Quaternary International 95–96 (2002) 75–85
Provinciality of ice rafting in the North Atlantic: application of
40
Ar/39Ar dating of individual ice rafted hornblende grains
Sidney R. Hemminga,*, Tore O. Vorrenb, Johan Klemanc
a
Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964, USA
b
Department of Geology, University of Tromsø, N-9037 Tromsø, Norway
c
Department of Physical Geography and Quaternary Geology, Stockholm University, S-106 91 Stockholm, Sweden
Abstract
The history of the Northern Hemisphere ice sheets is an important aspect of the paleoclimate system. The layered record of ice
rafted detritus (IRD) and other climate indicators preserved in deep-sea sediment cores provides the potential to unravel the
sequences of events surrounding important intervals during the last glacial cycle. The work presented here is a first step in the goal to
characterize the composition of glacially derived detritus in proximal locations near the Laurentide, Greenland and Scandinavian ice
sheets. The intent of this paper is to highlight the potential for using provenance studies of IRD in marine sediments, in particular
40
Ar/39Ar ages of hornblende grains in this case, to trace the history of ice sheets in the past.
Published work on land and at the glacial marine margins has mapped the margins of important ice sheets, including the locations
where glacial ice extended to the shelf-slope break. At these shelf-slope break locations, large glacial marine fans, termed trough
mouth fans (TMF), are known to occur in about 25 locations around the North Atlantic/Nordic/Arctic Oceans. By characterizing
the sediment sources in these TMFs, the major IRD components for deposition in deep-sea sediment cores are documented.
Although there is substantial overlap in the geological histories of the continental sources around the North Atlantic, there are
also some systematic variations that will allow distinction of different sources. Realization of the full potential awaits characterizing
the sources with multiple tracers as well as sedimentological studies, and documenting the geographic pattern of dispersal in marine
sediments within small time windows. r 2002 Published by Elsevier Science Ltd.
1. Introduction
Ice rafted detritus (IRD) is terrigenous clastic material
that is transported by drifting ice in the form of icebergs
and sea/lake ice. The occurrence of IRD indicates the
geographic extent of drifting ice in the past (Ruddiman,
1977; Smythe et al., 1985). In addition to the depositional pattern, the composition of IRD constrains the
origin of the ice and by extension can be used to
understand the past surface currents that controlled the
drifting trajectories. Accordingly IRD studies lend
themselves to constraining the inceptions of glaciers,
but only as the glaciers extended into the marine realm
and began to release icebergs. Ruddiman (1977) showed
that the flux of IRD during the last glacial cycle is
correlated to the extent of ice sheets in the northern
hemisphere, and his map of last glacial maximum flux is
shown in Fig. 1.
*Corresponding author. Tel.: +1-845-365-8417; fax: +1-845-3658155.
E-mail address: [email protected] (S.R. Hemming).
1040-6182/02/$ - see front matter r 2002 Published by Elsevier Science Ltd.
PII: S 1 0 4 0 - 6 1 8 2 ( 0 2 ) 0 0 0 2 9 - 0
The intent of this paper is to highlight the potential
for using provenance studies, in particular 40Ar/39Ar in
this case, to trace the history of ice sheets in the past.
Although data from time intervals that deal with
initiation, or extension beyond the coastline, of the ice
sheets are not currently available, the methods of
studying the last glacial would also apply to times of
ice sheet inception. Additionally, the characterization
of point sources applies in any time window.
1.1. Geochemical approaches to constraining provenance:
examples from the Heinrich layers
A particularly good set of examples of the application
of provenance approaches to IRD is the extensive
efforts to understand the origin of the so-called
‘‘Heinrich layers’’ due to excitement about their
glaciological significance and their apparent impact on
climate and ocean circulation. Heinrich layers are
associated with drastic changes in sea surface hydrography (Bard et al., 1987; Bond et al., 1992, 1993;
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
60
50
40
30
10
0
10
0
20 0
15 0
10
50
00
12
20
0
60
70
25
76
50
50
50
150
250
400
600
700
600
500
4000
30
2500
20
150
100
400
500
40
600
40
50
0
60 00
5 00 0
4 30 00 0
2 10
60
50
40
30
20
10
0
Fig. 1. Map of the North Atlantic from Ruddiman (1977) showing the pattern of flux variation during the last glacial maximum (25–14 ka). Flux
units are mg/cm2/kyr.
%IRD
H1 H2 H3
100
80
60
40
20
0
0
40Ar*
10-9 moles/g
100
H1 H2
H5
H6
H11
200
H4 H5
300
400
600
500
6
VM28-82
<63µm
4
2
0
H1
8
40Ar*
10-9 moles/g
H4
VM28-82
8
100
H2
200
H4
300
H5
ME68-89
2-16 µm
<2 µm
6
4
2
%IRD
0
K/Ar-age (Ga)
<2 µm 2-16 µm
Cortijo et al., 1997) and deep-water circulation (Oppo
and Lehman, 1995, Vidal et al., 1997) and have been
related to extreme and worldwide climate events
(Broecker, 1994). As first documented by Heinrich
(1988), they are recognized as prominent and repetitive
layers, exceptionally rich in IRD, that were deposited
during the last glacial period (Fig. 2). Heinrich layers
appear to have formed from massive discharges of
icebergs from the Hudson Strait, leaving high concentrations of lithic grains including abundant detrital
carbonate (Andrews and Tedesco, 1992; Bond et al.,
1992; Broecker et al., 1992). In addition to the
anomalous lithic contents, Heinrich layers have distinct
magnetic properties (e.g., Grousset et al., 1993;
Dowdeswell et al., 1995; Robinson et al., 1995; Stoner
et al., 1996). They are also outstanding by many different
measures of provenance based on inorganic and organic
geochemical and isotopic compositions. Huon and Ruch
(1992) and Jantschik and Huon (1992) showed that the
K/Ar age of the fine terrigenous detritus of H1, H2, H4
and H5 is approximately 1 Ga, compared to background
values of approximately 0.4 Ga. This 1 Ga age appears
to be the end member from the Labrador Sea (Hemming
et al., 2002). Grousset et al. (1993, 2000, 2001), Revel
et al. (1996) and Snoeckx et al. (1999) used Sr and Nd
isotopes to show that these Heinrich layers have
Archean heritage, and Grousset et al. (2000) showed
that they can be traced to at least the British part of the
European margin. Francois and Bacon (1994), Thomson et al. (1995), McManus et al. (1998), and Veiga-Pires
and Hillaire-Marcel (1999) showed that the flux of
detritus to these Heinrich layers was dramatically higher
than the ambient sediments. Gwiazda et al. (1996a–c)
and Hemming et al. (1998, 2000b) showed that ice rafted
100
80
60
40
20
0
H1
H2
100
H3
H4
200
H5 H6
300
400
H11
ME69-17
50
0
H1
1.2
1.0
0.8
0.6
0.4
0.8
0.6
0.4
150
100
H2
H4
200
250
H5
ME68-89
0
100
200
300
400
depth (cm)
Fig. 2. Records of ice rafting and K/Ar measurements from Dreizack
Sea Mount cores (Heinrich, 1988; Jantschik and Huon, 1992) and from
core V28-82 (Hemming et al., 2002). Percentages of IRD refer to the
>150 mm fraction, which is composed of IRD and foraminifera.
Locations of Dreizack Sea Mounts and V28-82 are shown in Fig. 3.
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
hornblende and feldspar grains have distinctive age and
isotopic characteristics in these Heinrich layers, that
indicate a strong Paleoproterozoic overprinting on their
Archean heritage. Rosell-Mele! et al. (1997) found that
Heinrich layers contain very distinctive biomarker
geopigments that derived from ancient sedimentary
rocks.
In summary, several approaches have been taken to
constrain the geochemical characteristics of Heinrich
Layer (and other) ice rafting sources. A primary
difference among the measurements concerns the size
fractions that are analyzed, and whether bulk analyses
or individual grain approaches are used. The o63 mm
fraction is considered to represent the integrated
erosional signature of ice-proximal sediments in Hudson
Strait and Cumberland Sound, important ice streams of
the former Northern Hemisphere ice sheets (Hughes,
1992; Clarke et al., 1999). However, there is concern that
in the deep ocean, sediments in o63 mm fraction can be
transported by a variety of processes. Accordingly,
several studies have selected the coarse fraction for
provenance studies (e.g., Gwiazda et al., 1996a–c;
Hemming et al., 1998, 2000b; Snoeckx et al., 1999;
Grousset et al., 2000, 2001; Hemming and Hajdas,
submitted). It appears in the case of Heinrich Layers
H1, H2, H4 and H5, within the IRD belt, that all the
size fractions are dominated by the same source
(Hemming et al., 1998, 2002).
1.2. Down core records
Testing the relative IRD contribution of the different
Northern Hemisphere ice sheets is required to understand better the relative timing of inceptions as well as
the mechanism(s) that led to an ice sheet collapse.
However, it is important to state up front that an
increase of IRD does not require or even suggest a
collapse of a particular ice sheet. Incorporation of
abundant basal debris in glacial ice may require a twostage process (e.g., Alley and MacAyeal, 1994). Two
important things are required to send icebergs across the
deep North Atlantic: (1) ice sheets must extend to the
margins so that calving ice can produce icebergs, and (2)
the surface water must be cold enough that the iceberg
can be transported far enough to drop its detritus in the
deep ocean. The only IRD events where evidence is
strong for catastrophic collapse of an ice sheet are the
Heinrich layers, particularly H1, H2, H4 and H5. The
many other documented ice rafting events are important
to understand ocean–ice–atmosphere interactions, but
care should be taken to not over-interpret their
significance.
The study of the composition of the IRD within high
IRD flux intervals has given evidence for the existence of
multiple sources of icebergs contributing to an increased
flux of IRD (e.g., Bond and Lotti, 1995; Revel et al.,
77
1996; Elliot et al., 1998; Grousset et al., 2001). More
interestingly there appears to be a succession in the
deposit of IRD originating from different source areas.
Bond and Lotti (1995) and Bond et al. (1999) found that
there was an increase of hematite stained grains, which
they attributed to a Gulf of St. Lawrence origin, and in
Icelandic glass that preceded the increase in detrital
carbonate. Finally, they suggested that the other sources
continued to contribute to the increase in IRD
throughout the events, but were strongly diluted by
the abundant detrital carbonate and related Heinrich
detritus. This is consistent with radiochemical evidence
for an immense increase in flux during the Heinrich
layers (Francois and Bacon, 1994; Thomson et al., 1995;
McManus et al., 1998; Veiga-Pires and Hillaire-Marcel,
1999), and by the near end-member radiogenic isotope
compositions (Grousset et al., 1993, 2001; Revel et al.,
1996; Gwiazda et al, 1996a; Hemming et al., 1998,
2000b, 2002).
1.3. Characterizing the dominant IRD sources
In this contribution we present new 40Ar/39Ar
hornblende data from marine cores in positions
proximal to potential iceberg sources (Fig. 3), including
core V17-203 (48.6831N, 60.01W, 335 m) in the Gulf of
St. Lawrence, core V17-193 (60.7331N, 45.751W, 413 m)
near the southern tip of Greenland, cores V29-207
(69.2671N, 19.5171W, 1454 m) and V28-23 (74.5171N,
13.1171W, 2325 m) from the eastern side of Greenland,
core JM98 624 (661430 N, 71420 E, 487 m) from the midNorwegian margin, and cores JM96 68/1 (731250 N,
141290 E, 914 m) and JM96-70/1 (741100 N 131000 E, 2042)
from the Bear Island trough mouth fan off the northern
Norwegian margin. Previously published results are
available from Baffin Island (Hemming et al., 2000a)
and off the Hudson Strait (Gwiazda et al., 1996b)
provide a sample of Hudson Strait compositions. This is
not a comprehensive sampling of potential sources, but
it provides an indication of the compositional variability
that may be expected.
2. Methods
Hornblende, biotite and muscovite grains were picked
from the >150 mm fraction of marine sediment core
samples, and co-irradiated with hornblende monitor
standard Mmhb (age=525 Ma, Samson and Alexander,
1987) in the Cd-lined, in core facility (CLICIT) at the
Oregon State reactor. 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
78
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
Fig. 3. Polar projection from 401 to 901 showing the extents of continental and sea ice in the Northern hemisphere, and cores presented and/or
discussed here. The locations of known trough mouth fans are shown as well as inferred ice streams of the Laurentide ice sheet. Trough mouth fans of
the Nordic Seas are from Vorren and Laberg (1997), those around Greenland are from Funder and Larsen (1989), and along the western Labrador
Sea are from Hesse et al. (1997). Arrows marking troughs across the Laurentide margin are based on mapping of J. Kleman. Arrow marking large
tough feeding the North Sea TMF is shown in Fig. 6, and in Stokes and Clark (2001), and the Barents Sea arrow indicating a large trough feeding the
Bear Island TMF is from Stokes and Clark (2001).
nuclear reactions, procedural blanks and atmospheric
Ar contamination.
3. Results and discussion
Results are reported in Table 1 and shown in Fig. 4.
In this contribution we explore the application of
40
Ar/39Ar dating of individual hornblende grains as a
means of characterizing the ages of basement source
contributions, and we also include limited biotite and
muscovite analyses in Table 1. Continental source
regions around the North Atlantic represent a variety
of geologic ages (Fig. 5). Although there is much overlap
of geologic ages on each side of the ocean due to their
pre-Atlantic juxtaposition, the details of the ice
dynamics allow us to expect some separation in the
predicted ages from the two sides.
3.1. 40Ar/39Ar ages of individual hornblende grains:
Laurentide margin
The east coast of North America is characterized by
generally increasing age terranes from south to north
(Fig. 5). Accordingly it is expected that IRD derived
from the northeastern portion of the Laurentide ice
sheet should be dominated by Paleoproterozoic and
Archean grains, while that derived from the southeastern portion should be dominated by Mesoproterozoic (Grenville) and Paleozoic grains. This expectation
is borne out by samples taken near the Hudson Strait
(Gwiazda et al., 1996b; Hemming et al., 2000a), which
have a dominant Paleoproterozoic hornblende age
population with a subordinate Late Archean population. In contrast samples from core V17-203 from the
Gulf of St. Lawrence have a dominant age population
between 0.9 and 1 Ga, thus dominated by Grenvillian
sources (Fig. 4).
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
79
Table 1
40
Ar/39Ar data for this study
Lab number
Sample
Ca/K
10553-09
10553-07
10553-01
10553-06
10553-05
10553-08
10553-02
10553-10
10553-03
10675-10
10675-03
10675-02
10675-09
10675-05
10675-07
10675-06
10675-04
10675-08
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
JM96
681/ 135–137
681/ 135–137
681/ 135–137
681/ 135–137
681/ 135–137
681/ 135–137
681/ 135–137
681/ 135–137
681/ 135–137
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
681/1 135–137 hb
10554-09
10554-12
10554-08
10554-07
10554-06
JM96
JM96
JM96
JM96
JM96
70/1
70/1
70/1
70/1
70/1
10555-01
10555-04
10555-05
10555-03
10555-02
10551-03
10671-08
10671-07
10671-03
10671-05
10671-04
10671-11
10671-02
10671-10
10671-01
10671-06
10671-21
10671-09
10671-17
10671-13
10671-20
10671-15
10671-14
10671-12
10671-19
10671-16
10671-18
10551-07
10551-08
10551-09
10551-05
10551-01
10551-04
10551-02
10670-01
10670-04
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
124–126
124–126
124–126
124–126
124–126
110–115
110–115
110–115
110–115
110–115
110–115
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115 bio
110–115
110–115
110–115
110–115
110–115
110–115
110–115
110–115 hb
110–115 hb
0.2
8.2
10.1
12.3
13.0
16.1
21.3
29.4
46.3
8.4675
8.9663
9.628
12.3828
14.5703
18.8834
28.7329
30.563
43.5991
0.0
0.0
0.0
0.1
7.9
0.0
0.0
0.0
0.1
0.3
0.6
0
0.0048
0.0106
0.0146
0.0156
0.0213
0.0345
0.0362
0.0464
0.0512
0.0532
0.1211
0.1222
0.1502
0.1524
0.1549
0.1804
0.1882
0.1941
0.2037
1.3857
5.0
14.2
16.1
18.1
18.5
24.8
26.3
0.018
0.1668
Age
7
Mineral
99.5
99.8
99.6
98.8
99.8
98.8
99.5
99.2
97.2
99.4
99.8
98.5
98.6
98.7
99.1
99.8
99.3
99.5
1249
1843
2053
1799
1767
1711
1796
1796
1914
1976
1957
1937
2241
1874
2050
1897
1993
2804
3
4
5
5
6
6
4
11
6
4.71
4.61
5.63
6.23
3.92
6.45
5.54
8
11.18
Biotite
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Pyroxene
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
102.4
99.8
73.3
98.2
99.6
560
382
225
317
1656
24
1
21
1
4
Biotite
Biotite
Biotite
Biotite
Hornblende
99.6
99.7
99.3
99.7
99.7
99.8
99.0
99.2
99.8
99.6
99.5
99.6
99.9
98.9
99.2
99.7
99.6
99.1
99.6
99.5
99.3
99.4
99.7
98.5
99.6
99.0
97.3
99.4
99.0
97.6
98.7
88.7
98.8
98.1
99.9
99.4
406
392
369
460
437
1553
374
379
375
382
377
455
516
370
361
381
373
396
372
367
401
550
383
382
411
408
503
515
402
386
426
423
501
504
487
553
%
40
Ar*
1
1
1
1
1
3
0.83
0.6
0.63
0.54
0.61
0.81
0.87
0.92
0.46
0.63
0.52
1
0.59
0.64
0.72
0.99
0.59
1.03
0.78
0.7
1.89
2
2
2
2
1
2
3
0.85
0.96
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
(continued on next page)
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
80
Table 1 (continued)
Lab number
Sample
10670-07
10670-11
10670-05
10670-06
10670-08
10670-03
10670-10
10672-02
10672-03
10672-04
10672-01
10551-06
10551-10
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
10552-05
10552-10
10674-16
10674-19
10674-12
10674-17
10674-20
10674-06
10674-18
10674-04
10674-07
10674-01
10674-05
10674-08
10674-10
10674-09
10674-11
10674-15
10674-02
10674-13
10674-14
10674-03
10552-07
10552-02
10552-08
10552-01
10552-04
10552-09
10673-02
10673-03
10673-04
10673-05
10673-07
10673-08
10673-09
10673-10
10673-11
10673-12
10673-13
10673-14
10552-06
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
JM98
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
624/1
10439-07
10439-15
10439-03
10439-11
10439-01
40508-04
10439-20
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193 (0 cm)
V17-193
7
Mineral
366
410
485
847
409
435
472
389
392
394
385
407
365
3.54
1.17
1.73
1.71
1.99
3.73
3.23
0.54
0.53
0.58
0.66
2
42
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Muscovite
Muscovite
Muscovite
Muscovite
Pyroxene
Pyroxene
99.6
98.4
99.6
99.7
99.7
94.3
99.2
99.6
99.8
99.8
99.8
98.8
99.8
75.3
99.7
99.6
99.6
99.8
95.4
97.2
99.3
99.7
99.3
99.2
99.9
93.5
95.3
74.5
86.9
98.0
99.7
94.5
98.1
97.8
98.6
98.0
98.0
95.1
99.1
97.9
98.2
365
449
375
449
603
361
360
401
402
426
409
419
389
367
380
396
435
750
453
404
353
389
499
639
393
429
421
568
422
429
474
366
915
422
478
411
440
627
421
428
3489
1
1
0.71
0.71
0.9
0.58
1.16
0.62
0.57
0.81
0.64
0.61
0.65
0.97
0.62
0.59
0.72
1.15
0.91
0.85
0.7
0.55
1
1
2
1
2
10
0.85
1.19
0.9
3.65
2.74
1.44
3.23
1.28
1.13
1.72
0.94
3.94
13
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Pyroxene
94.4
98.3
96.3
98.3
97.7
99.5
100.4
968
1637
1292
1306
1182
1253
1260
9
5
9
10
5
3
14
Biotite
Biotite
Biotite
Hornblende
Hornblende
Hornblende
Hornblende
40
Ca/K
%
110–115 hb
110–115 hb
110–115 hb
110–115 hb
110–115 hb
110–115 hb
110–115 hb
110–115 musc
110–115 musc
110–115 musc
110–115 musc
110–115
110–115
1.0676
10.0185
13.2372
17.6182
23.3388
30.2699
39.7398
0.0462
0.0595
0.1338
0.2121
40.1
147.5
85.2
100.2
98.0
98.7
94.1
91.2
99.0
99.8
99.8
99.8
99.4
97.7
61.3
45–50
45–50
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50 bio
45–50
45–50
45–50
45–50
45–50
45–50
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50 hb
45–50
0.0
0.0
0
0.0165
0.0269
0.0293
0.0329
0.0358
0.0407
0.0644
0.0709
0.0859
0.0982
0.119
0.1439
0.1589
0.1592
0.1994
0.234
0.2407
0.3051
0.6422
4.7
5.0
6.4
7.6
9.5
27.6
6.9053
12.1137
4.5805
29.5194
14.1629
9.118
20.5216
6.755
7.1998
11.0457
8.4522
20.318
128.5
0.1
0.2
0.5
1.4
2.7
3.0
4.5
Ar*
Age
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
81
Table 1 (continued)
Lab number
Sample
10439-05
40508-02
10439-04
40508-01
10439-19
40508-03
10439-08
10439-12
10439-16
10439-10
40508-05
10439-18
10439-06
10439-14
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
V17-193
10444-10
10444-03
10444-08
10444-02
10444-01
10444-07
10444-06
10444-09
10444-04
Ca/K
%
40
Ar*
Age
7
Mineral
4.9
5.1
8.0
8.6
11.6
11.7
12.3
13.7
14.2
15.6
16.9
26.4
26.6
50.7
96.4
98.9
98.2
99.1
100.1
99.2
99.1
96.5
94.9
100.1
93.1
78.2
92.9
92.5
1246
1248
1734
1818
1652
1767
1801
1731
1739
1843
1737
1746
1784
1421
9
4
9
5
41
5
15
45
15
27
8
75
50
49
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Pyroxene
V17-203
V17-203
V17-203
V17-203
V17-203
V17-203
V17-203
V17-203
V17-203
2.2
5.1
5.2
5.7
6.4
7.0
7.7
14.9
20.6
45.8
97.9
97.2
86.5
99.6
98.9
100.0
96.1
99.4
851
964
961
1458
969
1047
990
568
1018
72
3
3
38
2
3
7
4
7
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
10442-03
10442-06
10442-11
10442-10
10442-09
10442-04
10442-05
10442-08
10442-02
10442-07
10442-01
V28-23
V28-23
V28-23
V28-23
V28-23
V28-23
V28-23
V28-23
V28-23
V28-23
V28-23
0.0
2.0
6.9
9.0
10.0
10.2
12.7
13.0
15.4
20.5
140.6
99.4
94.1
96.3
86.3
94.5
98.6
98.1
96.9
65.2
93.7
97.3
389
553
425
488
476
1847
396
718
565
422
5038
1
2
1
2
1
5
1
4
4
1
43
Biotite
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Pyroxene
10445-03
10445-06
10445-04
10445-05
10445-01
10445-07
10445-02
V29-207
V29-207
V29-207
V29-207
V29-207
V29-207
V29-207
5.3
9.3
9.5
10.0
15.7
18.7
115.9
99.0
87.7
92.4
98.4
99.8
97.9
99.2
441
473
478
390
521
740
3198
2
4
6
4
4
14
47
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Hornblende
Pyroxene
(0 cm)
(0 cm)
(0 cm)
(0 cm)
3.2. 40Ar/39Ar ages of individual hornblende, biotite and
muscovite grains: Norwegian margin
The southern tip of Norway is underlain by Mesoproterozoic crust, and the west coast of Norway was
overprinted by Caledonian mountain building (Figs. 5
and 6). To the east of the Caledonides lie Paleoproterozoic and Archean crust. Hornblende grains from core
JM98-624, along the mid-Norwegian margin are
dominantly Paleozoic, ranging from 385 to 515 Ma
(n ¼ 33=40), and the rest are Neoproterozoic apparent
ages between 553 and 847 Ma. This is consistent with the
observations of the upstream geology, which consist of
Cambrian–Silurian metasediments intruded by Caledo-
nian igneous rocks. Hornblende grains are plentiful in
these samples. Biotite grains are also plentiful in these
samples, and a histogram of 49 grains is shown in Fig. 4.
Four muscovite grains yielded ages of 385–394 Ma
(Table 1).
The samples from the Bear Island trough mouth fan
are less plentiful in hornblende. The sample from core
JM96 681 contains a few, and they are virtually absent
in the sample from JM96-70/1, although one grain of
hornblende yielded a 1656 Ma age. The age distribution
from the Bear Island TMF samples is 1656–2053 Ma, i.e.
Paleoproterozoic, with an Archean grain (2804 Ma).
In addition to the scarcity of hornblende grains, the
Bear Island fan samples have abundant black shale
82
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
Fig. 4. Histograms of individual hornblende ages from proximal locations discussed in this paper: (A) Hornblende grains from Baffin Island tills
(Hemming et al., 2000a); (B) Hornblende grains from East Greenland cores; (C) Hornblende grains from the Gulf of St. Lawrence; (D) Hornblende
(black) and biotite (white) grains from the mid-Norwegian margin; (E) Hornblende grains from the South Greenland margin; and (F) Hornblende
grains from the Bear Island TMF.
fragments, and some black, disk-shaped carbonaceous
grains that are very distinctive. These fragments
and grains must be derived from the Cenozoic and
Mesozoic sedimentary rocks in the Barents Sea. The
samples are taken from debris flows. The lithology
of the debris flows is quite similar to the tills found
on the adjoining shelf (Laberg and Vorren, 1995).
The shelf tills as well as the debris flows contain
mostly sedimentary rocks but also some crystalline
rocks (e.g. Vassmyr and Vorren, 1990; Vorren et al.,
1998). We believe that the hornblende grains are
derived from the Paleoproterozoic rocks in northern
Fennoscandia and NW Russia. These areas partly fed
the Bear Island trough ice stream during LGM. Four
biotite grains from JM96-70/1 yielded Paleozoic
ages, consistent with a partial Caledonian overprint on
these ancient terranes.
3.3. 40Ar/39Ar ages of individual hornblende grains:
Greenland margin
Fig. 5. Generalized geologic map of the regions surrounding the
North Atlantic Ocean and Norwegian Sea, simplified from Hemming
et al. (1998).
There is also much overlap in geological ages with
Greenland, and it is expected that 40Ar/39Ar of
hornblende alone may not be sufficient to discriminate
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
83
3.4. Trough mouth fans as IRD point source archives
Fig. 6. Geologic basement and Fennoscandian ice sheet map to show
the probable locations of major sediment contributors from northern
Europe. The thin arrows represent the general flow directions on the
ice sheet, and the ice divide is marked by the heavy line. The heavy
arrows mark the locations of major troughs that appear to have
funneled ice streams from the Fennoscandian ice sheet.
Trough mouth fans are major glacial-marine sediment
fans that form when ice sheets extend to the shelf-slope
boundary (e.g., Vorren and Laberg, 1997). They are a
significant resource and archive for understanding the
potential compositional characteristics of sedimentladen icebergs, because they contain source-specific
detrital material in highly concentrated accumulations.
An important consideration that emphasizes the value
of this archive is the recognition that the compositions
of the debris flows mimic the tills in the source area and
therefore also the composition of the IRD in the calving
icebergs. Furthermore, most icebergs experience substantial melting close to the ice sheet margin, and the
sediments they carry tend to melt out before the iceberg
has reached the open ocean (Syvitski et al., 1996;
Andrews, 2000). Sediments deposited on the trough
mouth fans are not only good representative point
sources of the glacial drainage area, but they are also
located in positions of high iceberg production, that
increase the likelihood of an iceberg’s being carried to
deep marine environments in the surface ocean currents
before loosing its sediment load. Locations of documented trough mouth fans in the northern hemisphere
are shown in Fig. 3.
Many of the trough mouth fans have been sampled
during past coring cruises, and characterizing their
source compositions is a matter of organizing a group of
interested scientists.
4. Conclusions
sources. However, in combination with other methods,
this approach can provide important constraints. Three
cores near the Greenland margin were sampled for this
study. Core V17-193 from near the south tip of
Greenland (Fig. 3) contains a mixture of Paleoproterozoic ages that are indistinguishable from Churchill
sources and a Mesoproterozoic population (Fig. 4)
likely derived from the Gardar alkaline province. Cores
V29-207 and V28-23 are located along the northeastern
coast of Greenland (Fig. 3) are characterized by a
dominant Paleozoic hornblende population that cannot
be distinguished from the Norwegian Caledonian
(discussed above) or the Laurentide Appalachian
(Hemming et al., 2000b) age spectra. It is further
expected that some or all the ice stream sources that
drain into the Arctic Ocean from the Canadian Arctic
Islands (arrows in Fig. 3) will have a dominant
Paleozoic age spectrum. Application of more proxies
may allow distinguishing among these potential sources.
For a good example, see the work of Darby and Bischof
(1996) and Bischof and Darby (1999) on petrological
characterization of different Canadian Arctic Island
contributions to Arctic IRD.
The ice rafting history in the North Atlantic including
the Nordic Seas is a key aspect of its paleoclimate/
paleoceanography story and an important proxy for
understanding of the adjoining continental ice sheets. It
is thus important to understand the sources and
processes responsible for delivering it to the ocean floor.
Also, it is equally important to establish a framework in
which to document the relative timing of major iceberg
contributions from different sources. The work presented here is a step towards characterizing the
40
Ar/39Ar hornblende ages of major iceberg contributors
during the last glacial maximum. Future work should
focus on characterizing the provenance of trough mouth
fan deposits. Included in this characterization should be
the application of multiple tools in an integrated way,
including but not limited to petrological examination,
radiogenic isotopes of Nd–Pb–Sr–Ar, organic and
inorganic chemical analyses. An assessment should be
made of the grain size controls on the measured
compositions of bulk samples and in addition to
analyses of bulk sediment samples, ages and isotope
compositions of individual grains, including 40Ar/39Ar
84
S.R. Hemming et al. / Quaternary International 95–96 (2002) 75–85
ages of individual hornblende grains, need to be made to
assess the geological variability of the drainage basin as
recorded in ice rafted detritus.
Acknowledgements
This research was supported by NSF grant OCE 9907290. Millie Mendelson picked the hornblende grains
from the Norwegian margin samples. Discussions with
John Andrews, Donny Barber, Mary Elliot, Julian
Sachs, Lang Farmer, Anne Jennings, Jerry McManus,
Dennis Darby and Jens Bischof, and our attempt to get
a multiple proxy collaborative proposal funded to study
trough mouth fans, have had a great influence on the
.
views of SRH. Discussions with Torbjorn
Dahlgren,
Ralph Stea and others at the INCEPTIONS meeting
were also influential. Thanks go to Trond Dokken and
an anonymous reviewer for their reviews of the paper,
and to Arjen Stroeven for the editorial handling.
Support for the curating facilities of the Lamont–
Doherty Earth Observatory Deep-Sea Sample Repository is provided by the National Science Foundation
through Grant OCE94-02150 and the Office of Naval
Research through Grant N00014-I-0186. This is Lamont-Doherty Earth Observatory contribution XXXX.
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