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

Highlights: • The key CO₂ reservoir in karstic systems is CO₂-rich ground air. • Ventilation mixes external atmosphere with ground air to form cave air. • Temperature and wind drive seasonal regimes and rapid synoptic changes in cave air CO₂. • Ground air dynamics have implications for climatic interpretation of δ¹³C in speleothems. 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.