Although quantitative isotope 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 using the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally distributed speleothem d18O 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 the process of 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 isotope 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 isotope data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on d18O 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.
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Acknowledgements. SISAL (Speleothem Isotopes Synthesis and Analysis) is a working group of the Past Global Changes (PAGES) programme. We thank PAGES for their support for this activity. We thank the World Karst Aquifer Mapping project (WOKAM) team for providing us with the karst map presented in Fig. 1a. The authors would like to thank the following data contributors: Petra Bajo, Dominique Blamart, Russell Drysdale, Frank Mc-Dermott and Jean Riotte. Laia Comas-Bru and Sandy P. Harri- son acknowledge support from the ERC-funded project GC2.0 (Global Change 2.0: Unlocking the past for a clearer future, grant no. 694481). Sandy P. Harrison also acknowledges support from the JPI-Belmont project “PAleao-Constraints on Monsoon Evolution and Dynamics (PACMEDY)” through the UK Natural Environmental Research Council (NERC). Laia Comas-Bru also acknowledges support from the Geological Survey Ireland (Short Call 2017; grant number 2017-SC-056) and the Royal Irish Academy’s Charlemont Scholar award 2018. Cristina Veiga-Pires acknowledges funding from the Portuguese Science Foundation (FCT) through the CIMA research centre project UID/MAR/00350/2013. Kira Rehfeld was supported by Deutsche Forschungsgemeinschaft (DFG) grant no. RE3994/2-1.
Financial support. Financial support for SISAL activities that have lead to this research has been provided by the Past Global Changes (PAGES) programme; the European Geosciences Union (grant no. W2017/413); the Irish Centre for Research in Applied Geosciences (iCRAG); the European Association of Geochemistry (Early Career Ambassadors program 2017); the Quaternary Research Association UK; the Navarino Environmental Observatory, Stockholm University; University College Dublin (grant no. SF1428), Savillex (UK); John Cantle; Ibn Zohr University, Morocco; the University of Reading; the European Research Council (grant no. 694481); the Natural Environment Research Council (JPI-Belmont project “PAleao-Constraints on Monsoon Evolution and Dynamics (PACMEDY)”); the Geological Survey Ireland (grant no. 2017-SC-056); the Royal Irish Academy (Charlemont Scholar award 2018); the Portuguese Science Foundation (grant no. UID/MAR/00350/2013); and the Deutsche Forschungsgemein-schaft (grant no. RE3994/2-1).
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