External controls on CO2 in Gibraltar cave air and ground air: Implications for interpretation of δ13C in speleothems

Abstract

The principles of cave ventilation and ground air advection are outlined. CO2 concentrations were monitored every 2–4 h from 2006 to 2012 at twelve locations in two Gibraltar caves, New St. Michael's (240–275 m altitude) and Ragged Staff (0–70 m), providing detailed records of spatial distribution and temporal changes responding to transient meteorological conditions and seasonal cycles. Cave atmospheres are dynamic mixtures of external air and CO2-rich ground air containing soil-respired CO2 augmented by CO2 derived from oxidation of organic matter washed down into the vadose fracture porosity. The dominant feature of the cave air records is seasonal alternation between high and low CO2 states caused by reversals in ventilation and ground air advection into or away from the caves. The high and low pCO2 states are tightly coupled between the two caves in an antiphase relationship, with rapid switching in response to temperature-controlled density differences between external air and isothermal ground air at mean annual surface temperature (18 °C). The diurnal temperature cycle modulates this pattern, producing four ventilation-advection modes. Summer and winter modes occur when diurnal temperatures are respectively entirely above or below the core temperature of the Rock. During transitional modes the core temperature lies within the diurnal range and pCO2 fluctuates rapidly as the flow of low-CO2 external air through cave entrances alters in response. Strong easterly winds reduce CO2 concentrations in ground air entering the caves and progressively lower cave air CO2. The effects of the CO2 regime on δ13C in speleothem are restricted to variations of 2–3‰, suggesting that larger changes of up to 6‰ across major climatic transitions probably reflect changes in production, transport and oxidation rate of organic matter in the vadose zone. Direct observations of these processes would improve future understanding of the climatic significance of δ13C records in speleothems.