Quaternary Science Reviews 28 (2009) 3236–3245
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Quaternary Science Reviews
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Sediment sources of northern Québec and Labrador glacial deposits and the
northeastern sector of the Laurentide Ice Sheet during ice-rafting events
of the last glacial cycle
Martin Roy a, *, Sidney R. Hemming b, Michel Parent c
a
Département des Sciences de la Terre et de l’Atmosphère and GEOTOP Geochemistry and Geodynamics Research Center, Université du Québec à Montréal,
Succ. Centre-Ville, C.P. 8888, Montréal, QC, Canada H3C 3P8
b
Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964, USA
c
Geological Survey of Canada, 490 de la Couronne St., Québec City, QC, Canada G1K 9A9
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 December 2008
Received in revised form
10 August 2009
Accepted 10 August 2009
Provenance studies of anomalously high-flux layers of ice-rafted detritus (IRD) in North Atlantic sediments of the last glacial cycle show evidence for massive iceberg discharges coming from the Hudson
Strait region of the Laurentide Ice Sheet (LIS). Although these so-called Heinrich events (H events) are
commonly thought to be associated with abrupt drawdown of the LIS interior, uncertainties remain
regarding the sector(s) of this multi-domed ice sheet that conveyed ice through Hudson Strait. In
Northern Québec and Labrador (NQL), large-scale patterns of glacial lineations indicate massive ice flows
towards Ungava Bay and Hudson Strait that could reflect the participation of the Labrador–Québec ice
dome in H events. Here we evaluate this hypothesis by constraining the source of NQL glacial deposits,
which provide an estimate of the provenance characteristics of IRD originating from this sector.
Specifically, we use 40Ar/39Ar ages of detrital hornblende grains in 25 till samples distributed along
a latitudinal transect (lat. 58! ) extending east and west of Ungava Bay. The data show that tills located
west and southwest of the Ungava Bay region are largely dominated by hornblende grains with Archean
ages (>2.6 Ga), while tills located east of Ungava Bay are characterized by grains with early Paleoproterozoic ages (2.0–1.8 Ga), although most samples contain a few Archean-age grains. IRD derived from
the NQL region should thus be characterized by a large proportion of Archean-age detrital grains, which
contrasts significantly with the predominant Paleoproterozoic 40Ar/39Ar ages (1.8–1.6 Ga) typically
reported for the dominant age population of hornblende grains in H layers. Comparisons with IRD
through the last glacial cycle from a western North Atlantic core off Newfoundland do not show evidence
for any prominent ice-rafted event with the provenance characteristics of NQL glacial deposits, thereby
suggesting that significant ice-calving event(s) from the Labrador–Québec sector may have been limited
throughout that interval. Although these results tend to point towards a relative stability of this ice dome
during H events, our study also indicates that further provenance work is required on IRD proximal to the
Hudson Strait mouth in order to constrain with a greater confidence the sector(s) of the LIS that fed ice
into Hudson Strait during H events. Alternatively, these results and other paleogeographic considerations
tend to support models suggesting that part of the Ungava Bay glacial lineations could be associated with
a Late-Glacial ice flow across Hudson Strait.
! 2009 Elsevier Ltd. All rights reserved.
1. Introduction
The greater Hudson Strait region was subject to important icedynamic events during the last glacial cycle, notably in the form of
* Corresponding author. Tel.: þ1 514 987 3000; fax: þ1 514 987 7749.
E-mail addresses: [email protected] (M. Roy), [email protected]
(S.R. Hemming), [email protected] (M. Parent).
0277-3791/$ – see front matter ! 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2009.08.008
large drainage episodes of glacial ice through Hudson Strait (e.g.
Andrews, 1998), with concomitant massive iceberg discharges in the
North Atlantic ocean that have had a strong impact on climate (Bond
et al., 1992; Broecker et al., 1992). These so-called Heinrich events (H
events) are also significant in terms of ice sheet dynamics because
the composition and sedimentological characteristics of the associated ice-rafted detritus (IRD) imply sudden and episodic collapses of
the Laurentide Ice Sheet (LIS) (Andrews and Tedesco, 1992; Dowdeswell et al., 1995; Gwiazda et al.,1996; Hemming et al.,1998, 2000;
M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
see also Hemming, 2004 for a review). Other North Atlantic ice
sheets appear to have released icebergs in approximate synchroneity
(Grousset et al.,1993, 2000, 2001; Bond and Lotti,1995; Scourse et al.,
2000; Knutz et al., 2001, 2002, 2007; 2003; Jullien et al., 2006; Peck
et al., 2006, 2007), but their contribution to IRD concentrations was
small with respect to the LIS. Although multiple mechanisms have
been suggested to explain the cause/origin of H events (e.g.
Hemming, 2004), most glaciological and paleoclimatic proxy-based
models put forward to explain these large iceberg discharges involve
drawdown of large amount of ice from the central part of the LIS
(MacAyeal, 1993; Alley and MacAyeal, 1994; Alley et al., 2005;
Marshall and Koutnik, 2006). The spatial distribution of glacial
lineations and patterns of post-glacial rebound (e.g. Peltier, 2002;
Tarasov and Peltier, 2004), however, indicate that Hudson Bay was
not the area with the thickest ice, but was rather surrounded by three
important ice domes that fed the margins as well as the interior of
the LIS (Fig. 1a) (Dyke and Prest, 1987). Extensive mapping of glacial
striations on multifaceted rock outcrops and boulder tracing of
lithological indicators of glacial transport indicate large-scale
displacements of these ice domes during the last glacial cycle (Parent
et al., 1995; Veillette et al., 1999; Clark et al., 2000; Jansson et al.,
2002; McMartin and Henderson, 2004; Veillette, 2004). These
reorganizations of the ice divide system likely reflect important icesurface changes of the LIS, which could be related to the postulated
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ice sheet collapses during Heinrich events (Clark et al., 2000; Dyke
et al., 2002).
Additionally, the occurrence of major ice-flow patterns that
converge towards Hudson Bay and Hudson Strait could also reflect
disintegration paths associated with Heinrich-type drawdown
episodes of the LIS. This is well illustrated in Northern Québec and
Labrador (NQL) where the numerous fields of glacial lineations that
characterize the Ungava Bay region (Fig. 1b) were linked to a major
ice-draining event (or several synchronous events) of the northeastern sector of the LIS that may have contributed to the massive
iceberg discharges associated with H events (Jansson et al., 2003).
The contribution of the Labrador–Québec ice dome to H events
during the Last Glacial cycle and the significance of the NQL glacial
flutings in such events, however, remain largely unconstrained.
Overall, little is known on the sector(s) of the LIS and associated iceflow paths that conveyed ice to the Hudson Bay/Strait region during
H events, and this represents an important limitation in our
understanding of the LIS in the context of H events.
Here we document the provenance of NQL glacial sediments
(tills) representing the sediment source carried by Ungava Bay ice
streams and other regional ice-flow trajectories, which ultimately
delivered ice and associated IRD to the ocean. For this purpose, we
use the single-step laser fusion 40Ar/39Ar method to date 241
detrital hornblende grains from 25 samples roughly located on the
A
B
Fig. 1. A) Schematic configuration of the Laurentide ice sheet around the last glacial maximum (after Dyke and Prest, 1987; Dyke et al., 2002). Thin and discontinuous lines represent
glacial flow-lines coming from three main ice-dispersal centers located around Hudson Bay. Glacial flow-lines are based on the trend of glacial lineations that were mapped from
drumlins and glacially molded outcrops from aerial photographs and field observations. Continuous lines within the ice sheet mark the approximate extent of each sector. V23–14
indicates the name and location of the marine core used in this study. The letters A and G designate the Appalachians and the Gulf of St-Lawrence regions, respectively. B) Details of
glacial lineations in the study area (modified from Prest et al., 1968; Clark et al., 2000). The extent of the post-glacial marine limit is shown by a light gray zone around the coasts.
Location of till samples is marked by dark gray squares.
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M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
58th parallel and covering three main regions west, southwest, and
east of Ungava Bay (Figs. 1b and 2). We compare the results to
published 40Ar/39Ar ages obtained from H layers and other minor
IRD in a western North Atlantic core located east of Newfoundland
(Fig. 1a) (Hemming and Hajdas, 2003) to evaluate whether the
provenance signature of NQL glacial deposits can be recognized in
marine sediments of the last glacial cycle.
2. What sector of the LIS contributed to the ice discharges?
The identification of the LIS as the main source for the massive
iceberg discharges associated with H events comes primarily from
provenance work on anomalously high-flux layers of IRD in North
Atlantic deep-sea sediments of the last glacial cycle, which were
first recognized by the high concentrations of lithic grains in the
coarse fraction of IRD (Heinrich, 1988). The high detrital carbonate
concentration that characterizes the terrigenous sediment fraction
of H layers was attributed to a calving ice margin that overrode
a carbonate-floored substrate, likely corresponding to the one that
covers large areas of Hudson Bay and Hudson Strait (Andrews and
Tedesco, 1992; Bond et al., 1992; Broecker et al., 1992). The LIS origin
for H event was also reinforced by radiogenic provenance studies.
Lead and neodymium isotope systematics measured on bulk sediment suggested an Archean heritage of the terrigenous fraction of H
layers, while Pb isotopes of feldspar grains and 40Ar/39Ar ages of
hornblende grains indicated Paleoproterozoic metamorphism
(w1.8 Ga) of the source-rocks of H layers (Gwiazda et al., 1996;
Hemming et al., 1998, 2000; Hemming, 2004). These characteristics
further confined the source region of H layers to the rocks of the
Paleoproterozoic Churchill Province of the Canadian Shield, which
outcrop in the Hudson Strait area (Fig. 3). These results, along with
other sedimentological data, led to the interpretation that a large
fraction of the LIS in Hudson Bay drained repeatedly through an ice
stream flowing in Hudson Strait (e.g. Andrews, 1998).
Although the LIS was unequivocally the main contributor of
detrital material to H layers, the exact sector of the LIS that
conveyed ice through Hudson Strait has yet to be identified. Hudson Bay is commonly regarded as the source of the drawdown of
large amount of ice, mostly because this region is underlain by
Paleozoic carbonates. Unlike the coarse-grained crystalline rocks of
the Shield, this substrate provides soft-bedded conditions that are
necessary to generate the fast-flowing ice that has been invoked to
accommodate the massive iceberg discharges, in addition to
provide a source for the high detrital carbonate content of most H
layers (Andrews and Tedesco, 1992; Marshall and Clarke, 1997).
However, field-based paleogeographic reconstructions and
patterns of post-glacial uplift indicate that Hudson Bay was a site of
lower ice-surface elevation where ice flows were converging from
the three nearby centers of outflow (Dyke and Prest, 1987; Dyke,
2004; Tarasov and Peltier, 2004) (Fig. 1a). The difficulty in identifying the sector of the LIS that fed the Hudson Strait ice stream on
the basis of sedimentary and radiogenic data from ocean cores also
comes from the occurrence at the mouth of Hudson Strait of
a bedrock sill, which is composed of Paleoproterozoic crystalline
rocks of the Churchill Province (Fig. 3). Because this sill rises w300–
400 m above the seafloor, it likely acted as an important grounding
line for this ice stream (Andrews and MacLean, 2003; Hulbe et al.,
2004; Alley et al., 2005). The occurrence of a significant overdeepened zone located up-ice from the sill further suggests that it
may have been an area of important subglacial erosion during the
calving/streaming episodes that led to H events (Hemming, 2004).
Consequently, part of the geochemical signature of H layers likely
contain erosional products of this bedrock protuberance, thereby
complicating the identification of ice flow coming from the LIS
interior on the basis of IRD data alone. This points to the need of
Fig. 2. Location of till samples investigated in this study (red squares) with respect to the main ice-flow patterns in Northern Quebec and Labrador, shown here as flow-sets. Iceflow sets were defined by Clark et al. (2000) on the basis of detailed mapping of glacial lineations. Ice-flow set fsJ03 comes from Jansson et al. (2003). These ice-flow patterns are
here used to identify the main trajectories along which detrital material can be transported to the ocean, regardless of the age of formation of these features. Black arrows show
postulated ice flows within and around the Hudson Strait and Ungava Bay regions based from field evidence and inferred by glaciological modeling. Light gray areas around the
coasts and main river valleys show the approximate extent of the post-glacial marine invasion. For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.
M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
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Fig. 3. Bedrock geology map showing the main lithology mantling the floor of Hudson Strait and Ungava Bay (modified from Andrews and MacLean, 2003). Inset shows the location
of the Abloviak sill, which is composed of Paleoproterozoic crystalline basement rocks that raise w350 m above the Hudson Strait floor (MacLean, 2001).
better documenting the provenance of glacial deposits that are
associated with massive ice-flow paths extending from the
different ice domes into the Hudson Bay and Hudson Strait region
in order to further constrain the source of LIS ice for H events.
Nonetheless, in addition to large-scale patterns of glacial lineations that indicate massive ice flows towards Hudson Bay/Strait
(e.g. Clark et al., 2000; Jansson et al., 2003), Heinrich-type drawdown episodes are suggested by the occurrence of sequences of
shifting ice flows documented from rock outcrops all around
Hudson Bay (Parent et al., 1995; Veillette et al., 1999; McMartin and
Henderson, 2004; Veillette, 2004), mostly because these reorganizations of ice sheet profile require significant ice-surface
lowering of certain sectors of the LIS. These erosive and geomorphic
features, however, have yet to be documented within the context of
H events.
3. NQL glacial lineations and the northeastern sector of the
Laurentide Ice Sheet
The NQL region comprises a large fraction of the area formerly
occupied by the Labrador–Quebec ice dome, which was the main
center of ice outflow of the northeastern sector of the LIS (Fig. 1). The
landscape of this region is characterized by abundant drumlins,
crag-and-tails, glacially molded bedrock outcrops, and fluted till
plains with disseminated areas of crystalline rock outcrops (Fig. 4)
(Hughes, 1964; Prest et al., 1968; Lauriol, 1982; Gray and Lauriol,
1985; Bouchard and Marcotte, 1986; Gray et al., 1993; Parent et al.,
1995; Clark et al., 2000; Gray, 2001; Jansson et al., 2003). These
glacial lineations are associated with former glacial flow-lines that
were coming from this ice dome, presumably sometime after the
last glacial maximum (LGM) (Fig. 1). This region encompasses
Ungava Bay where the long axis of most glacial lineations shows an
impressive pattern of converging ice flows towards Ungava Bay
(Fig. 2) (Clark et al., 2000; Jansson et al., 2002). Detailed examination
of aerial photographs and satellite images showed that the Ungava
Bay glacial lineations can be grouped into discrete networks of iceflow sets (Clark et al., 2000; Jansson et al., 2003). These swarms of
glacial lineations were attributed to fast-flowing ice and interpreted
to reflect imprints of former ice streams. On the basis of crosscutting relationships, as evidenced by abrupt lateral margins of the
different ice-flow fans and other geomorphological features, a relative chronology describing a sequence of ice stream development
was further proposed. In this model, these landform systems would
have formed over a relatively broad time-interval spanning part of
the last glacial cycle (Jansson et al., 2003). There are, however,
almost no chronological constraints for these events. This lack of
constraints is at the origin of considerable contrasting interpretations regarding the age of these glacial lineations (e.g. Clark et al.,
2000; Jansson et al., 2002), ranging from relict landforms from an
older deglaciation preserved under cold-based ice during the last
glacial cycle (Kleman et al., 1994), to Late-Glacial features of the last
glaciation (Veillette et al., 1999; Dyke et al., 2002).
Of importance here is that the ice-dynamic conditions imposed
by the large-scale ice-flow patterns formed by the Ungava Bay glacial
lineations led to the suggestion that these Ungava Bay ice-flow
systems were likely the results of a major ice-flow event(s) that
drained the Labrador–Québec ice dome interior during H events,
supposedly through glacial flow-lines feeding the Hudson Strait ice
stream (Jansson et al., 2003). This link between the Labrador–Québec
sector and drawdown episodes of the LIS can also be seen in various
geological models and paleogeographic reconstructions on H events,
mainly in the form of a branch of the Hudson Strait ice stream (Fig. 2)
(Andrews, 1998; Andrews and MacLean, 2003; Hulbe et al., 2004).
Although involving the Labrador–Québec ice dome in Heinrich-type
events appears to be a logical mechanism to drain large amount of ice
from the deep interior of the ice sheet, there is little direct evidence
to support such a model. Involving the northeastern sector of the LIS
in the ice discharges of H events through Ungava Bay also requires
important constraints to be met, notably an ice-free Hudson Strait
(i.e. an ice stream retracted west of Ungava Bay), or, alternatively, the
presence of a Hudson Strait ice stream that has ice thickness/velocity
characteristics that allows ice from the Ungava ice stream network to
feed this main ice outlet (e.g. Kirby and Andrews, 1999; Andrews and
MacLean, 2003).
4. Approach and methods
Twenty-five till samples were collected along an east–west
transect roughly centered on latitude 58! North (Figs. 1 and 2). This
transect is broken into three sectors due to the presence of extensive marine deposits that mask the underlying glacial deposits in
low-lying areas around the coasts of Ungava Bay and eastern
Hudson Bay (Figs. 1b and 2). The surficial till samples used in this
study were collected above the post-glacial marine limit to avoid
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M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
Fig. 4. Aerial photograph (A) and oblique photograph (B) showing the glacial landscape of the study area, which is characterized by drumlin fields, fluted till plains, crag-and-tails,
molded bedrock outcrops, and disseminated outcrops of bare crystalline bedrock. Aerial photographs no. R6076-L-55, from National Airphoto Library, 1:60,000 scale. Photograph
taken nearby Lac Peters, west of Ungava Bay, 71!000 W, 59! 300 N; drumlins axis w065! N.
any possible reworking of the glacial deposits. The samples come
from glacially fluted till plain composed of a relatively thin mantle
(2–3 m) of glacial deposits that is broken in places by rock outcrops
(Fig. 4). Till in this area is characterized by a coarse (sandy) matrix
containing abundant clasts, reflecting the erosion of local crystalline lithologies (Parent et al., 2004).
Figs. 1 and 2 show the position of the three sectors of till
samples with respect to the main glacial flow-lines in this region.
These flow-lines are largely based on the geomorphologic and
striation records, for which chronological constraints are often
lacking. Although some of these flow-lines may be asynchronous,
and thus schematic, they provide important information on the
possible trajectories along which ice may have flowed from the ice
dome to the ocean. Within the context of the main ice-flow paths
in the NQL region (Clark et al., 2000; Jansson et al., 2003), the till
samples are expected to characterize the catchment areas of the
glacial debris entrained by ice flowing toward north-northeast and
northwest into Ungava Bay westard into northeastern Hudson Bay,
and to the eastward (into the Labrador Sea (Fig. 2). Although there
are no till samples located directly south of Ungava Bay in the area
comprising ice-flow sets extending northward into the bay, the
regional ice-flow patterns (Clark et al., 2000; Jansson et al., 2003)
indicate that the till samples of the middle sector lie on the
western edge of such former ice-flow paths (Fig. 2). The patterns of
regional ice flows also indicate that the till of the middle sector,
like the till south of Ungava Bay, orginates from ice taking its
source in the interior of Labrador–Quebec ice dome. Because of
their nature (till genesis), the composition of these deposits should
represent a mixture of the lithologies that were progressively
incorporated during glacial transport, and their provenance characteristics should thus reflect the general bedrock composition of
this region. The main geological divisions show that tills southwest
and south of Ungava Bay derive largely from the erosion of the
same source-rock, i.e. the Superior Province (Fig. 5). Consequently,
whether the middle sector till samples are associated with
erosion/deposition of a northeastward flowing ice (as suggested by
the dominant ice-flow pattern) or by northward-flowing ice (as
suggested by regional ice-flow patterns), the geological setting of
this area indicates that these tills should integrate the same
source-rock. Accordingly, the provenance characteristics of the
middle sector till samples can be considered representative of
sediments that would be carried by ice flowing northward into
Ungava Bay.
Individual hornblende grains of 125–250 mm in size were
extracted from the fine-grained matrix of till samples using
a binocular microscope. Hornblende grains were irradiated for ten
hours in the Cd-lined, in-core facility (CLICIT) of the TRIGA reactor at
Oregon State University, along with hornblende-monitor standard
Mmhb (age of 525 Ma; Samson and Alexander, 1987) and a sanidine
standard (Cima, 18.9 Ma, relative to MMhb-1 age of 523.2 # 2.3 Ma;
Spell and McDougall, 2003). Approximately 10 detrital hornblende
grains were dated in each sample using the 40Ar/39Ar method
through single-step laser fusion at Lamont–Doherty Earth Observatory (LDEO), for a total of 241 age-determinations. Ages were
calculated from Ar isotope ratios corrected for mass fractionation,
interfering nuclear reactions, procedural blanks, and atmospheric Ar
contamination.
The 40Ar/39Ar ages of detrital hornblendes are first evaluated
within the context of the numerical ages reported for the potential
rock-source areas (i.e. the main geological provinces found in the
region, described in the next section). The 40Ar/39Ar ages of hornblende correspond to either an estimate of igneous crystallization,
or, alternatively, the last time the source terrane was subjected to
high-temperature metamorphism (i.e. >450 ! C) (McDougall and
Harrison, 1999). Given the numerous orogens that characterize the
geological history of this part of the Canadian Shield, the 40Ar/39Ar
ages of hornblende grains from NQL tills should primarily reflect
the last time the source terranes were involved in a major metamorphic event. These 40Ar/39Ar ages provide information on the
provenance of the glacial sediments entrained by ice, and thus the
geochemical signature of ice-rafted material that would be associated with iceberg discharges coming from the NQL region.
The 40Ar/39Ar ages of hornblende grains of tills are also
compared to 40Ar/39Ar ages of IRD grains from H layers and minor
IRD layers of marine sediments spanning the time-interval
comprised between the early Holocene and 43 ka (Hemming and
Hajdas, 2003).
5. Geological setting of the Ungava Bay region
With the exception of the Paleozoic carbonate rocks covering
the floor of Hudson Strait and part of Ungava Bay, the rocks
underlying the study area are almost entirely composed of
Precambrian basement rocks of the Canadian Shield, which in the
Ungava Bay region, includes three structural provinces that were
assembled during various orogens: the Superior, Churchill, and
M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
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Fig. 5. Simplified bedrock geology of the Hudson Strait and Ungava Bay regions (based on the Geological Survey map 1860A by Wheeler et al., 1996).
Nain provinces (Fig. 5) (Hoffman, 1989). The rocks forming these
provinces, and thus rock-source regions for this study, can be
distinguished according to their dominant radiometric imprint,
which generally reflects the last major orogenic (metamorphic)
event to affect the province (i.e. age of thermotectonic resetting).
Specifically, the region west of Ungava Bay is composed of
Archean gneiss and other high-grade metamorphic rocks of the
Superior Province (Minto sub-province). Hornblende grains
derived from the glacial erosion of these rocks should thus be
characterized by 40Ar/39Ar ages of w >2.6 Ga (Card, 1990). The
northern tip of the Ungava Peninsula consists primarily in Paleoproterozoic (2.5–1.6 Ga) volcanic-arc terranes of the Cape Smith
thrust belt that were accreted during the New-Quebec orogen
(St-Onge et al., 1999). This orogen is also responsible for the
formation of the Labrador Trough that extends southward from
Ungava Bay, along a band of a few hundred kilometers wide. The
rocks east and southeast of Ungava Bay also belong to the eastern
Churchill (Rae) Province (Lewry and Stauffer, 1990) and although
they have an Archean heritage, they are primarily characterized by
a strong Paleoproterozoic imprint inherited from major episodes
of high-temperature metamorphism (Lewry and Stauffer, 1990),
notably during the Torngat Orogen that produced the prominent
Torngat mountain range that separates Ungava Bay from the Labrador Sea. In this area, the Nain Province shows up as a series of
Mesoproterozoic (1.6–0.9 Ga) intrusions that cover a large area
extending from the Labrador coast to the southernmost extent of
the Labrador Trough. Consequently, 40Ar/39Ar ages of the hornblende grains originating from the erosion of the Churchill (Rae)
rocks should cluster around 1.9–1.7 Ga, while tills derived from ice
advances having overridden the Nain sector should have
hornblende grains yielding 40Ar/39Ar ages of w1.6–1.4 Ga. Metasedimentary rocks of the Churchill Province also crop out at the
eastern end of Hudson Strait, within the Abloviak shear zone that
extents from the Torngat Mountains in a NW–SE trend, parallel to
the main regional structural boundaries (MacLean, 2001).
6. Results
The results of 40Ar/39Ar dating of individual hornblende grains
from tills are shown as individual probability plots in Fig. 6 where
data are grouped into 5 sectors according to the position of the
major ice-flow trajectories defined by the patterns of glacial flowlines in the region (Bouchard and Marcotte, 1986; Clark et al., 2000;
Jansson et al., 2003). Till samples from the eastern, west-central, and
western sectors should represent the characteristics of sediments
transported by ice converging into Ungava Bay (Fig. 2). Tills of the
westernmost sector represent sediment source from ice draining into
northeastern Hudson Bay, while tills from the easternmost sector
characterize material draining directly into the Labrador Sea (Fig. 2).
Tills from the westernmost and western sectors show detrital
hornblende grains with Archean ages, with 40Ar/39Ar ages ranging
predominantly from 2.60 to 2.75 Ga (Fig. 6), thus in agreement with
age of the source-rock from this region – the Superior Province. Tills
from the eastern and easternmost sectors are largely dominated by
40
Ar/39Ar ages ranging from 1.95 to 1.60 Ga (Fig. 6), reflecting the
erosion of the underlying Paleoproterozoic bedrock of the Churchill
and Nain provinces present in this area. A few grains with Archean
ages are also found in tills adjacent to the eastern Ungava Bay
coastline. This could reflect ice advance across the bay, from western
Ungava Bay, as suggested by erosional marks that indicate the
occurrence of such ice flow in this region (Paradis and Parent, 2002).
Alternatively, the occurrence of sparse Archean-age grains may also
reflect ice flow coming from the south. The ice-flow patterns in the
Ungava Bay region (Clark et al., 2000; Jansson et al., 2003). show that
some of the northward-flowing fans appear to take their source
deep enough into the Labrador–Quebec sector to reach rocks of the
Superior Province (Figs. 2 and 5). Tills from the west-central sector
show a large population of hornblende grains with Archean ages, as
well as abundant Paleoproterozoic-age grains (Fig. 6). This likely
reflect the geographic position of these samples, which are located
nearby the boundary between Archean and Paleoproterozoic
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M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
A
B
Fig. 6. A) Normalized probability plot showing 40Ar/39Ar ages of hornblende grains dated in till samples of the study area. Colors correspond to the different sectors discussed in
text. B) Stack histograms showing the distribution of 40Ar/39Ar ages of hornblendes for the 25 till samples of the different sectors. Note that Y axes are different in each histograms,
while X axes are all identical, with 50-Myr bins. The number (n) of dated hornblende grains and associated samples in each sector are given at top of each histogram. The complete
40
Ar/39Ar data set and samples coordinates are presented in Table DR1, as supplementary material in the on-line appendix.
M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
terranes (Fig. 5). The occurrence of Paleoproterozoic-age grains
could also be attributed to ice-flow paths taking their source slightly
to the east, on the Paleoproterozoic rocks of the Labrador Through,
as indicated by the regional ice-flow patterns (Fig. 2).
7. Discussion – conclusions
Sedimentary and radiogenic provenance studies indicate that
the detrital carbonate and crystalline rock fractions of IRD forming
H layers derive from the Hudson Strait region (Hemming, 2004).
Based on the age and geochemical composition of the IRD crystalline fraction, H layers show a clear link with the rocks forming
the Hudson Strait sill, but the carbonate clast content of H layers
implies a larger catchment area for which the extent is unconstrained. Ambient (non-Heinrich) IRD off Hudson Strait does not
include abundant carbonate clasts (e.g. Andrews and Tedesco,
1992), thereby indicating that the ice discharges related to H events
represent a different calving mechanism than those occurring
between H events. Taken together, these results support models
attributing the iceberg discharges of H events to an ice stream
draining the LIS interior and calving at the mouth of Hudson Strait.
How the LIS fed this ice stream is currently unknown, and an
objective of this study was to examine whether the Labrador–
Québec sector may have been a contributor. Large-scale patterns of
glacial lineations around Hudson Bay tend to agree with the
postulated disintegration models of the LIS, whereby they indicate
the convergence of massive ice flows from the three major icedispersal centers towards Hudson Bay/Strait (Dyke et al., 2002).
Problems in identifying which of these ice domes fed the ice flow
along the axis of Hudson Strait come from the fact that Churchillage rocks are found in the western and northern Hudson Bay
regions, directly under or adjacent to the former position of these
three major centers of ice outflow.
Another difficulty comes from uncertainty in interpreting the
significance of these major ice-flow patterns converging towards
Hudson Bay and Hudson Strait. This complexity is well exemplified
in the NQL region where multiple sets of glacial lineations indicating convergent ice flows towards Ungava Bay were interpreted
to reflect a drainage-event(s) of the Labrador-Quebec ice dome
interior during H events (Jansson et al., 2003). The data presented
in this paper suggest that it may be possible to evaluate this
hypothesis by constraining the rock-source of the glacial sediments
from three sectors located west, southwest, and east from Ungava
Bay, which should provide an estimate of the provenance characteristics of IRD deriving from ice exiting these sectors. Specifically,40Ar/39Ar ages of detrital hornblende grains from NQL glacial
sediments indicate that ice flow from the northeastern sector of the
LIS through northeastern Hudson Bay and Ungava Bay should have
incorporated debris characterized by a significant portion of
Archean-age detrital grains.
Insights on the occurrence of ice-draining event(s) from the
Labrador–Québec sector can be obtained from the provenance of
major (Heinrich) and minor (non-Heinrich) IRD deposits in North
Atlantic sediment cores located downstream from Hudson Strait.
The chronology of deep-sea record could further provide timing for
this event(s). Hemming and Hajdas (2003) report an extensive
record of near-continuous IRD sources in a core that spans the last
43 ka, an interval encompassing Heinrich event 0 (H0) through H4
(dated at w11, 14.5, 20.5, 27, 34 14C ka, respectively). Because of its
location off Newfoundland, in the western part of the Ruddiman
IRD belt, this core (V23–14) provides a record of LIS iceberg
contributions coming from the Gulf of St-Lawrence as well as the
Labrador Sea, thus including the Hudson Strait region (Fig. 1). The
40
Ar/39Ar ages of hornblende grains reported show a predominance
of Paleoproterozoic-age IRD during and between H events from 43
3243
to 30 ka, and after 17 ka. The data also document the a real evolution of the LIS through time, with IRD contribution from the
Grenville and Appalachians becoming more common from H3
(w30 ka) to H1 (w17 ka), as the ice sheet gained in volume and
expanded over southern latitudes (Fig. 7). Does this record contain
IRD with a strong Archean-age signature such as the one that would
be expected for ice draining the NQL region?
IRD layers with grains corresponding to source-rocks typical of
the Superior Province are present throughout this interval, but they
form at most 5–10% of the population of grains dated. No significant
ice-rafting event with a predominant Archean source-rock has
been identified throughout this interval. However, the overall
predominance of IRD derived from a Paleoproterozoic rock-source
may also be related to the bedrock sill of this age at the mouth of
Hudson Strait, as discussed earlier. If this site was subject to
important glacial erosion, this may have resulted in the production
of a large volume of sediments, which may have diluted the signal
of other rock-sources associated with ice flows coming from the ice
sheet interior. On the other hand, the occurrence of a prominent
dispersal train of distinctive glacial erratics (reddish rocks of the
Dubawnt Group) coming from Keewatin and going across northern
Hudson Bay (Shilts, 1980) suggests that important amount of
detrital material can be transported into the Strait, and possibly
into Labrador Sea. The presence of significant amount of carbonate
grains in IRD also suggests that detrital material up stream from the
sill can reach the ocean. The greater proximity of Ungava Bay to the
Fig. 7. Scatter plot of 40Ar/39Ar ages of detrital hornblende grains sampled from major
and minor IRD in core V23–14 in the northwest Atlantic ocean (43.4! N, 45.25! W,
3177 m; data from Hemming and Hajdas, 2003). The horizontal zones labeled H1–H4
indicate the position of the Heinrich layers; the vertical lines on each side of the C and
S letters delineate the age-range associated with the Churchill and Superior provinces,
respectively.
3244
M. Roy et al. / Quaternary Science Reviews 28 (2009) 3236–3245
Hudson Strait outlet thus tends to suggest that a drawdown from
this sector can potentially be reflected in the marine IRD record.
These issues clearly indicate that quantifying the amount of detrital
sediments produced at the mouth of Hudson Strait sill and differentiating these sediments from other rock-sources in North Atlantic
IRD represents an important challenge that should be the focus of
future studies. Although our results tend to preclude the occurrence of a major draining-event from the Labrador-Québec sector,
we believe that a detailed study involving large volume of IRD in
the Labrador Sea near the Hudson Strait is needed to provide
a more rigorous test of the contribution of NQL in H events, even
though the Archean-age fraction may end up to be diluted by
erosional processes at the mouth of Hudson Strait.
These results also leave open the question of when the Ungava
Bay glacial lineations were formed, and where was the outlet of the
ice involved? Direct interaction between the Labrador–Quebec
sector and the Labrador Sea through Ungava Bay is intimately
linked with the dynamics and position of the Hudson Strait ice
stream. Although interpretation of airphotos and satellite images
have led to the suggestion that parts of the Ungava Bay landform
swarm may have been formed over different time-intervals spanning part of the last glaciation (Clark et al., 2000; Jansson et al.,
2003), extensive field measurements of ice-flow erosional features
in NQL suggest that a large fraction of the Ungava Bay glacial
lineations were formed in Late-Glacial time, through the inland
propagation of a northward ice flow that was initially confined into
the Ungava Bay basin, and that eventually altered the radial outflow
of the Labrador–Québec ice dome (Veillette et al., 1999). This
capture event was attributed to the Gold Cove ice advance, which
crossed Hudson Strait from Ungava Bay and reached the southern
tip of Baffin Island at w10 14C ka BP (Miller and Kaufman, 1990;
Kaufman et al., 1993). If this scenario is correct, the northward
trending Ungava Bay glacial lineations could be associated with an
ice advance that took place either before or after H0 (wYounger
Dryas), during a period when the Hudson Strait ice stream had
retreated westward, away from the mouth of the Strait (Andrews,
1998; Kirby and Andrews, 1999; Clark et al., 2000; Dyke et al., 2002;
Andrews and MacLean, 2003). Another alternative would be that
the Ungava Bay glacial lineations were formed during H3, late in
Middle Wisconsinan time, because the provenance characteristics
of H3 layers differ significantly from the other H layers. This
suggests that through flow of ice along the eastern axis of Hudson
Strait was probably absent at that time, and this setting would then
allow across-the-strait flow from Ungava Bay. However, the relative
chronology deduced from the striation record does not appear to
support this hypothesis (e.g. Veillette et al., 1999).
In summary, the comparison of provenance characteristics of
glacial deposits underlying distinct flow patterns of the NQL region
and IRD of the North Atlantic marine record suggests that the
Labrador–Québec sector contribution to calving event(s) into the
North Atlantic appears to have been limited during the last glacial
cycle. Although these results suggest a relative stability of the
Labrador–Québec ice dome during H events, this interpretation
needs to be evaluated within the geological and geomorphological
context of the Hudson Strait region.This study indicates indeed that
further constraining the role of the Hudson Strait sill in the
production of detrital sediments is critical to our understanding of
the exact sector of the LIS (and other North Atlantic ice sheets) that
contributed to the marine IRD record throughout the last glacial
cycle. Similarly, the composition (radiogenic fingerprinting) of
glacial deposits associated with distinct flow patterns on land
should be further investigated in order to better define ice
sheet$ocean interactions and the role of ice sheets in climate
changes, as well as to bring additional information on the disintegration patterns of the LIS.
Acknowledgements
M. Charette contributed to sedimentological treatments of
samples and M. Laithier drafted some of the figures. An anonymous
reviewer provided helpful and constructive comments on the
manuscript. Funding for this project came from the LDEO Climate
Center (MR and SRH), NSERC (MR), NOAA (SRH) and the GSC (MP).
This is GEOTOP publication no. 2009-0019.
Appendix. Supplementary information
Supplementary data associated with this article can be found in
the version at doi:10.1016/j.quascirev.2009.08.008
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