Quaternary Geochronology 36 (2016) 104e119

Contents lists available at ScienceDirect

Quaternary Geochronology

journal homepage: www.elsevier.com/locate/quageo

Research paper

Methods for U-series dating of CaCO3 crusts associated with Palaeolithic cave art and application to Iberian sites

* Dirk L. Hoffmann a, b, c, , Alistair W.G. Pike d, Marcos García-Diez e, Paul B. Pettitt f, Joao~ Zilhao~ g, h a Max Planck Institute for Evolutionary Anthropology, Department of Human Evolution, Deutscher Platz 6, 04103, Leipzig, Germany b Bristol Isotope Group, School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK c Centro Nacional de Investigacion sobre la Evolucion Humana (CENIEH), Paseo Sierra de Atapuerca 3, 09002, Burgos, Spain d Department of Archaeology, University of Southampton, Avenue Campus, Highfield Road, Southampton, SO17 1BF, UK e Department of Geography, Prehistory and Archaeology, University of the Basque Country (UPV/EHU), c/ Tomas y Valiente s/n, 01006, Vitoria, Spain f Department of Archaeology, Durham University, South Road, Durham, DH1 3LE, UK g University of Barcelona, Departament de Prehistoria, Historia Antiga i Arqueologia (SERP), c/ Montalegre 6, 08001, Barcelona, Spain h Institucio Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010, Barcelona, Spain article info abstract

Article history: U-series dating is a precise and accurate geochronological tool which is widely applied to date secondary Received 18 February 2016 CaCO3 formation, for example in speleothem based palaeoclimate research. It can also be employed to Received in revised form provide chronological constraints for archaeological sites which have a stratigraphic relationship with 1 July 2016 speleothem formations. We present in detail our methods to conduct precise and accurate U-Th dating of Accepted 27 July 2016 calcite crusts that formed on top of cave paintings. Our protocols allow the application of U-series Available online 29 July 2016 measurements on small, thin calcite crusts covering cave art, which can be found in many sites, while taking care not to harm the art underneath. The method provides minimum ages for the covered art and, Keywords: fl Cave art where possible, also maximum ages by dating the owstone layer the art is painted on. We present U-Th dating dating results for crusts from two locality types in Spain, a typical cave environment (La Pasiega) and a

CaCO3 crust more open, rock shelter type cave (Fuente del Trucho). Minimum age © 2016 Published by Elsevier B.V. La Pasiega Fuente del Trucho

1. Introduction (often referred to as ‘cycles’), and the relationship of these to archaeologically recovered evidence of human occupation. But Cave art is found on almost every continent and, in addition to despite more than a century of study, cave art has proven to be one its bearing on the origins of art itself, it constitutes one of the few of the most difficult archaeological phenomena to date. sources of archaeological information about symbolic behaviour The chronology of cave art is often derived from contextual and and social interaction among early humans. It has been critical to stylistic evidence, usually assigned to a specific archaeological the definition of chrono-cultural groupings and successions that period on the basis of stylistic comparisons with portable art ob- form the methodological basis of archaeology, and continues to jects that have been recovered from dated archaeological deposits, play a major role in our interpretations of the world views and and through the construction of a relative stratigraphy of motifs behaviours of past cultures. Accurate chronological control of the that are demonstrably superimposed on cave walls (Breuil, 1952; emergence and/or execution of cave art is essential for under- García-Diez and Ochoa, 2013; Leroi-Gourhan, 1965). While stylis- standing the development and phasing of specific artistic periods tic grouping may be useful to identify connections between human groups in different geographical regions, a 'chronology' based on this method is potentially undermined by the possibility that spe- fi * Corresponding author. Max Planck Institute for Evolutionary Anthropology, ci c themes or styles could have been produced in different areas at Department of Human Evolution, Deutscher Platz 6, 04103, Leipzig, Germany. different times, and also that different styles could have co-existed E-mail addresses: [email protected] (D.L. Hoffmann), A.W.Pike@soton. at the same time. In short, one simply cannot assume a straight- ac.uk (A.W.G. Pike), [email protected] (M. García-Diez), paul.pettitt@durham. forward development that allows one to isolate well-defined and ac.uk (P.B. Pettitt), [email protected] (J. Zilhao).~ http://dx.doi.org/10.1016/j.quageo.2016.07.004 1871-1014/© 2016 Published by Elsevier B.V. D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 105 chronologically distinct artistic periods. Furthermore, since very in detail and present and discuss dating results for cave art in two few direct dates have been produced for cave art, assigning a date to different locality types, a typical cave environment (La Pasiega, a stylistic group runs the risk of circularity, whereby the age relies Puente Viesgo, Spain) and a more open, rock shelter type envi- on comparison with other stylistic groups rather than on criteria ronment (Fuente del Trucho, Huesca, Spain). Our results were ob- that are independent of style (von Petzinger and Nowell, 2011). In tained in three different laboratories with identical setups. The some cases the association underpinning the 'dating' is based on results published in Pike et al. (2012) were measured in the Bristol archaeological finds lying on the surface (floor) of a cave or in strata Isotope Group laboratory (Bristol, UK); those presented here were exposed by archaeological excavation. For these at least, a degree of measured in the U-series laboratory of the CENIEH (Burgos, Spain) chronological control exists (i.e. finds can be dated, for example, by and the U-series laboratory of the MPI for Human Evolution AMS radiocarbon measurement). However, the 'dating' of cave art (Leipzig, Germany). based on the assumption that the art is coeval with dated material recovered from elsewhere in the same cave is not based on an 2. General considerations unambiguous and demonstrable stratigraphic relationship, and hence does not constitute a direct, robust chronology. The conservation and protection of the precious and unique cave Radiocarbon dating, restricted as it is to the dating of organic art obviously has highest priority and is the underlying principle of materials, can be used for the direct dating of organic binders and our work. It is essential to avoid any damage to the cave art, and to pigments. This has been applied to a variety of cave art motifs, minimise our impact on the cave environment to ensure the mainly charcoal-based paintings, the result of which dates the preservation of this global heritage for future generations. Because production of the charcoal and thereby provides a maximum age of its fragility, we cooperate closely with the authorities and re- for the painting. In order to interpret the 14C age of charcoal as the searchers responsible for the management and curation of the sites age of the art, an assumption is necessary that this occurred where we work; we seek permission based on a detailed rationale meaningfully close to the time at which the charcoal was used to that is assessed by expert panels, and one or more delegates create the art. Archaeologically, the method is destructive: it is appointed by the relevant government agencies oversees our necessary to subsample material from the art itself. However, AMS sampling process, which is recorded and subsequently outlined in 14C dating of art from some of the key European cave art sites was a publications. major step forward (e.g. Valladas et al. (2001)), but many European In many cases, it is simply impossible to remove samples, either cave art motifs are engravings or were made with inorganic red because a potential risk of damage to the art is too great or our ochre, yellow ochre and/or black manganese and thus are not rigorous sampling criteria discussed below cannot be fulfilled 14 suitable for C dating. Even where suitable materials are available satisfactorily. In our approach, only CaCO3 that can be removed for radiocarbon dating, the understandable precedence of the without any impact on the art is considered appropriate for sam- conservation of the art necessitates that miniscule quantities of pling. However, impact on the CaCO3 which formed before or after pigment are removed for dating in order to keep visible damage to the painting was done (which is core to the dating method) cannot an absolute minimum. Sample sizes are therefore rarely ideal, be avoided. The impact nevertheless entails no damage to the cave rendering the samples more prone to problems of contamination art itself, and for this reason alone the method is preferable to the than the larger samples used for more routine measurement of removal of art pigments for radiocarbon dating. In order to ensure archaeological samples. Even where a sample of sufficient size can stratigraphic integrity (i.e. that all the calcite sampled had formed be removed, it is not always clear that the date of the organic on top of the targeted artistic motif) and avoid contamination of our fraction is contemporary with the execution of the art, e.g. in the dating sample with either pigment or cave wall, we do not remove case of old wood/charcoal used to make a black pigment (Pettitt all the CaCO3 and thus do not expose the covered pigment to the and Bahn, 2003). Thus, uncertainty remains in many cases, and cave atmosphere. additional methods for dating cave art are clearly required. All these considerations constrain the analytical techniques The U-series method cannot be used to date cave art directly, employed for sample characterisation and U-series dating. For since it cannot be applied to pigments, but in certain cases it can be example, in most cases it is impossible to obtain detailed infor- used to provide age constraints for the art through the dating of mation about CaCO3 fabrics or the stratigraphy of successive associated carbonate deposits. Schwarcz and Blackwell (1992) growth layers, all of which would require removing solid CaCO3 suggested U-series dating of calcite formations covering cave art pieces for detailed microscopy. Although such sampling has been as a means of providing minimum ages for the underlying art, but described and applied elsewhere (Aubert et al., 2007, 2014), we do at the time methodological constraints e especially sample sizes not support a strategy requiring the removal of chunks of calcite needed for alpha spectrometric U-series measurements e including the pigment layers. While this may have the advantage of hampered application of this method. The adoption of thermal providing access to calcite above and below the pigment, it is ionization mass spectrometry (TIMS) (Edwards et al., 1987) signif- destructive to the art itself. icantly reduced the sample size required and allowed the first U-Th The dating of cave art is a multidisciplinary endeavour which applications to cave art (Bischoff et al., 2003; Pike et al., 2005; requires a wide range of expertise ranging from Palaeolithic Plagnes et al., 2003). A further reduction of sample sizes was archaeology to scientific dating methods. A dating team should possible following the adoption of inductively coupled mass spec- therefore include experts in Palaeolithic archaeology, cave art, trometry (ICPMS) (Shen et al., 2002) and multi collector (MC) e speleogenesis, speleothems and dating/chronology. Each cave and ICPMS (e.g. Hellstrom (2003)) for U-series analyses. Thus, today it is its art is unique, and experts associated with specific sites need to possible to employ U-Th methods to a much wider range of sites be included in the planning and sampling process. The application and motifs due to another significant reduction of the sample size of the method presented in this study is limited to motifs where required for precise U-series measurements, using tailored MC- unambiguous stratigraphy of the art and overlying CaCO3 can be ICPMS protocols (Hoffmann, 2008; Hoffmann et al., 2007). This established. Generally, cave art made with pigments is more suit- enables the dating of very small carbonate samples scraped from able than engravings, because, with the latter, it can be difficult to CaCO3 (calcite or aragonite) formations covering cave art. It has define the boundary between the engraved bedrock and the over- been successfully applied in a number of recent studies, e.g. by Pike lying calcite, especially where the bedrock was speleothem- et al. (2012) or Aubert et al. (2014). Here, we describe our methods covered prior to the execution of the engraved motif. 106 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119

3. Sampling methods 3.2. Sampling documentation and tools

3.1. Sampling strategy The most suitable sampling material is chosen on the basis of the morphology of the CaCO3 and its stratigraphy relative to the art. The key to success of our methods lies in previous success in The CaCO3 formation needs to be inspected and documented in minimising the sample sizes needed for robust U-series isotope detail before the sampling starts: analyses (Hoffmann, 2008; Hoffmann et al., 2007, 2009). This en- ables us to deal with small CaCO3 formations and sampling re- The cave art panel/motif including its location in the cave should strictions due to preservation issues, and allows us to remove be described and documented (notes, photos, macro-photos) as multiple samples from the same calcite formation, and still achieve well as the exact position of the CaCO3 intended for sampling high quality dating results. It is possible to use CaCO3 samples as relative to the art. small as a few mg, and we typically use samples sizes in the range of The recent history of the panel needs to be investigated to find 5e10 mg. out whether it has undergone any restoration or cleaning efforts A carefully constructed sampling strategy and documentation of and e if so e whether any chemicals were applied that could all of its constituent steps are essential in order to obtain reliable have an impact on U-series dating results. results. Prior to sampling, it is essential that suitable CaCO3 for- The type of CaCO3 formation should be broadly classified (e.g. mations with an unambiguous stratigraphic relationship with the flowstone, stalactite, drapery, cauliflower/popcorn) and its art can be identified. Identifying such samples is the major morphology described (dense or porous carbonate; pristine or constraint for application of U-series dating to cave art. In many corroded) and whether any internal stratigraphy (e.g. layering cases, no such carbonates can be found. Even where suitable CaCO3 or inter-bedding) is visible. can be identified, risks of damage to the art while removing sam- The level of cleanliness of the CaCO3 needs to be described, i.e. ples (e.g. where the cave wall is fragile) may be too great and can whether the surface is visibly clean or whether there are therefore rule out sampling. Fig. 1 shows examples of CaCO3 for- obvious dirt inclusions in the CaCO3. Also, it is important to state mations on top of cave art found in La Pasiega. Suitable secondary whether the surface of the CaCO3 is dry, humid or wet. carbonates associated with its cave art are, for example, small It needs to be confirmed that there is no pigment on top of the cauliflower type deposits (Fig. 1a), thin flowstone films on cave CaCO3. walls (Fig. 1b), draperies (Fig. 1c) or stalactites (Fig. 1d).

Fig. 1. Examples for CaCO3 precipitates on top of cave art in La Pasiega. a) (PAS 30) Small ‘cauliflower’ type formations, b) (PAS 01) thin flowstone formation covering the whole painting, which therefore appears to be faded due to its covering, c) (PAS 16) cave wall drapery partly covering red pigment d) (PAS 27) combination of flowstone, cauliflower and stalactites covering the cave ceiling with underlying red pigment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 107

The stratigraphic relationship of the CaCO3 and pigment should be described and documented. To avoid removing unnecessary samples, it must be established that it is highly probable that pigment will be found under the CaCO3 before sampling commences.

The on-site documentation is mainly based on visual inspection of the CaCO3. For this, we recommend using magnifying lenses and good LED lights. High resolution photographic documentation is important, and pictures should be taken before the sampling and then after every sampling step. For the sampling procedure we use tools/equipment such as a scalpel with a round shaped blade (different sizes), a handheld micro drill with tungsten carbide drill bits (round head) or diamond cutting discs, a headband magnifier lens, dust blower, pre-cleaned sample tubes, clean tray and clean high quality brushes. All equipment and tools need to be as clean as possible and it is recommended that enough fresh scalpel blades are on hand to allow frequent changes of the blade. Similarly, several spare, pre-cleaned trays to collect sample powders and Fig. 2. Removing dust from the surface of the CaCO3 using a manual dust blower (PAS 30). several clean brushes for sample transfer into tubes will be needed. All equipment must be kept in closed containers which should only be opened when necessary and for as brief a time as possible. 3.4. Sampling Equipment and all other materials should be cleaned and stored away straight after use. For cleaning of equipment on site, directly In most cases samples are removed using a scalpel. After after use, we use clean tissues, methanol, distilled water and can- cleaning, sampling is done by gently scraping off CaCO3 from the ned air. Depending on the cave sites and motifs, additional pro- fresh surface of the formation directly into a pre-cleaned sample tective wear such as nitrile gloves, protective coveralls or face tube. The sample tube diameter is 1.5 cm and it is positioned masks may also be required. directly under the sampling spot and held by a second individual (Fig. 3). During pauses in sampling, the tube should be kept closed to prevent contamination by dust. In some cases, the cave topography and/or accessibility of the 3.3. Surface pre-treatment sampling position makes it impossible to position the sampling tube directly under the sampling spot. In such cases we recom- Dust is a potential contamination source for CaCO3 powder mend collection of the sample powder in a clean tray positioned samples. Particularly in dry caves and/or caves with frequent visi- under the sampling point. This allows collection of the sample from tors, the cave atmosphere can have elevated dust levels. The dust a greater distance or height. The powder can then be collected in can be deposited on cave surfaces including the cave art and CaCO3 the tray and transferred from there into a clean sample tube using a that formed on top of it. Thus, the surface of the CaCO3 must be clean, high quality brush. The tray should be prepared for the physically cleaned before sampling, by removing the surface transfer and have an appropriate opening under which the tube can (contaminated) layer of the CaCO3. This is done by scraping off the be placed (Fig. 4). outmost CaCO3 with the scalpel. The cleaning process also serves as For the purpose of quality control, a stratigraphic succession of fi a rst indication of CaCO3 hardness. In some cases, for larger calcite samples should be collected wherever possible (i.e. where the deposits, the surface can be cleaned by using a handheld portable micro drill with spherical tungsten carbide drill bits. A small round drill tip enables more uniform removal of an uneven surface. However, a drill must be used with great care; we recommend a slow spin speed to avoid spreading carbonate dust. Once removal of CaCO3 starts, constant vigilance must be kept for the potential of exposure of pigments, as the pigment layer can be found under or partially inside the CaCO3 crust at any depth. Throughout this procedure, and subsequent sampling, care is taken not to touch the cave wall anywhere near the vicinity of the painting. To facilitate this, a stick can be used to provide support to the sampler's arm if placed against the wall well away from the painting, or alterna- tively a tripod/monopod that avoids the wall altogether. After the outermost surface layer has been removed, the loose particles remaining on the surface are gently blown away with a manual dust blower (Fig. 2). The surface is then inspected to verify whether any pigment is visible under the CaCO3, and the cleaned surface is documented. We have to note that cleaning the surface will not necessarily remove all contamination since dust could have been previously incorporated in deeper CaCO3 layers. In addition, highly uneven surfaces cannot always be cleaned effectively as can Fig. 3. Sampling of CaCO using a scalpel (PAS 30). The sample is scraped off the fresh fi 3 be seen in Fig. 10b. Thus, the rst (outermost) sample is more likely surface of the CaCO3 formation directly into a pre-cleaned sample tube. Note that the to contain dirt particles. process requires two people. 108 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119

Fig. 4. Sampling of CaCO3 using a collecting tray. The sample is scraped off the fresh surface of the CaCO3 formation using a scalpel and collected in a clean tray (a). It is then transferred into a pre-cleaned sample tube with a clean high quality paintbrush (b). Note that a small hole has been drilled in the side of the tray to facilitate the transfer of the sample into the tube. calcite is thick enough). When the first (i.e. outermost) sub-sample reaches the required size, its tube should be sealed and labelled, and collection of a second sub-sample should be continued with a Fig. 5. Cutting off a small piece of stalactite using a handheld drill with a diamond- new tube. The process should be continued as long as it is possible, coated cutting disk. without exposing the pigment below. Collecting a sequence of sub- samples like this allows us to assess the integrity of the calcite through the dating of separate samples, and additionally provides concern. If we cannot demonstrate this stratigraphic relationship, alternative samples in the case that any are inadvertently the question as to whether the resulting dates have any bearing on contaminated. We recommend clear photographic documentation the age of the art will always remain open. Thus, only when of the surface of the sampling position, before, during and after pigment can be unambiguously identified directly underlying the each sub-sample (the latter with the inclusion of a photographic sampling position can a stratigraphic relationship between the scale) and at each stage a careful check should be made as to CaCO3 sample and the art be demonstrated. Then, and only then, whether pigment has started to become visible. The sample does the age of the CaCO3 represent a minimum age for the un- collection is stopped as soon as pigment can be clearly seen under derlying art. In cases where pigment does not after all become the translucent CaCO3, from which we infer that exposure is about visible after removing CaCO3 the sample must be rejected. Possible to occur. In most cases, it is possible to compare the appearance of causes for failure in establishing the stratigraphic relationship ex- the pigment under the CaCO3 and the parts of the same motif that pected at the outset of sampling may be that (a) the pigment was are not covered. originally on top of the calcite and has subsequently washed or For sequential sampling, sub-samples should be in some sort of eroded off or (b) the painting is for whatever reason discontinuous. stratigraphic order. However, one should be aware that the stra- In such cases, the age of the CaCO3 will provide no constraint for the tigraphy of the CaCO3 formation might be complex and difficult to art, and we recommend archiving such samples without analysis. assess, e.g., an uneven complex surface with potentially several Photographic evidence for the visible pigment at the spot of CaCO3 nucleus points and growths in different axes can lead to variable removal must be provided in order to demonstrate the strati- mixing of different phases of calcite growth in the samples. Sam- graphic association between the calcite and the art (Fig. 6). We ples are therefore not generally expected to be in a strict strati- cannot stress this point enough, especially as there have been a graphic age order. The very first (outermost) sample usually has a number of attempts to date calcite deposits near, but not in direct higher risk of dirt inclusion since the pre-cleaning procedure stratigraphic association with cave art (Arias and Ontan~on, 2008; cannot remove all surface contamination. We therefore recom- Gonzalez-Sainz and San Miguel, 2001; Gonzalez Sainz, 2003; mend taking three (or more) sub-samples wherever possible, the Ochoa and García-Diez, 2015). outermost (potentially dirtiest) sample and two subsequent (pre- sumably cleaner) samples. 3.6. Sample size In some cases, it is possible to collect solid CaCO3 pieces on site, which can then be sub-sampled in a laboratory. Fig. 5 shows a Sample sizes required for MC-ICPMS U-Th dating depend on stalactite formation which formed on top of pigment on the cave many factors such as U concentrations, age range (antiquity) of the ceiling in La Pasiega. The pigment is clearly visible through the samples, instrumentation and setup, and chemical separation ef- clean transparent CaCO layer and a small stalactite piece could be 3 ficiency. With a setup and instrumentation similar to Hoffmann cut off using a handheld drill with a 5 mm diameter diamond- et al. (2007) and Hoffmann (2008) a precise U isotope measure- coated cutting disk. ment can be achieved with about 0.5e1 ng U (or more) in the sample, which requires a sample of 5e10 mg CaCO3 with a U con- 3.5. Evidence for underlying pigment centration of 100 ng/g. For samples older than 10 ka, with this U concentration, 230Th can be measured at a precision in the range of It is of utmost importance that the CaCO3 sample to be dated has 1% (2 sigma) or better, and in many cases U concentrations are a demonstrable stratigraphic relationship with the cave art of higher. Thus masses of 5e10 mg for sub-samples are recommended D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 109

4. Analytical methods

4.1. Sample treatment

All samples need to be visually inspected for obvious particulate contamination. We use transparent sample tubes which allow in- spection of sample powders for particles with a microscope (Fig. 7). At this stage, dirt particles can be identified, described/documented and e if possible e removed from the sample before chemical treatment starts. The outside of the sample tubes is thoroughly cleaned before they are taken into laboratory space for further treatment of samples therein. When more than one sub-sample was taken from a specific CaCO3 crust, and no prior information about U concentration and expected age range is available, we initially analyse one sub- sample. The result can then be used to optimize sample prepara- tion and spike amount, for the expected U concentration, age, and detrital contamination of the remaining samples. The tube containing the sample powder is weighed and then Fig. 6. Magnified image of remaining parts of a calcite crust after sampling (PAS 30). MilliQ (MQ) water is added and the tube closed again. Water and The red pigment is visible (but not exposed) under the remaining CaCO3 in the centre of the sampling position. (For interpretation of the references to colour in this figure sample powder are mixed by gentle shaking and the resulting legend, the reader is referred to the web version of this article.) emulsion is poured into a clean PFA container. Usually, the powder is not completely transferred after the first attempt and the process is repeated until all of the sample has been transferred. The tube is as a standard size, yielding sub-percent precision of isotope mea- then placed in a dust free environment and left to dry. After the fi surements and age for most samples. Typically, a suf cient sample tube is completely dry, its mass is weighed without the sample can be removed from an area of between 5 and 10 mm in diameter powder, and from this the sample mass is ascertained. The sample (e.g. see Fig. 6), though this is dependent on the thickness of the in the PFA container is dissolved by adding just sufficient 7 M HNO3 calcite. (Romil™, supra purity (sp) grade) to completely dissolve the sam- Larger sample sizes obviously improve the precision of the re- ple. A mixed 229Th-236U tracer is then added to the solution. The sults and are encouraged when calcite crusts are thick enough to container is tightly closed and the solution is refluxed on a hotplate allow their removal. Prior information, e.g. on U concentration, at 80 C for several hours to equilibrate sample and spike. After the derived from other speleothems in the same cave, can help to reflux, the solution is inspected for any undissolved residuals. In optimise sample sizes. Without such prior information we recom- cases where particles remain undissolved, we recommend sepa- mend sub-samples of 5 mg as normal procedure, and increased rating them before further treatments. For this, the reflux is e sample sizes of 10 20 mg where CaCO3 formations are big enough. continued for at least 24 h and then the solution is completely dried Sequential sub-samples taken from one CaCO3 formation can be down. This guarantees complete equilibrium and the same oxida- combined into a larger size where very low U concentrations are tion state between sample and spike isotopes. The sample is then revealed after one of the sub-samples has been analysed. re-dissolved in 0.5 ml 7 M HNO3, transferred into micro-centrifuge tubes and centrifuged. The residue-free solution is pipetted back 3.7. Sample quality

The reliability of U-series results on carbonate crusts depends on many factors including sampling procedures in the cave, sample processing in the laboratory, and the ability to accurately measure U and Th isotopes in small samples. Small sample sizes not only pose analytical challenges, they also increase the relative influence of contamination during sampling and sample processing. It is therefore imperative that rigorous sampling procedures be fol- lowed, and quality control protocols must be in place. Taking samples whilst avoid contamination in a natural and dirty envi- ronment like a cave is a major challenge, and precautions must be taken to minimise contamination on site. Despite our many precautions, however, taking samples in a cave environment still bears a higher risk of increased blanks or contamination than if sub-sampling were undertaken in a clean laboratory. The additional contamination risk needs to be assessed by the collection of dedicated blank and 'standard' samples. For the blank assessment, sample tubes need to be opened and exposed to the cave environment at similar positions and for a similar length of time as for the sampling process. Additionally, standard carbonate powder samples with known U and Th concentrations and isotopic composition can be brought to the sampling locations in the cave and transferred into sample tubes using similar methods as for a sample. Fig. 7. A sample tube with collected CaCO3 powder (PAS 31b). 110 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 into the PFA container to be dried down again in order to continue 229Th-230Th-232Th standard to bracket Th isotope measurements. with the organic treatment step. In case that several subsamples Instrumental background, especially of the Aridus spray chamber, is with indissoluble components can be analysed for an isochron, we additionally reduced by employing a Cetac Quickwash system recommend different degrees of leaching using a variety of acid (Leipzig and Burgos) and background intensities are always strength for initial dissolution of the sub samples and total disso- measured after the washout procedure prior to sample and stan- lution for at least one of the sub samples. dard measurements. Measurements are corrected for instrumental If no residuals have been detected in the solution after sample biases, chemical blanks and spike contribution following Hoffmann dissolution, spiking and reflux, the solution is dried down and et al. (2007). Final results are reported as concentration or activity directly treated to remove any organics. For this, the sample is ratios. Activity ratios are calculated from isotope concentration dissolved with 50 ml concentrated HNO3 (Romil™ sp) plus 50 ml ratios using the following decay constants: l238 ¼ 10 1 concentrated H2O2 (Romil™ sp). The container is tightly closed and (1.55125 ± 0.0017)$10 a (Jaffey et al., 1971), l234 ¼ 6 then placed on a hotplate at 90 C for a minimum of 2 h. Then, 50 ml (2.826 ± 0.0056)$10 (Cheng et al., 2000), l232 ¼ (4.95 ± 0.035)$ 11 6 concentrated HCl (Romil™ sp) is added to the solution, the 10 (Holden, 1990), l230 ¼ (9.1577 ± 0.028)$10 (Cheng et al., container is tightly closed and then placed again on a hotplate at 2000). U-Th ages are iteratively calculated from the 230Th/238U 90 C for at least 12 h. The solution is then dried down and the and 234U/238U activity ratios using the above constants; all un- sample dissolved in 600 ml 6 M HCl for column chemistry. certainties are assessed and propagated by a Monte Carlo approach as described in Hoffmann et al. (2007). 4.2. Chemical separation and purification and U-series measurements 5. Reliability of U-Th dates

The chemical separation and purification protocols for anion U-Th dating is based on two fundamental assumptions: (a) only exchange resin column chemistry is modified from Hoffmann U and no Th is initially present when the CaCO3 forms and (b) the (2008). The main difference to Hoffmann (2008) is that here we CaCO3 remains a closed system, i.e. neither U nor Th is added or use a two resin procedure using BioRad™ AG 1 8 and Eichrom™ removed after precipitation. U-Th dating of speleothems is gener- UTEVA resins instead of using only one type of anion exchange ally considered to be reliable (Scholz and Hoffmann, 2008). How- resin (BioRad™ AG 1 8). All resin is pre-cleaned before it is used ever, both assumptions are susceptible to violation, and CaCO3 for sample processing. For AG 1 8 we use columns with 300 ml samples analysed for U-Th dating need to be checked for initial Th reservoir volume and for UTEVA the column volume is 75 ml. and closed system behaviour. The first step mainly separates U and Th into two fractions, which is done with AG 1 8. The pre-cleaned resin is conditioned 5.1. Closed system with 6 M HCl and the sample in 6 M HCl passed through the resin and collected. This fraction contains Th and also Ca, which is the It is unfortunately not possible to confirm closed system main matrix component in the solution. U is eluted from the resin behaviour of a CaCO3 sample where only one single U-Th age has and collected in a separate PFA container using 1 M HBr. Both been determined. There is no independent proxy within the U-Th fractions are evaporated to dryness. The Th fraction is re-dissolved isotope system that could confirm or disprove a closed system. with 7 M HNO3 and the U fraction with 3 M HNO3. The Th fraction is Thus, additional analyses are usually required to ensure reliability. purified twice, first with AG 1 8 followed by UTEVA, whereas the Speleothems that form with internal stratigraphic constraints U fraction, which does not contain major matrix elements, is pu- provide the possibility to check reliability by comparing ages of rified using UTEVA only. sub-samples along the growth axis of the calcite. A stalagmite, for For the first AG 1 8 purification of the Th fraction, the same example, must become progressively younger from bottom (i.e. columns are used as for the HCl separation step. The resin must be closest to the surface on which it first forms) to top. Since it is highly rinsed with HCl and MQ between the two passes and then condi- unlikely that leaching of U or incorporation of Th would simulta- tioned with 7 M HNO3. The Th fraction, in 7 M HNO3, is loaded onto neously affect all of its layers. Consistent multiple U-Th ages in the resin and matrix elements passed through the column with 7 M stratigraphic order can thus be taken to confirm the closed system HNO3. Th is eluted and collected with 6 M HCl. The solution is dried of the dated material. This has been commonly used as a reliability down and then dissolved in 3 M HNO3. check in U-Th dating of calcite for a wide range of applications from U and Th fractions are separately passed through UTEVA resin palaeoclimate studies to speleology (Hellstrom, 2006; Hoffmann for final purification. Pre-cleaned UTEVA is conditioned with 3 M et al., 2009; Ludwig et al., 1992; Scholz and Hoffmann, 2008; HNO3 and then the U fraction in 3 M HNO3 is loaded onto the resin Scholz et al., 2012). and the resin rinsed with 3 M HNO3. U is eluted from the resin with Most CaCO3 formations on top of cave art do not provide an 0.02 M HCl. The resin is rinsed with MQ water and then conditioned unambiguous internal structure. An uneven surface and potentially again with 3 M HNO3. The Th fraction in 3 M HNO3 is loaded onto multiple nucleus points with spherical calcite growth do not the resin and the resin rinsed with 3 M HNO3.Thisfinally eluted necessarily provide a well-defined stratigraphy from surface-to- with 3 M HCl. Both fractions (U and Th) are evaporated to dryness pigment, and it is not possible to check whether this is the case and then dissolved in 0.5 M HCl for analysis. Exact quantities for or not in the cave prior to sampling. However, it is still recom- rinsing the resin and eluting U or Th need to be determined by mended to perform U-Th analyses on multiple sub-samples. In- column calibration procedures prior to the chemistry. spection of the sampled area before and after a sub-sample is taken Procedures for MC-ICPMS measurements are detailed in is necessary in order to assess potential mixing of different growth Hoffmann et al. (2007) and Hoffmann (2008). In short, analyses are phases. Where results on multiple sub-samples are concordant or undertaken with a ThermoFinnigan Neptune MC-ICPMS. A 50 ml/ even in stratigraphic order, the system can be regarded as closed. min PFA nebulizer is used for sample introduction and a Cetac Environmental or climatic conditions such as humid events can Aridus (Bristol)/Aridus II (Leipzig and Burgos) system is used for influence the hydrology in the epikarst and lead to synchronous sample desolvation. U and Th are measured in separate runs, a calcite formation at multiple locations of a cave environment. sample e standard bracketing procedure is used with NBL 112a as Episodic growth phases relating to this can be identified by dating bracketing standard for U isotope measurements and an in-house stalagmites, stalactites, flowstones or calcite crusts on the cave D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 111 wall/ceiling including those found on top of cave art. Consistent ratio of 0.8 with 50% uncertainty and a 238U decay chain in the times of calcite formation between different sampling locations in a detrital component in secular equilibrium. For cave art-related cave can also be used as indication that the samples have remained calcite crusts, the detritus is usually dust/soil sourced and, where a closed system. measured, we typically found values that were within the range of In theory, the most rigorous closed system test is to employ the the bulk earth assumption, i.e. values between 0.4 and 1.2 (Pike alternative 235U-series and measure 231Pa/235U in addition to 230Th/ et al., 2012), making this the first choice for unconstrained U(Cheng et al., 1998). However, U-Pa requires much larger sample correction. In any case, both the measured and corrected activity sizes due to significantly smaller concentrations of 235U ratios and resulting ages of the calcite samples have to be reported (235U/238U ¼ 0.0073). The 235U decay constant is about or 6 times in order to demonstrate to what degree the correction influences higher than 238U and the 231Pa decay constant is about twice the the results. This also makes it possible that, where or when value for 230Th. Thus, for secular equilibrium 231Pa concentrations appropriate, a different correction factor can be applied in the are about 50 times smaller than 230Th and at least about 50 times future. larger samples would be required for 231Pa measurements with similar precision as 230Th. Thus, in most cases U-Pa is not a practical 5.3. Criteria for reliability option for cave art applications where sample size is highly limited, and in any case, problems of estimating initial 231Pa may prevent its Results on carbonate crusts are intended to constrain the age of application for even large samples. the art with which they are associated. To assess the reliability of results, as much information as possible should be established and 5.2. Initial Th published about the cave and its environment, the location of the art in the cave, as well as about the CaCO3 crusts and the sampling 230 The presence of initial (or detrital) Th is usually determined procedures. The quality of CaCO3 or the efficiency/success of the by measuring levels of common Th (232Th) in the sample. In case of sampling procedure can vary, and we suggest a suite of criteria to significant amounts of 232Th, a correction for initial 230Th must be describe and characterise samples and measurements. Overall, applied. As a rule of thumb, samples with a 230Th/232Th activity criteria warranting the reliability of results can be divided in two ratio greater than 200 are not affected by detrital Th; for samples parts, (a) the CaCO3 and sampling process and (b) analyses/mea- with a 230Th/232Th activity ratio smaller than 20 a correction is surement results. significant. However, the younger the sample the less radiogenic The sampling process: 230Th has built up, and thus the 230Th/232Th activity ratio is lower than for older samples with the same amount of detrital contam- The CaCO3 crystal morphology must be described and docu- ination. For young material, the 232Th/238U ratio is therefore an mented before and during sampling. Best results are expected additional proxy to assess presence of significant detritus. For sili- for dense, pristine CaCO3. 232 238 cates or 'bulk earth' samples the Th/ U activity ratios are close The CaCO3 must be characterised for cleanliness, based on its 232 238 to 1. Hence, CaCO3 samples with Th/ U activity ratio around 1 visual appearance in the cave. The best results are expected for 232 238 are dominated by detritus, and CaCO3 samples with Th/ U clean CaCO3. Dirt inclusions usually indicate that a detrital activity ratio values of more than 0.05 can be considered to be correction must be applied, which will render results less pre- significantly affected by detrital components. cise. Accuracy obviously depends on the constraints of the Obviously, for a correction we need to know the isotopic detrital component. composition of the detritus that was incorporated into the CaCO3 at Number of sub-samples: more than one sub-sample is required time of formation. This detritus not only carries 232Th, but also 238U, for best results and, ideally, a documented succession of sub- 234U and 230Th, all of which must be corrected for according to the samples should be taken. 232Th level found today, and the initial detrital isotopic composi- Sample size of about 5 mg or more ought to be collected for each tion. Detritus is usually a result of either the inclusion of dirt par- sub-sample. ticles into the CaCO3, or colloidal transport of Th with percolating No pigment should have been touched during sampling, and the waters (or both). In the case of silicate particle inclusion, an un- sampling process should have stopped before the pigment layer dissolved residue is often left behind after sample dissolution and a was reached/exposed. detrital composition close to the Earth's upper crust values ('bulk After sampling it should have been confirmed that pigment is earth') can be expected. To constrain the detrital correction, re- clearly present under the sampled CaCO3. sidual particles can be analysed for isotopic composition where large quantities (>1 mg) are found. Ideally, isochron analyses are Analyses and measurement results: employed to determine detrital endmember isotopic composition (Ludwig and Titterington,1994) and an isochron age. Unfortunately, The precise sub-sample sizes will be determined in the labora- the small size and the unconstrained geometry of most CaCO3 tory, and the samples will have been checked for residuals after formations associated with cave art do not usually allow the sub- dissolution. sampling of unambiguously coeval samples, especially since at The chemical separation and purification procedures will have least five or more sub-samples are required for reliable isochron been successful. analyses. Still, in some cases it is possible to generate 'pseudo' The mass spectrometric analyses will have been successful. isochrons based on sub-samples taken close to each other from an After data reduction, ages will have been determined, and re- area that is assumed to be coeval, but one has to keep in mind that sults for each of the sub-samples will be consistent. this assumption cannot be verified in situ. In case that no multiple sub-samples exist for one sampling In the absence of constraints for the detrital isotopic composi- spot, one will have checked for consistency with results on tion, a so-called 'bulk earth' value is commonly used for detrital additional samples removed from the same motif or related correction. This value is based on typical U and (common) Th motifs. concentrations found in the upper crust (Wedepohl, 1995), 232Th is a proxy for detrital components and should be negli- assumed to be representative for the detrital source in the calcite. In gible in the samples; high 232Th/238U indicates that a detrital practice, the twin assumptions are a detrital 238U/232Th activity correction must be applied. 112 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119

Where a detrital correction is required, an appropriate correc- that this level of success may in part also be related to the context of tion factor with associated uncertainty must be applied. the art, which in our study is largely in deep caves. Some studies (e.g. Plagnes et al. (2003)) have reported alleged problems with U- Strictly reliable age constraints for cave art are obtained by U-Th Th dating of calcite, but these derive from shallow caves and rock dating of CaCO3 crusts where the following criteria have been ful- shelters where the calcite is more likely to have been exposed to filled: (a) a suite of sub-samples (minimum of two samples) was weathering, and selection of suitable samples has therefore been taken on suitable CaCO3 clearly associated with cave art; (b) the more problematic. presence of underlying pigments has been confirmed; (c) the U-Th Samples with significant amounts of detrital Th usually yield results are analytically robust; (d) no or insignificant levels of less precise results due to the high uncertainties of the correction. detrital 232Th are detected in the samples and thus a detrital Accuracy of the corrected results obviously also depends on the correction is insignificant; and (e) results on the sub-samples are accuracy of the correction factor. It is recommended that some stratigraphically consistent. In this case, the oldest U-Th age of the effort is invested in constraining the isotopic composition of CaCO3 provides a reliable minimum age for the underlying art. detrital components where detrital contamination of CaCO3 for- The best case, of course, is that results strictly fall into correct mations is found to be high. This can be achieved by analysing stratigraphic order, with ages becoming older from the crust insoluble residue, isochron analyses, analyses of drip waters and/or (outermost) surface inwards towards the pigment. However, we determining today's composition of detritus from recent deposits. emphasise that a complicated geometry of the crust's layers e Isochrons can also be established on suitable (and similar) CaCO3 typical for spherically forming cauliflower type CaCO3 e can hinder crusts not directly associated (but close) to the cave art of concern. a strict stratigraphic order of results and lead to apparent age in- All of these methods help to constrain the expected range of detrital versions due to the different mixing of layers. Therefore, age in- U and Th isotopic composition in a specificcave. versions will not necessarily mean that results are unreliable, as discussed in more detail in section 6. 6. Applications of U-Th dating to Iberian cave art Where it has not been possible to collect multiple sub-samples it is still possible to check the consistency of dating results on 6.1. La Pasiega e cave site different CaCO3 formations from atop the same motif, or atop a (thematically or stylistically) related motif nearby on the same La Pasiega is one of five caves in the Monte Castillo, located in panel. Such results of course do not reach the same level of reli- Puente Viesgo (Cantabria, Spain). Four of the caves (El Castillo, Las ability as those for stratigraphic sub-samples, but where consistent Monedas, La Pasiega and Las Chimeneas) contain Palaeolithic cave results are found between different samples, more confidence can art (Gonzalez Sainz and Balbín, 2010) and are UNESCO World be placed in the resulting ages. Heritage sites. La Pasiega is representative of the cave sites we have A single date alone is not strictly reliable because a closed sys- sampled more widely in northern Spain. The cave has three main tem cannot be demonstrated. However, based on our results for sections, La Pasiega A, B and C. Fig. 8 shows a map of the galleries more than 170 samples in 17 caves open system behaviour of CaCO3 that are part of La Pasiega and the position of our sampling local- associated with cave art is clearly an exception rather than a rule, as ities in section La Pasiega C. is also the case for speleothems used in palaeoclimate research. The first results for samples collected in La Pasiega B and C were This demonstrates the success of our strict pre-selection criteria presented in Pike et al. (2012), who found that 14 out of 17 dated and, where followed, demonstrates the general reliability of U-Th CaCO3 formations associated with cave art formed during the Ho- age constraints for cave art. Thus, although not corroborated by locene. Two CaCO3 dates of 11.9 ± 0.5 and 12.6 ± 0.1 ka yielded additional sub-samples or repeat analyses of associated CaCO3, minimum ages of just before the start of the Holocene for the un- results can be assumed to be diagnostic where inspection of the derlying art. One CaCO3 sample taken in La Pasiega C on top of a red dated CaCO3 in the cave does not raise any doubts. Note, however, triangle was dated to 18.5 ± 0.1 ka. At the time we were still

Fig. 8. Map of La Pasiega and position of samples in this study. The Pasiega C area is highlighted in grey. D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 113 developing our sampling strategy and we were very cautious not to (Fig. 10b) before the first sample was taken. There were no traces of take our samples down too close to pigments, and thus for most pigments on top of the CaCO3 and there were good indications that CaCO3 crusts we did not take more than one sub-sample. We pre- the CaCO3 had formed on top of the art before we started sampling. sent here new dating results on CaCO3 that formed on top of five We took a total of seven sub-samples (PAS 30a e g) from the surface different cave art motifs in La Pasiega C (PAS 28e32). All of these towards the pigment. Fig. 10 aei shows the CaCO3 formation before results are based on multiple sub-samples, thereby allowing sampling, after initial surface treatment, and then after each sam- assessment of the reliability of our methods and of our previous pling step. All seven samples were analysed and results are shown results. in Table 1 (PAS 30a e g). The first sample likely included remains of For all results presented here, no residuals were found after surface dust, but all other six samples were visibly clean. Pigment dissolution and chemical sample preparation and purification, and was clearly revealed underlying the sampling spot after the last MC-ICPMS analyses were all successful. Overall, U concentrations of sample was taken (visible in PAS 30 h and PAS 30i). calcite crusts in La Pasiega C range between 0.4 and 1.5 mg/g, the There are no significant levels of detrital 232Th in the samples; same range for samples from La Pasiega reported in a previous all 232Th/238U activity ratios are smaller than 0.01. The outermost study (Pike et al. (2012); discussed in Pike et al. (in press)). The sample has the highest 232Th/238U activity ratio (0.0081 ± 0.00005) 234U/238U activity ratios are generally elevated, with values be- the remaining samples scatter around 0.004. The ages range from tween 3.4 and 4.9, and show no unusual behaviour. 232Th/238U 6.6 ± 0.1 ka (outermost sample) to 11.7 ± 0.1 ka (innermost). With activity ratios are generally smaller than 0.01 and detrital correc- the exception of the fourth sample, ages for each sub-sample are in tions are not significant, except for one outermost sub-sample (PAS good stratigraphic order, i.e. trending older from the outside in- 32a) which has an elevated ratio of 0.024. We discuss below the wards and towards the pigment. The small age inversion for sample results and implications in more detail with emphasis on (a) PAS 30d compared to Pas 30c can be explained by the spatially sequential samples on one crust, (b) comparison of previous results extended sample d compared to c as shown in Fig. 10 e and f. These on a crust with new results sampled on the same crust on a images illustrate that the spatial extent of the sampling spot was different spot, (c) sampling a second crust on a previously dated significantly increased (i.e. laterally) after PAS 30c was taken; thus, motif. given the complicated and in parts spherical geometry of the cauliflower type formations it is likely that the PAS 30d included a 6.1.1. Long sequential sampling sequence on a cauliflower type larger proportion of younger material from the surface layer, formation leading to the age inversion as schematically shown in Fig. 11. All sub-samples have a relatively high initial 234U/238U activity ratio of A relatively thick cauliflower type CaCO3 formation on top of a red horse (PAS 30, Fig. 9) was used to systematically investigate the 4.5, which is consistent with previously published results for consistency of a suite of sequential sub-samples. Here, several samples from La Pasiega (Pike et al., 2012) and our other results cauliflowers formed close to each other along a line across the neck presented here. Overall, the internal consistency of the series of of the horse painting. The cauliflowers eventually connected and results for the seven sub-samples of PAS 30 demonstrate the reli- today form a linear crust. Their surface geometry and internal ability of the dating method for such CaCO3 formations, where stratigraphy is therefore complex, since it is derived from several minor age inversions can result from differential mixing of layers spherical nucleation points as well as later infill of CaCO between due to the uneven surface and missing information about layer 3 fi initial nucleation points that can reverse stratigraphy. This type of structure at the time of sampling, but which are identi able and formation is often found on pigments and we investigated U-series have no affect on the resulting age interpretation. results of the samples taken as described above. The age of the sample closest to the pigment is therefore diag- nostic as a minimum age for the underlying art, and is strictly The dense, hard CaCO3 was clean, pristine and without obvious signs of recrystallisation, so we expected robust results. The surface reliable. The result indicates that the red horse has a minimum age ± was covered with a dust layer (Fig. 10a) which was partly removed of 11.7 0.1 ka (which, to incorporate the 2-sigma error, can also be reported as minimum age of 11.6 ka) and is therefore pre-Holocene in age. Most of this CaCO3 crust formed during the first half of the Holocene either at a very small growth rate or e more likely e episodically.

6.1.2. Re-sampling a previously dated calcite crust We revisited a triangular red sign in La Pasiega C (Fig. 12) for which we had previously obtained a minimum age of 18.5 ± 0.1 ka (O-72, Pike et al. (2012)). We sampled a different part of the same large CaCO3 formation analysed in the previous study. Here, we were able to take five sequential samples (PAS 28 a e e). The CaCO3 is dense and pristine with little dust on the surface. No indication of pigment was found on top of the CaCO3 and it was already confirmed that the calcite had formed on top of pigment by Pike et al. (2012). The surface was cleaned, five visibly clean sub- samples were taken, and pigment was then clearly revealed un- derlying the sampling spot. There are no significant levels of detrital 232Th in the samples; all 232Th/238U activity ratios are smaller than 0.01. We confirm relatively high initial 234U/238U ac- tivity ratios of 4.8e4.9 and find ages between 13.4 ± 0.1 ka for the fl Fig. 9. The red horse painting with cauli ower-type CaCO3 formations that have joined outermost sub-sample and 22.9 ± 0.2 ka for the sub-sample closest during formation to take on a linear aspect (PAS 30). Sampling is shown in Fig. 3, for to the pigment. This age range is in perfect agreement with the size reference of the marked zone in the inset see Fig. 6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of result of a single sample from the same crust analysed in (Pike et al., this article.) 2012). 114 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119

Fig. 10. Details of the sampling for PAS 30, showing the cauliflower type CaCO3 a) before sampling, b) after cleaning and c-i) after each sub-sample had been taken. The U-Th ages for the seven samples are: c) 6.6 ± 0.1 ka (PAS 30a); d) 7.5 ± 0.1 ka (PAS 30b); e) 8.1 ± 0.1 ka (PAS 30c); f) 7.6 ± 0.1 ka (PAS 30d); g) 9.3 ± 0.1 ka (PAS 30e); h) 10.8 ± 0.1 ka (PAS 30f); i) 11.7 ± 0.1 ka (PAS 30g). For size reference see Fig. 6.

Looking more into details, we find a small age inversion in PAS outermost sample, and closest to the pigment the CaCO3 has an age 28 similar to that of PAS 30. PAS 28c is apparently older than the of 11.8 ± 0.2 ka. The middle sample is slightly older with 12.2 ± 0.1 following sub samples PAS 28d and 28e. This is most likely a result ka. The CaCO3 had clearly started to form around the same time as of layers that had become mixed due to the spherical growth of the formation sampled for the red horse (PAS 30) which is less than cauliflowers, although, alternatively here the age inversion could 1 m next to the triangular sign. The minimum age of 11.6 ka is also indicate a local opening of the CaCO3 with respect to U-series. therefore strictly reliable. The crust shows episodic growth with an age of 17.5 ka for PAS 28b We additionally sampled a calcite crust overlying a yellow and around 23 ka for the following sub-samples. Thus, after an zoomorph in La Pasiega (PAS 29), from the same panel as the red approximately 5 ka long hiatus when the surface of the crust triangular sign (PAS 31). The top right corner of Fig. 13 also shows became wet again at around 17.5 ka and new calcite was deposited, the vertically positioned yellow animal (possibly a horse) with the extant surface could have been leached and become open CaCO3 formations on top of it. The missing head of the zoomorph system. However, even though the small age inversion (PAS 28c) may indicate that it underlies (and is therefore obscured by) the red could indicate a local open system, the consistent ages for PAS 28d sign, but we could not directly confirm a stratigraphic relationship and PAS 28e clearly confirm that the underlying calcite remained between the two motifs. We sampled the crust indicated in Fig. 13; closed. The minimum age for the red triangle of 22.7 ka is therefore the surface was cleaned and three sub-samples (PAS 29a e c) were considered strictly reliable. taken from the small clean and dense cauliflower type CaCO3 for- mation. All of these were visibly clean, and pigment was once again 6.1.3. U-Th dating of calcite crusts on previously dated motifs clearly revealed underlying the sampling spot. There are no sig- fi 232 232 238 Fig. 13 shows a red triangular sign consisting of two ovoid ni cant levels of detrital Th in the samples; all Th/ U ac- tivity ratios are smaller than 0.01. We obtained an age of 3.5 ± 0.06 symbols with CaCO3 formations on top of its upper right corner, where we sampled a cauliflower type CaCO formation (PAS 31). A ka for the outermost sample and closest to the pigment the CaCO3 3 ± previous date of 4.6 ± 0.02 ka was obtained for a formation on top has an age of 4.1 0.06 ka. Once again we found a small age ± of the sign's left side (O-73, Pike et al. (2012)). The surface of the inversion for the middle sample (4.47 0.05 ka) which seems to be a common feature for cauliflower-shaped crusts. Despite the small CaCO3 was cleaned and three sub-samples were taken (PAS 31a, b and c). All of these were visibly clean and pigment was clearly inversion, we consider the age of 4 ka as a reliable minimum age for revealed underlying the sampling spot. There are no significant the painting. levels of detrital 232Th in the samples; all 232Th/238U activity ratios Finally, we revisited a series of small red dots arranged in a fl are smaller than 0.01. We obtained an age of 10.5 ± 0.1 ka for the rectangular pattern (Fig. 14). A cauli ower-type calcite formation Table 1 U-series results for samples from Fuente del Trucho (FT) and La Pasiega (PAS). ID 238 U[mg/g] 232 Th [ng/g] 232 Th/238 U[x10 3 ] 230 Th/232 Th 230 Th/238 U 234 U/238 U Age [ka] 234 U/238 U initial 230 Th/238 U (corr) 234 U/238 U (corr) Age [ka] (corr) 234 U/238 U initial * ( ) FT 1a4.72 ± 0.07 56.98 ± 0.96 3.97 ± 0.01 50.1 ± 0.1 0.1979 ± 0.0008 0.9455 ± 0.0016 25.70 ± 0.12 0.9414 ± 0.0017 0.1953 ± 0.0015 0.9454 ± 0.0016 25.33 ± 0.22 0.9413 ± 0.0017 * FT 1b( ) 4.52 ± 0.05 23.21 ± 0.24 1.68 ± 0.004 120.4 ± 0.4 0.2023 ± 0.0009 0.9448 ± 0.0014 26.37 ± 0.13 0.9406 ± 0.0015 0.2012 ± 0.0010 0.9448 ± 0.0014 26.21 ± 0.16 0.9405 ± 0.0015 ** FT 2a( ) 3.48 ± 0.09 4.56 ± 0.13 0.43 ± 0.004 456.6 ± 3.9 0.1957 ± 0.0009 0.9468 ± 0.0019 25.35 ± 0.14 0.9428 ± 0.0020 0.1954 ± 0.0009 0.9467 ± 0.0019 25.31 ± 0.14 0.9428 ± 0.0020 * FT 2b( ) 3.21 ± 0.03 18.17 ± 0.19 1.86 ± 0.005 106.7 ± 0.3 0.1975 ± 0.0007 0.9495 ± 0.0014 25.52 ± 0.12 0.9457 ± 0.0015 0.1963 ± 0.0010 0.9494 ± 0.0014 25.35 ± 0.15 0.9456 ± 0.0015 * FT 2c( ) 3.49 ± 0.03 9.63 ± 0.13 0.90 ± 0.01 221.0 ± 2.2 0.1997 ± 0.0008 0.9466 ± 0.0014 25.94 ± 0.12 0.9425 ± 0.0015 0.1991 ± 0.0008 0.9466 ± 0.0014 25.85 ± 0.13 0.9425 ± 0.0015 * FT 3( ) 16.29 ± 0.14 8.57 ± 0.07 0.17 ± 0.0003 697.0 ± 1.2 0.1200 ± 0.0003 0.6195 ± 0.0010 24.07 ± 0.09 0.5927 ± 0.0011 0.1199 ± 0.0004 0.6194 ± 0.0010 24.04 ± 0.09 0.5927 ± 0.0011

* 104 (2016) 36 Geochronology Quaternary / al. et Hoffmann D.L. FT 4a( ) 19.84 ± 0.26 18.02 ± 0.23 0.30 ± 0.0008 403.9 ± 0.7 0.1200 ± 0.0004 0.6112 ± 0.0010 24.46 ± 0.11 0.5834 ± 0.0012 0.1198 ± 0.0005 0.6111 ± 0.0010 24.42 ± 0.12 0.5834 ± 0.0012 * FT 4b( ) 19.06 ± 0.12 8.27 ± 0.05 0.14 ± 0.0003 852.7 ± 1.6 0.1210 ± 0.0004 0.6116 ± 0.0010 24.67 ± 0.10 0.5835 ± 0.0011 0.1209 ± 0.0004 0.6115 ± 0.0010 24.65 ± 0.10 0.5835 ± 0.0011 * FT 5( ) 16.24 ± 0.18 6.66 ± 0.08 0.13 ± 0.0008 866.9 ± 5.4 0.1162 ± 0.0004 0.5133 ± 0.0008 29.32 ± 0.14 0.4713 ± 0.0010 0.1161 ± 0.0004 0.5133 ± 0.0008 29.30 ± 0.14 0.4713 ± 0.0010 * FT 6( ) 13.0 ± 0.1 5.58 ± 0.06 0.14 ± 0.001 865.4 ± 5.9 0.1214 ± 0.0005 0.5897 ± 0.0009 25.91 ± 0.12 0.5585 ± 0.0010 0.1213 ± 0.0005 0.5897 ± 0.0009 25.89 ± 0.12 0.5585 ± 0.0010 * FT 7( ) 12.12 ± 0.07 3.94 ± 0.02 0.11 ± 0.0002 1354.0 ± 2.3 0.1435 ± 0.0004 0.5933 ± 0.0009 31.30 ± 0.12 0.5556 ± 0.0010 0.1434 ± 0.0004 0.5932 ± 0.0009 31.29 ± 0.12 0.5556 ± 0.0010 * FT 8( ) 13.63 ± 0.13 5.02 ± 0.06 0.12 ± 0.0008 1021.7 ± 5.7 0.1231 ± 0.0004 0.5861 ± 0.0010 26.54 ± 0.12 0.5539 ± 0.0012 0.1231 ± 0.0004 0.5861 ± 0.0010 26.52 ± 0.12 0.5538 ± 0.0012 * FT 9( ) 11.31 ± 0.14 19.78 ± 0.23 0.57 ± 0.001 217.1 ± 0.4 0.1242 ± 0.0004 0.5839 ± 0.0008 26.94 ± 0.11 0.5509 ± 0.0010 0.1238 ± 0.0004 0.5837 ± 0.0008 26.85 ± 0.12 0.5509 ± 0.0010 * FT 10( ) 14.71 ± 0.16 27.29 ± 0.29 0.61 ± 0.001 232.7 ± 0.6 0.1412 ± 0.0005 0.6686 ± 0.0011 26.45 ± 0.12 0.6429 ± 0.0012 0.1408 ± 0.0006 0.6684 ± 0.0011 26.37 ± 0.13 0.6428 ± 0.0012 * FT 11( ) 6.27 ± 0.03 4.14 ± 0.04 0.22 ± 0.002 766.0 ± 4.8 0.1655 ± 0.0006 0.7520 ± 0.0011 27.53 ± 0.13 0.7319 ± 0.0013 0.1653 ± 0.0006 0.7519 ± 0.0011 27.50 ± 0.13 0.7319 ± 0.0013 * PAS 28a( ) 0.61 ± 0.01 11.47 ± 0.26 6.13 ± 0.04 94.0 ± 0.8 0.5772 ± 0.0036 4.9039 ± 0.0082 13.46 ± 0.09 5.0553 ± 0.0083 0.5751 ± 0.0037 4.9232 ± 0.0127 13.35 ± 0.10 5.0741 ± 0.0128 ** PAS 28b( ) 0.55 ± 0.02 7.39 ± 0.20 4.42 ± 0.03 169.5 ± 1.3 0.7471 ± 0.0042 4.8951 ± 0.0091 17.70 ± 0.11 5.0949 ± 0.0092 0.7462 ± 0.0042 4.9089 ± 0.0114 17.63 ± 0.11 5.1086 ± 0.0116 ** PAS 28c( ) 0.44 ± 0.02 5.63 ± 0.23 4.20 ± 0.03 232.4 ± 1.9 0.9735 ± 0.0063 4.8449 ± 0.0092 23.79 ± 0.18 5.1123 ± 0.0094 0.9734 ± 0.0064 4.8578 ± 0.0113 23.72 ± 0.18 5.1253 ± 0.0115 ** PAS 28d( ) 0.45 ± 0.01 4.61 ± 0.13 3.34 ± 0.02 283.3 ± 2.0 0.9424 ± 0.0045 4.8521 ± 0.0085 22.93 ± 0.13 5.1100 ± 0.0087 0.9422 ± 0.0045 4.8624 ± 0.0100 22.87 ± 0.13 5.1204 ± 0.0102 * PAS 28e( ) 0.46 ± 0.02 6.11 ± 0.24 4.33 ± 0.03 216.0 ± 1.8 0.9370 ± 0.0063 4.8142 ± 0.0095 22.99 ± 0.17 5.0702 ± 0.0098 0.9368 ± 0.0063 4.8275 ± 0.0116 22.91 ± 0.18 5.0835 ± 0.0119 * PAS 29a( ) 1.40 ± 0.07 10.35 ± 0.60 2.41 ± 0.03 46.5 ± 0.7 0.1123 ± 0.0015 3.4640 ± 0.0066 3.59 ± 0.05 3.4891 ± 0.0066 0.1106 ± 0.0017 3.4688 ± 0.0070 3.53 ± 0.06 3.4935 ± 0.0070 ** PAS 29b( ) 1.25 ± 0.04 10.54 ± 0.32 2.76 ± 0.02 49.8 ± 0.5 0.1370 ± 0.0013 3.3499 ± 0.0058 4.54 ± 0.04 3.3802 ± 0.0058 0.1350 ± 0.0016 3.3551 ± 0.0063 4.47 ± 0.05 3.3850 ± 0.0064 * PAS 29c( ) 1.34 ± 0.07 3.12 ± 0.22 0.76 ± 0.02 164.0 ± 3.2 0.1248 ± 0.0018 3.3702 ± 0.0061 4.10 ± 0.06 3.3979 ± 0.0062 0.1243 ± 0.0018 3.3717 ± 0.0062 4.08 ± 0.06 3.3992 ± 0.0062 * PAS 30a( ) 0.91 ± 0.03 22.58 ± 0.71 8.11 ± 0.05 32.8 ± 0.3 0.2665 ± 0.0019 4.4126 ± 0.0080 6.75 ± 0.05 4.4783 ± 0.0081 0.2617 ± 0.0031 4.4349 ± 0.0138 6.59 ± 0.08 4.4995 ± 0.0139 * PAS 30b( ) 1.12 ± 0.03 8.91 ± 0.28 2.59 ± 0.03 115.0 ± 1.3 0.2983 ± 0.0022 4.4558 ± 0.0074 7.50 ± 0.06 4.5299 ± 0.0074 0.2969 ± 0.0023 4.4630 ± 0.0082 7.45 ± 0.06 4.5368 ± 0.0083 * PAS 30c( ) 1.04 ± 0.02 16.49 ± 0.37 5.20 ± 0.09 62.6 ± 1.1 0.3261 ± 0.0019 4.4604 ± 0.0076 8.22 ± 0.05 4.5416 ± 0.0076 0.3233 ± 0.0023 4.4748 ± 0.0105 8.11 ± 0.06 4.5554 ± 0.0106 * PAS 30d( ) 1.00 ± 0.01 13.72 ± 0.19 4.48 ± 0.03 68.0 ± 0.6 0.3046 ± 0.0018 4.4407 ± 0.0069 7.69 ± 0.05 4.5164 ± 0.0070 0.3021 ± 0.0022 4.4531 ± 0.0093 7.61 ± 0.06 4.5281 ± 0.0094 (*) ± ± ± ± ± ± ± ± ± ± ± ± e

PAS 30e 0.79 0.01 7.41 0.15 3.08 0.03 119.9 1.1 0.3694 0.0020 4.4329 0.0073 9.40 0.05 4.5253 0.0074 0.3679 0.0021 4.4413 0.0085 9.34 0.06 4.5334 0.0086 119 * PAS 30f( ) 0.64 ± 0.02 6.46 ± 0.19 3.29 ± 0.03 128.5 ± 1.4 0.4231 ± 0.0030 4.4257 ± 0.0073 10.84 ± 0.08 4.5323 ± 0.0073 0.4216 ± 0.0031 4.4348 ± 0.0086 10.78 ± 0.09 4.5410 ± 0.0087 * PAS 30g( ) 0.71 ± 0.02 6.54 ± 0.19 2.99 ± 0.02 151.8 ± 1.3 0.4552 ± 0.0029 4.3867 ± 0.0074 11.80 ± 0.08 4.5016 ± 0.0074 0.4539 ± 0.0030 4.3948 ± 0.0084 11.74 ± 0.08 4.5094 ± 0.0085 * PAS 31a( ) 1.33 ± 0.02 12.56 ± 0.28 3.08 ± 0.02 102.4 ± 0.9 0.3158 ± 0.0016 3.3726 ± 0.0049 10.62 ± 0.06 3.4449 ± 0.0049 0.3141 ± 0.0019 3.3784 ± 0.0057 10.54 ± 0.07 3.4504 ± 0.0058 ** PAS 31b( ) 1.50 ± 0.03 7.87 ± 0.18 1.73 ± 0.01 211.0 ± 1.3 0.3633 ± 0.0015 3.3871 ± 0.0052 12.23 ± 0.06 3.4711 ± 0.0053 0.3624 ± 0.0016 3.3904 ± 0.0055 12.19 ± 0.06 3.4742 ± 0.0056 * PAS 31c( ) 1.43 ± 0.17 10.92 ± 1.40 2.50 ± 0.03 140.6 ± 2.0 0.3512 ± 0.0041 3.3626 ± 0.0078 11.90 ± 0.15 3.4434 ± 0.0079 0.3499 ± 0.0042 3.3673 ± 0.0082 11.84 ± 0.15 3.4478 ± 0.0083 * PAS 32a( ) 0.89 ± 0.04 65.85 ± 2.64 24.13 ± 0.05 14.6 ± 0.1 0.3535 ± 0.0029 3.9825 ± 0.0074 10.04 ± 0.09 4.0683 ± 0.0075 0.3408 ± 0.0072 4.0412 ± 0.0309 9.52 ± 0.22 4.1241 ± 0.0311 ** PAS 32b( ) 1.23 ± 0.06 19.72 ± 0.88 5.26 ± 0.03 70.2 ± 0.6 0.3684 ± 0.0024 3.8698 ± 0.0078 10.79 ± 0.08 3.9587 ± 0.0078 0.3657 ± 0.0028 3.8819 ± 0.0099 10.68 ± 0.09 3.9702 ± 0.0100 * PAS 32c( ) 0.98 ± 0.02 9.33 ± 0.19 3.11 ± 0.02 138.5 ± 1.1 0.4307 ± 0.0019 3.7177 ± 0.0056 13.26 ± 0.07 3.8215 ± 0.0056 0.4293 ± 0.0021 3.7245 ± 0.0065 13.18 ± 0.07 3.8279 ± 0.0066

All ratios are activity ratios. The degree of detrital 230 Th contamination is indicated by the measured 230 Th/232 Th activity ratio and corrections were calculated using a 238 U/232 Th activity ratio of 0.8 ± 0.4 and assuming secular 238 equilibrium of theU decay chain in the detritus. Analytical errors are at 95% confidence level.Activity ratios are calculated from isotope concentration ratios using decay constants according to Jaffey et al. (1971) ( l238 ), Cheng (*) (**) et al. (2000)l234 and( l230 )$and Holden (1990) ( l232 ).$ Analysed in CENIEH laboratory (Burgos). Analysed in MPI laboratory (Leipzig). 115 116 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119

Fig. 11. Schematic outline of a cauliflower type formation with two growth phases (12 ka and 6 ka). Sampling starts from the top which mostly consist of the younger phase. Sampling downwards towards the pigment can result in a fraction of the younger phase at the edge of a layer. Where the spatial extent of the sampling area is enlarged (sample d), a stratigraphically assumed 'older' sample can have a larger fraction of younger phase material than the sample above (sample c) and thus lead to apparent age inversion.

Fig. 12. Red triangle sign with CaCO3 formation in La Pasiega C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) on top of a red dot from the lower of two such rectangular patterns was sampled for this study (PAS 32). The new sample was taken ca. 15 cm to the right of a calcite crust overlying a red dot of the same arrangement that had been sampled in our previous study (O-98, Pike et al. (2012)). The surface was difficult to clean and the surface stratigraphy appeared complex. We obtained three sequential sub- samples which were all visibly clean, and pigment was clearly revealed underlying the sampling spot. The outermost sample (PAS 32a) has elevated detrital components with 232Th/238U activity Fig. 13. The red sign (PAS 31) and the vertically positioned yellow zoomorph (PAS-29, ratio of 0.024, but there are no significant levels of detrital 232Th in top right). (For interpretation of the references to colour in this figure legend, the the samples PAS 32 b and c. We obtained ages that lie in correct reader is referred to the web version of this article.) stratigraphic order with 9.5 ± 0.2 ka (PAS 32a, the outermost sub- sample), 10.7 ± 0.1 for the middle sub-sample (PAS 32b) and 13.2 ± 0.1 ka (PAS 32c, the innermost sub-sample). The result for 6.2. Fuente del Trucho e rock shelter-type cave the innermost sub-sample is a reliable minimum age for the red dot. In our previous study we found an age of 7.1 ± 0.04 ka for a Fuente del Trucho (Huesca, Spain) is located in the river Vero single sample (O-98, Pike et al. (2012)), a little younger than our basin at the southern side of the Pyrenees, and in contrast to the outermost calcite sample. Assuming that the rectangular arrange- deep cave of La Pasiega, is a shallow cave, more akin to a rock ment was created as a single motif (i.e. at the same time), our new shelter. The opening of the cave is 22 m wide and is oriented to the result of 13.1 ka provides a minimum age for all the red dots that SE. The entrance gives access to a wide hall that is about 24 m deep, comprise this motif, which is therefore demonstrably pre-Holocene divided into two asymmetrical lobes which are to some extent open in age. to daylight (Fig. 15). There are two main groups of Palaeolithic art in D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 117

Fig. 14. A series of small red dots created in a rectangular pattern. The tip of the scalpel Fig. 16. Red hand stencil in Panel III of Fuente del Trucho. Positions of the calcite crust indicates position of the calcite crust sampled for this study (PAS 32). The inset shows sample positions FT 1 and FT 2. Results are nearly identical for FT 1 and FT 2, and in details of the crust after the sampling. The arrow shows where the previous sample O- both cases sequential sub-samples yielded results that are in stratigraphic order. (For 98 (Pike et al., 2012) was taken. (For interpretation of the references to colour in this interpretation of the references to colour in this figure legend, the reader is referred to figure legend, the reader is referred to the web version of this article.) the web version of this article.)

The calcite crusts were relatively thin, but in three cases we were able to obtain several sub-samples. In all three cases the re- sults for each of the sub-samples are in stratigraphic order and thereby confirm the closed system of the calcite. Detrital correction is not significant for the samples even though Fuente del Trucho is a rock shelter-type cave and the surface of the crusts is covered with dirt. We find 230Th/232Th activity ratios generally greater than 100 and uncorrected and corrected dates agree within uncertainties for all samples except FT 1a, the outermost sample of FT 1, which has a 230Th/232Th activity ratio of 50. Additionally, we obtained almost identical results for the two crusts sampled for FT 1 and FT 2, which are a few cm away from each other (Fig.16). Results for FT 1 and FT 2 are thus consistent both within and between samples. The samples from Fuente del Trucho have relatively high U concentrations of more than 3 mg/g (Table 1). In more detail, U concentration as well as 234U/238U activity ratios are highly variable between samples from different sections of the cave wall. U con- centration varies between 3.2 mg/g (FT 2b, panel III on the wall) and 19.8 mg/g (FT 4a, panel VII on the ceiling). The lowest U concen- trations we obtained are found in calcite on Panel III, which was painted on the wall to the side of the cave. By contrast, calcite crusts on top of panels of the cave's frieze had higher U concentrations. The 234U/238U activity ratios are depleted and all smaller than unity; they vary between 0.95 and 0.5. Lower values for the 234U/238U activity ratio seem to be correlated with higher U con- Fig. 15. Map of Fuente del Trucho and the position of our samples. centrations. FT 1 and FT 2 (with the lowest U concentrations) have the highest 234U/238U around 0.95 whereas samples with high U the ‘cave’, one with engravings situated in the entrance and more concentrations towards the frieze/ceiling are highly depleted in than 100 painted motifs distributed in 21 panels all over the walls 234U, for example FT 5 has a U concentration of 16 mg/g and and ceiling in its inner part (Utrilla et al., 2014). Fourteen U-Th dates 234U/238U around 0.51. This indicates at least two different U on calcite pertaining to seven motifs, and their interpretation, are sources, each with a different 234U/238U signature. The U-Th ages presented in detail in Hoffmann et al. (in press), but the methods are neither correlated with highly variable U concentrations nor used and results are discussed briefly here. Great care was taken at with 234U/238U activity ratios found in crusts, confirming that this site to identify suitable calcite crusts and to assess the potential neither of them can be used to infer reliability (or lack thereof) of U- impact of weathering (or biological factors) on the cave wall/ceil- Th ages. ing, its art and calcite crusts. We found many calcite formations For Fuente del Trucho, all U-Th ages obtained on the eleven overlying pigments, and identified dense calcite crusts suitable for calcite crusts on top of cave art fall in the range 24 ka to 31 ka, the dating. The surface of the calcite was covered with more dust or dirt majority clustering between 25 ka and 27.5 ka, indicating increased than we typically observe in deeper caves, but surface pre- water availability in the epikarst system above the cave during this treatment revealed clean calcite beneath, and we obtained robust time. The results fall within a cold and wet period 29 to 25 ka in minimum ages for all seven motifs (Hoffmann et al., in press). central Spain between Heinrich Event 2 (H2) and H3 at the end of 118 D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119

MIS 3 (Dominguez-Villar et al., 2013). Higher precipitation and careful sample selection our methods are also applicable to a more lower temperature during this period lead to higher effective pre- open cave/rock-shelter site like Fuente del Trucho. cipitation, resulting in sufficient recharge in the epikarst to initiate The dated CaCO3 growth phase in Fuente del Trucho is consis- percolating water reaching the cave and crust precipitation in tent with a wet phase found for central Iberia 29 ka e 25 ka different, usually dry parts of the wall. Our dates are therefore (Dominguez-Villar et al., 2013). Thus, dating of calcite crusts can consistent with evidence for wetter conditions in central Spain at also provide palaeoenvironmental information about palae- this time as found in Eagle Cave (Dominguez-Villar et al., 2013). oenvironmental or hydrology changes. Overall, the dating results on crusts from Fuente del Trucho constrain the age of the art in Fuente del Trucho as older than 24 ka, Acknowledgements and in the case of at least one motif (set of red dots, panel VII, motif 54, sample FT 7) older than 31 ka. Another date also on one of the This study was funded by the National Environmental Research red dots of the same motif 54 yielded a minimum age of 25.8 ka. It Council (UK) (NE/F000510/1 and NE/K015184/1), the Spanish therefore seems likely that cave art in Fuente del Trucho can Ministerio de Cienca e Innovacion (MICINN, Plan Nacional research generally be considered to be older than 31 ka and that calcite grant CGL2011-27187) and the National Geographic Society (Grant precipitation on top of the art occurred between 31 and 24 ka. EC0603-12). We are grateful for fieldwork support by Daniel Gar- rido, Raúl Gutierrez, Carola Hoffmann, Roberto Ontan~on and Pilar 7. Summary and conclusions Utrilla.

We have provided here detailed information on our sampling References methods for CaCO3 crusts associated with cave art. These crusts can be used to determine minimum ages when they can be unambig- Arias, P., Ontan~on, R., 2008. Zona arqueologica de La Garma (Omono,~ Ribamontan al Monte). Campanas~ 2000-2003. In: Actuaciones Arqueologicas en Cantabria uously shown to have formed on top of (and thus subsequent to) 2000-2003. Consejería de Cultura, Turismo y Deporte, Gobierno de Cantabria, cave art, and when CaCO3 is demonstrably suitable for U-Th dating. Santander, pp. 43e60. However, the conservation and integrity of the cave art must be of Aubert, M., Brumm, A., Ramli, M., Sutikna, T., Saptomo, E.W., Hakim, B., highest priority, and samples are only taken where this is possible Morwood, M.J., van den Bergh, G.D., Kinsley, L., Dosseto, A., 2014. Pleistocene cave art from Sulawesi, Indonesia. Nature 514, 223e227. without any impact on the art itself. We believe that we have Aubert, M., O'Connor, S., McCulloch, M., Mortimer, G., Watchman, A., Richer- demonstrated that U-Th dating of calcites has considerable poten- LaFleche, M., 2007. Uranium-series dating rock art in east Timor. J. Archaeol. Sci. e tial to constrain the age of cave art, but we note that there are 34, 991 996. fi Bischoff, J., García-Diez, M., Gonzalez Morales, M.R., Sharp, W., 2003. Aplicacion del limitations in its application. The main obstacle is to nd suitable metodo de series de uranio al grafismo rupestre de estilo paleolitico: el caso de samples, i.e. calcite crusts that can be shown to have a demon- la cavidad de Covalanas. Veleia 20, 143e150. strable and unequivocal stratigraphic relationship with the art of Breuil, H., 1952. Quatre cents siecles d'art parietal. Centre d'Etudes de la Docu- mentation Prehistorique, Montignac. concern. Having established this, it must be possible to sample the Cheng, H., Edwards, R.L., Hoff, J., Gallup, C.D., Richards, D.A., Asmerom, Y., 2000. The crust without the risk of damaging the underlying art. Furthermore, half-lives of uranium-234 and thorium-230. Chem. Geol. 169, 17e33. Cheng, H., Edwards, R.L., Murrell, M.T., Benjamin, T.M., 1998. Uranium-thorium- the CaCO3 needs to be clean or at least possess low detrital protactinium dating systematics. Geochim. Cosmochim. Acta 62, 3437e3452. contamination, and a closed system needs to be demonstrated by Dominguez-Villar, D., Carrasco, R.M., Pedraza, J., Cheng, H., Edwards, R.L., sub-sampling. For this, the ability to undertake U-Th analyses on Willenbring, J.K., 2013. Early maximum extent of paleoglaciers from Mediter- small sample quantities is a critical prerequisite. ranean mountains during the last glaciation. Sci. Rep. 3, 1e6. fi Edwards, R.L., Chen, J.H., Wasserburg, G.J., 1987. U-238 U-234-Th-230-Th-232 sys- Our results demonstrate the ef cacy of our rigid and strict se- tematics and the precise measurement of time over the past 500000 years. lection criteria and sampling method. U-Th dating of CaCO3 crusts is Earth Planet. Sci. Lett. 81, 175e192. a powerful tool for constraining the age of underlying cave art, and García-Diez, M., Ochoa, B., 2013. Arte Prehistorico. In: García-Diez, M., Zapata, L. can be applied to art both in deep caves and in more open rock (Eds.), Metodos y Tecnicas de Analisis y Estudio en Arqueología Prehistorica. De lo tecnico a la reconstruccion de los grupos humanos. Universidad del País shelter type sites. We obtained consistent results on sequential Vasco, Bilbao, pp. 611e634. sub-samples, with the overwhelming majority of the results in Gonzalez-Sainz, C., San Miguel, C., 2001. Las cuevas del desfiladero. In: Arte rupestre fi stratigraphic order. The small number of minor and strati- paleolítico en el valle del des ladero del río Carranza. Universidad de Cantabria, Gobierno de Cantabria, Santander. graphically restricted age inversions that we observed probably Gonzalez Sainz, C., 2003. El conjunto parietal de la galería inferior de La Garma result from layer mixing, especially where surface of crusts is (Omono,~ Cantabria). Avance a su organizacion interna., El Arte Prehistorico composed of spherically grown deposits. desde los inicios del siglo XXI. In: Primer Symposium Internacional de Arte Prehistorico de Ribadesella, Ribadesella, pp. 201e222. The results obtained for La Pasiega provided in three cases Gonzalez Sainz, C., Balbín, R. de, 2010. La Pasiega, Las Cuevas Con Arte Paleolítico En minimum ages in the Holocene, and consequently have little sig- Cantabria. ACDPS, Santander, pp. 191e204. nificance for the archaeological argument about the particular age Hellstrom, J., 2003. Rapid and accurate U/Th dating using parallel ion-counting multi-collector ICP-MS. J. Anal. Atom. Spectrom. 18, 1346e1351. of the relevant art motifs. By contrast, sample PAS 28 is archaeo- Hellstrom, J., 2006. U-Th dating of speleotherns with high initial Th-230 using logically relevant, as the date indicates that we can reject the hy- stratigraphical constraint. Quat. Geochronol. 1, 289e295. pothesis that the motif was painted during the Magdalenian Hoffmann, D.L., 2008. Th-230 isotope measurements of femtogram quantities for U- series dating using multi ion counting (MIC) MC-ICPMS. Int. J. Mass Spectrom. (whose beginning in the region post-dates the minimum age of 275, 75e79. 22.7 ka obtained for the art), and conclude this triangular red sign Hoffmann, D.L., Prytulak, J., Richards, D.A., Elliott, T., Coath, C.D., Smart, P.L., was painted no later than the Upper Solutrean. Scholz, D., 2007. Procedures for accurate U and Th isotope measurements by e The archaeological implications of our results for Fuente del high precision MC-ICPMS. Int. J. Mass Spectrom. 264, 97 109. Hoffmann, D.L., Spotl,€ C., Mangini, A., 2009. Micromill and in situ laser ablation Trucho have already been discussed in detail in Hoffmann et al. (in sampling techniques for high spatial resolution MC-ICPMS U-Th dating of car- e press). The CaCO3 crusts all formed between 24 and 31 ka, placing bonates. Chem. Geol. 259, 253 261. ~ the underlying cave art generally older than 24 ka, and at least one Hoffmann, D.L., Utrilla, P., Bea, M., Pike, A.W.G., García-Diez, M., Zilhao, J., Domingo, R., 2016. U-series dating of Palaeolithic rock art at Fuente del Trucho motif older than 31 ka. There is no evidence for any open system (Aragon, Spain). Quat. Int. http://dx.doi.org/10.1016/j.quaint.2015.11.111 (in behaviour as shown by consistent results of a suite of sub-samples, press). and detritus is negligible. Thus, even though we obtained only Holden, N.E., 1990. Total half-lives for selected nuclides. Pure Appl. Chem. 62, 941e958. single samples for eight of the eleven crusts, all results can be Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Essling, A.M., 1971. Precision considered reliable minimum ages. This demonstrates that with measurement of half-lives and specific activities of U-235 and U-238. Phys. Rev. D.L. Hoffmann et al. / Quaternary Geochronology 36 (2016) 104e119 119

C 4, 1889-&. Eiszeitalt. Ggw. 57, 52e76. Leroi-Gourhan, A., 1965. Prehistoire de l'art occidental. Mazenod, Paris. Scholz, D., Hoffmann, D.L., Hellstrom, J., Ramsey, C.B., 2012. A comparison of Ludwig, K.R., Simmons, K.R., Szabo, B.J., Winograd, I.J., Landwehr, J.M., Riggs, A.C., different methods for speleothem age modelling. Quat. Geochronol. 14, 94e104. Hoffman, R.J., 1992. Mass-spectrometric TH-230-U-234-U-238 dating of the Schwarcz, H.P., Blackwell, B., 1992. Archaeological applications. In: Ivanovich, M., devils-hole calcite vein. Science 258, 284e287. Harmon, R.S. (Eds.), Uranium-series Disequilibrium: Applications to Earth, Ludwig, K.R., Titterington, D.M., 1994. Calculation of (230) Th/U isochrons, ages and Marine, and Environmental Sciences. Oxford University Press, Oxford, errors. Geochim. Cosmochim. Acta 58, 5031e5042. pp. 513e552. Ochoa, B., García-Diez, M., 2015. Chronology of western Pyrenean Paleolithic cave Shen, C.C., Edwards, R.L., Cheng, H., Dorale, J.A., Thomas, R.B., Moran, S.B., art: a critical examination. Quat. Int. 364, 272e282. Weinstein, S.E., Edmonds, H.N., 2002. Uranium and thorium isotopic and con- Pettitt, P., Bahn, P., 2003. Current problems in dating Palaeolithic cave art: candamo centration measurements by magnetic sector inductively coupled plasma mass and Chauvet. Antiquity 77, 134e141. spectrometry. Chem. Geol. 185, 165e178. Pike, A.W.G., Gilmour, M., Pettitt, P., Jacobi, R., Ripoll, S., Bahn, P., Munoz, F., 2005. Utrilla, P., Baldellou, V., Bea, M., Montes, L., Vinas,~ R., 2014. La cueva de la Fuente del Verification of the age of the Palaeolithic cave art at creswell crags, UK. Trucho (Asque-Colungo, Huesca). In: Sala, R. (Ed.), Los cazadores recolectores J. Archaeol. Sci. 32, 1649e1655. del Pleistoceno y del Holoceno en Iberia y el Estrecho de Gibraltar: estado actual Pike, A.W.G., Hoffmann, D.L., García-Diez, M., Pettitt, P.B., Alcolea, J., De Balbín, R., del conocimiento del registro arqueologico. Universidad de Burgos y Fundacion Gonzalez Sainz, C., de las Heras, C., Lasheras, J.A., Montes, R., Zilhao,~ J., 2012. U- Atapuerca, Burgos, Burgos, pp. 171e178. Series dating of Paleolithic art in 11 caves in Spain. Science 336, 1409e1413. Valladas, H., Tisnerat-Laborde, N., Cachier, H., Arnold, M., de Quiros, F.B., Cabrera- Pike, A.W.G., Hoffmann, D.L., Pettitt, P.B., García-Diez, M., Zilhao,~ J., 2016. Dating Valdes, V., Clottes, J., Courtin, J., Fortea-Perez, J.J., Gonzalez-Sainz, C., Moure- Palaeolithic cave art: why U-Th is the way to go. Quat. Int. http://dx.doi.org/ Romanillo, A., 2001. Radiocarbon AMS dates for paleolithic cave paintings. 10.1016/j.quaint.2015.12.013 (in press). Radiocarbon 43, 977e986. Plagnes, V., Causse, C., Fontugne, M., Valladas, H., Chazine, J.M., Fage, L.H., 2003. von Petzinger, G., Nowell, A., 2011. A question of style: reconsidering the stylistic Cross dating (Th/U-C-14) of calcite covering prehistoric paintings in Borneo. approach to dating Palaeolithic parietal art in France. Antiquity 85, 1165e1183. Quat. Res. 60, 172e179. Wedepohl, K.H., 1995. The composition of the continental-crust. Geochim. Cos- Scholz, D., Hoffmann, D., 2008. 230Th/U-dating of fossil corals and speleothems. mochim. Acta 59, 1217e1232.