A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem: Reconstructed drip water and relationship to local precipitation

Abstract

The response of a climate proxy against measured temperature, rainfall and atmospheric circulation patterns at sub-annual resolution is the ultimate test of proxy fidelity but very few data exist showing the level of correspondence between speleothem climate proxies and the instrumental climate record. Cave sites on the Gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with the longest known available precipitation isotopes and instrumental records. An actively growing speleothem sampled from New St. Michaels Cave in 2004 is composed of paired laminae consisting of light columnar calcite and a darker microsparitic calcite. Stable isotope analysis of samples micromilled in 100 μm steps at the equivalent of bi-monthly intervals reveals fabric-correlated annual cycles in carbon isotopes, oxygen isotopes and trace elements responding to seasonal changes in cave microclimate, hydrology and ventilation patterns. Calcite δ 13C values reach a minimum in the light columnar fabric and evidence from trace element behaviour and cave monitoring indicates that this grows under cave ‘winter’ conditions of highest pCO 2, whereas the dark microsparitic calcite, characterised by elevated δ 13C and δ 18O values grows under low ‘summer’ pCO 2 conditions. Drip water δ 13C DIC reaches a minimum in March–April, at which time the attenuated δ 18O signal becomes most representative of winter precipitation. An age model based on 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. The δ 18O characteristics of calcite deposited from drip water representing winter precipitation for each year can be derived from the seasonally resolved record and allows reconstruction of the δ 18O drip water representing winter precipitation for each year from 1951–2004. These data show an encouraging level of correspondence ( r 2 = 0.47) with the δ 18O of rainfall falling each year between October and March and on a decadal scale the δ 18O of reconstructed winter drip water mirrors secular change in mean winter temperatures.