Partly coeval flowstones formed in fractured gneiss and schist were studied
to test the palaeoclimate significance of this new type of speleothem archive
on a decadal-to-millennial timescale. The samples encompass a few hundred to
a few thousand years of the Late Glacial and the early Holocene. The
speleothem fabric is primarily comprised of columnar fascicular optic calcite
and acicular aragonite, both indicative of elevated Mg / Ca ratios in the
groundwater. Stable isotopes suggest that aragonite is more prone to
disequilibrium isotope fractionation driven by evaporation and prior
calcite/aragonite precipitation than calcite. Changes in mineralogy are
therefore attributed to these two internal fracture processes rather than to
palaeoclimate. Flowstones formed in the same fracture show similar
δ18O changes on centennial scales, which broadly correspond to
regional lacustrine δ18O records, suggesting that such
speleothems may provide an opportunity to investigate past climate conditions
in non-karstic areas. The shortness of overlapping periods in flowstone
growth and the complexity of in-aquifer processes, however, render the
establishment of a robust stacked δ18O record challenging.
Introduction
Speleothems from karst caves have contributed important information on
past climate change from orbital to seasonal timescales worldwide
(e.g. Johnson et al., 2006; Boch et al., 2011; Fohlmeister et al.,
2012; Wang et al., 2014; Webb et al., 2014; Luetscher et al., 2015;
Cheng et al., 2016). While the importance of these speleothems as
palaeoclimate archives is firmly established, very little is known
about the palaeoclimate potential of such deposits from non-carbonate
settings. Since only ∼20 % of the ice-free continental area
of the Earth consists of carbonate rocks prone to karstification (Ford
and Williams, 2007) speleothems from non-karstic settings may provide
relevant palaeoclimate archives for semi-arid (Koltai et al., 2017),
super-humid (Schmipf et al., 2011) and subglacial environments (Frisia
et al., 2017).
Oblique view of the Vinschgau valley. Red points show the occurrence
of vein-filling flowstones in the lower part of the south-facing slope
(Sonnenberg). Yellow points mark the three sampling sites.
In comparison to other palaeoclimate archives, a key advantage of
speleothems is that they can be dated with high precision by U–Th
techniques up to about half a million years (e.g. Richards and Dorale,
2003; Scholz and Hoffmann, 2008; Cheng et al., 2013). Since silicate
rocks usually contain much more uranium than limestones or dolostones
(Wedepohl, 1995), the 238U content of speleothems forming in
cavities and fractures of crystalline rocks is commonly orders of
magnitude higher, and therefore they require very small sample amounts to
yield high-resolution chronologies (Spötl et al., 2002; Koltai
et al., 2017).
The most widely used proxies of karst speleothems include stable
oxygen and carbon isotopes, trace elements, growth rate and fabric
changes (e.g. Frisia et al., 2003; McDermott, 2004; McMillan et al.,
2005; Wassenburg et al., 2012). Although the majority of
speleothem-based climate reconstructions utilized calcite speleothems,
several studies indicate that aragonite can also record past climate,
in particular rainfall variability (e.g. Polyak and Asmerom, 2001;
Ridley et al., 2015; Wassenburg et al., 2016).
Here, we present a well-dated, multi-proxy record of eight fast-growing
flowstones from a non-karstic setting in a dry inner-alpine setting (Vinschgau). Unlike speleothems formed in karst
caves, these calcite and aragonite flowstones were deposited in near-surface
fractures created by gravitational mass movements. The aim of this study was
to test the reliability of climate proxies preserved in this hitherto largely
neglected type of speleothem archive. To this end we chose the Late Glacial
to early Holocene time interval which is among the best characterized periods
in the late Quaternary of the Alps based on studies of lake sediments (e.g.
von Grafenstein et al., 1999, 2013; Magny et al., 2001; Ilyashuk et al.,
2009; Lauterbach et al., 2011; Heiri et al., 2014), palaeoglaciers (e.g.
Kerschner et al., 2000; Ivy-Ochs et al., 2008; Kerschner and Ivy-Ochs, 2008)
and speleothems (Wurth et al., 2004; Luetscher et al., 2016). By comparing
our speleothem data to these published records we explore the extent to which
these non-karstic speleothems register the well-documented succession of
rapid climate change during this time period.
Site description and samplesStudy site
The Vinschgau is a E–W trending inner-alpine valley shielded by high
mountain chains in the north, west and south. Although it is one of the
driest valleys of the eastern Alps today (Fliri, 1975; ZAMG, 2015), local
climate archives such as large debris-flow fans (which are largely inactive
today) point to periods of distinctly more humid climate during the Late
Glacial and Holocene in this valley.
The study area is comprised of paragneiss, orthogneiss and schists
that are heavily fractured as a result of deep-seated gravitational
mass movements along the Sonnenberg (Ostermann et al.,
2016). These fractures provide pathways for groundwater flow. Due
to pronounced water–rock interaction dominated by pyrite oxidation and
the presence of large internal mineral surfaces created by the mass
movements, these waters are highly mineralized. Their electric
conductivity varies between 730 and 2300 µScm-1 and
they are characterized by elevated molar Mg / Ca ratios
between 0.3 and 3.3. Due to evaporation and concomitant CO2
degassing groundwater reaches supersaturation on the south-facing
Sonnenberg slope, where springs are supersaturated with respect to
calcite and (except for one spring) aragonite. Speleothems form as
calcite and aragonite flowstones in the shallow subsurface and
calcitic freshwater tufa deposits are present on the surface
(Spötl et al., 2002; Koltai et al., 2017).
Speleothem samples
Eight vein-filling flowstones (Fig. S1 in the Supplement) were obtained from
three different fractures (Fig. 1). It must be emphasized that the samples
were found in the debris next to the fractures and their exact position
within the fractures is unknown. LAS 1, LAS 2 and LAS 21 were collected at
Törgltal (TR; 46.631∘ N, 10.680∘ E), while
LAS 6, LAS 34 and LAS 72 are from Stollenquelle (SQ; 46.630∘ N,
10.683∘ E). These two sites are less than 1 km apart. The
third site, Kortsch (KO; 46.635∘ N, 10.763∘ E), is situated
approximately 6 km east of SQ. Two samples (LAS 10 and 19) were
collected there.
Aragonite is present near the top of samples LAS 2, LAS 6 and LAS 21. Thus,
the uppermost 12, 15 and 6 mm of LAS 2, LAS 6 and LAS 21
respectively were not included in this study. As LAS 10 grew between 9.26±0.10 and 10.22±0.06 ka according to the depth–age model, the
present study focuses only on the lower 21.4 mm of this flowstone
(Fig. S1).
MethodsPetrography
Thin sections were analysed under transmitted-light and blue-light
epifluorescence microscopy in order to identify characteristic fabrics
and areas of replacement of aragonite by calcite. Furthermore, small
aliquots of carbonate powder were obtained from LAS 2, LAS 6 and
LAS 34 using a handheld dental drill in order to determine the
mineralogical composition by X-ray diffractometry (XRD).
Stable isotopes
Samples for stable oxygen and carbon isotope analyses were micromilled
at different resolutions. LAS 2 was sampled at 0.1 mm
intervals, LAS1, LAS 6, LAS 34 and LAS 72 were micromilled at
0.15 mm increments. In order to reach multi-annual
resolution (3 years) LAS 6 was also analysed by using three
2.5 mm long parallel tracks milled with a 0.8 mm
offset perpendicular to the lamination. LAS 10 and LAS 19 were
analysed at 0.2 mm, while LAS 21 was analysed at
0.25 mm resolution. Stable isotope measurements were performed
using a Thermo Fisher Scientific DELTAplusXL mass
spectrometer. Isotope values are reported against the VPDB scale and
the long-term analytical precision (1σ) of the
δ18O and δ13C measurements is 0.08 and
0.06 ‰ respectively (Spötl, 2011).
U–Th dating
A total of 77 powder samples were prepared for radiometric dating. If
present, primary aragonite was preferred over calcite, since
aragonite-to-calcite transformation may alter the geochemical composition
(e.g. Lachniet et al., 2012; Domínguez-Villar et al.,
2017). U–Th dates were divided amongst samples as follows: 11 dates measured
from LAS 2 and LAS 6, 10 from LAS 1, 8 from LAS 10 and LAS 21, 14
from LAS 19, 9 from LAS 72 and 6 from LAS 34. Aliquots were obtained
for U–Th dating from distinct growth layers using a handheld drill. The
weight of individual subsamples ranged between 1.0 and 9.0 mg.
The samples were analysed at the Xi`an Jiaotong University (China) following
standard chemistry procedures of Edwards et al. (1987) to separate uranium
and thorium. U and Th isotopes were analysed individually by using
a multicollector inductively coupled plasma mass spectrometer (Thermo Fisher
Neptune Plus) as described by Shen et al. (2012) and Cheng et al. (2013).
Final 230Th ages are given with their 2σ uncertainties as
years before AD 1950 (BP). Corrected and uncorrected results are given in
Table S1 in the Supplement. Corrected ages assume an initial
230Th/232Th ratio of 4.4±2.2×10-6 of
bulk Earth (Wedepohl, 1995). Separate age models for all samples were built
using the StalAge algorithm (Scholz and Hoffman, 2011).
Aragonite and calcite textures. (a) Boundary of fascicular
optic calcite (Cfo) and acicular aragonite (Aa), showing competing crystal
growth between the two polymorphs (blue line, sample LAS 2).
(b) Co-precipitation of primary calcite and aragonite (sample
LAS 21). (c) Complex fabric dominated by mosaic calcite (Cm) where
fascicular optic calcite polycrystals (Cfo) are locally present between
acicular aragonite (Aa) and fascicular optic calcite (Cfo, sample LAS 34).
Stable isotope composition of the Vinschgau flowstones.
δ18O (‰) δ13C (‰) SampleMineralogyMin.Max.MeanMin.Max.MeanTR LAS 1calcite–aragonite-12.8-11.1-11.9-2.24.3-0.2LAS 2calcite–aragonite-13.0-9.7-11.7-2.37.30.9LAS 21calcite–aragonite-13.1-9.8-12.1-2.32.4-0.7SQ LAS 6calcite-14.1-11.8-13.2-2.9-0.3-2.1LAS 34aragonite-12.8-9.9-11.3-1.06.22.2LAS 72aragonite-11.8-9.2-10.7-1.87.31.6KO LAS 10calcite-12.3-10.3-11.3-5.6-1.4-3.2LAS 19calcite-13.5-10.1-11.7-4.5-0.8-2.8ResultsPetrography
The crystal fabric of LAS 6, LAS 10 and LAS 19 is dominated by
columnar fascicular optic calcite (Cfo; Frisia, 2015), showing
undulose extinction due to the systematic change in the orientation of
the c axes (Kendall, 1985; Richter et al., 2011; Frisia,
2015). Detritus-rich layers are locally present in LAS 10 and
LAS 19. Their average thickness varies from 25 to 50 µm in
LAS 10 and from 50 to 75 µm in LAS 19, but in the
latter sample, detritus-rich layers up to 0.25 mm thick are
also present.
LAS 72 is comprised of white acicular aragonite, while translucent
calcite (Cfo) is also present in LAS 1, 2, 21 and 34 (Fig. 2a). XRD
results indicate 100 % calcite in the calcite layers of LAS 2 and
34. In LAS 2 and also in LAS 21 pristine aragonite islands are locally
present in the calcite fabric that show no sign of dissolution,
suggesting co-precipitation of aragonite and calcite (Fig. 2b).
Thin-section analyses of LAS 34 revealed the presence of mosaic
calcite (Fig. 2c), a fabric indicative of recrystallization
(e.g. Frisia, 2015). As recrystallization may have modified the
geochemical composition of the calcite, only the aragonite fabric is
discussed further in this study. None of the other samples show any
sign of diagenetic alteration.
From 60 until 110 mm distance from top (dft), LAS 6 exhibits annual calcite
lamina couplets which can be observed macroscopically as successive
white and translucent laminae. The white laminae are rich in opaque
particles, whose organic origin is confirmed by their strong
epifluorescence (Koltai et al., 2017). Similarly, the inclusion-rich
layers in the fascicular optic calcite of LAS 10 and LAS 19 show
excitation under epifluorescence. Furthermore, weakly fluorescent
laminae are present in LAS 72, while both calcite and aragonite layers
in LAS 2 and LAS 21 appear dull.
Stable isotope composition
The summarized results of the stable isotope analyses are presented in
Table 1. High-resolution stable isotope profiles of the flowstones collected
near TR show very similar values for δ18O, while carbon
isotope values are more diverse. Even though δ13C minima are
identical within the 1σ analytical error in these samples, the highest
carbon isotope values range from 2.4 to 7.3 ‰
(Table 1).
As Table 1 shows, the SQ samples are characterized by different values
regarding both δ18O and δ13C. The two
flowstones dominated by aragonite (LAS 34 and LAS 72) exhibit higher
isotope values, while the calcite samples from KO are characterized by
the lowest δ13C values.
The majority of the flowstones do not exhibit a correlation (R2<0.60) between δ13C and δ18O values,
with the exception of three samples (LAS 2, LAS 34 and LAS 72) that
show a significant correlation between the two isotopes with slopes of
regression varying from 2.5 to 3.4 (Fig. 3). The crystal fabric of
these samples is dominated by acicular aragonite. In LAS 2 the
covariance of δ13C and δ18O is
characterized by almost identical slopes of the regression lines for
both aragonite (δ13C=2.7×δ18O+32.4,
R2=0.79) and calcite (δ13C=2.9×δ18O+35.2, R2=0.60; Fig. 3a). On the
contrary, LAS 1 does not show any covariance for calcite (R2=0.33) and aragonite (R2=0.13). The number of stable isotopes
analyses (n=7) in the aragonite of LAS 21 was too small to
investigate the relationship of δ13C and
δ18O variability.
(a) Isotope crossplot for samples LAS 2, LAS 34 and LAS 72.
The highly significant correlation (R2≥0.60) between the two isotopes
suggests strong disequilibrium-controlled isotope fractionation.
(b) Calcite samples only show a weak correlation between
δ18O and δ13C (R2<0.60).
Carbon isotope values mostly follow the first-order changes of oxygen
isotopes in all samples except LAS 1. However, the relationship
between the two isotopes may vary within a given sample. In LAS 6 and
LAS 21 this relationship breaks down in the 13 and
7 mm dft respectively, while in LAS 19
rising δ18O values correspond to decreasing
δ13C levels from 66 to 54 mm (dft).
Major changes in δ18O values are observed in LAS 2 and LAS 19.
The former sample exhibits generally high oxygen isotope values in the
aragonite growth phase from the bottom until 50 mm (dft), interrupted
by periods of lower δ18O values. A 0.8 ‰ decrease is
seen between 74 and 72 mm (dft) coinciding with the presence of
a calcite layer. A gradual shift of 2.2 ‰ towards lower values is
observed from 52 to 47 mm (dft) and is independent of the fabric,
while a 1.6 ‰ rise in δ18O characterises the calcite
from 17 to 14 mm (dft). In LAS 19 a significant 3.2 ‰ shift
towards more positive δ18O values occurs from 66 to
54 mm (dft), followed by a decrease in carbon isotope values.
Moreover, in all samples δ18O values show high-frequency
changes of different amplitude (0.5 to 1.5 ‰), while no major trend
is observed.
Chronology
The Vinschgau samples show exceptionally high 238U concentrations,
ranging from ca. 1.5 to 1200 ppm (Table 1) and are among the most
U-rich speleothems ever reported (Spötl et al., 2002; Kelly et al.,
2003). δ234U ranges from 7 to 100 ‰. The
232Th content is highly variable and fluctuates between
61 ppt and 178 ppb (Table S1). Except for three subsamples of
LAS 19 all samples show high 230Th/232Th activity
ratios, and thus excess 230Th has no significant influence on the
final ages.
Of the 77 total dates measured, 74 are in stratigraphic order within their
2σ uncertainties. The three dated offsets (from samples L6-54, L6-79
and L1-38.5) are 5, 2 and 60 years beyond stratigraphic order respectively
(Table S1). As these differences represent less than 0.5, 0.25 and ∼2 ‰ age deviation for L6-54, L6-79 and LAS 34 respectively, we do
not consider these ages as outliers and include them in the age models
(Figs. S1–S2).
LAS 1 formed between 12.99±0.05 and 12.01±0.03 ka, while LAS 2
grew uninterruptedly between 14.18±0.03 and 12.12±0.03 ka. The
last sample from the TR site, LAS 21 initiated deposition at 12.28±0.03
and grew continuously until 11.68±0.02ka
(Figs. S2–S3).
LAS 6 from the SQ site formed between 12.06±0.04 and 11.68±0.03 ka. Similarly to LAS 2, growth of LAS 34 commenced 14.18±0.03 ka and ended at 12.54±0.03 ka, showing no major growth
interruptions. LAS 72 provides a record between 11.64±0.04 and 10.03±0.03 ka. The age model of LAS 72 shows that there may have been
a hiatus from 10.54 to 10.30 ka (modelled ages) corresponding to 13 to
14 mm on the depth scale (Fig. S3d). However, as thin-section analysis
provided no evidence for a growth interruption (e.g. corrosion layer) we
attribute this to an interval of slow growth rates.
The U–Th dates of the studied section of LAS 10 range from 9.94±0.03 to
10.21±0.06 ka, while LAS 19 started to form in the Younger Dryas (YD) at 11.98±0.05 ka and stopped growing in the early Holocene at 10.78±0.04 ka
(Figs. S2–S3).
DiscussionStable isotope systematicsδ18O
In karstic settings, speleothem δ18O values depend on
the δ18O composition of drip water and on the cave air
temperature, the latter influencing water–carbonate fractionation
factors for both calcite and aragonite (e.g. McDermott, 2004;
Lachniet, 2015). Modern spring monitoring in the Vinschgau suggests
that the speleothem-forming waters are part of a larger groundwater
system recharging at an elevation ranging from about 1200 to
2100 ma.s.l. Minimal variation in stable isotope composition
and low tritium content point to long mean residence times of up to
several decades (Spötl et al., 2002).
LAS 6 exhibits annual petrographic and geochemical lamination. Stable
isotope analyses and heat-transfer modelling indicate that its
δ18O oscillations are dominated by surface temperature
changes transmitted to the subsurface via heat conduction (Koltai
et al., 2017). Although δ18O provides a proxy for
seasonal fluctuations in surface temperature (Koltai et al., 2017),
its variability on a multi-annual timescale is well replicated by
LAS 19, implying that the two flowstones were deposited close to
isotopic equilibrium (Dorale and Liu, 2009).
As the well-developed petrographic lamination is due to traces of varying
amounts of humic and fulvic acids, the lack of such regular laminae can be
used as an indirect proxy for the depth of a given fracture. Thus we assume
that non-laminated flowstones formed at greater depths and the temperature in
these subsurface fractures most likely reflect the outside mean annual air
temperature. Spötl et al. (2002) reported that none of the nearby
perennial springs showed intra-annual temperature variability, supporting
this assumption. The water temperature of such a spring at the SQ site was
constant (12.8 ± 0.1 ∘C) during the 2-year monitoring period.
Water dripping from the slope breccia at KO, however, showed 3.7 ∘C
variability (Spötl et al., 2002), which can be explained by the seasonal
influence of the outside air. As we assume that the fractures were not
influenced by seasonal changes in air temperature the δ18O
signal of the Vinschgau flowstones is primarily regarded as a proxy for
δ18O of groundwater and local precipitation. In mid- and high
latitudes a well-established relationship exists between air temperature and
the oxygen isotopic composition of precipitation; i.e. a temperature rise of
1 ∘C leads to 0.59±0.09 ‰ higher isotope values
(Rozanski et al., 1992). This is partially counterbalanced in the speleothem
δ18O signal by the isotope fractionation during
calcite/aragonite formation. The temperature dependence of the oxygen isotope
fractionation during calcite precipitation is
-0.24 ‰∘C-1 based on experimental studies (Kim
and O'Neil, 1997), while a somewhat higher value
(-0.18 ‰∘C-1) was determined by a cave-based
study (Tremaine et al., 2011). Kim et al. (2007) reported a similar value
(-0.22 ‰∘C-1) for the temperature coefficient
for the oxygen isotope fractionation of aragonite. Consequently, for the
study area a positive (negative) net isotope change is expected in the
speleothem δ18O signal during climate amelioration
(deterioration). This signal, however, may be modified to a variable extent
by in-aquifer processes as discussed in 5.2.
δ13C
The interpretation of the carbon isotope signal of speleothems from
karst caves is commonly more challenging than that of
δ18O (e.g. McDermott, 2004), since δ13C
values can be affected by a variety of processes including carbon
dynamics of the soil and epikarst, subsurface air ventilation and
associated disequilibrium isotope fractionation. Today, the Sonnenberg
slope is mainly covered by sandy pararendzinas (Florineth, 1974) and
contains semi-arid vegetation. The strong soil moisture deficit on
the slopes (Della Chiesa et al., 2014) may limit the amount of solutes
entering the fracture system (Fairchild and Baker,
2012). Additionally, some of the carbonate is derived from the
crystalline host rock, in particular local occurrences of
Fe carbonates (Spötl et al., 2002).
Although carbon isotopes mostly follow the fluctuations of
δ18O, this relationship can vary within a given sample (e.g.
LAS 19). LAS 19 shows a δ18O rise of 3.2 ‰ at the
YD–Holocene transition, followed by a similar decrease in carbon isotopes
(Fig. S4), suggesting that during certain time intervals carbon isotopes may
reflect a soil signal, where lower δ13C values correspond to
an increase in soil bioproductivity (Genty et al., 2001; Fairchild and Baker,
2012; Borsato et al., 2015). As discussed below this signal is, however,
masked by in-aquifer processes.
δ18O variability of the TR samples LAS 1
(orange), LAS 21 (green) and LAS 2 (blue) in their overlapping
sections. All samples are plotted based on the modelled
ages. 230Th ages with their corresponding errors are
plotted above each δ18O time series. Pink rectangles
indicate aragonite fabric.
Internal aquifer processes
In a subsurface fracture system like the Sonnenberg where speleothems
form as vein-filling calcite and aragonite, prior calcite
precipitation (PCP) and/or prior aragonite precipitation (PAP) are
expected to influence the carbon isotope composition of speleothems
(e.g. Fairchild and Baker, 2012). However, recent laboratory experiments
indicate that PCP has an effect on the oxygen isotope systematics of
the precipitating calcite as well (Polag et al., 2010; Dreybrodt and
Scholz, 2011). Thus we propose that PCP may lead to progressively
higher oxygen isotope values along the flow path, even if this change
is of much smaller amplitude than that of
δ13C. Although similar experiments investigating the
influence of PAP on the stable isotope composition of the
precipitating aragonite are lacking, a simultaneous enrichment in
13C and 18O may be expected. Evaporation-induced
disequilibrium fractionation is also likely to have an effect on the
stable isotope values of calcite and aragonite flowstones since
evaporation and associated CO2 degassing are the primary
drivers of secondary carbonate deposition. Evaporation exerts
significant control on oxygen isotope values, resulting in the
enrichment of 18O in the remaining water and consequently
higher δ18O levels in the calcite/aragonite, while
CO2 degassing affects carbon levels. Stable isotope and
temperature monitoring of a shallow underground pool and its
associated actively forming calcite speleothem indicates that calcite
precipitation occurs close to isotopic equilibrium with respect to
δ18O, while δ13C levels strongly deviate
from equilibrium (Spötl et al., 2002). This is also supported by
the fact that even though carbon and oxygen isotopes show a covariance
in several flowstones, calcite samples and LAS 1 do not exhibit
co-varying δ13C and δ18O values.
Similarities in the absolute δ18O values of the three
TR samples (LAS 1, LAS 2 and LAS 21) further corroborate this,
suggesting that despite PCP/PAP occurrence along the flow path,
flowstone precipitation occurred close to isotopic
equilibrium. Therefore, we propose that disequilibrium fractionation
likely had a negligible influence on the δ18O of
speleothem calcite.
In contrast, given that δ13C and
δ18O values co-vary in the aragonite samples (LAS 34
and LAS 72) and also in LAS 2 independent of its mineralogy (Fig. 3),
these samples may have formed out of isotopic equilibrium. Although
the covariance of carbon and oxygen isotope values may result from
in-aquifer processes, it has been widely used as an indicator of
disequilibrium isotope fractionation (Hendy, 1971). The slope of
regression of Δδ13C/Δδ18O
varies between 2.5 and 3.4 in LAS 2, 24 and 72 (Fig. 3a). Such values
indicate that disequilibrium isotope fractionation occurred during
aragonite precipitation, whereby CO2 hydration and
hydroxylation reactions promoting oxygen isotope exchange between
HCO3 reservoir and H2O were not fast enough to
maintain isotopic equilibrium (Mickler et al., 2006). The lack of
a similarly strong correlation between the two isotopes in the flowstone
samples dominated by calcite (Fig. 3b), except for LAS 2, further
supports the influence of disequilibrium isotope effects over that
of in-aquifer processes. As none of these springs are presently
precipitating aragonite, it is hard to distinguish whether evaporation
and PCP/PAP or disequilibrium isotope fractionation, or a combination
of these processes, led to the covariation between
δ18O and δ13C in the flowstones.
To further analyse the potential influence of disequilibrium processes
and local hydrology on the isotopic composition of the Vinschgau
flowstones, coeval sections were compared. Unfortunately, in all cases
the common time window of deposition is too short to provide reliable
analyses using statistical methods (e.g. Fohlmeister,
2012). Nevertheless, as the deposition of the three TR samples partly
overlaps, intra-fracture variability can be tested on decadal-to-centennial scales despite differences in growth rate and hence proxy
data resolution. Given the similarities in the range of
δ18O variability (Fig. 4), mineralogy, and in the lack of
fluorescent lamination, we suggest that flowstones in a given fracture
form under very similar conditions and therefore most probably record
the local climate signal. Differences in the absolute values of
δ18O and δ13C are attributed to PAP and
PCP.
Stable isotope variability of LAS 72 (pink) and LAS 19 (blue)
between 12 and 10 ka. Dashed lines in the stable isotope time
series of LAS 72 represent the period characterized by slow growth rate.
Dashed tie lines indicate similarities between the δ18O
variability of the two flowstones. Note that LAS 72 is comprised of aragonite
as indicated by the pink rectangle, while LAS 19 is a calcitic flowstone.
Comparison of δ18O time series of
(a) benthic ostracods from Mondsee (Lauterbach et al.,
2011) and the Vinschgau flowstones (b–e). (b) shows the three samples from SQ site:
LAS 72 (bright pink), LAS 6 (turquoise) and LAS 34 (dark pink). The
δ18O variability of the TR samples is shown in
(c) and (d), where dark blue represents LAS 2,
green and orange mark the oxygen isotope record of LAS 21 and LAS 1
respectively. LAS 10 (yellow) and LAS 19 (light blue) from KO are
shown in (e). Note that the pink rectangles are indicative
of aragonite and the blue bar refers to the Younger Dryas.
A stronger influence of the local hydrology and hence a lower climate
signal / noise ratio is expected when comparing flowstones from
different sites (Fig. 5). Due to the inter-fracture variance in
PCP/PAP, calcite was deposited at KO (sample LAS 19), while at the same
time aragonite formed at the SQ site (sample LAS 72). Still, their
δ18O pattern shares some similarities within the
combined errors of the two age models, while δ13C in
the aragonite specimen shows a much larger amplitude (8.9 ‰)
than in the coeval calcitic one (3.6 ‰). This most likely
reflects the combined influence of disequilibrium isotopic
fractionation and the difference in the fractionation factor between
the two polymorphs (Morse and Mackenzie, 1990; Frisia et al.,
2002). Frisia et al. (2002) reported that carbon isotopes are 2 to
3.4 ‰ higher in aragonite than in calcite at Grotte de
Clamouse (southern France). δ13C values similar to that of
LAS 72 were reported from modern aragonite from Obstanser Eishöhle
(2.4 to 7.0 ‰), an alpine cave in southern Austria (Spötl
et al., 2016). Moreover, PCP (PAP) may have further increased the
δ13C values in the Vinschgau sites.
Potential as a palaeoclimate archive
Similar to speleothems from karst caves in semiarid settings
(e.g. Avigour et al., 1992; McMillan et al., 2005; Hoffmann et al.,
2016), fracture-filling flowstone from non-carbonate,
climate-sensitive settings may provide a useful record of
palaeoaridity and palaeohydrology. Annually laminated flowstones
(e.g. LAS 6) that formed at a few metres depth may provide insights
into changes in seasonality (Koltai et al., 2017), while those from
deeper fractures likely record changes at multi-decadal-to-centennial
resolution and thus provide a fragmented archive of local climate
history. This case study shows, however, that fracture-filling
speleothems also record the inherent heterogeneity of such fractured
aquifers, which may mask short-term climate signals.
As aragonite is metastable at Earth's surface conditions and hence
susceptible to diagenetic transformation, the possible alteration of the
geochemical signal has to be considered (e.g. Domínguez-Villar et al.,
2017 and references therein). Moreover, Lachniet (2015) emphasized that the
δ18O variability of aragonite speleothems should only be used
as a proxy if aragonite precipitation occurred close to isotopic equilibrium.
Thin section and XRD analyses indicate pristine aragonite in LAS 1, 2, 34 and
72. Aragonite preservation in these samples is further supported by the fact
that all 230Th ages are in stratigraphic order regardless of
mineralogy (Table S1). Nevertheless, flowstone deposition was most probably
influenced by disequilibrium isotope fractionation as suggested by the high
correlation between carbon and oxygen isotopes in LAS 2, 34 and 72 (Fig. 3).
Therefore, δ18O variability should be interpreted carefully in
these three samples.
Moreover, the timing of aragonite deposition does not show any
systematic relationship between the samples during the Late Glacial and
the early Holocene (Figs. 4–6). Instead petrographic analyses and
hydrochemistry data of modern springs (Spötl et al., 2002) suggest
that due to the high degree of total dissolved solids only small
changes in water chemistry give rise to either aragonite or calcite
precipitation, partly reflecting the heterogeneity of the fractured
aquifer. Similarly, changes in growth rate are first and foremost
driven by in-aquifer processes including PCP and/or PAP, as indicated
by the TR samples (Fig. 4). Therefore, calcite–aragonite transitions
and growth rate changes do not necessarily reflect an external
(climate) signal, unless coeval samples show a coherent pattern.
Carbon isotope data suggest a weak soil-derived signal for short time periods
only (e.g. at the YD–Holocene transition in LAS 19; Fig. S4), while most
values suggest buffering by inorganic carbon in conjunction with kinetic
isotope enrichment (Spötl et al., 2002).
The most prominent feature of δ18O proxy record is the
∼3.2 ‰ rise in LAS 19 at the YD–Holocene transition
(Fig. 6e). Moreover, the first-order pattern of the two aragonite
samples covering the Bølling–Allerød warm phase shows close
resemblance to the δ18O variability of the ostracod
record from Mondsee (Lauterbach et al., 2011), a lake in central
Austria, suggesting that centennial-to-orbital-scale large-amplitude
changes of the Northern Hemisphere climate system are recorded in the
δ18O variability of this archive.
During the YD a gradual ∼1.7 ‰ decline in δ18O
is observed in LAS 21 between 12.2 and
11.7 ka. Parts of this
shift are also captured by LAS 1 and LAS 19, implying a related cause.
Several terrestrial archives across Europe record a change in regional
climate mid-way through the YD (e.g. Brauer et al., 2008; Bakke et al., 2009;
Baldini et al., 2015; Belli et al., 2017). The onset of this transition was
time-transgressive (∼12.45 to
12.15 ka) across Europe
due to the gradual northward shift of the polar front driven by the
resumption of the North Atlantic overturning (Lane et al., 2013;
Bartolomé et al., 2015). Whether this shift towards lower values in our
δ18O record corresponds to the change in the regional climate
or to in-aquifer processes remains unclear given the lack of flowstone
samples covering the entire YD.
Conclusions
Petrographic and geochemical analyses of vein-filling calcite and
aragonite flowstones in near-surface fractures indicate that the
latter polymorph is more susceptible to disequilibrium processes
regarding both δ18O and δ13C. The two
most important in-aquifer processes modifying the geochemical
signature of these speleothems are evaporation and PCP/PAP. Both of
these processes are likely to govern variations in speleothem
mineralogy, as indicated by the deposition of coeval aragonite and
calcite flowstones. Accordingly, changes in speleothem mineralogy
cannot be used to constrain the timing of past episodes of high
vs. low precipitation in the Vinschgau.
δ18O variability has been proven to be the most reliable
climate proxy in the Vinschgau flowstones. Low-amplitude,
high-frequency (decadal-scale) variability in LAS 1, 6, 10, 19 and 21
is attributed to in-aquifer processes, while the centennial-scale
variability shows significant variation (e.g. 3.2 ‰)
suggesting changes in the δ18O of
precipitation. Although local factors, such as strong evaporation and
PCP/PAP can amplify these climate signatures, the δ18O
values show a broadly similar pattern to regional δ18O
lacustrine records (e.g. Mondsee).
Due to the lack of long overlapping sections of speleothem growth and the
complexity of in-aquifer processes this case study shows that it is highly
challenging to establish a robust stacked δ18O record of local
climate change on multi-millennial to orbital timescales using such
speleothems. However, it is possible that fracture-filling calcite and/or
aragonite from other areas have great potential as a climate archive if the
local hydrogeological conditions are well constrained. Our study also
emphasises that tight age control and a multi-proxy approach are essential
for the study of such non-karstic settings.
Data availability
Stable isotope data reported in this article is available
upon request from the authors.
The supplement related to this article is available online at: https://doi.org/10.5194/cp-14-369-2018-supplement.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This project was supported by the Autonome Provinz Bozen-Südtirol
(no. 16/40.3). Daniela Schmidmair is acknowledged for XRD analyses and
Kathleen Wendt for linguistic help and valuable comments. We thank
Ian J. Fairchild, Dana C. Riechelmann and an anonymous reviewer for
constructive and thorough comments that greatly helped to improve the
manuscript. Edited by: Marit-Solveig
Seidenkrantz Reviewed by: Ian J. Fairchild, Dana Riechelmann,
and one anonymous referee
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