JOURNAL OF QUATERNARY SCIENCE (2012) 27(1) 81–88
ISSN 0267-8179. DOI: 10.1002/jqs.1516
Timing of the Younger Dryas glacial maximum
in Western Norway
ØYSTEIN S. LOHNE,1* JAN MANGERUD1,2 and JOHN INGE SVENDSEN1,2
1
Department of Earth Science, University of Bergen, Allégaten 41, N-5007 Bergen, Norway
2
The Bjerknes Centre for Climate Research, Bergen, Norway
Received 11 February 2011; Revised 12 April 2011; Accepted 20 April 2011
ABSTRACT: This study precisely constrains the timing of the Younger Dryas (YD) glacial maximum in south-western
Norway by utilizing sediment records from lake basins. Two of the basins, located on the distal side of the mapped
Herdla–Halsnøy Moraine, received meltwater directly from the ice sheet only when the ice margin reached its
maximum extent during the YD. In the cores, the ice maximum is represented by well-defined units with meltwater
deposits, dominantly laminated silt. Plant macrofossils in the sediment sequences are common and we obtained 18
radiocarbon ages from one of the cores. By applying Bayesian age–depth modelling we obtained a precise date for this
meltwater event and thereby also for the timing of the YD glacial maximum. We conclude that the ice-sheet advance
culminated at the Halsnøy Moraine at 11 760 120 cal a BP, and that the ice margin stayed in this position for
170 120 years. The subsequent retreat started at 11 590 100 cal a BP, i.e. close to the YD/Holocene boundary.
Withdrawal was probably triggered by abrupt climatic warming at this time. Copyright # 2011 John Wiley & Sons, Ltd.
KEYWORDS: age–depth modelling; deglaciation; Hardangerfjorden; lake basins.
Introduction
The main objective of this study was to obtain a precise and
accurate age of the Younger Dryas (YD) moraines in western
Norway. These moraines are known as the Herdla–Halsnøy
Moraines, and mark the maximum position of a major ice sheet
advance that reached the outer coast (Aarseth and Mangerud,
1974). The YD end moraines are prominent glacial features that
can be traced around the former Scandinavian Ice Sheet
(Andersen et al., 1995a). Although the moraines are mapped
more or less continuously, their depositional timing is not
coeval (Mangerud, 2004). Thus, dating of the moraine in
different areas along the margin is important for understanding
ice sheet dynamics and how the Scandinavian Ice Sheet
responded to climate forcing. Our strategy was to core lake
basins that received meltwater and sediments directly from the
ice sheet at the time when the ice front reached its outermost
position at the mouth of Hardangerfjorden, one of the major
fjords in western Norway (Fig. 1). By applying Bayesian
statistical analysis to a series of 14C dates below, within and
above the meltwater sediments, we obtained a precise and
accurate age of the ice sheet advance to, and retreat from, the
YD end moraine. The age of the maximum ice sheet position
that we obtained has been utilized in a companion study aimed
at determining the production rate of the cosmogenic nuclide
10
Be, used for surface exposure dating, by sampling boulders on
the Halsnøy Moraine (Goehring et al., 2011).
Deglaciation history of SW Norway
When the Norwegian Channel offshore western Norway was
deglaciated, at about 18k cal a BP (Sejrup et al., 1995), the ice
sheet margin halted at a position close to the outer coast of
south-western Norway for about 4000 years (Mangerud et al.,
2011). Following this stillstand the outermost islands became
ice free at about 14.5k cal a BP, and the ice front then retreated
into the major fjords (Mangerud, 1970, 1977). A subsequent readvance started during the Allerød (Lohne et al., 2007) and
reached the Herdla–Halsnøy Moraine during the late YD
*Correspondence: Ø. S. Lohne, as above.
E-mail: [email protected]
Copyright ß 2011 John Wiley & Sons, Ltd.
(Andersen et al., 1995b; Bondevik and Mangerud, 2002;
Lohne, 2006). The culmination of the ice sheet advance
coincided with a relative sea-level rise and a subsequent
highstand that formed the marine limit in the coastal areas
outside the YD ice front position (Lohne et al., 2007).
The broad outline and approximate age of the Herdla–
Halsnøy Moraine was established in the early 1970 s
(Mangerud, 1970; Aarseth and Mangerud, 1974). However,
during the last decade the inferred ice sheet advance has been
questioned in several papers, and as an alternative interpretation it has been suggested that Hardangerfjorden remained ice
free throughout the YD (Helle et al., 1997, 2007; Bakke, 2004;
Helle, 2004, 2008; Bakke et al., 2005). If correct the latter
interpretation would imply that the Halsnøy Moraine (Fig. 1)
must be an older feature that does not belong to the YD end
moraine system in western Norway. This alternative view has
been much disputed over the years and was strongly opposed
by Mangerud (2000) and Lohne (2006). The assumption that
Hardangerfjorden was ice free during the YD was first and
foremost based on an undated deltaic sequence near the head
of the fjord (Helle, 2004) and poorly/ambiguously dated lake(Bakke et al., 2005) and isolation-basin cores from the fjord area
(Helle et al., 1997, 2007). Another and more indirect argument
was the occurrence of some undated cirque moraines inside the
Halsnøy Moraine postulated to be of YD age (Bakke et al.,
2005). However, in our opinion this hypothesis is not
substantiated by credible geochronological evidence. On the
contrary, we suggest that numerous observations and radiocarbon dates that have been obtained from shell-bearing tills
(Fig. 1; Supporting Information Table S1), lake stratigraphies
with meltwater sediments from Halsnøy (Lohne, 2006) and
Tysnes (present paper), and postglacial sediment from
Tørrvikbygd (Fig 1; Romundset et al., 2010) demonstrate that
the Scandinavian Ice Sheet extended to the Halsnøy Moraine
during the YD. This is also consistent with the mapping of the
moraine (Undås, 1963; Aarseth and Mangerud, 1974).
However, given the disagreement above, we further test the
age of the Halsnøy moraine, and some of the major findings are
documented in the present paper – all supporting the
interpretation that there was a major YD re-advance to the
Herdla–Halsnøy Moraine.
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JOURNAL OF QUATERNARY SCIENCE
Figure 1. Map (elevation model, ßNorge
digitalt) showing the Herdla–Halsnøy Moraine
(white line) that represents the maximum
extent of the Scandinavian Ice Sheet across
the mouth of the Hardangerfjord during the
Younger Dryas (Undås, 1963; Follestad, 1972;
Aarseth and Mangerud, 1974; Holtedahl,
1975). Numbered sites on southern Tysnes
refer to the sedimentary logs in Fig. 2. The
listed 14C dates were from mollusc shells found
in till or sub-till sediments (supporting Table
S1). The dates are all corrected for a marine
reservoir age of 380 years (Mangerud et al.,
2006), but it should be noted that the marine
reservoir correction for the YD interval may be
slightly higher (Bondevik et al., 2006). Note
that the fjord Langenuen is open to the coast in
the north-west. The inset map shows the YD
end moraine in southern Norway. This figure is
available in colour online at wileyonlinelibrary.
com.
The Halsnøy Moraine and research strategy
The Halsnøy Moraine, which crosses the mouth of the
Hardangerfjord (Fig. 1), consists of moraine ridges forming
an arc shape feature that can also be traced on the seafloor
(Undås, 1963; Holtedahl, 1967, 1975; Follestad, 1972; Aarseth
et al., 1997). The island of Halsnøy is in part made up by a
prominent moraine ridge, whereas only fragmentary deposits
are found on the islands of Huglo, Stord and Tysnes further
north.
The research strategy for this study was to core and
investigate lake basins located on the distal side of the
maximum YD ice-sheet margin, but close enough that they
received meltwater and glacially derived sediments directly
from the ice margin. This setting has enabled us to precisely
date the meltwater sediments, and thus the maximum ice
margin extension. The upper boundary of the meltwater
sediments marks the time horizon when the ice front retreated
to a position where the meltwater was re-routed away from the
lakes.
Methods
The stratigraphy in the lake basins was first described and
mapped by coring transects across the sites with a Russian peat
corer with diameters of 64 and 110 mm, which allows for easy
field examination. The main core site (Eplandmyr) was then
sampled by using a modified piston corer originally produced
Copyright ß 2011 John Wiley & Sons, Ltd.
by GEONOR (Oslo, Norway). The corer is operated by two
separate and integrated sets of steel rods, precisely controlling
both the coring tube and the piston. The sediments are sampled
in 2-m-long PVC core tubes with diameter of 110 mm. In the
laboratory the cores were split lengthwise, described and
analysed. Loss on ignition (LOI) was determined by ignition of
dried samples at 550 8C. Diatoms were used to identify salinity
variations. Samples for diatom analysis were prepared as smear
slides. Diatom valves were identified by using a standard light
microscope. Each diatom slide was inspected in 3–4 transects
and the most frequent (5–10) species in each slide were
identified. In all slides the diatoms unambiguously indicated
marine, brackish or lacustrine depositional environments.
Sediment slices, 1 or 2 cm thick, were sieved at 500 and
250 mm and plant remains were carefully retrieved. Under a
stereomicroscope terrestrial plant remains were identified,
picked and carefully cleaned as preparation for radiocarbon
dating. The selected plant remains were dried and submitted to
the Poznan Radiocarbon Laboratory in Poland for dating
(Table 1). Four samples that originally yielded dates with
unusually high standard deviations (T. Goslar, 2009, pers.
comm.) were re-dated. The two different ages that were
obtained for each sample were combined into a single date by
using the R_Combine function in OxCal v4.1 (Bronk Ramsey,
2010). For calibration and Bayesian analysis we used OxCal
v4.1 with the IntCal09 dataset (Reimer et al., 2009).
The Vedde Ash Bed (Mangerud et al., 1984) was recognized
as a dark grey sandy–silty layer in several of the studied basin
J. Quaternary Sci., Vol. 27(1) 81–88 (2012)
YOUNGER DRYAS GLACIAL MAXIMUM IN W NORWAY
83
Table 1. Radiocarbon dates of terrestrial plant macrofossils from Eplandmyr (59.959648N, 5.518038E).
Core
Depth (cm)
Material dated
Laboratory
number
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-116
505-26
505-26
505-26
505-26
505-26
505-26
505-26
505-26
505-26
505-26
505-26
505-26
505-26
629.5–630.5
629.5–630.5
Twig. Catkin scale and fruit (B)
Twig. Catkin scale and fruit (B)
Combined Poz-33264 and Poz-33421
Catkin scale, fruit (B). Leaf fragment. Twig
Moss (R). Leaf fragment
Moss (R). Leaf fragment
Combined Poz-33266 and Poz-33422
Twig. Catkin scale (B). Moss (R)
Twig. Catkin scale (B). Moss (R)
Combined Poz-33267 and Poz-33423
Twig
Twig
Combined Poz-33269 and Poz-33424
Moss (R). Straw
Moss (R). Leaf fragment
Moss (P). Leaf fragment
Leaf fragment (Sh)
Leaf fragment (Sh)
Leaf fragment (Sh,Sp)
Leaf fragment (Sh,Sp)
Terrestrial plant material
Moss (R). Leaf fragment (Sh)
Moss (R). Leaf fragment (Sh)
Moss (R, P). Leaf fragment
Leaf fragment (Sh,D). Moss (P,R)
Leaf fragment (Sh, D). Moss (P, R). Twigs
Poz-33264
Poz-33421
–
Poz-33265
Poz-33266
Poz-33422
–
Poz-33267
Poz-33423
–
Poz-33269
Poz-33424
–
Poz-35071
Poz-35072
Poz-35073
Poz-8041
Poz-8042
Poz-4945
Poz-4827
Poz-33270
Poz-33271
Poz-8043
Poz-8112
Poz-8045
Poz-8046
634.5–636.5
640.5–641.5
640.5–641.5
645.5–646.5
645.5–646.5
648.5–649.5
648.5–649.5
651.5–652.5
656.5–657.5
661.5–662.5
663.5–664.5
664.5–665.5
776.5–777.5
784.0–785.0
794.5–795.5
800–801
802.0–803.0
834.5–835.5
855.5–856.5
856.5–857.5
14
C age (a BP)
9280 80
9140 50
9460 90
9500 120
9610 60
9590 50
9500 100
9540 60
9530 50
9710 80
9650 60
9670 50
10600 60
9870 60
9960 100
10140 60
10050 60
9950 60
10050 50
10300 100
10090 110
10330 60
11070 60
12130 70
12050 70
Calibrated range
(95.4% level BP)
Posterior range
(95.4% level BP)
10670–10250
10490–10220
10490–10240
11110–10490
11190–10500
11180–10750
11150–10740
11170–10520
11130–10670
11100–10680
11250–10770
11210–10770
11220–10780
12660–12410
11600–11180
11950–11200
12050–11400
11960–11300
11700–11230
11820–11310
12530–11710
12050–11260
12420–11840
13120–12730
14180–13790
14090–13740
–
–
10560–10250
10740–10500
–
–
10940–10730
–
–
11090–10850
–
–
11200–10930
11300–11050
11480–11200
11680–11360
11760–11400
11760–11400
11970–11490
11980–11500
12010–11500
12030–11500
12040–11530
–
–
–
B, Betula; Sh, Salix herbacea; Sp, Salix polaris; D, Dryas; R, Racomitrium; P, Polytrichum.
sequences. This tephra layer represents an important time
marker and correlation horizon. It is identified as the Vedde Ash
based on its bimodal composition of volcanic glass shards
(rhyolitic and basaltic shards), shard morphology, stratigraphic
position and the fact that this is the only widespread visible ash
layer in western Norway.
All ages are given relative to AD 1950 (BP), for direct
comparison between calibrated 14C dates and ice core years.
Thus 50 years are subtracted from ice core years given as
‘‘before 2000’’ (b2k). The ‘‘total maximum counting error
(MCE)’’ given as error estimates for the events in the Greenland
ice cores (Rasmussen et al., 2006) is regarded as a 2s error
(Andersen et al., 2006), and is given in the text 1s (i.e. 0.5 MCE).
The YD glacial extent at southern Tysnes
The southern part of the island of Tysnes is one of the few areas
that contain frequent glacially eroded lake depressions along
the Halsnøy Moraine. However, there is no distinct marginal
moraine crossing this particular area. Therefore, in order to
locate the YD ice sheet extent several palaeolake basins were
investigated with a Russian peat corer along a profile from
Hardangerfjorden to Epland (Fig. 1, sites 1–6 and Epland).
Efforts were made to retrieve the deepest parts of the
stratigraphies. The six eastern basins (cores 1–6, Fig. 1) contain
only Holocene gyttja above the deglaciation sediments
(Fig. 2A) indicating they were overrun by glacial ice during
the YD. In the Epland area organic Allerød sediments overlain
by YD silt with the Vedde Ash Bed are present in all investigated
basins (presented below). We therefore conclude that the ice
margin was located between Epland and the six mentioned
cores, i.e. across the northern part of Lake Breidavatn (Figs 1
and 3A).
Copyright ß 2011 John Wiley & Sons, Ltd.
One could consider if the described ice margin shows the
extent of an ice cap centred on the mountains on northern
Tysnes (737-752 m a.s.l., Fig. 1). Such an assumption is
contradicted by a full lacustrine sequence of Allerød–YD
sediments in a lake (Stønatjørna, Figs 1 and 2B) between the
two areas.
Basins in the Epland area
In the Epland area three basins have been investigated (Figs 1
and 3). Eplandmyr and Stemmetjørn are drained by a small river
running towards the north-west and exiting into the Søreidevågen bay at Epland (Figs 1 and 3A). Breidamyr/Breidavatn drains
in the opposite direction into a larger drainage system at
southern Tysnes that flows into the sea at Flataråker. The two
drainage systems are separated by a saddle point between
Eplandmyr and Breidamyr with an elevation of 70–75 m a.s.l.
(Fig. 3A), i.e. some 10 m above Breidavatn (Fig. 4, upper panel).
Eplandmyr palaeolake
Eplandmyr (‘myr’ ¼ mire) is a 200-m-long and 80-m-wide mire
that fills in a rock basin that was formerly a lake (Fig. 3A). The
surface elevation of Eplandmyr is 58 m a.s.l., slightly below
the YD age marine limit (60 m a.s.l.) in this area (Lohne et al.,
2007). The basin is up to 9 m deep in the western part from
where the main core (110 mm) was collected. The main inflow
comes from the adjacent lake Stemmetjørn.
The basal sediments in the main core consist of greyish silty
sand overlain by <10 cm of bluish grey silt (Fig. 3B). Above the
silt is a sediment unit that consists of a brownish silty gyttja with
LOI values up to 30%. According to the 14C dates this unit
accumulated during the Allerød. At a depth of 835 cm the gyttja
grades into a greyish gyttja silt with a significantly lower content
of organic material. This unit is partly laminated and has LOI
J. Quaternary Sci., Vol. 27(1) 81–88 (2012)
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Figure 2. (A) Logs of sediment
sequences in lakes and palaeolakes where Allerød–YD sediments are missing, indicating
that the sites were overrun by
glacial ice during the YD. The
sites are presented along a profile
from
Hardangerfjorden
and
north-westwards across southern
Tysnes (Fig. 1). (B) The lake stratigraphy at Stønatjørna, Central
Tysnes (Fig. 1). The Allerød and
YD sequence, including the
Vedde Ash Bed, shows that the
Stønatjørna area was not covered
by glaciers during the YD. For
legend to the logs, see Fig. 3. This
figure is available in colour online
at wileyonlinelibrary.com.
values of only 6–8%. A 14C date indicates that the transition
between these two units represents the Allerød/YD boundary.
The Vedde Ash Bed (at 823 cm) is 1 cm thick. At around 815 cm
depth, the silt content increases slightly and the sediments
become more massive. The diatom content shows that this
boundary marks the transition from lacustrine to marine
sediments (Fig. 3B), thus reflecting a rising relative sea level
during the YD (Lohne et al., 2007).
At 801 cm depth there is a sharp boundary marked by an
abrupt drop in LOI, and a change to sterile, finely laminated silt
(Fig. 3B). Abundant diatom valves of diverse species are found
below the boundary, whereas only a few valves occur within
the 137-cm-long sequence above, and most of them are
fragmented. This upper unit consists of finely laminated, greyish
blue, clayey silt interbedded with several distinct sand layers.
LOI values are as low as 0.5–3%. We interpret this sequence as
ice-proximal, glaciomarine sediments. Some scattered plant
remains were found throughout the sequence, with concentrations in the sand layers. The 14C dates and the stratigraphic
position above the Vedde Ash Bed show that the entire
sequence accumulated during late YD. The upper boundary of
this unit marks the transition from marine to lacustrine
sediments. The lower 4 cm of the overlying organic sediments
is a brownish grey, gyttja silt that grades into the uppermost
sequence (660 cm) of dark-brown gyttja and peat.
We obtained radiocarbon ages from 18 levels in the core
(Table 1). These samples were retrieved from two parallel cores
collected with a Russian peat sampler (505-116) and a 110mm-diameter piston corer (505-26), respectively.
Breidamyr palaeolake
Breidamyr palaeolake is also presently a mire, 150 m long and
50 m wide, with surface elevation of 67 m a.s.l. (Fig. 3A).
Because of the site’s remote location, it was only cored with an
easy portable Russian peat corer. There are no inflow streams
entering the basin and its runoff drains into the adjacent lake
Breidavatn (Fig. 3A). As discussed below, the site was
connected to Lake Breidavatn when the advancing ice margin
Copyright ß 2011 John Wiley & Sons, Ltd.
blocked the outlet of Breidavatn (Figs 3A and 4). The basal
sediment in Breidamyr is bluish grey silt with a distinct sand
layer (Fig. 3B). This unit is overlain by brownish silty gyttja,
10 cm thick, with LOI values up to 8%. From about 780 cm
depth there is a distinct transition to a 15-cm-thick unit of
greyish gyttja silt marked by a drop in LOI values to about 4%.
The Vedde Ash Bed, which is 4 cm thick, occurs within this
unit. At 765 cm depth, there is a pronounced transition to
laminated, bluish grey clayey silt 240 cm thick. The upper part
of the glacial silt unit only has lacustrine diatoms, indicating
that Breidamyr was located above the marine limit. This unit
contains some distinct sand layers that are up to 20 cm thick.
There is a sharp upper boundary to a 5-m-thick, dark-brown
gyttja.
The core from Breidamyr has not been radiocarbon dated,
but the entire sequence can easily be correlated with
Eplandmyr, particularly the Vedde Ash Bed and the glacial
silt unit (Fig. 3B). The main differences are that the brownish
Allerød–YD deposits are thinner, and the YD glacial silt is
thicker in Breidamyr than in Eplandmyr.
Just north of Breidavatn (x in Fig. 3A) is a small (0.5 0.5 m)
outcrop showing finely laminated bluish silt and sand layers,
which we correlate with the glacial silt in the core.
Lake Stemmetjørn
The lake basin of Stemmetjørn is about 400 m long and 100 m
wide with a surface elevation of 79.5 m a.s.l. (Fig. 3A). The
northern part (0.3 ha) of the basin is filled with sediment and is
presently a bog. Sampling was carried out with a Russian peat
corer. The intention was to analyse a ‘control basin’ that is
located outside the reach of meltwater drainage from the ice
sheet.
The basal part (805–745 cm depth) of the core consists of
sand and bluish grey silt, evidently deposited during deglaciation. This unit is overlain by a greyish brown, silty gyttja (745–
722 cm depth), gradually becoming more organic upwards.
Above is a well-defined unit of greyish gyttja silt (722–714 cm
J. Quaternary Sci., Vol. 27(1) 81–88 (2012)
YOUNGER DRYAS GLACIAL MAXIMUM IN W NORWAY
85
Figure 3. (A) Digital elevation model for the Epland area (Fig. 1) showing the cored sites and the inferred YD ice sheet margin. The end points for the
topographic profile in Fig. 4 are shown by X and Y (500 m to the SE of the map). The elevation model is based on contour data with 5-m contour
interval, and all elevations are given in m a.s.l. (ßNorge digitalt). (B) Logs for one core from each of the three (palaeo-) lakes. Note the Vedde Ash Bed
for correlation between the sites and the thick sequence of glacially derived silt above the Vedde Ash Bed at Eplandmyr and Breidamyr. Diatom data for
the Eplandmyr site are presented as inferred environment at the levels of the diatom samples. The sample at 816 cm depth consists of a mixed diatom
flora inferred to be brackish. The two levels labelled with ‘0’ were barren of diatom valves. This figure is available in colour online at
wileyonlinelibrary.com.
depth) with a 1-cm-thick layer of Vedde Ash in the middle part.
The gyttja silt grades into a thick sequence of dark-brown gyttja.
The lateglacial sequence in Stemmetjørn shows a typical
succession for lakes in western Norway that are located outside
the YD ice sheet margin and that did not receive glacial
meltwater during the YD. In such basins the YD sediments
rarely exceed 10–20 cm in thickness (Kristiansen et al., 1988)
with the Vedde Ash Bed occurring near the mid point of the YD
unit (Mangerud et al., 1984).
Discussion
Interpretation of the YD silt unit
The interpretation of the bluish grey silt above the Vedde Ash
Bed in Eplandmyr and Breidamyr (Fig. 3B) is crucial for the
scope of this paper. The sediments have similar characteristics
at the two sites. The lower boundary is sharp, the organic
content very low, and the sediments are mostly finely laminated
with graded silt and clay layers. In Eplandmyr the sedimen-
Figure 4. Schematic profiles along the Epland
and Breidavatn drainage systems (Fig. 3A) showing drainage routing for the present day (upper)
and the late Younger Dryas (lower). End points
for the profiles are indicated in Fig. 3A. The
proglacial lake, meltwater overflow and deposition of glacial silt only occurred when the ice
margin was less than 400 m from the Breidamyr site. When the ice margin was >400m from
Breidamyr the meltwater was routed south-westwards. This figure is available in colour online at
wileyonlinelibrary.com.
Copyright ß 2011 John Wiley & Sons, Ltd.
J. Quaternary Sci., Vol. 27(1) 81–88 (2012)
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tation rate for the silt was up to 5 mm a1, as compared with
0.9 mm a1 for the unit between the lower boundary of YD and
the Vedde Ash Bed. This is also a much higher rate than during
the Early Holocene (0.3 mm a1). Based on these sedimentological criteria we conclude that the bluish grey silt is iceproximal meltwater sediment (e.g. Bondevik and Mangerud,
2002). The conclusion is supported by the large difference in
sediment thickness of the YD strata between the ‘non-glacial’
Stemmetjørn (8 cm) and the ‘glacial’ Eplandmyr (137 cm) and
Breidamyr (250 cm).
We can rule out the possibility that local glaciers on Tysnes
were the source of the YD glacial silt because the entire
drainage area of Breidamyr–Breidavatn (maximum altitude
120 m a.s.l.). is well below the equilibrium line altitude (ca.
350 m a.s.l.) in this area during the YD (Midtun, 2009). As
described above, the Stønatjørna site (Fig. 2B) shows that
glaciers from the Tysnes Mountains (737–752 m a.s.l.; Fig. 1)
did not extend to the Breidamyr–Breidavatn area. The only
feasible interpretation is that normal outlet from Breidavatn
(towards the east; Figs 1 and 3A) was blocked by the
Hardangerfjorden glacier and that meltwater from this ice
margin drained across the saddle point to Eplandmyr
(Figs 3A and 4). The glaciolacustrine silts north of Breidavatn
(Fig. 3A), correlated with glacial silt in the Breidamyr core,
demonstrate that there was a higher lake level when they were
deposited. That the silt sequence in Breidamyr is thicker than in
Eplandmyr supports this interpretation.
The Eplandmyr basin was connected to the sea during late
YD and theoretically one may argue that some of the glacial
sediment had entered the basin from the sea side (Fig. 1).
However, the submerged Epland valley was at that time a small
tributary inlet to the Søreidevågen bay (Fig. 1) where there was
no source for the glacial silt. The Eplandmyr basin was in fact
the innermost cove with a shallow sill (less than 2 m water
depth; Fig. 4). Because of its sheltered position, shallow sill
depth and lack of source for the glacial silt in Søreidevågen, we
find it unlikely that significant parts of the glacial sediments
recorded in the Eplandmyr cores entered the basin from the sea
side.
We conclude that the glacial silt in the Eplandmyr basin was
deposited from Breidavatn proglacial lake, as shown in Fig. 4.
Thus, the basin was a sensitive gauge for the maximum glacial
extension because meltwater would not be routed across the
saddle point (70–75 m a.s.l., Figs 3A and 4) until the ice front
was less than 400 m from the Breidamyr coring site.
of increments per depth unit, e.g. cm) determines how large
shifts in sedimentation rate the P_Sequence model will accept;
high numbers indicate a ‘stiff’ model. Based on layer thickness
Age of YD glacial maximum
A precise age for the YD glacial maximum at Tysnes is difficult
to obtain because this event falls on the 10k 14C a BP plateau. In
order to minimize this problem we have dated a series of
samples across the sediment transition. We used the 15 upper
dated levels and the Vedde Ash Bed, and applied a Bayesian
age–depth modelling approach by using the P_Sequence
function in the OxCal 4.1 calibration software package (Bronk
Ramsey, 2009a).
In the P_Sequence function the dated samples are postulated
to be in correct stratigraphic order and the sedimentation rate is
allowed to vary approximately proportionally to depth (Bronk
Ramsey, 2008). The P_Sequence function is realistic because it
allows for changes in sedimentation rate and utilizes the
information provided by the sample depth. Previous studies
have successfully applied this for construction of age–depth
models for lake sediments (Staff et al., 2010; Lane et al., 2011).
Outliers have been statistically handled by the Outlier_Model
with the ‘General’ setting, which objectively down-weight
suspicious dates (Bronk Ramsey, 2009b). The k-factor (number
Copyright ß 2011 John Wiley & Sons, Ltd.
Figure 5. Constructed age–depth model, based on Bayesian statistics
of the calibrated radiocarbon dates together with the Vedde Ash Bed,
for the cored sequence in the palaeolake basin of Eplandmyr. Details
about the model and results are given in Table 2 and Appendix S1. The
probability density for each calibrated age is shown in non-filled curves
and the modelled posterior probability functions are shown with filled
curves. The age–depth model is plotted at a 94.5% confidence level.
This figure is available in colour online at wileyonlinelibrary.com.
J. Quaternary Sci., Vol. 27(1) 81–88 (2012)
YOUNGER DRYAS GLACIAL MAXIMUM IN W NORWAY
87
Table 2. Constraints and results of the Bayesian age-depth modelling from the Eplandmyr core (Fig. 5).
Calculated ages of YD glacial silt (cal a BP)
Model
Eplandmyr
No. dates
in series
No. of ‘Boundaries’
(and depth in cm)
k-factor
Outlier
model
15
4 (815, 801, 663.5, 660)
2.5
‘General’
variations from three cores from Eplandmyr the k-factor has
been estimated to about 2.5 (four depositional events per cm),
using the method outlined in Bronk Ramsey (2008). There are
few dates below the Vedde Ash Bed and this part has been
omitted from the modelling. The Vedde Ash Bed is implemented as a calendar year date (C_Date) of 12 121 57 cal a
BP, following the GICC05 chronology of the NGRIP ice core
(Rasmussen et al., 2006).
According to our model (Fig. 5; Supporting infromation,
Appendix S1) the ice-proximal glacial silt started to accumulate
12 030–11 500 cal a BP (95.4% interval), with a mean of
11 760 120 cal a BP (Table 2). This corresponds to the time
when the ice margin advanced to a position less than 400 m
from its maximum extent. The age–depth model indicates a
95.4% age interval for the onset of ice sheet retreat of 11 760–
11 390 cal a BP, with mean age of 11 590 100 cal a BP
(Table 2). Accordingly, we estimate that the ice sheet margin
was located at its maximum position over a period that lasted
170 120 years (calculated by means and 1s; Table 2).
Comparison with other sites
In our opinion the best dated YD/Holocene boundary in
Norway is at Lake Kråkenes (Gulliksen et al., 1998), on the
northern tip of south-western Norway (Fig. 1). A re-calibration
using a similar Bayesian approach as for the Eplandmyr site
with the INTCAL09 data set (Reimer et al., 2009) indicates that
the YD/Holocene transition at Kråkenes occurred at
11 540 60 cal a BP. This is, within dating uncertainty, in
accordance with the 11 650 50 cal a BP age of the formal YD/
Holocene boundary obtained from the NGRIP ice core with the
GICC05 chronology (Walker et al., 2009). The age of the ice
sheet retreat from the Halsnøy moraine of 11 590 100 cal a
BP overlaps both these ages of the YD/Holocene boundary at
one standard deviation confidence level. Evidently the ice
margin started to retreat at or close to this transition, in
accordance with the chronology at Os some 30 km north
(Bondevik and Mangerud, 2002) (Fig. 1). The deglaciation was
probably initiated by the thermal amelioration at the YD/
Holocene transition.
Conclusions
During the YD the Scandinavian Ice Sheet in Hardangerfjorden extended over the southern part of Tysnes island
(Halsnøy Moraine stage).
The Scandinavian Ice Sheet advance during the YD in Hardangerfjorden culminated at the Halsnøy Moraine at
11 760 120 cal a BP, well after the deposition of the Vedde
Ash Bed.
The ice margin retreat from the Halsnøy Moraine started at
11 590 100 cal a BP, giving a duration of the maximum
position of 170 120 years.
The onset of retreat occurred, within dating uncertainty,
simultaneously with the YD/Holocene glacial boundary at
Copyright ß 2011 John Wiley & Sons, Ltd.
95.4% interval
Mean
s
Median
Upper boundary: 11 760–11 390
Lower boundary: 12 030–11 500
11 590
11 760
100
120
11 610
11 760
Kråkenes (11 540 60 cal a BP) and the YD/Holocene transition in Greenland ice cores (11 650 50 cal a BP).
Supporting information
Additional supporting information can be found in the online
version of this article:
Table S1. Radiocarbon dates of shells and shell-fragments
found in till or sub-till sediments from sites proximal to the
Halsnøy moraine in western Norway.
Appendix S1. Input code for P_Sequence with outlier
analysis used for the age–depth modelling of the Eplandmyr
stratigraphy in OxCal v4.1.7 (Bronk Ramsey, 2009a, b).
Please note: This supporting information is supplied by the
authors, and may be re-organized for online delivery, but is not
copy-edited or typeset by Wiley-Blackwell. Technical support
issues arising from supporting information (other than missing
files) should be addressed to the authors.
Acknowledgements. We thank Herbjørn P. Heggen, Dmitry Nazarov
and Anders Romundset for assistance during the fieldwork. The manuscript benefitted from comments by Brent M. Goehring, and journal
referees Lena Håkansson and Jason Briner. The study was supported
financially by the ICEHUS-project (Ice Age development and Human
Settlement in northern Eurasia), which is funded by The Research
Council of Norway.
Abbreviations. LOI, loss on ignition; YD, Younger Dryas.
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