LETTERS
Episodic reductions in bottom-water
currents since the last ice age
SUMMER K. PRAETORIUS*, JERRY F. MCMANUS, DELIA W. OPPO AND WILLIAM B. CURRY
Geology and Geophysics Department, Woods Hole Oceanographic Institution, MS 23, Woods Hole, Massachusetts 02543, USA
* e-mail: [email protected]
Published online: 15 June 2008; doi:10.1038/ngeo227
Past changes in the freshwater balance of the surface North
Atlantic Ocean are thought to have influenced the rate of
deep-water formation, and consequently climate1,2 . Although
water-mass proxies are generally consistent with an impact of
freshwater input on meridional overturning circulation3 , there
has been little dynamic evidence to support this linkage. Here
we present a 25,000 year record of variations in sediment
grain size from south of Iceland, which indicates vigorous
bottom-water currents during both the last glacial maximum
and the Holocene period. Together with reconstructions of North
Atlantic water-mass distribution, vigorous bottom currents
suggest a shorter residence time of northern-source waters
during the last glacial maximum, relative to the Holocene period.
The most significant reductions in flow strength occur during
periods that have been associated with freshening of the surface
North Atlantic. The short-term deglacial oscillations in bottom
current strength are closely coupled to changes in Greenland air
temperature, with a minimum during the Younger Dryas cold
reversal and a maximum at the time of rapid warming at the
onset of the Holocene. Our results support a strong connection
between ocean circulation and rapid climate change.
North Atlantic Deep Water (NADW) is an important
subsurface water mass, produced by the warm-to-cold conversion
of seawater in the upper limb of the Atlantic’s meridional
overturning circulation (MOC)4 . Changes in the volume of NADW
or its relative influence at a given location have been linked to
past climate variations1–3 . Previous studies using nutrient proxies
as water-mass tracers indicate that during the last glacial maximum
(LGM) the northern-source deep waters shoaled to an intermediate
water depth5–8 . This has led to the suggestion that the meridional
overturning was weaker during the glacial period, possibly as
a result of increased glacial melt water causing a reduction in
the surface density that contributes to deep-water convection7 .
However, water-mass composition alone cannot provide sufficient
constraints on the rate of overturning9 , which is a crucial element
in reconstructing the climatic influence of ocean circulation.
Additional constraints on this rate require proxies that provide
information on the motion of seawater, such as the strength of flows
associated with deep-water formation10 .
The grain size of sediments acts as a physical proxy
for bottom-current strength as a result of the depositional
sorting associated with variable flow speed11,12 . Because fine
particles tend to aggregate, the non-cohesive portion of the
silt size fraction (10–63 µm) has been proposed as the most
useful size range in examining past changes in flow speeds11,12 .
The mean size of this sortable silt (SS) has been widely
and successfully used to reconstruct the past strength of
bottom currents at different times and locations in the Atlantic
Ocean10,13 . By pairing grain-size measurements with a water-mass
chemistry tracer such as the δ13 C of benthic foraminifera shells
(δ13 C = [(13 C/12 C)sample /(13 C/12 C)standard ] − 1), a more complete
reconstruction can be made of changes that have occurred in
both the flow strength and water-mass configuration throughout
large-scale climate variations.
Ocean Drilling Program Site 984 (Fig. 1) is well situated to
evaluate changes in the vigour of the North Atlantic’s MOC during
the past 25,000 years. It is located at a depth of 1,648 m on a
rapidly accumulating sediment drift along the Reykjanes Ridge.
Today, Site 984 lies directly under the path of the Iceland–Scotland
Overflow Waters, which are a major component of the NADW.
These flows originate in the Nordic seas and spill over the shallow
Icelandic ridge, forming a Coriolis-influenced contour current that
flows southward along the Reykjanes Ridge4 . During the glacial
period, these overflows that now fill the deep Atlantic were replaced
by a shallower water mass known as the Glacial North Atlantic
Intermediate Water (GNAIW)3 , which probably formed south of
the Greenland–Iceland–Scotland ridge14 and was confined to the
upper 2,000 m of the Atlantic basin5–8 . The intermediate depth of
Site 984 is therefore ideally suited to monitor the past strength of
bottom currents along the ridge associated with the GNAIW, as
well as those deep currents originating as overflows from the Nordic
Seas to the north today.
The grain-size record shows some variations among the three
separate approaches applied to calculating the SS mean value
(see the Methods section); however, the major trends throughout
the record are robust and well defined (Fig. 2). The mean grainsize values are similar (∼19–20 µm) throughout the LGM and
Holocene (Fig. 3), indicating comparable bottom-flow strengths
at this location. Although the grain-size values are similar, the
δ13 C record suggests that these flows were associated with different
water masses. The glacial waters have consistently high values of
1.6h, the endmember value for GNAIW6,8 , indicating that the
flows are within the core of this water mass. The Holocene values
are more variable and average around 1.0h, the modern δ13 C
value of NADW15 . The grain-size values remain relatively stable
throughout most of the Holocene, whereas the glacial period shows
a series of larger millennial-scale oscillations. The SS% record also
shows increased variability during the LGM, ranging from 10% to
45% SS (Fig. 2).
Minima in the grain-size values occur near 24 kyr, 16 kyr and
12.7 kyr, and are accompanied by δ13 C minima. The reductions
at 24 kyr and 16 kyr occur during Heinrich events 1 and 2 (H1
and H2), and are characterized by large peaks in ice-rafted debris,
reflecting periods of massive iceberg discharge into the North
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LETTERS
Greenland
Labrador
Sea
Greenland
Sea
60° W
30° E
70° N
rre
X
Site 984
22
20
18
60
rw
eg
ia
u
nc
16
50
60° N
40
30
30° W
A tl a
n ti c c u rr e nt
2.0
50° N 0°
Figure 1 Schematic map of the North Atlantic circulation32 with the location of
ODP Site 984 (61◦ 250 N, 24◦ 040 W, 1.65 km). Red arrows indicate the warm saline
waters of the North Atlantic current, whereas blue arrows depict the return flow
paths of NADW. Site 984 lies directly in the path of the modern Iceland–Scotland
Overflow Waters.
δ 18O C.wuellerstorfi (‰)
rth
SS %
No
40° N
H2
LGM
nt
Norwegian
Sea
No
H1
24
Grain size (μm)
Labra
dor
60° E
YD BA
Holocene
26
80° N
20
2.5
10
3.0
0
3.5
4.0
4.5
5.0
0
2
4
6
8
10
12 14 16
Age (kyr)
18
20
22
24
26
Figure 2 The δ 18 O of benthic foraminifera plotted on an inverted axis with the
sediment-size data from Site 984. Black arrows indicate 14 C dates and the red
arrow indicates the Vedde Ash Horizon. The 10–63 µm, 10–45 µm and variable
100% size fractions are all plotted together with 2% error bars, on the basis of a
study of the precision of grain-size measurements33 . A three-point running average
was applied to each individual size fraction as well as the average of all three
brackets, shown by the bold black line. Black error bars are the standard error
between the three smoothed data sets. The SS% represents the amount of sediment
in the 10–63 µm range relative to the entire sample of 0.5–63 µm. Climate intervals
indicated: Holocene interglacial; Younger Dryas cold reversal (YD); Bølling–Allerød
warm period (BA); Heinrich events 1 and 2 (H1 and H2); last glacial maximum (LGM).
Atlantic16 . Ice-rafting events generally increase the grain size of
deep-sea sediments11,12 , so minima at these times do not result
from iceberg sediment delivery. The minimum at 12.7 kyr coincides
with a minimum in the δ13 C and the lowest air temperatures
in the GISP2 record during the Younger Dryas (YD), a period
marked by a rapid cooling to near-glacial conditions within a
trend towards interglacial warmth17 . It has been widely speculated
that the YD cooling was caused by a reduction in the MOC as
a result of a rerouting or catastrophic discharge of glacial melt
water originating from glacial Lake Agassiz, and routed either
through the St. Lawrence river18 or through the Arctic19 . A recently
hypothesized extraterrestrial impact on North America20 might
have had a similar meltwater impact on the MOC21 . The best
existing study of deglacial bottom currents in this region10 did not
fully resolve the YD reduction, but it is strongly evident in both SS
and δ13 C records from Site 984. The low benthic δ18 O values during
H1 and possibly the YD (Fig. 2) may indicate the subduction of
glacial melt water.
Directly following the YD minimum, the grain size increases
to a maximum at 11 kyr, the onset of the Holocene. This peak
in grain size includes the highest values within the record, as
well as the largest change in amplitude (1SS = 5 µm). The SS%
also doubles at this time, reflecting a switch from highly clay-rich
sediments during the glacial (∼15% SS) towards a siltier size
fraction during the Holocene (∼40% SS). The grain size record
remains relatively stable throughout most of the Holocene, with
the largest decrease occurring around 8.8 kyr, slightly preceding
the largest δ18 O decrease within the Holocene in the GISP2 record,
known as the ‘8.2 kyr event’22 .
Both glacial and interglacial periods show similarly high
grain size (∼20 µm), suggestive of strong current strengths.
However, the carbon isotopes indicate that these flows involved
different water masses. The high δ13 C values of 1.6h during
the LGM are characteristic of GNAIW6,8 , and when viewed in
conjunction with generally high grain size suggest that this
water mass maintained a relatively rapid rate of overturning.
Because the GNAIW was about half the volume of the modern
NADW5–8 , it must have had a significantly reduced residence
time. This is contrary to the idea of a weak glacial circulation,
and suggests that both glacial and interglacial modes can be
robust in the North Atlantic despite changes in the water-mass
configuration. Prevailing glacial conditions, including a generally
stronger wind field and greater tidal dissipation at depth, might
be expected to provide energy for the continued mechanical
mixing required to maintain the overturning circulation. Previous
studies utilizing scavenged radionuclides in Atlantic sediments
have indicated a significant export at depth during the LGM23,24 .
New records of radionuclides from shallower water depths imply
more rapid export in intermediate waters during the glacial
maximum25 . Combined with the evidence for diminished volume
of GNAIW compared to the modern NADW5–8 , the new Site 984
paired isotopic and grain-size evidence confirms that substantial
LGM meridional overturning and subsurface export occurred at
intermediate depths.
Changes in the grain size and carbon isotopes are closely
coupled in their timing and magnitude throughout the deglacial
period, and both show the largest decreases during H1, H2 and the
YD. Carbon isotopes may be influenced by a number of factors in
this region6 , including air–sea exchange and nutrient utilization.
Combined with the grain-size evidence for reductions in abyssal
flow rates, the sharp decreases in δ13 C at Site 984 are more likely to
represent reduced ventilation and/or greater mixing with low-δ13 C
southern-source waters, perhaps due to an even further shoaling
of GNAIW. These reductions in the depth and flow strength of
2
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LETTERS
–32
GISP2
YD
δ 18O (‰)
–34
H1
H2
–36
–38
24
–40
22
984
δ 13C C.wuellerstorfi (‰)
–44
20
2.0
18
Grain size (μm)
–42
984
1.6
16
1.2
0.8
Although the δ13 C and grain-size records generally correlate
very well during the deglacial period, there is considerable
variability within each proxy that does not show coeval changes on
millennial timescales. The δ13 C values remain fairly stable (∼1.6h)
during the glacial period despite some fluctuations in the current
strength (18–21 µm), whereas the δ13 C values fluctuate (0.8–1.5h)
in the Holocene despite a relatively stable current (19–20 µm).
This indicates that there can be changes in the current strength
without changes in the source water, whereas changes in the
endmember isotope values are not always associated with changes
in current strength. This highlights the importance of pairing
water-mass proxies with dynamic proxies to resolve the spatial and
temporal dynamics of the overturning circulation, both of which
are important factors in the overall climatic impact of meridional
overturning circulation.
0.4
METHODS
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26
Age (kyr)
Figure 3 GISP2 Greenland ice core δ 18 O record34 plotted with grain size and
carbon isotopes from Site 984. The grain-size record represents a three-point
running mean of the average of all three size-bracket calculations (shown as the
bold black line in Fig. 2). The δ 13 C record is composed of individual measurements
of benthic foraminifera C. wuellerstorfi. Shaded intervals indicate YD, Younger Dryas
cold reversal, and H1 and H2, Heinrich ice-rafting events 1 and 2: periods that have
been associated with large freshwater flux to the surface North Atlantic.
Grain-size measurements were made on the fine fraction (<63 µm) of sediment,
at intervals approximately 200 years apart. Samples were acidified with a 10%
acetic acid solution to remove the calcium carbonate fraction, primarily
planktonic microfossils that can be delivered through vertical particle rain as
opposed to being laterally transported by bottom currents. A Sedigraph 5100
was used to analyse the grain-size distribution of the remaining terrigenous
fraction. This system uses X-ray attenuation to measure the settling time of
particles falling out of suspension, and enables the calculation of the grain-size
distribution on the basis of Stoke’s settling principle, thus making it particularly
well suited to estimate the influence of hydrodynamics on sediment grain-size
spectra11,12 . The grain size was measured over a total range of 0.5–63 µm;
however, the <10 µm fraction was excluded from the grain-size calculations
to minimize artefacts related to the cohesive behaviour of fine particles. The
mean grain size was calculated using three separate approaches: the standard
SS fraction (10–63 µm), a modified size range of 10–45 µm and a size bracket
with a moving upper limit that was determined as the interval from which
the cumulative mass finer per cent began to systematically drop from 100%
(see Supplementary Information, Fig. S1). The two modified brackets were
calculated in order to reduce the analytical noise in some samples owing
to negligible material in the coarsest size range. The moving bracket was
thought to represent the most accurate calculation of the true mean, as it
isolated the fraction with the largest percentage of material within the specific
size distribution of each sample. The grain-size interval designated for each
sample in the variable bracket is listed in the Supplementary Information,
Set S1. The mean grain size for each size bracket was averaged together and a
three-point running mean was applied to the curve. The relative percentage
of SS (10–63 µm) to clay particles (0.5–10 µm) was also calculated for each
sample, which can indicate important aspects of the sediment delivery. The
chronology (see Supplementary Information, Table S1) was established using
18 radiocarbon dates and a correlation with the previously dated Vedde Ash
layer31 . Standard deviations of the respective calculations throughout the
majority of the record average around 0.65 µm, whereas the standard deviations
during the YD, H1 and H2 intervals are reduced by nearly half (σ = 0.37 µm),
strengthening the interpretation of these low grain-size values as periods with
reduced bottom-flow speeds.
northern-source waters occur during the coldest periods in the
GISP2 record, indicating a strong link between the circulation
system and the northern-hemisphere climate. Similarly, increases in
the current strength are seen during the Bølling–Allerød (BA) warm
period and during the time of rapid warming at the onset of the
Holocene interglacial period. Carbon-isotope values during both
of these warmings increase to around 1.0h. The peak in grain size
following the YD could indicate a sudden resumption of overflows
from the Norwegian Seas. The strong increase in current strength
in this period of rapid warming implies a close connection between
the re-initiation of the overturning circulation and the transition
into warm interglacial conditions.
The large increase in the SS% at the onset of the Holocene
is likely to reflect a change in the sediment source. Larger grains
typically represent a shorter distance of transport from the source
location owing to a shorter time of reworking, therefore the shift
to a larger overall grain size within the entire measured fraction
(0.5–63 µm) could suggest a more direct or proximal sediment
source to the core location. This would most likely be a result of the
initiation of the Iceland–Scotland Overflow Waters from the north,
which would entrain the previously deposited glacial sediments
lying on the Iceland shelf.
The decrease in grain size at approximately 8.8 kyr may be
linked with the ‘8.2 kyr’ cold event, which is evident as the coldest
Holocene interval in Greenland ice cores22 and North Atlantic
sediments26,27 . A series of sharp decreases in bottom-current
strength was also detected in the early portion of a nearby
Holocene record28 . Chronological uncertainty makes it difficult to
say whether these events are coeval, although the cooling during
this interval may have spanned several hundreds of years between 8
and 9 kyr (ref. 29). This cold spell has been suggested to be a result
of a damping of the MOC as a result of a catastrophic draining of
glacial Lake Agassiz through the Hudson Bay, thus introducing a
large amount of freshwater to the surface ocean30 .
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Received 15 February 2008; accepted 15 May 2008; published 15 June 2008.
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Acknowledgements
This study was improved by assistance and technical support from M. Jeglinski. Support for this
research was provided in part by the US-NSF, the WHOI-OCCI and the Comer Science and
Research Foundation.
Author information
Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions.
Correspondence and requests for materials should be addressed to S.K.P.
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