig. 2).
Cores from sites adjacent to outlet glaciers of the Fennoscandian,
Svalbard-Barents Sea and Greenland ice sheets (Fig. 1b) show that
there is little evidence for a systematic regional correlation of IRD
layers across the Nordic seas, nor is there a consistent linkage with
the North Atlantic Heinrich layers (Fig. 2). A detailed study of IRD
... ( Barber et al., 1999;Li et al., 2012;Törnqvist and Hijma, 2012). An alternative cause, a freshwater outburst from the collapsing ice saddle over Hudson Bay, has also been proposed (Gregoire et al., 2012;Matero et al., 2017). ...
... Additionally, there is evidence for multiple meltwater fluxes prior to and/or during the event (Hillaire-Marcel et al., 2001;Ellison et al., 2006;Lochte et al., 2019;Brouard et al., 2021). Whilst the exact trigger is still debated, the volume and routing of freshwater entering the North Atlantic Ocean was apparently sufficient to disrupt the Atlantic Meridional Overturning Circulation (AMOC), which transports heat from the tropics to the Arctic region (Barber et al., 1999;Ellison et al., 2006). ...
... Due to its proximity to the North Atlantic, south-western Europe is a climatologically important region for studying the impact of AMOC perturbations (Baldini et al., 2015). Several highquality speleothem records describe the Holocene climate of this region in terms of rainfall amount and/or seasonality (Domínguez-Villar et al., 2008;Railsback et al., 2011;Smith et al., 2016;Moreno et al., 2017;Baldini et al., 2019;Benson et al., 2021). Specifically, the regional climatic impact of the 8.2 ka event has mainly been described in terms of hydrological change from these studies. ...
Article
Full-text available
The 8.2 ka event is regarded as the most prominent climate anomaly of the Holocene and is thought to have been triggered by a meltwater release to the North Atlantic that was of sufficient magnitude to disrupt the Atlantic Meridional Overturning Circulation (AMOC). It is most clearly captured in Greenland ice-core records, where it is reported as a cold and dry anomaly lasting ∼ 160 years, from 8.25 ± 0.05 until 8.09 ± 0.05 ka (Thomas et al., 2007). It is also recorded in several archives in the North Atlantic region; however, its interpreted timing, evolution and impacts vary significantly. This inconsistency is commonly attributed to poorly constrained chronologies and/or inadequately resolved time series. Here we present a high-resolution speleothem record of early Holocene palaeoclimate from El Soplao Cave in northern Spain, a region pertinent to studying the impacts of AMOC perturbations on south-western Europe. We explore the timing and impact of the 8.2 ka event on a decadal scale by coupling speleothem stable carbon and oxygen isotopic ratios, trace element ratios (Mg / Ca and Sr / Ca), and growth rate. Throughout the entire speleothem record, δ18O variability is related to changes in effective recharge. This is supported by the pattern of changes in δ13C, Mg / Ca and growth rate. The 8.2 ka event is marked as a centennial-scale negative excursion in El Soplao δ18O, starting at 8.19 ± 0.06 ka and lasting until 8.05 ± 0.05 ka, suggesting increased recharge at the time. Although this is supported by the other proxies, the amplitude of the changes is minor and largely within the realm of variability over the preceding 1000 years. Further, the shift to lower δ18O leads the other proxies, which we interpret as the imprint of the change in the isotopic composition of the moisture source, associated with the meltwater flux to the North Atlantic. A comparison with other well-dated records from south-western Europe reveals that the timing of the 8.2 ka event was synchronous, with an error-weighted mean age for the onset of 8.23 ± 0.03 and 8.10 ± 0.05 ka for the end of the event. This compares favourably with the North Greenland Ice Core Project (NGRIP) record. The comparison also reveals that the El Soplao δ18O is structurally similar to the other archives in south-western Europe and the NGRIP ice-core record.
... This generates a small accommodation space, that limits the on-mound deposition of current-transported sediments (Wang et al., 2021), temporarily slowing mound formation. Climate simulations show that large parts of the Northern Hemisphere, including the NE Atlantic, were affected by periods of abrupt cooling of 1-3 • C at 8.2 ka (Barber et al., 1999;Thomas et al., 2007;Morrill et al., 2013), caused by the centennial meltwater pulse from the collapse of the Hudson Bay ice saddle (Carlson et al., 2008;Carlson et al., 2009;Gregoire et al., 2012;Wagner et al., 2013;Matero et al., 2017;Appah et al., 2020). Regionally, this short climactic shift has been observed in CWC mound records from the Porcupine Seabight and Rockall Trough, where mound formation slows due to decelerated bottom current speeds (O'Reilly et al., 2004;Frank et al., 2009). ...
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Full-text available
Within the Porcupine Bank Canyon (NE Atlantic), cold-water coral (CWC) mounds are mostly found clustered along the canyon lip, with individual disconnected mounds occurring nearby on the western Porcupine Bank. Remotely operated vehicle-mounted vibrocoring was utilized to acquire cores from both of these sites. This study is the first to employ this novel method when aiming to precisely sample two closely situated areas. Radiometric ages constrain the records from the early to mid-Holocene (9.1 to 5.6 ka BP). The cores were then subjected to 3D segmented computer tomography to capture mound formation stages. The cores were then further examined using stable isotopes and benthic foraminiferal assemblages, to constrain the paleoenvironmental variation that influenced CWC mound formation of each site. In total, mound aggradation rate in the Porcupine Bank Canyon and western Porcupine Bank was comparable to other Holocene CWC mounds situated off western Ireland. Results derived from multiproxy analysis, show that regional climatic shifts define the environmental conditions that allow positive coral mound formation. In addition, the aggradation rate of coral mounds is higher adjacent to the Porcupine Bank Canyon than on the western Porcupine Bank. Benthic foraminifera assemblages and planktic foraminiferal δ¹³C reveal that higher quality organic matter is more readily available closer to the canyon lip. As such, we hypothesize that coral mound formation in the region is likely controlled by an interplay between enhanced shelf currents and the existence of the Eastern North Atlantic Water-Mediterranean Outflow Water-Transition Zone. The geomorphology of the canyon promotes upwelling of these water masses that are enriched in particles, including food and sediment supply. The higher availability of these particles support the development and succession of ecological hotspots along the canyon lip and adjacent areas of the seafloor. These observations provide a glimpse into the role that submarine canyons play in influencing macro and micro benthic fauna distributions and highlights the importance of their conservation.
... When it finally broke through the ice dam in several places and poured up to 70,000 km 3 of water into the Atlantic Ocean over a six-month period, sea level rose by as much as 19 cm (Clarke et al. 2004), and the Gulf Stream, which carries enormous amounts of heat into the North Atlantic, was weakened. This led to cooling in Greenland and Europe (Barber et al. 1999) and caused Alpine glaciers to advance. ...
Chapter
There is a whole range of methods that can be used to reconstruct former glacier length. Many of them are aimed at the dating of moraines, but bogs and human traces also allow conclusions to be drawn about past extents. In the ice ages, warm periods (interglacials) alternated with cold periods (glacials) during which the glaciers were particularly large. The triggering and regulating mechanisms of these large climate fluctuations are subject to both terrestrial and extraterrestrial control. The most recent ice age, the Pleistocene, has left visible traces in the landforms of Europe and North America to this day. In the period since the last glacial there have also been fluctuations of climate and glaciers, but with a smaller amplitude than in the epoch before. At present, we are dealing with accelerated glacier retreat almost everywhere on Earth, which will continue in the future, and the consequences of which act on different spatial scales.
Article
The 4.2 ka event at the Mid- to Late-Holocene transition is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. The event is most clearly manifested in the Mediterranean and Middle East as a regional dry anomaly beginning abruptly at 4.26 kyr BP and extending until 3.97 kyr BP. Yet the impacts of this regional drought are often extended to other regions and sometimes globally. In particular, the nature and spatial extent of the 4.2 ka event in the tropics have not been established. Here, we present a new stalagmite stable isotope record from Anjohikely, northwest Madagascar. Growing between 5.22 and 2.00 kyr BP, stalagmite AK1 shows a hiatus between 4.31 and 3.93 kyr BP (±40 and ± 35 yrs), replicating a hiatus in another stalagmite from nearby Anjohibe, and therefore indicating a significant drying at the Mid- to Late-Holocene transition. This result is the opposite to wet conditions at the 8.2 ka event, suggesting fundamentally different forcing mechanisms. Dry conditions are also recorded in sediment cores in Lake Malawi, Lake Masoko and the Tatos Basin on Mauritius, also in the southeast African monsoon domain. However, no notable event is recorded at the northern (equatorial East Africa) and eastern (Rodrigues) peripheries of the monsoon domain, while a wet event is recorded in sediment cores at Lake Muzi and Mkhuze Delta to the south. The spatial pattern is largely consistent with the modern rainfall anomaly pattern associated a with weak Mozambique Channel Trough and a northerly austral summer Intertropical Convergence Zone position. Within age error, the observed peak climate anomalies overlap with the 4.2 ka event. However regional hydrological change consistently begins earlier than a 4.26 kyr BP event onset. Gradual hydrological change frequently begins around 4.5 kyr BP, raising doubt as to whether any coherent regional hydrological change is merely coincident with the 4.2 ka event or part of a global climatic anomaly.
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Chapter
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