The potential of speleothems from Western Europe as recorders of regional climate: a critical assessment of the SISAL database

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

Aged but withstanding: Maintenance of growth rates in old pines is not related to enhanced water-use efficiency

Growth of old trees in cold-limited forests may benefit from recent climate warming and rising atmospheric CO2 concentrations (ca) if age-related constraints do not impair wood formation. To test this hypothesis, we studied old Mountain pine trees at three Pyrenean high-elevation forests subjected to cold-wet (ORD, AIG) or warmer-drier (PED) conditions. We analyzed long-term trends (1450–2008) in growth (BAI, basal area increment), maximum (MXD) and minimum (MID) wood density, and tree-ring carbon (δ13C) and oxygen (δ18O) isotope composition, which were used as proxies for intrinsic water-use efficiency (iWUE) and stomatal conductance (gs), respectively. Old pines showed positive (AIG and ORD) or stable (PED) growth trends during the industrial period (since 1850) despite being older than 400 years. Growth and wood density covaried from 1850 onwards. In the cold-wet sites (AIG and ORD) enhanced photosynthesis through rising ca was likely responsible for the post-1850 iWUE improvement. However, uncoupling between BAI and iWUE indicated that increases in iWUE were not responsible for the higher growth but climate warming. A reduction in gs was inferred from increased δ18O for PED trees from 1960 onwards, the warmest site where the highest iWUE increase occurred (34%). This suggests that an emergent drought stress at warm-dry sites could trigger stomatal closure to avoid excessive transpiration. Overall, carbon acquisition as lasting woody pools is expected to be maintained in aged trees from cold and high-elevation sites where old forests constitute unique long-term carbon reservoirs.We are very grateful to several projects financed by “Organismo Autónomo de Parques Nacionales” (projects 12/2008 387/2011). E.G. was funded by a Juan de la Cierva post-doctoral research contract (FJCI-2014-19615, MEC, Spain). Spanish (AMB95-0160, CGL2011-26654) and EU projects ISONET (contract EV K2-2001-00237) and MILLENNIUM (017008–2) also supported this study by contributing additional datasets

Holocene hydro-climatic variability in the Mediterranean: A synthetic multi-proxy reconstruction

Here we identify and analyze proxy data interpreted to reflect hydro-climatic variability over the last 10,000 years from the Mediterranean region to (1) outline millennial and multi-centennial-scale trends and (2) identify regional patterns of hydro-climatic variability. A total of 47 lake, cave, and marine records were transformed to z-scores to allow direct comparisons between sites, put on a common time scale, and binned into 200-year time slices. Six different regions were identified based on numerical and spatial analyzes of z-scores: S Iberia and Maghreb, N Iberia, Italy, the Balkans, Turkey, and the Levant, and the overall hydro-climate history of each region was reconstructed. N Iberia is largely decoupled from the five other regions throughout the Holocene. Wetter conditions occur in the five other regions between 8500 and 6100 yr BP. After 6000 yr BP, climate oscillated until around 3000 ± 300 yr BP, which seems to have been the overall driest period in the eastern Mediterranean and North Africa. In contrast, Italy and N Iberia seem to have remained wetter during this period. In addition, non-metric multidimensional scaling (nMDS) was applied to 18 long, continuous climate z-score records that span the majority of the Holocene. nMDS axes 1 and 2 illustrate the main trends in the z-score data. The first axis captures a long-term development of drier condition in the Mediterranean from 7900 to 3700 yr BP. Rapid shifts occur in nMDS axis 2 at 6700–6300 BP, 4500–4300 BP, and 3500–3300 BP indicating centennial-scale climate change. Our synthesis highlights a dominant south/east versus north/west Mediterranean hydro-climate dipole throughout the Holocene and therefore confirms that there was no single climate trajectory characterizing the whole Mediterranean basin during the last 10 millennia

Pollen-inferred regional vegetation patterns and demographic change in Southern Anatolia through the Holocene

Southern Anatolia is a highly significant area within the Mediterranean, particularly in terms of understanding how agriculture moved into Europe from neighbouring regions. This study uses pollen, palaeoclimate and archaeological evidence to investigate the relationships between demography and vegetation change, and to explore how the development of agriculture varied spatially. Data from 21 fossil pollen records have been transformed into forested, parkland and open vegetation types using cluster analysis. Patterns of change have been explored using non-metric multidimensional scaling (nMDS) and through analysis of indicator groups, such as an Anthropogenic Pollen Index, and Simpson’s Diversity. Settlement data, which indicate population densities, and summed radiocarbon dates for archaeological sites have been used as a proxy for demographic change. The pollen and archaeological records confirm that farming can be detected earlier in Anatolia in comparison with many other parts of the Mediterranean. Dynamics of change in grazing indicators and the OJCV (Olea, Juglans, Castanea and Vitis) index for cultivated trees appear to match cycles of population expansion and decline. Vegetation and land use change is also influenced by other factors, such as climate change. Investigating the early impacts of anthropogenic activities (e.g. woodcutting, animal herding, the use of fire and agriculture) is key to understanding how societies have modified the environment since the mid–late Holocene, despite the capacity of ecological systems to absorb recurrent disturbances. The results of this study suggest that shifting human population dynamics played an important role in shaping land cover in central and southern Anatolia

SISALv2: A comprehensive speleothem isotope database with multiple age-depth models

Characterizing the temporal uncertainty in palaeoclimate records is crucial for analysing past climate change, correlating climate events between records, assessing climate periodicities, identifying potential triggers and evaluating climate model simulations. The first global compilation of speleothem isotope records by the SISAL (Speleothem Isotope Synthesis and Analysis) working group showed that age model uncertainties are not systematically reported in the published literature, and these are only available for a limited number of records (ca. 15 %, n = 107=691). To improve the usefulness of the SISAL database, we have (i) improved the database's spatiooral coverage and (ii) created new chronologies using seven different approaches for age depth modelling. We have applied these alternative chronologies to the records from the first version of the SISAL database (SISALv1) and to new records compiled since the release of SISALv1. This paper documents the necessary changes in the structure of the SISAL database to accommodate the inclusion of the new age models and their uncertainties as well as the expansion of the database to include new records and the qualitycontrol measures applied. This paper also documents the age depth model approaches used to calculate the new chronologies. The updated version of the SISAL database (SISALv2) contains isotopic data from 691 speleothem records from 294 cave sites and new age depth models, including age depth temporal uncertainties for 512 speleothems. SISALv2 is available at https://doi.org/10.17864/1947.256 (Comas-Bru et al., 2020a). © 2020 Author(s)

Evaluating model outputs using integrated global speleothem records of climate change since the last glacial

Although quantitative isotopic data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to use the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally-distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotopic values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model’s ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotopic data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on 18O values, the optimum period for the modern observational baseline, and the selection of an appropriate time-window for creating means of the isotope data for palaeo time slices

The SISAL database: a global resource to document oxygen and carbon isotope records from speleothems

Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide “out-of-sample” evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ 18O, δ 13C) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at https://doi.org/10.17864/1947.147

Climate – Tree-Growth Relationships in Central Sweden : An Evaluation of the Palmer Drought Severity Index as a Tool for Reconstructing Moisture Variability

A tree-ring width chronology from Scots pine (Pinus sylvestris L.) was constructed from a xeric site in Stockholm to investigate the relationships between climate and tree growth and to reconstruct past moisture variability. The measure of moisture conditions employed here is a self-calibrating Palmer Drought Severity Index (PDSI). The index is derived from temperature, precipitation, and available water capacity of the soil, and assesses the intensity and duration of drought. It is widely used in tree-ring based climate reconstructions, a method which has never before been tested in the Nordic countries. The comparison of the Stockholm tree-ring chronology with monthly temperature and precipitation data from a nearby meteorological station shows that tree growth is reduced by high summer temperatures, whereas high precipitation at the beginning of the growing season favours growth. The comparison with a PDSI calculated from this meteorological data shows that negative PDSI values are associated with narrow rings. Although tree growth in the humid climate of central Sweden is generally not limited by precipitation, the trees sampled for this study prove to be sensitive to changes in water supply. Their rings thus provide a record of past moisture variability and enable the reconstruction of precipitation and drought. The transfer function models for the reconstructions are calibrated using linear regression. A detailed verification of the results using the more than 200-year long meteorological record from Stockholm affirms the good model performance. May–June precipitation sums and the July PDSI could be reconstructed back to 1625. The Palmer Drought Severity Index is found to be a useful tool in a tree-ring based reconstruction of past moisture variability, approximating the fraction of rainfall which is actually available to the tree, by including soil moisture storage, runoff, and the influence of temperature on evapotranspiration. It cannot completely account for the combined temperature and precipitation forcing of tree growth, and the use of the index does not improve the reconstruction compared to using precipitation alone. However, a reconstruction of both precipitation and the PDSI is possible when selecting an adequate sample site

Variabilité climatique dans le sud-ouest de la France au cours des derniers 2000 ans : Calibration des proxies et reconstruction de sécheresses basée sur les isotopes stables des spéléothèmes et des cernes d’arbre

The characterization of natural climate variability is important in order to understand the climate response to natural forcings and to identify anthropogenic influences. The aim of this thesis is to reconstruct climate changes in the southwest of France, a region which is characterised by recurrent drought periods, where high resolution proxy records of the last millennia were lacking.The reconstruction is based on multiple proxies from two continental archives: speleothems and tree rings. Their combination can make use of the strengths of each archive while compensating their weaknesses. There are two principal objectives: first, to gain a better understanding of the climatic and non-climatic influences on each proxy; and second, to reconstruct drought periods in the past.The oxygen isotopic composition (δ18O) of speleothem fluid inclusions and tree ring cellulose is controlled to a large extent by the δ18O of precipitation, which can serve as a tracer of the atmospheric circulation. In order to interpret these proxies in terms of climate, it is necessary to understand how the climate signal becomes recorded in the proxy, and which processes modify the original signal during the formation of the archive.Measurements of δ18O in precipitation, cave drip water, and fluid inclusions in modern speleothem samples from Villars Cave demonstrated that the isotopic composition of cave drip water corresponds to the pluri-annual average precipitation. The speleothem fluid inclusions, in turn, preserve the isotopic composition of the drip water. Based on this calibration, it is possible to reconstruct drip water isotope variability using fluid inclusions in a more than 2000 year old stalagmite, which has been dated by laminae counting, as well as U-Th and 14C measurements. Changes in the cave environment, e.g. the vegetation cover, are indicated by other proxies from the same stalagmite (stable isotopes in calcite and trace element concentrations), but these changes do not seem to impact the fluid inclusion δ18O significantly.The isotopic composition of tree ring cellulose from Quercus spp. in the study area is strongly influenced by climate conditions during the summer. However, non-climatic influences on the isotopic composition of cellulose are identified. They are linked to the age of the trees and to site hydrology, and must be accounted for in the sampling and analytical procedures. Crossdated cores from living trees and timber wood in historic buildings near Angoulême are used to build an annually resolved chronology of cellulose δ18O. Significant correlations with meteorological data enable a calibration and a reconstruction of drought periods since 1360 AD.Lastly, this thesis explores a novel approach of integrating oxygen isotope records from speleothem fluid inclusions and tree ring cellulose from closely located sites to reconstruct both high- and low-frequency variability of droughts in the past.La caractérisation de la variabilité naturelle du climat est importante pour comprendre la réponse de celui-ci aux forçages naturels et pour identifier les influences anthropiques. L'objectif de cette thèse est de reconstituer les changements climatiques dans le sud-ouest de la France, une région qui se caractérise par des périodes de sécheresse récurrentes, où les reconstitutions millénaires basées sur des « proxies » à haute résolution font défaut.La reconstruction est basée sur plusieurs proxies dans deux archives continentales : les spéléothèmes et les cernes d'arbre. Leur combinaison permet de tirer avantage de leurs points forts tout en compensant leurs faiblesses. Il y a deux objectifs principaux : d’une part de mieux comprendre les influences climatiques et non climatiques qui s’exercent sur chaque proxy ; et, d'autre part, de reconstituer les périodes de sécheresse dans le passé.La composition isotopique en oxygène (δ18O) des inclusions fluides des spéléothèmes et de la cellulose des cernes d’arbre est contrôlée dans une large mesure par le δ18O des précipitations, qui peut servir de traceur de la circulation atmosphérique. Pour interpréter ces proxies en termes de climat, il est nécessaire de comprendre comment le signal climatique est enregistré dans le proxy, et quels processus modifient ce signal lors de la formation de l'archive.Les mesures de δ18O dans les précipitations, l'eau d’infiltration, et les inclusions fluides dans des échantillons de spéléothèmes modernes de la Grotte de Villars démontrent que la composition isotopique de l'eau d’infiltration dans la grotte correspond à une moyenne pluriannuelle des précipitations, et que les inclusions fluides des spéléothèmes préservent la composition isotopique de cette eau d’infiltration. Sur la base de cette calibration, il est possible de reconstituer les variations de la composition isotopique de l'eau d’infiltration à partir de celles des inclusions fluides d’une stalagmite vieille de plus de 2000 ans, datée par comptage de lamines, et par des mesures U-Th et 14C. Des changements dans l'environnement de la grotte, par exemple du couvert végétal, sont indiqués par d'autres proxies de la même stalagmite (les isotopes stables dans la calcite et les concentrations d'éléments traces), mais ces changements n'ont pas d'incidence significative sur le δ18O des inclusions fluides.La composition isotopique de la cellulose des cernes d'arbre de Quercus spp. dans la zone d'étude est fortement influencée par les conditions climatiques estivales. Cependant, des influences non-climatiques sur la composition isotopique de la cellulose sont identifiées ; elles sont liées à l'âge de l'arbre et à l'hydrologie du site, et doivent être prises en compte dans l'échantillonnage et dans les procédures analytiques. Grace à l’inter-datation de carottes d’arbres vivants et de poutre de bâtiments historiques des environs d'Angoulême, une chronologie de δ18O dans la cellulose à résolution annuelle a été établie. Des corrélations significatives avec les données météorologiques permettent une calibration des données isotopiques et une reconstruction des périodes de sécheresse depuis 1360 AD.Enfin, cette thèse explore une nouvelle approche intégrant les enregistrements isotopiques de l'oxygène dans les inclusions fluides des spéléothèmes et dans la cellulose des cernes d’arbre de sites proches pour reconstruire la variabilité à la fois haute et basse fréquence des sécheresses dans le passé