Cave atmosphere; a guide to calcification and a methane sink.

Studies of cave environments and speleothem growth are an important step towards quantitative speleothem palaeoclimate interpretation. Net accumulation of CaCO3 (speleothem growth) requires a perturbation to Gas-Aqueous-Solid equilibrium conditions in the cave environment (Aq. chem., T, P, pCO2). The largest equilibrium change in a ventilated cave environment causing speleothem growth is fluctuating pCO2 as a response to the cave air exchange, driven by external temperature. An intense 3-week field campaign in May 2008 (winter) using an FTIR spectrometer continuously measured (5 min) trace gases (CO2, CH4, N2O) H2O and δ13CCO2. Simultaneous drip-water pH, air flow, temperature, pressure, and relative humidity was logged by sensors in the cave together with external rainfall, temperature, pressure, and relative humidity. Drip water was sampled twice daily, coinciding with CO2 maxima and minima, for dissolved inorganic carbonate DIC, δ13CDIC, dissolved organic carbonate DOC, δ13CDOC, alkalinity, anions, and cations. Further spot samples were taken for drip-water stable isotopes, 14CDIC, and 3H. Low pCO2 in the morning cave air causes rapid speleothem growth with CO2 exsolved to the cave atmosphere lowering drip-water pH. pCO2 increases to an evening maxima and slows speleothem growth before early morning T induced ventilation decreases pCO2. δ13CCO2 has an antithetic relationship with CO2, with low pCO2 morning air the highest δ13CCO2 at -8 ‰ PDB. A Keeling analysis of end-member component mixing reveals the proportion of external air drawn into the cave and CO2 produced from speleothem formation through the diurnal cycle. Methane concentration in cave air also cycles through a diurnal pattern, negatively correlated with CO2. The methane concentration ranges from normal atmospheric 1700 ppb to