AMS dating Swedish varved clays of the last glacial/interglacial
transition and the potential/difficulties of calibrating Late Weichselian
‘absolute’ chronologies
BARBARA WOHLFARTH. SVANTE BJORCK. GORAN POSSNERT. GEOFFREY LEMDAHL, LARS BRUNNBERG.
JONAS ISING. SIV OLSSON A N D NILS-OLF SVENSSON
BOREAS
Wohlfarth. B.. Bjorck. S.. Possnert. G . . Lemdahl. G.. Brunnberg, L.. king. J.. Olsson. S. & Svensson. N.-0.
1993 (June): AMS dating Swedish varved clays of the last glacial/interglacial transition and the potential/
difficulties of calibrating Late Weichselian ‘absolute’ chronologies. Borecis. Vol. 22. pp. 113- 128. Oslo. ISSN
0300-9483.
The increasing focus on the chronology of environmental and climatic changes of the last glacial-interglacial
transition has led to several independent attempts to try to calibrate the “C time-scale beyond the Holocene.
The Late Weichselian Gotiglacial varved clays of the Swedish Time Scale could potentially be used for this
purpose. The reliability of the Swedish Time Scale is discussed as well as dil-rerent ways of using the Swedish
varved clays for calibrating the I4C chronology. The strategy and initial results from an ongoing calibration
project are presented. They show clearly that. if the right strategy is adopted. varved clay may be dated by
accelerator mass spectrometry ( AMS) I4C measurement of terrestrial macrofossils. A Late Weichselian ‘event
stratigraphy’. including the Vedde Ash fall-out, is established for south Scandinavia using three dating
categories: clay varve measurements, terrestrial macrofossil measurement, and lake sediment (including aquatic
mosses) measurements. It suggests that a I4C chronology based on terrestrial organic remains is not consistent
with the traditional Late Weichselian chronostratigraphy based on lake-sediment samples. and that ‘clay varve
years‘ exceed ‘terrestrial I4C years’ by c. 900 years at the end of, and by 1100- 1200 years at the beginning of
the Younger Dryas Chronozone. Further back in time. the time-scales appear to converge. These results are
compared with other recently published calibration studies.
Burharci Woldfirrih. Socinle Bj5rc.k. Geoffrey Le.mclulil. J0nU.S Ising, Sic Olsson und Nils-Olqf Suensson. Depurr nient of’ Quci/ernury Geologj. Luncl Universiiy. Torncic. 13. S-223 63 L i d . Sweden; Giirun Possnert. The
Sceclherg Lcihorciiory. Uppsnlr Uniuersitj~,Bo.1- 533. S - 751 21 Uppsnln. Swclen; Lnrs Brunnherg, Depurrmeni of
Quoierntirj~ Resecircl~.S/ockkolni Unioersiiy, Odeng. 63. S - I 13 2 Siockholm. Sweden; I 1111 December. I992
(rcvisecl 8117 Murch. 1993).
Attempts to calibrate the Late Weichselian I4C
chronology against other ‘absolute’ chronologies has
lately become a popular topic in Quaternary studies.
This stems largely from the intense interest in the
abrupt environmental changes of the last glacial/
interglacial transition, and from the need to tie these
changes to a calendar year chronology. During the
last few years major and important publications by
specialists from a wide variety of Late Quaternary
dating and correlation methods reveal that there are
significant differences between time-scales of different
origin, although several of these are supposed to be
based on calendar years (Bard & Broecker 1992).
Because of their regional validity and the ease with
which they can be radiocarbon dated, dendrochronology series are the perfect calibration tools. Unfortunately the most extensive series, the recently connected
German oak and pine chronologies (Becker &
Kromer 1986; Kromer & Becker 1992) cover only
the last 10,000 I4C years. Other seasonally based
chronologies are ice-core laminations, laminated lake
sediments, and the Swedish varved clays. Of these the
Swedish varve chronology, named the Swedish Time
Scale (De Geer 1940), is inherently similar to dendrochronology: it is based on the matching of an
immense number of sites and it has a regional validity
(Bjorck et ul. 1992a). Compared to the other types it
has the advantage of not having to rely on a complete
and measurable lamination series from a single site.
Laminated lake sediments have, however, been successfully radiocarbon dated (e.g. Lotter 1991; Lotter
et al. 1992). If that were the case for the Swedish Time
Scale, a useful calibration tool for the last glacial/
interglacial transition may emerge.
Background
Unless stated otherwise, we will in the following always refer to the chronozones of Mangerud et ul.
(1974), except for a revised age (12,000-12,150 BP)
for the Older Dryas Chronozone (Bjorck 1984; Bjorck
& Moller 1987).
The Swedish Time Scale, based on glacial and postglacial annually deposited varved clay, consists of
three parts: the youngest 9238 Postglacial varves (up
to AD 1950), 1191 Finiglacial varves, and the oldest
2500-3000 Gotiglacial varves (De Geer 1940; Cat0
1987). The Time Scale is built up using evidence from
thousands of varved clay sites of a fresh-water to
brackish environment along the Swedish east coast.
Any doubts about the validity of the Time Scale have
now been removed by Cat0 (1993) who has shown
that shore displacement curves in western Norrland,
based respectively on clay varves in deltas and isolations of lakes with lake laminations (cf. Renberg 1978;
Renberg & Segerstrom 1981) are more or less identical
during the last 9000 varve years. Cato (1985, 1987)
also demonstrated that the last 150 laminae in the
Angermanahen River mouth (Fig. I ) are annual when
he managed to link Liden’s (1938) glacial/postglacial
chronology with the present into what is now called
the revised Swedish Time Scale. This linkage showed
that the Fini- to Postglacial part of the Time Scale, i.e.
from the time that the receding ice-margin was situated just south of Stockholm and up to the present,
contains 10,430 years, with an estimated error of +35
to -205 years (Cato 1987; Stromberg 1989). On the
east coast. 910 older additional varves of the younger
part of the so-called Gotiglacial varves have been
linked to the Time Scale (Brunnberg 1988). while
Stromberg ( 1985a. 1989) prolonged the Time Scale in
south-central Sweden (Fig. 1 ) to I1,600-11,700 varve
years BP. These 1 1,600- I 1.700 years possibly correspond to and even exceed the time covered by the
German oak and pine dendrochronology (Kromer &
Becker 1992). Regarding the millennia preceding the
Younger Dryas-Holocene transition, the Swedish
varve chronology has one great advantage compared
to the tree-ring chronology: varves were undoubtedly
formed during the Younger Dryas, whereas tree
macrofossils are still absent from the main part of this
time period.
The Gotiglacial varves were deposited in southeastern Sweden during deglaciation from the province of
Skine up to Stockholm (Fig. I). Several attempts
have been made to connect the presumed ‘Allerrad
varves’ with the younger part of the Gotiglacial varves
(presumably Younger Dryas-Preboreal varves), but
without completely satisfactory results, mainly due to
connection problems within the Younger Dryas icemarginal zone. Strenuous attempts to solve this problem are in progress by collaborating research groups
in Lund and Stockholm to solve this problem. We do
know that the Gotiglacial part of the Time Scale
contains at least 2500 varves and possibly as many as
3000. So, even if the Gotiglacial varves are still not
perfectly connected to the present, they can be examined to assess the differences between the radiocarbon
chronology and calendar years during the Late Weichselian. Earlier calibration attempts between the revised
Swedish Time Scale and I4C years were made in
indirect ways, either by palaeomagnetic correlations
(Bjorck et ul. 1987; Sandgren 6’1 d.1988), by correlations of ice margins (Stromberg 1985b. 1989;
Lundqvist 1986; Bjorck ct uI. 19881, by biostratigraphic and palaeoclimatic correlations ( Bjorck &
Digerfeldt 1989) or by a combination of several
parameters (Bjorck & Moller 1987). The results vary
between studies (Bjorck rt ul. 1992a). but they all
indicate that varve years exceed I4C years by 200 - 700
years during the Aller~d-Younger Dryas chronozones and that the time scales might more or less
correspond to each other in the early B~lling(e.g.
Bjorck et ( I / . 1987). The main objection to these
calibrations has been that the varved clay has not
been dated directly using “C.
During numerous lake-sediment corings in Baltic
Ice Lake sediments over recent years, organic plant
macro remains were occasionally observed in the
varved clays, especially from lakes situated not too far
below the highest shoreline. Three small samples of
mainly Dryus and S u l i . ~remains from Younger Dryas
varved clays in Lake Mullsjoii (Bjorck & Digerfeldt
1989) were dated in 1986 by the accelerator at Gif sur
Yvette in Paris. Although the resulting ’ages’ (between
3400 and 8000 BP) were puzzling, this study provided
impetus for a study on a larger scale.
As the AMS technique (including preparation procedures) developed more and more, permitting measurements of smaller and smaller samples. and
concomitantly the interest in refining the chronology
of the Last Termination has increased, it was natural
to extend the use of AMS methods to “C date the
Gotiglacial part of the Swedish Time Scale. Attempts
to date the oldest parts of the Finiglacial part of the
Time Scale (9240-10.430 varve years BP) were recently published ( Brunnberg & Possnert 1992a.b). As
the onset of the Finiglacial varves represents a short
episode (c. 150 years) of brackish to marine influence
in the Stockholm area during the Yoldia Sea phase,
whole remains as well as fragments of the bivalve
Portlundiu urcticu and scattered finds of fish remains
occur. Eight samples of ditrerent fractions of the shells
yielded ages between 10,500 and 1 1,600 BP, whereas
three periostracum samples provided ages of 82009100 BP. A fish sample (Saltno dpirzus) gave an age of
c. 8700 BP. All samples, except the fish, derive from it
sequence of varved clay 10 cm thick dated to 10,37810,368 varve years BP. These dates undoubtedly pose
problems, but also point to the importance of securing
radiocarbon estimates from terrestrial macrofossils. In
contrast to the Finiglacial and Postglacial varves the
Gotiglacial varves were deposited in an environment
Fig. 1. Map of Europe and southern Sweden. The provinces are indicated along with the areas that have been studied for Gotiglacial varvr\
and chronologies; B = Brunnberg (1986. 1988). S = Stromberg (1985a. 1989). K = Kristiansson ( 1986). Ru = Rudmark ( 1975). and
Ri = Ringberg (1971. 1979. 1991). The locations of the 14 clay-varve sites used for the AMS datings are indicated by I 14 (see Tablc 3 )
as well as Kristiansson’s (1986) key site Fjlrdingstad and Bjorck et d . ’ s ( 1987) Rystad site. The location of the Angermanelven River ( A )
in northern Sweden is shown on the small map.
,
,
\
I
I
\
\
\
i
,
\
\
\
I
I
I
\
,,
\
Stockholm1
58"
57"
56"
12"
14"
16"E
116
Burharu Wohlfhrth et
(11.
BOREAS 22 (1993)
varves at 10,740 varve years BP. The most obvious
connection, based partly on unpublished studies in
western Ostergotland and by K.-E. Perhans ( B.
Stromberg pers. comm.), is to connect Kristansson’s
(
1986) colour change between the local years 707/708
Research strategy and feasibility
at the Fjardingstad site ( F in Fig. 1) with Brunnberg’s
This project required consideration of five problems ( 1986) colour change at 10,740 varve years BP. Such a
( A - E, below) before the sampling could begin.
connection implies that Bjorck et al.’s (1987) palaeomagnetic correlation is erroneous by 200- 300 years,
A. The reliubility of the Gotiglacial uarue chronologies or that the connection between the Rystad site
The Gotiglacial varves consist of four useful varve (Bjorck et al. 1987) and Kristiansson’s (1986)
chronology is erroneous. It also implies that the
chronologies:
1 . Stromberg’s (1985a. 1989) chronology is based on Younger Dryas stillstand in Ostergotland lasted sev51 sites and mainly situated between northern Lake eral hundreds of years longer than Kristiansson ( 1986)
Vittern, Mount Billingen and Lake Vanern (Fig. I). It had estimated.
Kristiansson ( 1986) has not himself connected his
covers more than 1000 varve years and has been
oldest
varves to the chronologies south of his study
connected to the Swedish Time Scale, but not to the
area
(Ringberg
1971; Rudmark 1975). However, a
other Gotiglacial varve chronologies.
2. Brunnberg’s varve chronology for the area south of preliminary connection by Bjorck & Moller ( 1987)
Stockholm (Fig. I), presented in Risberg et al. (1991), shows an overlap of c. 250 varve years between Kriscovers more than 900 varve years. It is based on 120 tiansson’s ( 1986) and Ringberg’s ( 1971) varve series.
sites. It is more or less synchronous with Stromberg’s
chronology and is very well-connected to the Swedish C . Suitable types of sites f o r mucrqfimd reitruins
Time Scale, whereas the connection to the older
Gotiglacial series of Kristiansson (1986), is more The glacial varved clays were all deposited in the large
Baltic Ice Lake. The configuration of its coastline was
problematic and remains uncertain.
3. Kristiansson’s (1986) varve chronology is based on quite similar to today: a mixture of archipelagos.
116 sites between the city of Norrkoping and the deeply indented narrow bays, and open flat coasts.
Oskarshamn area (Fig. I). It comprises c. 2300 varve Detailed information on the shore displacement of the
years. No convincing connection to the Swedish Time Baltic Ice Lake (Bjorck 1981, 1989; Svensson 1989,
Scale has been established, while a preliminary con- 1991; Bjorck & Digerfeldt 1991), enables reconstrucnection (Bjorck & Moller 1987) to the oldest tion of the rate and pattern of emergence.
To avoid dating redeposited organic remains, only
Gotiglacial varves (Ringberg 1971, 1979, 1991) in
typical
Late Weichselian terrestrial macrofossil plant
Blekinge and Skdne (Fig. 1) has been suggested.
4. Ringberg’s ( 1991) varve chronology covers c. 640 remains (cf. Liedberg Jonsson 1988) should be dated.
varve years. It is based on 186 sites. The youngest part The late-glacial environment, with its relatively sparse
may overlap with Kristiansson’s chronology. Accord- vegetation and its harsh and changeable climate,
ing to Ringberg & Rudmark (1985) there is also a must have caused considerable soil erosion and ingood overlap with Rudmark’s ( 1975) short ( 119 varve wash of organic matter from surrounding land areas.
Varved clays rich in terrestrial macroremains thereyears) chronology in southeastern Smdland.
These four chronologies are based on an integration fore would have been deposited preferentially in shalof the authors’ own investigations, previously pub- low water, close to the contemporaneous shore. To
lished studies, and unpublished varve diagrams within check this hypothesis, lakes and bogs which, during
each region. Each regional chronology is thus based deglaciation and some hundreds of years thereafter.
on a large number of sites and it is usually extremely were situated in ( 1 ) narrow and rather shallow ( 1 5 30m) bays, and ( 2 ) in deeper parts of the Baltic Ice
hard to find any ‘bad’ connections.
Lake were selected for coring. When the results were
compared, we found terrestrial macrofossils to be
B. Possible connections between the Gotiglacial
much more common in the shallower archipelago-like
vurve chronologies
areas.
Stromberg’s ( 1989) oldest varve dates to 11,680 varve
years BP, whereas Brunnberg’s oldest varve is 11,340
years old (Bjorck et ul. 1992a). Since all varves older D. Age differences between hulk seihents untl
than 10,430 are regarded as Gotiglacial varves, a fairly terrestrial organic remains
long part of the Swedish Time Scale stretches into Late Weichselian chronostratigraphies in northwest‘Gotiglacial time’.
ern Europe are mainly based on a combination of
Brunnberg’s (1986) varve series show a characteris- I4C dates and pollen stratigraphic correlations. The
tic change from thin brown varves into thick grey immense number of I4C dates in south Sweden have
(the Baltic Ice Lake) completely devoid of any marine
or brackish influence.
BOREAS 22 (1993)
been performed on carbonate-free bulk sediments or
on aquatic mosses. This means that the lake-sediment-based Late Weichselian chronostratigraphy of
southern Sweden may be subject to so-called ‘lake
reservoir effects’ (Olsson 1986). Compared to organic
remains of terrestrial origin, this effect may increase
the age by some 200-400 years. Examples of this
may be seen in the conflicting ages of the onset of the
Younger Dryas stadial. Accelerator mass spectrometry ( AMS) dates on terrestrial organic remains from
this boundary in the USA (Peteet 1992), Ireland
(Cwynar & Watts 1989). Switzerland (Ammann &
Lotter 1988), and Sweden (Hammarlund unpublished) indicate an age of 10,600 BP, while 25 bulk
dates in south Sweden on the same boundary gave a
mean age of 11,020 BP (Bjorck et ul. 1987). Another
example is the AMS date on pollen grains from the
elm decline, which gave an age of 4900 BP (RegneII
1992), whereas 1 1 carbonate-free bulk-sediment dates
were calculated to a mean age of 5300 BP. Svensson’s
( 1989) comparison between bulk-sediment ages of the
Ancylus transgression of the Baltic and previously
dated wood and peat in the associated beach ridges,
taking into account the possible contamination from
roots and the altitude of the dated wood (in relation to the beach crest), indicate the bulk dates to
be at least 200-400 years older than the peat and
wood.
The above-mentioned problems lead to the following conclusions:
1. Since the Late Weichselian pollen stratigraphy, as
well as the main part of any Late Weichselian-early
Holocene ‘event stratigraphy’ (Table I ) in south Sweden, is based on bulk-sediment dates from lakes
(mainly clay gyttjas) or aquatic mosses one may get
significant differences if the same events are dated
with pure terrestrial organic remains.
2. Any lake-reservoir effect presumably varies between different lakes and between different time periods: in some lakes it might be as much as 400-500
years, whereas in other lakes it is negligible.
3. Old C 0 2 from melting ice sheets or dead ice might
be a contamination factor in aquatic environments of
recently deglaciated areas.
Tuhk 1. A Late Weicheselian event sequence for southern Sweden.
from older to younger.
A.
B.
C.
D.
E.
F.
G.
H.
1.
Onset of Older Dryas cooling
End of Older Dryas and beginning of Allered warming
First drainage of the Baltic Ice Lake
End of Allered and beginning of Younger Dryas stadial
Vedde ash faall-out
End of Younger Dryas stadial climate
Second and final drainage of the Baltic Ice Lake
Distinct Younger Dryas-Preboredl climatic warming
Beginning of Yoldia salt-water ingression into the Baltic basin
J. Beginning of the Ancylus transgression
E. C~ironostrutigrupliic(I4C)positions for the
diferent Gotigluciul i:urve clzronologies
The Gotiglacial varves end with the salt-water ingression in Stockholm, and that event has been correlated
using I4C dates or pollen and lithostratigraphic comparisons to a saline event further south in the Baltic
(Svensson 1989; Bjorck et ul. 1990), at Mount Billingen (Bjorck & Digerfeldt 1986), and southwest of
Lake Vanern (Svedhage 1986). These correlations imply an age of 9900-10,000 I4C years for this saline
event, which means that the Gotiglacial varves are of
Late Weichselian age. By comparing I4C-based deglaciation chronologies with a tentative clay-varvebased deglaciation chronology it is possible to place
the different Gotiglacial varve chronologies along the
east coast in the different Late Weichselian chronozones (Table 2).
Methods
Fielihwk
Fieldwork has focused on selected sites (mainly lakes
and bogs) where varved clay was deposited in water
depths less than c. 30m in archipelago-like areas of
the Baltic Ice Lake (Fig. I), in view of the factors
influencing the concentrations of terrestrial macrofossils. The lakes were either cored from lake ice, bridges,
mire surfaces, or in shallow water. Strengthened Rus-
Tuhk 2. The relationship between the Gotiglacial varve chronologies and the Late Weichselian chronozones (Mangerud e~ d.1974). The
chronostratigraphic position of the Older Dryas Chronozone is. according to Bjorck & Moller (1987). placed at 12.150 12,000 BP.
? = uncertain correlation.
Varve chronologies
Region.
province( s)
Number of
varves
Chronostratigraphic
posit ion
( Mangerud ei t i / . 1974)
Ringberg ( 1991 )
Rudmark (1975)
Kristiansson ( 1986)
Stromberg (1989)
Brunn berg ( 1986)
Sklne- Blekinge
Southeast Smlland
Smlland Ostergotland
Eastern Vistergotland
Sodermanland
640
I I9
c. 2300
c. 1200
910
Be-OD-AI(?)
Bolling
Be(?)-OD-AI-YD
Al(‘?)-YD
YD
-
I 18 Burhuru Wohlfurth et (11.
sian peat samplers, 1 m long, with diameters of 4.5,
6.5 or IOcm, were used depending on the stiffness of
the sediments. All cores were overlapped by 50cm,
placed immediately in PVC tubes and wrapped with
plastic film to avoid any contamination.
Luhorwtorj* work
The cores were carefully cleaned and described in the
laboratory. Overlapping cores from each locality were
intercorrelated, mainly by using several diagnostic
varves. Varve measurements were carried out by attaching a paper strip to the core and marking each
summer and winter varve. The measurements were
stored on computer. Computer-drawn diagrams were
connected to the existing regional chronologies. Subsamples comprising 20- 70 varve years were abstracted
for detailed analysis.
The samples were stored overnight in water with a
few drops of 5% Na,P207 in order to disperse fine
particles. They were then washed and sieved using a
0.5 mm diameter mesh. Plant and animal remains
were carefully picked out and stored in distilled water
with a few drops of 2%)HCI in a cold room.
Determinations of macroscopic plant and animal
remains were carried out by comparison with the
reference collection at the Department of Quaternary
Geology in Lund. The atlas of fruits and seeds by
Beijerinck ( 1976) and the keys of Bertsch ( 1941) and
Frey (1964) were used for these determinations.
Macrofossils suitable for dating (including brown
mosses) were separated and rinsed with distilled water.
If, at this stage, it was obvious that a sample did not
contain sufficient material for AMS measurement, then
the underlying and/or overlying sample( s) were added.
In such cases, the samples covered 150-200 varve
years. Usually sets of samples were sent to Uppsala,
which meant that some samples had to be stored for a
shorter time in small glass tubes in distilled water, to
which a few drops of 2% HCI were added.
Of the clay-varve sites studied so far, only one
potentially covers the time period of the Vedde Ash
fall-out, namely Lake Mullsjon (14 in Fig. 1). In view
of the importance of this feature the lowermost cores
from that site that were previously studied by Bjorck
& Digerfeldt (1989) were divided into six samples.
Four of these contained 50 varves each, while two
covered 14 and 39 varves only. However, part of the
core (covering varves 114-202 from the bottom) was
missing as it had previously been used for other
purposes. Each sample was stored for some time in
water and 5‘%1Na,P,07 was added to disperse fines.
The samples were then washed and sieved through
three different meshes with diameters of 32, 63, and
125 pm. The coarsest fraction was saved for abstracting macro remains, while the two other fractions were
freeze-dried. The coarsest fraction (63- 125 pm) was
analysed under a stereo microscope with a magnifica-
ROREAS 22 (1993)
tion of up to 80 x . A small number of sharp, angular
isotropic particles as well as thin, cuspate. three- or
four-winged ‘butterfly-shaped’ glass particles ( Norddahl & Haflidason 1992) were noted. The latter type is
what we later call ‘typical Vedde Ash particles’ (VAP).
The finest fraction (32-63 pm) was analysed under a
polarizing microscope, equipped with a gypsum plate.
to better identify these two types of amorphous particles. Microprobe analysis was not possible. A sample
of VAP was used as reference material. The number of
sharp, angular isotropic particles as well as so-called
VAP found in each sample was related to the total
number (4000-5000 grains) of mineral particles
counted for each sample under the polarization microscope.
AMS ’‘C (luting
Radiocarbon was measured using the Uppsala ENtandem accelerator as an ultrasensitive mass spectrometer in the routine manner described by Possnert
(1990). The radiocarbon content of the samples is
determined by ‘‘C/”C measurements. the different
isotopes being injected sequentially into the accelerator. The absolute transmission of the system was
controlled by use of a National Bureau of Standards
oxalic acid I standard. The overall performance of the
accelerator system and chemistry pre-treatment procedure was checked by measurements of samples with
known ages and background samples at regular intervals in order to guarantee quality assurance according
to the recommendations of Long (1990). No deviations outside the normal counting statistics were observed.
In the present study, special demands were made on
the methods since the amount of organic material
available for the investigation was low (0.2- 1.5 mg).
The normal carbon-equivalent weight of samples
analysed in the laboratory is in the range 1-4mg,
which makes the chemical preparation a routine
nature. Special precautions had. however, to be undertaken in the present investigation in order to prevent
loss, destruction or contamination of the macro material separated from the varved clay.
The acid-base-acid ( AAA) pre-treatment normally
adopted to eliminate carbonates and humics was
avoided since more than 50% of the original material
can be lost with such a procedure. Since the clay
matrix, in which the macro fossils are found. is free
from carbonates and humic material, only a weak HCI
treatment ( p H = 3) at room temperature was applied
before the combustion to eliminate absorbed atmospheric CO,. Oxidation to CO, was effected by mixing
the fossil material with CuO and heating to 800 C for
5 minutes. The carbon dioxide was finally converted
to graphite by an Fe-catalyctic reduction in the presence of Hz, as described by Hut et UI. ( 1986). The
natural mass fractionation of cS”C was not measured
A M S dating varued clays
BOREAS 22 ( 1993)
but corrected for. A value of -25%, versus PDB was
used because the macrofossils mainly consist of terrestrial plant remains. A deviation of 3%, would only
change the obtained age by 24 years, which is of
minor importance in this study.
Results
So far 14 varved-clay sites have been cored and measured (Fig. l ) . Thirty-three samples were submitted to
the AMS laboratory and 18 provided meaningful
measurements. Some were inadequate in C content
I19
and had to be combined with an underlying or overlying sample.
All information concerning the dates are shown in
Tables 3 and 4. The macro remains used for dating
include taxa that were identified to genus or species
level. The majority of these taxa can be found today
in areas above the timberline in the Scandinavian
mountain range. Plants such as Salix reticulata, S.
polaris, S. herbacea, Betula nana and Dryas octopetala
were characteristic components of the Late Weichselian vegetation in southern Sweden (Liedberg Jonsson
1988; Lemdahl unpublished). Among the beetle remains recovered from Uvgol and Skirgol (Table 4),
Tubk 3. List of sites analysed for macrofossils. The sites numbered I - 13 have been cored exclusively for varved clays, whereas sites 14- 16 have been
previously described by Bjorck & Digerfeldt ( 1989) and Svensson ( 1989).
Site
number
I
Site name
Province
(map sheet)
Korsliden
2
3
4
5
5
6
7
8
Blekinge
3E NO
Farslycke
Blekinge
3E NO
Blekinge
Kroksjon
3 F NV
Horseberga- Smiland
5G NV
sjon
Toregol I
Smiland
SG NV
Toregol 11
SmHland
5G NV
Mabo Mosse N Smiland
7G NV
6stergot land
Lillsjon
7F NO
h e r g o t land
Hargsjon I
7F NO
Coordinates
N56 12'24"
El4 58'21"
N56 1406"
El4 55'00
N57 09'39"
El 5"59'00"
N57 09'03"
El6 07'12"
N57 09'03''
El6 07'12"
N58 01'29''
El6 04'15"
N58 03'55"
El5 46'45"
N58 0011"
El5 36'51"
Altitude Type of
m a.s.1. site
Number of
varves
Age of oldest
varve'
Bottom
varvel
AMSsample
Number of
varve years
-40
(Ringberg 1992)
Y
23
104- 29 1
f l l
Y
1
30
Peat bog
291
35-40
Peat bog
336
c.
(Ringberg 1992)
36
(Ringberg 1992)
2,690
( Kristiansson 1986)
2,645
(Kristiansson 1986)
2.643
(Kristiansson 1986)
+
Y
5&6
0-130
134- I97
70-227
Y
3
126-217
?Y
4
88-168
2
45.9
Lake
227
84.6
Lake
217
c.
80
Lake
168
c.
80
Lake
128
I 17.9
Peat bog
108
?
85.3
Lake
155
115.4
Lake
210
2096
n
(Kristiansson 1986)
2130
?Y
(Kristiansson 1986)
n
29. 30. 31
n
22
7&8
9
8
Hargsjon II
6stergotland
7 F NO
N58 '00'57'
E15'3651"
115.4
Lake
218
2058
?Y
(Kristiansson 1986)
9
S t Bergsjon
t)stergotland
7G NV
hergotland
8G sv
hergotland
8 F SO
Ostergotland
8 F SO
hergoland
8G SO
Vastergotland
8E SW
NSS"08'20"
E16"09'33"
NSS"1429
E I5"5 1'00"
N58"13'08"
El 5'45'30''
N58' 12'16"
78.2
Lake
180
94.0
Lake
141
89.4
Lake
90
105.0
Lake
156
1952
?Y
(Kristiansson 1986)
1850
?Y
(Kristiansson 1986)
1885
n
(Kristiansson 1986)
1840
?Y
(Kristiansson 1986)
N58' 18'23''
E I6 '2400"
N58 19'00"
El4 12'30"
57.0
Peat bog
133.0
Lake
Smiland
5G NO
Smiland
N57 13'20"
El6 18'00"
N57'12'50"
El6 15'20"
42.0
Peat bog
50-65
Lake
Nitvin
II
Satrasjon
12
Storsjon
13
Lillebosjon
14
Mullsjon
33
615
36
32
33
35
21
0 - 141
24
0- I56
18
0-160
210-260
310-410
410-510
510-615
?
1 1.600
(Stromberg 1992)
n
16 & 17
IS
13 & 14
12
15
Uvgol
16
Skirgol
Drainage
sediments
Drainage
sediments
' Corresponding 10 the local time-scale.
* y = bottom
134-156
0- 159
160-190
190- 2 10
113-190
70- I58
70- 158
200-2 18
10
10
88-128
varve reached: ?y =coring stopped by stone or stiff sediment; n = bottom varve not reached.
19
20
26
27
28
Fdrslycke
Fdrslycke
Kroksjon
Hargsjon
Hargsjon
Mullsjon
Mullsjon
Uvgol
Uvgol
Lillsjon
Korsliden
Skirgol
Skirgol
Skirgol
Toregol
Toregol
Toregol
Hargsjon
AMS-I
AMS-2
AMS-5 + 6
AMS-7, 8
AMS-I0
AMS-I2
AMS-16, 17
AMS-19
AMS-20
AMS-22
AMS-23
AMS-26
AMS-27
AMS-28
AMS-29
AMS-30
AMS-31
AMS-32
2
2
3
8
8
14
14
7
I
Allerad
Younger Dryas
Younger Dryas
Younger Dryas
Younger Dryas
34
16
32
19
15
I .5
0.2
0.8
1.o
4.40
1.23
2.47
20
105
50
Balling
Younger Dryas
Younger Dryas
Allered
Allerad
20
15
7
6
22
30
32
33
0.3
0.2
1 .o
0.5
I .o
0.5
1 .o
I .o
0.8
0.97
0.98
6.64
7.24
15.21
2.29
3.36
3.16
2.45
40
40
88
40
Drainage
Drainage
Drainage
186
22
Allerad
Allerad
Younger Dryas
Allerad
31
1.4
Drainage
Drainage
5.20
9.43
159
Allerad
Balling
Balling
27
30
1.19
wood fragment
brown mosses, insects
Salix or Betula (budscales,
L, R), Salix poluris (L. R),
Dryas oct. (R)
Dryas oct. (L, stem),
Leguminosa (seed)
?Dryas octopetala (L, R)
?Berula (L, R), ?Sali.r (L, R)
brown mosses (L, F, stems)
Insects, ?Oligocheta
Sa1i.u reticulala (L), insects,
?Oligocheia
Brown mosses (L). ?bark,
insects
Gramineae/Cyperaceae
(stems), ?Salix (twig), brown
mosses (L), insects
Dryus oct. (L), Betula or
Sa1i.r (L), insects,
?Oligochera
Dryus oct. (L, R), Sa1i.v
or Betula (L, R), brown moss
(L), insects, ?Oligochera
Berula nana (F), Sa1i.r or
Berula (L, 0, insects.
?Oligocheru
Dryas oct. (L, R), Berulu or
Sulis (L. R). insects, UP
D r y s oct. (L. R), Betulu or
Sulix (L. Q, insects. UP
D r y s OL'I. (L,K). Berula or
Su1i.v (L. R), insects. UP
?Berulu or Su1i.r (L. R), UP
Salix ?polaris (L), Dryas
oct. (L), Niiella ( S ) , Gramineae,
nana (R),
Salix reticulutu (L, R)
Salix ?polaris (L, IT)
?Sa/ix (L, f).Sa1i.u (L)
Salix herbacea ( L), Berula
Balling
0.3
29
0.38
Macro fossils
submitted'
Expected age
(chronozone)
1.12
Carbon
content ('XI)
Carbon
content (mg)
II
1.91
3.97
1.31
Weight before
burning (mg)
0.21
157
63
I30
Number of
v a n e years
' L = leaves, f = fragment. ff = fragments. F = fruits and seeds, UP = unidentified plant remains.
Locality
Sample
number
Site
number
Table 4. List of samples submitted for AMS dating. their macro fossil content and their AMS age.
Ua-2752
Ua-2753
11,820f 150
10.480 f I50
Ua-2751
Ua-2750
11,520 f 225
10,030 f 185
Ua-2749
Ua-2748
10,540 f 140
10,965 f 100
Ua-2747
Ua-2746
3,770 f 235
10,345 f 150
U a-2745
420 f 160
Ua-2741
Ua-2742
9,640 f 190
9,945 f 115
Ua-2743
Ua-2744
Ua-2544
11,405 f 145
11,915 f 130
8,560 f95
Ua-2543
9,910 f 140
4,610 f 130
Ua-2470
Ua-2469
12,740
150
Ua-2168
Laboratory
number
5.730 & 330
AMS I4C
age BP
3
F
C
A M S daring varved clajjs
BOREAS 22 (1993)
c
1
121
change in the background from 36,000 to 40,000 will
make a 10,000 BP sample only about 100 years
55000
younger (Possnert 1992). Some samples consisted exclusively of so-called typical late-glacial macrofossils.
Four samples did, however, also contain unidentified
remains, of which two, AMS-30 and AMS-32 (Table
4) provided lower ages than expected, as did some of
the other samples. The amount of carbon in the
samples was, generally, quite low, which greatly increases the effects of any contaminating recent carbon.
There are clearly a number of steps, from the field
t
30000 1
1
collection
to the accelerator, that involve risks of
0.1
1
10
contamination.
amount of carbon in sample (mg)
Until means are found of evaluating the effects of
Fig. 2. Carbon-14 background values as a function of different
these
problems, we regard all AMS dates from the
sample sizes for a chemically pretreated ‘dead’ spectroscopic
graphite (filled circles). and an infinitely old carbonate (open circles). varved clay as representing minimum ages only.
Therefore we do not discuss all the dates in Table 4 in
species such as Agabus arcticus, Helophorus gluciulis, detail, but concentrate on those measurements that we
Olophorum boreale and Hippodamia arctica were consider are crucial to the calibration project.
found. They occur frequently in south Swedish lake
Although the lakes and bogs studied in this project
sediments during the Younger Dryas (Lemdahl 1988, represent a slightly different sedimentation environ1991; Lemdahl & Persson 1989). Today they belong to ment than the valley sites, where the original local
arctic-alpine and subarctic-subalpine faunas.
varve chronologies were established, there was usually
The AMS results include several puzzling young no difficulty in connecting the new varve diagrams to
dates (Table 4). The most important factor to con- either Ringberg’s ( 1991) or Kristiansson’s ( 1986)
sider in the evaluation of the dates is the contribution chronology. In many cases, the bottom varve (consistof younger or older carbon to the samples. Possible ing of sand-silt) was obtained, which made it easy to
sources of such contamination can be related to find the best-fitting connection. However, typical
preparation procedures in the laboratory or even to varve sequences or distinct changes in varve character
reservoir effects during the lifetime of the organisms. were the main criteria for the connections (Fig. 3 ) .
In general, reservoir effects do not influence the age of These connections thus made it possible to relate each
terrestrial plant remains. The storage and preparation sampled and measured core and each submitted AMS
of the samples in the laboratory is therefore probably sample to local varve years and to the preliminary
the most critical source of error. We have tested the connected Time Scale (Fig. 4).
influence of modern carbon from the different prepaOne important task was to try to date some of the
ration steps in the AMS laboratory by using an al- marker events in Table 1, as, for example, the first
most I4C-free spectroscopic graphite. The I4C age of occurrence of particles from the Vedde Ash fall-out,
this graphite when introduced to the ion source with- which is I4C dated to 10,600 BP in lake sediments
out any chemical pre-treatment is 55,000 3000 BP. from Norway (Mangerud et a / . 1984) and Iceland
Modern atmospheric C 0 2 and any dust present in the (Bjorck et al. 1992b). Another important step was to
graphite do not give any ion current and therefore do try to directly I4C date the second and final Baltic Ice
not affect this background. However, if we apply a Lake lowering (drainage), which we correlate to the
complete pre-treatment (AAA-combustion-graphi- very distinct varve change at 10,740 varve years BP
tization) to this spectroscopic graphite, values in the (Table 5 and Fig. 4). The general conception is that
range of 35,000-45,000 BP for different sample sizes this drainage occurred at c. 10,300 I4C years BP
are obtained (Fig. 2). Corresponding ages for infi- ( Bjorck 1981 ; Svensson 1989), a conclusion based
nitely old carbon samples (Icelandic double spat) upon the evidence of bulk-sediment dates and pollen
leached by HCI are also included in Fig. 2. The stratigraphic correlations.
conclusion from these data is that the combustion step
introduces most of the background compared to the
graphitization, since this is the same for the carbonate I4C and varve ages for some regionally signlfcunt
and the organic material. In the interpretation of the events or changes
data, a background of 38,000 k 2000 BP was assumed. Onset of:Older Dryas cooling. - Subsample AMS-2 is
The chemicals used in the extraction of the macro- from a sample located slightly below event A, in
fossils from the sediment have similarly been checked Blekinge, i.e. in the uppermost part of the Berlling
with 6.2 mg spectroscopic graphite. A background according to Bjorck & Moller (1987). The assumed
value of 42,000 was measured, which excludes any age is c. 12,800 varve years BP (Fig. 5). The resulting
serious contamination during this procedure. A AMS date of 12,740 k 150 I4C years BP (Table 5) may
6oooo
I
*
122 Burbura Wohljhrth et al.
BOREAS 22 ( l Y Y 3 )
1
OSSY-SCIOA
2500
2660
2800
2650
TOREGOL II
AMS- 29 11,520 f225
AMS- 31.11.820 f150
Fig. 3. Clay-varve diagram from Toregol connected to Kristiansson’s ( 1986: plates 1 & 2) local chronology
VARVE
BRUNNBERG
STRdMBERG
YEARS BP
10.430*
-lie1
EVENT
-+
Yoldia ingression
KRISTIANSSON
10.740*
,707
11,000-
9
3
+Last Baltic Ice
Lake drainage
f
11,500-2400
Beginning of
+the Younger
DrYS
12,000-
12,500-
of
the Allered
+Beginning
13,000-
13.500-’
Fig. 4. The four main
Gotiglacial chronologies
connected to the Swedish
Time Scale (left). by connecting Brunnberg’s ( 1988)
colour change at 10,740
varve years BP with the
same change at the local
varve years 707 and 708 in
Kristiansson’s ( 1986) site
Fjirdingstad. Each sampled and analysed varve
sequence is related to the
Time Scale as well as to
each author’s chronology.
Some major events and
their chronostratigraphic
positions are shown to the
right.
A M S dating varved clays
BOREAS 22 (1993)
VARVE
BRUNNBERG
STRdWBERG
V k R S BP
EVENT
10.43om
KRISTIANSSON
10,740,
123
/
?
+Yoldia
ingression
9,260
AMS12
-10,020
11,0009,715
Ams14 17
4
10,175
11,500-
1760
12.000
Fig. 5. The same chronostratigraphy as in Fig. 4
with the consistent AMS
dates added and the varve
sequences from which they
were sampled. The stratigraphic position of each
AMS date is indicated as
well as the age range
(double standard error) of
each date (cf. Table 4).
The position of the oldest
presumed Vedde Ash
found, and the youngest
date of the Baltic Ice Lake
drainage are also shown.
-
12,5002560
+of
Beginning
the
Younger
Dryas
-
Beginning of
the Allered
2800
13,000-
13,500-
indicate that the connection between Ringberg's
(1991) and Kristiansson's (1986) varve chronologies is
erroneous, and that the SkHne-Blekinge chronology is
therefore older than assumed. Many more dates will
be carried out on the Blekinge varved clay in order to
establish its age.
(aquatic mosses) of c. 12,000 I4C years BP (Bjorck
1984).
Vedde Ash full-out. - In the 63-125 pm fraction only
two so-called VAP were found (in sample 4). The
thin-section analysis ( 32-63 pm) revealed, however, a
dramatic increase in isotropic glass particles in sample
End of Older Dryaslbeginning of Allerad warming. - 4 as well as the first and only appearance of VAP
The site Toregol in eastern SmHland is rich in macro- (Fig. 6). Since no other major ash bed is found in
fossils (Table 4) and three AMS dates close to event northwestern Europe in the middle of the Younger
B (i.e. near the lower Allersd boundary) were ob- Dryas we correlate this to the Vedde Ash and sample
tained from the same sequence of 40 varves (Fig. 3). 4 is dated to 203-242 varve years from the bottom of
Two of these (AMS-29 and 31) gave ages in the Lake Mullsjon (Fig. 6). Since we relate the end of
expected time range and a mean age of 11,670 f 125 varved clay formation in Lake Mullsjon with the final
I4C years BP, compared to an age of c. 12,600 varve lowering of the Baltic Ice Lake, and we date that to c.
years BP (Table 5). Until more ages are obtained this 10,740 varve years BP, the Vedde Ash is dated to the
date is regarded as the minimum AMS age of event interval 11,250- 1 1,100 varve years BP (Fig. 5 and
B to be compared with the corresponding bulk age Table 5).
124 Barbara Wohlfarrh et al.
BOREAS 22 (1993)
Tuhk 5. Comparisons between approximated clay varve ages, bulk
sediment ages (conventional I4C) and macrofossil ages (AMS) for
the events listed in Table 1. Note that the AMS ages include
double standard deviations.
Clay varve
years BP
Bulk sediment
I4C years BP
A
12,800
B
C
D
E
F
G
H
I
12.600
12,150
12,000
1 1.200
1 1,000
10,600
10,500
10,300
10.200
9900
9600
Event
Macrofossil
I4C years BP
~
I 1.800
1 1.200
10.740
10.430
J
12,500- 13.100
1 1,950- 1 1,450
10.175-9715
< 10.300
a
m
rQ
%
W
5
>
-
Second andjnal drainage of the Baltic Ice Lake. - The
apparently sudden 25-m lowering of the Baltic Ice
Lake produced sandy-silty layers in several lakes in
Smiland (Svensson 1989). The sand-silt is thought to
have been washed into depressions (e.g. lake basins)
during this rapid lowering as well as some time afterwards, when soils remained unstable and susceptible
to erosion. During the first phase of in-washing it is
very likely that redeposited organic material, consisting of a mixture of Aller~d-YoungerDryas soil material, was important. The later phase should, however,
provide sediments that are dominated by remains of
plants that colonized the unstable mineral soils. In
order to date the drainage event, ‘drainage sediments’
from two of Svensson’s (1989) sites were washed. Five
samples were submitted for AMS dating, but the dates
from one site were inconsistent: one was far too young
and the other was older than expected (AMS-20 and
AMS-19 in Table 4). This could indicate contamination from recent material and from redeposited organic matter, respectively. Three other dates from
another site, however, provided expected results,
which is interpreted as indicating diminishing effects
of contamination upwards in the sequence (AMS-28 is
the lowermost and AMS-26 the uppermost of these
dates). Sample AMS-26 thus gives a maximum age of
the drainage event of c. 10,300 BP (Fig. 5 and Table
5).
Beginning of (Yoldia) salt-water ingression into the
Baltic basin. - Brunnberg & Possnert’s (1992a,b)
AMS datings of the Yoldia ingression, event I, gave
very conflicting ages. The carbonate radiocarbon ages
can be disregarded, but the mean statistical age
(8615 i-90 BP) of the three periostracum samples and
the fish skeleton is also seriously in conflict with the
radiocarbon-dated (bulk sediments) pollen stratigraphy. The salt-water influence occurs clearly before the
arrival of Corylus, which is bulk dated in innumerable
south Swedish sites to 9700-9500 BP (Digerfeldt
1982). Approximately 1000 14C years later Ainus arrived (Digerfeldt 1982), at the same time as the dated
Portlandia (Yoldia) arctica molluscs are supposed to
have lived (according to the dated periostracum samples). At that time the Baltic had, however, been fresh
for more than 1000 years (e.g. Svensson 1989). This
difference of c. 1300 years between bulk and periostracum dates thus shows that there is still no ‘reliable’
AMS I4C age of event I.
1,100
1,200
1,300
-
Other I4C dated parts of the Gotiglucial uarues
1
2
3%
Fig. 6. The relative occurrence (as a percentage of total mineral
particles) of sharp, angular isotropic particles (non-filled bar) and
’typical Vedde Ash particles’ (filled part of the bar) in Lake
Mullsjon’s varved clay, related to Lake Mullsjon’s varves as well
as to varve years BP. Note that varves 114-202 could not be
analysed.
We have also attempted to date other parts of Ringberg’s (1991) and Kristiansson’s ( 1986) varve
chronologies. Most of these dates have, however, resulted in ages far too young to be considered even as
minimum ages. Examples of such dates are AMS5 + 6, AMS-7 + 8, AMS-22, and AMS-23 (Table 4).
BOREAS 22 (1993)
In addition to the seven more or less ‘reliable’, or
minimum, AMS dates listed above, only one more
date is regarded as belonging to that category, namely
AMS-10. The measured age is 1 1,405 f 145, which
should be compared with an assumed clay varve age
of c. 12,100 years (Fig. 5).
Discussion
The results obtained so far show that a great amount
of work remains to be done before any serious calibration attempt can be carried out, although they do
indicate that this is achievable. We have gained invaluable experience on how to proceed most efficiently
with the project. We now know (1) which type of
areas and which type of varved clay sites should be
selected, (2) in which parts of the clay sequences the
plant remains are most abundant, and (3) which types
of plant remains are preferred, and those that should
be avoided. We also aim to reduce, as much as
possible, the many steps required, each with risks for
contamination, between fieldwork and accelerator
measurement.
Regarding the AMS results obtained so far, we have
reason to believe that c. 50% can be regarded as
‘reliable’. During this first part of the project we
consider most of our dates (excluding the Baltic Ice
Lake drainage dates) as minimum ages, until the
contamination risks can be eliminated and more
dublicate dates have been obtained.
During previous attempts to compare the Late Weichselian I4C chronology based on bulk samples with
independant calendar year chronologies, possible lake
reservoir effects were often neglected. There are strong
reasons to believe that this effect has reduced the
actual differences between the chronologies, which is
supported by our AMS measurements. Comparison
between the chronologies should also take into account the three 14C plateaux dated to approximately
12,700, 10,000 and 9600-9500 radiocarbon years BP
(Lotter et al. 1992; Kromer & Becker 1992). Several of
the events in Tables 1 and 5 occur within the limits of
the last two plateau effects. This makes the detailed
AMS dating of these events quite difficult.
The following discussion on relationships between
chronologies tries to consider lake-reservoir effects ( a
few hundreds of years), although this is obviously
difficult to estimate in any great detail. There are
many similar difficulties, but we will try to relate all
comparisons to assumed terrestrial ages. When the
Late Weichselian chronozones are discussed we must,
however, use bulk ages.
There is now a general agreement that, during the
Late Weichselian- Holocene transition, the number of
calendar years exceeds I4C years by about 500- 1500
calendar years (e.g. Becker & Kromer 1986; Hammer
et al. 1986; Stuiver et al. 1986, 1991; Bjorck et al.
A M S dating varved clays
125
1987; Dansgaard et al. 1989; Becker et a/. 1991;
Johnsen et al. 1992; Kromer & Becker 1992; Rozanski
et af. 1992), which is also in line with the U-Th
datings of the Barbados corals (Bard et al. 1990a, b.
1992). Unfortunately, we can, so far, present only
AMS measures of the Baltic Ice Lake drainage from
this period. Since they, in contrast to the other AMS
dates, are regarded as maximum ages, they indicate a
difference of more than 400-500 years. The bulk age
(9900 BP) of the Yoldia ingression compared to the
varve age (10,430 BP) suggests a difference of c. 900
years, if a lake-reservoir effect of 300-400 years is
taken into consideration.
Further back in time we know much less about the
time-scale differences, which is mainly due to the fact
that the Preboreal pine chronology of the German oak
and pine dendrochronology is still not fixed, and that
pine material is missing from the main part of the
Younger Dryas. No pines have yet been dated to
between 10,800 and 10,200 I4C years BP (Kromer &
Becker 1992). One way to solve this shortcoming has
been to estimate the calendar year duration of the
Younger Dryas Chronozone. However, a possible I4C
plateau at around 10,000 I4C years BP (e.g. Ammann
& Lotter 1989; Lotter 1991; Kromer & Becker 1992;
Lotter et a/. 1992) makes precise dating of the end of
the Younger Dryas difficult. Nevertheless, these estimates have given many different answers on the duration, in calendar years, of the Younger Dryas
Chronozone. From laminated lake sediments, Lotter
et a/. (1992) find that these supposed 1000 I4C years
are covered by 900 varves in Swiss lakes, and a Polish
lake exhibits at /east 1200 Younger Dryas varves
(Rozanski et a/. 1992) and possibly 1600 varves (Ralska-Jasiewiczowa et a/. 1992), whereas a German lake
contains only 450 varves of the ‘Youner Dryas climatic deterioration’ (Zolitschka et a/. 1992; 89). In the
Greenland Summit ice-core the duration of the
Younger Dryas (chronozone or stadial?) is now provisionally estimated to 1 150 calendar years (Johnsen et
af. 1992). Based on U-Th ages, Bard et al. (1990a)
first concluded that the Younger Dryas may have
lasted as long as 1700 years, but there are now more
cautious as they conclude that the ‘Younger Dryas
Chronozone exceeded one millennium’ (Bard et a / .
1992; 109). The attempts to calibrate the Swedish
varved clay by palaeomagnetic stratigraphy indicated
that the Younger Dryas is represented by 900-1000
varves (Bjorck et a/. 1987). Correlations based on
secular variations are, however, not a very precise
correlation tool. This was the main reason for using
the varve colour and character change of varves 707
and 708 in Kristiansson’s (1986) chronology as the
most likely equivalent of Brunnberg’s (1986) comparable changes at varve year 10,740, providing a
provisional basis for connecting the whole Gotiglacial
varve series to the present. This procedure suggests
that 1 1,800 varve years correspond to c. 1 1,000 (bulk)
126 Barbara Wohlfarth et al.
I4C years (Table 5), i.e. perhaps 10,700-10,600
(AMS) 14C years. The difference is larger than at the
Younger Dryas-Preboreal transition, and the Vedde
Ash date supports a change during the Younger
Dryas. Our results therefore suggest that varve years
might exceed I4C years by 1100-1200 years at the
beginning of the Younger Dryas.
Interestingly, some of our more consistent dates
relate to an older period for which few calendar-yearbased studies are available. Lotter et al. (1992) find
that the Allerad Chronozone ( 12,000- 11,000 BP) covers a maximum of only 400 varves, whereas Bard et
d . ’ s (1990a) U-Th dates suggest an interval of c. 1000
calendar years. Bjorck & Moller (1987) estimated it to
800 varves, and since we have found no reason to alter
Kristiansson’s ( 1986) Allerad chronology, this estime
is still regarded as the most likely one. This implies
that the differences between the various time-scales are
reduced back in time, which is supported by the
AMS-I0 age ( 1 1,405 f 145), which corresponds to
12,100 varve years (Fig. 5 ) , and the combined age of
11,670 f 125 for AMS-29 and AMS-31, for the very
beginning of the Allerrad. Moreover, the 12,740 f 150
date of AMS-2 has a varve age of 12,800 years (Fig.
6). This apparent convergence of the time-scales in the
early Balling was postulated by Bjorck et al. (1987)
and Bjorck & Moller (1987) and is supported by
Lotter et al.’s (1992) lake-varve data. Such a dramatic
convergence is not supported by the Greenland Summit ice-core laminations (Johnsen et al. 1992), although their data may indicate a slight convergence
during the Balling. There are rather few U-Th dates
in the 11,000-13,000 time range (Bard et al. 1990a).
Yet, they also suggest a slight converging trend during
this time period. The problem on how the time-scales
behave beyond 12,000 I4C years BP is clearly an
important task to solve. We think we have a great
possibility to do so by AMS dating the relatively
plant-rich varved clays of western Blekinge (Table 4).
Conclusions
In the light of our experiences so far, some important
conclusions can be drawn:
1. If the right type of site is chosen it is possible to
find terrestrial macro remains in varved clay. However, even in the right type of site the lowermost
varves are usually barren, while the uppermost c. 100
varves are much richer in macro remains.
2. The relatively low proportion (c. 50’30) of ‘acceptable’ or ‘reliable’ AMS dates have led us to examine
different contamination risks as well as to modify the
original research strategy. Since some samples were
poor in terrestrial macro remains, we have in many
cases added brown mosses as well as other unidentified remains. However, this was done only when the
identifiable macro remains were of typical late-glacial
BOREAS 22 (1993)
plants. The sampling steps that may cause contamination, from fieldwork to the burning of samples at the
AMS laboratory, must be reduced.
3. There are strong indications that the Nordic Late
Weichselian chronostratigraphy suffers from lakereservoir effects, which makes comparisons with a
terrestrially based (mainly AMS) I4C chronology
difficult. These effects may be as much as 400-500
years, which may explain some unexpectedly young
AMS datings obtained here and in other studies.
4. It is possible to find Vedde Ash particles in the
Swedish varved clay. This will be a further calibration
tool between the time-scales, especially when the terrestrial 14C age has been established independently. It
seems, however, clear that its bulk-sediment age of c.
10,600 I4C years BP (e.g. Mangerud et ul. 1984;
Bjorck et al. 1992b) may be as much as 400-500 years
‘too old’.
5. The differences between I4C ages on terrestrial organic material and the Late Weichselian part of the
Swedish Time Scale seem to vary. Between 10,000 and
12,000 I4C years BP, varve age exceeds I4C age by
about 1000 years. The AI4C values seem to have
reached a maximum during the Younger Dryas
Chronozone, with slightly lower values during the
Allerad Chronozone. The time difference, and thus
also the AI4C values, are much less for the Balling
Chronozone.
Acknowledgemenis. Haflidi Haflidason ( Bergen) provided us with
a sample of Vedde Ash particles. SB and SO carried out the
thin-section analysis in search for the Vedde Ash. but their analyses
were independently checked by Lena Barnekow (Lund) on the
stereo-microscope. Lena Barnekow also freeze-dried and sieved some
of the samples. Ole Stilborg (Lund) made most of the thin-sections.
Goran Skog (Lund) was helpful in many ways, and the discussions
with him, Ingemar Cat0 (Uppsala), and especially Bo Stromberg
(Stockholm) have been very valuable. John Lowes’ (London) constructive review comments improved the paper in many ways. Tiit
Hang (Tallinn). Hui Jiang (Lund), Tor Olofsson (Lund) and Per
Sandgren (Lund) helped with the fieldwork. To all these people we
are very grateful. This project is financed by the Swiss National
Science Foundation (SNF), project no. 8220-0306917 and the
Swedish Natural Science Research Council ( N F R ) . This is a contribution to IGCP 253, ‘Termination of the Pleistocene’.
-
References
Ammann, B. & Lotter, A. F. 1989: Late-Glacial radiocarbon- and
palynostratigraphy on the Swiss Plateau. Boreus 18. 109 126.
Bard, E. & Broecker, W. S. (eds.) 1992: The Lust Degluciution: Ah.roluie
and Rudiocurhon Chronologies. NATO AS1 Series, Series I, Glohul
Enuironmenful Change, Volume 2, 344 pp. Springer-Verlag. Berlin.
Bard, E., Hamelin, B., Fairbanks, R. G . & Zindler. A. 1990a:
Calibration of the I4C timescale over the past 30.000 years using
mass spectrometric U - T h ages from the Barbados corals. Nutirre
345, 405-410.
Bard, E., Hamelin, B., Fairbanks. R. G., Zindler. A,. Mathieu. G .
& Arnold, M. 1990b: U/Th and I4C ages of corals from Barbados
and their use for calibrating the I4C time scale beyond 9000 years
B.P. Nucleur lnsirumenis and Met1iod.r in Physics Reseurch BSZ.
461 -468.
-
A M S dating varved clays
BOREAS 22 (1993)
Bard. E,, Fairbanks. R. G,, ~ ~ ~M,~ & l ~ d ~ , ~ B. ~1992:l
23qh/234U
and ‘“C ages obtained by mass spectrometry on corals
from Barbados (West Indies), Isabela (Galapagos) and Mururoa
(French Polynesia). In Bard. E. & Broecker. W. S . (eds.): The
Last Deglaciation: Absolute and Radiocarbon Chronologies.
NATO AS1 Series. Series I, Global Environmental Change, Volume 2. 103- 112. Springer-Verlag, Berlin.
Becker. B. & Kromer. B. 1986: Extension of the Holocene dendrochronology by the Preboreal pine series, 8800- 10,100 BP.
Radiocarhon 28. 96 I - 968.
Becker. B.. Kromer. B. & Trimborn, P. 1991: A stable isotope
tree-ring time scale of the Late Glacial/Holocene boundary. Nature 353. 647-649.
Beijerinck, W. 1976: Zadenatlas der Nederlandsche Flora, 31 6 pp.
Backhuys & Meesters. Amsterdam.
Bertsch. K. 1941: Friiclite und Samen. Ein Bestimmungshuch :ur
Pflanzenkunde der vorgeschichtlichen Zeit. Handbiicher der praktischen Vorgeschichtsforschung. Band 1. 247 pp. Verlag Ferdinand Enke, Stuttgart.
Bjorck. S. 1981: A stratigraphic study of Late Weichselian deglaciation. shore displacement and vegetation history in south-eastern
Sweden. Fossils and Strutu 14. 1-93.
Bjorck, S . 1984: Bio- and chronostratigraphic significance of the
Older Dryas Chronozone - on the basis of new radiocarbon
dates. Geologiska Foreningens i Stockholm Forhandlingar 106,
81 -91.
Bjorck. S. 1989: The geodynamic and geologic value of a regional
network of shore displacement curves: examples from southern
Sweden. Geologiska Foreningens i Stockholm Forhandlingar I I 1.
292- 295.
Bjorck. S. & Digerfeldt, G. 1986: Late Weichselian-Early Holocene
shore displacement west of Mt. Billingen, within the Middle
Swedish end-moraine zone. Boreas 15, 1 - 18.
Bjorck, S. & Digerfeldt, G . 1989: Lake Mullsjon - a key site for
understanding the final stage of the Baltic Ice Lake east of Mt.
Billingen. Boreas 18. 209-219.
Bjorck, S. & Digerfeldt, G . 1991: Allerod-Younger Dryas sea level
changes in southwestern Sweden and their relation to the Baltic
Ice Lake development. Boreas 20, 115-133.
Bjorck. S. & Moller, P. 1987: Late Weichselian environmental
history in southeastern Sweden during the deglaciation of the
Scandinavian Ice Sheet. Quaternary Research 28, 1-37.
Bjorck. S., Sandgren, P. & Holmqvist, B. 1987: A magnetostratigraphic comparison between I4C years and varve years during the
Late Weichselian, indicating significant differences between the
time-scales. Journal of Quaternary Science 2, 133- 140.
Bjorck. S., Berglund. B. E. & Digerfeldt, G. 1988. New aspects on
the deglaciation chronology of South Sweden. Geographicu
Polonica 55, 37-49.
Bjorck, S.. Dennegird, B. & Sandgren, P. 1990: The marine stratigraphy of the Hano Bay, SE Sweden, based on different sediment
stratigraphic methods. Geologiska Foreningens i Stockholm
Forhandlingar 112, 265-280.
Bjorck, S.. Cato, 1.. Brunnberg, L. & Stromberg, B. 1992a; The
clay-varve based Swedish Time Scale and its relation to the Late
Weichselian radiocarbon chronology. In Bard. E. & Broecker, W.
S . (eds.): The Last Deglaciation: Absolute and Radiocarbon
Chronologies. NATO AS1 Series, Series I, Global Environmental
Change, Volume 2, 25-44. Springer-Verlag, Berlin.
Bjorck. S., Ingolfsson, 0.. Haflidason, H., Hallsdottir, M. & Anderson, N. J. 1992b: Lake Torfadalsvatn: a high resolution
record of the North Atlantic ash zone I and the last glacial/interglacial environmental changes in Iceland. Boreas 21, 15-22,
Brunnberg, L. 1986: ’“C-daterade lervarv frin Stockholmsomridet.
I 7 Nordiska Grologmoret, Absfracts, 19. Helsinki.
Brunnberg, L. 1988: Diatoms in glacial clay in the Sodertorn area,
Stockholm Region. Proceedings of Nordic Diaromist Meeting,
Stockholm 1987. University of Stockholm, Department of Quaternary Research, Report 12, 7-9.
Brunnberg, L. & Possnert, G . 1992a: Radiocarbon dating of the
127
boundary of the Swedish Time Scale. Boreas 21,
i Goti-Finiglacial
~
,
89-96.
Brunnberg, L. & Possnert, G. 1992b: Strange results give rise to
strange discussions. Boreus 21, 372.
Cato, 1. 1985: The definitive connection of the Swedish geochronological time scale with the present. and the new date of the zero
year in Doviken, northern Sweden. Boreas 14, 117-122.
Cato, 1. 1987: On the definitive connection of the Swedish Time Scale
with the present. Sueriges Geologiska Undersokning Ca 68, 1-55.
Cato. 1. 1993: Shore displacement data based on lake isolations
confirm the postglacial part of the Swedish Geochronological
Time Scale. Sveriges Geologiska Undersokning, Ser. Ca 81, 75-80.
Cwynar. L. C. & Watts. W. A. 1989: Accelerator-mass spectrometer
ages for the late-glacial events at Ballybetagh, Ireland. Quaternary
Research 31, 377-380.
Dansgaard. W., White, J. W. C. & Johnsen, S. J. 1989: The abrupt
termination of the Younger Dryas climatic event. Nuture 339.
532--534.
De Geer, G . 1940: Geochronologica Sueccia Principles. Kungliga
Svenska Vetenskapsakudetniens Handlingnr 3. 18 (6). I 367.
Digerfeldt, G . 1982:The Holocene development of Lake Sambosjon.
1. The regional vegetation history. University 01‘Lund, Department
of Quaternary Geology. Report 23, 24 pp.
Frey, D. G. 1964: Remains of animals in Quaternary lake and bog
sediments and their interpretation. Archiv . f i r Hvdrobiologie.
Ergebnisse der Limnologie. Beihefte 2, I - I I . I - 114.
Johnsen, S. J. & Dansgaard. W. 1992: On flow model dating of stable
isotopes records from Greenland ice cores. In Bard, E. & Broecker.
W. S . (eds.): The Lust Drgluciation: Absolute and Radiocarbon
Chronologies. NATO AS1 Series, Series 1. Glohal Envirotimentul
Cliunge, Volume 2, 13-24. Springer-Verlag, Berlin.
Johnsen, S. J., Clausen, H. B., Dansgaard, W., Fuhrer, K.. Gundestrup, N., Hammer, C. U.. Iversen. P.. Jouzel, J.. Stauffer. B. &
Steffensen, J. P. 1992: Irregular glacial interstadials recorded in a
new Greenland ice core. Nuture 359, 31 1-313.
Hammer, C. U., Clausen. H. B. & Tauber. H. 1986: Ice-core dating
of the Pleistocene/Holocene boundary applied to a calibration of
the I4C time scale. Rudiocurbon 28. 284-291.
Hut. G . .Ostlund, H. G. & van der Borg, K. 1986: Fast and complete
CO, conversion for ’“C accelerator mass spectrometry. Radiocarbon 28, 186-190.
Kristiansson, J. 1986: The ice-recession in the south-eastern part of
Sweden. A varve-chronological time scale for the latest part of the
Late Weichselian. University of Stockholm, Department of Quaternary Research Report 7, I 132.
Kromer, B. & Becker. B. 1992: Tree-ring I4C calibration at
10,000BP. In Bard, E. & Broecker, W. S . (eds.): The Lasf
Deglariation: Ahsolute and Radiocarhon Chronologies. NATO AS1
Series, Series I, Glohal Environmental Change, Volume 2. 3- 12.
Springer-Verlag. Berlin.
Lemdahl, G . 1988: Late Weichselian insect assemblages from the
Kullen Peninsula, south Sweden - palaeoenvironmental interpretations. LUNDQUA Report 30, 1-55.
Lemdahl, G . 1991: A rapid climatic change at the end of the Younger
Dryas in south Sweden - palaeoclimatic and palaeoenvironmental
reconstructions based on fossil insect assemblages. Palaeogeography, Palaeoclimatology. Palaeoecology 83. 3 1 3 33 1 .
Lemdahl, G . & Persson, G . 1989: Late Weichselian biostratigraphy
at Mickelsmossen, southern Sweden. Geologiska Fureningens i
Srockholm Fiirhundlingar I 1 1. 25 1 259.
Lidkn, R. 1938: Den senkvartara strandforskjutningens forlopp och
kronologi i Angermanland. Geologiska Foreningens i Stockho~m
Forhandlingar 60. 397-404.
Liedberg Jonsson, B. 1988: The Late Weichselian macrofossil flora
in western Skine. southern Sweden. LUNDQUA Thesis 24, 12
pp. + two appendices.
Long, A. 1990: A quality assurance protocol for radiocarbon dating
laboratories. Radiocarbon 32. 109- 112.
Lotter, A. F. 1991: Absolute dating of the late-glacial period in
Switzerland using annually laminated sediments. Quaternary Research 35, 321 -330.
-
-
-
-
128 Barbara Wohlfarth et al.
BOREAS 22 (1993)
Lotter, A. F., Ammann, B., Beer, J., Hajdas, I . & Sturm, M. 1992:
A step towards an absolute time-scale for the Late Glacial:
annually laminated sediments from Soppensee (Switzerland). In
Bard, E. & Broecker, W. S . (eds.): The Last Deglaciation: Absolute
and Rudiocarbon Chronologies. NATO AS1 Series, Series I, Global
Environmental Change, Volume 2, 45-68. Springer-Verlag. Berlin.
Lundqvist, J . 1986: Late Weichselian glaciation and deglaciation in
Scandinavia. In Sibrava, V., Bowen, D. G. & Richmonds, G. M.
(eds.): Quaternary Glaciations in the Northern Hemisphere. Quaternary Science Reviews 5, 269-292.
Mangerud, J.. Andersen. S. T., Berglund, B. E. & Donner, J. J. 1974:
Quaternary stratigraphy of Norden, a proposal for terminology
and classification. Boreas 3, 109-127.
Mangerud, J., Lie, S. E.. Fumes, H., Kristiansen, I. L. & Lsmo, L.
1984: A Younger Dryas ash bed in western Norway with possible
correlations to the Norwegian Sea and the North Atlantic. Quaternary Research 21. 85- 104.
Norddahl, H. & Haflidason, H. 1992: The Skogar Tephra, a Younger
Dryas marker in North Iceland. Boreas 21, 23-41.
Olsson, 1. U. 1986: Radiocarbon dating. In Berglund, B. E. (ed.):
Handbook of Holocene Palaeoecology and Palaeohydrology. 275312. John Wiley. Chichester.
Peteet, D. M. 1992: The palynological expression and timing of the
Younger Dryas event - Europe versus Eastern North America. In
Bard, E. & Broecker, W. S . (eds.): The Last Deglaciation: Absolute
and Radiocarbon Chronologies. NATO AS1 Series, Series I, Global
Enttronmentui Change,Volume 2,327- 344. Springer-Verlag, Berlin.
Possnert. G. 1990: Radiocarbon dating by the accelerator technique.
Norwegian Archaeological Review 23, 30-37.
Possnert, G. 1992: I4C dating of samples aged 40,000BP by the
accelerator technique. In Andersen, B. G . & Konigsson, L.-K.
(eds.): Lute Quaternary Stratigraphy in the Nordic Countries
150.000- 15.OOO BP. Striae 34, 59-63.
Ralska-Jasiewiczowa, M., Van Geel, B., Goslar, T. & Kuc. T. 1992:
Duration of the Younger Dryas: evidence from varved sediments
in Poland. 8th Internarional Palynological Congress, Aix-enProvence. 6 - 12 September 1992, Abstracts. 119.
Regnell, J. 1992: Preparing pollen concentrates for AMS dating - a
methodological study from a hard-water lake in southern Sweden.
Boreas 21. 373-377.
Renberg, I. 1978: Palaeolimnology and varve counts of the annually
laminated lake sediment of Lake Rudetjlrn, Northern Sweden.
Eurlj, Norrlund I I , Archueological and Palaeoecologicul Studies in
Medelpud, N . Sweden, Kungligu Viiierheis Historie och Antikvitets
Akailemien, 63-92.
Renberg. 1. & Segerstrom, U. 1981: The initial points on a shoreline
displacement curve for Southern Vasterbotten, dated by varvecounts of lake sediments. In Konigsson, L.-K. & Paabo, K. (eds.):
Florilegium F1orini.s Dedicatum. Striue 14, 174- 176.
Ringberg, B. 197 I : Glacialgeologi och isavsmaltning i ostra Blekinge.
Sveriges Geologiska Undersokning C 661, I 174.
Ringberg. B. 1979: Varve chronology of the glacial sediments in
Blekinge and northeastern Skine, southeastern Sweden. Boreas 8,
209 -2 1 5.
-
Ringberg, B. 1991: Late Weichselian clay varve chronology and
glaciolacustrine environment during deglaciation in southeastern
Sweden. Sueriges Geologiska Undersokning Ca 79, 1-42,
Ringberg, B. & Rudmark, L. 1985: Varve chronology based upon
glacial sediments in the area between Karlskrona and Kalmar.
southeastern Sweden. Boreas 14, 107- 110.
Risberg, J., Miller, U. & Brunnberg. L. 1991: Deglaciation,
Holocene shore displacement and coastal settlements in eastern
Svealand, Sweden. Quaternary International 9, 33 37.
Rozanski, K., Goslar, T., Dulinski, M.. Kuc. T., Pazdur, M. F. &
Walanus, A. 1992: The late Glacial-Holocene transition in central Europe derived from isotope studies of laminated sediments
from Lake Gosciijz (Poland). In Bard, E. & Broecker, W. S.
(eds.): The Last Deglaciation: Absolute and Radiocarbon
Chronologies. NATO AS1 Series, Series I, Global Environmentul
Change, Volume 2, 69-80. Springer-Verlag, Berlin.
Rudmark, L. 1975: The deglaciation of Kalmarsund. southeastern
Sweden. Soeriges Geologiska Undersokning C 713. 1-88,
Sandgren. P., Bjorck. S., Brunnberg. L. & Kristiansson, J . 1988:
Palaeomagnetic records from two varved clay sequences in the
Middle Swedish ice marginal zone. Boreus 17, 215-227.
Stromberg. B. 1985a: New varve measurements in Vastergotland,
Sweden. Boreas 14. 1 I 1 - 1 15.
Stromberg, B. 1985b: Revision of the lateglacial Swedish varve
chronology. Boreas 14, 101- 105.
Stromberg, B. 1989: Late Weichselian deglaciation and clay varve
chronology in east-central Sweden. Sveriges Geologiska Undersokning Ca 73, 1 -70.
Stuiver, M., Kromer, B., Becker, B. & Ferguson, C. W. 1986:
Radiocarbon age calibration back to 13.300 years BP and the "C
age matching of the German oak and US Bristlecone pine
chronologies. Rudiocarbon 28. 969-979.
Stuiver, M., Braziunas, T. F., Becker, B. & Kromer, B. 1991:
Climatic, solar, oceanic and geomagnetic influences on LateGlacial and Holocene atmospheric ''C/'2C change. Quaiernary
Research 35, 1 -24.
Svedhage, K. 1986: Shore displacement during Late Weichselian
and early Holocene in the Risveden area, SW Sweden. Chalmers
tekniska hogskolalGoteborgs universiiet. Geologiska insiirurionen
Rapport 51, 1 - 11 1.
Svensson, N.-0. 1989: Late Weichselian and Early Holocene shore
displacement in the central Baltic, based on stratigraphical and
morphological records from eastern Smiland and Gotland, Sweden. LUNDQUA Thesis 25, 195 pp.
Svensson, N.-0. 1991: Late Weichselian and Early Holocene shore
displacement in the central Baltic Sea. Quuiernury Inrernationul 9,
7-26.
Zolitschka, B.. Haverkamp, B. & Negendank. J. F. W. 1992:
Younger Dryas oscillation - varve dated microstratigraphic.
palynological and paleomagnetic records from Lake Holzmaar.
Germany. In Bard, E. & Broecker. W. S . (eds.): The Last
Deglaciation: Absolute and Radiocarbon Chronologies. NATO AS1
Series, Series I, Global Environmeniul Change, Volume 2. 81 - 102.
Springer Verlag, Berlin.
-