Fidelity of speleothem climate proxies: an inter-annual calibration against the instrumental record in Gibraltar and prospects for climate hindcasting

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Speleothem (a name given to stalagtites, stalagmites and others types of calcium carbonate decorations found in caves) form by the action of rain water percolating through soil and bedrock into the cave environment and these deposits grow undisturbed for many thousands of years. Stalagmites grow as successive layers in response to seasonal changes in rainfall amount and can be accurately dated. Isotopes of oxygen, 18O and 16O, in stalagmite carbonate are inherited from the rainwater and the ratio 18O/16O in rainwater is an important indicator of the climatic environment, providing information on temperature and atmospheric circulation. Oxygen isotope variations record in speleothem are easily measured, and when combined with accurate dates provide unique information on environmental change going back many tens of thousands of years. Speleothem oxygen isotope records have great potential in reconstructing past climates yet surprisingly, the climate record deduced from speloethem has never been directly compared with real weather records to see how well they really work The objective of this project is to compare speleothem climate indicators with the longest combined meteorological and rainfall isotope records currently available: rainfall and temperature observations have been recorded in Gibraltar since 1791 and the oxygen isotope compositions of monthly rain isotope have been measured since 1961. A pilot study of active stalagmite formation in Gibraltar shows clear seasonal oscillations in isotopes and trace elements which allow the years to be counted and the climate signal directly with the historical weather record with great accuracy. The results of this test have will enable the natural cycles in the past climate of Gibraltar to be constructed with more confidence and will help create more accurate weather forecasts in the future.