Marine Geology 279 (2011) 188–198
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Marine Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o
Foraminifer isotope study of the Pleistocene Labrador Sea, northwest North Atlantic
(IODP Sites 1302/03 and 1305), with emphasis on paleoceanographical differences
between its “inner” and “outer” basins
Claude Hillaire-Marcel ⁎, Anne de Vernal, Jennifer McKay 1
GEOTOP, Université du Québec à Montréal, P.O. Box 8888, Montréal, Qc H3C 3P8 Canada
a r t i c l e
i n f o
Article history:
Received 10 March 2010
Received in revised form 31 October 2010
Accepted 1 November 2010
Available online 8 November 2010
Communicated by D.J.W. Piper
Keywords:
IODP
Pleistocene
isotope stratigraphy
Labrador Sea
foraminifers
a b s t r a c t
Cores raised during IODP Expedition 303 off southern Greenland (Eirik Ridge site 1305) and off the Labrador Coast
(Orphan Knoll site 1302/1303) were analyzed to establish an isotope stratigraphy, respectively for the “inner” and
“outer” basins of the Labrador Sea (LS). These isotopic data also provide information on the Atlantic Meridional
Overturning Circulation (AMOC), notably with regard to the intensity of the Western Boundary Under Current
(WBUC), which is tightly controlled by the production of Denmark Strait Overflow Water (DSOW), and the
production of Labrador Sea Water (LSW) in the inner basin through winter cooling and convection. The upper
184 m of sediment at Eirik Ridge spans marine isotope stages (MIS) 32 to 1. At this site, two distinct regimes are
observed: prior to MIS 20, the isotopic record resembles that of the open North Atlantic records of the interval,
whereas a more site-specific pattern is observed afterwards. This later pattern was characterized by i) high DSOW
production rates and strong WBUC during interglacial stages, as indicated by sedimentation rates, ii) large
amplitude δ18O-shifts from glacial stages to interglacial stages (N 2.5‰) and iii) an overall range of δ18O-values
significantly more positive than before. At Orphan Knoll, the 105 m record spans approximately 800 ka and
provides direct information on linkages between the northeastern sector of the Laurentide Ice Sheet and the
North Atlantic. At this site, a shift towards larger amplitude glacial/interglacial ranges of δ18O-values occurred
after MIS 13, although isotopic records bear a typical North Atlantic signature, particularly during MIS 5, in
contradiction to those of Eirik Ridge, where substages 5a to 5c are barely recognized. Closer examination of δ18Orecords in planktic and benthic foraminifera demonstrates the presence of distinct deep-water masses in the
inner vs. outer LS basins during MIS 11 and more particularly MIS 5e. Data confirm that the modern AMOC, with
LSW formation, seems mostly exclusive to the present interglacial, and also suggest some specificity of each
interglacial with respect to the production rate of DSOW and the AMOC, in general.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Sites drilled in the Labrador Sea during the Ocean Drilling Program
(ODP) Leg 105 and the Integrated Ocean Drilling Program (IODP)
Expedition 303 off southern Greenland (Sites 646 and 1305) and off
eastern Canada (Site 1302/1303) are ideally located to document the
climatic and glacial history of adjacent lands. Sites 646 and 1305 from
the Eirik Ridge yield sedimentary sequences recording ice and climate
variability over Greenland (de Vernal and Hillaire-Marcel, 2008),
whereas Site 1302/1303 from Orphan Knoll illustrate all major glacial
events in the northeastern sector of the Laurentide Ice Sheet (LIS) as
shown by piston cores from this area collected prior to IODP drilling
(cf. Hillaire-Marcel et al., 1994a; Stoner et al., 1996, 1998, 2000;
⁎ Corresponding author.
E-mail address: [email protected] (C. Hillaire-Marcel).
1
College of Oceanic and atmospheric sciences, Oregon State University 104 COAS
Admin Bldg Corvallis OR 97331-5503 USA.
0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.margeo.2010.11.001
Hillaire-Marcel and Bilodeau, 2000; Hiscott et al., 2001). Moreover,
the Eirik Ridge and Orphan Knoll sites are well situated to provide
information on the Atlantic Meridional Overturning Circulation
(AMOC), notably with regard to convection in the Labrador Sea and
to the intensity of the Western Boundary Under Current (WBUC),
which is tightly controlled by the production of Denmark Strait
Overflow Water (DSOW) (cf. Hunter et al., 2007). The two IODP sites
under investigation here are located at almost similar depths,
respectively on the northeastern side of Eirik Ridge off southern
Greenland (Site 1305 = 3463 m) and at the foot of Orphan Knoll off
the southern Labrador coast (Sites 1302 and 1303; 3561 and 3520 m,
respectively; Fig. 1). These two sites are bathed by deep currents
entering the “inner” Labrador Sea (NW basin; Site 1305) and
outflowing the “outer” Labrador Sea (SE sector of the basin, Sites
1302/1303) just before their merging into the open North Atlantic
Ocean, off the Great Banks (Fig.1). A survey of the literature, prior
1994, about the late Quaternary paleoceanography of the Labrador
Sea can be found in a special issue of the Canadian Journal of Earth
Sciences (cf. Hillaire-Marcel et al., 1994b) whereas more recent
C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
189
Fig. 1. Location map of study sites on Eirik Ridge in the inner Labrador Sea (LS), and Orphan Knoll in the outer Labrador Sea. Arrows illustrate major deep circulation components as
follows: Denmark Strait Overflow Water (DSOW), Norwegian Sea Overflow Water (NSOW), Iceland–Scotland Overflow Water (ISOW), Wyville–Thomson Ridge Overflow (WTRO),
North East Atlantic Deep Water (NEADW), Davis Strait Overflow (DSO), North Atlantic Deep Water (NADW), Western Boundary Under-Current (WBUC). Blue shaded areas
correspond to the modern production area of Labrador Sea Water (LSW). The Eirik Ridge site provides information on incoming deep water masses, notably the DSOW (green
arrows) whereas the Orphan Knoll site illustrate conditions along southward flowing deep water trajectories and records glacial surges (cf. Heinrich Events) from Hudson Strait, as
illustrated by the brown arrows.
studies are briefly reported in Hillaire-Marcel et al. (2001a–b), de
Vernal and Hillaire-Marcel (2006) and Fagel and Hillaire-Marcel
(2006), for example. Already drilled in 1985 (ODP Leg 105; Site 646),
the Eirik Ridge area was revisited in 2004 during IODP Expedition 303
in order to retrieve fresh material for higher resolution investigations
from the closely located Site 1305 (57°28.5′ N, 48° 31.8′ W; water
depth: 3460 m; cf. Channell et al., 2006a), slightly more to the south
than site 646. At Orphan Knoll, Sites 1302/1303 (~50°10 N, 45°38 W;
water depth: 3560 m; cf. Channel et al., 2006a) yielded the first highresolution sedimentary sequence spanning most of the Pleistocene,
down to approximately 800 ka (Channell et al., 2006a,b).
The objectives of our investigations and of the related analytical
work were multi-fold. Firstly, we aimed to produce a high-resolution
δ18O-stratigraphy from planktic foraminifers spanning the entire
Pleistocene with comparative δ18O-data from benthic foraminifers for
critical intervals. Secondly, we intended documenting changes in the
water mass structure of the Labrador Sea during interglacial stages
particularly marine isotope stages (MIS) 5e and 11, to complement
earlier investigations on a sequence from Eirik Ridge, by HillaireMarcel et al. (2001a), which documented MIS 5e conditions in the
“inner” basin. Unfortunately, earlier cores from the deep Orphan Knoll
site, representative of the “outer” basin (Fig. 1), were either too short
to reach this interval, or presented sampling artifacts making
impossible to recover undisturbed interglacial sequences. This was
notably the case of “Calypso” cores raised from the Marion Dufresne in
1995 and 1999 (cf. Hillaire-Marcel and Turon, 1999). Thirdly, we
intended to further document deep-current pathways and deepwater mass origins from clay mineral assemblages and their Nd and
Pb-isotopes and sea-surface conditions from dinoflagellate cyst
assemblages (e.g., de Vernal and Hillaire-Marcel, 2006). These data
are already published (e.g., de Vernal and Hillaire-Marcel, 2006; Fagel
and Hillaire-Marcel, 2006) or will be published elsewhere. In this
paper we focus on the first two objectives listed above. At Site 1305,
we analyzed the sequence from 0–184 mcd (meters composite
depth), which extents below the Jaramillo chronozone and encompasses the MIS 1–31 interval (Channell et al., 2006a). At site 1302/
1303, we analyzed most of the sequence that was drilled during
Expedition 303, down to 105 mcd.
2. Methods
In order to get a full recovery, several holes were drilled at each
site and then correlated using multi-sensor core logger data recovered
onboard (cf. Shipboard Scientific Party, 2005). Sub-sampling was
made at 5-cm interval on the composite sequences of sites 1305 and
1302/1303. It provided a theoretical resolution of about 500 a, as
documented below, i.e., within the mean age of mixed layers at the
modern water/sediment interface (Wu and Hillaire-Marcel, 1994a).
Stable isotope measurements on planktic foraminifer assemblages
of Neogloboquadrina pachyderma (left coiled), henceforth Npl, were
performed at 5 cm-intervals at both sites, and on Globigerina bulloides
(Gb) from a few intervals encompassing the warm interglacial
episodes of MIS 5e (both sites) and MIS 11 (Site 1302/1303 only).
At Site 1305, isotope measurements were difficult to make in
sediments of MIS 11 because of the poor preservation of the
foraminifer shells and the rarity of Gb specimens. We also performed
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C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
the surface layer (e.g., Bé and Tolderlund, 1971; Kuroyanagi and
Kawahata, 2004; Simstich et al., 2003). In comparison, the distribution
of Gb is often related to food availability and primary productivity in
the epipelagic layer (e.g., Reynolds and Thunell, 1985). In the North
Atlantic Gb is considered to be typical of spring bloom and it is usually
associated with shallow water habitat above the pycnocline (e.g.,
Ganssen and Kroon, 2000; Schiebel et al., 1997). However, in colder
environments, and notably during the last glacial cycle, its development
seems to take place during the warmer late summer months (e.g.,
Hillaire-Marcel and Bilodeau, 2000). We thus interpret the isotopic
compositions of Npl and Gb to reflect conditions from the top to the
bottom of the pycnocline between the surface and intermediate water
masses (i.e., from below the base of the surface mixed layer to the top of
the intermediate water layer), allowing for some seasonal discrepancies (cf. Hillaire-Marcel et al., 2001a; see also Jonkers et al., 2010,
concerning the seasonal fluxes and calcification depths of such planktic
foraminifers in modern environments of the northern North Atlantic).
isotopic analysis of the benthic foraminifers Cibicidoides wuellerstorfi
(Cw) in the interval spanning MIS 5e at both sites, and in sediment of
MIS 11 at Site 1302/1303. At Site 1305, there are insufficient benthic
foraminifers in MIS 11 for isotopic measurements. Sediment samples
were then sieved at 106 μm, using tap water (pH ~ 7) to avoid
dissolution. For isotopic analysis of planktic species, we used an
average of 12 shells (i.e., about 80 μg) hand-picked from the 150–
250 μm size fraction. For the benthic species (mostly Cw), 7 to 12
specimens were handpicked from the N 150 μm fraction, in the critical
intervals also selected for Gb. Benthic shells were heated at 250 °C
under vacuum for about one hour prior to further processing. For all
samples, the carbonate was reacted at 90 °C with 100% orthophosphoric acid, using a Multicarb™ preparation device online with a
dual inlet IsoPrime™ mass spectrometer. The standards used include
NBS 19 (e.g., Coplen, 1996) and our in-house UQ6 carbonate standard
calibrated against the Carrara Marble standard from Cambridge (cf.
Hillaire-Marcel et al., 1994a). The overall analytical uncertainty
determined from replicate measurements of UQ6, during each run,
is routinely better than 0.04‰ for both oxygen and carbon isotope
measurements. All results are reported as δ-values against VPDB
(Coplen, 1996). For Cw, δ18O values were corrected by 0.64‰,
following Shackleton and Opdyke (1973) to allow direct comparison
of its records with other foraminiferal records.
The analyses of two different planktic foraminifer species (Npl and
Gb) aimed at documenting the structure of the upper water masses.
The planktic foraminifer Npl, which is a cold-water species, can
inhabit a large range of depths, but occurs usually in the mesopelagic
layer, at or below the pycnocline, at sites with relatively low salinity in
1
Site 1305 - Eirik Ridge
"Inner" Labrador Sea
2
1
3. Results and discussion
3.1. Marine isotope stratigraphy and age models at Sites 1302/1303
and 1305
All data can be downloaded from the GEOTOP data base (http://
www.geotop.ca). The δ18O values of Npl assemblages at Sites 1305
and 1302/1303, in the inner and outer Labrador Sea, respectively, are
illustrated vs. depth on Fig. 2. Site 1302/1303 yields a nearly
“standard-shape” oxygen isotope stratigraphy, with all interglacial
7
9
11
15
13
17
31
25
21
5
19
29
23
3
27
e
4
a-d
M/B
0.78 Ma
5
0
20
40
60
80
100
120
Jaramillo
0.99-1.07 Ma
140
160
180
200
Depth (mcd)
Site 1302/1303 - Orphan Knoll
"Outer" Labrador Sea
1
1
5
7
IA/IB
11
9
13
15
17
2
I
II
III
IV
V
VI
VII
e
3
c
a
4
5
0
20
40
60
80
100
120
Depth (mcd)
Fig. 2. Oxygen isotope stratigraphy vs. depth at Site 1302/1303 (bottom) and Site1305 (top). Measurements were made on left-coiled Neogloboquadrina pachyderma (Npl) in the
150-250 μm size range. The depth of paleomagnetic Matuyama/Brunhes (M/B) Chron and Jaramillo Subchron at Site 1305 are indicated by grey bands in the top diagram. The depth
of the main lithostratigraphical transition (IA/IB) at Site 1302/1303 is indicated in the bottom diagram (Channell et al., 2006b). Roman numbers refer to terminations and italicized
Arabic numbers and letters refer to isotopic stages and substages. The dashed horizontal lines point to the main glacial-interglacial isotopic ranges. In the inner Labrador Sea (Site
1305; top diagram) significantly heavier interglacial and glacial values are recorded since MIS 20. In the outer Labrador Sea (Site 1302/1303), the change towards heavier glacial
values seems to have occurred later, during MIS 12 (about 450 ka).
C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
Table 1
Depth (mcd) and ages of the chronolostratigraphical boundaries at Sites 1305 and
1302/1303.
Site 1305
Site 1302/1303
Isotopic
Depth (mcd)
Depth (mcd)
Transition
10.03
13.00
17.00
18.50
2.71
4.70
10.00
11.20
13.20
1/2
2/3
3/4
4/5
5a peak
5b peak
5c peak
5d peak
5d/5e
5e peak
6/5e
6/7
7/8
8/9
9/10
10/11
11/12
12/13
13/14
14/15
15/16
16/17
17.40
24.90
32.00
35.09
48.31
56.00
61.99
65.15
78.13
80.58
96.50
99.10
109.00
112.00
122.50
127.50
127.9
132.00
133.50
141.00
149.00
153.20
154.51
19.80
21.00
23.40
30.00
37.90
43.50
50.50
55.20
61.00
68.00
76.00
79.00
86.10
94.00
98.00
a
b
Datum
Brunhes/Matuyama
19/20
20/21
21/22
22/23
23/24
24/25
25/26
26/27
27/28
Jaramillo (top)
28/29
29/30
30/31
Jaramillo (bottom)
31/32
32/33
cf. Lisiecki and Raymo (2005).
cf. Shipboard scientific Party (2005).
Age (ka)
b
17/18
18/19
157.2
161.89
165.91
168.35
170.80
Paleomagnetic
a
14
29
57
71
82
87
96
109
115
123
130
191
243
300
337
374
424
478
533
563
621
676
700
712
761
780
790
814
866
900
917
936
959
970
982
990
1014
1031
1062
1070
1081
1104
which correspond to the last 0.712 Ma. Below 97.5 mcd, the
sediments are interpreted as a debris flow deposit with equivocal
paleomagnetic properties (cf. Channell et al., 2006a).
3.2. Sediment accumulation rates at Sites 1302/1303 and 1305
The chronostratigraphical frame based on the isotope stratigraphy
at Sites 1302/1303 and 1305 (Table 1) permits the estimation of mean
sedimentation rates of 13.7 cm ka− 1 and 15.2 cm ka− 1, respectively
(Fig. 3) and thus mean resolutions of about 560 and 340 a,
respectively, with the 5-cm subsampling interval. The average
sedimentation rates are nearly similar at both sites. However, whereas
a relatively steady sedimentation rate is observed at Orphan Knoll, it
has been more variable at Eirik Ridge. This can be attributed to
changes in the intensity of the WBUC, whose modern high velocity
core lies slightly above Site 1305, on the lower Greenland Slope,
whereas Site 1302/03 is sheltered from direct impact of deep currents
by Orphan Knoll. At site 1305 interglacial stages depict significantly
higher sedimentation rates than glacial stages (Fig. 4), which seems to
be a consistent feature in the area of the Eirik Ridge (cf. HillaireMarcel et al., 1994a, 2001a). The high interglacial sedimentation rates
over Eirik Ridge can be due to a combination of factors: i) enhanced
sediment focusing under a higher velocity WBUC resulting from high
production of DSOW (e.g., Hunter et al. 2007), ii) enhanced biogenic
carbonate fluxes, and iii) important detrital supplies accompanying
ice-retreat phases on southern Greenland, especially during late
glacial/early interglacial intervals (see also Fagel and Hillaire-Marcel,
2006; Stoner et al., 2000).
3.3. Singularities of the Pleistocene 18O-stratigraphies at Sites 1302/1303
and 1305
Oxygen isotope records show some specific features. At Site 1305 a
shift in the glacial/interglacial isotopic range is seen at 132 mcd
(Fig. 2), which corresponds to MIS 20, i.e., slightly before 800 ka. This
shift might be part of the Mid-Pleistocene Transition (MPT; e.g.,
Sosdian and Rosenthal, 2009; Yu and Broecker, 2010). Prior to this
transition, glacial/interglacial δ18O values range 2.0 to 3.6‰, whereas
they vary from 2.3 to 4.5‰ afterward. This possibly illustrates a
regionally enhanced response to the transition from the obliquity- to
eccentricity-predominant forcings of the MPT. The absolute minimum
0
Isotopic transitions - Site 1302/1303-Depth mcd
Isotopic transitions - Site 1305
Paleomagnetic datums - Site 1305
50
Depth (mcd)
stages and substages (e.g., MIS 5a–e) easily distinguishable. To the
contrary, Site 1305 from the inner Labrador Sea, presents an unusual
stratigraphy with barely distinguishable substages 5a to 5d, for
example. Nevertheless, the δ18O values of Npl at both sites permit the
establishment of an isotope stratigraphy, which is correlated with the
LR04 stack benthic δ18O curve of Lisiecki and Raymo (2005), thus
providing a chronology with the unavoidable uncertainty resulting
from i) the uncertainty of tie-points between core 1305 and the LR04
stack, as explained above, and ii) the “transfer” of δ18O-signals
between ocean water masses (e.g., Wunsch and Heimbach, 2008).
Indeed, the precise timing of oxygen isotope shifts at Sites 1302/03
and 1305, near major meltwater sources, may not be synchronous
with δ18O changes observed elsewhere because of the time lag
between freshwater discharge and its mixing in the ocean (see also
Lisiecki and Raymo, 2009). At Site 1305, the stratigraphical scheme is
further constrained by the Matuyama/Brunhes Chron and the
Jaramillo Subchron (Channell et al., 2006a; 2009). The analyzed
sequence encompasses isotope stages 32 to 1, which correspond to
the last 1.1 Ma. At Site 1302/1303, the upper 97.5 m belongs to the
Brunhes and are thus necessarily younger than 0.78 Ma. This is
compatible with an isotope stratigraphy spanning stages 17 to 1,
191
100
150
200
0
200
400
600
800
1000
1200
Age (ka)
Fig. 3. Age vs. depth relationships at Sites 1302/1303 and 1305. The ages are
interpolated using the isotopic stage boundaries of Lisiecki and Raymo (2005). Note the
more steady sedimentation regime at Site 1302/1303 in comparison with Site 1305,
where higher sedimentation rates are observed during interglacial stages due to
sedimentary focusing below an enhanced WBUC, the high velocity core of which being
located a few hundred meters above the drilling site (cf. Hillaire-Marcel et al., 1994a).
192
C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
3.4. Oxygen isotope excursions, Heinrich events and ice rafted debris
δ18O (Npl)
5
4
3
2
1
0
I
10
20
II
30
40
III
50
IV
60
70
V
80
5
4
3
2
1
δ18O (Npl)
Fig. 4. Correlation between Site 1302/1303 and Site 1305 based on the isotope
stratigraphy. The diagram illustrates the enhanced interglacial sedimentation rates at
Site 1305 on Eirik Ridge in the inner Labrador Sea. It also shows that Site 1305 differs
from any global oxygen isotope stratigraphy with barely distinguishable isotopic
substages 5a-c, for example, as already noticed from cores collected at the vicinity (cf.
Hillaire-Marcel et al., 1994a, 2001a). One should also note that several terminations (at
least I, II, IV and V) at Site 1302/1303 notably, are characterized by high-amplitude light
isotopic excursions likely indicating ice surge events in the Hudson Strait, which can be
associated with some major Heinrich Events.
values recorded during interglacial stages, at this site, also constitute
another special feature. Prior to MIS 20, some interglacial stages (MIS
21, 25, 31) are characterized by particularly low δ18O values (b2‰).
After 800 ka, some interglacial stages yield values of ~ 2.5‰, similar to
those of the Holocene whereas other interglacial stages (e.g., MIS 7, 13
and 17) barely drop down to 3‰. In contrast, Site 1302/1303 exhibits
almost identical interglacial isotope oxygen values during MIS 1, 5e,
7c, 9, etc. but for very short and sharp excursions. The Site 1302/03
record also differs from that of Site 1305 by significantly reduced
glacial-interglacial shifts prior to MIS 13, with maximum values of
about + 4‰ compared to +4.5‰ during subsequent glacial stages.
Although they are relatively closely located, the two sites exhibit
distinct features in the oxygen isotope signature of mesopelagic
foraminifers, especially at Site 1305. This suggests distinct properties
in the inner vs. outer Labrador Sea, at least in the upper water column.
A special feature of both oxygen isotope records, although more
clearly depicted at Site 1302/1303, is the large amplitude isotopic
excursion during glacial stages that are linked to ice-margin
instabilities, in particular those associated with the Heinrich Events
(e.g., Clarke et al., 1999; Hiscott et al., 2001; Hodell et al., 2008;
Rasmussen et al., 2003; van Kreveld et al., 1996). We interpret these
excursions now more as a result of the addition of isotopically light
brines, resulting from sea-ice production, to the mesopelagic layer
inhabited by Npl (Hillaire-Marcel and de Vernal, 2008), than to direct
dilution of mesopelagic waters by meltwaters. During the HeinrichEvents (H-events), freshening and cooling in the surface water layer
likely resulted in enhanced sea-ice production, thus enhanced brine
extrusion and sinking in the water column, as seen today in the Arctic
Ocean or subarctic seas (Aagard, 1981; Strain and Tan, 1998). Another
point concerning these light isotopic excursions is the fact that they
characterize terminations I to IV notably, with very light δ18O values
peaking during glacial-interglacial transitions, as light or even lighter
than those recorded during the interglacial stages themselves (e.g.,
H1, termination; H11, termination II). This provides further evidence
of the enhanced isotopic response in Npl-shells to overall change in
North Atlantic seawater oxygen isotope composition as well as brine
extrusion events, more than to temperature changes.
As illustrated in earlier papers (e.g., Hillaire-Marcel et al., 1994a;
Hiscott et al., 2001; Stoner et al., 2000), such events are highlighted in
the Labrador Sea by an increased abundance of coarse-grained
material due to enhanced IRD supplies. This is further illustrated in
Fig. 5 for site 1302/1303, using the abundance of N106 μm material as
a proxy for these IRD supplies. This site is ideally situated along the
trajectory of icebergs from Hudson Strait, where some of the major
surges of the LIS occurred resulting in major H-events in the North
Atlantic as well as in more local sedimentological events (see HillaireMarcel and Bilodeau, 2000; Hiscott et al., 2001). Coarse fraction data
also illustrate a more or less continuous supply of IRD throughout the
sequence, thus indicating glacial activity in the Canadian Arctic and
Greenland for at least the last 0.712 Ma. A significant exception to this
pattern occurred during MIS 11 when the coarse fraction decreased to
less than 2% (Fig. 5). This small amount of coarse material is mostly
biogenic (i.e., foraminifer shells). During MIS 11, it is thus inferred that
the delivery of IRD to Site 1302/1303 essentially stopped. This
supports the hypothesis of a much reduced, nearly vanished
Greenland ice sheet during MIS 11 (cf. de Vernal and Hillaire-Marcel,
2008; Willerslev et al., 2007). To a lesser extent, MIS 5e sediment also
shows a reduced coarse fraction content, but not as drastically
reduced as in MIS 11 sediment.
3.5. δ13C signatures in planktic foraminifers
Carbon isotope compositions of Npl at Orphan Knoll show a typical
North Atlantic signature characterized here by values ranging
between minimums of about − 0.5‰ peaking during glacial intervals
to a maximum of nearly +1‰ during interglacials, the Holocene and
MIS 13 in particular (Fig. 6). As illustrated in Fig. 7 and in earlier
papers (e.g., de Vernal and Hillaire-Marcel, 2006), Npl is enriched by
approximately 1‰ in 13C relative to Gb (Fig. 7). This difference is due
to a combination of several processes, notably i) distinct growth
temperatures and water depths, ii) distinct specific fractionation, iii)
distinct carbon budgets (productivity vs. oxidation) in the mixed
surface layer (Gb) and along the pycnocline with the intermediate
water mass (Npl), and iv) distinct ventilation rates of the intermediate
water mass below the pycnocline. Planktic foraminifers from the
inner Labrador Sea (Eirik Ridge) generally show slightly higher δ13C
values than those from the outer Labrador Sea (Orphan Knoll).
However, when plotted against ages interpolated from oxygen
isotope transitions (cf. Lisiecki and Raymo, 2005) as in Fig. 6, the
C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
193
2.0
δ18O (Npl)
3.0
4.0
80
5.0
60
Depth (mcd)
56
57
58
59
60
40
61
62
60
MIS 11
40
20
20
0
0
10
20
30
40
50
60
70
80
Depth (mcd)
Fig. 5. Isotopic stratigraphy and coarse fraction (N 106 μm) at Site 1302/1303. The fraction N 106 μm is used as a proxy of ice rafted debris (IRD). Whereas most interglacial stages
show “residual” IRD activity, a blow-up of MIS 11 shows an almost totally absence of IRD supply. The small amount of N 106 μm material (b 2%) is mostly due to the biogenic remains.
Eirik Ridge δ13C-record shows large offsets, during glacial stages, with
its Orphan Knoll counterpart, due to the large variations in its
sedimentation rate. In such settings, a δ13C-peak to peak correlation
might provide a better way of comparing the two records, using the
interpolated chronology of Orphan Knoll, where steadier sedimentation rates are observed.
Aside major 13C-features also recorded elsewhere (e.g., the MIS 13
high 13C-event; see for example Raymo et al. 2004), a striking feature
from the LS δ13C records concerns the relatively low δ13C values of Npl
during most interglacial stages, except the Holocene and MIS 13
(Fig. 6). This suggests a lesser ventilation of the sub-surface
(intermediate) water mass in the northwest North Atlantic, assuming
similar isotopic compositions of atmospheric CO2 during interglacial
stages. This observation supports earlier findings about the absence of
intermediate LSW production during the last interglacial (substage
5e; Hillaire-Marcel et al., 2001a). Based on the present δ13C records,
this might have also been the case during most interglacial stages,
except for the Holocene and possibly MIS 13. However, δ13C-Npl
values are peaking positively during substage 5b, leaving open the
possibility of some convection in the Labrador Sea during this time
interval. The last glacial stage also differs from earlier ones with its
relatively less 13C-depleted values in Npl. Thus, although reduced in
comparison with modern conditions, some ventilation of intermediate waters was likely occurring throughout most of the last glacial,
eastward in the North Atlantic (e.g., de Vernal et al., 2002).
Negative δ13C excursions are observed during most H-events,
particularly during H1 to H5 and H11 (Fig. 8), which all bear a strong
geochemical signature linked to LIS surges in the Hudson Strait area
(Fig. 8). These features would appear to be an indication of lesserventilated North Atlantic waters, thus a very sluggish AMOC during
such intervals, as already demonstrated by many authors (e.g.,
Gherardi et al., 2009). Interestingly, H6 shows relatively high δ13CNpl values and differs significantly from the other H-events from this
viewpoint. A few other studies (e.g., Farmer et al., 2003) also noted
distinct features for this event that possibly had a lesser impact on the
AMOC as suggested here.
3.6. The last interglacial (MIS 5e) isotope records
As shown in Fig. 7, planktic and benthic δ18O records spanning the
last interglacial at the Orphan Knoll and the Eirik Ridge sites illustrate
the contrasting situations in the inner and outer Labrador Sea. At the
transition from MIS 6 to 5e, there is a light δ18O excursion that is
associated with Heinrich Event 11 (cf. Hodell et al., 2008; Rasmussen
et al., 2003; = H9 in Hiscott et al., 2001). It is visible at both sites but it
is much more pronounced at Orphan Knoll (Site 1302/1303), which
illustrates more closely the influence of LIS surges in the Hudson Strait
area.
The most peculiar feature in the MIS 5e records is seen in the very
narrow range of δ18O values between epipelagic (Gb), mesopelagic
MIS 5e
MIS 13
EIRIK RIDGE
0
5b
1
1
-1
0
-1
ORPHAN KNOLL
0
100
200
300
400
500
600
700
Age (ka)
Fig. 6. Carbon isotope composition of the mesopelagic species Neogloboquadrina pachyderma left-coiled at Sites 1302/1303 and 1305 for the interval spanning the last 800 ka.
Possible 13C tie-points are illustrated. They suggest that the interpolation of ages from δ18O data in 1305 (inner Labrador Sea), using mostly glacial termination ages from the
reference curve of Lisiecki and Raymo (2005), leads to large errors in interpolated ages, when compared to1302/1303 (outer Labrador Sea). This is simply due to the large amplitude
changes in sedimentation rate at Eirik Ridge (see Figs. 3 and 4).
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C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
Site 1302/1303 - Orphan Knoll
"Outer" Labrador Sea
a)
Core P13 - Eirik Ridge
"Inner" Labrador Sea
δ18O vs. VPDB
1
1
2
2
3
3
Npl (mesopelagic)
Cw (benthic)
H-event
Gb (epipelagic)
4
19
21
22
24
23
1250
1300
1350
1400
1450
δ13C vs. VPDB
1
0
0
H-event
1
H-event
b)
20
4
H-event
-1
-1
-2
19
20
21
22
23
24
Mean sample depth (mcd)
1250
1300
1350
1400
-2
1450
Mean sample depth (cm)
Fig. 7. The oxygen and carbon isotopic composition of benthic foraminifers (Cw = Cibicides wuellerstorfi) vs. epi- and mesopelagic planktic foraminifers (Gb = Globigerina bulloides;
Npl = Neogloboquadrina pachyderma left-coiled) at the study sites during the last interglacial (MIS 5e). a) δ18O data show large range of values in the Orphan Knoll record (Site 1302/
1303) suggesting strongly stratified water masses whereas the Eirik Ridge site (from core 90-013-013 –P13– near Site 1305; cf. Hillaire-Marcel et al., 2001a) is characterized by
narrow range of values in all species suggesting a drastically distinct thermohaline structure. Note the light isotopic excursions in Npl during termination II (H-event 11) at both sites.
b) δ13C data show opposite trends in benthic vs. epipelagic species and more variable values in the mesopelagic species at Orphan Knoll (Site 1302/1303). At Eirik Ridge (here from
core 90-013-013; cf. Hillaire-Marcel et al., 2001a), the trends are nearly similar in the three species, further illustrating the distinct water mass structure of the inner vs. outer
Labrador Sea. At both sites, the light oxygen isotope excursion of H-event 11 illustrated here by the densely dotted stripes does not show any change in δ13C values (within resolution
uncertainties) in contradictions with condition observed during termination I (H-event 1; cf. Fig. 8).
(Npl) and benthic (Cw) foraminifers, at the Eirik Ridge Site 1305 and in
the nearby core 90-013-013 (cf. also Hillaire-Marcel et al., 2001a–b),
which contrasts with the large spreading of values at the Orphan Knoll
Site, not unlike the modern gradients observed in the NW North
Atlantic during the late Holocene (de Vernal and Hillaire-Marcel,
2006). The isotopic offset between Gb-assemblages from the two IODP
sites is very large (~1.7‰ vs. ~ 2.2‰ in mid MIS-5e at Site 1302/1303
and Site 1305, respectively). This may relate, at least in part, to distinct
properties of the surface layer at the two sites, and/or in the seasonality
and depth habitat of Gb. Significant differences in dinocyst assemblages are also recorded between the two sites. A higher occurrence of
thermophilic species (Impagidinium aculeatum, I. paradoxum,
2
H6
3
V
4
1
V
H11
0
H1
0
20
40
60
80
100
120
140
Age (ka)
Fig. 8. Carbon and oxygen isotope composition of the mesopelagic species Neogloboquadrina pachyderma left-coiled at Site 1302/1303. The negative 13C-excursions of glacial stages
and terminations are significantly less pronounced during the last climatic cycle than previously. However, MIS 5d and MIS 4 values still suggest reduced ventilation of intermediate
water masses than during MIS 2. The light δ13C excursion during termination I matches the light δ18O excursion of H-event 1; a pattern also observed during older H-events, as
illustrated by grey stripes, but H6 (see arrow).
C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
I. patulum, I. spharicum, S. mirabilis) is observed at Site 1302/1303 than
at Site 1305. This indicates warmer sea-surface conditions in the outer
Labrador Sea than in the inner Labrador Sea (cf. de Vernal and HillaireMarcel, 2009). The quantitative reconstructions of Summer seasurface conditions indicate warmer temperatures and slightly lower
salinity at Site 1302/1303 (up to 17 °C; ~34) in comparison with Site
1305 (~ 13 °C; N34.5). This observation suggests that latitudinal
gradients of temperature in the epipelagic layer of the Labrador Sea
did prevail during the last interglacial. Nonetheless, the relatively high
δ18O–Gb values, which are close to those of Npl at Site 1305 in the inner
Labrador Sea, still have to be explained in view of conditions about 5 °C
warmer than at present, in the surface water layer (cf. Hillaire-Marcel
et al., 2001a). Such warmer conditions would have permitted Gb to
develop during an early seasonal window, for example during the late
spring bloom of diatoms (May–June), almost simultaneously with the
Npl bloom, i.e., when higher δ18O values characterize the surface water
layer. In the modern Labrador Sea, Gb develops mostly during the late
195
Summer, when a temperature of about 8 °C is reached in the surface
water layer (see Hillaire-Marcel and Bilodeau, 2000), whereas δ18O
values of Npl suggest a much earlier development, since this species
prefers cold waters. During the last interglacial, the overall 5 °C higher
mean temperatures would have permitted Gb to find suitable
conditions in the surface layer, much earlier in season, when food
was abundant. On another hand, Npl seems to tolerate temperatures of
up 8 °C (Wu and Hillaire-Marcel, 1994b). Therefore, both species
would have found the same temperature window during the spring
bloom of diatoms on which they feed.
The difference in the isotopic composition of Npl at the two sites
during the MIS 5e is relatively small (2.3‰ vs. 2.5‰ on the average at
Site 1302/1303 and Site 1305, respectively). This suggests a relatively
well-homogenized mesopelagic water mass throughout the inner and
outer Labrador Sea, assuming that both species of foraminifers
developed during MIS 5e on top of the pycnocline between surface
and intermediate waters, as they did during the Holocene (e.g., de
Fig. 9. Sketch of intermediate and deep circulation in the inner vs. outer basin of Labrador Sea during MIS 5e. Whereas we reconstruct at Orphan Knoll a water mass structure with a
deep-water mass circulating in the outer basin and in the North Atlantic in general, we propose one single sub-surface water mass occupying the inner basin (Site 1305) down to at
least 3400 m.
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C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
Vernal and Hillaire-Marcel, 2006). On the contrary, the differences in
the δ18O values of Cw are very large (~ 3.4 ‰ vs ~2.7‰ at Site 1302/
1303 and Site 1305, respectively) demonstrating that distinct deepwater mass bathed the two sites. At Site 1302/1303 in the outer
Labrador Sea, the Npl and Cw records suggest superimposition of two
distinct water masses in the water column: an intermediate mass
(Npl) and a deep colder and/or more saline water mass (Cw). On the
contrary, at Site 1305 (and core P13) in the inner Labrador Sea, the
almost identical Npl and Cw records rather suggest the presence of a
single water mass occupying the water column, from below the seasurface layer to the sea floor. Sea-surface density was previously
calculated and found to be incompatible with winter convection in the
basin (Hillaire-Marcel et al., 2001a). On another hand, sedimentological data at Eirik Ridge indicate that a relatively active WBUC existed
during MIS5e, although less intense than during the Holocene, thus
indicating some DSOW production (cf. Hillaire-Marcel et al., 1994b).
On these grounds, we infer production of intermediate to deep waters
in the Nordic Seas and/or the Arctic Ocean during the last interglacial.
We hypothesize here that these water masses have likely occupied
the intermediate to deep, inner Labrador Sea during MIS 5e as
schematically illustrated Fig. 9.
Carbon isotope data confirm the difference in the water properties at the two sites. Opposite trends at the two sites from the
beginning to the end of the interglacial characterize δ13C values in
Gb shells, suggesting different evolution of the surface water mass in
the inner vs. outer Labrador Sea, possibly in response to differences
in productivity. The trend of δ13C values in Npl follows that of Gb at
the inner Labrador Sea, except for the typical nearly one per mil
offset (e.g., Hillaire-Marcel and Bilodeau, 2000). In the outer
Labrador Sea, Npl shows a δ 13C maximum within the midinterglacial interval. Significantly heavier δ13C values (up to 0.3‰)
in the inner Labrador Sea, as compared to the outer Labrador Sea,
suggest that younger and more ventilated waters likely bathed the
northern part of the basin.
3.7. The isotope record of MIS 11 at the Orphan Knoll Site
As discussed by de Vernal and Hillaire-Marcel (2008), each
interglacial stage within the Pleistocene exhibits distinct features
both on land (cf. pollen) and at sea (cf. dinocysts at Eirik Ridge de
Vernal and Mudie, 1992). Among these, MIS 11 deserves special
attention in view of its long duration and because it probably
corresponds to an interglacial during which Greenland was largely
free of ice (e.g., de Vernal and Hillaire-Marcel, 2008; Willerslev et al.,
2007), and when significantly warmer conditions prevailed in the
surface water layer of the LS, as documented from transfer functions
using dinocysts (de Vernal and Hillaire-Marcel, 2008). Unfortunately,
the rarity of foraminifers at Eirik Ridge Site 1305 makes it difficult to
produce benthic records for this interglacial stage to be compared
with those of other sites. However, an isotopic record of MIS 11 at Site
1302/1303 in the outer Labrador Sea has been established (Fig. 10).
Significantly distinct isotopic values are recorded as compared to MIS
5e. Much heavier oxygen isotope values are observed in Npl and Cw,
whereas Gb shows its rather characteristic “light” interglacial values
(around 1.5‰). A tentative explanation would be to evoke a more
stratified ocean, during MIS 11, with more saline intermediate-deep
waters and, more importantly, a lesser influence of isotopically light
brines in these water masses compared to now (e.g., Frew et al., 2000).
4. Conclusion
Some of the initial objectives of this study were to establish a δ18O
stratigraphy for the inner and outer basins of the Labrador Sea and to
document water column properties during interglacial stages. The
first of these objectives was partly achieved. At Orphan Knoll (Site
1302/1303), the isotopic record resembles closely records from the
open North Atlantic and therefore led to the establishment of
SPECMAP derived chronostatigraphy (e.g. Martinson et al., 1987)
based on linkages with the reference curves of Lisiecki and Raymo
1
2
Gb (epipelagic)
3
Npl (mesopelagic)
4
Cw
5
Up
(benthic)
1
0
-1
-2
52
54
56
58
60
62
64
Mean sample depth (mcd)
Fig. 10. Oxygen and carbon isotope composition of benthic foraminifers (Cw = Cibicides wuellerstorfi) vs. epi- and mesopelagic plankic foraminifers (Gb = Globigerina bulloides;
Npl = Neogloboquadrina pachyderma left coiled) at Site 1302/1303 during MIS 11. δ18O offsets between Npl and Gb are not much different from those of MIS 5e. However, the almost
complete disappearance of Npl in the 56-59 mcd interval suggests much warmer overall conditions during MIS 11, at least in the upper to intermediate water masses, and the overall
shift of Npl and Cw isotopic compositions towards significantly heavier values (compared to the Holocene ones, for example), leads to infer drastically distinct salinity-18O properties
of the intermediate and deep water masses during this interval (see text).
C. Hillaire-Marcel et al. / Marine Geology 279 (2011) 188–198
(2005). However, some caveats must be raised. Due to the rarity of
benthic foraminifers, this stratigraphy relies mostly on oxygen isotopes
measured in the mesopelagic species Npl. Our experience with similar
measurements in several cores from the northwest North Atlantic leads
us to relate the mesopelagic (Npl) δ18O stratigraphy to isotopic and
physical properties of the subsurface to intermediate water layer. Npl
isotopic properties match closely those that can be estimated for calcite
precipitated at equilibrium under spring conditions (around May) at the
deepest end of the seasonal pycnocline (cf. Hillaire-Marcel et al., 2001b;
Jonkers et al., 2010). Furthermore, Npl is highly sensitive to episodes of
(isotopically light) brine production (Hillaire-Marcel and de Vernal,
2008) and shows high amplitude isotopic excursion relating to ice
margin instabilities and deglacial events. As a consequence, the δ18O
stratigraphy derived from Npl relates mostly to changes in intermediate
water properties of the northern North Atlantic. It may thus shows some
temporal millennial-scale offsets with the benthic stack reference curve
of Lisiecki and Raymo (2005) due to the ocean heterogeneity and mixing
properties (cf. Lisiecki and Raymo, 2009; Wunsch and Heimbach, 2008).
For example, the impact of isotopic excursions linked to ice-margin
instabilities during deglacial intervals may in part obscure isotopic
transitions commonly used as datums. This may add temporal offsets
with an age on the order of N103 years (Wunsch and Heimbach, 2008).
The fact that the ocean remains in transient stages at such time scales
results in inherent limitation of any oxygen isotope stratigraphy. This
caveat applies to other isotopic records from any sites, either established
from benthic or planktic foraminifers, but they are likely more critical
here, at near proximity of major ice-sheets. At Site 1305 in particular,
one must also consider the large irregularities in sedimentation rates
and isotopic recordings, resulting in an even lesser constrained isotopic
stratigraphy.
The second objective of documenting water mass structure in the
inner vs. outer Labrador Sea, especially during interglacials, has been met
for the last interglacial (MIS 5e; Fig. 7) and to a lesser extent MIS 11.
Combining the present information to that of de Vernal and HillaireMarcel (2008) leads to the conclusion that the thermohaline structure of
water masses in the Labrador Sea has been likely unique during each
interglacial, and was possibly distinct in the inner vs. outer basins, such as
during the last interglacial. As a consequence, the reconstruction of the
past ocean, especially at sites of importance for potential AMOC changes,
as it is the case here in the Labrador Sea where intermediate water
production has occurred since about 7.5 ka (Hillaire-Marcel et al., 2001a),
can only be made at basin- or even sub-basin scale. Changes in boundary
conditions at these scales may result in major reorganizations of the
world ocean. Progresses in our understanding of the modes of circulation
depend upon our ability to document them at such scales. This should
encourage ocean modelers to look at the ocean at a regional scale.
Acknowledgements
The authors owe special thanks to the scientific party of IODP leg
303 for helpful discussions. Julie Leduc, Sergio Mayor, Jena Zumaque
and Luc Baillargeon-Nadeau, all from GEOTOP, have to be thanked for
their help in the laboratory. This study is a contribution to the Polar
Climate Stability Network (PCSN) supported by the Canadian
Foundation for Climate and Atmospheric Sciences (CFCAS). Additional
support was provided by the Fonds Québécois de la Recherche sur la
Nature et les Technologies (FQRNT) and the Natural Sciences and
Engineering Research Council of Canada (NSERC). Comments from
two anonymous reviewers are also acknowledged. They helped to
clarify several aspects of this contribution.
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