Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
www.elsevier.nl/locate/palaeo
Stable isotope stratigraphy of Holocene speleothems: examples
from a cave system in Rana, northern Norway
H. Linge a, S.-E. Lauritzen a,*, J. Lundberg b, I.M. Berstad a
b
a
Department of Geology, University of Bergen, AlleÂgaten 41, N-5007Bergen, Norway
Department of Geography and Environmental Studies, Carleton University, Ottawa, Canada K1S 5B6
Received 27 August 1999; accepted for publication 8 September 2000
Abstract
High-precision TIMS U-series dates and continuous stable oxygen and carbon isotope pro®les of a 4000 year stalagmite
record from Rana, northern Norway, are presented and compared with data from two other speleothems from the same cave.
The dating results yield ages from 3875 ^ 34 to 296 ^ 3 years before AD2000, with 2s errors from 0.5 to 1%. The overall
growth rate is 35 mm/ka, corresponding to a temporal resolution of 29 years/mm. The stalagmite is tested for isotopic
equilibrium conditions, where all `Hendy' tests, except one, indicate isotopic equilibrium or quasi equilibrium deposition.
Both the stable oxygen and carbon isotope records reveal a strong and abrupt enrichment in the near-top measurements. This
corresponds in time to the opening of a second cave entrance in the late 1960s, which caused changes in the cave air circulation.
The stable oxygen isotope signal is enriched compared to the modern value over the last 300 years, indicating a negative
response to temperature changes. Likewise, the stable carbon isotope record is enriched in this period. However, both of the
stable isotope records are shown to be signi®cantly enriched compared to the isotope ranges displayed by other stalagmites in
the same cave, and this questions the reliability of the proxy records derived from the presented stalagmite. Still, a general good
correspondence of large scale ¯uctuations is found between the three stable oxygen isotope records from this cave. The stable
carbon isotope records show large variations within the cave and are believed to be governed by soil-zone conditions,
percolation pathways and possibly driprates. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Speleothems; TIMS; Holocene; Stable isotopes; Paleoclimate; Norway
1. Introduction
Speleothems or cave calcites are formed when
water, high in CO2 from the soil zone, enters a cave
where the CO2 degasses. If degassing proceeds
slowly, and in a temperature-stable environment,
calcite can be precipitated in isotopic equilibrium
with the parent drip water (Hendy, 1971). Characteristic features of equilibrium deposits are; i) insignif* Corresponding author. Fax: 147-55-58-9417.
E-mail address: [email protected] (S.-E. Lauritzen).
icant changes in the stable oxygen isotope
composition of calcite (d 18Oc) along a single growth
layer, and ii) that any slight variation in d 18Oc does not
correspond with changes in the stable carbon isotopic
composition of calcite (d 13Cc). When this test is
performed on a growth band or horizon, it is referred
to as the `Hendy' test. d 18Oc in such calcite is dictated
by the temperature in the cave and the isotopic
composition of the seepage water (Schwarcz, 1986;
Gascoyne, 1992). d 13Cc is more complex, as carbon
can be derived from both atmosphere, soil zone and
bedrock (Schwarcz, 1986), and it also changes
0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0031-018 2(00)00225-X
210
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
progressively during deposition, causing calcite
formed from the same water drop to become successively heavier (Dulinski and Rozanski, 1990). In addition to stable isotopes, speleothems also act as
archives of other climate proxies, e.g. pollen (Bastin,
1978; Lauritzen et al., 1990; Burney et al., 1994),
organic matter (Lauritzen et al., 1986; Ramseyer et
al., 1997), trace elements (Gascoyne, 1983; Goede
and Vogel, 1991; Roberts et al., 1998), and annual
growth laminae (Baker et al., 1993; Shopov et al.,
1994; Genty and Quinif, 1996). The growth frequency
(e.g. Gordon et al., 1989), as well as the internal
growth rate (e.g. Baker et al., 1998) also offer information. The various proxy records are tied to precise
timescales by uranium-series dating (e.g. Gascoyne,
1992).
If speleothems are to be considered as reliable
paleoclimate proxies then coeval samples from the
same cave should give the same paleoclimate information as should multiple traverses from the same
sample. The aim of the present study is to develop a
better understanding of high latitude speleothems and
to explore the internal agreement of paleoclimate
records from one cave system. This paper presents
stable isotope records from a late Holocene
speleothem that is compared with two other
speleothem records from the same cave system,
Sùylegrotta, in northern Norway.
Studies have suggested that the oxygen isotopic
composition of cave dripwater is constant through
the year, and approximately equal to the mean annual
precipitation outside the cave (Schwarcz et al., 1976;
Yonge et al., 1985); together with the commonly
observed fact that the deep cave temperature is close
to the mean annual temperature outside the cave
(Wigley and Brown, 1976), this yields the fundamental principle on which paleoclimatic studies on
speleothems are based. If deposited in isotopic equilibrium, changes in d 18Oc can be caused by both
temperature changes and changes in the isotopic
composition of seepage water. Variation in the
composition of the seepage water can be caused by
a number of factors. Gascoyne (1992) considered the
three most signi®cant ones to be: (a) changes in the
d 18O of the source of the meteoric precipitation, i.e.
seawater (d 18Osw or DSMOW), due to growth or
decay of continental ice sheets; (b) shift in wind
track causing more or less rain-out of 18O; (c) changes
in temperature of formation of water droplets (ice) in
the atmosphere.
Fluctuations in d 13Cc are often explained by
changes in vegetation (e.g. Dorale et al., 1992,
1998), the most obvious change relating to the photosynthetic pathways. However, high latitude vegetation uses only the C3 photosynthetic pathway and
thus changes in d 13Cc must be explained by other
mechanisms than ¯uctuations of C3±C4 plants.
Various models describing the evolution of d 13Cc
have been proposed (e.g. Hendy, 1971; Wigley et
al., 1978; Dreybrodt, 1982; Dulinski and Rozanski,
1990), and d 13Cc variations for pure C3 vegetation
sites are discussed by Baker et al. (1997). At high
latitudes, the short growing season will cause large
seasonal isotopic variation of the percolation waters,
which in turn complicates our calculation of annual
means in water and in calcite.
The formation of speleothems at high latitudes/altitudes requires suf®ciently ice free conditions to drain
the cave, and the presence of permafrost will impede
water percolation and thus prevent calcite deposition
(Lauritzen, 1995). Signi®cant speleothem growth
demands a certain soil cover as a source of CO2.
These factors causes speleothem deposition in high
latitude /altitude areas to be highly sensitive to
minor climate variations because they grow in environments where any slight climatic deterioration
approaches the limit of formation. In northern
Norway, most of the caves are situated inside the
Younger Dryas ice limit, and hence signi®cant postglacial speleothem growth is not expected to
commence until after this time.
The modern climate (data from DNMI, 1998) of the
study area is characterized by a mean annual temperature of 13 to 148C. November through March
normally have mean monthly temperatures below
08C, while mean monthly temperatures in June and
August are above 1108C. Due to the low temperatures
during winter time, the ground can be frozen for
several months. The mean annual meteoric precipitation is about 1400±1500 mm, and the typical precipitation pattern displays somewhat low monthly mean
rainfall from February through August, and heavy
precipitation from September through January.
Hence, the majority of the meteoric precipitation is
received both at the end of, and after the growth
season, and as snow. The combination of high
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
Fig. 1. a, Survey of Sùylegrotta (plan map) with sample locations, scale is 50 m; b, inset: map of Scandinavia, arrow indicates the location of the Rana area; c, inset: schematic sketch
of a vertical cross section of Sùylegrotta, indicating the growth positions of the SG-stalagmites. The size of the marble band is greatly exaggerated. The near-horizontal broken line
represents the waterlogged part of the cave, with a spring positioned at the solid circle. Vertical, thick broken lines, and the thick line in the contact between marble and schist,
indicate possible percolation pathways.
211
212
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
Fig. 2. a, Photo of SG95 stalagmite sampling slice; b, sketch of the sampling slice showing the vertical growth axis (mm scale).The hiatus at
,23 mm is indicated with a dash-dotted line, solid lines are growth horizons (A±D, S) sampled for the `Hendy' test. Broken lines indicate other
visible growth bands. Grey boxes represents the intervals dated with TIMS. A rock fragment at the base is represented by the heavily shaded
zone.
amounts of snow and frozen ground during the winter
is of importance for speleothems: it can cause discontinuous dripping in the cave during winter, and high
drip rates in spring related to snow melting. Moreover,
the low mean annual temperature causes the
speleothem growth to be overall slow.
2. Material and methods
2.1. Sample site and previous studies
The stalagmite SG95 presented in this study was
collected in 1995 from the middle part of Sùylegrotta
(`SG-cave', Fig. 1a), but since being monitored for
drip rate variations, calcite deposition was halted
from the summer of 1991. The dripping of the SG95
feeder was continuous in the period 1991±1993, with
an average drip rate of 14±16 drips/h. The cave
entrances are situated at 280 m a.s.l. in a small depression in the side of the Dunderlandsdalen, some 10 km
east of Mo i Rana, and approximately 20 km south of
the Arctic Circle (Fig. 1b). The cave is developed in a
2±10 m thick, steeply dipping band of calcite marble,
between rocks of ferruginious mica schist. The roof
thickness, and therefore the approximate length of the
percolation pathway, is between 0 and 100 m, depending on the position within the cave. Until the late
1960s, when a collapsed entrance was opened by
digging, the only natural entrance was through a
streamsink shaft and the cave was draught-free. The
opening of the second entrance created a weak seasonal draught in the upper parts of the cave. According
to Norwegian standards, the cave is well decorated
with speleothems.
Previous studies from the cave system include
both speleothem studies and monitoring of the cave
microclimate. Four speleothem samples from the
cave have been analysed with TIMS U-series dating
and stable isotopes (Berstad and Lauritzen, 1998;
Lauritzen and Lundberg, 1999). Two of these will
be presented later and compared with the present
Lab
no.
mm from
base
238
U conc. (ppm)
232
Th
conc.
(ppm)
234
U/ 238U
230
Th/ 234U
197
198
214
215
200
201
209
216
202
210
30±35
39±44
50±55
65±70
75±80
90±95
115±120
128±133
137±142
155±159
1.1055 ^ 0.0005
1.2754 ^ 0.0006
1.2589 ^ 0.0005
1.0586 ^ 0.0010
1.0836 ^ 0.0004
0.9246 ^ 0.0005
1.0567 ^ 0.0005
0.6254 ^ 0.0003
0.8686 ^ 0.0004
1.1132 ^ 0.0005
0.389
0.327
1.365
0.226
0.150
0.288
0.852
0.227
0.074
0.102
1.7142 ^ 0.0021
1.7156 ^ 0.0020
1.7057 ^ 0.0019
1.7012 ^ 0.0041
1.6955 ^ 0.0019
1.6991 ^ 0.0020
1.7098 ^ 0.0019
1.7192 ^ 0.0023
1.7414 ^ 0.0022
1.7142 ^ 0.0021
0.0351 ^ 0.0003
0.0321 ^ 0.0002
0.0290 ^ 0.0002
0.0258 ^ 0.0002
0.0228 ^ 0.0002
0.0196 ^ 0.0001
0.0156 ^ 0.0001
0.0090 ^ 0.0001
0.00575 ^ 0.00004
0.00270 ^ 0.00003
230
Th/ 232Th
23.1 ^ 0.3
29.0 ^ 0.3
11.7 ^ 0.1
52.0 ^ 0.5
38.2 ^ 0.4
14.6 ^ 0.2
8.6 ^ 0.1
10.9 ^ 0.1
15.8 ^ 0.2
13.2 ^ 0.2
234
U/ 232Th
658 ^ 9
905 ^ 9
404 ^ 4
2019 ^ 24
1680 ^ 20
745 ^ 10
553 ^ 6
1201 ^ 15
2751 ^ 39
4876 ^ 80
Age (years
before AD2000)
Age (years
AD/BC)
3875 ^ 34
3538 ^ 17
3201 ^ 19
2835 ^ 20
2501 ^ 17
2148 ^ 15
1712 ^ 7
989 ^ 7
629 ^ 5
296 ^ 3
1875 ^ 34BC
1538 ^ 17BC
1201 ^ 19BC
835 ^ 20BC
501 ^ 17BC
148 ^ 15BC
AD288 ^ 7
AD1011 ^ 7
AD1371 ^ 5
AD1704 ^ 3
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
Table 1
TIMS U-series dating results from sample SG95
213
214
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
Fig. 3. The positions and sizes of the dated intervals are plotted against the obtained ages. The analytical 2s errors are narrower than the
symbols except for the basal date. The broken line represents the overall growth rate. Symbols in circles indicate dates which are rejected from
the further work (see text for explanation).
study. Einevoll and Lauritzen (1994) found that,
beneath 50±100 m of rock, annual variations were
suf®ciently damped to give reliable mean annual estimates of surface temperature and stable isotopic
composition of the meteoric precipitation. The cave
temperature is 12.8 ^ 0.328C. The mean annual
temperature at a nearby meteorological station
(Nerdal, 31 m a.s.l., 1966±1989) is 13.6 ^ 1.08C,
with mean monthly temperatures ranging from
258C (January) to 113.08C (July) (DNMI, 1998).
Correction for the adiabatic lapse rate (0.68C/100m)
over the 150 m altitude difference between the
speleothem site and the meteorological station yields
a mean annual surface temperature of 2.7 ^ 1.08C. In
the period from 1991 to 1993, the average d 18O value
(SMOW) of stalactite drips (10 stations) was
210.46 ^ 0.20½, while the atmospheric precipitation collected at the surface varied from 23 to
221½ (SMOW) (Einevoll and Lauritzen, 1994).
2.2. Sample description
The SG95 stalagmite (Fig. 2a,b) is 168 mm along
the vertical growth axis, with a basal diameter of
about 70 mm. It is composed of calcite with a
dense, macrocrystalline fabric. The lower 23 mm is
distinctive with light brownish calcite displaying visible layering. A distinct hiatus at about 23 mm, where
the growth axis switches its position, separates the
basal part from the main part of the stalagmite. The
main part, 24±168 mm, is characterized by overall
translucent calcite with a few visible white, opaque
bands, except for the intervals 109±138 and 161±
165 mm which are white and completely opaque
(but still macrocrystalline). A little detritus is
commonly observed in the descending side layers,
but not in the horizontally layered central part. The
remains of an unidenti®ed beetle was found incorporated in descending layers corresponding to a growth
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
axis position of 115±120 mm. The upper 27 mm of
the stalagmite has previously been analysed in thin
section for ¯uorescent/luminescent lamina by Lauritzen and Kihle (1996), distinct laminae varying
between 10 and 40 mm are displayed for the whole
interval.
2.2.1. Analysis
Both stable isotope analysis and TIMS U-series
dating were performed on a single 10 mm thick,
polished centre slice. For stable isotope analysis,
more than 370 samples of approximately 0.1 mg
were drilled at 1 mm intervals with a 0.5 mm dental
drill. Two arbitrary traverses along the growth axis
were sampled to test for reproducibility. The distance
between the two traverses is 1 mm at the base and
increases to 3 mm at the top. The `Hendy' test has
been performed on ®ve individual growth horizons
labelled A±D and S (positions shown in Fig. 2b). A
minimum of four samples were analysed from each
horizon, with a total length of sampled interval from
40 to 120 mm. All stable isotope measurements were
performed at the GMS-laboratory at the University of
Bergen, using a Finnigan MAT 251 mass spectrometer and an automatic on-line carbonate preparation
device (`Kiel device'). Analytical reproducibility of
standard samples is ^0.06½ and ^0.07½ for d 13C
and d 18O respectively. Results are reported as ½ vs.
PDB, using the NIST (NBS) 19 standard as a reference.
Ten subsamples of 4 to 5 mm vertical thickness (grey
boxes, Fig. 2b), weighing 0.7±1.4 g have been dated by
the TIMS uranium-series dating technique, using
chemical preparation procedures and computer algorithms by Lundberg (1997) and Lauritzen and Lundberg
(1997). All subsamples were analysed at the University
of Bergen on a Finnigan MAT 262 mass spectrometer.
The U-series method yields age estimates in calendar
years before analysis year. In this paper, all ages are
reported as years before AD2000.
3. Results
3.1. U-series dating
3.1.1. Chronology
The TIMS U-series dating results are shown in
Table 1. Ratios are activity ratios, errors are 2s , and
215
ages are reported as years before AD2000. All ages
are in stratigraphic order, shown graphically in Fig. 3,
where the analytical 2s errors of 0.5 to 1% are
narrower than the size of the symbols. The dates are
all from positions above the hiatus (at 23 mm), and
range from 3875 ^ 34 to 296 ^ 3 years before
AD2000.
3.1.2. U-series systematics
The 238U concentration (Table 1) displays a rather
high variation, ranging from 0.6 to 1.3 ppm. It
decreases gradually from the base to 130 mm, then
it rises again in the two upper subsamples. The
234
U/ 238U ratio decreases from base to 77 mm, and
then increases up to 140 mm, before it again decreases
in the topmost subsample. Generally, decreasing
values are observed for both 238U content and
234
U/ 238U ratio from base to top in stalagmites, and
are interpreted as gradual leaching of U from the
bedrock surfaces in the ¯ow path. Hence, the observed
increasing trends suggest unstable percolation pathways. The concentration of 232Th, which usually
relates to detrital content, varies from 0.074 to
1.365 ppm in the analysed subsamples. The majority
of them have less than 0.4 ppm, only two subsamples
displaying high concentrations (0.852 and
1.365 ppm). The 234U/ 232Th ratio, which denotes the
proportion of Th to U, is in the range from 400±4880.
3.1.3. Detrital 232Th
The activity ratio of 230Th/ 232Th is between 9 and
52, and a few grains of mica particles are observed in
all subsamples. The 230Th/ 232Th ratio has traditionally
been used as a measure of detrital contamination (e.g.
Schwarcz, 1989). However, in Holocene speleothems
the ratio is not reliable because where 230Th levels are
low it is strongly affected by age. Low 230Th content
invariably give low 230Th/ 232Th ratios, even with
minor 232Th content. To avoid this, detritus in young
samples can be assessed by absolute 232Th content
(values above 1 ppm being considered suspect) and
by the 234U/ 232Th ratio, since it is the proportion of
Th to U which is most important for dating. A low
234
U/ 232Th ratio (e.g. less than 60) is indicative of
detrital content. From this point of view, all raw
ages are acceptable, except the two from subsamples
displaying high 232Th content. The dates are thus not
corrected for the presence of presumed allogenic
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H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
Fig. 4. Details of the variation in growth rate with time. Growth rates between dated intervals are connected. The corresponding con®dence
intervals, positioned between dated intervals, are calculated using the maximum and minimum difference in time and distance between two
subsamples.
230
Th because: (i) all ages are in stratigraphic order,
despite the close sampling and varying amounts of
232
Th, and (ii) commonly used correction factors for
initial Th contamination, such as 1.7 (Kaufman and
Broecker, 1965) and 1.5 (Gascoyne, 1979), may be
arbitrary for the present site, and even not constant
with time. Thus it is preferred to reject the two
samples high in 232Th (circled in Fig. 3), rather than
to use the corrected ages.
3.2. Growth rates and time resolution
The accepted `raw' or uncorrected ages are in
excellent stratigraphic order. This results in an age
model based on eight dates and the assumed date of
AD1991 of the top surface, where each age value is
taken to represent the centre of its respective subsample, and the growth rate between adjacent subsamples
is assumed to be linear.
The calculated growth rates range from 25 to 53 mm/
ka, yielding an overall growth rate of 35 mm/ka. The
corresponding time resolution then varies from 40 to 19
years/mm, with an average of 29 years/mm. The slope
angles in Fig. 3 show that the overall growth pattern is
more or less uniform. Further details of the changes in
growth rate can be seen in Fig. 4 where the growth rates
between dated intervals are shown graphically. High
growth rate is found between ca 2500 to 2150 and 650
to 300 years before AD2000. Low growth rate occurs
between ca 3900±3550, 2850±2500 and 1000±650
years before AD2000.
3.3. Stable isotopes
The measurements from the ®ve growth horizons
tested for isotopic equilibrium (`Hendy' tests) are
shown in Fig. 5. d 18Oc is plotted against mm from
the vertical growth axis, and against d 13Cc. The variation of d 18Oc along one single horizon, expressed as
Dd 18Oc, is 0.18, 0.31, 0.41, 0.6 and 0.75½ respectively for each horizon from the hiatus to top (horizons S, and A±D). A maximum tolerance of Dd 18Oc is
not generally agreed upon, but should be at least twice
the analytical error, i.e. 0.14½. Moreover, the
temporal resolution (growth rate and subsample
size) and the accuracy of the physical sampling (visibility of bands), causes internal variation along one
sampling horizon. For this particular sample, a maximum tolerance of Dd 18Oc should be at least 0.5½,
deduced from the correspondence between the two
traverses (see below). The upper `Hendy' test is therefore considered suspect, but the instability is believed
to be con®ned to the upper two mm, as will be
discussed later. Thus all horizons, except the upper
one, have acceptable Dd 18Oc values, and no signi®cant correlation between d 18Oc and d 13Cc is observed.
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
217
Fig. 5. `Hendy' tests for the ®ve growth horizons at (S) 27, (A) 49, (B) 116, (C) 137 and (D) 167 mm. These positions are also marked on Fig.
2b. d 18Oc is plotted against mm and d 13Cc, and reveals the variation along single horizons and possible correlations between d 18Oc and d 13Cc.
Note that the d 18Oc scales are inverted.
Hence, the SG95 stalagmite is considered to be deposited in isotopic equilibrium (or quasi equilibrium)
with its dripwater, and can thus be adopted for paleoclimatic studies.
As noted previously, two parallel traverses of
stable isotopes were measured at 1 mm intervals
along the growth axis. Fig. 6a shows the variation
of d 18Oc and d 13Cc for both traverses (traverse 1 bold
line, traverse 2 thin line) from base to top of the
stalagmite. In addition, the positions of the 'Hendy'
tests, and the positions, sizes and results of TIMS
analyses are shown. Theoretically, the two traverses
should be identical within twice the instrumental
error, provided they are replicates of exactly the
same calcite deposit. Fig. 6a shows that this was in
fact rarely the case, and this is even more evident
when comparing the 5 point running means from
the two traverses (traverse 1 bold line, traverse 2
thin line, Fig. 6b). However, the two traverses
display similar patterns for both isotopes. More
than 80% of the replicates display a difference of
less than 0.3½ for d 18Oc and less than 0.8½ for
d 13Cc. Furthermore, 97% of the d 18Oc measurements
differ with less than 0.5½. Inconsistency between
corresponding measurements can be explained by
inherent error in drilling position and/or the
morphology of the growth bands. The results of
this reproducibility test suggest that caution be exercised in detailed interpretation of high resolution
records, echoing Williams et al. (1999) warning
that individual variations are meaningless, one can
only trust the shift in the means. In the following
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H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
Fig. 6. Stable isotopes against mm along the growth axis from base to top. Note that both of the stable isotope scales show heavier values
downwards and lighter values upwards. Vertical dotted lines indicate the positions of the `Hendy' tests, and the hiatus at 23 mm is indicated by
a vertical broken line. The horizontal bars mark the position and width of the TIMS analyses. Ages (in years before AD2000) are given above
each bar. a, Raw data. The solid lines represent the ®rst traverse and the thin lines the second; b, 5 point running means for traverses 1 (thick
line) and 2 (thin line); c, the thin lines with circles are the combined curves of traverses 1 and 2 (in Fig. 6a), while the bold lines are the 5 point
running means of the combined data.
analysis, the two traverses were combined, so that
mean values for the two traverses are used for both
d 18Oc and d 13Cc (Fig. 6c).
The age of each isotope measure point is obtained by
interpolation between two dated positions (cf. growth
rate). The d 18Oc and d 13Cc trends with time are shown in
Fig. 7, where thin lines represent the combined records
from Fig. 6c, and the bold lines are 5 point running
means. Note that in Fig. 7 and in the following discussion, only data above the hiatus are included. The
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
219
Fig. 7. The stable isotopes against age above the hiatus. The horizontal line represents the modern d 18Oc value. Note that both of the stable
isotope scales show heavier values downwards and lighter values upwards. The two upper measurements are omitted in the running means.
modern d 18Oc value (horizontal line) is derived from
active stalactite tips from the cave (see discussion
below). In general, a close resemblance is observed
between the two isotope records, i.e. enriched and
depleted zones in both isotope records correspond in
time. The d 13Cc record ¯uctuates with a higher amplitude than d 18Oc, and is characterised by large scale
`depletion-enrichment cycles'. This pattern is not as
evident in the d 18Oc record.
The d 18Oc record (5 point running mean) above the
hiatus is in the range 27.4 to 27.1½ and characterized
by heavier than the modern value in the intervals
between ca 4200 and 4000, 2550±1700, 1500±1400,
and 1350±600 years before AD2000, and from 300
years before AD2000 to the top. The intervening intervals are lighter than the modern value. The distinct
enrichment in the two upper measurements in the raw
data is discussed later. The d 13Cc record (5 point running
mean) above the hiatus, is in the range 24.5 to 22.5½.
Speci®cally heavy intervals are evident between ca
4000±3250, 2400±1900, 1700±1500, and 300±150
years before AD2000. Similar to the d 18Oc record, but
less distinctive, a sharp enrichment is evident for the two
upper measurements.
4. Discussion
4.1. Interpretation of the stable oxygen isotope signal
Isotopic equilibrium (or quasi equilibrium) deposition is indicated by `Hendy'-tests throughout the
SG95 stalagmite, with the exception of the uppermost
part. Thus its stable oxygen signal is related either to
cave temperature, isotopic composition of the dripwater, or both. The present-day d 18Oc value in the
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H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
SG-cave is found to be 27.33½ in the deeper,
draught-free part of the cave (Lauritzen and Lundberg, 1999).
The two upper measurements in the SG95 record
show a strong and abrupt enrichment (Fig. 7), corresponding in time to ca 9 and 35 years before AD2000
(i.e. AD1991 and 1965), which is correlated to the
opening of the second (eastern) cave entrance in the
late 1960s. This most likely altered the cave air circulation. Hence, the most recently precipitated calcite,
i.e. uppermost part of SG95 and active stalactite tips
of the ventilated zone, does not represent the same
cave environment as prior to the human impact. In
order to interpret the d 18Oc record, the recent stable
isotopic enrichment trend in SG95 is disregarded and
removed from the running means. The effect of
changes in the isotopic composition of the source of
the meteoric precipitation, seawater, is believed to be
insigni®cant due to the near-uniform SMOW values
for the last 5000 years (Martinson et al., 1987; Fairbanks, 1989).
Calibration of the d 18Oc sensitivity with external
temperature in this site (using the `Little Ice Age'
temperature drop) suggests a negative d 18Oc response
with increasing temperature (Lauritzen and Lundberg,
1999). In the d 18Oc record of SG95 (Fig. 7), the period
from ca 310 to 80 years before AD2000 (AD
1690±1920) is heavier than the modern measured
value of 27.33½, while the period from ca 610 to
310 years before AD2000 (AD 1390±1690) is lighter.
This might re¯ect the large scale climatic variations,
commonly described as the `Little Ice Age' and the
Medieval Warm Period (e.g. Crowley and North,
1991). However, the timing of these isotope shifts
does not correspond with d 18Oc records from coeval
stalagmites from the same cave (see comparison
section below) or from a cave site further north
(Linge, unpublished), which both reveal lighter
d 18Oc values between ca 950 and 550 years before
AD2000 (AD1050±1450), and heavier d 18Oc values
between ca 550 and 100 years before AD2000
(AD1450±1900). These observations questions the
existence of a straightforward relationship between
the SG95 d 18Oc record and the large scale climate.
In addition, the d 18Oc signature of SG95 is signi®cantly enriched compared to other stalagmites from
the SG-cave. Further discussion is given in the
comparison section.
4.2. Interpretation of the stable carbon isotope signal
The d 13Cc signal in Norwegian speleothems is
believed to mainly re¯ect (summer) soil-zone conditions, i.e. its moisture and temperature variations,
which affect the chemical and isotopic composition
of the percolation water. Furthermore, most Norwegian caves are developed in the characteristic
`Streifen Karst' setting (Horn, 1937; Lauritzen,
1990), which greatly minimizes the fraction of
marble-derived carbon. During the growth season,
soil-CO2 production is high, provided suf®cient moisture is available. This causes high PCO2 in the soil zone,
with low d 13C values (Rightmire, 1978; Hesterberg
and Siegenthaler, 1991). Water that passes through
the soil will thus gain high PCO2 with a low d 13C
signature. In addition, the corrosion capacity of
these waters will be higher than for waters with low
PCO2. On the other hand, when the soil-CO2 production
is low, the corresponding d 13C will be less negative,
and thus enriched compared to the high production
situation. Low soil-CO2 production occurs outside
the growth season, but also during growth seasons
with prevailing cool conditions, or as a result of
heavy and/or frequent rainfalls (Rightmire, 1978) or
prolonged droughts (Hesterberg and Siegenthaler,
1991). Hence, if the d 13Cc signal is solely dictated
by soil-zone conditions, heavy values would designate
cold and/or very wet/dry conditions, while light
values would re¯ect warmer conditions with balanced
humidity level.
The d 13Cc record for the last 1000 years from SG95
show light values from 950 to 610 years before
AD2000 (AD1050±1390), heavier values from 610
to 350 years before AD2000 (AD1390±1650), and
very heavy values between 280 and 130 years before
AD2000 (AD1720±1870). The heavy values are
believed to re¯ect the `Little Ice Age'. Thus warmer
conditions yields lighter d 13Cc and cooler conditions
heavier, in correspondence with a soil-zone dictated
d 13Cc signal. However, the unusually elevated
average d 13Cc signal of SG95, compared to other
Norwegian speleothems, will be discussed below.
4.3. Comparison with other speleothem records from
the SG-cave
At the present, three Holocene stalagmites including
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
221
Fig. 8. Holocene stable oxygen and carbon isotope records from the SG-cave. All data sets are shown as 5 point running means. Bold black lines
represent the SG95 stalagmite, bold grey lines the SG93 stalagmite and thin black lines the SG92-4 stalagmite. Note that both of the stable
isotope scales show heavier values downwards and lighter values upwards.
this study, have been dated and analysed for stable
isotopes from the SG-cave. Their respective growth
positions are indicated in Fig. 1a.
The SG93 stalagmite (Lauritzen and Lundberg,
1998, 1999), collected from the deeper parts of the
cave, covers the period from about 10,000 to 140
years before AD 2000 (twelve TIMS dates). Over
the last 8000 years, it displays an isotope range (5
point running mean values) of 27.9 to 27.0½ for
d 18Oc and 29.0 to 26.0½ for d 13Cc. The stalagmite
300392-4 (Berstad, 1998), from now on named SG924, grew ca 50 m from the western cave entrance, and
represents a growth interval between ca 8000 to 4500
years before AD2000 (only three TIMS dates). Its
stable isotope range (5 point running mean values)
is between 27.8 and 27.0½ for d 18Oc and 210.5to
26.0½ for d 13Cc. Deduced from their growth positions within the cave (Fig. 1a), SG93 is expected to
have experienced the most stable cave microclimate.
Situated close to an entrance, SG92-4 would have
experienced larger seasonal and annual variations in
temperature/humidity, and possibly draughts. SG95,
growing more than 100 m from a former closed
entrance, should mirror cave conditions similar to
SG93. The stable oxygen and carbon isotope records
from the three SG-stalagmites are shown in Fig. 8.
A comparison of the three d 18Oc records reveals
that SG92-4 corresponds very well with SG93, both
in pattern and isotopic range, but has a greater amplitude. This is in accordance with the inferred difference
in cave microclimate at their respective growth positions. Apart from the overall heavier values of SG95,
a reasonably good correspondence is found between
SG95 and SG93 for the interval 3300±1300 and 600±
50 years before AD2000. The discrepancy in the
intervals 4200±3300 and 1300±600 years before
AD2000 can partly be caused by the difference in
temporal resolution, but cannot solely be explained
222
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
by chronological means; SG93 had no problems with
detritus so we are con®dent of the dates, while dates
affected by Th contamination are already rejected for
SG95. However, the well correlated interval coincides
with the interval of generally decreasing 238U content
in SG95 (Table 1), which is indicative of stable percolation pathways. The intervals of increasing 238U,
from the hiatus to 3540 years before AD2000 and
from 990 years before AD2000 to the top, can thus
be interpreted as periods of unstable or shifting percolation pathways, causing a variable chemical and
isotopic composition of the drip water. Nevertheless,
this does not explain the overall heavier d 18Oc signature of SG95 compared to the other records.
The patterns of the three d 13Cc records do not show
the same degree of correlation as seen for the d 18Oc
records. SG92-4 has somewhat lighter values than the
SG93 record, but both records have common large
scale features. SG95 on the other hand, with its overall
heavy d 13Cc signature, reveals highly differing ¯uctuations with time compared to SG93, e.g. in the interval 3500±2000 years before AD2000. Differences in
isotope ranges may be explained by the respective
growth positions of the three stalagmites within the
cave and the percolation pathways of their dripwaters
(Fig. 1c). SG92-4, displaying the most depleted
values, grew in a passage with schist roof, and its
relatively shallow depth below the surface yields a
short percolation path, which strongly reduces the
fraction of bedrock. SG93 has slightly heavier values,
it grew in a deep, horizontal passage, where the percolation path is assumed to be long, and the contact time
between seepage water and marble in the cave roof is
short. Finally, SG95, displays the heaviest values. The
possible percolation pathways of the SG95 dripwater
could be through ®ssures in the overlying schist, but
also along the contact interface between schist and
marble.
Since the SG95 stalagmite is considered to be
deposited in isotopic (quasi) equilibrium, the cause
of the observed enrichment must originate from
processes affecting the percolation water before it
drips into the cave passage. If the dripwater is
supplied from two aquifers of different ¯ow paths
and widely different amounts of carbonate rock, it
may be argued that mixing and exchange effects in
the two ¯ow paths may account for the observed
isotope shift. However, this can only be veri®ed by
further monitoring and analysis of dripwaters in the
cave.
5. Conclusions
The SG95 stalagmite is found to be deposited in
isotopic (quasi) equilibrium and hence to be suitable
for paleoclimatic studies. The d 18Oc signal (raw data)
in the interval 300±90 years before AD2000 (AD
1700±1910) is heavier than the modern value, indicating that there is a negative relationship between d 18Oc
and temperature. Heavy d 13Cc values in the same
interval may also be interpreted as re¯ecting cooler
conditions. However, such a positive correspondence
between the d 18Oc and d 13Cc records is not always
evident, as exempli®ed by the period from 950 to
400 years before AD2000 (AD1050±1600).
On comparison with two other Holocene stalagmites from the same cave system, a systematic shift
towards heavier stable isotope signatures is found for
SG95. The cause of this enrichment is not well understood, but it is probably related to processes affecting
the stable isotopic composition of the percolation
water before it enters the cave.
The overall correlation between the large scale ¯uctuations of the three Holocene d 18Oc records suggest
that they all re¯ect the local cave microclimate, and
also that they are signi®cant proxy records of the
external paleoclimate. Nevertheless, intervals with
lack of correlation and/or with large differences in
amplitude are dif®cult to explain and bear witness of
the complex nature of this proxy record. The d 13Cc
records from this cave show large variations both in
pattern and isotope range, and the d 13Cc signal is
believed to be governed by soil-zone conditions, and
local processes related to percolation pathways and
possibly driprates.
Speleothems are commonly considered to be appropriate for paleoclimatic studies if they are found to be
formed in isotopic equilibrium with the dripwater, and
have a simple growth form with regular crystals.
These properties are also found for the SG95 stalagmite. However, on comparison with other local
speleothem records, discrepancies are found in curve
patterns and stable isotope range. The main conclusions from this study are thus that (1) due to the spiky
nature of the stable isotope signal, only general trends
H. Linge et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 167 (2001) 209±224
can be considered signi®cantly meaningful, (2) stable
isotope patterns should be con®rmed from several
samples before rigorous interpretations are made
and (3) variations in percolation paths are sometimes
very signi®cant.
Acknowledgements
Financial support was provided by the Research
Council of Norway (NFR grants 110687/420 and
122226/720). The Royal Ministry of Environment
and `Statens Skoger' provided permission to sample
the speleothems. We thank R. SùraÊs and O. Hansen
for mass-spectrometric operation (stable isotopes).
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