Fidelity of climate recording by modern and ancient stalagmites:
Insight from cave monitoring in Gibraltar

ABSTRACT

Speleothems record fast, direct responses to local climate and provide rich archives of climate change that are dateable at very high precision. Lower resolution multi-millenial records are often interpreted in terms of relatively simple factors such as monsoon intensity in SE Asia, but over shorter time scales the d18O climate relations inevitably become increasingly complex, requiring a greater understanding of local processes that control speleothem growth. Cave sites on the Gibraltar peninsula, which has one of the longest instrumental records in Europe, provide a unique laboratory to examine speleothem climate recording in caves distributed through a 400m karst profile. Our early work in New St Michaels Cave (275m above sea level) focussed on the modern recording process by monitoring the soil groundwater cave speleothem system though regular sampling and continuous data logging techniques. Working in close collaboration with the local caving group we have tracked daily, monthly and seasonal variations over four annual cycles for water chemistry and isotopes, cave and soil air compositions, cave and external temperatures, cave ventilation mode, drip rates, amounts of rainfall and its daily isotopic composition. These data provides compelling evidence that the fabric, isotope and trace element cycles preserved in modern speleothem are annual features, and linked to specific seasons in the calendar year and cave ventilation, rather than water excess, is the main factor in controlling calcite growth. Gibraltar has undergone significant neotectonic uplift forming staircased marine terraces. Ragged Staff cave is one of the youngest known cave systems in the rock and is located at the top of the saturated zone, close to sea level, and contain deep lakes that pass through a halocline. Older Gibraltar cave chambers have similar architecture and have probably initially formed within the water table near sea level by groundwater mi xing corrosion processes and their present altitudes are derived as a result of sea level change and tectonic uplift. It is therefore likely that the oldest speleothem records, now located at higher elevations on the rock (e.g. in New St Michaels Cave), may have formed when the cave was at a different altitude under hydrological and ventilation schemes different to those seen today. Gibraltar provides an environment where we can better understand speleothem climate recording over many glacial cycles and current work is focussed on dating cave formation, and on the evolution of cave environments by monitoring of cave systems using networked instruments and multichannel CO2 logging. These data and observations will underpin the interpretation of very long Gibraltar speleothem proxy records and provide insight into the issues that may affect construction of climate records elsewhere where tectonics have controlled cave evolution.