Cryogenic cave carbonate – a new tool for estimation of the Last Glacial permafrost depth of the Central Europe

Introduction Conclusions References


Introduction
The southern limit of the continental glaciation of the central part of Europe during the Last Glacial (Weichselian) roughly followed the line defined by the present-day cities of Hamburg-Berlin-Pozna ń-Warszawa-Minsk (Poser, 1948;Svendsen et al., 2004).During the coldest phases of the Weichselian, a permafrost zone developed in the Figures

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Full 2001; Czudek, 2005).Permafrost is defined as ground (rock or soil, including ice and/or organic material) that remains at or below 0 • C for at least two consecutive years (cf. Permafrost Subcommittee NRC Canada, 1988;Dobinski, 2011).For this glacier-free zone located between the northern continental and the Alpine mountain glaciations, little information has been available about the distribution, duration and thickness of permafrost during the Weichselian.It is usually assumed that continuous permafrost existed in the northern part of this zone during the Last Glacial Maximum (LGM), while discontinuous or sporadic permafrost was probably typical for more southerly located areas in the lowlands (Czudek, 2005).
Caves represent a subsurface environment, which has so far rarely been employed for estimation of past permafrost depths.The effects of past freezing temperatures in caves are indicated by several kinds of observations.Typical results of freezing temperatures include cave roof frost shattering, features produced by frost and ice action in unconsolidated cave sediments (i.e.various kinds of size-sorting phenomena and patterned ground phenomena), speleothem damage by frost and ice action, and other phenomena.These features have been described in several caves in Central Europe by numerous authors (Kyrle, 1929(Kyrle, -1931;;Kempe, 1989Kempe, , 2004Kempe, , 2008;;Pielsticker, 1998Pielsticker, , 2000;;Wrede, 1999;Kempe and Rosendahl, 2003;Kempe et al., 2006).The main difficulty interpreting these observations results from the fact that some of these frost indicators can also be produced by other processes than frost and ice action.A further problem in relating them to surface climate change is represented by the difficulty in dating them.Probably the best evidence for former ice filling of cavities is represented by fragments of speleothems, called "ice attachments", attached by carbonate cement to steep sections of cave walls at sites where they could only be fixed due to the support of ice filling the cavity (or some other type of sediments filling the cavity at one time).These features were thoroughly studied in Germany (see references above) where extensive evidence of past ice filling of cavities has been collected.Another indicator of past temperatures at the freezing point in caves is represented by cryogenic cave carbonate (CCC).the depth below the surface of each locality, are interpreted from the point of view of the Weichselian minimum permafrost depth penetration and periods of permafrost existence and destruction.

CCC and its environmental setting
CCC is found in caves in two modes of occurrence, representing two different types of environment of its formation.Fine-crystalline cryogenic carbonate powder, formed during rapid freezing of thin water layers in strongly ventilated caves or cave sections, is rather common.Typical environments for the formation of cryogenic carbonate powder are either cave entrances with seasonal water freezing and icicle formation, or ice caves in the temperate climatic zone, where low temperatures are maintained due to specific cave morphology with trapping of dense cold winter air.According to our knowledge, the first author who noted and properly explained the presence of CCC powder on the surface of seasonal icicles in entrance sections of caves in limestone karst was Kunský (1939).Since then, powdery cryogenic precipitate has been observed in cave entrances and ice caves by many authors (e.g.Viehmann, 1960;Ek and Pissart, 1965;Savchenko, 1976;Lauriol et al., 1988;Pulina, 1990;Clark andLauriol, 1992, 2006a,b;Ž ák et al., 2004, 2008;Sp ötl, 2008;Lacelle et al., 2009;May et al., 2011).These powder CCC types, which form recently in strongly ventilated caves, are of no use for estimation of past permafrost depths.
The second mode of occurrence of CCC is represented by much larger crystals and crystal aggregates (ranging from <1 mm to about 40 mm), which were formed by segregation of the dissolved load during slow water freezing in pools.The recent formation of these coarse-crystalline CCC types has never been observed in caves.
Coarse-crystalline CCC of clearly Holocene origin has been reported only once (Onac et al., 2011), as gledonite-like, up to 4 cm large aggregates found in the ice of the and Poland.These specific carbonate forms have been recorded in caves in this area since the 1950's (Skřiv ánek, 1954;Tulis and Novotný, 1989;Urban and Złonkiewicz, 1989;Erlenmeyer and Schudelski, 1992;Schmidt, 1992;Durakiewicz et al., 1995) but their formation processes were not fully explained until the papers of Ž ák et al. ( 2004) and Richter and Niggemann (2005).Since then, coarse-crystalline CCC localities have been discovered in other caves in the periglacial zone of the Last Glacial in Europe (Orvošov á and Ž ák, 2007;Richter andRiechelmann, 2007, 2008;Richter et al., 2008Richter et al., , 2009aRichter et al., ,b,c, 2010aRichter et al., ,b, 2011Richter et al., , 2012;;Ž ák et al., 2008, 2011;Meissner et al., 2010).This specific type of speleothem frequently occurs in caves with limited or practically no ventilation.The only way how these cavities could have been cooled down to freezing conditions is, therefore, the occurrence of permafrost.

Morphology and mode of occurrence of coarse-crystalline CCC
Coarse-crystalline CCC usually occurs as accumulations of loose crystals and crystal aggregates freely deposited on the surface of the bottom of the cavity.CCC is only rarely covered by younger Holocene speleothems and/or clastic sediments.Its formation, therefore, represents one of the youngest events in the evolution of the studied caves.The accumulations (up to few cm thick, at maximum) usually cover an area of up to several square meters, located typically in the central parts of larger cavities, while they are usually not found in narrow corridors or near the cavity walls.CCC of this type never occurs near cave entrances or in a cave zone, which shows significant seasonal temperature changes.Instead, it is typically found in more remote cave sections.
The morphology of coarse-crystalline CCC is complex and differs from the usual secondary carbonate infillings and speleothems.The morphological variability led to development of many local terms used for description of CCC morphological types in individual countries.Generally, coarse-crystalline CCC types can be divided into several morphological, broadly-defined end-members, each with a typical position in the sequence of crystallization, and most probably slightly differing in the environment of their origin (see Sect. types include: (i) single crystals and their random or organized aggregates/intergrowth, (ii) crystalline raft-like aggregates, and (iii) fine-to coarse-crystalline spherical/globular forms.
The single crystals are usually small (from several tenths up to several millimeters) but relatively well-developed.Their stellate aggregates up to approximately 4 cm in size have also been documented (Urban and Złonkiewicz, 1989).More or less regular rhombohedra and, less frequently, scalenohedra are the most commonly detected crystal forms.The crystals are usually not perfectly developed, and various more or less skeletal-, hopper-, lathy-, fan-, serrated-forms and their various intergrowths are common.They are called "roses", "wreaths" and "twigs" in the local literature (Durakiewicz et al., 1995).
Raft-like aggregates are characterized by their generally flat appearance and by crystal arrangements very similar to classical floating carbonate rafts in non-iced caves (cf.Hill and Forti, 1997, p. 88).The basic building elements of cryogenic rafts usually consist in calcite scalenohedra, sometimes elongated in the direction of the vertical axis.The size of these crystals varies from several tenths of micrometers to several millimeters.Crystals of two generations have been observed at some occurrences of cryogenic raft samples (Na Javorce Cave, Czech Republic, Ž ák et al., 2011).
On the basis of the field observations, model of formation, and stable isotope studies ( Ž ák et al., 2004), it seems highly probable that the sequence of formation of these aggregates starts with sharp-edge aggregate types composed of rhombohedra, while the smooth-surface types are formed later in the precipitation sequence.The CCC precipitation sequence is controlled by increasing bulk salinity of the residual unfrozen portion of the solution during progressive freezing and by slightly decreasing temperature.The isotopically most evolved type (showing the lowest δ 18 O and highest δ 13 C data within each site, see below in Sect.2.2) formed during the final stage of freezinginduced crystallization is represented by more-or-less regular hemispheres.All these forms are today formed by low-Mg calcite.While the early rhombohedral and scalenohedral forms were probably already initially deposited as calcite, the smooth-surface types could have been originally formed by unstable carbonate phases like ikaite or vaterite (Killawee et al., 1998;Lacelle et al., 2009).They were probably all converted to calcite during the warmer Holocene climate.

C and O stable isotope systematics
Precipitation of carbonate minerals accompanied by water freezing exhibits large isotope fractionations, which have been frequently studied (Clark and Lauriol, 1992;Durakiewicz et al., 1995;Fairchild et al., 1996;Killawee et al., 1998;Ž ák et al., 2004, 2008, 2009, 2011;Richter and Niggemann, 2005;Lacelle et al., 2006Lacelle et al., , 2009;;Lauriol et al., 2006a;Orvošov á and Ž ák, 2007;Lacelle, 2007;Sp ötl, 2008;Richter et al., 2008Richter et al., , 2009aRichter et al., ,b,c, 2010a,b;,b;Richter andRiechelmann, 2007, 2008;Meissner et al., 2010;May et al., 2011;Onac et al., 2011).In the usual δ 13 C vs. δ 18 O plot, the data obtained on CCC samples cover a wide range from −2 to −24 ‰ in δ 18 O and −11 to +17 ‰ VPDB in δ 13 C. Their variability is obviously much larger than the variability stable isotope data of speleothems, which do not form due to freezing conditions (Fig. 1).Possible isotope fractionations (probably very small or none) related to conversion from eventually Figures originally present unstable carbonate phases to calcite are not known and, therefore, not evaluated.The C and O isotope systematics of individual CCC types depends on the dominance of either disequilibrium (largely kinetic) or equilibrium isotope fractionation, which is controlled by the precipitation rate (cf. DePaolo, 2011).The precipitation rate is controlled by several partly inter-dependent factors, such as thickness/depth of the layer of freezing water, freezing rate, ventilation of the cave, etc.The kinetic isotope fractionations largely dominate during formation of the cryogenic carbonate powder.Rapid freezing of thin water films and ice sublimation both lead to a faster supersaturation of the carbonate solution and thus to higher precipitation rates.The stable isotope data trends of the CCC powder in the δ 13 C vs. δ 18 O diagram are similar to those for speleothems deposited during evaporation of water or during rapid kinetic degassing of CO 2 from the drip water (cf.Hendy, 1971;Mickler et al., 2006;Dreybrodt and Scholz, 2011).Nevertheless, during freezing-triggered precipitation of CaCO 3 , the δ 13 C values can reach very high values, commonly above +10 ‰ VPDB (Fig. 1).
In case of the coarse-crystalline CCC and slow water freezing in pools, the dominant factor controlling δ 18 O values is equilibrium oxygen isotope fractionation between water and ice, which leads to progressive lowering of the oxygen isotope values (δ 18 O w values) of the residual, yet unfrozen solution (O'Neil, 1968;Jouzel and Souchez, 1982;Souchez and Jouzel, 1984).Kinetic isotope fractionation can occur during very rapid freezing and the resulting difference in δ 18 O between liquid water and ice may be smaller than the equilibrium value (Souchez et al., 2000).The stable isotope data trends for coarse-crystalline CCC illustrated in Fig. 1 2008).The CCC is formed during changing δ 18 O of the solution, the carbonate δ 18 O data itself are not the carrier of paleoclimatic information.The paleoclimatic information of this novel archive is contained only in the presence of CCC (which is a proof of freezing temperature) at a given site and at a given moment of the past.

Regional distribution of the known CCC locations
Central European caves with occurrences of coarse-crystalline CCC are located in a belt defined by latitudes between 47 • 30 and 51 • 30 N and longitudes between 7 • 30 and 20 • 30 E. Details for all studied caves including the references to the original publications are listed in Table 1.The locations of the caves are also shown in Fig. 2. The entrance elevations are mostly in the range between 175 and 500 m a.s.l.(hilly regions).A small number of the studied caves are located in mountainous areas with entrance elevations between 690 and 2230 m a.s.l.
The studied caves are grouped in several karst regions.In Germany, a group of eight caves containing CCC is located in the "Rheinisches Schiefergebrige", with morphologically complex caves formed mostly by corrosion in the phreatic or 2154 Figures

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Full epiphreatic zone in Devonian limestone.Caves dominated by vadose morphology are less common here (represented by some parts of the Herbstlabyrinth-Advent Cave System).The entrance elevations of these caves are in the range from 175 to 501 m a.s.l.One German cave with CCC occurrence (Riesenbergh öhle) is located in Jurassic limestone near the city of Hannover, and one mountain chasmal cave (Glaseish öhle), with an entrance elevation of 2230 m a.s.l., occurs in the Late Triassic limestone (Dachsteinkalk) of the Eastern Calcareous Alps (Fig. 2).
In the Czech Republic, the studied caves are mostly located in the Bohemian Karst, a small karst region developed in Silurian and Devonian limestone close to the city of Prague.CCC samples were collected here from four caves with entrance elevations between 244 and 402 m a.s.l.The caves in this region show a complex morphology (phreatic and/or epiphreatic maze caves are abundant).Several caves bear evidence of hydrothermal processes in the early phases of their evolution.One isolated locality in the Czech Republic (Javoříčko Caves, Fig. 2) with paleo-fluvial epipheratic to vadose levels is located in Devonian limestone in Central Moravia, with entrance elevations between 445 and 475 m a.s.l.
In Slovakia, the studied caves are generally located in the mountainous regions of the Western Carpathians and developed in several types of Mesozoic limestone.The entrance of morphologically complex and partly chasmal Hačova Cave, developed in Middle Triassic limestone, is at 690 m a.s.l. in the Lesser Carpathians, north of Bratislava.
The other localities are found in mountain ranges with entrance elevations between 995 and 1 767 m a.s.l.In all of them, the CCC sites occur hundreds of meters away from the cave entrance.
In Poland, only one cave system with several CCC sites has so far been found.The extensive Jama Chelosiowa -Jasikinia Jaworznicka Cave System, located close to town of Kielce, contains at least twelve CCC occurrences.The caves are of complex phreatic and epiphreatic morphology and formed in Devonian limestone below a small hill in relatively flat country, partly covered by a thin blanket of clastic Early Triassic sediments.The cave entrances elevation ranges from 257 to 288 m a.s.l.Introduction

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Full Cross sections (side projections) of selected localities with the positions of CCC sites indicated are shown in Fig. 3.The figure compiles those localities, which contain the deepest known CCC sites and the caves discussed in more detail.

Materials and methods
Cave maps and cave sections were obtained from the original publications as well as local caving clubs.For construction of the cave sections, the surface morphology was derived from detailed topographical maps (scale 1:10 000).Both the cave maps and surface shape above the caves were in several cases verified or measured more precisely by the authors, using common mapping methods for underground and surface areas.Each of the known or newly discovered CCC sites was then indicated on the cave maps and sections.In order to show the relationships to the surface, the cave sections used are generally side projections (projected profiles), not extended profiles (cf.Palmer, 2007, p. 12).Depths (distances from the closest surface above the site) were then estimated from these cave sections.The CCC sites were field-documented and photo-documented.The C and O stable isotope data were collected from the original publications (see description of concentration of the spike was calibrated against a secular equilibrium standard solution prepared from a 2 Ma old speleothem sample from Wilder Mann Cave, Austria (Meyer et al., 2009(Meyer et al., , 2011)), which has been shown to be in secular equilibrium (Hoffmann et al., 2007).The calibration was checked using another secular equilibrium solution (Harwell Uraninite, HU) that was used to calibrate the U-Th spike used at the Heidelberg Academy of Sciences (Hoffmann et al., 2007)

Results and discussion
Stable isotope data for individual coarse-crystalline CCC locations are shown in Fig. 1.
The field relationships, i.e. the depth of the CCC sites below the closest surface, are presented on selected cave cross sections (Fig. 3), and are summarized in Table 1.Table 1 also contains all available U-series ages of the CCC sites, with the data sources identified.New TIMS and MC-ICPMS U-series data are presented in Tables 2 and 3, respectively.

Evidence for cryogenic origin of the CCC
All the studied CCC samples exhibit a typical mode of occurrence in the field, ranking them among very young events in the evolution of the studied caves (see Sect.The C and O stable isotope data for each locality exhibit a typical trend (Fig. 1) towards extremely low δ 18 O values and slightly elevated δ 13 C values as precipitation proceeds (see above in Sect.2.2).The cryogenic origin of the studied samples can be considered as demonstrated.This is further confirmed by the determined U-series ages, which all (with one exception) suggest formation within glacials.

Ventilation of the studied caves
The typical environment of the occurrence of coarse-crystalline CCC is represented by cave sections remote from the entrances.Most caves with CCC occurrence are morphologically complex maze systems, open to the surface only via rather small entrances and/or very narrow and zigzag-shaped corridors.These caves maintain almost constant temperatures throughout the year, which are close to the present-day mean annual air temperature (MAAT).In several cases, the caves hosting CCC were discovered as isolated cavities by limestone quarrying, having no open connection to the surface (i.e.all connections were completely filled by pre-Weichselian sediments).
In several other cases, the cavities hosting CCC were discovered by cavers digging through the pre-Weichselian sediments that originally completely filled the corridors.described sites, the only way to cool the cavities hosting the CCC was through evolution of permafrost.

Age and model of formation of the studied CCC
In total, 44 U-series ages on coarse-crystalline CCC are currently available (  1).
The period with the most frequent occurrence of CCC ages between ∼40 and ∼21 ka BP corresponds to the younger, climatically unstable part of the MIS 3 and the first part of MIS 2. It is expected that this climatically unstable period induced oscillations in the permafrost depth, creating conditions suitable for CCC formation.Of the total data set of 44 age determinations, only three (less than 7 %) do not fit into the Weichselian.Two older dates (Apostelh öhle, Germany, 408 +42 −31 ka BP; Cold Wind Cave, Slovakia, 180 ± 6.3 ka BP) fit into the younger parts of the Elsterian and Saalian glacials, respectively.Findings of CCC from earlier glacials are rare, probably because these old CCC was covered during later cave evolution by clastic sediments or flowstone.From the whole data set, only one date corresponds to Holocene age (9.191 ± 0.072 ka BP; Na Javorce Cave, Bohemian Karst).This outlier may be interpreted in two ways.Either the U-series system of the sample was not closed (and the sample post-depositionally adsorbed U during the Holocene; the 234 U/ 238 U activity ratio of this sample is by far the highest in the whole data set), or the sample age indeed indicates the existence of relic permafrost in the Early Holocene.The first interpretation is supported by the sample character (aggregates of relatively small crystals with large surface area), the second by the position of the sampling site, which is the deepest (65 m below surface) within the low altitude localities.
Comparison of the U-series ages with the benthic δ 18 O stack from Lisiecki and Raymo (2005) and with NGRIP climate record (North Greenland Ice Core Project Members, 2004) is shown in the Fig. 4. The CCC ages mostly plot near transitions from colder to warmer periods.This is in agreement with the published models of the formation of ice fill in cavities during permafrost (Pielsticker, 2000) and CCC formation in these cavities (Richter and Riechelmann, 2008;Richter et al., 2010b).Deeper infiltration of groundwater is practically impossible during permafrost growth since infiltration is eliminated due to blocking by ice.Groundwater movement is mostly restricted to the near-surface permafrost layer during these periods.Deeper circulation of either highly mineralized groundwater within the permafrost or common groundwater in taliks is possible in karst environments (Ford and Williams, 2007, 421-427 pp.;Spektor and Spektor, 2009).Some karst conduits can be kept open by seasonally flowing water, transporting heat into the permafrost zone.Water with higher mineralization can flow within the permafrost at temperatures significantly below 0 • C. In Canada at 79 of mineralized saline groundwater with a temperature of −4 • C. Nevertheless, the available age data show that the periods of permafrost propagation to greater depths were usually not associated with intensive CCC formation.In contrast, the upper limit (0 • C isotherm) of the relic permafrost is lowered during the permafrost thawing.Thawing propagates to depth more rapidly along water-conducting clefts and karst channels.Because the zone above the upper limit of the deep relic permafrost is water-rich when a karst channel with liquid water penetrates into the top of a cavity, liquid water relatively rapidly fills the whole cavity.The cavity itself, however, is still in the permafrost zone.The water freezes slowly here (cf.Pielsticker, 2000, who came to an identical conclusion from the thermal modeling).The duration of the ice fill of the cavity is, therefore, only a short-term event.As the 0 • C isotherm moves deeper, the ice melts and CCC crystals and crystal aggregates are freely deposited on the bottom of the cavity.The CCC age data show that, depending on the oscillation of the surface climate, the permafrost depth oscillated during the Weichselian.One important period of repeated permafrost growth and destruction occurred between 40 and 21 ka BP, and the final permafrost destruction (at the depths of the studied caves) followed between 17.0 and 11.9 (possibly 9.2) ka BP.The basic principle of CCC formation in a cavity is depicted on the simple general model in Fig. 5.However, it is clear that for some of the studied caves more complex scenarios may be constructed, with repetition of water freezing periods (see Richter et al., 2010a) or possibly freezing of water streams and lakes in caves of the vadose type (see Richter et al., 2011).

Estimation of past permafrost thickness
Discontinuous permafrost typically starts to develop in areas with MAAT below approximately 1 • C (King, 1984).Continuous permafrost is typical for areas with subzero of earth materials, vegetation cover, quantity and duration of snow cover, topography including the aspect, which influences the intensity of insulation or existence of water bodies at the surface (French, 2007).An important factor for permafrost depth penetration is the duration of the climatic period with subzero MAAT.Construction and destruction (thawing) of deep permafrost is usually substantially delayed with respect to the evolution of the surface climate.Full thermal equilibration of the upper crust to the new surface climate conditions may take several thousand years ( Šafanda and Rajver, 2001).Temperature-depth profiles measured in deep boreholes in the area of past permafrost still exhibit thermal relics of the Weichselian permafrost in Europe even today ( Šafanda and Rajver, 2001;Šafanda et al., 2004).A deep relic zone with temperature close to 0 • C starting at a depth of 357 m below the surface and continuing down to the borehole base (450 m) was reported by Honczaruk and Śliwi ňski (2011) and Szewczyk and Nawrocki (2011) in Cretaceous sediments of North-eastern Poland.The temperature is controlled by water/ice transitions here.
Today, in Central Europe, permafrost exists at higher elevations in the Alps and sporadic permafrost patches are also assumed to exist in the highest parts of the Western Carpathians (High and Low Tatra Mts.) in the territory of Poland and Slovakia (Brown et al., 1997;Keller et al., 1998;Dobi ński, 2005).Recent permafrost in European mountain regions is generally more irregular than the permafrost, which developed in lowlands during glacials.In the Alps, the lower limit of occurrence of recent sporadic permafrost is at approximately 2100 m a.s.l., and discontinuous permafrost starts here at approx.2400 m a.s.l.(e.g.Keller et al., 1998).Sporadic or discontinuous permafrost is probably also present in the Tatra Mts. at elevations above approximately 1930 m a.s.l., but has so far been indicated only by ground-surface temperature measurement and ground-penetrating radar on the northern mountain slopes (e.g.Gadek and Grabiec, 2008).The present-day climate-related change of permafrost in Europe was studied by Harris et al. (2003Harris et al. ( , 2009)).Introduction

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Full The deepest cavities hosting CCC in the "Rheinisches Schiefergebrige" (Germany) and in the Bohemian Karst (western part of the Czech Republic) are at similar depths of 60 to 65 m below the surface.The data do not allow estimating whether the permafrost was discontinuous or continuous.The depth of 65 m can be considered as the minimum permafrost depth during the second half of the Weichselian in this area.Since the formation of CCC occurs predominantly during permafrost destruction, which certainly proceeds both from above and from below, the maximum permafrost depth was certainly greater.It should also be noted that the permafrost depth is typically lower in karst areas than the surrounding non-karst areas (Ford and Williams, 2007).One site in the Northern Calcareous Alps exhibits CCC at a depth of 229 m below the entrance (entrance elevation of 2230 m a.s.l.).Based on unpublished modeling (V.T. Balobayev and L. I. Schipiciny in Czudek, 2005), the maximum Weichselian permafrost depth of the Czech Republic was estimated to be in the range from approximately 50 to maximally 250 m, which is in agreement with our CCC depth data.
In the eastern part of the Czech Republic (Moravia), only one locality is known with CCC occurrence at the relatively shallow depth of 30 m below the surface (Javoříčko Caves).The cavity where the CCC occurs also hosts other indications of former ice fill (e.g.destroyed speleothems and "ice-attachments").In the large cave systems of the slightly more southerly located Moravian Karst, no CCC site is known.This CCC absence could possibly be due to the existence of abundant underground water streams in this karst region, which could produce extensive taliks.Relatively thick permafrost in the northern part of Moravia is indicated by the observation of R ůžičkov á and Zeman (1992), who described post-cryogenic structures down to a depth of 220 m in the Blahutovice borehole.Unfortunately, this observation is not accompanied by dating and, so it is not clear when the observed features were formed.
All the sites in Slovakia are located in mountainous areas.Greater depths for the CCC sites can be expected here since even today some of these caves exhibit relatively low temperatures.The deepest CCC site in Mesačný Tie ň Cave in the High Tatra Mts. is located at a depth of 285 m (surface elevation approx.1800 m a.s.l.).The Introduction

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Full present-day temperature of the cave interior varies between 1.0 and 3.5 • C (personal communication by B. Šmída; no more precise temperature measurements have been performed to date; the whole cave system was explored after 2004).This deep occurrence of CCC is in agreement with earlier estimates of Weichselian permafrost depth for the High Tatra Mts., with potential Weichselian permafrost depths down to 400 m below the surface (Dobi ński, 2004).

Further interpretation possibilities for CCC data
Since the formation of CCC proceeds at 0  equilibrium at a temperature around 0 • C by purely inorganic precipitation without any biological effects, CCC may be particularly useful for the calibration of the clumped isotope thermometer at low temperatures.

Conclusions
Cryogenic cave carbonate (CCC) formed by expulsion of the dissolved load during slow water freezing in caves during the Last Glacial (Weichselian) has been found in 20 caves located in the territories of Germany, the Czech Republic, Slovakia and Poland.The caves were located in the glacier-free zone between the northern continental glaciation and the Alpine mountain glaciation during the Weichselian.The CCC formed mainly in several restricted time periods, mostly between 40 and 21ka BP and 17.0 and 12 ka BP.CCC formation in deep caves mostly occurred during periods of permafrost thawing related to surface within-glacial warming periods (interstadials), when liquid water penetrated into empty cavities within the deeper relic permafrost and froze slowly.The CCC occurrences indicate the minimum permafrost depth during the younger half of the Weichselian in the studied area to be more than 65 m in the lowlands and uplands.In the High Tatra Mts., a permafrost depth of more than 285 m below surface is suggested.Since CCC is formed at a temperature of 0 • C or slightly less, they offer other, yet unexplored interpretation possibilities, such as estimates of the δ 18 O values of the source water or calibrations of "clumped isotope thermometry" at low temperatures.Cave system with three epiphreatic subhorizontal levels (CCC occurs in a main corridor of the middle level), limited ventilation before discovery.• 51 33 N 20

Supplementary material related to this article is available
• 29 56 E 257 m a.s.l.

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Full Discussion Paper | Discussion Paper | Discussion Paper | CCC is generally formed by carbonate Discussion Paper | Discussion Paper | Discussion Paper | Sc ǎris ¸oara Ice Cave, Romania.The Weichselian coarse-crystalline CCC localities are known in 20 caves located on the territories of Germany, the Czech Republic, Discussion Paper | Discussion Paper | Discussion Paper | 2.2 for stable isotope data).Three most common morphological Discussion Paper | Discussion Paper | Discussion Paper | . CCC of this type displays either smooth or fine to coarse crystalline surfaces and occurs as separate bodies or their random (sometimes planar) intergrowths/accumulations.It also often overgrows other types of coarsecrystalline CCC.Examples of the oriented intergrowth of two spherical bodies have also been documented.Visibly crystalline hemispheres (usually approximately half a sphere) with more or less exerted crystal edges and corners are usually formed of tightly intergrown, radially arranged crystals.Photographs of typical CCC field Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fig.1for data sources).230Th/U-dating was performed both by thermal ionization mass spectrometry (TIMS) and by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS).TIMS samples were prepared similarly as described inFrank et al. (2000)  orSchimpf et al. (2011).U-series isotopes were measured with an MAT 262 RPQ TIMS at the Heidelberg Academy of Sciences, Germany, using the double filament technique.MC-ICPMS samples were prepared similarly as described inHoffmann et al. (2007) and analyzed at the Max-Planck-Institute for Chemistry (MPIC), Mainz, Germany.The mixed 233 U-236 U-229 Th spike used at MPIC was calibrated against a gravimetric U standard solution and a secular equilibrium standard solution.The gravimetric U standard solution was prepared from a NBL-112a metal bar and also used to calibrate the U-Th spike used at the Heidelberg Academy of Sciences(Hoffmann et al., 2007).The 229 Discussion Paper | Discussion Paper | Discussion Paper | 2).Hydrothermal origin or origin by disintegration of the host limestone is excluded by the unique morphology of the crystals and crystal aggregates.Discussion Paper | Discussion Paper | Discussion Paper | Cooling of these cavities to subzero temperatures by air circulation was impossible.Only three localities are exceptions in this context.The chasmal Glasseish öhle (Northern Calcareous Alps) could have also been cooled by air exchange and not by permafrost evolution (present-day seasonal freezing inside the cave reaches a depth of 40 m, the CCC site occurs at a depth of 229 m).Recent temperatures slightly lower than the MAAT are also typical for the corridor hosting CCC in the Stratensk á Cave System, Slovakia.This inclined corridor leads above the CCC site through a rock collapse in the direction towards a large collapse structure, which is open to the surface and accumulates dense cold air.Finally, stronger cave winds are also characteristic for some sections of Hačova Cave, Slovakia, but most of this cave (including the cavity hosting CCC) has recent temperatures close to the MAAT.In summary, for 17 of 20 Discussion Paper | Discussion Paper | Discussion Paper | CCC age data corresponding to the Last Glacial Maximum (LGM; ∼26 to ∼19 ka BP; Clark et al., 2009) are generally scarce, and are only represented by ages for the Stratensk á Cave, Slovakia.Stratensk á Cave is one of three caves within the studied set, which could have also been cooled by air circulation.Another important group of dated samples from several caves is represented by ages between 17.0 and 11.9 ka BP, corresponding to periods of variable climate after the LGM and before the beginning of the Holocene when the permafrost thickness was generally decreasing.Discussion Paper | Discussion Paper | Discussion Paper | • N, Pollard et al. (1999) documented perennial discharge Discussion Paper | Discussion Paper | Discussion Paper | MAAT.Its depth penetration can be roughly calculated based on MAAT, using appropriate equations (e.g.French, 2007; Anderson and Anderson, 2010).Nevertheless, these model calculations represent only a rough estimate since the real permafrost thickness depends also on many other parameters such as thermal conductivity and diffusivity 2161 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | O w value closest to the initial value at the onset of freezing.The calculation is further complicated by rather imprecise calibration of the oxygen isotope fractionation between carbonate and water at 0 • C. The calculations using published calcite/water oxygen isotope fractionations at 0 • C differ by several ‰.For this calculation exercise, we use the equation of Friedman and O'Neil (1977), which suggests a calcite-water fractionation of 34.37 ‰ at 0 • C .Since the δ 18 O w value changes during freezing, the calculated data represent only the unilateral limit of the initial δ 18 O w value.This model calculation shows that the δ 18 O w value of Weichselian karst water from which the CCC started to grow was around −15 ‰ VSMOW in the lowlands and uplands, and around −20 ‰ VSMOW in the studied mountain regions.Formation of CCC at 0 • C offers some other analytical and interpretation possibilities.During the last few years, preliminary application of "clumped isotope thermometry" has been started (see the recent review by Eiler, 2011).Since the formation of CCC proceeds by slow precipitation of CaCO 3 probably under conditions of isotopic Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Dobi ński, W.: Permafrost in the Tatra Mts.: genesis, features, evolution, Przeglad Geograficzny, 76, 327-343, 2004.Dobi ński, W.: Permafrost of the Carpathian and Balkan Mountains, Eastern and Southeastern Europe, Permafrost Periglac., 16, 395-398, 2005.Dobinski, W.: Permafrost, Earth Sci.Rev., 108, 158-169, 2011Discussion Paper | Discussion Paper | Discussion Paper | Ž ák, K., Šmída, B., Filippi, M., Živor, R., Komaško, A., and Vybíral, S.: New localities of cryogenic cave carbonates in the Czech Republic and Slovakia, Speleof órum, Czech Speleol.Soc.Discussion Paper | Discussion Paper | Discussion Paper | dating: this work and Richter et al. (2012), dating lab: M 10 BUML Cave, Czech Rep. 49 • 56 37 N 14 • 07 42 E 246 m a.s.l.Phreatic cave with a large dome accessible only via long and narrow corridors, discovered by a quarry, limited ventilation.274, 31 45 m 25.95 ± 1.53 33.77 ± 1.77 CCC description and dating: Ž ák et al. (2004); dating lab: Discussion Paper | Discussion Paper | Discussion Paper | 50 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
Stable isotope data trends in this direction in the δ 13 C vs. δ 18 O plot are generally rare in nature.CCC localities found in larger cavities usually exhibit higher δ 13C values than CCC localities occurring in smaller and more isolated cavities.Since coarse-grained CCC is formed under closed (or semiclosed) system conditions and cryogenic powders under open system conditions, the different stable isotope trends of these two types can be explained easily( Ž ák et al., Hoffmann et al. (2007) andJochum et al. (2011)ndard-sample bracketing procedure to derive correction factors for mass fractionation and Faraday cup to ion counter gain, similarly as described inHoffmann et al. (2007) andJochum et al. (2011).All activity ratios reported for both laboratories were calculated using the decay con- librium.All ages are reported in years BP, i.e. before the year 1950.

Table 1 ,
Fig.4).Of these 44 dates, 41 (i.e. more than 93 %) fall within the Weichselian between 104.0 and 11.9 kaBP.The age data are not distributed randomly within the Weichselian, but show several age clusters, containing commonly in one age cluster samples derived from different caves of the study area.Ages from the first half of the Weichselian are less frequent, since the colder part of the Weichselian started since approximately 70 ka BP (cf.Fig.4).The oldest date within the Weichselian (104 ± 2.9 ka BP) falls into a relatively warm interval of Marine Isotope Stage (MIS) 5c.It was obtained for CCC sample from the Cold Wind Cave, Slovakia, located at a high elevation and at shallow depth below a mountain range.Dates from the first half of the Weichselian are more common for locations with higher elevation or with more northerly position within the study area (Cold Wind Cave, Slovakia; Jama Chelosiowa-Jaskinia Jaworznicka, Poland; Riesenbergh öhle, Germany; see data in Table • C (or slightly below, if the residual solution becomes more mineralized), the data allow direct calculation of the corresponding δ 18 O w values of the water from which the carbonate was formed.As stated in Sect.2.2, the δ 18 O w values of the residual, yet unfrozen portion of the solution are shifted to progressively lower values during freezing.Therefore, calculation using the temperature of 0 • C and the highest observed CCC δ 18 O value yields the δ 18

Table 1 .
Characteristics of the studied caves.The U-series ages have been obtained in three different laboratories: Heidelberger Akademie der Wissenschaften, Heidelberg (H), Institute of Geological Sciences of Polish Academy of Sciences, Warsaw (W), Max-Planck-Institute for Chemistry, Mainz (M).

Table 1 .
Continued.Extensive complex, multi-level cave system with large galleries and chambers, formed by allochthonous water streams.Could have larger ventilation in the past.Two CCC sites.

Table 2 .
New U-series data on CCC obtained in Heidelberg.

Table 3 .
New U-series data on CCC for the Riesenbergh öhle, Germany, obtained in Mainz.