Cave Monitoring and Calibration of a δ18O – Climate Transfer Function in Gibraltar

Cave sites on the Gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with long instrumental records extending back to 1792. An actively growing speleothem (Gib04a) sampled from New St. Michaels Cave in 2004 possesses exceptionally well defined δ¹³C and δ¹⁸O cycles that correlate with trace elements and paired fabrics. Monthly monitoring of microclimate, pCO₂, drip rate and drip water chemistry provides compelling evidence that the fabric, isotope and trace element cycles preserved in the speleothem are annual features, and linked to specific seasons in the calendar year, and that cave ventilation, rather than water excess, is the main factor in controlling calcite precipitation. The cause of δ¹⁸O cyclicity in speleothem calcite is a result of both annual recharge and evaporation effects but monitoring shows that drip water with lowest δ¹⁸O arriving at the Gib04a drip site in late spring is the best proxy representing the isotopic composition of the new season's winter rain. An age model based on δ¹³C cycle counting and the position of the ¹⁴C bomb carbon spike yields a precisely dated winter oxygen isotope proxy of cave seepage water for comparison with the GNIP and instrumental climate record for Gibraltar from 1951-2004. The δ¹⁸O of reconstructed winter drip water shows an encouraging level of correspondence (r²=0.47) with the δ¹⁸O of rainfall falling each year between October and March. Work in progress is examining the inter-annual variation in the δ¹⁸O of Gibraltar rainfall using modern regression methods on annual, monthly and daily resolution data (collected since 2005), to better understand the climatic significance of the winter rainfall δ¹⁸O record in Gibraltar speleothem records.