Earth and Planetary Science Letters 255 (2007) 106 – 116
www.elsevier.com/locate/epsl
FENNOSTACK and FENNORPIS: Varve dated Holocene
palaeomagnetic secular variation and relative
palaeointensity stacks for Fennoscandia
Ian Snowball a,⁎, Lovisa Zillén a , Antti Ojala b , Timo Saarinen c , Per Sandgren a
a
GeoBiosphere Science Centre, Department of Geology, Quaternary Sciences, Lund University, Sölvegatan 12, SE-223 63 Lund, Sweden
b
Geological Survey of Finland, PO Box 96, FIN-02151 Espoo, Finland
c
Department of Geology, University of Turku, FIN-20014 Turun yliopisto, Finland
Received 23 July 2006; received in revised form 11 December 2006; accepted 11 December 2006
Available online 15 December 2006
Editor: R.D. van der Hilst
Abstract
A Holocene palaeomagnetic secular variation master curve (FENNOSTACK) and a relative palaeointensity curve
(FENNORPIS) for Fennoscandia are presented. These curves were produced by stacking palaeomagnetic data obtained from
six annually laminated (varved) lake sediment sequences and one non-laminated sediment sequence. The six independent varve
chronologies were combined to form a timescale, which extends between the present year and 10,200 Cal yr BP. Smoothed
inclination and declination curves show trends that are similar to a palaeomagnetic secular variation curve for the United Kingdom,
except that the ages of most of the statistically significant features are a few hundred years younger in the Fennoscandian data set.
These differences are most likely due to excessively old ages of the United Kingdom features produced by the radiocarbon dating
of bulk sediments. The standardized relative palaeointensity stack shows that the intensity of the geomagnetic field in Fennoscandia
was highest at 2300 Cal yr BP and lowest at 7000 Cal yr BP. Three palaeointensity peaks (PIPs) and three palaeointensity lows
(PILs) are identified in FENNORPIS and indicate that the intensity of geomagnetic field varied significantly on millennial scales in
this region during the Holocene, but there is much scatter in the reconstruction prior to 8000 Cal yr BP. Higher frequency
palaeointensity features present in individual records are not significant in the stacked data set, which implies that these are a result
of local environmental bias. Significant discrepancies exist between the data and the output of the CALS7K.2 geomagnetic field
model for the last 7000 yr. In particular, the largest declination swing observed in the data at 2670 Cal yr BP (feature “f”) is not
produced by the model, which is most likely due to the influence of an inaccurately dated Icelandic secular variation curve on the
model result. Short term trends in standardized relative palaeointensity do differ from the model output, but it is difficult to make a
comparison because of the relative nature of the sediment record. Better absolute palaeointensity data are needed to reduce
uncertainties in reconstructions of past geomagnetic field strength.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Holocene; palaeomagnetism; secular variation; relative palaeointensity; Sweden; Finland; varves
⁎ Corresponding author. Tel.: +46 46 2223952; fax: +46 46 2224830.
E-mail addresses: [email protected] (I. Snowball), [email protected] (L. Zillén), [email protected] (A. Ojala), [email protected]
(T. Saarinen), [email protected] (P. Sandgren).
0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2006.12.009
I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
1. Introduction
Palaeomagnetic reconstructions, which are based on
the natural remanence acquired by igneous rocks,
archaeological artifacts and sediments at the time of
their formation, are the only independent means of
reconstructing the pre-historical variability of the
geomagnetic field. Recent achievements in ocean
sediment recovery [1] have improved reconstructions
of the geomagnetic field during the majority of the
Bruhnes chron (0–780,000 yr BP). Reconstructions of
field behaviour for the current Holocene epoch,
however, are dominated by studies of continental lake
sediments because they are relatively easier to recover
intact and can often be dated with better accuracy and
precision than marine sediments.
Palaeomagnetic intensity data [2] and the output of
time varying geomagnetic field models [3] have also
been used to correct radionuclide production for the
variable geomagnetic field, thus producing different
reconstructions of solar activity in the Holocene [4–6].
The largest source of uncertainty in these reconstructions
is our incomplete knowledge of how the geomagnetic
field varied in the past. New data are, therefore, required
to improve the confidence of palaeomagnetic reconstructions and constrain time varying geomagnetic field
models.
Two and a half decades ago, Turner and Thompson
[7] reconstructed a Holocene palaeomagnetic secular
variation master curve for Britain (United Kingdom)
which was based on multiple sediment cores collected
from three lakes. More recent studies of lake sediments
[e.g. 8] and marine sediments [e.g. 9] continue to
improve our understanding of changes in the direction
of the geomagnetic field. However, it is still rare that
reconstructions integrate data obtained from multiple
sites within one region and include a complete
reconstruction of the geomagnetic vector, i.e. of the
direction and intensity, the latter as relative palaeointensity (RPI). Over the past decade we have made an
intensive effort to reconstruct the behaviour of the
Holocene geomagnetic field by studying a series of
annually laminated (varved) lake sediments that exist in
Finland and Sweden [10–14]. These studies have
generated a set of seven varve dated records of
palaeomagnetic secular variation, which include varve
dated records of RPI that conform to established
reliability criteria [15,16]. The study of an eight,
partially varved site has provided a secular variation
and RPI data set dated by the radiocarbon method and
the transference of the PSV features from a varve dated
master curve [17]. We present here stacked palaeomag-
107
netic secular variation and RPI master curves of seven of
these sites, which are based on the multiple varve
chronologies (two of the varve chronologies are
supported by radiocarbon dating and tephrochronology). These data were not used to constrain the time
varying CALS7K.2 geomagnetic field model [18] and
thus can be used to test the validity of its results for
Fennoscandia.
2. Regional setting
The locations of the seven sites from which we have
used data are shown in Fig. 1. The inset in Fig. 1 shows
the location of these sites in relation to the three United
Kingdom sites studied by Turner and Thompson [7].
The Fennoscandian network extends between 12 and
25° East (13° of longitude) and between 57 and 64°
North (7° of latitude) and, therefore, covers a considerably larger area than the United Kingdom master
curve (2° of longitude and 4° of latitude). According to
the International Geomagnetic Reference Field (IGRF)
the geographical spread of Fennoscandian sites amounts
to about 5° in inclination and 4° in declination.
We have not included inclination and declination
data from Korttajärvi [14] because the sediment
accumulation rate was much faster and more variable
than in any of the chosen sites. With the exception of the
Byestadsjön sequence, which is partially varved in the
early Holocene, the sediments in the lakes are composed
of continuous biogenic/clastic varves. These varves
have formed since isostatic uplift during the Holocene
caused these basins to be isolated from much larger
freshwater and brackish basins that existed after the last
deglaciation. Petterson [19] has reviewed the processes
that lead to freshwater varve formation in this boreal
environment.
3. Materials and methods
The palaeomagnetic data sets used in this study are
listed in Table 1, which includes information about the
location, dating methods, the technique used to
reconstruct relative palaeointensity, the number of
cores from each site and the original reference. A total
of 14 sediment piston-cores contribute to the compilation. All palaeomagnetic measurements were carried out
on discrete samples (2 × 2 × 2 cm) with 2G-Enterprises
superconducting rock magnetometers at the Department
of Geology, University of Lund and the Geological
Survey of Finland, respectively. With the exception of
Byestadsjön, each site has an independent calendar year
timescale that is based on varve counting. The varve
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I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
Fig. 1. The map shows the positions of the seven lake sediment sequences in Sweden and Finland that contribute to this study. The inset shows the
positions of these sites with respect to the three sites that form the basis of a palaeomagnetic master curve for the United Kingdom [7].
chronologies are based on sites where varves continue to
form and are, therefore, fixed to the present year.
Sediment accumulation rates have ranged between 0.3
and 0.7 m/ka in these sites. Henceforth, we use an
annual timescale where the zero calendar year before
present (0 Cal yr BP) is AD 1950, which facilitates
comparison to calibrated radiocarbon ages. The total age
span of the stacked record is between 10,200 and −
30 Cal yr BP (where − 30 Cal yr BP is AD 2000). As the
aim of this study, however, was to produce a secular
variation master curve based on multiple sites, our final
results extend over a common base period for which
there are reliable data from at least three of the sites
(401–9714 Cal yr BP for inclination and declination,
and 161–9845 Cal yr BP for relative palaeointensity).
3.1. Compiled data set
The frequency histogram in Fig. 2 shows the
distribution of the samples according to bins of 500 yr.
The sample population (n) for the inclination and
declination sub-set data set (Appendix A) is 1891, while
Table 1
Sites and data references
Site name
Location
Dating methods
RPI method
No. of cores References
Sarsjön
Frängsjön
Nautajärvi
Alimmainen Savijärvi
Furskogstjärnet
Mötterudstjärnet
Byestadsjön
64°02′ N, 19°36′ E
64°01′ N, 19°42′ E
61°45′ N, 24°24′ E
61°48′ N, 24°41′ E
59°23′ N, 12°05′ E
59°40′ N, 12°40′ E
57°24′ N, 15°18′ E
Varve chronology
Varve chronology
Varve chronology
Varve chronology
Varve chronology, 14C, tephra
Varve chronology, 14C, tephra
14
C and PSV correlation
NRM/ARM at 40 mT
NRM/ARM at 40 mT
NRM/ARM at 20 mT
–
NRM/ARM at 40 mT
NRM/ARM at 40 mT
Pseudo-Thellier and NRM/ARM at 30 mT
2
2
1
1
2
2
4
[12]
[12]
[11]
[14]
[13]
[13]
[17]
I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
109
Fig. 2. Two histograms that show the number of palaeomagnetic sub-samples in each 500-year bin for the last 10,000 yr. (A) shows the number of
samples for palaeomagnetic directions and (B) shows the number of samples for relative palaeointensity. More than 75 data points contribute to each
500-year bin during the period between 9000 and 500 Cal yr BP.
the relative palaeointensity sample population is 1756
(Appendix A). The period between 8500 and 8000 Cal
yr BP contains the most data points (∼ 125) because of
relatively high sediment accumulation rates (thickest
varves) that occurred in the first centuries after lake
basin isolation, while the periods earlier than 9000 Cal
yr BP contain less than 70 data points due to the
restricted age of individual records.
3.2. Palaeomagnetic directions and statistics
Published palaeomagnetic directions (inclination and
declination) were available for 14 individual cores and
the raw data from each site are shown in equal area
stereo plots in Fig. 3. Lakes that are situated within 1° of
latitude and longitude of each other were considered as
single sites and these data form sub-sets. The cores were
not recovered with respect to a fixed azimuth and the
declination data were subsequently calculated as deviations from their respective core averages. References
given in Table 1 contain details of de-trending corrections made for probable core rotations. Statistics
relating to the individual data sets are shown in Table 2
(Palaeomag-Tools version 4.2a provided by Mark
Hounslow, Lancaster University). No corrections have
been made for potential shallow inclinations, which can
be expected to affect detrital remanent magnetizations,
because none of the inclination averages deviated more
than 5° from the inclination expected by a geo-axial
dipole (GAD) at the respective site latitude (Fig. 3 and
Table 2). It is not possible to determine what correction,
if necessary, should be applied and it is possible that
differences in the distributions in Fig. 3 could be due to
other processes (e.g. coring methods, sub-sampling bias
and analytical precision). We do not present a stacked
data set based on transformation to virtual geomagnetic
poles (VGPs) because this procedure would be based on
the assumption of a GAD and could provide a false
image about geomagnetic pole movement. As the
geographical spread of sites should lead to differences
in absolute inclination and declination values of a few
degrees the conversion to deviations from core averages
was made before the four sub-sets were amalgamated.
Thus, the inclination data were converted to deviations
from their respective core average and then the data
points for inclination and declination (n = 1891) were
stacked according to their varve ages.
3.3. Relative palaeointensity (RPI) and remanence
carriers
The mineral magnetic characteristics of the Swedish varved lake sediments suggest that single domain
magnetite crystals produced by magnetotactic bacteria
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I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
Fig. 3. Equal area stereoplots that show the distribution of the characteristic inclination and declination data obtained for the seven sites. Declination
data were available as deviations from the respective core averages. Data obtained for sites located within 1° of latitude and longitude are shown
together (A) Sar–Fra = Sarsjön and Frängsjön, (B) Fur–Mot = Furskogstjärnet and Mötterudstjärnet, (C) Naut–Ali = Nautajärvi and Alimmainen
Savijärvi, (D) Bye = Byestadsjön).
carry the majority of the natural remanent magnetization (NRM) in the post-isolation sediments [13,20,21].
No detailed mineral magnetic investigations of the
Finnish sites have been carried out, but the median
destructive fields of NRMs and anhysteretic remanent
magnetizations (ARMs) of pilot samples were approximately 40 mT, which indicates that similar sized
magnetite particles carry the NRM in these sites as
well. It is also notable that the NRM intensity of the
samples from all the sites was of similar magnitude to
the anhysteretic remanent magnetization (ARM)
acquired in a direct current bias field of 0.05 mT
superimposed on a peak alternating field of 100 mT,
which indicates that these sediments are efficient
recorders of the geomagnetic field. The RPI was
determined in all cases by normalization of the NRM
Table 2
Palaeomagnetic statistics
Sub-stack
N
Sar–Fra
Fur–Mot
Naut–Ali
Bye
549
495
387
458
Mean declination
Mean inclination
Fisher
Fisher
0.3
358.5
359.6
1.1
76.9
69.6
70.1
68.7
Alpha
95
Mean inclination
Alpha
95
0.5
0.5
0.6
0.5
76.0
69.2
69.4
68.3
0.6
1.0
0.8
0.5
Inclination
GAD
76.3
73.3
74.5
72.0
I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
to the ARM (precise details of the normalization
procedures are provided in the references in Table 1)
and relative palaeointensity estimates have been
published for 13 of the 14 sediment cores. These
data (n = 1756) were standardized according to the
mean and standard deviation associated with each core
and then stacked according to their varve age.
3.4. Smoothing routine
Scatter in stacked palaeomagnetic data obtained
from sediments can originate from true changes in
geomagnetic field direction and intensity, biased
recording of the NRM caused by environmental
changes and sedimentation processes, sediment coring, extrusion and analytical error. A further source of
error is also introduced when multiple records are
stacked according to the ages of individual samples,
because no geochronology is perfect and each varve
chronology used here contains some inaccuracy and
imprecision. However, for the purposes of this study
we implicitly trust the six independent varve chronol-
111
ogies and do not make any adjustments of the time–
depth relations based on inclination, declination and
RPI correlations (with the exception of the partially
varved Byestadsjön sequence). Thus, we avoid as far
as possible the potential reinforcement of high
frequency features that were not caused by geomagnetic field changes. We reduced the high frequency
scatter by using a smoothing routine designed
specifically for palaeomagnetic data, which provides
95% confidence limits [22]. The two data stacks (one
for directions and one for RPI) were smoothed using a
running time window of 150 yr, which was chosen
because it is equivalent to the maximum error in the
individual varve chronologies (see references in
Table 1) and 100–200 yr is considered to be a good
estimate of the average lock-in delay of the NRM
associated with the sediment sequences studied [e.g.
12]. The CALS7K.2 geomagnetic field model has a
resolution of approximately 100 yr [18] and although
the use of progressively longer period filters can
improve confidence, it also introduces the risk that
higher frequency secular variation is removed.
Fig. 4. Relative declinations (A) and relative inclinations (B) obtained for the seven sites plotted against their varve age (in Cal yr BP, where the 0 yr is
AD 1950). (C) shows the standardized RPI values obtained for six of the seven sites (see Table 1). The individual site data are shown as three point
moving averages in D, E and G (site labels as Fig. 3). Note the difference in the declination scale between A and D.
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I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
4. Results
4.1. FENNOSTACK and FENNORPIS: inclination and
declination, and relative palaeointensity data stacks for
Fennoscandia
Fig. 4 shows the stacked declination (A), inclination
(B) and standardized RPI (C) data sets plotted against
calendar years. Three point moving averages of the
individual site data are shown in Fig. 4D–G. The
majority of the raw declination data does not vary more
than ± 50° from the average of zero. Exceptions are
related to periods of high inclination at 8000, 3000 and
1000 Cal yr BP, respectively. Apparently anomalous
declinations at high latitude sites can be caused by
relatively small vertical offsets in core penetration and
sub-sampling, which can cause the declination to rotate
to the opposite hemisphere (see Fig. 3). The raw
inclination data (Fig. 4B) and individual site data
(Fig. 4E) clearly indicate that the steepest values
occurred during the last 3000 yr and the shallowest
values at approximately 9000 Cal yr BP. The standardized relative palaeointensity data (C) appear more
scattered than the directional data, but there is a distinct
maximum between 3000 and 2000 Cal yr BP and a
distinct minimum at approximately 7000 Cal yr BP.
Fig. 5 shows the result of applying the 150-year
smoothing routine to the stacked data, with 95%
confidence limits indicated. We call the smoothed
directional data set “FENNOSTACK” (Appendix A),
and the smoothed RPI stack “FENNORPIS” (Appendix
A). Despite a systematic age difference between the UK
master curve and the North Sweden master curve [12]
prior to 3000 Cal yr BP the pattern of secular variation
Fig. 5. The result of smoothing the data shown in Fig. 4 with a 150year running window. Grey lines show the 95% confidence limits
associated with the declination (A), inclination (B) and standardized
RPI (C). Solid and dashed vertical black lines show the positions of
maxima and minima, respectively, which have been labeled according
to the nomenclature of the United Kingdom palaeomagnetic secular
variation curve [7]. In (C) odd and even numbers represent significant
RPI peaks and lows, respectively.
over the common base period is the same as the United
Kingdom master curve and, therefore, we apply the
nomenclature of Turner and Thompson [7] to label the
Table 3
Ages of inclination and declination features in the Fennoscandian PSV master curve, their UK counterparts and the deviation between them
Declination
feature
a
b
c
d
e
f
g1
h
i
j
Fennostack
(Cal yr BP)
UK
(Cal yr BP)
Fennostack–UK
(yr)
Inclination
feature
850
2210
2670
5500
6610
7920
9410
150
450
600
1150
1950
2705
5550
7165
8400
–
–
–
–
− 300
− 260
− 35
− 50
− 555
− 480
α
β
γ
δ
ε1
ε
ζ
η
θ
ι
K
λ
μ
ν
Fennostack
(Cal yr BP)
UK age
(Cal yr BP)
Fennostack–UK
(yr)
1290
1840
2590
3090
–
–
4720
5400
6290
7890
9000
9780
250
650
1250
1650
?
3270
4000
4200
5200
6000
7000
8300
8800
–
–
–
40
190
−180
−480
−600
−610
−410
200
I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
statistically significant declination and inclination
features that we identify in FENNOSTACK. Table 3
and Fig. 6 compare the ages of these features and show
that the inclination features between 8000 and 4000 Cal
yr BP are approximately 700 yr younger in FENNOSTACK compared to the United Kingdom master curve.
The oldest inclination feature and the two youngest
features are slightly younger in the United Kingdom
master curve. We assign numbers to six significant
relative palaeointensity features, where odd numbers
denote palaeointensity peaks (PIPs) and even numbers
palaeointensity lows (PILs). The ages of these features
are shown in Table 4. We also label an earlier peak
(PIP7) but this should be treated cautiously because the
confidence is poor.
113
Table 4
Ages of relative palaeointensity features in FENNORPIS
Palaeointensity peak
(PIP)
FENNORPIS
(Cal yr BP)
1
900
1400
2300
5400
6300
7200
8800 (?)
3
5
7 (?)
Palaeointensity low
(PIL)
2
4
6
predictions for the last 7000 yr were standardized and
linear interpolation was applied to the FENNOSTACK
and FENNORPIS data to provide time points consistent
4.2. FENNOSTACK and FENNORPIS compared to
CALS7K.2
Fig. 7 shows the FENNOSTACK inclination and
declination data next to the results of the CALS7K.2
geomagnetic field model [18]. The modeled inclination
and declination data obtained for the four site positions
(Table 1) were converted to deviations from the average
direction for the last 7000 yr, which produced four
values for each time point at 10-year intervals (the range
of values is shown by the blue shaded band in Fig. 7).
The FENNORPIS data and the CALS7K.2 intensity
Fig. 6. A comparison between the ages of the inclination (crosses) and
declination (open circles) features identified in FENNOSTACK and
the UK palaeomagnetic master curve [7]. The grey dotted line shows
where the ages would be equal. The majority of the features identified
prior to 3000 Cal yr BP are between 400 and 600 yr younger in
FENNOSTACK compared to the UK data set (see Table 3).
Fig. 7. A comparison between the output of the CALS7K.2
geomagnetic field model [3] and the FENNOSTACK and FENNORPIS data for Fennoscandia. Inclination (A) and declination (A and B)
were calculated as deviations from their respective averages for the last
7000 yr, while the RPI data were standardized over the same period
(C). CALS7K.2 predicts a false declination feature between 4500 and
3900 Cal yr BP, and it fails to predict the most significant declination
feature (f) observed in FENNOSTACK at 2650 Cal yr BP. CALS7K.2
fails to predict the period of steepest inclinations between 3200 and
2700 Cal yr BP, and it underestimates inclination for much of the last
3000 yr. The multi-millennial scale trends in RPI are similar between
FENNORPIS and CALS7K.2, but there are differences at higher
frequencies. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
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I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
with the model output. The four model predictions
generated for each time point were averaged, and the
differences between the model and the data calculated.
In Fig. 8 we show the differences between the
CALS7K.2 model and FENNOSTACK (including
FENNORPIS) for the last 7000 yr. The average errors
in the model results for inclination and declination are
shown by dashed grey lines in Fig. 8A and B. Features
that exceed these limits cannot be explained by
uncertainties in the model or the data.
Figs. 7 and 8 show that CALS7K.2 produces an
easterly declination feature with maximum amplitude of
about 30° between 4500 and 3900 Cal yr BP. This is the
highest amplitude feature predicted by the model, but it
is not observed in the FENNOSTACK data. The
modeled declination does not produce a fit to the
westerly declination feature f, which is the most
significant feature in the data and it appears to
overestimate the range of inclination changes for the
past 3000 yr.
Fig. 8. The calculated differences between the CALS7K.2 model
output and FENNOSTACK declination (A), FENNOSTACK inclination (B) and standardized palaeointensity (C). The dashed grey lines in
(A) and (B) indicate the average errors included in the CALS7K.2
model [18]. The solid black line in (C) shows the difference between
the standardized global dipole moment predicted by CALS7K.2 and
FENNORPIS. The largest differences between observed RPI and the
model are reflected in both regional and global output.
It is difficult to compare RPI data obtained from
sediments to absolute values obtained from ceramics
and igneous rocks. It is important to note, however, that
the major differences between the standardized RPI data
obtained from FENNORPIS and the standardized results
of CALS7K.2 do not depend on the use of regional or
global model output (Fig. 8C). The largest consistent
difference occurs between 4500 and 3500 Cal yr BP.
5. Discussion
FENNOSTACK is the first Holocene palaeomagnetic
secular variation master curve that includes a relative
palaeointensity master curve (FENNORPIS) based on
multiple sites with independent dating. The timescale is
based on six independent and continuous varve
chronologies that are fixed to the present. The compiled
timescale also includes two varve chronologies (Furskogstjärnet and Mötterudstjärnet) that are supported by
tephrochronology and radiocarbon dating of terrestrial
macrofossils [23] and we consider the combined
timescale to have an accuracy of better than 2% at any
time point. Potential users of the stacked data must
consider that the geomagnetic signal was potentially
smoothed during natural lock-in of the natural remanent
magnetization and that we have used statistics to reduce
the scatter in the stacked records. These two smoothing
processes imply that the reconstructions underestimate
the amplitude of the real changes.
It is notable that many of the inclination and
declination features observed in FENNOSTACK are
significantly younger (by some centuries) than their
United Kingdom counterparts. These differences are
most likely due to the radiocarbon dating of bulk
sediments applied by Turner and Thompson [7], which
frequently leads to overestimated ages caused by a local
carbon reservoir effect. The age differences are too large
to be explained solely by westward drift of higher term
components of the geomagnetic field. It was also noted
that the interpolated calibrated bulk sediment radiocarbon ages of the PSV features in Byestadsjön were
slightly older than ages based on PSV correlation to the
North Sweden master curve, but that the majority of the
differences could be explained by dating uncertainties
[17] and that the differences were absent in the older
laminated sediments. An alternative explanation is that
the lock-in depth of the NRM is significantly deeper in
bioturbated (and therefore non-varved) lake sediments
compared to varved sites where bioturbation is not
detectable, leading to apparently older ages. Despite
these uncertainties, the FENNOSTACK/FENNORPIS
master curves provide an alternative method of dating
I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
lake and marine sediments in and around the Scandinavian and Baltic countries, but only if they contain a
valid palaeomagnetic signal.
There are significant differences between our stacked
data set and the results of the CALS7K.2 model for the last
7000 yr (Fig. 8). These differences must be discussed
because they show that the model cannot provide a good
fit for an area near central- and western Europe, which can
be considered to be relatively densely covered by data.
The differences imply either that (i) the data sets which
constrain the CALS7K.2 output are unsuitable due to poor
recording of the geomagnetic field and/or dating errors or
(ii) the model cannot solve the time evolution of the
magnetic field sufficiently to make predictions for regions
where there are limited data. Korte and Constable [18] do
reject some data sets because they do not fit the model
results, but this does not apply to Fennoscandia. Despite
the poor fits of inclination and declination at certain times,
we cannot logically reject any of the individual data used
to produce FENNOSTACK because the varve chronologies are independent and geologically robust, the natural
remanent magnetizations were strong and stable and the
confidence limits for the compiled palaeomagnetic data
sets are relatively small. Data from only two Fennoscandian lakes (Vuokonjärvi and Pohjajärvi) were used to
constrain the CALS7K.2 model [18]. Of these two,
Pohjajärvi was dated by varve counting but the record
extends only to 3200 Cal yr BP [10]. The Vuokonjärvi
timescale is based on pollen stratigraphy [24], which is
not an absolute dating technique and we would not
recommend that the palaeomagnetic data from this site are
used to constrain future models, unless the timescale is
improved. In the case of western Europe the United
Kingdom results [7] fit our data well, except that some
features are too old by a few centuries. Easterly
declination feature “f” is characterized by a large change
in declination and it is distinct in records that extend in
longitude from the United Kingdom [7], through
Fennoscandia [10] to Siberia [25]. This feature appears
to be muted in the stacked record produced from maar
lakes located in Germany further to the south [26].
However, our major concern is that the CALS7K.2 model
result for Fennoscandia is negatively influenced by data
obtained from the Icelandic lake of Vatnalsvatn [27],
which are dependent on a chronology based on the
radiocarbon dating of mineral rich bulk sediments.
Thompson and Turner [27] dated two cores and obtained
an age of approximately 3600 14C BP for feature “f”,
which calibrates to an age range of 3600–4200 Cal yr BP.
The FENNOSTACK varve age for feature “f” is 2650 Cal
yr BP and the oldest age in an individual data set included
in the stack is 3000 Cal yr BP [13]. We conclude that the
115
CALS7K.2 output for Fennoscandia is biased by
Icelandic lake sediment palaeomagnetic data that contain
significant dating errors. Alternatively, the model underestimates the degree of spatial variability in the
geomagnetic field between 5000 and 3000 Cal yr BP.
Inspection of Figs. 7C and 8C shows that there is good
agreement between the millennial to multi-millennial
scale trends in the observed and modeled standardized
palaeointensity estimates. However, our palaeointensity
data are relative and we are unable to discuss systematic
offsets in absolute intensity values, such as those known
to exist between the virtual axial dipole moment (VADM)
obtained from a global compilation of archaeomagnetic
data [1] and the dipole moment (DM) produced by
CALS7K.2. On the other hand, our data do suggest that
there are differences between the millennial scale trends
observed in our data and the results produced by the
model for Fennoscandia. Compared to the long term trend
for the past 7000 yr, our data suggest that the model
overestimates the intensity of the regional magnetic field
prior to 6500 Cal yr BP and over the last 2000 yr. It
significantly underestimates the geomagnetic field intensity between approximately 4500 and 3500 Cal yr BP.
These differences are essentially the same between
FENNORPIS and the global DM results of CALS7K.2.
Global compilations of palaeointensity data are heavily
biased by European data sets, but none of these have the
high temporal resolution provided by FENNORPIS. The
differences between the model and FENNORPIS need to
be solved if the model output is to be used for the
correction of atmospheric radionuclide production for
geomagnetic field changes at sub-millennial scales [6].
We emphasize that our stacked data set does not include
reproducible sub-millennial scale changes in RPI, which
have been used to make inter-hemispheric correlations
between individual marine sediment cores [28]. Correlation between individual cores located in different hemispheres is not justified by our RPI results, which indicate
that sub-millennial scale features in RPI in individual sites
are more likely to be a product of environmental bias than
the geomagnetic field. We have presented an independently dated record of geomagnetic field behaviour that
can improve geomagnetic field models and shown that the
stacking of RPI data from multiple sediment sequences
can remove high-frequency environmental bias often
present in individual sediment sequences.
Acknowledgements
This work has been supported by the Swedish
Research Council, the Nordic Council of Ministers,
the Academy of Finland and the Geological Survey of
116
I. Snowball et al. / Earth and Planetary Science Letters 255 (2007) 106–116
Finland. Raimund Muscheler is thanked for the critical
comments on parts of the submitted manuscript and Cor
Langereis provided suggestions for improvement.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
epsl.2006.12.009.
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