DOI: 10.25921/y73y-7990
title: NOAA/WDS Paleoclimatology - New St Michael's Cave Geochemical Data from 2 to -53 Cal Yr BP

description: 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 δ13C 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 pCO2, whereas the dark microsparitic calcite, characterised by elevated δ13C and δ18O values grows under low ‘summer’ pCO2 conditions. Drip water δ13CDIC reaches a minimum in March–April, at which time the attenuated δ18O 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 δ18O 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 δ18O drip water representing winter precipitation for each year from 1951–2004. These data show an encouraging level of correspondence (r2 = 0.47) with the δ18O of rainfall falling each year between October and March and on a decadal scale the δ18O of reconstructed winter drip water mirrors secular change in mean winter temperatures.
description: Data contributed as part of SISAL v3 Global Speleothem Isotope and Trace Element Database (https://doi.org/10.25921/jajq-qg60). Notes from the SISAL database: Vegetation: Upper Rock nature reserve is maquis/mattral (olive etc.); in the past was woodland. \\ Gib04a (SISAL entity_id = 182): The age model was based on counting back annual cycles in d13C which were known to be forced by reversing seasonal ventilation from cave monitoring. Annual cycles in d13C mapped onto the paired laminae. Minima in d13C represents calcite deposited in late winter (April). The age model was validated by the position of the radiocarbon bomb peak and a high correlation between the d18O of net winter calcite and the corresponding weighted mean of monthly rainfall from the GNIP record. Uncertainty of the age model varies from 0 to c. 1980 thereafter +/-1 year for the older part of the record.