Reconstructing palaeoprecipitation from an active cave flowstone

JOURNAL OF QUATERNARY SCIENCE (2011) 26(7) 675–687

ISSN 0267-8179. DOI: 10.1002/jqs.1490

Reconstructing palaeoprecipitation from an active cave flowstone ¨ TL RONNY BOCH* and CHRISTOPH SPO ¨ Institut fu¨r Geologie und Palaontologie, Universita¨t Innsbruck, Innrain 52, 6020 Innsbruck, Austria Received 7 January 2011; Revised 24 January 2011; Accepted 25 January 2011

ABSTRACT: Several drill cores were obtained from a laminated, actively forming flowstone from a shallow cave in Austria. Highly resolved petrographic and geochemical analyses combined with multi-annual cave monitoring reveal a distinct sensitivity of flowstone growth and composition to late Holocene meteoric precipitation. The regular sub-millimetre-scale lamination consists of thicker, translucent laminae and thinner (organic) inclusion-rich laminae. There is also a macroscopic millimetre-scale banding of darker and lighter bands comprising several laminae. Stable isotope analyses of drill cores and modern calcite precipitates show a pronounced positive covariation of d13C and d18O values indicative of kinetic isotope effects. Comparing the isotope values with petrography shows gradual changes across several of the annual laminae, i.e. changes of several per mille on a multi-annual to decadal timescale. The stable isotope and trace-element composition, as well as the flowstone petrography, are mainly controlled by the variable drip-water discharge controlling the water-film thickness and water residence time on the flowstone surface and consequently the intensity of CO2-degassing, kinetic isotope enrichment and concomitant calcite precipitation. Drill core PFU6 provides an isotope record of the last ca. 3000 years at near-annual resolution. A distinct phase of low C and O isotope values – interpreted as increased discharge and hence higher meteoric precipitation – occurred from ca. 300 to 140 a b2k (second half of the Little Ice Age) and another wet interval occurred around 700 a, corresponding to reported Medieval glacier advances. The Roman Warm Period was also dominated by relatively wet conditions, although significant decadal variability prevailed. Increased precipitation further characterized the intervals from ca. 2480 to 2430 and 2950 to 2770 a. Dry conditions persisted during the Medieval Climate Anomaly, although this trend towards reduced precipitation started earlier. The highest C isotope values of the last 2 ka are recorded around 750 a and another dry phase is centred at 1480 a. This new record shows that inter-annual to decadal oscillations are a dominant mode of variation during the last 3 ka in the Alps. Copyright # 2011 John Wiley & Sons, Ltd. KEYWORDS: flowstone; kinetic isotope fractionation; lamination; precipitation; speleothem.

Introduction Flowstone is a common variety of speleothem deposition in caves (Hill and Forti, 1997; Fairchild et al., 2006; Frisia and Borsato, 2010) originating from seepage water which precipitates calcite (less commonly other minerals) as sheet-like deposits covering walls, floors or joint planes. Carbonate is deposited layer by layer (often annually laminated, cf. Broecker et al., 1960). In contrast to stalagmites and stalactites, where the solution originates from a point-source (single drip site), seepage water feeding a flowstone commonly discharges from multiple drip sites or along fractures (Baker and Smart, 1995). As a consequence, the growth dynamics and morphology of these speleothems are often more complex (spatially and temporally) than those of stalagmites. Therefore, stalagmites have been strongly favoured over flowstone in palaeoclimatebased studies (Fairchild et al., 2006). Combining petrographic analysis, radiometric dating and modern environmental monitoring, however, can yield important insights into flowstone growth dynamics and allow to use these deposits as valuable palaeoenvironmental archives (Baker et al., 1995; Holzka¨mper et al., 2005; Meyer et al., 2008). In contrast to stalagmite-based studies which typically require the removal of the entire dripstone, flowstone sampling using drill cores is much less destructive and also feasible in highly protected caves. Studies using flowstone have yielded information about climate change on glacial–interglacial (Baker et al., 1995; Wainer et al., 2011) and stadial–interstadial timescales (Holzka¨mper et al., 2005; Meyer et al., 2008), droughts (Drysdale et al., 2006), vegetation history (Hellstrom et al., 1998), sea-level changes (Li et al., 1989), neotectonic activity (Gilli, 1999; Plan et al., 2010), as well as age constraints on * Correspondence: R. Boch, as above. E-mail: [email protected]

Copyright ß 2011 John Wiley & Sons, Ltd.

hominid occupation of cave sites (Walker et al., 2006; Bischoff et al., 2007; Dirks et al., 2010). In the study presented here a late Holocene flowstone has been investigated. The late Holocene has been considerably cooler and/or drier in many parts of the world compared with the early Holocene (Mayewski et al., 2004; Wanner et al., 2008) and includes climatically distinct intervals such as the Roman Warm Period (e.g. Holzhauser et al., 2005), the Medieval Warm Period (Broecker, 2001; Mangini et al., 2005) and the Little Ice Age (Shindell et al., 2001; Holzhauser et al., 2005; Denton and Broecker, 2008). Quantifying air temperatures during these last few millennia is of foremost importance to assess natural climate variability (e.g. Luterbacher et al., 2004; Moberg et al., 2005; IPCC, 2007; Mann et al., 2009). An even more challenging task is the reconstruction of palaeoprecipitation. Given the complex and highly variable spatial and temporal pattern of precipitation as shown by instrumental data (Efthymiadis et al., 2006; Pauling et al., 2006), high-fidelity records at high spatial resolution are needed to provide an accurate reconstruction of past precipitation variability. In this study of a flowstone from a small cave in southwestern Austria we combined petrographic and geochemical analyses with a multi-annual cave monitoring programme. The aim was to show how these proxy data can be used to constrain regional palaeoprecipitation over the last ca. 3 ka at nearannual resolution.

Site Klapferloch Cave (Austrian cave register no. 2132/5) is a small cave of tectonic origin which formed in carbonate-bearing metasediments on the steep south-facing flank of a gorge near Pfunds, Tyrol (1140 m a.s.l., 10833’E, 46857’N, Fig. 1). The cave consists of a single horizontal room 12 m long, 4–6 m wide and up to 5 m high. The entire cave is located within the twilight

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Figure 1. Location of Klapferloch Cave (asterisk; 1140 m a.s.l.) near the village of Pfunds in the upper Inn valley, Tyrol, Austria (upper image). The small cave developed in carbonate-bearing metasediments of the steep SW-facing flank of the Radurschl gorge. Lower image: plan view and crosssection of the cave showing the locations of the active flowstone, drill core samples, air temperature and drip-water sampling sites.

zone and direct sunlight only reaches the frontal part of the cave. Despite its shallow setting the cave hosts abundant speleothems including flowstone, stalactites, stalagmites, draperies and soda straws (Fig. 2). The most prominent formation is an active flowstone of brownish colour and convex morphology, 5 2 m at the base and 2.5 m in height (Fig. 2A). Water discharges at multiple drip points on the ceiling and maintains a water film on the flowstone surface year-round. This seepage water is part of a local aquifer whose groundwater infiltrates on the mountain flank north of the cave and flows toward the deepest point, the gorge. The cave is located in calc-siliciclastic metasediments of the Penninic Bu¨ndnerschiefer unit at the south-east rim of the tectonic Unterengadin Window (Oberhauser, 1980; Schmid et al., 2004). The climate in the area is characterized by pronounced seasons expressed both in air temperature and in precipitation. Mean annual air temperature (2005–2006) at the meteorological station Nauders (1360 m a.s.l., 9 km from the cave) is 4.8 8C. Mean annual precipitation (2005–2006) is ca. 600 mm (reflecting the relatively dry inneralpine setting) and intermittent snow cover occurs from October to April. Copyright ß 2011 John Wiley & Sons, Ltd.

Maximum precipitation sums occur during summer (related to thunderstorms). With regard to the large-scale atmospheric circulation, the dominant influence is exerted by Atlantic air masses advecting from north-west to west.

Methods Four cores, 25 mm in diameter and up to 50 cm long (PFU6, 7, 8, 9), were extracted from different parts of the flowstone using a battery-powered and water-cooled electric drill. The cores were embedded in epoxy resin and cut longitudinally. Thin sections were investigated using transmitted-light and epifluorescence microscopy, as well as reflected-light microscopy. Subsamples for U–Th dating were cut from the cores using a diamond-coated band saw and were analysed using a multicollector inductively-coupled plasma mass spectrometer (MCICP-MS, Nu Instruments) at the Institute of Geology in Berne, Switzerland. Typical sample size was 0.1 g. Activity ratios were calculated using the decay constants of Cheng et al. (2000) and were corrected for detrital Th. An initial 230Th/232Th activity J. Quaternary Sci., Vol. 26(7) 675–687 (2011)

RECONSTRUCTING PALAEOPRECIPITATION FROM AN ACTIVE CAVE FLOWSTONE

Figure 2. Views from Klapferloch Cave. (a) Active flowstone (arrow) from which drill cores were retrieved. (b) Drip-water monitoring site (discharge, temperature, electric conductivity). (c) Glass substrates were mounted into the drill core boreholes (image) in order to collect modern calcite precipitates. (d) Drill core embedded in epoxy resin. The active top is indicated by an arrow. Note the millimetre-scale bands, colour variations, macroscopic inclusions and bending of layers in the lower part resulting from the flowstone growth morphology. This figure is available in colour online at wileyonlinelibrary.com.

ratio of 0.8 was used (based on the mean Th/U ratio of the upper continental crust). Ages are quoted as ‘a b2k’ (i.e. years before the year AD 2000). A detailed description of the dating method and the preparative steps is given in Fleitmann et al. (2007). The U–Th-based age model was calculated using R (version 2.10.0; R Development Core Team, 2009) and an algorithm optimized for speleothems (Scholz and Hoffmann, 2011). The age–depth model and the corresponding 95% confidence intervals were calculated by superposition of ensembles of piecewise linear fits. In addition to the U–Th data points and the corresponding errors the algorithm also uses stratigraphic information. Stable carbon and oxygen isotopic compositions of drill core subsamples (milled at 0.10–0.15 mm resolution; 4250 individual samples) and modern calcite precipitates were analysed using a ThermoFisher DeltaplusXL isotope ratio mass spectrometer coupled to a ThermoFisher GasBench II. Results are reported relative to the VPDB standard and the precision of the d13C and d18O values expressed as the 1-sigma standard deviation is 0.06 and 0.08%, respectively (Spo¨tl and Vennemann, 2003). Cores PFU6 and PFU7 were sampled for their entire lengths (ca. 50 cm; PFU7 only at 1-mm resolution), whereas cores PFU8 and PFU9 were only studied for their most recent parts (uppermost 2 cm). Trace elemental analyses were Copyright ß 2011 John Wiley & Sons, Ltd.

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conducted at the Australian National University (Canberra; analyst: P. Treble) using 193-nm excimer laser-ablation inductively-coupled plasma mass spectrometry (Treble et al., 2003). Concentrations of Mg, Sr, Ba, U, Al and Pb were determined for two parallel, 50-mm-long tracks from the top (most recent part) of drill core PFU6. A cave monitoring programme was run between September 2003 and May 2007 and included bi-monthly visits as well as automatic data logging. Modern carbonate precipitates were collected on glass substrates and plastic foil bi-monthly (December 2003 to November 2006). Glass substrates were placed at the upper end of the boreholes and plastic foil was placed under one drip site. Cave air temperature was measured using an Optic StowAway logger ca. 4 m above the cave floor (Boch, 2008). Drip water temperature and drip rate were logged at drip water sampling site no. 1 (Figs 1 and 2b) using a tipping bucket. Attempts to log electrical conductivity (EC) failed because the probe became rapidly coated by calcite. From April until November 2006 drip rate was measured at this drip site using an acoustic drip counter (Collister and Mattey, 2008). At three additional drip sites (Fig. 1) EC, carbonate alkalinity and pH were manually measured in the field during the bimonthly visits. EC was measured using a WTW probe (referred to 25 8C) and pH was measured using a Mettler-Toledo glass electrode with a cut diaphragm calibrated on site using pH 7 and 9 buffer solutions ( 0.05 pH units). Carbonate alkalinity was titrated ( 0.5 mg L1 HCO3) using the Aquamerk carbonate hardness kit. Cations and anions were analysed using atomic absorption spectrometry at the University of Innsbruck and ion chromatography at the University of Keele, UK, respectively. Dissolved silica concentrations were measured by colorimetry at the Natural History Museum in Vienna. Stable C isotopic compositions of dissolved inorganic carbon (DIC) in drip water, as well as the H and O isotope values were determined for the three drip sites at bi-monthly intervals. C isotope values are reported on the VPDB scale, H and O isotope values on the VSMOW scale. The ThermoFisher DeltaplusXL mass spectrometer was calibrated using calcite standards for DIC and using VSMOW, SLAP and GISP for water isotopes (Spo¨tl, 2005; Boch, 2008). The calcite saturation index (SI) of the solutions was calculated using PHREEQC (Parkhurst and Appelo, 1999).

Results Flowstone petrography The flowstone consists of low-Mg calcite and shows a yellowish to brownish colour in different sections of the drill cores and on the active growth surface (Fig. 2A, D). Small segments of the cores consist of porous carbonate, while most consists of a compact, low-porosity fabric. Macroscopic inclusions, mostly cemented remains of insects, occur in some sections of the cores (Fig. 2D) and on the flowstone surface. Microscopic investigations of the cores revealed a regular sub-millimetre-scale lamination of the flowstone carbonate, visible both in transmitted light and epifluorescence (Fig. 3). The couplets are formed by typically thicker, translucent laminae and typically thinner (organic) inclusion-rich laminae. In transmitted light the former appear as light-coloured layers, while the latter appear dark. The inclusion-rich laminae are fluorescent, whereas the translucent laminae are not. Couplet thickness varies significantly but is typically in the order of 20– 300 mm. Curved laminae occur in some sections of the cores, documenting changes in the former surface morphology of the flowstone. In some sections the lamination is diffuse and difficult to resolve. J. Quaternary Sci., Vol. 26(7) 675–687 (2011)

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Figure 3. Flowstone petrography showing regular, sub-millimetre-scale lamination as well as macroscopic, millimetre-scale banding. The banding (lefthand image) consists of alternating darker (dark grey bars on top) and lighter (light grey) bands, each comprising several laminae. Transitions are gradual (intermediate grey). Lamina couplets consist of typically thicker, translucent laminae (white arrows) and typically thinner, organic inclusion-rich laminae (black arrows). The former appear as light-coloured layers in transmitted light (upper right-hand image), while the latter appear dark. Inclusion-rich laminae are fluorescent (light-blue layers in lower right-hand image). The bands are the result of variable thicknesses of the translucent laminae. Macroscopically dark (brownish) bands are dominated by thicker, translucent laminae, while the light bands are dominated by the organic inclusion-rich layers. The speleothem growth direction is from right to left in all images. This figure is available in colour online at wileyonlinelibrary.com.

In addition to the microscopic lamination a macroscopic, millimetre-scale banding of darker and lighter bands can be observed (Fig. 3). These bands comprise several laminae and are primarily due to the variable thickness of the translucent laminae. Macroscopically dark (brownish) bands are dominated by thicker, translucent laminae, while the light bands are bundles of significantly thinner, translucent laminae and therefore are macroscopically dominated by the (organic) inclusion-rich layers. The macroscopic appearance of these bands is determined by optical effects such as dispersion and reflection of the light depending on the occurrence and distribution of inclusions versus pure calcite. The macroscopically light bands appear dark (i.e. containing

abundant organic inclusions) under the transmitted-light microscope and vice versa.

Chronology Sixteen U–Th ages were determined along the growth axis of core PFU6, i.e. covering the uppermost ca. 50 cm of the flowstone stratigraphy (Fig. 4; Table 1). The analyses revealed high mean U concentrations of 2.5 p.p.m. with values of up to 4 p.p.m. in the oldest part of the core (Table 1). The calcite has low detrital Th concentrations (mean 1.2 ppb) and the correction for detrital Th results in a small shift of the final ages only (Table 1). All samples yielded late Holocene ages

Figure 4. Time versus distance plot (lefthand diagram) showing U–Th ages (with 2s uncertainties) obtained from drill core PFU6, as well as a growth model based on the algorithm (see Methods). The uppermost 50 cm formed during the last 3 ka and growth slowed down significantly during the last ca. 1200 years. Inset: a comparison of the U–Th-based age model for the top 4 mm versus the adjusted age model using petrographic information. Right-hand diagram: C and O isotope data versus distance from the active top. A 20-point running mean is superimposed on the highresolution data. Note the high degree of covariation of the C and O isotope curves. This figure is available in colour online at wileyonlinelibrary.com. Copyright ß 2011 John Wiley & Sons, Ltd.

J. Quaternary Sci., Vol. 26(7) 675–687 (2011)

8 30 49 33 83 42 73 78 71 79 77 63 84 76 53 60 122 294 667 723 1245 1373 1423 1766 2052 2311 2452 2749 2726 3012 2690 2983 8 15 45 15 46 31 46 46 61 46 46 46 61 46 46 46 123 305 676 732 1266 1389 1450 1785 2106 2319 2472 2762 2747 3021 2701 2991 0.0000 0.0001 0.0002 0.0001 0.0004 0.0002 0.0003 0.0003 0.0003 0.0003 0.0003 0.0003 0.0004 0.0003 0.0002 0.0003 0.0012 0.0028 0.0062 0.0067 0.0115 0.0126 0.0132 0.0162 0.0190 0.0210 0.0224 0.0250 0.0248 0.0273 0.0244 0.0270 0.4 1.3 2.1 1.4 1.8 2.7 1.3 2.4 0.7 9.9 1.2 2.8 1.9 2.7 2.3 5.6 0.0006 0.0008 0.0005 0.0010 0.0016 0.0018 0.0019 0.0013 0.0013 0.0026 0.0013 0.0011 0.0081 0.0011 0.0011 0.0009 0.001 0.006 0.002 0.006 0.011 0.004 0.015 0.011 0.032 0.004 0.021 0.009 0.013 0.011 0.011 0.007

0.976 0.952 1.002 1.011 0.989 0.990 0.986 0.981 0.975 0.962 0.965 0.956 0.991 1.003 1.016 1.033

0.001 0.002 0.001 0.001 0.002 0.004 0.004 0.003 0.002 0.003 0.003 0.002 0.016 0.002 0.002 0.002

0.9761 0.9518 1.0017 1.0112 0.9893 0.9898 0.9865 0.9812 0.9751 0.9625 0.9651 0.9560 0.9913 1.0026 1.0157 1.0331

17.2 26.1 58.8 63.2 53.1 164.6 49.8 103.3 34.7 478.4 72.4 191.4 131.4 191.3 199.4 400.4

(2s) (a b2k) (2s) (a b2k) (1s) (activity) (1s) (activity) (1s) (activity) (2s) (activity)

0.425 0.639 0.657 0.670 1.266 0.473 1.600 0.960 3.553 0.300 2.454 0.913 1.756 1.436 1.540 0.736 2.9 3.7 2.8 3.7 3.6 4.1 4.0 3.8 4.0 4.8 5.1 4.4 5.5 6.0 7.5 6.5 2122.0 2095.3 2069.8 2076.9 1958.1 2074.5 2032.4 2071.4 2197.9 2348.5 2717.4 2423.6 3108.2 3322.5 4096.3 3493.7 4.0 9.6 20.0 28.6 49.1 73.9 80.9 141.3 221.7 296.3 348.9 369.5 423.5 454.5 468.5 484.7 PFU6-0.4 PFU6-1.0 PFU6-2.0 PFU6-2.8 PFU6-4.8 PFU6-7.2 PFU6-8.0 PFU6-14.0 PFU6-22.0 PFU6-29.5 PFU6-34.0 PFU6-37.2 PFU6-42.5 PFU6-46.0 PFU6-47.0 PFU6-49.0

(p.p.b.) Depth (mm from Top) Sample

Table 1. U–Th age data of drill core PFU6.

U

(1s)

(p.p.b.)

Th

(1s)

234

U/238U initial

234

U/238U

230

Th/232Th

230

Th/234U

Age uncorr.

Age corr.



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ranging from 122 8 to 3012 76 a. A flowstone growth model was calculated (see Methods), which shows continuous deposition for the last ca. 3000 a, although the growth rate decreased significantly during the last ca. 1200 a (Fig. 4). Mean growth rates inferred from the growth model are 0.27 mm a1 and age uncertainties (2-sigma) range from 10 a in the most recent portion to 170 a around 3000 a. To compare instrumental climate data with stable isotope data, the age model of the top 4 mm of drill core PFU6 was improved by considering petrographic (annual laminae) information (see Supporting information, Fig. S1). Although a binary, annual lamination pattern prevails in the cores (see below), there are intervals where the lamination is diffuse and difficult to resolve. This is also the case in the top 4 mm, where several macroscopic bands of more condensed laminae (darkcoloured bands in transmitted light; Fig. 3 and supporting Fig. S1) occur and the typical thickness of annual couplets is 20 mm. In these top 4 mm annual couplets are best visible in these condensed bands and a growth rate of 20 mm a1 is considered as an approximation for the condensed portions. U–Th dating yielded an age of 122 8 a at ca. 4 mm distance from top (DFT), which serves as an anchor point of the (floating) lamina chronology. Taking into account the combined thickness of the condensed bands in the top 4 mm and their typical growth rate suggests that 92 years are represented in these bands (blue columns in supporting Fig. S1). Consequently, ca. 30 years of growth (122 minus 92) are represented by the remainder, faster growing intervals of the top 4 mm (light bands/orange columns in supporting Fig. S1). These 30 years represent an average growth rate of 78 mm a1. Altogether, based on annual growth rates of the bands observed and the U–Th anchor point it is possible to adjust and improve the internal chronology of the top 4 mm using petrographic information (Fig. 4; see also Fig. 9).

Stable isotopes and trace elements A high-resolution traverse along core PFU6 shows highamplitude variability in the isotope values: d13C values vary from 5.1 to 2.7% (mean 1.9%) and d18O values vary between 11.5 and 7.3 % (mean 10.0%). Drill core PFU7 shows similar values and ranges. A 20-mm-long section of the top of core PFU8 reveals the lowest C isotope values ranging from 7.5 to 4.2% (mean 6.2%) and O values from 11.1 to 9.2% (mean 10.2%). The 50-cm-long profile of PFU6 (2815 individual analyses) shows shifts of up to 7% in d13C and 3% in d18O (Fig. 4). These alternating high and low values occur on a multi-annual to decadal timescale (Figs 4 and 9). A more gradual (first-order) trend underlies this high-frequency (second-order) trend, e.g. relatively high values prevail from ca. 0 to 50 mm DFT and relatively low values dominate from ca. 100 to 200 mm, as well as from ca. 400 to 470 mm (Fig. 4). Most of the analysed sections show a strongly positive correlation between d13C and d18O values (Fig. 5). In the case of cores PFU6 and PFU7 a younger section (top ca. 20 cm) can be distinguished from an older section (ca. 20–50 cm) based on the degree of covariation between d13C and d18O (Fig. 5). Applying linear fits to these covarying data, the younger segments show Dd13C/Dd18O slopes of 1–1.5 and high coefficients of determination (R2). The older parts of the cores are characterized by a significantly lower degree of correlation and typically lower slopes. Linear fits of d13C vs. d18O in cores PFU8 and PFU9 show comparable slopes (1–1.5) but typically lower coefficients (in particular PFU8). A comparison of the stable isotope data with the laminated petrography reveals a gradual change of the isotope values across several laminae (inter-lamina variability). Macroscopically, the light (whitish) bands show high C and O isotope J. Quaternary Sci., Vol. 26(7) 675–687 (2011)

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Figure 5. C versus O isotope plots of drill core samples and modern calcite precipitates. All measured samples show a distinct positive correlation, although of different slope and coefficient of determination (R2). Upper left: measurements of four drill cores. Upper right: a younger portion (top ca. 200 mm) can be distinguished from an older portion in drill cores PFU6 and PFU7 based on the different slopes and R2 values. Lower left: modern calcite precipitates collected bi-monthly on glass substrates installed at the top of four boreholes. Lower right: calcite precipitates collected bi-monthly on plastic foil. For detailed explanations see text.

values, whereas the dark (brownish) bands show low values (Fig. 6). Transmitted-light thin section analyses further show a correspondence of the high isotope values with bands dominated by inclusion-rich laminae and the low isotope values with bands dominated by translucent calcite (Fig. 6). These relationships occur in all of the investigated cores. To test these relationships across laminae and to preclude artefacts (aliasing) due to the isotopic sampling procedure, samples were micromilled from the same traverse at different increments (0.05, 0.1 and 0.2 mm; Boch, 2004). A spatial resolution of 0.05 mm equals 2–6 subsamples within a single couplet. Modern calcite precipitates exhibit a strongly positive correlation of C and O isotope values (Fig. 5). Mean d13C values of calcite on plastic foil are 4.7% and mean d18O values are 9.3%. Carbonate on glass substrates show mean d13C values of 2.8, 4.7, 3.6 and 3.7% for the collection sites PFU6, PFU8, PFU9, and PFU0, respectively (Fig. 5). Mean d18O values are 8.5, 9.3, 8.9 and 8.9 %, respectively. Thus, PFU6 shows the highest mean C and O isotope values, while PFU8 and the plastic foil samples show the lowest. This is in good agreement with the observations from the drill core isotopic analyses. No clear seasonal trends, i.e. no significant difference between warm and cold season, can be inferred from the C and O isotope data of the modern precipitates (supporting Fig. S2). A plot of the C versus O compositions of all modern calcite samples collected on plastic foil reveals a slope of 1.60 and a coefficient of determination of 0.87 for the corresponding linear fit (Fig. 5). Plotting the data for all glass substrate samples (collection sites PFU6, 8, 9 and 0) reveals a slope of 2.24 and a R2 of 0.74. The slopes for PFU6 and PFU8 are 2.78 and 1.50, respectively, and the corresponding coefficients of determination are 0.82 and 0.53, respectively. The collection sites PFU9 and PFU0 yielded slope and R2 values intermediate between those for sites PFU6 and PFU8 (Fig. 5). Consequently, modern calcite from glass substrates in drill hole PFU6 shows the steepest slope and highest coefficient of determination, whereas samples from PFU8 exhibit a smaller slope and a lower coefficient. Trace-elemental analysis of the top portion of core PFU6 shows a pronounced variability (supporting Fig. S3): Mg concentrations vary from 1857 to 14429 p.p.m. (mean ¼ 6184 p.p.m.), Sr from 726 to 1636 p.p.m. (mean ¼ 1121 p.p.m.), Ba from 16 to 56 p.p.m. (mean ¼ 31 p.p.m.) and U from 1 to 28 p.p.m. (mean ¼ 3 p.p.m.). A distinct positive covariation occurs between Mg, Sr and Ba. Moreover, the latter elements also exhibit a positive relationship with the C and O isotope values,

Figure 6. Stable C and O isotopes versus flowstone petrography (banding and lamination) in two different sections (42–50 and 0–3 mm DFT) of drill core PFU6. Left-hand diagram: gradual alternations of light and dark bands associated with relatively high and low isotope values, respectively. Righthand diagram: the macroscopically dark bands appear light (with low isotope values) in transmitted light, whereas the macroscopically light bands are dark (dominated by inclusion-rich laminae) and show higher isotope values. This figure is available in colour online at wileyonlinelibrary.com. Copyright ß 2011 John Wiley & Sons, Ltd.



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Cave air- and drip water temperatures exhibit pronounced seasonal variations (Fig. 7). During the warm season (March– November) cave air temperature shows strong diurnal fluctuations and fluctuations due to the prevailing weather conditions. Compared with the air temperature at a nearby meteorological station (Nauders), the cave air temperature shows attenuated amplitudes. During the cold season (December–February) air exchange between the cave and its surrounding is strongly restricted. This is manifested by a lack of diurnal cave air temperature fluctuations and no longer-term trends due to the prevailing weather conditions. During the cold season the cave therefore acts as a trap of warm, less dense air in its upper part. The measured drip water temperatures are

mostly slightly lower than the corresponding cave air temperatures, in particular during the cold season (Fig. 7). The water temperature shows a higher variability than cave air temperature during the cold season, i.e. some influence of the prevailing weather conditions. This can be explained by the location of the drip water temperature logger, which was installed near the bottom of the chamber, whereas the air temperature was logged at a height of 4 m. The cave air- and drip water temperatures also show a persistent cooling during the cold season, most likely due to conductive cooling of the trapped air and surrounding host rock in close contact with the outside atmosphere. Drip-rate measurements revealed seasonal variations (Fig. 7). During winter the drip rates are relatively low, most likely due to the reduced infiltration at the base of the snow pack (Fig. 7). High drip rates are observed following spring snow melt. Summer is characterized by typically lower values, probably due to increased evapotranspiration. Intense rainfall events, however, occasionally exert an influence on the drip rates in summer (Fig. 7). Increased drip rates typically occur again in autumn. Drip water shows rather constant and very similar hydrochemical and stable isotopic compositions (Fig. 8). The waters

Figure 7. Cave monitoring data compared with instrumental meteorological data for 2003–2007. (a) Air temperature at the meteorological station Nauders (9 km from the cave; operating since June 2004). (b) Cave air and drip-water temperature. Note the different modes during the warm and cold seasons. (c) Daily precipitation (rainfall and snow) at Nauders. (d) Water discharge (drip rates) measured with two different types of data logger in Klapferloch. This figure is available in colour online at wileyonlinelibrary.com.

Figure 8. Drip-water chemical and stable isotopic composition monitored at three different drip sites. (a) Electric conductivity (EC) shows overall high values and some seasonal variation of relatively high values in autumn and typically lower values during spring and summer. (b) The waters are supersaturated with respect to calcite year-round and the saturation index varies mainly on inter-annual scales. d18O (c) and dD (d) reveal similar and rather constant values and the d13C values of DIC primarily vary on an inter-annual scale. Squares represent drip water sampling site no. 1 (cf. Fig. 1), triangles no. 2, circles no. 3. This figure is available in colour online at wileyonlinelibrary.com.

although the sampling resolution of the isotopes is lower. Consequently, the trace elements also show a relationship with the flowstone petrography, i.e. macroscopically light bands, characterized by relatively thin translucent laminae, correspond to higher C and O isotope values and hence increased Mg, Sr and Ba contents.

Cave air temperature and drip water monitoring



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Figure 9. Comparison of mean annual instrumental precipitation data (upper diagram; HISTALP dataset – station Nauders; Auer et al., 2007) with flowstone stable isotope data (lower diagram, inverted scales) for the period AD 1890–2008. Times of increased precipitation are reflected in relatively low C and O isotope values (yellow bars), i.e. within the uncertainties of the flowstone age model in this section based on annual lamina observation (see supporting Fig. S1). This figure is available in colour online at wileyonlinelibrary. com.

are supersaturated with respect to calcite year-round (SI values from 0.3 to 1.1; mean ¼ 0.8) and the SIs vary mainly on interannual scales (Fig. 8). Major cations and anions as well as EC also show inter-annual rather than intra-annual trends. The latter shows overall high values (589–738 mS cm1) and some weak seasonal variation: relatively high values prevail in the autumn, while spring and summer values are typically lower. The monitored drip sites show similar and rather constant d18O and dD values (Fig. 8). Mean values are 12.7 and 96.5%, respectively. Mean values of precipitation measured at the meteorological station La¨ngenfeld (1179 m a.s.l., 30 km from the cave) are 13.3 and 100.3% (mean 1973–2009) for d18O and dD, respectively (Umweltbundesamt, 2010). Thus, drip water and meteoric precipitation values are comparable. The d13C values of DIC primarily vary on an inter-annual scale, i.e. between 10.3 and 8.0% (Fig. 8).

Discussion Flowstone growth and proxy interpretation Stable isotope fractionation Calcite deposition in Klapferloch Cave is continuous throughout the year. This is supported by uninterrupted drip-water supply and high SI values, by calcite precipitation on glass substrates, and also by the U–Th growth model of core PFU6. Variations in carbonate precipitation rate and isotopic and chemical compositions, however, occur on both seasonal and inter-annual timescales. The pronounced positive correlation between C and O isotope values (Fig. 5) strongly suggests kinetic isotope fractionation during CO2-degassing of the drip water and concomitant calcite precipitation (cf. Mickler et al., 2004; Romanov et al., 2008; Mu¨hlinghaus et al., 2009). Preferential removal of the light isotopes 12C and 16O from the drip water in Copyright ß 2011 John Wiley & Sons, Ltd.

the course of CO2-degassing results in a Rayleigh distillation process and thus in the successive enrichment of 13C and 18 O. Both the C and O isotopic compositions can be affected by kinetic enrichment via pronounced (fast) CO2-degassing, resulting in a distinct positive covariance of these isotopes (Hendy, 1971; Mu¨hlinghaus et al., 2009). This effect is more pronounced for the C isotopes, and in the case of slow degassing the O isotopic composition is buffered (cf. Mickler et al., 2006; Wiedner et al., 2008). Evaporation can also lead to fractionation in drip water and calcite (Hendy, 1971; Mickler et al., 2006). Given the extremely shallow setting of Klapferloch, significant evaporation effects are to be expected, in particular during the warm season when exchange between the cave and the outside atmosphere is highest (see above). A comparison of the drill core d18O values with calculated modern O isotopic equilibrium values based on monitoring data reveals O isotopic compositions close to isotopic equilibrium only for the calcite d18O minima. Mean drip water d18O values are 12.7 0.2% and the average water temperature (in 2004) was 7.2 8C. Calcite precipitated in O isotopic equilibrium with this water should show d18O values of 10.7 0.2% (using the fractionation factors of Friedman and O’Neil, 1977) or 11.3 0.2% (after Kim and O’Neil, 1997). The lowest measured O isotope values of calcite from the cores are 11.5, 11.9, 11.1 and 10.7% for cores PFU6, PFU7, PFU8 and PFU9, respectively, and are therefore close to modern equilibrium values. Calcite precipitated in C isotopic equilibrium with the drip water (mean d13C of DIC ¼ 9.5 %) exhibits d13C values of ca. 8.5% (after Romanek et al., 1992). The lowest measured C isotope values of drill core calcite are 5.1, 6.0, 7.5 and 5.0% for PFU6, 7, 8 and 9, respectively. Thus, even the lowest measured C isotope values are significantly higher than the equilibrium values. Modern calcite precipitates also show a pronounced positive correlation of the C and O isotope values (Fig. 5), strongly J. Quaternary Sci., Vol. 26(7) 675–687 (2011)


supporting kinetic isotope effects. Comparing the monitored isotope values of precipitates and drip water allows us to quantify isotopic fractionation during modern carbonate deposition. Mean d18O values of calcite collected on plastic foil are 9.3 0.9% and the water temperature inferred from this composition in comparison with the observed drip water values (d18O ¼ 12.7 0.2%; measured water temperature ¼ 7.2 8C) is 1.5 8C (after Friedman and O’Neil, 1977) and 1.6 8C (after Kim and O’Neil, 1997). Thus, the calculated water temperature is far too low. In other words, the average d18O value of calcite on plastic foil is enriched by 1.5% (Friedman and O’Neil, 1977) to 2.1% (Kim and O’Neil, 1997). The mean d13C of calcite on plastic foil is 4.7% and the mean d13C value of DIC is 9.5%. This translates into a C isotopic enrichment of ca. 3.9% (Romanek et al., 1992). A similar picture emerges from modern calcite collected on glass substrates. Calcite from glass substrates placed at the PFU6 coring site shows the highest mean C and O isotope values (2.8 and 8.5%, respectively) and also maximum C and O isotopic enrichment (6.2 and 2.3%, respectively). In contrast, the lowest mean C and O isotope values (4.7 and 9.3%) and the smallest enrichment (3.7 and 1.8%) were observed in the flowstone section from which core PFU8 was retrieved (Figs 1 and 5). Consequently, the observations from the glass substrates correspond well to those of the drill cores.

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and kinetic enrichment of the speleothem carbonate. A thin water-film and increased time to degas, both resulting from diminished discharge, favour CO2-degassing as well as evaporation and hence kinetic enrichment. Modern calcite precipitates do not suggest a significant influence of evaporation on the isotopic composition, not even during the warm season. Trace elements also support a major control by the amount of drip-water supply: the positive correlation of Mg, Sr and Ba, as well as the positive relationship with d13C suggest the occurrence of prior calcite precipitation in the course of degassing in the aquifer and/or on the cave ceiling, which is known to be strongly dependent on discharge (e.g. Fairchild et al., 2001; McMillan et al., 2005). Again, inter-annual changes dominate as shown by the lack of a seasonal signal in the drip-water trace element concentrations (Boch, 2004). This behaviour and the seasonally constant isotopic compositions strongly suggest that most of the water discharging into this cave has a mean residence time well in excess of one year. Routes of more direct and rapid seepage transmission, however, might exist (e.g. affecting lamina formation; see below). In essence, there is compelling evidence that the carbonate C and O isotopic as well as trace elemental (Mg, Sr, Ba) compositions primarily reflect the amount of drip-water discharge on multiannual timescales. Annual lamination

Intra-annual vs. inter-annual variations Comparing the flowstone isotopic composition with the petrography reveals a pattern of inter-lamina variability, i.e. the isotope values change gradually across several laminae (Fig. 6). This suggests a multi-annual isotopic signal, assuming an annual origin of the lamination. Micromilling at different spatial resolution confirmed the gradual, inter-annual nature of isotopic variability (Boch, 2004). This is also consistent with the lack of a seasonal (high-frequency) variation in the modern calcite C and O isotope values (supporting Fig. S2) as well as in their kinetic enrichment, i.e. a multi-annual signal is supported by the modern precipitates. Cave air temperature, by contrast, follows a seasonal pattern. Intensive air exchange with the outside atmosphere during the warm season results in a high-frequency temperature variability and probably also in variable cave air CO2 concentrations, further governing CO2-degassing of the drip water (e.g. Spo¨tl et al., 2005; Mattey et al., 2008; Boch et al., 2011). Evaporation is probably enhanced during summer due to higher air temperatures. Evaporation leads to C and O isotope enrichment in drip water and calcite (Mickler et al., 2006; Wiedner et al., 2008), but no clear seasonal isotopic signal was observed at the cave site, either in the monitored drip water or in the modern calcite (Fig. 8; supporting Fig. S2). Calcite precipitation is also promoted by enhanced degassing and seasonally varying cave air exchange (e.g. length and intensity of the warm season) could thus – to some degree – affect the thickness of the (translucent) laminae. The stable isotopic composition, however, is clearly not affected by the seasonally changing cave ventilation. Drip rates in this cave are related to recharge in the catchment area and are a major factor controlling carbonate ion supply and thus calcite precipitation and flowstone growth (e.g. Dreybrodt, 1981; Genty et al., 2001; Mu¨hlinghaus et al., 2007). The drip-water discharge also controls the water-film thickness and water residence time on the flowstone surface, both controlling CO2-degassing (Baldini, 2001; Banner et al., 2007; Mu¨hlinghaus et al., 2007; Wiedner et al., 2008). As discussed above, the amount (Rayleigh distillation) and rate (fast/slow) of CO2-degassing regulates the isotopic composition Copyright ß 2011 John Wiley & Sons, Ltd.

The lamination is the result of a distinct seasonal signal: the typically thin, organic-rich laminae (dark in transmitted light and strongly UV-fluorescent) form during times of a higher proportion of soil organic matter in the drip water and on the flowstone surface, which typically occurs in autumn (Baker et al., 2002; Frisia et al., 2003; Verheyden et al., 2008). The thicker translucent laminae form during times of enhanced vegetation activity and/or snow cover, i.e. reduced soil detrital input. Increased drip-water supply and cave air exchange (mainly during the warm season) result in relatively thick translucent layers. An annual origin of the lamination is supported by the regular nature of the laminae and the observed seasonal control on cave air and some drip-water parameters. Moreover, the thickness of the annual couplets inferred from petrographic studies and from the U–Th growth model correspond well. Together, the variable thickness both of the annual laminae and of the multi-annual banding primarily reflect the amount of drip-water discharge and to some minor degree also the intensity of cave air exchange. Flowstone morphology Flowstone-specific (geometric) growth effects have to be considered in interpreting the stable isotopic composition and petrography of the drill cores. The morphology, and in particular the growth variability and switching of flow routes in different sections of the active growth surface, can significantly impact the degree of kinetic enrichment as well as the lamina thickness in individual segments of the speleothem (cf. Meyer et al., 2009). For example, the distribution and thickness of the water film varies both temporally and spatially and this controls CO2-degassing and hence kinetic isotope fractionation as well as lamina formation. Different parts of the flowstone therefore show different sensitivities to changes in water supply. This is manifested by variable degrees of isotope enrichment reflected by different slopes of the linear regressions, as well as coefficients of determination when plotting C versus O isotope values (Fig. 5). The flowstone segment from which cores PFU6 and PFU7 were retrieved (Fig. 1) is relatively sensitive to the processes regulating isotopic enrichment, i.e. the high slopes and R2 values (e.g. PFU6 modern calcite: 2.78 and 0.82, J. Quaternary Sci., Vol. 26(7) 675–687 (2011)

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respectively; Fig. 5) in the cores and modern calcite reflect pronounced isotopic enrichment as a result of effective degassing from a thin water film. This section also shows high variability, i.e. high amplitudes of stable C and O values (Figs 4 and 5). In contrast, the flowstone segments where cores PFU8 and PFU9 (Fig. 1) were drilled proved to be less sensitive, i.e. lower slopes and R2 values (e.g. PFU8 modern calcite: 1.50 and 0.53, respectively). These sites exhibit a lower amplitude in C and O isotope values. Differences in the overall magnitude of kinetic fractionation not only exist between different segments of the flowstone surface (i.e. spatially), but also along individual drill cores (i.e. temporally) reflecting the dynamics of flowstone accretion. For example, the topmost 20 cm in drill cores PFU6 and PFU7 show an overall higher degree of kinetic fractionation compared with the older part. The slope in the younger and older parts of both cores is 1.5 and 0.85–1.0, respectively (Fig. 5). Likewise, the R2 values of PFU6 and 7 are 0.75 and 0.82 in the younger part, compared with 0.19 and 0.29 in the older part, respectively (Fig. 5). Flowstone-specific growth effects therefore influence the isotopic composition on a long-term scale, i.e. over centuries to millennia, and are primarily reflected in the first-order trends of the isotope curves (Figs 4 and 9). On a decadal timescale, however, drip-water supply dominates the isotopic signal (second-order trends; cf. Figs 4 and 9) and provides a link to meteoric precipitation in the recharge area. Empirical vs. experimental observations Our observations in Klapferloch agree well with results from studies of kinetic isotope fractionation conducted in the laboratory and in situ in caves. Mickler et al. (2006) collected calcite on artificial substrates in a cave and found that a vertical slope in a Dd13C versus Dd18O plot reflects complete O isotope buffering between the precipitating carbonate and drip water due to CO2-hydration and hydroxylation reactions. A slope of 0.52 indicates no buffering and intermediate slopes reflect partial buffering of the O isotopes and significant nonequilibrium isotope effects (Mickler et al., 2006). The intermediate slopes observed in Klapferloch thus represent kinetic fractionation of both the C and the O isotopes. Wiedner et al. (2008) conducted laboratory experiments to verify kinetic isotope effects as predicted by Hendy (1971). They distinguished a ‘slow degassing’ mode resulting in kinetically enriched C isotopes but buffered (equilibrated) O isotopes from a ‘fast degassing’ mode where both C and O isotopes are

kinetically enriched. Typical Dd13C/Dd18O slopes of fast degassing are 1.4 0.6 (Wiedner et al., 2008). Similar slopes are observed in the flowstone cores (PFU6: 0.9–1.5, PFU7: 1.0– 1.5, PFU8: 1.4, PFU9: 1.0) and in the modern precipitates (mean of all glass substrates: 2.2; mean of plastic foil samples: 1.6). The active flowstone in Klapferloch thus constitutes a natural example of speleothem formation reflecting the kinetic processes suggested by laboratory experiments and carbonate precipitates on artificial substrates.

Regional precipitation changes in the late Holocene There is strong evidence that the flowstone’s stable isotopic and trace elemental composition, as well as the variable lamina thickness primarily reflect the amount of drip-water discharge and hence past precipitation in the catchment on a multiannual timescale. A climate (precipitation) signal is apparently amplified in the shallow cave. In addition, a seasonal signal is reflected in the binary lamination pattern. On longer timescales, however, the flowstone’s growth dynamics have to be considered. The kinetically controlled C and O isotopic enrichment is used as a proxy for palaeoprecipitation and core PFU6 proved to be a sensitive site on the flowstone surface. A comparison of the isotope curves with instrumental climate data, i.e. the homogenized HISTALP precipitation dataset of the alpine region (Auer et al., 2007), is not trivial, as it suffers from the chronological uncertainty of the top portion of the flowstone record. Annual and homogenized precipitation data of the nearby station Nauders, however, reveal good correspondence with observed C and O isotope changes on a decadal timescale, considering the age uncertainties and different temporal resolution (Fig. 9). Times of increased meteoric precipitation are reflected by relatively low C and O isotope values (yellow bars in Fig. 9). During the most recent decades of the late Holocene the PFU6 isotope record shows distinct ups and downs and low C and O values match within the dating uncertainties some of the recent glacier advances (cf. Fig. 10) in the Alps (Veit, 2002; Holzhauser et al., 2005), e.g. at 20 a b2k (1980 AD), 80 a (1920 AD) and 150 a (1850 AD). The isotopes show intermediate values compared with the last ca. 1000 years. Going further back in time our record indicates a distinct phase of increased precipitation from ca. 300 to 140 a, with a pronounced peak (drop in isotope values) around 160 a (1840 AD). This falls

Figure 10. Stable isotope record of the last ca. 3 ka primarily reflecting palaeoprecipitation in the region compared with (a) known climate intervals in the Alps (from Veit, 2002; RGA, Recent Glacier Advances; LIA, Little Ice Age; MGA, Medieval Glacier Advances; MWP, Medieval Warm Period; DACP, Dark Ages Cold Period; RWP, Iron Age/Roman Warm Period), (b) Swiss glacier advances and high lake levels (Holzhauser et al., 2005) and glacier recessions in Switzerland (Hormes et al., 2001 (c); Joerin et al., 2006 (d)). Some prominent intervals of the isotope record are highlighted by yellow bars. Low isotope values represent intervals of increased dripwater discharge and meteoric precipitation (see text for details). This figure is available in colour online at wileyonlinelibrary.com. Copyright ß 2011 John Wiley & Sons, Ltd.



within the second half of the Little Ice Age (LIA), a period of prevailing cool climate between ca. 500 and 150 a (Veit, 2002; Wanner et al., 2008), whose global significance is being discussed (Keigwin and Boyle, 2000; Mayewski et al., 2004; Mann et al., 2009) and a mean Northern Hemisphere cooling of 0.7 8C was inferred (Moberg et al., 2005). In the Alps cold and wet summers and very cold winters were frequent during the LIA (Veit, 2002; Mangini et al., 2005; Bu¨ntgen et al., 2006). Pronounced glacier advances occurred between 700 and 140 a (1300–1860 AD) and culminated in a highstand around 150 a (Holzhauser et al., 2005; Joerin et al., 2006). Prominent temperature depressions are also recorded in tree-ring data during the 1810 s and 1820 s (Bu¨ntgen et al., 2006). Some warmer phases, however, also occurred during the LIA (Veit, 2002) and this pattern of a high climate variability corresponds well to the flowstone isotope record. Another distinguished interval in late Holocene climate is the Medieval Warm Period (MWP) or, more generally, the Medieval Climate Anomaly (cf. Moberg et al., 2005; Mann et al., 2009). Relatively warm and dry climate conditions were associated with this interval lasting from ca. 1150 to 750 a (Veit, 2002; Mangini et al., 2005; Wanner et al., 2008; Fig. 10) and a global extent is debated (Broecker, 2001; Mann et al., 2009). The flowstone isotope record supports overall dry conditions during this time in the Alps. The isotope values increase gradually during the MWP, although this development towards drier conditions starts earlier (Fig. 10). Glacier advances in Switzerland as well as high lake levels in France occurred between ca. 1200 and 1100 a (Holzhauser et al., 2005) and a distinct negative isotope peak centred at ca. 1180 a (Fig. 10) in the flowstone record suggests high precipitation. The highest C isotope values of the last 2 ka and thus pronounced dry conditions are recorded around 750 a. Tree-ring records from the Alps further suggest high summer temperatures around 800–750 a (early 13th century; Bu¨ntgen et al., 2006). At the end and immediately after the Medieval Climate Anomaly, i.e. from ca. 800 to 660 a (Veit, 2002), distinct glacier advances (Fig. 10) are documented. A prominent negative isotope peak representing wet conditions occurs around 700 a in our record. From ca. 1550 to 1200 a (Veit, 2002) a cooling interval often referred to as Dark Ages Cold Period (DACP; e.g. McDermott et al., 2001) occurred and the mean Northern Hemisphere temperature was ca. 0.5 8C below the 1961–1990 AD average (Moberg et al., 2005). Veit (2002) attributed cool and wet climate conditions in the Alps to the DACP. The flowstone record suggests a distinct phase of dry conditions centred at 1480 a, i.e. at the beginning of the DACP (Fig. 10). Subsequently, a relatively wet interval lasted for a few decades, which is also confirmed by Swiss glacier advances between 1500 and 1400 a (Holzhauser et al., 2005). The latter interval was followed by a distinct drying trend towards the MWP, also recorded by major glacier recession periods in the Alps, e.g. from 1450 to 1250 a (Fig. 10; Joerin et al., 2006; Hormes et al., 2001). It should be noted, however, that glacier recession phases as recorded by organic remains lag behind climate change by up to 100–200 years (glacier response and vegetation colonization; cf. Hormes et al., 2001). An overall warm climate, referred to as the Roman Warm Period (RWP), prevailed from ca. 2300 to 1600 a in the Alps (e.g. Veit, 2002; Vollweiler et al., 2006). In the flowstone record this time is manifested as a relatively wet interval, although significant inter-annual and in particular decadal variations occurred (Fig. 10). A distinct negative peak reflecting increased precipitation is centred at 1670 a, i.e. at the end of the RWP (Fig. 10). As discussed above the drill core section from 20– 50 cm distance from top, i.e. older than ca. 2000 years, is characterized by overall smaller kinetic isotope enrichment. Copyright ß 2011 John Wiley & Sons, Ltd.

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This renders the interpretation of the climate record on centennial and millennial timescales more complex, as it could reflect overall increased meteoric precipitation and discharge and/or an overall thicker water-film on the flowstone surface in the section of core PFU6, resulting from a changing flowstone geometry. Interestingly, there is evidence that the North Atlantic Oscillation, playing a major role in winter precipitation in the Alps, experienced a significant change about 2 ka ago (Wanner et al., 2008), supporting a dominant discharge control also in this older part of the drill core, i.e. when looking at first-order trends. The second-order (interannual and decadal) isotope variations, however, most likely reflect a precipitation signal also in the older part. Going even further back in time (before the RWP) the isotope record suggests distinct phases of relatively increased precipitation from ca. 2480 to 2430 a and from ca. 2950 to 2770 a (Fig. 10). These phases are also reflected in advances of Swiss glaciers and high lake levels in eastern France (Holzhauser et al., 2005; Luetscher et al., 2010; Fig. 10).

Conclusions This study discusses the environmental control of kinetic C and O isotope fractionation in an active flowstone formation and its application as a proxy of palaeorainfall. Kinetic enrichment (and also the petrography) proved to be sensitive to changes of water-film thickness on the flowstone surface, which modulates CO2-degassing and concomitant calcite precipitation. Our observations agree well with results from studies conducted both in the laboratory and in situ in caves. Different parts of the flowstone revealed different sensitivities of the isotopic enrichment caused by environmental changes, reflected by different Dd13C/Dd18O slopes and coefficients of determination of the d13C versus d18O values. Sensitivity changes further occur along individual drill cores (i.e. temporally), reflecting the longer-term flowstone growth dynamics. This means that flowstone-specific (geometric) growth effects have to be considered when interpreting the stable isotopic composition and petrography of drill cores in terms of palaeoclimate changes, i.e. changing flow routes can significantly modify the degree of kinetic enrichment and the lamina thickness. Such effects are particularly important on timescales of several centuries to millennia, primarily reflected in the first-order trends of the isotope curves. The second-order (decadal) isotope variations are less prone to be influenced by such growth effects. This issue can be addressed by obtaining several long drill cores from different flowstone sections.

Supporting information Additional supporting information can be found in the online version of this article: Fig. S1: Improvement of the internal chronology of the top 4 mm of drill core PFU6 based on petrographic observations Fig. S2: Stable C and O isotope values of modern calcite precipitates collected on glass substrates and plastic foil Fig. S3: Comparison of trace elements (Mg, Sr, Ba), petrography and stable isotopes in the top 50 mm of drill core PFU6 Please note: As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer-reviewed, and may be reorganized for online delivery, but are not copy-edited or typeset by Wiley-Blackwell. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. J. Quaternary Sci., Vol. 26(7) 675–687 (2011)

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Acknowledgements. We are grateful to L. Schuchter, who introduced us to Klapferloch Cave. We also thank J. Kramers for his advice with U– Th dating and P. Treble for trace-element measurements. R. Bo¨hm is acknowledged for providing instrumental climate data and R. Pavuza and I. Kershaw performed some water analyses. Financial support was granted by the Austrian Science Fund (project Y122-GEO).

Abbreviations. DACP, Dark Ages Cold Period; DIC, dissolved inorganic carbon; EC, electrical conductivity; LIA, Little Ice Age; MWP, Medieval Warm Period; RWP, Roman Warm Period; SI, saturation index.

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