Quaternary Science Reviews 49 (2012) 82e94
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Quaternary Science Reviews
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Quantifying kinetic fractionation in Bunker Cave speleothems using D47
Tobias Kluge*, Hagit P. Affek
Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 January 2012
Received in revised form
18 June 2012
Accepted 22 June 2012
Available online
Isotopic signals in speleothems are used for investigating paleoclimate variability on land and are useful
to constrain the dating of prominent climate events. A quantitative use, however, is limited by an
incomplete understanding of parameters contributing to the carbon and oxygen isotope signals. These
include external and environmental parameters such as d18O of cave drip waters as well as internal
parameters associated with speleothem formation, such as the presence of non-equilibrium effects and
especially the magnitude of their isotopic shifts.
We explore the use of clumped isotopes as a new tool for investigating the kinetic isotope effect in
speleothems. Holocene and modern speleothems from Bunker Cave (Germany) as well as modern
material from the adjacent Dechen Cave are all offset from the equilibrium relationship due to kinetic
fractionation. This kinetic offset in clumped isotopes is observed in a stalagmite despite mostly negative
Hendy tests, providing a sensitive indicator for kinetic fractionation in cave carbonates. The temperature
dependence of the clumped isotope values (0.005& per C) is low compared to the observed magnitude
of kinetic offsets (between 0.021 and 0.075&), so that the mean offsets in apparent temperatures due
to kinetic isotope effects are on the order of 10 C. As a result clumped isotopes are useful in identifying
temporal variations in the kinetic fractionation in a stalagmite, when the temperatures during the
speleothem growth period are either relatively constant (variations <2 C) or can be independently
constrained.
The variations in the kinetic isotope fractionation in Bunker Cave are associated with changing drip
water super saturation with periods of stronger prior calcite precipitation associated with lower kinetic
offsets in the speleothem calcite. In contrast, stalagmite growth rates show no direct correlation with the
degree of kinetic fractionation in the investigated range (13e1500 mm/a).
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Stalagmite
Clumped isotopes
Equilibrium fractionation
Holocene
Hendy test
Growth rate
1. Introduction
Stable isotopes in carbonates are widely used as proxies for
paleoclimate and paleoenvironmental reconstruction. Speleothems
are cave carbonate deposits that provide a valuable archive for
terrestrial climatic conditions (Schwarcz, 1986; Gascoyne, 1992;
McDermott, 2004). Stalagmites from caves in the mid-latitudes and
the Mediterranean reveal details of climate changes in the Holocene (Dorale et al., 1992; McDermott et al., 2001, 2011; Frisia et al.,
2005; Mangini et al., 2005), during the Last glacial (Harmon et al.,
1979; Bar-Matthews et al., 1997; Dorale et al., 1998; Fleitmann
et al., 2009; Boch et al., 2011) and the Last Interglacial (Duplessy
et al., 1970; Spötl et al., 2002; Meyer et al., 2008; Couchoud et al.,
2009; Wainer et al., 2011). Speleothems from low-latitude sites
provide unique information about monsoon activity (Neff et al.,
* Corresponding author. Tel.: þ1 203 432 3761; fax: þ1 203 432 3134.
E-mail addresses: [email protected] (T. Kluge), [email protected]
(H.P. Affek).
0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.quascirev.2012.06.013
2001; Yuan et al., 2004; Fleitmann et al., 2007; Wang et al.,
2008). Furthermore, the robust and precise U/Th disequilibrium
dating of speleothems allows precise age estimates of prominent
climatic events (Spötl and Mangini, 2002; Genty et al., 2003;
Drysdale et al., 2005; Wang et al., 2008).
A limited number of studies directly infer paleotemperatures or
estimate absolute paleotemperature changes from the oxygen
isotope signal in the cave calcite. These are based on combining the
temperature dependence of the carbonateewater equilibrium
fractionation with the temperature dependence of the rainfall d18O
at high latitudes (Duplessy et al., 1970; Dorale et al., 1992) or on
observation-based transfer functions between stalagmite d18O and
temperature tie points (Lauritzen and Lundberg, 1999; Mangini
et al., 2005). In general, though, a quantitative interpretation of
the oxygen isotope signal in speleothems with regard to temperature is hampered by the inability to partition the d18O signal into its
temperature and water composition components. The main
obstacle is the absence of independent estimates of d18O of the drip
water feeding the stalagmite (Baldini et al., 2006; Lachniet, 2009;
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
Wackerbarth et al., 2010). Furthermore, d18O thermometry relies on
carbonate formation at isotopic equilibrium with the drip water.
However, speleothems are often influenced by non-equilibrium
conditions and d18O thermometry is therefore limited by incomplete understanding of kinetic isotope effects during speleothem
calcite precipitation (Mickler et al., 2004; Dreybrodt, 2008;
Mühlinghaus et al., 2009; Scholz et al., 2009; Dreybrodt and Scholz,
2011).
The search for isotopic equilibrium through d18O is ambiguous,
first due to the drip water composition that is often not well constrained, and second, due to the value of the oxygen equilibrium
fractionation factor between the solution and the precipitated
carbonate being recently questioned. Experimental studies (Dietzel
et al., 2009; Day and Henderson, 2011; Gabitov et al., 2012) and
results from slow growing natural carbonates (Coplen, 2007) point
towards a fractionation factor that is significantly larger than the
currently accepted d18O thermometer calibration (e.g., Kim and
O’Neil, 1997). Quantifying disequilibrium in speleothems therefore requires a less ambiguous alternative.
Understanding of the fractionations associated with calcite
precipitation in general and speleothem formation in particular is
a key to the ability to use cave deposits for paleoclimate reconstruction. If the drip water composition is known, for example
through fluid inclusion analysis (Schwarcz et al., 1976; van
Breukelen et al., 2008; Zhang et al., 2008), and the stalagmite was
deposited under conditions of isotopic equilibrium with this water,
temperatures can be calculated from the calcite d18O. Alternatively,
if the temperature is known, e.g., through noble gas analysis (Kluge
et al., 2008; Scheidegger et al., 2011) or clumped isotopes
measurements (Affek et al., 2008; Meckler et al., 2009; Wainer
et al., 2011), calcite d18O may be used to reconstruct the water
composition. These cases, however, require that calcite has been
formed under isotopic equilibrium conditions, or that the deviation
from the equilibrium is well understood.
Speleothems are typically assumed to grow under isotopic
equilibrium. However, recent studies have shown that this is often
not the case (McDermott et al., 2006, 2011; Mickler et al., 2006;
Demény et al., 2010; Tremaine et al., 2011), highlighting the need
to characterize kinetic isotope effects in speleothems. Commonly,
‘Hendy tests’ (Hendy, 1971) are performed to investigate if
a stalagmite grew at isotopic equilibrium. Simultaneous increases
in d18O and d13C values along a growth layer are taken as indication for disequilibrium, whereas constant d18O values along the
growth layer are considered as indicator for equilibrium conditions. Recent studies suspect, however, that Hendy tests are
ambiguous and not sufficient (Spötl and Mangini, 2002;
Dreybrodt, 2008; Dorale and Liu, 2009; Mühlinghaus et al., 2009);
a negative Hendy test (namely, non-correlated d18O and d13C
increases along the growth layer) is not necessarily a reliable
evidence for equilibrium deposition (Mühlinghaus et al., 2009)
and the Hendy test in general is possibly not sensitive enough to
identify disequilibrium.
Dorale and Liu (2009) suggested replication tests as a more
reliable alternative. If isotopic trends and signals of different
stalagmites (in one cave) of similar age are significantly different
from each other then they are likely to reflect site-specific
processes that are not necessarily climate-related, and typically
the result of non-equilibrium deposition. In contrast, well dated
and replicated or highly similar d13C and d18O trends are considered
as high-fidelity paleoclimate records and are therefore assumed to
reflect isotopic equilibrium (e.g., Dorale et al., 1998; Dong et al.,
2010). However, even in these records kinetic fractionation may
occur, but vary uniformly for a set of speleothems within the cave
(for example due to changes in cave ventilation or by fundamental
changes in the vadose zone).
83
In this paper we examine clumped isotopes in speleothems as
a tool for identifying and quantifying disequilibrium. Carbonate
clumped isotopes (reported as D47) are a measure of chemical
bonding between 13C and 18O in the carbonate lattice, with preferential binding of 13C and 18O at low temperature leading to higher
D47 values. At thermodynamic equilibrium, D47 reflects the
carbonate precipitation temperature, providing a new paleotemperature proxy with typical values of 0.6e0.7& at Earth’s
surface temperatures (Eiler and Schauble, 2004; Ghosh et al., 2006;
Eiler, 2007, 2011). Studies of clumped isotopes in modern speleothems, however, reveal a clear deviation from the nominal equilibrium D47 e T relationship (Affek et al., 2008; Meckler et al.,
2009; Daëron et al., 2011; Wainer et al., 2011). These deviations
are likely caused by kinetic isotope effects associated with CO2
degassing out of the precipitating solution (discussed from theoretical perspective by Guo, 2008). Deviations from equilibrium have
been observed in several caves, including caves where most
stalagmites have passed both the Hendy and replication tests such
as Soreq Cave (Israel; Bar-Matthews et al., 1996) and Villars Cave
(France; Genty et al., 2006; Wainer et al., 2009), suggesting that D47
may be an especially sensitive indicator for kinetic fractionation in
speleothems. Using two Holocene stalagmites from the Bunker
Cave (Germany) and modern material from the adjacent Dechen
Cave we assess the potential of clumped isotopes for identifying
and quantifying disequilibrium and its variability over time in
comparison to the commonly used Hendy test.
2. Study site and samples
2.1. Study site
The study was conducted using samples from Bunker Cave and
Dechen Cave, both located in the Rhenish-Slate Mountains
(Western Germany). The caves are overlaid by 15e30 m of Middle
to Upper Devonian limestone and are located at about 180 m above
sea level. The distance between both caves is about 1300 m. They
are located 250 km south of the North Sea and 700e900 km east of
the Atlantic Ocean. The study area is characterized by a temperate
climate with precipitation throughout the year (annual mean
900 mm, 1961e1990) and a mean annual air temperature of 9.5 C
(1961e1990). Present-day temperatures in the Bunker Cave region
reflect the recent increase in mean annual air temperature to
10.5 C (1988e2007), consistent with in-situ measurements of
10.6 C inside Bunker cave (monitoring 2006e2009; Riechelmann
et al., 2011). Dechen Cave is accessible to the public and exhibits
a mean temperature of 10.6 C (Pflitsch et al., 2000). Holocene
pollen-based temperature reconstructions (Davis et al., 2003)
suggest constant temperatures over the last 8 ka and regional
temperatures that are cooler by 1 C between 8 and 10 ka relative to
1961e1990.
Bunker Cave is monitored for the isotopic composition of drip
water, for drip rates, and for CO2 concentrations at a monthly
resolution since 2006 as part of the DAPHNE project (dated
speleothems e archives of the paleoenvironment; e.g.,
Riechelmann, 2010; Riechelmann et al., 2011). The mean drip
water d18O value is 8.0 0.2& (VSMOW) for most of the
investigated sites within Bunker Cave and specifically at the
locations of the stalagmites used in this study (Riechelmann
et al., 2011). This value reflects the isotopic composition of the
infiltration-weighted mean of the rainfall. As no stable isotope
measurements have been carried out on drip water from Dechen
Cave we assume it to be similar to Bunker Cave values. This is
justified by the proximity and the similar hydrological setting of
both caves, resulting in comparable percolation times on the
order of few years (Kluge et al., 2010a,b).
84
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
2.2. Samples
3. Methods
Modern calcite precipitates and Holocene samples from two
stalagmites (BU1, BU4) were collected in Bunker Cave. Modern
material was deposited in Bunker Cave on seven watch glasses
(convex side up) that were placed below water drip sites at
different seasons during the monitoring period, or was sampled
from the top of the stalagmite BU4 that was actively growing before
the removal. Typically, watch glass material has been collected for
about 3 months. Modern material was collected also from Dechen
Cave. The samples (DC-1, 2) grew on an electric cable at two
different locations and are younger than 40 years. The two stalagmites (BU1 and BU4) grew approximately 10 m apart, in adjacent
cave chambers of Bunker Cave, with growth phases from about 9 ka
to today (BU4) or almost until today (BU1). BU4 grew continuously
over the whole time period, with accelerated growth in the late
Holocene (at 2e3 ka growth rate increased from 13e30 mm/a to
70e270 mm/a; Fohlmeister et al., 2012) and has a total length of
21 cm. The growth pattern of BU1 is generally more complex with
at a major hiatus between 4 ka and 2 ka. It stopped growing several
decades before modern times. BU1 grew generally faster, with
growth rates of up to several 100 mm/a (Fohlmeister et al., 2012) and
is therefore considerably taller (total length of the Holocene part
w63 cm). The stalagmite width was determined on the cut halves
from rim to rim and yields a typical radius of about 3.5 cm in both
cases. Both stalagmites are fed by rather slow drips with presentday drip discharge rates of 0.03 0.02 ml/min for BU1 and
0.001 0.001 ml/min for BU4 (Riechelmann et al., 2011). The drip
height measured from the stalagmite tops is 1.25 m for BU1 and
2.1 m for BU4, dripping on the stalagmites from a small (6e10 cm
long) stalactite or soda straw, respectively. Both stalagmites have
been sampled along the growth axis with nine samples taken from
each (Table 1).
3.1. The D47 thermometer
The carbonate clumped isotope thermometer is based on the
abundance of isotopologues with two rare isotopes, e.g., 13C and 18O
in CO2 gas produced by acid digestion of carbonates. The isotopologues of nominal mass 47, consisting mainly of 13C18O16O, are
measured to define the D47 parameter. Its value is calculated by
comparing the abundance ratio of mass 47 to mass 44 (R47) to that
expected for random distribution of the isotopes among all isotopologues, following:
"
D47 ¼
R47
2
2R13 $R18 þ2R17 $R18 þR13 $ R17
#
R45
þ1 $1000
2 13
R þ2R17
2R18 þ2R13 $R17 þ R17
R46
(1)
The denominator values are derived by calculating R13
(¼ C/12C) and R18 (¼18O/16O) from the measured R45 and R46. R17 is
calculated from R18 assuming a mass-dependent relationship
between 18O and 17O. Details are given in Affek and Eiler (2006) and
Huntington et al. (2009).
Carbonates that were formed at Earth surface conditions are
enriched in 13Ce18O bonds resulting in typical D47 values of
0.6e0.7&. The advantage of this thermometer over, for example,
d18O is that D47 is an internal parameter of the carbonate lattice and
is therefore independent of the d18O and d13C of the solution in
which the carbonate is formed. An empirical temperature calibration for D47 in carbonates was determined by laboratory calcite
precipitation experiments, and confirmed by measurements of
biogenic carbonates that grew at known temperatures (Ghosh et al.,
13
Table 1
Sample details and isotopic composition. ‘Distance from top’ refers to the sample position with regard to the top of the stalagmite. DC-1 and DC-2 are from Dechen Cave, all other
samples are from Bunker Cave. Samples U I-4 e U VII-15 are calcites precipitated on watch glasses. Uncertainties in D47 and d18O are given as standard errors of replicate analysis.
For single measurements the standard deviation of replicate analysis of standards is used as error estimate. d13C and d18O values are reported using the VPDB scale. D47 values are
reported using the original reference frame (see supplementary data for absolute reference frame). Ages refer to an UeTh stalagmite age model (Fohlmeister et al., 2012).
Sample
Modern set
DC-1
DC-2
U I-4
U I-16
U IV-15
U VII-5
U VII-8
U VII-14
U VII-15
Stalagmite BU1
1
2
3
4
5
6
7
8
9
Stalagmite BU4
1
2
3
4
5
6
7
8
9
d18O (&)
d13C (&)
0.005
0.004
0.020
0.012
0.020
0.020
0.020
0.020
0.008
5.79
5.64
6.33
6.12
5.52
6.38
5.66
6.26
5.85
0.07
0.03
0.20
0.03
0.20
0.20
0.20
0.20
0.04
9.05
9.92
9.72
10.49
5.27
7.35
6.22
7.69
8.16
0.21
0.11
0.20
0.12
0.20
0.20
0.20
0.20
0.06
0.007
0.002
0.009
0.002
0.005
0.003
0.008
0.007
0.002
5.59
5.70
6.30
5.90
5.80
5.31
5.32
6.16
5.76
0.01
0.06
0.02
0.04
0.12
0.02
0.07
0.11
0.08
9.85
11.18
10.43
10.34
8.88
7.87
8.23
8.42
9.12
0.12
0.03
0.04
0.10
0.13
0.21
0.04
0.14
0.04
0.007
0.003
0.003
0.007
0.006
0.004
0.004
0.005
0.005
5.57
5.07
5.74
6.14
5.92
5.73
5.67
6.05
6.13
0.14
0.04
0.06
0.04
0.07
0.11
0.05
0.06
0.05
8.32
8.94
9.47
9.89
9.61
9.13
8.75
8.89
9.12
0.85
0.02
0.03
0.04
0.05
0.14
0.08
0.04
0.22
Distance from top (mm)
Age (ka)
Replicates
D47 (&)
e
e
e
e
e
e
e
e
e
0e0.04
0e0.04
Recent
Recent
Recent
Recent
Recent
Recent
Recent
7
7
1
4
1
1
1
1
2
0.648
0.657
0.668
0.666
0.638
0.670
0.653
0.675
0.658
15e19
52e55
84e86
137e140
249e251
347e351
388e392
502e504
565e570
0.64e0.76
1.00 0.15
1.22e1.24
1.40 0.15
4e5
5.04 0.10
5.93e6.03
7.0 0.3
7.5 0.3
5
5
7
5
5
5
5
6
5
0.659
0.673
0.670
0.685
0.676
0.679
0.678
0.681
0.693
0e0.5
11e13.5
20e28
38e42
42e45
91e94
94e96
135e140
210e214
0e0.05
0.5 0.3
1e1.2
1.25e1.35
1.30 0.15
2.6e2.8
2.8e3
5.1e5.3
8.2 0.2
5
5
5
6
5
5
5
5
5
0.668
0.677
0.671
0.669
0.677
0.667
0.676
0.692
0.705
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
2006; Eiler, 2011). We use this empirical D47 e T calibration line
(Ghosh et al., 2006) as a reference for nominal isotopic equilibrium
D47, equil following:
D47;equil ðTÞ ¼ 59200=T 2 0:02 ðD47 in &; T in KÞ
(2)
The temperature dependence of equilibrium D47 values is
therefore 0.005e0.006& per 1 C at the range of typical Earth
surface temperatures.
3.2. Measurement procedure
The sample preparation and mass spectrometric measurement
for clumped isotope analysis follow the procedures described by
Ghosh et al. (2006), Affek et al. (2008), Huntington et al. (2009), and
Zaarur et al. (2011). Briefly, CO2 was extracted from 4 to 6 mg of
speleothem calcite by overnight reaction with 105% phosphoric
acid at 25 C. CO2 was purified using a GC column (Supelco Q plot,
530 mm diameter, 30 m length) held at 20 C, to remove hydrocarbons and other contaminants affecting the mass spectrometric
analysis. The measurements were done using a dual-inlet gassource isotope ratio mass spectrometer (Thermo Fisher MAT 253)
with 6 Faraday cups aligned to detect simultaneously mass 44 to
mass 49. The bellow pressure was adjusted to reach 16 V for the
mass 44 signal (typically w40 mbar bellow pressure). The
measurement was split into 11 sequences whereof 2 were used to
measure the baseline signal in the presence of CO2, by adjusting the
acceleration voltage to measure off-peak. The other 9 sequences
analyze alternately sample and reference gas (10 cycles per
sequence) with 20 s integration time for each recording. To correct
for non-linearity and to monitor D47 scale compression effects, CO2
of variable d13C and d18O values, that was heated to 1000 C to
achieve random distribution, was measured multiple times per
week following the procedures described by Huntington et al.
(2009). CO2 equilibrated with water at different temperatures
(10 C, 25 C, 50 C) was measured to calibrate scale compression
(Dennis et al., 2011). Carrara marble and cylinder CO2 were
analyzed to test for system stability and for inter-laboratory
comparison.
Samples were measured throughout the years 2010 and 2011 to
exclude systematic errors associated with uncharacterized shortterm fluctuations in mass spectrometric and sample preparation
parameters. Samples were measured in 5e7 replicates (Table 1)
with the exception of most watch glass samples that could only be
measured once due to the limited amount of precipitated calcite. A
Carrara marble laboratory standard (d13C ¼ 2.4 0.1& (1s) VPDB,
d18O ¼ 1.84 0.08& (1s) VPDB, n ¼ 66, measured during 2010
and 2011) was prepared and processed using the same procedure as
the speleothem samples and analyzed at least once per week. The
mass spectrometer performance was further monitored by weekly
measurements of CO2 cylinder gas internal laboratory standard
(‘Corn CO2’: d13C ¼ 10.00 0.14& (1s) VPDB,
d18O ¼ 12.93 0.21& (1s) VPDB, D47 ¼ 0.871 0.022& (1s),
n ¼ 117, measured during 2010 and 2011). The mean D47 value of
the Carrara Marble is 0.358 0.024& (1s), comparable to a published value of 0.352 0.019& (NBS-19; Ghosh et al., 2006). An
inter-laboratory comparison showed this in-house Carrara marble
value to be consistent with results from Johns Hopkins University,
California Institute of Technology, and Harvard University (Dennis
et al., 2011). The uncertainty of single D47 measurements is
w0.02& (1s) based on analysis of standard materials (Carrara
marble, NBS-19, Corn CO2), but shows a better reproducibility for
speleothem samples (0.012&). Replicate measurements of the
speleothems samples lead to a substantial reduction of the uncertainty to typically 0.005& (standard error, SE).
85
Carbon and oxygen isotopes were measured together with the
D47 analysis. Absolute d13C and d18O values were defined using
a pre-calibrated Oztech (Safford, AZ, USA) reference gas
(d13C ¼ 3.64& VPDB, d18O ¼ 15.80& VPDB) and verified by
regular measurements of NBS-19 that was processed as a sample.
Considering a fractionation factor of 10.25& for the phosphoric
acid digestion of calcite at 25 C, the mean d18O of all NBS-19
measurements throughout 2010 and 2011 is 2.17 0.04& (1s,
n ¼ 12, VPDB), in agreement with the NIST reference value
of 2.2&. The d13C value is slightly higher (2.11 0.13& VPDB (1s,
n ¼ 12), versus 1.95& of the NIST reference). Typical uncertainties
in d18O and d13C for replicate sample analysis are 0.06& (SE) and
0.15& (SE), respectively.
3.3. Calculation of d18O and D47 values
D47 is calculated according to Eq. (1) and is reported in the text
using the same reference frame of the Ghosh et al. (2006) D47-T
calibration in which CO2 extracted from Carrara marble has a D47
value of 0.352&. In the supplementary data (Table EA1) we also
report D47 data using the new absolute reference frame (D47,abs)
that is defined based on CO2 equilibrated at different temperatures
through isotope exchange with water (Dennis et al., 2011), in which
Carrara marble has a D47 value of 0.395&.
d13C and d18O are reported using VPDB as a reference frame. The
expected calcite d18O value is estimated based on the measured
drip water d18O, the measured or estimated growth temperature
(T), and the equilibrium fractionation factor given by Kim and
O’Neil (1997) corrected for the acid reaction fractionation (Böhm
et al., 2000; Affek et al., 2008):
1000lnacalciteH2 O ¼ 18:03$103 =T 32:17
(3)
4. Results
4.1. D47 results
The modern samples varied in D47 values between
0.638 0.020& and 0.675 0.020& (Table 1), averaging
0.660 0.011&. These values are significantly lower than the
expected D47 of 0.715& for equilibrium conditions at the modern
cave temperature of 10.6 C. Mean D47 values of both stalagmites
over the Holocene are identical to each other and slightly higher
than modern values (BU1: 0.677 0.010&, BU4: 0.678 0.012&).
The two stalagmites exhibit a trend in D47 over the Holocene, with
higher values in the early and mid-Holocene (0.683 0.011&
averaged over both stalagmites, Table 1) and values closer to
modern samples in the late Holocene (0.672 0.007&), corresponding to increasing offsets (Fig. 1).
4.2. d13C and d18O signals
Both d13C and d18O were measured together with each clumped
isotopes analysis, as part of the D47 analysis. In the modern samples
d18O values varied only in a limited range, between 5.5
and 6.4&, whereas the range of variation in d13C (5.3
to 10.5&) is large, and is comparable to the whole range of
variations observed in the two stalagmites throughout the Holocene. The enrichment ranges from 0.49& to 1.77& (Table 2),
assuming that the relationship in Equation (3) reflects equilibrium
(Kim and O’Neil, 1997; Böhm et al., 2000).
Our data is consistent with the high resolution d13C and d18O
measurements obtained for these stalagmites at the University of
Innsbruck and is discussed in detail elsewhere (Fohlmeister et al.,
86
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
(Fohlmeister et al., 2012). These high resolution records show no
significant correlation between d13C and d18O along the growth axis
(R2 ¼ 0.20 for BU1 and 0.18 for BU4).
Hendy tests have been reported previously for stalagmite BU4
(Riechelmann, 2010) as a traditional test for equilibrium precipitation. BU1 could not be investigated due to growth layers that are
not clearly visible. Four of the six Hendy tests show no correlated
enrichment of 18O and 13C, but rather constant values or even
a slight depletion (Fig. 2). Only one Hendy test, at 22 mm from the
stalagmite tip (w1 ka), shows some enrichment in both 18O and 13C
towards the flanks and a significant correlation (R2 ¼ 0.77).
5. Discussion
5.1. Offset in D47 compared to the equilibrium line
Fig. 1. Deviations of the measured D47 (D47 offset) from the expected equilibrium
values over time and the resulting offsets in apparent temperatures (right axis)
observed in two stalagmites in Bunker Cave (Germany) and a series of modern deposits
in Bunker Cave and nearby Dechen Cave. Lower offsets correspond to a smaller kinetic
isotope fractionation. The expected equilibrium values are calculated following the
clumped isotope thermometer calibration of Ghosh et al. (2006).
2012). Briefly, late Holocene d18O (0e3 ka, BU1: 6.0 0.4&,
BU4: 5.7 0.3&) is indistinguishable from the mid-Holocene
mean (3e9 ka, BU1: 5.7 0.3&, BU4: 5.6 0.3&) whereas
d13C differs between these time periods with higher values in the
mid-Holocene (BU1: 9.0 0.5&, BU4: 8.3 0.7&) relative to
the late Holocene (BU1: 10.6 0.5&, BU4: 9.4 0.7&)
Most carbonate D47 values, especially those of marine biogenic
carbonates, are consistent with the nominal equilibrium calibration
(Eiler, 2007, 2011; Tripati et al., 2010). However, results from surface
corals (Ghosh et al., 2006; Saenger et al., 2011) and speleothems
(Affek et al., 2008; Meckler et al., 2009; Daëron et al., 2011) indicate
that effects other than temperature may influence the clumped
isotopes values in cases where isotopic equilibrium is not established. The offsets from the expected equilibrium value that we
observe in Bunker and Dechen Cave are consistent with these
previous speleothem studies.
Assuming the modern temperature (9.5 C, 1961e1990) to be
a reasonable first order estimate for the late Holocene and using
pollen-based reconstructions (Davis et al., 2003) for longer-term
and early Holocene temperatures, deviations in D47 can be estimated (Table 2). For the modern samples DC-1 and DC-2 and the
watch glass samples we use the cave chamber temperature of 10.6
Table 2
Temperatures (Tequil) calculated from the sample D47 using the nominal equilibrium calibration relationship (Ghosh et al., 2006). The expected temperatures T are based on
instrumental measurements or independent paleoclimate studies (Davis et al., 2003) anchored at the 1961e1990 air-temperature mean. Offsets in D47 and d18O refer to the
difference from nominal equilibrium. Growth rates and ages are estimated based on stalagmite age models (Fohlmeister et al., 2012).
Sample
Modern set
DC-1
DC-2
U I-4
U I-16
U IV-15
U VII-5
U VII-8
U VII-14
U VII-15
Stalagmite BU1
1
2
3
4
5
6
7
8
9
Stalagmite BU4
1
2
3
4
5
6
7
8
9
Age (ka)
D47 (&)
0e0.04
0e0.04
Recent
Recent
Recent
Recent
Recent
Recent
Recent
0.648
0.657
0.668
0.666
0.638
0.670
0.653
0.675
0.658
0.64e0.76
1.00 0.15
1.22e1.24
1.40 0.15
4e5
5.04 0.10
5.93e6.03
7.0 0.3
7.5 0.3
0.659
0.673
0.670
0.685
0.676
0.679
0.678
0.681
0.693
0e0.05
0.5 0.3
1e1.2
1.25e1.35
1.30 0.15
2.6e2.8
2.8e3
5.1e5.3
8.2 0.2
0.668
0.677
0.671
0.669
0.677
0.667
0.676
0.692
0.705
Tequil
( C)
T
( C)
Offset
D47 (&)
Offset
d18O (&)
Growth rate
(mm/a)
0.005
0.004
0.020
0.012
0.020
0.020
0.020
0.020
0.008
24.5
22.5
20.2
20.6
26.8
19.8
23.4
18.7
22.5
10.6
10.6
11.0
11.0
11.0
11.0
11.0
12.0
11.0
0.067
0.058
0.045
0.047
0.075
0.043
0.060
0.033
0.056
þ1.30
þ1.44
þ0.85
þ1.05
þ1.66
þ0.80
þ1.51
þ1.14
þ1.32
125e340
175e340
e
e
e
e
e
e
e
0.007
0.002
0.009
0.002
0.005
0.003
0.008
0.007
0.002
22.1
19.2
19.9
16.5
18.5
18.0
18.0
17.5
15.0
9.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
0.062
0.048
0.051
0.036
0.045
0.042
0.043
0.040
0.028
þ1.25
þ1.13
þ0.54
þ0.95
þ1.04
þ1.53
þ1.52
þ0.68
þ1.08
e
135
e
300
e
120e1500
e
280
80
0.007
0.003
0.003
0.007
0.006
0.004
0.004
0.005
0.005
20.2
18.2
19.5
19.9
18.2
20.3
18.4
15.3
12.6
9.5
9.5
9.5
9.5
9.5
9.5
9.5
9.5
8.5
0.053
0.044
0.050
0.052
0.044
0.054
0.045
0.029
0.021
þ1.27
þ1.77
þ1.10
þ0.70
þ0.92
þ1.11
þ1.17
þ0.79
þ0.49
e
14
270
70
70
13
13
30
30
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
87
Fig. 2. Deviations of measured D47 (D47 offset) from the expected equilibrium D47 along the growth axis of stalagmite BU4 in comparison with Hendy tests (HAeHF; Riechelmann,
2010) of the same stalagmite. Only Hendy tests HA and HB show correlated enrichment of d18O (closed square) and d13C (open circle). The insets show d18O vs. d13C along the growth
layer.
and 11 C (for U VII-14: 12 C), respectively. D47 values that are
expected at equilibrium are 0.721& at 9.5 C and 0.713& at 11 C.
The observed speleothem D47 data always tend towards lower
values, showing D47 offsets associated with disequilibrium that
vary between 0.021 and 0.075& (Table 2, Fig. 1). Modern calcite
precipitated on an electric cable from Dechen Cave (DC-1) and on
one of the watch glasses from Bunker Cave (U IV-15) yield the
largest deviations from equilibrium (0.067& and 0.075&),
whereas the early Holocene stalagmite samples BU4-8, BU4-9 and
BU1-9 show significantly lower offsets (0.029&, 0.021&
and 0.028&, respectively). Using the temperature sensitivity
of 0.005&/ C (Eq. (2)) these offsets correspond to temperatures
that are too high by 4e16 C (mean offset of 10 3 C) (Fig. 1).
The mean D47 offset during the Holocene is 0.044 0.010& for
BU1 and 0.043 0.011& for BU4. Both stalagmites, however,
show a temporal change with larger mean offsets at 0e2 ka
(0.049 0.007&) and 2e4 ka (0.048 0.005&), a smaller
offset at 4e6 ka (0.038 0.008&) and a noticeably smaller offset
at 6e9 ka (0.030 0.010&). This temporal change is more
pronounced in BU4 than in BU1 (Figs. 1 and 2).
which is usually considered to reflect equilibrium conditions,
creates the notion of nominal equilibrium for the D47-T calibration.
The support for this D47-T calibration line reflecting isotopic equilibrium comes from the large variety of natural, mostly biogenic,
calcite and aragonite that conform to the same relationship. Deviations from this relationship are therefore likely to be caused by
kinetic isotope effects that are associated with CO2 degassing in
speleothem formation (Guo, 2008). This conclusion is supported by
the observed enrichment of 18O (0.49e1.77&) relative to the value
expected at equilibrium (Fig. 3) and the co-variation of D47 and d18O
offsets, that are both expected when disequilibrium is associated
with CO2 degassing (Guo, 2008; Daëron et al., 2011). The correlation
between D47 and d18O for the watch glasses collected during the
monitoring period (Fig. 3) yields a slope of 0.04& decrease in D47
per 1& increase in d18O (R2 ¼ 0.97, the error-weighted linear fit
includes the data point representing d18O and D47 equilibrium). The
Holocene stalagmites follow the same co-variation supporting it as
a robust signal for the conditions of this cave system.
5.2. D47 e d18O co-variation
We interpret the variation of the offsets along a d18O e D47 covariation slope as reflecting a varying degree of kinetic fractionation, with the D47 offsets used as a proxy for quantifying it.
Variations in the D47 offset of modern material and within the
stalagmites (Fig. 1) indicate variability in the kinetic isotope fractionation on different time scales. At the seasonal scale, watch glass
precipitates show variations in d18O (w0.9&) and D47 (w0.04&)
All speleothem D47 values are falling clearly below the Ghosh
et al. (2006) calibration line, that has been determined using
synthetic calcite produced by degassing of CO2 out of a saturated
Ca(HCO3)2 solution, following the method of Kim and O’Neil (1997)
for d18O. The link with the Kim and O’Neil d18O-T relationship,
5.3. Kinetic effects determined with D47
88
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
Fig. 3. D47 offset plotted versus the offset in d18O. All samples are depleted in D47 and
enriched in d18O. The expected equilibrium values are calculated following the clumped isotope thermometer calibration of Ghosh et al. (2006). The expected equilibrium
d18O is calculated following the calibration of Kim and O’Neil (1997). The slope (0.04)
is calculated for the modern samples including the calcite precipitates on watch
glasses. The linear fit assumes no offset for either D47 or d18O at equilibrium (offset of
both is zero) and is error-weighted using the uncertainty in the D47 values.
corresponding to short-term changes in kinetic isotope fractionation. Potentially, some of the variability in d18O (though not in D47)
may be related to variations in drip water through the season,
although stable drip water d18O values in monitoring during the last
five years (8.0 0.2&) suggest that this is not likely the case.
The investigated stalagmites give insight into the long-term
variations of the disequilibrium effects, revealing significant variations in kinetic isotope fractionation in speleothems over time.
The rather uniform offsets observed in the younger parts of
stalagmites BU4 and BU1 (<2 ka) suggest a constant kinetic isotope
fractionation that is comparable to that of modern material. In
contrast, significantly lower offsets for the older growth periods of
stalagmite BU4 (5e9 ka) suggest a major decrease in the kinetic
isotope fractionation (potential mechanisms are discussed below).
5.4. D47 as a new test for equilibrium vs. kinetic fractionation in
speleothems
The Hendy test, that looks for co-variation of d18O and d13C along
a growth lamina, is commonly used to assess whether a stalagmite
was precipitated in isotopic equilibrium. However, recent studies
suspect that the Hendy test is ambiguous due to difficulties to
identify a single lamina and that it might not be sufficiently
sensitive in the sense that although 18O and 13C enrichment and covariation is only possible under conditions of disequilibrium
precipitation, the lack of co-variation does not necessarily indicate
equilibrium (Mühlinghaus et al., 2009). Furthermore, theoretical
and experimental studies showed the possibility of equilibrium
deposition at the stalagmite apex despite a positive Hendy test
(namely the presence of d18Oed13C covariance) along the stalagmite flanks (Spötl and Mangini, 2002; Dreybrodt, 2008; Romanov
et al., 2008; Wiedner et al., 2008).
Whereas Hendy tests in BU4 are mostly negative, two layers
show increasing isotope values and a positive d18Oed13C correlation. These are in agreement with the D47 offsets (Fig. 2). The
positive Hendy-test BU4-HA and also the less significant BU4-HB
correspond to a time window of relatively high D47 offsets in BU4
(0.049 0.004&, 0e3 ka). The four negative Hendy tests at the
mid and early Holocene part of BU4 correspond to reduced D47
offsets (0.025 0.006&), indicating a general agreement of the
degree of kinetic isotope fractionation (quantified by the D47 offset)
and the results of the Hendy test measurements.
However, significant D47 offsets were observed (0.021
to 0.029&) also in the cases in which Hendy tests do not suggest
disequilibrium. Yet, these D47 offsets coincide with 18O enrichment
with respect to the expected equilibrium values. In other words,
Hendy tests seem to detect kinetic fractionation control only when the
effect is relatively large (offsets of D47 w 0.05& and d18O w þ 1.1&),
but is not sensitive enough to detect relatively small kinetic fractionations. D47 is a more sensitive indicator of kinetic isotope fractionation mainly because the kinetic D47 offsets are large (in this
study 0.021 to 0.075&, average: 0.044 0.010&) compared to
temperature-related uncertainties (0.005e0.006&/1 C).
In case where a strict Hendy test is impossible due to rapidly
thinning stalagmite flanks and blurry layers, the co-variation of
d18O and d13C along the growth axis has been suggested as an
alternative test for kinetic fractionation (Hendy and Wilson, 1968;
Desmarchelier et al., 2000; Cosford et al., 2008; Jo et al., 2011).
However, this test is problematic as co-variation of d18O and d13C
along the growth axis may reflect climatic changes that affect both
isotopes rather than kinetic isotope effects (Burns et al., 2002;
Mattey et al., 2008; Dorale and Liu, 2009).
The results of the Bunker Cave stalagmites show no significant
co-variation between d18O and d13C along the growth axis of BU1
and BU4 (Fohlmeister et al., 2012). Thus, this test would suggest no
kinetic fractionation, in sharp contradiction to the clumped isotope
results and the offsets from the equilibrium d18O values, casting
serious doubt on the usefulness of d13Ced18O co-variation along the
growth axis as a test for disequilibrium.
A more direct approach to examine kinetic fractionation in
modern samples is by comparison of modern calcite d18O to the drip
water d18O where the cave temperature is known (Mickler et al.,
2006; Verheyden et al., 2008; Tremaine et al., 2011). This
approach may be applied also to Holocene stalagmites where
assumptions about drip-water d18O and cave temperature can be
made, or to older speleothems in cases where d18O values of drip
water can be constrained using fluid inclusions. However, using only
d18O to determine the magnitude of kinetic fractionation is difficult
for several reasons. First, the nominal equilibrium relationship of
d18O versus temperature has been recently questioned, resulting in
significant uncertainty regarding the correct thermometer calibration. For example, the relationship determined by laboratory
experiments either by O’Neil et al. (1969) or by Kim and O’Neil
(1997) are commonly used as the nominal equilibrium relationship of the calcite-water system and therefore as d18O-temperature
calibration. Whereas the difference between these two relationships is small and similar to theoretical expectations (Horita and
Clayton, 2007), they are very different from values derived from
mammillary calcite in Devils Hole that is thought to have been
naturally precipitated in equilibrium, due to its extremely slow
growth from slightly super saturated ground water (Coplen, 2007).
At 34 C Devils Hole calcite differs by 1.5& from the value expected if
Kim and O’Neil (1997) is the relevant calibration. Second, for premodern calcite the equilibrium d18O reference value is uncertain
because it depends on the drip water d18O that might have changed
over time due to changes in the precipitation patterns, the main
infiltration period, and the general storm track paths. Third, the
temperature dependence of the equilibrium oxygen fractionation is
rather strong (0.20e0.24& change per 1 C), introducing considerable uncertainty in the calculation of the expected d18O due to small
uncertainties in Holocene temperature variations.
The use of D47 as an indicator for disequilibrium has several
advantages. In contrast to d18O, the equilibrium D47 value of DIC
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
entering the cave is only determined by the epikarst temperature
and is independent of the drip water d18O. Thus, the starting point
for D47 in the initial solution is known, given a sufficiently long
residence time in the overlying karst bedrock (longer than several
days) and similar temperatures in bedrock and cave. For example,
in the Bunker Cave area the water residence time in the epikarst is
in the order of few years (Kluge et al., 2010a,b) and in many other
caves the residence time is at least a few weeks, which is sufficient
for reaching isotopic equilibrium in the solution. Furthermore, the
disequilibrium offsets observed in speleothems (0.021
to 0.075& in this study, and within the same range for caves
examined by Affek et al. (2008) and Daëron et al. (2011)) is about an
order of magnitude larger than the temperature sensitivity of D47
(0.005e0.006& per 1 C) making it less sensitive than d18O to
uncertainties in Holocene temperatures.
The investigation of kinetic fractionation in stalagmites is not
limited to the Holocene. In older speleothems, temperatures have
to be constrained independently in order to investigate the degree
of kinetic fractionation from clumped isotopes. This can be estimated from other paleoclimatic archives in the region (e.g., marine
and lacustrine sediments, ice cores, tree rings) or from the stalagmites themselves, for example, through noble gas concentrations in
fluid inclusions (Kluge et al., 2008; Scheidegger et al., 2011).
Furthermore, Wainer et al. (2011) used water d18O in speleothem
fluid inclusions together with the co-variance between d18O and
D47 offsets to estimate paleotemperatures. D47 can thus replace or
complement the Hendy test in identifying and quantifying the
degree of kinetic fractionation, as long as reliable paleotemperature
estimates are available.
5.5. Possible mechanisms for D47 variations
Irrespective of direct attempts of paleoclimate reconstruction,
the variability observed in D47 offsets may be used to explore the
mechanisms controlling isotopic disequilibrium in speleothems.
The offset from equilibrium reflects the balance between three
processes:
(1) degassing of CO2 from the drip water solution, that creates
super saturation and leads to an increase in d13C and d18O and to
a decrease in D47 in the DIC; (2) buffering of both d18O and D47 by
oxygen isotope exchange between dissolved CO2 and water, that
pulls the DIC back towards equilibrium values; and (3) CaCO3
precipitation that records the DIC composition of the solution that
may be either in equilibrium or disequilibrium state. Degassing
proceeds quickly at a time scale of seconds. Buffering through
isotope exchange is slow, at a time scale of hours (several 10,000 s),
and the precipitation of calcite occurs at an intermediate rate that
depends on temperature, on the order of minutes (several 100 s to
2000 s; Dreybrodt and Scholz, 2011). Considering these timescales,
fast calcite precipitation is expected to result in unbuffered or
partially buffered DIC with respect to both oxygen isotopes and
clumped isotopes.
5.5.1. Influence of the drip interval
As precipitation of calcite proceeds fast compared to the time
scale of isotopic buffering, d18O and D47 values are modulated by
the drip interval. The drip interval affects the evolution of the DIC
composition through mixing with fresh solution brought by the
next drip that is likely to be at equilibrium (Mühlinghaus et al.,
2009). At relatively slow drips (drip intervals >100 s), DIC
undergoes Rayleigh-type isotopic evolution that is related to the
fraction of DIC that has been precipitated, and always tend
towards enrichment of the precipitated calcite (Mickler et al.,
2004; Dreybrodt and Scholz, 2011). Oxygen isotope exchange
with water is too slow to enable full buffering and return to
89
isotopic equilibrium. Fast drips, however, with very short drip
intervals (few seconds) have less time for degassing-related
enrichment which is further diluted by mixing with fresh solution. These drips should result in calcite that precipitates from
a marginally super saturated solution that is closer to isotopic
equilibrium.
In relatively slow drips, the degree of 18O enrichment in the DIC
is therefore related to the degree of super saturation (caused by the
initial degassing of CO2 from the solution and affected by the soilto-cave CO2 gradient) and the fraction of CaCO3 precipitated from
DIC, via the Rayleigh-type evolution of the drip-water solution. In
the case of high super saturation, i.e. high initial CO2 gradient
between solution and cave atmosphere, the large fraction of DIC
undergoing degassing leads to a large kinetic effect.
5.5.2. Influence of vadose zone processes
The isotopic composition of stalagmites can also be affected by
processes acting before the solution reaches the stalagmite, such as
prior calcite precipitation (PCP; e.g., Fairchild et al., 2000; Spötl
et al., 2005; Mattey et al., 2010) and potentially through retarded
precipitation, which starts hours after the initial super saturation
was reached. Retarded calcite precipitation may lead to d18O and
D47 values closer to equilibrium as it enables buffering, which
drives the DIC back towards equilibrium. An example can be water
in the vadose zone undergoing partial degassing (e.g., by flow
through seasonally ventilated void spaces, Sherwin and Baldini,
2011) and becomes slightly super saturated. In the absence of
nucleation sites, such as already abundant calcite crystals, precipitation may not begin until higher super saturation is reached by
additional degassing, or until the solution reaches the active
nucleation sites at the surface of speleothems. This has been
observed in laboratory and field experiments, where no mineral
was formed under low saturation levels in the absence of nucleation sites (Suarez, 1983; Dreybrodt et al., 1992, 1997), reflecting
a critical super saturation threshold that is required for the nucleation processes (Berner, 1980).
Long drip intervals (as observed during the monitoring period in
Bunker Cave, 100e600 s for BU1 and about 1000 s for BU4;
Riechelmann, 2010; Riechelmann et al., 2011) enable a Rayleightype evolution of the solution (see 5.5.1.), which can explain the
relatively large offsets in d18O and D47 found in the modern material
of both stalagmites (Table 1). However, disequilibrium effects have
been also detected for short drip intervals (e.g., in Katerloch Cave,
Baron Cave, and Villars Cave; Daëron et al., 2011) suggesting that
the “history” of the drip water prior reaching the stalagmite top,
such as prior-calcite precipitation, may be an important controlling
factor. The observed temporal change in kinetic fractionation
during the Holocene growth of Bunker Cave stalagmites BU1 and
BU4 may thus be related to a temporal change in drip rates, super
saturation, precipitation rates (that is related to both super saturation and drip rate), and variations in PCP (that affects super
saturation). These processes are explored in the following
paragraphs.
5.5.3. Influence of growth rate
Laboratory experiments of calcite precipitation in a bulk solution setup suggest a strong influence of precipitation rate on the
fractionation factor, with reduced 18O fractionation for high
precipitation rates (Dietzel et al., 2009). Note, that super saturation
in the Dietzel et al. (2009) setup is not controlled by degassing. Guo
(2008) also observed a correlation between precipitation rate and
offsets in D47, with increased precipitation rates related to lower D47
values, corresponding to a stronger disequilibrium.
Growth rate is therefore the most obvious parameter to explore
with respect to equilibrium precipitation. For BU1 and BU4 we
90
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
estimate growth rates from a UeTh age model (Fohlmeister et al.,
2012). This approach can only give average growth rates for each
time interval between dating points, whereas instantaneous
mineral precipitation rate may vary considerably. It can nevertheless give valuable information on general growth conditions and on
control by parameters like drip water supply and super saturation.
The two stalagmites and the modern stalagmite-like precipitates
span the typical range of stalagmite growth rates (Table 2; Figs. 4
and 5). Stalagmite BU4 with a mean growth rate of 23 mm/
a (between 13 mm/a and 270 mm/a) provides an example of slow
growth. Stalagmite BU1 grew faster, with an average growth rate of
100 mm/a (between 80 and 1500 mm/a). The modern stalagmite-like
calcites DC-1 and DC-2 grew faster than both stalagmites, with
growth rates of w270 mm/a. Whereas the large D47 offset of the
modern samples may be related to their high growth rate, no direct
correlation is observed between growth rates and the D47 offset in
the two stalagmites (Fig. 5). The average D47 offset of stalagmites
BU1 and BU4 is very similar in spite of their mean growth rates
being different by a factor of 5. Furthermore, the D47 offset of the
very slowly growing part of BU4 (w13 mm/a, 0.047 0.005&, 1s,
n ¼ 3) is indistinguishable from the fast growing parts of BU1 and
from DC1, DC2 (360 mm/a, 0.049 0.013&, 1s, n ¼ 5).
Growth rates, however, depend on both drip water supply and
super saturation, each having a potentially distinct effect on the
isotopic composition, so that the variation of both parameters has
to be taken into account. The modern-day drip rate of the fastgrowing BU1 is an order of magnitude higher than that of BU4.
On the other hand, the modern-day saturation indices of the two
stalagmite sites are practically identical (0.34 0.15 and
0.30 0.18; Riechelmann et al., 2011), implying that the differences
in growth rate are likely related to the water supply. Late Holocene
D47 offsets are identical for both stalagmites indicating that the
Fig. 4. D47 offsets, growth rates, carbon isotopes and radii of stalagmites BU1 and BU4
shown versus time. High resolution d13C record of BU1 (grey curve) and BU4 (black
curve) is adapted from Fohlmeister et al. (2012) and is shown for comparison.
water supply is not the main control on the degree of D47 offset.
Variations in D47 offsets are therefore related to changes in the
super saturation, at least for rather slow drips as in case of BU1 and
BU4 (see 5.5.1.).
5.5.4. Influence of super saturation
We further investigate the interplay between drip rate and
super saturation using additional constraints from stalagmite
morphology together with an assessment of changes in the drip
interval. The stalagmite radius (r, cm) is related to the drip interval
Tdrip (s) when drip water supply is continuous (Dreybrodt, 1999):
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
V
r ¼
p$ap $Tdrip
(4)
where V is the droplet volume (cm3); ap is a chemical kinetics
constant (cm s1), which in itself depends on the chemical parameters involved in the precipitation of calcite from a thin film such as
temperature, film thickness, and super saturation. ap relates
stalagmite elongation rate (growth rate along the growth axis) to
solution super saturation. Typically, short drip intervals lead to
a larger stalagmite radius than longer drip intervals at the same
super saturation. As the chemical kinetics parameter ap cannot be
assumed to be constant, it is impossible to extract climatic information from stalagmite morphology, but it can be used to constrain
findings from other proxies. Stalagmite BU4 has a radius of
2.7e2.9 cm at the top part where growth is relatively fast (0e2.5 ka;
growth in this context refers to the stalagmite elongation rate), and
3.4e3.5 cm in the lower part where growth is slower (3e8.2 ka).
Stalagmite BU1 shows similar radius variations at the same time
periods (2.1e4.0 cm, Fig. 4). Considering that four times the radius is
required in axial direction for a faithful recording of an equilibrium
shape (Dreybrodt, 1999), the smaller radii in the late Holocene are
likely to reflect the equilibrium shape following a change in
precipitation conditions. The reduced radius of 3 cm in BU1 starts
already 15 cm from top. BU4 marginally satisfies the requirement
(due to the lower growth rates), with the reduced radius starting
8 cm from the top. The stalagmites shape therefore indicates two
phases in growth regime: the late Holocene (<2.5 ka) with narrow
radii, and the early to mid Holocene with larger radii.
Precipitation rate can be estimated as a combination of elongation rate and radius, and is controlled by both drip water supply
and solution saturation levels. When comparing the two stalagmites during either the late or the early to mid Holocene, BU1
precipitates faster (namely, deposits more material) than BU4 at
both time intervals. The increase in elongation rates with time is
much larger than the decrease in radii, so that precipitation rates
are higher in the late Holocene, in particular in BU4 (wtwo-fold
increase).
Higher water supply (shorter drip intervals) in the late Holocene
is consistent with suggested wet late Holocene conditions in the
region. Wetter conditions during the last 2 ka and generally dry
conditions interspersed with short rainy periods between 5 and
8 ka were suggested based on an analysis of crystal fabrics in both
stalagmites, together with d13C and Mg/Ca values (Riechelmann,
2010). This interpretation is supported by studies of other stalagmites from the same region (Niggemann et al., 2003) and from the
northern Alps (Wurth et al., 2004). A clear climatic distinction
between the early to mid Holocene (8.6e5.5 ka) and the last 2.6 ka
is observed also in the wider regional sense, such as in the carbon
isotope record of lake sediments in Western Germany (Lücke et al.,
2003) or in the alkenone-derived SSTs of the Norwegian Sea (Calvo
et al., 2002) and the North Atlantic (Moros et al., 2004). The climate
in Germany is linked to these parts of the Atlantic via the North
Atlantic Oscillation.
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
91
Fig. 5. D47 offsets versus growth rates (A) and radius (B) of stalagmite BU1, BU4 and modern deposits (DC-1, DC-2). The growth rates of BU1 and BU4 are based on the age model of
Fohlmeister et al. (2012). The growth rate of the sample marked with an asterisk is highly uncertain.
The hydrological changes between early/mid and late Holocene
are likely to affect drip rates in Bunker Cave, resulting in shorter
drip intervals in the late Holocene and contributing to the higher
precipitation rate. However, a full drip rate control requires a large
increase in water supply in BU4 but hardly any change in BU1.
Although not impossible, it is unlikely that a change related to the
regional hydro-climate would affect the two drip sites so differently, especially given that the modern-day drip rate in BU4 is slow,
and should have been much slower in the early Holocene. It is more
likely that the large increase in precipitation rate in late Holocene
BU4 reflects a small change is water supply, combined with large
increase in super saturation.
The modulation of the super saturation results from changes in
both soil biological activity and in prior calcite precipitation (PCP,
which is observed in modern precipitates at the BU4 site;
Riechelmann, 2010). It is likely that climatic changes (e.g., wetter
conditions in the late Holocene) are accompanied by changes in both
soil activity and PCP. An increased soil activity would lead to 13Cdepleted DIC (Genty et al., 2006) and to higher super saturation,
whereas PCP would cause 13C enrichment and reduced super saturation (Baker et al., 1997). In addition, cave ventilation may also
affect super saturation and 13C enrichment through CO2 degassing.
At least one of these processes must have changed significantly
given the large variations in d13C observed in both stalagmites. d13C
shifted from relatively high values of 9.0& (BU1) and 8.3& (BU4)
at 5e8 ka to 10.6& and 9.4&, respectively, in the last 2 ka (Fig. 4).
Assuming that the cave ventilation regime has not changed significantly during the Holocene, d13C is likely controlled by the soil CO2
concentrations and by changes in PCP. PCP is expected to be stronger
during dry conditions, leading to reduced super saturation at the
stalagmite surface, whereas during wet periods, the pore-space of
the vadose zone is mostly water-filled and PCP is reduced. Therefore,
d13C may be interpreted as a scenario of increased PCP in the dryer
mid Holocene, possibly combined with lower soil biological activity.
Both mechanisms would result in relatively low drip water super
saturation and higher d13C values. The Mg/Ca ratios in stalagmite are
higher in the mid Holocene (Riechelmann, 2010), also supporting
PCP as main controlling mechanism.
We therefore suggest that the degree of kinetic fractionation, as
observed by D47 offsets in BU1 and BU4, is closely linked to the
super saturation at the stalagmite surface. Following the discussion
above, similar late Holocene D47 offsets in both stalagmites are
consistent with similar super saturation levels, whereas the
reduced super saturation inferred for BU4 in the early Holocene
results in an observed D47 value that is closer to equilibrium
(namely, a smaller offset).
6. Conclusion and outlook
D47 values in modern and Holocene speleothems show significant deviations from the expected equilibrium values. The robust
co-variation of d18O and D47 offsets suggests that the D47 deviation
from nominal equilibrium is a sensitive measure for kinetic fractionation in speleothems. It may thus enable identifying and
quantifying kinetic isotope fractionation during speleothem
precipitation based on well-controlled laboratory experiments or
in-situ cave sampling at sites with long-term monitoring. The
addition of clumped isotope measurements may add constraints to
elucidate some of the fundamental questions related to calcite
precipitation, such as under what conditions isotope equilibrium
can be expected.
The finding of spatial and temporal variability in the kinetic
isotope fractionation has important consequences for paleoclimatic
studies using C and O isotope signals in speleothems. Isotopic
signals in speleothems are commonly interpreted assuming equilibrium in which the transfer function between speleothem
composition and environmental conditions is constant over time,
such that isotopic signals are interpreted to reflect climatic variations only. Using clumped isotopes we identify disequilibrium that
shows significant temporal variability. The examined Bunker Cave
stalagmites show high kinetic fractionation in the late Holocene
and reduced kinetic effects in the mid to early Holocene. The kinetic
isotope fractionation is related to growth rate only indirectly,
whereas the main controlling mechanism is linked to changes in
prior-calcite precipitation (PCP) and related shifts in super saturation at the stalagmite surface. Time periods of drier climate, with
increased amount of air-filled pore space in the vadose zone are
likely to favour PCP, and hence to reduce the super saturation in the
drip water reaching the stalagmite, leading to a lower degree of the
kinetic fractionation.
Paleoclimate research using stalagmite d18O as a proxy for
temperature and rainfall should include an independent
92
T. Kluge, H.P. Affek / Quaternary Science Reviews 49 (2012) 82e94
assessment of the degree of kinetic fractionation throughout the
stalagmite record in order to distinguish between variability that is
intrinsic to the cave system (such as the degree of kinetic fractionation) vs. climatic parameters. Clumped isotopes are especially
valuable for this purpose due to the well-defined equilibrium
relationship and the small effect of the temperature uncertainty on
the equilibrium reference. For meaningful application of clumped
isotopes as a proxy for kinetic fractionation to pre-Holocene calcite
the growth temperature should be reasonably well known (at the
2 C level). Alternatively, paleotemperatures can be determined
from calcite and drip water d18O in combination with clumped
isotopes analyses to constrain kinetic isotope fractionation also in
case of speleothems with unknown growth temperatures, using
independent constraints on drip water compositions (Daëron et al.,
2011; Wainer et al., 2011).
Acknowledgement
The research was funded by the German science foundation DFG
(Forschungsstipendium KL2391/1-1 to TK) and the National Science
foundation (NSF-EAR-0842482 to HPA). We are grateful for samples
provided by the DAPHNE group and thank Christoph Spötl for stable
isotope data and Denis Scholz, Dana Riechelmann, Sylvia Riechelmann, Stefan Niggemann, Andrea Schröder-Ritzrau, Jens Fohlmeister, René Eichstädter, and Augusto Mangini for background
information and discussion. We thank the Earth System Center for
Stable Isotope Studies of the Yale Institute for Biospheric Studies. The
manuscript greatly benefited from comments and suggestions of
A.N. Meckler and an anonymous reviewer.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.quascirev.2012.
06.013.
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