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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
. To perform such a
reconstruction, Chapman
devised a Fienup-type
iterative algo-
rithm. Using a strengthened form of this, Miao et al.
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
can be relaxed somewhat for the higher-dimensional
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
. 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
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
. 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
Gy without observable
morphological damage
. Finally, to improve the resolution with-
out sacrificing specimen size, a CCD detector with more pixelswould
be needed: such detectors are now commercially available.
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
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
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.
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
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
. Abrupt climate fluctuations that occurred during both
Holocene and Late Pleistocene times have been linked to changes
in ocean circulation
, but their causes remain uncertain. One of
the largest such events in the Holocene occurred between 8,400
and 8,000 calendar years ago
C years ago), when
the temperature dropped by 48 8C in central Greenland
1.53 8C at marine
and terrestrial
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
C reservoir
for Hudson Bay which, in combination with other regional data,
indicate that the glacial lakes Agassiz and Ojibway
dammed by a remnant of the Laurentide ice sheet) drained
catastrophically ,8,470 calendar years ago; this would have
released .10
of fresh water into the Labrador Sea. This
finding supports the hypothesis
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
(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
© 1999 Macmillan Magazines Ltd
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22 JULY 1999
| 345
Before its demise, the Hudson Bay ice mass and the associated
proglacial lakes contained a combined volume
estimated at 5 ×
; 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
before its abrupt northward drainage was 10
. The
volume of Lake Agassiz at that time is not well constrained, but we
follow Veillette
in assuming that the volume approximately equalled
that of Ojibway. Thus the total freshwater volume released by
drainage of both lakes is ,2 × 10
. Before drainage, the lake
surfaces stood >175 m above the contemporaneous sea level
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
(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
(Fig. 1). Regional stratigraphic correlations and provenance studies
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
. 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
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
to predict the distribution of sediment
Table 1 Dates on drainage of lakes Agassiz and Ojibway
Sample site Interval
Laboratory no. Ref.* Lat., lon.†
(8N, 8W)
Radiocarbon age‡
CyrBP 6 1j)
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
C ages (no reservoir correction).
§ See Table 2, Fig.1 and text for derivation of local DR values.
k Calibrated date (cal. yr) converted
from weighted mean
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
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
C dates.
Hudson Strait
Agassiz & Ojibway
8.9 cal kyr
ice margin
8.2 cal kyr
ice margin
Figure 1 Northeastern Canada and adjacent seas. Former ice-sheet margins
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
. 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
C age
contemporaneous surface ocean
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
; site 2, west Hudson Strait cores HU85027-068
, 90023-
, -099
; site 3, east Hudson Strait cores HU85027-057
, 90023-
, -064
, 93034-004
; site 4, Cartwright saddle
cores HU87033-017, -018; and
site 5, Orphan knoll
cores HU91045-094 and MD95-2024.
© 1999 Macmillan Magazines Ltd
letters to nature
VOL 400
22 JULY 1999
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
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
O of planktonic foraminifera
between 8.5 and 8.3 cal. kyr ago (,7.87.6
. Farther
southeast, piston cores from the flank of Orphan knoll (Fig. 1) show
increased offsets between the d
O compositions of planktonic
foraminifera Globigerina bulloides and left-coiling Neogloboquadrina
pachyderma (shallow- and deep-dwelling species, respectively)
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
. 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
and the Hudson Strait
(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)
. 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
, 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
Conversion of
C dates to the calendar-year timescale allows
comparison with events in ice-core chronologies
produced by
counting of annual layers, but precise calibration cannot be per-
formed without a correction for the local marine
C reservoir
. 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
. 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
C ages of live-collected Hudson Bay shells
Collection site
(lat. 8N, Ion. 8W)*
Collection year (AD) Radiocarbon age
(years 6 1j)†
Model age
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
C ages.
Model-derived mean surface ocean ages for the year of collection
§ DR = measured shell radiocarbon age minus modelled surface ocean age. Regional means of the DR values are used in calibration
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
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
, 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
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
C-free bicarbonate
; (2) persistent
sea-ice coverage inhibits airsea
C equilibration. Modelling
suggests that the 78 months of seasonal sea ice in the Hudson
could cause an apparent ageing of the marine
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
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
(Fig. 1) probably lengthened
the sea-ice season
. Additionally,
C-free bicarbonate input was
probably higher due to the abundance of fresh, glacially abraded
Palaeozoic carbonate
. 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
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
. 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
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
(Fig. 2).
The cataclysmic release of 2 ×10
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
), 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
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
. 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
O values and increased ice-rafted detritus at
both Cartwright saddle
and Orphan knoll
(Fig. 1), apparently
resulted from the advance and subsequent breakup of a partly
marine-based northern Labrador ice sheet
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
. The
present northward ocean heat transport associated with formation
of LSW is 0.3 PW (1 PW = 10
W), half that due to formation of
NADW (0.6 PW)
. 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
C kyr
Calendar kyr
Cariaco Basin
sediment greyscale
Summit, Greenland
8.2 cal kyr
Cold Event
Figure 2 Climate proxy records of the ‘8.2-kyr’ cold event. Both
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
. 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:// Lower curve shows bidecadal d
O values of ice
from the GISP2 ice core
, 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
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
VOL 400
22 JULY 1999
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
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
C kyr
BP)directly supports the hypothesis
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
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
Correspondence and requests for materials should be addressed to D.B. (e-mail: barberdc@ucsub.
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
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
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
. 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
. 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
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
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
. 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
(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. ...
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). ...
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. ...
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.
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.
Full-text available
The Ningbo Plain on the East China coast is an important center of Neolithic culture, and associated settlements were influenced by changing sea levels and the geomorphological and hydrological environments of the palaeo-Ningbo Bay, the details of which are still subject to debate. This study is based on two well-dated sediment cores obtained from the Ningbo Plain, and here we report analyses of their sedimentology and foraminifera to reveal the infilling history of the palaeo-Ningbo Bay and its association with Neolithic occupation. The lithology of the largely muddy sediments and the dominance of euryhaline and brackish water foraminiferal species are indicative of an intertidal to a subtidal environment in the palaeo-bay during the early to mid-Holocene. Abrupt coarsening of sediment grain size and a corresponding increase in the abundance of foraminiferal species of inner and middle shelf environments occurred at ca. 8.8 cal. kyr BP and 7.6 cal. kyr BP, reflecting two major events of strengthened marine transgression that correspond to the rapid global sea-level rise events of Meltwater pulses (MWPs) 1C and 1D, respectively. A marked increase in the relative abundance of Ammonia annectens and Ammonia compressiuscula during ca. 7.5–7.1 cal. kyr BP further indicates frequent storm surges at that time. Between the two rapid transgression events, aggradation of tidal flats prevailed after ca. 8.0 cal. kyr BP, which provided a suitable setting for Neolithic settlements, as indicated by the recently discovered Jingtoushan site. However, the transgression sequence associated with the latter, the MWP-1D event, caused a regional cultural interruption at ca. 7.6 cal. kyr BP. Infilling and coastal marsh development in the palaeo-Ningbo Bay occurred progressively after ca. 7.0 cal. kyr BP and are associated with the emergence of the Hemudu culture.
Living in a Dangerous Climate provides a journey through human and Earth history, showing how a changing climate has affected human evolution and society. Is it possible for humanity to evolve quickly, or is slow, gradual, genetic evolution the only way we change? Why did all other Homo species go extinct while Homo sapiens became dominant? How did agriculture, domestication and the use of fossil fuels affect humanity's growing dominance? Do today's dominant societies – devoted as they are to Darwinism and 'survival of the fittest' – contribute to our current failure to meet the hazards of a dangerous climate? Unique and thought provoking, the book links scientific knowledge and perspectives of evolution, climate change and economics in a way that is accessible and exciting for the general reader. The book is also valuable for courses on climate change, human evolution and environmental science.
His thesis, combining a dual geomorphological and sedimentological approach coupled with different dating methods, has deciphered the deglaciation sequences of the eastern margin of the Laurentide Ice Sheet since the Last Glacial Maximum, about 21,000 years ago. Two systems with different characteristics, but representative of their region, were investigated: 1) the Clyde fjord-cross-shelf trough system and eastern Baffin Island; and 2) the Churchill River valley and eastern Quebec-Labrador. Overall, this thesis allows redefining the deglaciation chronology of these two key areas and clarifies the main factors influencing the retreat of the ice margin. In addition, these results provide a near-complete temporal coverage of the last deglaciation for the eastern fringe of the Laurentide Ice Sheet, from its maximum extent and initial retreat of a floating ice shelf off Clyde Trough to its terrestrial disintegration recorded by the Churchill River fluvio-deltaic system during the Holocene.
The geography of the Earth at the end of the Tertiary, with the new arrangement of continents, oceans and the distribution of mountain ranges, especially since the opening of the Drake Strait and the closing of the Isthmus of Panama, favoured a global cooling trend that culminated in the Quaternary. In addition to the ice sheets of Antarctica and Greenland, glaciers during the Quaternary tended to expand, especially on the Northern Hemisphere continents and in the mountains. The expansion ended abruptly for short periods, of about 10 ka, during which these glaciers largely disappeared. These periods are called terminations and mark the end of different glacial cycles. In the first half of the Quaternary, terminations occurred every 41 ka, but in the last 800 ka, terminations have been delayed, whilst glaciers could extend over larger areas, occurring every 100 ka. The onset of the terminations and their dynamics remains a mystery, but it coincides with a series of processes, where it is difficult to know what the cause is and what is the effect. It appears that when the glaciers in the Northern Hemisphere reach their maximum extent, an increase in insolation in the mid-latitudes of the Northern Hemisphere causes the onset of global termination. Once termination has begun, a series of temperature changes take place, intense in the Northern Hemisphere and milder in the Southern Hemisphere, but with inverse trends, and in direct relation to changes in the intensity of the Atlantic Meridional Overturning Circulation (AMOC) and to latitudinal changes in atmospheric circulation. Despite these changes in temperature, CO2 in the atmosphere increases throughout the termination, albeit with varying intensity. Once the balance between the AMOC and the proportion of CO2 in the atmosphere is in equilibrium, the temperature stabilises and the termination ends, leading to the onset of an interglacial optimum. This occurs when the northern continental ice sheets have disappeared or have reduced their extension.
Full-text available
Specific components of the Earth system may abruptly change their state in response to gradual changes in forcing. This possibility has attracted great scientific interest in recent years, and has been recognized as one of the greatest threats associated with anthropogenic climate change. Examples of such components, called tipping elements, include the Atlantic Meridional Overturning Circulation, the polar ice sheets, the Amazon rainforest, as well as the tropical monsoon systems. The mathematical language to describe abrupt climatic transitions is mainly based on the theory of nonlinear dynamical systems and, in particular, on their bifurcations. Applications of this theory to nonautonomous and stochastically forced systems are a very active field of climate research. The empirical evidence that abrupt transitions have indeed occurred in the past stems exclusively from paleoclimate proxy records. In this review, we explain the basic theory needed to describe critical transitions, summarize the proxy evidence for past abrupt climate transitions in different parts of the Earth system, and examine some candidates for future abrupt transitions in response to ongoing anthropogenic forcing. Predicting such transitions remains difficult and is subject to large uncertainties. Substantial improvements in our understanding of the nonlinear mechanisms underlying abrupt transitions of Earth system components are needed. We argue that such an improved understanding requires combining insights from (a) paleoclimatic records; (b) simulations using a hierarchy of models, from conceptual to comprehensive ones; and (c) time series analysis of recent observation-based data that encode the dynamics of the present-day Earth system components that are potentially prone to tipping.
According to Philostratus, Indians had founded sixty cities in sub-Saharan Africa already before the ancient times, and according to Juba of Numidia, there was an Indian colony in West Africa. According to Cornelius Nepos, an Indian tribe had sailed to Germania to do commerce, and according to Scymnus, the land of the Indians and Celts was located west from Sardinia. All these pieces of evidence have been traditionally considered invented, fictional, and unreliable. This book provides genetic, archaeological, historical, and linguistic evidence that supports the India–Africa–Europe theory, according to which the Indian culture expanded to Uganda 4000–3300 BCE and spread to West Africa and Europe 3000–150 BCE, most likely already before 650 BCE. This theory was tested against data that was obtained from and The research data provided additional evidence concerning the migrations from India to Western Europe via West Africa. Out of the 22 studied ancient name sets, the Ancient West African, Berber and Libu, Sea Peoples, Etruscan, and Mitanni name sets were found to have their likely urheimat at the Indian Peninsula. Although the name study did not provide dates of the migrations, supplementary data concerning the migration dates and routes was obtained from the Genetic Atlas of Human Admixture History.
Full-text available
Airgun and high resolution Huntec seismic reflection profiles are interpreted to show up to 130 m of glacial, glaciomarine and postglacial sediments overlying bedrock. In a basin at the eastern entrance to Hudson Strait most of the surficial sediment was deposited during the last déglaciation, but in western Hudson Strait multiple till sequences from previous glaciations are recognized. Five acoustic units were identified, at least three of which were penetrated with piston cores. Foraminifera of the stratigraphically deepest core in the eastern basin indicate a proximal glaciomarine environment and a likely presence of an ice shelf. A 14C date of 8060 ± 70 yBP (TO 750) on molluscan shells gives a minimum age for the top of the acoustically laminated distal glaciomarine sediments. The early postglacial foraminifera suggest a period of increased influence of offshore bottom waters restricted to the deep eastern basin. The surface sediments of all cores contain species indigenous of colder and fresher inshore waters of the present time. The ratio of 18CV16O in the benthic foraminifer Cibicides lobatulus is herein related to bottom salinity. Downcore measurements of 8'8O on C. lobatulus tests indicate bottom paleosalinities lower by about 0.5%o shortly before the dated horizon of 8000 yBP. By this time Hudson Strait was sufficiently clear of glacial ice for establishment of the present tidal regime. The lower bottom salinities indicate that tidal mixing took place between glacial meltwater leaving Hudson Bay and the offshore counterflow. This process is thought to have reduced the sharpness of the salinity difference between the offshore water and the surface plume of Laurentide meltwater as it entered the ocean.
Full-text available
The detailed radiocarbon age vs calibrated (cal) age studies of tree rings reported in this Calibration Issue provide a unique data set for precise 14C age calibration of materials formed in isotopic equilibrium with atmospheric CO2. The situation is more complex for organisms formed in other reservoirs such as lakes and oceans. Here the initial specific 14C activity may differ from that of the contemporaneous atmosphere. The measured remaining 14C activity of samples formed in such reservoirs not only reflects 14C decay but also the reservoir 14C activity. Model calibrations are made for the global marine response for surface (0-75m) thermocline (75-1000m) and deep (1000-3800m) waters. Model calculations yield information of atmospheric Δ14C values, production rates, Q, and alternative changes in oceanic mixing rates Kz, and demonstrate the validity of the production modulation approach to calibration. -from Authors
Till containing over 10% matrix carbonate extends in a belt 200-300 km wide south of the Hudson Bay Paleozoic basin source. The southern boundary is represented by the "carbonate line'. Higher silt content and lighter color are associated with the higher carbonate till. Glaciolacustrine sediments have higher carbonate content than nearby till and similarly form widespread sediment blankets even beyond that of the carbonate-rich till. -from Author
A high-resolution transmission x-ray microscope (TXM) has been used for threedimensional imaging. As a first test-object sheaths of the bacteria Leptothrix Ochracea have been visualized. 33 tilted-view images have been taken at tilt angles between 0 and 160 degrees in steps of 5 degrees. Before reconstruction the images were flat-field corrected and aligned. The rotation axis was determined and the images were rotated accordingly. For tomographic reconstruction a fast MART (Multiplicative Algebraic Reconstruction Technique) algorithm was used. Reconstruction of a volume with 2563 voxels from 33 projections of 2562 pixels takes about 20 to 40 minutes on a Silicon Graphics Workstation with a 200 MHz MIPS R4400 processor. In the reconstructed volume structures of sizes around 50 nm are made visible. This new technique gives access to a wide range of new investigations like quantitative measurement of surface, volume and distance. Since tomographic reconstruction returns the linear absorption coefficient of the material included in the objects, it is possible to gain new information on objects, like e.g. the chemical concentration in special cell features. The x-ray microscope, which is routinely used as a 2D investigation-tool in biology, medical research, soil-science and colloid chemistry promises to become a valuable tool for three-dimensional examination.
It is demonstrated that solutions to physically interesting Fourier phase problems, in more than one dimension, are effectively unique, almost always, for localized (in a very wide sense) images such as occur in optics, astronomy and most other branches of image processing. Simple algorithms for generating the aforesaid solutions are outlined here. Practical details of the algorithms and illustrative computational examples are presented in companion papers. It is contended that all this puts recent work by ″maximum entropists″ , , extenders of Gerchberg's algorithm″ and by Bruck and Sodin, Huiser and van Toorn, and most importantly Fienup, into a general practical context.
Les relevés de terrain et l’étude photo-géomorphologique révèlent que le secteur glaciaire du Labrador s’est scindé en deux au droit de la moraine d’Harricana et que par la suite ces deux calottes résiduelles, appelées respectivement glacier du Nouveau-Québec et glacier d’Hudson, se sont retirées l’une vers le nord-est et l’autre vers le nord-ouest au contact des eaux profondes du lac Ojibway. Cette masse d’eau a eu pour effet d’accélérer le retrait des glaciers dont la marge flottait localement à la manière des plates-formes de glace. Le retrait du glacier du Nouveau-Québec fut entrecoupé de courtes pauses et d’un arrêt majeur défini par la moraine de Sakami qui recoupe l’extrémité nord-est des basses terres de la baie de James. Trois récurrences du glacier d’Hudson ont affecté le secteur sud-ouest des basses terres et deux de ces mouvements appartiennent aux réavancées de Cochrane. L’étude de séquences varvées et les caractéristiques du till indiquent qu’il s’agit de crues glaciaires d’une glace partiellement flottée. Le lac Ojibway s’est étendu vers l’est jusqu’à la moraine de Sakami et au-delà de la Grande Rivière vers le nord. Il s’est probablement drainé vers le nord à la hauteur du 80° de longitude ouest. La mer de Tyrrell, qui a submergé les basses terres de la baie de James et de la mer d’Hudson, a atteint l’altitude de 290 m sur le versant est et de 198 m sur le versant sud. La vidange du lac, l’invasion marine et l’arrêt de Sakami sont datés à 7900 ans BP, alors que les réavancées de Cochrane I et II ont atteint leur position maximale, il y a 8200 ans et 7975 ans.
The temporal and geographical roles of meltwater discharge (from the Laurentide ice sheet) on North Atlantic Deep Water (NADW) production are investigated utilizing a global, realistic geometry, coupled climate model which does not require the use of flux adjustments. Model results suggest that preconditioning by meltwater discharge (to the Mississippi) prior to the Younger Dryas (YD) is capable of pushing NADW beyond the limit of its sustainability. The diversion of meltwater to the St. Lawrence then merely serves to completely inhibit NADW production. The modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the YD and is intimately linked to changes in NADW formation. The global thermohaline circulation provides an interhemispheric teleconnection with the Southern Oceans, while changes in the atmospheric heat transport (reacting to a global redistribution of oceanic heat transport) provide a mechanism for interbasin teleconnection. Although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model, in the absence of additional feedbacks an equilibrium without the presence of NADW is possible. The inclusion of the wind stress/speed feedback is found to significantly contribute to the resumption of NADW production, as suggested by previous studies. Contrary to these same studies, however, the coupled model indicates an advective spin-up timescale is required for resumption of NADW production and hence the termination of the modeled YD-like climate event (as opposed to a decadal-century timescale). The reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies, or the duration, strength, and geographical location of the imposed meltwater applied.