Global and Planetary Change 54 (2006) 225 – 238
www.elsevier.com/locate/gloplacha
Contrasting conditions preceding MIS3 and MIS2 Heinrich events
Elsa Jullien a,⁎, Francis E. Grousset a,b , Sidney R. Hemming b , Victoria L. Peck c ,
Ian R. Hall c , Cédric Jeantet a , Isabelle Billy a
a
EPOC, UMR 5805, Université Bordeaux I, Avenue des Facultés, 33405 Talence, France
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA
School of Earth, Ocean and Planetary Science, Cardiff University, Cardiff CF10 3YE, UK
b
c
Received 4 August 2005; accepted 15 June 2006
Available online 3 October 2006
Abstract
This paper presents an integrated multi-tracer study performed on piston cores recovered in the glacial ice-rafted detritus belt,
stretching from Newfoundland to the Irish margin across the North Atlantic (40–55°N), in order to compare in detail the internal
structure of each Heinrich event (HE). These tracers are IRD counts (quartz, dolomite, volcanic grains), their Nd isotopic
composition and Ar–Ar datings of individual hornblende grains. A focus on the detailed structure of HE confirms that all intervals
of massive sediment flux, specifically Heinrich layers HL1-to-5 (HLs), were dominated by North American, Laurentide ice-sheet
surges from Hudson Strait, that are evident as far east as the Bay of Biscay (European margin). The sequences of events leading up
to the HLs, however, present significant dissimilarities. One important difference is that HL2 and HL1 were preceded by “precursor
events” (increases in the number of lithic grains per gram from non-Laurentide sediment sources). Sediment debris derived from
near-simultaneous iceberg releases originating from the European ice-sheet are only detectable close to the European margin. In
contrast there are no comparable precursor events before HL5 and HL4. This observation implies that precursor events are unlikely
to be mechanistically linked to the triggering of HEs. The similarity of the HLs, against contrasting background conditions, is a
significant observation that should add constraints to their origin.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Heinrich events; ice-rafted detritus; Nd isotopes;
40
Ar/39Ar; Last glacial; North Atlantic
1. Introduction
Over the last fifteen years or so, more than twohundred articles have been published on the so-called
“Heinrich events” (HE) and their apparent global correlatives in oceans and continental climate archives (Heinrich, 1988 and see reviews by Andrews et al., 1998;
Broecker and Hemming, 2001; Voelker, 2002; Hem⁎ Corresponding author. Tel.: +33 540 008 438; fax: +33 556 840
848.
E-mail address: [email protected] (E. Jullien).
0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.gloplacha.2006.06.021
ming, 2004). These events are characterized by abrupt
collapses of the Laurentide ice-sheet, which delivered
armadas of icebergs to the North Atlantic. Their melting
resulted in ice-rafted detritus accumulation, the Heinrich
layers (HL), identified by horizons rich in coarse lithic
grains (N150 μm). The large bibliography and references
therein mentioned above attest to the importance of these
events for understanding mechanisms of rapid climate
change; however, it is frustrating that a consensus conclusion on their origin has yet to be reached. A few
theories concerning the origin of Heinrich layers have
been proposed, but it must be admitted that none of them
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E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
is entirely satisfactory. In particular, the question of the
process(es) that produced HEs is still a matter of debate,
and of particular importance is the question of whether
they are internally or externally driven. A summary of
the main hypotheses for HEs, glaciological binge-purge
(MacAyeal, 1993), including earthquake induced destabilization, Hudson Bay jökulhlaups (Johnson and Lauritzen, 1995) and ice shelf break-up, was recently
reviewed by Hemming (2004). A more recent external
forcing mechanism, destabilizing the Laurentide icesheet rim with exceptionally high tides has been
proposed by Arbic et al. (2004). The events following
seem to be implied for each HE: (i). something (internal
or external?) generated the iceberg armadas from Hudson Strait, (ii). iceberg armadas invaded the North
Atlantic and melted in a west–east latitudinal band (40–
55°N) delivering their ice-rafted detritus; (iii). Atlantic
meridional overturning circulation (MOC) shoaled
dramatically or shut down, (iv). atmospheric teleconnections spread the climatic signal across the Northern
Hemisphere, and perhaps the globe. However, Heinrich
events are not identical. Before HL1 and HL2, precursor
events are identified (Snoeckx et al., 1999; Grousset
et al., 2001).
From correlations of ice core and marine core data,
the pattern of occurrences strongly implies climate con-
trol on the origin of Heinrich layers, and thus a purely
internal mechanism is unlikely. However, the sequence
of events is not clear and requires further attention for
interpretation of linkages. For example: 1) asynchronous surges of both Fennoscandian and Icelandic icesheets reveal that these European ice-sheets may have
independently responded to their own internal dynamics
(Dowdeswell et al., 1999; but see Elliot et al., 1998); 2)
in the western side of the North Atlantic, the Laurentide
could have generated its own precursors with very similar Nd and Sr isotope compositions to European
sources (Farmer et al., 2003); 3) Timing of the
meridional overturning circulation shutdown does not
clearly postdate arrival of icebergs (fresh water) (Vidal
et al., 1997).
There are several important questions that have not
been satisfactorily answered concerning the origin and
impacts of Heinrich events. Among them are: (1) what
were the ambient conditions associated with them and
were they the same for all of them? (2) how much freshwater was released? (3) what was their duration, was it
the same for each? (4) did they lead to a shutdown in
MOC or did that come first? In this study, we attempt to
better constrain the first question with regard to the
behaviour of ice-sheets as captured by Ice Rafted Debris
(IRD) composition. The goal is to test if the different
Fig. 1. Core locations across the IRD (ice-rafted detritus) belt: main core SU90-11 (closed circle), other cores already studied (open circles) and cores
mentioned in the text (crosses). Dotted line represents the extension of the ice-sheets during HE4 and HE5, whereas the thin continuous line
represents the maximum extension of the Laurentide ice-sheet at LGM (− 130 m).
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
HEs have exactly the same precursory structure. Multiple parameters are documented from the same core,
including magnetic susceptibility, X-radiography, isotopic (Nd and Sr composition of bulk coarse fraction as
well as some quartz separates, 40Ar/39Ar of individual
hornblende grains) tracers, and coarse fraction counts of
mineral/rock fragment abundance (quartz, tephra, dolomite, etc). This study is focused on core SU90-11, a core
recovered from a seamount located near the western end
of the Ruddiman belt (preferential accumulation zone of
IRD between 40° and 55°N; Ruddiman, 1977), close to
the Laurentide ice-sheet (LIS), which should have recorded a particularly pure Laurentide source signal. Nd
isotope results are compared to a series of cores exclusively located within the Ruddiman belt, along a W–E
transect from close to the Laurentide iceberg source
(≈ 40°W) to the Bay of Biscay (≈ 10°W).
2. Materials and methods
Piston-core SU90-11 (Fig. 1) was recovered from the
top of the Milnes Seamount located on the western part of
the Mid-Atlantic Ridge (44°04′N, 40°2′W, 3645 m water
depth). This site is located near the western end of the
IRD belt (Ruddiman, 1977), and is about 1000 m above
the surrounding abyssal plain (≈4700 m). Accordingly,
the site is not likely to have received sediment from the
Canadian margin by bottom current advection, and the
site can be assumed to have collected only surfacederived particles in the coarse fraction, such as planktic
microfossils and ice-rafted detritus. δ18O isotopic analysis
on the planktic foraminifera Neogloboquadrina pachy-
227
derma (left coiling), along with AMS-14C ages are
previously published (Labeyrie et al., 1995), allowing us
to constrain the age model (Fig. 2, Table 1).
Five other cores are used for comparison: SU90-08
(43°50′N, 30°40′W, 3080 m water depth) and SU90-09
(43°08′N, 31°08′W, 3375 m water depth) were recovered from the Mid-Atlantic Ridge and they have been
extensively studied (Grousset et al., 1993; Revel et al.,
1996; Cortijo et al., 1997; Vidal et al., 1997; Elliot et al.,
1998; Huon et al., 2001; Grousset et al., 2000, 2001).
Three cores located at the eastern extent of the IRD belt
(around 15°W), were also studied: MD01-2448 (44°77′
N, 11°27′W, 3460 m water depth) from the top of the
Biscay Seamount, MD95-2002 (47°45′N, 8°53′W,
2174 m water depth, on the Celtic margin) (Grousset
et al., 2000) and MD01-2461 (51°45′N, 12°55′W,
1153 m water depth, on the Porcupine Bank) (Peck et al.,
2006).
The present study focuses on the upper three meters
of core SU90-11 (covering the last 60 kyr), which
contain the six most recent Heinrich Layers. Whole-core
low-field magnetic susceptibility was measured aboard
ship with a pass-through Bartington magnetometer. This
instrument integrates the sediment section within the
coil (12.5 cm ∅). Resolution is of the order of 2 cm. The
sedimentary sequences, lithologic facies, and the geometry of physical and biological structures were observed using a digital SCOPIX X-ray device, coupling a
high-energy brightness amplifier and a digital camera
(Migeon et al., 1999).
The sediment core was sliced at 1 cm intervals. For
each slice a 2-cm3 aliquot was washed and sieved to
Fig. 2. Age model for core SU90-11. The accumulation rate was dramatically greater during HLs (≈ 12 cm/ky) (grey lines), whereas between HLs,
accumulation rate was three times lower (≈4 cm/ky) (dark lines).
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E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
Table 1
Age constraints used for building the SU90-11 age model. Sources: 1.
this work; 2. Labeyrie et al. (1995); 3. Grousset et al. (2001). Calendar
ages, after Bard (1988)
Depth
(cm)
14
C ages
(kyr)
Pointers
Source
Calendar ages
(kyr)
12
14
11.1
11.6
1
2
12.938
13.549
17
13.4
3
15.735
42
15
3
17.661
45
16.4
2
19.334
65
20
3
23.582
89
22
3
25.908
93
22.3
2
26.255
105
23.7
3
27.866
110
26.4
3
30.941
115
27.3
2
31.956
124
29.2
2
34.083
135
33.8
3
39.142
3. Results
166
36.1
3
41.516
195
44
3
44.934
220
46
3
46.387
239
54
Ash Zone I
AMS-14
C dating
Tuned on
SU90-09/08
Tuned on
SU90-09/08
AMS-14
C dating
Tuned on
SU90-09/08
Tuned on
SU90-09/08
AMS-14
C dating
Tuned on
SU90-09/08
tuned on
SU90-09/08
AMS-14
C dating
AMS-14
C dating
Tuned on
SU90-09/08
Tuned on
SU90-09/08
Tuned on
SU90-09/08
Tuned on
SU90-09/08
Ash Zone II
the Toulouse University. Standard results for the period of
these analyses are 0.710245(20) (n = 6) for Sr standard
NBS987 and 0.511846(15) (n = 5) for Nd standard La
Jolla. For convenience, Nd isotopic ratios are presented as
follows: εNd(o) = (( 143 Nd/ 144 Nd) / 0.512636 − 1) ⁎ 104 ,
where 0.512636 is the Chondritic Uniform Reservoir
value (Jacobsen and Wasserburg, 1980).
Hornblende grains were picked from the N150 μm
fraction and processed as reported in Gwiazda et al.
(1996a), Hemming et al. (1998, 2000) and Hemming
and Hajdas (2003). An effort was made to select at least
15 hornblende grains per sample; however in some
cases this was not possible, and all the grains were
picked and analysed. Samples were co-irradiated with a
monitor standard of known age in the Cd-lined in core
facility (CLICIT) at the Oregon State reactor. Ages of
hornblende grains are consistent with 525 Ma for hornblende monitor standard Mmhb (Samson and Alexander, 1987). Analyses were made in the Ar geochronology
laboratory and Lamont-Doherty Earth Observatory.
Individual grains were fused with a CO2 laser, and
ages were calculated from Ar isotope ratios corrected for
mass discrimination, interfering nuclear reactions,
procedural blanks and atmospheric Ar contamination.
1
analyse the coarse fraction (N150 μm); IRD (including
quartz, carbonate and volcanic particles) were counted
under a binocular microscope (Table A1, accessible
through the electronic supplement). Counts were made
at each centimeter between 5 and 104 cm, and 144 and
235 cm, every other sample was counted between 104
and 144 cm depth. For each sample, more than 300
grains were counted. Samples with large numbers were
split to yield (300–1300) grains.
Nd and Sr isotopic compositions were measured on the
carbonate-free, coarse (N150 μm) fraction. Carbonate was
removed by HCl 1 N. Samples were processed in a “US
class-1000” clean laboratory at Université Bordeaux. The
coarse fraction was dissolved with acid solutions (HCl,
HF, HClO4). Sr and Nd were separated through cationic
and anionic chromatographic columns following the
procedure of Grousset et al. (2001). Samples were
analysed on a Finnigan MAT 261 mass spectrometer at
3.1. Stratigraphy
In addition to the δ18O and AMS-14C results from
Labeyrie et al. (1995), there are some specific markers that
allow greater age constraints on core SU90-11. At 12–
13 cm depth, abundant translucent-to-light yellow,
bubble-wall glass shards reveal the presence of the well
known Ash Layer 1. The maximum occurrence of these
tephra was observed at 13 cm depth, providing thus a well
defined age: 11,100 years BP (Lacasse et al., 1996 and
references therein). Translucent tephra characterizing the
Ash Zone II were identified at a 239 cm depth, providing
another age control: 54 ka (Lacasse et al., 1996 and
references therein). Along with these two tephrachronologic dates, six AMS-14 C ages were obtained on
foraminifera N. pachyderma (lc) (N 150 μm) (Labeyrie
et al., 1995). These ages have to be considered with care,
as a constant age-reservoir correction could be inaccurate,
especially for levels close to the HLs (Waelbroeck et al.,
2001). They are, however, consistent with ages classically
attributed to HL boundaries (see review in Hemming,
2004). The apparent age at the core top is ∼8000 years BP
and thus we assume that upper Holocene sediments might
have been destroyed during coring or that Holocene
sedimentation rates were very low.
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
229
Fig. 3. Core SU90-11: a. X-radiograph (HL: Heinrich Layers; p: precursor); b. magnetic susceptibility (10− 6 emu); IRD counts (grains/g. of dry
sediment): quartz grains (c), dolomite grains (d), volcanic grains (e); (f). Nd isotopic composition (εNd(o)) of the carbonate-free, coarse (N150 μm)
IRD fraction; (g). δ18O (PDB) of planktonic foraminifera Neogloboquadrina pachyderma (left coiling) (from Labeyrie et al., 1995). HLs are
highlighted by light grey bands and dark grey bands for precursors (p).
230
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
Fig. 4. IRD counts (grains/g. of dry sediment) of quartz (thin grey line), dolomite (thin black line), volcanic grains (dotted line) in cores SU90-11,
SU90-09 and MD01-2448. Heinrich layers and precursors are highlighted by light and dark grey bands, respectively.
Whereas HL3 and HL6 are barely visible on bulk
sediment measurements, the main HLs (HL1, HL2, HL4
and HL5) can be easily identified, based on magnetic
susceptibility and X-ray density records (Fig. 3). The
less apparent presence of HL3 is consistent with the
early survey of HLs (Grousset et al., 1993). Using the
positions of their boundaries and the age model developed from tephra and 14C, the following approximate
14
C ages are implied for the Heinrich events: HL1:
13,095–15,000 yrs; HL2: 20,360–22,380 yrs; HL3:
25,950–27,260 yrs; HL4: 34,050–36,310 yrs; HL5:
44,050–46,900 yrs (Table 1, Fig. 2). The age range
implied is greater than the duration of the events because
a linear sedimentation rate is used to make the estimates.
Whereas the mean accumulation rate of the ambient
sediment is 4 cm/ky, during the most prominent HLs
(HL1, 2, 4, 5) it is multiplied by at least a factor of 3
(≈12 cm/ky). This increase of the accumulation rate
during the HEs is confirmed by a parallel systematic
decrease by a factor 5 of the 230 Thexcess (C. Lalou,
unpublished data, pers. comm.). In the ambient sediments, however, such low accumulation rates prevent
the identification of abrupt, secondary lithic peaks – if
any – , such as IRD events previously observed in
contourite drifts (Bond and Lotti, 1995) or in deep sea
fan systems (Knutz et al., 2001, 2002).
3.2. Petrology, geochemistry, isotopic composition and
proxy meanings
The goal of this study is to combine multiple compositional parameters in order to better determine the
geographic origin of the IRD, and therefore to identify
the ice-sheet(s) responsible for their transport (Fig. 3).
In the coarse fraction (N 150 μm) quartz grains are 5
to 7 times more abundant during the HLs than in the
ambient sediments, along with the increase in accumulation rate, suggesting a substantial increase in the IRD
flux delivered to the site. Quartz is a ubiquitous mineral
in continental sources, and thus it is a measure of the
quantity of IRD delivered, but does not constrain the
origin except to eliminate basaltic sources. The dolomite
(CaMg(CO3)2) grain record parallels the quartz record.
Dolomite grains comprise up to 12% of HLs and are
very sparse in between. Dolomite is an important sedimentary and metamorphic mineral found in limestones,
marbles and serpentinites. Although dolomitic limestone is a relatively common component of sedimentary
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
Table 2
Nd isotope data expressed as εNd(o) and mean Ar–Ar ages (My)
Cores
Depth
(cm)
Events
εNd
(o)
SU90-11
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
MD01-2448
"
"
"
"
"
"
"
"
"
"
"
12
20
35
40
43
50
73
88
94
97
100
110
142
160
163
168
172
125
129
131
136
140
142
184
199
202
204
206
216
Ash Zone 1
HL1
HL1
Precursor
Precursor
Precursor
HL2
HL2
HL2
Precursor
Precursor
HL3
HL4
HL4
HL4
HL4–HL5
HL4–HL5
HL2
HL2
HL2
HL2
Precursor
Precursor
HL3–HL4
HL4
HL4
HL4
HL4
HL4–HL5
−16.3
−36.6
−26.2
−14.12
− 4.9
−13.9
−29.2
−30.5
−16
− 9.5
−12.8
−24.1
−29.4
−31.5
−31.2
−24
−21.5
−12.9
−24.1
−18.9
−22.12
− 8.66
−12.4
−15.9
−26.3
−33.36
−34.23
−29.57
−14.02
Ar–Ar ages
(My)
2373
2581
710
937
1668
1750
outcrops (Bond et al., 1992), the increase in detrital
carbonate in Heinrich layers is coincident with a Hudson
Strait source of icebergs (Andrews and Tedesco, 1992).
Thus, these grains are inferred to reflect the erosion of
dolomite-rich limestones, located in the Baffin Island
and Hudson Bay area. The maximum concentration of
dolomite is reached later, a few cm after the quartz
maximum, consistent with the deposition sequence
reported by Grousset et al. (2001) for core SU90-09
(Fig. 4). Micas and rock fragments (e.g. granite, gneiss,
schist) are also observed within the IRD assemblage and
are counted as other grains as they are not discriminant
enough versus sources (Table A1).
The abundances of quartz and detrital carbonate
grains clearly mimic the magnetic susceptibility record
and X-ray image (Fig. 3), which reflect the abundance of
magnetite associated with feldspars through the core
(Grousset et al., 1993). Sharp basal contacts are apparent
in the X-ray images for HL1, HL2, HL4 and HL5
(Fig. 3). HL3 is visible, but the basal contact is not as
clear and the layer is no more than 3 cm thick. The
increase in IRD content begins before the onset of the
231
Hudson Strait (sharp basal contact and detrital
carbonate) flux during HL1 and HL2. The precursory
interval prior to HL2 is visible approximately 3 cm
below the sharp contact with the dark X-ray image for
HL2. The increase in IRD content for HL4 and HL5 is
very sharp, and quartz and volcanic grains increase
together at the beginning of HLs and decrease abruptly
at the end, whereas dolomite is restricted to the inner
part of the IRD layers.
The abundance of volcanic grains (pumice, dark
obsidian and basaltic fragments) displays a different
behaviour to the crustal fragments. Before HL3 and after
HL1, the abundance of volcanic grains follows those of
quartz and dolomite (Fig. 3). In between, the abundance
of volcanic grains increases significantly (×2) just prior
to HL2 and HL1. A similar pattern was reported for core
SU90-09 (Fig. 4) and considered as “European
precursor events” (Grousset et al., 2001). In the North
Atlantic province, the volcanic grains may be sourced
from Iceland, the Thulean province of the Eastern
Greenland, the Faeroes and even by British Isles (Knutz
et al., 2002). The isotopic signatures of these different
volcanic provinces are too similar and thus cannot be
distinguished. Considering its significant size and location, the Icelandic province seems to be the most credible source for volcanic particles. Some of these
volcanic grains were isolated in the neighbour core
SU90-09 (Grousset et al., 2001) and their isotopic
composition (87Sr/86Sr = 0.70367; εNd(o) = + 7.4) matches the composition of the Cenozoïc basaltic rocks of
Iceland and Eastern Greenland: (87Sr/86Sr ≈ 0.7035; εNd
(o) ≈ + 7.6, Revel et al., 1996). During the last glacial
period, Iceland was entirely covered by an ice-sheet that
extended across the shelf, at least down to − 120 m
(Siddall et al., 2003). Thus, the volcanic grain record is
interpreted to reflect the temporal behaviour of the
Icelandic ice-sheet.
The Sr and Nd isotopic composition of IRD may help
to fingerprint sediment sources: basically, when 87Sr/
86
Sr ≈ 0.704 and 0 b εNd(o) b +10, sediments are derived
from young volcanic rocks (≈ 0–1 Ga); on the other
hand, when 87Sr/86Sr N 0.72 and − 40 b εNd(o) b − 10,
sediments are derived from older crustal rocks (≈ 1–
3 Ga). Around the northern North Atlantic Ocean, such
old outcrops only exist on the North Canadian Shield.
Our isotopic data are consistent with results from other
cores (Grousset et al., 1993; Revel et al., 1996; Hemming et al., 1998; Snoeckx et al., 1999; Grousset et al.,
2000, 2001). Within the HEs most of the εNd(o) values
range from −20 to − 40, along with radiogenic Sr ratios
(0.72 b 87Sr/86Sr b 0.73), reflecting Archean heritage.
However, just prior to H2 and H1 – within the precursor
232
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
Fig. 5. Mean Ar–Ar ages (Ga) of hornblendes compared to the εNd(o) values (both measured in the same carbonate-free, coarse (N150 μm) IRD
fraction). Inset histograms show the distributions of ages, where: a. b50 Ma indicates Icelandic (broadly, including the hotspot in general); b. 200–
600 Ma indicates Appalachian/Caledonian ages; c. 0.8–1.2 Ga indicates Grenville ages; d. 1.6–1.9 Ga indicates “Churchill” (this is the Canadian
Shield term) or contemporaneous crystalline sources from e.g., Greenland; e. 2.4–2.8 Ga indicates “Superior” (this is the Canadian Shield term) or
contemporaneous crystalline sources from e.g., Greenland; f. “other” indicates grains that fall outside of these groupings, and could be derived from
crustal provinces not included here, partially reset grains from the provinces listed, or complex systematics such as excess Ar. The apparent ages of the
“other” grains are listed above that bar on the histograms.
events – εNd(o) range from − 5 to − 15 and 87Sr/86Sr
ratios range from 0.715 and 0.72. Such high εNd(o)
values were interpreted as reflecting a European origin
(Grousset et al., 2000, 2001). A radiogenic εNd(o) value
of − 16.1 at 12–13 cm depth represents the signature of
Ash Zone I (Table 2).
Ar–Ar ages of hornblendes were measured from the
same levels for which we obtained εNd(o) values.
Individual data are reported in a supplementary table.
Results presented in Fig. 5 are the averaged value of the
measurements made in each sample, and histograms are
included to show the age populations represented. The
oldest mean ages were obtained for HL2 (≈ 2.5 Ga) and
HL3 (≈ 1.75 Ga). They correspond to the most unradiogenic εNd(o) values, which is consistent with a
Canada/Greenland origin. HL3 could also be derived
from Europe: in Scandinavia, Early Proterozoic crust is
observed but samples from the Bear Island “trough
mouth fans” have only few hornblendes and are clearly
dominantly derived from sedimentary sources. Along
the Norwegian margin, the crystalline crust is dominated
by Caledonian ages with the possible exception of the
very southern tip of Norway which has Grenville outcrops. So, in this core, it is reasonable to exclude a
European source. The Laurentide-derived origin of HL3
challenges an entirely European origin for HL3 and HL6
(e.g., Snoeckx et al., 1999); however, the geographical
and spatial distribution of Sr isotopes compiled by
Hemming (2004) is consistent with a European origin in
the eastern basin and a Laurentide (and Greenland)
origin in the western basin. Both precursor events and
embedding sediments display more radiogenic εNd(o)
and youngest ages (≈0.5 to ≈1.5 Ga) that are consistent
ages previously observed in the North Atlantic sediment
background (Huon and Jantschik, 1993) (Table 3).
4. Discussion
4.1. HL1 and HL2
The internal structure of HL1 and HL2 is similar
(Fig. 3): a rapid increase in IRD associated with a
sudden decrease of the δ18O composition of planktic
foraminifera. The interval with lighter δ18O is shown to
have a classic Hudson Strait signature of high magnetic
susceptibility, high IRD content, dominated by quartz
and detrital carbonate and very unradiogenic Nd. These
two main layers are characteristic of the typical Heinrich
layers, extensively described in the North Atlantic since
1988, especially within the IRD belt (e.g., Bond et al.,
1992; Grousset et al., 1993; Hemming, 2004 and
references therein).
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
Table 3
Individual Ar–Ar ages (My) obtained on single hornblende grains. 12,
17, 17, 6, 9, 12 grains were analysed at 73 cm, 88 cm, 94 cm, 97 cm,
100 cm and 110 cm, respectively
Depth
(cm)
Ar–Ar age
(My)
of single grains
73
"
"
"
"
"
"
"
"
"
"
"
88
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
94
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
97
"
"
"
"
"
100
"
1282.9
1664.5
1716.5
1731.5
1752.5
1881.5
2358.3
2570.1
2847
2907
3096.4
4664.3
1441.8
1661.9
1668.8
1718.6
1726.7
1730.5
1747.5
1782.9
1883.4
2579.1
5216.3
3894.4
2091.9
1915.1
3012.4
6634.2
3172.1
25.1
302.9
383.6
391.2
403.4
411.2
452.5
463.2
488.7
510.2
988.4
1484.1
1723.7
1945
498.3
675.4
917.1
4.3
16.0
455.4
1603.1
1719.9
1823.1
18.3
1647.1
Ar–Ar age
(My)
mean value
2373
233
Table 3 (continued)
Depth
(cm)
Ar–Ar age
(My)
of single grains
"
"
"
"
"
"
"
110
"
"
"
"
"
"
"
"
"
"
"
4520.6
1861.6
970.8
1750.7
2073.2
1573.9
592.2
1307.2
1395.1
1626.9
1749.9
1753.1
2232.3
2272.6
2529.9
1503.8
1980.4
2342.9
306.0
Ar–Ar age
(My)
mean value
1668
1750
Prior to the major Laurentide surges during HL1 and
HL2, precursor events are observed in many cores from
the IRD belt:
2581
710
937
– In the northeastern Atlantic basin, it has been clearly
shown that precursor events reflected early discharge
of icebergs by the European ice-sheets. For example,
such early emissions of icebergs coming from British
Isles ice-sheet have been described in cores MD012448 (this work), MD95-2002 (Grousset et al.,
2000), OMEX-4K (Scourse et al., 2000), DAPC2
(Knutz et al., 2002) and MD01-2461 (Peck et al.,
2006) (Fig. 1).
– In the northwestern Atlantic basin, precursor events
occurring prior HL1 and HL2 are revealed by thin
volcanic-rich layers, displaying more radiogenic Nd
compositions (εNd(o) N − 17; Grousset et al., 2001).
This is the case in both SU90-09 and SU90-11 cores,
where these characteristics suggest a significant
increase of iceberg release from the Icelandic icesheet and/or from other volcanogenic provinces such
as western Greenland or even the southeastern
Canadian Monteregian Hills province.
How can we explain the geographic pattern of these
precursors? Along the 45°N latitude, the thickness of
the HL1-2 precursors decreases eastward: 7–9 cm in
SU90-11, 4–5 cm in SU90-09 and they are barely
visible in MD01-2448 (Figs. 1 and 4). Off the Irish
margin, however, at about 52°N (core VM23-81),
234
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
precursor events are still visible prior to HL1 and HL2
(Bond and Lotti, 1995). Furthermore, in the ambient
sediments separating HL3, HL2 and HL1, a few
distinct, discrete IRD pulses were also identified by
these authors. These precursors are enriched in basaltic
grains which may come from the British Tertiary
Province (Knutz et al., 2001) or from Iceland (most
likely), and increases in hematite stained grains in these
intervals have been attributed to a Laurentide (Gulf of
St. Lawrence) origin (e.g., Bond and Lotti, 1995).
Additionally, their Nd isotope composition allow a Gulf
of St. Lawrence source (Farmer et al., 2003) and the
Ar–Ar ages measured in hornblendes deposited right
before HL2 in SU90-11 (Fig. 5) have a large Paleozoic
(Appalachian/Caledonian) signature.
Indeed, during ambient glacial conditions preceding
HL1 and HL2, the different pan-Atlantic ice-sheets
released icebergs that melted close to their borders: the
British Isles ice-sheet along the European margins, the
Laurentide/Greenland over the western Atlantic basin.
However, the Laurentide IRD supply during HL1 and
HL2 is clearly dominant. LIS sourced icebergs were
presumably distributed by the prevailing westerly
winds and eastward currents, as far east as the
European margin. Such a supply has been identified
in the eastern Atlantic basin: for example, in core
VM23-81 (Bond and Lotti, 1995), in core MD95-2002
(Grousset et al., 2000), in core DAPC2 (Knutz et al.,
2002) or in core MD01-2461 (Peck et al., 2006).
Along the European margins, whereas HL1 and HL2
are dominated by European IRD, the central section of
these Heinrich layers is dominated by a Canadian
imprint. During these two HLs, Laurentide surges became so massive, that icebergs invaded the entire
surface of the IRD belt, as far east as the Bay of Biscay
(Grousset et al., 2000).
4.2. The “atypical” HL3 event
Because previous works on HL3 have been mostly
conducted on cores located on the eastern side of the
North Atlantic, it had been claimed that HL3 was
different from the other events and that its origin was
more likely European (Grousset et al., 1993; Gwiazda
et al., 1996b; Snoeckx et al., 1999). In fact, this
statement has to be tempered. In cores DSDP-609 and
VM23-81, H3 clearly reveals a succession of pulses of
1) hematite-coated grains that could be derived from the
“red beds” in the Gulf of St. Lawrence and 2) detrital
carbonate grains from the Hudson Strait (Bond and
Lotti, 1995). We may argue that there are several
possible hematite-grained sources around the North
Atlantic Ocean and thus, hematite-grained does not
appear to be a reliable tracer for the LIS origin. A North
American signature is confirmed, however, by our
findings in core SU90-11: in this western core, HL3
displays a Laurentide-derived isotopic composition (εNd
(o) = − 24.1). Although HL3 remains an “atypical” HL
(Snoeckx et al., 1999), it is clear that it cannot be
considered as an HL solely derived from Europe
(Fennoscandia and/or British Isles). We suggest that,
HL3 was probably generated by reduced iceberg
releases concerning all the different pan-Atlantic icesheets, their respective influence being recorded only
on a regional scale. This is reflected in the map pattern
of Sr isotope compositions (Hemming, 2004).
As HE3 and HE6 present very similar characteristics
(Huon and Jantschik, 1993; Gwiazda et al., 1996b;
Grousset et al., 2000), they might have been generated
by similar processes. Thus, in order to check its
resemblance to HE3, HE6 should be better studied in
the future.
4.3. HL4 and HL5
Like HL2, both HL4 and HL5 in core SU90-11 are
characterized by a sharp basal contact of the IRD
deposits and the underlying glacial sediments (Fig. 3),
implying a very sudden onset of IRD deposition. Before
HL2, the ice volume of all North Atlantic ice-sheets was
considerably reduced, compared to the HL2–LGM–
HL1 interval. Accordingly, during HL1 and HL2, the
contribution of IRD and probably fresh water were
approximatively equal to those during HL4 or HL5. It
implies that ice-sheets do not need to be fully developed
to create “typical” Heinrich events but presumably the
collapse of the smaller ice-sheets present at that time
was probably larger during HL4 or HL5. It would also
seem to place constraints on the volume of ice that could
be assumed to have been purged from the Laurentide
ice-sheet. The late appearance of dolomite in HL4 may
indicate a precursory event as well, but the Nd
composition indicates ancient sources, unlike before
HL1 and HL2. The dolomite content of HL5 is very low
compared to the other layers. In core V28-82, HL4 and
HL5 contain similar percents of detrital carbonate, 15%
and 16% (Hemming et al., 1998). Thus the lesser
content at SU90-11 might reflect a more northern
iceberg melting path during HL5, more like that
followed by HL2 and HL1, as traced by magnetic
susceptibility patterns (Grousset et al., 1993).
As suggested by the X-ray image (clearer sections on
Fig. 3), it appears that before, between and after HL5
and HL4, the ambient sediments contain finer
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
terrigenous components. Prior to HL4, we do not
observe any precursor event similar to the precursors
observed prior to HL1 and HL2. Instead, a very small
increase of quartz and volcanic grains can be observed
(Fig. 3): such a pattern cannot be interpreted as a typical
precursor event. Some IRD are present between HLs at a
very low level, revealing that pan-Atlantic ice-sheets
(especially Laurentide and Greenland) released icebergs
continuously. Because they could have been incorporated into the anticlockwise surface current gyre
suggested by Ruddiman (1977), during these intermediate periods, icebergs discharged from the European
ice-sheets could have also reached the western part of
the IRD belt. If so, their abundance was very limited and
thoroughly diluted by the Laurentide/Greenland inputs.
Moreover, the low accumulation rate characterizing this
core might preclude any identification of secondary IRD
events previously observed in high accumulation rate
cores (Bond and Lotti, 1995; Elliot et al., 1998).
A similar scenario is observed in other cores from
across the North Atlantic within the IRD belt, the HL
thickness during both HL4 and HL5 decreases from
west to east: 30 cm in SU90-11, 20 cm in SU90-09
and 10–12 cm (only for HL4) in MD01-2448 (Fig. 4)
and increases again along the European margin
(Grousset et al., 1993). On the eastern side of the
IRD belt (core MD01-2448), the isotopic composition
of the glacial background is typically European (εNd(o) N
− 16 / − 17).
The contrast between HL4–HL5 and HL1–HL2 has
also been suggested by other studies. For example, the
Nd–Sr isotopes in core ODP-609 show variations
consistent with those reported here (Vance and Archer,
2002). Additionally, both the δ13C and δ15N of the
organic matter (Huon et al., 2001) and biomarker
distribution (Kornikova et al., pers. comm.) in core
SU90-09 show profoundly different character in HL1–
HL2 and HL4–HL5.
What is the origin of the difference between HL1/
HL2 different from HL4/HL5? During the LGM
(between 25 and 14 ka), the sea level was particularly
low (oscillating then between −80 and − 130 m; Siddall
et al., 2003) and the Northern Hemisphere ice-sheets
were extended to their maximum positions, with many
terminating at the shelf–slope boundary.
4.4. Spatial and temporal variability
Two aspects of the LGM were more favourable for
depositing IRD with distinctive composition into the
North Atlantic. One is that the termination of the ice at
the shelf–slope boundary would have made it more
235
likely for the IRD of icebergs to survive melting until
they were carried into the open ocean currents (e.g.,
Syvitski et al., 1996; Hemming et al., 2002). The
second is that the ice covered regions with different
geological ages. This is particularly clear off Nova
Scotia and Newfoundland (Fig. 1) and could explain the
particular composition of sediments imbedding HL1,
HL2 and HL3 in core SU90-11. Hemming et al. (2000)
and Hemming and Hajdas (2003) have showed that in
the interval between the end of HL3 and the end of
HL1, the non-Heinrich layer IRD has a very large
fraction of hornblende grains with 40Ar/39Ar ages
reflecting erosion of Appalachian/Caledonian and
Grenville age provinces. In contrast, after HL1 and
before HL3 and down to HL5 (the end of the studied
interval), there is little evidence of icebergs derived
from terrains with such ages, and instead the ages are
dominated by Paleoproterozoic grains (Hemming and
Hajdas, 2003). These authors have interpreted the
variations in hornblende ages from these cores near the
Laurentide margin to reflect the evolution of ice
advancing to the ocean on Laurentide. Specifically,
the results would imply that the LIS expanded to the
Gulf of St. Lawrence region at about 30,000 years BP
(∼ HL3) and remained there until about 17,000 years
BP (∼ HL1). Before HL3, it is likely that Greenland and
possibly northern Laurentia were the dominant sources
of hornblende grains, and after HL1 it is likely that the
importance of Greenland sources increased again
(Hemming et al., 2000; Hemming and Hajdas, 2003).
We would suspect a similar progression for Iceland and
European ice-sheets and this is consistent with studies
of the abundance of IRD in the Nordic Seas (see
references in Hemming, 2004). In parallel, around the
LGM, the simultaneous change in surface circulation
associated with the southward shift of the polar front,
could have modified the iceberg trajectories, explaining thus the change in their IRD load composition.
It is clear that in all sites studied in the North Atlantic
Ocean, a LIS signal is identified for typical HLs
(− 40 b εNd(o) b − 17) (Fig. 6). For HL3, it seems to be
more complex because we observe limited evidence of a
LIS contribution based on Sr and Nd isotopes and Ar/Ar.
Close to the source of icebergs at the western entrance of
the IRD belt, HLs are exclusively represented by debris
of Canadian origin. In this region, the HL thickness may
reach 50 cm (Grousset et al., 1993). Further east, as
icebergs melted progressively, the Laurentide sourced
IRD became progressively less prominent: about 10 cm
thick on the Mid-Atlantic Ridge and only a few
centimeters along the European margin (Scourse et al.,
2000; Grousset et al., 2000). The eastward transport of
236
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
Fig. 6. Spatial variability of the εNd(o) values measured on the carbonate-free, coarse (N150 μm) IRD fractions of the Heinrich layers in five different
cores (see location on Fig. 1). Records are aligned along a west–east transect, inside the IRD belt. εNd(o) values more negative than − 17 (right side of
the dotted vertical lines) point to a Canada/Greenland origin (sub-horizontal grey bands).
Laurentide-derived icebergs was driven by the oceanic
and atmospheric circulations. Thus, the Greenland and
Labrador currents brought them through the entrance
gate of the IRD belt. Stronger westerlies transported
them eastward (Keefer et al., 1988), as far east as the
European margin (e.g. core MD95-2002). Based on
AMS-14C datings from within HL2 in core SU90-09
(Grousset et al., 2001), we could suggest that the
Hudson Strait-derived icebergs crossed the Mid-Atlantic
Ridge eastward, for only one to a few centuries,
consistent with the compilation of Hemming (2004) as
well as modelling results to simulate the δ18O anomaly
generated by the melting of icebergs during HE4 (Roche
et al., 2004). Unfortunately, due to great uncertainties in
reservoir age corrections applied to these AMS-14C
ages, it is clear that the dating uncertainties could be as
great as 1000 years (Waelbroeck et al., 2001).
In the eastern North Atlantic, Hudson Strait
Heinrich events are embedded within IRD of European origin (Fig. 6). It is clear that the European icesheets (McCabe et al., 1998; Bowen et al., 2002)
released their own icebergs during the entire duration
of the HEs (1-to-2 thousand years). The Laurentide
surge thus appears to have occurred briefly within this
period of release of European icebergs, as already
suggested for HL1 along the Irish margin (Knutz et al.,
2002).
The absence of precursors prior to HL4 and HL5 is
an important observation. It suggests that precursor
events are not a necessary step required for triggering
the main Laurentide surges. Rather, the precursory
intervals near HL2 and HL1 are more likely simple
periods of intensified releases of icebergs by all the panAtlantic ice-sheets.
5. Conclusions
The accumulation of IRD forming the Heinrich
Layers that are observed within the Ruddiman IRD
belt is mostly due to anomalous contributions from the
Hudson Strait portion of the Laurentide ice-sheet. During
Heinrich events, broadly defined, European ice-sheets
(Fennoscandia, British Isles) released icebergs too. Their
contribution, however, is only clearly identifiable near
their margins due to a combination of lesser flux, greater
required transport distance to the IRD belt and prevailing
E. Jullien et al. / Global and Planetary Change 54 (2006) 225–238
surface circulation. Icelandic-derived icebergs deposited
a distinctive IRD assemblage and thus can give an
indication of the extent of movement of icebergs within
the surface currents of the North Atlantic. Icelandic
icebergs seem to have been transported over a much
broader area and are thus visible within the entire IRD
belt; however, their spatial and temporal pattern of
deposition is complex, because they could have been
incorporated in the anticlockwise surface currents.
It is also clear that the HLs are not similar to each
other in detail. It was understood very early that HE3 and
HE6 were different and less dramatic (e.g., Bond et al.,
1992, 1993; Grousset et al., 1993). But the large Heinrich
events also have some key distinctions that may be
crucial for constraining their origin. For example, HE2
and HE1 are more similar to each other and occur against
different background conditions compared to HE5 and
HE4. However, HE1 is different in detail from HE2 due
to its occurrence at the termination of the LGM. Precursory iceberg discharges precede the occurrence of the
major Hudson Strait sediment pulse of HE2 and HE1. In
contrast, there is no precursory increase in IRD before
H4 and H5. This distinction is important and allows us to
claim that precursors, as originally conceived, must not
have been a necessary initial step for triggering the major
Laurentide surges.
The fact that precursor events occur only prior to HE1
and HE2 could be attributed to the fact that, during that
period (≈13.5 to ≈28 kyr B.P.), the sea level was much
lower (between −80 and −130 m) and the ice-sheets were
accordingly much more extended, with many terminations at the shelf–slope boundary.
Acknowledgements
The cores studied here were collected by the IFRE
MER R/V Le Suroit and IMAGES cores were recovered
by the N/V Marion-Dufresne (French Polar Institute). J.
Saint-Paul, O. Weber and P. Brunet provided invaluable
technical assistance. Laurent Labeyrie kindly provided
AMS-14C ages and δ18O records. We have benefited
from the fruitful reviews of Paul Knutz and Jens Bischof.
Financial contribution from the Centre National de la
Recherche Scientifique ECLIPSE programme is acknowledged. SRH acknowledges support from a grants/
cooperative agreement from the National Oceanic and
Atmospheric Administration. The views expressed
herein are those of the authors and do not necessarily
reflect the views of NOAA or any of its sub-agencies. IH,
VP and FG are grateful to NERC and the Royal Society
for support. This is UMR 5805 EPOC contribution 1566
and L-DEO contribution 6822.
237
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
gloplacha.2006.06.021.
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