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Review

The Potential of Speleothems from Western Europe as Recorders of Regional Climate: A Critical Assessment of the SISAL Database

by
Franziska A. Lechleitner
1,*,
Sahar Amirnezhad-Mozhdehi
2,
Andrea Columbu
3,
Laia Comas-Bru
2,4,
Inga Labuhn
5,
Carlos Pérez-Mejías
6 and
Kira Rehfeld
7
1
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
2
UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland
3
Department of Biological, Geological and Environmental Sciences, University of Bologna, Via Zamboni 67, 40126 Bologna, Italy
4
Centre for Past Climate Change and School of Archaeology, Geography & Environmental Sciences, Reading University, Whiteknights, Reading RG6 6AH, UK
5
Institute of Geography, University of Bremen, Celsiusstrasse 2, 28359 Bremen, Germany
6
Department of Geoenvironmental Processes and Global Change, Pyrenean Institute of Ecology (IPE–CSIC), Avda. Montañana 1005, 50059 Zaragoza, Spain
7
Institute of Environmental Physics, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 229, 69210 Heidelberg, Germany
*
Author to whom correspondence should be addressed.
Quaternary 2018, 1(3), 30; https://doi.org/10.3390/quat1030030
Submission received: 31 August 2018 / Revised: 27 November 2018 / Accepted: 30 November 2018 / Published: 7 December 2018
(This article belongs to the Special Issue Speleothem Records and Climate)
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Abstract

:
Western Europe is the region with the highest density of published speleothem δ18O (δ18Ospel) records worldwide. Here, we review these records in light of the recent publication of the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database. We investigate how representative the spatial and temporal distribution of the available records is for climate in Western Europe and review potential sites and strategies for future studies. We show that spatial trends in precipitation δ18O are mirrored in the speleothems, providing means to better constrain the factors influencing δ18Ospel at a specific location. Coherent regional δ18Ospel trends are found over stadial-interstadial transitions of the last glacial, especially in high altitude Alpine records, where this has been attributed to a strong temperature control of δ18Ospel. During the Holocene, regional trends are less clearly expressed, due to lower signal-to-noise ratios in δ18Ospel, but can potentially be extracted with the use of statistical methods. This first assessment highlights the potential of the European region for speleothem palaeoclimate reconstruction, while underpinning the importance of knowing local factors for a correct interpretation of δ18Ospel.

1. Introduction

Speleothems (secondary cave carbonates) are a widely used archive for the reconstruction of past terrestrial climate, and particularly for the investigation of high resolution climate variability, owing to their often exceptional chronological control [1,2]. The first version of the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database (SISAL_v1) contains 376 speleothem records from across the globe [3]. About a quarter of these records (92) are from Western Europe, making it the region with the highest density of published speleothem datasets worldwide [3]. This paper reviews these records within the wider (palaeo-)climatic context of Western Europe, with the objective to identify and promote the potential of cave sites in the region for future palaeoclimate studies. Moreover, we test the suitability of a large compilation of speleothem records to reveal the existence of regional trends in space and time [4].
While early studies on Western European speleothems principally focused on their availability for temperature reconstruction using δ18O of the carbonate [5,6], it was quickly recognised that δ18Ospel is driven by a complex interplay of regional and site-specific factors, such as moisture source and circulation dynamics, amount of precipitation, dripwater residence time in the overlying karst, cave temperature and ventilation dynamics, and potential kinetic effects during carbonate deposition [7,8]. Due to the filtering effect of the soil-karst system, the δ18Ospel signal is usually strongly temporally attenuated compared to precipitation δ18O, and affected to varying degrees by local noise [4]. This is particularly pronounced at mid-latitude sites and over the Holocene, when ranges in δ18Ospel are typically small (average standard deviation of Western European δ18Ospel in SISAL_v1 is 0.36‰) and reflect only moderate climate shifts, as is apparent from other palaeoclimatic evidence (e.g., [9]). Over glacial-interglacial timescales, changes in the seasonality of precipitation and the spatial stationarity of climate patterns need to be considered as additional external drivers for variability in δ18Ospel [10,11]. These, along with the fact that the signal-to-noise ratio with respect to underlying climatic variations of a given speleothem-based record depends on the local climate, geology, hydrology, and vegetation, underpins the importance of interpreting speleothem records in their specific context. Western Europe possesses the highest density of Global Network of Isotopes in Precipitation (GNIP; [12]) stations worldwide, which provide information on climatic drivers of precipitation δ18O in the region. Moreover, many cave sites are well studied, with continuous, high-resolution multiannual cave monitoring time series, and a good understanding of local climatic conditions. Monitoring data from caves and their surface environment provide a basis for understanding speleothem growth conditions in a specific setting (e.g., [13,14,15,16,17]). These present-day observations are enormously useful to delineate and characterise processes influencing speleothem geochemistry, and of critical importance for the calibration of recent speleothem records against meteorological data.
Speleothems can provide precisely dated, high-resolution palaeoclimate records, and thus they are powerful archives for examining changes in climate variability and modes, and refine the picture obtained by other palaeoclimate archives. Focusing on Western Europe, where a large amount of palaeoclimate records from both speleothems and other archives is available, provides an opportunity to disentangle changes in mean climate state and variability.
Here, we provide an overview of climate in Western Europe, before describing the available data in SISAL_v1 [3]. This study is focused at highlighting the potential of this new database for reconstructing regional trends in δ18Ospel, as well as identifying common issues encountered with speleothem records from this region, in particular with respect to mixed climatic controls (temperature, moisture source, precipitation amount) that often affect Western European δ18Ospel.

2. Study Region and Climate

We define Western Europe roughly as the region between 11 and 16° E, and 36 and 71° N, based on political borders, and subdivide it into Southern Europe (<45° N; Iberian Peninsula, Southern France, Italy except the alpine region), Northern Europe (>45° N; Germany, Northern France, Belgium, Netherlands, Scandinavia), Great Britain and Ireland, and the Alpine region (Austria, Switzerland, Italian Alps; Figure 1; [18]). Soluble lithologies, notably carbonates and evaporites, are present throughout the region, with the exception of most of Scandinavia, Northern Germany, the Netherlands, and parts of the Iberian Peninsula (Figure 1).
The present-day climate is characterised by strong spatial heterogeneity (Figure 2), and climate conditions become increasingly continental moving eastward from the Atlantic Ocean, as a result of predominant westerly moisture transport [21,22,23]. A latitudinal gradient is also present between the temperate-humid climate of Northern Europe and the seasonally arid climate of the Mediterranean region. These spatial gradients are reflected in the regional patterns of precipitation δ18O, and are predominantly related to the continental effect, with lower δ18O due to progressive rainout and Rayleigh distillation with increasing distance from the Atlantic coast [4,24]. Deviations from this trend are found in Southern Europe, where the influence of water vapour from the Mediterranean Sea can lead to higher δ18O values [25], and the Alpine region, where the altitude effect lowers precipitation δ18O [24,26].
The most important driver of interannual climate variability is the North Atlantic Oscillation (NAO), which describes surface sea-level pressure differences between the Icelandic Low and the Azores High [27]. Variations in the NAO strongly influence winter surface temperatures, precipitation patterns, and storminess in the North Atlantic realm and in Western Europe [27,28,29], and modulate precipitation δ18O [30,31,32,33,34,35]. On top of shifts in NAO polarity, changes in the location and geographical extension of the NAO’s centres of action can occur, in particular related to the influence of other modes of climate variability in the North Atlantic region (e.g., the East Atlantic and Scandinavian patterns [36,37,38]). In the Mediterranean region, precipitation is largely controlled by the Western Mediterranean Oscillation (WeMO), understood as an East-West dipole of sea-level pressures between the Azores High and the Ligurian Low [39].
Reconstructing regional and temporal variability in climate conditions through palaeoclimate records is challenging, as they are unevenly distributed in time and space and the accuracy of their chronologies varies widely across records. Thus, the potential of a large database of speleothem records lies in the possibility of extracting regional climate patterns from local responses at individual cave sites.

3. Western European Records in SISAL_v1

3.1. Spatio-Temporal Coverage and Regional Potential

In total, 146 published speleothem isotope records have been identified in Western Europe. Almost a hundred of these records (80 stalagmites, seven flowstones, and five composites) from 41 caves are currently included in SISAL_v1 [3], with 11 records from Great Britain and Ireland, 24 from Southern Europe, 22 from Northern Europe, and 35 from the Alpine region (Figure 1, Table 1).
Regional distribution of records with respect to the occurrence of soluble lithologies (carbonate and evaporite rocks) is patchy, in particular in central France and Germany, eastern central Spain, and central-Southern Italy, where few records are published. While soluble lithologies do not necessarily contain caves and speleothems, as speleogenesis also depends on other factors, these regions are potential targets for future speleothem-based palaeoclimate investigations. However, many sites in the region are protected, either due to the rich speleothem decorations or the presence of archaeological and/or palaeontological remains, requiring minimal sampling impact and close collaboration with other researchers and local caving communities.
Temporal coverage of the Western European records in SISAL_v1 reaches back to ~400 ka, with stalagmite CC-1 from Antro del Corchia in Italy providing the longest record (~265 kyrs, i.e., kilo-years, including growth stops; [41]; Figure 3). Note that, in this study, ka BP is defined as thousand years before present, with the present referring to 1950 CE. The majority of the records however only cover the Holocene, with eight speleothems deposited during the last millennium (Figure 3B). Temporal coverage of speleothem records beyond the Holocene steadily decreases with increasing age, with 17% of all records starting to grow during the Last Glacial period (~12–80 ka), 15% during Marine Isotope Stage 5 (MIS 5; ~80–135 ka), and 10% before MIS 5. The average length of the records is ~16 kyrs, including growth interruptions (min: ~50 years; max: ~265 kyrs; median: ~6.7 kyrs). Many records (33%), especially the longest ones, contain at least one hiatus (Figure 3). The longest uninterrupted record, stalagmite CC-5 from Antro del Corchia, Italy, covers ~80 kyrs [42]. The average chronological resolution over all records is 17 years, gradually decreasing with record length/antiquity (Figure 2). The vast majority of records deposited since the end of the Last Glacial Maximum (LGM; 21 ka) have annual to multi-decadal resolution over most of their length (Figure 3B). Some records have seasonal (e.g., Gib04a from St. Michaels Cave, Gibraltar; [43]) or annual resolution (e.g., ER76 from Ernesto Cave, Italy; [44]), but others only provide multi-decadal and centennial information (e.g., CC-1 from Antro del Corchia, Italy; [41]). A tendency towards more speleothem growth during interstadials and interglacials is apparent, especially at high latitudes (Figure 3A), highlighting the dominance of temperature control on hydroclimate and vegetation/soil dynamics in this region.
Records not yet included in the SISAL database, due to difficulties in retrieving the original data [20], will improve the temporal coverage of the region. For example, records from Scandinavia covering past interglacials [45,46,47] will shed light on the temperature control of speleothem growth at high latitudes (Figure 2). Nevertheless, a bias towards more recent post-LGM reconstructions is apparent in our assessment of SISAL_v1, mainly because speleothems suffer from natural attrition [48]: recent deposits are usually more numerous as they are less likely to be lost and/or destroyed by processes such as floods, underground collapses, in-cave sedimentation covering the speleothems. Recent deposits also tend to be more suitable for geochemical analyses (e.g., less weathered and chemically altered) than older material. Additionally, there has been a strong interest of the community for very recent reconstructions that allow calibration with instrumental records, as this could pave the way toward quantitative reconstructions of past climatic variations (precipitation amount, temperature, etc.). Given the ability of speleothems to provide high resolution and very precisely dated records of past terrestrial climate, speleothem records from this region spanning further back in time have enormous potential, e.g., to better constrain conditions during past interglacials where climatic conditions were similar or warmer than today’s [49]. This would be particularly useful for time periods beyond the range of radiocarbon dating (~50 ka), where chronological control in other archives becomes increasingly tenuous.

3.2. Dating Methods and Chronologies

Any palaeoclimatic interpretation hinges on the accuracy and precision of the dating method and age modelling technique applied. Speleothems are known for their precise U-Th chronologies, but other dating methods (e.g., layer counting, radiocarbon) are applied as well, and methodologies are often combined. SISAL_v1 contains 1189 ages from Western European speleothems, 96% of which were obtained with the U-Th method. U-Th ratios were measured by Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS; 62% of the total) or Thermal Ionisation Mass Spectrometry (TIMS; 34% of the total). For the calculation of corrected ages, the most recent publications use the U-Th decay constants published in [131,132], while older datasets mostly refer to [133]. This is because decay constants have been updated over time as a result of increasing analytical precision and methodological developments. Although not having crucial implications on the final ages, it highlights the efforts for a better understanding of the U-Th systematics. Other dating approaches such as 14C analyses (e.g., [43,130]), layer counting (e.g., [44,134]) or approaches combining multiple methods each represent ≤2% of the total. The median two sigma (2σ) uncertainty related to single ages is 1.2% (with respect to the final corrected ages), ranging between 0% and 131%; uncertainties of 0% refer only to the top of actively growing speleothems. The dating uncertainties vary according to the method used (Figure 4), with MC-ICP-MS dates being the most precise (median 0.9%; min 0.2%; max 131%), followed by TIMS (1.5%; 0.3%; 67%), layer counting (2%; 0%; 13%), combination of multiple approaches (4%; 1%; 7%) and 14C (17%; 4%; 42%). It should be noted that U-Th uncertainty increases in younger samples (i.e., last millennia), reflecting the difficulties in measuring the low radiogenic 230Th content for recent speleothems [135] (Figure 4). For these young speleothems, other dating methods such as 14C and layer counting might be more suitable and result in less overall uncertainty, as demonstrated by Mattey et al. [43], who used a combination of 14C measurements and counting of minima in seasonal stable carbon isotope ratio (δ13C) cycles to derive a precise chronology for a 53-year old stalagmite (Gib04a) from Gibraltar. For laminae counting, uncertainties are usually attributed after a series of independent counts (e.g., [61]), and offsets between band-counted and radiometric ages might occur if annual layers are missing (undercounting) and/or if intra-annual lamination is present (overcounting; [136,137]).
A variety of methods and algorithms are available to convert single dates to an age-depth model and, in most cases, calculate the propagation of age uncertainties through time. For Western European speleothems in SISAL_v1, the modelling procedures used are: StalAge (25%; [138]), linear interpolation (19%), Bayesian (not including OxCal, Bacon, BChron, and COPRA; 11%), polynomial fit (10%), OxCal (3%; [139]) and others or a combination of methods (30%). Here, “combination of methods” usually stems from the use of different dating techniques, e.g., 14C and laminae counting in Gib04a from Gibraltar [43] or U-Th analysis and laminae counting in speleothems in Uamh an Tartair [134] and Larshullet Cave [103]. In our dataset, 13% of all ages were excluded from the final age-depth models in the original publications, mostly because they were not in the correct stratigraphic order and were thus considered unreliable. In radiometric dating, out-of-sequence anomalous ages can occur due to diagenetic phenomena affecting the carbonate fabric, and/or sources of error during sample preparation and analysis (e.g., cross-contamination, environmental contamination, spike calibration problems; [65,140,141,142,143]).
All age modelling protocols have their advantages and shortcomings [144], and emphasise different aspects of chronology development. In many cases, the age model for a single speleothem needs to take into account specific properties of the sample, e.g., petrographic and geochemical anomalies, and is thus to a certain extent subjective to the decisions of the user. This is important to ascertain the quality of the age-depth model for an individual speleothem but might introduce biases. An updated age modelling technique intercomparison study appears timely with the publication of SISAL_v1 and would be helpful to determine common strengths and weaknesses in the models.

3.3. Availability of Environmental and Monitoring Data

Local climate and cave monitoring data are useful for the characterisation of the karst system and the processes acting on geochemical signatures recorded in speleothems. With sufficiently long time series of outside and in-cave temperature, and isotopic composition of precipitation and drip water, it is possible to estimate the extent to which δ18Ospel records are representative of present-day external environmental signals, i.e., temperature and precipitation δ18O. This is important, as fluid transfer through the karst system results in lagging, attenuation, or modification of the original precipitation signature. It must be noted, however, that this modern information needs to be carefully evaluated in the context of past climate, as the processes driving them might be temporally non-stationary and responding to climatic changes themselves (e.g., vegetation and soil cover, precipitation seasonality, changes in the temperature gradient between cave and exterior) or influenced by anthropogenic activities (e.g., land use changes, cave tourism, alteration of cave passages).
In Western Europe, temperature and precipitation data is widely available from meteorological stations for at least a few decades. Moreover, Europe has some of the longest meteorological records in the world, which go back to the 18th century (e.g., [145,146,147]). Gridded reanalysis or observational datasets provide continuous spatial coverage at high resolution since the late 19th century [40] and over 250 GNIP sites [12] provide monthly or event-based measurements of isotopes in precipitation, including the station with the longest record worldwide (Vienna-Hohe Warte, period covered: 1960–2016). Roughly 37% of these GNIP sites have been active for at least 10 years, and 63% for at least 5 years, providing an invaluable source of data to understand variability in precipitation and speleothem δ18O (e.g., [148,149]). In-cave monitoring data, in contrast, is usually acquired within the context of specific speleothem studies (e.g., [16,17,150]), and is thus often limited by short-term, project-based funding or limited accessibility to the cave. Of the 41 Western European cave systems included in the SISAL_v1 database, 22 have been monitored over at least one season (53% of all caves in SISAL_v1 of the region), as caves are often easily accessible, and more research funding is available than in some other regions.

3.4. Climate Controls on Speleothem Growth

In Western Europe, the response of speleothems to climate can generally be considered hierarchical, with growth presence/absence being the response to large climatic variations (glacial-interglacial changes), while millennial to seasonal variations are often recorded more specifically through geochemical variations. Speleothem growth is promoted during the warmer and more humid interglacials and interstadial periods (Figure 3A), aided by higher soil pCO2 and the availability of infiltrating water. At high latitude/altitude locations, the most straightforward climatic influence on speleothem growth is the presence of ice or permafrost above the cave, which prevent fluid percolation through the soil-karst system. Presence/absence of growth and growth rate can thus both be powerful proxies for palaeoclimate conditions (e.g., [134,151,152]), with growth cessation and/or slow growth rates indicative of drier/colder stages, such as glacials and stadials (Figure 3A). This is observed at the Scandinavian cave sites, where speleothem growth is limited to interglacial time periods [47,107]. Conversely, evidence from high Alpine caves has shown that “subglacial” speleothem growth is possible if carbonate dissolution is promoted by sulphide oxidation, for instance seen in stalagmites from Milchbach and Sieben Hengste Caves, Switzerland [104,114] and Spannagel Cave, Austria [119].
Changing soil and vegetation activity is another mechanism influencing speleothem growth, as soil pCO2 drives carbonate dissolution in the karst. Examples for such sites sensitive to changes in soil pCO2 are Villars Cave in France, where cold phases during the last glacial are reflected by hiatuses in stalagmite Vil-stm9 [126], and Han-sur-Lesse Cave in Belgium, where stalagmite Han-9 stops growing after a period of drastic vegetation changes (shift to a more grass-dominated vegetation) and aridification synchronous with Greenland stadial 26 [85]. Milder stadial/glacial climate conditions at lower latitudes (e.g., Southern Italy and Southern Spain) appear to have allowed speleothem deposition at some sites [153], but so far these regions are poorly represented in the literature on interglacial-glacial transitions (Figure 1). Given the range of processes that can cause speleothem growth cessation, a climatic interpretation of growth presence/absence and growth rates hinges on an assessment of potential site-specific controls, e.g., drip pathway changes, tectonic activity, or anthropogenic influences. Growth presence/absence as a response to climatic change therefore needs to be ideally verified through replication with a number of samples from the same cave.

3.5. Controls on δ18Ospel

A multitude of factors can influence δ18Ospel, from local effects such as cave temperature and karst infiltration dynamics, to processes driving precipitation δ18O (air temperature, precipitation amount and seasonality, moisture source and circulation dynamics). This is especially pronounced at the mid-latitude sites of Western Europe, where competing influences from several of these processes and a generally weaker climate control (especially during the Holocene) require detailed evaluation of the drivers of δ18Ospel.
The δ18Ospel at sites included in SISAL_v1 is interpreted as dominantly reflecting air temperature (17 sites), precipitation amount (six sites), moisture source (one site), or a mixed signal of temperature and amount/moisture source (nine sites). The interpretation of δ18Ospel remains unclear for eight sites, as the original publications had a different focus.
A dominant temperature signal is found principally at high-altitude sites in the Alps (Baschg [58], Entrische Kirche [78], Grotta di Ernesto [44], Hölloch im Mahdtal [87], Katerloch [93], Kleegruben [96], Schafsloch [112], Schneckenloch [58,88], Sieben Hengste [114]). At these sites, δ18Ospel is understood to closely reflect precipitation δ18O, which is highly correlated to changes in air temperature during moisture condensation, with higher (lower) δ18O reflecting warmer (colder) conditions (e.g., [87]). The authors of the original studies however emphasise that factors such as rainfall seasonality [58,78,93,112] or moisture source changes [87,88,117] play an additional role in the modulation of δ18Ospel, preventing a quantitative reconstruction of surface air temperatures. Interestingly, an opposite δ18O-temperature relationship (higher δ18Ospel corresponding to lower temperature) has been reported for some alpine sites, most prominently for the Holocene portion of the Spannagel Cave record, where δ18Ospel has been interpreted in terms of variable dripwater sources (snowmelt water vs. rainfall), with more negative δ18O values reflecting a larger contribution of melt waters and less negative δ18O explained by a stronger share of meteoric waters due to glacier retreat during interglacials [120,122]. The high-latitude Scandinavian caves (Søylegrotta [45], Okshola [107], Labyrintgrottan [98], Korallgrottan [97] and Larshullet [103]) show the same reversed δ18O-temperature relationship, corroborated by temperature calibration studies [6,45], and are also explained by seasonally selective infiltration during snowmelt [98,107]. Only three sites at lower altitudes report a dominant temperature effect on δ18Ospel (Han-sur-Lesse [85], Clamouse [68,69], and Crag [69]).
Sites with a dominant influence of precipitation amount on δ18Ospel are widely spread throughout the region (Antro del Corchia [41,42,51,52], Buca della Renella [62,63], Bue Marino [65], Burgeois-Delaunay [59], Cueva de Asiul [74], Klapferloch [95]). In all these cases, higher (lower) δ18Ospel is interpreted to reflect lower (higher) rainfall amount, as a consequence of isotopic depletion of precipitation during large storms [154]. Correlating speleothem δ18O and carbon isotope ratios (δ13C) often helps discerning whether δ18Ospel is driven by precipitation amount, as enhanced rainfall intensifies soil activity and increase the drip rate, resulting in lower speleothem δ13C (e.g., [59,65]).
Several sites (Bunker [66], Cova da Arcoia [71], El Pindal [109], Grotta di Carburangeli [80], Molinos [77], Uamh an Tartair [123,124], Villars [67,126,127,128,129]) show a mixed signal of temperature and precipitation amount controls, which complicates their interpretation.
Moisture source is the third main driver for δ18Ospel in Western Europe, as precipitation in some areas (mainly southern Europe) can originate from either the Atlantic or the Mediterranean, with the latter exhibiting higher δ18O values [25,155,156] and wide range of total δ18O variability. The differences in salinity and temperature between the two bodies of water determine the δ18O of the moisture throughout the cloud trajectory, although other effects such as continentality, rainout effect or altitude also contribute to the final δ18O signal. Moreover, changes in atmospheric circulation patterns can lead to shifts in moisture trajectories and seasonality of precipitation at a site (e.g., [157]). These effects have been recognised as partially influencing many of the records in SISAL_v1, mostly as second-order controls (e.g., Baschg [58], Ejulve [11,77], Grotta di Ernesto [44], Schafsloch [112], Schneckenloch [58,88], Seso [113]). The LGM portion of the composite record from Sieben Hengste (7H) Cave, is the only instance where δ18Ospel was interpreted as principally reflecting changes between northerly and southerly moisture transport, informing on shifts in the meridional position of the North Atlantic storm track [114].
Due to these different factors potentially influencing δ18Ospel in Western Europe, other climate proxies are often used to better constrain the palaeoclimatic interpretation of the records (e.g., δ13C or trace element ratios) and their use should be promoted in the future. One possibility to quantitatively reconstruct the isotopic signature of the “parent” precipitation δ18O is through fluid inclusion stable isotope analysis in speleothems [128,130]. Fluid inclusions provide a more direct record of precipitation δ18O than speleothem calcite and allow the reconstruction of palaeo-temperatures when used in combination with carbonate δ18O, if this has precipitated at isotopic equilibrium [158]. To date, however, very few paleoclimate records based on speleothem fluid inclusions exist (e.g., [129,130,159]) mostly because of the considerably larger effort required for their analysis and interpretation compared to carbonate δ18O. Much larger sample sizes are typically needed (~100 mg carbonate, compared to few tens of micrograms for carbonate δ18O), depending on the stalagmite growth rate and water content. Moreover, the analytical uncertainty is much higher than for carbonate (0.5‰ for δ18O and 2‰ for δD, compared to 0.05–0.1‰ for carbonate δ18O; e.g., [160,161]). Thus, fluid inclusion isotope records remain less available for available for very high-resolution (up to subannual) studies that compose one of the main strengths of speleothems.

4. Regional Patterns in δ18Ospel Records Through Time

4.1. Spatial Trends and Comparison to Observations

We use the SISAL database to detect whether present-day regional trends in δ18Ospel values exist, and how they compare to trends in precipitation δ18O, taking advantage of the dense network of GNIP stations with long-term measurement records (up to 50 years). All analyses were referenced to the period 1958–2015. The average δ18Ospel over this time period was calculated from the SISAL_v1 database (n = 18), while GNIP data was averaged over the entire year, after tests showed that averaging for winter months only gave the same result (n = 211; Figure 5). For a more direct comparison of SISAL_v1 δ18Ospel and local precipitation data, we used the gridded interpolated precipitation data that is based on the GNIP network (background maps in Figure 5; [162,163]).
Our comparison between SISAL sites and the GNIP interpolated precipitation data reveals moderate correlation (r2 = 0.48, 18 sites, under the assumption of full independence between sites).
It is apparent from this comparison that high latitude SISAL sites are strongly offset from their corresponding precipitation δ18O values (Figure 5B). The present-day spatial trends in δ18Ospel reflect the dominant climatic processes reflected in precipitation δ18O and provide the opportunity to establish “base lines” of δ18Ospel, to which high-frequency changes can be compared, as well as to check whether single isotope records might be anomalous within their regional climatic context [4]. On the whole, the spatial trend in δ18O is very similar for precipitation and speleothems (Figure 5), and reflects increasing rainout away from the Atlantic (continental effect, [24]). Smaller-scale trends, such as the high δ18O values found in the circum-Mediterranean region, and the altitude effect apparent in the Alpine region, are mirrored by the SISAL data.
Despite this good spatial agreement over Western Europe, discrepancies are apparent at some sites. Local conditions are known to affect speleothem geochemistry and need to be taken into account when developing transfer functions for climate reconstruction. The isotopically effective recharge [8], related to the dominant infiltration season and the degree of mixing in the karst aquifer, can substantially affect the correlation between δ18Ospel and precipitation δ18O [149,164]. Small-scale variability in mountain climate that is not captured by the interpolation approach used for the GNIP data is likely the reason for the offset between SISAL sites and the GNIP data in the Alpine region and northern Scandinavia (Figure 5; [32]).

4.2. Last Glacial Period

The SISAL_v1 database contains 24 records that cover the last glacial time period, i.e., the period between ~11.7 and ~115 ka (Figure 6). Many records from high altitude alpine cave sites show a very clear response to millennial-scale forcing from the North Atlantic (e.g., 7H, SPA 49, SPA 126, HOL-7, HOL-10, HOL-16-17, HOL-comp, BA-1, BA-1b, BA-2; Figure 6B). This strong synchronicity and similarity in the climatic response of the northern Alpine region and Greenland suggests a tight coupling between the two regions, which is likely related to the strong temperature control in these high altitude speleothems [87], and supported by data from other archives (e.g., [165]).
Speleothems from other cave sites in the region show a much less consistent pattern over the last glacial period, probably as a result of the complex interplay of processes affecting δ18Ospel at mid-latitudes (Figure 6C). In some of these cases, the original authors used other geochemical proxies, such as δ13C, for the palaeoclimate interpretation. Stalagmites from Villars and Chauvet caves in France, for example, suggest a complex combination of temperature, precipitation amount and source changes affecting and muting their δ18Ospel, whereas δ13C appears to be more sensitive to stadial-interstadial forcing. The authors interpret these rapid shifts in δ13C as reflecting changes in soil CO2 production, which is linked to temperature and humidity [67,126,127].

4.3. Holocene Climate Variability

In our first assessment, we find no consistent regional trends in δ18Ospel in SISAL_v1 records from Western Europe spanning the entire Holocene period. This is partly due to age modelling uncertainties and low temporal resolution in some records, which prevents the detection of climatic shifts in δ18Ospel during periods with low signal-to-noise ratios. Although Holocene climate conditions are more stable than during the last glacial period, recent evidence suggests significant variability, challenging the notion of a “very stable Holocene” [170]. The 8.2 ka event, a significant North Atlantic focused temperature event, can be used as a benchmark to test the sensitivity of our records for millennial-scale climate change. In SISAL_v1, 21 records from Western Europe cover the time period around 8.2 ka, and nine of them were interpreted by the original authors as recording evidence for a climatic perturbation at that time (Figure 7). Another four show changes in their growth and petrography (i.e., hiatuses, erosional surfaces, changes in calcite fabrics) that can tentatively be related to climate change around 8.2 ka. For most of the records, however, chronological uncertainty and/or temporal resolution remain an issue, and the detailed structure and timing of the 8.2 ka event often cannot be resolved (Figure 7). As a result of the paucity of available datasets it is also not possible at this stage to assess any regional trends in the expression of the 8.2 ka event in stalagmites from Western Europe. Recent advances in analytical and sampling methods provide an opportunity for future studies to obtain more detailed insights into this event, both in previously sampled records and at new sites. Such investigations might also help in disentangling the 8.2 ka event in Western Europe from underlying low-frequency climatic events at that time [171].

4.4. The Last Two Millennia

We evaluated the regional coherency of δ18Ospel over the last two millennia using the data in SISAL_v1 by stacking all records covering at least 1/3 of the interval (−50–2000 yr BP) at reasonable resolution (≥10 data points), and for which an age model is available. Eighteen records fulfil these requirements: Stalagmites Vil-stm1 and Vil-stm6 from Villars Cave, France [130]; stalagmites LH-70s-2 and LH-70s-3 from Lancaster Hole, England [102]; stalagmite SU967 from Uamh an Tartair Cave, Scotland [123]; stalagmite FM3 from Okshola Cave, Norway [107]; stalagmite L03 from Larshullet Cave, Norway [103]; stalagmite SG05 from Soylegrotta Cave, Norway [45]; stalagmite K11 from Korallgrottan Cave, Sweden [97]; the composite record from the Austrian Alps COMNISPAII [116], stalagmite CC3 from Crag Cave, Ireland [69]; flowstone PFU6 from Klapferloch Cave, Austria [95]; stalagmite CL26 from Clamouse Cave, France [69]; the composite record from Bunker Cave, Germany [66]; stalagmites ASR and ASM from Cueva de Asiul, Spain [74]; flowstone RL4 from Buca della Renella, Italy [63]; and stalagmite ESP03 from Cova da Arcoia, Spain [71]. After a Gaussian smoothing on a 100-year timescale was performed for all records, they were interpolated to 5-year timescales. For the stack, the unweighted average of all records was used, and no weighting based on location, correlation strength, or uncertainty was performed.
The stacked record for the last 2000 years shows a long-term trend towards more positive δ18Ospel between 2000–550 yr BP, followed by a reversal (Figure 8). The highest δ18Ospel values of the stack at 550 yr BP fall within the Little Ice Age (LIA; [175]). However there is no clear indication of systematic changes in δ18Ospel corresponding to the Roman Warm Period (RWP; [177]), the Late Antique Little Ice Age (LALIA; [177]), or the Medieval Climate Anomaly (MCA; [176]), periods of significant temperature change in Europe. This failure may reflect the high uncertainties related to the degree of noise in the single speleothem records, as well as the different climate signals recorded by the individual speleothems. It is important to note that the stack captures the mean δ18Ospel signal in Western Europe, and not a single climate process, e.g., temperature or precipitation amount. A screening of the records based on their response to climate was not possible at present, since most records are not calibrated against instrumental data. If more records that fulfil this requirement become available, a stack based on the recorded climate process might become feasible. In addition, future improvements of this procedure should tackle a regional assessment of the trends using a higher number of records and possibly including proxy data from other archives, checking correlations between nearby records, incorporating age uncertainties, and the integration of the signal’s interpretation by the original authors.

5. Improvements to SISAL for Western Europe

Only 60% of the known records from Western Europe are in the SISAL_v1 database and the remaining records not yet in SISAL are widely distributed across Europe (Austria: 22, Belgium: 3, France: 7, Germany: 5, Italy: 3, Norway: 5, Spain: 9, Sweden: 1, Switzerland: 2). We have shown, for the current version of the database, that the speleothem records from Western Europe have potential to document some aspects of past climate change. However, it is clear that outstanding issues need to be addressed first, which in Western Europe can be summarised as (i) improvement of temporal coverage, (ii) improvement of spatial coverage, especially with records calibrated against modern climate conditions, and (iii) a more comprehensive use of statistical approaches to extract underlying modes from spatially distributed records, a key aim of the SISAL working group.
Overall, the paucity of records spanning beyond the LGM presents an opportunity for future studies to target speleothems covering previous time periods, especially given their often more precise U-Th chronologies at these time scales compared to ice cores and marine sediment records [11,178,179]. Climate variability over the last glacial and beyond remains poorly constrained and speleothems could provide detailed information from vast range of different environments (coastal, continental, high altitude/latitude, etc.).
Modern records that contribute to improving our assessment of the spatial robustness of δ18Ospel in Western Europe need to be calibrated against modern conditions, taking advantage of the GNIP network and climate model simulations. This is particularly important for the goals outlined by SISAL, as δ18O is by far the most reported speleothem geochemical proxy, and the only one that has a direct parameter equivalent in climate model simulations with isotopic tracers. Instrumental and modelling data have recently allowed to define regions within Western Europe that are particularly suited for reconstructions of certain (hydro-)climatic conditions, especially with respect to spatio-temporal non-stationarities of the NAO [29,31,32]. In particular, variable sensitivities to the NAO have been implied for Central Europe, the Iberian Peninsula, the Baltic Sea, the British Isles, and the circum-Mediterranean region [32]. Record coverage in these areas is still patchy (Figure 1), and should be targeted by future efforts, ideally including long-term cave microclimate monitoring to define present day surface to cave transport processes, assessment of local hydroclimate variability (through the isotopic analysis of precipitation samples from the cave site), and careful sampling practices.
Competing influences on δ18Ospel and low signal-to-noise ratios at mid-latitude sites still prevent a quantitative interpretation of δ18O. As recently demonstrated by Deininger et al. [33], who used Monte Carlo based Principal Component Analysis on several δ18Ospel records in the same region, sophisticated statistics methods can extract a common mode of climate variability from these datasets. Our approach of stacking different records is also useful to determine robust regional trends in a quantitative manner, but needs to be refined by future studies. Such statistical approaches hold great promise in the context of a large database like SISAL and could be used to better constrain spatial trends in precipitation δ18O over time, and improve our understanding of local and regional influences on δ18Ospel.

6. Future Directions

This first assessment of the Western European data compiled in the SISAL_v1 database highlights interesting spatial and temporal trends in stalagmite δ18O, but these will need to be validated and better constrained by future work on the database. Specifically, we encourage:
  • Inclusion of missing records, which were not available to us for SISAL_v1 as they had not been archived in the supplementary information or on public repositories, and where no/limited contact with the original authors could be established. This is crucial for the assessment of temporal and spatial coverage of speleothem records in Western Europe and helps defining future target regions and time periods for new studies. This could be a starting point for revisiting sites and speleothems that have shown great sensitivity for climate reconstruction, but where resolution and/or chronological precision could be improved. It could also be of interest for a better definition of short-lived events such as the 8.2 ka event or the 4.2 ka event, and to improve chronological controls of speleothems previously dated with TIMS.
  • Addition and use of other types of data. For example, fluid inclusion δ18O measurements on speleothems would provide important direct information on past precipitation δ18O. Similarly, speleothem δ13C data, already included in the database, should be evaluated, as many sites highlight its importance as (qualitative) proxy for soil activity and hydroclimate [44,61,126].
  • Inclusion of more information about cave monitoring. The SISAL_v1 database only includes a yes/no/unknown entry for cave monitoring, which is often not sufficient when evaluating the extent of knowledge of modern cave conditions.

7. Conclusions

By assessing the speleothem data collected in the SISAL_v1 database for Western Europe, we describe regional trends in δ18O, and evaluate the potential of this large compilation of records for palaeoclimate studies in this region. Western Europe has the largest number of published speleothem palaeoclimate records worldwide, many of which (>60% of the identified records) are currently included in SISAL_v1. Moreover, climate conditions are well understood, due to the availability of a dense network of GNIP stations, some of the longest meteorological records worldwide, and global modelling and reanalysis datasets. This is a great advantage for the interpretation of δ18Ospel records, which at mid-latitude sites is often difficult because of competing effects from precipitation and cave processes.
In this review of Western European data included in SISAL_v1, we find that (i) present-day spatial trends in δ18Ospel from Western European caves generally mirror the trends in precipitation δ18O. (ii) Over the late Quaternary, site-specific noise in δ18Ospel presents the main issue for the extraction of a regional climate signal, especially over the Holocene. (iii) Encouraging results can be obtained through the use of statistical methods, which allow the extraction of regional climate modes.
The SISAL database is a valuable tool for the intercomparison of δ18Ospel records over the Western European region. We believe this will provide an important resource of palaeoclimatic input for modelling studies and improve our understanding of the speleothem archive at mid-latitude regions.

Author Contributions

All authors contributed in the collection of data and liaison with authors of the original studies included here. F.A.L. coordinated the study and wrote the manuscript. All authors analysed data, reviewed the literature, drafted and edited figures, and contributed to the text. All authors discussed manuscript ideas and edited earlier manuscript versions, and approved the final manuscript.

Funding

F.A.L. acknowledges funding by the Swiss National Science Foundation (SNSF) grant P2EZP2_172213. S.A.-M. and L.C.-B. acknowledge funding from the Geological Survey Ireland (Short Call 2017; grant number 2017-SC-056) and from the ERC-funded project GC2.0 (Global Change 2.0: Unlocking the past for a clearer future, grant number 694481). C.P.-M. acknowledges funding by the Government of Aragón predoctoral research grant B158/13 and CGL2016-77479-R (SPYRIT) project. K.R. acknowledges financial support by the German Research Foundation (DFG) grant RE3994-1/1.

Acknowledgments

We thank everybody involved in SISAL for fruitful discussions and collaboration on the preparation of this manuscript. SISAL is a working group of the Past Global Changes (PAGES) programme and we thank PAGES for their support of this activity. We thank three anonymous reviewers and Prof. Andy Baker for constructive criticism on our manuscript. We are grateful to Sandy Harrison for reviewing and giving feedback on the manuscript and academic editorial handling. We also thank the editorial team at Quaternary for their support and help. We thank the World Karst Aquifer Mapping project (WOKAM) team for providing us with the karst region map presented in Figure 1.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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Figure 1. Map showing distribution of carbonate and evaporite rocks in Western Europe, provided by the World Karst Aquifer Mapping project (WOKAM; [19]). Purple circles show the sites included in SISAL_v1, while green triangles indicate study sites in the region identified by the Speleothem Isotopes Synthesis and AnaLysis (SISAL) working group, but not yet included in SISAL_v1 [20].
Figure 1. Map showing distribution of carbonate and evaporite rocks in Western Europe, provided by the World Karst Aquifer Mapping project (WOKAM; [19]). Purple circles show the sites included in SISAL_v1, while green triangles indicate study sites in the region identified by the Speleothem Isotopes Synthesis and AnaLysis (SISAL) working group, but not yet included in SISAL_v1 [20].
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Figure 2. Map of the study region depicting mean annual surface air temperatures (MAT) (A) and total annual precipitation (TAP) (B) for the period 1958–2013, using the CRU-TS4.01 dataset [40]. The time period used was selected to match with the data extracted from the Global Network of Isotopes in Precipitation (GNIP; [12]) network and the SISAL_v1 records (see Figure 4).
Figure 2. Map of the study region depicting mean annual surface air temperatures (MAT) (A) and total annual precipitation (TAP) (B) for the period 1958–2013, using the CRU-TS4.01 dataset [40]. The time period used was selected to match with the data extracted from the Global Network of Isotopes in Precipitation (GNIP; [12]) network and the SISAL_v1 records (see Figure 4).
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Figure 3. Temporal coverage of the Western Europe records included in SISAL_v1. (A) Records covering pre-Holocene time intervals. If a record extends into the Holocene, that part is shown in the next panel. (B) Records covering the last 22 ka. Marine Isotope Stage (MIS) and glacial terminations (T) timings according to Lisiecki and Raymo [50]. Hiatuses in individual records are shown by blank spaces. Records are sorted by latitude, with the northernmost site at the top.
Figure 3. Temporal coverage of the Western Europe records included in SISAL_v1. (A) Records covering pre-Holocene time intervals. If a record extends into the Holocene, that part is shown in the next panel. (B) Records covering the last 22 ka. Marine Isotope Stage (MIS) and glacial terminations (T) timings according to Lisiecki and Raymo [50]. Hiatuses in individual records are shown by blank spaces. Records are sorted by latitude, with the northernmost site at the top.
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Figure 4. Dating methods, ages and age uncertainties. Shown are results in SISAL_v1 for Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) and Thermal Ionisation Mass Spectrometry (TIMS) U-Th dating, 14C dating, laminae counting, and combinations of these methods (mostly U-Th or 14C combined with laminae counting). Note that, for simplicity, ages with 0% uncertainty (i.e., top of actively growing speleothems) were excluded.
Figure 4. Dating methods, ages and age uncertainties. Shown are results in SISAL_v1 for Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) and Thermal Ionisation Mass Spectrometry (TIMS) U-Th dating, 14C dating, laminae counting, and combinations of these methods (mostly U-Th or 14C combined with laminae counting). Note that, for simplicity, ages with 0% uncertainty (i.e., top of actively growing speleothems) were excluded.
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Figure 5. Comparison between: (A) GNIP station data, (B) average SISAL δ18O values for the period 1958–2015 with interpolated GNIP data (background map: mean annual weighted δ18O data from waterisotopes.org [163]). GNIP station data reflects calculated long-term annual means from stations with at least 10 months of data per year and five or more years of data (n = 211) and is shown as ‰ VSMOW. SISAL records (filled circles; n = 18) are averages over the period 1958–2013 and shown as ‰ VPDB. The SISAL records were not filtered to have a minimum number of data points, but only calcitic and aragonitic samples converted to calcite were included to avoid bias from mineralogy. Cave sites are numbered according to latitude: 1—Okshola [107], 2—Soylegrotta [45], 3—Korallgrottan [97], 4—Uamh an Tartair [123,124], 5—Crag [69,73], 6—Brown’s Folly Mine [60,61], 7—Bunker [66], 8—Han-sur-Lesse [84], 9—Spannagel [116], 10—Klapferloch [95], 11—Villars [130], 12—Cueva de Asiul [74], 13—New St. Michael’s [43,106].
Figure 5. Comparison between: (A) GNIP station data, (B) average SISAL δ18O values for the period 1958–2015 with interpolated GNIP data (background map: mean annual weighted δ18O data from waterisotopes.org [163]). GNIP station data reflects calculated long-term annual means from stations with at least 10 months of data per year and five or more years of data (n = 211) and is shown as ‰ VSMOW. SISAL records (filled circles; n = 18) are averages over the period 1958–2013 and shown as ‰ VPDB. The SISAL records were not filtered to have a minimum number of data points, but only calcitic and aragonitic samples converted to calcite were included to avoid bias from mineralogy. Cave sites are numbered according to latitude: 1—Okshola [107], 2—Soylegrotta [45], 3—Korallgrottan [97], 4—Uamh an Tartair [123,124], 5—Crag [69,73], 6—Brown’s Folly Mine [60,61], 7—Bunker [66], 8—Han-sur-Lesse [84], 9—Spannagel [116], 10—Klapferloch [95], 11—Villars [130], 12—Cueva de Asiul [74], 13—New St. Michael’s [43,106].
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Figure 6. Examples of Western European records covering the last glacial period in SISAL_v1. To ease intercomparison of δ18Ospel, all records were normalised as z-scores. The onset of Greenland Interstadials (GIs) is indicated by the grey triangles [166,167]. (A) North Greenland Ice Core Project (NGRIP) ice core δ18O on the layer-counted GICC05modelext time scale for the last 60 kyrs, and extended further back on the ss09sea06bm time scale [167,168,169]. (B) Records from the Alpine region (colour coded): 7H—Composite record from Sieben Hengste Cave, Switzerland [114], SPA—Stalagmites SPA 49 and SPA 126, Kleegruben Cave, Austria [96], HOL—Stalagmites HOL-7 and HOL-10, composite records HOL-16-17 and HOL-comp from Hölloch Cave, Austria [87,88], BA—Stalagmites BA-1, BA-1b, and BA-2 from Baschg Cave, Austria [58], SCH—Stalagmites SCH-5 and SCH-7 from Schneckenloch Cave, Austria [58,88], TKS—Flowstone TKS from Entrische Kirche Cave, Austria [78]. Note that TKS is shown on a different y-axis to ease visual comparison. (C) Records from other parts of Western Europe (colour coded): SESO—Stalagmite SE09-6 from Seso Cave, Spain [113], PIN—Stalagmite Candela from El Pindal Cave, Spain [109,110], CHAU—Stalagmite Chau-stm6 from Chauvet Cave, France [67], VIL—Stalagmites Vil-stm9 and Vil-stm27 from Villars Cave, France [126,127], HAN—Stalagmite Han-9 from Han-sur-Lesse Cave, Belgium [85], BMS—Stalagmite BMS1 from Bue Marino Cave, Italy [65]. (D) Chronological uncertainty of the records: Speleothem U-Th ages are shown in colour coded dots. If available, the uncertainty of the age model is shown instead, as a more accurate measure of the time series uncertainty (lines). The black line indicates the maximum counting error of the layer-counted part of the ice core chronology [169].
Figure 6. Examples of Western European records covering the last glacial period in SISAL_v1. To ease intercomparison of δ18Ospel, all records were normalised as z-scores. The onset of Greenland Interstadials (GIs) is indicated by the grey triangles [166,167]. (A) North Greenland Ice Core Project (NGRIP) ice core δ18O on the layer-counted GICC05modelext time scale for the last 60 kyrs, and extended further back on the ss09sea06bm time scale [167,168,169]. (B) Records from the Alpine region (colour coded): 7H—Composite record from Sieben Hengste Cave, Switzerland [114], SPA—Stalagmites SPA 49 and SPA 126, Kleegruben Cave, Austria [96], HOL—Stalagmites HOL-7 and HOL-10, composite records HOL-16-17 and HOL-comp from Hölloch Cave, Austria [87,88], BA—Stalagmites BA-1, BA-1b, and BA-2 from Baschg Cave, Austria [58], SCH—Stalagmites SCH-5 and SCH-7 from Schneckenloch Cave, Austria [58,88], TKS—Flowstone TKS from Entrische Kirche Cave, Austria [78]. Note that TKS is shown on a different y-axis to ease visual comparison. (C) Records from other parts of Western Europe (colour coded): SESO—Stalagmite SE09-6 from Seso Cave, Spain [113], PIN—Stalagmite Candela from El Pindal Cave, Spain [109,110], CHAU—Stalagmite Chau-stm6 from Chauvet Cave, France [67], VIL—Stalagmites Vil-stm9 and Vil-stm27 from Villars Cave, France [126,127], HAN—Stalagmite Han-9 from Han-sur-Lesse Cave, Belgium [85], BMS—Stalagmite BMS1 from Bue Marino Cave, Italy [65]. (D) Chronological uncertainty of the records: Speleothem U-Th ages are shown in colour coded dots. If available, the uncertainty of the age model is shown instead, as a more accurate measure of the time series uncertainty (lines). The black line indicates the maximum counting error of the layer-counted part of the ice core chronology [169].
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Figure 7. Examples of Western European records in SISAL showing evidence for a climatic perturbation over the 8.2 ka event. To facilitate intercomparison of δ18Ospel, all records were normalised as z-scores. The duration of the 8.2 ka event is shown with the grey bar [166]. K1, K3—Stalagmites from Katerloch Cave, Austria [93], CR1—Stalagmite from Grotta di Carburangeli, Italy [80], BuStack—Composite record from Bunker Cave, Germany [66], WSC-97-10-5—Stalagmite from White Scar Cave, England [111], YD01—Stalagmite from Pippikin Pot Cave, England [111]. Ages with respective 2σ uncertainties are shown below each record. For WSC-97-10-5 and YD01, the uncertainties of the age model are also shown (shaded error bars underlying the time series). Note that the uncertainty of the Bunker Cave composite record is likely smaller than suggested by the single U-Th ages, due to overlap of single stalagmite records, but this information was not available in SISAL_v1 [66]. Stalagmites with petrographic or growth rate evidence for a climatic event are shown in grey bars at the top: A glacier advance is suggested by petrographic changes in stalagmites from Milchbach Cave, Switzerland [104], growth cessation/start around the event is recorded by stalagmites ASR and ASM from Cueva de Asiul, Spain [74], and an erosional surface is found in stalagmite ESP03 from Cova da Arcoia, Spain [71].
Figure 7. Examples of Western European records in SISAL showing evidence for a climatic perturbation over the 8.2 ka event. To facilitate intercomparison of δ18Ospel, all records were normalised as z-scores. The duration of the 8.2 ka event is shown with the grey bar [166]. K1, K3—Stalagmites from Katerloch Cave, Austria [93], CR1—Stalagmite from Grotta di Carburangeli, Italy [80], BuStack—Composite record from Bunker Cave, Germany [66], WSC-97-10-5—Stalagmite from White Scar Cave, England [111], YD01—Stalagmite from Pippikin Pot Cave, England [111]. Ages with respective 2σ uncertainties are shown below each record. For WSC-97-10-5 and YD01, the uncertainties of the age model are also shown (shaded error bars underlying the time series). Note that the uncertainty of the Bunker Cave composite record is likely smaller than suggested by the single U-Th ages, due to overlap of single stalagmite records, but this information was not available in SISAL_v1 [66]. Stalagmites with petrographic or growth rate evidence for a climatic event are shown in grey bars at the top: A glacier advance is suggested by petrographic changes in stalagmites from Milchbach Cave, Switzerland [104], growth cessation/start around the event is recorded by stalagmites ASR and ASM from Cueva de Asiul, Spain [74], and an erosional surface is found in stalagmite ESP03 from Cova da Arcoia, Spain [71].
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Figure 8. Stacked record of SISAL entities covering the last 2000 years, compared to global climate forcings: Global Volcanic Forcing (GVF; [172]), solar forcing (total solar insolation anomalies, dTSI; [173]), and CO2 concentrations from combined ice core records [174]. Important climate periods are indicated, and defined according to the following references: Little Ice Age (LIA; [175]), Medieval Climate Anomaly (MCA; [176]), Late Antique Little Ice Age (LALIA), and Roman Warm Period (RWP; [177]).
Figure 8. Stacked record of SISAL entities covering the last 2000 years, compared to global climate forcings: Global Volcanic Forcing (GVF; [172]), solar forcing (total solar insolation anomalies, dTSI; [173]), and CO2 concentrations from combined ice core records [174]. Important climate periods are indicated, and defined according to the following references: Little Ice Age (LIA; [175]), Medieval Climate Anomaly (MCA; [176]), Late Antique Little Ice Age (LALIA), and Roman Warm Period (RWP; [177]).
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Table
Table 1. Summary of all records currently included in SISAL_v1 for Western Europe and records identified but not yet included (i.e., all records that do not have a SISAL entity_id). Information on records not included in SISAL_v1 was derived from the original publications. * no interpolated ages available.
Table 1. Summary of all records currently included in SISAL_v1 for Western Europe and records identified but not yet included (i.e., all records that do not have a SISAL entity_id). Information on records not included in SISAL_v1 was derived from the original publications. * no interpolated ages available.
site_namesite_idCountryLatitude (N)Longitude (E)entity_nameentity_idMin. Year (BP)Max. Year (BP)Reference
Antro del Corchia145Italy43.9810.22CC-1_2004313125,432.63393,407.69[41]
Antro del Corchia145Italy43.9810.22CC-5_200531488,347170,549.44[42]
Antro del Corchia145Italy43.9810.22CC-2831595,191.16117,497.82[51]
Antro del Corchia145Italy43.9810.22CC-1_2009316127,997148,970[52]
Antro del Corchia145Italy43.9810.22CC-5_2009317117,965156,957[52]
Antro del Corchia145Italy43.9810.22CC-7318121,333126,805[52]
Antro del Corchia145Italy43.9810.22COR-1 [53]
Antro del Corchia145Italy43.9810.22CC-26 75011,260[54]
Atta Germany50.807.44STAL-AH-1 17632723[55]
Atta Germany50.807.44AH-1 8608430[56]
B7 Germany51.377.65STAL-B7-1 619612,405[57]
B7 Germany51.377.65STAL-B7-5 58508810[57]
B7 Germany51.377.65STAL-B7-7 54017,230[57]
Baschg15Austria47.259.67BA-1b7075,492.4480,896.94[58]
Baschg15Austria47.259.67BA-17180,982.0689,489[58]
Baschg15Austria47.259.67BA-27288,609.9889,723.31[58]
Beatus Switzerland46.387.49EXC-3 100,940110,000[58]
Beatus Switzerland46.387.49EXC-4 77,450107,080[58]
Bourgeois Delaunay73France45.670.51BDinf162121,339128,151[59]
Brown’s Folly mine96England51.38−2.37Boss192−4732[60,61]
Brown’s Folly mine96England51.38−2.37BFM-9193−4721[60,61]
Brown’s Folly mine96England51.38−2.37F2194−4613[60,61]
Buca della Renella133Italy44.0810.21RL4_20062821215.566928.36[62]
Buca della Renella133Italy44.0810.21RL4_20162831150.377262[63]
Buca della Renella133Italy44.0810.21RL4_20183811024.117277.24[64]
Bue Marino97Italy40.259.62BMS1195110,207112,881[65]
Bunker117Germany51.377.66Bu12401376644.9[66]
Bunker117Germany51.377.66Bu22417497.610,723.5[66]
Bunker117Germany51.377.66Bu4242−57.38162.8[66]
Bunker117Germany51.377.66Bu6243874910,258.2[66]
Bunker117Germany51.377.66BuStack244−57.410,723.5[66]
Chauvet77France44.234.26Chau-stm616611,41534,183[67]
Clamouse108France43.713.55Cla421174,460187,405[68]
Clamouse108France43.713.55CL26212142.3311,178.79[69]
Clamouse108France43.713.55Cla-stm5 432,000611,000[70]
Cova da Arcoia143Spain42.61−7.09ESP033103409440[71]
Cova de Cala Falco Spain39.53.3CCF-03-03-01 48,000112,000[72]
Crag98Ireland52.25−9.43CC3196−4710,132[69,73]
Cueva de Asiul119Spain43.32−3.59ASR248488.6112,160.96[74]
Cueva de Asiul119Spain43.32−3.59ASM249−627776.64[74]
Cueva del Cobre Spain42.98−4.37C11 772614[75]
Cueva Mayor Spain42.37−3.51SLX1 621513[75]
Cueva Rosa Spain43.43−5.13Romeo 52948097[76]
Ejulve120Spain40.45−0.35ARTEMISA251218,975.67257,426.49[11]
Ejulve120Spain40.45−0.35HOR2502708.446071.82[77]
Entrische Kirche121Austria47.1613.15TKS252113,389.5126,889.04[78]
Entrische Kirche121Austria47.1613.15ENT-10 114,000127,000[78]
Excentrica40Portugal37.10−7.77GEX-SPA1165329.366565.22[79]
Gitana Spain37.44−2.02GC-01-05-02 58,000274,000[72]
Grotta di Carburangeli129Italy38.1713.16CR1277947.738373.72[80,81]
Grotta di Ernesto131Italy45.9711.65ER762792511.487969.05[44]
Grotta Savi Italy45.6113.88SV-1 132516,799[82]
Hamarnes Norway66.4214.02Ham-85.2 4510123,000[45]
Han-sur-Lesse16Belgium50.125.19Han-stm173477810,949[83]
Han-sur-Lesse16Belgium50.125.19Han-stm5b74−4416[84]
Han-sur-Lesse16Belgium50.125.19Han-975106,499.65125,343.05[85]
Han-sur-Lesse16Belgium50.125.19Proserpine −51471[86]
Hölloch im Mahdtal115Austria47.3810.15HOL-723040,10548,664[87]
Hölloch im Mahdtal115Austria47.3810.15HOL-1623136,70163,546[87]
Hölloch im Mahdtal115Austria47.3810.15HOL-1723235,83264,934[87]
Hölloch im Mahdtal115Austria47.3810.15HOL-1823352,50957,283[87]
Hölloch im Mahdtal115Austria47.3810.15HOL-16-1723435,70537,578[87]
Hölloch im Mahdtal115Austria47.3810.15HOL-comp23549,06364,498[87]
Hölloch im Mahdtal115Austria47.3810.15HOL-10236110,844131,765[88]
Hölloch im Mahdtal115Austria47.3810.15Stal-Hoel-1 138012,690[89]
Hötting Breccia Austria47.2811.39HOT-1 73,90098,700[90]
Hötting Breccia Austria47.2811.39HOT-2 70,30073,800[90]
Hotton Belgium50.255.45 275011,150[91]
Kaite Spain42.94−3.57comp. 3949569[92]
Kaite Spain42.94−3.57LV5 3933885[75]
Katerloch100Austria47.0815.55K32007786.6210,027.08[93]
Katerloch100Austria47.0815.55K-2 [94]
Katerloch100Austria47.0815.55K-4 [94]
Katerloch100Austria47.0815.55K-5 [94]
Katerloch100Austria47.0815.55K-7 [94]
Katerloch100Austria47.0815.55K-8 [94]
Katerloch100Austria47.0815.55K-RZ6-072007 [53]
Katerloch100Austria47.0815.55K-Top3-Cl [53]
Katerloch100Austria47.0815.55K11997079.510,324[93]
Katerloch100Austria47.0815.55K-6 [94]
Klapferloch101Austria46.9510.55PFU6201−472943.04[95]
Klapferloch101Austria46.9510.55PFU-7 [95]
Klapferloch101Austria46.9510.55PFU-8 [95]
Klapferloch101Austria46.9510.55PFU-9 [95]
Klaus-Cramer Austria47.269.52KC-1 54,56071,940[58]
Kleegruben132Austria47.0811.67SPA_12628047,39655,966[96]
Kleegruben132Austria47.0811.67SPA_4928147,81658,266[96]
Korallgrottan102Sweden64.8814.00K11202−553791.88[97]
Korallgrottan102Sweden64.8814.15K1 60708629[98]
La Faurie France45.131.18Fra-stm-6 [53]
La Garma Spain43.43−3.66GAR-01 10,14213,757[99]
La Garma Spain43.43−3.66GAR-02 [100]
Labyrintgrottan46Sweden66.0614.68L41227347.59565.1[98]
Lancaster Hole8England54.22−2.52LH-70s-1503456.2212,717.56[101]
Lancaster Hole8England54.22−2.52LH-70s-251261.869735.67[102]
Lancaster Hole8England54.22−2.52LH-70s-352945.798462.88[102]
Laphullet Norway66.3114.18PL-6 380,000502,000[47]
Larshullet47Norway66.0014.00L031231303920.52[103]
Milchbach123Switzerland46.628.08MB-22553248.656830[104]
Milchbach123Switzerland46.628.08MB-32561986.959025.84[104]
Milchbach123Switzerland46.628.08MB-6258**[104]
Milchbach123Switzerland46.628.08MB-52573889.717245.91[104]
Molinos109Spain40.79−0.45MO-721732536812[77,105]
Molinos109Spain40.79−0.45MO-1216472711,334.76[77]
New St Michael’s89Gibraltar36.13−5.35Gib04a182−53.62[43,106]
Okshola26Norway67.0015.00FM395−477515.2[107]
Okshola26Norway67.0015.00Oks8296500610,327.64[107]
Pere Noel Belgium50.135.16PN-stm-95-5 180012,900[91,108]
Pindal87Spain43.40−4.53Candela18011,640.3229,339.98[109,110]
Pippikin Pot53England54.21−2.51YD011294205.589478.94[101,111]
Schafsloch125Switzerland47.239.38MF-3260130,050137,390[112]
Schneckenloch105Austria47.439.87SCH-5206115,340134,085[88]
Schneckenloch105Austria47.439.87SCH-7207111,588.73118,314.57[58]
Seso106Spain42.460.04SE09-620811,61612,995[113]
Sieben Hengste55Switzerland46.757.817H13314,62029,873[114]
Sieben Hengste55Switzerland46.757.817H-313514,639.4523,536.89[114]
Sieben Hengste55Switzerland46.757.817H-213417,137.1729,940.32[114]
Soylegrotta57Norway66.0014.00SG95137−434141.22[45]
Soylegrotta57Norway66.0014.00SG92-4 45008000[115]
Soylegrotta57Norway66.0014.00SG-92-2 320,000630,000[46]
Soylegrotta57Norway66.0014.00SG93 25310,409[6]
Spannagel58Austria47.0811.67SPA12138605043[116]
Spannagel58Austria47.0811.67SPA7013945499894[116]
Spannagel58Austria47.0811.67SPA12814025206140[116]
Spannagel58Austria47.0811.67SPA12714127378449[116]
Spannagel58Austria47.0811.67COMNISPA II142−139930.6[116]
Spannagel58Austria47.0811.67SPA1331549636.510,796.1[116]
Spannagel58Austria47.0811.67SPA121261187,290242,070[117]
Spannagel58Austria47.0811.67SPA-4 265,700353,900[118,119]
Spannagel58Austria47.0811.67SPA-59 52,900261,400[120]
Spannagel58Austria47.0811.67SPA-12 152040[121]
Spannagel58Austria47.0811.67SPA-119 220,500226,900[119]
Spannagel58Austria47.0811.67SPA-52 91,100204,100[122]
Spannagel58Austria47.0811.67SPA-11 117,000202,800[122]
Uamh an Tartair21Scotland58.14−4.93SU96785−35892[123]
Uamh an Tartair21Scotland58.14−4.93SU03286−53271[124]
Villars4France45.430.78Vil-stm627−438657[125]
Villars4France45.430.78Vil-stm92831,437.9182,854.5[126,127]
Villars4France45.430.78Vil-stm1129536115,875[67]
Villars4France45.430.78Vil-stm143028,892.6852,156.42[127,128]
Villars4France45.430.78Vil-stm273131,340.949,663.24[126]
Villars4France45.430.78Vil-car1321055178,002[129]
Villars4France45.430.78Vil#10B [53]
Villars4France45.430.78Vil#1A [53]
Villars4France45.430.78VilGal#1B [53]
Villars4France45.430.78VilPlq-8 [53]
Villars4France45.430.78Vil-stm133−382333[130]
Villars4France45.430.78Vil-stm24 102,800113,600[70]
White Scar66England54.17−2.44WSC-97-10-51507347.8711,190.74[101,111]

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MDPI and ACS Style

Lechleitner, F.A.; Amirnezhad-Mozhdehi, S.; Columbu, A.; Comas-Bru, L.; Labuhn, I.; Pérez-Mejías, C.; Rehfeld, K. The Potential of Speleothems from Western Europe as Recorders of Regional Climate: A Critical Assessment of the SISAL Database. Quaternary 2018, 1, 30. https://doi.org/10.3390/quat1030030

AMA Style

Lechleitner FA, Amirnezhad-Mozhdehi S, Columbu A, Comas-Bru L, Labuhn I, Pérez-Mejías C, Rehfeld K. The Potential of Speleothems from Western Europe as Recorders of Regional Climate: A Critical Assessment of the SISAL Database. Quaternary. 2018; 1(3):30. https://doi.org/10.3390/quat1030030

Chicago/Turabian Style

Lechleitner, Franziska A., Sahar Amirnezhad-Mozhdehi, Andrea Columbu, Laia Comas-Bru, Inga Labuhn, Carlos Pérez-Mejías, and Kira Rehfeld. 2018. "The Potential of Speleothems from Western Europe as Recorders of Regional Climate: A Critical Assessment of the SISAL Database" Quaternary 1, no. 3: 30. https://doi.org/10.3390/quat1030030

APA Style

Lechleitner, F. A., Amirnezhad-Mozhdehi, S., Columbu, A., Comas-Bru, L., Labuhn, I., Pérez-Mejías, C., & Rehfeld, K. (2018). The Potential of Speleothems from Western Europe as Recorders of Regional Climate: A Critical Assessment of the SISAL Database. Quaternary, 1(3), 30. https://doi.org/10.3390/quat1030030

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